Turbulent kinetic energy equation and free mixing

NASA Technical Reports Server (NTRS)

Morel, T.; Torda, T. P.; Bradshaw, P.

1973-01-01

Calculation of free shear flows was carried out to investigate the usefulness of several concepts which were previously successfully applied to wall flows. The method belongs to the class of differential approaches. The turbulence is taken into account by the introduction of one additional partial differential equation, the transport equation for the turbulent shear stress. The structure of turbulence is modeled after Bradshaw et al. This model was used successfully in boundary layers and its applicability to other flows is demonstrated. The work reported differs substantially from that of an earlier attempt to use this approach for calculation of free flows. The most important difference is that the region around the center line is treated by invoking the interaction hypothesis (concerning the structure of turbulence in the regions separated by the velocity extrema). The compressibility effects on shear layer spreading at low and moderate Mach numbers were investigated. In the absence of detailed experiments in free flows, the evidence from boundary layers that at low Mach numbers the structure of turbulence is unaffected by the compressibility was relied on. The present model was tested over a range of self-preserving and developing flows including pressure gradients using identical empirical input. The dependence of the structure of turbulence on the spreading rate of the shear layer was established.

Bandwidth turbulence control based on flow community structure in the Internet

NASA Astrophysics Data System (ADS)

Wu, Xiaoyu; Gu, Rentao; Ji, Yuefeng

2016-10-01

Bursty flows vary rapidly in short period of time, and cause fierce bandwidth turbulence in the Internet. In this letter, we model the flow bandwidth turbulence process by constructing a flow interaction network (FIN network), with nodes representing flows and edges denoting bandwidth interactions among them. To restrain the bandwidth turbulence in FIN networks, an immune control strategy based on flow community structure is proposed. Flows in community boundary positions are immunized to cut off the inter-community turbulence spreading. By applying this control strategy in the first- and the second-level flow communities separately, 97.2% flows can effectively avoid bandwidth variations by immunizing 21% flows, and the average bandwidth variation degree reaches near zero. To achieve a similar result, about 70%-90% immune flows are needed with targeted control strategy based on flow degrees and random control strategy. Moreover, simulation results showed that the control effect of the proposed strategy improves significantly if the immune flow number is relatively smaller in each control step.

Effects of polymers on the spatial structure of turbulent flows

NASA Astrophysics Data System (ADS)

Sinhuber, Michael; Ballouz, Joseph G.; Ouellette, Nicholas T.

2017-11-01

It is well known that the addition of minor amounts of polymers to a turbulent water flow can significantly change its properties. One of the most prominent effects is the observed drastic reduction of drag in wall-bounded flows that is utilized in many engineering applications. Much of the research on polymers in turbulence has focused on their influence on the turbulent energy cascade and their interaction with the energy transfer processes. Much less investigated are their effects on the spatial structure of turbulent flows. In a classical von-Kárman swirling flow setup, we used Lagrangian particle tracking to obtain three-dimensional particle trajectories, velocities, and accelerations and find that polymers have a significant effect on Lagrangian measures of the turbulence structure such as radial distribution functions and the curvature of particle trajectories. We find that not only do the statistical distributions change, but also that polymers appear to affect the spatial statistics well beyond the size of the polymers themselves.

A new energy transfer model for turbulent free shear flow

NASA Technical Reports Server (NTRS)

Liou, William W.-W.

1992-01-01

A new model for the energy transfer mechanism in the large-scale turbulent kinetic energy equation is proposed. An estimate of the characteristic length scale of the energy containing large structures is obtained from the wavelength associated with the structures predicted by a weakly nonlinear analysis for turbulent free shear flows. With the inclusion of the proposed energy transfer model, the weakly nonlinear wave models for the turbulent large-scale structures are self-contained and are likely to be independent flow geometries. The model is tested against a plane mixing layer. Reasonably good agreement is achieved. Finally, it is shown by using the Liapunov function method, the balance between the production and the drainage of the kinetic energy of the turbulent large-scale structures is asymptotically stable as their amplitude saturates. The saturation of the wave amplitude provides an alternative indicator for flow self-similarity.

Turbulent structures in wall-bounded shear flows observed via three-dimensional numerical simulators. [using the Illiac 4 computer

NASA Technical Reports Server (NTRS)

Leonard, A.

1980-01-01

Three recent simulations of tubulent shear flow bounded by a wall using the Illiac computer are reported. These are: (1) vibrating-ribbon experiments; (2) study of the evolution of a spot-like disturbance in a laminar boundary layer; and (3) investigation of turbulent channel flow. A number of persistent flow structures were observed, including streamwise and vertical vorticity distributions near the wall, low-speed and high-speed streaks, and local regions of intense vertical velocity. The role of these structures in, for example, the growth or maintenance of turbulence is discussed. The problem of representing the large range of turbulent scales in a computer simulation is also discussed.

Experimental Investigation of the Near Wall Flow Structure of a Low Reynolds Number 3-D Turbulent Boundary Layer

NASA Technical Reports Server (NTRS)

Fleming, J. L.; Simpson, R. L.

1997-01-01

Laser Doppler velocimetry (LDV) measurements and hydrogen bubble flow visualization techniques were used to examine the near-wall flow structure of 2D and 3D turbulent boundary layers (TBLs) over a range of low Reynolds numbers. The goals of this research were (1) an increased understanding of the flow physics in the near wall region of turbulent boundary layers,(2) to observe and quantify differences between 2D and 3D TBL flow structures, and (3) to document Reynolds number effects for 3D TBLs. The LDV data have provided results detailing the turbulence structure of the 2D and 3D TBLs. These results include mean Reynolds stress distributions, flow skewing results, and U and V spectra. Effects of Reynolds number for the 3D flow were also examined. Comparison to results with the same 3D flow geometry but at a significantly higher Reynolds number provided unique insight into the structure of 3D TBLs. While the 3D mean and fluctuating velocities were found to be highly dependent on Reynolds number, a previously defined shear stress parameter was discovered to be invariant with Reynolds number. The hydrogen bubble technique was used as a flow visualization tool to examine the near-wall flow structure of 2D and 3D TBLs. Both the quantitative and qualitative results displayed larger turbulent fluctuations with more highly concentrated vorticity regions for the 2D flow.

Toward a structural understanding of turbulent drag reduction: nonlinear coherent states in viscoelastic shear flows.

PubMed

Stone, Philip A; Waleffe, Fabian; Graham, Michael D

2002-11-11

Nontrivial steady flows have recently been found that capture the main structures of the turbulent buffer layer. We study the effects of polymer addition on these "exact coherent states" (ECS) in plane Couette flow. Despite the simplicity of the ECS flows, these effects closely mirror those observed experimentally: Structures shift to larger length scales, wall-normal fluctuations are suppressed while streamwise ones are enhanced, and drag is reduced. The mechanism underlying these effects is elucidated. These results suggest that the ECS are closely related to buffer layer turbulence.

Structure of turbulent shear flows

NASA Technical Reports Server (NTRS)

Hussain, A. K. M. F.; Jeong, J.; Kim, J.

1987-01-01

Activities are summarized for each of the three topics discussed: eduction of coherent structures; measurement of propagation velocities of perturbation in turbulent flows; and direct evaluation of the Taylor hypothesis.

Prediction of the structure of fuel sprays in gas turbine combustors

NASA Technical Reports Server (NTRS)

Shuen, J. S.

1985-01-01

The structure of fuel sprays in a combustion chamber is theoretically investigated using computer models of current interest. Three representative spray models are considered: (1) a locally homogeneous flow (LHF) model, which assumes infinitely fast interphase transport rates; (2) a deterministic separated flow (DSF) model, which considers finite rates of interphase transport but ignores effects of droplet/turbulence interactions; and (3) a stochastic separated flow (SSF) model, which considers droplet/turbulence interactions using random sampling for turbulence properties in conjunction with random-walk computations for droplet motion and transport. Two flow conditions are studied to investigate the influence of swirl on droplet life histories and the effects of droplet/turbulence interactions on flow properties. Comparison of computed results with the experimental data show that general features of the flow structure can be predicted with reasonable accuracy using the two separated flow models. In contrast, the LHF model overpredicts the rate of development of the flow. While the SSF model provides better agreement with measurements than the DSF model, definitive evaluation of the significance of droplet/turbulence interaction is not achieved due to uncertainties in the spray initial conditions.

Three-dimensional structure of dominant instabilities in turbulent flow over smooth and rough boundaries

NASA Astrophysics Data System (ADS)

Grass, A. J.; Stuart, R. J.; Mansour-Tehrani, M.

1991-01-01

The current status of knowledge regarding coherent vortical structures in turbulent boundary layers and their role in turbulence generation are reviewed. The investigations reported in the study concentrate attention on rough-wall flows prevailing in the geophysical environment and include an experiment determining the three-dimensional form of the turbulence structures linked to the ejection and inrush events observed over rough walls and an experiment concerned with measuring the actual spanwise scale of the near-wall structures for boundary conditions ranging from hydrodynamically smooth to fully rough. It is demonstrated that horseshoe vortical structures are present and play an important role in rough-wall flows and they increase in scale with increasing wall distance, while a dominant spanwise wavelength occurs in the instantaneous cross-flow distribution of streamwise velocity close to the rough wall.

Eddy, drift wave and zonal flow dynamics in a linear magnetized plasma

PubMed Central

Arakawa, H.; Inagaki, S.; Sasaki, M.; Kosuga, Y.; Kobayashi, T.; Kasuya, N.; Nagashima, Y.; Yamada, T.; Lesur, M.; Fujisawa, A.; Itoh, K.; Itoh, S.-I.

2016-01-01

Turbulence and its structure formation are universal in neutral fluids and in plasmas. Turbulence annihilates global structures but can organize flows and eddies. The mutual-interactions between flow and the eddy give basic insights into the understanding of non-equilibrium and nonlinear interaction by turbulence. In fusion plasma, clarifying structure formation by Drift-wave turbulence, driven by density gradients in magnetized plasma, is an important issue. Here, a new mutual-interaction among eddy, drift wave and flow in magnetized plasma is discovered. A two-dimensional solitary eddy, which is a perturbation with circumnavigating motion localized radially and azimuthally, is transiently organized in a drift wave – zonal flow (azimuthally symmetric band-like shear flows) system. The excitation of the eddy is synchronized with zonal perturbation. The organization of the eddy has substantial impact on the acceleration of zonal flow. PMID:27628894

High-speed holocinematographic velocimeter for studying turbulent flow control physics

NASA Technical Reports Server (NTRS)

Weinstein, L. M.; Beeler, G. B.; Lindemann, A. M.

1985-01-01

Use of a dual view, high speed, holographic movie technique is examined for studying turbulent flow control physics. This approach, which eliminates some of the limitations of previous holographic techniques, is termed a holocinematographic velocimeter (HCV). The data from this system can be used to check theoretical turbulence modeling and numerical simulations, visualize and measure coherent structures in 'non-simple' turbulent flows, and examine the mechanisms operative in various turbulent control/drag reduction concepts. This system shows promise for giving the most complete experimental characterization of turbulent flows yet available.

Coherent structures in wall-bounded turbulence.

PubMed

Dennis, David J C

2015-01-01

The inherent difficulty of understanding turbulence has led to researchers attacking the topic in many different ways over the years of turbulence research. Some approaches have been more successful than others, but most only deal with part of the problem. One approach that has seen reasonable success (or at least popularity) is that of attempting to deconstruct the complex and disorganised turbulent flow field into to a set of motions that are in some way organised. These motions are generally called "coherent structures". There are several strands to this approach, from identifying the coherent structures within the flow, defining their characteristics, explaining how they are created, sustained and destroyed, to utilising their features to model the turbulent flow. This review considers research on coherent structures in wall-bounded turbulent flows: a class of flow which is extremely interesting to many scientists (mainly, but not exclusively, physicists and engineers) due to their prevalence in nature, industry and everyday life. This area has seen a lot of activity, particularly in recent years, much of which has been driven by advances in experimental and computational techniques. However, several ideas, developed many years ago based on flow visualisation and intuition, are still both informative and relevant. Indeed, much of the more recent research is firmly indebted to some of the early pioneers of the coherent structures approach. Therefore, in this review, selected historical research is discussed along with the more contemporary advances in an attempt to provide the reader with a good overview of how the field has developed and to highlight the perspicacity of some of the early researchers, as well as providing an overview of our current understanding of the role of coherent structures in wall-bounded turbulent flows.

Preferential particle concentration in wall-bounded turbulence with zero skin friction

NASA Astrophysics Data System (ADS)

Yang, Kun; Zhao, Lihao; Andersson, Helge I.

2017-11-01

Inertial particles dispersed in turbulence distribute themselves unevenly. Besides their tendency to segregate near walls, they also concentrate preferentially in wall-parallel planes. We explore the latter phenomenon in a tailor-made flow with the view to examine the homogeneity and anisotropy of particle clustering in the absence of mean shear as compared with conventional, i.e., sheared, wall turbulence. Inertial particles with some different Stokes numbers are suspended in a turbulent Couette-Poiseuille flow, in which one of the walls moves such that the shear rate vanishes at that wall. The anisotropies of the velocity and vorticity fluctuations are therefore qualitatively different from those at the opposite non-moving wall, along which quasi-coherent streaky structures prevail, similarly as in turbulent pipe and channel flows. Preferential particle concentration is observed near both walls. The inhomogeneity of the concentration is caused by the strain-vorticity selection mechanism, whereas the anisotropy originates from coherent flow structures. In order to analyse anisotropic clustering, a two-dimensional Shannon entropy method is developed. Streaky particle structures are observed near the stationary wall where the flow field resembles typical wall-turbulence, whereas particle clusters near the moving friction-free wall are similar to randomly oriented clusters in homogeneous isotropic turbulence, albeit with a modest streamwise inclination. In the absence of mean-shear and near-wall streaks, the observed anisotropy is ascribed to the imprint of large-scale flow structures which reside in the bulk flow and are global in nature.

Studying Turbulence Using Numerical Simulation Databases, 2. Proceedings of the 1988 Summer Program

NASA Technical Reports Server (NTRS)

1988-01-01

The focus of the program was on the use of direct numerical simulations of turbulent flow for study of turbulence physics and modeling. A special interest was placed on turbulent mixing layers. The required data for these investigations were generated from four newly developed codes for simulation of time and spatially developing incompressible and compressible mixing layers. Also of interest were the structure of wall bounded turbulent and transitional flows, evaluation of diagnostic techniques for detection of organized motions, energy transfer in isotropic turbulence, optical propagation through turbulent media, and detailed analysis of the interaction of vortical structures.

Experimental study of turbulent structures over hairy poro-elastic surfaces

NASA Astrophysics Data System (ADS)

Couliou, Marie; Hansson, Jonas; van der Wijngaart, Wouter; Lundell, Fredrik; Bagheri, Shervin

2016-11-01

Flows over slender, deformable and dense structures are ubiquitous in both nature and technological applications, ranging from the atmospheric flow over trees to the flow over the over the skin of organisms. In order to create a fundamental understanding of how poro-elatic surface can be used for flow control purposes, our work focuses on the behaviour of wall-bounded turbulent flows over fibrous poro-elastic surfaces. We fabricate the coatings using Off-Stoichiometry-Thiolene-Epoxy (OSTE+) polymers and multidirectional UV-lithography which enables us to design arrays of flexible pillars with various geometrical parameters (aspect ratio, pitch, inclinaison, etc.). We assess the effects of these coatings on an overlying low-Reynolds number turbulent flow using a water-table facility and PIV measurements. In particular, we focus on the modification of near wall turbulent structures in both space and time due to the presence of the poro-elastic coatings.

Modification in drag of turbulent boundary layers resulting from manipulation of large-scale structures

NASA Technical Reports Server (NTRS)

Corke, T. C.; Guezennec, Y.; Nagib, H. M.

1981-01-01

The effects of placing a parallel-plate turbulence manipulator in a boundary layer are documented through flow visualization and hot wire measurements. The boundary layer manipulator was designed to manage the large scale structures of turbulence leading to a reduction in surface drag. The differences in the turbulent structure of the boundary layer are summarized to demonstrate differences in various flow properties. The manipulator inhibited the intermittent large scale structure of the turbulent boundary layer for at least 70 boundary layer thicknesses downstream. With the removal of the large scale, the streamwise turbulence intensity levels near the wall were reduced. The downstream distribution of the skin friction was also altered by the introduction of the manipulator.

Observations, theoretical ideas and modeling of turbulent flows: Past, present and future

NASA Technical Reports Server (NTRS)

Chapman, G. T.; Tobak, M.

1985-01-01

Turbulence was analyzed in a historical context featuring the interactions between observations, theoretical ideas, and modeling within three successive movements. These are identified as predominantly statistical, structural and deterministic. The statistical movement is criticized for its failure to deal with the structural elements observed in turbulent flows. The structural movement is criticized for its failure to embody observed structural elements within a formal theory. The deterministic movement is described as having the potential of overcoming these deficiencies by allowing structural elements to exhibit chaotic behavior that is nevertheless embodied within a theory. Four major ideas of this movement are described: bifurcation theory, strange attractors, fractals, and the renormalization group. A framework for the future study of turbulent flows is proposed, based on the premises of the deterministic movement.

Rough-pipe flows and the existence of fully developed turbulence

NASA Astrophysics Data System (ADS)

Gioia, G.; Chakraborty, Pinaki; Bombardelli, Fabián A.

2006-03-01

It is widely believed that at high Reynolds number (Re) all turbulent flows approach a limiting state of "fully developed turbulence" in which the statistics of the velocity fluctuations are independent of Re. Nevertheless, direct measurements of the velocity fluctuations have failed to yield firm empirical evidence that even the second-order structure function becomes independent of Re at high Re, let alone structure functions of higher order. Here we relate the friction coefficient (f) of rough-pipe flows to the second-order structure function. Then we show that in light of experimental measurements of f our results yield unequivocal evidence that the second-order structure function becomes independent of Re at high Re, compatible with the existence of fully developed turbulence.

Turbulent Flow and Sand Dune Dynamics: Identifying Controls on Aeolian Sediment Transport

NASA Astrophysics Data System (ADS)

Weaver, C. M.; Wiggs, G.

2007-12-01

Sediment transport models are founded on cubic power relationships between the transport rate and time averaged flow parameters. These models have achieved limited success and recent aeolian and fluvial research has focused on the modelling and measurement of sediment transport by temporally varying flow conditions. Studies have recognised turbulence as a driving force in sediment transport and have highlighted the importance of coherent flow structures in sediment transport systems. However, the exact mechanisms are still unclear. Furthermore, research in the fluvial environment has identified the significance of turbulent structures for bedform morphology and spacing. However, equivalent research in the aeolian domain is absent. This paper reports the findings of research carried out to characterise the importance of turbulent flow parameters in aeolian sediment transport and determine how turbulent energy and turbulent structures change in response to dune morphology. The relative importance of mean and turbulent wind parameters on aeolian sediment flux was examined in the Skeleton Coast, Namibia. Measurements of wind velocity (using sonic anemometers) and sand transport (using grain impact sensors) at a sampling frequency of 10 Hz were made across a flat surface and along transects on a 9 m high barchan dune. Mean wind parameters and mass sand flux were measured using cup anemometers and wedge-shaped sand traps respectively. Vertical profile data from the sonic anemometers were used to compute turbulence and turbulent stress (Reynolds stress; instantaneous horizontal and vertical fluctuations; coherent flow structures) and their relationship with respect to sand transport and evolving dune morphology. On the flat surface time-averaged parameters generally fail to characterise sand transport dynamics, particularly as the averaging interval is reduced. However, horizontal wind speed correlates well with sand transport even with short averaging times. Quadrant analysis revealed that turbulent events with a positive horizontal component, such as sweeps and outward interactions, were responsible for the majority of sand transport. On the dune surface results demonstrate the development and modification of turbulence and sediment flux in key regions: toe, crest and brink. Analysis suggests that these modifications are directly controlled by streamline curvature and flow acceleration. Conflicting models of dune development, morphology and stability arise when based upon either the dynamics of measured turbulent flow or mean flow.

Turbulence study in the vicinity of piano key weir: relevance, instrumentation, parameters and methods

NASA Astrophysics Data System (ADS)

Tiwari, Harinarayan; Sharma, Nayan

2017-05-01

This research paper focuses on the need of turbulence, instruments reliable to capture turbulence, different turbulence parameters and some advance methodology which can decompose various turbulence structures at different levels near hydraulic structures. Small-scale turbulence research has valid prospects in open channel flow. The relevance of the study is amplified as we introduce any hydraulic structure in the channel which disturbs the natural flow and creates discontinuity. To recover this discontinuity, the piano key weir (PKW) might be used with sloped keys. Constraints of empirical results in the vicinity of PKW necessitate extensive laboratory experiments with fair and reliable instrumentation techniques. Acoustic Doppler velocimeter was established to be best suited within range of some limitations using principal component analysis. Wavelet analysis is proposed to decompose the underlying turbulence structure in a better way.

Polymer stress tensor in turbulent shear flows.

PubMed

L'vov, Victor S; Pomyalov, Anna; Procaccia, Itamar; Tiberkevich, Vasil

2005-01-01

The interaction of polymers with turbulent shear flows is examined. We focus on the structure of the elastic stress tensor, which is proportional to the polymer conformation tensor. We examine this object in turbulent flows of increasing complexity. First is isotropic turbulence, then anisotropic (but homogenous) shear turbulence, and finally wall bounded turbulence. The main result of this paper is that for all these flows the polymer stress tensor attains a universal structure in the limit of large Deborah number De > 1. We present analytic results for the suppression of the coil-stretch transition at large Deborah numbers. Above the transition the turbulent velocity fluctuations are strongly correlated with the polymer's elongation: there appear high-quality "hydroelastic" waves in which turbulent kinetic energy turns into polymer potential energy and vice versa. These waves determine the trace of the elastic stress tensor but practically do not modify its universal structure. We demonstrate that the influence of the polymers on the balance of energy and momentum can be accurately described by an effective polymer viscosity that is proportional to the cross-stream component of the elastic stress tensor. This component is smaller than the streamwise component by a factor proportional to De2. Finally we tie our results to wall bounded turbulence and clarify some puzzling facts observed in the problem of drag reduction by polymers.

Turbulent structure in low-concentration drag-reducing channel flows

NASA Technical Reports Server (NTRS)

Luchik, T. S.; Tiederman, W. G.

1988-01-01

A two-component laser-Doppler velocimeter was used to obtain simultaneous measurements of the velocity components parallel and normal to the wall in two fully developed well-mixed low-concentration drag-reducing channel flows and one turbulent channel flow. For the drag-reducing flows, the average time between bursts was found to increase. Although the basic structure of the fundamental momentum transport event is shown to be the same in these drag-reducing flows, the lower-threshold Reynolds-stress-producing motions were found to be damped, while the higher-threshold motions were not. It is suggested that some strong turbulent motions are needed to maintain extended polymer molecules, which produce a solution with properties that can damp lower threshold turbulence and thereby reduce viscous drag.

Experimental search for Exact Coherent Structures in turbulent small aspect ratio Taylor-Couette flow

NASA Astrophysics Data System (ADS)

Crowley, Christopher J.; Krygier, Michael; Grigoriev, Roman O.; Schatz, Michael F.

2017-11-01

Recent theoretical and experimental work suggests that the dynamics of turbulent flows are guided by unstable nonchaotic solutions to the Navier-Stokes equations. These solutions, known as exact coherent structures (ECS), play a key role in a fundamentally deterministic description of turbulence. In order to quantitatively demonstrate that actual turbulence in 3D flows is guided by ECS, high resolution, 3D-3C experimental measurements of the velocity need to be compared to solutions from direct numerical simulation of the Navier-Stokes equations. In this talk, we will present experimental measurements of fully time resolved, velocity measurements in a volume of turbulence in a counter-rotating, small aspect ratio Taylor-Couette flow. This work is supported by the Army Research Office (Contract # W911NF-16-1-0281).

Anisotropic Stochastic Vortex Structure Method for Simulating Particle Collision in Turbulent Shear Flows

NASA Astrophysics Data System (ADS)

Dizaji, Farzad; Marshall, Jeffrey; Grant, John; Jin, Xing

2017-11-01

Accounting for the effect of subgrid-scale turbulence on interacting particles remains a challenge when using Reynolds-Averaged Navier Stokes (RANS) or Large Eddy Simulation (LES) approaches for simulation of turbulent particulate flows. The standard stochastic Lagrangian method for introducing turbulence into particulate flow computations is not effective when the particles interact via collisions, contact electrification, etc., since this method is not intended to accurately model relative motion between particles. We have recently developed the stochastic vortex structure (SVS) method and demonstrated its use for accurate simulation of particle collision in homogeneous turbulence; the current work presents an extension of the SVS method to turbulent shear flows. The SVS method simulates subgrid-scale turbulence using a set of randomly-positioned, finite-length vortices to generate a synthetic fluctuating velocity field. It has been shown to accurately reproduce the turbulence inertial-range spectrum and the probability density functions for the velocity and acceleration fields. In order to extend SVS to turbulent shear flows, a new inversion method has been developed to orient the vortices in order to generate a specified Reynolds stress field. The extended SVS method is validated in the present study with comparison to direct numerical simulations for a planar turbulent jet flow. This research was supported by the U.S. National Science Foundation under Grant CBET-1332472.

The structure of evaporating and combusting sprays: Measurements and predictions

NASA Technical Reports Server (NTRS)

Shuen, J. S.; Solomon, A. S. P.; Faeth, F. M.

1983-01-01

The structure of particle-laden jets and nonevaporating and evaporating sprays was measured in order to evaluate models of these processes. Three models are being evaluated: (1) a locally homogeneous flow model, where slip between the phases is neglected and the flow is assumed to be in local thermodynamic equilibrium; (2) a deterministic separated flow model, where slip and finite interphase transport rates are considered but effects of particle/drop dispersion by turbulence and effects of turbulence on interphase transport rates are ignored; and (3) a stochastic separated flow model, where effects of interphase slip, turbulent dispersion and turbulent fluctuations are considered using random sampling for turbulence properties in conjunction with random-walk computations for particle motion. All three models use a k-e-g turbulence model. All testing and data reduction are completed for the particle laden jets. Mean and fluctuating velocities of the continuous phase and mean mixture fraction were measured in the evaporating sprays.

Turbulence modeling and combustion simulation in porous media under high Peclet number

NASA Astrophysics Data System (ADS)

Moiseev, Andrey A.; Savin, Andrey V.

2018-05-01

Turbulence modelling in porous flows and burning still remains not completely clear until now. Undoubtedly, conventional turbulence models must work well under high Peclet numbers when porous channels shape is implemented in details. Nevertheless, the true turbulent mixing takes place at micro-scales only, and the dispersion mixing works at macro-scales almost independent from true turbulence. The dispersion mechanism is characterized by the definite space scale (scale of the porous structure) and definite velocity scale (filtration velocity). The porous structure is stochastic one usually, and this circumstance allows applying the analogy between space-time-stochastic true turbulence and the dispersion flow which is stochastic in space only, when porous flow is simulated at the macro-scale level. Additionally, the mentioned analogy allows applying well-known turbulent combustion models in simulations of porous combustion under high Peclet numbers.

Self-sustaining turbulence in a restricted nonlinear model of plane Couette flow

DOE Office of Scientific and Technical Information (OSTI.GOV)

Thomas, Vaughan L.; Gayme, Dennice F.; Lieu, Binh K.

2014-10-15

This paper demonstrates the maintenance of self-sustaining turbulence in a restricted nonlinear (RNL) model of plane Couette flow. The RNL system is derived directly from the Navier-Stokes equations and permits higher resolution studies of the dynamical system associated with the stochastic structural stability theory (S3T) model, which is a second order approximation of the statistical state dynamics of the flow. The RNL model shares the dynamical restrictions of the S3T model but can be easily implemented by reducing a DNS code so that it retains only the RNL dynamics. Comparisons of turbulence arising from DNS and RNL simulations demonstrate thatmore » the RNL system supports self-sustaining turbulence with a mean flow as well as structural and dynamical features that are consistent with DNS. These results demonstrate that the simplified RNL system captures fundamental aspects of fully developed turbulence in wall-bounded shear flows and motivate use of the RNL/S3T framework for further study of wall-turbulence.« less

Numerical study of axial turbulent flow over long cylinders

NASA Technical Reports Server (NTRS)

Neves, J. C.; Moin, P.; Moser, R. D.

1991-01-01

The effects of transverse curvature are investigated by means of direct numerical simulations of turbulent axial flow over cylinders. Two cases of Reynolds number of about 3400 and layer-thickness-to-cylinder-radius ratios of 5 and 11 were simulated. All essential turbulence scales were resolved in both calculations, and a large number of turbulence statistics were computed. The results are compared with the plane channel results of Kim et al. (1987) and with experiments. With transverse curvature the skin friction coefficient increases and the turbulence statistics, when scaled with wall units, are lower than in the plane channel. The momentum equation provides a scaling that collapses the cylinder statistics, and allows the results to be interpreted in light of the plane channel flow. The azimuthal and radial length scales of the structures in the flow are of the order of the cylinder diameter. Boomerang-shaped structures with large spanwise length scales were observed in the flow.

Turbulent boundary layer in high Rayleigh number convection in air.

PubMed

du Puits, Ronald; Li, Ling; Resagk, Christian; Thess, André; Willert, Christian

2014-03-28

Flow visualizations and particle image velocimetry measurements in the boundary layer of a Rayleigh-Bénard experiment are presented for the Rayleigh number Ra=1.4×1010. Our visualizations indicate that the appearance of the flow structures is similar to ordinary (isothermal) turbulent boundary layers. Our particle image velocimetry measurements show that vorticity with both positive and negative sign is generated and that the smallest flow structures are 1 order of magnitude smaller than the boundary layer thickness. Additional local measurements using laser Doppler velocimetry yield turbulence intensities up to I=0.4 as in turbulent atmospheric boundary layers. From our observations, we conclude that the convective boundary layer becomes turbulent locally and temporarily although its Reynolds number Re≈200 is considerably smaller than the value 420 underlying existing phenomenological theories. We think that, in turbulent Rayleigh-Bénard convection, the transition of the boundary layer towards turbulence depends on subtle details of the flow field and is therefore not universal.

Experimental measurements of unsteady turbulent boundary layers near separation

NASA Technical Reports Server (NTRS)

Simpson, R. L.

1982-01-01

Investigations conducted to document the behavior of turbulent boundary layers on flat surfaces that separate due to adverse pressure gradients are reported. Laser and hot wire anemometers measured turbulence and flow structure of a steady free stream separating turbulent boundary layer produced on the flow of a wind tunnel section. The effects of sinusoidal and unsteadiness of the free stream velocity on this separating turbulent boundary layer at a reduced frequency were determined. A friction gage and a thermal tuft were developed and used to measure the surface skin friction and the near wall fraction of time the flow moves downstream for several cases. Abstracts are provided of several articles which discuss the effects of the periodic free stream unsteadiness on the structure or separating turbulent boundary layers.

3D Measurements of coupled freestream turbulence and secondary flow effects on film cooling

NASA Astrophysics Data System (ADS)

Ching, David S.; Xu, Haosen H. A.; Elkins, Christopher J.; Eaton, John K.

2018-06-01

The effect of freestream turbulence on a single round film cooling hole is examined at two turbulence levels of 5 and 8% and compared to a baseline low freestream turbulence case. The hole is inclined at 30° and has length to diameter ratio L/D=4 and unity blowing ratio. Turbulence is generated with grid upstream of the hole in the main channel. The three-dimensional, three-component mean velocity field is acquired with magnetic resonance velocimetry (MRV) and the three-dimensional temperature field is acquired with magnetic resonance thermometry (MRT). The 8% turbulence grid produces weak mean secondary flows in the mainstream (peak crossflow velocities are 7% of U_bulk) which push the jet close to the wall and significantly change the adiabatic effectiveness distribution. By contrast, the 5% grid has a simpler structure and does not produce a measurable secondary flow structure. The grid turbulence causes little change to the temperature field, indicating that the turbulence generated in the shear layers around the jet dominates the freestream turbulence. The results suggest that secondary flows induced by complex turbulence generators may have caused some of the contradictory results in previous works.

Temporal and spatial intermittencies within Newtonian turbulence

NASA Astrophysics Data System (ADS)

Kushwaha, Anubhav; Graham, Michael

2015-11-01

Direct numerical simulations of a pressure driven turbulent flow are performed in a large rectangular channel. Intermittent high- and low-drag regimes within turbulence that have earlier been found to exist temporally in minimal channels have been observed both spatially and temporally in full-size turbulent flows. These intermittent regimes, namely, ''active'' and ''hibernating'' turbulence, display very different structural and statistical features. We adopt a very simple sampling technique to identify these intermittent intervals, both temporally and spatially, and present differences between them in terms of simple quantities like mean-velocity, wall-shear stress and flow structures. By conditionally sampling of the low wall-shear stress events in particular, we show that the Maximum Drag Reduction (MDR) velocity profile, that occurs in viscoelastic flows, can also be approached in a Newtonian-fluid flow in the absence of any additives. This suggests that the properties of polymer drag reduction are inherent to all flows and their occurrence is just enhanced by the addition of polymers. We also show how the intermittencies within turbulence vary with Reynolds number. The work was supported by AFOSR grant FA9550-15-1-0062.

Turbulence production near walls: The role of flow structures with spanwise asymmetry

NASA Technical Reports Server (NTRS)

Alfredsson, P. Henrik; Johansson, Arne V.; Kim, John

1988-01-01

Space-time evolution of near wall flow structures is described by conditional sampling methods, in which conditional averages are formed at various stages of development of shear layer structures. The development of spanwise asymmetry of the structures was found to be important in the creation of the structures and for the process of turbulence production.

Annual Research Briefs

NASA Technical Reports Server (NTRS)

Spinks, Debra (Compiler)

1997-01-01

This report contains the 1997 annual progress reports of the research fellows and students supported by the Center for Turbulence Research (CTR). Titles include: Invariant modeling in large-eddy simulation of turbulence; Validation of large-eddy simulation in a plain asymmetric diffuser; Progress in large-eddy simulation of trailing-edge turbulence and aeronautics; Resolution requirements in large-eddy simulations of shear flows; A general theory of discrete filtering for LES in complex geometry; On the use of discrete filters for large eddy simulation; Wall models in large eddy simulation of separated flow; Perspectives for ensemble average LES; Anisotropic grid-based formulas for subgrid-scale models; Some modeling requirements for wall models in large eddy simulation; Numerical simulation of 3D turbulent boundary layers using the V2F model; Accurate modeling of impinging jet heat transfer; Application of turbulence models to high-lift airfoils; Advances in structure-based turbulence modeling; Incorporating realistic chemistry into direct numerical simulations of turbulent non-premixed combustion; Effects of small-scale structure on turbulent mixing; Turbulent premixed combustion in the laminar flamelet and the thin reaction zone regime; Large eddy simulation of combustion instabilities in turbulent premixed burners; On the generation of vorticity at a free-surface; Active control of turbulent channel flow; A generalized framework for robust control in fluid mechanics; Combined immersed-boundary/B-spline methods for simulations of flow in complex geometries; and DNS of shock boundary-layer interaction - preliminary results for compression ramp flow.

Studying Turbulence Using Numerical Simulation Databases. Part 6; Proceedings of the 1996 Summer Program

NASA Technical Reports Server (NTRS)

1996-01-01

Topics considered include: New approach to turbulence modeling; Second moment closure analysis of the backstep flow database; Prediction of the backflow and recovery regions in the backward facing step at various Reynolds numbers; Turbulent flame propagation in partially premixed flames; Ensemble averaged dynamic modeling. Also included a study of the turbulence structures of wall-bounded shear flows; Simulation and modeling of the elliptic streamline flow.

Evolution and dynamics of shear-layer structures in near-wall turbulence

NASA Technical Reports Server (NTRS)

Johansson, Arne V.; Alfredsson, P. H.; Kim, John

1991-01-01

Near-wall flow structures in turbulent shear flows are analyzed, with particular emphasis on the study of their space-time evolution and connection to turbulence production. The results are obtained from investigation of a database generated from direct numerical simulation of turbulent channel flow at a Reynolds number of 180 based on half-channel width and friction velocity. New light is shed on problems associated with conditional sampling techniques, together with methods to improve these techniques, for use both in physical and numerical experiments. The results clearly indicate that earlier conceptual models of the processes associated with near-wall turbulence production, based on flow visualization and probe measurements need to be modified. For instance, the development of asymmetry in the spanwise direction seems to be an important element in the evolution of near-wall structures in general, and for shear layers in particular. The inhibition of spanwise motion of the near-wall streaky pattern may be the primary reason for the ability of small longitudinal riblets to reduce turbulent skin friction below the value for a flat surface.

Bounded energy states in homogeneous turbulent shear flow: An alternative view

NASA Technical Reports Server (NTRS)

Bernard, Peter S.; Speziale, Charles G.

1990-01-01

The equilibrium structure of homogeneous turbulent shear flow is investigated from a theoretical standpoint. Existing turbulence models, in apparent agreement with physical and numerical experiments, predict an unbounded exponential time growth of the turbulent kinetic energy and dissipation rate; only the anisotropy tensor and turbulent time scale reach a structural equilibrium. It is shown that if vortex stretching is accounted for in the dissipation rate transport equation, then there can exist equilibrium solutions, with bounded energy states, where the turbulence production is balanced by its dissipation. Illustrative calculations are present for a k-epsilon model modified to account for vortex stretching. The calculations indicate an initial exponential time growth of the turbulent kinetic energy and dissipation rate for elapsed times that are as large as those considered in any of the previously conducted physical or numerical experiments on homogeneous shear flow. However, vortex stretching eventually takes over and forces a production-equals-dissipation equilibrium with bounded energy states. The validity of this result is further supported by an independent theoretical argument. It is concluded that the generally accepted structural equilibrium for homogeneous shear flow with unbounded component energies is in need of re-examination.

Experimental studies on flow visualization and velocity field of compression ramp with different incoming boundary layers

NASA Astrophysics Data System (ADS)

Wu, Yu; Yi, Shi-He; He, Lin; Chen, Zhi; Zhu, Yang-Zhu

2014-11-01

Experimental studies which focus on flow visualization and the velocity field of a supersonic laminar/turbulent flow over a compression ramp were carried out in a Mach 3.0 wind tunnel. Fine flow structures and velocity field structures were obtained via NPLS (nanoparticle-tracer planar laser scattering) and PIV (particle image velocimetry) techniques, time-averaged flow structures were researched, and spatiotemporal evolutions of transient flow structures were analyzed. The flow visualization results indicated that when the ramp angles were 25°, a typical separation occurred in the laminar flow, some typical flow structures such as shock induced by the boundary layer, separation shock, reversed flow and reattachment shock were visible clearly. While a certain extent separation occurred in turbulent flow, the separation region was much smaller. When the ramp angles were 28°, laminar flow separated further, and the separation region expanded evidently, flow structures in the separation region were complex. While a typical separation occurred in turbulent flow, reversed flow structures were significant, flow structures in the separation region were relatively simple. The experimental results of velocity field were corresponding to flow visualization, and the velocity field structures of both compression ramp flows agreed with the flow structures well. There were three layered structures in the U component velocity, and the V component velocity appeared like an oblique “v”. Some differences between these two compression ramp flows can be observed in the velocity profiles of the shear layer and the shearing intensity.

Turbulence regeneration in pipe flow at moderate Reynolds numbers.

PubMed

Hof, Björn; van Doorne, Casimir W H; Westerweel, Jerry; Nieuwstadt, Frans T M

2005-11-18

We present the results of an experimental investigation into the nature and structure of turbulent pipe flow at moderate Reynolds numbers. A turbulence regeneration mechanism is identified which sustains a symmetric traveling wave within the flow. The periodicity of the mechanism allows comparison to the wavelength of numerically observed exact traveling wave solutions and close agreement is found. The advection speed of the upstream turbulence laminar interface in the experimental flow is observed to form a lower bound on the phase velocities of the exact traveling wave solutions. Overall our observations suggest that the dynamics of the turbulent flow at moderate Reynolds numbers are governed by unstable nonlinear traveling waves.

Studying Turbulence Using Numerical Simulation Databases. Proceedings of the 1987 Summer Program

NASA Technical Reports Server (NTRS)

Moin, Parviz (Editor); Reynolds, William C. (Editor); Kim, John (Editor)

1987-01-01

The focus was on the use of databases obtained from direct numerical simulations of turbulent flows, for study of turbulence physics and modeling. Topics addressed included: stochastic decomposition/chaos/bifurcation; two-point closure (or k-space) modeling; scalar transport/reacting flows; Reynolds stress modeling; and structure of turbulent boundary layers.

Shear-layer structures in near-wall turbulence

NASA Technical Reports Server (NTRS)

Johansson, A. V.; Alfredsson, P. H.; Kim, J.

1987-01-01

The structure of internal shear layer observed in the near-wall region of turbulent flows is investigated by analyzing flow fields obtained from numerical simulations of channel and boundary-layer flows. It is found that the shear layer is an important contributor to the turbulence production. The conditionally averaged production at the center of the structure was almost twice as large as the long-time mean value. The shear-layer structure is also found to retain its coherence over streamwise distances on the order of a thousand viscous length units, and propagates with a constant velocity of about 10.6 u sub rho throughout the near wall region.

Analysis of coherent dynamical processes through computer vision

NASA Astrophysics Data System (ADS)

Hack, M. J. Philipp

2016-11-01

Visualizations of turbulent boundary layers show an abundance of characteristic arc-shaped structures whose apparent similarity suggests a common origin in a coherent dynamical process. While the structures have been likened to the hairpin vortices observed in the late stages of transitional flow, a consistent description of the underlying mechanism has remained elusive. Detailed studies are complicated by the chaotic nature of turbulence which modulates each manifestation of the process and which renders the isolation of individual structures a challenging task. The present study applies methods from the field of computer vision to capture the time evolution of turbulent flow features and explore the associated physical mechanisms. The algorithm uses morphological operations to condense the structure of the turbulent flow field into a graph described by nodes and links. The low-dimensional geometric information is stored in a database and allows the identification and analysis of equivalent dynamical processes across multiple scales. The framework is not limited to turbulent boundary layers and can also be applied to different types of flows as well as problems from other fields of science.

On the connection between Maximum Drag Reduction and Newtonian fluid flow

NASA Astrophysics Data System (ADS)

Whalley, Richard; Park, Jae-Sung; Kushwaha, Anubhav; Dennis, David; Graham, Michael; Poole, Robert

2014-11-01

To date, the most successful turbulence control technique is the dissolution of certain rheology-modifying additives in liquid flows, which results in a universal maximum drag reduction (MDR) asymptote. The MDR asymptote is a well-known phenomenon in the turbulent flow of complex fluids; yet recent direct numerical simulations of Newtonian fluid flow have identified time intervals showing key features of MDR. These intervals have been termed ``hibernating turbulence'' and are a weak turbulence state which is characterised by low wall-shear stress and weak vortical flow structures. Here, in this experimental investigation, we monitor the instantaneous wall-shear stress in a fully-developed turbulent channel flow of a Newtonian fluid with a hot-film probe whilst simultaneously measuring the streamwise velocity at various distances above the wall with laser Doppler velocimetry. We show, by conditionally sampling the streamwise velocity during low wall-shear stress events, that the MDR velocity profile is approached in an additive-free, Newtonian fluid flow. This result corroborates recent numerical investigations, which suggest that the MDR asymptote in polymer solutions is closely connected to weak, transient Newtonian flow structures.

Statistical assessment of optical phase fluctuations through turbulent mixing layers

NASA Astrophysics Data System (ADS)

Gardner, Patrick J.; Roggemann, Michael C.; Welsh, Byron M.; Bowersox, Rodney D.

1995-09-01

A lateral shearing interferometer is used to measure the slope of perturbed wavefronts after propagating through turbulent shear flows. This provides a two-dimensional flow visualization technique which is nonintrusive. The slope measurements are used to reconstruct the phase of the turbulence-corrupted wave front. Experiments were performed on a plane shear mixing layer of helium and nitrogen gas at fixed velocities, for five locations in the flow development. The two gases, having a density ratio of approximately seven, provide an effective means of simulating compressible shear layers. Statistical autocorrelation functions and structure functions are computed on the reconstructed phase maps. The autocorrelation function results indicate that the turbulence-induced phase fluctuations are not wide-sense stationary. The structure functions exhibit statistical homogeneity, indicating the phase fluctuation are stationary in first increments. However, the turbulence-corrupted phase is not isotropic. A five-thirds power law is shown to fit one-dimensional, orthogonal slices of the structure function, with scaling coefficients related to the location in the flow.

Plane waves and structures in turbulent channel flow

NASA Technical Reports Server (NTRS)

Sirovich, L.; Ball, K. S.; Keefe, L. R.

1990-01-01

A direct simulation of turbulent flow in a channel is analyzed by the method of empirical eigenfunctions (Karhunen-Loeve procedure, proper orthogonal decomposition). This analysis reveals the presence of propagating plane waves in the turbulent flow. The velocity of propagation is determined by the flow velocity at the location of maximal Reynolds stress. The analysis further suggests that the interaction of these waves appears to be essential to the local production of turbulence via bursting or sweeping events in the turbulent boundary layer, with the additional suggestion that the fast acting plane waves act as triggers.

Bounded energy states in homogeneous turbulent shear flow - An alternative view

NASA Technical Reports Server (NTRS)

Bernard, P. S.; Speziale, C. G.

1992-01-01

The equilibrium structure of homogeneous turbulent shear flow is investigated from a theoretical standpoint. Existing turbulence models, in apparent agreement with physical and numerical experiments, predict an unbounded exponential time growth of the turbulent kinetic energy and dissipation rate; only the anisotropy tensor and turbulent time scale reach a structural equilibrium. It is shown that if a residual vortex stretching term is maintained in the dissipation rate transport equation, then there can exist equilibrium solutions, with bounded energy states, where the turbulence production is balanced by its dissipation. Illustrative calculations are presented for a k-epsilon model modified to account for net vortex stretching.

Numerical investigation of exact coherent structures in turbulent small-aspect-ratio Taylor-Couette flow

NASA Astrophysics Data System (ADS)

Krygier, Michael; Crowley, Christopher J.; Schatz, Michael F.; Grigoriev, Roman O.

2017-11-01

As suggested by recent theoretical and experimental studies, fluid turbulence can be described as a walk between neighborhoods of unstable nonchaotic solutions of the Navier-Stokes equation known as exact coherent structures (ECS). Finding ECS in an experimentally-accessible setting is the first step toward rigorous testing of the dynamical role of ECS in 3D turbulence. We found several ECS (both relative periodic orbits and relative equilibria) in a weakly turbulent regime of small-aspect-ratio Taylor-Couette flow with counter-rotating cylinders. This talk will discuss how the geometry of these solutions guides the evolution of turbulent flow in the simulations. This work is supported by the Army Research Office (Contract # W911NF-15-1-0471).

NASA research on viscous drag reduction

NASA Technical Reports Server (NTRS)

Petersen, R. H.; Maddalon, D. V.

1982-01-01

Research on natural laminar flow, laminar flow control by suction, and turbulent drag reduction is discussed. Preliminary results suggest that a significant amount of natural laminar flow can be achieved on small, straight wing airplanes. On larger, swept wing aircraft, laminar flow control by distributed suction is expected to result in significant fuel savings. The area over which laminar flow control is applied depends on tradeoffs involving structural complexity, maintenance, and cost. Several methods of reducing turbulent skin friction by altering the turbulence structure itself have shown promise in exploratory testing. The status of these technologies and the benefits of applying them to future aircraft are reviewed.

Large-eddy-simulation approach in understanding flow structures of 2D turbulent density currents over sloping surfaces

NASA Astrophysics Data System (ADS)

Nayamatullah, M.; Rao Pillalamarri, Narasimha; Bhaganagar, Kiran

2018-04-01

A numerical investigation was performed to understand the flow dynamics of 2D density currents over sloping surfaces. Large eddy simulation was conducted for lock-exchange (L-E) release currents and overflows. 2D Navier-Stokes equations were solved using the Boussinesq approximation. The effects of the lock aspect-ratio (height/length of lock), slope, and Reynolds number on the flow structures and turbulence mixing have been analyzed. Results have confirmed buoyancy within the head of the two-dimensional currents is not conserved which contradicts the classical thermal theory. The lock aspect-ratio dictates the fraction of initial buoyancy which is carried by the head of the current at the beginning of the slumping (horizontal) and accelerating phase (over a slope), which has important implications on turbulence kinetic energy production, and hence mixing in the current. For L-E flows over a slope, increasing slope angle enhances the turbulence production. Increasing slope results in shear reversal within the density current resulting in shear-instabilities. Differences in turbulence production mechanisms and flow structures exist between the L-E and constant-flux release currents resulting in significant differences in the flow characteristics between different releases.

Studying Turbulence Using Numerical Simulation Databases. 3: Proceedings of the 1990 Summer Program

NASA Technical Reports Server (NTRS)

Spinks, Debra (Compiler)

1990-01-01

Papers that cover the following topics are presented: subgrid scale modeling; turbulence modeling; turbulence structure, transport, and control; small scales mixing; turbulent reacting flows; and turbulence theory.

Interaction between turbulent flow and sea breeze front over urban-like coast in large-eddy simulation

NASA Astrophysics Data System (ADS)

Jiang, Ping; Wen, Zhiping; Sha, Weiming; Chen, Guixing

2017-05-01

Turbulent flow and its interaction with a sea breeze front (SBF) over an urban-like coast with a regular block array were investigated using a building-resolving computational fluid dynamics model. It was found that during daytime with an offshore ambient flow, streaky turbulent structures tended to grow within the convective boundary layer (CBL) over a warm urban surface ahead of the SBF. The structures were organized as streamwise streaks at an interval of a few hundred meters, which initiated at the rooftop level with strong wind shear and strengthens in the CBL with moderate buoyancy. The streaks then interacted with the onshore-propagating SBF as it made landfall. The SBF, which was initially characterized as a shallow and quasi-linear feature over the sea, developed three-dimensional structures with intensified updrafts at an elevated frontal head after landfall. Frontal updrafts were locally enhanced at intersections where the streaks merged with the SBF, which greatly increased turbulent fluxes at the front. The frontal line was irregular because of merging, tilting, and transformation effects of vorticity associated with streaky structures. Inland penetration of the SBF was slowed by the frictional effect of urban-like surfaces and turbulent flow on land. The overall SBF intensity weakened after the interaction with turbulent flow. These findings aid understanding of local weather over coastal cities during typical sea breeze conditions.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zentgraf, Florian; Baum, Elias; Dreizler, Andreas

Planar particle image velocimetry (PIV) and tomographic PIV (TPIV) measurements are utilized to analyze turbulent statistical theory quantities and the instantaneous turbulence within a single-cylinder optical engine. Measurements are performed during the intake and mid-compression stroke at 800 and 1500 RPM. TPIV facilitates the evaluation of spatially resolved Reynolds stress tensor (RST) distributions, anisotropic Reynolds stress invariants, and instantaneous turbulent vortical structures. The RST analysis describes distributions of individual velocity fluctuation components that arise from unsteady turbulent flow behavior as well as cycle-to-cycle variability (CCV). A conditional analysis, for which instantaneous PIV images are sampled by their tumble center location,more » reveals that CCV and turbulence have similar contributions to RST distributions at the mean tumble center, but turbulence is dominant in regions peripheral to the tumble center. Analysis of the anisotropic Reynolds stress invariants reveals the spatial distribution of axisymmetric expansion, axisymmetric contraction, and 3D isotropy within the cylinder. Findings indicate that the mid-compression flow exhibits a higher tendency toward 3D isotropy than the intake flow. A novel post-processing algorithm is utilized to classify the geometry of instantaneous turbulent vortical structures and evaluate their frequency of occurrence within the cylinder. Findings are coupled with statistical theory quantities to provide a comprehensive understanding of the distribution of turbulent velocity components, the distribution of anisotropic states of turbulence, and compare the turbulent vortical flow distribution that is theoretically expected to what is experimentally observed. The analyses reveal requisites of important turbulent flow quantities and discern their sensitivity to the local flow topography and engine operation.« less

Modeling the stochastic dynamics of moving turbulent spots over a slender cone at Mach 5 during laminar-turbulent transition

NASA Astrophysics Data System (ADS)

Robbins, Brian; Field, Rich; Grigoriu, Mircea; Jamison, Ryan; Mesh, Mikhail; Casper, Katya; Dechant, Lawrence

2016-11-01

During reentry, a hypersonic vehicle undergoes a period in which the flow about the vehicle transitions from laminar to turbulent flow. During this transitional phase, the flow is characterized by intermittent formations of localized turbulent behavior. These localized regions of turbulence are born at the onset of transition and grow as they move to the aft end of the flight vehicle. Throughout laminar-turbulent transition, the moving turbulent spots cause pressure fluctuations on the outer surface of the vehicle, which leads to the random vibration of the structure and its internal components. In light of this, it is of great interest to study the dynamical response of a flight vehicle undergoing transitional flow so that aircraft can be better designed to prevent structural failure. In this talk, we present a statistical model that calculates the birth, evolution, and pressure field of turbulent spots over a generic slender cone structure. We then illustrate that the model appropriately quantifies intermittency behavior and pressure loading by comparing the intermittency and root-mean-square pressure fluctuations produced by the model with theory and experiment. Finally, we present results pertaining to the structural response of a housing panel on the slender cone. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Tracking coherent structures in massively-separated and turbulent flows

NASA Astrophysics Data System (ADS)

Rockwood, Matthew; Huang, Yangzi; Green, Melissa

2018-01-01

Coherent vortex structures are tracked in simulations of massively-separated and turbulent flows. Topological Lagrangian saddle points are found using intersections of the positive and negative finite-time Lyapunov exponent ridges, and these points are then followed in order to track individual coherent structure motion both in a complex interacting three-dimensional flow (turbulent channel) and during vortex formation (two-dimensional bluff body shedding). For a simulation of wall-bounded turbulence in a channel flow, tracking Lagrangian saddles shows that the average structure convection speed exhibits a similar trend as a previously published result based on velocity and pressure correlations, giving validity to the method. When this tracking method is applied in a study of a circular cylinder in cross-flow it shows that Lagrangian saddles rapidly accelerate away from the cylinder surface as the vortex sheds. This saddle behavior is compared with the time-resolved static pressure distribution on the circular cylinder, yielding locations on a cylinder surface where common sensors could detect this phenomenon, which is not available from force measurements or vortex circulation calculations. The current method of tracking coherent structures yields insight into the behavior of the coherent structures in both of the diverse flows presented, highlighting the breadth of its potential application.

Data-driven spectral filters for decomposing the streamwise turbulent kinetic energy in turbulent boundary layers

NASA Astrophysics Data System (ADS)

Baars, Woutijn J.; Hutchins, Nicholas; Marusic, Ivan

2017-11-01

An organization in wall-bounded turbulence is evidenced by the classification of distinctly different flow structures, including large-scale motions such as hairpin packets and very large-scale motions or superstructures. In conjunction with less organized turbulence, these flow structures all contribute to the streamwise turbulent kinetic energy . Since different class structures comprise dissimilar scalings of their overlapping imprints in the streamwise velocity spectra, their coexistence complicates the interpretation of the wall-normal trend in and its Reynolds number dependence. Via coherence analyses of two-point data in boundary layers we derive spectral filters for stochastically decomposing the streamwise spectra into sub-components, representing different types of statistical flow structures. It is also explored how the decomposition reflects the spectral break-down following the modeling attempts of Perry et al. 1986 and Marusic & Perry 1995. In the process we reveal a universal wall-scaling for a portion of the outer-region turbulence that is coherent with the near-wall region for Reτ O(103) to O(106) , which is described as a wall-attached self-similar structure embedded within the logarithmic region.

A priori evaluation of the Pantano and Sarkar model in compressible homogeneous shear flows

NASA Astrophysics Data System (ADS)

Khlifi, Hechmi; Abdallah, J.; Aïcha, H.; Taïeb, L.

2011-01-01

In this study, a Reynolds stress closure, including the Pantano and Sarkar model of the mean part of the pressure-strain correlation is used for the computation of compressible homogeneous at high-speed shear flow. Several studies concerning the compressible homogeneous shear flow show that the changes of the turbulence structures are principally due to the structural compressibility effects which significantly affect the pressure field and then the pressure-strain correlation. Eventually, this term appears as the main term responsible for the changes in the magnitude of the Reynolds stress anisotropies. The structure of the gradient Mach number is similar to that of turbulence, therefore this parameter may be appropriate to study the changes in turbulence structures that arise from structural compressibility effects. Thus, the incompressible model of the pressure strain correlation and its corrected form by using the turbulent Mach turbulent only, fail to correctly evaluate the compressibility effects at high shear flow. An extension of the widely used incompressible Launder, Reece and Rodi model on compressible homogeneous shear flow is the major aim of the present work. From this extension, the standard coefficients C become a function of the extra compressibility parameters (the turbulent Mach number M and the gradient Mach number M) through the Pantano and Sarkar model. Application of the model on compressible homogeneous shear flow by considering various initial conditions shows reasonable agreement with the DNS results of Simone et al. and Sarkar. The observed trend of the dramatic increase in the normal Reynolds stress anisotropies, the significant decrease in the Reynolds shear stress anisotropy and the increase of the turbulent kinetic energy amplification rate with increasing the gradient Mach number are well predicted by the model. The ability of the model to predict the equilibrium states for the flow in cases A to A from DNS results of Sarkar is examined, the results appear to be very encouraging. Thus, both parameters M and M should be used to model significant structural compressibility effects at high-speed shear flow.

Wave models for turbulent free shear flows

NASA Technical Reports Server (NTRS)

Liou, W. W.; Morris, P. J.

1991-01-01

New predictive closure models for turbulent free shear flows are presented. They are based on an instability wave description of the dominant large scale structures in these flows using a quasi-linear theory. Three model were developed to study the structural dynamics of turbulent motions of different scales in free shear flows. The local characteristics of the large scale motions are described using linear theory. Their amplitude is determined from an energy integral analysis. The models were applied to the study of an incompressible free mixing layer. In all cases, predictions are made for the development of the mean flow field. In the last model, predictions of the time dependent motion of the large scale structure of the mixing region are made. The predictions show good agreement with experimental observations.

Dynamics of Secondary Large-Scale Structures in ETG Turbulence Simulations

NASA Astrophysics Data System (ADS)

Li, Jiquan; Y, Kishimoto; Dong, Jiaqi; N, Miyato; T, Matsumoto

2006-01-01

The dynamics of secondary large-scale structures in electron-temperature-gradient (ETG) turbulence is investigated based on gyrofluid simulations in sheared slab geometry. It is found that structural bifurcation to zonal flow dominated or streamer-like states depends on the spectral anisotropy of turbulent ETG fluctuation, which is governed by the magnetic shear. The turbulent electron transport is suppressed by enhanced zonal flows. However, it is still low even if the streamer is formed in ETG turbulence with strong shears. It is shown that the low transport may be related to the secondary excitation of poloidal long-wavelength mode due to the beat wave of the most unstable components or a modulation instability. This large-scale structure with a low frequency and a long wavelength may saturate, or at least contribute to the saturation of ETG fluctuations through a poloidal mode coupling. The result suggests a low fluctuation level in ETG turbulence.

Suppression of flow pulsation activity by relaxation process of additive effect on viscous media transport

NASA Astrophysics Data System (ADS)

Kharlamov, S.; Dedeyev, P.; Meucci, L.; Shenderova, I.; Manastirniy, A.; Usenko, M.

2015-11-01

The article presents the analysis of the processes occurring together with the turbulent transfer of impulse in mixture of hydrocarbon fluid and polymer solutions (anti-turbulent additives). The study evaluates complex shear flows by popular theoretical and practical methods. Understanding of hydrodynamic and dissipative effects of laminar-turbulent transition tightening and turbulence suppression is provided. The peculiarities of "thin" flow structure in pipeline zones with complex shape walls are evaluated. Recommendations to forecast the local flow parameters, calculation of hydraulic resistance are given.

Contributions to the understanding of large-scale coherent structures in developing free turbulent shear flows

NASA Technical Reports Server (NTRS)

Liu, J. T. C.

1986-01-01

Advances in the mechanics of boundary layer flow are reported. The physical problems of large scale coherent structures in real, developing free turbulent shear flows, from the nonlinear aspects of hydrodynamic stability are addressed. The presence of fine grained turbulence in the problem, and its absence, lacks a small parameter. The problem is presented on the basis of conservation principles, which are the dynamics of the problem directed towards extracting the most physical information, however, it is emphasized that it must also involve approximations.

Elliptical, parabolic, and hyperbolic exchanges of energy in drag reducing plane Couette flows

NASA Astrophysics Data System (ADS)

Pereira, Anselmo S.; Mompean, Gilmar; Thompson, Roney L.; Soares, Edson J.

2017-11-01

In the present paper, we investigate the polymer-turbulence interaction by discriminating between the mechanical responses of this system to three different subdomains: elliptical, parabolic, and hyperbolic, corresponding to regions where the magnitude of vorticity is greater than, equal to, or less than the magnitude of the rate of strain, respectively, in accordance with the Q-criterion. Recently, it was recognized that hyperbolic structures play a crucial role in the drag reduction phenomenon of viscoelastic turbulent flows, thanks to the observation that hyperbolic structures, as well as vortical ones, are weakened by the action of polymers in turbulent flows in a process that can be referred to as flow parabolization. We employ direct numerical simulations of a viscoelastic finite extensible nonlinear elastic model with the Peterlin approximation to examine the transient evolution and statistically steady regimes of a plane Couette flow that has been perturbed from a laminar flow at an initial time and developed a turbulent regime as a result of this perturbation. We have found that even more activity is located within the confines of the hyperbolic structures than in the elliptical ones, which highlights the importance of considering the role of hyperbolic structures in the drag reduction mechanism.

Large-scale structures in turbulent Couette flow

NASA Astrophysics Data System (ADS)

Kim, Jung Hoon; Lee, Jae Hwa

2016-11-01

Direct numerical simulation of fully developed turbulent Couette flow is performed with a large computational domain in the streamwise and spanwise directions (40 πh and 6 πh) to investigate streamwise-scale growth mechanism of the streamwise velocity fluctuating structures in the core region, where h is the channel half height. It is shown that long streamwise-scale structures (> 3 h) are highly energetic and they contribute to more than 80% of the turbulent kinetic energy and Reynolds shear stress, compared to previous studies in canonical Poiseuille flows. Instantaneous and statistical analysis show that negative-u' structures on the bottom wall in the Couette flow continuously grow in the streamwise direction due to mean shear, and they penetrate to the opposite moving wall. The geometric center of the log layer is observed in the centerline with a dominant outer peak in streamwise spectrum, and the maximum streamwise extent for structure is found in the centerline, similar to previous observation in turbulent Poiseuille flows at high Reynolds number. Further inspection of time-evolving instantaneous fields clearly exhibits that adjacent long structures combine to form a longer structure in the centerline. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2057031).

Sand transportation and reverse patterns over leeward face of sand dune

NASA Astrophysics Data System (ADS)

Jiang, Hong; Dun, Hongchao; Tong, Ding; Huang, Ning

2017-04-01

Sand saltation has complex interactions with turbulent flow and dune form. Most models of wind-blown sand consider ideal circumstances such as steady wind velocity and a flat surface, and the bulk of data on wind flow and sand transport over an individual dune has focused mostly on the influence of dune shape or inter-dune space on the wind flow, neglecting the effect of morphology on sand saltation, particularly airflow and sand transportation over the leeward slope. Wind flow structures over the leeward slope of sand dunes have a fundamental influence on the organization of sand dunes. In order to understand sand dune dynamics, lee face airflow and sediment transportation should be paid more attention. Previous field observations could not measure turbulent flow structure well because of the limited observation points and the influence of experiment structure on wind field. In addition, the reverse sand particles over leeward face could not be collected by sand trap in field. Numerous field observations could not measure turbulent flow structure because of the limited observation points and the influence of experimental structures on the wind field. In addition, the reverse transport of sand particles over leeward face could not be collected by sand traps in field. Therefore, this paper aims to investigate the turbulent flow structure and sand transport pattern over the leeward slope. A numerical model of sand saltation over slope terrain is constructed, which also considers the coupling effects between air flow and sand particles. The large eddy simulation method is used to model turbulent flow. Sand transport is simulated by tracking the trajectory of each sand particle. The results show that terrain significantly alters the turbulent air flow structure and wind-blown sand movement, especially over the leeward slope. Here, mass flux increases initially and then decreases with height in the reversed flow region in the direction of wind flow, and the mass flux decreases with height in the reversed direction. The height of 0.5 H is the height of vortex core in the reversed flow region. The vortex core is a critical point in the flow region where few particles are transited. In the reversed region, the reversed mass flux of sand particles is 25% of the mass flux in the flow direction. This research may contribute to scientific understanding of the mechanisms of sand motion and wind flow over leeward of dune and it is likely to be significant in desertification control.

Vortices and turbulence (The 23rd Lanchester Memorial Lecture)

NASA Astrophysics Data System (ADS)

Lilley, G. M.

1983-12-01

A comprehensive discussion is presented concerning the phenomena characteristically treated in vortex and turbulence theory, as well as the degree of success achieved by various computation and visualization methods and theoretical models developed for vortex flow behavior prediction. Note is taken of the pioneering research conducted by F. W. Lanchester in 1893-1907, and attention is given to vortex tip and edge generation by rectangular and delta wings, the cool core effect of the Ranque-Hilsch vortex tube, the modeling of shear flows by means of vortex array methods, the classification and modelling of turbulent flows (together with a taxonomy of their calculation methods), and NASA ILLIAC IV computations of two-dimensional channel flow. Also noted are recent results concerning the boundary layer coherent structure of a flat plate at zero pressure gradient, including the regeneration structure and flow distortion and breakdown of a turbulent boundary layer.

Study of compressible turbulent flows in supersonic environment by large-eddy simulation

NASA Astrophysics Data System (ADS)

Genin, Franklin

The numerical resolution of turbulent flows in high-speed environment is of fundamental importance but remains a very challenging problem. First, the capture of strong discontinuities, typical of high-speed flows, requires the use of shock-capturing schemes, which are not adapted to the resolution of turbulent structures due to their intrinsic dissipation. On the other hand, low-dissipation schemes are unable to resolve shock fronts and other sharp gradients without creating high amplitude numerical oscillations. Second, the nature of turbulence in high-speed flows differs from its incompressible behavior, and, in the context of Large-Eddy Simulation, the subgrid closure must be adapted to the modeling of compressibility effects and shock waves on turbulent flows. The developments described in this thesis are two-fold. First, a state of the art closure approach for LES is extended to model subgrid turbulence in compressible flows. The energy transfers due to compressible turbulence and the diffusion of turbulent kinetic energy by pressure fluctuations are assessed and integrated in the Localized Dynamic ksgs model. Second, a hybrid numerical scheme is developed for the resolution of the LES equations and of the model transport equation, which combines a central scheme for turbulent resolutions to a shock-capturing method. A smoothness parameter is defined and used to switch from the base smooth solver to the upwind scheme in regions of discontinuities. It is shown that the developed hybrid methodology permits a capture of shock/turbulence interactions in direct simulations that agrees well with other reference simulations, and that the LES methodology effectively reproduces the turbulence evolution and physical phenomena involved in the interaction. This numerical approach is then employed to study a problem of practical importance in high-speed mixing. The interaction of two shock waves with a high-speed turbulent shear layer as a mixing augmentation technique is considered. It is shown that the levels of turbulence are increased through the interaction, and that the mixing is significantly improved in this flow configuration. However, the region of increased mixing is found to be localized to a region close to the impact of the shocks, and that the statistical levels of turbulence relax to their undisturbed levels some short distance downstream of the interaction. The present developments are finally applied to a practical configuration relevant to scramjet injection. The normal injection of a sonic jet into a supersonic crossflow is considered numerically, and compared to the results of an experimental study. A fair agreement in the statistics of mean and fluctuating velocity fields is obtained. Furthermore, some of the instantaneous flow structures observed in experimental visualizations are identified in the present simulation. The dynamics of the interaction for the reference case, based on the experimental study, as well as for a case of higher freestream Mach number and a case of higher momentum ratio, are examined. The classical instantaneous vortical structures are identified, and their generation mechanisms, specific to supersonic flow, are highlighted. Furthermore, two vortical structures, recently revealed in low-speed jets in crossflow but never documented for high-speed flows, are identified during the flow evolution.

Detached eddy simulation for turbulent fluid-structure interaction of moving bodies using the constraint-based immersed boundary method

NASA Astrophysics Data System (ADS)

Nangia, Nishant; Bhalla, Amneet P. S.; Griffith, Boyce E.; Patankar, Neelesh A.

2016-11-01

Flows over bodies of industrial importance often contain both an attached boundary layer region near the structure and a region of massively separated flow near its trailing edge. When simulating these flows with turbulence modeling, the Reynolds-averaged Navier-Stokes (RANS) approach is more efficient in the former, whereas large-eddy simulation (LES) is more accurate in the latter. Detached-eddy simulation (DES), based on the Spalart-Allmaras model, is a hybrid method that switches from RANS mode of solution in attached boundary layers to LES in detached flow regions. Simulations of turbulent flows over moving structures on a body-fitted mesh incur an enormous remeshing cost every time step. The constraint-based immersed boundary (cIB) method eliminates this operation by placing the structure on a Cartesian mesh and enforcing a rigidity constraint as an additional forcing in the Navier-Stokes momentum equation. We outline the formulation and development of a parallel DES-cIB method using adaptive mesh refinement. We show preliminary validation results for flows past stationary bodies with both attached and separated boundary layers along with results for turbulent flows past moving bodies. This work is supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1324585.

Role of turbulence fluctuations on uncertainties of acoutic Doppler current profiler discharge measurements

USGS Publications Warehouse

Tarrab, Leticia; Garcia, Carlos M.; Cantero, Mariano I.; Oberg, Kevin

2012-01-01

This work presents a systematic analysis quantifying the role of the presence of turbulence fluctuations on uncertainties (random errors) of acoustic Doppler current profiler (ADCP) discharge measurements from moving platforms. Data sets of three-dimensional flow velocities with high temporal and spatial resolution were generated from direct numerical simulation (DNS) of turbulent open channel flow. Dimensionless functions relating parameters quantifying the uncertainty in discharge measurements due to flow turbulence (relative variance and relative maximum random error) to sampling configuration were developed from the DNS simulations and then validated with field-scale discharge measurements. The validated functions were used to evaluate the role of the presence of flow turbulence fluctuations on uncertainties in ADCP discharge measurements. The results of this work indicate that random errors due to the flow turbulence are significant when: (a) a low number of transects is used for a discharge measurement, and (b) measurements are made in shallow rivers using high boat velocity (short time for the boat to cross a flow turbulence structure).

Couette-Poiseuille flow experiment with zero mean advection velocity: Subcritical transition to turbulence

NASA Astrophysics Data System (ADS)

Klotz, L.; Lemoult, G.; Frontczak, I.; Tuckerman, L. S.; Wesfreid, J. E.

2017-04-01

We present an experimental setup that creates a shear flow with zero mean advection velocity achieved by counterbalancing the nonzero streamwise pressure gradient by moving boundaries, which generates plane Couette-Poiseuille flow. We obtain experimental results in the transitional regime for this flow. Using flow visualization, we characterize the subcritical transition to turbulence in Couette-Poiseuille flow and show the existence of turbulent spots generated by a permanent perturbation. Due to the zero mean advection velocity of the base profile, these turbulent structures are nearly stationary. We distinguish two regions of the turbulent spot: the active turbulent core, which is characterized by waviness of the streaks similar to traveling waves, and the surrounding region, which includes in addition the weak undisturbed streaks and oblique waves at the laminar-turbulent interface. We also study the dependence of the size of these two regions on Reynolds number. Finally, we show that the traveling waves move in the downstream (Poiseuille) direction.

Challenges in Scale-Resolving Simulations of turbulent wake flows with coherent structures

NASA Astrophysics Data System (ADS)

Pereira, Filipe S.; Eça, Luís; Vaz, Guilherme; Girimaji, Sharath S.

2018-06-01

The objective of this work is to investigate the challenges encountered in Scale-Resolving Simulations (SRS) of turbulent wake flows driven by spatially-developing coherent structures. SRS of practical interest are expressly intended for efficiently computing such flows by resolving only the most important features of the coherent structures and modelling the remainder as stochastic field. The success of SRS methods depends upon three important factors: i) ability to identify key flow mechanisms responsible for the generation of coherent structures; ii) determine the optimum range of resolution required to adequately capture key elements of coherent structures; and iii) ensure that the modelled part is comprised nearly exclusively of fully-developed stochastic turbulence. This study considers the canonical case of the flow around a circular cylinder to address the aforementioned three key issues. It is first demonstrated using experimental evidence that the vortex-shedding instability and flow-structure development involves four important stages. A series of SRS computations of progressively increasing resolution (decreasing cut-off length) are performed. An a priori basis for locating the origin of the coherent structures development is proposed and examined. The criterion is based on the fact that the coherent structures are generated by the Kelvin-Helmholtz (KH) instability. The most important finding is that the key aspects of coherent structures can be resolved only if the effective computational Reynolds number (based on total viscosity) exceeds the critical value of the KH instability in laminar flows. Finally, a quantitative criterion assessing the nature of the unresolved field based on the strain-rate ratio of mean and unresolved fields is examined. The two proposed conditions and rationale offer a quantitative basis for developing "good practice" guidelines for SRS of complex turbulent wake flows with coherent structures.

Hairpin vortices in the outer and near wall regions of the canonical turbulent boundary layer

NASA Astrophysics Data System (ADS)

Wallace, James; Wu, Xiaohua; Moin, Parviz

2016-11-01

While the dominance of hairpin vortices and their significance for transport processes in the transitional and early turbulent regions of the canonical turbulent boundary layer has been widely accepted, opinion is divided about the developed flow downstream. Here we investigate the representative vortical structures in the outer and near wall regions for the momentum thickness Reynolds number, Reθ , of up to 3000 using the DNS database described in. This boundary layer grows spatially from a laminar state at Reθ = 80 beneath a freestream of continuous and nearly isotropic turbulence decaying from an intensity of 3 to 0.8%. The vortical structures are visualized with the swirling strength, λci. In the outer region hairpin vortices appear and are advected over distances corresponding to about 300 - 400 in Reθ within the fully turbulent region, demonstrating that they are not remnants of transitional structures. The near wall vortical structures are more difficult to visualize and require careful tuning of the swirling strength and making invisible the flow above the near wall region of the flow. The hairpins in this region occur in intermittent clusters that have features remarkably similar to transitional turbulent spots.

Sound production due to large-scale coherent structures. [and identification of noise mechanisms in turbulent shear flow

NASA Technical Reports Server (NTRS)

Gatski, T. B.

1979-01-01

The sound due to the large-scale (wavelike) structure in an infinite free turbulent shear flow is examined. Specifically, a computational study of a plane shear layer is presented, which accounts, by way of triple decomposition of the flow field variables, for three distinct component scales of motion (mean, wave, turbulent), and from which the sound - due to the large-scale wavelike structure - in the acoustic field can be isolated by a simple phase average. The computational approach has allowed for the identification of a specific noise production mechanism, viz the wave-induced stress, and has indicated the effect of coherent structure amplitude and growth and decay characteristics on noise levels produced in the acoustic far field.

Regeneration of near-wall turbulence structures

NASA Technical Reports Server (NTRS)

Hamilton, James M.; Kim, John J.; Waleffe, Fabian A.

1993-01-01

An examination of the regeneration mechanisms of near-wall turbulence and an attempt to investigate the critical Reynolds number conjecture of Waleffe & Kim is presented. The basis is an extension of the 'minimal channel' approach of Jimenez and Moin which emphasizes the near-wall region and further reduces the complexity of the turbulent flow. Reduction of the flow Reynolds number to the minimum value which will allow turbulence to be sustained has the effect of reducing the ratio of the largest scales to the smallest scales or, equivalently, of causing the near-wall region to fill more of the area between the channel walls. In addition, since each wall may have an active near-wall region, half of the channel is always somewhat redundant. If a plane Couette flow is instead chosen as the base flow, this redundancy is eliminated: the mean shear of a plane Couette flow has a single sign, and at low Reynolds numbers, the two wall regions share a single set of structures. A minimal flow with these modifications possesses, by construction, the strongest constraints which allow sustained turbulence, producing a greatly simplified flow in which the regeneration process can be examined.

Comparison of turbulence in a transitional boundary layer to turbulence in a developed boundary layer*

NASA Astrophysics Data System (ADS)

Park, G. I.; Wallace, J.; Wu, X.; Moin, P.

2010-11-01

Using a recent DNS of a flat-plate boundary layer, statistics of turbulence in transition at Reθ= 500 where spots merge (distributions of the mean velocity, rms velocity and vorticity fluctuations, Reynolds shear stress, kinetic energy production and dissipation rates and enstrophy) have been compared to these statistics for the developed boundary layer turbulence at Reθ= 1850. When the distributions in the transitional region, determined in narrow planes 0.03 Reθ wide, exclude regions and times when the flow is not turbulent, they closely resemble those in the developed turbulent state at the higher Reynolds number, especially in the buffer and sublayers. The skin friction coefficient, determined in this conditional manner in the transitional flow is, of course, much larger than that obtained by including both turbulent and non-turbulent information there, and is consistent with a value obtained by extrapolating from the developed turbulent region. We are attempting to perform this data analysis even further upstream in the transitioning flow at Reθ= 300 where the turbulent spots are individuated. These results add further evidence to support the view that the structure of a developed turbulent boundary layer is little different from its structure in its embryonic form in turbulent spots. *CTR 2010 Summer Program research.

Numerical investigation of an internal layer in turbulent flow over a curved hill

NASA Technical Reports Server (NTRS)

Kim, S-W.

1989-01-01

The development of an internal layer in a turbulent boundary layer flow over a curved hill is investigated numerically. The turbulence field of the boundary layer flow over the curved hill is compared with that of a turbulent flow over a symmetric airfoil (which has the same geometry as the curved hill except that the leading and trailing edge plates were removed) to study the influence of the strongly curved surface on the turbulence field. The turbulent flow equations are solved by a control-volume based finite difference method. The turbulence is described by a multiple-time-scale turbulence model supplemented with a near-wall turbulence model. Computational results for the mean flow field (pressure distributions on the walls, wall shearing stresses and mean velocity profiles), the turbulence structure (Reynolds stress and turbulent kinetic energy profiles), and the integral parameters (displacement and momentum thicknesses) compared favorably with the measured data. Computational results show that the internal layer is a strong turbulence field which is developed beneath the external boundary layer and is located very close to the wall. Development of the internal layer was more obviously observed in the Reynolds stress profiles and in the turbulent kinetic energy profiles than in the mean velocity profiles. In this regard, the internal layers is significantly different from wall-bounded simple shear layers in which the mean velocity profile characterizes the boundary layer most distinguishably. Development of such an internal layer, characterized by an intense turbulence field, is attributed to the enormous mean flow strain rate caused by the streamline curvature and the strong pressure gradient. In the turbulent flow over the curved hill, the internal layer begin to form near the forward corner of the hill, merges with the external boundary layer, and develops into a new fully turbulent boundary layer as the fluid flows in the downstream direction. For the flow over the symmetric airfoil, the boundary layer began to form from almost the same location as that of the curved hill, grew in its strength, and formed a fully turbulent boundary layer from mid-part of the airfoil and in the downstream region. Computational results also show that the detailed turbulence structure in the region very close to the wall of the curved hill is almost the same as that of the airfoil in most of the curved regions except near the leading edge. Thus the internal layer of the curved hill and the boundary layer of the airfoil were also almost the same. Development of the wall shearing stress and separation of the boundary layer at the rear end of the curved hill mostly depends on the internal layer and is only slightly influenced by the external boundary layer flow.

A Particle Representation Model for the Deformation of Homogeneous Turbulence

NASA Technical Reports Server (NTRS)

Kassinos, S. C.; Reynolds, W. C.

1996-01-01

In simple flows, where the mean deformation rates are mild and the turbulence has time to come to equilibrium with the mean flow, the Reynolds stresses are determined by the applied strain rate. Hence in these flows, it is often adequate to use an eddy-viscosity representation. The modern family of kappa-epsilon models has been very useful in predicting near equilibrium turbulent flows, where the rms deformation rate S is small compared to the reciprocal time scale of the turbulence (epsilon/kappa). In modern engineering applications, turbulence models are quite often required to predict flows with very rapid deformations (large S kappa/epsilon). In these flows, the structure takes some time to respond and eddy viscosity models are inadequate. The response of turbulence to rapid deformations is given by rapid distortion theory (RDT). Under RDT the nonlinear effects due to turbulence-turbulence interactions are neglected in the governing equations, but even when linearized in this fashion, the governing equations are unclosed at the one-point level due to the non-locality of the pressure fluctuations.

Turbulent Heat Transfer in Curved Pipe Flow

NASA Astrophysics Data System (ADS)

Kang, Changwoo; Yang, Kyung-Soo

2013-11-01

In the present investigation, turbulent heat transfer in fully-developed curved pipe flow with axially uniform wall heat flux has been numerically studied. The Reynolds numbers under consideration are Reτ = 210 (DNS) and 1,000 (LES) based on the mean friction velocity and the pipe radius, and the Prandtl number (Pr) is 0.71. For Reτ = 210 , the pipe curvature (κ) was fixed as 1/18.2, whereas three cases of κ (0.01, 0.05, 0.1) were computed in the case of Reτ = 1,000. The mean velocity, turbulent intensities and heat transfer rates obtained from the present calculations are in good agreement with the previous numerical and experimental results. To elucidate the secondary flow structures due to the pipe curvature, the mean quantities and rms fluctuations of the flow and temperature fields are presented on the pipe cross-sections, and compared with those of the straight pipe flow. To study turbulence structures and their influence on turbulent heat transfer, turbulence statistics including but not limited to skewness and flatness of velocity fluctuations, cross-correlation coefficients, an Octant analysis, and turbulence budgets are presented and discussed. Based on our results, we attempt to clarify the effects of Reynolds number and the pipe curvature on turbulent heat transfer. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0008457).

Large Eddy Simulation of Turbulent Flow in a Ribbed Pipe

NASA Astrophysics Data System (ADS)

Kang, Changwoo; Yang, Kyung-Soo

2011-11-01

Turbulent flow in a pipe with periodically wall-mounted ribs has been investigated by large eddy simulation with a dynamic subgrid-scale model. The value of Re considered is 98,000, based on hydraulic diameter and mean bulk velocity. An immersed boundary method was employed to implement the ribs in the computational domain. The spacing of the ribs is the key parameter to produce the d-type, intermediate and k-type roughness flows. The mean velocity profiles and turbulent intensities obtained from the present LES are in good agreement with the experimental measurements currently available. Turbulence statistics, including budgets of the Reynolds stresses, were computed, and analyzed to elucidate turbulence structures, especially around the ribs. In particular, effects of the ribs are identified by comparing the turbulence structures with those of smooth pipe flow. The present investigation is relevant to the erosion/corrosion that often occurs around a protruding roughness in a pipe system. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0008457).

Discrete-vortex simulation of pulsating flow on a turbulent leading-edge separation bubble

NASA Technical Reports Server (NTRS)

Sung, Hyung Jin; Rhim, Jae Wook; Kiya, Masaru

1992-01-01

Studies are made of the turbulent separation bubble in a two-dimensional semi-infinite blunt plate aligned to a uniform free stream with a pulsating component. The discrete-vortex method is applied to simulate this flow situation because this approach is effective for representing the unsteady motions of the turbulent shear layer and the effect of viscosity near the solid surface. The numerical simulation provides reasonable predictions when compared with the experimental results. A particular frequency with a minimum reattachment is related to the drag reduction. The most effective frequency is dependent on the amplified shedding frequency. The turbulent flow structure is scrutinized. This includes the time-mean and fluctuations of the velocity and the surface pressure, together with correlations between the fluctuating components. A comparison between the pulsating flow and the non-pulsating flow at the particular frequency of the minimum reattachment length of the separation bubble suggests that the large-scale vortical structure is associated with the shedding frequency and the flow instabilities.

Simulation analysis of air flow and turbulence statistics in a rib grit roughened duct.

PubMed

Vogiatzis, I I; Denizopoulou, A C; Ntinas, G K; Fragos, V P

2014-01-01

The implementation of variable artificial roughness patterns on a surface is an effective technique to enhance the rate of heat transfer to fluid flow in the ducts of solar air heaters. Different geometries of roughness elements investigated have demonstrated the pivotal role that vortices and associated turbulence have on the heat transfer characteristics of solar air heater ducts by increasing the convective heat transfer coefficient. In this paper we investigate the two-dimensional, turbulent, unsteady flow around rectangular ribs of variable aspect ratios by directly solving the transient Navier-Stokes and continuity equations using the finite elements method. Flow characteristics and several aspects of turbulent flow are presented and discussed including velocity components and statistics of turbulence. The results reveal the impact that different rib lengths have on the computed mean quantities and turbulence statistics of the flow. The computed turbulence parameters show a clear tendency to diminish downstream with increasing rib length. Furthermore, the applied numerical method is capable of capturing small-scale flow structures resulting from the direct solution of Navier-Stokes and continuity equations.

Effects of forcing time scale on the simulated turbulent flows and turbulent collision statistics of inertial particles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rosa, B., E-mail: bogdan.rosa@imgw.pl; Parishani, H.; Department of Earth System Science, University of California, Irvine, California 92697-3100

2015-01-15

In this paper, we study systematically the effects of forcing time scale in the large-scale stochastic forcing scheme of Eswaran and Pope [“An examination of forcing in direct numerical simulations of turbulence,” Comput. Fluids 16, 257 (1988)] on the simulated flow structures and statistics of forced turbulence. Using direct numerical simulations, we find that the forcing time scale affects the flow dissipation rate and flow Reynolds number. Other flow statistics can be predicted using the altered flow dissipation rate and flow Reynolds number, except when the forcing time scale is made unrealistically large to yield a Taylor microscale flow Reynoldsmore » number of 30 and less. We then study the effects of forcing time scale on the kinematic collision statistics of inertial particles. We show that the radial distribution function and the radial relative velocity may depend on the forcing time scale when it becomes comparable to the eddy turnover time. This dependence, however, can be largely explained in terms of altered flow Reynolds number and the changing range of flow length scales present in the turbulent flow. We argue that removing this dependence is important when studying the Reynolds number dependence of the turbulent collision statistics. The results are also compared to those based on a deterministic forcing scheme to better understand the role of large-scale forcing, relative to that of the small-scale turbulence, on turbulent collision of inertial particles. To further elucidate the correlation between the altered flow structures and dynamics of inertial particles, a conditional analysis has been performed, showing that the regions of higher collision rate of inertial particles are well correlated with the regions of lower vorticity. Regions of higher concentration of pairs at contact are found to be highly correlated with the region of high energy dissipation rate.« less

Numerical investigation of turbulent channel flow

NASA Technical Reports Server (NTRS)

Moin, P.; Kim, J.

1981-01-01

Fully developed turbulent channel flow was simulated numerically at Reynolds number 13800, based on centerline velocity and channel halt width. The large-scale flow field was obtained by directly integrating the filtered, three dimensional, time dependent, Navier-Stokes equations. The small-scale field motions were simulated through an eddy viscosity model. The calculations were carried out on the ILLIAC IV computer with up to 516,096 grid points. The computed flow field was used to study the statistical properties of the flow as well as its time dependent features. The agreement of the computed mean velocity profile, turbulence statistics, and detailed flow structures with experimental data is good. The resolvable portion of the statistical correlations appearing in the Reynolds stress equations are calculated. Particular attention is given to the examination of the flow structure in the vicinity of the wall.

Numerical study of entropy generation due to coupled laminar and turbulent mixed convection and thermal radiation in an enclosure filled with a semitransparent medium.

PubMed

Goodarzi, M; Safaei, M R; Oztop, Hakan F; Karimipour, A; Sadeghinezhad, E; Dahari, M; Kazi, S N; Jomhari, N

2014-01-01

The effect of radiation on laminar and turbulent mixed convection heat transfer of a semitransparent medium in a square enclosure was studied numerically using the Finite Volume Method. A structured mesh and the SIMPLE algorithm were utilized to model the governing equations. Turbulence and radiation were modeled with the RNG k-ε model and Discrete Ordinates (DO) model, respectively. For Richardson numbers ranging from 0.1 to 10, simulations were performed for Rayleigh numbers in laminar flow (10⁴) and turbulent flow (10⁸). The model predictions were validated against previous numerical studies and good agreement was observed. The simulated results indicate that for laminar and turbulent motion states, computing the radiation heat transfer significantly enhanced the Nusselt number (Nu) as well as the heat transfer coefficient. Higher Richardson numbers did not noticeably affect the average Nusselt number and corresponding heat transfer rate. Besides, as expected, the heat transfer rate for the turbulent flow regime surpassed that in the laminar regime. The simulations additionally demonstrated that for a constant Richardson number, computing the radiation heat transfer majorly affected the heat transfer structure in the enclosure; however, its impact on the fluid flow structure was negligible.

Numerical Study of Entropy Generation due to Coupled Laminar and Turbulent Mixed Convection and Thermal Radiation in an Enclosure Filled with a Semitransparent Medium

PubMed Central

Goodarzi, M.; Safaei, M. R.; Oztop, Hakan F.; Karimipour, A.; Sadeghinezhad, E.; Dahari, M.; Kazi, S. N.; Jomhari, N.

2014-01-01

The effect of radiation on laminar and turbulent mixed convection heat transfer of a semitransparent medium in a square enclosure was studied numerically using the Finite Volume Method. A structured mesh and the SIMPLE algorithm were utilized to model the governing equations. Turbulence and radiation were modeled with the RNG k-ε model and Discrete Ordinates (DO) model, respectively. For Richardson numbers ranging from 0.1 to 10, simulations were performed for Rayleigh numbers in laminar flow (104) and turbulent flow (108). The model predictions were validated against previous numerical studies and good agreement was observed. The simulated results indicate that for laminar and turbulent motion states, computing the radiation heat transfer significantly enhanced the Nusselt number (Nu) as well as the heat transfer coefficient. Higher Richardson numbers did not noticeably affect the average Nusselt number and corresponding heat transfer rate. Besides, as expected, the heat transfer rate for the turbulent flow regime surpassed that in the laminar regime. The simulations additionally demonstrated that for a constant Richardson number, computing the radiation heat transfer majorly affected the heat transfer structure in the enclosure; however, its impact on the fluid flow structure was negligible. PMID:24778601

Free compressible jet investigation

NASA Astrophysics Data System (ADS)

De Gregorio, Fabrizio

2014-03-01

The nozzle pressure ratio (NPR) effect on a supersonic turbulent jet was investigated. A dedicated convergent/divergent nozzle together with a flow feeding system was designed and manufactured. A nozzle Mach exit of M j = 1.5 was selected in order to obtain a convective Mach number of M c = 0.6. The flow was investigated for over-expanded, correctly expanded and under-expanded jet conditions. Mach number, total temperature and flow velocity measurements were carried out in order to characterise the jet behaviour. The inlet conditions of the jet flow were monitored in order to calculate the nozzle exit speed of sound and evaluate the mean Mach number distribution starting from the flow velocity data. A detailed analysis of the Mach results obtained by a static Pitot probe and by a particle image velocimetry measurement system was carried out. The mean flow velocity was investigated, and the axial Mach decay and the spreading rate were associated with the flow structures and with the compressibility effects. Aerodynamics of the different jet conditions was evaluated, and the shock cells structures were detected and discussed correlating the jet structure to the flow fluctuation and local turbulence. The longitudinal and radial distribution of the total temperature was investigated, and the temperature profiles were analysed and discussed. The total temperature behaviour was correlated to the turbulent phenomena and to the NPR jet conditions. Self-similarity condition was encountered and discussed for the over-expanded jet. Compressibility effects on the local turbulence, on the turbulent kinetic energy and on the Reynolds tensor were discussed.

Investigation of the asymptotic state of rotating turbulence using large-eddy simulation

NASA Technical Reports Server (NTRS)

Squires, Kyle D.; Chasnov, Jeffrey R.; Mansour, Nagi N.; Cambon, Claude

1993-01-01

Study of turbulent flows in rotating reference frames has long been an area of considerable scientific and engineering interest. Because of its importance, the subject of turbulence in rotating reference frames has motivated over the years a large number of theoretical, experimental, and computational studies. The bulk of these previous works has served to demonstrate that the effect of system rotation on turbulence is subtle and remains exceedingly difficult to predict. A rotating flow of particular interest in many studies, including the present work, is examination of the effect of solid-body rotation on an initially isotropic turbulent flow. One of the principal reasons for the interest in this flow is that it represents the most basic turbulent flow whose structure is altered by system rotation but without the complicating effects introduced by mean strains or flow inhomogeneities. The assumption of statistical homogeneity considerably simplifies analysis and computation. The principal objective of the present study has been to examine the asymptotic state of solid-body rotation applied to an initially isotropic, high Reynolds number turbulent flow. Of particular interest has been to determine the degree of two-dimensionalization and the existence of asymptotic self-similar states in homogeneous rotating turbulence.

Conference on Complex Turbulent Flows: Comparison of Computation and Experiment, Stanford University, Stanford, CA, September 14-18, 1981, Proceedings. Volume 2 - Taxonomies, reporters' summaries, evaluation, and conclusions

NASA Technical Reports Server (NTRS)

Kline, S. J. (Editor); Cantwell, B. J. (Editor); Lilley, G. M.

1982-01-01

Computational techniques for simulating turbulent flows were explored, together with the results of experimental investigations. Particular attention was devoted to the possibility of defining a universal closure model, applicable for all turbulence situations; however, conclusions were drawn that zonal models, describing localized structures, were the most promising techniques to date. The taxonomy of turbulent flows was summarized, as were algebraic, differential, integral, and partial differential methods for numerical depiction of turbulent flows. Numerous comparisons of theoretically predicted and experimentally obtained data for wall pressure distributions, velocity profiles, turbulent kinetic energy profiles, Reynolds shear stress profiles, and flows around transonic airfoils were presented. Simplifying techniques for reducing the necessary computational time for modeling complex flowfields were surveyed, together with the industrial requirements and applications of computational fluid dynamics techniques.

Interactive wall turbulence control

NASA Technical Reports Server (NTRS)

Wilkinson, Stephen P.

1990-01-01

After presenting boundary layer turbulence physics in a manner that emphasizes the possible modification of structural surfaces in a way that locally alters the production of turbulent flows, an account is given of the hardware that could plausibly be employed to implement such a turbulence-control scheme. The essential system components are flow sensors, electronic processors, and actuators; at present, actuator technology presents the greatest problems and limitations. High frequency/efficiency actuators are required to handle three-dimensional turbulent motions whose frequency and intensity increases in approximate proportion to freestream speed.

Impact of Ocean Surface Waves on Air-Sea Momentum Flux

NASA Astrophysics Data System (ADS)

Tamura, H.; Drennan, W. M.; Collins, C. O., III; Graber, H. C.

2016-02-01

In this study, we investigated the structure of turbulent air flow over ocean waves. Observations of wind and waves were retrieved by air-sea interaction spar (ASIS) buoys during the shoaling waves experiment (SHOWEX) in Duck, NC in 1999. It is shown that the turbulent velocity spectra and co-spectra for pure wind sea conditions follow the universal forms estimated by Miyake et al [1970]. In the presence of strong swells, the wave boundary layer was extended and the universal spectral scaling of u'w' broke down [Drennan et al, 1999]. On the other hand, the use of the peak wave frequency (fp) to reproduce the "universal spectra" succeeded at explaining the spectral structure of turbulent flow field. The u'w' co-spectra become negative near the fp, which suggests the upward momentum transport (i.e., negative wind stress) induced by ocean waves. Finally, we propose three turbulent flow structures for different wind-wave regimes.

Numerical study of turbulent secondary flows in curved ducts

NASA Technical Reports Server (NTRS)

Hur, N.; Thangam, S.; Speziale, C. G.

1990-01-01

The pressure driven, fully-developed turbulent flow of an incompressible viscous fluid in curved ducts of square-section is studied numerically by making use of a finite volume method. A nonlinear Kappa - Iota model is used to represent the turbulence. The results for both straight and curved ducts are presented. For the case of fully-developed turbulent flow in straight and curved ducts, the secondary flow is characterized by an eight-vortex structure for which the computed flowfield is shown to be in good agreement with available experimental data. The introduction of moderate curvature is shown to cause a substantial increase in the strength of the secondary flow and to change the secondary flow pattern to either a double-vortex or a four-vortex configuration.

Numerical study of turbulent secondary flows in curved ducts

NASA Technical Reports Server (NTRS)

Hur, N.; Thangam, S.; Speziale, C. G.

1989-01-01

The pressure driven, fully-developed turbulent flow of an incompressible viscous fluid in curved ducts of square cross-section is studied numerically by making use of a finite volume method. A nonlinear Kappa - Iota model is used to represent the turbulence. The results for both straight and curved ducts are presented. For the case of fully-developed turbulent flow in straight ducts, the secondary flow is characterized by an eight-vortex structure for which the computed flowfield is shown to be in good agreement with available experimental data. The introduction of moderate curvature is shown to cause a substantial increase in the strength of the secondary flow and to change the secondary flow pattern to either a double-vortex or a four-vortex configuration.

The Analysis of Turbulent Flow by Hot Wire Signals. Ph.D. Thesis - Physikalische Ingenieurvissenschaft der Technischen Univ., 1981

NASA Technical Reports Server (NTRS)

Bartenwerfer, M.

1982-01-01

When measuring velocities in turbulent gas flow, approximation signal analysis with hot wire anemometers having one and two wire probes are used. A numeric test of standard analyses shows the resulting systemmatic error increases quickly with increasing turbulent intensity. Since it also depends on the turbulence structure, it cannot be corrected. The use of such probes is thus restricted to low turbulence. By means of three wire probes (in two dimensional flows with X wire probes) in principle, instantaneous values of velocity can be determined, and an asymmetric arrangement of wires has a theoretical advantage.

Dynamics and structures of transitional viscoelastic turbulence in channel flow

NASA Astrophysics Data System (ADS)

Shekar, Ashwin; Wang, Sung-Ning; Graham, Michael

2017-11-01

Introducing a trace amount of polymer into turbulent flows can result in a substantial reduction of drag. However, the mechanism is not fully understood at high levels of drag reduction. In this work we perform direct numerical simulations (DNS) of viscoelastic channel flow turbulence using a scheme that guarantees the positive-definiteness of polymer conformation tensor without artificial diffusion. Here we present the results of two parametric studies with the bulk Reynolds number fixed at 2000. First, the Weissenberg number (Wi) is kept at 100 and we vary the viscosity ratio (ratio ratio of the solvent viscosity and the total viscosity). Maximum drag reduction (MDR) is observed with viscosity ratio <0.95. As we decrease the viscosity ratio, i.e. increase polymer concentration, the mean velocity profile is almost invariant. However, this is accompanied by a decrease in velocity fluctuations but the flow stays turbulent. Turbulent kinetic energy budget analysis shows that, in this parameter regime, polymer becomes the major source of velocity fluctuations, replacing the energy transfer from the mean flow. In the second study, we fix the viscosity ratio at 0.95 and trace the Wi up to this regime and present the accompanying changes in flow quantities and structures.

Contributions of numerical simulation data bases to the physics, modeling and measurement of turbulence

NASA Technical Reports Server (NTRS)

Moin, Parviz; Spalart, Philippe R.

1987-01-01

The use of simulation data bases for the examination of turbulent flows is an effective research tool. Studies of the structure of turbulence have been hampered by the limited number of probes and the impossibility of measuring all desired quantities. Also, flow visualization is confined to the observation of passive markers with limited field of view and contamination caused by time-history effects. Computer flow fields are a new resource for turbulence research, providing all the instantaneous flow variables in three-dimensional space. Simulation data bases also provide much-needed information for phenomenological turbulence modeling. Three dimensional velocity and pressure fields from direct simulations can be used to compute all the terms in the transport equations for the Reynolds stresses and the dissipation rate. However, only a few, geometrically simple flows have been computed by direct numerical simulation, and the inventory of simulation does not fully address the current modeling needs in complex turbulent flows. The availability of three-dimensional flow fields also poses challenges in developing new techniques for their analysis, techniques based on experimental methods, some of which are used here for the analysis of direct-simulation data bases in studies of the mechanics of turbulent flows.

Modifications to intermittent turbulent structures by sheared flow in LAPD

NASA Astrophysics Data System (ADS)

Rossi, Giovanni; Schaffner, David; Carter, Troy; Guice, Danny; Bengtson, Roger

2012-10-01

Turbulence in the edge of the Large Plasma Device is generally observed to be intermittent with the production of filamentary structures. Density-enhancement events (called ``blobs'') are localized to the region radially outside the edge of the cathode source while density-depletion events (called ``holes'') are localized to the region radially inward. A flow-shear layer is also observed to be localized to this same spatial region. Control over the edge flow and shear in LAPD is now possible using a biasable limiter. Edge intermittency is observed to be strongly affected by variations in the edge flow, with intermittency (as measured by skewness of the fluctuation amplitude PDF) increasing with edge flow (in either direction) and reaching a minimum when spontaneous edge flow is zeroed-out using biasing. This trend is counter to the observed changes in turbulent particle flux, which peaks at low flow/shear. Two-dimensional cross-conditional averaging confirms the blobs to be detached filamentary structures with a clear dipolar potential structure and a geometry also dependent on the magnitude of sheared flow. More detailed measurements are made to connect the occurrence of these blobs to observed flow-driven coherent modes and their contribution to radial particle flux.

Inside pyroclastic density currents - uncovering the enigmatic flow structure and transport behaviour in large-scale experiments

NASA Astrophysics Data System (ADS)

Breard, Eric C. P.; Lube, Gert

2017-01-01

Pyroclastic density currents (PDCs) are the most lethal threat from volcanoes. While there are two main types of PDCs (fully turbulent, fully dilute pyroclastic surges and more concentrated pyroclastic flows encompassing non-turbulent to turbulent transport) pyroclastic flows, which are the subject of the present study, are far more complex than dilute pyroclastic surges and remain the least understood type despite their far greater hazard, greater runout length and ability to transport vast quantities of material across the Earth's surface. Here we present large-scale experiments of natural volcanic material and gas in order to provide the missing quantitative view of the internal structure and gas-particle transport mechanisms in pyroclastic flows. We show that the outer flow structure with head, body and wake regions broadly resembles current PDC analogues of dilute gravity currents. However, the internal structure, in which lower levels consist of a concentrated granular fluid and upper levels are more dilute, contrasts significantly with the internal structure of fully dilute gravity currents. This bipartite vertical structure shows strong analogy to current conceptual models of high-density turbidity currents, which are responsible for the distribution of coarse sediment in marine basins and of great interest to the hydrocarbon industry. The lower concentrated and non-turbulent levels of the PDC (granular-fluid basal flow) act as a fast-flowing carrier for the more dilute and turbulent upper levels of the current (ash-cloud surge). Strong kinematic coupling between these flow parts reduces viscous dissipation and entrainment of ambient air into the lower part of the ash-cloud surge. This leads to a state of forced super-criticality whereby fast and destructive PDCs can endure even at large distances from volcanoes. Importantly, the basal flow/ash-cloud surge coupling yields a characteristically smooth rheological boundary across the non-turbulent/turbulent interface, as well as vertical velocity and density profiles in the ash-cloud surge, which strongly differ from current theoretical predictions. Observed generation of successive pulses of high dynamic pressure within the upper dilute levels of the PDC may be important to understand the destructive potential of PDCs. The experiments further show that a wide range in the degree of coupling between particle and gas phases is critical to the vertical and longitudinal segregation of the currents into reaches that have starkly contrasting sediment transport capacities. In particular, the formation of mesoscale turbulence clusters under strong particle-gas feedback controls vertical stratification inside the turbulent upper levels of the current (ash-cloud surge) and triggers significant transfers of mass and momentum from the ash-cloud surge onto the granular-fluid basal flow. These results open up new pathways to advance current computational PDC hazard models and to describe and interpret PDCs as well as other types of high-density gravity currents transported across the surfaces of Earth and other planets and across marine basins.

Simultaneous PIV and PLIF measurement of passive scalar mixing in a confined planar jet

NASA Astrophysics Data System (ADS)

Feng, Hua

2005-11-01

Simultaneous velocity and concentration fields in a confined liquid-phase planar jet with a Reynolds number based on hydraulic diameter of 50,000 were obtained using combined particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF). Data at six downstream locations were analyzed for flow statistics such as mean velocity, Reynolds stresses, turbulent kinetic energy, concentration mean and variance, turbulent fluxes, turbulent viscosity and diffusivity, and turbulent Schmidt number. Spatial correlation fields of turbulent fluxes and concentration were then determined. The Ru'φ' correlation was elliptical in shape with a major axis tilted downward with respect to the streamwise axis, whereas the Rv'φ' correlation was a horizontally oriented ellipse. The Rφ'φ' correlation field was found to be an ellipse with the major axis inclined at about 45-degrees with respect to the streamwise direction. Linear stochastic estimation was used to determine conditional flow structures. Large-scale structures were observed in the conditional velocity fields that are elliptical in shape with a streamwise major axis. The size of the structure initially increased linearly with respect to downstream distance, but then grew more slowly as the flow evolved towards channel flow.

The effect of hydrodynamic conditions on the phenotype of Pseudomonas fluorescens biofilms.

PubMed

Simões, Manuel; Pereira, Maria O; Sillankorva, Sanna; Azeredo, Joana; Vieira, Maria J

2007-01-01

This study investigated the phenotypic characteristics of monoculture P. fluorescens biofilms grown under turbulent and laminar flow, using flow cells reactors with stainless steel substrata. The cellular physiology and the overall biofilm activity, structure and composition were characterized, and compared, within hydrodynamically distinct conditions. The results indicate that turbulent flow-generated biofilm cells were significantly less extensive, with decreased metabolic activity and a lower protein and polysaccharides composition per cell than those from laminar flow-generated biofilms. The effect of flow regime did not cause significantly different outer membrane protein expression. From the analysis of biofilm activity, structure and composition, turbulent flow-generated biofilms were metabolically more active, had twice more mass per cm(2), and higher cellular density and protein content (mainly cellular) than laminar flow-generated biofilms. Conversely, laminar flow-generated biofilms presented higher total and matrix polysaccharide contents. Direct visualisation and scanning electron microscopy analysis showed that these different flows generate structurally different biofilms, corroborating the quantitative results. The combination of applied methods provided useful information regarding a broad spectrum of biofilm parameters, which can contribute to control and model biofilm processes.

Effects of spanwise rotation on the structure of two-dimensional fully developed turbulent channel flow.

NASA Technical Reports Server (NTRS)

Johnston, J. P.; Halleen, R. M.; Lezius, D. K.

1972-01-01

Experiments on fully developed turbulent flow in a channel which is rotating at a steady rate about a spanwise axis are described. The Coriolis force components in the region of two-dimensional mean flow affect both local and global stability. Three stability-related phenomena were observed or inferred: (1) the reduction (increase) of the rate of wall-layer streak bursting in locally stabilized (destabilized) wall layers; (2) the total suppression of transition to turbulence in a stabilized layer; (3) the development of large-scale roll cells on the destabilized side of the channel by growth of a Taylor-Gortler vortex instability. Local effects of rotational stabilization, such as reduction of the turbulent stress in wall layers, can be related to the local Richardson number in a simple way. This paper not only investigates this effect, but also, by methods of flow visualization, exposes some of the underlying structure changes caused by rotation.-

Experiments in free shear flows: Status and needs for the future

NASA Technical Reports Server (NTRS)

Kline, S. J.; Coles, D. E.; Eggers, J. M.; Harsha, P. T.

1973-01-01

Experiments in free turbulent flows are recommended with the primary concern placed on classical flows in order to augment understanding and for model building. Five classes of experiments dealing with classical free turbulent flows are outlined and proposed as being of particular significance for the near future. These classes include the following: (1) Experiments clarifying the effect of density variation owing to use of different gases, with and without the additional effect of density variation due to high Mach number or other effects; (2) experiments clarifying the role and importance of various parameters which determine the behavior of the near field as well as the condictions under which any of these parameters can be neglected; (3) experiments determining the cumulative effect of initial conditions in terms of distance to fully established flow; (4) experiments for cases where two layers of distinctly different initial turbulence structure flow side by side at the same mean speed; and (5) experiment using contemporary experimental techniques to study structure in free turbulent shear flows in order to compliment and support contemporary work on boundary layers.

The effect of wall geometry in particle-laden turbulent flow

NASA Astrophysics Data System (ADS)

Abdehkakha, Hoora; Iaccarino, Gianluca

2016-11-01

Particle-laden turbulent flow plays a significant role in various industrial applications, as turbulence alters the exchange of momentum and energy between particles and fluid flow. In wall-bounded flows, inhomogeneity in turbulent properties is the primary cause of turbophoresis that leads the particles toward the walls. Conversely, shear-induced lift force on the particles can become important if large scale vortical structures are present. The objective of this study is to understand the effects of geometry on fluid flows and consequently on particles transport and concentration. Direct numerical simulations combined with point particle Lagrangian tracking are performed for several geometries such as a pipe, channel, square duct, and squircle (rounded-corners duct). In non-circular ducts, anisotropic and inhomogeneous Reynolds stresses are the most influential phenomena that produce the secondary flows. It has been shown that these motions can have a significant impact on transporting momentum, vorticity, and energy from the core of the duct to the corners. The main focus of the present study is to explore the effects of near the wall structures and secondary flows on turbophoresis, lift, and particle concentration.

Measurements and Modeling of the Mean and Turbulent Flow Structure in High-Speed Rough-Wall Non-Equilibrium Boundary Layers

DTIC Science & Technology

2010-01-25

study builds on three basic bodies of knowledge: (1) supersonic rough wall boundary layers, (2) distorted supersonic turbulent boundary layers, and...with the boundary layer turbulence . The present study showed that secondary distortions associated with such waves significantly affect the transport...38080 14. ABSTRACT The response of a supersonic high Reynolds number turbulent boundary layer flow subjected to mechanical distortions was

An extended algebraic variational multiscale-multigrid-multifractal method (XAVM4) for large-eddy simulation of turbulent two-phase flow

NASA Astrophysics Data System (ADS)

Rasthofer, U.; Wall, W. A.; Gravemeier, V.

2018-04-01

A novel and comprehensive computational method, referred to as the eXtended Algebraic Variational Multiscale-Multigrid-Multifractal Method (XAVM4), is proposed for large-eddy simulation of the particularly challenging problem of turbulent two-phase flow. The XAVM4 involves multifractal subgrid-scale modeling as well as a Nitsche-type extended finite element method as an approach for two-phase flow. The application of an advanced structural subgrid-scale modeling approach in conjunction with a sharp representation of the discontinuities at the interface between two bulk fluids promise high-fidelity large-eddy simulation of turbulent two-phase flow. The high potential of the XAVM4 is demonstrated for large-eddy simulation of turbulent two-phase bubbly channel flow, that is, turbulent channel flow carrying a single large bubble of the size of the channel half-width in this particular application.

Separated flows receptivity for external disturbances

NASA Astrophysics Data System (ADS)

Zanin, B. Yu.

2017-10-01

Results of experimental investigations of the flow over a straight-wing model in a low-turbulence wind tunnel are reported. The influence of a turbulent wake due to a thin filament on the structure of boundary layer on the model surface was examined. Also the fishing line was installed in the test section of the wind tunnel and the effect of line on the boundary-layer flow structure is considered. Flow visualization in boundary layer and hot-wire measurements were performed. The wake and the grid substantially modified the boundary layer flow pattern: the separation disappeared from the wing surface, and the formation of longitudinal structures was observed.

Swirl effect on flow structure and mixing in a turbulent jet

NASA Astrophysics Data System (ADS)

Kravtsov, Z. D.; Sharaborin, D. K.; Dulin, V. M.

2018-03-01

The paper reports on experimental study of turbulent transport in the initial region of swirling turbulent jets. The particle image velocimetry and planar laser-induced fluorescence techniques are used to investigate the flow structure and passive scalar concentration, respectively, in free air jet with acetone vapor. Three flow cases are considered, viz., non-swirling jets and swirling jets with and without vortex breakdown and central recirculation zone. Without vortex breakdown, the swirl is shown to promote jet mixing with surrounding air and to decrease the jet core length. The vortex core breakdown further enhances mixing as the jet core disintegrates at the nozzle exit.

Large eddy simulation study of spanwise spacing effects on secondary flows in turbulent channel flow

NASA Astrophysics Data System (ADS)

Aliakbarimiyanmahaleh, Mohammad; Anderson, William

2015-11-01

The structure of turbulent flow over a complex topography composed of streamwise-aligned rows of cones with varying spanwise spacing, s is studied with large-eddy simulation (LES). Similar to the experimental study of Vanderwel and Ganapathisubramani, 2015: J. Fluid Mech., we investigate the relationship between secondary flow and s, for 0 . 25 <= s / δ <= 5 . For cases with s / δ > 2 , domain-scale rollers freely exist. These had previously been called ``turbulent secondary flows'' (Willingham et al., 2014: Phys. Fluids; Barros and Christensen, 2014: J. Fluid Mech.; Anderson et al., 2015: J. Fluid Mech.), but closer inspection of the statistics indicates these are a turbulent tertiary flow: they only remain ``anchored'' to the conical roughness elements for s / δ > 2 . For s / δ < 2 , turbulent tertiary flows are prevented from occupying the domain by virtue of proximity to adjacent, counter-rotating tertiary flows. Turbulent secondary flows are associated with the conical roughness elements. These turbulent secondary flows emanate from individual conical topographic elements and set the roughness sublayer depth. The turbulent secondary flows remain intact for large and small spacing. For s / δ < 1 , a mean tertiary flow is not present. This work was supported by the Air Force Office of Sci. Research, Young Inv. Program (PM: Dr. R. Ponnoppan and Ms. E. Montomery) under Grant # FA9550-14-1-0394. Computational resources were provided by the Texas Adv. Comp. Center at the Univ. of Texas.

Structure analysis of turbulent liquid phase by POD and LSE techniques

DOE Office of Scientific and Technical Information (OSTI.GOV)

Munir, S., E-mail: shahzad-munir@comsats.edu.pk; Muthuvalu, M. S.; Siddiqui, M. I.

2014-10-24

In this paper, vortical structures and turbulence characteristics of liquid phase in both single liquid phase and two-phase slug flow in pipes were studied. Two dimensional velocity vector fields of liquid phase were obtained by Particle image velocimetry (PIV). Two cases were considered one single phase liquid flow at 80 l/m and second slug flow by introducing gas at 60 l/m while keeping liquid flow rate same. Proper orthogonal decomposition (POD) and Linear stochastic estimation techniques were used for the extraction of coherent structures and analysis of turbulence in liquid phase for both cases. POD has successfully revealed large energymore » containing structures. The time dependent POD spatial mode coefficients oscillate with high frequency for high mode numbers. The energy distribution of spatial modes was also achieved. LSE has pointed out the coherent structured for both cases and the reconstructed velocity fields are in well agreement with the instantaneous velocity fields.« less

Transition in Pulsatile Pipe Flow

NASA Astrophysics Data System (ADS)

Vlachos, Pavlos; Brindise, Melissa

2016-11-01

Transition has been observed to occur in the aorta, and stenotic vessels, where pulsatile flow exists. However, few studies have investigated the characteristics and effects of transition in oscillating or pulsatile flow and none have utilized a physiological waveform. In this work, we explore transition in pipe flow using three pulsatile waveforms which all maintain the same mean and maximum flow rates and range to zero flow, as is physiologically typical. Velocity fields were obtained using planar particle image velocimetry for each pulsatile waveform at six mean Reynolds numbers ranging between 500 and 4000. Turbulent statistics including turbulent kinetic energy (TKE) and Reynolds stresses were computed. Quadrant analysis was used to identify characteristics of the production and dissipation of turbulence. Coherent structures were identified using the λci method. We developed a wavelet-Hilbert time-frequency analysis method to identify high frequency structures and compared these to the coherent structures. The results of this study demonstrate that the different pulsatile waveforms induce different levels of TKE and high frequency structures, suggesting that the rates of acceleration and deceleration influence the onset and development of transition.

Control of Flow Structure in Square Cross-Sectioned U Bend using Numerical Modeling

NASA Astrophysics Data System (ADS)

Yavuz, Mehmet Metin; Guden, Yigitcan

2014-11-01

Due to the curvature in U-bends, the flow development involves complex flow structures including Dean vortices and high levels of turbulence that are quite critical in considering noise problems and structural failure of the ducts. Computational fluid dynamic (CFD) models are developed using ANSYS Fluent to analyze and to control the flow structure in a square cross-sectioned U-bend with a radius of curvature Rc/D = 0.65. The predictions of velocity profiles on different angular positions of the U-bend are compared against the experimental results available in the literature and the previous numerical studies. The performances of different turbulence models are evaluated to propose the best numerical approach that has high accuracy with reduced computation time. The numerical results of the present study indicate improvements with respect to the previous numerical predictions and very good agreement with the available experimental results. In addition, a flow control technique is utilized to regulate the flow inside the bend. The elimination of Dean vortices along with significant reduction in turbulence levels in different cross flow planes are successfully achieved when the flow control technique is applied. The project is supported by Meteksan Defense Industries, Inc.

Bioinspired surfaces for turbulent drag reduction

PubMed Central

Golovin, Kevin B.; Gose, James W.; Perlin, Marc; Ceccio, Steven L.; Tuteja, Anish

2016-01-01

In this review, we discuss how superhydrophobic surfaces (SHSs) can provide friction drag reduction in turbulent flow. Whereas biomimetic SHSs are known to reduce drag in laminar flow, turbulence adds many new challenges. We first provide an overview on designing SHSs, and how these surfaces can cause slip in the laminar regime. We then discuss recent studies evaluating drag on SHSs in turbulent flow, both computationally and experimentally. The effects of streamwise and spanwise slip for canonical, structured surfaces are well characterized by direct numerical simulations, and several experimental studies have validated these results. However, the complex and hierarchical textures of scalable SHSs that can be applied over large areas generate additional complications. Many studies on such surfaces have measured no drag reduction, or even a drag increase in turbulent flow. We discuss how surface wettability, roughness effects and some newly found scaling laws can help explain these varied results. Overall, we discuss how, to effectively reduce drag in turbulent flow, an SHS should have: preferentially streamwise-aligned features to enhance favourable slip, a capillary resistance of the order of megapascals, and a roughness no larger than 0.5, when non-dimensionalized by the viscous length scale. This article is part of the themed issue ‘Bioinspired hierarchically structured surfaces for green science’. PMID:27354731

Bioinspired surfaces for turbulent drag reduction.

PubMed

Golovin, Kevin B; Gose, James W; Perlin, Marc; Ceccio, Steven L; Tuteja, Anish

2016-08-06

In this review, we discuss how superhydrophobic surfaces (SHSs) can provide friction drag reduction in turbulent flow. Whereas biomimetic SHSs are known to reduce drag in laminar flow, turbulence adds many new challenges. We first provide an overview on designing SHSs, and how these surfaces can cause slip in the laminar regime. We then discuss recent studies evaluating drag on SHSs in turbulent flow, both computationally and experimentally. The effects of streamwise and spanwise slip for canonical, structured surfaces are well characterized by direct numerical simulations, and several experimental studies have validated these results. However, the complex and hierarchical textures of scalable SHSs that can be applied over large areas generate additional complications. Many studies on such surfaces have measured no drag reduction, or even a drag increase in turbulent flow. We discuss how surface wettability, roughness effects and some newly found scaling laws can help explain these varied results. Overall, we discuss how, to effectively reduce drag in turbulent flow, an SHS should have: preferentially streamwise-aligned features to enhance favourable slip, a capillary resistance of the order of megapascals, and a roughness no larger than 0.5, when non-dimensionalized by the viscous length scale.This article is part of the themed issue 'Bioinspired hierarchically structured surfaces for green science'. © 2016 The Author(s).

Phenomenological and statistical analyses of turbulence in forced convection with temperature-dependent viscosity under non-Boussinesq condition.

PubMed

Yahya, S M; Anwer, S F; Sanghi, S

2013-10-01

In this work, Thermal Large Eddy Simulation (TLES) is performed to study the behavior of weakly compressible Newtonian fluids with anisotropic temperature-dependent viscosity in forced convection turbulent flow. A systematic analysis of variable-viscosity effects, isolated from gravity, with relevance to industrial cooling/heating applications is being carried out. A LES of a planar channel flow with significant heat transfer at a low Mach number was performed to study effects of fluid property variation on the near-wall turbulence structure. In this flow configuration the top wall is maintained at a higher temperature (T hot ) than the bottom wall (T cold ). The temperature ratio (R θ = T hot /T cold ) is fixed at 1.01, 2 and 3 to study the effects of property variations at low Mach number. Results indicate that average and turbulent fields undergo significant changes. Compared with isothermal flow with constant viscosity, we observe that turbulence is enhanced in the cold side of the channel, characterized by locally lower viscosity whereas a decrease of turbulent kinetic energy is found at the hot wall. The turbulent structures near the cold wall are very short and densely populated vortices but near the hot wall there seems to be a long streaky structure or large elongated vortices. Spectral study reveals that turbulence is completely suppressed at the hot side of the channel at a large temperature ratio because no inertial zone is obtained (i.e. index of Kolmogorov scaling law is zero) from the spectra in these region.

Numerical Simulation of Flow Features and Energy Exchange Physics in Near-Wall Region with Fluid-Structure Interaction

NASA Astrophysics Data System (ADS)

Zhang, Lixiang; Wang, Wenquan; Guo, Yakun

Large eddy simulation is used to explore flow features and energy exchange physics between turbulent flow and structure vibration in the near-wall region with fluid-structure interaction (FSI). The statistical turbulence characteristics in the near-wall region of a vibrating wall, such as the skin frictional coefficient, velocity, pressure, vortices, and the coherent structures have been studied for an aerofoil blade passage of a true three-dimensional hydroturbine. The results show that (i) FSI greatly strengthens the turbulence in the inner region of y+ < 25; and (ii) the energy exchange mechanism between the flow and the vibration depends strongly on the vibration-induced vorticity in the inner region. The structural vibration provokes a frequent action between the low- and high-speed streaks to balance the energy deficit caused by the vibration. The velocity profile in the inner layer near the vibrating wall has a significant distinctness, and the viscosity effect of the fluid in the inner region decreases due to the vibration. The flow features in the inner layer are altered by a suitable wall vibration.

Three-dimensional turbulent near-wall flows in streamwise corners: Current state and questions

NASA Astrophysics Data System (ADS)

Kornilov, V. I.

2017-10-01

Current advances in experimental and computational studies of three-dimensional (3-D) near-wall turbulent flows in streamwise corners (SC) including the boundary-layer transition are reviewed. The focus is the structure, properties and main regularities of such flows in a wide range of variable conditions and basic parameters. A variety of different kinds of near-wall streamwise corner flows is displayed. Analysis of approaches for modeling of the near-wall corner flow in laboratory experiment is given. The problem of simulation of such flows where some ambiguities remain is discussed. The main factors on the structure of the flow in streamwise corners are analyzed. Also, the effectiveness of flow control by streamwise vortices in the junction regions of aerodynamic surfaces is shown. Finally, some important properties of the modified near-wall turbulent corner flows which have been revealed experimentally, in particular, for the flow near the wing/body junction (WBJ), can be used as an attractive alternative for real applications.

Coherent structure dynamics and identification during the multistage transitions of polymeric turbulent channel flow

NASA Astrophysics Data System (ADS)

Zhu, Lu; Xi, Li

2018-04-01

Drag reduction induced by polymer additives in wall-bounded turbulence has been studied for decades. A small dosage of polymer additives can drastically reduce the energy dissipation in turbulent flows and alter the flow structures at the same time. As the polymer-induced fluid elasticity increases, drag reduction goes through several stages of transition with drastically different flow statistics. While much attention in the area of polymer-turbulence interactions has been focused on the onset and the asymptotic stage of maximum drag reduction, the transition between the two intermediate stages – low-extent drag reduction (LDR) and high-extent drag reduction (HDR) – likely reflects a qualitative change in the underlying vortex dynamics according to our recent study [1]. In particular, we proposed that polymers start to suppress the lift-up and bursting of vortices at HDR, leading to the localization of turbulent structures. To test our hypothesis, a statistically robust conditional sampling algorithm, based on Jenong and Hussain [2]’s work, was adopted in this study. The comparison of conditional eddies between the Newtonian and the highly elastic turbulence shows that (i) the lifting “strength” of vortices is suppressed by polymers as reflected by the decreasing lifting angle of the conditional eddy and (ii) the curvature of vortices is also eliminated as the orientation of the head of the conditional eddy changes. In summary, the results of conditional sampling support our hypothesis of polymer-turbulence interactions during the LDR-HDR transition.

Turbulent structure of stably stratified inhomogeneous flow

NASA Astrophysics Data System (ADS)

Iida, Oaki

2018-04-01

Effects of buoyancy force stabilizing disturbances are investigated on the inhomogeneous flow where disturbances are dispersed from the turbulent to non-turbulent field in the direction perpendicular to the gravity force. Attaching the fringe region, where disturbances are excited by the artificial body force, a Fourier spectral method is used for the inhomogeneous flow stirred at one side of the cuboid computational box. As a result, it is found that the turbulent kinetic energy is dispersed as layered structures elongated in the streamwise direction through the vibrating motion. A close look at the layered structures shows that they are flanked by colder fluids at the top and hotter fluids at the bottom, and hence vertically compressed and horizontally expanded by the buoyancy related to the countergradient heat flux, though they are punctuated by the vertical expansion of fluids at the forefront of the layered structures, which is related to the downgradient heat flux, indicating that the layered structures are gravity currents. However, the phase between temperature fluctuations and vertical velocity is shifted by π/2 rad, indicating that temperature fluctuations are generated by the propagation of internal gravity waves.

Zombie Turbulence and More in Stratified Couette Flow

NASA Astrophysics Data System (ADS)

Marcus, Philip; Barranco, Joe; Pei, Suyang; Jiang, Chung-Hsiang

2016-11-01

Zombie turbulence occurs in rotating, shearing vertically-stratified flows such as stratified Couette flows. The turbulence is triggered by a neutrally-stable eigenmode with a critical layer receptive to finite-amplitude perturbations. Once excited, the critical layer becomes a vortex layer pair that rolls up into discrete vortices. Those vortices excite new critical layers, and the process repeats ad infinitum. When the vortex amplitudes become sufficiently large, the flow becomes turbulent. Although possessing a mid-range energy spectrum with E (k) k - 5 / 3 , the turbulence is non-Kolmogorov, highly anisotropic, and with large turbulent, but coherent, structures that retain the length scales of the spacing between the critical layers. The motivation for this study is protoplanetary disks (PPDs) where new stars form. In the PPD the Brunt-Vaisala frequency N increases as a function of distance from the midplane where it is zero. We cannot trigger the initial finite amplitude instability where N is small (close to the midplane). However, computations in PPDs and Couette flows show that zombie turbulence forms where N is large, and then a new type of turbulence, that is neither zombie nor Kolmogorov turbulence, fills in the remainder of the domain even where N = 0 .

The effects of local blowing perturbations on thermal turbulent structures

NASA Astrophysics Data System (ADS)

Liu, Can; Araya, Guillermo; Leonardi, Stefano; Castillo, Luciano

2013-11-01

Blowing is an active flow control technique with several industrial applications, particularly in film cooling of turbine blades. In the past, the effects of localized blowing have been mostly analyzed on the velocity field and its influence of the flow parameters and turbulence structures (Krogstad and Kourakine, 2000). However, little literature can be found on the effects of blowing on the coherent thermal structures. In the present study, an incompressible turbulent channel flow with given steady blowing at the wall is simulated via DNS by means of five spanwise holes. The Reynolds number based on the friction velocity and half channel height is approximately Re = 394 and the molecular Prandtl number is Pr = 0.71. Temperature is considered a passive scalar with isothermal conditions at the wall. Different blowing amplitudes and perturbing angles (with respect to the streamwise direction) are applied to find out their effects on the turbulent thermal structures by means of a two-point correlation analysis. In addition, local reduction and increase of drag are connected to vorticity. The corresponding influence of perturbing amplitudes and angles on the energy budget of thermal fluctuations and turbulent Prandtl numbers are also shown and discussed.

Instability of the roll-streak structure induced by background turbulence in pretransitional Couette flow

NASA Astrophysics Data System (ADS)

Farrell, Brian F.; Ioannou, Petros J.; Nikolaidis, Marios-Andreas

2017-03-01

Although the roll-streak structure is ubiquitous in both observations and simulations of pretransitional wall-bounded shear flow, this structure is linearly stable if the idealization of laminar flow is made. Lacking an instability, the large transient growth of the roll-streak structure has been invoked to explain its appearance as resulting from chance occurrence in the background turbulence of perturbations configured to optimally excite it. However, there is an alternative interpretation for the role of free-stream turbulence in the genesis of the roll-streak structure, which is that the background turbulence interacts with the roll-streak structure to destabilize it. Statistical state dynamics (SSD) provides analysis methods for studying instabilities of this type that arise from interaction between the coherent and incoherent components of turbulence. SSD in the form of a closure at second order is used in this work to analyze the cooperative eigenmodes arising from interaction between the coherent streamwise invariant component and the incoherent background component of turbulence. In pretransitional Couette flow a manifold of stable modes with roll-streak form is found to exist in the presence of low-intensity background turbulence. The least stable mode of this manifold is destabilized at a critical value of a parameter controlling the background turbulence intensity and a finite-amplitude roll-streak structure arises from this instability through a bifurcation in this parameter. Although this bifurcation has analytical expression only in the infinite ensemble formulation of second order SSD, referred in this work as the S3T system, it is closely reflected in numerical simulations of both the dynamically similar quasilinear system, referred to as the restricted nonlinear (RNL) system, as well as in the full Navier-Stokes equations. This correspondence is verified using ensemble implementations of the RNL system and the Navier-Stokes equations. The S3T system also predicts a second bifurcation at a higher value of the turbulent excitation parameter that results in destabilization of the finite-amplitude roll-streak equilibria. This second bifurcation is shown to lead first to time dependence of the roll-streak structure in the S3T system and then to chaotic fluctuation corresponding to minimal channel turbulence. This transition scenario is also verified in simulations of the RNL system and of the Navier-Stokes equations. This bifurcation from a finite-amplitude roll-streak equilibrium provides a direct route to the turbulent state through the S3T roll-streak instability.

Flow Topology Transition via Global Bifurcation in Thermally Driven Turbulence

NASA Astrophysics Data System (ADS)

Xie, Yi-Chao; Ding, Guang-Yu; Xia, Ke-Qing

2018-05-01

We report an experimental observation of a flow topology transition via global bifurcation in a turbulent Rayleigh-Bénard convection. This transition corresponds to a spontaneous symmetry breaking with the flow becomes more turbulent. Simultaneous measurements of the large-scale flow (LSF) structure and the heat transport show that the LSF bifurcates from a high heat transport efficiency quadrupole state to a less symmetric dipole state with a lower heat transport efficiency. In the transition zone, the system switches spontaneously and stochastically between the two long-lived metastable states.

A criterion for delimiting active periods within turbulent flows

NASA Astrophysics Data System (ADS)

Keylock, C. J.

2008-06-01

Delimiting effectively the extent of the major motions in atmospheric, tidal and fluvial turbulent flows is an important task for studies of mixing and particle transport. The most common method for this (quadrant analysis) is closely linked to the turbulent stresses but subdivides active periods in the flow into separate events. A method based on the pointwise Hölder characteristics of the velocity data is introduced in this paper and applied to extract the whole duration of the active periods, within which turbulence intensities and stresses are high for some of the time. The cross-correlation structure of the Hölder series permits a simple threshold to be adopted. The technique is tested on data from a turbulent wake in a wind tunnel and flow over a forest canopy.

Rapid distortion theory and the 'problems' of turbulence

NASA Astrophysics Data System (ADS)

Hunt, J. C. R.; Carruthers, D. J.

1990-03-01

This paper describes some developments in the techniques of the rapid distortion theory (RDT) and in the general understanding of how it can be used. It is noted in particular that the theory provides a rational basis for analyzing rapidly changing turbulent flows (RCT), and a heuristic method for estimating certain features of slowly changing turbulent flows (SCT). Recent developments of the RDT are reviewed, including criteria for its validity and new solutions allowing for the effects of inhomogeneities and boundaries. The problems associated with analyzing different kinds of turbulent flow and different methods of solution are classified and discussed with reference to how the turbulent structure in a flow domain depends on the scale and geometry of the domain's boundary, and on the information provided in the boundary conditions.

Generation of Turbulent Inflow Conditions for Pipe Flow via an Annular Ribbed Turbulator

NASA Astrophysics Data System (ADS)

Moallemi, Nima; Brinkerhoff, Joshua

2016-11-01

The generation of turbulent inflow conditions adds significant computational expense to direct numerical simulations (DNS) of turbulent pipe flows. Typical approaches involve introducing boxes of isotropic turbulence to the velocity field at the inlet of the pipe. In the present study, an alternative method is proposed that incurs a lower computational cost and allows the anisotropy observed in pipe turbulence to be physically captured. The method is based on a periodic DNS of a ribbed turbulator upstream of the inlet boundary of the pipe. The Reynolds number based on the bulk velocity and pipe diameter is 5300 and the blockage ratio (BR) is 0.06 based on the rib height and pipe diameter. The pitch ratio is defined as the ratio of rib streamwise spacing to rib height and is varied between 1.7 and 5.0. The generation of turbulent flow structures downstream of the ribbed turbulator are identified and discussed. Suitability of this method for accurate representation of turbulent inflow conditions is assessed through comparison of the turbulent mean properties, fluctuations, Reynolds stress profiles, and spectra with published pipe flow DNS studies. The DNS results achieve excellent agreement with the numerical and experimental data available in the literature.

The Effect of Spanwise System Rotation on Turbulent Poiseuille Flow at Very-Low-Reynolds Number

NASA Astrophysics Data System (ADS)

Iida, Oaki; Fukudome, K.; Iwata, T.; Nagano, Y.

Direct numerical simulations (DNSs) with a spectral method are performed with large and small computational domains to study the effects of spanwise rotation on a turbulent Poiseuille flow at the very low-Reynolds numbers. In the case without system rotation, quasi-laminar and turbulent states appear side by side in the same computational domain, which is coined as laminar-turbulence pattern. However, in the case with system rotation, the pattern disappears and flow is dominated by quasi-laminar region including very long low-speed streaks coiled by chain-like vortical structures. Increasing the Reynolds number can not generate the laminar-turbulence pattern as long as system rotation is imposed.

Rayleigh Light Scattering for Concentration Measurements in Turbulent Flows

NASA Technical Reports Server (NTRS)

Pitts, William M.

1996-01-01

Despite intensive research over a number of years, an understanding of scalar mixing in turbulent flows remains elusive. An understanding is required because turbulent mixing has a pivotal role in a wide variety of natural and technologically important processes. As an example, the mixing and transport of pollutants in the atmosphere and in bodies of water are often dependent on turbulent mixing processes. Turbulent mixing is also central to turbulent combustion which underlies most hydrocarbon energy use in modern societies as well as in unwanted fire behavior. Development of models for combusting flows is therefore crucial, however, an understanding of scalar mixing is required before useful models of turbulent mixing and, ultimately, turbulent combustion can be developed. An important subset of turbulent flows is axisymmetric turbulent jets and plumes because they are relatively simple to generate, and because the provide an appropriate test bed for the development of general theories of turbulent mixing which can be applied to more complex geometries and flows. This paper focuses on a number of experimental techniques which have been developed at the National Institute of Standards and Development for measuring concentration in binary axisymmetric turbulent jets. In order to demonstrate the value of these diagnostics, some of the more important results from earlier and on-going investigations are summarized. Topics addressed include the similarity behavior of variable density axisymmetric jets, the behavior of absolutely unstable axisymmetric helium jets, and the role of large scale structures and scalar dissipation in these flows.

Transport Of Passive Scalars In A Turbulent Channel Flow

NASA Technical Reports Server (NTRS)

Kim, John; Moin, Parviz

1990-01-01

Computer simulation of transport of passive scalars in turbulent channel flow described in report. Shows flow structures and statistical properties. As used here, "passive scalars" means scalar quantities like fluctuations in temperature or concentrations of contaminants that do not disturb flow appreciably. Examples include transport of heat in heat exchangers, gas turbines, and nuclear reactors and dispersal of pollution in atmosphere.

An Experimental Study of a Separated/Reattached Flow Behind a Backward-Facing Step. Re(sub h) = 37,000

NASA Technical Reports Server (NTRS)

Jovic, Srba

1996-01-01

An experimental study was carried out to investigate turbulent structure of a two-dimensional incompressible separating/reattaching boundary layer behind a backward-facing step. Hot-wire measurement technique was used to measure three Reynolds stresses and higher-order mean products of velocity fluctuations. The Reynolds number, Re(sub h), based on the step height, h, and the reference velocity, U(sub 0), was 37,000. The upstream oncoming flow was fully developed turbulent boundary layer with the Re(sub theta) = 3600. All turbulent properties, such as Reynolds stresses, increase dramatically downstream of the step within an internally developing mixing layer. Distributions of dimensionless mean velocity, turbulent quantities and antisymmetric distribution of triple velocity products in the separated free shear layer suggest that the shear layer above the recirculating region strongly resembles free-shear mixing layer structure. In the reattachment region close to the wall, turbulent diffusion term balances the rate of dissipation since advection and production terms appear to be negligibly small. Further downstream, production and dissipation begin to dominate other transport processes near the wall indicating the growth of an internal turbulent boundary layer. In the outer region, however, the flow still has a memory of the upstream disturbance even at the last measuring station of 51 step-heights. The data show that the structure of the inner layer recovers at a much faster rate than the outer layer structure. The inner layer structure resembles the near-wall structure of a plane zero pressure-gradient turbulent boundary layer (plane TBL) by 25h to 30h, while the outer layer structure takes presumably over 100h.

A Concept of Cross-Ferroic Plasma Turbulence

PubMed Central

Inagaki, S.; Kobayashi, T.; Kosuga, Y.; Itoh, S.-I.; Mitsuzono, T.; Nagashima, Y.; Arakawa, H.; Yamada, T.; Miwa, Y.; Kasuya, N.; Sasaki, M.; Lesur, M.; Fujisawa, A.; Itoh, K.

2016-01-01

The variety of scalar and vector fields in laboratory and nature plasmas is formed by plasma turbulence. Drift-wave fluctuations, driven by density gradients in magnetized plasmas, are known to relax the density gradient while they can generate flows. On the other hand, the sheared flow in the direction of magnetic fields causes Kelvin-Helmholtz type instabilities, which mix particle and momentum. These different types of fluctuations coexist in laboratory and nature, so that the multiple mechanisms for structural formation exist in extremely non-equilibrium plasmas. Here we report the discovery of a new order in plasma turbulence, in which chained structure formation is realized by cross-interaction between inhomogeneities of scalar and vector fields. The concept of cross-ferroic turbulence is developed, and the causal relation in the multiple mechanisms behind structural formation is identified, by measuring the relaxation rate and dissipation power caused by the complex turbulence-driven flux. PMID:26917218

Hairpin exact coherent states in channel flow

NASA Astrophysics Data System (ADS)

Graham, Michael; Shekar, Ashwin

2017-11-01

Questions remain over the role of hairpin vortices in fully developed turbulent flows. Studies have shown that hairpins play a role in the dynamics away from the wall but the question still persists if they play any part in (near wall) fully developed turbulent dynamics. In addition, the robustness of the hairpin vortex regeneration mechanism is still under investigation. Recent studies have shown the existence of nonlinear traveling wave solutions to the Navier-Stokes equations, also known as exact coherent states (ECS), that capture many aspects of near-wall turbulent structures. Previously discovered ECS in channel flow have a quasi-streamwise vortex structure, with no indication of hairpin formation. Here we present a family of traveling wave solutions for channel flow that displays hairpin vortices. They have a streamwise vortex-streak structure near the wall with a spatially localized hairpin head near the channel centerline, attached to and sustained by the near wall structures. This family of solutions emerges through a transcritical bifurcation from a branch of traveling wave solutions with y and z reflectional symmetry. We also look into the instabilities that lead to the development of hairpins also explore its connection to turbulent dynamics.

Dynamics of hairpin vortices and polymer-induced turbulent drag reduction.

PubMed

Kim, Kyoungyoun; Adrian, Ronald J; Balachandar, S; Sureshkumar, R

2008-04-04

It has been known for over six decades that the dissolution of minute amounts of high molecular weight polymers in wall-bounded turbulent flows results in a dramatic reduction in turbulent skin friction by up to 70%. First principles simulations of turbulent flow of model polymer solutions can predict the drag reduction (DR) phenomenon. However, the essential dynamical interactions between the coherent structures present in turbulent flows and polymer conformation field that lead to DR are poorly understood. We examine this connection via dynamical simulations that track the evolution of hairpin vortices, i.e., counter-rotating pairs of quasistreamwise vortices whose nonlinear autogeneration and growth, decay and breakup are centrally important to turbulence stress production. The results show that the autogeneration of new vortices is suppressed by the polymer stresses, thereby decreasing the turbulent drag.

The Rapid Distortion of Two-Way Coupled Particle-Laden Turbulence

NASA Astrophysics Data System (ADS)

Kasbaoui, Mohamed; Koch, Donald; Desjardins, Olivier

2017-11-01

The modulation of sheared turbulence by dispersed particles is addressed in the two-way coupling regime. The preferential sampling of the straining regions of the flow by inertial particles in turbulence leads to the formation of clusters. These fast sedimenting particle structures cause the anisotropic alteration of turbulence at small scales in the direction of gravity. These effects are investigated in a revisited Rapid Distortion Theory (RDT), extended for two-way coupled particle-laden flows. To make the analysis tractable, we assume that particles have small but non-zero inertia. In the classical results for single-phase flows, the RDT assumption of fast shearing compared to the turbulence time scales leads to the distortion of ``frozen'' turbulence. In particle-laden turbulence, the coupling between the two phases remains strong even under fast shearing and leads to a dynamic modulation of the turbulence spectrum. Turbulence statistics obtained from RDT are compared with Euler-Lagrange simulations of homogeneously sheared particle-laden turbulence.

Modification of Turbulence Structures in a Channel Flow by Uniform Magnetic Fluxes

NASA Astrophysics Data System (ADS)

Lee, D.; Choi, H.; Kim, J.

1997-11-01

Effects of electromagnetic forcing on the near-wall turbulence are investigated by applying a uniform magnetic flux in a turbulent channel flow in the streamwise and spanwise directions, respectively. The base flow is a fully developed turbulent channel flow and the direct numerical simulation technique is used. The electromagnetic force induced from the magnetic fluxes reduces the intensity of the wall-layer structures and thus drag is significantly reduced. The wall-normal and spanwise velocity fluctuations and the Reynolds shear stress decrease with the increased magnetic flux in both directions. The streamwise velocity fluctuations increase with the streamwise magnetic flux, whereas they decrease with the spanwise magnetic flux. It is also shown that the spanwise magnetic flux is much more effective than the streamwise magnetic flux in reducing the skin-friction drag. Instantaneous Lorentz force vectors show that the flow motions by the near-wall vortices are directly inhibited by the spanwise magnetic flux, while they are less effectively inhibited by the streamwise magnetic flux. Other turbulence statistics that reveal the effects of the applied magnetic forcing will be presented. ^* Supported by KOSEF Contract No. 965-1008-003-2 and ONR Grant No. N00014-95-1-0352.

Effects of elevated line sources on turbulent mixing in channel flow

NASA Astrophysics Data System (ADS)

Nguyen, Quoc; Papavassiliou, Dimitrios

2016-11-01

Fluids mixing in turbulent flows has been studied extensively, due to the importance of this phenomena in nature and engineering. Convection effects along with motion of three-dimensional coherent structures in turbulent flow disperse a substance more efficiently than molecular diffusion does on its own. We present here, however, a study that explores the conditions under which turbulent mixing does not happen, when different substances are released into the flow field from different vertical locations. The study uses a method which combines Direct Numerical Simulation (DNS) with Lagrangian Scalar Tracking (LST) to simulate a turbulent channel flow and track the motion of passive scalars with different Schmidt numbers (Sc). The particles are released from several instantaneous line sources, ranging from the wall to the center region of the channel. The combined effects of mean velocity difference, molecular diffusion and near-wall coherent structures lead to the observation of different concentrations of particles downstream from the source. We then explore in details the conditions under which particles mixing would not happen. Results from numerical simulation at friction Reynolds number of 300 and 600 will be discussed and for Sc ranging from 0.1 to 2,400.

Concentration Measurements in Self-Excited, Momentum-Dominated Helium Jets

NASA Technical Reports Server (NTRS)

Yildirim, Bekir Sedat

2004-01-01

Flow structure of momentum-dominated pure helium jets discharged vertically into ambient air was investigated using high-speed rainbow schlieren deflectometry (RSD) technique. Effects of the operating parameters, i.e., Reynolds number (Re) and Richardson number (Ri), on the oscillatory behavior of the flow were examined over a range of experimental conditions. To seek the individual effect of these parameters, one of them was fixed and the other was varied with certain constraints. Measurements revealed highly periodic oscillations in the laminar region as well as high regularity in transition and turbulent regions. Maximum spectral power profiles at different axial locations indicated the oscillation amplitude increasing until the breakdown of the jet in the turbulent regime. The transition from the laminar to turbulent flow was also investigated. Fast Fourier transform analysis performed in the transition regime showed that the flow oscillates at a unique frequency, which was the same in the upstream laminar flow region. Measured deflection angle data were used in Abel inversion algorithm to construct the helium concentration fields. Instantaneous helium concentration contours revealed changes in the flow structure and evolution of vortical structures during an oscillation cycle. Temporal evolution plots of helium concentration at different axial location showed repeatable oscillations at all axial and radial locations up to the turbulent regime. A cross-correlation technique, applied to find the spatial displacements of the vortical structures, provided correlation coefficient peaks between consecutive schlieren images. Results show that the vortical structure convected and accelerated only in the axial direction.

The mechanism by which nonlinearity sustains turbulence in plane Couette flow

NASA Astrophysics Data System (ADS)

Nikolaidis, M.-A.; Farrell, B. F.; Ioannou, P. J.

2018-04-01

Turbulence in wall-bounded shear flow results from a synergistic interaction between linear non-normality and nonlinearity in which non-normal growth of a subset of perturbations configured to transfer energy from the externally forced component of the turbulent state to the perturbation component maintains the perturbation energy, while the subset of energy-transferring perturbations is replenished by nonlinearity. Although it is accepted that both linear non-normality mediated energy transfer from the forced component of the mean flow and nonlinear interactions among perturbations are required to maintain the turbulent state, the detailed physical mechanism by which these processes interact in maintaining turbulence has not been determined. In this work a statistical state dynamics based analysis is performed on turbulent Couette flow at R = 600 and a comparison to DNS is used to demonstrate that the perturbation component in Couette flow turbulence is replenished by a non-normality mediated parametric growth process in which the fluctuating streamwise mean flow has been adjusted to marginal Lyapunov stability. It is further shown that the alternative mechanism in which the subspace of non-normally growing perturbations is maintained directly by perturbation-perturbation nonlinearity does not contribute to maintaining the turbulent state. This work identifies parametric interaction between the fluctuating streamwise mean flow and the streamwise varying perturbations to be the mechanism of the nonlinear interaction maintaining the perturbation component of the turbulent state, and identifies the associated Lyapunov vectors with positive energetics as the structures of the perturbation subspace supporting the turbulence.

Flow Field Measurements of Methane-Oxygen Turbulent Nonpremixed Flames at High Pressure

NASA Astrophysics Data System (ADS)

Iino, Kimio; Kikkawa, Hoshitaka; Akamatsu, Fumiteru; Katsuki, Masashi

We carried out the flow field measurement of methane-oxygen turbulent nonpremixed flame in non-combusting and combusting situations at high pressures using LDV. The main objectives are to study the influences of combustion on the turbulence structure at high pressures and to provide detailed data on which numerical predictions on such flows can rely. Direct observation and CH* chemiluminescence detection are conducted at high pressures up to 1.0MPa. It was found that the flame length at elevated pressures became constant. From flow field measurements, the following features of flames at elevated pressure were found: (1) the existence of flame suppressed turbulence in the upstream region of the jet and enhanced it in the downstream region with increasing pressure; (2) Turbulence in the flame was more anisotropic than in the corresponding cold jet in all regions of the flow with increasing pressure; (3) Reynolds shear stresses did not change at elevated pressure; (4) Combustion processes had a marked influence on the turbulence macroscale under high pressures, however, the turbulence macroscale was not changed even with the increase in pressure.

Multiscale plant wakes, turbulence and non linear scaling flexible effects

NASA Astrophysics Data System (ADS)

Vila, Teresa; Redondo, Jose M.; Velasco, David

2010-05-01

We present velocity ADV measurements and flow visualization of the turbulent wakes behind plant arrays, as these are often fractal in nature, we compare the multifractal spectra and the turbulence structure behind the wakes. Both statistical measures allowing to calculate integral lengthscales and their profiles modified by the plant cannopies [1,2] as well as intermittency and spectral behaviour are also measured [3,4]. We distinguish several momentum transfer mechanisms between the cannopy and the flow, an internal one where lateral turbulent tensions are dominant, and another one just above the plant average height dominated by vertical Reynolds stresses. Visualization of flow over individual plant models show the role of coherent vortices triggered by plant elasticity. The deformation rate of the plants and their Youngs modulus may be correlated with overal plant drag and geometry. This is modified strongly in fractal canopies. Large turbulent integral scales are linked to rugosity and the scaling of the waves.[5,6] Pearlescence experiments where local shear is visualized and numerical simulations of Fractal grids are compared following [7]. [1] Nepf,H.M. Drag, turbulence and diffusion in flow through emergent vegetation. Water Resources Res. 35(2)(1999) [2] Ben Mahjoub,O., Redondo J.M. and Babiano A. Jour.Structure functions in complex flows. Flow Turbulence and Combustion 59, 299-313. [3] El-Hakim, O. Salama, M. Velocity distribution inside and above branched flexible roughness. ASCE Journal of Irrigation and Drainage Engineering, Vol. 118, No 6, (November/December 1992) 914-927. [4] Finnigan,J. Turbulence in plant canopies. Annu. Rev. Fluid Mech. 2000 , Vol. 32: 519-571. [5] Ikeda, S., Kanazawa, M. Three- dimensional organized vortices above flexible water plants. ASCE Journal of Hydraulic Engineering, Vol. 122, No 11, (1996) 634-640. [6] Velasco, D.,Bateman A.,Redondo J.M and Medina V. An open channel flow experimental and theorical study of resistance and turbulent characterization over flexible vegetated linnings. Flow, Turbulence and Combustion. 70, 69-88. [7] Layzet S. Vassilicos C. Vila T and Redondo J.M. I.O.P Fractal grids and wakes, Proceedings on Earth sciences (2009)

Large-eddy simulations of the restricted nonlinear system

NASA Astrophysics Data System (ADS)

Bretheim, Joel; Gayme, Dennice; Meneveau, Charles

2014-11-01

Wall-bounded shear flows often exhibit elongated flow structures with streamwise coherence (e.g. rolls/streaks), prompting the exploration of a streamwise-constant modeling framework to investigate wall-turbulence. Simulations of a streamwise-constant (2D/3C) model have been shown to produce the roll/streak structures and accurately reproduce the blunted turbulent mean velocity profile in plane Couette flow. The related restricted nonlinear (RNL) model captures these same features but also exhibits self-sustaining turbulent behavior. Direct numerical simulation (DNS) of the RNL system results in similar statistics for a number of flow quantities and a flow field that is consistent with DNS of the Navier-Stokes equations. Aiming to develop reduced-order models of wall-bounded turbulence at very high Reynolds numbers in which viscous near-wall dynamics cannot be resolved, this work presents the development of an RNL formulation of the filtered Navier-Stokes equations solved for in large-eddy simulations (LES). The proposed LES-RNL system is a computationally affordable reduced-order modeling tool that is of interest for studying the underlying dynamics of high-Reynolds wall-turbulence and for engineering applications where the flow field is dominated by streamwise-coherent motions. This work is supported by NSF (IGERT, SEP-1230788 and IIA-1243482).

Role of large-scale motions to turbulent inertia in turbulent pipe and channel flows

NASA Astrophysics Data System (ADS)

Hwang, Jinyul; Lee, Jin; Sung, Hyung Jin

2015-11-01

The role of large-scale motions (LSMs) to the turbulent inertia (TI) term (the wall-normal gradient of the Reynolds shear stress) is examined in turbulent pipe and channel flows at Reτ ~ 930 . The TI term in the mean momentum equation represents the net force of inertia exerted by the Reynolds shear stress. Although the turbulence statistics characterizing the internal turbulent flows are similar close to the wall, the TI term differs in the logarithmic region due to the different characteristics of LSMs (λx > 3 δ) . The contribution of the LSMs to the TI term and the Reynolds shear stress in the channel flow is larger than that in the pipe flow. The LSMs in the logarithmic region act like a mean momentum source (where TI >0) even the TI profile is negative above the peak of the Reynolds shear stress. The momentum sources carried by the LSMs are related to the low-speed regions elongated in the downstream, revealing that momentum source-like motions occur in the upstream position of the low-speed structure. The streamwise extent of this structure is relatively long in the channel flow, whereas the high-speed regions on the both sides of the low-speed region in the channel flow are shorter and weaker than those in the pipe flow. This work was supported by the Creative Research Initiatives (No. 2015-001828) program of the National Research Foundation of Korea (MSIP) and partially supported by KISTI under the Strategic Supercomputing Support Program.

Turbulent structures in cylindrical density currents in a rotating frame of reference

NASA Astrophysics Data System (ADS)

Salinas, Jorge S.; Cantero, Mariano I.; Dari, Enzo A.; Bonometti, Thomas

2018-06-01

Gravity currents are flows generated by the action of gravity on fluids with different densities. In some geophysical applications, modeling such flows makes it necessary to account for rotating effects, modifying the dynamics of the flow. While previous works on rotating stratified flows focused on currents of large Coriolis number, the present work focuses on flows with small Coriolis numbers (i.e. moderate-to-large Rossby numbers). In this work, cylindrical rotating gravity currents are investigated by means of highly resolved simulations. A brief analysis of the mean flow evolution to the final state is presented to provide a complete picture of the flow dynamics. The numerical results, showing the well-known oscillatory behavior of the flow (inertial waves) and a final state lens shape (geostrophic adjustment), are in good agreement with experimental observations and theoretical models. The turbulent structures in the flow are visualized and described using, among others, a stereoscopic visualization and videos as supplementary material. In particular, the structure of the lobes and clefts at the front of the current is presented in association to local turbulent structures. In rotating gravity currents, the vortices observed at the lobes front are not of hairpin type but are rather of Kelvin-Helmholtz type.

Determination of billows and other turbulent structures, part 4.1A

NASA Technical Reports Server (NTRS)

Rastogi, P. K.

1984-01-01

Billows are regular, wave-like arrays of cross-flow vortices that develop in stratified oceanic or atmospheric flows with large shear. Atmospheric billows can become manifest through condensation. Billows are frequently seen in their characteristic cloud forms in the lower atmosphere. Under suitable viewing conditions, billows can also be seen in noctilucent clouds that form near the polar mesosphere during the summer months. Other turbulent structures -- related to billows -- are the Kelvin-Helmholtz instability (KHI) and cat's eye structures that occur in fully developed turbulent shear flows. Shear flows may contain perturbations at many different horizontal wavelengths and vertical scales. Realistic theoretical models have been constructed to study the stability and growth of these perturbations. The extent to which billows and Kelvin-Helmholtz instability have been observed in the atmosphere with the use of radars is outlined. Most of these observations are confined to the troposphere. Suggestions are made for improved radar experiments that are required to detect these structures at higher altitudes.

Homoclinic tangle on the edge of shear turbulence.

PubMed

van Veen, Lennaert; Kawahara, Genta

2011-09-09

Experiments and simulations lend mounting evidence for the edge state hypothesis on subcritical transition to turbulence, which asserts that simple states of fluid motion mediate between laminar and turbulent shear flow as their stable manifolds separate the two in state space. In this Letter we describe flows homoclinic to a time-periodic edge state that display the essential properties of turbulent bursting. During a burst, vortical structures and the associated energy dissipation are highly localized near the wall, in contrast with the familiar regeneration cycle.

Computation of Large Turbulence Structures and Noise of Supersonic Jets

NASA Technical Reports Server (NTRS)

Tam, Christopher

1996-01-01

Our research effort concentrated on obtaining an understanding of the generation mechanisms and the prediction of the three components of supersonic jet noise. In addition, we also developed a computational method for calculating the mean flow of turbulent high-speed jets. Below is a short description of the highlights of our contributions in each of these areas: (a) Broadband shock associated noise, (b) Turbulent mixing noise, (c) Screech tones and impingement tones, (d) Computation of the mean flow of turbulent jets.

Direct numerical simulation of turbulent plane Couette flow under neutral and stable stratification

NASA Astrophysics Data System (ADS)

Mortikov, Evgeny

2017-11-01

Direct numerical simulation (DNS) approach was used to study turbulence dynamics in plane Couette flow under conditions ranging from neutral stability to the case of extreme stable stratification, where intermittency is observed. Simulations were performed for Reynolds numbers, based on the channel height and relative wall speed, up to 2 ×105 . Using DNS data, which covers a wide range of stability conditions, parameterizations of pressure correlation terms used in second-order closure turbulence models are discussed. Particular attention is also paid to the sustainment of intermittent turbulence under strong stratification. Intermittent regime is found to be associated with the formation of secondary large-scale structures elongated in the spanwise direction, which define spatially confined alternating regions of laminar and turbulent flow. The spanwise length of this structures increases with the increase in the bulk Richardson number and defines and additional constraint on the computational box size. In this work DNS results are presented in extended computational domains, where the intermittent turbulence is sustained for sufficiently higher Richardson numbers than previously reported.

The Statistical Mechanics of Ideal MHD Turbulence

NASA Technical Reports Server (NTRS)

Shebalin, John V.

2003-01-01

Turbulence is a universal, nonlinear phenomenon found in all energetic fluid and plasma motion. In particular. understanding magneto hydrodynamic (MHD) turbulence and incorporating its effects in the computation and prediction of the flow of ionized gases in space, for example, are great challenges that must be met if such computations and predictions are to be meaningful. Although a general solution to the "problem of turbulence" does not exist in closed form, numerical integrations allow us to explore the phase space of solutions for both ideal and dissipative flows. For homogeneous, incompressible turbulence, Fourier methods are appropriate, and phase space is defined by the Fourier coefficients of the physical fields. In the case of ideal MHD flows, a fairly robust statistical mechanics has been developed, in which the symmetry and ergodic properties of phase space is understood. A discussion of these properties will illuminate our principal discovery: Coherent structure and randomness co-exist in ideal MHD turbulence. For dissipative flows, as opposed to ideal flows, progress beyond the dimensional analysis of Kolmogorov has been difficult. Here, some possible future directions that draw on the ideal results will also be discussed. Our conclusion will be that while ideal turbulence is now well understood, real turbulence still presents great challenges.

Characterization of structural response to hypersonic boundary-layer transition

DOE PAGES

Riley, Zachary B.; Deshmukh, Rohit; Miller, Brent A.; ...

2016-05-24

The inherent relationship between boundary-layer stability, aerodynamic heating, and surface conditions makes the potential for interaction between the structural response and boundary-layer transition an important and challenging area of study in high-speed flows. This paper phenomenologically explores this interaction using a fundamental two-dimensional aerothermoelastic model under the assumption of an aluminum panel with simple supports. Specifically, an existing model is extended to examine the impact of transition onset location, transition length, and transitional overshoot in heat flux and fluctuating pressure on the structural response of surface panels. Transitional flow conditions are found to yield significantly increased thermal gradients, and theymore » can result in higher maximum panel temperatures compared to turbulent flow. Results indicate that overshoot in heat flux and fluctuating pressure reduces the flutter onset time and increases the strain energy accumulated in the panel. Furthermore, overshoot occurring near the midchord can yield average temperatures and peak displacements exceeding those experienced by the panel subject to turbulent flow. Lastly, these results suggest that fully turbulent flow does not always conservatively predict the thermo-structural response of surface panels.« less

Self similarity of two point correlations in wall bounded turbulent flows

NASA Technical Reports Server (NTRS)

Hunt, J. C. R.; Moin, P.; Moser, R. D.; Spalart, P. R.

1987-01-01

The structure of turbulence at a height y from a wall is affected by the local mean shear at y, by the direct effect of the wall on the eddies, and by the action of other eddies close to or far from the wall. Some researchers believe that a single one of these mechanisms is dominant, while others believe that these effects have to be considered together. It is important to understand the relative importance of these effects in order to develop closure models, for example for the dissipation or for the Reynolds stress equation, and to understand the eddy structure of cross correlation functions and other measures. The specific objective was to examine the two point correlation, R sub vv, of the normal velocity component v near the wall in a turbulent channel flow and in a turbulent boundary layer. The preliminary results show that even in the inhomogeneous turbulent boundary layer, the two-point correlation function may have self similar forms. The results also show that the effects of shear and of blocking are equally important in the form of correlation functions for spacing normal to the wall. But for spanwise spacing, it was found that the eddy structure is quire different in these near flows. So any theory for turbulent structure must take both these effects into account.

Coherent structures in wall-bounded turbulence

NASA Astrophysics Data System (ADS)

Jiménez, Javier

2018-05-01

This article discusses the description of wall-bounded turbulence as a deterministic high-dimensional dynamical system of interacting coherent structures, defined as eddies with enough internal dynamics to behave relatively autonomously from any remaining incoherent part of the flow. The guiding principle is that randomness is not a property, but a methodological choice of what to ignore in the flow, and that a complete understanding of turbulence, including the possibility of control, requires that it be kept to a minimum. After briefly reviewing the underlying low-order statistics of flows at moderate Reynolds numbers, the article examines what two-point statistics imply for the decomposition of the flow into individual eddies. Intense eddies are examined next, including their temporal evolution, and shown to satisfy many of the properties required for coherence. In particular, it is shown that coherent structures larger than the Corrsin scale are a natural consequence of the shear. In wall-bounded turbulence, they can be classified into coherent dispersive waves and transient bursts. The former are found in the viscous layer near the wall and as very-large structures spanning the boundary layer thickness. Although they are shear-driven, these waves have enough internal structure to maintain a uniform advection velocity. Conversely, bursts exist at all scales, are characteristic of the logarithmic layer, and interact almost linearly with the shear. While the waves require a wall to determine their length scale, the bursts are essentially independent from it. The article concludes with a brief review of our present theoretical understanding of turbulent structures, and with a list of open problems and future perspectives.

Implementation of Flow Tripping Capability in the USM3D Unstructured Flow Solver

NASA Technical Reports Server (NTRS)

Pandya, Mohagna J.; Abdol-Harrid, Khaled S.; Campbell, Richard L.; Frink, Neal T.

2006-01-01

A flow tripping capability is added to an established NASA tetrahedral unstructured parallel Navier-Stokes flow solver, USM3D. The capability is based on prescribing an appropriate profile of turbulence model variables to energize the boundary layer in a plane normal to a specified trip region on the body surface. We demonstrate this approach using the k-e two-equation turbulence model of USM3D. Modification to the solution procedure primarily consists of developing a data structure to identify all unstructured tetrahedral grid cells located in the plane normal to a specified surface trip region and computing a function based on the mean flow solution to specify the modified profile of the turbulence model variables. We leverage this data structure and also show an adjunct approach that is based on enforcing a laminar flow condition on the otherwise fully turbulent flow solution in user specified region. The latter approach is applied for the solutions obtained using other one- and two-equation turbulence models of USM3D. A key ingredient of the present capability is the use of a graphical user-interface tool PREDISC to define a trip region on the body surface in an existing grid. Verification of the present modifications is demonstrated on three cases, namely, a flat plate, the RAE2822 airfoil, and the DLR F6 wing-fuselage configuration.

Implementation of Flow Tripping Capability in the USM3D Unstructured Flow Solver

NASA Technical Reports Server (NTRS)

Pandya, Mohagna J.; Abdol-Hamid, Khaled S.; Campbell, Richard L.; Frink, Neal T.

2006-01-01

A flow tripping capability is added to an established NASA tetrahedral unstructured parallel Navier-Stokes flow solver, USM3D. The capability is based on prescribing an appropriate profile of turbulence model variables to energize the boundary layer in a plane normal to a specified trip region on the body surface. We demonstrate this approach using the k-epsilon two-equation turbulence model of USM3D. Modification to the solution procedure primarily consists of developing a data structure to identify all unstructured tetrahedral grid cells located in the plane normal to a specified surface trip region and computing a function based on the mean flow solution to specify the modified profile of the turbulence model variables. We leverage this data structure and also show an adjunct approach that is based on enforcing a laminar flow condition on the otherwise fully turbulent flow solution in user-specified region. The latter approach is applied for the solutions obtained using other one-and two-equation turbulence models of USM3D. A key ingredient of the present capability is the use of a graphical user-interface tool PREDISC to define a trip region on the body surface in an existing grid. Verification of the present modifications is demonstrated on three cases, namely, a flat plate, the RAE2822 airfoil, and the DLR F6 wing-fuselage configuration.

The dynamics of coherent flow structures within a submerged permeable bed

NASA Astrophysics Data System (ADS)

Blois, G.; Best, J.; Sambrook Smith, G.; Hardy, R. J.; Lead, J.

2009-12-01

The existence of complex 3D coherent vortical structures in turbulent boundary layers has been widely reported from experimental observations (Adrian et al., 2007, Christensen and Adrian, 2001) and investigations of natural open channel flows (e.g. Kostaschuk and Church, 1993; Best, 2005). The interaction between these flow structures and the solid boundary that is responsible for their generation is also receiving increasing attention due to the central role played by turbulence in governing erosion-deposition processes. Yet, for the majority of studies, the bed roughness has been represented using rough impermeable surfaces. While not inherently acknowledged, most research in this area is thus only strictly applicable to those natural river beds composed either of bedrock or clay, or that have armoured, impermeable, surfaces. Recently, many researchers have noted the need to account for the role of bed permeability in order to accurately reproduce the true nature of flow over permeable gravel-bed rivers. For these cases, the near-bed flow is inherently and mutually linked to the interstitial-flow occurring in the porous solid matrix. This interaction is established through turbulence mechanisms occurring across the interface that may be important for influencing the incipient motion of cohesionless sediment. However, the nature of this turbulence and the formation of coherent structures within such permeable beds remain substantially unresolved due to the technical challenges of collecting direct data in this region. In this paper, we detail the existence and dynamic nature of coherent vortical structures within the individual pore spaces of a permeable bed submerged by a free stream flow. Laboratory experiments are reported in which a permeable flume bed was constructed using spheres packed in an offset cubic arrangement. We applied a high resolution E-PIV (Endoscopic Particle Image Velocimetry) approach in order to fully resolve the instantaneous structure of flow within the permeable bed, which allowed visualisation of coherent vortices in the pore space, and investigation of their formative mechanisms and spatio-temporal evolution. The spatial scale of these structures is found to be of the order of the pore space, with jet flows occurring between interconnected pores and interacting with the spherical particles constituting the solid matrix. Such jets are hypothesized to be triggered by the interstitial pressure gradients between interconnected pores, which in turn are linked to large-scale coherent flow structures in the free-flow that advect and propagate through the permeable bed. As the jet flow interacts with the matrix around the pore space, coherent flow structures are generated with both clockwise and anticlockwise rotation. The nature of these subsurface turbulent flow patterns will be presented, which allows new insight into flows within permeable beds and the hydrodynamic processes triggering the motion of sediment.

Turbulent Compressible Convection with Rotation. Part 1; Flow Structure and Evolution

NASA Technical Reports Server (NTRS)

Brummell, Nicholas H.; Hurlburt, Neal E.; Toomre, Juri

1996-01-01

The effects of Coriolis forces on compressible convection are studied using three-dimensional numerical simulations carried out within a local modified f-plane model. The physics is simplified by considering a perfect gas occupying a rectilinear domain placed tangentially to a rotating sphere at various latitudes, through which a destabilizing heat flux is driven. The resulting convection is considered for a range of Rayleigh, Taylor, and Prandtl (and thus Rossby) numbers, evaluating conditions where the influence of rotation is both weak and strong. Given the computational demands of these high-resolution simulations, the parameter space is explored sparsely to ascertain the differences between laminar and turbulent rotating convection. The first paper in this series examines the effects of rotation on the flow structure within the convection, its evolution, and some consequences for mixing. Subsequent papers consider the large-scale mean shear flows that are generated by the convection, and the effects of rotation on the convective energetics and transport properties. It is found here that the structure of rotating turbulent convection is similar to earlier nonrotating studies, with a laminar, cellular surface network disguising a fully turbulent interior punctuated by vertically coherent structures. However, the temporal signature of the surface flows is modified by inertial motions to yield new cellular evolution patterns and an overall increase in the mobility of the network. The turbulent convection contains vortex tubes of many scales, including large-scale coherent structures spanning the full vertical extent of the domain involving multiple density scale heights. Remarkably, such structures align with the rotation vector via the influence of Coriolis forces on turbulent motions, in contrast with the zonal tilting of streamlines found in laminar flows. Such novel turbulent mechanisms alter the correlations which drive mean shearing flows and affect the convective transport properties. In contrast to this large-scale anisotropy, small-scale vortex tubes at greater depths are randomly orientated by the rotational mixing of momentum, leading to an increased degree of isotropy on the medium to small scales of motion there. Rotation also influences the thermodynamic mixing properties of the convection. In particular, interaction of the larger coherent vortices causes a loss of correlation between the vertical velocity and the temperature leaving a mean stratification which is not isentropic.

The minimal flow unit in near-wall turbulence

NASA Technical Reports Server (NTRS)

Jimeez, Javier; Moin, Parviz

1991-01-01

Direct numerical simulations of unsteady channel flow were performed at low to moderate Reynolds numbers on computational boxes chosen small enough so that the flow consists of a doubly periodic (in x and z) array of identical structures. The goal is to isolate the basic flow unit, to study its morphology and dynamics, and to evaluate its contribution to turbulence in fully developed channels. For boxes wider than approximately 100 wall units in the spanwise direction, the flow is turbulent, and the low-order turbulence statistics are in good agreement with experiments in the near-wall region. For a narrow range of widths below that threshold, the flow near only one wall remains turbulent, but its statistics are still in fairly good agreement with experimental data when scaled with the local wall stress. For narrower boxes only laminar solutions are found. In all cases, the elementary box contains a single low-velocity streak, consisting of a longitudinal strip on which a thin layer of spanwise vorticity is lifted away from the wall.

Effect of external plasma flows on the interaction between turbulence and convective cells

NASA Astrophysics Data System (ADS)

Uzawa, Ken; Li, Jiquan

2005-10-01

It is widely recognized that large scale structures, such as zonal flows, streamers and also long wavelength Kelvin-Helmholtz modes are nonlinearly generated from maternal turbulence through modulational instability process and play a crucial role in regulating the transport in tokamaks. In order to control the transport, it is desirable to control such structures and/or modulational process. One of control parameters may be mean flow which intrinsically exists in tokamak plasmas. Besides the direct influence on the transport through vortex decorrelation, the mean flow may indirectly change the zonal flow generation by acting on the modulational process itself. In this work, we theoretically investigate the characteristics of zonal flow generation due to the electron temperature gradient (ETG) turbulence in the presence of long wavelength ITG driven zonal flow. This was done by extending our previous modulational analyses[1]. We have numerically analyzed the influence of mean flow on zonal flow generation. The main result is that the zonal flow generation is suppressed by the presence of the mean flow. [1]J. Li, Y. Kishimoto, Physics of Plasmas, 9, 1241 (2002)

Cell structures caused by settling particles in turbulent Rayleigh-Bénard convection

NASA Astrophysics Data System (ADS)

Lee, Changhoon; Park, Sangro

2016-11-01

Turbulent thermal convection is an important phenomenon frequently found in nature and industrial processes, often with laden particles. In the last several decades, the vast majority of studies have addressed single phase convective flow with focus on the scaling relation of flow parameters associated with heat transfer. Particle-laden Rayleigh-Bénard convection, however, has not been sufficiently studied. In this study, modulation of cell structures by settling particles in turbulent Rayleigh-Bénard convection in a doubly periodic square channel is investigated using direct numerical simulation with a point particle approach. Flow parameters are fixed at Rayleigh number=106, Prandtl number=0.7, the aspect ratio=6, and Froude number=0.19. We report from the simulations that settling heavy particles modulate irregular large-scale thermal plume structures into organized polygonal cell structures. Different shapes of flow structures are obtained for different particle diameters and mass loadings. We found that polygonal cell structures arise due to asymmetric feedback force exerted by particles onto hot and cold plumes. Increasing the number of particles augments the asymmetry and the polygonal cell structures become smaller, eventually going to the hexagonal structures.

Effects of Fetch on Turbulent Flow and Pollutant Dispersion Within a Cubical Canopy

NASA Astrophysics Data System (ADS)

Michioka, Takenobu; Takimoto, Hiroshi; Ono, Hiroki; Sato, Ayumu

2018-03-01

The effects of fetch on turbulent flow and pollutant dispersion within a canopy formed by regularly-spaced cubical objects is investigated using large-eddy simulation. Six tracer gases are simultaneously released from a ground-level continuous pollutant line source placed parallel to the spanwise axis at the first, second, third, fifth, seventh and tenth rows. Beyond the seventh row, the standard deviations of the fluctuations in the velocity components and the Reynolds shear stresses reach nearly equivalent states. Low-frequency turbulent flow is generated near the bottom surface around the first row and develops as the fetch increases. The turbulent flow eventually passes through the canopy at a near-constant interval. The mean concentration within the canopy reaches a near-constant value beyond the seventh row. In the first and second rows, narrow coherent structures frequently affect the pollutant escape from the top of the canopy. These structures increase in width as the fetch increases, and they mainly affect the removal of pollutants from the canopy.

Topographic effect on the inclination angle of ramp like structures in rough wall, turbulent channel flow

NASA Astrophysics Data System (ADS)

Awasthi, Ankit; Anderson, William

2015-11-01

We have studied variation in structural inclination angle of coherent structures responding to a topography with abrupt spanwise heterogeneity. Recent results have shown that such a topography induces a turbulent secondary flow due to spanwise-wall normal heterogeneity of the Reynolds stresses (Anderson et al., 2015: J. Fluid Mech.). The presence of these spanwise alternating low and high momentum pathways (which are flanked by counter rotating, domain-scale vortices, Willingham et al., 2014: Phys. Fluids; Barros and Christensen, 2014: J. Fluid Mech.) are primarily due to the spanwise heterogeneity of the complex roughness under consideration. Results from the present research have been used to explore structural attributes of the hairpin packet paradigm in the presence of a turbulent secondary flow. Vortex visualization in the streamwise-wall normal plane above the crest (high drag) and trough (low drag) demonstrate variation in the inclination angle of coherent structures. The inclination angle of structures above the crest was approximately 45 degrees, much larger than the ``canonical'' value of 15 degrees. Thus, we present evidence that the hairpin packet concept is preserved - but modified - when a turbulent secondary flow is present. This work was supported by the Air Force Office of Sci. Research, Young Inv. Program (PM: Dr. R. Ponnoppan and Ms. E. Montomery) under Grant # FA9550-14-1-0394. Computational resources were provided by the Texas Adv. Comp. Center at Univ. of Texas.

Turbulence, flow and transport: hints from reversed field pinch

NASA Astrophysics Data System (ADS)

Vianello, N.; Antoni, V.; Spada, E.; Spolaore, M.; Serianni, G.; Cavazzana, R.; Bergsåker, H.; Cecconello, M.; Drake, J. R.

2006-04-01

The interplay between sheared E × B flows and turbulence has been experimentally investigated in the edge region of the Extrap-T2R reversed field pinch experiment. Electrostatic fluctuations are found to rule the momentum balance equation representing the main driving term for sheared flows which counterbalances anomalous viscous damping. The driving role of electrostatic fluctuations is proved by the spatial structure of the Reynolds stress and by the time behaviour of the mean energy production term which supports the existence of an energy exchange from the small scales of turbulence to the larger scales of the mean flow.

Lagrangian turbulence near walls: Structures and mixing in admissible model flows

NASA Astrophysics Data System (ADS)

Ottino, J. M.

1989-05-01

The general objective of work during this period was to bridge the gap between modern ideas from dynamical systems and chaos and more traditional approaches to turbulence. In order to reach this objective we conducted theoretical and computational work on two systems: a perturbed Kelvin cat eyes flow, and prototype solutions of the Navier-Stokes equations near solid walls. The main results obtained are two-fold: production flows capable of producing complex distributions of vorticity, and constructed flow fields, based on solutions of the Navier Stokes equations, which are capable of displaying both Eulerian and Lagrangian turbulence.

Sediment and plankton lift off recirculations in strong synthetic turbulence (KS)

NASA Astrophysics Data System (ADS)

Redondo, Jose M.; Castilla, Roberto; Sekula, Emil; Furmanek, Petr

2014-05-01

The study of particle diffusion and of turbulent sedimentation is of great importance in many geophysical fields, such as in Environmental Science or Oceanography as well as in Bio-environmental and industrial processes. For a long time, the study of diffusion was numerically computed with random free paths, which gives Brownian behavior. (Richardson 1929). These stochastics methods have the objection that do not take into account the flow profile. On the other hand, there are many ways to simulate a fluid flow, but when this is turbulent our aim is to simulate the behaviour of neutral or heavy and inertial particles of biological or geological nature in a turbulent flow, in a simple way with a kinematically simulated model and to validate the results. We use the Kinematic Simulation (KS) model, also known as Synthetic Turbulence, suggested by Kraichnan (1966) and developed further by Castilla et al.(2007), Nicolleau et al.(2012). In this model, velocity field is generated through a Fourier series of random modes. The typical scales and the energy spectrum of the turbulence are inputs of the model. As we do not solve the flow in a discrete grid, but use a random predictive expression, we can simulate the flow at the smallest scales. In an unstratified flow, a KS flow field consists of a random, truncated Fourier representation in space and time, subject to constraints associated with incompressibility, and a prescribed initial energy spectrum. For stratified calculations, two further constraints are imposed, associated with the internal wave field in stratified flows, and the tendency of density variations to suppress vertical motion. With these model modifications, good agreement is found between KS and DNS with regard to the confinement in the vertical direction characteristic of stratified turbulence. Since stratifed flows exhibit this vertical confinement, KS in strictly two dimensions was considered as a first step to understanding dispersion within a stratified flow. The properties of ensemble averages of the separation between two particles in a 2D turbulent flow were considered, and the KS approach was found to give satisfactory answers, with good comparison to experiment. We also compare structure and intermittency between KS and DNS. And experiments (Redondo 1988) The dynamical processes associated with the stably stratified atmospheric boundary layer or in the ocean thermocline are less well understood than those of its convective counterparts. This is due to its complexity, and the fact that buoyancy reduces entrainment across density interfaces. We present results on a numerical simulation of homogeneous and density stratified fluids and of comparable laboratory experiments where a sharp density interface generated by either salt concentration or heat, advances due to grid stirred turbulence Redondo (1988, 1990). The appearance of bursts of turbulence in very stable conditions due to breaking up of the internal waves, confers a sporadic character to the turbulence; these conditions of non-fully developed turbulence could explain this unusual behaviour of the scaling exponents. (Mahjoub et al. 1998, 20009 The structure functions show, in the inertial range, a potential law . The relation is concave in strong mixing situations (instability with fully developed turbulence), and convex in very stable situations (in which the breaking up of the interval waves confers a sporadic character to the turbulence).The multifractal model can not be used to represent situations of non-fully developed turbulence but the use of structure function analysis allows the investigation of intermittent and scale to scale energy transfer even in local non equilibrium flows. The relative diffusion of tracers is strongly dependent on the slope of the energy spectra which tends to Richardson's law also for very steep spectra. (Castilla et al. 2007) Local turbulence is used to establish the geometry of the turbulence mixing, changes in the equilibrium (or not) cascade may lead to more physically realistic (and understandable) models to paramerize sub-grid scaling. Care has to be taken when interpreting the direct 3D Kolmogorov cascade and the Inverse 2D Kraichnan Cascade. It is very interesting to use ESS and the third order structure functions (p=3) to investigate the scale to scale transfer of energy (and enstrophy) A parameter space based on Richardson numbers, Rossby numbers and Reynolds Numbers can be used to determine the dominant instability with different intermittencies in a complex full stratified-rotating flow. Intermittency diminishes as spectral slope increases between 5/3 (Kolmogorov's local energy balance) and 3 (Kraichnan's local enstrophy balance) like near a boundary. (Rodriguez et al 1999, Redondo et al. 1993)(Gabaldon and Redondo 2001) Helicity local balance leads to a 7/3 Energy spectra that may be strongly affected by intermittency. It should also depend on the length scale. So in K62, Kolmogorov introduced the notion of intermittency, and he would transpose the universality character of his previous constant to the universality of several parameters, the intermittence which is generalized to higher orders p, μ(p). We know that μ is not universal, as it varies from approximately 0.2 to 0.7, according to different experiments. The new energy spectra, E(k,p), has a correction term in its power: -5/3 becomes -5/3-μ(p)/9, thus, the global form of the spectra is E(k) ~ k -β(p), The different simulations produce very different spatial distributions of the bio-tracers. Gabaldon J., Redondo J.M. (2009) Plankton vertical distribution in the ocean, CUM, XTDFTG in Advances in Environmental Turbulence. UPC, Barcelona. 212. Kraichnan, R.H.: (1966), 'Dispersion of particle pairs in homogeneous turbulence', Physics Fluids, 9, 1728. Kolmogorov, A. N. (1941). The local structure of turbulence in Incompressible viscous fluid at very large Reynolds numbers. C. R. Acad. Sci. URSS 30:301. Richardson, L. F. (1929). A search forr the law of atmosferic diffusion. Beitr. Phys. frei. Atmos. 15:24. Redondo J M (1991). The structure of density interfaces, Ph. D. Thesis, CUP, University of Cambridge. Rodríguez A, Sánchez-Arcilla A, Redondo JM, Mosso C (1999) Macroturbulence measurements with electromagnetic and ultrasonic sensors: a comparison under high-turbulent flows. Exp Fluids 27:31-42 Redondo J M (1987). Effects of ground proximity on dense gas entrainment, Journal of Hazardous Materials, 16, 381-393. Redondo J M, Sanchez M A and Cantalapiedra I R (1995). Turbulent Mechanisms in Stratified Flows, Dynamics of Atmospheres and Oceans, 23, 454-462. Mahjoub O.B., Redondo J.M. and Babiano A.(1998), Structure functions in complex flows, Applied Scientific Research, 59, 299. Mahjoub O.B., Redondo J.M. and Babiano A.,(2000) Hyerarchy flux in nonhomogeneous flows in Turbulent diffusion in the environment Eds. Redondo J.M. and Babiano A. 249-260. Redondo J.M., (1988) Difusion turbulenta por rejilla oscilante. Revista de Geofisica 44, 163-174,. Vindel, J. M., Yagüe, C., & Redondo, J. M. (2008). Structure function analysis and intermittency in the atmospheric boundary layer. Nonlinear Processes in Geophysics, 15(6), 915-929.

Effect of the prosthetic mitral valve on vortex dynamics and turbulence of the left ventricular flow

NASA Astrophysics Data System (ADS)

Querzoli, G.; Fortini, S.; Cenedese, A.

2010-04-01

Mechanical heart valves implanted in mitral position have a great effect on the ventricular flow. Changes include alteration of the dynamics of the vortical structures generated during the diastole and the onset of turbulence, possibly affecting the efficiency of the heart pump or causing blood cell damage. Modifications to the hemodynamics in the left ventricle, when the inflow through the mitral orifice is altered, were investigated in vitro using a silicone rubber, flexible ventricle model. Velocity fields were measured in space and time by means of an image analysis technique: feature tracking. Three series of experiments were performed: one with a top hat inflow velocity profile (schematically resembling physiological conditions), and two with mechanical prosthetic valves of different design, mounted in mitral position—one monoleaflet and the other bileaflet. In each series of runs, two different cardiac outputs have been examined by changing the stroke volume. The flow was investigated in terms of phase averaged velocity field and second order moments of turbulent fluctuations. Results show that the modifications in the transmitral flow change deeply the interaction between the coherent structures generated during the first phase of the diastole and the incoming jet during the second diastolic phase. Top hat inflow gives the coherent structures which are optimal, among the compared cases, for the systolic function. The flow generated by the bileaflet valve preserves most of the beneficial features of the top hat inflow, whereas the monoleaflet valve generates a strong jet which discourages the permanence of large coherent structures at the end of the diastole. Moreover, the average shear rate magnitudes induced by the smoother flow pattern of the case of top hat inflow are nearly halved in comparison with the values measured with the mechanical valves. Finally, analysis of the turbulence statistics shows that the monoleaflet valves yield higher turbulence intensity in comparison with the bileaflet and, with top hat inflow, there is not a complete transition to turbulence.

Bacterial turbulence in motion

NASA Astrophysics Data System (ADS)

Rusconi, Roberto; Smriga, Steven; Stocker, Roman; Secchi, Eleonora; Buzzaccaro, Stefano; Piazza, Roberto

2014-11-01

Dense suspensions of motile bacteria exhibit collective dynamics akin to those observed in classic, high Reynolds number turbulence, yet this analogy has remained largely qualitative. Here we present experiments in which a dense suspension of Bacillus subtilis bacteria was flown through narrow microchannels and the velocity statistics of the flowing suspension were accurately quantified with a recently developed velocimetry technique. This revealed a robust intermittency phenomenon, whereby the average velocity profile of the flowing suspension oscillated between a plug-like flow and a parabolic flow. This intermittency is a hallmark of classic turbulence and was associated with the presence of collective structures in the suspension. Furthermore, quantification of the Reynolds stress profile revealed a direct link between the turbulent nature of the suspension and its anomalous viscosity.

Subgrid-scale models for large-eddy simulation of rotating turbulent channel flows

NASA Astrophysics Data System (ADS)

Silvis, Maurits H.; Bae, Hyunji Jane; Trias, F. Xavier; Abkar, Mahdi; Moin, Parviz; Verstappen, Roel

2017-11-01

We aim to design subgrid-scale models for large-eddy simulation of rotating turbulent flows. Rotating turbulent flows form a challenging test case for large-eddy simulation due to the presence of the Coriolis force. The Coriolis force conserves the total kinetic energy while transporting it from small to large scales of motion, leading to the formation of large-scale anisotropic flow structures. The Coriolis force may also cause partial flow laminarization and the occurrence of turbulent bursts. Many subgrid-scale models for large-eddy simulation are, however, primarily designed to parametrize the dissipative nature of turbulent flows, ignoring the specific characteristics of transport processes. We, therefore, propose a new subgrid-scale model that, in addition to the usual dissipative eddy viscosity term, contains a nondissipative nonlinear model term designed to capture transport processes, such as those due to rotation. We show that the addition of this nonlinear model term leads to improved predictions of the energy spectra of rotating homogeneous isotropic turbulence as well as of the Reynolds stress anisotropy in spanwise-rotating plane-channel flows. This work is financed by the Netherlands Organisation for Scientific Research (NWO) under Project Number 613.001.212.

Navier-Stokes simulations of slender axisymmetric shapes in supersonic, turbulent flow

NASA Astrophysics Data System (ADS)

Moran, Kenneth J.; Beran, Philip S.

1994-07-01

Computational fluid dynamics is used to study flows about slender, axisymmetric bodies at very high speeds. Numerical experiments are conducted to simulate a broad range of flight conditions. Mach number is varied from 1.5 to 8 and Reynolds number is varied from 1 X 10(exp 6)/m to 10(exp 8)/m. The primary objective is to develop and validate a computational and methodology for the accurate simulation of a wide variety of flow structures. Accurate results are obtained for detached bow shocks, recompression shocks, corner-point expansions, base-flow recirculations, and turbulent boundary layers. Accuracy is assessed through comparison with theory and experimental data; computed surface pressure, shock structure, base-flow structure, and velocity profiles are within measurement accuracy throughout the range of conditions tested. The methodology is both practical and general: general in its applicability, and practicaal in its performance. To achieve high accuracy, modifications to previously reported techniques are implemented in the scheme. These modifications improve computed results in the vicinity of symmetry lines and in the base flow region, including the turbulent wake.

The Numerical Analysis of a Turbulent Compressible Jet. Degree awarded by Ohio State Univ., 2000

NASA Technical Reports Server (NTRS)

DeBonis, James R.

2001-01-01

A numerical method to simulate high Reynolds number jet flows was formulated and applied to gain a better understanding of the flow physics. Large-eddy simulation was chosen as the most promising approach to model the turbulent structures due to its compromise between accuracy and computational expense. The filtered Navier-Stokes equations were developed including a total energy form of the energy equation. Subgrid scale models for the momentum and energy equations were adapted from compressible forms of Smagorinsky's original model. The effect of using disparate temporal and spatial accuracy in a numerical scheme was discovered through one-dimensional model problems and a new uniformly fourth-order accurate numerical method was developed. Results from two- and three-dimensional validation exercises show that the code accurately reproduces both viscous and inviscid flows. Numerous axisymmetric jet simulations were performed to investigate the effect of grid resolution, numerical scheme, exit boundary conditions and subgrid scale modeling on the solution and the results were used to guide the three-dimensional calculations. Three-dimensional calculations of a Mach 1.4 jet showed that this LES simulation accurately captures the physics of the turbulent flow. The agreement with experimental data was relatively good and is much better than results in the current literature. Turbulent intensities indicate that the turbulent structures at this level of modeling are not isotropic and this information could lend itself to the development of improved subgrid scale models for LES and turbulence models for RANS simulations. A two point correlation technique was used to quantify the turbulent structures. Two point space correlations were used to obtain a measure of the integral length scale, which proved to be approximately 1/2 D(sub j). Two point space-time correlations were used to obtain the convection velocity for the turbulent structures. This velocity ranged from 0.57 to 0.71 U(sub j).

Transition to subcritical turbulence in a tokamak plasma

NASA Astrophysics Data System (ADS)

van Wyk, F.; Highcock, E. G.; Schekochihin, A. A.; Roach, C. M.; Field, A. R.; Dorland, W.

2016-12-01

Tokamak turbulence, driven by the ion-temperature gradient and occurring in the presence of flow shear, is investigated by means of local, ion-scale, electrostatic gyrokinetic simulations (with both kinetic ions and electrons) of the conditions in the outer core of the Mega-Ampere Spherical Tokamak (MAST). A parameter scan in the local values of the ion-temperature gradient and flow shear is performed. It is demonstrated that the experimentally observed state is near the stability threshold and that this stability threshold is nonlinear: sheared turbulence is subcritical, i.e. the system is formally stable to small perturbations, but, given a large enough initial perturbation, it transitions to a turbulent state. A scenario for such a transition is proposed and supported by numerical results: close to threshold, the nonlinear saturated state and the associated anomalous heat transport are dominated by long-lived coherent structures, which drift across the domain, have finite amplitudes, but are not volume filling; as the system is taken away from the threshold into the more unstable regime, the number of these structures increases until they overlap and a more conventional chaotic state emerges. Whereas this appears to represent a new scenario for transition to turbulence in tokamak plasmas, it is reminiscent of the behaviour of other subcritically turbulent systems, e.g. pipe flows and Keplerian magnetorotational accretion flows.

The significance of turbulent flow representation in single-continuum models

USGS Publications Warehouse

Reimann, T.; Rehrl, C.; Shoemaker, W.B.; Geyer, T.; Birk, S.

2011-01-01

Karst aquifers exhibit highly conductive features caused from rock dissolution processes. Flow within these structures can become turbulent and therefore can be expressed by nonlinear gradient functions. One way to account for these effects is by coupling a continuum model with a conduit network. Alternatively, turbulent flow can be considered by adapting the hydraulic conductivity within the continuum model. Consequently, the significance of turbulent flow on the dynamic behavior of karst springs is investigated by an enhanced single-continuum model that results in conduit-type flow in continuum cells (CTFC). The single-continuum approach CTFC represents laminar and turbulent flow as well as more complex hybrid models that require additional programming and numerical efforts. A parameter study is conducted to investigate the effects of turbulent flow on the response of karst springs to recharge events using the new CTFC approach, existing hybrid models, and MODFLOW-2005. Results reflect the importance of representing (1) turbulent flow in karst conduits and (2) the exchange between conduits and continuum cells. More specifically, laminar models overestimate maximum spring discharge and underestimate hydraulic gradients within the conduit. It follows that aquifer properties inferred from spring hydrographs are potentially impaired by ignoring flow effects due to turbulence. The exchange factor used for hybrid models is necessary to account for the scale dependency between hydraulic properties of the matrix continuum and conduits. This functionality, which is not included in CTFC, can be mimicked by appropriate use of the Horizontal Flow Barrier package for MODFLOW. Copyright 2011 by the American Geophysical Union.

An experimental study of combustion: The turbulent structure of a reacting shear layer formed at a rearward-facing step. Ph.D. Thesis. Final Report

NASA Technical Reports Server (NTRS)

Pitz, R. W.

1981-01-01

A premixed propane-air flame is stabilized in a turbulent free shear layer formed at a rearward-facing step. The mean and rms averages of the turbulent velocity flow field were determined by LDV for both reacting and non-reacting flows. The reaching flow was visualized by high speed schlieren photography. Large scale structures dominate the reacting shear layer. The growth of the large scale structures is tied to the propagation of the flame. The linear growth rate of the reacting shear layer defined by the mean velocity profiles is unchanged by combustion but the virtual origin is shifted downstream. The reacting shear layer based on the mean velocity profiles is shifted toward the recirculation zone and the reattachments lengths are shortened by 30%.

Turbulence accelerates the growth of drinking water biofilms.

PubMed

Tsagkari, E; Sloan, W T

2018-06-01

Biofilms are found at the inner surfaces of drinking water pipes and, therefore, it is essential to understand biofilm processes to control their formation. Hydrodynamics play a crucial role in shaping biofilms. Thus, knowing how biofilms form, develop and disperse under different flow conditions is critical in the successful management of these systems. Here, the development of biofilms after 4 weeks, the initial formation of biofilms within 10 h and finally, the response of already established biofilms within 24-h intervals in which the flow regime was changed, were studied using a rotating annular reactor under three different flow regimes: turbulent, transition and laminar. Using fluorescence microscopy, information about the number of microcolonies on the reactor slides, the surface area of biofilms and of extracellular polymeric substances and the biofilm structures was acquired. Gravimetric measurements were conducted to characterise the thickness and density of biofilms, and spatial statistics were used to characterise the heterogeneity and spatial correlation of biofilm structures. Contrary to the prevailing view, it was shown that turbulent flow did not correlate with a reduction in biofilms; turbulence was found to enhance both the initial formation and the development of biofilms on the accessible surfaces. Additionally, after 24-h changes of the flow regime it was indicated that biofilms responded to the quick changes of the flow regime. Overall, this work suggests that different flow conditions can cause substantial changes in biofilm morphology and growth and specifically that turbulent flow can accelerate biofilm growth in drinking water.

Wind Tunnel Investigation of the Near-wake Flow Dynamics of a Horizontal Axis Wind Turbine

NASA Astrophysics Data System (ADS)

Hashemi-Tari, P.; Siddiqui, K.; Refan, M.; Hangan, H.

2014-06-01

Experiments conducted in a large wind tunnel set-up investigate the 3D flow dynamics within the near-wake region of a horizontal axis wind turbine. Particle Image Velocimetry (PIV) measurements quantify the mean and turbulent components of the flow field. Measurements are performed in multiple adjacent horizontal planes in order to cover the area behind the rotor in a large radial interval, at several locations downstream of the rotor. The measurements were phase-locked in order to facilitate the re-construction of the threedimensional flow field. The mean velocity and turbulence characteristics clearly correlate with the near-wake vortex dynamics and in particular with the helical structure of the flow, formed immediately behind the turbine rotor. Due to the tip and root vortices, the mean and turbulent characteristics of the flow are highly dependent on the azimuth angle in regions close to the rotor and close to the blade tip and root. Further from the rotor, the characteristics of the flow become phase independent. This can be attributed to the breakdown of the vortical structure of the flow, resulting from the turbulent diffusion. In general, the highest levels of turbulence are observed in shear layer around the tip of the blades, which decrease rapidly downstream. The shear zone grows in the radial direction as the wake moves axially, resulting in velocity recovery toward the centre of the rotor due to momentum transport.

Air-flow distortion and turbulence statistics near an animal facility

NASA Astrophysics Data System (ADS)

Prueger, J. H.; Eichinger, W. E.; Hipps, L. E.; Hatfield, J. L.; Cooper, D. I.

The emission and dispersion of particulates and gases from concentrated animal feeding operations (CAFO) at local to regional scales is a current issue in science and society. The transport of particulates, odors and toxic chemical species from the source into the local and eventually regional atmosphere is largely determined by turbulence. Any models that attempt to simulate the dispersion of particles must either specify or assume various statistical properties of the turbulence field. Statistical properties of turbulence are well documented for idealized boundary layers above uniform surfaces. However, an animal production facility is a complex surface with structures that act as bluff bodies that distort the turbulence intensity near the buildings. As a result, the initial release and subsequent dispersion of effluents in the region near a facility will be affected by the complex nature of the surface. Previous Lidar studies of plume dispersion over the facility used in this study indicated that plumes move in complex yet organized patterns that would not be explained by the properties of turbulence generally assumed in models. The objective of this study was to characterize the near-surface turbulence statistics in the flow field around an array of animal confinement buildings. Eddy covariance towers were erected in the upwind, within the building array and downwind regions of the flow field. Substantial changes in turbulence intensity statistics and turbulence-kinetic energy (TKE) were observed as the mean wind flow encountered the building structures. Spectra analysis demonstrated unique distribution of the spectral energy in the vertical profile above the buildings.

Multi-fluid Dynamics for Supersonic Jet-and-Crossflows and Liquid Plug Rupture

NASA Astrophysics Data System (ADS)

Hassan, Ezeldin A.

Multi-fluid dynamics simulations require appropriate numerical treatments based on the main flow characteristics, such as flow speed, turbulence, thermodynamic state, and time and length scales. In this thesis, two distinct problems are investigated: supersonic jet and crossflow interactions; and liquid plug propagation and rupture in an airway. Gaseous non-reactive ethylene jet and air crossflow simulation represents essential physics for fuel injection in SCRAMJET engines. The regime is highly unsteady, involving shocks, turbulent mixing, and large-scale vortical structures. An eddy-viscosity-based multi-scale turbulence model is proposed to resolve turbulent structures consistent with grid resolution and turbulence length scales. Predictions of the time-averaged fuel concentration from the multi-scale model is improved over Reynolds-averaged Navier-Stokes models originally derived from stationary flow. The response to the multi-scale model alone is, however, limited, in cases where the vortical structures are small and scattered thus requiring prohibitively expensive grids in order to resolve the flow field accurately. Statistical information related to turbulent fluctuations is utilized to estimate an effective turbulent Schmidt number, which is shown to be highly varying in space. Accordingly, an adaptive turbulent Schmidt number approach is proposed, by allowing the resolved field to adaptively influence the value of turbulent Schmidt number in the multi-scale turbulence model. The proposed model estimates a time-averaged turbulent Schmidt number adapted to the computed flowfield, instead of the constant value common to the eddy-viscosity-based Navier-Stokes models. This approach is assessed using a grid-refinement study for the normal injection case, and tested with 30 degree injection, showing improved results over the constant turbulent Schmidt model both in mean and variance of fuel concentration predictions. For the incompressible liquid plug propagation and rupture study, numerical simulations are conducted using an Eulerian-Lagrangian approach with a continuous-interface method. A reconstruction scheme is developed to allow topological changes during plug rupture by altering the connectivity information of the interface mesh. Rupture time is shown to be delayed as the initial precursor film thickness increases. During the plug rupture process, a sudden increase of mechanical stresses on the tube wall is recorded, which can cause tissue damage.

Resolution requirements for aero-optical simulations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mani, Ali; Wang Meng; Moin, Parviz

2008-11-10

Analytical criteria are developed to estimate the error of aero-optical computations due to inadequate spatial resolution of refractive index fields in high Reynolds number flow simulations. The unresolved turbulence structures are assumed to be locally isotropic and at low turbulent Mach number. Based on the Kolmogorov spectrum for the unresolved structures, the computational error of the optical path length is estimated and linked to the resulting error in the computed far-field optical irradiance. It is shown that in the high Reynolds number limit, for a given geometry and Mach number, the spatial resolution required to capture aero-optics within a pre-specifiedmore » error margin does not scale with Reynolds number. In typical aero-optical applications this resolution requirement is much lower than the resolution required for direct numerical simulation, and therefore, a typical large-eddy simulation can capture the aero-optical effects. The analysis is extended to complex turbulent flow simulations in which non-uniform grid spacings are used to better resolve the local turbulence structures. As a demonstration, the analysis is used to estimate the error of aero-optical computation for an optical beam passing through turbulent wake of flow over a cylinder.« less

Letter: The link between the Reynolds shear stress and the large structures of turbulent Couette-Poiseuille flow

NASA Astrophysics Data System (ADS)

Gandía-Barberá, Sergio; Hoyas, Sergio; Oberlack, Martin; Kraheberger, Stefanie

2018-04-01

The length and width of the long and wide structures appearing in turbulent Couette flows are studied by means of a new dataset of direct numerical simulation covering a stepped transition from pure Couette flow to pure Poiseuille one, at Reτ ≈ 130, based on the stationary wall. The existence of these structures is linked to the averaged Reynolds stress, u v ¯ : as soon as in any part of the channel u v ¯ changes its sign, the structures disappear. The length and width of the rolls are found to be, approximately, 50h and 2.5h, respectively. For this Reynolds number, simulations with a domain shorter than 100h cannot properly describe the behaviour of the longest structures of the flow.

Experimental Investigation of Vortex Structures in Wake of Hyperboloid-Shaped Model by Means of 2D Particle Image Velocimetry Measurement

NASA Astrophysics Data System (ADS)

Barraclough, V.; Novotný, J.; Šafařík, P.

2018-06-01

This paper deals with flow around a bluff body of hyperboloid shape. It consists of results gathered in the course of research by means of Particle Image Velocimetry (PIV). The experiments were carried out by means of low-frequency 2D PIV in a range of Reynolds numbers from 40000 to 50000. A hyperboloid-shaped model was measured in a wind tunnel with a modelled atmospheric boundary layer (and additionally, in a low-speed wind tunnel with low turbulence). The model was tested in a subcritical range of Reynolds numbers and various planes in a wake of the model were captured with the intention of getting an estimation of 3D flow structures. The tunnel with the modelled atmospheric boundary layer has a high rate of turbulence, so the influence of the turbulence of incoming flow on the wake could be outlined. The ratio of the height of the model to a thickness of the modelled boundary layer in the tunnel was 1/3, meaning the turbulence in the boundary layer strongly influenced the flow around the model; it suppresses the wake which leads to a lot shorter area of recirculation than low turbulence incoming flow would cause.

Zonal Flows and Long-lived Axisymmetric Pressure Bumps in Magnetorotational Turbulence

NASA Astrophysics Data System (ADS)

Johansen, A.; Youdin, A.; Klahr, H.

2009-06-01

We study the behavior of magnetorotational turbulence in shearing box simulations with a radial and azimuthal extent up to 10 scale heights. Maxwell and Reynolds stresses are found to increase by more than a factor of 2 when increasing the box size beyond two scale heights in the radial direction. Further increase of the box size has little or no effect on the statistical properties of the turbulence. An inverse cascade excites magnetic field structures at the largest scales of the box. The corresponding 10% variation in the Maxwell stress launches a zonal flow of alternating sub- and super-Keplerian velocity. This, in turn, generates a banded density structure in geostrophic balance between pressure and Coriolis forces. We present a simplified model for the appearance of zonal flows, in which stochastic forcing by the magnetic tension on short timescales creates zonal flow structures with lifetimes of several tens of orbits. We experiment with various improved shearing box algorithms to reduce the numerical diffusivity introduced by the supersonic shear flow. While a standard finite difference advection scheme shows signs of a suppression of turbulent activity near the edges of the box, this problem is eliminated by a new method where the Keplerian shear advection is advanced in time by interpolation in Fourier space.

Noise produced by turbulent flow into a rotor: Users manual for noise calculation

NASA Technical Reports Server (NTRS)

Amiet, R. K.; Egolf, C. G.; Simonich, J. C.

1989-01-01

A users manual for a computer program for the calculation of noise produced by turbulent flow into a helicopter rotor is presented. These inputs to the program are obtained from the atmospheric turbulence model and mean flow distortion calculation, described in another volume of this set of reports. Descriptions of the various program modules and subroutines, their function, programming structure, and the required input and output variables are included. This routine is incorporated as one module of NASA's ROTONET helicopter noise prediction program.

Worm-like micelles of CTAB and sodium salicylate under turbulent flow.

PubMed

Rodrigues, Roberta K; da Silva, Marcelo A; Sabadini, Edvaldo

2008-12-16

Polymers with high molecular weight and worm-like micelles are drag-reducing agents under turbulent flow. However, in contrast to the polymeric systems, the worm-like micelles do not undergo mechanical degradation due to the turbulence, because their macromolecular structure can be spontaneously restored. This very favorable property, together with their drag-reduction capability, offer the possibility to use such worm-like micelles in heating and cooling systems to recirculate water while expending less energy. The formation, growth, and stability of worm-like micelles formed by cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) were investigated using the self-fluorescence of salicylate ions and the ability of the giant micelles to promote hydrodynamic drag reduction under turbulent flow. The turbulence in solutions of CTAB-Sal was produced within the double-gap cell of a rotational rheometer. Detailed diagrams were obtained for different ratios of Sal and CTAB, which revealed transitions associated with the thermal stability of giant micelles under turbulent flow.

The structure of turbulent channel flow with passive scalar transport

NASA Technical Reports Server (NTRS)

Guezennec, Y.; Stretch, D.; Kim, J.

1990-01-01

The simulation of turbulent channel flow, with various passive markers, was examined to investigate the local mechanisms of passive scalar transport. We found significant differences between the local transport of heat and momentum, even when the molecular and turbulent Prandtl numbers are of order one. These discrepancies can be attributed to the role of the pressure. We also found that the heat is a poor marker of the vorticity field outside of the near wall region and that scalar transport over significant distances results from the aggregate effect of many turbulent eddies.

Prospectus: towards the development of high-fidelity models of wall turbulence at large Reynolds number

PubMed Central

Klewicki, J. C.; Chini, G. P.; Gibson, J. F.

2017-01-01

Recent and on-going advances in mathematical methods and analysis techniques, coupled with the experimental and computational capacity to capture detailed flow structure at increasingly large Reynolds numbers, afford an unprecedented opportunity to develop realistic models of high Reynolds number turbulent wall-flow dynamics. A distinctive attribute of this new generation of models is their grounding in the Navier–Stokes equations. By adhering to this challenging constraint, high-fidelity models ultimately can be developed that not only predict flow properties at high Reynolds numbers, but that possess a mathematical structure that faithfully captures the underlying flow physics. These first-principles models are needed, for example, to reliably manipulate flow behaviours at extreme Reynolds numbers. This theme issue of Philosophical Transactions of the Royal Society A provides a selection of contributions from the community of researchers who are working towards the development of such models. Broadly speaking, the research topics represented herein report on dynamical structure, mechanisms and transport; scale interactions and self-similarity; model reductions that restrict nonlinear interactions; and modern asymptotic theories. In this prospectus, the challenges associated with modelling turbulent wall-flows at large Reynolds numbers are briefly outlined, and the connections between the contributing papers are highlighted. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’. PMID:28167585

Prospectus: towards the development of high-fidelity models of wall turbulence at large Reynolds number.

PubMed

Klewicki, J C; Chini, G P; Gibson, J F

2017-03-13

Recent and on-going advances in mathematical methods and analysis techniques, coupled with the experimental and computational capacity to capture detailed flow structure at increasingly large Reynolds numbers, afford an unprecedented opportunity to develop realistic models of high Reynolds number turbulent wall-flow dynamics. A distinctive attribute of this new generation of models is their grounding in the Navier-Stokes equations. By adhering to this challenging constraint, high-fidelity models ultimately can be developed that not only predict flow properties at high Reynolds numbers, but that possess a mathematical structure that faithfully captures the underlying flow physics. These first-principles models are needed, for example, to reliably manipulate flow behaviours at extreme Reynolds numbers. This theme issue of Philosophical Transactions of the Royal Society A provides a selection of contributions from the community of researchers who are working towards the development of such models. Broadly speaking, the research topics represented herein report on dynamical structure, mechanisms and transport; scale interactions and self-similarity; model reductions that restrict nonlinear interactions; and modern asymptotic theories. In this prospectus, the challenges associated with modelling turbulent wall-flows at large Reynolds numbers are briefly outlined, and the connections between the contributing papers are highlighted.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'. © 2017 The Author(s).

Investigations of the role of nonlinear couplings in structure formation and transport regulation in plasma turbulence

NASA Astrophysics Data System (ADS)

Holland, Christopher George

Studies of nonlinear couplings and dynamics in plasma turbulence are presented. Particular areas of focus are analytic studies of coherent structure formation in electron temperature gradient turbulence, measurement of nonlinear energy transfer in simulations of plasma turbulence, and bispectral analysis of experimental and computational data. The motivation for these works has been to develop and expand the existing theories of plasma transport, and verify the nonlinear predictions of those theories in simulation and experiment. In Chapter II, we study electromagnetic secondary instabilities of electron temperature gradient turbulence. The growth rate for zonal flow generation via modulational instability of electromagnetic ETG turbulence is calculated, as well as that for zonal (magnetic) field generation. In Chapter III, the stability and saturation of streamers in ETG turbulence is considered, and shown to depend sensitively upon geometry and the damping rates of the Kelvin-Helmholtz mode. Requirements for a credible theory of streamer transport are presented. In addition, a self-consistent model for interactions between ETG and ITG (ion temperature gradient) turbulence is presented. In Chapter IV, the nonlinear transfer of kinetic and internal energy is measured in simulations of plasma turbulence. The regulation of turbulence by radial decorrelation due to zonal flows and generation of zonal flows via the Reynolds stress are explicitly demonstrated, and shown to be symmetric facets of a single nonlinear process. Novel nonlinear saturation mechanisms for zonal flows are discussed. In Chapter V, measurements of fluctuation bicoherence in the edge of the DIII-D tokamak are presented. It is shown that the bicoherence increases transiently before a L-H transition, and decays to its initial value after the barrier has formed. The increase in bicoherence is localized to the region where the transport barrier forms, and shows strong coupling between well-separated frequencies. These results are qualitatively reproduced in a simple numerical "thought experiment," described in Chapter VI, which suggests that zonal flows may trigger the L-H transition.

Turbulent pipe flows subjected to temporal decelerations

NASA Astrophysics Data System (ADS)

Jeong, Wongwan; Lee, Jae Hwa

2016-11-01

Direct numerical simulations of temporally decelerating turbulent pipe flows were performed to examine effects of temporal decelerations on turbulence. The simulations were started with a fully developed turbulent pipe flow at a Reynolds number, ReD =24380, based on the pipe radius (R) and the laminar centerline velocity (Uc 0). Three different temporal decelerations were imposed to the initial flow with f= | d Ub / dt | =0.00127, 0.00625 and 0.025, where Ub is the bulk mean velocity. Comparison of Reynolds stresses and turbulent production terms with those for steady flow at a similar Reynolds number showed that turbulence is highly intensified with increasing f due to delay effects. Furthermore, inspection of the Reynolds shear stress profiles showed that strong second- and fourth-quadrant Reynolds shear stresses are greatly increased, while first- and third-quadrant components are also increased. Decomposition of streamwise Reynolds normal stress with streamwise cutoff wavelength (λx) 1 R revealed that the turbulence delay is dominantly originated from delay of strong large-scale turbulent structures in the outer layer, although small-scale motions throughout the wall layer adjusted more rapidly to the temporal decelerations. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2057031).

The structure of dilute combusting sprays

NASA Technical Reports Server (NTRS)

Shuen, J. S.; Solomon, A. S. P.; Faeth, F. M.

1985-01-01

An experimental and theoretical study of drop processes in a turbulent flame is described. The experiments involved a monodisperse (105 and 180 micro m initial diameter) stream of methanol drops injected at the base of a turbulent methane-fueled diffusion flame burning in still air. The following measurements were made: mean and fluctuating phase velocities, mean drop number flux, drop-size distributions and mean gas-phase temperatures. Measurements were compared with predictions of two separated flow models: (1) deterministic separated flow, where drop-turbulence interactions are ignored; and (2) stochastic separated flow, where drop-turbulence interactions are considered using random-walk computations. The stochastic separated flow analysis yielded best agreement with measurements, since it provides for turbulent dispersion of drops which was important for present test conditions (and probably for most combusting sprays as well). Distinguishing the presence or absence of envelope flames around the drops, however, was relatively unimportant for present test conditions, since the drops spent most of their lifetime in fuel-rich regions of the flow where this distinction is irrelevant.

Enhancement of fine-scale mixing for fuel-rich plume combustion

NASA Astrophysics Data System (ADS)

Schadow, K. C.; Gutmark, E.; Parr, T. P.; Parr, D. M.; Wilson, K. J.; Ferrell, G. B.

1987-01-01

The effect of enhancing small-scale turbulent structures on the combustion intensity and flame stability was studied in nonreacting and reacting flows. Hot-wire anemometry was used to map the mean and turbulent flow fields of the nonreacting flows. Reacting flows were studied in a free flame and in a ducted gas-generator fuel-rich plume using Planar Laser Induced Fluorescence, a rake of thermocouples and high speed photography. A modified circular nozzle having several backward facing steps upstream of its exit was used to introduce numerous inflection points in the initial mean velocity profiles, thus producing multiple corresponding sources of small-scale turbulence generators. Cold flow tests showed turbulence increases of up to six times the initial turbulence level relative to a circular nozzle. The ensuing result was that the flame of this nozzle was more intense with a homogeneous heat release. The fuel-rich plume was stable even in supersonic speeds, and secondary ignition was obtained under conditions that prevented sustained afterburning using the circular nozzle.

How pattern is selected in drift wave turbulence: Role of parallel flow shear

NASA Astrophysics Data System (ADS)

Kosuga, Y.

2017-12-01

The role of parallel shear flow in the pattern selection problem in drift wave turbulence is discussed. Patterns of interest here are E × B convective cells, which include poloidally symmetric zonal flows and radially elongated streamers. The competition between zonal flow formation and streamer formation is analyzed in the context of modulational instability analysis, with the parallel flow shear as a parameter. For drift wave turbulence with k⊥ρs ≲ O (1 ) and without parallel flow coupling, zonal flows are preferred structures. While increasing the magnitude of parallel flow shear, streamer growth overcomes zonal flow growth. This is because the self-focusing effect of the modulational instability becomes more effective for streamers through density and parallel velocity modulation. As a consequence, the bursty release of free energy may result as the parallel flow shear increases.

Turbulent particulate transportation during electrostatic precipitation

NASA Astrophysics Data System (ADS)

Choi, Bum Seog

The generation of secondary flows and turbulence by a corona discharge influences particle transport in an electrostatic precipitator (ESP), and is known to play an important role in the particle collection process. However, it is difficult to characterise theoretically and experimentally the ``turbulent'' fluctuations of the gas flow produced by negative tuft corona. Because of this difficulty, only limited studies have been undertaken previously to understand the structure of corona-induced turbulence and its influence on particle transport in ESPs. The present study is aimed at modelling electrohydrodynamic turbulent flows and particle transport, and at establishing an unproved understanding of them. For a multiply interactive coupling of electrostatics, fluid dynamics and particle dynamics, a strongly coupled system of the governing equations has been solved. The present computer model has considered the most important interaction mechanisms including an ionic wind, corona- induced turbulence and the particle space charge effect. Numerical simulations have been performed for the extensive validation of the numerical and physical models. To account for electrically excited turbulence associated with the inhomogeneous and unsteady characteristics of negative corona discharges, a new turbulence model (called the electrostatic turbulence model) has been developed. In this, an additional production or destruction term is included into the turbulent kinetic energy and dissipation rate equations. It employs a gradient type model of the current density and an electrostatic diffusivity concept. The results of the computation show that the electrostatic turbulence model gives much better agreement with the experimental data than the conventional RNG k-ɛ turbulence model when predicting turbulent gas flows and particle distributions in an ESP. Computations of turbulent particulate two-phase flows for both mono-dispersed and poly-dispersed particles have been performed. The effects of coriona-induced turbulence and the particle space charge on particle transport and the collection process have been investigated. The calculated results for the poly-dispersed particulate flow were compared with those of the mono-dispersed particulate flow, and significant differences were demonstrated. It is established that effective particle- particle interaction occurs, due to the influence of the particle space charge, even for dilute gas-particle flows that occur in ESPs.

Shark-skin surfaces for fluid-drag reduction in turbulent flow: a review.

PubMed

Dean, Brian; Bhushan, Bharat

2010-10-28

The skin of fast-swimming sharks exhibits riblet structures aligned in the direction of flow that are known to reduce skin friction drag in the turbulent-flow regime. Structures have been fabricated for study and application that replicate and improve upon the natural shape of the shark-skin riblets, providing a maximum drag reduction of nearly 10 per cent. Mechanisms of fluid drag in turbulent flow and riblet-drag reduction theories from experiment and simulation are discussed. A review of riblet-performance studies is given, and optimal riblet geometries are defined. A survey of studies experimenting with riblet-topped shark-scale replicas is also given. A method for selecting optimal riblet dimensions based on fluid-flow characteristics is detailed, and current manufacturing techniques are outlined. Due to the presence of small amounts of mucus on the skin of a shark, it is expected that the localized application of hydrophobic materials will alter the flow field around the riblets in some way beneficial to the goals of increased drag reduction.

Flow turbulence topology in regular porous media: From macroscopic to microscopic scale with direct numerical simulation

NASA Astrophysics Data System (ADS)

Chu, Xu; Weigand, Bernhard; Vaikuntanathan, Visakh

2018-06-01

Microscopic analysis of turbulence topology in a regular porous medium is presented with a series of direct numerical simulation. The regular porous media are comprised of square cylinders in a staggered array. Triply periodic boundary conditions enable efficient investigations in a representative elementary volume. Three flow patterns—channel with sudden contraction, impinging surface, and wake—are observed and studied quantitatively in contrast to the qualitative experimental studies reported in the literature. Among these, shear layers in the channel show the highest turbulence intensity due to a favorable pressure gradient and shed due to an adverse pressure gradient downstream. The turbulent energy budget indicates a strong production rate after the flow contraction and a strong dissipation on both shear and impinging walls. Energy spectra and pre-multiplied spectra detect large scale energetic structures in the shear layer and a breakup of scales in the impinging layer. However, these large scale structures break into less energetic small structures at high Reynolds number conditions. This suggests an absence of coherent structures in densely packed porous media at high Reynolds numbers. Anisotropy analysis with a barycentric map shows that the turbulence in porous media is highly isotropic in the macro-scale, which is not the case in the micro-scale. In the end, proper orthogonal decomposition is employed to distinguish the energy-conserving structures. The results support the pore scale prevalence hypothesis. However, energetic coherent structures are observed in the case with sparsely packed porous media.

Statistical anisotropy in free turbulence for mixing layers at high Reynolds numbers

NASA Astrophysics Data System (ADS)

Gardner, Patrick J.; Roggemann, Michael C.; Welsh, Byron M.; Bowersox, Rodney D.; Luke, Theodore E.

1996-08-01

A lateral shearing interferometer was used to measure the slope of perturbed wave fronts after propagating through free turbulent mixing layers. Shearing interferometers provide a two-dimensional flow visualization that is nonintrusive. Slope measurements were used to reconstruct the phase of the turbulence-corrupted wave front. The random phase fluctuations induced by the mixing layer were captured in a large ensemble of wave-front measurements. Experiments were performed on an unbounded, plane shear mixing layer of helium and nitrogen gas at fixed velocities and high Reynolds numbers for six locations in the flow development. Statistical autocorrelation functions and structure functions were computed on the reconstructed phase maps. The autocorrelation function results indicated that the turbulence-induced phase fluctuations were not wide-sense stationary. The structure functions exhibited statistical homogeneity, indicating that the phase fluctuations were stationary in first increments. However, the turbulence-corrupted phase was not isotropic. A five-thirds power law is shown to fit orthogonal slices of the structure function, analogous to the Kolmogorov model for isotropic turbulence. Strehl ratios were computed from the phase structure functions and compared with classical estimates that assume isotropy. The isotropic models are shown to overestimate the optical degradation by nearly 3 orders of magnitude compared with the structure function calculations.

On the freestream matching condition for stagnation point turbulent flows

NASA Technical Reports Server (NTRS)

Speziale, C. G.

1989-01-01

The problem of plane stagnation point flow with freestream turbulence is examined from a basic theoretical standpoint. It is argued that the singularity which arises from the standard kappa-epsilon model is not due to a defect in the model but results from the use of an inconsistent freestream boundary condition. The inconsistency lies in the implementation of a production equals dissipation equilibrium hypothesis in conjunction with a freestream mean velocity field that corresponds to homogeneous plane strain - a turbulent flow which does not reach such a simple equilibrium. Consequently, the adjustment that has been made in the constants of the epsilon-transport equation to eliminate this singularity is not self-consistent since it is tantamount to artificially imposing an equilibrium structure on a turbulent flow which is known not to have one.

Effects of biofilm on flow over and through a permeable bed

NASA Astrophysics Data System (ADS)

Kazemifar, Farzan; Blois, Gianluca; Aybar, Marcelo; Perez-Calleja, Patricia; Nerenberg, Robert; Sinha, Sumit; Hardy, Richard; Best, James; Sambrook-Smith, Gregory; Christensen, Kenneth

2016-11-01

Biofilms constitute an important form of bacterial life in aquatic environments and are present at the interface of fluids and solids, such as riverbeds. Biofilms are permeable, heterogeneous, and deformable structures that can influence the flow and mass/momentum transport, yet their interaction with flow is not fully understood in part due to technical obstacles impeding quantitative experimental investigations. The porosity of river beds results in the generation of a diverse mosaic of 'suction' and 'ejection' events that are far removed from typical assumptions of turbulent flow structure over an impermeable bed. In this work, the effect of biofilm on bed permeability is studied. Experiments are conducted in a closed water channel equipped with 4-cm-deep permeable bed models consisting of horizontal cylinders normal to the bulk flow direction, forming an idealized two-dimensional permeable bed. Prior to conducting flow experiments, the models are placed within an independent biofilm reactor to initiate and control the biofilm growth. Once a targeted biofilm growth stage is achieved, the models are transferred to the water channel and subjected to transitional and turbulent flows. Long-distance microscopic particle image velocimetry measurements are performed to quantify the effect of biofilm on the turbulence structure of the free flow as well as the freestream-subsurface flow interaction.

Effects of biofilm on flow over and through a permeable bed

NASA Astrophysics Data System (ADS)

Kazemifar, F.; Blois, G.; Aybar, M.; Perez Calleja, P.; Nerenberg, R.; Sinha, S.; Hardy, R. J.; Best, J.; Sambrook Smith, G.; Christensen, K. T.

2016-12-01

Biofilms constitute an important form of bacterial life in aquatic environments and are present at the fluid-solid interfaces, such as riverbeds. Biofilms are permeable, heterogeneous, and deformable structures that can influence the flow and mass/momentum transport, yet their interaction with flow is not fully understood in part due to technical obstacles impeding quantitative experimental investigations. The porosity of river beds results in the generation of a diverse mosaic of `suction' and `ejection' events that are far removed from typical assumptions of turbulent flow structure over an impermeable bed. In this work, the effect of biofilm on bed permeability is studied. Experiments are conducted in a closed water channel equipped with 4-cm-deep permeable bed models consisting of horizontal cylinders normal to the bulk flow direction, forming an idealized two-dimensional permeable bed (Figure 1). Prior to conducting flow experiments, the models are placed within an independent biofilm reactor to initiate and accurately control the biofilm growth. Once a targeted biofilm growth stage is achieved, the models are transferred to the water channel and subjected to transitional and turbulent flows. Long-distance microscopic particle image velocimetry measurements are performed to quantify the effect of biofilm on the turbulence structure of the free flow as well as the freestream-subsurface flow interaction.

The structure of turbulence in fully developed pipe flow

NASA Technical Reports Server (NTRS)

Laufer, John

1954-01-01

Measurements, principally with a hot-wire anemometer, were made in fully developed turbulent flow in a 10-inch pipe at speeds of approximately 10 and 100 feet per second. Emphasis was placed on turbulence and conditions near the wall. The results include relevant mean and statistical quantities, such as Reynolds stresses, triple correlations, turbulent dissipation, and energy spectra. It is shown that rates of turbulent-energy production, dissipation, and diffusion have sharp maximums near the edge of the laminar sublayer and that there exist a strong movement of kinetic energy away from this point and an equally strong movement of pressure energy toward it.

A new mixed subgrid-scale model for large eddy simulation of turbulent drag-reducing flows of viscoelastic fluids

NASA Astrophysics Data System (ADS)

Li, Feng-Chen; Wang, Lu; Cai, Wei-Hua

2015-07-01

A mixed subgrid-scale (SGS) model based on coherent structures and temporal approximate deconvolution (MCT) is proposed for turbulent drag-reducing flows of viscoelastic fluids. The main idea of the MCT SGS model is to perform spatial filtering for the momentum equation and temporal filtering for the conformation tensor transport equation of turbulent flow of viscoelastic fluid, respectively. The MCT model is suitable for large eddy simulation (LES) of turbulent drag-reducing flows of viscoelastic fluids in engineering applications since the model parameters can be easily obtained. The LES of forced homogeneous isotropic turbulence (FHIT) with polymer additives and turbulent channel flow with surfactant additives based on MCT SGS model shows excellent agreements with direct numerical simulation (DNS) results. Compared with the LES results using the temporal approximate deconvolution model (TADM) for FHIT with polymer additives, this mixed SGS model MCT behaves better, regarding the enhancement of calculating parameters such as the Reynolds number. For scientific and engineering research, turbulent flows at high Reynolds numbers are expected, so the MCT model can be a more suitable model for the LES of turbulent drag-reducing flows of viscoelastic fluid with polymer or surfactant additives. Project supported by the China Postdoctoral Science Foundation (Grant No. 2011M500652), the National Natural Science Foundation of China (Grant Nos. 51276046 and 51206033), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20112302110020).

A numerical study of the complex flow structure in a compound meandering channel

NASA Astrophysics Data System (ADS)

Moncho-Esteve, Ignacio J.; García-Villalba, Manuel; Muto, Yasu; Shiono, Koji; Palau-Salvador, Guillermo

2018-06-01

In this study, we report large eddy simulations of turbulent flow in a periodic compound meandering channel for three different depth conditions: one in-bank and two overbank conditions. The flow configuration corresponds to the experiments of Shiono and Muto (1998). The predicted mean streamwise velocities, mean secondary motions, velocity fluctuations, turbulent kinetic energy as well as mean flood flow angle to meandering channel are in good agreement with the experimental measurements. We have analyzed the flow structure as a function of the inundation level, with particular emphasis on the development of the secondary motions due to the interaction between the main channel and the floodplain flow. Bed shear stresses have been also estimated in the simulations. Floodplain flow has a significant impact on the flow structure leading to significantly different bed shear stress patterns within the main meandering channel. The implications of these results for natural compound meandering channels are also discussed.

Numerical Simulation of Hydrogen Air Supersonic Coaxial Jet

NASA Astrophysics Data System (ADS)

Dharavath, Malsur; Manna, Pulinbehari; Chakraborty, Debasis

2017-10-01

In the present study, the turbulent structure of coaxial supersonic H2-air jet is explored numerically by solving three dimensional RANS equations along with two equation k-ɛ turbulence model. Grid independence of the solution is demonstrated by estimating the error distribution using Grid Convergence Index. Distributions of flow parameters in different planes are analyzed to explain the mixing and combustion characteristics of high speed coaxial jets. The flow field is seen mostly diffusive in nature and hydrogen diffusion is confined to core region of the jet. Both single step laminar finite rate chemistry and turbulent reacting calculation employing EDM combustion model are performed to find the effect of turbulence-chemistry interaction in the flow field. Laminar reaction predicts higher H2 mol fraction compared to turbulent reaction because of lower reaction rate caused by turbulence chemistry interaction. Profiles of major species and temperature match well with experimental data at different axial locations; although, the computed profiles show a narrower shape in the far field region. These results demonstrate that standard two equation class turbulence model with single step kinetics based turbulence chemistry interaction can describe H2-air reaction adequately in high speed flows.

Transition to turbulence in Taylor-Couette ferrofluidic flow

PubMed Central

Altmeyer, Sebastian; Do, Younghae; Lai, Ying-Cheng

2015-01-01

It is known that in classical fluids turbulence typically occurs at high Reynolds numbers. But can turbulence occur at low Reynolds numbers? Here we investigate the transition to turbulence in the classic Taylor-Couette system in which the rotating fluids are manufactured ferrofluids with magnetized nanoparticles embedded in liquid carriers. We find that, in the presence of a magnetic field transverse to the symmetry axis of the system, turbulence can occur at Reynolds numbers that are at least one order of magnitude smaller than those in conventional fluids. This is established by extensive computational ferrohydrodynamics through a detailed investigation of transitions in the flow structure, and characterization of behaviors of physical quantities such as the energy, the wave number, and the angular momentum through the bifurcations. A finding is that, as the magnetic field is increased, onset of turbulence can be determined accurately and reliably. Our results imply that experimental investigation of turbulence may be feasible by using ferrofluids. Our study of transition to and evolution of turbulence in the Taylor-Couette ferrofluidic flow system provides insights into the challenging problem of turbulence control. PMID:26065572

Experimental investigation on aero-optical aberration of shock wave/boundary layer interactions

NASA Astrophysics Data System (ADS)

Ding, Haolin; Yi, Shihe; Fu, Jia; He, Lin

2016-10-01

After streaming through the flow field which including the expansion, shock wave, boundary, etc., the optical wave would be distorted by fluctuations in the density field. Interactions between laminar/turbulent boundary layer and shock wave contain large number complex flow structures, which offer a condition for studying the influences that different flow structures of the complex flow field have on the aero-optical aberrations. Interactions between laminar/turbulent boundary layer and shock wave are investigated in a Mach 3.0 supersonic wind tunnel, based on nanoparticle-tracer planar laser scattering (NPLS) system. Boundary layer separation/attachment, induced suppression waves, induced shock wave, expansion fan and boundary layer are presented by NPLS images. Its spatial resolution is 44.15 μm/pixel. Time resolution is 6ns. Based on the NPLS images, the density fields with high spatial-temporal resolution are obtained by the flow image calibration, and then the optical path difference (OPD) fluctuations of the original 532nm planar wavefront are calculated using Ray-tracing theory. According to the different flow structures in the flow field, four parts are selected, (1) Y=692 600pixel; (2) Y=600 400pixel; (3) Y=400 268pixel; (4) Y=268 0pixel. The aerooptical effects of different flow structures are quantitatively analyzed, the results indicate that: the compressive waves such as incident shock wave, induced shock wave, etc. rise the density, and then uplift the OPD curve, but this kind of shock are fixed in space position and intensity, the aero-optics induced by it can be regarded as constant; The induced shock waves are induced by the coherent structure of large size vortex in the interaction between turbulent boundary layer, its unsteady characteristic decides the induced waves unsteady characteristic; The space position and intensity of the induced shock wave are fixed in the interaction between turbulent boundary layer; The boundary layer aero-optics are induced by the coherent structure of large size vortex, which result in the fluctuation of OPD.

Comparison of Noise Source Localization Data with Flow Field Data Obtained in Cold Supersonic Jets and Implications Regarding Broadband Shock Noise

NASA Technical Reports Server (NTRS)

Podboy, Gary; Wernet, Mark; Clem, Michelle; Fagan, Amy

2013-01-01

Phased array noise source localization have been compared with 2 types of flow field data (BOS and PIV). The data show that: 1) the higher frequency noise in a BBSN hump is generated further downstream than the lower frequency noise. This is due to a) the shock spacing decreasing and b) the turbulent structure size increasing with distance downstream. 2) BBSN can be created by very weak shocks. 3) BBSN is not created by the strong shocks just downstream of the nozzle because the turbulent structures have not grown large enough to match the shock spacing. 4) The point in the flow where the shock spacing equals the average size of the turbulent structures is a hot spot for shock noise. 5) Some of the shocks responsible for producing the first hump also produce the second hump.

Wind noise spectra in small Reynolds number turbulent flows.

PubMed

Zhao, Sipei; Cheng, Eva; Qiu, Xiaojun; Burnett, Ian; Liu, Jacob Chia-Chun

2017-11-01

Wind noise spectra caused by wind from fans in indoor environments have been found to be different from those measured in outdoor atmospheric conditions. Although many models have been developed to predict outdoor wind noise spectra under the assumption of large Reynolds number [Zhao, Cheng, Qiu, Burnett, and Liu (2016). J. Acoust. Soc. Am. 140, 4178-4182, and the references therein], they cannot be applied directly to the indoor situations because the Reynolds number of wind from fans in indoor environments is usually much smaller than that experienced in atmospheric turbulence. This paper proposes a pressure structure function model that combines the energy-containing and dissipation ranges so that the pressure spectrum for small Reynolds number turbulent flows can be calculated. The proposed pressure structure function model is validated with the experimental results in the literature, and then the obtained pressure spectrum is verified with the numerical simulation and experiment results. It is demonstrated that the pressure spectrum obtained from the proposed pressure structure function model can be utilized to estimate wind noise spectra caused by turbulent flows with small Reynolds numbers.

A Novel Multi-scale Simulation Strategy for Turbulent Reacting Flows

DOE Office of Scientific and Technical Information (OSTI.GOV)

James, Sutherland C.

In this project, a new methodology was proposed to bridge the gap between Direct Numerical Simulation (DNS) and Large Eddy Simulation (LES). This novel methodology, titled Lattice-Based Multiscale Simulation (LBMS), creates a lattice structure of One-Dimensional Turbulence (ODT) models. This model has been shown to capture turbulent combustion with high fidelity by fully resolving interactions between turbulence and diffusion. By creating a lattice of ODT models, which are then coupled, LBMS overcomes the shortcomings of ODT, which are its inability to capture large scale three dimensional flow structures. However, by spacing these lattices significantly apart, LBMS can avoid the cursemore » of dimensionality that creates untenable computational costs associated with DNS. This project has shown that LBMS is capable of reproducing statistics of isotropic turbulent flows while coarsening the spacing between lines significantly. It also investigates and resolves issues that arise when coupling ODT lines, such as flux reconstruction perpendicular to a given ODT line, preservation of conserved quantities when eddies cross a course cell volume and boundary condition application. Robust parallelization is also investigated.« less

Turbulence Resolving Flow Simulations of a Francis Turbine in Part Load using Highly Parallel CFD Simulations

NASA Astrophysics Data System (ADS)

Krappel, Timo; Riedelbauch, Stefan; Jester-Zuerker, Roland; Jung, Alexander; Flurl, Benedikt; Unger, Friedeman; Galpin, Paul

2016-11-01

The operation of Francis turbines in part load conditions causes high fluctuations and dynamic loads in the turbine and especially in the draft tube. At the hub of the runner outlet a rotating vortex rope within a low pressure zone arises and propagates into the draft tube cone. The investigated part load operating point is at about 72% discharge of best efficiency. To reduce the possible influence of boundary conditions on the solution, a flow simulation of a complete Francis turbine is conducted consisting of spiral case, stay and guide vanes, runner and draft tube. As the flow has a strong swirling component for the chosen operating point, it is very challenging to accurately predict the flow and in particular the flow losses in the diffusor. The goal of this study is to reach significantly better numerical prediction of this flow type. This is achieved by an improved resolution of small turbulent structures. Therefore, the Scale Adaptive Simulation SAS-SST turbulence model - a scale resolving turbulence model - is applied and compared to the widely used RANS-SST turbulence model. The largest mesh contains 300 million elements, which achieves LES-like resolution throughout much of the computational domain. The simulations are evaluated in terms of the hydraulic losses in the machine, evaluation of the velocity field, pressure oscillations in the draft tube and visual comparisons of turbulent flow structures. A pre-release version of ANSYS CFX 17.0 is used in this paper, as this CFD solver has a parallel performance up to several thousands of cores for this application which includes a transient rotor-stator interface to support the relative motion between the runner and the stationary portions of the water turbine.

Experimental analysis of the vorticity and turbulent flow dynamics of a pitching airfoil at realistic flight (helicopter) conditions

NASA Astrophysics Data System (ADS)

Sahoo, Dipankar

Improved basic understanding, predictability, and controllability of vortex-dominated and unsteady aerodynamic flows are important in enhancement of the performance of next generation helicopters. The primary objective of this research project was improved understanding of the fundamental vorticity and turbulent flow physics for a dynamically stalling airfoil at realistic helicopter flight conditions. An experimental program was performed on a large-scale (C = 0.45 m) dynamically pitching NACA 0012 wing operating in the Texas A&M University large-scale wind tunnel. High-resolution particle image velocimetry data were acquired on the first 10-15% of the wing. Six test cases were examined including the unsteady (k>0) and steady (k=0) conditions. The relevant mechanical, shear and turbulent time-scales were all of comparable magnitude, which indicated that the flow was in a state of mechanical non-equilibrium, and the expected flow separation and reattachment hystersis was observed. Analyses of the databases provided new insights into the leading-edge Reynolds stress structure and the turbulent transport processes. Both of which were previously uncharacterized. During the upstroke motion of the wing, a bubble structure formed in the leading-edge Reynolds shear stress. The size of the bubble increased with increasing angle-of-attack before being diffused into a shear layer at full separation. The turbulent transport analyses indicated that the axial stress production was positive, where the transverse production was negative. This implied that axial turbulent stresses were being produced from the axial component of the mean flow. A significant portion of the energy was transferred to the transverse stress through the pressure-strain redistribution, and then back to the transverse mean flow through the negative transverse production. An opposite trend was observed further downstream of this region.

A Nonlinear Dynamic Subscale Model for Partially Resolved Numerical Simulation (PRNS)/Very Large Eddy Simulation (VLES) of Internal Non-Reacting Flows

NASA Technical Reports Server (NTRS)

Shih, Tsan-Hsing; Liu, nan-Suey

2010-01-01

A brief introduction of the temporal filter based partially resolved numerical simulation/very large eddy simulation approach (PRNS/VLES) and its distinct features are presented. A nonlinear dynamic subscale model and its advantages over the linear subscale eddy viscosity model are described. In addition, a guideline for conducting a PRNS/VLES simulation is provided. Results are presented for three turbulent internal flows. The first one is the turbulent pipe flow at low and high Reynolds numbers to illustrate the basic features of PRNS/VLES; the second one is the swirling turbulent flow in a LM6000 single injector to further demonstrate the differences in the calculated flow fields resulting from the nonlinear model versus the pure eddy viscosity model; the third one is a more complex turbulent flow generated in a single-element lean direct injection (LDI) combustor, the calculated result has demonstrated that the current PRNS/VLES approach is capable of capturing the dynamically important, unsteady turbulent structures while using a relatively coarse grid.

Sand waves in environmental flows: Insights gained by coupling large-eddy simulation with morphodynamics

NASA Astrophysics Data System (ADS)

Sotiropoulos, Fotis; Khosronejad, Ali

2016-02-01

Sand waves arise in subaqueous and Aeolian environments as the result of the complex interaction between turbulent flows and mobile sand beds. They occur across a wide range of spatial scales, evolve at temporal scales much slower than the integral scale of the transporting turbulent flow, dominate river morphodynamics, undermine streambank stability and infrastructure during flooding, and sculpt terrestrial and extraterrestrial landscapes. In this paper, we present the vision for our work over the last ten years, which has sought to develop computational tools capable of simulating the coupled interactions of sand waves with turbulence across the broad range of relevant scales: from small-scale ripples in laboratory flumes to mega-dunes in large rivers. We review the computational advances that have enabled us to simulate the genesis and long-term evolution of arbitrarily large and complex sand dunes in turbulent flows using large-eddy simulation and summarize numerous novel physical insights derived from our simulations. Our findings explain the role of turbulent sweeps in the near-bed region as the primary mechanism for destabilizing the sand bed, show that the seeds of the emergent structure in dune fields lie in the heterogeneity of the turbulence and bed shear stress fluctuations over the initially flatbed, and elucidate how large dunes at equilibrium give rise to energetic coherent structures and modify the spectra of turbulence. We also discuss future challenges and our vision for advancing a data-driven simulation-based engineering science approach for site-specific simulations of river flooding.

Constructal Law of Vascular Trees for Facilitation of Flow

PubMed Central

Razavi, Mohammad S.; Shirani, Ebrahim; Salimpour, Mohammad Reza; Kassab, Ghassan S.

2014-01-01

Diverse tree structures such as blood vessels, branches of a tree and river basins exist in nature. The constructal law states that the evolution of flow structures in nature has a tendency to facilitate flow. This study suggests a theoretical basis for evaluation of flow facilitation within vascular structure from the perspective of evolution. A novel evolution parameter (Ev) is proposed to quantify the flow capacity of vascular structures. Ev is defined as the ratio of the flow conductance of an evolving structure (configuration with imperfection) to the flow conductance of structure with least imperfection. Attaining higher Ev enables the structure to expedite flow circulation with less energy dissipation. For both Newtonian and non-Newtonian fluids, the evolution parameter was developed as a function of geometrical shape factors in laminar and turbulent fully developed flows. It was found that the non-Newtonian or Newtonian behavior of fluid as well as flow behavior such as laminar or turbulent behavior affects the evolution parameter. Using measured vascular morphometric data of various organs and species, the evolution parameter was calculated. The evolution parameter of the tree structures in biological systems was found to be in the range of 0.95 to 1. The conclusion is that various organs in various species have high capacity to facilitate flow within their respective vascular structures. PMID:25551617

Large-Scale Flow Structure in Turbulent Nonpremixed Flames under Normal- And Low-Gravity Conditions

NASA Technical Reports Server (NTRS)

Clemens, N. T.; Idicheria, C. A.; Boxx, I. G.

2001-01-01

It is well known that buoyancy has a major influence on the flow structure of turbulent nonpremixed jet flames. Buoyancy acts by inducing baroclinic torques, which generate large-scale vortical structures that can significantly modify the flow field. Furthermore, some suggest that buoyancy can substantially influence the large-scale structure of even nominally momentum-dominated flames, since the low velocity flow outside of the flame will be more susceptible to buoyancy effects. Even subtle buoyancy effects may be important because changes in the large-scale structure affects the local entrainment and fluctuating strain rate, and hence the structure of the flame. Previous studies that have compared the structure of normal- and micro-gravity nonpremixed jet flames note that flames in microgravity are longer and wider than in normal-gravity. This trend was observed for jet flames ranging from laminar to turbulent regimes. Furthermore, imaging of the flames has shown possible evidence of helical instabilities and disturbances starting from the base of the flame in microgravity. In contrast, these characteristics were not observed in normal-gravity. The objective of the present study is to further advance our knowledge of the effects of weak levels of buoyancy on the structure of transitional and turbulent nonpremixed jet flames. In later studies we will utilize the drop tower facilities at NASA Glenn Research Center (GRC), but the preliminary work described in this paper was conducted using the 1.25-second drop tower located at the University of Texas at Austin. A more detailed description of these experiments can be found in Idicheria et al.

Stably Stratified Atmospheric Boundary Layers

NASA Astrophysics Data System (ADS)

Mahrt, L.

2014-01-01

Atmospheric boundary layers with weak stratification are relatively well described by similarity theory and numerical models for stationary horizontally homogeneous conditions. With common strong stratification, similarity theory becomes unreliable. The turbulence structure and interactions with the mean flow and small-scale nonturbulent motions assume a variety of scenarios. The turbulence is intermittent and may no longer fully satisfy the usual conditions for the definition of turbulence. Nonturbulent motions include wave-like motions and solitary modes, two-dimensional vortical modes, microfronts, intermittent drainage flows, and a host of more complex structures. The main source of turbulence may not be at the surface, but rather may result from shear above the surface inversion. The turbulence is typically not in equilibrium with the nonturbulent motions, sometimes preventing the formation of an inertial subrange. New observational and analysis techniques are expected to advance our understanding of the very stable boundary layer.

Stability and Interaction of Coherent Structure in Supersonic Reactive Wakes

NASA Technical Reports Server (NTRS)

Menon, Suresh

1983-01-01

A theoretical formulation and analysis is presented for a study of the stability and interaction of coherent structure in reacting free shear layers. The physical problem under investigation is a premixed hydrogen-oxygen reacting shear layer in the wake of a thin flat plate. The coherent structure is modeled as a periodic disturbance and its stability is determined by the application of linearized hydrodynamic stability theory which results in a generalized eigenvalue problem for reactive flows. Detailed stability analysis of the reactive wake for neutral, symmetrical and antisymmetrical disturbance is presented. Reactive stability criteria is shown to be quite different from classical non-reactive stability. The interaction between the mean flow, coherent structure and fine-scale turbulence is theoretically formulated using the von-Kaman integral technique. Both time-averaging and conditional phase averaging are necessary to separate the three types of motion. The resulting integro-differential equations can then be solved subject to initial conditions with appropriate shape functions. In the laminar flow transition region of interest, the spatial interaction between the mean motion and coherent structure is calculated for both non-reactive and reactive conditions and compared with experimental data wherever available. The fine-scale turbulent motion determined by the application of integral analysis to the fluctuation equations. Since at present this turbulence model is still untested, turbulence is modeled in the interaction problem by a simple algebraic eddy viscosity model. The applicability of the integral turbulence model formulated here is studied parametrically by integrating these equations for the simple case of self-similar mean motion with assumed shape functions. The effect of the motion of the coherent structure is studied and very good agreement is obtained with previous experimental and theoretical works for non-reactive flow. For the reactive case, lack of experimental data made direct comparison difficult. It was determined that the growth rate of the disturbance amplitude is lower for reactive case. The results indicate that the reactive flow stability is in qualitative agreement with experimental observation.

Large-eddy simulation of turbulent cavitating flow in a micro channel

DOE Office of Scientific and Technical Information (OSTI.GOV)

Egerer, Christian P., E-mail: christian.egerer@aer.mw.tum.de; Hickel, Stefan; Schmidt, Steffen J.

2014-08-15

Large-eddy simulations (LES) of cavitating flow of a Diesel-fuel-like fluid in a generic throttle geometry are presented. Two-phase regions are modeled by a parameter-free thermodynamic equilibrium mixture model, and compressibility of the liquid and the liquid-vapor mixture is taken into account. The Adaptive Local Deconvolution Method (ALDM), adapted for cavitating flows, is employed for discretizing the convective terms of the Navier-Stokes equations for the homogeneous mixture. ALDM is a finite-volume-based implicit LES approach that merges physically motivated turbulence modeling and numerical discretization. Validation of the numerical method is performed for a cavitating turbulent mixing layer. Comparisons with experimental data ofmore » the throttle flow at two different operating conditions are presented. The LES with the employed cavitation modeling predicts relevant flow and cavitation features accurately within the uncertainty range of the experiment. The turbulence structure of the flow is further analyzed with an emphasis on the interaction between cavitation and coherent motion, and on the statistically averaged-flow evolution.« less

Characterization of turbulent coherent structures in square duct flow

NASA Astrophysics Data System (ADS)

Atzori, Marco; Vinuesa, Ricardo; Lozano-Durán, Adrián; Schlatter, Philipp

2018-04-01

This work is aimed at a first characterization of coherent structures in turbulent square duct flows. Coherent structures are defined as connected components in the domain identified as places where a quantity of interest (such as Reynolds stress or vorticity) is larger than a prescribed non-uniform threshold. Firstly, we qualitatively discuss how a percolation analysis can be used to assess the effectiveness of the threshold function, and how it can be affected by statistical uncertainty. Secondly, various physical quantities that are expected to play an important role in the dynamics of the secondary flow of Prandtl’s second kind are studied. Furthermore, a characterization of intense Reynolds-stress events in square duct flow, together with a comparison of their shape for analogous events in channel flow at the same Reynolds number, is presented.

Non-normality and classification of amplification mechanisms in stability and resolvent analysis

NASA Astrophysics Data System (ADS)

Symon, Sean; Rosenberg, Kevin; Dawson, Scott T. M.; McKeon, Beverley J.

2018-05-01

Eigenspectra and pseudospectra of the mean-linearized Navier-Stokes operator are used to characterize amplification mechanisms in laminar and turbulent flows in which linear mechanisms are important. Success of mean flow (linear) stability analysis for a particular frequency is shown to depend on whether two scalar measures of non-normality agree: (1) the product between the resolvent norm and the distance from the imaginary axis to the closest eigenvalue and (2) the inverse of the inner product between the most amplified resolvent forcing and response modes. If they agree, the resolvent operator can be rewritten in its dyadic representation to reveal that the adjoint and forward stability modes are proportional to the forcing and response resolvent modes at that frequency. Hence the real parts of the eigenvalues are important since they are responsible for resonant amplification and the resolvent operator is low rank when the eigenvalues are sufficiently separated in the spectrum. If the amplification is pseudoresonant, then resolvent analysis is more suitable to understand the origin of observed flow structures. Two test cases are studied: low Reynolds number cylinder flow and turbulent channel flow. The first deals mainly with resonant mechanisms, hence the success of both classical and mean stability analysis with respect to predicting the critical Reynolds number and global frequency of the saturated flow. Both scalar measures of non-normality agree for the base and mean flows, and the region where the forcing and response modes overlap scales with the length of the recirculation bubble. In the case of turbulent channel flow, structures result from both resonant and pseudoresonant mechanisms, suggesting that both are necessary elements to sustain turbulence. Mean shear is exploited most efficiently by stationary disturbances while bounds on the pseudospectra illustrate how pseudoresonance is responsible for the most amplified disturbances at spatial wavenumbers and temporal frequencies corresponding to well-known turbulent structures. Some implications for flow control are discussed.

Parametrization of turbulence models using 3DVAR data assimilation in laboratory conditions

NASA Astrophysics Data System (ADS)

Olbert, A. I.; Nash, S.; Ragnoli, E.; Hartnett, M.

2013-12-01

In this research the 3DVAR data assimilation scheme is implemented in the numerical model DIVAST in order to optimize the performance of the numerical model by selecting an appropriate turbulence scheme and tuning its parameters. Two turbulence closure schemes: the Prandtl mixing length model and the two-equation k-ɛ model were incorporated into DIVAST and examined with respect to their universality of application, complexity of solutions, computational efficiency and numerical stability. A square harbour with one symmetrical entrance subject to tide-induced flows was selected to investigate the structure of turbulent flows. The experimental part of the research was conducted in a tidal basin. A significant advantage of such laboratory experiment is a fully controlled environment where domain setup and forcing are user-defined. The research shows that the Prandtl mixing length model and the two-equation k-ɛ model, with default parameterization predefined according to literature recommendations, overestimate eddy viscosity which in turn results in a significant underestimation of velocity magnitudes in the harbour. The data assimilation of the model-predicted velocity and laboratory observations significantly improves model predictions for both turbulence models by adjusting modelled flows in the harbour to match de-errored observations. Such analysis gives an optimal solution based on which numerical model parameters can be estimated. The process of turbulence model optimization by reparameterization and tuning towards optimal state led to new constants that may be potentially applied to complex turbulent flows, such as rapidly developing flows or recirculating flows. This research further demonstrates how 3DVAR can be utilized to identify and quantify shortcomings of the numerical model and consequently to improve forecasting by correct parameterization of the turbulence models. Such improvements may greatly benefit physical oceanography in terms of understanding and monitoring of coastal systems and the engineering sector through applications in coastal structure design, marine renewable energy and pollutant transport.

A stochastic vortex structure method for interacting particles in turbulent shear flows

NASA Astrophysics Data System (ADS)

Dizaji, Farzad F.; Marshall, Jeffrey S.; Grant, John R.

2018-01-01

In a recent study, we have proposed a new synthetic turbulence method based on stochastic vortex structures (SVSs), and we have demonstrated that this method can accurately predict particle transport, collision, and agglomeration in homogeneous, isotropic turbulence in comparison to direct numerical simulation results. The current paper extends the SVS method to non-homogeneous, anisotropic turbulence. The key element of this extension is a new inversion procedure, by which the vortex initial orientation can be set so as to generate a prescribed Reynolds stress field. After validating this inversion procedure for simple problems, we apply the SVS method to the problem of interacting particle transport by a turbulent planar jet. Measures of the turbulent flow and of particle dispersion, clustering, and collision obtained by the new SVS simulations are shown to compare well with direct numerical simulation results. The influence of different numerical parameters, such as number of vortices and vortex lifetime, on the accuracy of the SVS predictions is also examined.

Entrainment of solid particles over irregular wavy walls

NASA Astrophysics Data System (ADS)

Milici, Barbara

2017-11-01

The distribution of inertial particles in turbulent flows is highly nonuniform and is governed by the dynamics of turbulent structures of the underlying carrier flow field which, in turn, is affected by the presence of a loading of dispersed particles. The issue is discussed here focusing on the coupling between near-bed coherent structures and suspended solid particles dynamics, in wall-bounded turbulent multiphase flows, bounded by rough boundaries. The friction Reynolds number of the unladen flow is Reτ=180 and the dispersed phase spans one order of magnitude of particle diameter. The analysis takes into account fluid-particle interaction (two-way coupling) in the frame of the Particle-Source-In-Cell (PSIC) method, using Direct Numerical Simulations (DNS) for the carrier phase coupled with Lagrangian Particle Tracking (LPT) for the dispersed phase. The effect of the wall's roughness is taken into account modelling the elastic rebound of particles onto it, instead of using a virtual rebound model.

Direct numerical simulation of axisymmetric turbulence

NASA Astrophysics Data System (ADS)

Qu, Bo; Bos, Wouter J. T.; Naso, Aurore

2017-09-01

The dynamics of decaying, strictly axisymmetric, incompressible turbulence is investigated using direct numerical simulations. It is found that the angular momentum is a robust invariant of the system. It is further shown that long-lived coherent structures are generated by the flow. These structures can be associated with stationary solutions of the Euler equations. The structures obey relations in agreement with predictions from selective decay principles, compatible with the decay laws of the system. Two different types of decay scenarios are highlighted. The first case results in a quasi-two-dimensional flow with a dynamical behavior in the poloidal plane similar to freely decaying two-dimensional turbulence. In a second regime, the long-time dynamics is dominated by a single three-dimensional mode.

On the interaction of small and large eddies in two dimensional turbulent flows

NASA Technical Reports Server (NTRS)

Foias, C.; ate work.

1987-01-01

Some results concerning the interaction of small and large eddies to two dimensional turbulent flows are presented. It is shown that the amplitude of small structures decays exponentially to a small value, and from this is inferred a simplified interaction law of small and large eddies. Beside their intrinsic interest for the understanding of the physics of turbulence, these results lead to new numerical schemes to be studied in a separate work.

Emergence and equilibration of jets in planetary turbulence

NASA Astrophysics Data System (ADS)

Constantinou, Navid; Ioannou, Petros; Farrell, Brian

2013-04-01

Spatially and temporally coherent large scale jets that are not forced directly at the jet scale are prominent feature of rotating turbulence. A familiar example is the midlatitude jet in the Earth's atmosphere and the banded winds of the giants planets. These jets arise and are supported by the systematic organisation of the turbulent Reynolds stresses. Understanding the mechanism producing the required eddy momentum flux convergence, and how the jets and associated eddy field mutually adjust to maintain a steady jet structure at finite amplitude, constitute fundamental theoretical problems. Stochastic Structural Stability Theory (SSST) gives an explanation for jet formation that is fundamentally based on the interaction between jets and their associated field of turbulent eddies. SSST combines the full dynamics of the zonal mean flow with the second order statistics of the turbulent field obtained from a stochastic turbulence model (STM). The quasi-linear (QL) approximation to the full nonlinear dynamics (NL) results when the perturbation-perturbation interactions are parameterized in the perturbation equations, while interaction between the perturbations and the zonal mean flow is retained in the zonal mean equation. SSST consists of an infinite ensemble of perturbations evolving under QL. Therefore, SSST provides a set of dynamical equations for the mean flow and the second order statistics of the second cummulant of the perturbation vorticity field, which are autonomous and fluctuation free and can facilitate analytic study of turbulent equilibria and their stability as a function of parameters. Thus, jet formation in homogeneous beta-turbulence can be identified with an SSST structural instability of a homogeneous (mean flow free) SSTT equilibrium. We investigate the emergence and equilibration of jets from homogeneous barotropic beta-plane turbulence in the absence of coherent external forcing. SSST predicts that infinitesimal perturbations with zonal jet form organise homogeneous turbulence to produce systematic upgradient fluxes, giving rise to exponential jet growth and eventually to the establishment of finite amplitude equilibrium jets. We compare these predictions with simulations of the NL equations and their QL approximation in order to examine further the mechanism of emergence and equilibration of jets from turbulence. We concentrate on the effects of perturbation-perturbation nonlinearity on jet bifurcation and equilibration, and on the influence of perturbations in exciting the manifold of SSST modes with jet structure. We find that the bifurcation structure predicted by SSST for the emergence of zonal jets from a homogeneous turbulent state is confirmed by both QL and NL simulations. Moreover, we show that the finite amplitude equilibrium jets found in NL and QL simulations are as predicted by the fixed point solutions of SSST. Obtaining this agreement between NL and both SSST and QL simulations required in some cases that the modification of the turbulent spectrum caused by the perturbation-perturbation nonlinearity in NL be accounted for in the specification of the stochastic forcing in QL and SSST. These results confirm that jet emergence in barotropic beta-plane turbulence can be traced to the cooperative mean flow/perturbation instability that is captured by SSST.

Comments on Reynolds number effects in wall-bounded shear layers

NASA Technical Reports Server (NTRS)

Bandyopadhyay, Promode R.

1991-01-01

The effect of Reynolds number on the structure of turbulent boundary layers and channel flows is discussed. Published data are reexamined in light of the following questions: (1) does the boundary layer turbulence structure change after the well known Reynolds number limit viz, when Re(theta) is greater than 6000?; (2) is it possible to disturb a high Reynolds number flat plate turbulent boundary layer near the wall such that the recovery length is O(100 delta)?; and (3) how close is the numerically simulated low Reynolds number flat plate turbulence structure to that observed experimentally? The turbulence structure appears to change continuously with Reynolds number virtually throughout the bounday layer and sometimes in unexpected manners at high Reynolds numbers.

In situ rheology of yeast biofilms.

PubMed

Brugnoni, Lorena I; Tarifa, María C; Lozano, Jorge E; Genovese, Diego

2014-01-01

The aim of the present work was to investigate the in situ rheological behavior of yeast biofilms growing on stainless steel under static and turbulent flow. The species used (Rhodototula mucilaginosa, Candida krusei, Candida kefyr and Candida tropicalis) were isolated from a clarified apple juice industry. The flow conditions impacted biofilm composition over time, with a predominance of C. krusei under static and turbulent flow. Likewise, structural variations occurred, with a tighter appearance under dynamic flow. Under turbulent flow there was an increase of 112 μm in biofilm thickness at 11 weeks (p < 0.001) and cell morphology was governed by hyphal structures and rounded cells. Using the in situ growth method introduced here, yeast biofilms were determined to be viscoelastic materials with a predominantly solid-like behavior, and neither this nor the G'0 values were significantly affected by the flow conditions or the growth time, and at large deformations their weak structure collapsed beyond a critical strain of about 1.5-5%. The present work could represent a starting point for developing in situ measurements of yeast rheology and contribute to a thin body of knowledge about fungal biofilm formation.

Prospectus: towards the development of high-fidelity models of wall turbulence at large Reynolds number

NASA Astrophysics Data System (ADS)

Klewicki, J. C.; Chini, G. P.; Gibson, J. F.

2017-03-01

Recent and on-going advances in mathematical methods and analysis techniques, coupled with the experimental and computational capacity to capture detailed flow structure at increasingly large Reynolds numbers, afford an unprecedented opportunity to develop realistic models of high Reynolds number turbulent wall-flow dynamics. A distinctive attribute of this new generation of models is their grounding in the Navier-Stokes equations. By adhering to this challenging constraint, high-fidelity models ultimately can be developed that not only predict flow properties at high Reynolds numbers, but that possess a mathematical structure that faithfully captures the underlying flow physics. These first-principles models are needed, for example, to reliably manipulate flow behaviours at extreme Reynolds numbers. This theme issue of Philosophical Transactions of the Royal Society A provides a selection of contributions from the community of researchers who are working towards the development of such models. Broadly speaking, the research topics represented herein report on dynamical structure, mechanisms and transport; scale interactions and self-similarity; model reductions that restrict nonlinear interactions; and modern asymptotic theories. In this prospectus, the challenges associated with modelling turbulent wall-flows at large Reynolds numbers are briefly outlined, and the connections between the contributing papers are highlighted.

Numerical simulation of turbulent Taylor-Couette flow between conducting cylinders in an axial magnetic field at low magnetic Reynolds number

NASA Astrophysics Data System (ADS)

Leng, Xueyuan; Kolesnikov, Yurii B.; Krasnov, Dmitry; Li, Benwen

2018-01-01

The effect of an axial homogeneous magnetic field on the turbulence in the Taylor-Couette flow confined between two infinitely long conducting cylinders is studied by the direct numerical simulation using a periodic boundary condition in the axial direction. The inner cylinder is rotating, and the outer one is fixed. We consider the case when the magnetic Reynolds number Rem ≪ 1, i.e., the influence of the induced magnetic field on the flow is negligible that is typical for industry and laboratory study of liquid metals. Relevance of the present study is based on the similarity of flow characteristics at moderate and high magnetic field for the cases with periodic and end-wall conditions at the large flow aspect ratio, as proven in the earlier studies. Two sets of Reynolds numbers 4000 and 8000 with several Hartmann numbers varying from 0 to 120 are employed. The results show that the mean radial induced electrical current, resulting from the interaction of axial magnetic field with the mean flow, leads to the transformation of the mean flow and the modification of the turbulent structure. The effect of turbulence suppression is dominating at a strong magnetic field, but before reaching the complete laminarization, we capture the appearance of the hairpin-like structures in the flow.

Mach Number effects on turbulent superstructures in wall bounded flows

NASA Astrophysics Data System (ADS)

Kaehler, Christian J.; Bross, Matthew; Scharnowski, Sven

2017-11-01

Planer and three-dimensional flow field measurements along a flat plat boundary layer in the Trisonic Wind Tunnel Munich (TWM) are examined with the aim to characterize the scaling, spatial organization, and topology of large scale turbulent superstructures in compressible flow. This facility is ideal for this investigation as the ratio of boundary layer thickness to test section spanwise extent ratio is around 1/25, ensuring minimal sidewall and corner effects on turbulent structures in the center of the test section. A major difficulty in the experimental investigation of large scale features is the mutual size of the superstructures which can extend over many boundary layer thicknesses. Using multiple PIV systems, it was possible to capture the full spatial extent of large-scale structures over a range of Mach numbers from Ma = 0.3 - 3. To calculate the average large-scale structure length and spacing, the acquired vector fields were analyzed by statistical multi-point methods that show large scale structures with a correlation length of around 10 boundary layer thicknesses over the range of Mach numbers investigated. Furthermore, the average spacing between high and low momentum structures is on the order of a boundary layer thicknesses. This work is supported by the Priority Programme SPP 1881 Turbulent Superstructures of the Deutsche Forschungsgemeinschaft.

Effects of sedimenting particles on the turbulence structure in a horizontal channel flow

NASA Astrophysics Data System (ADS)

Tay, Godwin F. K.; Kuhn, David C. S.; Tachie, Mark F.

2015-02-01

This work presents the results of experiments conducted in a horizontal channel to characterize low Reynolds number turbulent flows in the presence of small solid particles. The particle diameter relative to the integral length scale, dp/Λx, is approximately 0.02. Particles and fluid turbulence characteristics are measured for three average solid volume fractions of approximately ϕv = 2.0 × 10-4, 4.0 × 10-4, and 8.0 × 10-4 under conditions where the particle number density is evolving due to deposition. The results indicate that the mean slip between particles and the fluid is important only close to the wall. Away from the wall, the particles and unladen fluid mean velocities are similar. Differences between particles and the unladen fluid statistics are more pronounced in the wall-normal velocity fluctuations than the streamwise velocity fluctuations and Reynolds shear stress due to the stronger effect of the gravitational force in the wall-normal direction. The fluid turbulent intensities show no dependency on loading, but the peak Reynolds shear stress is significantly reduced. A quadrant decomposition of the Reynolds shear stress revealed a corresponding reduction in the ejections and sweeps for the laden flow in comparison with the unladen flow. Swirling strength and vorticity root-mean-square fluctuations decayed due to the damping effect of particles. The influence of particles on the turbulence structure was examined using two-point correlations of the velocity fluctuations and swirling strength, where it was demonstrated that the wall structures are attached eddies which are more extensive (much larger) in the particle-laden flow compared to the unladen flow.

A compressibility correction of the pressure strain correlation model in turbulent flow

NASA Astrophysics Data System (ADS)

Klifi, Hechmi; Lili, Taieb

2013-07-01

This paper is devoted to the second-order closure for compressible turbulent flows with special attention paid to modeling the pressure-strain correlation appearing in the Reynolds stress equation. This term appears as the main one responsible for the changes of the turbulence structures that arise from structural compressibility effects. From the analysis and DNS results of Simone et al. and Sarkar, the compressibility effects on the homogeneous turbulence shear flow are parameterized by the gradient Mach number. Several experiment and DNS results suggest that the convective Mach number is appropriate to study the compressibility effects on the mixing layers. The extension of the LRR model recently proposed by Marzougui, Khlifi and Lili for the pressure-strain correlation gives results that are in disagreement with the DNS results of Sarkar for high-speed shear flows. This extension is revised to derive a turbulence model for the pressure-strain correlation in which the compressibility is included in the turbulent Mach number, the gradient Mach number and then the convective Mach number. The behavior of the proposed model is compared to the compressible model of Adumitroiae et al. for the pressure-strain correlation in two turbulent compressible flows: homogeneous shear flow and mixing layers. In compressible homogeneous shear flows, the predicted results are compared with the DNS data of Simone et al. and those of Sarkar. For low compressibility, the two compressible models are similar, but they become substantially different at high compressibilities. The proposed model shows good agreement with all cases of DNS results. Those of Adumitroiae et al. do not reflect any effect of a change in the initial value of the gradient Mach number on the Reynolds stress anisotropy. The models are used to simulate compressible mixing layers. Comparison of our predictions with those of Adumitroiae et al. and with the experimental results of Goebel et al. shows good qualitative agreement.

Key vortical structure causing laminar-turbulent transition in a boundary layer disturbed by a short-duration jet

NASA Astrophysics Data System (ADS)

Yoshikawa, Joe; Nishio, Yu; Izawa, Seiichiro; Fukunishi, Yu

2018-01-01

Numerical simulations are carried out to discover the flow structure that plays an important role in the laminar-turbulent transition process of a boundary layer on a flat plate. The boundary layer is destabilized by ejecting a short-duration jet from a hole in the surface. When the jet velocity is set to 20% of the uniform-flow velocity, a laminar-turbulent transition takes place, whereas in the 18% case, the disturbances created by the jet decay downstream. It is found that in both cases, hairpin vortices are generated; however, these first-generation hairpins do not directly cause the transition. Only in the 20% case does a new hairpin vortex of a different shape with wider distance between the legs appear. The new hairpin grows with time and evokes the generation of vortical structures one after another around it, turning the flow turbulent. It is found that the difference between the two cases is whether or not one of the first-generation hairpin vortices gets connected with the nearby longitudinal vortices. Only when the connection is successful is the new hairpin vortex with wider distance between the legs created. For each of several cases tested with changing jet-ejecting conditions, no difference is found in the importance of the role of the hairpin structure. Therefore, we conclude that the hairpin vortex with widespread legs is a key structure in the transition to turbulence.

Vibroacoustic response of structures and perturbation Reynolds stress near structure-turbulence interface

NASA Technical Reports Server (NTRS)

Maekawa, S.; Lin, Y. K.

1977-01-01

The interaction between a turbulent flow and certain types of structures which respond to its excitation is investigated. One-dimensional models were used to develop the basic ideas applied to a second model resembling the fuselage construction of an aircraft. In the two-dimensional case a simple membrane, with a small random variation in the membrane tension, was used. A decaying turbulence was constructed by superposing infinitely many components, each of which is convected as a frozen pattern at a different velocity. Structure-turbulence interaction results are presented in terms of the spectral densities of the structural response and the perturbation Reynolds stress in the fluid at the vicinity of the interface.

The turbulence structure of katabatic flows below and above wind-speed maximum

NASA Astrophysics Data System (ADS)

Grachev, Andrey; Leo, Laura; Di Sabatino, Silvana; Fernando, Harindra; Pardyjak, Eric; Fairall, Christopher

2015-04-01

Measurements of atmospheric small-scale turbulence made over the complex-terrain at the US Army Dugway Proving Grounds in Utah during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program are used to describe the turbulence structure of katabatic flows. Turbulent and mean meteorological data were continuously measured at multiple levels (up to seven) on four towers deployed along East lower slope (2-4 degrees) of Granite Mountain. The multi-level, multi-tower observations obtained during a 30-day long MATERHORN-Fall field campaign in September-October 2102 allow studying temporal and spatial structure of nocturnal slope flows in detail. In this study, we focus on the various statistics (fluxes, variances, spectra, cospectra, etc.) of the small-scale turbulence of katabatic winds. Observed vertical profiles of velocity, turbulent fluxes, and other quantities show steep gradients near the surface but in the layer above the slope jet these variables vary with height more slowly than near the surface. It is found that vertical momentum flux and horizontal heat (buoyancy) flux in a slope-following coordinate system change their sign below and above the wind maximum of a katabatic flow. The vertical momentum flux is directed downward (upward) whereas the horizontal heat flux is downslope (upslope) below (above) the wind maximum. Our study, therefore, suggests that a position of the jet speed maximum can be derived from linear interpolation between positive and negative values of the momentum flux (or the horizontal heat flux) and determination of a height where a flux becomes zero. It is shown that the standard deviations of all wind speed components (and therefore the turbulent kinetic energy) and the dissipation rate of turbulent kinetic energy have a local minimum, whereas the standard deviation of air temperature has an absolute maximum at the height of wind speed maximum. We report several cases when the destructive effect of vertical heat (buoyancy) flux is completely cancelled by the generation of turbulence due to the horizontal heat (buoyancy) flux. Turbulence in the layer above the wind-speed maximum is decoupled from the surface and it is consistent with the classical local z-less predictions for stably stratified boundary layer.

In-cylinder flows of a motored four-stroke engine with flat-crown and slightly concave-crown pistons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huang, R.F.; Yang, H.S.; Yeh, C.-N.

2008-04-15

The temporal and spatial evolution processes of the in-cylinder flow structures and turbulence intensities in the symmetry and offset planes of a motored four-valve, four-stroke engine during the intake and compression strokes are diagnosed by using a particle image velocimeter. Two pistons of different crown shapes (flat-crown and slightly concave-crown pistons) are studied. The inception, establishment, and evolution of the tumbling vortical flow structures during the intake and compression strokes are clearly depicted. Quantitative strengths of the rotating vortical flow motions are presented by a dimensionless parameter, the tumble ratio, which can represent the mean angular velocity of the vorticesmore » in the target plane. The turbulence intensity of the in-cylinder flow is also calculated by using the measured time-varying velocity data. The results show that the flat-crown piston induces higher bulk-averaged tumble ratio and turbulence intensity than the slightly concave-crown piston does because the tumble ratio and turbulence generated by the flat-crown piston in the offset planes during the compression stroke are particularly large. The engine with the flat-crown piston also presents larger torque and power outputs and lower hydrocarbon emission than that with the slightly concave-crown piston. This might be caused by the enhanced combustion in the engine cylinder due to the stronger tumble ratio and turbulence intensity. (author)« less

Study of the Transition from MRI to Magnetic Turbulence via Parasitic Instability by a High-order MHD Simulation Code

NASA Astrophysics Data System (ADS)

Hirai, Kenichiro; Katoh, Yuto; Terada, Naoki; Kawai, Soshi

2018-02-01

Magnetic turbulence in accretion disks under ideal magnetohydrodynamic (MHD) conditions is expected to be driven by the magneto-rotational instability (MRI) followed by secondary parasitic instabilities. We develop a three-dimensional ideal MHD code that can accurately resolve turbulent structures, and carry out simulations with a net vertical magnetic field in a local shearing box disk model to investigate the role of parasitic instabilities in the formation process of magnetic turbulence. Our simulations reveal that a highly anisotropic Kelvin–Helmholtz (K–H) mode parasitic instability evolves just before the first peak in turbulent stress and then breaks large-scale shear flows created by MRI. The wavenumber of the enhanced parasitic instability is larger than the theoretical estimate, because the shear flow layers sometimes become thinner than those assumed in the linear analysis. We also find that interaction between antiparallel vortices caused by the K–H mode parasitic instability induces small-scale waves that break the shear flows. On the other hand, at repeated peaks in the nonlinear phase, anisotropic wavenumber spectra are observed only in the small wavenumber region and isotropic waves dominate at large wavenumbers unlike for the first peak. Restructured channel flows due to MRI at the peaks in nonlinear phase seem to be collapsed by the advection of small-scale shear structures into the restructured flow and resultant mixing.

Variability of stratification according to operation of the tidal power plant in Lake Sihwa, South Korea.

NASA Astrophysics Data System (ADS)

Woo, S. B.; Song, J. I.; Jang, T. H.; Park, C. J.; Kwon, H. K.

2017-12-01

Artificial forcing according to operation of the tidal power plant (TPP) affects the physical environmental changes near the power plant. Strong turbulence by generation is expected to change the stratification structure of the Lake Sihwa inside. In order to examine the stratification changes by the power plant operation, ship bottom mounted observation were performed for 13 hours using an acoustic Doppler current profiler (ADCP) and Conductivity-Temperature-Depth (CTD) in Lake Sihwa at near TPP. The strong stratification in Sihwa Lake is maintained before TPP operation. The absence of external forces and freshwater inflow from the land forms the stratification in the Lake. Strong winds in a stratification statement lead to two-layer circulation. After wind event, multi-layer velocity structure is formed which lasted for approximately 4 h. After TPP operation, the jet flow was observed in entire water column at the beginning of the power generation. Vortex is formed by strong jet flow and maintained throughout during power generation period. Strong turbulence flow is generated by the turbine blades, enhancing vertical mixing. External forces, which dominantly affect Lake Sihwa, have changed from the wind to the turbulent flow. The stratification was extinguished by strong turbulent flow and becomes fully-mixed state. Changes in stratification structure are expected to affect material transport and ecological environment change continuously.

Laminar Flow in the Ocean Ekman Layer

NASA Astrophysics Data System (ADS)

Woods, J. T. H.

INTRODUCTION THE EFFECT OF A STABLE DENSITY GRADIENT THE FATAL FLAW FLOW VISUALIZATION THE DISCOVERY OF LAMINAR FLOW FINE STRUCTURE WAVE-INDUCED SHEAR INSTABILITY BILLOW TURBULENCE REVERSE TRANSITION REVISED PARADIGM ONE-DIMENSIONAL MODELLING OF THE UPPER OCEAN DIURNAL VARIATION BUOYANT CONVECTION BILLOW TURBULENCE IN THE DIURNAL THERMOCLINE CONSEQUENCES FOR THE EKMAN CURRENT PROFILE SOLAR RADIATION APPLICATIONS Slippery Seas of Acapulco Pollution Afternoon Effect in Sonar Patchiness Fisheries Climate DISCUSSION CONCLUSION REFERENCES

Study of Varying Boundary Layer Height on Turret Flow Structures

DTIC Science & Technology

2011-06-01

fluid dynamics. The difficulties of the problem arise in modeling several complex flow features including separation, reattachment, three-dimensional...impossible. In this case, the approach is to create a model to calculate the properties of interest. The main issue with resolving turbulent flows...operation and their effect is modeled through subgrid scale models . As a result, the the most important turbulent scales are resolved and the

Dynamical Connections in a Turbulent Fluid: Experiment and Simulation

NASA Astrophysics Data System (ADS)

Kageorge, Logan; Suri, Balachandra; Tithof, Jeff; Grigoriev, Roman; Schatz, Michael

2017-11-01

Embedded in the state space of a turbulent flow there exist invariant solutions to the Navier-Stokes equation called Exact Coherent Structures (ECS). Recent studies have demonstrated that the geometry of the ECS locally describes the evolution of the turbulent flow. Theory suggests that global connections may serve to guide the flow from the neighborhood of one ECS to that of another. We present here a numerical model of a Kolmogorov-like two-dimensional flow in which such connections have been calculated. Moreover, we present an experimental quasi-two-dimensional realization of this flow in which these connections prove dynamically relevant. This work is supported by the National Science Foundation (NSF CMMI 12-34436) and DARPA (HR0011-16-2-0033 subcontract to Georgia Tech).

Marginal turbulent state of viscoelastic fluids: A polymer drag reduction perspective.

PubMed

Xi, Li; Bai, Xue

2016-04-01

The laminar-turbulent (LT) transition of dilute polymer solutions is of great interest not only for the complex transition dynamics itself, but also for its potential link to the maximum drag reduction (MDR) phenomenon. We present an in-depth investigation of the edge state (ES), an asymptotic solution on the LT boundary, in viscoelastic channel flow. For given Re and simulation domain size, mean flow statistics of the ES do not vary with the introduction of polymers, proving that there is a region of turbulent states not susceptible to polymer drag reduction effects. The dynamics of the ES features low-frequency fluctuations and in the longer domains we studied it is nearly periodic with regular bursts of turbulent activities separated by extended quiescent periods. Its flow field is dominated by elongated vortices and streaks, with very weak extensional and rotational flow motions. Polymer stretching is almost exclusively contributed by the mean shear and polymer-turbulence interaction is minimal. Flow structures and the kinematics of the ES match hibernating turbulence, an MDR-like phase intermittently occurring in turbulent dynamics. Its observation now seems to result from recurrent visits to certain parts of the ES. The ES offers explanations for the existence and universality of MDR, the quantitative magnitude of which, however, still remains unsolved.

Marginal turbulent state of viscoelastic fluids: A polymer drag reduction perspective

NASA Astrophysics Data System (ADS)

Xi, Li; Bai, Xue

2016-04-01

The laminar-turbulent (LT) transition of dilute polymer solutions is of great interest not only for the complex transition dynamics itself, but also for its potential link to the maximum drag reduction (MDR) phenomenon. We present an in-depth investigation of the edge state (ES), an asymptotic solution on the LT boundary, in viscoelastic channel flow. For given Re and simulation domain size, mean flow statistics of the ES do not vary with the introduction of polymers, proving that there is a region of turbulent states not susceptible to polymer drag reduction effects. The dynamics of the ES features low-frequency fluctuations and in the longer domains we studied it is nearly periodic with regular bursts of turbulent activities separated by extended quiescent periods. Its flow field is dominated by elongated vortices and streaks, with very weak extensional and rotational flow motions. Polymer stretching is almost exclusively contributed by the mean shear and polymer-turbulence interaction is minimal. Flow structures and the kinematics of the ES match hibernating turbulence, an MDR-like phase intermittently occurring in turbulent dynamics. Its observation now seems to result from recurrent visits to certain parts of the ES. The ES offers explanations for the existence and universality of MDR, the quantitative magnitude of which, however, still remains unsolved.

Comparison of PDF and Moment Closure Methods in the Modeling of Turbulent Reacting Flows

NASA Technical Reports Server (NTRS)

Norris, Andrew T.; Hsu, Andrew T.

1994-01-01

In modeling turbulent reactive flows, Probability Density Function (PDF) methods have an advantage over the more traditional moment closure schemes in that the PDF formulation treats the chemical reaction source terms exactly, while moment closure methods are required to model the mean reaction rate. The common model used is the laminar chemistry approximation, where the effects of turbulence on the reaction are assumed negligible. For flows with low turbulence levels and fast chemistry, the difference between the two methods can be expected to be small. However for flows with finite rate chemistry and high turbulence levels, significant errors can be expected in the moment closure method. In this paper, the ability of the PDF method and the moment closure scheme to accurately model a turbulent reacting flow is tested. To accomplish this, both schemes were used to model a CO/H2/N2- air piloted diffusion flame near extinction. Identical thermochemistry, turbulence models, initial conditions and boundary conditions are employed to ensure a consistent comparison can be made. The results of the two methods are compared to experimental data as well as to each other. The comparison reveals that the PDF method provides good agreement with the experimental data, while the moment closure scheme incorrectly shows a broad, laminar-like flame structure.

Flow topologies and turbulence scales in a jet-in-cross-flow

DOE PAGES

Oefelein, Joseph C.; Ruiz, Anthony M.; Lacaze, Guilhem

2015-04-03

This study presents a detailed analysis of the flow topologies and turbulence scales in the jet-in-cross-flow experiment of [Su and Mungal JFM 2004]. The analysis is performed using the Large Eddy Simulation (LES) technique with a highly resolved grid and time-step and well controlled boundary conditions. This enables quantitative agreement with the first and second moments of turbulence statistics measured in the experiment. LES is used to perform the analysis since experimental measurements of time-resolved 3D fields are still in their infancy and because sampling periods are generally limited with direct numerical simulation. A major focal point is the comprehensivemore » characterization of the turbulence scales and their evolution. Time-resolved probes are used with long sampling periods to obtain maps of the integral scales, Taylor microscales, and turbulent kinetic energy spectra. Scalar-fluctuation scales are also quantified. In the near-field, coherent structures are clearly identified, both in physical and spectral space. Along the jet centerline, turbulence scales grow according to a classical one-third power law. However, the derived maps of turbulence scales reveal strong inhomogeneities in the flow. From the modeling perspective, these insights are useful to design optimized grids and improve numerical predictions in similar configurations.« less

Temperature, Velocity, and Mean Turbulence Structure in Stongly-Heated Internal Gas Flows

DOE Office of Scientific and Technical Information (OSTI.GOV)

McEligot, Donald Marinus; Mikielewicz, D. P.; Shehata, A. M.

2002-10-01

The main objective of the present study is to examine whether "simple" turbulence models (i.e., models requiring two partial differential equations or less for turbulent transport) are suitable for use under conditions of forced flow of gas at low Reynolds numbers in tubes with intense heating, leading to large variations of fluid properties and considerable modification of turbulence. Eleven representative models are considered. The ability of such models to handle such flows was assessed by means of computational simulations of the carefully designed experiments of Shehata and McEligot (IJHMT 41 (1998) 4297) at heating rates of q+in˜0.0018, 0.0035 and 0.0045,more » yielding flows ranging from essentially turbulent to laminarized. The resulting comparisons of computational results with experiments showed that the model by Launder and Sharma (Lett. Heat Transfer 1 (1974) 131) performed best in predicting axial wall temperature profiles. Overall, agreement between the measured velocity and temperature distributions and those calculated using the Launder–Sharma model is good, which gives confidence in the values forecast for the turbulence quantities produced. These have been used to assist in arriving at a better understanding of the influences of intense heating, and hence strong variation of fluid properties, on turbulent flow in tubes.« less

The role of turbulence-flow interactions in L- to H-mode transition dynamics: recent progress

NASA Astrophysics Data System (ADS)

Schmitz, L.

2017-02-01

Recent experimental and simulation work has substantially advanced the understanding of L-mode plasma edge turbulence and plasma flows and their mutual interaction across the L-H transition. Flow acceleration and E   ×   B shear flow amplification via the turbulent Reynolds stress have been directly observed in multiple devices, using multi-tip probe arrays, Doppler backscattering, beam emission spectroscopy, and gas puff imaging diagnostics. L-H transitions characterized by limit-cycle oscillations (LCO) allow probing of the trigger dynamics and the synergy of turbulence-driven and pressure-gradient-driven flows with high spatio-temporal resolution. L-mode turbulent structures exhibit characteristic changes in topology (tilting) and temporal and radial correlation preceding the L-H transition. Long-range toroidal flow correlations increase preceding edge-transport-barrier formation. The energy transfer from the turbulence spectrum to large-scale axisymmetric flows has been quantified in L-LCO and fast L-H transitions in several devices. After formation of a transient barrier, the increasing ion pressure gradient (via the E   ×   B flow shear associated with diamagnetic flow) sustains fluctuation suppression and secures the transition to H-mode. Heuristic models of the L-H trigger dynamics have progressed from 0D predator-prey models to 1D extended models, including neoclassical ion flow-damping and pressure-gradient evolution. Initial results from 2D and 3D reduced fluid models have been obtained for high-collisionality regimes.

An Examination of Parameters Affecting Large Eddy Simulations of Flow Past a Square Cylinder

NASA Technical Reports Server (NTRS)

Mankbadi, M. R.; Georgiadis, N. J.

2014-01-01

Separated flow over a bluff body is analyzed via large eddy simulations. The turbulent flow around a square cylinder features a variety of complex flow phenomena such as highly unsteady vortical structures, reverse flow in the near wall region, and wake turbulence. The formation of spanwise vortices is often times artificially suppressed in computations by either insufficient depth or a coarse spanwise resolution. As the resolution is refined and the domain extended, the artificial turbulent energy exchange between spanwise and streamwise turbulence is eliminated within the wake region. A parametric study is performed highlighting the effects of spanwise vortices where the spanwise computational domain's resolution and depth are varied. For Re=22,000, the mean and turbulent statistics computed from the numerical large eddy simulations (NLES) are in good agreement with experimental data. Von-Karman shedding is observed in the wake of the cylinder. Mesh independence is illustrated by comparing a mesh resolution of 2 million to 16 million. Sensitivities to time stepping were minimized and sampling frequency sensitivities were nonpresent. While increasing the spanwise depth and resolution can be costly, this practice was found to be necessary to eliminating the artificial turbulent energy exchange.

Three-Dimensional Navier-Stokes Simulations with Two-Equation Turbulence Models of Intersecting Shock-Waves/Turbulent Boundary Layer at Mach 8.3

NASA Technical Reports Server (NTRS)

Bardina, J. E.; Coakley, T. J.

1994-01-01

An investigation of the numerical simulation with two-equation turbulence models of a three-dimensional hypersonic intersecting (SWTBL) shock-wave/turbulent boundary layer interaction flow is presented. The flows are solved with an efficient implicit upwind flux-difference split Reynolds-averaged Navier-Stokes code. Numerical results are compared with experimental data for a flow at Mach 8.28 and Reynolds number 5.3x10(exp 6) with crossing shock-waves and expansion fans generated by two lateral 15 fins located on top of a cold-wall plate. This experiment belongs to the hypersonic database for modeling validation. Simulations show the development of two primary counter-rotating cross-flow vortices and secondary turbulent structures under the main vortices and in each corner singularity inside the turbulent boundary layer. A significant loss of total pressure is produced by the complex interaction between the main vortices and the uplifted jet stream of the boundary layer. The overall agreement between computational and experimental data is generally good. The turbulence modeling corrections show improvements in the predictions of surface heat transfer distribution and an increase in the strength of the cross-flow vortices. Accurate predictions of the outflow flowfield is found to require accurate modeling of the laminar/turbulent boundary layers on the fin walls.

Study of Turbulent Premixed Flame Propagation using a Laminar Flamelet Model

NASA Technical Reports Server (NTRS)

Im, H. G.

1995-01-01

The laminar flamelet concept in turbulent reacting flows is considered applicable to many practical combustion systems (Linan & Williams 1993). For turbulent premixed combustion, the laminar flamelet regime is valid when turbulent Karlovitz number is less than unity, which is equivalent to stating that the characteristic thickness of the flame is less than that of a Kolmogorov eddy; this is known as the Klimov-Williams criterion (Williams 1985). In such a case, the flame maintains its laminar structure, and the effect of turbulent flow is merely to wrinkle and strain the flame front. The propagating wrinkled premixed flame can then be described as an infinitesimally thin surface dividing the unburnt fresh mixture and the burnt product.

Annual Research Briefs, 1998

NASA Technical Reports Server (NTRS)

Spinks, Debra (Compiler)

1998-01-01

The topics contained in this progress report are direct numerical simulation of turbulent non-premixed combustion with realistic chemistry; LES of non-premixed turbulent reacting flows with conditional source term estimation; measurements of the three-dimensional scalar dissipation rate in gas-phase planar turbulent jets; direct simulation of a jet diffusion flame; on the use of interpolating wavelets in the direct numerical simulation of combustion; on the use of a dynamically adaptive wavelet collocation algorithm in DNS (direct numerical simulation) of non-premixed turbulent combustion; 2D simulations of Hall thrusters; computation of trailing-edge noise at low mach number using LES and acoustic analogy; weakly nonlinear modeling of the early stages of bypass transition; interactions between freestream turbulence and boundary layers; interfaces at the outer boundaries of turbulent motions; largest scales of turbulent wall flows; the instability of streaks in near-wall turbulence; an implementation of the v(sup 2) - f model with application to transonic flows; heat transfer predictions in cavities; a structure-based model with stropholysis effects; modeling a confined swirling coaxial jet; subgrid-scale models based on incremental unknowns for large eddy simulations; subgrid scale modeling taking the numerical error into consideration; towards a near-wall model for LES of a separated diffuser flow; on the feasibility of merging LES with RANS (Reynolds Averaging Numerical simulation) for the near-wall region of attached turbulent flows; large-eddy simulation of a separated boundary layer; numerical study of a channel flow with variable properties; on the construction of high order finite difference schemes on non-uniform meshes with good conservation properties; development of immersed boundary methods for complex geometries; and particle methods for micro and macroscale flow simulations.

Lagrangian turbulence: Structures and mixing in admissible model flows

NASA Astrophysics Data System (ADS)

Ottino, Julio M.

1991-12-01

The goal of our research was to bridge the gap between modern ideas from dynamical systems and chaos and more traditional approaches to turbulence. In order to reach this objective we conducted theoretical and computational work on two systems: (1) a perturbed-Kelvin cat eyes flow, and (2) prototype solutions of the Navier-Stokes equations near solid walls. The main results obtained are two-fold: we have been able to produce flows capable of producing complex distributions of vorticity, and we have been able to construct flowfields, based on solutions of the Navier-Stokes equations, which are capable of displaying both Eulerian and Lagrangian turbulence. These results exemplify typical mechanisms of mixing enhancement in transitional flows.

On streak spacing in wall-bounded turbulent flows

NASA Technical Reports Server (NTRS)

Hamilton, James M.; Kim, John J.

1993-01-01

The present study is a continuation of the examination by Hamilton et al. of the regeneration mechanisms of near-wall turbulence and an attempt to investigate the conjecture of Waleffe et al. The basis of this study is an extension of the 'minimal channel' approach of Jimenez and Moin that emphasizes the near-wall region and reduces the complexity of the turbulent flow by considering a plane Couette flow of near minimum Reynolds number and stream-wise and span-wise extent. Reduction of the flow Reynolds number to the minimum value which will allow turbulence to be sustained has the effect of reducing the ratio of the largest scales to the smallest scales or, equivalently, of causing the near-wall region to fill more of the area between the channel walls. A plane Couette flow was chosen for study since this type of flow has a mean shear of a single sign, and at low Reynolds numbers, the two wall regions are found to share a single set of structures.

Aeroacoustics of Turbulent High-Speed Jets

NASA Technical Reports Server (NTRS)

Rao, Ram Mohan; Lundgren, Thomas S.

1996-01-01

Aeroacoustic noise generation in a supersonic round jet is studied to understand in particular the effect of turbulence structure on the noise without numerically compromising the turbulence itself. This means that direct numerical simulations (DNS's) are needed. In order to use DNS at high enough Reynolds numbers to get sufficient turbulence structure we have decided to solve the temporal jet problem, using periodicity in the direction of the jet axis. Physically this means that turbulent structures in the jet are repeated in successive downstream cells instead of being gradually modified downstream into a jet plume. Therefore in order to answer some questions about the turbulence we will partially compromise the overall structure of the jet. The first section of chapter 1 describes some work on the linear stability of a supersonic round jet and the implications of this for the jet noise problem. In the second section we present preliminary work done using a TVD numerical scheme on a CM5. This work is only two-dimensional (plane) but shows very interesting results, including weak shock waves. However this is a nonviscous computation and the method resolves the shocks by adding extra numerical dissipation where the gradients are large. One wonders whether the extra dissipation would influence small turbulent structures like small intense vortices. The second chapter is an extensive discussion of preliminary numerical work using the spectral method to solve the compressible Navier-Stokes equations to study turbulent jet flows. The method uses Fourier expansions in the azimuthal and streamwise direction and a 1-D B-spline basis representation in the radial direction. The B-spline basis is locally supported and this ensures block diagonal matrix equations which are solved in O(N) steps. A very accurate highly resolved DNS of a turbulent jet flow is expected.

3D WindScanner lidar measurements of wind and turbulence around wind turbines, buildings and bridges

NASA Astrophysics Data System (ADS)

Mikkelsen, T.; Sjöholm, M.; Angelou, N.; Mann, J.

2017-12-01

WindScanner is a distributed research infrastructure developed at DTU with the participation of a number of European countries. The research infrastructure consists of a mobile technically advanced facility for remote measurement of wind and turbulence in 3D. The WindScanners provide coordinated measurements of the entire wind and turbulence fields, of all three wind components scanned in 3D space. Although primarily developed for research related to on- and offshore wind turbines and wind farms, the facility is also well suited for scanning turbulent wind fields around buildings, bridges, aviation structures and of flow in urban environments. The mobile WindScanner facility enables 3D scanning of wind and turbulence fields in full scale within the atmospheric boundary layer at ranges from 10 meters to 5 (10) kilometers. Measurements of turbulent coherent structures are applied for investigation of flow pattern and dynamical loads from turbines, building structures and bridges and in relation to optimization of the location of, for example, wind farms and suspension bridges. This paper presents our achievements to date and reviews briefly the state-of-the-art of the WindScanner measurement technology with examples of uses for wind engineering applications.

Development of an Aero-Optics Software Library and Integration into Structured Overset and Unstructured Computational Fluid Dynamics (CFD) Flow Solvers

DTIC Science & Technology

2011-04-01

some similarities to the far- field (i.e. atmospheric ) propagation, but due to the interactions between turbulence length scales, beam wavelengths...equivalently, phase differences, have been used to characterize the beam distortion caused by the unsteady turbulent flow field. A Partially-Averaged Navier...A., Wang, M., and Moin, P., “Computational Study of Aero-Optical Distortion by Turbulent Wake,” AIAA Paper 2005-4655. [11] Mani, A., Wang, M., and

Surface roughness effects on turbulent Couette flow

NASA Astrophysics Data System (ADS)

Lee, Young Mo; Lee, Jae Hwa

2017-11-01

Direct numerical simulation of a turbulent Couette flow with two-dimensional (2-D) rod roughness is performed to examine the effects of the surface roughness. The Reynolds number based on the channel centerline laminar velocity (Uco) and channel half height (h) is Re =7200. The 2-D rods are periodically arranged with a streamwise pitch of λ = 8 k on the bottom wall, and the roughness height is k = 0.12 h. It is shown that the wall-normal extent for the logarithmic layer is significantly shortened in the rough-wall turbulent Couette flow, compared to a turbulent Couette flow with smooth wall. Although the Reynolds stresses are increased in a turbulent channel flow with surface roughness in the outer layer due to large-scale ejection motions produced by the 2-D rods, those of the rough-wall Couette flow are decreased. Isosurfaces of the u-structures averaged in time suggest that the decrease of the turbulent activity near the centerline is associated with weakened large-scale counter-rotating roll modes by the surface roughness. This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1A09000537) and the Ministry of Science, ICT & Future Planning (NRF-2017R1A5A1015311).

Wall turbulence control

NASA Technical Reports Server (NTRS)

Wilkinson, Stephen P.; Lindemann, A. Margrethe; Beeler, George B.; Mcginley, Catherine B.; Goodman, Wesley L.; Balasubramanian, R.

1986-01-01

A variety of wall turbulence control devices which were experimentally investigated are discussed; these include devices for burst control, alteration of outer flow structures, large eddy substitution, increased heat transfer efficiency, and reduction of wall pressure fluctuations. Control of pre-burst flow was demonstrated with a single, traveling surface depression which is phase-locked to elements of the burst production process. Another approach to wall turbulence control is to interfere with the outer layer coherent structures. A device in the outer part of a boundary layer was shown to suppress turbulence and reduce drag by opposing both the mean and unsteady vorticity in the boundary layer. Large eddy substitution is a method in which streamline curvature is introduced into the boundary layer in the form of streamwise vortices. Riblets, which were already shown to reduce turbulent drag, were also shown to exhibit superior heat transfer characteristics. Heat transfer efficiency as measured by the Reynolds Analogy Factor was shown to be as much as 36 percent greater than a smooth flat plate in a turbulent boundary layer. Large Eddy Break-Up (LEBU) which are also known to reduce turbulent drag were shown to reduce turbulent wall pressure fluctuation.

Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surface-mounted circular cylinder

NASA Astrophysics Data System (ADS)

Kirkil, Gokhan; Constantinescu, George

2014-11-01

The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder's base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely-long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. A detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (Re = 16,000, subcritical flow regime) and Reynolds numbers at which transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (Re = 500,000, supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices, vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed.

Complexity of the laminar-turbulent boundary in pipe flow

NASA Astrophysics Data System (ADS)

Budanur, Nazmi Burak; Hof, Björn

2018-05-01

Over the past decade, the edge of chaos has proven to be a fruitful starting point for investigations of shear flows when the laminar base flow is linearly stable. Numerous computational studies of shear flows demonstrated the existence of states that separate laminar and turbulent regions of the state space. In addition, some studies determined invariant solutions that reside on this edge. In this paper, we study the unstable manifold of one such solution with the aid of continuous symmetry reduction, which we formulate here for the simultaneous quotiening of axial and azimuthal symmetries. Upon our investigation of the unstable manifold, we discover a previously unknown traveling-wave solution on the laminar-turbulent boundary with a relatively complex structure. By means of low-dimensional projections, we visualize different dynamical paths that connect these solutions to the turbulence. Our numerical experiments demonstrate that the laminar-turbulent boundary exhibits qualitatively different regions whose properties are influenced by the nearby invariant solutions.

Nobody knew turbulent transition could be so complicated

NASA Astrophysics Data System (ADS)

Barkley, Dwight

2017-11-01

Explaining the route to turbulence in wall-bounded shear flows has been a long and tortuous journey. After years of missteps, controversies, and uncertainties, we are at last converging on a unified and fascinating picture of transition in flows such as pipes, channels, and ducts. Classically, subcritical transition (such as in a pipe), was thought to imply a discontinuous route to turbulence. We now know that this is not the case - subcritical shear flows may, and often do, exhibit continuous transition. I will discuss recent developments in experiments, simulations, and theory that have established a deep connection between transition in subcritical shear flows and a class of non-equilibrium statistical phase transitions known as directed percolation. From this we understand how to define precise critical points for systems without linear instabilities and how to characterize the onset of turbulence in terms of non-trivial, but universal power laws. I will discuss the physics responsible for the complex turbulent structures ubiquitously observed near transition and end with thoughts on outstanding open questions.

Computation of turbulent reacting flow in a solid-propellant ducted rocket

NASA Astrophysics Data System (ADS)

Chao, Yei-Chin; Chou, Wen-Fuh; Liu, Sheng-Shyang

1995-05-01

A mathematical model for computation of turbulent reacting flows is developed under general curvilinear coordinate systems. An adaptive, streamline grid system is generated to deal with the complex flow structures in a multiple-inlet solid-propellant ducted rocket (SDR) combustor. General tensor representations of the k-epsilon and algebraic stress (ASM) turbulence models are derived in terms of contravariant velocity components, and modification caused by the effects of compressible turbulence is also included in the modeling. The clipped Gaussian probability density function is incorporated in the combustion model to account for fluctuations of properties. Validation of the above modeling is first examined by studying mixing and reacting characteristics in a confined coaxial-jet problem. This is followed by study of nonreacting and reacting SDR combustor flows. The results show that Gibson and Launder's ASM incorporated with Sarkar's modification for compressible turbulence effects based on the general curvilinear coordinate systems yields the most satisfactory prediction for this complicated SDR flowfield.

Computation of turbulent reacting flow in a solid-propellant ducted rocket

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chao, Y.; Chou, W.; Liu, S.

1995-05-01

A mathematical model for computation of turbulent reacting flows is developed under general curvilinear coordinate systems. An adaptive, streamline grid system is generated to deal with the complex flow structures in a multiple-inlet solid-propellant ducted rocket (SDR) combustor. General tensor representations of the k-epsilon and algebraic stress (ASM) turbulence models are derived in terms of contravariant velocity components, and modification caused by the effects of compressible turbulence is also included in the modeling. The clipped Gaussian probability density function is incorporated in the combustion model to account for fluctuations of properties. Validation of the above modeling is first examined bymore » studying mixing and reacting characteristics in a confined coaxial-jet problem. This is followed by study of nonreacting and reacting SDR combustor flows. The results show that Gibson and Launder`s ASM incorporated with Sarkar`s modification for compressible turbulence effects based on the general curvilinear coordinate systems yields the most satisfactory prediction for this complicated SDR flowfield. 36 refs.« less

Direct numerical simulation of turbulence in injection-driven plane channel flows

NASA Astrophysics Data System (ADS)

Venugopal, Prem; Moser, Robert D.; Najjar, Fady M.

2008-10-01

Compressible turbulent flow in a periodic plane channel with mass injecting walls is studied as a simplified model for core flow in a solid-propellant rocket motor with homogeneous propellant and other injection-driven internal flows. In this model problem, the streamwise direction was asymptotically homogenized by assuming that at large distances from the closed end, both the mean and rms of turbulent fluctuations evolve slowly in the streamwise direction when compared to the turbulent fluctuations themselves. The Navier-Stokes equations were then modified to account for this slow growth. A direct numerical simulation of the homogenized compressible injection-driven turbulent flow was then conducted for conditions occurring at a streamwise location situated 40 channel half-widths from the closed off end and at an injection Reynolds number of approximately 190. The turbulence in this model flow was found to be only weakly compressible, although significant compressibility existed in the mean flow. As in nontranspired channels, turbulence resulted in increased near-wall shear for the mean streamwise velocity. When normalized by the average rate of turbulence production, the magnitudes of near-wall velocity fluctuations were similar to those in the log region of nontranspired wall-bounded turbulence. However, the sharp peak in streamwise velocity fluctuations observed in nontranspired channels was absent. While streaks and inclined vortices were observed in the near-wall region, their structure was very similar to those observed in the log region of nontranspired channels. These differences are attributed to the absence of a viscous sublayer in the transpired case which in turn is the result of the fact that the no-slip condition for the transpired case is an inviscid boundary condition. That is, unlike nontranspired walls, with transpiration, zero tangential velocity boundary conditions can be imposed at the wall for the Euler (inviscid) equations. The results of this study have important implications on the ability of turbulence models to predict this flow.

Velocity and pressure fields associated with near-wall turbulence structures

NASA Technical Reports Server (NTRS)

Johansson, Arne V.; Alfredsson, P. Henrik; Kim, John

1990-01-01

Computer generated databases containing velocity and pressure fields in three-dimensional space at a sequence of time-steps, were used for the investigation of near-wall turbulence structures, their space-time evolution, and their associated pressure fields. The main body of the results were obtained from simulation data for turbulent channel flow at a Reynolds number of 180 (based on half-channel height and friction velocity) with a grid of 128 x 129 x and 128 points. The flow was followed over a total time of 141 viscous time units. Spanwise centering of the detected structures was found to be essential in order to obtain a correct magnitude of the associated Reynolds stress contribution. A positive wall-pressure peak is found immediately beneath the center of the structure. The maximum amplitude of the pressure pattern was, however, found in the buffer region at the center of the shear-layer. It was also found that these flow structures often reach a maximum strength in connection with an asymmetric spanwise motion, which motivated the construction of a conditional sampling scheme that preserved this asymmetry.

PSE-3D Instability Analysis and Application to Flow Over an Elliptic Cone

DTIC Science & Technology

2015-04-01

Since the seminal experiments conducted by Crow and Champagne [1971], it is generally accepted that turbulent flows are organized by orderly quasi...2007. C. Crow and F.H. Champagne . Orderly structure in jet turbulence. J. Fluid Mech., 48:547–591, 1971. S.C. Crow. Stability theory for a pair of

Four-way coupled simulations of small particles in turbulent channel flow: The effects of particle shape and Stokes number

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhao, F.; Wachem, B. G. M. van, E-mail: berend.van.wachem@gmail.com; George, W. K.

2015-08-15

This paper investigates the effects of particle shape and Stokes number on the behaviour of non-spherical particles in turbulent channel flow. Although there are a number of studies concerning spherical particles in turbulent flows, most important applications occurring in process, energy, and pharmaceutical industries deal with non-spherical particles. The computation employs a unique and novel four-way coupling with the Lagrangian point-particle approach. The fluid phase at low Reynolds number (Re{sub τ} = 150) is modelled by direct numerical simulation, while particles are tracked individually. Inter-particle and particle-wall collisions are also taken into account. To explore the effects of particles onmore » the flow turbulence, the statistics of the fluid flow such as the fluid velocity, the terms in the turbulence kinetic energy equation, the slip velocity between the two phases and velocity correlations are analysed considering ellipsoidal particles with different inertia and aspect ratio. The results of the simulations show that the turbulence is considerably attenuated, even in the very dilute regime. The reduction of the turbulence intensity is predominant near the turbulence kinetic energy peak in the near wall region, where particles preferentially accumulate. Moreover, the elongated shape of ellipsoids strengthens the turbulence attenuation. In simulations with ellipsoidal particles, the fluid-particle interactions strongly depend on the orientation of the ellipsoids. In the near wall region, ellipsoids tend to align predominantly within the streamwise (x) and wall-normal (y) planes and perpendicular to the span-wise direction, whereas no preferential orientation in the central region of the channel is observed. Important conclusions from this work include the effective viscosity of the flow is not affected, the direct dissipation by the particles is negligible, and the primary mechanism by which the particles affect the flow is by altering the turbulence structure around the turbulence kinetic energy peak.« less

Structure and statistics of turbulent flow over riblets

NASA Astrophysics Data System (ADS)

Henderson, R. D.; Crawford, C. H.; Karniadakis, G. E.

1993-01-01

In this paper we present comparisons of turbulence statistics obtained from direct numerical simulation of flow over streamwise aligned triangular riblets with experimental results. We also present visualizations of the instantaneous velocity field inside and around the riblet valleys. In light of the behavior of the statistics and flowfields inside the riblet valleys, we investigate previously reported physical mechanisms for the drag reducing effect of riblets; our results here support the hypothesis of flow anchoring by the riblet valleys and the corresponding inhibition of spanwise flow motions.

Study of the structure of turbulent shear flows at supersonic speeds and high Reynolds number

NASA Technical Reports Server (NTRS)

Smits, A. J.; Bogdonoff, S. M.

1984-01-01

A major effort to improve the accuracies of turbulence measurement techniques is described including the development and testing of constant temperature hot-wire anemometers which automatically compensate for frequency responses. Calibration and data acquisition techniques for normal and inclined wires operated in the constant temperature mode, flow geometries, and physical models to explain the observed behavior of flows are discussed, as well as cooperation with computational groups in the calculation of compression corner flows.

Turbulent convective flows in a cubic cavity at high Prandtl number

NASA Astrophysics Data System (ADS)

Vasiliev, A.; Sukhanovskii, A.; Frick, P.

2016-10-01

Characteristics of turbulent convective flows in a cubic cell is studied experimentally for high values of Prandtl number. The first set was carriied out with propylene glycol (Pr = 64 and the second one with 25% water solution of propylene glycol (Pr = 24). It was found that increasing of Pr from 6.1 to 24 leads only to the slight change of intensity of the flow but during the next increasing of Pr from 24 to 64 the flow changes its structure.

Direct numerical simulation of curved turbulent channel flow

NASA Technical Reports Server (NTRS)

Moser, R. D.; Moin, P.

1984-01-01

Low Reynolds number, mildly curved, turbulent channel flow has been simulated numerically without subgrid scale models. A new spectral numerical method developed for this problem was used, and the computations were performed with 2 million degrees of freedom. A variety of statistical and structural information has been extracted from the computed flow fields. These include mean velocity, turbulence stresses, velocity skewness, and flatness factors, space time correlations and spectra, all the terms in the Reynolds stress balance equations, and contour and vector plots of instantaneous velocity fields. The effects of curvature on this flow were determined by comparing the concave and convex sides of the channel. The observed effects are consistent with experimental observations for mild curvature. The most significant difference in the turbulence statistics between the concave and convex sides was in the Reynolds shear stress. This was accompanied by significant differences in the terms of the Reynolds shear stress balance equations. In addition, it was found that stationary Taylor-Gortler vortices were present and that they had a significant effect on the flow by contributing to the mean Reynolds shear stress, and by affecting the underlying turbulence.

Turbulent intermittent structure in non-homogeneous non-local flows

NASA Astrophysics Data System (ADS)

Mahjoub, O. B.; Castilla, R.; Vindel, J. M.; Redondo, J. M.

2010-05-01

Data from SABLES98 experimental campaign have been used in order to study the influence of stability (from weak to strong stratification) on intermittency [1]. Standard instrumentation, 14 thermocouples and 3 sonic anemometers at three levels (5.8, 13.5 and 32 m) were available in September 1998 and calculations are done in order to evaluate structure functions and the scale to scale characteristics. Using BDF [2-4] as well as other models of cascades, the spectral equilibrium values were used to calculate fluxes of momentum and heat as well as non-homogeneous models and the turbulent mixing produced. The differences in structure and higher order moments between stable, convective and neutral turbulence were used to identify differences in turbulent intermittent mixing and velocity PDF's. The intermittency of atmospheric turbulence in strongly stable situations affected by buoyancy and internal waves are seen to modify the structure functions exponents and intermittency, depending on the modulus of the Richardson's number,Ri, as well as of the Monin-Obukhov and Ozmidov lengthscales. The topological aspects of the turbulence affected by stratification reduce the vertical length-scales to a maximum described by the Thorpe and the Ozmidov lenth-scales, but intermittency, Kurtosis and other higher order descriptors of the turbulence based on spectral wavelet analysis are also affected in a complex way [5,6]. The relationship between stratification, intermittency, µ(Ri) and the fractal dimension of the stable flows and between the dispersion, the fractal dimension are discussed. The data analyzed is from the campaign SABLES-98 at the north-west Iberian Peninsula plateau.(Cuxart et al. 2000). Conditional statistics of the relationship between µ(Ri) are confirmed as in (Vindel et al 2008)[4] and compared with laboratory experiments and with 2D-3D aspects of the turbulence cascade. The use of BDF [3] model comparing the corresponding relative scaling exponents which are estimated from two characteristic parameters(D,b). For unstable or neutral situations, it is possible to find values for these parameters that represent the empirical scaling exponents D and b obtained from [1]. When D increases, the order smaller than 3 relative scaling exponents also increases (but for orders higher than 3, they decrease) linearly. On the contrary, for a certain value of D, when b increases the behavior of the relative scaling exponents is the opposite and non-linear. [1]Ben-Mahjoub O., Babiano A. y Redondo J.M. Velocity structure and Extended Self Similarity in nonhomogeneous Turbulent Jets and Wakes. Journal of flow turbulence and combustion. 59 , 299-313. 1998. [2]Ben-Mahjoub O., Redondo J.M., and R. Alami. Turbulent Structure Functions in Geophysical Flows, Rapp. Comm. int. Mer Medit., 35, 126-127. 1998 [3]Babiano, A., Dubrulle, B., Frick, P. Some properties of two-dimensional inverse energy cascade dynamics, Phys. Rev. E. 55, 2693, 1997. [4]Vindel J.M., Yague C. and J.M. Redondo, Structure function analysis and intermittency in the ABL, NonLin. Proc. Geophys. 15, 6. 915-929. 2009. [5]Cuxart, J., Yagüe, C., Morales, G., Terradellas, E., Orbe, J., Calvo, J., Fernández, A., Soler, M. R., Infante, C., Buenestado, P., Espinalt, A., Joergensen, H. E., Rees, J. M., Vila, J., Redondo, J. M., Cantalapiedra, I. R., Conangla L., Bound-Layer Meteor. 96, 337-370 2000. [6]Rodríguez, A., Sánchez-Arcilla, A., Redondo, J. M., Mosso, C.: Macroturbulence measurements with electromagnetic and ultrasonic sensors: a comparison under high-turbulent flows, Experiments in Fluids, 27, 31-42. 1999.

Modification of homogeneous and isotropic turbulence by solid particles

NASA Astrophysics Data System (ADS)

Hwang, Wontae

2005-12-01

Particle-laden flows are prevalent in natural and industrial environments. Dilute loadings of small, heavy particles have been observed to attenuate the turbulence levels of the carrier-phase flow, up to 80% in some cases. We attempt to increase the physical understanding of this complex phenomenon by studying the interaction of solid particles with the most fundamental type of turbulence, which is homogeneous and isotropic with no mean flow. A flow facility was developed that could create air turbulence in a nearly-spherical chamber by means of synthetic jet actuators mounted on the corners. Loudspeakers were used as the actuators. Stationary turbulence and natural decaying turbulence were investigated using two-dimensional particle image velocimetry for the base flow qualification. Results indicated that the turbulence was fairly homogeneous throughout the measurement domain and very isotropic, with small mean flow. The particle-laden flow experiments were conducted in two different environments, the lab and in micro-gravity, to examine the effects of particle wakes and flow structure distortion caused by settling particles. The laboratory experiments showed that glass particles with diameters on the order of the turbulence Kolmogorov length scale attenuated the fluid turbulent kinetic energy (TKE) and dissipation rate with increasing particle mass loadings. The main source of fluid TKE production in the chamber was the speakers, but the loss of potential energy of the settling particles also resulted in a significant amount of production of extra TKE. The sink of TKE in the chamber was due to the ordinary fluid viscous dissipation and extra dissipation caused by particles. This extra dissipation could be divided into "unresolved" dissipation caused by local velocity disturbances in the vicinity of the small particles and dissipation caused by large-scale flow distortions from particle wakes and particle clusters. The micro-gravity experiments in NASA's KC-135 showed that the absence of particle potential energy loss and particle wakes caused greater levels of turbulence attenuation since there was no additional production due to mean particle motion. The relatively stationary dispersion of particles acted like a series of screens which produced forces opposing turbulent motions.

The effects of streamwise concave curvature on turbulent boundary layer structure

NASA Astrophysics Data System (ADS)

Jeans, A. H.; Johnston, J. P.

1982-06-01

Concave curvature has a relatively large, unpredictable effect on turbulent boundary layers. Some, but not all previous studies suggest that a large-scale, stationary array of counter-rotating vortices exists within the turbulent boundary layer on a concave wall. The objective of the present study was to obtain a qualitative model of the flow field in order to increase our understanding of the underlying physics. A large free-surface water channel was constructed in order to perform a visual study of the flow. Streamwise components of mean velocity and turbulence intensity were measured using a hot film anemometer. The upstream boundary was spanwise uniform with a momentum thickness to radius of curvature of 0.05. Compared to flat wall flow, large-scale, randomly distributed sweeps and ejections were seen in the boundary layer on the concave wall. The sweeps appear to suppress the normal mechanism for turbulence production near the wall by inhibiting the bursting process. The ejections appear to enhance turbulence production in the outer layers as the low speed fluid convected from regions near the wall interacts with the higher speed fluid farther out. The large-scale structures did not occur at fixed spanwise locations, and could not be called roll cells or vortices.

Turbulent Compressible Convection with Rotation. 2; Mean Flows and Differential Rotation

NASA Technical Reports Server (NTRS)

Brummell, Nicholas H.; Hurlburt, Neal E.; Toomre, Juri

1998-01-01

The effects of rotation on turbulent, compressible convection within stellar envelopes are studied through three-dimensional numerical simulations conducted within a local f-plane model. This work seeks to understand the types of differential rotation that can be established in convective envelopes of stars like the Sun, for which recent helioseismic observations suggest an angular velocity profile with depth and latitude at variance with many theoretical predictions. This paper analyzes the mechanisms that are responsible for the mean (horizontally averaged) zonal and meridional flows that are produced by convection influenced by Coriolis forces. The compressible convection is considered for a range of Rayleigh, Taylor, and Prandtl (and thus Rossby) numbers encompassing both laminar and turbulent flow conditions under weak and strong rotational constraints. When the nonlinearities are moderate, the effects of rotation on the resulting laminar cellular convection leads to distinctive tilts of the cell boundaries away from the vertical. These yield correlations between vertical and horizontal motions that generate Reynolds stresses that can drive mean flows, interpretable as differential rotation and meridional circulations. Under more vigorous forcing, the resulting turbulent convection involves complicated and contorted fluid particle trajectories, with few clear correlations between vertical and horizontal motions, punctuated by an evolving and intricate downflow network that can extend over much of the depth of the layer. Within such networks are some coherent structures of vortical downflow that tend to align with the rotation axis. These yield a novel turbulent alignment mechanism, distinct from the laminar tilting of cellular boundaries, that can provide the principal correlated motions and thus Reynolds stresses and subsequently mean flows. The emergence of such coherent structures that can persist amidst more random motions is a characteristic of turbulence with symmetries broken by rotation and stratification. Such structure is here found to play a crucial role in defining the mean zonal and meridional flows that coexist with the convection. Though they are subject to strong inertial oscillations, the strength and type of the mean flows are determined by a combination of the laminar tilting and the turbulent alignment mechanisms. Varying the parameters produces a wide range of mean motions. Among these, some turbulent solutions exhibit a mean zonal velocity profile that is nearly constant with depth, much as deduced by helioseismology at midlatitudes within the Sun. The solutions exhibit a definite handedness, with the direction of the persistent mean flows often prescribing a spiral with depth near the boundaries, also in accord with helioseismic deductions. The mean helicity has a profile that is positive in the upper portion of the domain and negative in the lower portion, a property bearing on magnetic dynamo processes that may be realized within such rotating layers of turbulent convection.

3D Numerical simulation of bed morphological responses to complex in-streamstructures

NASA Astrophysics Data System (ADS)

Xu, Y.; Liu, X.

2017-12-01

In-stream structures are widely used in stream restoration for both hydraulic and ecologicalpurposes. The geometries of the structures are usually designed to be extremely complex andirregular, so as to provide nature-like physical habitat. The aim of this study is to develop anumerical model to accurately predict the bed-load transport and the morphological changescaused by the complex in-stream structures. This model is developed in the platform ofOpenFOAM. In the hydrodynamics part, it utilizes different turbulence models to capture thedetailed turbulence information near the in-stream structures. The technique of immersedboundary method (IBM) is efficiently implemented in the model to describe the movable bendand the rigid solid body of in-stream structures. With IBM, the difficulty of mesh generation onthe complex geometry is greatly alleviated, and the bed surface deformation is able to becoupled in to flow system. This morphodynamics model is firstly validated by simple structures,such as the morphology of the scour in log-vane structure. Then it is applied in a more complexstructure, engineered log jams (ELJ), which consists of multiple logs piled together. Thenumerical results including turbulence flow information and bed morphological responses areevaluated against the experimental measurement within the exact same flow condition.

Wall Turbulence.

ERIC Educational Resources Information Center

Hanratty, Thomas J.

1980-01-01

This paper gives an account of research on the structure of turbulence close to a solid boundary. Included is a method to study the flow close to the wall of a pipe without interferring with it. (Author/JN)

Analysis of turbulent synthetic jet by dynamic mode decomposition

NASA Astrophysics Data System (ADS)

Hyhlík, Tomáš; Netřebská, Hana; Devera, Jakub; Kalinay, Radomír

The article deals with the analysis of CFD results of the turbulent synthetic jet. The numerical simulation of Large Eddy Simulation (LES) using commercial solver ANSYS CFX has been performed. The unsteady flow field is studied from the point of view of identification of the moving vortex ring, which has been identified both on the snapshots of flow field using swirling-strength criterion and using the Dynamic Mode Decomposition (DMD) of five periods. It is shown that travelling vortex ring vanishes due to interaction with vortex structures in the synthesised turbulent jet. DMD modes with multiple of the basic frequency of synthetic jet, which are connected with travelling vortex structure, have largest DMD amplitudes.

Numerical simulation of turbulence in the presence of shear

NASA Technical Reports Server (NTRS)

Shaanan, S.; Ferziger, J. H.; Reynolds, W. C.

1975-01-01

The numerical calculations are presented of the large eddy structure of turbulent flows, by use of the averaged Navier-Stokes equations, where averages are taken over spatial regions small compared to the size of the computational grid. The subgrid components of motion are modeled by a local eddy-viscosity model. A new finite-difference scheme is proposed to represent the nonlinear average advective term which has fourth-order accuracy. This scheme exhibits several advantages over existing schemes with regard to the following: (1) the scheme is compact as it extends only one point away in each direction from the point to which it is applied; (2) it gives better resolution for high wave-number waves in the solution of Poisson equation, and (3) it reduces programming complexity and computation time. Examples worked out in detail are the decay of isotropic turbulence, homogeneous turbulent shear flow, and homogeneous turbulent shear flow with system rotation.

Physical experiments and analysis on the generation and evolution of tsunami-induced turbulent coherent structures

NASA Astrophysics Data System (ADS)

Kalligeris, Nikos; Lynett, Patrick

2017-11-01

Numerous historical accounts describe the formation of ``whirpools'' inside ports and harbors during tsunami events, causing port operation disruptions. Videos from the Japan 2011 tsunami revealed complex nearshore flow patters, resulting from the interaction of tsunami-induced currents with the man-made coastline, and the generation of large eddies (or turbulent coherent structures) in numerous ports and harbors near the earthquake epicenter. The aim of this work is to study the generation and evolution of tsunami-induced turbulent coherent structures (TCS) in a well-controlled environment using realistic scaling. A physical configuration is created in the image of a port entrance at a scale of 1:27 and a small-amplitude, long period wave creates a transient flow through the asymmetric harbor channel. A separated region forms, which coupled with the transient flow, leads to the formation of a stable monopolar TCS. The surface flow is examined through mono- and stereo-PTV techniques to extract surface velocity vectors. Surface velocity maps and vortex flow profiles are used to study the experimental TCS generation and evolution, and characterize the TCS structure. Analytical tools are used to describe the TCS growth rate and kinetic energy decay. This work was funded by the National Science Foundation NEES Research program, with Award Number 1135026.

Biofilm Effect on Flow Structure over a Permeable Bed

NASA Astrophysics Data System (ADS)

Kazemifar, F.; Blois, G.; Aybar, M.; Perez-Calleja, P.; Nerenberg, R.; Sinha, S.; Hardy, R. J.; Best, J.; Sambrook Smith, G.; Christensen, K. T.

2017-12-01

Biofilms constitute an important form of bacterial life in aquatic environments and are present at the fluid-solid interfaces in natural and industrial settings, such as water distribution systems and riverbeds among others. The permeable, heterogeneous, and deformable structure of biofilms can influence mass and momentum transport between the subsurface and freestream. However, this interaction is not fully understood, in part due to technical obstacles impeding quantitative experimental investigations. In this work, the effect of biofilm on flow structure over a permeable bed is studied. Experiments are conducted in a closed water channel equipped with an idealized two-dimensional permeable bed. Prior to conducting flow experiments, the models are placed within an independent recirculating reactor for biofilm growth. Once a targeted biofilm growth stage is achieved, the models are transferred to the water channel and subjected to transitional and turbulent flows. Long-distance microscopic particle image velocimetry measurements are performed to quantify the effect of biofilm on the turbulence structure of the free flow as well as the freestream-subsurface flow interaction.

Experimental and theoretical study of combustion jet ignition

NASA Technical Reports Server (NTRS)

Chen, D. Y.; Ghoniem, A. F.; Oppenheim, A. K.

1983-01-01

A combustion jet ignition system was developed to generate turbulent jets of combustion products containing free radicals and to discharge them as ignition sources into a combustible medium. In order to understand the ignition and the inflammation processes caused by combustion jets, the studies of the fluid mechanical properties of turbulent jets with and without combustion were conducted theoretically and experimentally. Experiments using a specially designed igniter, with a prechamber to build up and control the stagnation pressure upstream of the orifice, were conducted to investigate the formation processes of turbulent jets of combustion products. The penetration speed of combustion jets has been found to be constant initially and then decreases monotonically as turbulent jets of combustion products travel closer to the wall. This initial penetration speed to combustion jets is proportional to the initial stagnation pressure upstream of the orifice for the same stoichiometric mixture. Computer simulations by Chorin's Random Vortex Method implemented with the flame propagation algorithm for the theoretical model of turbulent jets with and without combustion were performed to study the turbulent jet flow field. In the formation processes of the turbulent jets, the large-scale eddy structure of turbulence, the so-called coherent structure, dominates the entrainment and mixing processes. The large-scale eddy structure of turbulent jets in this study is constructed by a series of vortex pairs, which are organized in the form of a staggered array of vortex clouds generating local recirculation flow patterns.

Numerical investigation of hub clearance flow in a Kaplan turbine

NASA Astrophysics Data System (ADS)

Wu, H.; Feng, J. J.; Wu, G. K.; Luo, X. Q.

2012-11-01

In this paper, the flow field considering the hub clearance flow in a Kaplan turbine has been investigated through using the commercial CFD code ANSYS CFX based on high-quality structured grids generated by ANSYS ICEM CFD. The turbulence is simulated by k-ω based shear stress transport (SST) turbulence model together with automatic near wall treatments. Four kinds of simulations have been conducted for the runner geometry without hub clearance, with only the hub front clearance, with only the rear hub clearance, and with both front and rear clearance. The analysis of the obtained results is focused on the flow structure of the hub clearance flow, the effect on the turbine performance including hydraulic efficiency and cavitation performance, which can improve the understanding on the flow field in a Kaplan turbine.

Wind turbine wakes in forest and neutral plane wall boundary layer large-eddy simulations

NASA Astrophysics Data System (ADS)

Schröttle, Josef; Piotrowski, Zbigniew; Gerz, Thomas; Englberger, Antonia; Dörnbrack, Andreas

2016-09-01

Wind turbine wake flow characteristics are studied in a strongly sheared and turbulent forest boundary layer and a neutral plane wall boundary layer flow. The reference simulations without wind turbine yield similar results as earlier large-eddy simulations by Shaw and Schumann (1992) and Porte-Agel et al. (2000). To use the fields from the homogeneous turbulent boundary layers on the fly as inflow fields for the wind turbine wake simulations, a new and efficient methodology was developed for the multiscale geophysical flow solver EULAG. With this method fully developed turbulent flow fields can be achieved upstream of the wind turbine which are independent of the wake flow. The large-eddy simulations reproduce known boundary-layer statistics as mean wind profile, momentum flux profile, and eddy dissipation rate of the plane wall and the forest boundary layer. The wake velocity deficit is more asymmetric above the forest and recovers faster downstream compared to the velocity deficit in the plane wall boundary layer. This is due to the inflection point in the mean streamwise velocity profile with corresponding turbulent coherent structures of high turbulence intensity in the strong shear flow above the forest.

Interaction of monopoles, dipoles, and turbulence with a shear flow

NASA Astrophysics Data System (ADS)

Marques Rosas Fernandes, V. H.; Kamp, L. P. J.; van Heijst, G. J. F.; Clercx, H. J. H.

2016-09-01

Direct numerical simulations have been conducted to examine the evolution of eddies in the presence of large-scale shear flows. The numerical experiments consist of initial-value-problems in which monopolar and dipolar vortices as well as driven turbulence are superposed on a plane Couette or Poiseuille flow in a periodic two-dimensional channel. The evolution of the flow has been examined for different shear rates of the background flow and different widths of the channel. Results found for retro-grade and pro-grade monopolar vortices are consistent with those found in the literature. Boundary layer vorticity, however, can significantly modify the straining and erosion of monopolar vortices normally seen for unbounded domains. Dipolar vortices are shown to be much more robust coherent structures in a large-scale shear flow than monopolar eddies. An analytical model for their trajectories, which are determined by self-advection and advection and rotation by the shear flow, is presented. Turbulent kinetic energy is effectively suppressed by the shearing action of the background flow provided that the shear is linear (Couette flow) and of sufficient strength. Nonlinear shear as present in the Poiseuille flow seems to even increase the turbulence strength especially for high shear rates.

The effect of fuel inlet turbulence intensity on H2/CH4 flame structure of MILD combustion using the LES method

NASA Astrophysics Data System (ADS)

Afarin, Yashar; Tabejamaat, Sadegh

2013-06-01

Large eddy simulations (LES) are employed to investigate the effect of the inlet turbulence intensity on the H2/CH4 flame structure in a hot and diluted co-flow stream which emulates the (Moderate or Intense Low-oxygen Dilution) MILD combustion regime. In this regard, three fuel inlet turbulence intensity profiles with the values of 4%, 7% and 10% are superimposed on the annular mixing layer. The effects of these changes on the flame structure under the MILD condition are studied for two oxygen concentrations of 3% and 9% (by mass) in the oxidiser stream and three hot co-flow temperatures 1300, 1500 and 1750 K. The turbulence-chemistry interaction of the numerically unresolved scales is modelled using the (Partially Stirred Reactor) PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influences of the turbulence intensity on the flame structure under the MILD condition are studied by using the profile of temperature, CO and OH mass fractions in both physical and mixture fraction spaces at two downstream locations. Also, the effects of this parameter are investigated by contours of OH, HCO and CH2O radicals in an area near the nozzle exit zone. Results show that increasing the fuel inlet turbulence intensity has a profound effect on the flame structure particularly at low oxygen mass fraction. This increment weakens the combustion zone and results in a decrease in the peak values of the flame temperature and OH and CO mass fractions. Furthermore, increasing the inlet turbulence intensity decreases the flame thickness, and increases the MILD flame instability and diffusion of un-burnt fuel through the flame front. These effects are reduced by increasing the hot co-flow temperature which reinforces the reaction zone.

Effect of free-stream turbulence on boundary layer transition.

PubMed

Goldstein, M E

2014-07-28

This paper is concerned with the transition to turbulence in flat plate boundary layers due to moderately high levels of free-stream turbulence. The turbulence is assumed to be generated by an (idealized) grid and matched asymptotic expansions are used to analyse the resulting flow over a finite thickness flat plate located in the downstream region. The characteristic Reynolds number Rλ based on the mesh size λ and free-stream velocity is assumed to be large, and the turbulence intensity ε is assumed to be small. The asymptotic flow structure is discussed for the generic case where the turbulence Reynolds number εRλ and the plate thickness and are held fixed (at O(1) and O(λ), respectively) in the limit as [Formula: see text] and ε→0. But various limiting cases are considered in order to explain the relevant transition mechanisms. It is argued that there are two types of streak-like structures that can play a role in the transition process: (i) those that appear in the downstream region and are generated by streamwise vorticity in upstream flow and (ii) those that are concentrated near the leading edge and are generated by plate normal vorticity in upstream flow. The former are relatively unaffected by leading edge geometry and are usually referred to as Klebanoff modes while the latter are strongly affected by leading edge geometry and are more streamwise vortex-like in appearance. © 2014 The Author(s) Published by the Royal Society. All rights reserved.

Multifractal Modeling of Turbulent Mixing

NASA Astrophysics Data System (ADS)

Samiee, Mehdi; Zayernouri, Mohsen; Meerschaert, Mark M.

2017-11-01

Stochastic processes in random media are emerging as interesting tools for modeling anomalous transport phenomena. Applications include intermittent passive scalar transport with background noise in turbulent flows, which are observed in atmospheric boundary layers, turbulent mixing in reactive flows, and long-range dependent flow fields in disordered/fractal environments. In this work, we propose a nonlocal scalar transport equation involving the fractional Laplacian, where the corresponding fractional index is linked to the multifractal structure of the nonlinear passive scalar power spectrum. This work was supported by the AFOSR Young Investigator Program (YIP) award (FA9550-17-1-0150) and partially by MURI/ARO (W911NF-15-1-0562).

A wind model for an elevated STOL-port configuration

NASA Technical Reports Server (NTRS)

Peterka, J. A.; Cermak, J. E.

1974-01-01

Measurements of mean velocity magnitude and direction as well as three-dimensional turbulence intensity were made in the flow over a model of an elevated STOL-port. A 1:300 scale model was placed in a wind tunnel flow simulating the mean velocity profile and turbulence characteristics of atmospheric winds over a typical city environment excluding detailed wake structures of possible nearby buildings. Hot-wire anemometer measurements of velocity and turbulence were made along approach and departure paths of aircraft operating on the runway centerline and at specified lateral distances from the centerline. Approach flow directions simulated were 0 and 30 degrees to the runway centerline.

Turbulent Premixed Flame Propagation in Microgravity

NASA Technical Reports Server (NTRS)

Menon, S.; Disseau, M.; Chakravarthy, V. K.; Jagoda, J.

1997-01-01

A facility in which turbulent Couette flow could be generated in a microgravity environment was designed and built. To fit into the NASA Lewis drop tower the device had to be very compact. This means that edge effects and flow re-circulation were expected to affect the flow. The flow was thoroughly investigated using LDV and was found to be largely two dimensional away from the edges with constant turbulence intensities in the core. Slight flow asymmetries are introduced by the non symmetric re-circulation of the fluid outside the test region. Belt flutter problems were remedied by adding a pair of guide plates to the belt. In general, the flow field was found to be quite similar to previously investigated Couette flows. However, turbulence levels and associated shear stresses were higher. This is probably due to the confined re-circulation zone reintroducing turbulence into the test section. An estimate of the length scales in the flow showed that the measurements were able to resolve nearly all the length scales of interest. Using a new LES method for subgrid combustion it has been demonstrated that the new procedure is computational feasible even on workstation type environment. It is found that this model is capable of capturing the propagation of the premixed names by resolving the flame in the LES grid within 2-3 grid points. In contrast, conventional LES results in numerical smearing of the flame and completely inaccurate estimate of the turbulent propagation speed. Preliminary study suggests that there is observable effect of buoyancy in the 1g environment suggesting the need for microgravity experiments of the upcoming experimental combustion studies. With the cold flow properties characterized, an identical hot flow facility is under construction. It is assumed that the turbulence properties ahead of the flame in this new device will closely match the results obtained here. This is required since the hot facility will not enable LDV measurements. The reacting flow facility is also being constructed with planned drop tower experiments in mind. Therefore, issues related to safety and structural integrity are being take into account. Further development of the numerical model will also be carried out to include finite-rate kinetics for representative premixed cases. More detail analysis of the flame structure and propagation nature will be investigated. Simulations will also be compared to the flame properties observed in the experiments.

Flexible Inhibitor Fluid-Structure Interaction Simulation in RSRM.

NASA Astrophysics Data System (ADS)

Wasistho, Bono

2005-11-01

We employ our tightly coupled fluid/structure/combustion simulation code 'Rocstar-3' for solid propellant rocket motors to study 3D flows past rigid and flexible inhibitors in the Reusable Solid Rocket Motor (RSRM). We perform high resolution simulations of a section of the rocket near the center joint slot at 100 seconds after ignition, using inflow conditions based on less detailed 3D simulations of the full RSRM. Our simulations include both inviscid and turbulent flows (using LES dynamic subgrid-scale model), and explore the interaction between the inhibitor and the resulting fluid flow. The response of the solid components is computed by an implicit finite element solver. The internal mesh motion scheme in our block-structured fluid solver enables our code to handle significant changes in geometry. We compute turbulent statistics and determine the compound instabilities originated from the natural hydrodynamic instabilities and the inhibitor motion. The ultimate goal is to studdy the effect of inhibitor flexing on the turbulent field.

Effect of Turbulence Modeling on an Excited Jet

NASA Technical Reports Server (NTRS)

Brown, Clifford A.; Hixon, Ray

2010-01-01

The flow dynamics in a high-speed jet are dominated by unsteady turbulent flow structures in the plume. Jet excitation seeks to control these flow structures through the natural instabilities present in the initial shear layer of the jet. Understanding and optimizing the excitation input, for jet noise reduction or plume mixing enhancement, requires many trials that may be done experimentally or computationally at a significant cost savings. Numerical simulations, which model various parts of the unsteady dynamics to reduce the computational expense of the simulation, must adequately capture the unsteady flow dynamics in the excited jet for the results are to be used. Four CFD methods are considered for use in an excited jet problem, including two turbulence models with an Unsteady Reynolds Averaged Navier-Stokes (URANS) solver, one Large Eddy Simulation (LES) solver, and one URANS/LES hybrid method. Each method is used to simulate a simplified excited jet and the results are evaluated based on the flow data, computation time, and numerical stability. The knowledge gained about the effect of turbulence modeling and CFD methods from these basic simulations will guide and assist future three-dimensional (3-D) simulations that will be used to understand and optimize a realistic excited jet for a particular application.

Influence of Zostera marina canopies on unidirectional flow, hydraulic roughness and sediment movement

NASA Astrophysics Data System (ADS)

Lefebvre, A.; Thompson, C. E. L.; Amos, C. L.

2010-09-01

Seagrasses develop extensive or patchy underwater meadows in coastal areas around the world, forming complex, highly productive ecosystems. Seagrass canopies exert strong effects on water flow inside and around them, thereby affecting flow structure, sediment transport and benthic ecology. The influence of Zostera marina canopies on flow velocity, turbulence, hydraulic roughness and sediment movement was evaluated through laboratory experiments in 2 flumes and using live Z. marina and a mobile sand bed. Profiles of instantaneous velocities were measured and sediment movement was identified upstream, within and downstream of patches of different sizes and shoot density and at different free-stream velocities. Flow structure was characterised by time-averaged velocity, turbulence intensity and Turbulent Kinetic Energy (TKE). When velocity data were available above the canopy, they were fitted to the Law of the Wall and shear velocities and roughness lengths were calculated. When a seagrass canopy was present, three layers were distinguishable in the water column: (1) within canopy represented by low velocities and high turbulence; (2) transition zone around the height of the canopy, where velocities increased, turbulence decreased and TKE was high; and (3) above canopy where velocities were equal or higher than free-stream velocities and turbulence and TKE were lower than below. Shoot density and patch-width influenced this partitioning of the flow when the canopy was long enough (based on flume experiments, at least more than 1 m-long). The enhanced TKE observed at the canopy/water interface suggests that large-scale turbulence is generated at the canopy surface. These oscillations, likely to be related to the canopy undulations, are then broken down within the canopy and high-frequency turbulence takes place near the bed. This turbulence 'cascade' through the canopy may have an important impact on biogeochemical processes. The velocity above the canopy generally followed a logarithmic profile. Roughness lengths were higher above the canopy than over bare sand and increased with increasing distance from the leading edge of the canopy; however, they were still small (<1 cm) compared to other studies in the literature. Within and downstream of the canopy, sediment movement was observed at velocities below the threshold of motion. It was likely caused by the increased turbulence at those positions. This has large implications for sediment transport in coastal zones where seagrass beds develop.

Description and detection of burst events in turbulent flows

NASA Astrophysics Data System (ADS)

Schmid, P. J.; García-Gutierrez, A.; Jiménez, J.

2018-04-01

A mathematical and computational framework is developed for the detection and identification of coherent structures in turbulent wall-bounded shear flows. In a first step, this data-based technique will use an embedding methodology to formulate the fluid motion as a phase-space trajectory, from which state-transition probabilities can be computed. Within this formalism, a second step then applies repeated clustering and graph-community techniques to determine a hierarchy of coherent structures ranked by their persistencies. This latter information will be used to detect highly transitory states that act as precursors to violent and intermittent events in turbulent fluid motion (e.g., bursts). Used as an analysis tool, this technique allows the objective identification of intermittent (but important) events in turbulent fluid motion; however, it also lays the foundation for advanced control strategies for their manipulation. The techniques are applied to low-dimensional model equations for turbulent transport, such as the self-sustaining process (SSP), for varying levels of complexity.

PIV measurement of high-Reynolds-number homogeneous and isotropic turbulence in an enclosed flow apparatus with fan agitation

NASA Astrophysics Data System (ADS)

Dou, Zhongwang; Pecenak, Zachary K.; Cao, Lujie; Woodward, Scott H.; Liang, Zach; Meng, Hui

2016-03-01

Enclosed flow apparatuses with negligible mean flow are emerging as alternatives to wind tunnels for laboratory studies of homogeneous and isotropic turbulence (HIT) with or without aerosol particles, especially in experimental validation of Direct Numerical Simulation (DNS). It is desired that these flow apparatuses generate HIT at high Taylor-microscale Reynolds numbers ({{R}λ} ) and enable accurate measurement of turbulence parameters including kinetic energy dissipation rate and thereby {{R}λ} . We have designed an enclosed, fan-driven, highly symmetric truncated-icosahedron ‘soccer ball’ airflow apparatus that enables particle imaging velocimetry (PIV) and other whole-field flow measurement techniques. To minimize gravity effect on inertial particles and improve isotropy, we chose fans instead of synthetic jets as flow actuators. We developed explicit relations between {{R}λ} and physical as well as operational parameters of enclosed HIT chambers. To experimentally characterize turbulence in this near-zero-mean flow chamber, we devised a new two-scale PIV approach utilizing two independent PIV systems to obtain both high resolution and large field of view. Velocity measurement results show that turbulence in the apparatus achieved high homogeneity and isotropy in a large central region (48 mm diameter) of the chamber. From PIV-measured velocity fields, we obtained turbulence dissipation rates and thereby {{R}λ} by using the second-order velocity structure function. A maximum {{R}λ} of 384 was achieved. Furthermore, experiments confirmed that the root mean square (RMS) velocity increases linearly with fan speed, and {{R}λ} increases with the square root of fan speed. Characterizing turbulence in such apparatus paves the way for further investigation of particle dynamics in particle-laden homogeneous and isotropic turbulence.

Coherent Structures and Extreme Events in Rotating Multiphase Turbulent Flows

NASA Astrophysics Data System (ADS)

Biferale, L.; Bonaccorso, F.; Mazzitelli, I. M.; van Hinsberg, M. A. T.; Lanotte, A. S.; Musacchio, S.; Perlekar, P.; Toschi, F.

2016-10-01

By using direct numerical simulations (DNS) at unprecedented resolution, we study turbulence under rotation in the presence of simultaneous direct and inverse cascades. The accumulation of energy at large scale leads to the formation of vertical coherent regions with high vorticity oriented along the rotation axis. By seeding the flow with millions of inertial particles, we quantify—for the first time—the effects of those coherent vertical structures on the preferential concentration of light and heavy particles. Furthermore, we quantitatively show that extreme fluctuations, leading to deviations from a normal-distributed statistics, result from the entangled interaction of the vertical structures with the turbulent background. Finally, we present the first-ever measurement of the relative importance between Stokes drag, Coriolis force, and centripetal force along the trajectories of inertial particles. We discover that vortical coherent structures lead to unexpected diffusion properties for heavy and light particles in the directions parallel and perpendicular to the rotation axis.

Streamwise-Localized Solutions with natural 1-fold symmetry

NASA Astrophysics Data System (ADS)

Altmeyer, Sebastian; Willis, Ashley; Hof, Björn

2014-11-01

It has been proposed in recent years that turbulence is organized around unstable invariant solutions, which provide the building blocks of the chaotic dynamics. In direct numerical simulations of pipe flow we show that when imposing a minimal symmetry constraint (reflection in an axial plane only) the formation of turbulence can indeed be explained by dynamical systems concepts. The hypersurface separating laminar from turbulent motion, the edge of turbulence, is spanned by the stable manifolds of an exact invariant solution, a periodic orbit of a spatially localized structure. The turbulent states themselves (turbulent puffs in this case) are shown to arise in a bifurcation sequence from a related localized solution (the upper branch orbit). The rather complex bifurcation sequence involves secondary Hopf bifurcations, frequency locking and a period doubling cascade until eventually turbulent puffs arise. In addition we report preliminary results of the transition sequence for pipe flow without symmetry constraints.

Turbulence generation by waves

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kaftori, D.; Nan, X.S.; Banerjee, S.

The interaction between two-dimensional mechanically generated waves, and a turbulent stream was investigated experimentally in a horizontal channel, using a 3-D LDA synchronized with a surface position measuring device and a micro-bubble tracers flow visualization with high speed video. Results show that although the wave induced orbital motion reached all the way to the wall, the characteristics of the turbulence wall structures and the turbulence intensity close to the wall were not altered. Nor was the streaky nature of the wall layer. On the other hand, the mean velocity profile became more uniform and the mean friction velocity was increased.more » Close to the free surface, the turbulence intensity was substantially increased as well. Even in predominantly laminar flows, the introduction of 2-D waves causes three dimensional turbulence. The turbulence enhancement is found to be proportional to the wave strength.« less

Large-scale Density Structures in Magneto-rotational Disk Turbulence

NASA Astrophysics Data System (ADS)

Youdin, Andrew; Johansen, A.; Klahr, H.

2009-01-01

Turbulence generated by the magneto-rotational instability (MRI) is a strong candidate to drive accretion flows in disks, including sufficiently ionized regions of protoplanetary disks. The MRI is often studied in local shearing boxes, which model a small section of the disk at high resolution. I will present simulations of large, stratified shearing boxes which extend up to 10 gas scale-heights across. These simulations are a useful bridge to fully global disk simulations. We find that MRI turbulence produces large-scale, axisymmetric density perturbations . These structures are part of a zonal flow --- analogous to the banded flow in Jupiter's atmosphere --- which survives in near geostrophic balance for tens of orbits. The launching mechanism is large-scale magnetic tension generated by an inverse cascade. We demonstrate the robustness of these results by careful study of various box sizes, grid resolutions, and microscopic diffusion parameterizations. These gas structures can trap solid material (in the form of large dust or ice particles) with important implications for planet formation. Resolved disk images at mm-wavelengths (e.g. from ALMA) will verify or constrain the existence of these structures.

Numerical study of 3D flow structure near a cylinder piercing turbulent free-convection boundary layer on a vertical plate

NASA Astrophysics Data System (ADS)

Levchenya, A. M.; Smirnov, E. M.; Zhukovskaya, V. D.

2018-05-01

The present contribution covers RANS-based simulation of 3D flow near a cylinder introduced into turbulent vertical-plate free-convection boundary layer. Numerical solutions were obtained with a finite-volume Navier-Stokes code of second-order accuracy using refined grids. Peculiarities of the flow disturbed by the obstacle are analyzed. Cylinder-diameter effect on the horseshoe vortex size and its position is evaluated.

Modeling near-wall turbulent flows

NASA Astrophysics Data System (ADS)

Marusic, Ivan; Mathis, Romain; Hutchins, Nicholas

2010-11-01

The near-wall region of turbulent boundary layers is a crucial region for turbulence production, but it is also a region that becomes increasing difficult to access and make measurements in as the Reynolds number becomes very high. Consequently, it is desirable to model the turbulence in this region. Recent studies have shown that the classical description, with inner (wall) scaling alone, is insufficient to explain the behaviour of the streamwise turbulence intensities with increasing Reynolds number. Here we will review our recent near-wall model (Marusic et al., Science 329, 2010), where the near-wall turbulence is predicted given information from only the large-scale signature at a single measurement point in the logarithmic layer, considerably far from the wall. The model is consistent with the Townsend attached eddy hypothesis in that the large-scale structures associated with the log-region are felt all the way down to the wall, but also includes a non-linear amplitude modulation effect of the large structures on the near-wall turbulence. Detailed predicted spectra across the entire near- wall region will be presented, together with other higher order statistics over a large range of Reynolds numbers varying from laboratory to atmospheric flows.

Analysis and modeling of localized invariant solutions in pipe flow

NASA Astrophysics Data System (ADS)

Ritter, Paul; Zammert, Stefan; Song, Baofang; Eckhardt, Bruno; Avila, Marc

2018-01-01

Turbulent spots surrounded by laminar flow are a landmark of transitional shear flows, but the dependence of their kinematic properties on spatial structure is poorly understood. We here investigate this dependence in pipe flow for Reynolds numbers between 1500 and 5000. We compute spatially localized relative periodic orbits in long pipes and show that their upstream and downstream fronts decay exponentially towards the laminar profile. This allows us to model the fronts by employing the linearized Navier-Stokes equations, and the resulting model yields the spatial decay rate and the front velocity profiles of the periodic orbits as a function of Reynolds number, azimuthal wave number, and propagation speed. In addition, when applied to a localized turbulent puff, the model is shown to accurately approximate the spatial decay rate of its upstream and downstream tails. Our study provides insight into the relationship between the kinematics and spatial structure of localized turbulence and more generally into the physics of localization.

Remote visualization and scale analysis of large turbulence datatsets

NASA Astrophysics Data System (ADS)

Livescu, D.; Pulido, J.; Burns, R.; Canada, C.; Ahrens, J.; Hamann, B.

2015-12-01

Accurate simulations of turbulent flows require solving all the dynamically relevant scales of motions. This technique, called Direct Numerical Simulation, has been successfully applied to a variety of simple flows; however, the large-scale flows encountered in Geophysical Fluid Dynamics (GFD) would require meshes outside the range of the most powerful supercomputers for the foreseeable future. Nevertheless, the current generation of petascale computers has enabled unprecedented simulations of many types of turbulent flows which focus on various GFD aspects, from the idealized configurations extensively studied in the past to more complex flows closer to the practical applications. The pace at which such simulations are performed only continues to increase; however, the simulations themselves are restricted to a small number of groups with access to large computational platforms. Yet the petabytes of turbulence data offer almost limitless information on many different aspects of the flow, from the hierarchy of turbulence moments, spectra and correlations, to structure-functions, geometrical properties, etc. The ability to share such datasets with other groups can significantly reduce the time to analyze the data, help the creative process and increase the pace of discovery. Using the largest DOE supercomputing platforms, we have performed some of the biggest turbulence simulations to date, in various configurations, addressing specific aspects of turbulence production and mixing mechanisms. Until recently, the visualization and analysis of such datasets was restricted by access to large supercomputers. The public Johns Hopkins Turbulence database simplifies the access to multi-Terabyte turbulence datasets and facilitates turbulence analysis through the use of commodity hardware. First, one of our datasets, which is part of the database, will be described and then a framework that adds high-speed visualization and wavelet support for multi-resolution analysis of turbulence will be highlighted. The addition of wavelet support reduces the latency and bandwidth requirements for visualization, allowing for many concurrent users, and enables new types of analyses, including scale decomposition and coherent feature extraction.

Turbulence sources, character, and effects in the stable boundary layer: Insights from multi-scale direct numerical simulations and new, high-resolution measurements

NASA Astrophysics Data System (ADS)

Fritts, Dave; Wang, Ling; Balsley, Ben; Lawrence, Dale

2013-04-01

A number of sources contribute to intermittent small-scale turbulence in the stable boundary layer (SBL). These include Kelvin-Helmholtz instability (KHI), gravity wave (GW) breaking, and fluid intrusions, among others. Indeed, such sources arise naturally in response to even very simple "multi-scale" superpositions of larger-scale GWs and smaller-scale GWs, mean flows, or fine structure (FS) throughout the atmosphere and the oceans. We describe here results of two direct numerical simulations (DNS) of these GW-FS interactions performed at high resolution and high Reynolds number that allow exploration of these turbulence sources and the character and effects of the turbulence that arises in these flows. Results include episodic turbulence generation, a broad range of turbulence scales and intensities, PDFs of dissipation fields exhibiting quasi-log-normal and more complex behavior, local turbulent mixing, and "sheet and layer" structures in potential temperature that closely resemble high-resolution measurements. Importantly, such multi-scale dynamics differ from their larger-scale, quasi-monochromatic gravity wave or quasi-horizontally homogeneous shear flow instabilities in significant ways. The ability to quantify such multi-scale dynamics with new, very high-resolution measurements is also advancing rapidly. New in-situ sensors on small, unmanned aerial vehicles (UAVs), balloons, or tethered systems are enabling definition of SBL (and deeper) environments and turbulence structure and dissipation fields with high spatial and temporal resolution and precision. These new measurement and modeling capabilities promise significant advances in understanding small-scale instability and turbulence dynamics, in quantifying their roles in mixing, transport, and evolution of the SBL environment, and in contributing to improved parameterizations of these dynamics in mesoscale, numerical weather prediction, climate, and general circulation models. We expect such measurement and modeling capabilities to also aid in the design of new and more comprehensive future SBL measurement programs.

Modeling Coherent Structures in Canopy Flows

NASA Astrophysics Data System (ADS)

Luhar, Mitul

2017-11-01

It is well known that flows over vegetation canopies are characterized by the presence of energetic coherent structures. Since the mean profile over dense canopies exhibits an inflection point, the emergence of such structures is often attributed to a Kelvin-Helmholtz instability. However, though stability analyses provide useful mechanistic insights into canopy flows, they are limited in their ability to generate predictions for spectra and coherent structure. The present effort seeks to address this limitation by extending the resolvent formulation (McKeon and Sharma, 2010, J. Fluid Mech.) to canopy flows. Under the resolvent formulation, the turbulent velocity field is expressed as a superposition of propagating modes, identified via a gain-based (singular value) decomposition of the Navier-Stokes equations. A key advantage of this approach is that it reconciles multiple mechanisms that lead to high amplification in turbulent flows, including modal instability, transient growth, and critical-layer phenomena. Further, individual high-gain modes can be combined to generate more complete models for coherent structure and velocity spectra. Preliminary resolvent-based model predictions for canopy flows agree well with existing experiments and simulations.

On the structure of the turbulent vortex

NASA Technical Reports Server (NTRS)

Roberts, L.

1985-01-01

The trailing vortex generated by a lifting surface, the structure of its turbulent core and the influence of axial flow within the vortex on its initial persistence and on its subsequent decay are described. Similarity solutions of the turbulent diffusion equation are given in closed form and results are expressed in sufficiently simple terms that the influence of the lifting surface parameters on the length of persistence and the rate of decay of the vortex can be evaluated.

Study of the Structure of Turbulence in Accelerating Transitional Boundary Layers.

DTIC Science & Technology

1987-12-23

be sufficient to relaminarize even fully turbulent boundary layers. Since local heat transfer rates are very sensitive to the state of the boundary...was calibrated for velocity and angular sensitivity in a low- .’ turbulence 1 1/2-in. dia. jet flow for approximately twenty jet flow speeds "-’ ranging...intersection of the wires of the x. The angular sensitivity of the wires was assumed to conform to Champagne’s k2 law (Ref. 20), UE2 (0) = U2(0 = 0) (cos 2

Intermittency and Topology of Shock Induced Mixing

NASA Astrophysics Data System (ADS)

Tellez, Jackson; Redondo, Jose M.; Ben Mahjoub, Otman; Malik, Nadeem; Vila, Teresa

2016-04-01

The advance of a Rayleigh-Taylor front is described in Linden & Redondo (1991),[1-3] and may be shown to follow a quadratic law in time where the width of the growing region of instability depends on the local mixing efficiency of the different density fluids that accelerate against each other g is the acceleration and A is the Atwood number defined as the diference of densities divided by their sum. This results show the independence of the large amplitude structures on the initial conditions the width of the mixing region depends also on the intermittency of the turbulence. Then dimensional analysis may also depend on the relevant reduced acceleration driven time and the molecular reactive time akin to Damkholer number and the fractal structure of the contact zone [2,4]. Detailed experiments and simulations on RT and RM shock induced fronts analized with respect to structure functions are able to determine which mechanisms are most effective in local mixing which increase the effective fractal dimension, as well as the effect of higher order geometrical parameters, such as the structure functions, in non-homogeneous fluids (Mahjoub et al 1998)[5]. The structure of a Mixing blob shows a relatively sharp head with most of the mixing taking place at the sides due to what seems to be shear instability very similar to the Kelvin-Helmholtz instabilities, but with sideways accelerations. The formation of the blobs and spikes with their secondary instabilities produces a turbulent cascade, evident just after about 1 non-dimensional time unit, from a virtual time origin that takes into account the linear growth phase, as can be seen by the growth of the fractal dimension for different volume fractions. Two-dimensional cuts of the 3D flow also show that vortex flows have closed or spiral streamlines around their core. Examples of such flows can be also seen in the laboratory, for example at the interface of atwo-layer stratified fluid in a tank in which case streamlines are more regular. Mixing in turbulent flows remains less well understood, and in spite research some basic problems are still virtually unexplored. Th e indications suggest that mixing in non-decaying and accelerating turbulent flows are different from those in vortical and steady flows. Fluid element pairs separate, neither linearly nor exponentially but according to a generalized intermittent Richardson's law. Fractal analysis in the laboratory shows that fluid element pairs travel close to each other for a long time until they separate quite suddenly suggest that straining regions around hyperbolic points play an important role in the violent turbulent stirring and in the mechanisms by which turbulence causes fluid element pairs to move apart [6,7]. So the eddies that are most effective in separating fluid elements are those that have a size comparable to the instantaneous separation between the two fluid elements. This is seen in both RT and RM instabilities. For a constant acceleration, the RT instability is found to grow self -similarly according to mixing coefficients which when measured over a comprehensive range of density ratio (Atwood nubers)show that the results are found applicable to supernova exlposions.For an impulsive acceleration (RM), there are two components. The RM impulse from a shock is greatly reduced at high Mach number due to compressive effects in reasonable agreement with linear theory. The ensuing motion is essentially incompressible and described by a power law However, the exponents obtained from the compressible RM experiments are larger than those obtained from incompressible RT experiments. The discrepancy is not well understood but intermittency differences could explain the role of compressibility in fractal media. [1] Linden P.F., Redondo J.M. and Youngs D. (1994) Molecular mixing in Rayleigh-Taylor Instability. Jour. Fluid Mech. 265, 97-124. [2] Redondo, J.M., 1990. The structure of density interfaces. Ph.D. Thesis. DAMTP, University of Cambridge. Cambridge [3] Redondo J.M. (1996) Vertical microstructure and mixing in stratified flows. Advances in Turbulence VI. Eds. S. Gavrilakis et al. 605-608. [4] Redondo J.M.,M.A. Sanchez y R. Castilla (2000) Vortical structures in stratified turbulent flows, Turbulent diffusion in the environment. Eds. Redondo J.M. and Babiano A. 113-120. [5] Mahjoub, O. B., Babiano A. and Redondo, J. M.: Structure functions in complex flows, Flow, Turbulence and Combustion, 59,299-313, 1998. [6] Malik, N.A. Vassilicos, J.C. 1999 A Lagrangian model of turbulent dispersion with turbulent-like flow structure: comparison with direct numerical simulation for two-particle statistics. Phys. Fluids, 11, 1572-1580. [7] Fung, J.C.H., Hunt, J.C.R., Malik, N.A. and Perkins, R.J.(1992. Kinematic simulation of homogeneous turbulence by unsteady random Fourier modes. J. Fluid Mech.236-281. [8] Tarquis, A. M., Platonov, A., Matulka, A., Grau, J., Sekula, E., Diez, M., & Redondo, J. M. (2014). Application of multifractal analysis to the study of SAR features and oil spills on the ocean surface. Nonlinear Processes in Geophysics, 21(2), 439-450. [9] Fraunie, P., Berreba, S., Chashechkin, Y. D., Velasco, D., & Redondo, J. M. (2008). Large eddy simulation and laboratory experiments on the decay of grid wakes in strongly stratified flows. Nuovo Cimento C, 31, 909-930.

Hierarchy compensation of non-homogeneous intermittent atmospheric turbulence

NASA Astrophysics Data System (ADS)

Redondo, Jose M.; Mahjoub, Otman B.; Cantalapiedra, Inma R.

2010-05-01

In this work a study both the internal turbulence energy cascade intermittency evaluated from wind speed series in the atmospheric boundary layer, as well as the role of external or forcing intermittency based on the flatness (Vindel et al 2008)is carried out. The degree of intermittency in the stratified ABL flow (Cuxart et al. 2000) can be studied as the deviation, from the linear form, of the absolute scaling exponents of the structure functions as well as generalizing for non-isotropic and non-homogeneous turbulence, even in non-inertial ranges (in the Kolmogorov-Kraichnan sense) where the scaling exponents are not constant. The degree of intermittency, evaluated in the non-local quasi-inertial range, is explained from the variation with scale of the energy transfer as well as the dissipation. The scale to scale transfer and the structure function scaling exponents are calculated and from these the intermittency parametres. The turbulent diffusivity could also be estimated and compared with Richardson's law. Some two point correlations and time lag calculations are used to investigate the time and spatial integral length scales obtained from both Lagrangian and Eulerian correlations and functions, and we compare these results with both theoretical and laboratory data. We develop a theoretical description of how to measure the different levels of intermittency following (Mahjoub et al. 1998, 2000) and the role of locality in higher order exponents of structure function analysis. Vindel J.M., Yague C. and Redondo J.M. (2008) Structure function analysis and intermittency in the ABL. Nonlin. Processes Geophys., 15, 915-929. Cuxart J, Yague C, Morales G, Terradellas E, Orbe J, Calvo J, Fernández A, Soler M R, Infante C, Buenestado P, Espinalt A, Joergensen H E, Rees J M, Vilá J, Redondo J M, Cantalapiedra R and Conangla L (2000): Stable atmospheric boundary-layer experiment in Spain (Sables 98): a report, Boundary-Layer Meteorology 96, 337-370 Mahjoub O.B., Babiano A. and Redondo J.M. (1998) Structure functions in complex flows. Journal of Flow Turbulence and Combustion. 59, 299-313. Mahjoub O.B., Redondo J.M. and Babiano A. (2000) Self similarity and intermittency in a turbulent non-homogeneous wake. Proceedings of the Eighth European Turbulence Conference. (Eds. Dopazo et al.) CIMNE, Barcelona, 783-786. Mahjoub O.B., Redondo J.M., and R. Alami, (1998) Turbulent Structure Functions in Geophysical Flows, Rapp. Comm. int. Mer Medit., 35, 126-127.

Statistical features of rapidly rotating decaying turbulence: Enstrophy and energy spectra and coherent structures

NASA Astrophysics Data System (ADS)

Sharma, Manohar K.; Kumar, Abhishek; Verma, Mahendra K.; Chakraborty, Sagar

2018-04-01

In this paper, we investigate the properties of rapidly rotating decaying turbulence using numerical simulations and phenomenological modeling. We find that as the turbulent flow evolves in time, the Rossby number decreases to ˜10-3, and the flow becomes quasi-two-dimensional with strong coherent columnar structures arising due to the inverse cascade of energy. We establish that a major fraction of energy is confined in Fourier modes (±1, 0, 0) and (0, ±1, 0) that correspond to the largest columnar structure in the flow. For wavenumbers (k) greater than the enstrophy dissipation wavenumber (kd), our phenomenological arguments and numerical study show that the enstrophy flux and spectrum of a horizontal cross section perpendicular to the axis of rotation are given by ɛω⁡exp (-C (k/kd ) 2 ) and C ɛω2 /3k-1⁡exp (-C (k/kd ) 2 ) , respectively; for this 2D flow, ɛω is the enstrophy dissipation rate, and C is a constant. Using these results, we propose a new form for the energy spectrum of rapidly rotating decaying turbulence: E (k ) =C ɛω2 /3k-3⁡exp (-C (k/kd ) 2 ) . This model of the energy spectrum is based on wavenumber-dependent enstrophy flux, and it deviates significantly from power law energy spectrum reported earlier.

Confined Turbulent Swirling Recirculating Flow Predictions. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Abujelala, M. T.

1984-01-01

Turbulent swirling flow, the STARPIC computer code, turbulence modeling of turbulent flows, the k-xi turbulence model and extensions, turbulence parameters deduction from swirling confined flow measurements, extension of the k-xi to confined swirling recirculating flows, and general predictions for confined turbulent swirling flow are discussed.

Computation of Large-Scale Structure Jet Noise Sources With Weak Nonlinear Effects Using Linear Euler

NASA Technical Reports Server (NTRS)

Dahl, Milo D.; Hixon, Ray; Mankbadi, Reda R.

2003-01-01

An approximate technique is presented for the prediction of the large-scale turbulent structure sound source in a supersonic jet. A linearized Euler equations code is used to solve for the flow disturbances within and near a jet with a given mean flow. Assuming a normal mode composition for the wave-like disturbances, the linear radial profiles are used in an integration of the Navier-Stokes equations. This results in a set of ordinary differential equations representing the weakly nonlinear self-interactions of the modes along with their interaction with the mean flow. Solutions are then used to correct the amplitude of the disturbances that represent the source of large-scale turbulent structure sound in the jet.

A minimal model of self-sustaining turbulence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Thomas, Vaughan L.; Gayme, Dennice F.; Farrell, Brian F.

2015-10-15

In this work, we examine the turbulence maintained in a Restricted Nonlinear (RNL) model of plane Couette flow. This model is a computationally efficient approximation of the second order statistical state dynamics obtained by partitioning the flow into a streamwise averaged mean flow and perturbations about that mean, a closure referred to herein as the RNL{sub ∞} model. The RNL model investigated here employs a single member of the infinite ensemble that comprises the covariance of the RNL{sub ∞} dynamics. The RNL system has previously been shown to support self-sustaining turbulence with a mean flow and structural features that aremore » consistent with direct numerical simulations (DNS). Regardless of the number of streamwise Fourier components used in the simulation, the RNL system’s self-sustaining turbulent state is supported by a small number of streamwise varying modes. Remarkably, further truncation of the RNL system’s support to as few as one streamwise varying mode can suffice to sustain the turbulent state. The close correspondence between RNL simulations and DNS that has been previously demonstrated along with the results presented here suggest that the fundamental mechanisms underlying wall-turbulence can be analyzed using these highly simplified RNL systems.« less

Effect of Swirl on Turbulent Structures in Supersonic Jets

NASA Technical Reports Server (NTRS)

Rao, Ram Mohan; Lundgren, Thomas S.

1998-01-01

Direct Numerical Simulation (DNS) is used to study the mechanism of generation and evolution of turbulence structures in a temporally evolving supersonic swirling round jet and also to examine the resulting acoustic radiations. Fourier spectral expansions are used in the streamwise and azimuthal directions and a 1-D b-spline Galerkin representation is used in the radial direction. Spectral-like accuracy is achieved using this numerical scheme. Direct numerical simulations, using the b-spline spectral method, are carried out starting from mean flow initial conditions which are perturbed by the most unstable linear stability eigenfunctions. It is observed that the initial helical instability waves evolve into helical vortices which eventually breakdown into smaller scales of turbulence. 'Rib' structures similar to those seen in incompressible mixing layer flow of Rogers and Moserl are observed. The jet core breakdown stage exhibits increased acoustic radiations.

Large Scale Turbulent Structures in Supersonic Jets

NASA Technical Reports Server (NTRS)

Rao, Ram Mohan; Lundgren, Thomas S.

1997-01-01

Jet noise is a major concern in the design of commercial aircraft. Studies by various researchers suggest that aerodynamic noise is a major contributor to jet noise. Some of these studies indicate that most of the aerodynamic jet noise due to turbulent mixing occurs when there is a rapid variation in turbulent structure, i.e. rapidly growing or decaying vortices. The objective of this research was to simulate a compressible round jet to study the non-linear evolution of vortices and the resulting acoustic radiations. In particular, to understand the effect of turbulence structure on the noise. An ideal technique to study this problem is Direct Numerical Simulations(DNS), because it provides precise control on the initial and boundary conditions that lead to the turbulent structures studied. It also provides complete 3-dimensional time dependent data. Since the dynamics of a temporally evolving jet are not greatly different from those, of a spatially evolving jet, a temporal jet problem was solved, using periodicity ill the direction of the jet axis. This enables the application of Fourier spectral methods in the streamwise direction. Physically this means that turbulent structures in the jet are repeated in successive downstream cells instead of being gradually modified downstream into a jet plume. The DNS jet simulation helps us understand the various turbulent scales and mechanisms of turbulence generation in the evolution of a compressible round jet. These accurate flow solutions will be used in future research to estimate near-field acoustic radiation by computing the total outward flux across a surface and determine how it is related to the evolution of the turbulent solutions. Furthermore, these simulations allow us to investigate the sensitivity of acoustic radiations to inlet/boundary conditions, with possible application to active noise suppression. In addition, the data generated can be used to compute various turbulence quantities such as mean velocities, turbulent stresses, etc. which will aid in turbulence modeling. This report will be presented in two chapters. The first chapter describes some work on the linear stability of a supersonic round jet and the implications of this for the jet noise problem. The second chapter is an extensive discussion of numerical work using the spectral method which we use to solve the compressible Navier-Stokes equations to study turbulent jet flows. The method uses Fourier expansions in the azimuthal and streamwise direction and a 1-D B-spline basis representation in the radial direction. The B-spline basis is locally supported and this ensures block diagonal matrix equations which can be solved in O(N) steps. This is a modification of a boundary layer code developed by Robert Moser. A very accurate highly resolved Direct Numerical Simulation (DNS) of a turbulent jet flow is produced.

Large Scale Turbulent Structures in Supersonic Jets

NASA Technical Reports Server (NTRS)

Rao, Ram Mohan; Lundgren, Thomas S.

1997-01-01

Jet noise is a major concern in the design of commercial aircraft. Studies by various researchers suggest that aerodynamic noise is a major contributor to jet noise. Some of these studies indicate that most of the aerodynamic jet noise due to turbulent mixing occurs when there is a rapid variation in turbulent structure, i.e. rapidly growing or decaying vortices. The objective of this research was to simulate a compressible round jet to study the non-linear evolution of vortices and the resulting acoustic radiations. In particular, to understand the effect of turbulence structure on the noise. An ideal technique to study this problem is Direct Numerical Simulations (DNS), because it provides precise control on the initial and boundary conditions that lead to the turbulent structures studied. It also provides complete 3-dimensional time dependent data. Since the dynamics of a temporally evolving jet are not greatly different from those of a spatially evolving jet, a temporal jet problem was solved, using periodicity in the direction of the jet axis. This enables the application of Fourier spectral methods in the streamwise direction. Physically this means that turbulent structures in the jet are repeated in successive downstream cells instead of being gradually modified downstream into a jet plume. The DNS jet simulation helps us understand the various turbulent scales and mechanisms of turbulence generation in the evolution of a compressible round jet. These accurate flow solutions will be used in future research to estimate near-field acoustic radiation by computing the total outward flux across a surface and determine how it is related to the evolution of the turbulent solutions. Furthermore, these simulations allow us to investigate the sensitivity of acoustic radiations to inlet/boundary conditions, with possible appli(,a- tion to active noise suppression. In addition, the data generated can be used to compute, various turbulence quantities such as mean velocities, turbulent stresses, etc. which will aid in turbulence modeling. This report will be presented in two chapters. The first chapter describes some work on the linear stability of a supersonic round jet and the implications of this for the jet noise problem. The second chapter is an extensive discussion of numerical work using the spectral method which we use to solve the compressible Navier-Stokes equations to study turbulent jet flows. The method uses Fourier expansions in the azimuthal and streamwise direction and a 1-D B-spline basis representation in the radial direction. The B-spline basis is locally supported and this ensures block diagonal matrix equations which can be solved in O(N) steps. This is a modification of a boundary layer code developed by Robert Moser. A very accurate highly resolved DNS of a turbulent jet flow is produced.

Aspects of turbulent-shear-layer dynamics and mixing

NASA Astrophysics Data System (ADS)

Slessor, Michael David

Experiments have been conducted in the GALCIT Supersonic Shear Layer Facility to investigate some aspects of high-Reynolds-number, turbulent, shearlayer flows in both incompressible- and compressible-flow regimes. Experiments designed to address several issues were performed; effects of inflow boundary conditions, freestream conditions (supersonic/subsonic flow), and compressibility, on both large-scale dynamics and small-scale mixing, are described. Chemically-reacting and non-reacting flows were investigated, the former relying on the (H2 + NO/F2) chemical system, in the fast-kinetic regime, to infer the structure and amount of molecular-scale mixing through use of "flip" experiments. A variety of experimental techniques, including a color-schlieren visualization system developed as part of this work, were used to study the flows. Both inflow conditions and compressibility are found to have significant effects on the flow. In particular, inflow conditions are "remembered" for long distances downstream, a sensitivity similar to that observed in low-dimensionality, non-linear (chaotic) systems. The global flowfields (freestreams coupled by the shear layer) of transonic flows exhibit a sensitivity to imposed boundary conditions, i. e., local area ratios. A previously-proposed mode-selection rule for turbulent-structure convection speeds, based on the presence of a lab-frame subsonic freestream, was experimentally demonstrated to be incorrect. Compressibility, when decoupled from all other parameters, e.g., Reynolds number, velocity and density ratios, etc., reduces laxge-scale entrainment and turbulent growth, but slightly enhances smallscale mixing, with an associated change in the structure of the molecularly-mixed fluid. This reduction in shear-layer growth rate is examined and a new parameter that interprets compressibility as an energy-exchange mechanism is proposed. The parameter reconciles and collapses experimentally-observed growth rates.

Aspects of turbulent-shear-layer dynamics and mixing

NASA Astrophysics Data System (ADS)

Slessor, Michael David

Experiments have been conducted in the GALCIT Supersonic Shear Layer Facility to investigate some aspects of high-Reynolds-number, turbulent, shear-layer flows in both incompressible- and compressible-flow regimes. Experiments designed to address several issues were performed; effects of inflow boundary conditions, freestream conditions (supersonic/subsonic flow), and compressibility, on both large-scale dynamics and small-scale mixing, are described. Chemically-reacting and non-reacting flows were investigated, the former relying on the (Hsb2 + NO)/Fsb2 chemical system, in the fast-kinetic regime, to infer the structure and amount of molecular-scale mixing through use of "flip" experiments. A variety of experimental techniques, including a color-schlieren visualization system developed as part of this work, were used to study the flows. Both inflow conditions and compressibility are found to have significant effects on the flow. In particular, inflow conditions are "remembered" for long distances downstream, a sensitivity similar to that observed in low-dimensionality, non-linear (chaotic) systems. The global flowfields (freestreams coupled by the shear layer) of transonic flows exhibit a sensitivity to imposed boundary conditions, a.e., local area ratios. A previously-proposed mode-selection rule for turbulent-structure convection speeds, based on the presence of a lab-frame subsonic freestream, was experimentally demonstrated to be incorrect. Compressibility, when decoupled from ail other parameters, e.g., Reynolds number, velocity and density ratios, etc., reduces large-scale entrainment and turbulent growth, but slightly enhances small-scale mixing, with an associated change in the structure of the molecularly-mixed fluid. This reduction in shear-layer growth rate is examined and a new parameter that interprets compressibility as an energy-exchange mechanism is proposed. The parameter reconciles and collapses experimentally-observed growth rates.

A Comparison of Hybrid Reynolds Averaged Navier Stokes/Large Eddy Simulation (RANS/LES) and Unsteady RANS Predictions of Separated Flow for a Variable Speed Power Turbine Blade Operating with Low Inlet Turbulence Levels

DTIC Science & Technology

2017-10-01

Facility is a large-scale cascade that allows detailed flow field surveys and blade surface measurements.10–12 The facility has a continuous run ...structured grids at 2 flow conditions, cruise and takeoff, of the VSPT blade . Computations were run in parallel on a Department of Defense...RANS/LES) and Unsteady RANS Predictions of Separated Flow for a Variable-Speed Power- Turbine Blade Operating with Low Inlet Turbulence Levels

Effects of energetic coherent motions on the power and wake of an axial-flow turbine

NASA Astrophysics Data System (ADS)

Chamorro, L. P.; Hill, C.; Neary, V. S.; Gunawan, B.; Arndt, R. E. A.; Sotiropoulos, F.

2015-05-01

A laboratory experiment examined the effects of energetic coherent motions on the structure of the wake and power fluctuations generated by a model axial-flow hydrokinetic turbine. The model turbine was placed in an open-channel flow and operated under subcritical conditions. The incoming flow was locally perturbed with vertically oriented cylinders of various diameters. An array of three acoustic Doppler velocimeters aligned in the cross-stream direction and a torque transducer were used to collect high-resolution and synchronous measurements of the three-velocity components of the incoming and wake flow as well as the turbine power. A strong scale-to-scale interaction between the large-scale and broadband turbulence shed by the cylinders and the turbine power revealed how the turbulence structure modulates the turbine behavior. In particular, the response of the turbine to the distinctive von Kármán-type vortices shed from the cylinders highlighted this phenomenon. The mean and fluctuating characteristics of the turbine wake are shown to be very sensitive to the energetic motions present in the flow. Tip vortices were substantially dampened and the near-field mean wake recovery accelerated in the presence of energetic motions in the flow. Strong coherent motions are shown to be more effective than turbulence levels for triggering the break-up of the spiral structure of the tip-vortices.

Effects of Buoyancy on Lean Premixed V-Flames Part I: Laminar and Turblent Flame Structure

NASA Technical Reports Server (NTRS)

Cheng, Robert K.; Bedat, Benoit; Kostiuk, Larry W.

1998-01-01

Laser schlieren and planar laser-induced fluorescence techniques have been used to investigate laminar and turbulent v-flames in +g, -g, and micro g under flow conditions that span the regimes of momentum domination (Ri < 0. 1) and buoyancy domination (Ri > 0.1). Overall flame features shown by schlieren indicate that buoyancy dominates the entire flow field for conditions close to Ri = 1. With decreasing Ri, buoyancy effects are observed only in the far-field regions. Analyses of the mean flame angles demonstrate that laminar and turbulent flames do not have similar responses to buoyancy. Difference in the laminar +g and -g flame angles decrease with Ri (i.e., increasing Re) and converge to the microgravity flame angle at the momentum limit (Ri - 0). This is consistent with the notion that the effects of buoyancy diminish with increasing flow momentum. The +g and -g turbulent flame angles, however, do not converge at Ri = 0. As shown by OH-PLIF images, the inconsistency in +g and -g turbulent flame angles is associated with the differences in flame wrinkles. Turbulent flame wrinkles evolve more slowly in +g than in -g. The difference in flame wrinkle structures, however, cannot be explained in terms of buoyancy effects on flame instability mechanisms. It seems to be associated with the field effects of buoyancy that stretches the turbulent flame brushes in +g and compresses the flame brush in -g. Flame wrinkling offers a mechanism through which the flame responds to the field effects of buoyancy despite increasing flow momentum. These observations point to the need to include both upstream and downstream contributions in theoretical analysis of flame turbulence interactions.

Dynamics of intrinsic axial flows in unsheared, uniform magnetic fields

DOE Office of Scientific and Technical Information (OSTI.GOV)

Li, J. C.; Diamond, P. H.; Xu, X. Q.

2016-05-15

A simple model for the generation and amplification of intrinsic axial flow in a linear device, controlled shear decorrelation experiment, is proposed. This model proposes and builds upon a novel dynamical symmetry breaking mechanism, using a simple theory of drift wave turbulence in the presence of axial flow shear. This mechanism does not require complex magnetic field structure, such as shear, and thus is also applicable to intrinsic rotation generation in tokamaks at weak or zero magnetic shear, as well as to linear devices. This mechanism is essentially the self-amplification of the mean axial flow profile, i.e., a modulational instability.more » Hence, the flow development is a form of negative viscosity phenomenon. Unlike conventional mechanisms where the residual stress produces an intrinsic torque, in this dynamical symmetry breaking scheme, the residual stress induces a negative increment to the ambient turbulent viscosity. The axial flow shear is then amplified by this negative viscosity increment. The resulting mean axial flow profile is calculated and discussed by analogy with the problem of turbulent pipe flow. For tokamaks, the negative viscosity is not needed to generate intrinsic rotation. However, toroidal rotation profile gradient is enhanced by the negative increment in turbulent viscosity.« less

The Slug and Churn Turbulence Characteristics of Oil-Gas-Water Flows in a Vertical Small Pipe

NASA Astrophysics Data System (ADS)

Liu, Weixin; Han, Yunfeng; Wang, Dayang; Zhao, An; Jin, Ningde

2017-08-01

The intention of the present study was to investigate the slug and churn turbulence characteristics of a vertical upward oil-gas-water three-phase flow. We firstly carried out a vertical upward oil-gas-water three-phase flow experiment in a 20-mm inner diameter (ID) pipe to measure the fluctuating signals of a rotating electric field conductance sensor under different flow patterns. Afterwards, typical flow patterns were identified with the aid of the texture structures in a cross recurrence plot. Recurrence quantitative analysis and multi-scale cross entropy (MSCE) algorithms were applied to investigate the turbulence characteristics of slug and churn flows with the varying flow parameters. The results suggest that with cross nonlinear analysis, the underlying dynamic characteristics in the evolution from slug to churn flow can be well understood. The present study provides a novel perspective for the analysis of the spatial-temporal evolution instability and complexity in oil-gas-water three-phase flow.

Scalar gradient trajectory measurements using high-frequency cinematographic planar Rayleigh scattering

NASA Astrophysics Data System (ADS)

Gampert, Markus; Narayanaswamy, Venkat; Peters, Norbert

2013-12-01

In this work, we perform an experimental investigation into statistics based on scalar gradient trajectories in a turbulent jet flow, which have been suggested as an alternative means to analyze turbulent flow fields by Wang and Peters (J Fluid Mech 554:457-475, 2006, 608:113-138, 2008). Although there are several numerical simulations and theoretical works that investigate the statistics along gradient trajectories, only few experiments in this area have been reported. To this end, high-frequency cinematographic planar Rayleigh scattering imaging is performed at different axial locations of a turbulent propane jet issuing into a CO2 coflow at nozzle-based Reynolds numbers Re 0 = 3,000-8,600. Taylor's hypothesis is invoked to obtain a three-dimensional reconstruction of the scalar field in which then the corresponding scalar gradient trajectories can be computed. These are then used to examine the local structure of the mixture fraction with a focus on the scalar turbulent/non-turbulent interface. The latter is a layer that is located between the fully turbulent part of the jet and the outer flow. Using scalar gradient trajectories, we partition the turbulent scalar field into these three regions according to an approach developed by Mellado et al. (J Fluid Mech 626:333-365, 2009). Based on the latter, we investigate the probability to find the respective regions as a function of the radial distance to the centerline, which turns out to reveal the meandering nature of the scalar T/NT interface layer as well as its impact on the local structure of the turbulent scalar field.

FLOW STRUCTURE AND TURBULENT DIFFUSION AROUND A THREE-DIMENSIONAL HILL. FLUID MODELING STUDY ON EFFECTS OF STRATIFICATION. PART I. FLOW STRUCTURE

EPA Science Inventory

This research program was initiated with the overall objective of gaining understanding of the flow and diffusion of pollutants in complex terrain under both neutral and stably stratified conditions. This report covers the first phase of the project; it describes the flow structu...

Visualization of the structural response of a hypersonic turbulent boundary layer to convex curvature

NASA Astrophysics Data System (ADS)

Humble, R. A.; Peltier, S. J.; Bowersox, R. D. W.

2012-10-01

The effects of convex curvature on the outer structure of a Mach 4.9 turbulent boundary layer (Reθ = 4.7 × 104) are investigated using condensate Rayleigh scattering and analyzed using spatial correlations, intermittency, and fractal theory. It is found that the post-expansion boundary layer structure morphology appears subtle, but certain features exhibit a more obvious response. The large-scale flow structures survive the initial expansion, appearing to maintain the same physical size. However, due to the nature of the expansion fan, a differential acceleration effect takes place across the flow structures, causing them to be reoriented, leaning farther away from the wall. The onset of intermittency moves closer towards the boundary layer edge and the region of intermittent flow decreases. It is likely that this reflects the less frequent penetration of outer irrotational fluid into the boundary layer, consistent with a boundary layer that is losing its ability to entrain freestream fluid. The fractal dimension of the turbulent/nonturbulent interface decreases with increasing favorable pressure gradient, indicating that the interface's irregularity decreases. Because fractal scale similarity does not encompass the largest scales, this suggests that the change in fractal dimension is due to the action of the smaller-scales, consistent with the idea that the small-scale flow structures are quenched during the expansion in response to bulk dilatation.

Thermal non-equilibrium effect of small-scale structures in compressible turbulence

NASA Astrophysics Data System (ADS)

Li, Shi-Yi; Li, Qi-Bing

2018-05-01

The thermal non-equilibrium effect of the small-scale structures in the canonical two-dimensional turbulence is studied. Comparative studies of Unified Gas Kinetic Scheme (UGKS) and GKS-Navier-Stokes (NS) for Taylor-Green flow with initial Ma = 1, Kn = 0.01 and decaying isotropic turbulence with initial Mat = 1, Reλ = 20 show that the discrepancy exists both in small and large scales, even beyond the dissipation range to 10η with accuracy to 8% in the SGS energy transfer of the decaying isotropic turbulence, illustrating the necessity for resolving the kinetic scales even at moderated Reλ = 20.

Environmental turbulence and climate-weather scaling

NASA Astrophysics Data System (ADS)

Ben Mahjoub, Otman; Cherubini, Claudia; Jebbad, Raghda; Mosso, Cessar; Benjamin, Juan Jose; Jorge, Joan; Diez, Margarita; Redondo, Jose M.

2017-04-01

Climate changes in Harbours, coastal areas and ROFI are key to Environmental flows. Ocean and Atmospheric turbulence is an energetic, eddying state of motion that disperses material at rates far higher than those of molecular processes alone; The role of intermittency and understanding of how turbulence is modified at Climatic and Weather scales in shallow seas, the deep ocean, and in the mixed layers is of great importance and practical applications. The larger-scale and time coherent structures associated with large Stommel diagram processes akin to turbulence that also have intermittency. With the aid of remote sensing we also use surface signatures[1,2] that can be detected and used to infer ocean parameters. Such effects dominate mesoscale vorticity, the role of Rossby deformation radius, Spiral eddies, convective cells, or the spacing of Langmuir turbulence, related to the depth of the mixed layer, or to cloud tops. The dominant instability processes can generate different intermittency , detected often as bursts or in variations in the scale to scale transfer of turbulence. We include climatic scales where Extended Self Simmilarity is used also in these scales in a fractal way. Global experiments, even with a wide range of new configurations are possible[3-6]. Such complex flows are known to generate nonequilbrium and non-local turbulence which produces different turbulence properties and varying intermittency. Applications to enhanced mixing and drag reduction are still being investigated [6, 7], and how do the turbulence and mixing properties change in Lagrangian and Eulerian descriptors with generalized Rayleigh, Rossby, Richardson and Reynolds numbers? in complex Poincare like, parameter spaces. [1]. Redondo J.M., Mixing efficiencies of different kinds of turbulent processes and instabilities, Applications to the environment in Turbulent mixing in geophysical flows. Eds. Linden P.F. and Redondo J.M. 131-157. 2002. [2]. Ben Mahjoub, Redondo J.M.and Babiano A. Structure functions in complex flows . Applied Scientific Research 59, 299.1998. [3]. Castilla R., Onate E. and Redondo J.M. Models, Experiments and Computations in Turbulence. CIMNE, Barcelona. 2007. P. 255. [4]. Nicolleau, F.C.G.A.; Cambon, C.; Redondo, J.M.; Vassilicos, J.C.; Reeks, M.; Nowakowski,A.F. (Eds.)(2012) New Approaches in Modeling Multiphase Flows and Dispersion in Turbulence, Fractal Methods and Synthetic Turbulence. ERCOFTAC Series. [5]. Fraunie P., Berreba S. Chashechkin Y., Velasco D. and Redondo J.M. (2008) LES and laboratory experiments on the decay of grid wakes in strongly stratied fows. Il Nuovo Cimento 31, 909-930 [6]. Gonzlez-Nieto, P., Cano J.L., and J. M. Redondo. (2008) Buoyant Mixing Processes Generated in Turbulent Plume Arrays. Fsica de la Tierra 19, 2008: 205-217. [7]. Redondo J.M. and Babiano A.: Turbulent Diffusion in the Environment, 2001, Fragma, Madrid.

Experimental investigation of supersonic flow over elliptic surface

NASA Astrophysics Data System (ADS)

Zhang, Qinghu; Yi, Shihe; He, Lin; Zhu, Yangzhu; Chen, Zhi

2013-11-01

The coherent structures of flow over a compression elliptic surface are experimentally investigated in a supersonic low-noise wind tunnel at Mach Number 3 using nano-tracer planar laser scattering (NPLS) and particle image velocimetry (PIV) techniques. High spacial resolution images and the average velocity profiles of both laminar inflow and turbulent inflow over the testing model were captured. From statistically significant ensembles, spatial correlation analysis of both cases is performed to quantify the mean size and orientation of large structures. The results indicate that the mean structure is elliptical in shape and structure angles in separated region of laminar inflow are slightly smaller than that of turbulent inflow. Moreover, the structure angle of both cases increases with its distance away from from the wall. POD analysis of velocity and vorticity fields is performed for both cases. The energy portion of the first mode for the velocity data is much larger than that for the vorticity field. For vorticity decompositions, the contribution from the first mode for the laminar inflow is slightly larger than that for the turbulent inflow and the cumulative contributions for laminar inflow converges slightly faster than that for turbulent inflow

Structure of turbulence in three-dimensional boundary layers

NASA Technical Reports Server (NTRS)

Subramanian, Chelakara S.

1993-01-01

This report provides an overview of the three dimensional turbulent boundary layer concepts and of the currently available experimental information for their turbulence modeling. It is found that more reliable turbulence data, especially of the Reynolds stress transport terms, is needed to improve the existing modeling capabilities. An experiment is proposed to study the three dimensional boundary layer formed by a 'sink flow' in a fully developed two dimensional turbulent boundary layer. Also, the mean and turbulence field measurement procedure using a three component laser Doppler velocimeter is described.

Gradient-driven flux-tube simulations of ion temperature gradient turbulence close to the non-linear threshold

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peeters, A. G.; Rath, F.; Buchholz, R.

2016-08-15

It is shown that Ion Temperature Gradient turbulence close to the threshold exhibits a long time behaviour, with smaller heat fluxes at later times. This reduction is connected with the slow growth of long wave length zonal flows, and consequently, the numerical dissipation on these flows must be sufficiently small. Close to the nonlinear threshold for turbulence generation, a relatively small dissipation can maintain a turbulent state with a sizeable heat flux, through the damping of the zonal flow. Lowering the dissipation causes the turbulence, for temperature gradients close to the threshold, to be subdued. The heat flux then doesmore » not go smoothly to zero when the threshold is approached from above. Rather, a finite minimum heat flux is obtained below which no fully developed turbulent state exists. The threshold value of the temperature gradient length at which this finite heat flux is obtained is up to 30% larger compared with the threshold value obtained by extrapolating the heat flux to zero, and the cyclone base case is found to be nonlinearly stable. Transport is subdued when a fully developed staircase structure in the E × B shearing rate forms. Just above the threshold, an incomplete staircase develops, and transport is mediated by avalanche structures which propagate through the marginally stable regions.« less

Further studies of turbulence structure resulting from interactions between embedded vortices and wall jets at high blowing ratios

NASA Astrophysics Data System (ADS)

Doner, William D.

1989-12-01

Interactions of wall jets and vortices embedded in turbulent layers commonly occur near gas turbine blades and endwalls where film cooling is employed. These interactions frequently result in undesirable heat transfer effects at blade and endwall surfaces. In this thesis, a crossed hot-wire probe is used to measure the turbulence structure resulting from this type of interaction. The vortex is generated using a half delta-wing vortex generator mounted 12 deg with respect to a 10 m/s mean velocity flow over a flat plate. A single injection hole, 0.95 cm in diameter, inclined 30 deg to the horizontal, is positioned 59.3 cm downstream of the vortex generator. The vortex generator is positioned so that vortex upwash and downwash could be located over the injection hole. Streamwise development of the turbulent boundary layer was investigated for the following cases: (1) boundary layer with jet only (m = 1.5), and (2) boundary layer with vortex only. Measurement of interaction between the boundary layer, vortex upwash, and the wall jet was made at one station with various blowing ratios. At low blowing ratios (m = 0.5 and 1.5) the vortex dominates the flow. Significant alterations to the turbulent structure are seen in the Reynolds stress components, vorticity distributions and mean velocities. At higher blowing ratios (m = 2.5 and 3.5) the jet dominates the flow, the vortex is blown away from the wall, and its turbulence effects are dispersed over a larger area.

Simulations of Turbine Cooling Flows Using a Multiblock-Multigrid Scheme

NASA Technical Reports Server (NTRS)

Steinthorsson, Erlendur; Ameri, Ali A.; Rigby, David L.

1996-01-01

Results from numerical simulations of air flow and heat transfer in a 'branched duct' geometry are presented. The geometry contains features, including pins and a partition, as are found in coolant passages of turbine blades. The simulations were performed using a multi-block structured grid system and a finite volume discretization of the governing equations (the compressible Navier-Stokes equations). The effects of turbulence on the mean flow and heat transfer were modeled using the Baldwin-Lomax turbulence model. The computed results are compared to experimental data. It was found that the extent of some regions of high heat transfer was somewhat under predicted. It is conjectured that the underlying reason is the local nature of the turbulence model which cannot account for upstream influence on the turbulence field. In general, however, the comparison with the experimental data is favorable.

Suppression of turbulent energy cascade due to phase separation in homogenous binary mixture fluid

NASA Astrophysics Data System (ADS)

Takagi, Youhei; Okamoto, Sachiya

2015-11-01

When a multi-component fluid mixture becomes themophysically unstable state by quenching from well-melting condition, phase separation due to spinodal decomposition occurs, and a self-organized structure is formed. During phase separation, free energy is consumed for the structure formation. In our previous report, the phase separation in homogenous turbulence was numerically simulated and the coarsening process of phase separation was discussed. In this study, we extended our numerical model to a high Schmidt number fluid corresponding to actual polymer solution. The governing equations were continuity, Navier-Stokes, and Chan-Hiliard equations as same as our previous report. The flow filed was an isotropic homogenous turbulence, and the dimensionless parameters in the Chan-Hilliard equation were estimated based on the thermophysical condition of binary mixture. From the numerical results, it was found that turbulent energy cascade was drastically suppressed in the inertial subrange by phase separation for the high Schmidt number flow. By using the identification of turbulent and phase separation structure, we discussed the relation between total energy balance and the structures formation processes. This study is financially supported by the Grand-in-Aid for Young Scientists (B) (No. T26820045) from the Ministry of Education, Cul-ture, Sports, Science and Technology of Japan.

Simulation of a turbulent supersonic underexpanded jet flowing into a submerged space with the help of a shear stress transfer model

NASA Astrophysics Data System (ADS)

Isaev, S. A.; Lipnitskii, Yu. M.; Baranov, P. A.; Panasenko, A. V.; Usachov, A. E.

2012-11-01

We have calculated the flow of an axisymmetric turbulent supersonic underexpanded jet into a submerged space with the help of the VP2/3 package as part of the generalized pressure correction procedure. The shear stress transfer model modified with account for the curvature of streamlines has been verified on the basis of comparison with V. I. Zapryagaev's data obtained at the S. A. Khristianovich Institute of Theoretical and Applied Mechanics, Siberian Branch of the Russian Academy of Sciences. The influence of the generated vortex viscosity on the shock-wave structure of the jet, the field of flow parameters, and the turbulence characteristics has been analyzed.

Laboratory Experiments On Continually Forced 2d Turbulence

NASA Astrophysics Data System (ADS)

Wells, M. G.; Clercx, H. J. H.; Van Heijst, G. J. F.

There has been much recent interest in the advection of tracers by 2D turbulence in geophysical flows. While there is a large body of literature on decaying 2D turbulence or forced 2D turbulence in unbounded domains, there have been very few studies of forced turbulence in bounded domains. In this study we present new experimental results from a continuously forced quasi 2D turbulent field. The experiments are performed in a square Perspex tank filled with water. The flow is made quasi 2D by a steady background rotation. The rotation rate of the tank has a small (<8 %) sinusoidal perturbation which leads to the periodic formation of eddies in the corners of the tank. When the oscillation period of the perturbation is greater than an eddy roll-up time-scale, dipole structures are observed to form. The dipoles can migrate away from the walls, and the interior of the tank is continually filled with vortexs. From experimental visualizations the length scale of the vortexs appears to be largely controlled by the initial formation mechanism and large scale structures are not observed to form at large times. Thus the experiments provide a simple way of cre- ating a continuously forced 2D turbulent field. The resulting structures are in contrast with most previous laboratory experiments on 2D turbulence which have investigated decaying turbulence and have observed the formations of large scale structure. In these experiments, decaying turbulence had been produced by a variety of methods such as the decaying turbulence in the wake of a comb of rods (Massen et al 1999), organiza- tion of vortices in thin conducting liquids (Cardoso et al 1994) or in rotating systems where there are sudden changes in angular rotation rate (Konijnenberg et al 1998). Results of dye visualizations, particle tracking experiments and a direct numerical simulation will be presented and discussed in terms of their oceanographic application. Bibliography Cardoso,O. Marteau, D. &Tabeling, P. Quantitative experimental study of the free decay of quasi-two-dimensional turbulence Phys. Rev. E 49, 454 (1994) Maassen, S.R., H.J.H. Clercx &G.J.F. van Heijst - Decaying quasi-2D turbulence in a stratified fluid with circular boundaries. Europhys. Lett. 46, 339-345 (1999). Konijnenberg, J.A. van de, J.B. Flor &G.J.F. van Heijst - Decaying quasi-two- dimensional viscous flow on a square domain. Phys. Fluids 10, 595-606 (1998).

Influence of turbulence, orientation, and site configuration on the response of buildings to extreme wind.

PubMed

Aly, Aly Mousaad

2014-01-01

Atmospheric turbulence results from the vertical movement of air, together with flow disturbances around surface obstacles which make low- and moderate-level winds extremely irregular. Recent advancements in wind engineering have led to the construction of new facilities for testing residential homes at relatively high Reynolds numbers. However, the generation of a fully developed turbulence in these facilities is challenging. The author proposed techniques for the testing of residential buildings and architectural features in flows that lack fully developed turbulence. While these methods are effective for small structures, the extension of the approach for large and flexible structures is not possible yet. The purpose of this study is to investigate the role of turbulence in the response of tall buildings to extreme winds. In addition, the paper presents a detailed analysis to investigate the influence of upstream terrain conditions, wind direction angle (orientation), and the interference effect from the surrounding on the response of high-rise buildings. The methodology presented can be followed to help decision makers to choose among innovative solutions like aerodynamic mitigation, structural member size adjustment, and/or damping enhancement, with an objective to improve the resiliency and the serviceability of buildings.

Influence of Turbulence, Orientation, and Site Configuration on the Response of Buildings to Extreme Wind

PubMed Central

2014-01-01

Atmospheric turbulence results from the vertical movement of air, together with flow disturbances around surface obstacles which make low- and moderate-level winds extremely irregular. Recent advancements in wind engineering have led to the construction of new facilities for testing residential homes at relatively high Reynolds numbers. However, the generation of a fully developed turbulence in these facilities is challenging. The author proposed techniques for the testing of residential buildings and architectural features in flows that lack fully developed turbulence. While these methods are effective for small structures, the extension of the approach for large and flexible structures is not possible yet. The purpose of this study is to investigate the role of turbulence in the response of tall buildings to extreme winds. In addition, the paper presents a detailed analysis to investigate the influence of upstream terrain conditions, wind direction angle (orientation), and the interference effect from the surrounding on the response of high-rise buildings. The methodology presented can be followed to help decision makers to choose among innovative solutions like aerodynamic mitigation, structural member size adjustment, and/or damping enhancement, with an objective to improve the resiliency and the serviceability of buildings. PMID:24701140

Influences of the Darrieus-Landau instability on premixed turbulent flames

NASA Astrophysics Data System (ADS)

Patyal, Advitya; Matalon, Moshe

2017-11-01

The propagation of turbulent flames in three-dimensional turbulent flows is studied within the context of the hydrodynamic theory. The flame is treated as a surface of density discontinuity with the flow modified by gas expansion resulting from heat released during combustion. The flame is tracked using a level-set method with a propagation speed that depends on the local flame stretch, modulated by a Markstein length. Impact of the Darrieus-Landau instability on the topology of the flame surface is studied. It is shown that similar to passive interfaces, flames under the influence of the hydrodynamic instability resort to cylindrical structures with increasing turbulence intensity, even in 3D. The mechanism of modification of vortical structures in the burned gas is identified in terms of the alignments between the vorticity vector, flame surface normal and eigenvectors of the strain rate tensor. The results indicate that the strain rate tensor is intricately coupled with the normal to the flame surface and creates anisotropy in the orientation of vortical structures, which begins to weaken as the turbulent intensity increases. Furthermore, vorticity budgets are used to highlight the relative importance of baroclinic torque due to Darrieus-Landau instability.

Two-dimensional dynamics of elasto-inertial turbulence and its role in polymer drag reduction

NASA Astrophysics Data System (ADS)

Sid, S.; Terrapon, V. E.; Dubief, Y.

2018-02-01

The goal of the present study is threefold: (i) to demonstrate the two-dimensional nature of the elasto-inertial instability in elasto-inertial turbulence (EIT), (ii) to identify the role of the bidimensional instability in three-dimensional EIT flows, and (iii) to establish the role of the small elastic scales in the mechanism of self-sustained EIT. Direct numerical simulations of viscoelastic fluid flows are performed in both two- and three-dimensional straight periodic channels using the Peterlin finitely extensible nonlinear elastic model (FENE-P). The Reynolds number is set to Reτ=85 , which is subcritical for two-dimensional flows but beyond the transition for three-dimensional ones. The polymer properties selected correspond to those of typical dilute polymer solutions, and two moderate Weissenberg numbers, Wiτ=40 ,100 , are considered. The simulation results show that sustained turbulence can be observed in two-dimensional subcritical flows, confirming the existence of a bidimensional elasto-inertial instability. The same type of instability is also observed in three-dimensional simulations where both Newtonian and elasto-inertial turbulent structures coexist. Depending on the Wi number, one type of structure can dominate and drive the flow. For large Wi values, the elasto-inertial instability tends to prevail over the Newtonian turbulence. This statement is supported by (i) the absence of typical Newtonian near-wall vortices and (ii) strong similarities between two- and three-dimensional flows when considering larger Wi numbers. The role of small elastic scales is investigated by introducing global artificial diffusion (GAD) in the hyperbolic transport equation for polymers. The aim is to measure how the flow reacts when the smallest elastic scales are progressively filtered out. The study results show that the introduction of large polymer diffusion in the system strongly damps a significant part of the elastic scales that are necessary to feed turbulence, eventually leading to flow laminarization. A sufficiently high Schmidt number (weakly diffusive polymers) is necessary to allow self-sustained turbulence to settle. Although EIT can withstand a low amount of diffusion and remains in a nonlaminar chaotic state, adding a finite amount of GAD in the system can have an impact on the dynamics and lead to important quantitative changes, even for Schmidt numbers as large as 102. The use of GAD should therefore be avoided in viscoelastic flow simulations.

Scaling Properties of Particle Density Fields Formed in Simulated Turbulent Flows

NASA Technical Reports Server (NTRS)

Hogan, Robert C.; Cuzzi, Jeffrey N.; Dobrovolskis, Anthony R.; DeVincenzi, Donald (Technical Monitor)

1998-01-01

Direct numerical simulations (DNS) of particle concentrations in fully developed 3D turbulence were carried out in order to study the nonuniform structure of the particle density field. Three steady-state turbulent fluid fields with Taylor microscale Reynolds numbers (Re(sub lambda)) of 40, 80 and 140 were generated by solving the Navier-Stokes equations with pseudospectral methods. Large scale forcing was used to drive the turbulence and maintain temporal stationarity. The response of the particles to the fluid was parameterized by the particle Stokes number St, defined as the ratio of the particle's stopping time to the mean period of eddies on the Kolmogorov scale (eta). In this paper, we consider only passive particles optimally coupled to these eddies (St approx. = 1) because of their tendency to concentrate more than particles with lesser or greater St values. The trajectories of up to 70 million particles were tracked in the equilibrated turbulent flows until the particle concentration field reached a statistically stationary state. The nonuniform structure of the concentration fields was characterized by the multifractal singularity spectrum, f(alpha), derived from measures obtained after binning particles into cells ranging from 2(eta) to 15(eta) in size. We observed strong systematic variations of f(alpha) across this scale range in all three simulations and conclude that the particle concentration field is not statistically self similar across the scale range explored. However, spectra obtained at the 2(eta), 4(eta), and 8(eta) scales of each flow case were found to be qualitatively similar. This result suggests that the local structure of the particle concentration field may be flow-Independent. The singularity spectra found for 2n-sized cells were used to predict concentration distributions in good agreement with those obtained directly from the particle data. This Singularity spectrum has a shape similar to the analogous spectrum derived for the inertial-range energy dissipation fields of experimental turbulent flows at Re(sub lambda) = 110 and 1100. Based on this agreement, and the expectation that both dissipation and particle concentration are controlled by the same cascade process, we hypothesize that singularity spectra similar to the ones found in this work provide a good characterization of the spatially averaged statistical properties of preferentially concentrated particles in higher Re(sub lambda) turbulent flows.

Viewing inside Pyroclastic Flows - Large-scale Experiments on hot pyroclast-gas mixture flows

NASA Astrophysics Data System (ADS)

Breard, E. C.; Lube, G.; Cronin, S. J.; Jones, J.

2014-12-01

Pyroclastic density currents are the largest threat from volcanoes. Direct observations of natural flows are persistently prevented because of their violence and remain limited to broad estimates of bulk flow behaviour. The Pyroclastic Flow Generator - a large-scale experimental facility to synthesize hot gas-particle mixture flows scaled to pyroclastic flows and surges - allows investigating the physical processes behind PDC behaviour in safety. The ability to simulate natural eruption conditions and to view and measure inside the hot flows allows deriving validation and calibration data sets for existing numerical models, and to improve the constitutive relationships necessary for their effective use as powerful tools in hazard assessment. We here report on a systematic series of large-scale experiments on up to 30 ms-1 fast, 2-4.5 m thick, 20-35 m long flows of natural pyroclastic material and gas. We will show high-speed movies and non-invasive sensor data that detail the internal structure of the analogue pyroclastic flows. The experimental PDCs are synthesized by the controlled 'eruption column collapse' of variably diluted suspensions into an instrumented channel. Experiments show four flow phases: mixture acceleration and dilution during free fall; impact and lateral blasting; PDC runout; and co-ignimbrite cloud formation. The fully turbulent flows reach Reynolds number up to 107 and depositional facies similar to natural deposits. In the PDC runout phase, the shear flows develop a four-partite structure from top to base: a fully turbulent, strongly density-stratified ash cloud with average particle concentrations <<1vol%; a transient, turbulent dense suspension region with particle concentrations between 1 and 10 vol%; a non-turbulent, aerated and highly mobile dense underflows with particle concentrations between 40 and 50 vol%; and a vertically aggrading bed of static material. We characterise these regions and the exchanges of energy and momentum through their interfaces via vertical time-series profiles of velocity, particle concentration, gas and particle transport directionality and turbulent eddy characteristics. We highlight the importance of each region for the PDC runout dynamics and introduce a new transport and sedimentation model for downslope evolving pyroclastic flows.

Intermittent nature of acceleration in near wall turbulence.

PubMed

Lee, Changhoon; Yeo, Kyongmin; Choi, Jung-Il

2004-04-09

Using direct numerical simulation of a fully developed turbulent channel flow, we investigate the behavior of acceleration near a solid wall. We find that acceleration near the wall is highly intermittent and the intermittency is in large part associated with the near wall organized coherent turbulence structures. We also find that acceleration of large magnitude is mostly directed towards the rotation axis of the coherent vortical structures, indicating that the source of the intermittent acceleration is the rotational motion associated with the vortices that causes centripetal acceleration.

Turbulence in nature and in the laboratory

PubMed Central

Warhaft, Z.

2002-01-01

Fluid turbulence has attracted the attention of physicists, mathematicians, and engineers for over 100 years, yet it remains an unsolved problem. The reasons for the difficulties are outlined and recent advances in describing its small-scale statistical structure are described. Contrary to traditional notions, new experimental evidence indicates that the small scales are anisotropic, reflecting the overall character of the flow. The consequences of this finding with regard to the long-held postulate of the universality of the small-scale turbulence structure are discussed. PMID:11875199

Application of the TEMPEST computer code for simulating hydrogen distribution in model containment structures. [PWR; BWR

DOE Office of Scientific and Technical Information (OSTI.GOV)

Trent, D.S.; Eyler, L.L.

In this study several aspects of simulating hydrogen distribution in geometric configurations relevant to reactor containment structures were investigated using the TEMPEST computer code. Of particular interest was the performance of the TEMPEST turbulence model in a density-stratified environment. Computed results illustrated that the TEMPEST numerical procedures predicted the measured phenomena with good accuracy under a variety of conditions and that the turbulence model used is a viable approach in complex turbulent flow simulation.

Spatio-temporal dynamics of turbulence trapped in geodesic acoustic modes

NASA Astrophysics Data System (ADS)

Sasaki, M.; Kobayashi, T.; Itoh, K.; Kasuya, N.; Kosuga, Y.; Fujisawa, A.; Itoh, S.-I.

2018-01-01

The spatio-temporal dynamics of turbulence with the interaction of geodesic acoustic modes (GAMs) are investigated, focusing on the phase-space structure of turbulence, where the phase-space consists of real-space and wavenumber-space. Based on the wave-kinetic framework, the coupling equation between the GAM and the turbulence is numerically solved. The turbulence trapped by the GAM velocity field is obtained. Due to the trapping effect, the turbulence intensity increases where the second derivative of the GAM velocity (curvature of the GAM) is negative. While, in the positive-curvature region, the turbulence is suppressed. Since the trapped turbulence propagates with the GAMs, this relationship is sustained spatially and temporally. The dynamics of the turbulence in the wavenumber spectrum are converted in the evolution of the frequency spectrum, and the simulation result is compared with the experimental observation in JFT-2M tokamak, where the similar patterns are obtained. The turbulence trapping effect is a key to understand the spatial structure of the turbulence in the presence of sheared flows.

Effect of Resonant Magnetic Perturbations on secondary structures in Drift-Wave turbulence

NASA Astrophysics Data System (ADS)

Leconte, Michael

2011-10-01

In this work, we study the effects of RMPs on turbulence, flows and confinement, in the framework of two paradigmatic models, resistive ballooning and resistive drift waves. For resistive ballooning turbulence, we use 3D global numerical simulations, including RMP fields and (externally-imposed) sheared rotation profile. Without RMPs, relaxation oscillations of the pressure profile occur. With RMPs, results show that long-lived convection cells are generated by the combined effects of pressure modulation and toroidal curvature coupling. These modify the global structure of the turbulence and eliminate relaxation oscillations. This effect is due mainly to a modification of the pressure profile linked to the presence of residual magnetic island chains. Hence convection-cell generation increases for increasing δBr/B0. For RMP effect on zonal flows in drift wave turbulence, we extend the Hasegawa-Wakatani model to include RMP fields. The effect of the RMPs is to induce a linear coupling between the zonal electric field and the zonal density gradient, which drives the system to a state of electron radial force balance for large δBr/B0. Both the vorticity flux (Reynolds stress), and particle flux are modulated. We derive an extended predator prey model which couples zonal potential and density dynamics to the evolution of turbulence intensity. This model has both turbulence drive and RMP amplitude as control parameters, and predicts a novel type of transport bifurcation in the presence of RMPs. We find a novel set of system states that are similar to the Hmode-like state of the standard predator-prey model, but for which the power threshold is now a function of the RMP strength. For small RMP amplitude and low collisionality, both the ambient turbulence and zonal flow energy increase with δBr/B0. For larger RMP strength, the turbulence energy increases, but the energy of zonal flows decreases with δBr/B0, corresponding to a damping of zonal flows. At high collisionnality, zonal flow damping occurs even at small RMP amplitude. Finally, for very strong values of δBr/B0, the system bifurcates back to an Lmode-like state. This work was supported by the World Class Institute (WCI) Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology of Korea (MEST).

Three Dimensional Energetics of Left Ventricle Flows Using Time-Resolved DPIV

NASA Astrophysics Data System (ADS)

Pierrakos, Olga; Vlachos, Pavlos

2006-11-01

Left ventricular (LV) flows in the human heart are very complex and in the presence of unhealthy or prosthetic heart valves (HV), the complexity of the flow is further increased. Yet to date, no study has documented the complex 3D hemodynamic characteristics and energetics of LV flows. We present high sampling frequency Time Resolved DPIV results obtained in a flexible, transparent LV documenting the evolution of eddies and turbulence. The purpose is to characterize the energetics of the LV flow field in the presence of four orientations of the most commonly implanted mechanical bileaflet HV and a porcine valve. By decomposing the energy scales of the flow field, the ultimate goal is to quantify the total energy losses associated with vortex ring formation and turbulence dissipation. The energies associated to vortex ring formation give a measure of the energy trapped within the structure while estimations of the turbulence dissipation rate (TDR) give a measure of the energy dissipated at the smaller scales. For the first time in cardiovascular applications, an LES-based PIV method, which overcomes the limitations of conventional TDR estimation methods that assume homogeneous isotropic turbulence, was employed. We observed that energy lost at the larger scales (vortex ring) is much higher than the energy lost at the smaller scales due to turbulence dissipation.

The effect of neutrally buoyant finite-size particles on channel flows in the laminar-turbulent transition regime

NASA Astrophysics Data System (ADS)

Loisel, Vincent; Abbas, Micheline; Masbernat, Olivier; Climent, Eric

2013-12-01

The presence of finite-size particles in a channel flow close to the laminar-turbulent transition is simulated with the Force Coupling Method which allows two-way coupling with the flow dynamics. Spherical particles with channel height-to-particle diameter ratio of 16 are initially randomly seeded in a fluctuating flow above the critical Reynolds number corresponding to single phase flow relaminarization. When steady-state is reached, the particle volume fraction is homogeneously distributed in the channel cross-section (ϕ ≅ 5%) except in the near-wall region where it is larger due to inertia-driven migration. Turbulence statistics (intensity of velocity fluctuations, small-scale vortical structures, wall shear stress) calculated in the fully coupled two-phase flow simulations are compared to single-phase flow data in the transition regime. It is observed that particles increase the transverse r.m.s. flow velocity fluctuations and they break down the flow coherent structures into smaller, more numerous and sustained eddies, preventing the flow to relaminarize at the single-phase critical Reynolds number. When the Reynolds number is further decreased and the suspension flow becomes laminar, the wall friction coefficient recovers the evolution of the laminar single-phase law provided that the suspension viscosity is used in the Reynolds number definition. The residual velocity fluctuations in the suspension correspond to a regime of particulate shear-induced agitation.

Spatial organisation of ecologically-relevant high frequency flow properties and implications for habitat assessment.

NASA Astrophysics Data System (ADS)

Trinci, G.; Harvey, G.; Henshaw, A.; Bertoldi, W.

2016-12-01

Turbulence plays a crucial role in the life cycle of river plants and animals. Turbulent flow facilitates access to food, maintenance of adequate oxygen levels, removal of wastes, locomotion and predator evasion, but can also act as a stressor, leading to dislodgement from habitats, increased energy costs, physiological damage and even mortality. Despite this, hydraulic habitat assessments for river appraisal and restoration design have largely focused on temporally and spatially averaged flow properties rather than more complex descriptors of turbulence (turbulence intensity, and the periodicity, orientation and scale of coherent flow structures) that are known to directly influence aquatic organisms. Contrasting relationships between turbulence and mean flow velocity have been reported and there is a pressing need to improve understanding of the hydraulic environment provided by mesoscale river features, such as geomorphic units (e.g. riffles, pools, steps), upon which river management and restoration often focuses. We undertook high frequency velocity surveys within three river reaches (low, medium and high gradient) using a 3-dimensional Acoustic Doppler Velocimeter, combined with detailed surveys of bed topography and visual assessments of the spatial organisation of geomorphic units. Using a combination of multivariate statistical analysis (Principal Components Analysis, Cluster Analysis and GLMs) and geostatistics (semi-variance), the paper explores the spatial organisation of key turbulence parameters across the reaches and linkages with mean flow velocity and characteristic roughness elements. The ability of `higher order' turbulence properties to distinguish between visually identified geomorphic units is also assessed. The findings provide insights into scales of variability in turbulence properties that have direct ecological relevance, helping to inform river assessment and restoration efforts.

A statistical state dynamics approach to wall turbulence.

PubMed

Farrell, B F; Gayme, D F; Ioannou, P J

2017-03-13

This paper reviews results obtained using statistical state dynamics (SSD) that demonstrate the benefits of adopting this perspective for understanding turbulence in wall-bounded shear flows. The SSD approach used in this work employs a second-order closure that retains only the interaction between the streamwise mean flow and the streamwise mean perturbation covariance. This closure restricts nonlinearity in the SSD to that explicitly retained in the streamwise constant mean flow together with nonlinear interactions between the mean flow and the perturbation covariance. This dynamical restriction, in which explicit perturbation-perturbation nonlinearity is removed from the perturbation equation, results in a simplified dynamics referred to as the restricted nonlinear (RNL) dynamics. RNL systems, in which a finite ensemble of realizations of the perturbation equation share the same mean flow, provide tractable approximations to the SSD, which is equivalent to an infinite ensemble RNL system. This infinite ensemble system, referred to as the stochastic structural stability theory system, introduces new analysis tools for studying turbulence. RNL systems provide computationally efficient means to approximate the SSD and produce self-sustaining turbulence exhibiting qualitative features similar to those observed in direct numerical simulations despite greatly simplified dynamics. The results presented show that RNL turbulence can be supported by as few as a single streamwise varying component interacting with the streamwise constant mean flow and that judicious selection of this truncated support or 'band-limiting' can be used to improve quantitative accuracy of RNL turbulence. These results suggest that the SSD approach provides new analytical and computational tools that allow new insights into wall turbulence.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'. © 2017 The Author(s).

A statistical state dynamics approach to wall turbulence

PubMed Central

Gayme, D. F.; Ioannou, P. J.

2017-01-01

This paper reviews results obtained using statistical state dynamics (SSD) that demonstrate the benefits of adopting this perspective for understanding turbulence in wall-bounded shear flows. The SSD approach used in this work employs a second-order closure that retains only the interaction between the streamwise mean flow and the streamwise mean perturbation covariance. This closure restricts nonlinearity in the SSD to that explicitly retained in the streamwise constant mean flow together with nonlinear interactions between the mean flow and the perturbation covariance. This dynamical restriction, in which explicit perturbation–perturbation nonlinearity is removed from the perturbation equation, results in a simplified dynamics referred to as the restricted nonlinear (RNL) dynamics. RNL systems, in which a finite ensemble of realizations of the perturbation equation share the same mean flow, provide tractable approximations to the SSD, which is equivalent to an infinite ensemble RNL system. This infinite ensemble system, referred to as the stochastic structural stability theory system, introduces new analysis tools for studying turbulence. RNL systems provide computationally efficient means to approximate the SSD and produce self-sustaining turbulence exhibiting qualitative features similar to those observed in direct numerical simulations despite greatly simplified dynamics. The results presented show that RNL turbulence can be supported by as few as a single streamwise varying component interacting with the streamwise constant mean flow and that judicious selection of this truncated support or ‘band-limiting’ can be used to improve quantitative accuracy of RNL turbulence. These results suggest that the SSD approach provides new analytical and computational tools that allow new insights into wall turbulence. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’. PMID:28167577

Measurement of turbulent spatial structure and kinetic energy spectrum by exact temporal-to-spatial mapping

NASA Astrophysics Data System (ADS)

Buchhave, Preben; Velte, Clara M.

2017-08-01

We present a method for converting a time record of turbulent velocity measured at a point in a flow to a spatial velocity record consisting of consecutive convection elements. The spatial record allows computation of dynamic statistical moments such as turbulent kinetic wavenumber spectra and spatial structure functions in a way that completely bypasses the need for Taylor's hypothesis. The spatial statistics agree with the classical counterparts, such as the total kinetic energy spectrum, at least for spatial extents up to the Taylor microscale. The requirements for applying the method are access to the instantaneous velocity magnitude, in addition to the desired flow quantity, and a high temporal resolution in comparison to the relevant time scales of the flow. We map, without distortion and bias, notoriously difficult developing turbulent high intensity flows using three main aspects that distinguish these measurements from previous work in the field: (1) The measurements are conducted using laser Doppler anemometry and are therefore not contaminated by directional ambiguity (in contrast to, e.g., frequently employed hot-wire anemometers); (2) the measurement data are extracted using a correctly and transparently functioning processor and are analysed using methods derived from first principles to provide unbiased estimates of the velocity statistics; (3) the exact mapping proposed herein has been applied to the high turbulence intensity flows investigated to avoid the significant distortions caused by Taylor's hypothesis. The method is first confirmed to produce the correct statistics using computer simulations and later applied to measurements in some of the most difficult regions of a round turbulent jet—the non-equilibrium developing region and the outermost parts of the developed jet. The proposed mapping is successfully validated using corresponding directly measured spatial statistics in the fully developed jet, even in the difficult outer regions of the jet where the average convection velocity is negligible and turbulence intensities increase dramatically. The measurements in the developing region reveal interesting features of an incomplete Richardson-Kolmogorov cascade under development.

Determination of the turbulence integral model parameters for a case of a coolant angular flow in regular rod-bundle

NASA Astrophysics Data System (ADS)

Bayaskhalanov, M. V.; Vlasov, M. N.; Korsun, A. S.; Merinov, I. G.; Philippov, M. Ph

2017-11-01

Research results of “k-ε” turbulence integral model (TIM) parameters dependence on the angle of a coolant flow in regular smooth cylindrical rod-bundle are presented. TIM is intended for the definition of efficient impulse and heat transport coefficients in the averaged equations of a heat and mass transfer in the regular rod structures in an anisotropic porous media approximation. The TIM equations are received by volume-averaging of the “k-ε” turbulence model equations on periodic cell of rod-bundle. The water flow across rod-bundle under angles from 15 to 75 degrees was simulated by means of an ANSYS CFX code. Dependence of the TIM parameters on flow angle was as a result received.

Density Effects on Post-shock Turbulence Structure

NASA Astrophysics Data System (ADS)

Tian, Yifeng; Jaberi, Farhad; Livescu, Daniel; Li, Zhaorui; Michigan State University Collaboration; Los Alamos National Laboratory Collaboration; Texas A&M University-Corpus Christi Collaboration

2017-11-01

The effects of density variations due to mixture composition on post-shock turbulence structure are studied using turbulence-resolving shock-capturing simulations. This work extends the canonical Shock-Turbulence Interaction (STI) problem to involve significant variable density effects. The numerical method has been verified using a series of grid and LIA convergence tests, and is used to generate accurate post-shock turbulence data for a detailed flow study. Density effects on post-shock turbulent statistics are shown to be significant, leading to an increased amplification of turbulent kinetic energy (TKE). Eulerian and Lagrangian analyses show that the increase in the post-shock correlation between rotation and strain is weakened in the case with significant density variations (referred to as the ``multi-fluid'' case). Similar to previous single-fluid results and LIA predictions, the shock wave significantly changes the topology of the turbulent structures, exhibiting a symmetrization of the joint PDF of second and third invariant of the deviatoric part of velocity gradient tensor. In the multi-fluid case, this trend is more significant and mainly manifested in the heavy fluid regions. Lagrangian data are also used to study the evolution of turbulence structure away from the shock wave and assess the accuracy of Lagrangian dynamical models.

Flame-conditioned turbulence modeling for reacting flows

NASA Astrophysics Data System (ADS)

Macart, Jonathan F.; Mueller, Michael E.

2017-11-01

Conventional approaches to turbulence modeling in reacting flows rely on unconditional averaging or filtering, that is, consideration of the momentum equations only in physical space, implicitly assuming that the flame only weakly affects the turbulence, aside from a variation in density. Conversely, for scalars, which are strongly coupled to the flame structure, their evolution equations are often projected onto a reduced-order manifold, that is, conditionally averaged or filtered, on a flame variable such as a mixture fraction or progress variable. Such approaches include Conditional Moment Closure (CMC) and related variants. However, recent observations from Direct Numerical Simulation (DNS) have indicated that the flame can strongly affect turbulence in premixed combustion at low Karlovitz number. In this work, a new approach to turbulence modeling for reacting flows is investigated in which conditionally averaged or filtered equations are evolved for the momentum. The conditionally-averaged equations for the velocity and its covariances are derived, and budgets are evaluated from DNS databases of turbulent premixed planar jet flames. The most important terms in these equations are identified, and preliminary closure models are proposed.

Low flow fume hood

DOEpatents

Bell, Geoffrey C.; Feustel, Helmut E.; Dickerhoff, Darryl J.

2002-01-01

A fume hood is provided having an adequate level of safety while reducing the amount of air exhausted from the hood. A displacement flow fume hood works on the principal of a displacement flow which displaces the volume currently present in the hood using a push-pull system. The displacement flow includes a plurality of air supplies which provide fresh air, preferably having laminar flow, to the fume hood. The displacement flow fume hood also includes an air exhaust which pulls air from the work chamber in a minimally turbulent manner. As the displacement flow produces a substantially consistent and minimally turbulent flow in the hood, inconsistent flow patterns associated with contaminant escape from the hood are minimized. The displacement flow fume hood largely reduces the need to exhaust large amounts of air from the hood. It has been shown that exhaust air flow reductions of up to 70% are possible without a decrease in the hood's containment performance. The fume hood also includes a number of structural adaptations which facilitate consistent and minimally turbulent flow within a fume hood.

Characterizing Turbulent Events at a Tidal Energy Site from Acoustic Doppler Velocity Observations

NASA Astrophysics Data System (ADS)

McCaffrey, Katherine; Fox-Kemper, Baylor; Hamlington, Peter

2013-11-01

As interest in marine renewable energy increases, observations are crucial to understanding the environments encountered by energy conversion devices. Data obtained from an acoustic Doppler current profiler and an acoustic Doppler velocimeter at two locations in the Puget Sound, WA are used to perform a detailed analysis of the turbulent environment that is expected to be present at a turbine placed in a tidal strait. Metrics such as turbulence intensity, structure functions, probability density functions, intermittency, coherent turbulence kinetic energy, anisotropy invariants, and linear combinations of eigenvalues are used to characterize the turbulence. The results indicate that coherent turbulence kinetic energy and turbulence intensity can be used to identify and parameterize different turbulent events in the flow. An analysis of the anisotropy characteristics leads to a physical description of turbulent events (defined using both turbulence intensity and coherent turbulent kinetic energy) as being dominated by one component of the Reynolds stresses. During non-turbulent events, the flow is dominated by two Reynolds stress components. The importance of these results for the development of realistic models of energy conversion devices is outlined. Cooperative Institute for Research in Environmental Sciences, Department of Atmospheric and Oceanic Sciences.

Time-Filtered Navier-Stokes Approach and Emulation of Turbulence-Chemistry Interaction

NASA Technical Reports Server (NTRS)

Liu, Nan-Suey; Wey, Thomas; Shih, Tsan-Hsing

2013-01-01

This paper describes the time-filtered Navier-Stokes approach capable of capturing unsteady flow structures important for turbulent mixing and an accompanying subgrid model directly accounting for the major processes in turbulence-chemistry interaction. They have been applied to the computation of two-phase turbulent combustion occurring in a single-element lean-direct-injection combustor. Some of the preliminary results from this computational effort are presented in this paper.

MHD Instability and Turbulence in the Tachocline

NASA Technical Reports Server (NTRS)

Werne, Joe; Wagner, William J. (Technical Monitor)

2003-01-01

The focus of this project was to study the physical processes that govern tachocline dynamics and structure. Specific features explored included stratification, shear, waves, and toroidal and poloidal background fields. In order to address recent theoretical work on anisotropic mixing and dynamics in the tachocline, we were particularly interested in such anisotropic mixing for the specific tachocline processes studied. Transition to turbulence often shapes the largest-scale features that appear spontaneously in a flow during the development of turbulence. The resulting large-scale straining field can control the subsequent dynamics; therefore, anticipation of the large-scale straining field that results for individual realizations of the transition to turbulence can be important for subsequent dynamics, flow morphology, and transport characteristics. As a result, we paid particular attention to the development of turbulence in the stratified and sheared environment of the tachocline. This is complicated by the fact that the linearly stability of sheared MHD flows is non-self-adjoint, implying that normal asymptotic linear stability theory may not be relevant.

Modeling of Fine-Particle Formation in Turbulent Flames

NASA Astrophysics Data System (ADS)

Raman, Venkat; Fox, Rodney O.

2016-01-01

The generation of nanostructured particles in high-temperature flames is important both for the control of emissions from combustion devices and for the synthesis of high-value chemicals for a variety of applications. The physiochemical processes that lead to the production of fine particles in turbulent flames are highly sensitive to the flow physics and, in particular, the history of thermochemical compositions and turbulent features they encounter. Consequently, it is possible to change the characteristic size, structure, composition, and yield of the fine particles by altering the flow configuration. This review describes the complex multiscale interactions among turbulent fluid flow, gas-phase chemical reactions, and solid-phase particle evolution. The focus is on modeling the generation of soot particles, an unwanted pollutant from automobile and aircraft engines, as well as metal oxides, a class of high-value chemicals sought for specialized applications, including emissions control. Issues arising due to the numerical methods used to approximate the particle number density function, the modeling of turbulence-chemistry interactions, and model validation are also discussed.

Modeling the pressure-strain correlation of turbulence: An invariant dynamical systems approach

NASA Technical Reports Server (NTRS)

Speziale, Charles G.; Sarkar, Sutanu; Gatski, Thomas B.

1990-01-01

The modeling of the pressure-strain correlation of turbulence is examined from a basic theoretical standpoint with a view toward developing improved second-order closure models. Invariance considerations along with elementary dynamical systems theory are used in the analysis of the standard hierarchy of closure models. In these commonly used models, the pressure-strain correlation is assumed to be a linear function of the mean velocity gradients with coefficients that depend algebraically on the anisotropy tensor. It is proven that for plane homogeneous turbulent flows the equilibrium structure of this hierarchy of models is encapsulated by a relatively simple model which is only quadratically nonlinear in the anisotropy tensor. This new quadratic model - the SSG model - is shown to outperform the Launder, Reece, and Rodi model (as well as more recent models that have a considerably more complex nonlinear structure) in a variety of homogeneous turbulent flows. Some deficiencies still remain for the description of rotating turbulent shear flows that are intrinsic to this general hierarchy of models and, hence, cannot be overcome by the mere introduction of more complex nonlinearities. It is thus argued that the recent trend of adding substantially more complex nonlinear terms containing the anisotropy tensor may be of questionable value in the modeling of the pressure-strain correlation. Possible alternative approaches are discussed briefly.

Modelling the pressure-strain correlation of turbulence - An invariant dynamical systems approach

NASA Technical Reports Server (NTRS)

Speziale, Charles G.; Sarkar, Sutanu; Gatski, Thomas B.

1991-01-01

The modeling of the pressure-strain correlation of turbulence is examined from a basic theoretical standpoint with a view toward developing improved second-order closure models. Invariance considerations along with elementary dynamical systems theory are used in the analysis of the standard hierarchy of closure models. In these commonly used models, the pressure-strain correlation is assumed to be a linear function of the mean velocity gradients with coefficients that depend algebraically on the anisotropy tensor. It is proven that for plane homogeneous turbulent flows the equilibrium structure of this hierarchy of models is encapsulated by a relatively simple model which is only quadratically nonlinear in the anisotropy tensor. This new quadratic model - the SSG model - is shown to outperform the Launder, Reece, and Rodi model (as well as more recent models that have a considerably more complex nonlinear structure) in a variety of homogeneous turbulent flows. Some deficiencies still remain for the description of rotating turbulent shear flows that are intrinsic to this general hierarchy of models and, hence, cannot be overcome by the mere introduction of more complex nonlinearities. It is thus argued that the recent trend of adding substantially more complex nonlinear terms containing the anisotropy tensor may be of questionable value in the modeling of the pressure-strain correlation. Possible alternative approaches are discussed briefly.

Study on steady state wind and turbulence environments. [structure of wakes near buildings

NASA Technical Reports Server (NTRS)

Brundidge, K. C.

1977-01-01

The structure of wakes and how this structure is related to the size and shape of buildings and other obstacles, and to ambient winds, was investigated. Mean values of natural atmospheric flow were obtained and used in conjunction with theoretical relationships developed by dimensional analysis to establish a model of the flow in the wake. Results indicate that conventional and V/STOL aircraft passing through the wake during takeoff and landing would experience not only a change in turbulence level, but also a change in mean wind speed of a magnitude roughly equivalent to that of the eddy components.

The Effect of Impingement on Transitional Behavior in Underexpanded Jets

NASA Technical Reports Server (NTRS)

Inman, Jennifer A.; Danehy, Paul M.; Nowak, Robert J.; Alderfer, David W.

2009-01-01

An investigation into the development of flow unsteadiness in impinging axisymmetric underexpanded jets has been conducted at NASA Langley Research Center. The study has examined the effect of an impingement target placed at various distances and angles on transitional behavior of such jets. Two nozzles, with exit Mach numbers of 1.0 and 2.6, were used in this investigation. Planar laser-induced fluorescence of nitric oxide (NO PLIF) has been used to identify flow unsteadiness and to image transitional and turbulent flow features. Measurements of the location of the onset of various degrees of unsteady flow behavior have been made using these PLIF images. Both qualitative and quantitative comparisons are presented to demonstrate the observed effects of impingement and flow parameters on the process of the transition to turbulence. The presence of the impingement target was found to significantly shorten the distance to transition to turbulence by up to a factor of approximately three, with closer targets resulting in slightly shorter distance to transition and turbulence. The location at which the flow first exhibits unsteadiness was found to have a strong dependence on the presence and location of key flow structures. This paper presents quantitative results on transition criteria for free and impinging jets.

Biofilm effect on flow structure over a permeable bed

NASA Astrophysics Data System (ADS)

Kazemifar, Farzan; Blois, Gianluca; Aybar, Marcelo; Perez-Calleja, Patricia; Nerenberg, Robert; Sinha, Sumit; Hardy, Richard; Best, James; Sambrook Smith, Gregory; Christensen, Kenneth

2017-11-01

Biofilms constitute an important form of bacterial life in aquatic environments and are present at the fluid-solid interfaces in natural and industrial settings, such as water distribution systems and riverbeds among others. The permeable, heterogeneous, and deformable structure of biofilms can influence mass and momentum transport between the subsurface and freestream. However, this interaction is not fully understood, in part due to technical obstacles impeding quantitative experimental investigations. In this work, the effect of biofilm on flow structure over a permeable bed is studied. Experiments are conducted in a closed water channel equipped with an idealized two-dimensional permeable bed. Prior to conducting flow experiments, the models are placed within an independent recirculating reactor for biofilm growth. Once a targeted biofilm growth stage is achieved, the models are transferred to the water channel and subjected to transitional and turbulent flows. Long-distance microscopic particle image velocimetry measurements are performed to quantify the effect of biofilm on the turbulence structure of the free flow as well as the freestream-subsurface flow interaction. Funded by UK Natural Environment Research Council.

Comparison of direct numerical simulation databases of turbulent channel flow at Reτ = 180

NASA Astrophysics Data System (ADS)

Vreman, A. W.; Kuerten, J. G. M.

2014-01-01

Direct numerical simulation (DNS) databases are compared to assess the accuracy and reproducibility of standard and non-standard turbulence statistics of incompressible plane channel flow at Reτ = 180. Two fundamentally different DNS codes are shown to produce maximum relative deviations below 0.2% for the mean flow, below 1% for the root-mean-square velocity and pressure fluctuations, and below 2% for the three components of the turbulent dissipation. Relatively fine grids and long statistical averaging times are required. An analysis of dissipation spectra demonstrates that the enhanced resolution is necessary for an accurate representation of the smallest physical scales in the turbulent dissipation. The results are related to the physics of turbulent channel flow in several ways. First, the reproducibility supports the hitherto unproven theoretical hypothesis that the statistically stationary state of turbulent channel flow is unique. Second, the peaks of dissipation spectra provide information on length scales of the small-scale turbulence. Third, the computed means and fluctuations of the convective, pressure, and viscous terms in the momentum equation show the importance of the different forces in the momentum equation relative to each other. The Galilean transformation that leads to minimum peak fluctuation of the convective term is determined. Fourth, an analysis of higher-order statistics is performed. The skewness of the longitudinal derivative of the streamwise velocity is stronger than expected (-1.5 at y+ = 30). This skewness and also the strong near-wall intermittency of the normal velocity are related to coherent structures.

On the application of a hairpin vortex model of wall turbulence to trailing edge noise prediction

NASA Technical Reports Server (NTRS)

Liu, N. S.; Shamroth, S. J.

1985-01-01

The goal is to develop a technique via a hairpin vortex model of the turbulent boundary layer, which would lead to the estimation of the aerodynamic input for use in trailing edge noise prediction theories. The work described represents an initial step in reaching this goal. The hairpin vortex is considered as the underlying structure of the wall turbulence and the turbulent boundary layer is viewed as an ensemble of typical hairpin vortices of different sizes. A synthesis technique is examined which links the mean flow and various turbulence quantities via these typical vortices. The distribution of turbulence quantities among vortices of different scales follows directly from the probability distribution needed to give the measured mean flow vorticity. The main features of individual representative hairpin vortices are discussed in detail and a preliminary assessment of the synthesis approach is made.

Bursting and critical layer frequencies in minimal turbulent dynamics and connections to exact coherent states

NASA Astrophysics Data System (ADS)

Park, Jae Sung; Shekar, Ashwin; Graham, Michael D.

2018-01-01

The dynamics of the turbulent near-wall region is known to be dominated by coherent structures. These near-wall coherent structures are observed to burst in a very intermittent fashion, exporting turbulent kinetic energy to the rest of the flow. In addition, they are closely related to invariant solutions known as exact coherent states (ECS), some of which display nonlinear critical layer dynamics (motions that are highly localized around the surface on which the streamwise velocity matches the wave speed of ECS). The present work aims to investigate temporal coherence in minimal channel flow relevant to turbulent bursting and critical layer dynamics and its connection to the instability of ECS. It is seen that the minimal channel turbulence displays frequencies very close to those displayed by an ECS family recently identified in the channel flow geometry. The frequencies of these ECS are determined by critical layer structures and thus might be described as "critical layer frequencies." While the bursting frequency is predominant near the wall, the ECS frequencies (critical layer frequencies) become predominant over the bursting frequency at larger distances from the wall, and increasingly so as Reynolds number increases. Turbulent bursts are classified into strong and relatively weak classes with respect to an intermittent approach to a lower branch ECS. This temporally intermittent approach is closely related to an intermittent low drag event, called hibernating turbulence, found in minimal and large domains. The relationship between the strong burst and the instability of the lower branch ECS is further discussed in state space. The state-space dynamics of strong bursts is very similar to that of the unstable manifolds of the lower branch ECS. In particular, strong bursting processes are always preceded by hibernation events. This precursor dynamics to strong turbulence may aid in development of more effective control schemes by a way of anticipating dynamics such as intermittent hibernating dynamics.

Velocity-Vorticity Correlation Structure in Turbulent Channel Flow

NASA Astrophysics Data System (ADS)

Chen, J.; Pei, J.; She, Z. S.; Hussain, F.

2011-09-01

We present a new definition of statistical structure — velocity-vorticity correlation structure (VVCS) — based on amplitude distributions of the tensor field of normalized velocity-vorticity correlation (uiωj), and show that it displays the geometry of the statistical structure relevant to a given reference point, and it effectively captures coherent motions in inhomogeneous shear flows. The variation of the extracted objects moving with the reference point yr+ then presents a full picture of statistical structures for the flow, which goes beyond the traditional view of searching for reference-independent structures. Application to turbulent channel flow simulation data at Reτ = 180 demonstrates that the VVCS successfully captures, qualitatively and quantitatively, the near-wall streaks, the streamwise vortices [1,2], and their extensions up to yr+ = 110 with variations of their length and inclination angle. More interestingly, the VVCS associated with the streamwise velocity component (particularly (uωx ( and (uωz) displays topological change at four distances from the wall (with transitions at yr+≈20,40,60,110), giving rise to a geometrical interpretation of the multi-layer structure of wall-bounded turbulence. Specifically, we find that the VVCS of (uωz( bifurcates at yr+ = 40 with one attached to the wall and the other near the reference location. The VVCS of (uωx) is blob-like in the center region, quite different from a pair of elongated and inclined objects near the wall. The propagation speeds of the velocity components in the near-wall region, y+ ≤ 10, is found to be characterized by the same stream-wise correlation structures of (uωx) and (uωz), whose core is located at y+≈20. As a result, the convection of the velocity fluctuations always reveal the constant propagation speeds in the near-wall region. The coherent motions parallel to the wall plays an important role in determining the propagation of the velocity fluctuations. This study suggests that a variable set of geometrical structures should be invoked for the study of turbulence structures and for modeling mean flow properties in terms of structures. The method and the concept presented here are general for the study of other flow systems (like boundary or mixing layer), as long as ensemble averaging is well-defined.

Geometry of tracer trajectories in rotating turbulent flows

NASA Astrophysics Data System (ADS)

Alards, Kim M. J.; Rajaei, Hadi; Del Castello, Lorenzo; Kunnen, Rudie P. J.; Toschi, Federico; Clercx, Herman J. H.

2017-04-01

The geometry of passive tracer trajectories is studied in two different types of rotating turbulent flows; rotating Rayleigh-Bénard convection (RBC; experiments and direct numerical simulations) and rotating electromagnetically forced turbulence (EFT; experiments). This geometry is fully described by the curvature and torsion of trajectories, and from these geometrical quantities we can subtract information on the typical flow structures at different rotation rates. Previous studies, focusing on nonrotating homogeneous isotropic turbulence (HIT), show that the probability density functions (PDFs) of curvature and torsion reveal pronounced power laws. However, the set-ups studied here involve inhomogeneous turbulence, and in RBC the flow near the horizontal plates is definitely anisotropic. We investigate how the typical shapes of the curvature and torsion PDFs, including the pronounced scaling laws, are influenced by this level of anisotropy and inhomogeneity and how this effect changes with rotation. A first effect of rotation is observed as a shift of the curvature and torsion PDFs towards higher values in the case of RBC and towards lower values in the case of EFT. This shift is related to the length scale of typical vortical structures that decreases with rotation in RBC, but increases with rotation in EFT, explaining the opposite shifts of the curvature (and torsion) PDFs. A second remarkable observation is that in RBC the HIT scaling laws are always recovered, as long as the boundary layer (BL) is excluded. This suggests that these scaling laws are very robust and hold as long as we measure in the turbulent bulk. In the BL of the RBC cell, however, the scaling deviates from the HIT prediction for lower rotation rates. This scaling behavior is found to be consistent with the coupling between the boundary layer dynamics and the bulk flow, which changes under rotation. In particular, it is found that the active coupling of the Ekman-type boundary layer with the bulk flow suppresses anisotropy in the BL region for increasing rotation rates.

Flow and turbulence structure in a hypertidal estuary with the world's biggest tidal bore

NASA Astrophysics Data System (ADS)

Tu, Junbiao; Fan, Daidu

2017-04-01

Turbulent and flow structure associated with breaking tidal bores are deliberately investigated on the basis of field measurements. High-resolution velocity and hydrographic data are collected in the middle Qiantang Estuary by a vertical array of acoustic Doppler velocimeters and optical backscatter sensors, collaborated with a bottom-mounted acoustic Doppler current profiler. Besides obvious variations in diurnal and spring-neap tidal cycles, the estuarine dynamics is featured by extreme asymmetry in flood and ebb tides. The flood tide is abnormally accelerated to generate tidal bores at the first 10 min or more, with breaking or undular configurations at the front. The occurrence of peak flow velocity, turbulent kinetic energy (TKE), and TKE dissipation rate (ɛ) is definitely associated with breaking bores, with their values several times to 2 orders of magnitude larger than the corresponding secondary peak values during the maximum ebb flows. Flow and turbulence dynamics are significantly affected by the tidal-bore Froude number. A sandwich ɛ structure is clear exhibited with the maximum value at the surface, secondary maximum near the bed, and the minimum at the intermediate. Dual TKE sources are indicated by an approximate local balance between shear production and dissipation near the bottom, and a top-down TKE dissipation using the modified Froude scaling in the vertical water column. The highly elevated dissipation by breaking bores is comparable to that by intense breaking waves in the surf zone, and the former potentially penetrates the entire water column to produce extreme sediment-resuspension events in combination with intense bottom shear stress.

Sound production due to large-scale coherent structures

NASA Technical Reports Server (NTRS)

Gatski, T. B.

1979-01-01

The acoustic pressure fluctuations due to large-scale finite amplitude disturbances in a free turbulent shear flow are calculated. The flow is decomposed into three component scales; the mean motion, the large-scale wave-like disturbance, and the small-scale random turbulence. The effect of the large-scale structure on the flow is isolated by applying both a spatial and phase average on the governing differential equations and by initially taking the small-scale turbulence to be in energetic equilibrium with the mean flow. The subsequent temporal evolution of the flow is computed from global energetic rate equations for the different component scales. Lighthill's theory is then applied to the region with the flowfield as the source and an observer located outside the flowfield in a region of uniform velocity. Since the time history of all flow variables is known, a minimum of simplifying assumptions for the Lighthill stress tensor is required, including no far-field approximations. A phase average is used to isolate the pressure fluctuations due to the large-scale structure, and also to isolate the dynamic process responsible. Variation of mean square pressure with distance from the source is computed to determine the acoustic far-field location and decay rate, and, in addition, spectra at various acoustic field locations are computed and analyzed. Also included are the effects of varying the growth and decay of the large-scale disturbance on the sound produced.

Complex Convective Thermal Fluxes and Vorticity Structure

NASA Astrophysics Data System (ADS)

Redondo, Jose M.; Tellez, Jackson; Sotillos, Laura; Lopez Gonzalez-Nieto, Pilar; Sanchez, Jesus M.; Furmanek, Petr; Diez, Margarita

2015-04-01

Local Diffusion and the topological structure of vorticity and velocity fields is measured in the transition from a homogeneous linearly stratified fluid to a cellular or layered structure by means of convective cooling and/or heating[1,2]. Patterns arise by setting up a convective flow generated by an array of Thermoelectric devices (Peltier/Seebeck cells) these are controlled by thermal PID generating a buoyant heat flux [2]. The experiments described here investigate high Prandtl number mixing using brine and fresh water in order to form density interfaces and low Prandtl number mixing with temperature gradients. The set of dimensionless parameters define conditions of numeric and small scale laboratory modeling of environmental flows. Fields of velocity, density and their gradients were computed and visualized [3,4]. When convective heating and cooling takes place the combination of internal waves and buoyant turbulence is much more complicated if the Rayleigh and Reynolds numbers are high in order to study entrainment and mixing. Using ESS and selfsimilarity structures in the velocity and vorticity fieds and intermittency [3,5] that forms in the non-homogeneous flow is related to mixing and stiring. The evolution of the mixing fronts are compared and the topological characteristics of the merging of plumes and jets in different configurations presenting detailed comparison of the evolution of RM and RT, Jets and Plumes in overall mixing. The relation between structure functions, fractal analysis and spectral analysis can be very useful to determine the evolution of scales. Experimental and numerical results on the advance of a mixing or nonmixing front occurring at a density interface due to body forces [6]and gravitational acceleration are analyzed considering the fractal and spectral structure of the fronts like in removable plate experiments for Rayleigh-Taylor flows. The evolution of the turbulent mixing layer and its complex configuration is studied taking into account the dependence on the initial modes at the early stages and its spectral, self-similar information [3,7-9]. Spectral and Fractal analysis on the images has been used in order to estimate dominant mixing structures as well as the dispersion relations of basic instabilities [4,8. Comparison of the range of entrainment values from laboratory experiments with those ocurring in nature, both in the atmosphere and ocean or in Astrophysics shows the importance of modeling correctly the integral lengthscales of the turbulence. The Entrainment may actually be related to the ratio of the flux to gradient Richardson numbers as well as the Turbulent Schmidt or Prandtl number [6,8] and their structure functions [5]. Turbulent mixing diagnostics are based on schlieren and shadowgraph visualization, planar laser sheet, laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). We are interested by the influence of initial conditions [9](flux gradients of various sizes) on the transition to turbulent mixing, the influence of subsequent vortices and waves, initially in a one dimensional or plane configuration and furthermore in a two-three dimensional configuration with the interface oblique with respect to the horizontal. [1] Redondo J.M. (1992) Termodinámica de los procesos irreversibles, efectos termoeléctricos Rev. Termoelectricidad 2, 16-29.AIT.Pamplona. [2] Redondo J.M., Sanchez J.M. Pascual I. Noriega,G.F.(1995).Thermoelectric regulation for electric cabinets. XVI International Conference in Thermoelectrics, 02-G20. Ed. V. Vedernikov. 456-468. St. Petersburg, Russia. [3] Nicolleau, F.C.G.A.; Cambon, C.; Redondo, J.M.; Vassilicos, J.C.; Reeks, M.; Nowakowski, A.F. (Eds.)(2012) New Approaches in Modeling Multiphase Flows and Dispersion in Turbulence, Fractal Methods and Synthetic Turbulence. ERCOFTAC Series. [4] Redondo J M, Sanchez M A and Cantalapiedra I R (1995). Turbulent Mechanisms in Stratified Flows, Dynamics of Atmospheres and Oceans, 23, 454-462. [5] Mahjoub O.B., Redondo J.M. and Babiano A.(2000) Hyerarchy flux in nonhomogeneous flows in Turbulent diffusion in the environment Eds. Redondo J.M. and Babiano A. 249-260. [6] Matulka A., Redondo J.M. and Carrillo A.(2008) Experiments in stratified and rotating decaying 2D flows, Il Nuovo Cimento 31, 5-6, 757-770. 2008. [7]Fraunie P., Berreba S. Chashechkin Yu.D., Velasco D. and Redondo J.M. (2008) Large eddy simulation and laboratory experiments on the decay of grid wakes in strongly stratified flows. Il Nuovo Cimento C 31, 909-930. [8]Matulka, A., López, P., Redondo, J. M., and Tarquis, A.(2014) On the entrainment coefficient in a forced plume: quantitative effects of source parameters, Nonlin. Processes Geophys., 21, 269-278. [9] Castilla R., Oñate E. and Redondo J.M. (2007) Models, Experiments and Computations in Turbulence. CIMNE, Barcelona, 255.

Downstream influence of swept slot injection in hypersonic turbulent flow

NASA Technical Reports Server (NTRS)

Hefner, J. N.; Cary, A. M., Jr.; Bushnell, D. B.

1977-01-01

Results of an experimental and numerical investigation of tangential swept slot injection into a thick turbulent boundary layer at Mach 6 are presented. Film cooling effectiveness, skin friction, and flow structure downstream of the swept slot injection were investigated. The data were compared with that for unswept slots, and it was found that cooling effectiveness and skin friction reductions are not significantly affected by sweeping the slot.

Soap film flows: Statistics of two-dimensional turbulence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Vorobieff, P.; Rivera, M.; Ecke, R.E.

1999-08-01

Soap film flows provide a very convenient laboratory model for studies of two-dimensional (2-D) hydrodynamics including turbulence. For a gravity-driven soap film channel with a grid of equally spaced cylinders inserted in the flow, we have measured the simultaneous velocity and thickness fields in the irregular flow downstream from the cylinders. The velocity field is determined by a modified digital particle image velocimetry method and the thickness from the light scattered by the particles in the film. From these measurements, we compute the decay of mean energy, enstrophy, and thickness fluctuations with downstream distance, and the structure functions of velocity,more » vorticity, thickness fluctuation, and vorticity flux. From these quantities we determine the microscale Reynolds number of the flow R{sub {lambda}}{approx}100 and the integral and dissipation scales of 2D turbulence. We also obtain quantitative measures of the degree to which our flow can be considered incompressible and isotropic as a function of downstream distance. We find coarsening of characteristic spatial scales, qualitative correspondence of the decay of energy and enstrophy with the Batchelor model, scaling of energy in {ital k} space consistent with the k{sup {minus}3} spectrum of the Kraichnan{endash}Batchelor enstrophy-scaling picture, and power-law scalings of the structure functions of velocity, vorticity, vorticity flux, and thickness. These results are compared with models of 2-D turbulence and with numerical simulations. {copyright} {ital 1999 American Institute of Physics.}« less

Pressure measurements in a rapidly sheared turbulent wall layer

NASA Astrophysics Data System (ADS)

Diwan, Sourabh; Morrison, Jonathan

2014-11-01

The aim of the present work is to improve understanding of the role of pressure fluctuations in the generation of coherent structures in wall-bounded turbulent flows, with particular regard to the rapid and slow source terms. The work is in part motivated by the recent numerical simulations of Sharma et al. (Phy. Fluids, 23, 2011), which showed the importance of pressure fluctuations (and their spatial gradients) in the dynamics of large-scale turbulent motions. Our experimental design consists of first generating a shearless boundary layer in a wind tunnel by passing a grid-generated turbulent flow over a moving floor whose speed is matched to the freestream velocity, and then shearing it rapidly by passing it over a stationary floor further downstream. Close to the leading edge of the stationary floor, the resulting flow is expected to satisfy the approximations of the Rapid Distortion Theory and therefore would be an ideal candidate for studying linear processes in wall turbulence. We carry out pressure measurements on the wall as well as within the flow - the former using surface mounted pressure transducers and the latter using a static pressure probe similar in design to that used by Tsuji et al. (J. Fluid. Mech. 585, 2007). We also present a comparison between the rapidly sheared flow and a more conventional boundary layer subjected to a turbulent free stream. We acknowledge the financial support from EPSRC (Grant No. EP/I037938).

Hydrodynamics of larval settlement: The influence of turbulent stress events at potential recruitment sites

USGS Publications Warehouse

Crimaldi, John P.; Thompson, Janet K.; Rosman, Johanna H.; Lowe, Ryan J.; Koseff, Jeffrey R.

2002-01-01

We describe a laboratory investigation into the effect of turbulent hydrodynamic stresses on clam larvae in the settlement phase of the recruitment process. A two-component laser-Doppler anemometer (LDA) was used to measure time histories of the instantaneous turbulence structure at potential recruitment sites within reconstructed beds of the adult Asian clam, Potamocorbula amurensis. Measurements were made for two flow speeds over beds with three different clam densities and two different clam heights. We analyze the statistical effect of the turbulence on the larval flux to the bed and on the probability of successful anchoring to the substrate. It is shown that the anchoring probability depends on the nature of the instantaneous stress events rather than on mean stresses. The instantaneous turbulence structure near the bed is altered by the flow rate and the spacing and height of adult clams living in the substrate. The ability to anchor quickly is therefore extremely important, since the time sequence of episodic turbulent stress events influences larval settlement success. The probability of successful larval settlement is predicted to decrease as the spacing between adults decreases, implying that the hydrodynamics impose negative feedback on clam bed aggregation dynamics.

Laser Doppler velocimeter measurements of boundary layer velocity and turbulent intensities in Mach 2.5 flow

NASA Technical Reports Server (NTRS)

Sewell, Jesse; Chew, Larry

1994-01-01

In recent years, the interest in developing a high-speed civil transport has increased. This has led to an increase in research activity on compressible supersonic flows, in particular the boundary layer. The structure of subsonic boundary layers has been extensively documented using conditional sampling techniques which exploit the knowledge of both u and v velocities. Researchers using these techniques have been able to explore some of the complex three-dimensional motions which are responsible for Reynolds stress production and transport in the boundary layer. As interest in turbulent structure has grown to include supersonic flows, a need for simultaneous multicomponent velocity measurements in these flows has developed. The success of conditional analysis in determining the characteristics of coherent motions and structures in the boundary layer relies on accurate, simultaneous measurement of two instantaneous velocity components.

Review Article: Advances in modeling of bed particle entrainment sheared by turbulent flow

NASA Astrophysics Data System (ADS)

Dey, Subhasish; Ali, Sk Zeeshan

2018-06-01

Bed particle entrainment by turbulent wall-shear flow is a key topic of interest in hydrodynamics because it plays a major role to govern the planetary morphodynamics. In this paper, the state-of-the-art review of the essential mechanisms governing the bed particle entrainment by turbulent wall-shear flow and their mathematical modeling is presented. The paper starts with the appraisal of the earlier multifaceted ideas in modeling the particle entrainment highlighting the rolling, sliding, and lifting modes of entrainment. Then, various modeling approaches of bed particle entrainment, such as deterministic, stochastic, and spatiotemporal approaches, are critically analyzed. The modeling criteria of particle entrainment are distinguished for hydraulically smooth, transitional, and rough flow regimes. In this context, the responses of particle size, particle exposure, and packing condition to the near-bed turbulent flow that shears the particles to entrain are discussed. From the modern experimental outcomes, the conceptual mechanism of particle entrainment from the viewpoint of near-bed turbulent coherent structures is delineated. As the latest advancement of the subject, the paper sheds light on the origin of the primitive empirical formulations of bed particle entrainment deriving the scaling laws of threshold flow velocity of bed particle motion from the perspective of the phenomenological theory of turbulence. Besides, a model framework that provides a new look on the bed particle entrainment phenomenon stemming from the stochastic-cum-spatiotemporal approach is introduced. Finally, the future scope of research is articulated with open questions.

Characterization of Mechanical Properties of Microbial Biofilms

NASA Astrophysics Data System (ADS)

Callison, Elizabeth; Gose, James; Perlin, Marc; Ceccio, Steven

2017-11-01

The physical properties of microbial biofilms grown subject to shear flows determine the form and mechanical characteristics of the biofilm structure, and consequently, the turbulent interactions over and through the biofilm. These biofilms - sometimes referred to as slime - are comprised of microbial cells and extracellular polymeric substance (EPS) matrices that surround the multicellular communities. Some of the EPSs take the form of streamers that tend to oscillate in flows, causing increased turbulent mixing and drag. As the presence of EPS governs the compliance and overall stability of the filamentous streamers, investigation of the mechanical properties of biofilms may also inform efforts to understand hydrodynamic performance of fouled systems. In this study, a mixture of four diatom genera was grown under turbulent shear flow on test panels. The mechanical properties and hydrodynamic performance of the biofilm were investigated using rheology and turbulent flow studies in the Skin-Friction Flow Facility at the University of Michigan. The diatoms in the mixture of algae were identified, and the elastic and viscous moduli were determined from small-amplitude oscillations, while a creep test was used to evaluate the biofilm compliance.

Flowing biofilms as a transport mechanism for biomass through porous media under laminar and turbulent conditions in a laboratory reactor system.

PubMed

Stoodley, P; Dodds, I; De Beer, D; Scott, H Lappin; Boyle, J D

2005-01-01

Fluid flow has been shown to be important in influencing biofilm morphology and causing biofilms to flow over surfaces in flow cell experiments. However, it is not known whether similar effects may occur in porous media. Generally, it is assumed that the primary transport mechanism for biomass in porous media is through convection, as suspended particulates (cells and flocs) carried by fluid flowing through the interstices. However, the flow of biofilms over the surfaces of soils and sediment particles, may represent an important flux of biomass, and subsequently affect both biological activity and permeability. Mixed species bacterial biofilms were grown in glass flow cells packed with 1 mm diameter glass beads, under laminar or turbulent flow (porous media Reynolds number = 20 and 200 respectively). The morphology and dynamic behavior reflected those of biofilms grown in the open flow cells. The laminar biofilm was relatively uniform and after 23 d had inundated the majority of the pore spaces. Under turbulent flow the biofilm accumulated primarily in protected regions at contact points between the beads and formed streamers that trailed from the leeward face. Both biofilms caused a 2 to 3-fold increase in friction factor and in both cases there were sudden reductions in friction factor followed by rapid recovery, suggesting periodic sloughing and regrowth events. Time-lapse microscopy revealed that under both laminar and turbulent conditions biofilms flowed over the surface of the porous media. In some instances ripple structures formed. The velocity of biofilm flow was on the order of 10 mum h(-1) in the turbulent flow cell and 1.0 mum h(-1) in the laminar flow cell.

On the aeroacoustic and flow structures developed on a flat plate with a serrated sawtooth trailing edge

NASA Astrophysics Data System (ADS)

Chong, Tze Pei; Vathylakis, Alexandros

2015-10-01

Results of an experimental study on turbulent flow over a flat plate with a serrated sawtooth trailing edge are presented in this paper. After tripping the boundary layer to become turbulent, the broadband noise sources at the sawtooth serrated trailing edge is studied by several experimental techniques. Broadband noise reduction by the serrated sawtooth trailing edge can be realistically achieved in the flat plate configuration. The variations of wall pressure power spectral density and the spanwise coherence (which relates to the spanwise correlation length) in a sawtooth trailing edge play a minor role in the mechanisms underpinning the reduction of self noise radiation. Conditional-averaging technique was applied in the boundary layer data where a pair of pressure-driven oblique vortical structures near the sawtooth side edges is identified. In the current flat plate configuration, the interaction between the vortical structures and the local turbulent boundary layer results in a redistribution of the momentum transport and turbulent shear stress near the sawtooth side edges as well as the sawtooth tip, thus affecting the efficiency of self noise radiation.

Large Eddy Simulation of a Film Cooling Flow Injected from an Inclined Discrete Cylindrical Hole into a Crossflow with Zero-Pressure Gradient Turbulent Boundary Layer

NASA Technical Reports Server (NTRS)

Johnson, Perry L.; Shyam, Vikram

2012-01-01

A Large Eddy Simulation (LES) is performed of a high blowing ratio (M = 1.7) film cooling flow with density ratio of unity. Mean results are compared with experimental data to show the degree of fidelity achieved in the simulation. While the trends in the LES prediction are a noticeable improvement over Reynolds-Averaged Navier-Stokes (RANS) predictions, there is still a lack a spreading on the underside of the lifted jet. This is likely due to the inability of the LES to capture the full range of influential eddies on the underside of the jet due to their smaller structure. The unsteady structures in the turbulent coolant jet are also explored and related to turbulent mixing characteristics

Turbulent solutions of the equations of fluid motion

NASA Technical Reports Server (NTRS)

Deissler, R. G.

1984-01-01

Some turbulent solutions of the unaveraged Navier-Stokes equations (equations of fluid motion) are reviewed. Those equations are solved numerically in order to study the nonlinear physics of incompressible turbulent flow. Initial three-dimensional cosine velocity fluctuations and periodic boundary conditions are used in most of the work considered. The three components of the mean-square velocity fluctuations are initially equal for the conditions chosen. The resulting solutions show characteristics of turbulence such as the linear and nonlinear excitation of small-scale fluctuations. For the stronger fluctuations, the initially nonrandom flow develops into an apparently random turbulence. Thus randomness or turbulence can arise as a consequence of the structure of the Navier-Stokes equations. The cases considered include turbulence which is statistically homogeneous or inhomogeneous and isotropic or anisotropic. A mean shear is present in some cases. A statistically steady-state turbulence is obtained by using a spatially periodic body force. Various turbulence processes, including the transfer of energy between eddy sizes and between directional components, and the production, dissipation, and spatial diffusion of turbulence, are considered. It is concluded that the physical processes occurring in turbulence can be profitably studied numerically.

The structure of the solution obtained with Reynolds-stress-transport models at the free-stream edges of turbulent flows

NASA Astrophysics Data System (ADS)

Cazalbou, J.-B.; Chassaing, P.

2002-02-01

The behavior of Reynolds-stress-transport models at the free-stream edges of turbulent flows is investigated. Current turbulent-diffusion models are found to produce propagative (possibly weak) solutions of the same type as those reported earlier by Cazalbou, Spalart, and Bradshaw [Phys. Fluids 6, 1797 (1994)] for two-equation models. As in the latter study, an analysis is presented that provides qualitative information on the flow structure predicted near the edge if a condition on the values of the diffusion constants is satisfied. In this case, the solution appears to be fairly insensitive to the residual free-stream turbulence levels needed with conventional numerical methods. The main specific result is that, depending on the diffusion model, the propagative solution can force turbulence toward definite and rather extreme anisotropy states at the edge (one- or two-component limit). This is not the case with the model of Daly and Harlow [Phys. Fluids 13, 2634 (1970)]; it may be one of the reasons why this "old" scheme is still the most widely used, even in recent Reynolds-stress-transport models. In addition, the analysis helps us to interpret some difficulties encountered in computing even very simple flows with Lumley's pressure-diffusion model [Adv. Appl. Mech. 18, 123 (1978)]. A new realizability condition, according to which the diffusion model should not globally become "anti-diffusive," is introduced, and a recalibration of Lumley's model satisfying this condition is performed using information drawn from the analysis.

Structure and modeling of turbulence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Novikov, E.A.

The {open_quotes}vortex strings{close_quotes} scale l{sub s} {approximately} LRe{sup -3/10} (L-external scale, Re - Reynolds number) is suggested as a grid scale for the large-eddy simulation. Various aspects of the structure of turbulence and subgrid modeling are described in terms of conditional averaging, Markov processes with dependent increments and infinitely divisible distributions. The major request from the energy, naval, aerospace and environmental engineering communities to the theory of turbulence is to reduce the enormous number of degrees of freedom in turbulent flows to a level manageable by computer simulations. The vast majority of these degrees of freedom is in the small-scalemore » motion. The study of the structure of turbulence provides a basis for subgrid-scale (SGS) models, which are necessary for the large-eddy simulations (LES).« less

The third-order structure function in two dimensions: The Rashomon effect

NASA Astrophysics Data System (ADS)

Cerbus, Rory T.; Chakraborty, Pinaki

2017-11-01

We study the third-order longitudinal structure function, S3(r), in two-dimensional turbulence. In three dimensions, there is considerable theoretical, experimental, and numerical consensus regarding the validity of Kolmogorov's arch-famous " /4 5 th law" for S3(r). By contrast, in two dimensions, two disparate cascades, changed dissipation anomalies, a large-scale drag, and other factors conspire to create several versions of the S3(r) "law." This single quantity can vary considerably when viewed from different perspectives, reminiscent of the "Rashomon effect" in anthropology. After reviewing the history and usage of S3(r) in two-dimensional turbulence, we show that S3(r) generically embodies a mixture of energy and enstrophy fluxes. Building on this result, we derive S3(r) laws for freely decaying and forced two-dimensional turbulent flows, where we also account for the effects of a large-scale drag, an inextricable feature of quasi two-dimensional turbulence in experimental and atmospheric flows. We draw attention to the caution needed in interpreting S3(r) in two-dimensional turbulence.

Near-field acoustic radiation by high-speed turbulence: amplitude, structure, gas-stiffness, and dilatational dissipation

NASA Astrophysics Data System (ADS)

Buchta, David; Freund, Jonathan

2017-11-01

High-speed (supersonic) turbulent shear flows are well-known to radiate pressure-wave patterns that have higher positive peaks than negative valleys, which yields a notable skewness, usually with Sk > 0.4 . Direct numerical simulations (DNS) of planar turbulent mixing layers at different Mach numbers (M) are used to examine this. The baseline simulations, of an air-like gas at speeds up to M = 3.5 , reproduced the observed behavior of jets. Simulations initialized with corresponding instability modes show that Sk increases linearly with the velocity amplitude (Mt =√{ui' ui'} /co), reflecting the M dependence of the DNS, which can be related to simpler gas dynamic flows. Simulations with a stiffened-gas equation of state (often used to model liquids) show essentially the same Mach-number dependence, despite the nominally greater resistance to compressibility. Turbulence simulations with an artificial energy reallocation mechanism, imposed to alter its structure, show little change in Sk. Finally, we also consider significantly increased bulk viscosity to suppress dilatation. In this case, Sk diminishes along with the sound-field intensity, though the turbulence stresses themselves are nearly unchanged.

A low-dissipation monotonicity-preserving scheme for turbulent flows in hydraulic turbines

NASA Astrophysics Data System (ADS)

Yang, L.; Nadarajah, S.

2016-11-01

The objective of this work is to improve the inherent dissipation of the numerical schemes under the framework of a Reynolds-averaged Navier-Stokes (RANS) simulation. The governing equations are solved by the finite volume method with the k-ω SST turbulence model. Instead of the van Albada limiter, a novel eddy-preserving limiter is employed in the MUSCL reconstructions to minimize the dissipation of the vortex. The eddy-preserving procedure inactivates the van Albada limiter in the swirl plane and reduces the artificial dissipation to better preserve vortical flow structures. Steady and unsteady simulations of turbulent flows in a straight channel and a straight asymmetric diffuser are demonstrated. Profiles of velocity, Reynolds shear stress and turbulent kinetic energy are presented and compared against large eddy simulation (LES) and/or experimental data. Finally, comparisons are made to demonstrate the capability of the eddy-preserving limiter scheme.

Sheared E×B flow and plasma turbulence viscosity in a Reversed Field Pinch

NASA Astrophysics Data System (ADS)

Vianello, N.; Antoni, V.; Spada, E.; Spolaore, M.; Serianni, G.; Regnoli, G.; Zuin, M.; Cavazzana, R.; Bergsåker, H.; Cecconello, M.; Drake, J. R.

2004-11-01

The relationship between electromagnetic turbulence and sheared plasma flow in Reversed Field Pinch configuration is addressed. The momentum balance equation for a compressible plasma is considered and the terms involved are measured in the outer region of Extrap-T2R RFP device. It results that electrostatic fluctuations determine the plasma flow through the electrostatic component of Reynolds Stress tensor. This term involves spatial and temporal scales comparable to those of MHD activity. The derived experimental perpendicular viscosity is consistent with anomalous diffusion, the latter being discussed in terms of electrostatic turbulence background and coherent structures emerging from fluctuations. The results indicate a dynamical interplay between turbulence, anomalous transport and mean E×B profiles. The momentum balance has been studied also in non-stationary condition during the application of Pulsed Poloidal Current Drive, which is known to reduce the amplitude of MHD modes.

Improved turbulence models based on large eddy simulation of homogeneous, incompressible turbulent flows

NASA Technical Reports Server (NTRS)

Bardino, J.; Ferziger, J. H.; Reynolds, W. C.

1983-01-01

The physical bases of large eddy simulation and subgrid modeling are studied. A subgrid scale similarity model is developed that can account for system rotation. Large eddy simulations of homogeneous shear flows with system rotation were carried out. Apparently contradictory experimental results were explained. The main effect of rotation is to increase the transverse length scales in the rotation direction, and thereby decrease the rates of dissipation. Experimental results are shown to be affected by conditions at the turbulence producing grid, which make the initial states a function of the rotation rate. A two equation model is proposed that accounts for effects of rotation and shows good agreement with experimental results. In addition, a Reynolds stress model is developed that represents the turbulence structure of homogeneous shear flows very well and can account also for the effects of system rotation.

The structure of evaporating and combusting sprays: Measurements and predictions

NASA Technical Reports Server (NTRS)

Shuen, J. S.; Solomon, A. S. P.; Faeth, G. M.

1982-01-01

An apparatus was constructed to provide measurements in open sprays with no zones of recirculation, in order to provide well-defined conditions for use in evaluating spray models. Measurements were completed in a gas jet, in order to test experimental methods, and are currently in progress for nonevaporating sprays. A locally homogeneous flow (LHF) model where interphase transport rates are assumed to be infinitely fast; a separated flow (SF) model which allows for finite interphase transport rates but neglects effects of turbulent fluctuations on drop motion; and a stochastic SF model which considers effects of turbulent fluctuations on drop motion were evaluated using existing data on particle-laden jets. The LHF model generally overestimates rates of particle dispersion while the SF model underestimates dispersion rates. The stochastic SF flow yield satisfactory predictions except at high particle mass loadings where effects of turbulence modulation may have caused the model to overestimate turbulence levels.

An experimental and numerical study of diffusion flames in cross-flow and quiescent environment at smoke point condition

NASA Astrophysics Data System (ADS)

Goh, Sien Fong

An experimental and numerical study of a turbulent smoke point diffusion flame in a quiescent and cross-flow condition was performed. The fuel mass flow rate of a turbulent smoke point flame was determined at a quiescent condition and in cross-flow with velocity ranging from 2 to 4 m/s. This fuel mass flow rate is defined as the Critical Fuel Mass Flow Rate (CFMFR). At a fuel mass flow rate below the CFMFR the flame produces smoke. In the dilution study, an amount of inert gas (nitrogen) was added to the fuel stream to achieve the smoke point condition for ten different fractions of CFMFR. From this dilution study, three regions were defined, the chemically-dominated region, transition region, and momentum-dominated region. The first objective of this study was to determine the factors behind the distinction of these three regions. The second objective was to understand the effect of cross-flow velocity on the smoke point flame structure. The flame temperature, radiation, geometrical dimension of flame, velocity, and global emissions and in-flame species concentration were measured. The third objective was to study a numerical model that can simulate the turbulent smoke point flame structure. The dilution study showed that the flames in quiescent condition and in the 3.5 and 4 m/s cross-flow condition had the chemically-dominated region at 5% to 20% CFMFR, the transition region at 20% to 40% CFMFR, and the momentum-dominated region at 40% to 100% CFMFR. On the other hand, the flame in cross-flow of 2 to 3 m/s showed the chemically-dominated region at 5% to 10% CFMFR, the transition region at 10% to 30% CFMFR, and the momentum-dominated region at 30% to 100% CFMFR. The chemically-dominated flame had a sharp dual-peak structure for the flame temperature, CO2 and NO concentration profiles at 25% and 50% flame length. However, the momentum-dominated region flame exhibited a dual peak structure only at 25% flame length. The decrease of flow rate from 30% to 10% CFMFR showed an increase of flame length. The LII study showed that the soot concentration increased with the decrease of the turbulence intensity in the momentum dominated region (tested on the 100% and 60% CFMFR). The cross-flow velocity had a non-monotonic effects on the flame. The evidences could be observed from the flame length and the soot concentration results. The flame length showed a decrease when the cross-flow velocity increased from 2 to 3 m/s. The numerical model was fairly adequate in qualitatively predicting a smoke point turbulent diffusion flame structure in a cross-flow and quiescent condition. The model failed in the prediction of a laminar flame. The model showed a good agreement between experimental and numerical results for O 2 concentration and flame temperature. (Abstract shortened by UMI.)

Connections between density, wall-normal velocity, and coherent structure in a heated turbulent boundary layer

NASA Astrophysics Data System (ADS)

Saxton-Fox, Theresa; Gordeyev, Stanislav; Smith, Adam; McKeon, Beverley

2015-11-01

Strong density gradients associated with turbulent structure were measured in a mildly heated turbulent boundary layer using an optical sensor (Malley probe). The Malley probe measured index of refraction gradients integrated along the wall-normal direction, which, due to the proportionality of index of refraction and density in air, was equivalently an integral measure of density gradients. The integral output was observed to be dominated by strong, localized density gradients. Conditional averaging and Pearson correlations identified connections between the streamwise gradient of density and the streamwise gradient of wall-normal velocity. The trends were suggestive of a process of pick-up and transport of heat away from the wall. Additionally, by considering the density field as a passive marker of structure, the role of the wall-normal velocity in shaping turbulent structure in a sheared flow was examined. Connections were developed between sharp gradients in the density and flow fields and strong vertical velocity fluctuations. This research is made possible by the Department of Defense through the National Defense & Engineering Graduate Fellowship (NDSEG) Program and by the Air Force Office of Scientific Research Grant # FA9550-12-1-0060.

The Impact of Unsteady Reconfiguration on Turbulence Structure within a Flexible Canopy: Large-Eddy Simulation Study of a Cornfield

NASA Astrophysics Data System (ADS)

Chamecki, M.; Pan, Y.; Nepf, H. M.; Follett, E.

2014-12-01

Flexible plants bend in response to fluid motion and this reconfiguration mechanism allows plants to minimize the increase of drag force with increasing velocity, ensuring survival in flow-dominated habitats. The effect of reconfiguration on the flow field can be modeled by introducing a drag coefficient that decreases with increasing velocity. Typically, a power-law decrease of the drag coefficient with increasing velocity is used, and the negative exponent is known as the Vogel number. In practice, the Vogel number is a function of canopy rigidity and flow conditions. In this work we show that accounting for the effect of reconfiguration is required for large-eddy simulation (LES) models to reproduce the skewness of the streamwise and vertical velocity components and the distribution of sweeps and ejections observed in a large cornfield. Additional LES runs are conducted to investigate the structure of turbulence in different reconfiguration regimes, with mean vertical momentum flux constrained by measurements. The change of the Vogel number has negligible effects on LES predictions of the total vertical momentum flux and the components of turbulent kinetic energy, but produces profound changes in the mechanisms of momentum transport. This work demonstrates the necessity to model the effect of reconfiguration in LES studies of canopy flows. It also highlights the impacts of reconfiguration on the structure of turbulence and the dynamics of momentum fluxes, as well as any other process that depends on velocity fluctuations above and within the canopy region.

Flow in Atherosclerotic Blood Vessels

NASA Astrophysics Data System (ADS)

Berger, Stanley A.; Stroud, Jenn S.

2000-11-01

Atherosclerotic lesions occur in arteries where there are major changes in flow structure, e.g. bifurcations and junctions. The reduction of vessel lumen alters the flow, including the mechanical forces on the walls. We have examined the flow in carotid artery bifurcations with realistic plaque contours. The unsteady, incompressible, Navier-Stokes equations are solved in finite-volume form. Steady and pulsatile flows have been analyzed for laminar and turbulent flows, using for the latter a low-Reynolds number k- ɛ model and a k-ω model. Non-Newtonian viscosity is also considered using a power-law model. In general the very irregular contours of the vessels lead to recirculating regions, strong spatial variations of wall shear stresses, and in some cases, vortex shedding. Even steady inlet flow exhibits fluctuating, unsteady behavior. Neither turbulence models captures all the physics of the flow. The flow, in fact, appears to be transitional and not fully turbulent. For unsteady flow, there are also strong temporal variations of normal and shear stresses, which together with the strong spatial variations, has important implications for the onset and progression of atherosclerotic disease.

The role of zonal flows and predator–prey oscillations in triggering the formation of edge and core transport barriers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Schmitz, Lothar; Zeng, Lei; Rhodes, Terry L.

2014-04-24

Here, we present direct evidence of low frequency, radially sheared, turbulence-driven flows (zonal flows (ZFs)) triggering edge transport barrier formation preceding the L- to H-mode transition via periodic turbulence suppression in limit-cycle oscillations (LCOs), consistent with predator–prey dynamics. The final transition to edge-localized mode-free H-mode occurs after the equilibrium E × B flow shear increases due to ion pressure profile evolution. ZFs are also observed to initiate formation of an electron internal transport barrier (ITB) at the q = 2 rational surface via local suppression of electron-scale turbulence. Multi-channel Doppler backscattering (DBS) has revealed the radial structure of the ZF-induced shear layer and the E × B shearing rate, ω E×B, in both barrier types. During edge barrier formation, the shearing rate lags the turbulence envelope during the LCO by 90°, transitioning to anti-correlation (180°) when the equilibrium shear dominates the turbulence-driven flow shear due to the increasing edge pressure gradient. The time-dependent flow shear and the turbulence envelope are anti-correlated (180° out of phase) in the electron ITB. LCOs with time-reversed evolution dynamics (transitioning from an equilibrium-flow dominated to a ZF-dominated state) have also been observed during the H–L back-transition and are potentially of interest for controlled ramp-down of the plasma stored energy and pressure (normalized to the poloidal magnetic field)more » $$\\beta_{\\theta} =2\\mu_{0} n{( {T_{{\\rm e}} +T_{{\\rm i}}})}/{B_{\\theta}^{2}}$$ in ITER.« less

The role of zonal flows and predator-prey oscillations in triggering the formation of edge and core transport barriers

NASA Astrophysics Data System (ADS)

Schmitz, L.; Zeng, L.; Rhodes, T. L.; Hillesheim, J. C.; Peebles, W. A.; Groebner, R. J.; Burrell, K. H.; McKee, G. R.; Yan, Z.; Tynan, G. R.; Diamond, P. H.; Boedo, J. A.; Doyle, E. J.; Grierson, B. A.; Chrystal, C.; Austin, M. E.; Solomon, W. M.; Wang, G.

2014-07-01

We present direct evidence of low frequency, radially sheared, turbulence-driven flows (zonal flows (ZFs)) triggering edge transport barrier formation preceding the L- to H-mode transition via periodic turbulence suppression in limit-cycle oscillations (LCOs), consistent with predator-prey dynamics. The final transition to edge-localized mode-free H-mode occurs after the equilibrium E × B flow shear increases due to ion pressure profile evolution. ZFs are also observed to initiate formation of an electron internal transport barrier (ITB) at the q = 2 rational surface via local suppression of electron-scale turbulence. Multi-channel Doppler backscattering (DBS) has revealed the radial structure of the ZF-induced shear layer and the E × B shearing rate, ωE×B, in both barrier types. During edge barrier formation, the shearing rate lags the turbulence envelope during the LCO by 90°, transitioning to anti-correlation (180°) when the equilibrium shear dominates the turbulence-driven flow shear due to the increasing edge pressure gradient. The time-dependent flow shear and the turbulence envelope are anti-correlated (180° out of phase) in the electron ITB. LCOs with time-reversed evolution dynamics (transitioning from an equilibrium-flow dominated to a ZF-dominated state) have also been observed during the H-L back-transition and are potentially of interest for controlled ramp-down of the plasma stored energy and pressure (normalized to the poloidal magnetic field) \\beta_{\\theta} =2\\mu_{0} n{( {T_{e} +T_{i}})}/{B_{\\theta}^{2}} in ITER.

Theoretical study of turbulent channel flow - Bulk properties, pressure fluctuations, and propagation of electromagnetic waves

NASA Technical Reports Server (NTRS)

Canuto, V. M.; Hartke, G. J.; Battaglia, A.; Chasnov, J.; Albrecht, G. F.

1990-01-01

In this paper, we apply two theoretical turbulence models, DIA and the recent GISS model, to study properties of a turbulent channel flow. Both models provide a turbulent kinetic energy spectral function E(k) as the solution of a non-linear equation; the two models employ the same source function but different closures. The source function is characterized by a rate n sub s (k) which is derived from the complex eigenvalues of the Orr-Sommerfeld (OS) equation in which the basic flow is taken to be of a Poiseuille type. The O-S equation is solved for a variety of Reynolds numbers corresponding to available experimental data. A physical argument is presented whereby the central line velocity characterizing the basic flow, U0 sup L, is not to be identified with the U0 appearing in the experimental Reynolds number. The theoretical results are compared with two types of experimental data: (1) turbulence bulk properties, and (2) properties that depend strongly on the structure of the turbulence spectrum at low wave numbers. The only existing analytical expression for Pi (k) cannot be used in the present case because it applies to the case of a flat plate, not a finite channel.

Turbulent flow structures and aeolian sediment transport over a barchan sand dune

NASA Astrophysics Data System (ADS)

Wiggs, G. F. S.; Weaver, C. M.

2012-03-01

The turbulent structure of airflow over a barchan sand dune is determined using quadrant analysis of wind velocity data derived from sonic anemometers. Results indicate an increased frequency of ejection and sweep events in the toe region of the dune, characteristic of the turbulent bursting process. In contrast, at the crest there was a significant increase in the occurrence of outward interactions. Combined with high frequency saltation data our analyses show that turbulent structures characterised by a positive streamwise fluctuating velocity (+u‧ sweeps at the toe and outward interactions at the crest) have a dominant influence on sand transport on the dune, together accounting for up to 83% and 95% of transporting events at the toe and crest respectively.

Imaging of supersonic flow over a double elliptic surface

NASA Astrophysics Data System (ADS)

Zhang, Qing-Hu; Yi, Shi-He; He, Lin; Zhu, Yang-Zhu; Chen, Zhi

2013-11-01

The coherent structures of flow over a double elliptic surface are experimentally investigated in a supersonic low-noise wind tunnel at Mach number 3 using nano-tracer planar laser scattering (NPLS) and particle image velocimetry (PIV) techniques. High spatiotemporal resolution images and velocity fields of both laminar and turbulent inflows over the test model are captured. Based on the time-correlation images, the spatial and temporal evolutionary characteristics of the coherent structures are investigated. The flow structures in the NPLS images are in good agreement with the velocity fluctuation fields by PIV. From statistically significant ensembles, spatial correlation analysis of both cases is performed to quantify the mean size and the orientation of coherent structures. The results indicate that the mean structure is elliptical in shape and the structural angles in the separated region of laminar inflow are slightly smaller than that of turbulent inflow. Moreover, the structural angles of both cases increase with their distance away from the wall.

Ion-scale turbulence in MAST: anomalous transport, subcritical transitions, and comparison to BES measurements

NASA Astrophysics Data System (ADS)

van Wyk, F.; Highcock, E. G.; Field, A. R.; Roach, C. M.; Schekochihin, A. A.; Parra, F. I.; Dorland, W.

2017-11-01

We investigate the effect of varying the ion temperature gradient (ITG) and toroidal equilibrium scale sheared flow on ion-scale turbulence in the outer core of MAST by means of local gyrokinetic simulations. We show that nonlinear simulations reproduce the experimental ion heat flux and that the experimentally measured values of the ITG and the flow shear lie close to the turbulence threshold. We demonstrate that the system is subcritical in the presence of flow shear, i.e., the system is formally stable to small perturbations, but transitions to a turbulent state given a large enough initial perturbation. We propose that the transition to subcritical turbulence occurs via an intermediate state dominated by low number of coherent long-lived structures, close to threshold, which increase in number as the system is taken away from the threshold into the more strongly turbulent regime, until they fill the domain and a more conventional turbulence emerges. We show that the properties of turbulence are effectively functions of the distance to threshold, as quantified by the ion heat flux. We make quantitative comparisons of correlation lengths, times, and amplitudes between our simulations and experimental measurements using the MAST BES diagnostic. We find reasonable agreement of the correlation properties, most notably of the correlation time, for which significant discrepancies were found in previous numerical studies of MAST turbulence.

Nature, theory and modelling of geophysical convective planetary boundary layers

NASA Astrophysics Data System (ADS)

Zilitinkevich, Sergej

2015-04-01

Geophysical convective planetary boundary layers (CPBLs) are still poorly reproduced in oceanographic, hydrological and meteorological models. Besides the mean flow and usual shear-generated turbulence, CPBLs involve two types of motion disregarded in conventional theories: 'anarchy turbulence' comprised of the buoyancy-driven plumes, merging to form larger plumes instead of breaking down, as postulated in conventional theory (Zilitinkevich, 1973), large-scale organised structures fed by the potential energy of unstable stratification through inverse energy transfer in convective turbulence (and performing non-local transports irrespective of mean gradients of transporting properties). C-PBLs are strongly mixed and go on growing as long as the boundary layer remains unstable. Penetration of the mixed layer into the weakly turbulent, stably stratified free flow causes turbulent transports through the CPBL outer boundary. The proposed theory, taking into account the above listed features of CPBL, is based on the following recent developments: prognostic CPBL-depth equation in combination with diagnostic algorithm for turbulence fluxes at the CPBL inner and outer boundaries (Zilitinkevich, 1991, 2012, 2013; Zilitinkevich et al., 2006, 2012), deterministic model of self-organised convective structures combined with statistical turbulence-closure model of turbulence in the CPBL core (Zilitinkevich, 2013). It is demonstrated that the overall vertical transports are performed mostly by turbulence in the surface layer and entrainment layer (at the CPBL inner and outer boundaries) and mostly by organised structures in the CPBL core (Hellsten and Zilitinkevich, 2013). Principal difference between structural and turbulent mixing plays an important role in a number of practical problems: transport and dispersion of admixtures, microphysics of fogs and clouds, etc. The surface-layer turbulence in atmospheric and marine CPBLs is strongly enhanced by the velocity shears in horizontal branches of organised structures. This mechanism (Zilitinkevich et al., 2006), was overlooked in conventional local theories, such as the Monin-Obukhov similarity theory, and convective heat/mass transfer law: Nu~Ra1/3, where Nu and Ra are the Nusselt number and Raleigh numbers. References Hellsten A., Zilitinkevich S., 2013: Role of convective structures and background turbulence in the dry convective boundary layer. Boundary-Layer Meteorol. 149, 323-353. Zilitinkevich, S.S., 1973: Shear convection. Boundary-Layer Meteorol. 3, 416-423. Zilitinkevich, S.S., 1991: Turbulent Penetrative Convection, Avebury Technical, Aldershot, 180 pp. Zilitinkevich S.S., 2012: The Height of the Atmospheric Planetary Boundary layer: State of the Art and New Development - Chapter 13 in 'National Security and Human Health Implications of Climate Change', edited by H.J.S. Fernando, Z. Klaić, J.L. McKulley, NATO Science for Peace and Security Series - C: Environmental Security (ISBN 978-94-007-2429-7), Springer, 147-161. Zilitinkevich S.S., 2013: Atmospheric Turbulence and Planetary Boundary Layers. Fizmatlit, Moscow, 248 pp. Zilitinkevich, S.S., Hunt, J.C.R., Grachev, A.A., Esau, I.N., Lalas, D.P., Akylas, E., Tombrou, M., Fairall, C.W., Fernando, H.J.S., Baklanov, and A., Joffre, S.M., 2006: The influence of large convective eddies on the surface layer turbulence. Quart. J. Roy. Met. Soc. 132, 1423-1456. Zilitinkevich S.S., Tyuryakov S.A., Troitskaya Yu. I., Mareev E., 2012: Theoretical models of the height of the atmospheric planetary boundary layer and turbulent entrainment at its upper boundary. Izvestija RAN, FAO, 48, No.1, 150-160 Zilitinkevich, S.S., Elperin, T., Kleeorin, N., Rogachevskii, I., Esau, I.N., 2013: A hierarchy of energy- and flux-budget (EFB) turbulence closure models for stably stratified geophysical flows. Boundary-Layer Meteorol. 146, 341-373.

Drag reduction induced by superhydrophobic surfaces in turbulent pipe flow

NASA Astrophysics Data System (ADS)

Costantini, Roberta; Mollicone, Jean-Paul; Battista, Francesco

2018-02-01

The drag reduction induced by superhydrophobic surfaces is investigated in a turbulent pipe flow. Wetted superhydrophobic surfaces are shown to trap gas bubbles in their asperities. This stops the liquid from coming in direct contact with the wall in that location, allowing the flow to slip over the air bubbles. We consider a well-defined texture with streamwise grooves at the walls in which the gas is expected to be entrapped. This configuration is modeled with alternating no-slip and shear-free boundary conditions at the wall. With respect to the classical turbulent pipe flow, a substantial drag reduction is observed which strongly depends on the grooves' dimension and on the solid fraction, i.e., the ratio between the solid wall surface and the total surface of the pipe's circumference. The drag reduction is due to the mean slip velocity at the wall which increases the flow rate at a fixed pressure drop. The enforced boundary conditions also produce peculiar turbulent structures which on the contrary decrease the flow rate. The two concurrent effects provide an overall flow rate increase as demonstrated by means of the mean axial momentum balance. This equation provides the balance between the mean pressure gradient, the Reynolds stress, the mean flow rate, and the mean slip velocity contributions.

Internal and external 2-d boundary layer flows

NASA Technical Reports Server (NTRS)

Crawford, M. E.; Kays, W. M.

1978-01-01

Computer program computes general two dimensional turbulent boundary-layer flow using finite-difference techniques. Structure allows for user modification to accommodate unique problems. Program should prove useful in many applications where accurate boundary-layer flow calculations are required.

Low-dimensional representations of exact coherent states of the Navier-Stokes equations from the resolvent model of wall turbulence.

PubMed

Sharma, Ati S; Moarref, Rashad; McKeon, Beverley J; Park, Jae Sung; Graham, Michael D; Willis, Ashley P

2016-02-01

We report that many exact invariant solutions of the Navier-Stokes equations for both pipe and channel flows are well represented by just a few modes of the model of McKeon and Sharma [J. Fluid Mech. 658, 336 (2010)]. This model provides modes that act as a basis to decompose the velocity field, ordered by their amplitude of response to forcing arising from the interaction between scales. The model was originally derived from the Navier-Stokes equations to represent turbulent flows and has been used to explain coherent structure and to predict turbulent statistics. This establishes a surprising new link between the two distinct approaches to understanding turbulence.

Low-dimensional representations of exact coherent states of the Navier-Stokes equations from the resolvent model of wall turbulence

NASA Astrophysics Data System (ADS)

Sharma, Ati S.; Moarref, Rashad; McKeon, Beverley J.; Park, Jae Sung; Graham, Michael D.; Willis, Ashley P.

2016-02-01

We report that many exact invariant solutions of the Navier-Stokes equations for both pipe and channel flows are well represented by just a few modes of the model of McKeon and Sharma [J. Fluid Mech. 658, 336 (2010), 10.1017/S002211201000176X]. This model provides modes that act as a basis to decompose the velocity field, ordered by their amplitude of response to forcing arising from the interaction between scales. The model was originally derived from the Navier-Stokes equations to represent turbulent flows and has been used to explain coherent structure and to predict turbulent statistics. This establishes a surprising new link between the two distinct approaches to understanding turbulence.

Numerical Modeling of Turbulent Combustion

NASA Technical Reports Server (NTRS)

Ghoneim, A. F.; Chorin, A. J.; Oppenheim, A. K.

1983-01-01

The work in numerical modeling is focused on the use of the random vortex method to treat turbulent flow fields associated with combustion while flame fronts are considered as interfaces between reactants and products, propagating with the flow and at the same time advancing in the direction normal to themselves at a prescribed burning speed. The latter is associated with the generation of specific volume (the flame front acting, in effect, as the locus of volumetric sources) to account for the expansion of the flow field due to the exothermicity of the combustion process. The model was applied to the flow in a channel equipped with a rearward facing step. The results obtained revealed the mechanism of the formation of large scale turbulent structure in the wake of the step, while it showed the flame to stabilize on the outer edges of these eddies.

Hairpin vortices in turbulent boundary layers

NASA Astrophysics Data System (ADS)

Eitel-Amor, G.; Örlü, R.; Schlatter, P.; Flores, O.

2015-02-01

The present work presents a number of parallel and spatially developing simulations of boundary layers to address the question of whether hairpin vortices are a dominant feature of near-wall turbulence, and which role they play during transition. In the first part, the parent-offspring regeneration mechanism is investigated in parallel (temporal) simulations of a single hairpin vortex introduced in a mean shear flow corresponding to either turbulent channels or boundary layers (Reτ ≲ 590). The effect of a turbulent background superimposed on the mean flow is considered by using an eddy viscosity computed from resolved simulations. Tracking the vortical structure downstream, it is found that secondary hairpins are only created shortly after initialization, with all rotational structures decaying for later times. For hairpins in a clean (laminar) environment, the decay is relatively slow, while hairpins in weak turbulent environments (10% of νt) dissipate after a couple of eddy turnover times. In the second part, the role of hairpin vortices in laminar-turbulent transition is studied using simulations of spatial boundary layers tripped by hairpin vortices. These vortices are generated by means of specific volumetric forces representing an ejection event, creating a synthetic turbulent boundary layer initially dominated by hairpin-like vortices. These hairpins are advected towards the wake region of the boundary layer, while a sinusoidal instability of the streaks near the wall results in rapid development of a turbulent boundary layer. For Reθ > 400, the boundary layer is fully developed, with no evidence of hairpin vortices reaching into the wall region. The results from both the parallel and spatial simulations strongly suggest that the regeneration process is rather short-lived and may not sustain once a turbulent background is developed. From the transitional flow simulations, it is conjectured that the forest of hairpins reported in former direct numerical simulation studies is reminiscent of the transitional boundary layer and may not be connected to some aspects of the dynamics of the fully developed wall-bounded turbulence.

Reynolds number scaling of straining motions in turbulence

NASA Astrophysics Data System (ADS)

Elsinga, Gerrit; Ishihara, T.; Goudar, M. V.; da Silva, C. B.; Hunt, J. C. R.

2017-11-01

Strain is an important fluid motion in turbulence as it is associated with the kinetic energy dissipation rate, vorticity stretching, and the dispersion of passive scalars. The present study investigates the scaling of the turbulent straining motions by evaluating the flow in the eigenframe of the local strain-rate tensor. The analysis is based on DNS of homogeneous isotropic turbulence covering a Reynolds number range Reλ = 34.6 - 1131. The resulting flow pattern reveals a shear layer containing tube-like vortices and a dissipation sheet, which both scale on the Kolmogorov length scale, η. The vorticity stretching motions scale on the Taylor length scale, while the flow outside the shear layer scales on the integral length scale. These scaling results are consistent with those in wall-bounded flow, which suggests a quantitative universality between the different flows. The overall coherence length of the vorticity is 120 η in all directions, which is considerably larger than the typical size of individual vortices, and reflects the importance of spatial organization at the small scales. Transitions in flow structure are identified at Reλ 45 and 250. Below these respective Reynolds numbers, the small-scale motions and the vorticity stretching motions appear underdeveloped.

Wind-tunnel experiments of turbulent flow over a surface-mounted 2-D block in a thermally-stratified boundary layer

NASA Astrophysics Data System (ADS)

Zhang, Wei; Markfort, Corey; Porté-Agel, Fernando

2014-11-01

Turbulent flows over complex surface topography have been of great interest in the atmospheric science and wind engineering communities. The geometry of the topography, surface roughness and temperature characteristics as well as the atmospheric thermal stability play important roles in determining momentum and scalar flux distribution. Studies of turbulent flow over simplified topography models, under neutrally stratified boundary-layer conditions, have provided insights into fluid dynamics. However, atmospheric thermal stability has rarely been considered in laboratory experiments, e.g., wind-tunnel experiments. Series of wind-tunnel experiments of thermally-stratified boundary-layer flow over a surface-mounted 2-D block, in a well-controlled boundary-layer wind tunnel, will be presented. Measurements using high-resolution PIV, x-wire/cold-wire anemometry and surface heat flux sensors were conducted to quantify the turbulent flow properties, including the size of the recirculation zone, coherent vortex structures and the subsequent boundary layer recovery. Results will be shown to address thermal stability effects on momentum and scalar flux distribution in the wake, as well as dominant mechanism of turbulent kinetic energy generation and consumption. The authors gratefully acknowledge funding from the Swiss National Foundation (Grant 200021-132122), the National Science Foundation (Grant ATM-0854766) and NASA (Grant NNG06GE256).

Modifications to the Conduit Flow Process Mode 2 for MODFLOW-2005

USGS Publications Warehouse

Reimann, T.; Birk, S.; Rehrl, C.; Shoemaker, W.B.

2012-01-01

As a result of rock dissolution processes, karst aquifers exhibit highly conductive features such as caves and conduits. Within these structures, groundwater flow can become turbulent and therefore be described by nonlinear gradient functions. Some numerical groundwater flow models explicitly account for pipe hydraulics by coupling the continuum model with a pipe network that represents the conduit system. In contrast, the Conduit Flow Process Mode 2 (CFPM2) for MODFLOW-2005 approximates turbulent flow by reducing the hydraulic conductivity within the existing linear head gradient of the MODFLOW continuum model. This approach reduces the practical as well as numerical efforts for simulating turbulence. The original formulation was for large pore aquifers where the onset of turbulence is at low Reynolds numbers (1 to 100) and not for conduits or pipes. In addition, the existing code requires multiple time steps for convergence due to iterative adjustment of the hydraulic conductivity. Modifications to the existing CFPM2 were made by implementing a generalized power function with a user-defined exponent. This allows for matching turbulence in porous media or pipes and eliminates the time steps required for iterative adjustment of hydraulic conductivity. The modified CFPM2 successfully replicated simple benchmark test problems. ?? 2011 The Author(s). Ground Water ?? 2011, National Ground Water Association.

Melt Flow before Crystal Seeding in Cz Si Growth with Transversal MF

NASA Astrophysics Data System (ADS)

Iizuka, Masaya; Mukaiyama, Yuji; Demina, S. E.; Kalaev, V. V.

2017-06-01

Industrial Cz growth of Si crystal of 300 mm and higher diameter usually requires DC magnetic fields (MFs) to suppress turbulence in the melt. We present 3D unsteady analysis of melt turbulent convection in an industrial Cz system coupled with the effect of the transversal MF for different argon gas flow rates for the stage before crystal seeding. We have performed detailed 2D axisymmetric modeling of global heat transfer in the whole Cz furnace. Radiative heat fluxes obtained in 2D modeling have been used in detailed 3D steady and unsteady modeling of crystallization zone. LES method is applied as a predictive approach for modeling of turbulent flow of silicon melt. We have obtained flow structure and temperature distribution in the melt, which were different from previously reported data. We have observed a well-fixed dark spike which includes low temperature melt area on the melt free surface in MF cases. These results indicates that MF and argon flow rate conditions are important to achieve stable positioning of the dark spike on the melt free surface for optimized crystal seeding without uncontrollable meltdown and single crystal structure loss.

MHD Modeling of the Solar Wind with Turbulence Transport and Heating

NASA Technical Reports Server (NTRS)

Goldstein, M. L.; Usmanov, A. V.; Matthaeus, W. H.; Breech, B.

2009-01-01

We have developed a magnetohydrodynamic model that describes the global axisymmetric steady-state structure of the solar wind near solar minimum with account for transport of small-scale turbulence associated heating. The Reynolds-averaged mass, momentum, induction, and energy equations for the large-scale solar wind flow are solved simultaneously with the turbulence transport equations in the region from 0.3 to 100 AU. The large-scale equations include subgrid-scale terms due to turbulence and the turbulence (small-scale) equations describe the effects of transport and (phenomenologically) dissipation of the MHD turbulence based on a few statistical parameters (turbulence energy, normalized cross-helicity, and correlation scale). The coupled set of equations is integrated numerically for a source dipole field on the Sun by a time-relaxation method in the corotating frame of reference. We present results on the plasma, magnetic field, and turbulence distributions throughout the heliosphere and on the role of the turbulence in the large-scale structure and temperature distribution in the solar wind.

On some structure-turbulence interaction problems

NASA Technical Reports Server (NTRS)

Maekawa, S.; Lin, Y. K.

1976-01-01

The interactions between a turbulent flow structure; responding to its excitation were studied. The turbulence was typical of those associated with a boundary layer, having a cross-spectral density indicative of convection and statistical decay. A number of structural models were considered. Among the one-dimensional models were an unsupported infinite beam and a periodically supported infinite beam. The fuselage construction of an aircraft was then considered. For the two-dimensional case a simple membrane was used to illustrate the type of formulation applicable to most two-dimensional structures. Both the one-dimensional and two-dimensional structures studied were backed by a cavity filled with an initially quiescent fluid to simulate the acoustic environment when the structure forms one side of a cabin of a sea vessel or aircraft.

Evaluation of wind regimes and their impact on vertical mixing and coupling in a moderately dense forest

NASA Astrophysics Data System (ADS)

Wunder, Tobias; Ehrnsperger, Laura; Thomas, Christoph

2017-04-01

In the last decades much attention has been devoted to improving our understanding of organized motions in plant canopies. Particularly the impact of coherent structures on turbulent flows and vertical mixing in near-neutral conditions has been the focus of many experimental and modeling studies. Despite this progress, the weak-wind subcanopy airflow in concert with stable or weak-wind above-canopy conditions remains poorly understood. In these conditions, evidence is mounting that larger-scale motions, so called sub-meso motions which occupy time scales from minutes to hours and spatial scales from tens of meters to kilometers, dominate transport and turbulent mixing particularly in the subcanopy, because of generally weaker background flow as a result of the enhanced friction due to the plant material. We collected observations from a network of fast-response sensor across the vertical and horizontal dimensions during the INTRAMIX experiment at the Fluxnet site Waldstein/ Weidenbrunnen (DE-Bay) in a moderately dense Norway spruce (Picea Abies) forest over a period of ten weeks. Its main goal was to investigate the role of the submeso-structures on the turbulent wind field and the mixing mechanisms including coherent structures. In a first step, coupling regimes differentiating between weak and strong flows and day- and nighttime-conditions are determined. Subsequently, each of the regimes is analyzed for its dominant flow dynamics identified by wavelet analysis. It is hypothesized that strong vertical wind directional shear does not necessarily indicate a decoupling of vertical layers, but on the contrary may create situations of significant coupling of the sub-canopy with the canopy layers above. Moreover, rapid changes of wind direction or even reversals may generate substantial turbulence and induce intermittent coupling on a variety of time scales. The overarching goal is to improve diagnostics for vertical mixing in plant canopies incorporating turbulence and submeso-motions and to develop a classification of flow modes capable of representing the main driving mechanisms of mixing in forest canopies.

A classification scheme for turbulent flows based on their joint velocity-intermittency structure

NASA Astrophysics Data System (ADS)

Keylock, C. J.; Nishimura, K.; Peinke, J.

2011-12-01

Kolmogorov's classic theory for turbulence assumed an independence between velocity increments and the value for the velocity itself. However, this assumption is questionable, particularly in complex geophysical flows. Here we propose a framework for studying velocity-intermittency coupling that is similar in essence to the popular quadrant analysis method for studying near-wall flows. However, we study the dominant (longitudinal) velocity component along with a measure of the roughness of the signal, given mathematically by its series of Hölder exponents. Thus, we permit a possible dependence between velocity and intermittency. We compare boundary layer data obtained in a wind tunnel to turbulent jets and wake flows. These flow classes all have distinct velocity-intermittency characteristics, which cause them to be readily distinguished using our technique. Our method is much simpler and quicker to apply than approaches that condition the velocity increment statistics at some scale, r, on the increment statistics at a neighbouring, larger spatial scale, r+Δ, and the velocity itself. Classification of environmental flows is then possible based on their similarities to the idealised flow classes and we demonstrate this using laboratory data for flow in a parallel-channel confluence where the region of flow recirculation in the lee of the step is discriminated as a flow class distinct from boundary layer, jet and wake flows. Hence, using our method, it is possible to assign a flow classification to complex geophysical, turbulent flows depending upon which idealised flow class they most resemble.

Drift waves, intense parallel electric fields, and turbulence associated with asymmetric magnetic reconnection at the magnetopause

NASA Astrophysics Data System (ADS)

Ergun, R. E.; Chen, L.-J.; Wilder, F. D.; Ahmadi, N.; Eriksson, S.; Usanova, M. E.; Goodrich, K. A.; Holmes, J. C.; Sturner, A. P.; Malaspina, D. M.; Newman, D. L.; Torbert, R. B.; Argall, M. R.; Lindqvist, P.-A.; Burch, J. L.; Webster, J. M.; Drake, J. F.; Price, L.; Cassak, P. A.; Swisdak, M.; Shay, M. A.; Graham, D. B.; Strangeway, R. J.; Russell, C. T.; Giles, B. L.; Dorelli, J. C.; Gershman, D.; Avanov, L.; Hesse, M.; Lavraud, B.; Le Contel, O.; Retino, A.; Phan, T. D.; Goldman, M. V.; Stawarz, J. E.; Schwartz, S. J.; Eastwood, J. P.; Hwang, K.-J.; Nakamura, R.; Wang, S.

2017-04-01

Observations of magnetic reconnection at Earth's magnetopause often display asymmetric structures that are accompanied by strong magnetic field (B) fluctuations and large-amplitude parallel electric fields (E||). The B turbulence is most intense at frequencies above the ion cyclotron frequency and below the lower hybrid frequency. The B fluctuations are consistent with a thin, oscillating current sheet that is corrugated along the electron flow direction (along the X line), which is a type of electromagnetic drift wave. Near the X line, electron flow is primarily due to a Hall electric field, which diverts ion flow in asymmetric reconnection and accompanies the instability. Importantly, the drift waves appear to drive strong parallel currents which, in turn, generate large-amplitude ( 100 mV/m) E|| in the form of nonlinear waves and structures. These observations suggest that turbulence may be common in asymmetric reconnection, penetrate into the electron diffusion region, and possibly influence the magnetic reconnection process.

Compressibility effect on thermal coherent structures in spatially-developing turbulent boundary layers via DNS

NASA Astrophysics Data System (ADS)

Araya, Guillermo; Jansen, Kenneth

2017-11-01

DNS of compressible spatially-developing turbulent boundary layers is performed at a Mach number of 2.5 over an isothermal flat plate. Turbulent inflow information is generated by following the concept of the rescaling-recycling approach introduced by Lund et al. (J. Comp. Phys. 140, 233-258, 1998); although, the proposed methodology is extended to compressible flows. Furthermore, a dynamic approach is employed to connect the friction velocities at the inlet and recycle stations (i.e., there is no need of an empirical correlation as in Lund et al.). Additionally, the Morkovin's Strong Reynolds Analogy (SRA) is used in the rescaling process of the thermal fluctuations from the recycle plane. Low/high order flow statistics is compared with direct simulations of an incompressible isothermal ZPG boundary layer at similar Reynolds numbers and temperature regarded as a passive scalar. Focus is given to the effect assessment of flow compressibility on the dynamics of thermal coherent structures. AFOSR #FA9550-17-1-0051.

Studies on spectral analysis of randomly sampled signals: Application to laser velocimetry data

NASA Technical Reports Server (NTRS)

Sree, David

1992-01-01

Spectral analysis is very useful in determining the frequency characteristics of many turbulent flows, for example, vortex flows, tail buffeting, and other pulsating flows. It is also used for obtaining turbulence spectra from which the time and length scales associated with the turbulence structure can be estimated. These estimates, in turn, can be helpful for validation of theoretical/numerical flow turbulence models. Laser velocimetry (LV) is being extensively used in the experimental investigation of different types of flows, because of its inherent advantages; nonintrusive probing, high frequency response, no calibration requirements, etc. Typically, the output of an individual realization laser velocimeter is a set of randomly sampled velocity data. Spectral analysis of such data requires special techniques to obtain reliable estimates of correlation and power spectral density functions that describe the flow characteristics. FORTRAN codes for obtaining the autocorrelation and power spectral density estimates using the correlation-based slotting technique were developed. Extensive studies have been conducted on simulated first-order spectrum and sine signals to improve the spectral estimates. A first-order spectrum was chosen because it represents the characteristics of a typical one-dimensional turbulence spectrum. Digital prefiltering techniques, to improve the spectral estimates from randomly sampled data were applied. Studies show that the spectral estimates can be increased up to about five times the mean sampling rate.

A New View of the Dynamics of Reynolds Stress Generation in Turbulent Boundary Layers

NASA Technical Reports Server (NTRS)

Cantwell, Brian J.; Chacin, Juan M.

1998-01-01

The structure of a numerically simulated turbulent boundary layer over a flat plate at Re(theta) = 670 was studied using the invariants of the velocity gradient tensor (Q and R) and a related scalar quantity, the cubic discriminant (D = 27R(exp 2)/4 + Q(exp 3)). These invariants have previously been used to study the properties of the small-scale motions responsible for the dissipation of turbulent kinetic energy. In addition, these scalar quantities allow the local flow patterns to be unambiguously classified according to the terminology proposed by Chong et al. The use of the discriminant as a marker of coherent motions reveals complex, large-scale flow structures that are shown to be associated with the generation of Reynolds shear stress -u'v'(bar). These motions are characterized by high spatial gradients of the discriminant and are believed to be an important part of the mechanism that sustains turbulence in the near-wall region.

Asymptotic behavior of solutions of the renormalization group K-epsilon turbulence model

NASA Technical Reports Server (NTRS)

Yakhot, A.; Staroselsky, I.; Orszag, S. A.

1994-01-01

Presently, the only efficient way to calculate turbulent flows in complex geometries of engineering interest is to use Reynolds-average Navier-Stokes (RANS) equations. As compared to the original Navier-Stokes problem, these RANS equations posses much more complicated nonlinear structure and may exhibit far more complex nonlinear behavior. In certain cases, the asymptotic behavior of such models can be studied analytically which, aside from being an interesting fundamental problem, is important for better understanding of the internal structure of the models as well as to improve their performances. The renormalization group (RNG) K-epsilon turbulence model, derived directly from the incompresible Navier-Stokes equations, is analyzed. It has already been used to calculate a variety of turbulent and transitional flows in complex geometries. For large values of the RNG viscosity parameter, the model may exhibit singular behavior. In the form of the RNG K-epsilon model that avoids the use of explicit wall functions, a = 1, so the RNG viscosity parameter must be smaller than 23.62 to avoid singularities.

Reynolds stress structures in a self-similar adverse pressure gradient turbulent boundary layer at the verge of separation.

NASA Astrophysics Data System (ADS)

Atkinson, C.; Sekimoto, A.; Jiménez, J.; Soria, J.

2018-04-01

Mean Reynolds stress profiles and instantaneous Reynolds stress structures are investigated in a self-similar adverse pressure gradient turbulent boundary layer (APG-TBL) at the verge of separation using data from direct numerical simulations. The use of a self-similar APG-TBL provides a flow domain in which the flow gradually approaches a constant non-dimensional pressure gradient, resulting in a flow in which the relative contribution of each term in the governing equations is independent of streamwise position over a domain larger than two boundary layer thickness. This allows the flow structures to undergo a development that is less dependent on the upstream flow history when compared to more rapidly decelerated boundary layers. This APG-TBL maintains an almost constant shape factor of H = 2.3 to 2.35 over a momentum thickness based Reynolds number range of Re δ 2 = 8420 to 12400. In the APG-TBL the production of turbulent kinetic energy is still mostly due to the correlation of streamwise and wall-normal fluctuations, 〈uv〉, however the contribution form the other components of the Reynolds stress tensor are no longer negligible. Statistical properties associated with the scale and location of sweeps and ejections in this APG-TBL are compared with those of a zero pressure gradient turbulent boundary layer developing from the same inlet profile, resulting in momentum thickness based range of Re δ 2 = 3400 to 3770. In the APG-TBL the peak in both the mean Reynolds stress and the production of turbulent kinetic energy move from the near wall region out to a point consistent with the displacement thickness height. This is associated with a narrower distribution of the Reynolds stress and a 1.6 times higher relative number of wall-detached negative uv structures. These structures occupy 5 times less of the boundary layer volume and show a similar reduction in their streamwise extent with respect to the boundary layer thickness. A significantly lower percentage of wall-attached structures is observed in the present case when compared with a similar investigation of a rapidly decelerating APG-TBL, suggesting that these wall-attached features could be the remanent from the lower pressure gradient domain upstream.

Turbulent Suspension Mechanics in Sediment-Laden Boundary Layers

NASA Astrophysics Data System (ADS)

Kiger, K.

2013-05-01

Accurate prediction of benthic sediment transport is a challenging problem due the two-phase nature of the flow near the mobile bed, as well as the large difference in scales between the meso-scale flow and smaller-scale structures interacting with the sediment bed. Of particular importance is the parameterization of the physics at the bottom boundary. This requires estimation of key quantities such as effective bed stress and sediment flux based on the on the outer regional-scale velocity field. An appropriate turbulence/sediment parameterization is needed to specify the correct bottom momentum and sediment flux. Prior work has shown the shortcoming of standard models to properly predict such behavior, which is speculated to result from the dominant role played by large-scale coherent structures in the generation of the bed morphology, suspension of particulates, and important particle-fluid coupling effects. The goal of the current work is to elucidate such relationships through a combination of direct simulation and laboratory-scale experiment, the latter of which will be the primary focus of this paper. Specifically, two-phase PIV is used to provide a novel quantitative description of both phases, allowing for a detailed examination of the flow behavior and particle-turbulence coupling. Experiments were conducted in both a steady, fully-developed turbulent channel flow and an oscillatory boundary layer in order to examine the fundamental behaviour of the suspension and particle coupling mechanisms. The turbulent channel flow measurements indicated an increase in the effective wall stress due to the presence of the sediment on the order of 7%. The sediment suspension was directly correlated with the ejection dynamics of prototypical hairpin structures, but were found to settle back towards the bed in a manner uncorrelated with the fluid structure. In contrast, the measurements of the oscillatory flow reveal it to be dominated by alternating streaming motions and the ejection of a large-scale vortex at flow reversal. The vortex formation is initiated by the separation from the lee side of the dune during the relaxation of the favourable pressure gradient approaching the peak velocity. Through the deceleration phase, the recirculation region strengthens and grows, detaching into a free vortex as flow reversal is approached. Examining the fluctuating component of Reynolds stress show the vortex to be the dominant source of turbulent transport into the outer flow, which gradually decays as it is transported over the dunes. This vortex is also seen to be the major source of sediment transport into the outer flow region, with the time-averaged sediment flux streaming in a recirculating pattern emanating from the dune crests. The recirculation region is continually populated by particles scoured from the high-shear region on the upstream stoss slope, and upon flow reversal are ejected into the outer flow. Comparison of particle a fluid velocity shows significant slip in the vortex/particle cloud, with the particles settling relative to the fluid at close to 2 cm/s. In other regions of the flow, the mean slip magnitude is generally small, but negative, as one might expect owing to the net settling influence exerted by gravity.

Three-dimensional localized coherent structures of surface turbulence: Model validation with experiments and further computations.

PubMed

Demekhin, E A; Kalaidin, E N; Kalliadasis, S; Vlaskin, S Yu

2010-09-01

We validate experimentally the Kapitsa-Shkadov model utilized in the theoretical studies by Demekhin [Phys. Fluids 19, 114103 (2007)10.1063/1.2793148; Phys. Fluids 19, 114104 (2007)]10.1063/1.2793149 of surface turbulence on a thin liquid film flowing down a vertical planar wall. For water at 15° , surface turbulence typically occurs at an inlet Reynolds number of ≃40 . Of particular interest is to assess experimentally the predictions of the model for three-dimensional nonlinear localized coherent structures, which represent elementary processes of surface turbulence. For this purpose we devise simple experiments to investigate the instabilities and transitions leading to such structures. Our experimental results are in good agreement with the theoretical predictions of the model. We also perform time-dependent computations for the formation of coherent structures and their interaction with localized structures of smaller amplitude on the surface of the film.

Another look at zonal flows: Resonance, shearing, and frictionless saturation

NASA Astrophysics Data System (ADS)

Li, J. C.; Diamond, P. H.

2018-04-01

We show that shear is not the exclusive parameter that represents all aspects of flow structure effects on turbulence. Rather, wave-flow resonance enters turbulence regulation, both linearly and nonlinearly. Resonance suppresses the linear instability by wave absorption. Flow shear can weaken the resonance, and thus destabilize drift waves, in contrast to the near-universal conventional shear suppression paradigm. Furthermore, consideration of wave-flow resonance resolves the long-standing problem of how zonal flows (ZFs) saturate in the limit of weak or zero frictional drag, and also determines the ZF scale. We show that resonant vorticity mixing, which conserves potential enstrophy, enables ZF saturation in the absence of drag, and so is effective at regulating the Dimits up-shift regime. Vorticity mixing is incorporated as a nonlinear, self-regulation effect in an extended 0D predator-prey model of drift-ZF turbulence. This analysis determines the saturated ZF shear and shows that the mesoscopic ZF width scales as LZ F˜f3 /16(1-f ) 1 /8ρs5/8l03 /8 in the (relevant) adiabatic limit (i.e., τckk‖2D‖≫1 ). f is the fraction of turbulence energy coupled to ZF and l0 is the base state mixing length, absent ZF shears. We calculate and compare the stationary flow and turbulence level in frictionless, weakly frictional, and strongly frictional regimes. In the frictionless limit, the results differ significantly from conventionally quoted scalings derived for frictional regimes. To leading order, the flow is independent of turbulence intensity. The turbulence level scales as E ˜(γL/εc) 2 , which indicates the extent of the "near-marginal" regime to be γL<εc , for the case of avalanche-induced profile variability. Here, εc is the rate of dissipation of potential enstrophy and γL is the characteristic linear growth rate of fluctuations. The implications for dynamics near marginality of the strong scaling of saturated E with γL are discussed.

Extreme-scale motions in turbulent plane Couette flows

NASA Astrophysics Data System (ADS)

Lee, Myoungkyu; Moser, Robert D.

2018-05-01

We study the size of large-scale motions in turbulent plane Couette flows at moderate Reynolds number up to $Re_\\tau$ = 500. Direct numerical simulation domains were as large as $100\\pi\\delta\\times2\\delta\\times5\\pi\\delta$, where $\\delta$ is half the distance between the walls. The results indicate that there are structures with streamwise extent, as measured by the wavelength, as long as 78$\\delta$ and at least 310$\\delta$ at $Re_\\tau$ = 220 and 500, respectively. The presence of these very long structures is apparent in the spectra of all three velocity components and the Reynolds stress. In DNS using a smaller domain, the large structures are constrained, eliminating the streamwise variations present in the larger domain. Effects of a smaller domain are also present in the mean velocity and the streamwise velocity variance in the outer flow.

MODELING STATISTICAL PROPERTIES OF SOLAR ACTIVE REGIONS THROUGH DIRECT NUMERICAL SIMULATIONS OF 3D-MHD TURBULENCE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Malapaka, Shiva Kumar; Mueller, Wolf-Christian

Statistical properties of the Sun's photospheric turbulent magnetic field, especially those of the active regions (ARs), have been studied using the line-of-sight data from magnetograms taken by the Solar and Heliospheric Observatory and several other instruments. This includes structure functions and their exponents, flatness curves, and correlation functions. In these works, the dependence of structure function exponents ({zeta}{sub p}) of the order of the structure functions (p) was modeled using a non-intermittent K41 model. It is now well known that the ARs are highly turbulent and are associated with strong intermittent events. In this paper, we compare some of themore » observations from Abramenko et al. with the log-Poisson model used for modeling intermittent MHD turbulent flows. Next, we analyze the structure function data obtained from the direct numerical simulations (DNS) of homogeneous, incompressible 3D-MHD turbulence in three cases: sustained by forcing, freely decaying, and a flow initially driven and later allowed to decay (case 3). The respective DNS replicate the properties seen in the plots of {zeta}{sub p} against p of ARs. We also reproduce the trends and changes observed in intermittency in flatness and correlation functions of ARs. It is suggested from this analysis that an AR in the onset phase of a flare can be treated as a forced 3D-MHD turbulent system in its simplest form and that the flaring stage is representative of decaying 3D-MHD turbulence. It is also inferred that significant changes in intermittency from the initial onset phase of a flare to its final peak flaring phase are related to the time taken by the system to reach the initial onset phase.« less

Experimental Investigation of the Behavior of Sub-Grid Scale Motions in Turbulent Shear Flow

NASA Technical Reports Server (NTRS)

Cantwell, Brian

1992-01-01

Experiments have been carried out on a vertical jet of helium issuing into a co-flow of air at a fixed exit velocity ratio of 2.0. At all the experimental conditions studied, the flow exhibits a strong self excited periodicity. The natural frequency behavior of the jet, the underlying fine-scale flow structure, and the transition to turbulence have been studied over a wide range of flow conditions. The experiments were conducted in a variable pressure facility which made it possible to vary the Reynolds number and Richardson number independently. A stroboscopic schlieren system was used for flow visualization and single-component Laser Doppler Anemometry was used to measure the axial component of velocity. The flow exhibits several interesting features. The presence of co-flow eliminates the random meandering typical of buoyant plumes in a quiescent environment and the periodicity of the helium jet under high Richardson number conditions is striking. Under these conditions transition to turbulence consists of a rapid but highly structured and repeatable breakdown and intermingling of jet and freestream fluid. At Ri = 1.6 the three-dimensional structure of the flow is seen to repeat from cycle to cycle. The point of transition moves closer to the jet exit as either the Reynolds number or the Richardson number increases. The wavelength of the longitudinal instability increases with Richardson number. At low Richardson numbers, the natural frequency scales on an inertial time scale. At high Richardson number the natural frequency scales on a buoyancy time scale. The transition from one flow regime to another occurs over a narrow range of Richardson numbers from 0.7 to 1. A buoyancy Strouhal number is used to correlate the high Richardson number frequency behavior.

Schlieren image velocimetry measurements in a rocket engine exhaust plume

NASA Astrophysics Data System (ADS)

Morales, Rudy; Peguero, Julio; Hargather, Michael

2017-11-01

Schlieren image velocimetry (SIV) measures velocity fields by tracking the motion of naturally-occurring turbulent flow features in a compressible flow. Here the technique is applied to measuring the exhaust velocity profile of a liquid rocket engine. The SIV measurements presented include discussion of visibility of structures, image pre-processing for structure visibility, and ability to process resulting images using commercial particle image velocimetry (PIV) codes. The small-scale liquid bipropellant rocket engine operates on nitrous oxide and ethanol as propellants. Predictions of the exhaust velocity are obtained through NASA CEA calculations and simple compressible flow relationships, which are compared against the measured SIV profiles. Analysis of shear layer turbulence along the exhaust plume edge is also presented.

Interaction of viscous and inviscid instability modes in separation-bubble transition

NASA Astrophysics Data System (ADS)

Brinkerhoff, Joshua R.; Yaras, Metin I.

2011-12-01

This paper describes numerical simulations that are used to examine the interaction of viscous and inviscid instability modes in laminar-to-turbulent transition in a separation bubble. The results of a direct numerical simulation are presented in which separation of a laminar boundary-layer occurs in the presence of an adverse streamwise pressure gradient. The simulation is performed at low freestream-turbulence levels and at a flow Reynolds number and pressure distribution approximating those typically encountered on the suction side of low-pressure turbine blades in a gas-turbine engine. The simulation results reveal the development of a viscous instability upstream of the point of separation which produces streamwise-oriented vortices in the attached laminar boundary layer. These vortices remain embedded in the flow downstream of separation and are carried into the separated shear layer, where they are amplified by the local adverse pressure-gradient and contribute to the formation of coherent hairpin-like vortices. A strong interaction is observed between these vortices and the inviscid instability that typically dominates the shear layer in the separated zone. The interaction is noted to determine the spanwise extent of the vortical flow structures that periodically shed from the downstream end of the separated shear layer. The structure of the shed vortical flow structures is examined and compared with the coherent structures typically observed within turbulent boundary layers.

Compressible turbulent mixing: Effects of Schmidt number.

PubMed

Ni, Qionglin

2015-05-01

We investigated by numerical simulations the effects of Schmidt number on passive scalar transport in forced compressible turbulence. The range of Schmidt number (Sc) was 1/25∼25. In the inertial-convective range the scalar spectrum seemed to obey the k(-5/3) power law. For Sc≫1, there appeared a k(-1) power law in the viscous-convective range, while for Sc≪1, a k(-17/3) power law was identified in the inertial-diffusive range. The scaling constant computed by the mixed third-order structure function of the velocity-scalar increment showed that it grew over Sc, and the effect of compressibility made it smaller than the 4/3 value from incompressible turbulence. At small amplitudes, the probability distribution function (PDF) of scalar fluctuations collapsed to the Gaussian distribution whereas, at large amplitudes, it decayed more quickly than Gaussian. At large scales, the PDF of scalar increment behaved similarly to that of scalar fluctuation. In contrast, at small scales it resembled the PDF of scalar gradient. Furthermore, the scalar dissipation occurring at large magnitudes was found to grow with Sc. Due to low molecular diffusivity, in the Sc≫1 flow the scalar field rolled up and got mixed sufficiently. However, in the Sc≪1 flow the scalar field lost the small-scale structures by high molecular diffusivity and retained only the large-scale, cloudlike structures. The spectral analysis found that the spectral densities of scalar advection and dissipation in both Sc≫1 and Sc≪1 flows probably followed the k(-5/3) scaling. This indicated that in compressible turbulence the processes of advection and dissipation except that of scalar-dilatation coupling might deferring to the Kolmogorov picture. It then showed that at high wave numbers, the magnitudes of spectral coherency in both Sc≫1 and Sc≪1 flows decayed faster than the theoretical prediction of k(-2/3) for incompressible flows. Finally, the comparison with incompressible results showed that the scalar in compressible turbulence with Sc=1 lacked a conspicuous bump structure in its spectrum, but was more intermittent in the dissipative range.

Direct numerical simulation of supersonic turbulent boundary layer subjected to a curved compression ramp

NASA Astrophysics Data System (ADS)

Tong, Fulin; Li, Xinliang; Duan, Yanhui; Yu, Changping

2017-12-01

Numerical investigations on a supersonic turbulent boundary layer over a longitudinal curved compression ramp are conducted using direct numerical simulation for a free stream Mach number M∞ = 2.9 and Reynolds number Reθ = 2300. The total turning angle is 24°, and the concave curvature radius is 15 times the thickness of the incoming turbulent boundary layer. Under the selected conditions, the shock foot is transferred to a fan of the compression wave because of the weaker adverse pressure gradient. The time-averaged flow-field in the curved ramp is statistically attached where the instantaneous flow-field is close to the intermittent transitory detachment state. Studies on coherent vortex structures have shown that large-scale vortex packets are enhanced significantly when the concave curvature is aligned in the spanwise direction. Consistent with findings of previous experiments, the effect of the concave curvature on the logarithmic region of the mean velocity profiles is found to be small. The intensity of the turbulent fluctuations is amplified across the curved ramp. Based on the analysis of the Reynolds stress anisotropy tensor, the evolutions of the turbulence state in the inner and outer layers of the boundary layer are considerably different. The curvature effect on the transport mechanism of the turbulent kinetic energy is studied using the balance analysis of the contributing terms in the transport equation. Furthermore, the Görtler instability in the curved ramp is quantitatively analyzed using a stability criterion. The instantaneous streamwise vorticity confirms the existence of the Görtler-like structures. These structures are characterized by an unsteady motion. In addition, the dynamic mode decomposition analysis of the instantaneous flow field at the spanwise/wall-normal plane reveals that four dynamical relevant modes with performance loss of 16% provide an optimal low-order representation of the essential characteristics of the numerical data. The spatial structures of the dominated low-frequency dynamic modes are found to be similar to that of the Görtler-like vortices.

Radial transfer effects for poloidal rotation

NASA Astrophysics Data System (ADS)

Hallatschek, Klaus

2010-11-01

Radial transfer of energy or momentum is the principal agent responsible for radial structures of Geodesic Acoustic Modes (GAMs) or stationary Zonal Flows (ZF) generated by the turbulence. For the GAM, following a physical approach, it is possible to find useful expressions for the individual components of the Poynting flux or radial group velocity allowing predictions where a mathematical full analysis is unfeasible. Striking differences between up-down symmetric flux surfaces and asymmetric ones have been found. For divertor geometries, e.g., the direction of the propagation depends on the sign of the ion grad-B drift with respect to the X-point, reminiscent of a sensitive determinant of the H-mode threshold. In nonlocal turbulence computations it becomes obvious that the linear energy transfer terms can be completely overwhelmed by the action of the turbulence. In contrast, stationary ZFs are governed by the turbulent radial transfer of momentum. For sufficiently large systems, the Reynolds stress becomes a deterministic functional of the flows, which can be empirically determined from the stress response in computational turbulence studies. The functional allows predictions even on flow/turbulence states not readily obtainable from small amplitude noise, such as certain transport bifurcations or meta-stable states.

Organized turbulent motions in a hedgerow vineyard: effect of evolving canopy structure

NASA Astrophysics Data System (ADS)

Vendrame, Nadia; Tezza, Luca; Tha Paw U, Kyaw; Pitacco, Andrea

2017-04-01

Vegetation-atmosphere exchanges are determined by functional and structural properties of the plants together with environmental forcing. However, a fundamental aspect is the interaction of the canopy with the lower atmosphere. The vegetation deeply alters the composition and physical properties of the air flow, exchanging energy, matter and momentum with it. These processes take place in the bottom part of the atmospheric boundary layer where turbulence is the main mechanism transporting within-canopy air towards the mid- and upper atmospheric boundary layer and vice versa. Canopy turbulence is highly influenced by vegetation drag elements, determining the vertical profile of turbulent moments within the canopy. Canopies organized in rows, like vineyards, show peculiar turbulent transport dynamics. In addition, the morphological structure (phenology) of the vineyard is greatly variable seasonally, shifting from an empty canopy during vine dormancy to dense foliage in summer. The understanding of the canopy ventilation regime is related to several practical applications in vineyard management. For example, within-canopy turbulent motion is very important to predict small particles dispersion, like fungal spores, and minimize infection studying the effect on leaf wetness duration. Our study aims to follow the continuous evolution of turbulence characteristics and canopy structure during the growing season of a hedgerow vineyard, from bud break to fully developed canopy. The field experiment was conducted in a flat extensive vineyard in North-Eastern Italy, using a vertical array of five synchronous sonic anemometers within and above the canopy. Turbulent flow organization was greatly influenced by canopy structure. Turbulent coherent structures involved in momentum transport have been investigated using the classical quadrant analysis and a novel approach to identify dominant temporal scales. Momentum transport in the canopy was dominated by downward gusts showing increasing importance throughout the growing season. At the same time, transport intermittency increased with developing leaf density. The contribution by interaction terms, acting opposite to downward momentum flux, increased in the lower canopy. The analysis of event time scales revealed that momentum transport in the vineyard was dominated by sweeps of 2-4 s duration and ejections of 4-6 s duration, which can be summed to estimate an average duration of dominating coherent structures in the order of 6-10 s. The evolution of canopy morphology did not have any clear influence on structure duration.

Direct Numerical Simulation of a Weakly Stratified Turbulent Wake

NASA Technical Reports Server (NTRS)

Redford, J. A.; Lund, T. S.; Coleman, Gary N.

2014-01-01

Direct numerical simulation (DNS) is used to investigate a time-dependent turbulent wake evolving in a stably stratified background. A large initial Froude number is chosen to allow the wake to become fully turbulent and axisymmetric before stratification affects the spreading rate of the mean defect. The uncertainty introduced by the finite sample size associated with gathering statistics from a simulation of a time-dependent flow is reduced, compared to earlier simulations of this flow. The DNS reveals the buoyancy-induced changes to the turbulence structure, as well as to the mean-defect history and the terms in the mean-momentum and turbulence-kinetic-energy budgets, that characterize the various states of this flow - namely the three-dimensional (essentially unstratified), non-equilibrium (or 'wake-collapse') and quasi-two-dimensional (or 'two-component') regimes observed elsewhere for wakes embedded in both weakly and strongly stratified backgrounds. The wake-collapse regime is not accompanied by transfer (or 'reconversion') of the potential energy of the turbulence to the kinetic energy of the turbulence, implying that this is not an essential feature of stratified-wake dynamics. The dependence upon Reynolds number of the duration of the wake-collapse period is demonstrated, and the effect of the details of the initial/near-field conditions of the wake on its subsequent development is examined.

Turbulent Transfer Between Street Canyons and the Overlying Atmospheric Boundary Layer

NASA Astrophysics Data System (ADS)

Salizzoni, Pietro; Marro, Massimo; Soulhac, Lionel; Grosjean, Nathalie; Perkins, Richard J.

2011-12-01

The turbulent exchange of momentum between a two-dimensional cavity and the overlying boundary layer has been studied experimentally, using hot-wire anemometry and particle image velocimetry (PIV). Conditions within the boundary layer were varied by changing the width of the canyons upstream of the test canyon, whilst maintaining the square geometry of the test canyon. The results show that turbulent transfer is due to the coupling between the instabilities generated in the shear layer above the canyons and the turbulent structures in the oncoming boundary layer. As a result, there is no single, unique velocity scale that correctly characterizes all the processes involved in the turbulent exchange of momentum across the boundary layer. Similarly, there is no single velocity scale that can characterize the different properties of the turbulent flow within the canyon, which depends strongly on the way in which turbulence from the outer flow is entrained into the cavity and carried round by the mean flow. The results from this study will be useful in developing simple parametrizations for momentum exchange in the urban canopy, in situations where the street geometry consists principally of relatively long, uniform streets arranged in grid-like patterns; they are unlikely to be applicable to sparse geometries composed of isolated three-dimensional obstacles.

A classification scheme for turbulence based on the velocity-intermittency structure with an application to near-wall flow and with implications for bed load transport

NASA Astrophysics Data System (ADS)

Keylock, C. J.; Nishimura, K.; Peinke, J.

2012-03-01

Kolmogorov's classic theory for turbulence assumed an independence between velocity increments and the value for the velocity itself. However, recent work has called this assumption in to question, which has implications for the structure of atmospheric, oceanic and fluvial flows. Here we propose a conceptually simple analytical framework for studying velocity-intermittency coupling that is similar in essence to the popular quadrant analysis method for studying near-wall flows. However, we study the dominant (longitudinal) velocity component along with a measure of the roughness of the signal, given mathematically by its series of Hölder exponents. Thus, we permit a possible dependence between velocity and intermittency. We compare boundary layer data obtained in a wind tunnel to turbulent jets and wake flows. These flow classes all have distinct characteristics, which cause them to be readily distinguished using our technique and the results are robust to changes in flow Reynolds numbers. Classification of environmental flows is then possible based on their similarities to the idealized flow classes and we demonstrate this using laboratory data for flow in a parallel-channel confluence. Our results have clear implications for sediment transport in a range of geophysical applications as they suggest that the recently proposed impulse-based methods for studying bed load transport are particularly relevant in domains such as gravel bed river flows where the boundary layer is disrupted and wake interactions predominate.

Turbulent water flow in a channel at Reτ = 400 laden with 0.25 mm diameter air-bubbles clustered near the wall

NASA Astrophysics Data System (ADS)

Lakehal, D.; Métrailler, D.; Reboux, S.

2017-06-01

This paper presents Direct Numerical Simulation (DNS) results of a turbulent water flow in a channel at Reτ = 400 laden with 0.25 mm diameter air bubbles clustered near the wall (maximum void fraction of α = 8% at y+ ˜ 20). The bubbles were fully resolved using the level set approach built within the CFD/CMFD code TransAT. The fluid properties (air and water) were kept real, including density, viscosity, and surface tension coefficient. The aim of this work is to understand the effects of the bubbles on near-wall turbulence, paving the way towards convective wall-boiling flow studies. The interactions between the gas bubbles and the water stream were studied through an in-depth analysis of the turbulence statistics. The near-wall flow is overall affected by the bubbles, which act like roughness elements during the early phase, prior to their departure from the wall. The average profiles are clearly altered by the bubbles dynamics near the wall, which somewhat contrasts with the findings from similar studies [J. Lu and G. Tryggvason, "Dynamics of nearly spherical bubbles in a turbulent channel upflow," J. Fluid Mech. 732, 166 (2013)], most probably because the bubbles were introduced uniformly in the flow and not concentrated at the wall. The shape of the bubbles measured as the apparent to initial diameter ratio is found to change by a factor of at least two, in particular at the later stages when the bubbles burst out from the boundary layer. The clustering of the bubbles seems to be primarily localized in the zone populated by high-speed streaks and independent of their size. More importantly, the bubbly flow seems to differ from the single-phase flow in terms of turbulent stress distribution and energy exchange, in which all the stress components seem to be increased in the region very close to the wall, by up to 40%. The decay in the energy spectra near the wall was found to be significantly slower for the bubbly flow than for a single-phase flow, which confirms that the bubbles increase the energy at smaller scales. The coherent structures in the boundary layer are broken by the bubbles, which disrupts the formation of long structures, reducing the streamwise integral length scale.

Turbulent flow reduces oxygen consumption in the labriform swimming shiner perch, Cymatogaster aggregata.

PubMed

van der Hoop, Julie M; Byron, Margaret L; Ozolina, Karlina; Miller, David L; Johansen, Jacob L; Domenici, Paolo; Steffensen, John F

2018-06-12

Fish swimming energetics are often measured in laboratory environments which attempt to minimize turbulence, though turbulent flows are common in the natural environment. To test whether the swimming energetics and kinematics of shiner perch, Cymatogaster aggregata (a labriform swimmer), were affected by turbulence, two flow conditions were constructed in a swim-tunnel respirometer. A low-turbulence flow was created using a common swim-tunnel respirometry setup with a flow straightener and fine-mesh grid to minimize velocity fluctuations. A high-turbulence flow condition was created by allowing large velocity fluctuations to persist without a flow straightener or fine grid. The two conditions were tested with particle image velocimetry to confirm significantly different turbulence properties throughout a range of mean flow speeds. Oxygen consumption rate of the swimming fish increased with swimming speed and pectoral fin beat frequency in both flow conditions. Higher turbulence also caused a greater positional variability in swimming individuals (versus low-turbulence flow) at medium and high speeds. Surprisingly, fish used less oxygen in high-turbulence compared with low-turbulence flow at medium and high swimming speeds. Simultaneous measurements of swimming kinematics indicated that these reductions in oxygen consumption could not be explained by specific known flow-adaptive behaviours such as Kármán gaiting or entraining. Therefore, fish in high-turbulence flow may take advantage of the high variability in turbulent energy through time. These results suggest that swimming behaviour and energetics measured in the lab in straightened flow, typical of standard swimming respirometers, might differ from that of more turbulent, semi-natural flow conditions. © 2018. Published by The Company of Biologists Ltd.

Controlling Heat Transport and Flow Structures in Thermal Turbulence Using Ratchet Surfaces

NASA Astrophysics Data System (ADS)

Jiang, Hechuan; Zhu, Xiaojue; Mathai, Varghese; Verzicco, Roberto; Lohse, Detlef; Sun, Chao

2018-01-01

In this combined experimental and numerical study on thermally driven turbulence in a rectangular cell, the global heat transport and the coherent flow structures are controlled with an asymmetric ratchetlike roughness on the top and bottom plates. We show that, by means of symmetry breaking due to the presence of the ratchet structures on the conducting plates, the orientation of the large scale circulation roll (LSCR) can be locked to a preferred direction even when the cell is perfectly leveled out. By introducing a small tilt to the system, we show that the LSCR orientation can be tuned and controlled. The two different orientations of LSCR give two quite different heat transport efficiencies, indicating that heat transport is sensitive to the LSCR direction over the asymmetric roughness structure. Through a quantitative analysis of the dynamics of thermal plume emissions and the orientation of the LSCR over the asymmetric structure, we provide a physical explanation for these findings. The current work has important implications for passive and active flow control in engineering, biofluid dynamics, and geophysical flows.

The effects of free stream turbulence on the flow field through a compressor cascade

NASA Astrophysics Data System (ADS)

Muthanna Kolera, Chittiappa

The flow through a compressor cascade with tip leakage has been studied experimentally. The cascade of GE rotor B section blades had an inlet angle of 65.1°, a stagger angle of 56.9°, and a solidity of 1.08. The final turning angle of the cascade was 11.8°. This compressor configuration was representative of the core compressor of an aircraft engine. The cascade was operated with a tip gap of 1.65%, and operated at a Reynolds number based on the chord length (0.254 m) of 388,000. Measurements were made at 8 axial locations to reveal the structure of the flow as it evolved through the cascade. Measurements were also made to reveal the effects of grid generated turbulence on this flow. The data set is unique in that not only does it give a comparison of elevated free stream turbulence effects, but also documents the developing flow through the blade row of a compressor cascade with tip leakage. Measurements were made at a total of 8 locations 0.8, 0.23 axial chords upstream and 0, 0.27, 0.48, 0.77, 0.98, and 1.26 axial chords downstream of the leading edge of the blade row for both inflow turbulence cases. The measurements revealed the formation and development of the tip leakage vortex within the passage. The tip leakage vortex becomes apparent at approximately X/ca = 0.27 and dominated much of the endwall flow. The tip leakage vortex is characterized by high streamwise velocity deficits, high vorticity and high turbulence kinetic energy levels. The result showed that between 0.77 and 0.98 axial chords downstream of the leading edge, the vortex structure and behavior changes. The effects of grid generated turbulence were also documented. The results revealed significant effects on the flow field. The results showed a 4% decrease in the blade loading and a 20% reduction in the vorticity levels within tip leakage vortex. There was also a shift in the vortex path, showing a shift close to the suction side with grid generated turbulence, indicating the strength of the vortex was decreased. Circulation calculations showed this reduction, and also indicated that the tip leakage vortex increased in size by about 30%. The results revealed that overall, the turbulence kinetic energy levels in the tip leakage vortex were increased, with the most drastic change occurring at X/ca = 0.77.

Analysis of turbulent transport and mixing in transitional Rayleigh–Taylor unstable flow using direct numerical simulation data

DOE PAGES

Schilling, Oleg; Mueschke, Nicholas J.

2010-10-18

Data from a 1152X760X1280 direct numerical simulation (DNS) of a transitional Rayleigh-Taylor mixing layer modeled after a small Atwood number water channel experiment is used to comprehensively investigate the structure of mean and turbulent transport and mixing. The simulation had physical parameters and initial conditions approximating those in the experiment. The budgets of the mean vertical momentum, heavy-fluid mass fraction, turbulent kinetic energy, turbulent kinetic energy dissipation rate, heavy-fluid mass fraction variance, and heavy-fluid mass fraction variance dissipation rate equations are constructed using Reynolds averaging applied to the DNS data. The relative importance of mean and turbulent production, turbulent dissipationmore » and destruction, and turbulent transport are investigated as a function of Reynolds number and across the mixing layer to provide insight into the flow dynamics not presently available from experiments. The analysis of the budgets supports the assumption for small Atwood number, Rayleigh/Taylor driven flows that the principal transport mechanisms are buoyancy production, turbulent production, turbulent dissipation, and turbulent diffusion (shear and mean field production are negligible). As the Reynolds number increases, the turbulent production in the turbulent kinetic energy dissipation rate equation becomes the dominant production term, while the buoyancy production plateaus. Distinctions between momentum and scalar transport are also noted, where the turbulent kinetic energy and its dissipation rate both grow in time and are peaked near the center plane of the mixing layer, while the heavy-fluid mass fraction variance and its dissipation rate initially grow and then begin to decrease as mixing progresses and reduces density fluctuations. All terms in the transport equations generally grow or decay, with no qualitative change in their profile, except for the pressure flux contribution to the total turbulent kinetic energy flux, which changes sign early in time (a countergradient effect). The production-to-dissipation ratios corresponding to the turbulent kinetic energy and heavy-fluid mass fraction variance are large and vary strongly at small evolution times, decrease with time, and nearly asymptote as the flow enters a self-similar regime. The late-time turbulent kinetic energy production-to-dissipation ratio is larger than observed in shear-driven turbulent flows. The order of magnitude estimates of the terms in the transport equations are shown to be consistent with the DNS at late-time, and also confirms both the dominant terms and their evolutionary behavior. Thus, these results are useful for identifying the dynamically important terms requiring closure, and assessing the accuracy of the predictions of Reynolds-averaged Navier-Stokes and large-eddy simulation models of turbulent transport and mixing in transitional Rayleigh-Taylor instability-generated flow.« less

Experimental modeling of gravity underflow in submarine channels

NASA Astrophysics Data System (ADS)

Islam, Mohammad Ashraful

Active and relic meandering channels are common on the seafloor adjacent to continental margins. These channels and their associated submarine fan deposits are products of the density-driven gravity flows known as turbidity currents. Unlike natural rivers, few attempts have been made to explore the process of channel meandering in the submarine environment. This research focuses on resolving the flow field of submarine channels by conducting experiments in a large laboratory basin. Saline and particulate density flows were studied in a straight channel, a single bend sinuous channel with vertical sidewalls and a multiple-bend sinuous channel with sloping sidewalls. Instantaneous velocities in steady developed currents were measured using 3-component acoustic Doppler velocity probes. Excess fractional density was measured at selected locations by collecting water sample using a siphon rake. Turbulent kinetic energy and Reynolds stress components are derived from the instantaneous velocity data of the straight channel experiments. Structure functions for mean velocity, Reynolds stress and turbulent kinetic energy profiles are derived by fitting normalized data. The normalized Reynolds-averaged velocity shows excellent similarity collapse while the Reynolds-stress and the turbulent kinetic energy profiles display reasonable similarity. Vertical profiles of the turbulent kinetic energy display two peaks separated by a zone of low turbulence; the ratio of the maximum to the depth-averaged turbulent kinetic energy is approximately 1.5. Theoretical profile of turbulent kinetic energy is derived. Comparisons of experimentally and theoretically derived turbulent kinetic energy profiles show reasonable agreement except at the position of velocity maximum where the theoretical profile displays a very small value. Velocity profiles derived from the measurements with confined flow in the single bend channel reveal that channel curvature drives two helical flow cells, one stacked upon the other. The lower cell forms near the channel bed surface and has a circulation pattern similar to fluvial channels where a near-bed flow is directed inward. The other circulation cell forms in the upper part of the gravity flow and has a streamwise vorticity opposite to the lower cell. The lower circulation cell can be reasonably approximated by open channel flow theory. The curvature induced mixing is found to shift the position of the maximum streamwise velocity in the upward direction. Experiments conducted in the multiple-bend channel reveals that the channel side slope does not alter the structure of the secondary flow as long as the flow remains confined within the channel. However, if flow spilling occurs at the channel bend, the lateral convection suppresses the upper circulation cell. The lateral slope promotes high superelevation of the dense-light fluid interface at a channel bend and the current almost entirely separates from the inner bank. Compared with the saline flow, the silt-laden flow has larger thickness and thus easily experiences spilling at the bend apex. The overbank flow approximately follows the pre-bend direction of the in-channel flow. Unlike the flow in the channel with vertical sidewalls, the maximum velocity position does not experience an upward shift. This may be attributed to the highly superelevated current interface. The saline flow experiences little reduction in flow velocity while the velocity of the particulate flow drops significantly in the downstream direction primarily due to in-channel sediment deposit.

Irregular wall roughness in turbulent Taylor-Couette flow

NASA Astrophysics Data System (ADS)

Berghout, Pieter; Zhu, Xiaojue; Verzicco, Roberto; Lohse, Detlef; Stevens, Richard

2017-11-01

Many wall bounded flows in nature, engineering and transport are affected by surface roughness. Often, this has adverse effects, e.g. drag increase leading to higher energy costs. A major difficulty is the infinite number of roughness geometries, which makes it impossible to systematically investigate all possibilities. Here we present Direct Numerical Simulations (DNS) of turbulent Taylor-Couette flow. We focus on the transitionally rough regime, in which both viscous and pressure forces contribute to the total wall stress. We investigate the effect of the mean roughness height and the effective slope on the roughness function, ΔU+ . Also, we present simulations of varying Ta (Re) numbers for a constant mean roughness height (kmean+). Alongside, we show the behavior of the large scale structures (e.g. plume ejection, Taylor rolls) and flow structures in the vicinity of the wall.

Experimental scaling law for the subcritical transition to turbulence in plane Poiseuille flow.

PubMed

Lemoult, Grégoire; Aider, Jean-Luc; Wesfreid, José Eduardo

2012-02-01

We present an experimental study of the transition to turbulence in a plane Poiseuille flow. Using a well-controlled perturbation, we analyze the flow by using extensive particle image velocimetry and flow visualization (using laser-induced fluorescence) measurements, and use the deformation of the mean velocity profile as a criterion to characterize the state of the flow. From a large parametric study, four different states are defined, depending on the values of the Reynolds number and the amplitude of the perturbation. We discuss the role of coherent structures, such as hairpin vortices, in the transition. We find that the minimal amplitude of the perturbation triggering transition scales asymptotically as Re(-1).

Phenomenological study of subsonic turbulent flow over a swept rearward-facing step. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Selby, G. V.

1982-01-01

The phenomenology of turbulent, subsonic flow over a swept, rearward-facing step was studied. Effects of variations in step height, sweep angle, base geometry, and end conditions on the 3-D separated flow were examined. The separated flow was visualized using smoke wire, oil drop, and surface tuft techniques. Measurements include surface pressure, reattachment distance and swirl angle. Results indicate: (1) model/test section coupling affects the structure of the separated flow, but spanwise end conditions do not; (2) the independence principle is evidently valid for sweep angles up to 38 deg; (3) a sweep angle/swirl angle correlation exists; and (4) base modifications can significantly reduce the reattachment distance.

The Hydrodynamic Distinctiveness of Living Organisms: Communication in Complex Hydraulic Environments

NASA Astrophysics Data System (ADS)

Johnson, M.

2015-12-01

Animals make decisions about the suitability of habitat and their reaction to other organisms based on the sensory information that they first obtain. This information is transmitted, masked and filtered by fluvial processes, such as turbulent flow. Despite governing how animals interact with the environment, limited attention has been paid to the controls on the propagation of sensory signals through rivers. Some animals interpret hydraulic events and use the characteristics of wakes to sense the presence of other organisms. This implies that at least some animals can differentiate turbulent flow generated by the presence of living organisms from ambient environmental turbulence. We investigate whether there are specific flow characteristics, distinct from the ambient environment, that potentially flag the presence of organisms to other animals. ADV and PIV measurements in a series of laboratory flume experiments quantified the flow around living Signal Crayfish (Pacifastacus leniusculus) and two inanimate objects of equivalent shape and size. Experiments were repeated across a gradient of turbulence intensities generated over nine combinations of flow velocity and relative submergence. Flows downstream of living crayfish were distinct from inanimate objects, with greater turbulent intensities, higher energy in low- to intermediate frequencies, and flow structures that were less coherent in comparison to those measured downstream of inanimate objects. However, the hydrodynamic signature of crayfish became masked as the intensity of ambient turbulence exceeded that generated by living crayfish. These results demonstrate the importance of the fluvial processes in the transmission of sensory information and suggest that the ability of animals to perceive hydraulic signatures is likely to be limited in many situations in rivers. Thus, animals may need to rely on other senses, such as sight or hearing, especially where depth is shallow relative to grain size.

Wave-induced coherent turbulence structures and sediment resuspension in the nearshore of a prototype-scale sandy barrier beach

NASA Astrophysics Data System (ADS)

Kassem, Hachem; Thompson, Charlotte E. L.; Amos, Carl L.; Townend, Ian H.

2015-10-01

The suspension of sediments by oscillatory flows is a complex case of fluid-particle interaction. The aim of this study is to provide insight into the spatial (time) and scale (frequency) relationships between wave-generated boundary layer turbulence and event-driven sediment transport beneath irregular shoaling and breaking waves in the nearshore of a prototype sandy barrier beach, using data collected through the Barrier Dynamics Experiment II (BARDEX II). Statistical, quadrant and spectral analyses reveal the anisotropic and intermittent nature of Reynolds' stresses (momentum exchange) in the wave boundary layer, in all three orthogonal planes of motion. The fractional contribution of coherent turbulence structures appears to be dictated by the structural form of eddies beneath plunging and spilling breakers, which in turn define the net sediment mobilisation towards or away from the barrier, and hence ensuing erosion and accretion trends. A standing transverse wave is also observed in the flume, contributing to the substantial skewness of spanwise turbulence. Observed low frequency suspensions are closely linked to the mean flow (wave) properties. Wavelet analysis reveals that the entrainment and maintenance of sediment in suspension through a cluster of bursting sequence is associated with the passage of intermittent slowly-evolving large structures, which can modulate the frequency of smaller motions. Outside the boundary layer, small scale, higher frequency turbulence drives the suspension. The extent to which these spatially varied perturbation clusters persist is associated with suspension events in the high frequency scales, decaying as the turbulent motion ceases to supply momentum, with an observed hysteresis effect.

Simultaneous determination of micellar structure and drag reduction in a surfactant solution flow using the fluorescence probe method

NASA Astrophysics Data System (ADS)

Wakimoto, Tatsuro; Araga, Koichi; Katoh, Kenji

2018-03-01

As widely known, the addition of a specific type of surfactant to water reduces drag in a pipe flow. This effect is considered to be a result of the suppression of turbulent transition caused by the ordered structure of rod-like micelles that is referred to as a shear-induced structure (SIS). However, it is typically difficult to determine the SIS since it is necessary to noninvasively detect the SIS with several hundred nanometers in the actual moving flow. In this study, we used the fluorescence probe method to locally determine the SIS in a pipe flow. When hydrophobic fluorescence molecules are added to the surfactant solution, the fluorescence molecules are trapped in micelles. Thus, fluorescence intensity varies based on the change in the micellar structure. We verified the applicability of the fluorescence probe method to the SIS detection and determined the relationship between the micellar structure and the drag reduction in the pipe flow by simultaneously measuring the fluorescence intensity and pipe friction factor. The experimental result demonstrates that the SIS formation in the near-wall region is closely correlated with the drag reduction and suggests that the near-wall SIS suppresses the turbulent transition.

Target Lagrangian kinematic simulation for particle-laden flows.

PubMed

Murray, S; Lightstone, M F; Tullis, S

2016-09-01

The target Lagrangian kinematic simulation method was motivated as a stochastic Lagrangian particle model that better synthesizes turbulence structure, relative to stochastic separated flow models. By this method, the trajectories of particles are constructed according to synthetic turbulent-like fields, which conform to a target Lagrangian integral timescale. In addition to recovering the expected Lagrangian properties of fluid tracers, this method is shown to reproduce the crossing trajectories and continuity effects, in agreement with an experimental benchmark.

Effects of extensional rates on characteristic scales of two-dimensional turbulence in polymer solutions

NASA Astrophysics Data System (ADS)

Hidema, R.

2014-08-01

In order to study the effects of extensional viscosities on turbulent drag reduction, experimental studies using two-dimensional turbulence have been made. Anisotropic structures and variations of energy transfer induced by polymers are considered. Polyethyleneoxide and hydroxypropyl cellulose having different flexibility, which is due to different characteristics of extensional viscosity, are added to 2D turbulence. Variations of the turbulence were visualized by interference patterns of 2D flow, and were analysed by an image processing. The effects of polymers on turbulence in the streamwise and normal directions were also analysed by 2D Fourier transform. In addition, characteristic scales in 2D turbulence were analysed by wavelet transform.

Effect of atmospheric turbulence on wind turbine wakes: An LES study

NASA Astrophysics Data System (ADS)

Wu, Y. T.; Porté-Agel, F.

2012-04-01

A comprehensive numerical study of atmospheric turbulence effect on wind-turbine wakes is presented. Large-eddy simulations of neutrally-stratified boundary layers developed over different flat surfaces (forest, farmland, grass, and snow) are performed to investigate the structure of turbine wakes in cases where the incident flows to the wind turbine have the same mean velocity at the hub height but different mean wind shears and turbulence intensity levels. The simulation results show that the different wind shears and turbulence intensity levels of the incoming flow lead to considerable influence on the spatial distribution of the mean velocity deficit, turbulence intensity, and turbulent shear stress in the wake region downstream of the turbine. In general, the recovery of the turbine-induced wake (velocity deficit) is faster and the turbulence intensity level is higher and has its maximum closer to the turbine for wakes of turbines over rougher terrain. In order to isolate the effect of turbulence intensity from that of wind shear, simulations have also been performed with synthetic inflow velocity fields that have the same mean wind shear but different turbulence intensity levels. We find that the effect of the inflow turbulence intensity on the wake recovery and turbulence levels is stronger than that of the mean shear.

Characteristics Of Turbulent Nonpremixed Jet-Flames And Jet-Flames In Crossflow In Normal- And Low-Gravity

NASA Technical Reports Server (NTRS)

Clemens, N. T.; Boxx, I. G.; Idicheria, C. A.

2003-01-01

It is well known that buoyancy has a major influence on the flow structure of turbulent nonpremixed jet flames. For example, previous studies have shown that transitional and turbulent jet flames exhibit flame lengths that are as much as a factor of two longer in microgravity than in normal gravity. The objective of this study is to extend these previous studies by investigating both mean and fluctuating characteristics of turbulent nonpremixed jet flames under three different gravity levels (1 g, 20 mg and 100 micrograms). This work is described in more detail elsewhere. In addition, we have recently initiated a new study into the effects of buoyancy on turbulent nonpremixed jet flames in cross-flow (JFICF). Buoyancy has been observed to play a key role in determining the centerline trajectories of such flames.6 The objective of this study is to use the low gravity environment to study the effects of buoyancy on the turbulent characteristics of JFICF.

DNS and LES/FMDF of turbulent jet ignition and combustion

NASA Astrophysics Data System (ADS)

Validi, Abdoulahad; Jaberi, Farhad

2014-11-01

The ignition and combustion of lean fuel-air mixtures by a turbulent jet flow of hot combustion products injected into various geometries are studied by high fidelity numerical models. Turbulent jet ignition (TJI) is an efficient method for starting and controlling the combustion in complex propulsion systems and engines. The TJI and combustion of hydrogen and propane in various flow configurations are simulated with the direct numerical simulation (DNS) and the hybrid large eddy simulation/filtered mass density function (LES/FMDF) models. In the LES/FMDF model, the filtered form of the compressible Navier-Stokes equations are solved with a high-order finite difference scheme for the turbulent velocity and the FMDF transport equation is solved with a Lagrangian stochastic method to obtain the scalar field. The DNS and LES/FMDF data are used to study the physics of TJI and combustion for different turbulent jet igniter and gas mixture conditions. The results show the very complex and different behavior of the turbulence and the flame structure at different jet equivalence ratios.

Global characteristics of zonal flows due to the effect of finite bandwidth in drift wave turbulence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Uzawa, K.; Li Jiquan; Kishimoto, Y.

2009-04-15

The spectral effect of the zonal flow (ZF) on its generation is investigated based on the Charney-Hasegawa-Mima turbulence model. It is found that the effect of finite ZF bandwidth qualitatively changes the characteristics of ZF instability. A spatially localized (namely, global) nonlinear ZF state with an enhanced, unique growth rate for all spectral components is created under a given turbulent fluctuation. It is identified that such state originates from the successive cross couplings among Fourier components of the ZF and turbulence spectra through the sideband modulation. Furthermore, it is observed that the growth rate of the global ZF is determinedmore » not only by the spectral distribution and amplitudes of turbulent pumps as usual, but also statistically by the turbulence structure, namely, their probabilistic initial phase factors. A ten-wave coupling model of the ZF modulation instability involving the essential effect of the ZF spectrum is developed to clarify the basic features of the global nonlinear ZF state.« less

Development of Large-Eddy Interaction Model for inhomogeneous turbulent flows

NASA Technical Reports Server (NTRS)

Hong, S. K.; Payne, F. R.

1987-01-01

The objective of this paper is to demonstrate the applicability of a currently proposed model, with minimum empiricism, for calculation of the Reynolds stresses and other turbulence structural quantities in a channel. The current Large-Eddy Interaction Model not only yields Reynolds stresses but also presents an opportunity to illuminate typical characteristic motions of large-scale turbulence and the phenomenological aspects of engineering models for two Reynolds numbers.

The self-preservation of dissipation elements in homogeneous isotropic decaying turbulence

NASA Astrophysics Data System (ADS)

Gauding, Michael; Danaila, Luminita; Varea, Emilien

2017-11-01

The concept of self-preservation has played an important role in shaping the understanding of turbulent flows. The assumption of complete self-preservation imposes certain constrains on the dynamics of the flow, allowing to express statistics by choosing an appropriate unique length scale. Another approach in turbulence research is to study the dynamics of geometrical objects, like dissipation elements (DE). DE appear as coherent space-filling structures in turbulent scalar fields and can be parameterized by the linear length between their ending points. This distance is a natural length scale that provides information about the local structure of turbulence. In this work, the evolution of DE in decaying turbulence is investigated from a self-preservation perspective. The analysis is based on data obtained from direct numerical simulations (DNS). The temporal evolution of DE is governed by a complex process, involving cutting and reconnection events, which change the number and consequently also the length of DE. An analysis of the evolution equation for the probability density function of the length of DE is carried out and leads to specific constraints for the self-preservation of DE, which are justified from DNS. Financial support was provided by Labex EMC3 (under the Grant VAVIDEN), Normandy Region and FEDER.

Experimental Study of Transitional Flow Behavior in a Simulated Low Pressure Turbine

NASA Technical Reports Server (NTRS)

Sohn, Ki Hyeon; DeWitt, Kenneth J.

1998-01-01

A detailed investigation of the flow physics occurring on the suction side of a simulated Low Pressure Turbine (LPT) blade was performed. A contoured upper wall was designed to simulate the pressure distribution of an actual LPT airfoil onto a flat lower plate. The experiments were carried out for the Reynolds numbers of 35,000, 70,000, 100,000 and 250,000 with four levels of freestream turbulence ranging from 1% to 4%. For the three lower Reynolds numbers, the boundary layer on the flat plate was separated and formed a bubble. The size of laminar separation bubble was measured to be inversely proportional to the freestream turbulence levels and Reynolds numbers. However, no separation was observed for the Re = 250,000 case. The transition on a separated flow was found to proceed through the formation of turbulent spots in the free shear layer as evidenced in the intermittency profiles for Re = 35,000, 70,000 and 100,000. Spectral data show no evidence of Kelvin-Helmholtz or Tollmien-Schlichting instability waves in the free shear layer over a separation bubble (bypass transition). However, the flow visualization revealed the large vortex structures just outside of the bubble and their development to turbulent flow for Re = 50,000, which is similar to that in the free shear layer (separated-flow transition). Therefore, it is fair to say that the bypass and separated-flow transition modes coexist in the transitional flows over the separation bubble for certain conditions. Transition onset and end locations and length determined from intermittency profiles decrease as Reynolds number and freestream turbulence levels increase.

Experimental Study of Transitional Flow Behavior in a Simulated Low Pressure Turbine

NASA Technical Reports Server (NTRS)

Sohn, Ki Hyeon; DeWitt, Kenneth J.

2007-01-01

A detailed investigation of the flow physics occurring on the suction side of a simulated Low Pressure Turbine (LPT) blade was performed. A contoured upper wall was designed to simulate the pressure distribution of an actual LPT airfoil onto a flat lower plate. The experiments were carried out for the Reynolds numbers of 35,000, 70,000, 100,000, and 250,000 with four levels of freestream turbulence ranging from 1 to 4 percent. For the three lower Reynolds numbers, the boundary layer on the flat plate was separated and formed a bubble. The size of laminar separation bubble was measured to be inversely proportional to the freestream turbulence levels and Reynolds numbers. However, no separation was observed for the Re = 250,000 case. The transition on a separated flow was found to proceed through the formation of turbulent spots in the free shear layer as evidenced in the intermittency profiles for Re = 35,000, 70,000, and 100,000. Spectral data show no evidence of Kelvin-Helmholtz of Tollmien-Schlichting instability waves in the free shear layer over a separation bubble (bypass transition). However, the flow visualization revealed the large vortex structures just outside of the bubble and their development to turbulent flow for Re = 50,000, which is similar to that in the free shear layer (separated-flow transition). Therefore, it is fair to say that the bypass and separated-flow transition modes coexist in the transition flows over the separation bubble of certain conditions. Transition onset and end locations and length determined from intermittency profiles decreased as Reynolds number and freestream turbulence levels increase.

Power laws and fragility in flow networks.

PubMed

Shore, Jesse; Chu, Catherine J; Bianchi, Matt T

2013-01-01

What makes economic and ecological networks so unlike other highly skewed networks in their tendency toward turbulence and collapse? Here, we explore the consequences of a defining feature of these networks: their nodes are tied together by flow. We show that flow networks tend to the power law degree distribution (PLDD) due to a self-reinforcing process involving position within the global network structure, and thus present the first random graph model for PLDDs that does not depend on a rich-get-richer function of nodal degree. We also show that in contrast to non-flow networks, PLDD flow networks are dramatically more vulnerable to catastrophic failure than non-PLDD flow networks, a finding with potential explanatory power in our age of resource- and financial-interdependence and turbulence.

Separated and Recovering Turbulent Boundary Layer Flow Behind a Backward Facing Step For Different Reynolds Numbers

NASA Technical Reports Server (NTRS)

Jovic, Srba; Kutler, Paul F. (Technical Monitor)

1994-01-01

Experimental results for a two-dimensional separated turbulent boundary layer behind a backward facing step for five different Reynolds numbers are reported. Results are presented in the form of tables, graphs and a floppy disk for an easy access of the data. Reynolds number based on the step height was varied by changing the reference velocity upstream of the step, U(sub o), and the step height, h. Hot-wire measurement techniques were used to measure three Reynolds stresses and four triple-velocity correlations. In addition, surface pressure and skin friction coefficients were measured. All hot-wire measurements were acquired in a measuring domain which excluded recirculating flow region due to the directional insensitivity of hot-wires. The downstream extent of the domain from the step was 51 h for the largest and I 14h for the smallest step height. This significant downstream length permitted extensive study of the flow recovery. Prediction of perturbed flows and their recovery is particularly attractive for popular turbulence models since variations of turbulence length and time scales and flow interactions in different regions are generally inadequately predicted. The data indicate that the flow in the free shear layer region behaves like the plane mixing layer up to about 2/3 of the mean reattachment length when the flow interaction with the wall commences the flow recovery to that of an ordinary turbulent boundary layer structure. These changes of the flow do not occur abruptly with the change of boundary conditions. A reattachment region represents a transitional region where the flow undergoes the most dramatic adjustments to the new boundary conditions. Large eddies, created in the upstream free-shear layer region, are being torn, recirculated, reentrained back into the main stream interacting with the incoming flow structure. It is foreseeable that it is quite difficult to describe the physics of this region in a rational and quantitative manner other than statistical. Downstream of the reattachment point the flow recovers at different rates near the wall, in the newly developing internal boundary layer, and in the outer part of the flow. It appears that Reynolds stresses do not fully recover up to the longest recovery length of 114 h.

Numerical Simulation of Complex Turbomachinery Flows

NASA Technical Reports Server (NTRS)

Chernobrovkin, A. A.; Lakshiminarayana, B.

1999-01-01

An unsteady, multiblock, Reynolds Averaged Navier Stokes solver based on Runge-Kutta scheme and Pseudo-time step for turbo-machinery applications was developed. The code was validated and assessed against analytical and experimental data. It was used to study a variety of physical mechanisms of unsteady, three-dimensional, turbulent, transitional, and cooling flows in compressors and turbines. Flow over a cylinder has been used to study effects of numerical aspects on accuracy of prediction of wake decay and transition, and to modify K-epsilon models. The following simulations have been performed: (a) Unsteady flow in a compressor cascade: Three low Reynolds number turbulence models have been assessed and data compared with Euler/boundary layer predictions. Major flow features associated with wake induced transition were predicted and studied; (b) Nozzle wake-rotor interaction in a turbine: Results compared to LDV data in design and off-design conditions, and cause and effect of unsteady flow in turbine rotors were analyzed; (c) Flow in the low-pressure turbine: Assessed capability of the code to predict transitional, attached and separated flows at a wide range of low Reynolds numbers and inlet freestream turbulence intensity. Several turbulence and transition models have been employed and comparisons made to experiments; (d) leading edge film cooling at compound angle: Comparisons were made with experiments, and the flow physics of the associated vortical structures were studied; and (e) Tip leakage flow in a turbine. The physics of the secondary flow in a rotor was studied and sources of loss identified.

An investigation of turbulence structure in a low-Reynolds-number incompressible turbulent boundary

NASA Technical Reports Server (NTRS)

White, B. R.; Strataridakis, C. J.

1987-01-01

An existing high turbulence intensity level (5%) atmospheric boundary-layer wind tunnel has been successfully converted to a relatively low level turbulence (0.3%) wind tunnel through extensive modification, testing, and calibration. A splitter plate was designed, built, and installed into the wind-tunnel facility to create thick, mature, two-dimensional turbulent boundary layer flow at zero pressure gradient. Single and cross hot-wire measurements show turbulent boundary layer characteristics of good quality with unusually large physical size, i.e., viscous sublayer of the order of 1 mm high. These confirm the potential ability of the tunnel to be utilized for future high-quality near-wall turbulent boundary layer measurements. It compares very favorably with many low turbulence research tunnels.

An investigation of the flow characteristics in the blade endwall corner region

NASA Technical Reports Server (NTRS)

Hazarika, Birinchi K.; Raj, Rishi S.

1987-01-01

Studies were undertaken to determine the structure of the flow in the blade end wall corner region simulated by attaching two uncambered airfoils on either side of a flat plate with a semicircular leading edge. Detailed measurements of the corner flow were obtained with conventional pressure probes, hot wire anemometry, and flow visualization. The mean velocity profiles and six components of the Reynolds stress tensor were obtained with an inclined single sensor hot wire probe whereas power spectra were obtained with a single sensor oriented normal to the flow. Three streamwise vortices were identified based on the surface streamlines, distortion of total pressure profiles, and variation of mean velocity components in the corner. A horseshoe vortex formed near the leading edge of the airfoil. Within a short distance downstream, a corner vortex was detected between the horseshoe vortex and the surfaces forming the corner. A third vortex was formed at the rear portion of the corner between the corner vortex and the surface of the flat plate. Turbulent shear stress and production of turbulence are negligibly small. A region of negative turbulent shear stress was also observed near the region of low turbulence intensity from the vicinity of the flat plate.

Elasto-Inertial Turbulence: From Subcritical Turbulence to Maximum Drag Reduction

NASA Astrophysics Data System (ADS)

Dubief, Yves; Sid, Samir; Egan, Raphael; Terrapon, Vincent

2015-11-01

Elasto Inertial Turbulence (EIT) is a turbulence state found so far in polymer solutions. Upon the appropriate initial perturbation, an autonomous regeneration cycle emerges between polymer dynamics, pressure and velocity fluctuations. This cycle is best explained by the Poisson equation derived from viscoelastic flow models such as FENE-P (used in this study). This presentation provides an overview of the structure of EIT in 2D channel flows for Reynolds numbers ranging from Reτ = 10 to 100 and for 3D simulations up to Ret au = 300 . For flows below the Newtonian critical Reynolds number, EIT increases the drag. For higher Reynolds numbers, EIT is surmised to be the energetic bound of Maximum Drag Reduction (MDR), the asymptotic state of drag reduction in polymer solutions. The very existence of EIT at low Reynolds numbers (Reτ < 60) highlights a backward energy transfer from the small scale polymer dynamics to larger flow scales. Similar dynamics is identified at higher Reynolds numbers, which could explain why polymer flows never become fully laminar. The authors acknowledge computational resources from CÉCI (F.R.S.-FNRS grant No.2.5020.11), the PRACE infrastructure, and the Vermont Advanced Computing Core.

Asymptotic theory of two-dimensional trailing-edge flows

NASA Technical Reports Server (NTRS)

Melnik, R. E.; Chow, R.

1975-01-01

Problems of laminar and turbulent viscous interaction near trailing edges of streamlined bodies are considered. Asymptotic expansions of the Navier-Stokes equations in the limit of large Reynolds numbers are used to describe the local solution near the trailing edge of cusped or nearly cusped airfoils at small angles of attack in compressible flow. A complicated inverse iterative procedure, involving finite-difference solutions of the triple-deck equations coupled with asymptotic solutions of the boundary values, is used to accurately solve the viscous interaction problem. Results are given for the correction to the boundary-layer solution for drag of a finite flat plate at zero angle of attack and for the viscous correction to the lift of an airfoil at incidence. A rational asymptotic theory is developed for treating turbulent interactions near trailing edges and is shown to lead to a multilayer structure of turbulent boundary layers. The flow over most of the boundary layer is described by a Lighthill model of inviscid rotational flow. The main features of the model are discussed and a sample solution for the skin friction is obtained and compared with the data of Schubauer and Klebanoff for a turbulent flow in a moderately large adverse pressure gradient.

Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surface-mounted circular cylinder

NASA Astrophysics Data System (ADS)

Kirkil, Gokhan; Constantinescu, George

2015-07-01

The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on the results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder's base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. This is because even at very high cylinder Reynolds numbers, ReD, the flow regime remains subcritical in the vicinity of the bed surface due to the reduction of the incoming flow velocity within the bottom boundary layer. The paper provides a detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (ReD = 16 000, subcritical flow regime) and Reynolds numbers at which the transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (ReD = 5 ∗ 105, supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices, vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed. Being able to quantitatively and qualitatively describe these changes is critical to understand Reynolds-number-induced scale effects on sediment erosion mechanisms around cylinders mounted on a loose bed, which is a problem of great practical relevance (e.g., for pier scour studies).

The Numerical Simulation of Time Dependent Flow Structures Over a Natural Gravel Surface.

NASA Astrophysics Data System (ADS)

Hardy, R. J.; Lane, S. N.; Ferguson, R. I.; Parsons, D. R.

2004-05-01

Research undertaken over the last few years has demonstrated the importance of the structure of gravel river beds for understanding the interaction between fluid flow and sediment transport processes. This includes the observation of periodic high-speed fluid wedges interconnected by low-speed flow regions. Our understanding of these flows has been enhanced significantly through a series of laboratory experiments and supported by field observations. However, the potential of high resolution three dimensional Computational Fluid Dynamics (CFD) modeling has yet to be fully developed. This is largely the result of the problems of designing numerically stable meshes for use with complex bed topographies and that Reynolds averaged turbulence schemes are applied. This paper develops two novel techniques for dealing with these issues. The first is the development and validation of a method for representing the complex surface topography of gravel-bed rivers in high resolution three-dimensional computational fluid dynamic models. This is based upon a porosity treatment with a regular structured grid and the application of a porosity modification to the mass conservation equation in which: fully blocked cells are assigned a porosity of zero; fully unblocked cells are assigned a porosity of one; and partly blocked cells are assigned a porosity of between 0 and 1, according to the percentage of the cell volume that is blocked. The second is the application of Large Eddy Simulation (LES) which enables time dependent flow structures to be numerically predicted over the complex bed topographies. The regular structured grid with the embedded porosity algorithm maintains a constant grid cell size throughout the domain implying a constant filter scale for the LES simulation. This enables the prediction of coherent structures, repetitive quasi-cyclic large-scale turbulent motions, over the gravel surface which are of a similar magnitude and frequency to those previously observed in both flume and field studies. These structures are formed by topographic forcing within the domain and are scaled with the flow depth. Finally, this provides the numerical framework for the prediction of sediment transport within a time dependent framework. The turbulent motions make a significant contribution to the turbulent shear stress and the pressure fluctuations which significantly affect the forces acting on the bed and potentially control sediment motion.

Multiscale image-based modeling and simulation of gas flow and particle transport in the human lungs

PubMed Central

Tawhai, Merryn H; Hoffman, Eric A

2013-01-01

Improved understanding of structure and function relationships in the human lungs in individuals and sub-populations is fundamentally important to the future of pulmonary medicine. Image-based measures of the lungs can provide sensitive indicators of localized features, however to provide a better prediction of lung response to disease, treatment and environment, it is desirable to integrate quantifiable regional features from imaging with associated value-added high-level modeling. With this objective in mind, recent advances in computational fluid dynamics (CFD) of the bronchial airways - from a single bifurcation symmetric model to a multiscale image-based subject-specific lung model - will be reviewed. The interaction of CFD models with local parenchymal tissue expansion - assessed by image registration - allows new understanding of the interplay between environment, hot spots where inhaled aerosols could accumulate, and inflammation. To bridge ventilation function with image-derived central airway structure in CFD, an airway geometrical modeling method that spans from the model ‘entrance’ to the terminal bronchioles will be introduced. Finally, the effects of turbulent flows and CFD turbulence models on aerosol transport and deposition will be discussed. CFD simulation of airflow and particle transport in the human lung has been pursued by a number of research groups, whose interest has been in studying flow physics and airways resistance, improving drug delivery, or investigating which populations are most susceptible to inhaled pollutants. The three most important factors that need to be considered in airway CFD studies are lung structure, regional lung function, and flow characteristics. Their correct treatment is important because the transport of therapeutic or pollutant particles is dependent on the characteristics of the flow by which they are transported; and the airflow in the lungs is dependent on the geometry of the airways and how ventilation is distributed to the peripheral tissue. The human airway structure spans more than 20 generations, beginning with the extra-thoracic airways (oral or nasal cavity, and through the pharynx and larynx to the trachea), then the conducting airways, the respiratory airways, and to the alveoli. The airways in individuals and sub-populations (by gender, age, ethnicity, and normal vs. diseased states) may exhibit different dimensions, branching patterns and angles, and thickness and rigidity. At the local level, one would like to capture detailed flow characteristics, e.g. local velocity profiles, shear stress, and pressure, for prediction of particle transport in an airway (lung structure) model that is specific to the geometry of an individual, to understand how inter-subject variation in airway geometry (normal or pathological) influences the transport and deposition of particles. In a systems biology – or multiscale modeling – approach, these local flow characteristics can be further integrated with epithelial cell models for the study of mechanotransduction. At the global (organ) level, one would like to match regional ventilation (lung function) that is specific to the individual, thus ensuring that the flow that transports inhaled particles is appropriately distributed throughout the lung model. Computational models that do not account for realistic distribution of ventilation are not capable of predicting realistic particle distribution or targeted drug deposition. Furthermore, the flow in the human lung can be transitional or turbulent in the upper and proximal airways, and becomes laminar in the distal airways. The flows in the laminar, transitional and turbulent regimes have different temporal and spatial scales. Therefore, modeling airway structure and predicting gas flow and particle transport at both local and global levels require image-guided multiscale modeling strategies. In this article, we will review the aforementioned three key aspects of CFD studies of the human lungs: airway structure (conducting airways), lung function (regional ventilation and boundary conditions), and flow characteristics (modeling of turbulent flow and its effect on particle transport). For modeling airway structure, we will focus on the conducting airways, and review both symmetric vs. asymmetric airway models, idealized vs. CT-based airway models, and multiscale subject-specific airway models. Imposition of physiological subject-specific boundary conditions (BCs) in CFD is essential to match regional ventilation in individuals, which is also critical in studying preferential deposition of inhaled aerosols in sub-populations, e.g. normals vs. asthmatics that may exhibit different ventilation patterns. Subject-specific regional ventilation defines flow distributions and characteristics in airway segments and bifurcations, which subsequently determines the transport and deposition of aerosols in the entire lungs. Turbulence models are needed to capture the transient and turbulent nature of the gas flow in the human lungs. Thus, the advantages and disadvantages of different turbulence models as well as their effects on particle transport will be discussed. The ultimate goal of the development is to identify sensitive structural and functional variables in sub-populations of normal and diseased lungs for potential clinical applications. PMID:23843310

Variation of turbulence in a coastal thermal internal boundary layer

DOE Office of Scientific and Technical Information (OSTI.GOV)

SethuRaman, S.; Raynor, G.S.; Brown, R.M.

1981-01-01

Internal boundary layers (IBL) form when an air mass encounters a change in surface characteristics. There are essentially two types of internal boundary layers - one caused by the change in surface roughness and the other by the variation in surface heating. The former is known as the aerodynamic internal boundary layer (AIBL) and the latter the thermal internal boundary layer (TIBL). Change in shear stress generally characterizes the AIBL and change in turbulence the TIBL. Results of some observations of the vertical component of turbulence made in a coastal TIBL over Long Island, New York from 1974 to 1978more » are reported. Vertical turbulence measured by a simple sail plane variometer in a thermal internal boundary layer over Long Island with onshore flows indicates the structure to depend significantly on the land-water temperature difference. The position of the vertical velocity fluctuation maximum seems to vary from one test to another but its variation could not be correlated to other parameters due to lack of a sufficient number of tests. The structure of vertical turbulence was found to be different for sea breeze flows as compared to gradient winds.« less

Development of a Regional Structured and Unstructured Grid Methodology for Chemically Reactive Turbulent Flows

NASA Astrophysics Data System (ADS)

Stefanski, Douglas Lawrence

A finite volume method for solving the Reynolds Averaged Navier-Stokes (RANS) equations on unstructured hybrid grids is presented. Capabilities for handling arbitrary mixtures of reactive gas species within the unstructured framework are developed. The modeling of turbulent effects is carried out via the 1998 Wilcox k -- o model. This unstructured solver is incorporated within VULCAN -- a multi-block structured grid code -- as part of a novel patching procedure in which non-matching interfaces between structured blocks are replaced by transitional unstructured grids. This approach provides a fully-conservative alternative to VULCAN's non-conservative patching methods for handling such interfaces. In addition, the further development of the standalone unstructured solver toward large-eddy simulation (LES) applications is also carried out. Dual time-stepping using a Crank-Nicholson formulation is added to recover time-accuracy, and modeling of sub-grid scale effects is incorporated to provide higher fidelity LES solutions for turbulent flows. A switch based on the work of Ducros, et al., is implemented to transition from a monotonicity-preserving flux scheme near shocks to a central-difference method in vorticity-dominated regions in order to better resolve small-scale turbulent structures. The updated unstructured solver is used to carry out large-eddy simulations of a supersonic constrained mixing layer.

MHD Instability and Turbulence in the Tachocline

NASA Technical Reports Server (NTRS)

Werne, Joseph

2001-01-01

In this quarter we have begun simulations on the Cray T3E at PSC and we are debugging our code on the TSC. The PSC simulations are examining stratified shear turbulence with a flow-aligned magnetic field and passive tracer particles. We have conducted analysis of neutral simulations to establish a firm basis of comparison. Second-order structure functions have been computed, fit, and compared to theoretical expressions relating the dissipation fields and the structure-function-fit parameters. Agreement with high-Reynolds number observations is excellent, giving us confidence that the lower-Re simulations are relevant to higher-Re flows. We have also evaluated the neutral layer anisotropy.

Application of turbulence modeling to predict surface heat transfer in stagnation flow region of circular cylinder

NASA Technical Reports Server (NTRS)

Wang, Chi R.; Yeh, Frederick C.

1987-01-01

A theoretical analysis and numerical calculations for the turbulent flow field and for the effect of free-stream turbulence on the surface heat transfer rate of a stagnation flow are presented. The emphasis is on the modeling of turbulence and its augmentation of surface heat transfer rate. The flow field considered is the region near the forward stagnation point of a circular cylinder in a uniform turbulent mean flow. The free stream is steady and incompressible with a Reynolds number of the order of 10 to the 5th power and turbulence intensity of less than 5 percent. For this analysis, the flow field is divided into three regions: (1) a uniform free-stream region where the turbulence is homogeneous and isotropic; (2) an external viscid flow region where the turbulence is distorted by the variation of the mean flow velocity; and, (3) an anisotropic turbulent boundary layer region over the cylinder surface. The turbulence modeling techniques used are the kappa-epsilon two-equation model in the external flow region and the time-averaged turbulence transport equation in the boundary layer region. The turbulence double correlations, the mean velocity, and the mean temperature within the boundary layer are solved numerically from the transport equations. The surface heat transfer rate is calculated as functions of the free-stream turbulence longitudinal microlength scale, the turbulence intensity, and the Reynolds number.

Transition to turbulence under low-pressure turbine conditions.

PubMed

Simon, T W; Kaszeta, R W

2001-05-01

In this paper, the topic of laminar to turbulent flow transition, as applied to the design of gas turbines, is discussed. Transition comes about when a flow becomes sufficiently unstable that the orderly vorticity structure of the laminar layer becomes randomly oriented. Vorticity with a streamwise component leads to rapid growth of eddies of a wide range of sizes and eventually to turbulent flow. Under "natural" transition, infinitesimal disturbances of selected frequencies grow. "Bypass transition" is a term coined to describe a similar process, but one driven by strong external disturbances. Transition proceeds so rapidly that the processes associated with "natural" transition seem to be "bypassed." Because the flow environment in the turbine is disturbed by wakes from upstream airfoils, eddies from combustor flows, jets from film cooling, separation zones on upstream airfoils and steps in the duct walls, transition is of the bypass mode. In this paper, we discuss work that has been done to characterize and model bypass transition, as applied to the turbine environment.

Numerical simulations of current generation and dynamo excitation in a mechanically forced turbulent flow.

PubMed

Bayliss, R A; Forest, C B; Nornberg, M D; Spence, E J; Terry, P W

2007-02-01

The role of turbulence in current generation and self-excitation of magnetic fields has been studied in the geometry of a mechanically driven, spherical dynamo experiment, using a three-dimensional numerical computation. A simple impeller model drives a flow that can generate a growing magnetic field, depending on the magnetic Reynolds number Rm=micro0sigmaVa and the fluid Reynolds number Re=Vanu of the flow. For Re<420, the flow is laminar and the dynamo transition is governed by a threshold of Rmcrit=100, above which a growing magnetic eigenmode is observed that is primarily a dipole field transverse to the axis of symmetry of the flow. In saturation, the Lorentz force slows the flow such that the magnetic eigenmode becomes marginally stable. For Re>420 and Rm approximately 100 the flow becomes turbulent and the dynamo eigenmode is suppressed. The mechanism of suppression is a combination of a time varying large-scale field and the presence of fluctuation driven currents (such as those predicted by the mean-field theory), which effectively enhance the magnetic diffusivity. For higher Rm, a dynamo reappears; however, the structure of the magnetic field is often different from the laminar dynamo. It is dominated by a dipolar magnetic field aligned with the axis of symmetry of the mean-flow, which is apparently generated by fluctuation-driven currents. The magnitude and structure of the fluctuation-driven currents have been studied by applying a weak, axisymmetric seed magnetic field to laminar and turbulent flows. An Ohm's law analysis of the axisymmetric currents allows the fluctuation-driven currents to be identified. The magnetic fields generated by the fluctuations are significant: a dipole moment aligned with the symmetry axis of the mean-flow is generated similar to those observed in the experiment, and both toroidal and poloidal flux expulsion are observed.

Premixed Turbulent Flame Propagation in Microgravity

NASA Technical Reports Server (NTRS)

Menon, Suresh

1999-01-01

A combined numerical-experimental study has been carried out to investigate the structure and propagation characteristics of turbulent premixed flames with and without the influence of buoyancy. Experimentally, the premixed flame characteristics are studied in the wrinkled regime using a Couette flow facility and an isotropic flow facility in order to resolve the scale of flame wrinkling. Both facilities were chosen for their ability to achieve sustained turbulence at low Reynolds number. This implies that conventional diagnostics can be employed to resolve the smallest scales of wrinkling. The Couette facility was also built keeping in mind the constraints imposed by the drop tower requirements. Results showed that the flow in this Couette flow facility achieves full-developed turbulence at low Re and all turbulence statistics are in good agreement with past measurements on large-scale facilities. Premixed flame propagation studies were then carried out both using the isotropic box and the Couette facility. Flame imaging showed that fine scales of wrinkling occurs during flame propagation. Both cases in Ig showed significant buoyancy effect. To demonstrate that micro-g can remove this buoyancy effect, a small drop tower was built and drop experiments were conducted using the isotropic box. Results using the Couette facility confirmed the ability to carry out these unique reacting flow experiments at least in 1g. Drop experiments at NASA GRC were planned but were not completed due to termination of this project.

Defect-mediated turbulence in ribbons of viscoelastic Taylor-Couette flow.

PubMed

Latrache, Noureddine; Abcha, Nizar; Crumeyrolle, Olivier; Mutabazi, Innocent

2016-04-01

Transition to defect-mediated turbulence in the ribbon patterns observed in a viscoelastic Taylor-Couette flow is investigated when the rotation rate of the inner cylinder is increased while the outer cylinder is fixed. In four polymer solutions with different values of the elasticity number, the defects appear just above the onset of the ribbon pattern and trigger the appearance of disordered oscillations when the rotation rate is increased. The flow structure around the defects is determined and the statistical properties of these defects are analyzed in the framework of the complex Ginzburg-Landau equation.

On the Two Components of Turbulent Mixing Noise from Supersonic Jets

NASA Technical Reports Server (NTRS)

Tam, Christopher K. W.; Golebiowski, Michel; Seiner, J. M.

1996-01-01

It is argued that because of the lack of intrinsic length and time scales in the core part of the jet flow, the radiated noise spectrum of a high-speed jet should exhibit similarity. A careful analysis of all the axisymmetric supersonic jet noise spectra in the data-bank of the Jet Noise Laboratory of the NASA Langley Research Center has been carried out. Two similarity spectra, one for the noise from the large turbulence structures/instability waves of the jet flow, the other for the noise from the fine-scale turbulence, are identified. The two similarity spectra appear to be universal spectra for axisymmetric jets. They fit all the measured data including those from subsonic jets. Experimental evidence are presented showing that regardless of whether a jet is supersonic or subsonic the noise characteristics and generation mechanisms are the same. There is large turbulence structures/instability waves noise from subsonic jets. This noise component can be seen prominently inside the cone of silence of the fine-scale turbulence noise near the jet axis. For imperfectly expanded supersonic jets, a shock cell structure is formed inside the jet plume. Measured spectra are provided to demonstrate that the presence of a shock cell structure has little effect on the radiated turbulent mixing noise. The shape of the noise spectrum as well as the noise intensity remain practically the same as those of a fully expanded jet. However, for jets undergoing strong screeching, there is broadband noise amplification for both turbulent mixing noise components. It is discovered through a pilot study of the noise spectrum of rectangular and elliptic supersonic jets that the turbulent mixing noise of these jets is also made up of the same two noise components found in axisymmetric jets. The spectrum of each individual noise component also fits the corresponding similarity spectrum of axisymmetric jets.

Investigations of turbulent scalar fields using probability density function approach

NASA Technical Reports Server (NTRS)

Gao, Feng

1991-01-01

Scalar fields undergoing random advection have attracted much attention from researchers in both the theoretical and practical sectors. Research interest spans from the study of the small scale structures of turbulent scalar fields to the modeling and simulations of turbulent reacting flows. The probability density function (PDF) method is an effective tool in the study of turbulent scalar fields, especially for those which involve chemical reactions. It has been argued that a one-point, joint PDF approach is the one to choose from among many simulation and closure methods for turbulent combustion and chemically reacting flows based on its practical feasibility in the foreseeable future for multiple reactants. Instead of the multi-point PDF, the joint PDF of a scalar and its gradient which represents the roles of both scalar and scalar diffusion is introduced. A proper closure model for the molecular diffusion term in the PDF equation is investigated. Another direction in this research is to study the mapping closure method that has been recently proposed to deal with the PDF's in turbulent fields. This method seems to have captured the physics correctly when applied to diffusion problems. However, if the turbulent stretching is included, the amplitude mapping has to be supplemented by either adjusting the parameters representing turbulent stretching at each time step or by introducing the coordinate mapping. This technique is still under development and seems to be quite promising. The final objective of this project is to understand some fundamental properties of the turbulent scalar fields and to develop practical numerical schemes that are capable of handling turbulent reacting flows.

Embedded function methods for supersonic turbulent boundary layers

NASA Technical Reports Server (NTRS)

He, J.; Kazakia, J. Y.; Walker, J. D. A.

1990-01-01

The development of embedded functions to represent the mean velocity and total enthalpy distributions in the wall layer of a supersonic turbulent boundary layer is considered. The asymptotic scaling laws (in the limit of large Reynolds number) for high speed compressible flows are obtained to facilitate eventual implementation of the embedded functions in a general prediction method. A self-consistent asymptotic structure is derived, as well as a compressible law of the wall in which the velocity and total enthalpy are logarithmic within the overlap zone, but in the Howarth-Dorodnitsyn variable. Simple outer region turbulence models are proposed (some of which are modifications of existing incompressible models) to reflect the effects of compressibility. As a test of the methodology and the new turbulence models, a set of self-similar outer region profiles is obtained for constant pressure flow; these are then coupled with embedded functions in the wall layer. The composite profiles thus obtained are compared directly with experimental data and good agreement is obtained for flows with Mach numbers up to 10.

Validation of a Computational Fluid Dynamics (CFD) Code for Supersonic Axisymmetric Base Flow

NASA Technical Reports Server (NTRS)

Tucker, P. Kevin

1993-01-01

The ability to accurately and efficiently calculate the flow structure in the base region of bodies of revolution in supersonic flight is a significant step in CFD code validation for applications ranging from base heating for rockets to drag for protectives. The FDNS code is used to compute such a flow and the results are compared to benchmark quality experimental data. Flowfield calculations are presented for a cylindrical afterbody at M = 2.46 and angle of attack a = O. Grid independent solutions are compared to mean velocity profiles in the separated wake area and downstream of the reattachment point. Additionally, quantities such as turbulent kinetic energy and shear layer growth rates are compared to the data. Finally, the computed base pressures are compared to the measured values. An effort is made to elucidate the role of turbulence models in the flowfield predictions. The level of turbulent eddy viscosity, and its origin, are used to contrast the various turbulence models and compare the results to the experimental data.

Modeling the Effects of Turbulence in Rotating Detonation Engines

NASA Astrophysics Data System (ADS)

Towery, Colin; Smith, Katherine; Hamlington, Peter; van Schoor, Marthinus; TESLa Team; Midé Team

2014-03-01

Propulsion systems based on detonation waves, such as rotating and pulsed detonation engines, have the potential to substantially improve the efficiency and power density of gas turbine engines. Numerous technical challenges remain to be solved in such systems, however, including obtaining more efficient injection and mixing of air and fuels, more reliable detonation initiation, and better understanding of the flow in the ejection nozzle. These challenges can be addressed using numerical simulations. Such simulations are enormously challenging, however, since accurate descriptions of highly unsteady turbulent flow fields are required in the presence of combustion, shock waves, fluid-structure interactions, and other complex physical processes. In this study, we performed high-fidelity three dimensional simulations of a rotating detonation engine and examined turbulent flow effects on the operation, performance, and efficiency of the engine. Along with experimental data, these simulations were used to test the accuracy of commonly-used Reynolds averaged and subgrid-scale turbulence models when applied to detonation engines. The authors gratefully acknowledge the support of the Defense Advanced Research Projects Agency (DARPA).

Numerical flow simulation and efficiency prediction for axial turbines by advanced turbulence models

NASA Astrophysics Data System (ADS)

Jošt, D.; Škerlavaj, A.; Lipej, A.

2012-11-01

Numerical prediction of an efficiency of a 6-blade Kaplan turbine is presented. At first, the results of steady state analysis performed by different turbulence models for different operating regimes are compared to the measurements. For small and optimal angles of runner blades the efficiency was quite accurately predicted, but for maximal blade angle the discrepancy between calculated and measured values was quite large. By transient analysis, especially when the Scale Adaptive Simulation Shear Stress Transport (SAS SST) model with zonal Large Eddy Simulation (ZLES) in the draft tube was used, the efficiency was significantly improved. The improvement was at all operating points, but it was the largest for maximal discharge. The reason was better flow simulation in the draft tube. Details about turbulent structure in the draft tube obtained by SST, SAS SST and SAS SST with ZLES are illustrated in order to explain the reasons for differences in flow energy losses obtained by different turbulence models.

Wall-pressure fluctuations beneath a spatially evolving turbulent boundary layer

NASA Astrophysics Data System (ADS)

Mahesh, Krishnan; Kumar, Praveen

2016-11-01

Wall-pressure fluctuations beneath a turbulent boundary layer are important in applications dealing with structural deformation and acoustics. Simulations are performed for flat plate and axisymmetric, spatially evolving zero-pressure-gradient turbulent boundary layers at inflow Reynolds number of 1400 and 2200 based on momentum thickness. The simulations generate their own inflow using the recycle-rescale method. The results for mean velocity and second-order statistics show excellent agreement with the data available in literature. The spectral characteristics of wall-pressure fluctuations and their relation to flow structure will be discussed. This work is supported by ONR.