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Sample records for les jets turbulents

  1. 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.

  2. LES/FMDF of turbulent jet ignition in a rapid compression machine

    NASA Astrophysics Data System (ADS)

    Validi, Abdoulahad; Schock, Harold; Toulson, Elisa; Jaberi, Farhad; CFD; Engine Research Labs, Michigan State University Collaboration

    2015-11-01

    Turbulent Jet Ignition (TJI) is an efficient method for initiating and controlling combustion in combustion systems, e.g. internal combustion engines. It enables combustion in ultra-lean mixtures by utilizing hot product turbulent jets emerging from a pre-chamber combustor as the ignition source for the main combustion chamber. Here, we study the TJI-assisted ignition and combustion of lean methane-air mixtures in a Rapid Compression Machine (RCM) for various flow/combustion conditions with the hybrid large eddy simulation/filtered mass density function (LES/FMDF) computational model. In the LES/FMDF model, the filtered form of compressible Navier-Stokes equations are solved with a high-order finite difference scheme for the turbulent velocity, while the FMDF transport equation is solved with a Lagrangian stochastic method to obtain the scalar (species mass fraction and temperature) field. The LES/FMDF data are used to study the physics of TJI and combustion in RCM. The results show the very complex behavior of the reacting flow and the flame structure in the pre-chamber and RCM.

  3. Modeling studies of a turbulent pulsed jet flame using LES/PDF

    NASA Astrophysics Data System (ADS)

    Zhang, Pei; Wang, Haifeng

    2015-11-01

    The combustion field in a pulsed turbulent piloted jet flame is studied using an advanced large eddy simulation (LES) / probability density function (PDF) method. Measurement data with a joint OH-PLIF/OH* chemiluminescence/LDV system are available including the temporal series of the axial velocity and planar OH images. A time-dependent inflow condition is specified based on the measurement data. A direct comparison of the mean and rms velocities from the calculations and from the measurement shows a satisfactory prediction of the flow fields by using the employed modeling methods. The predicted OH mass fractions are compared qualitatively with the measured OH images at selected temporal and spatial locations. The comparison shows a good agreement. Conditional quantities and flame index are extracted from the simulations to examine the bimodal and multi-regime combustion dynamics in the flame. This paper is based upon work supported by the National Science Foundation under Grant No. CBET-1336075.

  4. Validating LES for Jet Aeroacoustics

    NASA Technical Reports Server (NTRS)

    Bridges, James; Wernet, Mark P.

    2011-01-01

    Engineers charged with making jet aircraft quieter have long dreamed of being able to see exactly how turbulent eddies produce sound and this dream is now coming true with the advent of large eddy simulation (LES). Two obvious challenges remain: validating the LES codes at the resolution required to see the fluid-acoustic coupling, and the interpretation of the massive datasets that are produced. This paper addresses the former, the use of advanced experimental techniques such as particle image velocimetry (PIV) and Raman and Rayleigh scattering, to validate the computer codes and procedures used to create LES solutions. This paper argues that the issue of accuracy of the experimental measurements be addressed by cross-facility and cross-disciplinary examination of modern datasets along with increased reporting of internal quality checks in PIV analysis. Further, it argues that the appropriate validation metrics for aeroacoustic applications are increasingly complicated statistics that have been shown in aeroacoustic theory to be critical to flow-generated sound, such as two-point space-time velocity correlations. A brief review of data sources available is presented along with examples illustrating cross-facility and internal quality checks required of the data before it should be accepted for validation of LES.

  5. Validating LES for Jet Aeroacoustics

    NASA Technical Reports Server (NTRS)

    Bridges, James

    2011-01-01

    Engineers charged with making jet aircraft quieter have long dreamed of being able to see exactly how turbulent eddies produce sound and this dream is now coming true with the advent of large eddy simulation (LES). Two obvious challenges remain: validating the LES codes at the resolution required to see the fluid-acoustic coupling, and the interpretation of the massive datasets that result in having dreams come true. This paper primarily addresses the former, the use of advanced experimental techniques such as particle image velocimetry (PIV) and Raman and Rayleigh scattering, to validate the computer codes and procedures used to create LES solutions. It also addresses the latter problem in discussing what are relevant measures critical for aeroacoustics that should be used in validating LES codes. These new diagnostic techniques deliver measurements and flow statistics of increasing sophistication and capability, but what of their accuracy? And what are the measures to be used in validation? This paper argues that the issue of accuracy be addressed by cross-facility and cross-disciplinary examination of modern datasets along with increased reporting of internal quality checks in PIV analysis. Further, it is argued that the appropriate validation metrics for aeroacoustic applications are increasingly complicated statistics that have been shown in aeroacoustic theory to be critical to flow-generated sound.

  6. DNS of autoigniting turbulent jet flame

    NASA Astrophysics Data System (ADS)

    Asaithambi, Rajapandiyan; Mahesh, Krishnan

    2014-11-01

    Direct numerical simulation of a round turbulent hydrogen jet injected into vitiated coflowing air is performed at a jet Reynolds number of 10,000 and the results are discussed. A predictor-corrector density based method for DNS/LES of compressible chemically reacting flows is developed and used on a cylindrical grid. A novel strategy to remove the center-line stiffness is developed. A fully developed turbulent pipe flow simulation is prescribed as the velocity inlet for the fuel jet. The flame base is observed to be stabilized primarily by autoignition. Further downstream the flame exhibits a diffusion flame structure with regions of rich and lean premixed regimes flanking the central diffusion flame. The lift-off height is well predicted by a simple relation between the ignition delay of the most-reactive mixture fraction and the streamwise velocity of the jet and coflow.

  7. Towards LES Models of Jets and Plumes

    NASA Technical Reports Server (NTRS)

    Webb, A. T.; Mansour, N. N.

    2000-01-01

    As pointed out by Rodi standard integral solutions for jets and plumes developed for discharge into infinite, quiescent ambient are difficult to extend to complex situations, particularly in the presence of boundaries such as the sea floor or ocean surface. In such cases the assumption of similarity breaks down and it is impossible to find a suitable entrainment coefficient. The models are also incapable of describing any but the most slowly varying unsteady motions. There is therefore a need for full time-dependent modeling of the flow field for which there are three main approaches: (1) Reynolds averaged numerical simulation (RANS), (2) large eddy simulation (LES), and (3) direct numerical simulation (DNS). Rodi applied RANS modeling to both jets and plumes with considerable success, the test being a match with experimental data for time-averaged velocity and temperature profiles as well as turbulent kinetic energy and rms axial turbulent velocity fluctuations. This model still relies on empirical constants, some eleven in the case of the buoyant jet, and so would not be applicable to a partly laminar plume, may have limited use in the presence of boundaries, and would also be unsuitable if one is after details of the unsteady component of the flow (the turbulent eddies). At the other end of the scale DNS modeling includes all motions down to the viscous scales. Boersma et al. have built such a model for the non-buoyant case which also compares well with measured data for mean and turbulent velocity components. The model demonstrates its versatility by application to a laminar flow case. As its name implies, DNS directly models the Navier-Stokes equations without recourse to subgrid modeling so for flows with a broad spectrum of motions (high Re) the cost can be prohibitive - the number of required grid points scaling with Re(exp 9/4) and the number of time steps with Re(exp 3/4). The middle road is provided by LES whereby the Navier-Stokes equations are formally

  8. Impulsively started incompressible turbulent jet

    SciTech Connect

    Witze, P O

    1980-10-01

    Hot-film anemometer measurements are presented for the centerline velocity of a suddenly started jet of air. The tip penetration of the jet is shown to be proportional to the square-root of time. A theoretical model is developed that assumes the transient jet can be characterized as a spherical vortex interacting with a steady-state jet. The model demonstrates that the ratio of nozzle radius to jet velocity defines a time constant that uniquely characterizes the behavior and similarity of impulsively started incompressible turbulent jets.

  9. Multiwave Interactions in Turbulent Jets

    NASA Technical Reports Server (NTRS)

    Mankbadi, Reda R.

    1989-01-01

    Nonlinear wave-wave interactions in turbulent jets were investigated based on the integrated energy of each scale of motion in a cross section of the jet. The analysis indicates that two frequency components in the axisymmetric mode can interact with other background frequencies in the axisymmetric mode, thereby amplifying an enormous number of other frequencies. Two frequency components in a single helical mode cannot, by themselves, amplify other frequency components. But combinations of frequency components of helical and axisymmetric modes can amplify other frequencies in other helical modes. The present computations produce several features consistent with experimental observations such as: (1) dependency of the interactions on the initial phase differences, (2) enhancement of the momentum thickness under multifrequency forcing, and (3) the increase in background turbulence under forcing. In a multifrequency-excited jet, mixing enhancement was found to be a result of the turbulence enhancement rather than simply the amplification of forced wave components. The excitation waves pump energy from the mean flow to the turbulence, thus enhancing the latter. The high frequency waves enhance the turbulence close to the jet exit, but, the low frequency waves are most effective further downstream.

  10. CHEMICALLY REACTING TURBULENT JETS

    EPA Science Inventory

    The paper reports additional experimental evidence supporting a new description of the mechanism of turbulent entrainment, mixing, and chemical reactions that is emerging from experiments in the last few years which reveal the presence of large scale structures in turbulent shear...

  11. NON-PREMIXED TURBULENT JET FLAMES

    EPA Science Inventory

    The paper, part of a general investigation of mixing and chemical reaction in turbulent jets, concerns the length of non-premixed turbulent jet flames in a stationary environment. Experimental results for the turbulent flame length of chemically reacting jets in water show both i...

  12. 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.

  13. Flash evaporation from turbulent water jets

    NASA Astrophysics Data System (ADS)

    Bharathan, D.; Penney, T.

    1983-02-01

    Results of an experimental investigation of flash evaporation from turbulent planar and axisymmetric water jets are reported. In the range of jet thicknesses tested, for planar jets, due to shattering, evaporation is found to be nearly independent of the jet thickness. Evaporation from the planar jets was found to be dependent on the initial level of turbulence in the water supply manifold. An approximate analysis to model the evaporation process based on the physical phenomena and experimental observations is outlined. Comparisons between the experimental data and analytical predictions of the liquid temperature variation along the jet are included. Use of screens in the water jet are shown to be effective for enhancing evaporation.

  14. Effects of core turbulence on jet excitability

    NASA Technical Reports Server (NTRS)

    Mankbadi, Reda R.; Raman, Ganesh; Rice, Edward J.

    1989-01-01

    The effects of varying freestream core turbulence on the evolution of a circular jet with and without tonal excitation are examined. Measurements are made on an 8.8 cm diameter jet at a Mach number of 0.3. The jet is excitated by plane waves at Strouhal number 0.5. For the excited and unexcited cases the turbulence level is varied by screens and grids placed upstream of the nozzle exit. The experiment results are compared with a theoretical model which incorporates a variable core turbulence and considers the energy interactions between the mean flow, the turbulence and the forced component. Both data and theory indicate that increasing the freestream turbulence diminishes the excitability of the jet and reduces the effect of excitation on the spreading rate of the jet.

  15. 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.

  16. Turbulent wall jet in a coflowing stream

    NASA Technical Reports Server (NTRS)

    Campbell, J. F.

    1975-01-01

    A theoretical investigation was undertaken to develop a relatively simple model of a two-dimensional, turbulent wall jet in a coflowing stream. The incompressible jet flow was modeled by using an integral method which includes turbulent shear stress, entrainment, and heat transfer. The method solves the conservation equations for the average jet flow properties and uses the velocity profile suggested by Escudier and Nicoll to obtain detailed characteristics of the jet on a flat plate. The analytical results compare favorably with experimental data for a range of injection velocities, which demonstrates the usefulness of the theory for estimating jet growth, velocity decay, and wall skin friction. The theory, which was applied to a Coanda jet on a circular cylinder, provided estimates of suction pressures aft of the jet exit that were in close agreement with experimental values.

  17. The numerical analysis of a turbulent compressible jet

    NASA Astrophysics Data System (ADS)

    Debonis, James Raymond

    2000-10-01

    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. Sub-grid 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 sub-grid 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 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 sub-grid 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 ½Dj. 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 Uj.

  18. Large Eddy Simulation of Multiple Turbulent Round Jets

    NASA Astrophysics Data System (ADS)

    Balajee, G. K.; Panchapakesan, Nagangudy

    2015-11-01

    Turbulent round jet flow was simulated as a large eddy simulation with OpenFoam software package for a jet Reynolds number of 11000. The intensity of the fluctuating motion in the incoming nozzle flow was adjusted so that the initial shear layer development compares well with available experimental data. The far field development of averages of higher order moments up to fourth order were compared with experiments. The agreement is good indicating that the large eddy motions were being computed satisfactorily by the simulation. Turbulent kinetic energy budget as well as the quality of the LES simulations were also evaluated. These conditions were then used to perform a multiple turbulent round jets simulation with the same initial momentum flux. The far field of the flow was compared with the single jet simulation and experiments to test approach to self similarity. The evolution of the higher order moments in the development region where the multiple jets interact were studied. We will also present FTLE fields computed from the simulation to educe structures and compare it with those educed by other scalar measures. Support of AR&DB CIFAAR, and VIRGO cluster at IIT Madras is gratefully acknowledged.

  19. 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

  20. 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

  1. Adaptive resolution LES of a reacting non-premixed jet

    NASA Astrophysics Data System (ADS)

    Pantano, Carlos; Deiterding, Ralf; Hill, David; Pullin, Dale

    2004-11-01

    We present results of a turbulent reactive non-premixed jet using Large-Eddy Simulation (LES) performed within the blockstructured adaptive mesh refinement infrastructure AMROC. A fully compressible formulation of the transport equations and the stretched-vortex subgrid-stress model of Misra & Pullin (1997) are integrated with the assumed Beta subgrid pdf model for non-premixed combustion. Flamelet libraries are precomputed with the Cantera chemistry package. The modeling technique has been previously used and validated/verified in prior work, primarily for incompressible flows. One difficulty commonly encountered for these unstationary flows is the need to resolve certain regions of the flow field more finely than others. These can include thin shear layers and regions of steep density gradients produced by combustion. We show that adaptive resolution can be used successfully in the context of LES. This work is part of Caltech's ASC center supported by the Department of Energy (DOE).

  2. Transverse jet injection into a supersonic turbulent cross-flow

    NASA Astrophysics Data System (ADS)

    Rana, Z. A.; Thornber, B.; Drikakis, D.

    2011-04-01

    Jet injection into a supersonic cross-flow is a challenging fluid dynamics problem in the field of aerospace engineering which has applications as part of a rocket thrust vector control system for noise control in cavities and fuel injection in scramjet combustion chambers. Several experimental and theoretical/numerical works have been conducted to explore this flow; however, there is a dearth of literature detailing the instantaneous flow which is vital to improve the efficiency of the mixing of fluids. In this paper, a sonic jet in a Mach 1.6 free-stream is studied using a finite volume Godunov type implicit large eddy simulations technique, which employs fifth-order accurate MUSCL (Monotone Upstream-centered Schemes for Conservation Laws) scheme with modified variable extrapolation and a three-stage second-order strong-stability-preserving Runge-Kutta scheme for temporal advancement. A digital filter based turbulent inflow data generation method is implemented in order to capture the physics of the supersonic turbulent boundary layer. This paper details the averaged and instantaneous flow features including vortex structures downstream of the jet injection, along with the jet penetration, jet mixing, pressure distributions, turbulent kinetic energy, and Reynolds stresses in the downstream flow. It demonstrates that Kelvin-Helmholtz type instabilities in the upper jet shear layer are primarily responsible for mixing of the two fluids. The results are compared to experimental data and recently performed classical large eddy simulations (LES) with the same initial conditions in order to demonstrate the accuracy of the numerical methods and utility of the inflow generation method. Results here show equivalent accuracy for 1/45th of the computational resources used in the classical LES study.

  3. A theory for turbulent curved wall jets

    NASA Technical Reports Server (NTRS)

    Roberts, Leonard

    1986-01-01

    A simple theoretical model is proposed to describe the flow of a turbulent wall jet along a curved surface into a quiescent atmosphere. An integral method is used to solve the momentum equation and identifies three contributions to the spreading rate of the wall jet: (1) turbulent diffusion in the wall jet; (2) wall curvature; and (3) rate of change of wall curvature. Closed from approximate solutions are found for the case of a plane wall, a circular cylinder, and a logarithmic spiral surface. Comparison with experimental data for these three cases is made showing good agreement.

  4. Large-eddy simulation of turbulent circular jet flows

    SciTech Connect

    Jones, S. C.; Sotiropoulos, F.; Sale, M. J.

    2002-07-01

    This report presents a numerical method for carrying out large-eddy simulations (LES) of turbulent free shear flows and an application of a method to simulate the flow generated by a nozzle discharging into a stagnant reservoir. The objective of the study was to elucidate the complex features of the instantaneous flow field to help interpret the results of recent biological experiments in which live fish were exposed to the jet shear zone. The fish-jet experiments were conducted at the Pacific Northwest National Laboratory (PNNL) under the auspices of the U.S. Department of Energy’s Advanced Hydropower Turbine Systems program. The experiments were designed to establish critical thresholds of shear and turbulence-induced loads to guide the development of innovative, fish-friendly hydropower turbine designs.

  5. Synchrotron brightness distribution of turbulent radio jets

    NASA Technical Reports Server (NTRS)

    Henriksen, R. N.; Bridle, A. H.; Chan, K. L.

    1981-01-01

    Radio jets are considered as turbulent mixing regions and it is proposed that the essential small scale viscous dissipation in these jets is by emission of MHD waves and by their subsequent strong damping due, at least partly, to gyro-resonant acceleration of supra-thermal particles. A formula relating the synchrotron surface brightness of a radio jet to the turbulent power input is deduced from physical postulates, and is tested against the data for NGC315 and 3C31 (NGC383). The predicted brightness depends essentially on the collimation behavior of the jet, and, to a lesser extent, on the CH picture of a 'high' nozzle with accelerating flow. The conditions for forming a large scale jet at a high nozzle from a much smaller scale jet are discussed. The effect of entrainment on the prediction is discussed with the use of similarity solutions. Although entrainment is inevitably associated with the turbulent jet, it may or may not be a dominant factor depending on the ambient density profile.

  6. Microgravity Turbulent Gas-Jet Diffusion Flames

    NASA Technical Reports Server (NTRS)

    1996-01-01

    A gas-jet diffusion flame is similar to the flame on a Bunsen burner, where a gaseous fuel (e.g., propane) flows from a nozzle into an oxygen-containing atmosphere (e.g., air). The difference is that a Bunsen burner allows for (partial) premixing of the fuel and the air, whereas a diffusion flame is not premixed and gets its oxygen (principally) by diffusion from the atmosphere around the flame. Simple gas-jet diffusion flames are often used for combustion studies because they embody the mechanisms operating in accidental fires and in practical combustion systems. However, most practical combustion is turbulent (i.e., with random flow vortices), which enhances the fuel/air mixing. These turbulent flames are not well understood because their random and transient nature complicates analysis. Normal gravity studies of turbulence in gas-jet diffusion flames can be impeded by buoyancy-induced instabilities. These gravitycaused instabilities, which are evident in the flickering of a candle flame in normal gravity, interfere with the study of turbulent gas-jet diffusion flames. By conducting experiments in microgravity, where buoyant instabilities are avoided, we at the NASA Lewis Research Center hope to improve our understanding of turbulent combustion. Ultimately, this could lead to improvements in combustor design, yielding higher efficiency and lower pollutant emissions. Gas-jet diffusion flames are often researched as model flames, because they embody mechanisms operating in both accidental fires and practical combustion systems (see the first figure). In normal gravity laboratory research, buoyant air flows, which are often negligible in practical situations, dominate the heat and mass transfer processes. Microgravity research studies, however, are not constrained by buoyant air flows, and new, unique information on the behavior of gas-jet diffusion flames has been obtained.

  7. Ray Traces Through Unsteady Jet Turbulence

    NASA Technical Reports Server (NTRS)

    Freund, J. B.; Fleischman, T. G.

    2002-01-01

    Results of an ongoing effort to quantify the role turbulence in scattering sound in jets are reported. Using a direct numerical simulation database to provide the flow data, ray paths traced through the mean flow are compared with those traced through the actual time evolving turbulent flow. Significant scattering by the turbulence is observed. The most notable effect is that upstream traveling waves that are trapped in the potential core by the mean flow, which acts as a wave guide, easily escape in the turbulent flow. A crude statistical estimate based on ray number density suggests that directivity is modified by the turbulence, but no rigorous treatment of non-uniformities in the high-frequency approximation is attempted.

  8. Formation and inflammation of a turbulent jet

    NASA Technical Reports Server (NTRS)

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

    1984-01-01

    The formation and inflammation of a planar, turbulent jet in an incompressible medium is modeled numerically by the use of the random vortex method amended by a flame propagation algorithm. The results demonstrate the dominant influence of turbulent eddies and their interactions upon the development of the jet. Its growth is shown to consist of three stages: formation of small eddies, pairing of eddies with the same sign of circulation, and pairing of eddies of opposite signs. On this basis a number of features of the jet mechanism are revealed, namely penetration, engulfment, entrainment, and intermittency. Two cases of inflammation are considered. In one, the jet is ignited at the center of the orifice, the solution tracing its own inflammation. In the other, combustion is initiated across its full cross section, the results modeling the action of a turbulent torch as it spreads the flame into the combustible surroundings. In both cases the flow field is still dominated by the turbulent eddies and their interactions. However, the coherence among them is encumbered as a consequence of expansion due to the exothermicity of the combustion process.

  9. Modelisations des effets de surface sur les jets horizontaux subsoniques d'hydrogene et de methane

    NASA Astrophysics Data System (ADS)

    Gomez, Luis Fernando

    Le developpement des codes et de normes bases sur une methodologie scientifique requiert la capacite de predire l'etendue inflammable de deversements gazeux d'hydrogene sous differentes conditions. Des etudes anterieures ont deja etabli des modeles bases sur les lois de conservation de la mecanique des fluides basees sur des correlations experimentales qui permettent de predire la decroissance de la concentration et de la vitesse d'un gaz le long de l'axe d'un jet libre vertical. Cette etude s'interesse aux effets de proximite a une surface horizontale parallele sur un jet turbulent. Nous nous interessons a son impact sur l'etendue du champ de la concentration et sur l'enveloppe inflammable en particulier. Cette etude est comparative : l'hydrogene est compare au methane. Ceci permet de degager l'influence des effets de difference de la densite sur le comportement du jet, et de comparer le comportement de l'hydrogene aux correlations experimentales, qui ont ete essentiellement etablies pour le methane. Un modele decrivant l'evolution spatio-temporelle du champ de concentration du gaz dilue est propose, base sur la mecanique des fluides computationnelle. Cette approche permet de varier systematiquement les conditions aux frontieres (proximite du jet a la surface, par exemple) et de connaitre en detail les proprietes de l'ecoulement. Le modele est implemente dans le code de simulations par volumes finis de FLUENT. Les resultats des simulations sont compares avec les lois de similitudes decoulant de la theorie des jets d'ecoulements turbulents libres ainsi qu'avec les resultats experimentaux disponibles. L'effet de la difference des masses molaires des constituantes du jet et des constituantes du milieu de dispersion est egalement etudie dans le contexte du comportement d'echelle de la region developpee du jet.

  10. Multiple Mode Actuation of a Turbulent Jet

    NASA Technical Reports Server (NTRS)

    Pack, LaTunia G.; Seifert, Avi

    2001-01-01

    The effects of multiple mode periodic excitation on the evolution of a circular turbulent jet were studied experimentally. A short, wide-angle diffuser was attached to the jet exit. Streamwise and cross-stream excitations were introduced at the junction between the jet exit and the diffuser inlet on opposing sides of the jet. The introduction of high amplitude, periodic excitation in the streamwise direction enhances the mixing and promotes attachment of the jet shear-layer to the diffuser wall. Cross-stream excitation applied over a fraction of the jet circumference can deflect the jet away from the excitation slot. The two modes of excitation were combined using identical frequencies and varying the relative phase between the two actuators in search of an optimal response. It is shown that, for low and moderate periodic momentum input levels, the jet deflection angles depend strongly on the relative phase between the two actuators. Optimum performance is achieved when the phase difference is pi +/- pi/6. The lower effectiveness of the equal phase excitation is attributed to the generation of an azimuthally symmetric mode that does not produce the required non-axisymmetric vectoring. For high excitation levels, identical phase becomes more effective, while phase sensitivity decreases. An important finding was that with proper phase tuning, two unsteady actuators can be combined to obtain a non-linear response greater than the superposition of the individual effects.

  11. The Aeroacoustics of Turbulent Coanda Wall Jets

    NASA Astrophysics Data System (ADS)

    Lubert, Caroline; Fox, Jason

    2007-11-01

    Turbulent Coanda wall jets have become increasingly widely used in a variety of industrial applications in recent years, due to the substantial flow deflection that they afford. A related characteristic is the enhanced turbulence levels and entrainment they offer, compared with conventional jet flows. This characteristic is, however, generally accompanied by a significant increase in the noise levels associated with devices employing this effect. As a consequence, the potential offered by Coanda devices is yet to be fully realized. This problem provides the impetus for the research detailed in this poster. To date, some work has been done on developing a mathematical model of the Turbulent Mixing Noise emitted by such a device, assuming that the surface adjoining the turbulent flow was essentially 2-D. This poster extends this fundamental model, through a combination of mathematical modeling and acoustical and optical experiments. The effect of a variety of parameters, including nozzle configuration and jet exit velocity will be discussed, and ways of reducing or attenuating the noise generated by such flow, whilst still maintaining the crucial flow characteristics, will be presented.

  12. LES/RANS Simulation of a Supersonic Reacting Wall Jet

    NASA Technical Reports Server (NTRS)

    Edwards, Jack R.; Boles, John A.; Baurle, Robert A.

    2010-01-01

    This work presents results from large-eddy / Reynolds-averaged Navier-Stokes (LES/RANS) simulations of the well-known Burrows-Kurkov supersonic reacting wall-jet experiment. Generally good agreement with experimental mole fraction, stagnation temperature, and Pitot pressure profiles is obtained for non-reactive mixing of the hydrogen jet with a non-vitiated air stream. A lifted flame, stabilized between 10 and 22 cm downstream of the hydrogen jet, is formed for hydrogen injected into a vitiated air stream. Flame stabilization occurs closer to the hydrogen injection location when a three-dimensional combustor geometry (with boundary layer development resolved on all walls) is considered. Volumetric expansion of the reactive shear layer is accompanied by the formation of large eddies which interact strongly with the reaction zone. Time averaged predictions of the reaction zone structure show an under-prediction of the peak water concentration and stagnation temperature, relative to experimental data and to results from a Reynolds-averaged Navier-Stokes calculation. If the experimental data can be considered as being accurate, this result indicates that the present LES/RANS method does not correctly capture the cascade of turbulence scales that should be resolvable on the present mesh. Instead, energy is concentrated in the very largest scales, which provide an over-mixing effect that excessively cools and strains the flame. Predictions improve with the use of a low-dissipation version of the baseline piecewise parabolic advection scheme, which captures the formation of smaller-scale structures superimposed on larger structures of the order of the shear-layer width.

  13. Turbulent jet patterns in accelerating flows

    NASA Technical Reports Server (NTRS)

    Lipshitz, A.; Greber, I.

    1981-01-01

    Results of flow visualization experiments, and a semi-empirical model of a single turbulent jet injected perpendicularly to a different density cross flow are presented. The model is based on integral conservation equations, including the pressure terms appropriate to accelerating flow. It uses an entrainment correlation obtained from previous experiments of a jet in a cross stream. The results show trajectories and spreading rates, and are typified by a set of three parameters: momentum ratio, Froude number and density ratio. Agreement between test and calculated results is encouraging, but tend to be poorer with increasing momentum ratio.

  14. Simultaneous computation of jet turbulence and noise

    NASA Technical Reports Server (NTRS)

    Berman, C. H.; Ramos, J. I.

    1989-01-01

    The existing flow computation methods, wave computation techniques, and theories based on noise source models are reviewed in order to assess the capabilities of numerical techniques to compute jet turbulence noise and understand the physical mechanisms governing it over a range of subsonic and supersonic nozzle exit conditions. In particular, attention is given to (1) methods for extrapolating near field information, obtained from flow computations, to the acoustic far field and (2) the numerical solution of the time-dependent Lilley equation.

  15. Extended LES-PaSR model for simulation of turbulent combustion

    NASA Astrophysics Data System (ADS)

    Sabelnikov, V.; Fureby, C.

    2013-03-01

    In this work, a novel model for Large Eddy Simulations (LES) of high Reynolds moderate Damköhler number turbulent flames is proposed. The development is motivated by the need for more accurate and versatile LES combustion models for engineering applications such as jet engines. The model is based on the finite rate chemistry approach in which the filtered species equations of a reduced reaction mechanism are solved prior to closure modeling. The modeling of the filtered reaction rate provides the challenge: as most of the chemical activity, and thus also most of the exothermicity occurs on the subgrid scales, this model needs to be based on the properties of fine-scale turbulence and mixing and Arrhenius chemistry. The model developed here makes use of the similarities with the mathematical treatment of multiphase flows together with the knowledge of fine-scale turbulence and chemistry obtained by Direct Numerical Simulation (DNS) and experiments. In the model developed, equations are proposed for the fine-structure composition and volume fraction that are solved together with the LES equations for the resolved scales. If subgrid convection can be neglected, the proposed model simplifies to the Partially Stirred Reactor (PaSR) model. To validate the proposed LES model, comparisons with experimental data and other LES results are made, using other turbulence chemistry interaction models, for a lean premixed bluff-body stabilized flame.

  16. Modeling of Turbulence Generated Noise in Jets

    NASA Technical Reports Server (NTRS)

    Khavaran, Abbas; Bridges, James

    2004-01-01

    A numerically calculated Green's function is used to predict jet noise spectrum and its far-field directivity. A linearized form of Lilley's equation governs the non-causal Green s function of interest, with the non-linear terms on the right hand side identified as the source. In this paper, contributions from the so-called self- and shear-noise source terms will be discussed. A Reynolds-averaged Navier-Stokes solution yields the required mean flow as well as time- and length scales of a noise-generating turbulent eddy. A non-compact source, with exponential temporal and spatial functions, is used to describe the turbulence velocity correlation tensors. It is shown that while an exact non-causal Green's function accurately predicts the observed shift in the location of the spectrum peak with angle as well as the angularity of sound at moderate Mach numbers, at high subsonic and supersonic acoustic Mach numbers the polar directivity of radiated sound is not entirely captured by this Green's function. Results presented for Mach 0.5 and 0.9 isothermal jets, as well as a Mach 0.8 hot jet conclude that near the peak radiation angle a different source/Green's function convolution integral may be required in order to capture the peak observed directivity of jet noise.

  17. Large Eddy Simulation of a Turbulent Jet

    NASA Technical Reports Server (NTRS)

    Webb, A. T.; Mansour, Nagi N.

    2001-01-01

    Here we present the results of a Large Eddy Simulation of a non-buoyant jet issuing from a circular orifice in a wall, and developing in neutral surroundings. The effects of the subgrid scales on the large eddies have been modeled with the dynamic large eddy simulation model applied to the fully 3D domain in spherical coordinates. The simulation captures the unsteady motions of the large-scales within the jet as well as the laminar motions in the entrainment region surrounding the jet. The computed time-averaged statistics (mean velocity, concentration, and turbulence parameters) compare well with laboratory data without invoking an empirical entrainment coefficient as employed by line integral models. The use of the large eddy simulation technique allows examination of unsteady and inhomogeneous features such as the evolution of eddies and the details of the entrainment process.

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

    NASA Astrophysics Data System (ADS)

    Ruiz, A. M.; Lacaze, G.; Oefelein, J. C.

    2015-04-01

    This paper presents a detailed analysis of the flow topologies and turbulence scales in the jet-in-cross-flow experiment of Su and Mungal ["Simultaneous measurements of scalar and velocity field evolution in turbulent crossflowing jets," J. Fluid Mech. 513(1), 1-45 (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 comprehensive 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.

  19. The deterministic chaos and random noise in turbulent jet

    SciTech Connect

    Yao, Tian-Liang; Liu, Hai-Feng Xu, Jian-Liang; Li, Wei-Feng

    2014-06-01

    A turbulent flow is usually treated as a superposition of coherent structure and incoherent turbulence. In this paper, the largest Lyapunov exponent and the random noise in the near field of round jet and plane jet are estimated with our previously proposed method of chaotic time series analysis [T. L. Yao, et al., Chaos 22, 033102 (2012)]. The results show that the largest Lyapunov exponents of the round jet and plane jet are in direct proportion to the reciprocal of the integral time scale of turbulence, which is in accordance with the results of the dimensional analysis, and the proportionality coefficients are equal. In addition, the random noise of the round jet and plane jet has the same linear relation with the Kolmogorov velocity scale of turbulence. As a result, the random noise may well be from the incoherent disturbance in turbulence, and the coherent structure in turbulence may well follow the rule of chaotic motion.

  20. Numerical Study of High-Temperature Jet Flow Using RANS/LES and PANS Formulations

    NASA Technical Reports Server (NTRS)

    Abdol-Hamid, Khaled S.; Elmiligui, Alaa

    2005-01-01

    Two multi-scale-type turbulence models are implemented in the PAB3D solver. The models are based on modifying the Reynolds Averaged Navier-Stokes (RANS) equations. The first scheme is a hybrid RANS/LES model utilizing the two-equation (k(epsilon)) model with a RANS/LES transition function dependent on grid spacing and the computed turbulence length scale. The second scheme is a modified version of the Partially Averaged Navier-Stokes (PANS) model, where the unresolved kinetic energy parameter (f(sub k)) is allowed to vary as a function of grid spacing and the turbulence length scale. This parameter is estimated based on a novel two-stage procedure to efficiently estimate the level of scale resolution possible for a given flow on a given grid for Partial Averaged Navier-Stokes (PANS). It has been found that the prescribed scale resolution can play a major role in obtaining accurate flow solutions. The parameter f(sub k) varies between zero and one and equal to one in the viscous sub layer, and when the RANS turbulent viscosity becomes smaller than the LES viscosity. The formulation, usage methodology, and validation examples are presented to demonstrate the enhancement of PAB3D's time-accurate and turbulence modeling capabilities. The accurate simulations of flow and turbulent quantities will provide valuable tool for accurate jet noise predictions. Solutions from these models are compared to RANS results and experimental data for high-temperature jet flows. The current results show promise for the capability of hybrid RANS/LES and PANS in simulating such flow phenomena.

  1. Laser beam probing of jet exhaust turbulence.

    PubMed

    Hogge, C B; Visinsky, W L

    1971-04-01

    A He-Ne (6328-A) laser beam was passed through the highly turbulent region in the exhaust of a jet engine (J-57 with afterburner). Estimates of a structure constant that would characterize the turbulence in the exhaust are made from the beam spread of focused and collimated beams. The structure constant obtained in this manner is then compared with that determined from scintillation measurements of a (10.6-micro) beam and with the results of hot-wire anemometer readings taken in the exhaust. The various methods yield results for the structure constant that are in good agreement (typically a structure constant of the order of 3 x 10(-5) m(-?)). PMID:20094556

  2. Turbulence measurements in axisymmetric jets of air and helium. I - Air jet. II - Helium jet

    NASA Astrophysics Data System (ADS)

    Panchapakesan, N. R.; Lumley, J. L.

    1993-01-01

    Results are presented of measurements on turbulent round jets of air and of helium of the same nozzle momentum efflux, using, for the air jets, x-wire hot-wire probes mounted on a moving shuttle and, for He jets, a composite probe consisting of an interference probe of the Way-Libby type and an x-probe. Current models for scalar triple moments were evaluated. It was found that the performance of the model termed the Full model, which includes all terms except advection, was very good for both the air and the He jets.

  3. Design of swirled axisymmetric turbulent jets

    NASA Astrophysics Data System (ADS)

    Zhumayev, Z. S.; Abramov, A. A.; Fayziyev, R. A.

    1984-10-01

    The problem of swirling a jet for such applications as turbulization of the flame from burners in industrial furnaces or combustion chambers is treated on the basis of steady state turbulent straight axisymmetric flow of an incompressible fluid with the addition of a tangential velocity component. In the corresponding system of four partial differential equations there appears an exchange coefficient proportional to the mixing half width squared. After reduction to dimensionless form, this system of nonlinear equations is solved for the appropriate boundary conditions with constant momentum flow and constant moment of momentum. Radial profiles of both axial and tangential velocity components at various distances from the nozzle throat have been calculated numerically by the method of finite differences with an implicit scheme. The maxima of both velocity components are found to decrease and to shift toward the jet axis with increasing distance from the nozzle. A sharp swirl can give rise to a positive pressure gradient sufficiently large to produce a backcurrent. The results agree qualitatively with experimental data.

  4. Behavior of turbulent gas jets in an axisymmetric confinement

    NASA Technical Reports Server (NTRS)

    So, R. M. C.; Ahmed, S. A.

    1985-01-01

    The understanding of the mixing of confined turbulent jets of different densities with air is of great importance to many industrial applications, such as gas turbine and Ramjet combustors. Although there have been numerous studies on the characteristics of free gas jets, little is known of the behavior of gas jets in a confinement. The jet, with a diameter of 8.73 mm, is aligned concentrically in a tube of 125 mm diameter, thus giving a confinement ratio of approximately 205. The arrangement forms part of the test section of an open-jet wind tunnel. Experiments are carried out with carbon dioxide, air and helium/air jets at different jet velocities. Mean velocity and turbulence measurements are made with a one-color, one-component laser Doppler velocimeter operating in the forward scatter mode. Measurements show that the jets are highly dissipative. Consequently, equilibrium jet characteristics similar to those found in free air jets are observed in the first two diameters downstream of the jet. These results are independent of the fluid densities and velocities. Decay of the jet, on the other hand, is a function of both the jet fluid density and momentum. In all the cases studied, the jet is found to be completely dissipated in approximately 30 jet diameters, thus giving rise to a uniform flow with a very high but constant turbulence field across the confinement.

  5. A numerical study of confined turbulent jets

    NASA Technical Reports Server (NTRS)

    Zhu, J.; Shih, T.-H.

    1993-01-01

    A numerical investigation is reported of turbulent incompressible jets confined in two ducts, one cylindrical and the other conical with a 5 degree divergence. In each case, three Craya-Curtet numbers are considered which correspond, respectively, to flow situations with no moderate and strong recirculation. Turbulence closure is achieved by using the k-epsilon model and a recently proposed realizable Reynolds stress algebraic equation model that relates the Reynolds stresses explicitly to the quadratic terms of the mean velocity gradients and ensures the positiveness of each component of the turbulent kinetic energy. Calculations are carried out with a finite-volume procedure using boundary-fitted curvilinear coordinates. A second-order accurate, bounded convection scheme and sufficiently fine grids are used to prevent the solutions from being contaminated by numerical diffusion. The calculated results are compared extensively with the available experimental data. It is shown that the numerical methods presented are capable of capturing the essential flow features observed in the experiments and that the realizable Reynolds stress algebraic equation model performs much better than the k-epsilon model for this class of flows of great practical importance.

  6. Survey of Turbulence Models for the Computation of Turbulent Jet Flow and Noise

    NASA Technical Reports Server (NTRS)

    Nallasamy, N.

    1999-01-01

    The report presents an overview of jet noise computation utilizing the computational fluid dynamic solution of the turbulent jet flow field. The jet flow solution obtained with an appropriate turbulence model provides the turbulence characteristics needed for the computation of jet mixing noise. A brief account of turbulence models that are relevant for the jet noise computation is presented. The jet flow solutions that have been directly used to calculate jet noise are first reviewed. Then, the turbulent jet flow studies that compute the turbulence characteristics that may be used for noise calculations are summarized. In particular, flow solutions obtained with the k-e model, algebraic Reynolds stress model, and Reynolds stress transport equation model are reviewed. Since, the small scale jet mixing noise predictions can be improved by utilizing anisotropic turbulence characteristics, turbulence models that can provide the Reynolds stress components must now be considered for jet flow computations. In this regard, algebraic stress models and Reynolds stress transport models are good candidates. Reynolds stress transport models involve more modeling and computational effort and time compared to algebraic stress models. Hence, it is recommended that an algebraic Reynolds stress model (ASM) be implemented in flow solvers to compute the Reynolds stress components.

  7. Coupling Turbulence in Hybrid LES-RANS Techniques

    NASA Technical Reports Server (NTRS)

    Woodruff, Stephen L.

    2011-01-01

    A formulation is proposed for hybrid LES-RANS computations that permits accurate computations during resolution changes, so that resolution may be changed at will in order to employ only as much resolution in each subdomain as is required by the physics. The two components of this formulation, establishing the accuracy of a hybrid model at constant resolutions throughout the RANS-to-LES range and maintaining that accuracy when resolution is varied, are demonstrated for decaying, homogeneous, isotropic turbulence.

  8. Direct Numerical Simulations of Turbulent Autoigniting Hydrogen Jets

    NASA Astrophysics Data System (ADS)

    Asaithambi, Rajapandiyan

    Autoignition is an important phenomenon and a tool in the design of combustion engines. To study autoignition in a canonical form a direct numerical simulation of a turbulent autoigniting hydrogen jet in vitiated coflow conditions at a jet Reynolds number of 10,000 is performed. A detailed chemical mechanism for hydrogen-air combustion and non-unity Lewis numbers for species transport is used. Realistic inlet conditions are prescribed by obtaining the velocity eld from a fully developed turbulent pipe flow simulation. To perform this simulation a scalable modular density based method for direct numerical simulation (DNS) and large eddy simulation (LES) of compressible reacting flows is developed. The algorithm performs explicit time advancement of transport variables on structured grids. An iterative semi-implicit time advancement is developed for the chemical source terms to alleviate the chemical stiffness of detailed mechanisms. The algorithm is also extended from a Cartesian grid to a cylindrical coordinate system which introduces a singularity at the pole r = 0 where terms with a factor 1/r can be ill-defined. There are several approaches to eliminate this pole singularity and finite volume methods can bypass this issue by not storing or computing data at the pole. All methods however face a very restrictive time step when using a explicit time advancement scheme in the azimuthal direction (theta) where the cell sizes are of the order DelrDeltheta. We use a conservative finite volume based approach to remove the severe time step restriction imposed by the CFL condition by merging cells in the azimuthal direction. In addition, fluxes in the radial direction are computed with an implicit scheme to allow cells to be clustered along the jet's shear layer. This method is validated and used to perform the large scale turbulent reacting simulation. The resulting flame structure is found to be similar to a turbulent diusion flame but stabilized by autoignition at the

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

    DOE PAGESBeta

    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

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

    SciTech Connect

    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 comprehensive 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.

  11. Turbulence modeling for impinging jet flows

    NASA Technical Reports Server (NTRS)

    Childs, Robert E.; Rodman, Laura C.; Bradshaw, Peter; Bott, Donald M.; Shoemaker, William C.

    1992-01-01

    The objective of the present work is to improve the accuracy of the k-epsilon turbulence model for flows involving one or more jets impinging on a plate in a crossflow which generate a horseshoe vortex. The k-epsilon model is modified by adding source terms to the epsilon equation, which enables it to more accurately predict the shear stress in flows subject to streamline curvature and vortex stretching (or lateral divergence). Calculations with the modified model predict the ground vortex core to be about 15 percent upstream of its experimental location. This is a significant improvement over the standard model which yields higher errors for calculation of the vortex-core location.

  12. LES of turbulent lifted CH4 /H2 flames using a novel FGM-PDF model

    NASA Astrophysics Data System (ADS)

    Abtahizadeh, S. Ebrahim; van Oijen, Jeroen; Bastiaans, Rob; de Goey, Philip

    2014-11-01

    This study reports on numerical investigations of preferential diffusion effects on flame stabilization of turbulent lifted flames using LES with a FGM-PDF approach. The experimental test case is the Delft JHC burner to study Mild combustion; a clean combustion concept. In this burner, CH4 based fuel has been enriched from 0 to 25% of H2. Since the main stabilization mechanism of these turbulent flames is autoignition, the developed numerical model should be able to predict this complex event. Furthermore, addition of hydrogen makes modeling even more challenging due to its preferential diffusion effects. These effects are increasingly important since autoignition is typically initiated at very small mixture fractions where molecular diffusion is comparable to turbulence transport (eddy viscosity). In this study, first, a novel numerical model is developed based on the Flamelet Generated Manifolds (FGM) to account for preferential diffusion effects in autoignition. Afterwards, the developed FGM approach is implemented in LES of the H2 enriched turbulent lifted jet flames. Main features of these turbulent lifted flames such as the formation of ignition kernels and stabilization mechanisms are thoroughly analyzed and compared with the measurements of OH chemiluminescence. The authors gratefully acknowledge the financial support of the Dutch Technology Foundation (STW) under Project No. 10414.

  13. Characteristics of Turbulent/non-turbulent Interface in a Turbulent Planar Jet with a Chemical Reaction

    NASA Astrophysics Data System (ADS)

    Watanabe, Tomoaki; Sakai, Yasuhiko; Nagata, Kouji; Terashima, Osamu; Ito, Yasumasa; Hayase, Toshiyuki

    2013-11-01

    Characteristics of chemical reaction (A + B --> P) near the turbulent/non-turbulent (T/NT) interface are investigated by using the direct numerical simulation of reactive planar jet. The reactants A and B are separately premixed into the jet and ambient flows, respectively. DNS is performed at three different Damköhler numbers. The conditional statistics conditioned on the distance from the T/NT interface is used to investigate the chemical reaction near the T/NT interface. The conditional mean concentration of product P shows a sharp jump near the T/NT interface, and the product P hardly exists in the non-turbulent region. This implies that the chemical reaction takes place in the turbulent region after the reactant B in the ambient flow is entrained into the turbulent region. The conditional mean scalar dissipation rate of mixture fraction has a large peak value slightly inside the T/NT interface. At the same point, the chemical reaction rate also has a peak value in the case of large Damköhler number. On the other hand, when the Damköhler number is small, the chemical reaction rate near the T/NT interface is smaller than that in the turbulent region. This work was carried out under the Collaborative Research Project of the Institute of Fluid Science, Tohoku University. Part of this work was supported by JSPS KAKENHI Grant Number 25002531 and MEXT KAKENHI Grant Numbers 25289030, 25289031, 2563005.

  14. Turbulence Associated With Broadband Shock Noise in Hot Jets

    NASA Technical Reports Server (NTRS)

    Bridges, James E.; Wernet, Mark P.

    2008-01-01

    Time-Resolved Particle Image Velocimetry (TRPIV) has been applied to a series of jet flows to measure turbulence statistics associated with broadband shock associated noise (BBSN). Data were acquired in jets of Mach numbers 1.05, 1.185, and 1.4 at different temperatures. Both convergent and ideally expanded nozzles were tested, along with a convergent nozzle modified to minimize screech. Key findings include the effect of heat on shock structure and jet decay, the increase in turbulent velocity when screech is present, and the relative lack of spectral detail associated with the enhanced turbulence.

  15. Secondary Peak in Nusselt Number for Jet Impinging Flows: LES Study

    NASA Astrophysics Data System (ADS)

    Dutta, Rabijit; Dewan, Anupam; Srinivasan, Balaji

    2013-11-01

    Jet impingement heat transfer is widely studied because of its industrial and as well as fundamental relevance. A secondary peak in Nusselt number at some distance away from the stagnation point is observed for both round and slot jet impingement flows at a small nozzle-to-plate spacing. Although various researchers have studied the reason behind this secondary peak, it is still an open question in the literature. We present large eddy simulation (LES) of turbulent slot jet impingement heat transfer to gain further insight into the phenomenon of secondary peak in Nusselt number. Profiles of mean velocities, turbulent fluctuating velocities and Nusselt numbers have been analyzed along the impingement plate. A sudden increase in the wall normal turbulence fluctuations have been observed in the region where the secondary peak in Nusselt number occurs. Further, an analysis of vortex structures of the flow showed that the increase in the wall normal turbulence could be associated with the secondary vortex observed near the impingement wall. PhD candidate.

  16. Characteristics of transitional and turbulent jet diffusion flames in microgravity

    NASA Technical Reports Server (NTRS)

    Bahadori, Yousef M.; Small, James F., Jr.; Hegde, Uday G.; Zhou, Liming; Stocker, Dennis P.

    1995-01-01

    This paper presents the ground-based results obtained to date in preparation of a proposed space experiment to study the role of large-scale structures in microgravity transitional and turbulent gas-jet diffusion flames by investigating the dynamics of vortex/flame interactions and their influence on flame characteristics. The overall objective is to gain an understanding of the fundamental characteristics of transitional and turbulent gas-jet diffusion flames. Understanding of the role of large-scale structures on the characteristics of microgravity transitional and turbulent flames will ultimately lead to improved understanding of normal-gravity turbulent combustion.

  17. Multiscale turbulence effects in supersonic jets exhausting into still air

    NASA Technical Reports Server (NTRS)

    Abdol-Hamid, Khaled S.; Wilmoth, Richard G.

    1987-01-01

    A modified version of the multiscale turbulence model of Hanjalic has been applied to the problem of supersonic jets exhausting into still air. In particular, the problem of shock-cell decay through turbulent interaction with the mixing layer has been studied for both mildly interacting and strongly resonant jet conditions. The modified Hanjalic model takes into account the nonequilibrium energy transfer between two different turbulent spectral scales. The turbulence model was incorporated into an existing shock-capturing, parabolized Navier-Stokes computational model in order to perform numerical experiments. The results show that the two-scale turbulence model provides significant improvement over one-scale models in the prediction of plume shock structure for underexpanded supersonic (Mach 2) and sonic (Mach 1) jets. For the supersonic jet, excellent agreement with experiment was obtained for the centerline shock-cell pressure decay up to 40 jet radii. For the sonic jet, the agreement with experiment was not so good, but the two-scale model still showed significant improvement over the one-scale model. It is shown that by relating some of the coefficients in the turbulent-transport equations to the relative time scale for transfer of energy between scales the two-scale model can provide predictions that bound the measured shock-cell decay rate for the sonic jet.

  18. Modélisation du bruit des jets turbulents libres et subsoniques à température ambiante

    NASA Astrophysics Data System (ADS)

    Béchara, W.; Lafon, P.; Candel, S. M.

    1993-03-01

    Based on the theoretical model of Goldstein for a round free jet, we establish analytical expressions (G_a model) for the noise radiation from a turbulent jet, depending on the local statistical properties of the flow. These characteristics are calculated by solving the Reynolds average Navier-Stokes equations with a numerical code based on a K - \\varepsilon turbulence closure model. A comparison between the numerical results and the experimental data for a simple jet and two coaxial jets shows that this model correctly predicts the evolution of the acoustic radiation. The G_a model developed in the case of axisymmetric turbulence superimposed over a mean flow is found to be more suitable than the Ribner model associated to an isotropic turbulence. A comparison between differents models issued from Lighthill theory, shows that the G_a model yields the best directivity of the acoustic intensity at high jet exit velocities. À partir du modèle théorique proposé initialement par Goldstein pour un jet libre turbulent circulaire, on établit des expressions analytiques (modèle G_a) permettant le calcul du bruit émis à partir des grandeurs statistiques locales du jet turbulent. Ces grandeurs sont estimées par résolution des équations de Navier-Stokes moyennées à l'aide d'un code numérique de turbulence utilisant un modèle de fermeture de type K - \\varepsilon. La comparaison entre les résultats numériques obtenus et les données expérimentales, pour un jet simple et deux jets coaxiaux, montre que ce modèle estime correctement l'évolution des grandeurs acoustiques étudiées. Les calculs indiquent que le modèle G_a développé pour une turbulence axisymétrique convectée par un écoulement moyen est mieux adapté que celui de Ribner associé à une turbulence isotrope. De plus, une comparaison entre différents modèles basés sur la théorie de Lighthill indique que le modèle G_a donne la meilleure directivité de l'intensité acoustique aux

  19. 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

  20. Turbulence effects on hemolysis by revisiting experiments with LES computations

    NASA Astrophysics Data System (ADS)

    Ozturk, Mesude; O'Rear, Edgar; Papavassiliou, Dimitrios

    2015-11-01

    Determining mechanically stimulated red blood cell trauma as a function of turbulence properties is required to design prosthetic heart devices. Because blood is typically exposed to turbulence in such devices, the design of prosthetic heart devices depends on determining the effect of turbulent stresses on hemolysis. While turbulent stresses increase hemolysis when cells are exposed to them, turbulent flow characteristics in the vicinity of lysed blood cells, and the mechanism of cell damage remains uncertain. In this work, LES computations are used to investigate the effect of turbulent eddy structure on cell damage. The flow was simulated for classic Couette and capillary tube experiments, in order to examine the relation between hemolysis turbulence properties related to the dissipation of turbulent kinetic energy. The hypothesis tested is that eddies that are close in size with the erythrocytes are the ones that are responsible for hemolysis, rather than Reynolds stresses or viscous stresses. We define extensive measures, like the eddy areas for small eddies comparable to the size of the red blood cells, to provide a more general understanding of the mechanical cause of blood trauma.

  1. Micro-jets in confined turbulent cross flow

    SciTech Connect

    Kelman, J.B.; Greenhalgh, D.A.; Whiteman, M.

    2006-03-01

    The mixing of sub-millimetre diameter jets issuing into a turbulent cross flow is examined with a combination of laser diagnostic techniques. The cross flow stream is in a confined duct and the micro-jet issue from the sides of injector vanes. A range of cross jet momentum ratios, cross flow temperatures and turbulence intensities are investigated to examine the influence on the jet mixing. Methane, seeded with acetone, was used to measure the concentrations of the jets and the mixing of the jet fluid in the duct. Unlike previous jet in cross flow work, mixing appears to be dominated by the free stream turbulence, rather than the cross jet momentum ratios. Temperature increases in the free stream appear to increase the rate of mixing in the duct, despite the associated decrease in the Reynolds number. The dominance of the free stream turbulence in controlling the mixing is of particular interest in respect of gas turbine injection systems, as the cross jet momentum ratio is insufficient in defining the mixing process. (author)

  2. Naturally occurring and forced azimuthal modes in a turbulent jet

    NASA Technical Reports Server (NTRS)

    Raman, Ganesh; Rice, Edward J.; Reshotko, Eli

    1991-01-01

    Naturally occurring instability modes in an axisymmetric jet were studied using the modal frequency technique. The evolution of the modal spectrum was obtained for a jet with a Reynolds number based on a diameter of 400,000 for both laminar and turbulent nozzle boundary layers. In the early evolution of the jet the axisymmetric mode was predominant, with the azimuthal modes growing rapidly but dominating only the end of the potential core. The growth of the azimuthal was observed closer to the nozzle exit for the jet in the laminar boundary layer case than for the turbulent. Target modes for efficient excitation of the jet were determined and two cases of excitation were studied. First, a jet was excited simultaneously by two helical modes, m equals plus 1 and m equals minus 1 at a Strouhal number based on jet diameter of 0.15 and the axisymmetric mode, m equals 0 at a jet diameter of 0.6. Second, m equals plus one and m equals minus 1 at jet diameter equals 0.3 and m equals 0 at jet diameter equals 0.6 were excited simultaneously. The downstream evolution of the hydrodynamic modes and the spreading rate of the jet were documented for each case. Higher jet spreading rates, accompanied by distorted jet cross sections were observed for the cases where combinations of axisymmetric and helical forcings were applied.

  3. Turbulence measurements in axisymmetric jets of air and helium

    NASA Astrophysics Data System (ADS)

    Panchapakesan, N. R.

    Turbulent axisymmetric jets of air helium with the same nozzle momentum flux were studied experimentally using hot-wire probes. An X-wire hot-wire probe was used in the air jet and a composite probe consisting of an X-wire and an interference probe of the Way-Libby type was used in the helium jet to measure the helium concentration and two velocity components. Moments of turbulent fluctuations, up to fourth order, were calculated to characterize turbulent transport in the jet and to evaluate current models for triple moments that occur in the Reynolds stress equation. In the air jet, the momentum flux across the jet was found to be within +/- 5 percent of the nozzle input and the integral of the radial diffusive flux of the turbulent kinetic energy across the jet was found to be close to zero indicating consistency of measurements with the equations of motion. The fourth moments were very well described in terms of the second moments by the quasi-Gaussian approximation across the entire jet. Profiles of third moments were found to be significantly different from earlier measurements - (u(v exp 2)) (u(w exp 2)) and ((u exp 2)v) were found to be negative near the axis of the jet. The measurements in the helium jet were in the intermediate region between the non-buoyant jet and the plume regions. The helium mass flux across the jet was found to be within +/- 0 percent of the nozzle input. The far field behavior was in accord with the expected plume scalings. The near field behavior of the mean velocity along the axis of the jet follows the scaling expressed by the effective diameter but the mean concentration decay has a different density ratio dependence. The radical profiles of mean velocity and concentration indicate a turbulent Schmidt number of 0.7, the same as for passive scalars in round jets. Turbulent intensity of axial velocity fluctuations was significantly higher than that observed in the air jet while the radial and azimuthal intensities are virtually

  4. On integrating LES and laboratory turbulent flow experiments

    SciTech Connect

    Grinstein, Fernando Franklin

    2008-01-01

    Critical issues involved in large eddy simulation (LES) experiments relate to the treatment of unresolved subgrid scale flow features and required initial and boundary condition supergrid scale modelling. The inherently intrusive nature of both LES and laboratory experiments is noted in this context. Flow characterization issues becomes very challenging ones in validation and computational laboratory studies, where potential sources of discrepancies between predictions and measurements need to be clearly evaluated and controlled. A special focus of the discussion is devoted to turbulent initial condition issues.

  5. Interaction of Pulsed Vortex Generator Jets with Turbulent Boundary Layers

    NASA Astrophysics Data System (ADS)

    McManus, K. R.; Johari, H.

    1996-11-01

    Vortex Generator Jets (VGJ) have been proposed as a means for active control of turbulent boundary layer separation by Johnston footnote AIAA J. 28, 989 (1990). It has been shown that a vortex generator jet can create weak longitudinal vorticity of a single sign when the surface-mounted jets are pitched and skewed with respect to the solid surface. The primary advantages of VGJs when compared to solid vortex generators are their lack of parasitic drag when the jets are off and the ability to rapidly activate and deactivate the jets for dynamic control. Pulsing of the jets is proposed as a way of increasing the turbulent mixing and therefore, improving the performance of vortex generator jets. Initial experiments with jets pitched at 45 deg and skewed at 90 deg degrees in air have indicated that large-scale turbulent structures are formed by the pulsed VGJs. Subsequent flow visualization experiments in a water tunnel suggest that fully-modulated jets embedded in a flat plate boundary layer result in a series of puffs which penetrate through the boundary layer. The influence of jet velocity, diameter, pulsing frequency and duty-cycle will be discussed. * Supported by NSF and PSI.

  6. Modeling of Turbulence Effect on Liquid Jet Atomization

    NASA Technical Reports Server (NTRS)

    Trinh, H. P.

    2007-01-01

    Recent studies indicate that turbulence behaviors within a liquid jet have considerable effect on the atomization process. Such turbulent flow phenomena are encountered in most practical applications of common liquid spray devices. This research aims to model the effects of turbulence occurring inside a cylindrical liquid jet to its atomization process. The two widely used atomization models Kelvin-Helmholtz (KH) instability of Reitz and the Taylor analogy breakup (TAB) of O'Rourke and Amsden portraying primary liquid jet disintegration and secondary droplet breakup, respectively, are examined. Additional terms are formulated and appropriately implemented into these two models to account for the turbulence effect. Results for the flow conditions examined in this study indicate that the turbulence terms are significant in comparison with other terms in the models. In the primary breakup regime, the turbulent liquid jet tends to break up into large drops while its intact core is slightly shorter than those without turbulence. In contrast, the secondary droplet breakup with the inside liquid turbulence consideration produces smaller drops. Computational results indicate that the proposed models provide predictions that agree reasonably well with available measured data.

  7. Adaptive LES Methodology for Turbulent Flow Simulations

    SciTech Connect

    Oleg V. Vasilyev

    2008-06-12

    Although turbulent flows are common in the world around us, a solution to the fundamental equations that govern turbulence still eludes the scientific community. Turbulence has often been called one of the last unsolved problem in classical physics, yet it is clear that the need to accurately predict the effect of turbulent flows impacts virtually every field of science and engineering. As an example, a critical step in making modern computational tools useful in designing aircraft is to be able to accurately predict the lift, drag, and other aerodynamic characteristics in numerical simulations in a reasonable amount of time. Simulations that take months to years to complete are much less useful to the design cycle. Much work has been done toward this goal (Lee-Rausch et al. 2003, Jameson 2003) and as cost effective accurate tools for simulating turbulent flows evolve, we will all benefit from new scientific and engineering breakthroughs. The problem of simulating high Reynolds number (Re) turbulent flows of engineering and scientific interest would have been solved with the advent of Direct Numerical Simulation (DNS) techniques if unlimited computing power, memory, and time could be applied to each particular problem. Yet, given the current and near future computational resources that exist and a reasonable limit on the amount of time an engineer or scientist can wait for a result, the DNS technique will not be useful for more than 'unit' problems for the foreseeable future (Moin & Kim 1997, Jimenez & Moin 1991). The high computational cost for the DNS of three dimensional turbulent flows results from the fact that they have eddies of significant energy in a range of scales from the characteristic length scale of the flow all the way down to the Kolmogorov length scale. The actual cost of doing a three dimensional DNS scales as Re{sup 9/4} due to the large disparity in scales that need to be fully resolved. State-of-the-art DNS calculations of isotropic turbulence

  8. Turbulence and heat excited noise sources in single and coaxial jets

    NASA Astrophysics Data System (ADS)

    Koh, Seong Ryong; Schröder, Wolfgang; Meinke, Matthias

    2010-03-01

    The generation of noise in subsonic high Reynolds number single and coaxial turbulent jets is analyzed by a hybrid method. The computational approach is based on large-eddy simulations (LES) and solutions of the acoustic perturbation equations (APE). The method is used to investigate the acoustic fields of one isothermal single stream jet at a Mach number 0.9 and a Reynolds number 400,000 based on the nozzle diameter and two coaxial jets whose Mach number and Reynolds number based on the secondary jet match the values of the single jet. One coaxial jet configuration possesses a cold primary flow, whereas the other configuration has a hot primary jet. Thus, the configurations allow in a first step the analysis of the relationship of the flow and acoustic fields of a single and a cold coaxial jet and in a second step the investigation of the differences of the fluid mechanics and aeroacoustics of cold and hot coaxial jets. For the isothermal single jet the present hybrid acoustic computation shows convincing agreement with the direct acoustic simulation based on large-eddy simulations. The analysis of the acoustic field of the coaxial jets focuses on two noise sources, the Lamb vector fluctuations and the entropy sources of the APE equations. The power spectral density (PSD) distributions evidence the Lamb vector fluctuations to represent the major acoustic sources of the isothermal jet. Especially the typical downstream and sideline acoustic generations occur on a cone-like surface being wrapped around the end of the potential core. Furthermore, when the coaxial jet possesses a hot primary jet, the acoustic core being characterized by the entropy source terms increases the low frequency acoustics by up to 5 dB, i.e., the sideline acoustics is enhanced by the pronounced temperature gradient.

  9. Establishing Consensus Turbulence Statistics for Hot Subsonic Jets

    NASA Technical Reports Server (NTRS)

    Bridges, James; Werner, Mark P.

    2010-01-01

    Many tasks in fluids engineering require knowledge of the turbulence in jets. There is a strong, although fragmented, literature base for low order statistics, such as jet spread and other meanvelocity field characteristics. Some sources, particularly for low speed cold jets, also provide turbulence intensities that are required for validating Reynolds-averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) codes. There are far fewer sources for jet spectra and for space-time correlations of turbulent velocity required for aeroacoustics applications, although there have been many singular publications with various unique statistics, such as Proper Orthogonal Decomposition, designed to uncover an underlying low-order dynamical description of turbulent jet flow. As the complexity of the statistic increases, the number of flows for which the data has been categorized and assembled decreases, making it difficult to systematically validate prediction codes that require high-level statistics over a broad range of jet flow conditions. For several years, researchers at NASA have worked on developing and validating jet noise prediction codes. One such class of codes, loosely called CFD-based or statistical methods, uses RANS CFD to predict jet mean and turbulent intensities in velocity and temperature. These flow quantities serve as the input to the acoustic source models and flow-sound interaction calculations that yield predictions of far-field jet noise. To develop this capability, a catalog of turbulent jet flows has been created with statistics ranging from mean velocity to space-time correlations of Reynolds stresses. The present document aims to document this catalog and to assess the accuracies of the data, e.g. establish uncertainties for the data. This paper covers the following five tasks: Document acquisition and processing procedures used to create the particle image velocimetry (PIV) datasets. Compare PIV data with hotwire and laser Doppler

  10. Spectra and Diffusion in a Round Turbulent Jet

    NASA Technical Reports Server (NTRS)

    Corrsin, Stanley; Uberoi, Mahinder S

    1951-01-01

    In a round turbulent jet at room temperature, measurement of the shear correlation coefficient as a function of frequency (through band-pass filters) has given a rather direct verification of Kolmogoroff's local-isotropy hypothesis. One-dimensional power spectra of velocity and temperature fluctuations, measured in unheated and heated jets, respectively, have been contrasted. Under the same conditions, the two corresponding transverse correlation functions have been measured and compared. Finally, measurements have been made of the mean thermal wakes behind local (line) heat sources in the unheated turbulent jet, and the order of magnitude of the temperature fluctuations has been determined.

  11. Spectrums and Diffusion in a Round Turbulent Jet

    NASA Technical Reports Server (NTRS)

    Corrsin, Stanley; Uberoi, Mahinder S

    1950-01-01

    In a round turbulent jet at room temperature, measurement of the shear correlation coefficient as a function of frequency (through bandpass filters) has given a rather direct verification of Kolmogoroff's local-isotropy hypothesis. One-dimensional power spectrums of velocity and temperature fluctuations, measured in unheated and heated jets, respectively, have been contrasted. Under the same conditions, the two corresponding transverse correlation functions have been measured and compared. Finally, measurements have been made of the mean thermal wakes behind local (line) heat sources in the unheated turbulent jet, and the order of magnitude of the temperature fluctuations has been determined. (author)

  12. Studies of turbulent round jets through experimentation, simulation, and modeling

    NASA Astrophysics Data System (ADS)

    Keedy, Ryan

    This thesis studies the physics of the turbulent round jet. In particular, it focuses on three different problems that have the turbulent round jet as their base flow. The first part of this thesis examines a compressible turbulent round jet at its sonic condition. We investigate the shearing effect such a jet has when impinging on a solid surface that is perpendicular to the flow direction. We report on experiments to evaluate the jet's ability to remove different types of explosive particles from a glass surface. Theoretical analysis revealed trends and enabled modeling to improve the predictability of particle removal for various jet conditions. The second part of thesis aims at developing a non-intrusive measurement technique for free-shear turbulent flows in nature. Most turbulent jet investigations in the literature, both in the laboratory and in the field, required specialized intrusive instrumentation and/or complex optical setups. There are many situations in naturally-occurring flows where the environment may prove too hostile or remote for existing instrumentation. We have developed a methodology for analyzing video of the exterior of a naturally-occurring flow and calculating the flow velocity. We found that the presence of viscosity gradients affects the velocity analysis. While these effects produce consistent, predictable changes, we became interested in the mechanism by which the viscosity gradients affect the mixing and development of the turbulent round jet. We conducted a stability analysis of the axisymmetric jet when a viscosity gradient is present. Finally, the third problem addressed in this thesis is the growth of liquid droplets by condensation in a turbulent round jet. A vapor-saturated turbulent jet issues into a cold, dry environment. The resulting mixing produces highly inhomogeneous regions of supersaturation, where droplets grow and evaporate. Non-linear interactions between the droplet growth rate and the supersaturation field make

  13. Input-output analysis of high-speed turbulent jet noise

    NASA Astrophysics Data System (ADS)

    Jeun, Jinah; Nichols, Joseph W.

    2015-11-01

    We apply input-output analysis to predict and understand the aeroacoustics of high-speed isothermal turbulent jets. We consider axisymmetric linear perturbations about Reynolds-averaged Navier-Stokes solutions of ideally expanded turbulent jets with Mach numbers 0 . 6 jets, the optimal response closely resembles a wavepacket in both the nearfield and the farfield such as those obtained by the parabolized stability equations (PSE), and this mode dominates the response. For subsonic jets, however, the singular values indicate that the contributions of suboptimal modes to noise generation are nearly equal to that of the optimal mode, explaining why PSE misses some of the farfield sound in this case. Finally, high-fidelity large eddy simulation (LES) is used to assess the prevalence of suboptimal modes in the unsteady data. By projecting LES data onto the corresponding input modes, the weighted gain of each mode is examined.

  14. Understanding and predicting soot generation in turbulent non-premixed jet flames.

    SciTech Connect

    Wang, Hai; Kook, Sanghoon; Doom, Jeffrey; Oefelein, Joseph Charles; Zhang, Jiayao; Shaddix, Christopher R.; Schefer, Robert W.; Pickett, Lyle M.

    2010-10-01

    This report documents the results of a project funded by DoD's Strategic Environmental Research and Development Program (SERDP) on the science behind development of predictive models for soot emission from gas turbine engines. Measurements of soot formation were performed in laminar flat premixed flames and turbulent non-premixed jet flames at 1 atm pressure and in turbulent liquid spray flames under representative conditions for takeoff in a gas turbine engine. The laminar flames and open jet flames used both ethylene and a prevaporized JP-8 surrogate fuel composed of n-dodecane and m-xylene. The pressurized turbulent jet flame measurements used the JP-8 surrogate fuel and compared its combustion and sooting characteristics to a world-average JP-8 fuel sample. The pressurized jet flame measurements demonstrated that the surrogate was representative of JP-8, with a somewhat higher tendency to soot formation. The premixed flame measurements revealed that flame temperature has a strong impact on the rate of soot nucleation and particle coagulation, but little sensitivity in the overall trends was found with different fuels. An extensive array of non-intrusive optical and laser-based measurements was performed in turbulent non-premixed jet flames established on specially designed piloted burners. Soot concentration data was collected throughout the flames, together with instantaneous images showing the relationship between soot and the OH radical and soot and PAH. A detailed chemical kinetic mechanism for ethylene combustion, including fuel-rich chemistry and benzene formation steps, was compiled, validated, and reduced. The reduced ethylene mechanism was incorporated into a high-fidelity LES code, together with a moment-based soot model and models for thermal radiation, to evaluate the ability of the chemistry and soot models to predict soot formation in the jet diffusion flame. The LES results highlight the importance of including an optically-thick radiation model

  15. Large Eddy Simulation Of Gravitational Effects In Transitional And Turbulent Gas-Jet Diffusion Flames

    NASA Technical Reports Server (NTRS)

    Jaberi, Farhad A.; Givi, Peyman

    2003-01-01

    The influence of gravity on the spatial and the compositional structures of transitional and turbulent hydrocarbon diffusion flames are studies via large eddy simulation (LES) and direct numerical simulation (DNS) of round and planar jets. The subgrid-scale (SGS) closures in LES are based on the filtered mass density function (FMDF) methodology. The FMDF represents the joint probability density function (PDF) of the SGS scalars, and is obtained by solving its transport equation. The fundamental advantage of LES/FMDF is that it accounts for the effects of chemical reaction and buoyancy exactly. The methodology is employed for capturing some of the fundamental influences of gravity in equilibrium flames via realistic chemical kinetic schemes. Some preliminary investigation of the gravity effects in non-equilibrium flames is also conducted, but with idealized chemical kinetics models.

  16. Large eddy simulation of spark ignition in a turbulent methane jet

    SciTech Connect

    Lacaze, G.; Richardson, E.; Poinsot, T.

    2009-10-15

    Large eddy simulation (LES) is used to compute the spark ignition in a turbulent methane jet flowing into air. Full ignition sequences are calculated for a series of ignition locations using a one-step chemical scheme for methane combustion coupled with the thickened flame model. The spark ignition is modeled in the LES as an energy deposition term added to the energy equation. Flame kernel formation, the progress and topology of the flame propagating upstream, and stabilization as a tubular edge flame are analyzed in detail and compared to experimental data for a range of ignition parameters. In addition to ignition simulations, statistical analysis of nonreacting LES solutions is carried out to discuss the ignition probability map established experimentally. (author)

  17. Interaction of a round turbulent jet with a thermocline

    NASA Astrophysics Data System (ADS)

    Ezhova, Ekaterina; Cenedese, Claudia; Brandt, Luca

    2016-04-01

    Vertical turbulent jets serve as the models of numerous flows both in nature and industry including convective cloud flows in the atmosphere, effluents from submerged wastewater outfall systems in the ocean, pollutant discharge from industrial chimneys, subglacial discharge. We investigate the dynamics of an axisymmetric vertical turbulent jet in a stratified fluid with two layers of different temperature separated by a thermocline. This configuration is a typical model of the upper thermocline layer of lakes and pycnocline in oceans as well as thermal inversions in the atmosphere. In general, turbulent jets in nature and industry originate from the mixed sources of buoyancy and momentum. However, when the source is located far enough from the pycnocline, the jet mixes effectively with the surrounding fluid and the density of the flow at the pycnocline entrance tends to the density of the lower layer of stratification. Dynamics of such a flow in the pycnocline can be modelled employing a neutrally buoyant turbulent jet with the positive vertical momentum. We study the behaviour of a vertical round turbulent jet in an unconfined stratified environment by means of well-resolved large eddy simulation. We consider two cases: when the thermocline width is small and of the same order with the jet diameter at the thermocline entrance. Mean jet penetration, stratified turbulent entrainment and jet oscillations as well as the generation of internal waves are quantified. The mean jet penetration is predicted well by a simple model based on the conservation of the jet volume, momentum and buoyancy fluxes. The entrainment coefficient for the thin thermocline is consistent with the theoretical model for a two-layer stratification with a sharp interface, while for the thick thermocline entrainment is larger at low Froude numbers. For the thick thermocline we demonstrate the presence of a secondary horizontal flow in the upper thermocline, resulting in the entrainment of fluid

  18. Large Eddy Simulation of a Near Sonic Turbulent Jet and Its Radiated Noise

    NASA Technical Reports Server (NTRS)

    Constantinescu, G. S.; Lele, S. K.

    2001-01-01

    In this paper numerical simulations are used to calculate the turbulence dynamics simultaneously with the sound field for a high-speed near-sonic (Ma=0.9) compressible jet at two Reynolds numbers of 3,600 and 72,000. LES (Large Eddy Simulation) in conjunction with accurate numerical schemes is used to calculate the unsteady flow and sound in the near field of the jet. It is shown that the jet mean parameters, mean velocity fields and turbulence statistics are in good agreement with experimental data and results from other simulations. The sound in the near-field is calculated directly from the simulations. The calculations are shown to capture the peak in the dilatation and pressure spectra around a Strouhal number St=0.25-0.3, in agreement with typical jet-noise spectra measured in experiments. Dilatation contours in the near-field show the formation of acoustic waves with a dominant wavelength of 3.2-4 jet diameters, corresponding to the peak in the dilatation spectra. As expected, the non-compact noise sources are found to be most dominant in the region corresponding to the end of the potential core. The contribution of the LES model to the radiated noise appears to be weak and does not contaminate the sound field with spurious high-frequency noise. However, the frequency spectra of the sound show a rapid falloff away from the peak frequency. This is attributed to the quasi-laminar state of the shear-layers in the region prior to potential core closure, and a possible effect of insufficient azimuthal resolution at the observed location. Further analysis of the effect of the LES model, especially at high frequencies, is needed.

  19. Characteristics of 3D turbulent jets in crossflow

    NASA Technical Reports Server (NTRS)

    Demuren, A. O.

    1991-01-01

    Three dimensional turbulent jets in crossflow at low to medium jet-to-crossflow velocity ratios are computed with a finite volume numerical procedure which utilizes a second-moment closure model to approximate the Reynolds stresses. A multigrid method is used to accelerate the convergence rate of the procedure. Comparison of the computations to measured data show good qualitative agreement. All trends are correctly predicted, though there is some uncertainty on the height of penetration of the jet. The evolution of the vorticity field is used to explore the jet-crossflow interaction.

  20. A Jet-Stirred Apparatus for Turbulent Combustion Experiments

    NASA Astrophysics Data System (ADS)

    Davani, Abbasali; Ronney, Paul

    2015-11-01

    A novel jet-stirred combustion chamber is designed to study turbulent premixed flames. In the new approach, multiple impinging turbulent jets are used to stir the mixture. It is well known that pair of counterflowing turbulent jets produces nearly a constant intensity (u') along the jet axes. In this study, different numbers of impinging jets in various configurations are used to produce isotropic turbulence intensity. FLUENT simulations have been conducted to assess the viability of the proposed chamber. In order to be able to compare different configurations, three different non dimensional indices are introduces. Mean flow index; Homogeneity index, and Isotropicity index. Using these indices one can compare various chambers including conventional Fan-stirred Reactors. Results show that a concentric inlet/outlet chamber (CAIO) with 8 inlets and 8 outlets with inlet velocity of 20 m/s and initial intensity of 15% produces near zero mean flow and 2.5 m/s turbulence intensity which is much more higher than reported values for Fan-stirred chamber. This research was sponsored by National Science Foundation.

  1. Mass and Momentum Turbulent Transport Experiments with Confined Coaxial Jets

    NASA Technical Reports Server (NTRS)

    Johnson, B. V.; Bennett, J. C.

    1981-01-01

    Downstream mixing of coaxial jets discharging in an expanded duct was studied to obtain data for the evaluation and improvement of turbulent transport models currently used in a variety of computational procedures throughout the propulsion community for combustor flow modeling. Flow visualization studies showed four major shear regions occurring; a wake region immediately downstream of the inlet jet inlet duct; a shear region further downstream between the inner and annular jets; a recirculation zone; and a reattachment zone. A combination of turbulent momentum transport rate and two velocity component data were obtained from simultaneous measurements with a two color laser velocimeter (LV) system. Axial, radial and azimuthal velocities and turbulent momentum transport rate measurements in the r-z and r-theta planes were used to determine the mean value, second central moment (or rms fluctuation from mean), skewness and kurtosis for each data set probability density function (p.d.f.). A combination of turbulent mass transport rate, concentration and velocity data were obtained system. Velocity and mass transport in all three directions as well as concentration distributions were used to obtain the mean, second central moments, skewness and kurtosis for each p.d.f. These LV/LIF measurements also exposed the existence of a large region of countergradient turbulent axial mass transport in the region where the annular jet fluid was accelerating the inner jet fluid.

  2. Turbulent Mixing of an Angled Jet in Various Mainstream Conditions

    NASA Astrophysics Data System (ADS)

    Ryan, Kevin; Coletti, Filippo; Elkins, Christopher; Eaton, John

    2013-11-01

    The angled jet in crossflow has been studied in detail with specific emphasis on the turbulent mixing of the jet fluid with the mainstream flow. The interaction of the upstream boundary layer with the jet shear layer results in complex vortex patterns that cause large mean distortion of the jet and rapid turbulent mixing. Most previous studies have been conducted in flat plate flows with little attention paid to the characteristics of the boundary layer. The present study examines the effect of mainstream geometric changes on the jet trajectory, counter-rotating vortex pair strength, and turbulent mixing. Seven cases were examined including flat plate boundary layers with three different thicknesses, adverse and favorable pressure gradient cases, and flows with concave and convex streamwise curvature. Full field, 3D mean velocity and scalar concentration fields were measured using magnetic resonance imaging (MRI) techniques in a water flow. The distortion of the streamtube initiated at the hole exit was examined for each of the seven cases. The degree of mixing was quantified by measuring the amount of mainstream fluid entrained into the jet as well as the turbulent diffusivity as a function of streamwise position.

  3. High speed turbulent reacting flows: DNS and LES

    NASA Technical Reports Server (NTRS)

    Givi, Peyman

    1990-01-01

    Work on understanding the mechanisms of mixing and reaction in high speed turbulent reacting flows was continued. Efforts, in particular, were concentrated on taking advantage of modern computational methods to simulate high speed turbulent flows. In doing so, two methodologies were used: large eddy simulations (LES) and direct numerical simulations (DNS). In the work related with LES the objective is to study the behavior of the probability density functions (pdfs) of scalar properties within the subgrid in reacting turbulent flows. The data base obtained by DNS for a detailed study of the pdf characteristics within the subgrid was used. Simulations are performed for flows under various initializations to include the effects of compressibility on mixing and chemical reactions. In the work related with DNS, a two-dimensional temporally developing high speed mixing layer under the influence of a second-order non-equilibrium chemical reaction of the type A + B yields products + heat was considered. Simulations were performed with different magnitudes of the convective Mach numbers and with different chemical kinetic parameters for the purpose of examining the isolated effects of the compressibility and the heat released by the chemical reactions on the structure of the layer. A full compressible code was developed and utilized, so that the coupling between mixing and chemical reactions is captured in a realistic manner.

  4. The role of POD modes in turbulent jet noise

    NASA Astrophysics Data System (ADS)

    Colonius, Tim; Freund, Jonathan

    2001-11-01

    We examine the acoustic radiation and structure of the POD modes of a turbulent, Re=3600, M=0.9 jet. The POD modes were computed using the method of snapshots with 2333 samples from a three-dimensional DNS database. The jet mean flow, turbulence statistics, and acoustic radiation were previously validated against experiments. We compute vector-valued POD modes for using several different physically meaningful norms and analyze the extent to which the most energetic modes (for the given norm) contribute to the overall acoustic radiation. This enables a detailed characterization of the contribution of large-scale turbulent structures to the radiated acoustic field. We also quantify the interaction of different modes by correlations of their variation in time as well as correlations with acoustic signals in the far field. We also examine the connection between the POD modes and the linear instability eigenfunctions of a slowly spreading jet.

  5. Temperature-Corrected Model of Turbulence in Hot Jet Flows

    NASA Technical Reports Server (NTRS)

    Abdol-Hamid, Khaled S.; Pao, S. Paul; Massey, Steven J.; Elmiligui, Alaa

    2007-01-01

    An improved correction has been developed to increase the accuracy with which certain formulations of computational fluid dynamics predict mixing in shear layers of hot jet flows. The CFD formulations in question are those derived from the Reynolds-averaged Navier-Stokes equations closed by means of a two-equation model of turbulence, known as the k-epsilon model, wherein effects of turbulence are summarized by means of an eddy viscosity. The need for a correction arises because it is well known among specialists in CFD that two-equation turbulence models, which were developed and calibrated for room-temperature, low Mach-number, plane-mixing-layer flows, underpredict mixing in shear layers of hot jet flows. The present correction represents an attempt to account for increased mixing that takes place in jet flows characterized by high gradients of total temperature. This correction also incorporates a commonly accepted, previously developed correction for the effect of compressibility on mixing.

  6. Effects of forward velocity on turbulent jet mixing noise

    NASA Technical Reports Server (NTRS)

    Plumblee, H. E., Jr. (Editor)

    1976-01-01

    Flight simulation experiments were conducted in an anechoic free jet facility over a broad range of model and free jet velocities. The resulting scaling laws were in close agreement with scaling laws derived from theoretical and semiempirical considerations. Additionally, measurements of the flow structure of jets were made in a wind tunnel by using a laser velocimeter. These tests were conducted to describe the effects of velocity ratio and jet exit Mach number on the development of a jet in a coflowing stream. These turbulence measurements and a simplified Lighthill radiation model were used in predicting the variation in radiated noise at 90 deg to the jet axis with velocity ratio. Finally, the influence of forward motion on flow-acoustic interactions was examined through a reinterpretation of the 'static' numerical solutions to the Lilley equation.

  7. Modellingthe Turbulent Mixing Noise Associated with Coanda Jets

    NASA Astrophysics Data System (ADS)

    Smith, Caroline

    2004-11-01

    Turbulent Mixing Noise (TMN) is a primary high-frequency noise source in aeronautical and aerospace applications that utilize the Coanda effect, due to the enhanced turbulence levels and entrainment that devices employing this effect generally offer when compared with conventional jet flows. A theory, previously developed to predict the TMN emitted by unit volume of jet-type shear-layer turbulence close to a rigid plane, is extended to predict the aeroacoustic characteristics of a three-dimensional turbulent flow over a particular Coanda surface. The ability to accurately predict this significant source of high frequency acoustic radiation will allow investigation of modifications to basic Coanda devices, so that the benefits of such devices can be fully exploited, without this unfortunate side effect.

  8. The interaction of synthetic jets with turbulent boundary layers

    NASA Astrophysics Data System (ADS)

    Cui, Jing

    In recent years, a promising approach to the control of wall bounded as well as free shear flows, using synthetic jet (oscillatory jet with zero-net-mass-flux) actuators, has received a great deal of attention. A variety of impressive flow control results have been achieved experimentally by many researchers including the vectoring of conventional propulsive jets, modification of aerodynamic characteristics of bluff bodies, control of lift and drag of airfoils, reduction of skin-friction of a flat plate boundary layer, enhanced mixing in circular jets, and control of external as well as internal flow separation and of cavity oscillations. More recently, attempts have been made to numerically simulate some of these flowfields. Numerically several of the above mentioned flow fields have been simulated primarily by employing the Unsteady Reynolds-Averaged Navier Stokes (URANS) equations with a turbulence model and a limited few by Direct Numerical Simulation (DNS). In simulations, both the simplified boundary conditions at the exit of the jet as well as the details of the cavity and lip have been included. In this dissertation, I describe the results of simulations for several two- and three-dimensional flowfields dealing with the interaction of a synthetic jet with a turbulent boundary layer and control of separation. These simulations have been performed using the URANS equations in conjunction with either one- or a two-equation turbulence model. 2D simulations correspond to the experiments performed by Honohan at Georgia Tech. and 3D simulations correspond to the CFD validation test cases proposed in the NASA Langley Research Center Workshop---"CFD Validation of Synthetic Jets and Turbulent Separation Control" held at Williamsburg VA in March 2004. The sources of uncertainty due to grid resolution, time step, boundary conditions, turbulence modeling etc. have been examined during the computations. Extensive comparisons for various flow variables are made with the

  9. Turbulence in sound excited jets - Measurements and theory

    NASA Technical Reports Server (NTRS)

    Morris, P. J.; Baltas, C.

    1981-01-01

    The mechanisms by which the turbulent structure of an axisymmetric jet is modified by the presence of an acoustic excitation are examined. A model is described in which the excitation triggers instability waves at the jet exit. As these waves propagate downstream they extract energy from the mean flow and transfer it to the random turbulence. This results in an increase of the random turbulence levels and a more rapid mixing and spreading for the jet. Models are introduced for the Reynolds stress and the 'wave-induced stress'. It is shown that at high frequencies the presence of the instability wave may reduce the random turbulence levels. Numerical calculations are presented for both the radial and axial variation in the time-averaged properties of the flow as a function of excitation conditions. The calculations are compared with measurements of fluctuating velocity and pressure in a round jet with a Reynolds number of 375,000, based on jet diameter and exit velocity.

  10. Development of turbulent variable density mixing in jets with coflow

    NASA Astrophysics Data System (ADS)

    Charonko, John; Prestridge, Kathy

    2015-11-01

    Fully turbulent jets with coflow at two density ratios (At=0.1 & 0.6) were studied as a statistically stationary system for improving our understanding of variable density mixing in turbulent flows. The exit Reynolds number was matched for both flows at ~19,000 and simultaneous planar PIV and acetone PLIF measurements were acquired so the coupled evolution of the velocity and density statistics could be examined in terms of density-weighted average quantities. Measurements were taken over 10,000 snapshots of the flow at three locations to insure statistical convergence, and the spatial resolution (288 μm) was at or below the Taylor microscale. In agreement with our previous work at lower Reynolds numbers, for large density ratios turbulent kinetic energy and Reynolds stresses are preserved or increased with downstream distance, contrasting with the behavior at low density ratios. Furthermore, in regions where the buoyancy effects began dominating the initial momentum-driven flow (~30 jet diameters), the jet is still not developing toward a self-similar state. Instead, a region of homogeneous turbulence appeared to establish itself in the center of the jet even for the lower density ratio condition, in contrast with classical results for single-fluid jets.

  11. Flow field topology of submerged jets with fractal generated turbulence

    NASA Astrophysics Data System (ADS)

    Cafiero, Gioacchino; Discetti, Stefano; Astarita, Tommaso

    2015-11-01

    Fractal grids (FGs) have been recently an object of numerous investigations due to the interesting capability of generating turbulence at multiple scales, thus paving the way to tune mixing and scalar transport. The flow field topology of a turbulent air jet equipped with a square FG is investigated by means of planar and volumetric particle image velocimetry. The comparison with the well-known features of a round jet without turbulence generators is also presented. The Reynolds number based on the nozzle exit section diameter for all the experiments is set to about 15 000. It is demonstrated that the presence of the grid enhances the entrainment rate and, as a consequence, the scalar transfer of the jet. Moreover, due to the effect of the jet external shear layer on the wake shed by the grid bars, the turbulence production region past the grid is significantly shortened with respect to the documented behavior of fractal grids in free-shear conditions. The organization of the large coherent structures in the FG case is also analyzed and discussed. Differently from the well-known generation of toroidal vortices due to the growth of azimuthal disturbances within the jet shear layer, the fractal grid introduces cross-wise disturbs which produce streamwise vortices; these structures, although characterized by a lower energy content, have a deeper streamwise penetration than the ring vortices, thus enhancing the entrainment process.

  12. Mixing by turbulent buoyant jets in slender containers

    NASA Astrophysics Data System (ADS)

    Voropayev, S. I.; Nath, C.; Fernando, H. J. S.

    2012-10-01

    A turbulent buoyant jet injected vertically into a slender cylinder containing a stratified fluid is investigated experimentally. The working fluid is water, and salt is used to change its density to obtain either a positively or negatively buoyant jet. The interest is the vertical density distribution in container and its dependence on time and other parameters. For each case (lighter or heavier jet) the experimental data could be collapsed into a ‘universal’ time dependent behavior, when properly non-dimensionalized. A theoretical model is advanced to explain the results. Possible applications include refilling of crude oil into U.S. strategic petroleum reserves caverns.

  13. Effects of inclined jets on turbulent oxy-flame characteristics in a triple jet burner

    SciTech Connect

    Boushaki, T.; Mergheni, M.A.; Sautet, J.C.; Labegorre, B.

    2008-07-15

    The reactants are generally injected into the industrial furnaces by jets. An effective method to act on combustion in such systems is to control the way injection jets. The present study concerns the control of turbulent flames by the jets deflection in a natural gas-oxygen burner with separated jets. The burner of 25 kW power is constituted with three aligned jets, one central natural gas jet surrounded by two oxygen jets. The principal idea is to confine the fuel jet by oxygen jets to favour the mixing in order to improve the flame stability and consequently to reduce the pollutant emissions like NO{sub x}. The flame stability and its structural properties are analyzed by the OH chemiluminescence. The Particle Image Velocimetry technique has been used to characterize the dynamic field. Results show that the control by inclined jets has a considerable effect on the dynamic behaviour and flame topology. Indeed, the control by incline of oxygen jets towards fuel jet showed a double interest: a better stabilization of flame and a significant reduction of nitrogen oxides. Measurements showed that the deflection favours the mixing and accelerates the fusion of jets allowing the flame stabilization. (author)

  14. Noise prediction of a subsonic turbulent round jet using the lattice-Boltzmann method

    PubMed Central

    Lew, Phoi-Tack; Mongeau, Luc; Lyrintzis, Anastasios

    2010-01-01

    The lattice-Boltzmann method (LBM) was used to study the far-field noise generated from a Mach, Mj=0.4, unheated turbulent axisymmetric jet. A commercial code based on the LBM kernel was used to simulate the turbulent flow exhausting from a pipe which is 10 jet radii in length. Near-field flow results such as jet centerline velocity decay rates and turbulence intensities were in agreement with experimental results and results from comparable LES studies. The predicted far field sound pressure levels were within 2 dB from published experimental results. Weak unphysical tones were present at high frequency in the computed radiated sound pressure spectra. These tones are believed to be due to spurious sound wave reflections at boundaries between regions of varying voxel resolution. These “VR tones” did not appear to bias the underlying broadband noise spectrum, and they did not affect the overall levels significantly. The LBM appears to be a viable approach, comparable in accuracy to large eddy simulations, for the problem considered. The main advantages of this approach over Navier–Stokes based finite difference schemes may be a reduced computational cost, ease of including the nozzle in the computational domain, and ease of investigating nozzles with complex shapes. PMID:20815448

  15. A Hybrid RANS/LES Approach for Predicting Jet Noise

    NASA Technical Reports Server (NTRS)

    Goldstein, Marvin E.

    2006-01-01

    Hybrid acoustic prediction methods have an important advantage over the current Reynolds averaged Navier-Stokes (RANS) based methods in that they only involve modeling of the relatively universal subscale motion and not the configuration dependent larger scale turbulence. Unfortunately, they are unable to account for the high frequency sound generated by the turbulence in the initial mixing layers. This paper introduces an alternative approach that directly calculates the sound from a hybrid RANS/LES flow model (which can resolve the steep gradients in the initial mixing layers near the nozzle lip) and adopts modeling techniques similar to those used in current RANS based noise prediction methods to determine the unknown sources in the equations for the remaining unresolved components of the sound field. The resulting prediction method would then be intermediate between the current noise prediction codes and previously proposed hybrid noise prediction methods.

  16. Evolution of turbulent jets in low aspect ratio containers

    NASA Astrophysics Data System (ADS)

    Pol, S.; Nath, C.; Gest, D.; Voropayev, S.; Fernando, H. J. S.; Webb, S.

    2009-11-01

    The evolution of homogeneous and buoyant turbulent jets released into a low aspect ratio (width/height) container was investigated experimentally using PIV, MSCT probing and digital imaging. The motivation was to understand mixing process occurring in U.S. Strategic Petroleum Reserves (SPR), where crude oil is stored in salt caverns of low aspect ratio. During maintenance or filling, oil is introduced as a jet from the top of the caverns. This study is focussed on mean and turbulent flow characteristics as well as global flow instability and periodic oscillations intrinsic to jets in low aspect ratio containers. Scaling arguments were advanced for salient flow parameters, which included the characteristic length (container width D) and velocity (for homogeneous jets, J^1/2D, where J is the momentum flux at the jet exit) scales. For buoyant jets, the buoyancy flux B needs to be introduced as an additional parameter. Such jet flows do not reach a steady state, but bifurcate periodically with a frequency scale J^1/2/ D^2 while enhancing global mixing.

  17. Splattering during turbulent liquid jet impingement on solid targets

    SciTech Connect

    Bhunia, S.K.; Lienhard, J.H. V . Dept. of Mechanical Engineering)

    1994-06-01

    In turbulent liquid jet impingement, a spray of droplets often breaks off of the liquid layer formed on the target. This splattering of liquid alters the efficiencies of jet impingement heat transfer processes and chemical containment safety devices, and leads to problems of aerosol formation in jet impingement cleaning processes. In this paper, the authors present a more complete study of splattering and improved correlations that extend and supersede the previous reports on this topic. The authors report experimental results on the amount of splattering for jets of water, isopropanol-water solutions, and soap-water mixtures. Jets were produced by straight tube nozzles of diameter 0.8--5.8 mm, with fully developed turbulent pipe-flow upstream of the nozzle exist. These experiments cover Weber numbers between 130--31,000, Reynolds numbers between 2,700--98,000, and nozzle-to-target separations of 0.2 [<=]l/d[<=]125. Splattering of up to 75 percent of the incoming jet liquid is observed. The results show that only the Weber number and l/d affect the fraction of jet liquid splattered. The presence of surfactants does not alter the splattering. A new correlation for the onset condition for splattering is given. In addition, the authors establish the range of applicability of the model of Lienhard et al. and the authors provide a more accurate set of coefficients for their correlation.

  18. Disturbance convection velocity in turbulent jets under aeroacoustic excitation

    NASA Astrophysics Data System (ADS)

    Pimshtein, V. G.

    2007-09-01

    The velocity of propagation of toroidal and oblique vortices formed in subsonic and supersonic turbulent jets under longitudinal internal and transverse external excitation by finite-amplitude saw-tooth acoustic waves is studied experimentally. It is demonstrated that the convection velocity of vortices is not constant, and the character of its variation depends on the vortex shape.

  19. The development of an axisymmetric curved turbulent wall jet

    NASA Astrophysics Data System (ADS)

    Gregory-Smith, D. G.; Hawkins, M. J.

    1991-12-01

    An experimental study has been carried out of the low speed Coanda wall jet with both streamwise and axisymmetric curvature. A single component laser Doppler technique was used, and by taking several orientations at a given point, values of the three mean velocities and five of the six Reynolds stresses were obtained. The lateral divergence and convex streamwise curvature both enhanced the turbulence in the outer part of the jet compared with a plane two-dimensional wall jet. The inner layer exhibited a large separation of the positions of maximum velocity and zero shear stress. It was found that the streamwise mean velocity profile became established very rapidly downstream of the slot exit. The profile appeared fairly similar at later downstream positions, but the mean radial velocity and turbulence parameters showed the expected nonself preservation of the flow. Removal of the streamwise curvature resulted in a general return of the jet conditions toward those expected of a plane wall jet. The range and accuracy of the data may be used for developing turbulence models and computational techniques for this type of flow.

  20. The effects of Gravity on Transitional and Turbulent Jet Flames

    NASA Astrophysics Data System (ADS)

    Mehravaran, Kian; Jaberi, Farhad

    2002-11-01

    The effects of gravity on compositional and physical structure of transitional and turbulent jet flames are studied via analysis of the data generated by direct numerical simulation (DNS) of a planar jet flame at various gravity conditions. A fully-compressible finite-difference computational solver is used together with a single step Arrhenius model for chemical reaction. The results of our non-reacting flow simulations are in good agreement with available experimental data for planar jets. The results of our reacting simulations are also consistent with previous findings and indicate that at zero- (or micro-) gravity condition combustion damps the flow instability; hence reduces ``turbulence production'' and jet growth. However, in ``normal'' gravity condition, combustion generated density variations and buoyancy effects promotes vorticity generation and enhances the otherwise damped turbulence by heat of reaction. Buoyancy generated vorticity and strain field leads to more jet entrainment as well as better mixing and combustion. Both large and small scale flow structures are modified by gravity; resulting in variation of the spatial and the compositional flame structures. The analysis of compositional flame structures suggest that finite-rate chemistry effects and localized flame extinction are more significant in normal gravity conditions than in zero-gravity.

  1. Intermittency in non-homogeneous Wake and Jet Turbulence

    NASA Astrophysics Data System (ADS)

    Mahjoub, O. B.; Sekula, E.; Redondo, J. M.

    2010-05-01

    The scale to scale transfer and the structure functions are calculated and from these the intermittency parametres [1[3]. The estimates of turbulent diffusivity could also be measured. Some two point correlations and time lag calculations are used to investigate the local mixedness [4,5] and the temporal and spatial integral length scales obtained from both Lagrangian and Eulerian correlations and functions. We compare these results with both theoretical and experimental ones in the Laboratory with a wind tunnel at the wake of a grid or cillinder with and withoutand a near Wall. The a theoretical description of how to simulate intermittency following the model of Babiano et al. (1996) and the role of locality in higher order exponents is applied to the different flows. The information about turbulent jets is needed in several configurations providing basic information about the turbulent free jet, the circular jet and the turbulent wall jet. The experimental measurements of turbulent velocity is based on Acoustic Doppler Velocimeter measurements of the jet centerline and off centered radial positions in the tank at several distances from the wall. Spectral and structure function analysis are useful to determine the flow mixing ability using also flow visualization [6,7]. Results of experiments include the velocity distribution, entrainment angle of the jets, jet and wake average and fluctuating velocity, PDF's, Skewness and Kurthosis, velocity and vorticity standard deviation, boundary layers function and turbulence intensity . Different range of Wake and Jet flows show a maximum of turbulent intensity at a certain distance from the wall as it breaks the flow simmetry and adds large scale vorticity in the different experiments, these efects are also believed to occur in Geo-Astrophysical flows. [1] Babiano, A. (2002), On Particle dispersion processes in two-dimensional turbulence. In Turbulent mixing in geophysical flows. Eds. Linden P.F. and Redondo J.M., p. 2

  2. Theoretical study of reactive and nonreactive turbulent coaxial jets

    NASA Technical Reports Server (NTRS)

    Gupta, R. N.; Wakelyn, N. T.

    1976-01-01

    The hydrodynamic properties and the reaction kinetics of axisymmetric coaxial turbulent jets having steady mean quantities are investigated. From the analysis, limited to free turbulent boundary layer mixing of such jets, it is found that the two-equation model of turbulence is adequate for most nonreactive flows. For the reactive flows, where an allowance must be made for second order correlations of concentration fluctuations in the finite rate chemistry for initially inhomogeneous mixture, an equation similar to the concentration fluctuation equation of a related model is suggested. For diffusion limited reactions, the eddy breakup model based on concentration fluctuations is found satisfactory and simple to use. The theoretical results obtained from these various models are compared with some of the available experimental data.

  3. Numerical calculation of two-phase turbulent jets

    SciTech Connect

    Saif, A.A.

    1995-05-01

    Two-phase turbulent round jets were numerically simulated using a multidimensional two-phase CFD code based on the two-fluid model. The turbulence phenomena were treated with the standard k-{epsilon} model. It was modified to take into account the additional dissipation of turbulent kinetic energy by the dispersed phase. Within the context of the two-fluid model it is more appropriate and physically justified to treat the diffusion by an interfacial force in the momentum equation. In this work, the diffusion force and the additional dissipation effect by the dispersed phase were modeled starting from the classical turbulent energy spectrum analysis. A cut-off frequency was proposed to decrease the dissipation effect by the dispersed phase when large size particles are introduced in the flow. The cut-off frequency combined with the bubble-induced turbulence effect allows for an increase in turbulence for large particles. Additional care was taken in choosing the right kind of experimental data from the literature so that a good separate effect test was possible for their models. The models predicted the experimental data very closely and they were general enough to predict extreme limit cases: water-bubble and air-droplet jets.

  4. Turbulence Statistics of a Buoyant Jet in a Stratified Environment

    NASA Astrophysics Data System (ADS)

    McCleney, Amy Brooke

    Using non-intrusive optical diagnostics, turbulence statistics for a round, incompressible, buoyant, and vertical jet discharging freely into a stably linear stratified environment is studied and compared to a reference case of a neutrally buoyant jet in a uniform environment. This is part of a validation campaign for computational fluid dynamics (CFD). Buoyancy forces are known to significantly affect the jet evolution in a stratified environment. Despite their ubiquity in numerous natural and man-made flows, available data in these jets are limited, which constrain our understanding of the underlying physical processes. In particular, there is a dearth of velocity field data, which makes it challenging to validate numerical codes, currently used for modeling these important flows. Herein, jet near- and far-field behaviors are obtained with a combination of planar laser induced fluorescence (PLIF) and multi-scale time-resolved particle image velocimetry (TR-PIV) for Reynolds number up to 20,000. Deploying non-intrusive optical diagnostics in a variable density environment is challenging in liquids. The refractive index is strongly affected by the density, which introduces optical aberrations and occlusions that prevent the resolution of the flow. One solution consists of using index matched fluids with different densities. Here a pair of water solutions - isopropanol and NaCl - are identified that satisfy these requirements. In fact, they provide a density difference up to 5%, which is the largest reported for such fluid pairs. Additionally, by design, the kinematic viscosities of the solutions are identical. This greatly simplifies the analysis and subsequent simulations of the data. The spectral and temperature dependence of the solutions are fully characterized. In the near-field, shear layer roll-up is analyzed and characterized as a function of initial velocity profile. In the far-field, turbulence statistics are reported for two different scales, one

  5. Evaluation of Turbulence-Model Performance as Applied to Jet-Noise Prediction

    NASA Technical Reports Server (NTRS)

    Woodruff, S. L.; Seiner, J. M.; Hussaini, M. Y.; Erlebacher, G.

    1998-01-01

    The accurate prediction of jet noise is possible only if the jet flow field can be predicted accurately. Predictions for the mean velocity and turbulence quantities in the jet flowfield are typically the product of a Reynolds-averaged Navier-Stokes solver coupled with a turbulence model. To evaluate the effectiveness of solvers and turbulence models in predicting those quantities most important to jet noise prediction, two CFD codes and several turbulence models were applied to a jet configuration over a range of jet temperatures for which experimental data is available.

  6. Lagrangian statistics across the turbulent-nonturbulent interface in a turbulent plane jet

    NASA Astrophysics Data System (ADS)

    Taveira, Rodrigo R.; Diogo, José S.; Lopes, Diogo C.; da Silva, Carlos B.

    2013-10-01

    Lagrangian statistics from millions of particles are used to study the turbulent entrainment mechanism in a direct numerical simulation of a turbulent plane jet at Reλ≈110. The particles (tracers) are initially seeded at the irrotational region of the jet near the turbulent shear layer and are followed as they are drawn into the turbulent region across the turbulent-nonturbulent interface (TNTI), allowing the study of the enstrophy buildup and thereby characterizing the turbulent entrainment mechanism in the jet. The use of Lagrangian statistics following fluid particles gives a more correct description of the entrainment mechanism than in previous works since the statistics in relation to the TNTI position involve data from the trajectories of the entraining fluid particles. The Lagrangian statistics for the particles show the existence of a velocity jump and a characteristic vorticity jump (with a thickness which is one order of magnitude greater than the Kolmogorov microscale), in agreement with previous results using Eulerian statistics. The particles initially acquire enstrophy by viscous diffusion and later by enstrophy production, which becomes “active” only deep inside the turbulent region. Both enstrophy diffusion and production near the TNTI differ substantially from inside the turbulent region. Only about 1% of all particles find their way into pockets of irrotational flow engulfed into the turbulent shear layer region, indicating that “engulfment” is not significant for the present flow, indirectly suggesting that the entrainment is largely due to “nibbling” small-scale mechanisms acting along the entire TNTI surface. Probability density functions of particle positions suggests that the particles spend more time crossing the region near the TNTI than traveling inside the turbulent region, consistent with the particles moving tangent to the interface around the time they cross it.

  7. Buoyancy Effects in Turbulent Jet Flames in Crossflow

    NASA Astrophysics Data System (ADS)

    Boxx, Isaac; Idicheria, Cherian; Clemens, Noel

    2003-11-01

    The aim of this study is to investigate the effects of buoyancy on the structure of turbulent, non-premixed hydrocarbon jet-flames in crossflow (JFICF). This was accomplished using a small jet-in-crossflow facility which can be oriented at a variety of angles with respect to the gravity vector. This facility enables us to alter the relative influence of buoyancy on the JFICF without altering the jet-exit Reynolds number, momentum flux ratio or the geometry of the system. Results are compared to similar, but non-buoyant, JFICF studied in microgravity. Departures of jet-centerline trajectory from the well-known power-law scaling of turbulent JFICF were used to explore the transition from a buoyancy-influenced regime to a momentum dominated one. The primary diagnostic was CCD imaging of soot-luminosity. We present results on ethylene jet flames with jet-exit Reynolds numbers of 1770 to 8000 and momentum flux ratios of 5 to 13.

  8. Multigrid acceleration and turbulence models for computations of 3D turbulent jets in crossflow

    NASA Technical Reports Server (NTRS)

    Demuren, A. O.

    1992-01-01

    A multigrid method is presented for the calculation of three-dimensional turbulent jets in crossflow. Turbulence closure is achieved with either the standard k-epsilon model or a Reynolds stress model (RSM). Multigrid acceleration enables convergence rates which are far superior to that for a single grid method to be obtained with both turbulence models. With the k-epsilon model the rate approaches that for laminar flow, but with RSM it is somewhat slower. The increased stiffness of the system of equation in the latter may be responsible. Computed results with both turbulence models are compared to experimental data for a pair of opposed jets in crossflow. Both models yield reasonable agreement for the mean flow velocity, but RSM yields better predictions of the Reynolds stresses.

  9. Strategies for comparing LES and experimental data of urban turbulence

    NASA Astrophysics Data System (ADS)

    Hertwig, D.; Nguyen van yen, R.; Patnaik, G.; Leitl, B.

    2012-12-01

    Unsteady flow within and above built environments is an important example of the complex nature of near-surface atmospheric turbulence. Typically, obstacle-resolving micro-scale meteorological models based on the Reynolds-averaged conservation equations are adopted to investigate and predict the mean-states of urban flow phenomena. The rapid advancements in computer capacities, however, fostered the use of time-resolved approaches like large-eddy simulation (LES) for applications on the urban micro-scale. LES has the potential to provide a realistic picture of the spatio-temporal behavior of turbulent flows within and above the urban canopy layer, which cannot be easily achieved with classic in-situ micro-meteorological measurements. The further success of eddy-resolving techniques, however, is coupled to the critical assessment of the model performance in terms of a rigorous validation against suitable reference data. This task is particularly challenging with regard to the time-dependent nature of the problem and the need to verify whether the model predicts turbulence structures in a realistic way. In this study, a hierarchy of validation strategies for urban LES flow fields is formulated and systematically applied. The test case is neutrally stratified turbulent flow in the inner city of Hamburg, Germany. The LES computations were conducted by the U.S. Naval Research Laboratory on the basis of a monotone integrated LES methodology. Reference experiments in terms of single-point, high resolution time-series measurements were carried out in the boundary-layer wind-tunnel facility at the University of Hamburg. The wind-tunnel model was built on a scale of 1:350 and included building structures with a full-scale spatial resolution of 0.5 m. Benchmark parameters for the congruent representation of atmospheric inflow conditions in the physical and the numerical model were obtained from long-term sonic anemometer measurements at a suburban meteorological field site

  10. Linear global modes in a high Reynolds number Mach 0.9 turbulent jet

    NASA Astrophysics Data System (ADS)

    Schmidt, Oliver; Towne, Aaron; Colonius, Tim

    2015-11-01

    A global linear stability and resolvent analysis of the mean flow from a carefully validated Mach 0 . 9 turbulent jet large eddy simulation (LES) is conducted. Spatiotemporal Fourier decomposition of the simulation data reveals the presence of large scale coherent structures at small azimuthal wavenumbers. The latter wave packets appear as discrete sets of lightly dampened modes in the linear global stability analysis. Their common feature is a spatial separation into an upstream traveling acoustic perturbation in the potential core region, and a Kelvin-Helmholtz-like vortical perturbation which is advected downstream. The least stable branch of discrete modes observed at Strouhal numbers 0 . 38 < St < 0 . 42 exhibits the same acoustic super-directivity as found in the LES and various experimental studies, and hence establishes a direct link between global linear instabilities and low-angle acoustic radiation. Branches at higher frequencies and azimuthal wavenumbers show multi-directive acoustic emission patterns. This observation is of particular interest since high angle, broadband radiation is commonly attributed to stochastic fluctuations of the turbulent jet shear layer.

  11. Variability in Active Galactic Nuclei from Propagating Turbulent Relativistic Jets

    NASA Astrophysics Data System (ADS)

    Pollack, Maxwell; Pauls, David; Wiita, Paul J.

    2016-03-01

    We use the Athena hydrodynamics code to model propagating two-dimensional relativistic jets as approximations to the growth of radio-loud active galactic nuclei for various input jet velocities and jet-to-ambient matter density ratios. Using results from these simulations we estimate the changing synchrotron emission by summing the fluxes from a vertical strip of zones behind the reconfinement shock, which is nearly stationary, and from which a substantial portion of the flux variability should arise. We explore a wide range of timescales by considering two light curves from each simulation; one uses a relativistic turbulence code with bulk velocities taken from our simulations as input, while the other uses the bulk velocity data to compute fluctuations caused by variations in the Doppler boosting due to changes in the direction and the speed of the flow through all zones in the strip. We then calculate power spectral densities (PSDs) from the light curves for both turbulent and bulk velocity origins for variability. The range of the power-law slopes of the PSDs for the turbulence induced variations is -1.8 to -2.3, while for the bulk velocity produced variations this range is -2.1 to -2.9 these are in agreement with most observations. When superimposed, these power spectra span a very large range in frequency (about five decades), with the turbulent fluctuations yielding most of the shorter timescale variations and the bulk flow changes dominating the longer periods.

  12. Mean dynamics of a turbulent plane wall jet

    NASA Astrophysics Data System (ADS)

    Mehdi, Faraz; Klewicki, Joseph

    2015-11-01

    Experimental and large-eddy simulation data are used to investigate the balances between viscous and inertial forces in plane turbulent wall jets. In recent years, analysis of the mean momentum balance in its unintegrated form has been shown to provide a mathematically and physically useful means for clarifying the leading order mean dynamics as a function of the transverse coordinate. Distinct from its laminar counterpart, each of the terms in the appropriately simplified form of the mean dynamical equation for the planar turbulent wall jet is leading order somewhere, but not everywhere, across the flow domain. Similar to what is observed in the canonical turbulent wall-flows, there is a wall region where the mean viscous force retains leading order. The wall jet, however, contains two peaks of opposite sign in its Reynolds stress profile. With distance from the wall, the first peak is associated with the loss of a leading order viscous force, while the outer peak is akin to the wholly inertial balance exchange that occurs in shear-wake flows. The physics of these balances exchanges are described, the scaling behaviors of the leading order balance layers are estimated, and the present findings are compared with previous models of planar wall jet structure.

  13. Measurements of turbulent inclined plane dual jets

    NASA Astrophysics Data System (ADS)

    Wang, C. S.; Lin, Y. F.; Sheu, M. J.

    1993-11-01

    Measurements of mean velocities, flow direction, velocity fluctuations and Reynolds shear stress were made with a split film probe of hot wire anemometer to investigate the interactions created by two air jets issuing from two identical plane inclined nozzles. The reverse flow was detected by using the split film probe and observed by flow visualization. Experimental results with an inclined angle of 9° are presented in the paper. Some experimental results with an inclined angle of 27° are presented to investigate the effect of inclination on the flow field. Mean velocities approach self-preservation in both the converging region and the combining region. Velocity fluctuations and Reynolds shear stress approach self-preservation in the combining region only. The spreads of jet and the square of the decay of maximum mean velocity increase linearly as the distance from the nozzle exit increases.

  14. Modeling of Turbulence Effects on Liquid Jet Atomization and Breakup

    NASA Technical Reports Server (NTRS)

    Trinh, Huu P.; Chen, C. P.

    2005-01-01

    Recent experimental investigations and physical modeling studies have indicated that turbulence behaviors within a liquid jet have considerable effects on the atomization process. This study aims to model the turbulence effect in the atomization process of a cylindrical liquid jet. Two widely used models, the Kelvin-Helmholtz (KH) instability of Reitz (blob model) and the Taylor-Analogy-Breakup (TAB) secondary droplet breakup by O Rourke et al, are further extended to include turbulence effects. In the primary breakup model, the level of the turbulence effect on the liquid breakup depends on the characteristic scales and the initial flow conditions. For the secondary breakup, an additional turbulence force acted on parent drops is modeled and integrated into the TAB governing equation. The drop size formed from this breakup regime is estimated based on the energy balance before and after the breakup occurrence. This paper describes theoretical development of the current models, called "T-blob" and "T-TAB", for primary and secondary breakup respectivety. Several assessment studies are also presented in this paper.

  15. Swirling jet turbulent mixing and combustion computations

    NASA Technical Reports Server (NTRS)

    Rubel, A.

    1973-01-01

    Computations are presented describing the mixing and combustion of swirling jets in a coaxial stream. It is demonstrated that the boundary layer equations represent the flow reasonably well until reversed flow is imminent. For the range of parameters investigated indications are that the edge velocity has little effect on the behavior of the flow. Furthermore, confining the flow with a constant pressure wall, or impressing a favorable pressure gradient on the coaxial flow, acts to reduce the severity of the centerline adverse pressure gradient created by the swirl decay. A simple scalar eddy viscosity model, including a potential core formulation, is shown to described the behavior of weak swirling flow in the far region but is only in fair agreement with observations in the near region. The effects of swirl on a burning hydrocarbon jet exhausting into a cold coaxial stream are shown to be intensified by the reduction of the density due to combustion. The enhanced mixing properties of high swirl flow produce rapid diffusion of the burning gases into the cold edge flow causing early cessation of the NO producing reactions. Computations show that doubling the initial jet swirl could reduce the NO production by 25 percent.

  16. Evaluation of Turbulence-Model Performance in Jet Flows

    NASA Technical Reports Server (NTRS)

    Woodruff, S. L.; Seiner, J. M.; Hussaini, M. Y.; Erlebacher, G.

    2001-01-01

    The importance of reducing jet noise in both commercial and military aircraft applications has made jet acoustics a significant area of research. A technique for jet noise prediction commonly employed in practice is the MGB approach, based on the Lighthill acoustic analogy. This technique requires as aerodynamic input mean flow quantities and turbulence quantities like the kinetic energy and the dissipation. The purpose of the present paper is to assess existing capabilities for predicting these aerodynamic inputs. Two modern Navier-Stokes flow solvers, coupled with several modern turbulence models, are evaluated by comparison with experiment for their ability to predict mean flow properties in a supersonic jet plume. Potential weaknesses are identified for further investigation. Another comparison with similar intent is discussed by Barber et al. The ultimate goal of this research is to develop a reliable flow solver applicable to the low-noise, propulsion-efficient, nozzle exhaust systems being developed in NASA focused programs. These programs address a broad range of complex nozzle geometries operating in high temperature, compressible, flows. Seiner et al. previously discussed the jet configuration examined here. This convergent-divergent nozzle with an exit diameter of 3.6 inches was designed for an exhaust Mach number of 2.0 and a total temperature of 1680 F. The acoustic and aerodynamic data reported by Seiner et al. covered a range of jet total temperatures from 104 F to 2200 F at the fully-expanded nozzle pressure ratio. The aerodynamic data included centerline mean velocity and total temperature profiles. Computations were performed independently with two computational fluid dynamics (CFD) codes, ISAAC and PAB3D. Turbulence models employed include the k-epsilon model, the Gatski-Speziale algebraic-stress model and the Girimaji model, with and without the Sarkar compressibility correction. Centerline values of mean velocity and mean temperature are

  17. Scanning tomographic particle image velocimetry applied to a turbulent jet

    NASA Astrophysics Data System (ADS)

    Casey, T. A.; Sakakibara, J.; Thoroddsen, S. T.

    2013-02-01

    We introduce a modified tomographic PIV technique using four high-speed video cameras and a scanning pulsed laser-volume. By rapidly illuminating adjacent subvolumes onto separate video frames, we can resolve a larger total volume of velocity vectors, while retaining good spatial resolution. We demonstrate this technique by performing time-resolved measurements of the turbulent structure of a round jet, using up to 9 adjacent volume slices. In essence this technique resolves more velocity planes in the depth direction by maintaining optimal particle image density and limiting the number of ghost particles. The total measurement volumes contain between 1 ×106 and 3 ×106 velocity vectors calculated from up to 1500 reconstructed depthwise image planes, showing time-resolved evolution of the large-scale vortical structures for a turbulent jet of Re up to 10 000.

  18. 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.

  19. 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 Moser are observed. The jet core breakdown stage exhibits increased acoustic radiations.

  20. Large-eddy simulation of axially-rotating, turbulent pipe and particle-laden swirling jet flows

    NASA Astrophysics Data System (ADS)

    Castro, Nicolas D.

    The flows of fully-developed turbulent rotating pipe and particle-laden swirling jet emitted from the pipe into open quiescent atmosphere are investigated numerically using Large-Eddy Simulation (LES). Simulations are performed at various rotation rates and Reynolds numbers, based on bulk velocity and pipe diameter, of 5.3x103, 12x103, and 24x103, respectively. Time-averaged LES results are compared with experimental and simulation data from previous studies. Pipe flow results confirm observations in previous studies, such as the deformation of the turbulent mean axial velocity profile towards the laminar Poiseuille-profile, with increased rotation. The Reynolds stress anisotropy tensor shows a redistribution due to pipe rotation. The axial component near the wall is suppressed, whereas the tangential component is amplified, as rotation is increased. The anisotropy invariant map also shows a movement away from the one-component limit in the viscous sublayer, with increased rotation. Exit conditions for the pipe flow simulation are utilized as inlet conditions for the jet flow simulation. Jet flow without swirl and at a swirl rate of S=0.5 is investigated. Swirl is observed to change the characteristics of the jet flow field, leading to an increase in jet spread and velocity decay and a corresponding decrease in the jet potential core. Lagrangian tracking with one way coupling is used to analyze particle dispersion in the jet flow. Three particle diameter sizes are investigated: 10, 100, and 500μm, which correspond to Stokes numbers of 0.06, 6, and 150, respectively. Particles are injected with an initial velocity set equal to the instantaneous fluid phase flow velocities at the jet inlet. The results show that, in the absence of swirl, particle dispersion is inversely proportional to particle size. With the addition of swirl, particle evolution is much more complicated. Largely unaffected by turbulent structures, the largest particles maintain their initial radial

  1. Evaluation of Liquid Fuel Spray Models for Hybrid RANS/LES and DLES Prediction of Turbulent Reactive Flows

    NASA Astrophysics Data System (ADS)

    Afshar, Ali

    An evaluation of Lagrangian-based, discrete-phase models for multi-component liquid sprays encountered in the combustors of gas turbine engines is considered. In particular, the spray modeling capabilities of the commercial software, ANSYS Fluent, was evaluated. Spray modeling was performed for various cold flow validation cases. These validation cases include a liquid jet in a cross-flow, an airblast atomizer, and a high shear fuel nozzle. Droplet properties including velocity and diameter were investigated and compared with previous experimental and numerical results. Different primary and secondary breakup models were evaluated in this thesis. The secondary breakup models investigated include the Taylor analogy breakup (TAB) model, the wave model, the Kelvin-Helmholtz Rayleigh-Taylor model (KHRT), and the Stochastic secondary droplet (SSD) approach. The modeling of fuel sprays requires a proper treatment for the turbulence. Reynolds-averaged Navier-Stokes (RANS), large eddy simulation (LES), hybrid RANS/LES, and dynamic LES (DLES) were also considered for the turbulent flows involving sprays. The spray and turbulence models were evaluated using the available benchmark experimental data.

  2. Multigrid acceleration and turbulence models for computations of 3D turbulent jets in crossflow

    NASA Technical Reports Server (NTRS)

    Demuren, A. O.

    1991-01-01

    A multigrid method is presented for the calculation of three-dimensional turbulent jets in crossflow. Turbulence closure is achieved with either the standard k-epsilon model or a Reynolds Stress Model (RSM). Multigrid acceleration enables convergence rates which are far superior to that for a single grid method. With the k-epsilon model the rate approaches that for laminar flow, but with RSM it is somewhat slower. The increased stiffness of the system of equations in the latter may be responsible. Computed results with both turbulence models are compared with experimental data for a pair of opposed jets in crossflow. Both models yield reasonable agreement with mean flow velocity but RSM yields better prediction of the Reynolds stresses.

  3. Experimental study of highly turbulent isothermal opposed-jet flows

    NASA Astrophysics Data System (ADS)

    Coppola, Gianfilippo; Gomez, Alessandro

    2010-10-01

    Opposed-jet flows have been shown to provide a valuable means to study a variety of combustion problems, but have been limited to either laminar or modestly turbulent conditions. With the ultimate goal of developing a burner for laboratory flames reaching turbulence regimes of relevance to practical systems, we characterized highly turbulent, strained, isothermal, opposed-jet flows using particle image velocimetry (PIV). The bulk strain rate was kept at 1250 s-1 and specially designed and properly positioned turbulence generation plates in the incoming streams boosted the turbulence intensity to well above 20%, under conditions that are amenable to flame stabilization. The data were analyzed with proper orthogonal decomposition (POD) and a novel statistical analysis conditioned to the instantaneous position of the stagnation surface. Both POD and the conditional analysis were found to be valuable tools allowing for the separation of the truly turbulent fluctuations from potential artifacts introduced by relatively low-frequency, large-scale instabilities that would otherwise partly mask the turbulence. These instabilities cause the stagnation surface to wobble with both an axial oscillation and a precession motion about the system axis of symmetry. Once these artifacts are removed, the longitudinal integral length scales are found to decrease as one approaches the stagnation line, as a consequence of the strained flow field, with the corresponding outer scale turbulent Reynolds number following a similar trend. The Taylor scale Reynolds number is found to be roughly constant throughout the flow field at about 200, with a value virtually independent of the data analysis technique. The novel conditional statistics allowed for the identification of highly convoluted stagnation lines and, in some cases, of strong three-dimensional effects, that can be screened, as they typically yield more than one stagnation line in the flow field. The ability to lock on the

  4. Numerical Simulation of Liquid Jet Atomization Including Turbulence Effects

    NASA Technical Reports Server (NTRS)

    Trinh, Huu P.; Chen, C. P.; Balasubramanyam, M. S.

    2005-01-01

    This paper describes numerical implementation of a newly developed hybrid model, T-blob/T-TAB, into an existing computational fluid dynamics (CFD) program for primary and secondary breakup simulation of liquid jet atomization. This model extend two widely used models, the Kelvin-Helmholtz (KH) instability of Reitz (blob model) and the Taylor-Analogy-Breakup (TAB) secondary droplet breakup by O'Rourke and Amsden to include turbulence effects. In the primary breakup model, the level of the turbulence effect on the liquid breakup depends on the characteristic scales and the initial flow conditions. For the secondary breakup, an additional turbulence force acted on parent drops is modeled and integrated into the TAB governing equation. Several assessment studies are presented and the results indicate that the existing KH and TAB models tend to under-predict the product drop size and spray angle, while the current model provides superior results when compared with the measured data.

  5. Broadband sound generation by confined turbulent jets.

    PubMed

    Zhang, Zhaoyan; Mongeau, Luc; Frankel, Steven H

    2002-08-01

    Sound generation by confined stationary jets is of interest to the study of voice and speech production, among other applications. The generation of sound by low Mach number, confined, stationary circular jets was investigated. Experiments were performed using a quiet flow supply, muffler-terminated rigid uniform tubes, and acrylic orifice plates. A spectral decomposition method based on a linear source-filter model was used to decompose radiated nondimensional sound pressure spectra measured for various gas mixtures and mean flow velocities into the product of (1) a source spectral distribution function; (2) a function accounting for near field effects and radiation efficiency; and (3) an acoustic frequency response function. The acoustic frequency response function agreed, as expected, with the transfer function between the radiated acoustic pressure at one fixed location and the strength of an equivalent velocity source located at the orifice. The radiation efficiency function indicated a radiation efficiency of the order (kD)2 over the planar wave frequency range and (kD)4 at higher frequencies, where k is the wavenumber and D is the tube cross sectional dimension. This is consistent with theoretical predictions for the planar wave radiation efficiency of quadrupole sources in uniform rigid anechoic tubes. The effects of the Reynolds number, Re, on the source spectral distribution function were found to be insignificant over the range 20002.5. The influence of a reflective open tube termination on the source function spectral distribution was found to be insignificant, confirming the absence of a feedback mechanism. PMID:12186047

  6. Probing Turbulence and Acceleration at Relativistic Shocks in Blazar Jets

    NASA Astrophysics Data System (ADS)

    Baring, Matthew G.; Boettcher, Markus; Summerlin, Errol J.

    2016-04-01

    Acceleration at relativistic shocks is likely to be important in various astrophysical jet sources, including blazars and other radio-loud active galaxies. An important recent development for blazar science is the ability of Fermi-LAT data to pin down the power-law index of the high energy portion of emission in these sources, and therefore also the index of the underlying non-thermal particle population. This paper highlights how multiwavelength spectra including X-ray band and Fermi data can be used to probe diffusive acceleration in relativistic, oblique, MHD shocks in blazar jets. The spectral index of the non-thermal particle distributions resulting from Monte Carlo simulations of shock acceleration, and the fraction of thermal particles accelerated to non-thermal energies, depend sensitively on the particles' mean free path scale, and also on the mean magnetic field obliquity to the shock normal. We investigate the radiative synchrotron/Compton signatures of thermal and non-thermal particle distributions generated from the acceleration simulations. Important constraints on the frequency of particle scattering and the level of field turbulence are identified for the jet sources Mrk 501, AO 0235+164 and Bl Lacertae. Results suggest the interpretation that turbulence levels decline with remoteness from jet shocks, with a significant role for non-gyroresonant diffusion.

  7. Microburst Simulation via Vortex-Ring and Turbulent Jet Models.

    NASA Astrophysics Data System (ADS)

    Wan, Tung

    Microbursts, suggested as primary causes of many aircraft fatal crashes, are the subject of this research. A microburst, or low-level intense wind shear, is generated by a thunderstorm or a small rain cloud, and presents hazardous conditions for aircraft during take-off and landing maneuvers. Recently released data show that a microburst resembles a transient vortex ring. Three microburst models have been constructed in this study. First, the turbulent jet model encompasses a free jet at high altitude and a wall jet near the ground surface. Second, the vortex ring model is a combination of a primary and an image vortex ring, with an inviscid -viscous interaction at the central axial and surface regions. An unsteady version of this model is also provided by solving the trajectory equation with the Direct Formal Integration (DFI) method or with the Runge-Kutta method. Third and finally, the complete unsteady microburst model equations (conservation of mass, momentum, and energy), or what has been referred to as the Navier-Stokes model formulation, are solved by the successive over relaxation method. Results show that the microburst can be simulated accurately by impulsive turbulent jet at high altitude and a transient vortex ring in mid-air and near the ground surface. In addition to improved understanding of the physical nature of microbursts, the models presented here can also be used for flight simulation and the pilot training purposes.

  8. PAB3D: Its History in the Use of Turbulence Models in the Simulation of Jet and Nozzle Flows

    NASA Technical Reports Server (NTRS)

    Abdol-Hamid, Khaled S.; Pao, S. Paul; Hunter, Craig A.; Deere, Karen A.; Massey, Steven J.; Elmiligui, Alaa

    2006-01-01

    This is a review paper for PAB3D s history in the implementation of turbulence models for simulating jet and nozzle flows. We describe different turbulence models used in the simulation of subsonic and supersonic jet and nozzle flows. The time-averaged simulations use modified linear or nonlinear two-equation models to account for supersonic flow as well as high temperature mixing. Two multiscale-type turbulence models are used for unsteady flow simulations. These models require modifications to the Reynolds Averaged Navier-Stokes (RANS) equations. The first scheme is a hybrid RANS/LES model utilizing the two-equation (k-epsilon) model with a RANS/LES transition function, dependent on grid spacing and the computed turbulence length scale. The second scheme is a modified version of the partially averaged Navier-Stokes (PANS) formulation. All of these models are implemented in the three-dimensional Navier-Stokes code PAB3D. This paper discusses computational methods, code implementation, computed results for a wide range of nozzle configurations at various operating conditions, and comparisons with available experimental data. Very good agreement is shown between the numerical solutions and available experimental data over a wide range of operating conditions.

  9. 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.

  10. Computations of Complex Three-Dimensional Turbulent Free Jets

    NASA Technical Reports Server (NTRS)

    Wilson, Robert V.; Demuren, Ayodeji O.

    1997-01-01

    Three-dimensional, incompressible turbulent jets with rectangular and elliptical cross-sections are simulated with a finite-difference numerical method. The full Navier- Stokes equations are solved at low Reynolds numbers, whereas at high Reynolds numbers filtered forms of the equations are solved along with a sub-grid scale model to approximate the effects of the unresolved scales. A 2-N storage, third-order Runge-Kutta scheme is used for temporary discretization and a fourth-order compact scheme is used for spatial discretization. Although such methods are widely used in the simulation of compressible flows, the lack of an evolution equation for pressure or density presents particular difficulty in incompressible flows. The pressure-velocity coupling must be established indirectly. It is achieved, in this study, through a Poisson equation which is solved by a compact scheme of the same order of accuracy. The numerical formulation is validated and the dispersion and dissipation errors are documented by the solution of a wide range of benchmark problems. Three-dimensional computations are performed for different inlet conditions which model the naturally developing and forced jets. The experimentally observed phenomenon of axis-switching is captured in the numerical simulation, and it is confirmed through flow visualization that this is based on self-induction of the vorticity field. Statistical quantities such as mean velocity, mean pressure, two-point velocity spatial correlations and Reynolds stresses are presented. Detailed budgets of the mean momentum and Reynolds stresses are presented. Detailed budgets of the mean momentum and Reynolds stress equations are presented to aid in the turbulence modeling of complex jets. Simulations of circular jets are used to quantify the effect of the non-uniform curvature of the non-circular jets.

  11. Oscillations of a Turbulent Jet Incident Upon an Edge

    SciTech Connect

    J.C. Lin; D. Rockwell

    2000-09-19

    For the case of a jet originating from a fully turbulent channel flow and impinging upon a sharp edge, the possible onset and nature of coherent oscillations has remained unexplored. In this investigation, high-image-density particle image velocimetry and surface pressure measurements are employed to determine the instantaneous, whole-field characteristics of the turbulent jet-edge interaction in relation to the loading of the edge. It is demonstrated that even in absence of acoustic resonant or fluid-elastic effects, highly coherent, self-sustained oscillations rapidly emerge above the turbulent background. Two clearly identifiable modes of instability are evident. These modes involve large-scale vortices that are phase-locked to the gross undulations of the jet and its interaction with the edge, and small-scale vortices, which are not phase-locked. Time-resolved imaging of instantaneous vorticity and velocity reveals the form, orientation, and strength of the large-scale concentrations of vorticity approaching the edge in relation to rapid agglomeration of small-scale vorticity concentrations. Such vorticity field-edge interactions exhibit rich complexity, relative to the simplified pattern of vortex-edge interaction traditionally employed for the quasi-laminar edgetone. Furthermore, these interactions yield highly nonlinear surface pressure signatures. The origin of this nonlinearity, involving coexistence of multiple frequency components, is interpreted in terms of large- and small-scale vortices embedded in distributed vorticity layers at the edge. Eruption of the surface boundary layer on the edge due to passage of the large-scale vortex does not occur; rather apparent secondary vorticity concentrations are simply due to distension of the oppositely-signed vorticity layer at the tip of the edge. The ensemble-averaged turbulent statistics of the jet quickly take on an identity that is distinct from the statistics of the turbulent boundary layer in the channel

  12. Control of coherent structure in coaxial swirling turbulent jets

    NASA Astrophysics Data System (ADS)

    Lee, Wonjoong

    The purpose of this investigation is to explore the swirl jet characteristics and the possibility of using artificial means for excitation of shear layers with the application as swirl jet control. For this purpose, a subsonic jet facility and a mechanical excitation device are designed and fabricated for the low speed and plain perturbations. The major system components consist of concentric subsonic nozzles, swirl generators, and the excitation devices with straight lobes. The experiments are carried out at various swirl flow conditions and excitation modes. Three components of mean velocity and turbulence fluctuation measurements are carried out with wave excitation using a stereoscopic particle image velocimetry. The acquired data are presented in cubic plots and two-dimensional contour plots. Furthermore, the numerical analysis is performed to investigate the helical excitation effects on a relatively high speed region. The computed data are presented in two-dimensional contour pictures and the trace plots of particles. Including extracted vorticity, both the experimental and computational results are compared with the baseline at various conditions, and with the values reported in the existing literature. In general, axisymmetric swirling jets are unstable in the near field to all the excitation modes examined. It is shown that the overall response of the swirling jet to excitation is not only dependent on the wave mode number, but also strongly on its sign; meaning the spiral direction of the convex lobes with respect to the swirling jet. This confirms the previous theoretical results. Excitation at both plain and helical perturbations simultaneously affects the flow property distributions in the vortex core and the shear layer at the jet periphery. Especially negative helical wave excitation is considered as the effective way of mixing enhancement for swirling jets compared to the straight lobe perturbation. The preferred mode is the second negative helical

  13. Turbulence Statistics of a Buoyant Jet in a Stratified Environment

    NASA Astrophysics Data System (ADS)

    McCleney, Amy Brooke

    Using non-intrusive optical diagnostics, turbulence statistics for a round, incompressible, buoyant, and vertical jet discharging freely into a stably linear stratified environment is studied and compared to a reference case of a neutrally buoyant jet in a uniform environment. This is part of a validation campaign for computational fluid dynamics (CFD). Buoyancy forces are known to significantly affect the jet evolution in a stratified environment. Despite their ubiquity in numerous natural and man-made flows, available data in these jets are limited, which constrain our understanding of the underlying physical processes. In particular, there is a dearth of velocity field data, which makes it challenging to validate numerical codes, currently used for modeling these important flows. Herein, jet near- and far-field behaviors are obtained with a combination of planar laser induced fluorescence (PLIF) and multi-scale time-resolved particle image velocimetry (TR-PIV) for Reynolds number up to 20,000. Deploying non-intrusive optical diagnostics in a variable density environment is challenging in liquids. The refractive index is strongly affected by the density, which introduces optical aberrations and occlusions that prevent the resolution of the flow. One solution consists of using index matched fluids with different densities. Here a pair of water solutions - isopropanol and NaCl - are identified that satisfy these requirements. In fact, they provide a density difference up to 5%, which is the largest reported for such fluid pairs. Additionally, by design, the kinematic viscosities of the solutions are identical. This greatly simplifies the analysis and subsequent simulations of the data. The spectral and temperature dependence of the solutions are fully characterized. In the near-field, shear layer roll-up is analyzed and characterized as a function of initial velocity profile. In the far-field, turbulence statistics are reported for two different scales, one

  14. Transition to turbulence and noise radiation in heated coaxial jet flows

    NASA Astrophysics Data System (ADS)

    Gloor, Michael; Bühler, Stefan; Kleiser, Leonhard

    2016-04-01

    Laminar-turbulent transition and noise radiation of a parametrized set of subsonic coaxial jet flows with a hot primary (core) stream are investigated numerically by Large-Eddy Simulation (LES) and direct noise computation. This study extends our previous research on local linear stability of heated coaxial jet flows by analyzing the nonlinear evolution of initially laminar flows disturbed by a superposition of small-amplitude unstable eigenmodes. First, a baseline configuration is studied to shed light on the flow dynamics of coaxial jet flows. Subsequently, LESs are performed for a range of Mach and Reynolds numbers to systematically analyze the influences of the temperature and the velocity ratios between the primary and the secondary (bypass) stream. The results provide a basis for a detailed analysis of fundamental flow-acoustic phenomena in the considered heated coaxial jet flows. Increasing the primary-jet temperature leads to an increase of fluctuation levels and to an amplification of far-field noise, especially at low frequencies. Strong mixing between the cold bypass stream and the hot primary stream as well as the intermittent character of the flow field at the end of the potential core lead to a pronounced noise radiation at an aft angle of approximately 35∘. The velocity ratio strongly affects the shear-layer development and therefore also the noise generation mechanisms. Increasing the secondary-stream velocity amplifies the dominance of outer shear-layer perturbations while the disturbance growth rates in the inner shear layer decrease. Already for rmic > 40R1, where rmic is the distance from the end of the potential core and R1 is the core-jet radius, a perfect 1/rmic decay of the sound pressure amplitudes is observed. The potential-core length increases for higher secondary-stream velocities which leads to a shift of the center of the dominant acoustic radiation in the downstream direction.

  15. The effect of boundary-layer turbulence on mixing in heated jets

    NASA Astrophysics Data System (ADS)

    Strykowski, P. J.; Russ, S.

    1992-05-01

    The mixing properties of a heated axisymmetric jet at a density ratio of 0.55 were examined for initially laminar and turbulent separated boundary layers. Initially laminar jets displayed large intermittent spread rates with half-angles up to 45° and a corresponding rapid decay of the streamwise velocity and temperature on the jet axis. When the boundary layer was disturbed upstream of the nozzle exit, creating an initially turbulent separated layer, the jet mixing was significantly reduced. Flow visualization revealed that the turbulent conditions eliminated the intermittent nature of the jet spreading, producing constant spreading rates at half-angles near 10°.

  16. The effect of boundary-layer turbulence on mixing in heated jets

    NASA Astrophysics Data System (ADS)

    Strykowski, P. J.; Russ, S.

    1992-05-01

    The mixing properties of a heated axisymmetric jet at a density ratio of 0.55 were examined for initially laminar and turbulent separated boundary layers. Initially laminar jets displayed large intermittent spread rates with half-angles up to 45 deg and a corresponding rapid decay af the streamwise velocity and temperature on the jet axis. When the boundary layer was disturbed upstream of the nozzle exit, creating an initially turbulent separated layer, the jet mixing was significantly reduced. Flow visualization revealed that the turbulent conditions eliminated the intermittent nature of the jet spreading, producing constant spreading rates at half-angles near 10 deg.

  17. 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.

  18. DNS of a turbulent lifted DME jet flame

    DOE PAGESBeta

    Minamoto, Yuki; Chen, Jacqueline H.

    2016-05-07

    A three-dimensional direct numerical simulation (DNS) of a turbulent lifted dimethyl ether (DME) slot jet flame was performed at elevated pressure to study interactions between chemical reactions with low-temperature heat release (LTHR), negative temperature coefficient (NTC) reactions and shear generated turbulence in a jet in a heated coflow. By conditioning on mixture fraction, local reaction zones and local heat release rate, the turbulent flame is revealed to exhibit a “pentabrachial” structure that was observed for a laminar DME lifted flame [Krisman et al., (2015)]. The propagation characteristics of the stabilization and triple points are also investigated. Potential stabilization points, spatialmore » locations characterized by preferred temperature and mixture fraction conditions, exhibit autoignition characteristics with large reaction rate and negligible molecular diffusion. The actual stabilization point which coincides with the most upstream samples from the pool of potential stabilization points fovr each spanwise location shows passive flame structure with large diffusion. The propagation speed along the stoichiometric surface near the triple point is compared with the asymptotic value obtained from theory [Ruetsch et al., (1995)]. At stoichiometric conditions, the asymptotic and averaged DNS values of flame displacement speed deviate by a factor of 1.7. However, accounting for the effect of low-temperature species on the local flame speed increase, these two values become comparable. In conclusion, this suggests that the two-stage ignition influences the triple point propagation speed through enhancement of the laminar flame speed in a configuration where abundant low-temperature products from the first stage, low-temperature ignition are transported to the lifted flame by the high-velocity jet.« less

  19. Assessment of stretched vortex subgrid-scale models for LES of incompressible inhomogeneous turbulent flow.

    PubMed

    Shetty, Dinesh A; Frankel, Steven H

    2013-09-20

    The physical space version of the stretched vortex subgrid scale model [Phys. Fluids 12, 1810 (2000)] is tested in large eddy simulations (LES) of the turbulent lid driven cubic cavity flow. LES is carried out using a higher order finite-difference method [J. Comput. Phys. 229, 8802 (2010)]. The effects of different vortex orientation models and subgrid turbulence spectrums are assessed through comparisons of the LES predictions against direct numerical simulations (DNS) [Phys. Fluids 12, 1363 (2000)]. Three Reynolds numbers 12000, 18000, and 22000 are studied. Good agreement with the DNS data for the mean and fluctuating quantities is observed. PMID:24187423

  20. Aeroacoustics of Turbulent Jets: Flow Structure, Noise Sources, and Control

    NASA Astrophysics Data System (ADS)

    Gutmark, Ephraim Jeff; Callender, Bryan William; Martens, Steve

    The paper reviews research performed to advance the understanding of state-of-the-art technologies capable of reducing coaxial jet noise simulating the exhaust flow of turbofan engines. The review focuses on an emerging jet noise passive control technology known as chevron nozzles. The fundamental physical mechanisms responsible for the acoustic benefits provided by these nozzles are discussed. Additionally, the relationship between these physical mechanisms and some of the primary chevron geometric parameters are highlighted. Far-field acoustic measurements over a wide range of nozzle operating conditions illustrated the ability of the chevron nozzles to provide acoustic benefits. Detailed mappings of the acoustic near-field provided more insight into the chevron noise suppression mechanisms by successfully identifying two primary chevron effects consistent with the results of the far-field measurements: chevrons penetration and shear velocity across them. Mean and turbulence data identified the physical flow mechanisms responsible for the effects documented in the far- and near-field studies.

  1. Numerical Simulation of Turbulent Jets with Rectangular Cross-Section

    NASA Technical Reports Server (NTRS)

    Wilson, Robert V.; Demuren, Ayodeji O.

    1997-01-01

    Three-dimensional turbulent jets with rectangular cross-section are simulated with a finite-difference numerical method. The full Navier-Stokes equations are solved at low Reynolds numbers, whereas at the high Reynolds numbers filtered forms of the equations are solved along with a sub-grid scale model to approximate effects of the unresolved scales. A 2-N storage, third-order Runge-Kutta scheme is used for temporal discretization and a fourth-order compact scheme is used for spatial discretization. Computations are performed for different inlet conditions which represent different types of jet forcing. The phenomenon of axis-switching is observed, and it is confirmed that this is based on self-induction of the vorticity field. Budgets of the mean streamwise velocity show that convection is balanced by gradients of the Reynolds stresses and the pressure.

  2. Turbulent jet flow in a duct with a circulation zone

    NASA Astrophysics Data System (ADS)

    Glebov, G. A.; Petrov, V. N.

    An approximation method is proposed for calculating flows resulting from the interaction between a turbulent jet and a slipstream inside a duct, including the case where a back stream is formed near the wall. In accordance with the approach proposed here, the velocity profile in the mixing region is determined using the well known method of the polynomial approximation of the Reynolds shear stress profile in the duct cross-sections. The flow parameters are then determined using integral equations of flow rate and momentum. The results obtained using the approximation method are found to be in good agreement with experiment data.

  3. Turbulent jet flow in a channel with a circulation region

    NASA Astrophysics Data System (ADS)

    Glebov, G. A.; Petrov, V. N.

    1985-01-01

    An approximation method is proposed for calculating flows resulting from the interaction between a turbulent jet and a slipstream inside a duct, including the case where a back stream is formed near the wall. In accordance with the approach proposed here, the velocity profile in the mixing region is determined using the well known method of the polynomial approximation of the Reynolds shear stress profile in the duct cross-sections. The flow parameters are then determined using integral equations of flow rate and momentum. The results obtained using the approximation method are found to be in good agreement with experiment data.

  4. Numerical simulation of turbulent jet noise, part 1

    NASA Technical Reports Server (NTRS)

    Metcalfe, R. W.; Orszag, S. A.

    1975-01-01

    Flow characteristics, such as quadrupole moments are examined in order to study generation of aerodynamic noise. The mean flow quantities are set in accordance with experimental data and the incompressible Navier-Stokes are solved numerically. Isolated downstream sections of a turbulent jet are modelled separately with the mean flow characteristics held constant in time. The flows are allowed to evolve until the fluctuating velocity components reach a statistically steady state. Cross section contour plots of the velocity components and the quadrupole moments at three different downstream positions are presented.

  5. Computational aeroacoustics of turbulent high-speed jets

    NASA Astrophysics Data System (ADS)

    Nichols, Joseph W.

    2014-11-01

    Despite significant scientific investigation, jet noise remains a large component of the overall noise generated by supersonic aircraft. Experiments show that alterations to nozzle geometry, such as the addition of chevrons to the nozzle lip, can significantly reduce jet noise. In this talk, we assess unstructured large eddy simulation as a tool for predicting and understanding the aeroacoustic effects of complex geometry upon supersonic jets. Body-fitted, adaptive meshes are used to simulate the flow inside, around and through complicated nozzles, and results are validated against experimental measurements. High-fidelity simulations utilizing as many as one million processors simultaneously will be discussed, allowing for a detailed description of interactions between turbulence, shocks, and acoustics. This includes observations of the phenomenon of ``crackle'' noise in heated supersonic jets. We will briefly discuss challenges met and overcome along this frontier of com putational science, and describe how information extracted from the high-fidelity simulations can be used to construct accurate reduced-order models useful for aeroacoustic design. Computational resources were provided by the Argonne Leadership Computing Facility at Argonne National Laboratory and the ERDC and AFRL supercomputing centers.

  6. Experiments with Turbulent Jets at Mach Number 0.9

    NASA Technical Reports Server (NTRS)

    Agui, Juan; Andreopoulos, Yiannis; Davis, David O. (Technical Monitor)

    2001-01-01

    A systematic investigation of the structure of turbulent jets before their interaction with shock or expansion waves was undertaken during the last year. In particular compressibility and density effects in circular jets issuing in still air were investigated experimentally. Jets with nitrogen, helium, and krypton gases at 0.3, 0.6, and 0.9 Mach numbers were investigated in detail. Particle Image Velocimetry technique was developed, tested, and used to obtain qualitative information of the two-dimensional velocity field on a plane inside the flow field, which was illuminated by a laser sheet. The motion of particles was recorded by a CCD camera, which was appropriately triggered to capture two images within a fraction of a microsecond. Statistical averaging of the data at each location reduced the large amount of acquired data. It was found that the spreading rate of the jets was reduced with increased Mach numbers or increased density ratio. It was also found that decay rates of centerline Mach numbers are higher in gases with reduced density ratio. Mach number fluctuations appear to decrease with increasing Mach number of the flow. It has been proposed that the reason for this behavior is the reduction of vortex stretching activities with increased Mach number.

  7. Turbulent acidic jets and plumes injected into an alkaline environment

    NASA Astrophysics Data System (ADS)

    Ulpre, Hendrik

    2012-11-01

    The characteristics of a strong acidic turbulent jet or plume injected into an alkaline environment comprising of a weak/strong base are examined theoretically and experimentally. A chemistry model is developed to understand how the pH of a fluid parcel of monoprotic acid changes as it is diluted and reacts with the ambient fluid. A standard fluid model, based on a top-hat model for acid concentration and velocity is used to express how the dilution of acid varies with distance from the point of discharge. These models are applied to estimate the point of neutralisation and the travel time with distance within the jet/plume. An experimental study was undertaken to test the theoretical results. These experiments involved injecting jets or vertical plumes of dilute nitric acid into a large tank containing a variety of base salts dissolved in water. The injected fluid contained litmus indicator dye which showed a change in colour from red to blue close to the point of neutralisation. In order to obtain a range of neutralisation distances, additional basic salts were added to the water to increase its pH buffering capacity. The results are applied to discuss the environmental implications of an acidic jet/plume injected into the sea off the South East coast of Great Britain.

  8. The radiated noise from isotropic turbulence and heated jets

    NASA Technical Reports Server (NTRS)

    Lilley, G. M.

    1995-01-01

    Our understanding of aerodynamic noise has its foundations in the work of Sir James Lighthill (1952), which was the first major advance in acoustics since the pioneering work of Lord Rayleigh in the last century. The combination of Lighthill's theory of aerodynamic noise as applied to turbulent flows and the experimental growing database from the early 1950's was quickly exploited by various jet propulsion engine designers in reducing the noise of jet engines at takeoff and landing to levels marginally acceptable to communities living in the neighborhoods of airports. The success in this noise containment led to the rapid growth of fast economical subsonic civil transport aircraft worldwide throughout the 1960's and has continued to the present day. One important factor in this success story has been the improvements in the engine cycle that have led to both reductions in specific fuel consumption and noise. The second is the introduction of Noise Certification, which specifies the maximum noise levels at takeoff and landing that all aircraft must meet before they can be entered on the Civil Aircraft Register. The growing interest in the development of a new supersonic civil transport to replace 'Concorde' in the early years of the next century has led to a resurgence of interest in the more challenging problem of predicting the noise of hot supersonic jets and developing means of aircraft noise reduction at takeoff and landing to meet the standards now accepted for subsonic Noise Certification. The prediction of aircraft noise to the accuracy required to meet Noise Certification requirements has necessitated reliance upon experimental measurements and empirically derived laws based on the available experimental data bases. These laws have their foundation in the results from Lighthill's theory, but in the case of jet noise, where the noise is generated in the turbulent mixing region with the external ambient fluid, the complexity of the turbulent motion has

  9. Manipulation of Turbulent Boundary Layers Using Synthetic Jets

    NASA Astrophysics Data System (ADS)

    Berger, Zachary; Gomit, Guillaume; Lavoie, Philippe; Ganapathisubramani, Bharath

    2015-11-01

    This work focuses on the application of active flow control, in the form of synthetic jet actuators, of turbulent boundary layers. An array of 2 synthetic jets are oriented in the spanwise direction and located approximately 2.7 meters downstream from the leading edge of a flat plate. Actuation is applied perpendicular to the surface of the flat plate with varying blowing ratios and reduced frequencies (open-loop). Two-component large window particle image velocimetry (PIV) was performed at the University of Southampton, in the streamwise-wall-normal plane. Complementary stereo PIV measurements were performed at the University of Toronto Institute for Aerospace Studies (UTIAS), in the spanwise-wall-normal plane. The freestream Reynolds number is 3x104, based on the boundary layer thickness. The skin friction Reynolds number is 1,200 based on the skin friction velocity. The experiments at Southampton allow for the observation of the control effects as the flow propagates downstream. The experiments at UTIAS allow for the observation of the streamwise vorticity induced from the actuation. Overall the two experiments provide a 3D representation of the flow field with respect to actuation effects. The current work focuses on the comparison of the two experiments, as well as the effects of varying blowing ratios and reduced frequencies on the turbulent boundary layer. Funded Supported by Airbus.

  10. An experimental study of turbulent flow in attachment jet combustors by LDV

    NASA Astrophysics Data System (ADS)

    Li, Jun; Wu, Cheng-Kang

    1993-12-01

    Flame stabilization in attachment jet combustors is based on the existence of the high temperature recirculation zone, provided by the Coanda effect of an attachment jet. The single attachment jet in a rectangular channel is a fundamental form of this type of flow. In this paper, the detailed characteristics of turbulent flow of a single attachment jet were experimentally studied by using a 2-D LDV. The flowfield consists of a forward flow and two reverse flows. The forward one is composed of a curved and a straight section. The curved section resembles a bent turbulent free jet, and the straight part is basically a section of turbulent wall jet. A turbulent counter-gradient transport region exists at the curved section. According to the results, this kind of combustor should have a large sudden enlargement ratio and not too narrow in width.

  11. Computation of confined coflow jets with three turbulence models

    NASA Technical Reports Server (NTRS)

    Zhu, J.; Shih, T. H.

    1993-01-01

    A numerical study of confined jets in a cylindrical duct is carried out to examine the performance of two recently proposed turbulence models: an RNG-based K-epsilon model and a realizable Reynolds stress algebraic equation model. The former is of the same form as the standard K-epsilon model but has different model coefficients. The latter uses an explicit quadratic stress-strain relationship to model the turbulent stresses and is capable of ensuring the positivity of each turbulent normal stress. The flow considered involves recirculation with unfixed separation and reattachment points and severe adverse pressure gradients, thereby providing a valuable test of the predictive capability of the models for complex flows. Calculations are performed with a finite-volume procedure. Numerical credibility of the solutions is ensured by using second-order accurate differencing schemes and sufficiently fine grids. Calculations with the standard K-epsilon model are also made for comparison. Detailed comparisons with experiments show that the realizable Reynolds stress algebraic equation model consistently works better than does the standard K-epsilon model in capturing the essential flow features, while the RNG-based K-epsilon model does not seem to give improvements over the standard K-epsilon model under the flow conditions considered.

  12. Turbulence in a microscale planar confined impinging-jets reactor.

    PubMed

    Liu, Ying; Olsen, Michael G; Fox, Rodney O

    2009-04-21

    Confined impinging-jets reactors (CIJR) offer many advantages for rapid chemical processing at the microscale in applications such as precipitation and the production of organic nanoparticles. It has been demonstrated that computational fluid dynamics (CFD) is a promising tool for "experiment-free" design and scale-up of such reactors. However, validation of the CFD model used for the microscale turbulence applications requires detailed experimental data on the unsteady flow, the availability of which has until now been very limited. In this work, microscopic particle-image velocimetry (microPIV) techniques were employed to measure the instantaneous velocity field for various Reynolds numbers in a planar CIJR. In order to illustrate the validation procedure, the performance of a particular CFD model, the two-layer k-epsilon model, was evaluated by comparing the predicted flow field with the experimental data. To our knowledge, this study represents the first attempt to directly measure and quantify velocity and turbulence in a microreactor and to use the results to validate a CFD model for microscale turbulent flows. PMID:19350093

  13. Computation of confined coflow jets with three turbulence models

    NASA Astrophysics Data System (ADS)

    Zhu, J.; Shih, T. H.

    1993-07-01

    A numerical study of confined jets in a cylindrical duct is carried out to examine the performance of two recently proposed turbulence models: an RNG-based K-epsilon model and a realizable Reynolds stress algebraic equation model. The former is of the same form as the standard K-epsilon model but has different model coefficients. The latter uses an explicit quadratic stress-strain relationship to model the turbulent stresses and is capable of ensuring the positivity of each turbulent normal stress. The flow considered involves recirculation with unfixed separation and reattachment points and severe adverse pressure gradients, thereby providing a valuable test of the predictive capability of the models for complex flows. Calculations are performed with a finite-volume procedure. Numerical credibility of the solutions is ensured by using second-order accurate differencing schemes and sufficiently fine grids. Calculations with the standard K-epsilon model are also made for comparison. Detailed comparisons with experiments show that the realizable Reynolds stress algebraic equation model consistently works better than does the standard K-epsilon model in capturing the essential flow features, while the RNG-based K-epsilon model does not seem to give improvements over the standard K-epsilon model under the flow conditions considered.

  14. The Formation of Turbulent Vortex Rings by Synthetic Jets

    NASA Astrophysics Data System (ADS)

    Lawson, John; Dawson, James

    2013-11-01

    Vortex rings formed by synthetic jets are found in many engineering and biological flows. For vortex rings formed both periodically and in isolation, a constraint on vortex formation (``pinch-off'') has been observed which is relevant to unsteady propulsion. However, there is no clear consensus on the physical mechanism of this constraint. We present analysis of time resolved, 2D Particle Image Velocimetry measurements of the velocity and material acceleration field in an axisymmetric, turbulent synthetic jet in air at maximum stroke ratios Lm / D = 2 - 15 . Using the acceleration field, pinch-off may be identified in a manner which is frame invariant and consistent with previous studies. An adverse pressure gradient behind the ring and induced by it plays a role in the pinch-off and separation of the ring from the jet. Recognising this, we revise an existing model for pinch-off: this revision fits our data well. Additionally, we show that as the ring forms, hydrodynamic impulse is delivered via two equally important mechanisms: a material flux and a vortex force. For large Lm / D , this vortex force may deliver a substantial impulse to the ring after pinch-off. This has implications for unsteady propulsion, models of vortex ring formation and existing explanations for pinch-off.

  15. Analysis of the injection of a heated turbulent jet into a cross flow

    NASA Technical Reports Server (NTRS)

    Campbell, J. F.; Schetz, J. A.

    1973-01-01

    The development of a theoretical model is investigated of the incompressible jet injection process. The discharge of a turbulent jet into a cross flow was mathematically modeled by using an integral method which accounts for natural fluid mechanisms such as turbulence, entrainment, buoyancy, and heat transfer. The analytical results are supported by experimental data and demonstrate the usefulness of the theory for estimating the trajectory and flow properties of the jet for a variety of injection conditions. The capability of predicting jet flow properties, as well as two- and three-dimensional jet paths, was enhanced by obtaining the jet cross-sectional area during the solution of the conservation equations. Realistic estimates of temperature in the jet fluid were acquired by accounting for heat losses in the jet flow due to forced convection and to entrainment of free-stream fluid into the jet.

  16. Large eddy simulation of a lifted turbulent jet flame

    SciTech Connect

    Ferraris, S.A.; Wen, J.X.

    2007-09-15

    The flame index concept for large eddy simulation developed by Domingo et al. [P. Domingo, L. Vervisch, K. Bray, Combust. Theory Modell. 6 (2002) 529-551] is used to capture the partially premixed structure at the leading point and the dual combustion regimes further downstream on a turbulent lifted flame, which is composed of premixed and nonpremixed flame elements each separately described under a flamelet assumption. Predictions for the lifted methane/air jet flame experimentally tested by Mansour [M.S. Mansour, Combust. Flame 133 (2003) 263-274] are made. The simulation covers a wide domain from the jet exit to the far flow field. Good agreement with the data for the lift-off height and the mean mixture fraction has been achieved. The model has also captured the double flames, showing a configuration similar to that of the experiment which involves a rich premixed branch at the jet center and a diffusion branch in the outer region which meet at the so-called triple point at the flame base. This basic structure is contorted by eddies coming from the jet exit but remains stable at the lift-off height. No lean premixed branches are observed in the simulation or and experiment. Further analysis on the stabilization mechanism was conducted. A distinction between the leading point (the most upstream point of the flame) and the stabilization point was made. The later was identified as the position with the maximum premixed heat release. This is in line with the stabilization mechanism proposed by Upatnieks et al. [A. Upatnieks, J. Driscoll, C. Rasmussen, S. Ceccio, Combust. Flame 138 (2004) 259-272]. (author)

  17. Effect of noncircular orifice plates on the near flow field of turbulent free jets

    NASA Astrophysics Data System (ADS)

    Xu, Min-Yi; Tong, Xing-Qing; Yue, Dan-Ting; Zhang, Jian-Peng; Mi, Jian-Chun; Nathan, G. J.; Kalt, P. A. M.

    2014-12-01

    In this paper, we experimentally investigate the near-field flow characteristics of turbulent free jets respectively issued from circular, triangular, diamond, rectangular, and notched-rectangular orifice plates into air surroundings. All the orifice plates have identical opening areas or equivalent diameters (De) and their aspect ratios (AR) range from 1 to 6.5. Planar particle image velocimetry (PIV) is used to measure the velocity field at the same Reynolds number of Re = 5 × 104, where Re = UeDe/v with Ue being the exit bulk velocity and v the kinematic viscosity of fluid. The mean and turbulent velocity fields of all the five jets are compared in detail. Results show that the noncircular jets can enhance the entrainment rate, reflected by the higher acceleration rates of mean velocity decay and spread, shorten the length of the unmixed core, expedite the increase of turbulent intensity compared with the circular counterpart shortened unmixed core, and increase turbulent intensity comparing to the circular counterpart. Among the five jets, the rectangular jet (AR = 6.5) produces the greatest decay rate of the near-field mean velocity, postpones the position at which the `axis-switching' phenomenon occurs. This supports that axis switching phenomenon strongly depends on jet initial conditions. In addition, the hump in the centerline variation of the turbulence intensity is observed in the rectangular and triangular jets, but not in the circular jet, nor in diamond jet nor in notched-rectangular jet.

  18. A study of sound generation from turbulent heated round jets using three-dimensional large eddy simulation

    NASA Astrophysics Data System (ADS)

    Lew, Phoi-Tack

    Improvements in computing speed over the past decade have made Large Eddy Simulations (LES) amenable to the study of jet noise. The study of turbulent hot jets is required jets since all jet engines fitted on aircraft operate at hot exhaust conditions. The primary goal of this research was to further advance the science of jet noise prediction with a specific emphasis on heated jets using 3-D LES. For the 3-D LES methodology, spatial filtering is used as an implicit subgrid scale (SGS) model in place of an explicit SGS model, such as the classical Smagorinsky or Dynamic Smagorinsky models. To study the far-field noise, the porous FfowcsWilliams-Hawkings (FWH) surface integral acoustic formulation is employed. Results obtained for the heated jets in terms of jet development are in good agreement with other LES results and experimental data. The predicted overall sound pressure level (OASPL) values for heated jets exhibited the same trend as experimental data. The levels were over-predicted by approximately 3 dB, which was deemed satisfactory. An investigation of noise sources for heated jets was also performed within the framework of Lighthill's acoustic analogy. It is discovered that when a high-speed is jet heated, significant cancellations occur between shear and entropy noise sources compared to an unheated high speed jet. This could explain why a high speed heated jet is quieter than an unheated jet at the same ambient Mach number. High-order compact finite difference schemes along with high-order filters are used extensively in LES, especially for aeroacoustics problems, since these schemes have very high accuracy and spectral-like resolution as well as low-dispersion and diffusion errors. Due to the implicit nature of compact schemes, one technique of parallelization is based on the data transposition strategy. However, such transposition strategy is near impossible to apply to jets with complex geometries. Hence, an alternative parallelization methodology

  19. Mean velocity, turbulence intensity, and scale in a subsonic turbulent jet impinging normal to a large flat plate

    NASA Technical Reports Server (NTRS)

    Boldman, D. R.; Brinich, P. F.

    1977-01-01

    To explain the increase in noise when a jet impinges on a large flat plate, mean velocity, turbulence intensity, and scale were measured at nominal nozzle-exit velocities of 61, 138, and 192 meters per second with the plate located 7.1 nozzle-exit diameters from the nozzle. The maximum turbulence intensities in free and impinging jets were about the same; however, the integral length scale near the plate surface was only about one-half the free jet scale. The measured intensities and length scales, in conjunction with a contemporary theory of aerodynamic noise, provided a good explanation for the observed increase in noise associated with the impinging jet. An increase in the volume of highly turbulent flow could be the principal reason for the increase in noise.

  20. Protostellar jets and magnetised turbulence with smoothed particle magnetohydrodynamics

    NASA Astrophysics Data System (ADS)

    Tricco, Terrence

    2016-01-01

    Magnetic fields are an integral component of the formation of stars. During my thesis work, I built new methods to model magnetic fields in smoothed particle magnetohydrodynamics which enforce the divergence-free constraint on the magnetic field and reduce numerical dissipation of the magnetic field. Using these methods, we have performed simulations of isolated protostar formation, studying the production of jets and outflows of material and their effect on transporting angular momentum away from the protostar and reducing the efficiency of star formation. A major code comparison project on the small-scale turbulent dynamo amplification of magnetic fields was performed, using conditions representative of molecular clouds, the formation site of stars. The results were compared against results from grid-based methods, finding excellent agreement on their statistics and qualitative behaviour. I will outline the numerical methods developed, and present the results from our protostar and molecular cloud simulations.

  1. LES, DNS and RANS for the analysis of high-speed turbulent reacting flows

    NASA Technical Reports Server (NTRS)

    Givi, Peyman

    1994-01-01

    The objective of this research is to continue our efforts in advancing the state of knowledge in Large Eddy Simulation (LES), Direct Numerical Simulation (DNS), and Reynolds Averaged Navier Stokes (RANS) methods for the analysis of high-speed reacting turbulent flows. In the first phase of this research, conducted within the past six months, focus was in three directions: RANS of turbulent reacting flows by Probability Density Function (PDF) methods, RANS of non-reacting turbulent flows by advanced turbulence closures, and LES of mixing dominated reacting flows by a dynamics subgrid closure. A summary of our efforts within the past six months of this research is provided in this semi-annual progress report.

  2. Large-eddy simulations of a turbulent Coanda jet on a circulation control airfoil

    NASA Astrophysics Data System (ADS)

    Nishino, Takafumi; Hahn, Seonghyeon; Shariff, Karim

    2010-12-01

    Large-eddy simulations are performed of a turbulent Coanda jet separating from a rounded trailing edge of a simplified circulation control airfoil model. The freestream Reynolds number based on the airfoil chord is 0.49×106, the jet Reynolds number based on the jet slot height is 4470, and the ratio of the peak jet velocity to the freestream velocity is 3.96. Three different grid resolutions are used to show that their effect is very small on the mean surface pressure distribution, which agrees very well with experiments, as well as on the mean velocity profiles over the Coanda surface. It is observed that the Coanda jet becomes fully turbulent just downstream of the jet exit, accompanied by asymmetric alternating vortex shedding behind a thin (but blunt) jet blade splitting the jet and the external flow. A number of "backward-tilted" hairpin vortices (i.e., the head of each hairpin being located upstream of the legs) are observed around the outer edge of the jet over the Coanda surface. These hairpins create strong upwash between the legs and weak downwash around them, contributing to turbulent mixing of the high-momentum jet below the hairpins and the low-momentum external flow above them. The probability density distribution of velocity fluctuations is shown to be highly asymmetric in this region, consistent with the observation that the hairpin vortices create strong upwash and weak downwash. Turbulent structures inside the jet, its spreading rate, and self-similarity are also discussed.

  3. Numerical modeling of normal turbulent plane jet impingement on solid wall

    NASA Astrophysics Data System (ADS)

    Guo, C.-Y.; Maxwell, W. H. C.

    1984-10-01

    Attention is given to a numerical turbulence model for the impingement of a well developed normal plane jet on a solid wall, by means of which it is possible to express different jet impingement geometries in terms of different boundary conditions. Examples of these jets include those issuing from VTOL aircraft, chemical combustors, etc. The two-equation, turbulent kinetic energy-turbulent dissipation rate model is combined with the continuity equation and the transport equation of vorticity, using an iterative finite difference technique in the computations. Peak levels of turbulent kinetic energy occur not only in the impingement zone, but also in the intermingling zone between the edges of the free jet and the wall jet.

  4. Numerical modeling of normal turbulent plane jet impingement on solid wall

    SciTech Connect

    Guo, C.Y.; Maxwell, W.H.C.

    1984-10-01

    Attention is given to a numerical turbulence model for the impingement of a well developed normal plane jet on a solid wall, by means of which it is possible to express different jet impingement geometries in terms of different boundary conditions. Examples of these jets include those issuing from VTOL aircraft, chemical combustors, etc. The two-equation, turbulent kinetic energy-turbulent dissipation rate model is combined with the continuity equation and the transport equation of vorticity, using an iterative finite difference technique in the computations. Peak levels of turbulent kinetic energy occur not only in the impingement zone, but also in the intermingling zone between the edges of the free jet and the wall jet. 20 references.

  5. Emission of sound from axisymmetric turbulence convected by a mean flow with application to jet noise

    NASA Technical Reports Server (NTRS)

    Goldstein, M. E.; Rosenbaum, B. M.

    1972-01-01

    A model, based on Lighthill's theory, for predicting aerodynamic noise from a turbulent shear flow is developed. This model is a generalization of the one developed by Ribner. Unlike Ribner's model, it does not require that the turbulent correlations factor into space and time-dependent parts. It replaces his assumption of isotropic. turbulence by the more realistic one of axisymmetric turbulence. The implications of the model for jet noise are discussed.

  6. The Prediction of Noise Due to Jet Turbulence Convecting Past Flight Vehicle Trailing Edges

    NASA Technical Reports Server (NTRS)

    Miller, Steven A. E.

    2014-01-01

    High intensity acoustic radiation occurs when turbulence convects past airframe trailing edges. A mathematical model is developed to predict this acoustic radiation. The model is dependent on the local flow and turbulent statistics above the trailing edge of the flight vehicle airframe. These quantities are dependent on the jet and flight vehicle Mach numbers and jet temperature. A term in the model approximates the turbulent statistics of single-stream heated jet flows and is developed based upon measurement. The developed model is valid for a wide range of jet Mach numbers, jet temperature ratios, and flight vehicle Mach numbers. The model predicts traditional trailing edge noise if the jet is not interacting with the airframe. Predictions of mean-flow quantities and the cross-spectrum of static pressure near the airframe trailing edge are compared with measurement. Finally, predictions of acoustic intensity are compared with measurement and the model is shown to accurately capture the phenomenon.

  7. ANALYSIS OF TURBULENT MIXING JETS IN LARGE SCALE TANK

    SciTech Connect

    Lee, S; Richard Dimenna, R; Robert Leishear, R; David Stefanko, D

    2007-03-28

    Flow evolution models were developed to evaluate the performance of the new advanced design mixer pump for sludge mixing and removal operations with high-velocity liquid jets in one of the large-scale Savannah River Site waste tanks, Tank 18. This paper describes the computational model, the flow measurements used to provide validation data in the region far from the jet nozzle, the extension of the computational results to real tank conditions through the use of existing sludge suspension data, and finally, the sludge removal results from actual Tank 18 operations. A computational fluid dynamics approach was used to simulate the sludge removal operations. The models employed a three-dimensional representation of the tank with a two-equation turbulence model. Both the computational approach and the models were validated with onsite test data reported here and literature data. The model was then extended to actual conditions in Tank 18 through a velocity criterion to predict the ability of the new pump design to suspend settled sludge. A qualitative comparison with sludge removal operations in Tank 18 showed a reasonably good comparison with final results subject to significant uncertainties in actual sludge properties.

  8. Entrainment of Vertical Jets in Turbulent Cross Flow

    NASA Astrophysics Data System (ADS)

    Freedland, Graham; Roberts, Karen; Mastin, Larry; Solovitz, Stephen; Cal, Raul

    2015-11-01

    Volcanic eruptions produce high concentrations of ash that produce clouds in the atmosphere that are hazardous for private and commercial aviation. Without accurate models to predict ash concentrations, air traffic is unable to safely navigate ash clouds downwind of an eruption as critical concentrations are difficult to identify visually. Current models rely on inputs such as plume height, eruptive dissipation and cross-flow wind speeds as well as empirical parameters such as the entrainment ratio between the cross-flow and the plume velocity. A wind tunnel experiment has been designed to investigate these models by injecting air orthogonally into a cross-flow. The ratio of the cross-flow and jet velocities is varied to simulate a weak plume and flow response is measured using particle image velocimetry. Grids upstream of the plume create different turbulence intensities, which, combined with different jet geometries, allow us to study the flow field, mean and second order moments and thereby obtain information to accurately model volcanic ash concentrations in the atmosphere.

  9. Effect of liquid droplets on turbulence structure in a round gaseous jet

    NASA Technical Reports Server (NTRS)

    Mostafa, A. M.; Elghobashi, S. E.

    1984-01-01

    A proposed two equation turbulence model for incompressible dilute two phase flows was validated and extended for steady incompressible two phase flow including phase change. The model was tested for the flow of a turbulent axisymmetric gaseous jet laden with multisize evaporating liquid droplets. Predicted results include distributions of the mean velocity; volume fractions of different phases concentration of the evaporated material in the carrier phase; turbulence intensity and shear stress of the carrier phase; droplet diameter distribution; and the jet spreading rate. Results are analyzed based on a qualitative comparison with the corresponding single phase jet flow.

  10. Turbulent Deflagrated Flame Interaction with a Fluidic Jet Flow for Deflagration-to-Detonation Flame Acceleration

    NASA Astrophysics Data System (ADS)

    Chambers, Jessica; McGarry, Joseph; Ahmed, Kareem

    2015-11-01

    Detonation is a high energetic mode of pressure gain combustion. Detonation combustion exploits the pressure rise to augment high flow momentum and thermodynamic cycle efficiencies. The driving mechanism of deflagrated flame acceleration to detonation is turbulence generation and induction. A fluidic jet is an innovative method for the production of turbulence intensities and flame acceleration. Compared to traditional obstacles, the jet reduces the pressure losses and heat soak effects while providing turbulence generation control. The investigation characterizes the turbulent flame-flow interactions. The focus of the study is on classifying the turbulent flame dynamics and the temporal evolution of turbulent flame regime. The turbulent flame-flow interactions are experimentally studied using a LEGO Detonation facility. Advanced high-speed laser diagnostics, particle image velocimetry (PIV), planar laser induced florescence (PLIF), and Schlieren imaging are used in analyzing the physics of the interaction and flame acceleration. Higher turbulence induction is observed within the turbulent flame after contact with the jet, leading to increased flame burning rates. The interaction with the fluidic jet results in turbulent flame transition from the thin reaction zones to the broken reaction regime.

  11. Fluorescence imaging study of free and impinging supersonic jets: Jet structure and turbulent transition

    NASA Astrophysics Data System (ADS)

    Inman, Jennifer Ann

    A series of experiments into the behavior of underexpanded jet flows has been conducted at NASA Langley Research Center. This work was conducted in support of the Return to Flight effort following the loss of the Columbia. The tests involved simulating flow through a hypothetical breach in the leading edge of the Space Shuttle Orbiter along its reentry trajectory, with the goal of generating a data set with which other researchers can test and validate computational modeling tools. Two nozzles supplied with high-pressure gas were used to generate axisymmetric underexpanded jets exhausting into a low-pressure chamber. These nozzles had exit Mach numbers of 1 and 2.6. Reynolds numbers based on nozzle exit conditions ranged from about 200 to 35,000, and nozzle exit-to-ambient jet pressure ratios ranged from about 1 to 37. Both free and impinging jets were studied, with impingement distances ranging from 10 to 40 nozzle diameters, and impingement angles of 45°, 60°, and 90°. For the majority of cases, the jet fluid was a mixture of 99.5% nitrogen seeded with 0.5% nitric oxide (NO). Planar laser-induced fluorescence (PLIF) of NO was used to non-intrusively visualize the flow with a temporal resolution on the order of lets. PLIF images were used to identify and measure the location and size of flow structures. PLIF images were further used to identify unsteady jet behavior in order to quantify the conditions governing the transition to turbulent flow. This dissertation will explain the motivation behind the work, provide details of the laser system and test hardware components, discuss the theoretical aspects of laser-induced fluorescence, give an overview of the spectroscopy of nitric oxide, and summarize the governing fluid mechanical concepts. It will present measurements of the size and location of flow structures, describe the basic mechanisms and origins of unsteady behavior in these flows, and discuss the dependence of such behavior on particular flow

  12. Development of a Hybrid RANS/LES Method for Turbulent Mixing Layers

    NASA Technical Reports Server (NTRS)

    Georgiadis, Nicholas J.; Alexander, J. Iwan D.; Reshotko, Eli

    2001-01-01

    Significant research has been underway for several years in NASA Glenn Research Center's nozzle branch to develop advanced computational methods for simulating turbulent flows in exhaust nozzles. The primary efforts of this research have concentrated on improving our ability to calculate the turbulent mixing layers that dominate flows both in the exhaust systems of modern-day aircraft and in those of hypersonic vehicles under development. As part of these efforts, a hybrid numerical method was recently developed to simulate such turbulent mixing layers. The method developed here is intended for configurations in which a dominant structural feature provides an unsteady mechanism to drive the turbulent development in the mixing layer. Interest in Large Eddy Simulation (LES) methods have increased in recent years, but applying an LES method to calculate the wide range of turbulent scales from small eddies in the wall-bounded regions to large eddies in the mixing region is not yet possible with current computers. As a result, the hybrid method developed here uses a Reynolds-averaged Navier-Stokes (RANS) procedure to calculate wall-bounded regions entering a mixing section and uses a LES procedure to calculate the mixing-dominated regions. A numerical technique was developed to enable the use of the hybrid RANS-LES method on stretched, non-Cartesian grids. With this technique, closure for the RANS equations is obtained by using the Cebeci-Smith algebraic turbulence model in conjunction with the wall-function approach of Ota and Goldberg. The LES equations are closed using the Smagorinsky subgrid scale model. Although the function of the Cebeci-Smith model to replace all of the turbulent stresses is quite different from that of the Smagorinsky subgrid model, which only replaces the small subgrid turbulent stresses, both are eddy viscosity models and both are derived at least in part from mixing-length theory. The similar formulation of these two models enables the RANS

  13. Interaction of a turbulent-jet noise source with transverse modes in a rectangular duct

    NASA Technical Reports Server (NTRS)

    Succi, G. P.; Baumeister, K. J.; Ingard, K. U.

    1978-01-01

    A turbulent jet was used to excite transverse acoustic modes in a rectangular duct. The pressure spectrum showed asymmetric singularities (pressure spikes) at the resonant frequencies of the duct modes. This validates previously published theoretical results. These pressure spikes occurred over a range of jet velocities, orientations, and inlet turbulence levels. At the frequency of the spike, the measured transverse pressure shape matched the resonant mode shape.

  14. Numerical investigation of heat transfer under confined impinging turbulent slot jets

    SciTech Connect

    Tzeng, P.Y.; Soong, C.Y.; Hsieh, C.D. )

    1999-06-01

    Impinging jet systems are extensively used to provide rapid heating, cooling, or drying in diverse industrial applications. Among these applications are the annealing of metals and plastic sheets; tempering and shaping of glass; drying of textiles, veneer, paper, and film materials; and cooling of combustion walls, turbine blades, and electronic components. This work numerically investigates confined impinging turbulent slot jets. Eight turbulence models, including one standard and seven low-Reynolds-number [kappa]-[epsilon] models, are employed and tested to predict the heat transfer performance of multiple impinging jets. Validation results indicate that the prediction by each turbulence model depends on grid distribution and numerical scheme used in spatial discretization. In addition, spent fluid exits are set between impinging jets to reduce the cross-flow effect in degradation of the heat transfer of downstream impinging jets. The overall heat transfer performance can be enhanced by proper spent fluid removal.

  15. Laminar and turbulent nozzle-jet flows and their acoustic near-field

    SciTech Connect

    Bühler, Stefan; Obrist, Dominik; Kleiser, Leonhard

    2014-08-15

    We investigate numerically the effects of nozzle-exit flow conditions on the jet-flow development and the near-field sound at a diameter-based Reynolds number of Re{sub D} = 18 100 and Mach number Ma = 0.9. Our computational setup features the inclusion of a cylindrical nozzle which allows to establish a physical nozzle-exit flow and therefore well-defined initial jet-flow conditions. Within the nozzle, the flow is modeled by a potential flow core and a laminar, transitional, or developing turbulent boundary layer. The goal is to document and to compare the effects of the different jet inflows on the jet flow development and the sound radiation. For laminar and transitional boundary layers, transition to turbulence in the jet shear layer is governed by the development of Kelvin-Helmholtz instabilities. With the turbulent nozzle boundary layer, the jet flow development is characterized by a rapid changeover to a turbulent free shear layer within about one nozzle diameter. Sound pressure levels are strongly enhanced for laminar and transitional exit conditions compared to the turbulent case. However, a frequency and frequency-wavenumber analysis of the near-field pressure indicates that the dominant sound radiation characteristics remain largely unaffected. By applying a recently developed scaling procedure, we obtain a close match of the scaled near-field sound spectra for all nozzle-exit turbulence levels and also a reasonable agreement with experimental far-field data.

  16. Vorticity, turbulence production, and turbulence induced accelerations in a rectangular jet as measured using 3-D LDA

    NASA Technical Reports Server (NTRS)

    Morrison, Gerald L.; Swan, David H.

    1990-01-01

    The flow field of a rectangular jet with a 4:1 aspect ratio (50.4 x 12.7 mm) was studied at a Reynolds number of 100,000 (Mach number 0.09) using a 3-D laser Doppler anemometer system. Measurements were performed along the major and minor axis planes and at various downstream cross-sections of the jet. The mean velocity vector and entire Reynolds stress tensor were measured and presented in a previous publication. The present work presents the vorticity vector, turbulence production, and turbulence induced acceleration vector distributions which were calculated from the previously presented data.

  17. Structure of three-dimensional turbulent offset jets with small offset distances

    NASA Astrophysics Data System (ADS)

    Agelin-Chaab, Martin; Tachie, Mark

    2009-11-01

    An offset jet is a jet that discharges into a medium above a wall which is offset by a certain distance. The ``Coanda effect'' forces the offset jet to deflect towards the wall and eventually attaches itself to the wall. The only detailed study of three-dimensional offset jets (3DOJs) did not report the flow field in the region from the jet exit to the point where the jet attaches itself to the wall. In this region flow reversal is expected. Velocity measurements of 3DOJs were conducted using particle image velocimetry. The 3DOJs have different jet exit offset distances (h) normalized by the jet exit diameter (d) of h/d = 0.5 to 4. The Reynolds numbers based on the jet exit velocities and jet exit diameters were 5000, 10000 and 20000. The detailed flow fields of the 3DOJs were examined in terms of mean velocities, and one-point turbulence statistics. In view of the wide range of length and temporal scales that are present in turbulent flows, multi-point turbulence statistics such as two-point velocity correlations and proper orthogonal decomposition are used to document the salient features of 3DOJs.

  18. Assessment of regularization models for LES of high-Re turbulent flows

    NASA Astrophysics Data System (ADS)

    Chandy, Abhilash; Frankel, Steven

    2008-11-01

    Regularization-based SGS turbulence models for LES are quantitatively assessed for decaying homogeneous turbulence (DHT) and transition to turbulence for the Taylor-Green vortex (TGV) through comparisons to laboratory measurements and DNS respectively. LES predictions using the Leray-α, LANS-α, and Clark-α regularization-based SGS models are compared to the classic non-dynamic Smagorinsky model. Regarding the regularization models, this work represents their first application to relatively high Re decaying turbulence with comparison to the active-grid-generated decaying turbulence measurements of Kang et al. (JFM, 2003) at Reλ 720 and the Re=3000 DNS of transition to turbulence in the TGV of Drikakis et al. (J. Turb., 2007). For DHT the non-dynamic Smagorinsky model is in excellent agreement with measurements for t.k.e., but higher-order moments show slight discrepancies and for TGV, the energy decay rates agree reasonably well with DNS. Regarding the regularization models stable results are not obtained as compared to Smagorinsky at the same grid resolution for various values of α, and at higher resolutions, they are in worse agreement. However, with additional dissipation such as in mixed α-Smagorinsky models, results are acceptable, but show slight deviations from Smagorinsky.

  19. Large Eddy Simulation/Probability Density Function Modeling of a Turbulent CH4/H2/N2 Jet Flame

    SciTech Connect

    Wang, Haifeng; Pope, Stephen B.

    2011-01-01

    In this work, we develop the large-eddy simulation (LES)/probability density function (PDF) simulation capability for turbulent combustion and apply it to a turbulent CH{sub 4}/H{sub 2}/N{sub 2} jet flame (DLR Flame A). The PDF code is verified to be second-order accurate with respect to the time-step size and the grid size in a manufactured one-dimensional test case. Three grids (64×64×16,192×192×48,320×320×80)(64×64×16,192×192×48,320×320×80) are used in the simulations of DLR Flame A to examine the effect of the grid resolution. The numerical solutions of the resolved mixture fraction, the mixture fraction squared, and the density are duplicated in the LES code and the PDF code to explore the numerical consistency between them. A single laminar flamelet profile is used to reduce the computational cost of treating the chemical reactions of the particles. The sensitivity of the LES results to the time-step size is explored. Both first and second-order time splitting schemes are used for integrating the stochastic differential equations for the particles, and these are compared in the jet flame simulations. The numerical results are found to be sensitive to the grid resolution, and the 192×192×48192×192×48 grid is adequate to capture the main flow fields of interest for this study. The numerical consistency between LES and PDF is confirmed by the small difference between their numerical predictions. Overall good agreement between the LES/PDF predictions and the experimental data is observed for the resolved flow fields and the composition fields, including for the mass fractions of the minor species and NO. The LES results are found to be insensitive to the time-step size for this particular flame. The first-order splitting scheme performs as well as the second-order splitting scheme in predicting the resolved mean and rms mixture fraction and the density for this flame.

  20. Streamwise vortex production by pitched and skewed jets in a turbulent boundary layer

    NASA Astrophysics Data System (ADS)

    Compton, Debora A.; Johnston, James P.

    1991-01-01

    Weak longitudinal (streamwise) vortices produced by the interaction of simple round wall jets with a two-dimensional flow comprising a turbulent boundary layer were studied experimentally. Like the jets used in the Vortex Generator Jet (VGJ) method of stall control by Johnston and Nushi (1989), the jets in this study were pitched up at 45 degrees and skewed relative to the free stream as they entered from the wall. Skew angles of 90 to 45 degrees produced the strongest vortices, and vortex strength increased with jet speed over the range studied, VR = U(j)U(infinity) = 0.7 to 1.3. It is shown that the vortices produced by the jets are significantly different from those produced by a solid vortex generator, despite the fact that both types have proven effective in enhancement of mixing across a separated or separating turbulent boundary layer.

  1. Evaluating vortex generator jet experiments for turbulent flow separation control

    NASA Astrophysics Data System (ADS)

    von Stillfried, F.; Kékesi, T.; Wallin, S.; Johansson, A. V.

    2011-12-01

    Separating turbulent boundary-layers can be energized by streamwise vortices from vortex generators (VG) that increase the near wall momentum as well as the overall mixing of the flow so that flow separation can be delayed or even prevented. In general, two different types of VGs exist: passive vane VGs (VVG) and active VG jets (VGJ). Even though VGs are already successfully used in engineering applications, it is still time-consuming and computationally expensive to include them in a numerical analysis. Fully resolved VGs in a computational mesh lead to a very high number of grid points and thus, computational costs. In addition, computational parameter studies for such flow control devices take much time to set-up. Therefore, much of the research work is still carried out experimentally. KTH Stockholm develops a novel VGJ model that makes it possible to only include the physical influence in terms of the additional stresses that originate from the VGJs without the need to locally refine the computational mesh. Such a modelling strategy enables fast VGJ parameter variations and optimization studies are easliy made possible. For that, VGJ experiments are evaluated in this contribution and results are used for developing a statistical VGJ model.

  2. Transport of inertial particles in a turbulent premixed jet flame

    NASA Astrophysics Data System (ADS)

    Battista, F.; Picano, F.; Troiani, G.; Casciola, C. M.

    2011-12-01

    The heat release, occurring in reacting flows, induces a sudden fluid acceleration which particles follow with a certain lag, due to their finite inertia. Actually, the coupling between particle inertia and the flame front expansion strongly biases the spatial distribution of the particles, by inducing the formation of localized clouds with different dimensions downstream the thin flame front. A possible indicator of this preferential localization is the so-called Clustering Index, quantifying the departure of the actual particle distribution from the Poissonian, which would correspond to a purely random spatial arrangement. Most of the clustering is found in the flame brush region, which is spanned by the fluctuating instantaneous flame front. The effect is significant also for very light particles. In this case a simple model based on the Bray-Moss-Libby formalism is able to account for most of the deviation from the Poissonian. When the particle inertia increases, the effect is found to increases and persist well within the region of burned gases. The effect is maximum when the particle relaxation time is of the order of the flame front time scale. The evidence of this peculiar source of clustering is here provided by data from a direct numerical simulation of a turbulent premixed jet flame and confirmed by experimental data.

  3. The formation of turbulent vortex rings by synthetic jets

    NASA Astrophysics Data System (ADS)

    Lawson, J. M.; Dawson, J. R.

    2013-10-01

    An investigation is made into the mechanism of pinch-off for turbulent vortex rings formed by a synthetic jet using time resolved particle image velocimetry measurements in air. During formation, measurements of the material acceleration field show a trailing pressure maximum (TPM) forms behind the vortex core. The adverse pressure gradient behind this TPM inhibits vorticity transport into the ring and the TPM is spatially coincident with the termination of vorticity flux into a control volume moving with the ring. A Lagrangian Coherent Structures (LCS) analysis is shown to be in agreement with the role of the TPM in pinch-off and in identifying the vortex ring before separation. The LCS analysis provides physical insights which form the basis of a revised model of pinch-off, based on kinematics, which predicts the time of formation (formation number) well for the present dataset. The delivery of impulse to the vortex ring is also considered. Two equally important mechanisms are shown to play a role: a material flux and a vortex force. In the case of long maximum stroke ratio, it is demonstrated that a vortex force continues to deliver impulse to the ring after the material flux is terminated at pinch-off and that this contribution may be substantial. This shows that the pinch-off and separation process cannot be considered impulse invariant, which has important implications for unsteady propulsion, present models of vortex ring formation, and existing explanations for vortex ring pinch-off.

  4. Estimation of Turbulent Wall Jet Velocity Fields for Noise Prediction

    NASA Astrophysics Data System (ADS)

    Nickels, Adam; Ukeiley, Lawrence; Reger, Robert; Cattafesta, Louis

    2015-11-01

    Estimation of the time-dependent turbulent velocity field of a planar wall jet based on discrete surface pressure measurements is performed using stochastic estimation in both the time and frequency domain. Temporally-resolved surface pressure measurements are measured simultaneously with planar Particle Image Velocimetry (PIV) snapshots, obtained at a relatively reduced rate. Proper Orthogonal Decomposition (POD) is then applied to both the surface pressure probes and the PIV snapshots, allowing for the isolation of portions of the wall pressure and velocity field signals that are well correlated. Using the time-varying pressure expansion coefficients as unconditional variables, velocity expansion coefficients are estimated and used to produce reconstructed estimates of the velocity field. Optimization in terms of number of unconditional probes employed, location of probes, and effects of PIV discretization are investigated with regards to the resulting estimates. Coupled with this analysis, Poisson's equation for fluctuating pressure is solved such that the necessary source terms of an acoustic analogy can be calculated for estimates of the far-field acoustics. Specifically in this work, the effects of using estimated velocity fields to solve for the hydrodynamic pressure and acoustic pressure will be studied.

  5. Generation of Alfvenic Waves and Turbulence in Magnetic Reconnection Jets

    NASA Astrophysics Data System (ADS)

    Hoshino, M.

    2014-12-01

    The magneto-hydro-dynamic (MHD) linear stability for the plasma sheet with a localized bulk plasma flow parallel to the neutral sheet is investigated. We find three different unstable modes propagating parallel to the anti-parallel magnetic field line, and we call them as "streaming tearing'', "streaming sausage'', and "streaming kink'' mode. The streaming tearing and sausage modes have the tearing mode-like structure with symmetric density fluctuation to the neutral sheet, and the streaming kink mode has the asymmetric fluctuation. The growth rate of the streaming tearing mode decreases with increasing the magnetic Reynolds number, while those of the streaming sausage and kink modes do not strongly depend on the Reynolds number. The wavelengths of these unstable modes are of the order of the thickness of plasma sheet, which behavior is almost same as the standard tearing mode with no bulk flow. Roughly speaking the growth rates of three modes become faster than the standard tearing mode. The situation of the plasma sheet with the bulk flow can be realized in the reconnection exhaust with the Alfvenic reconnection jet, and the unstable modes may be regarded as one of the generation processes of Alfvenic turbulence in the plasma sheet during magnetic reconnection.

  6. Selectivity of competitive-consecutive reactions depending on the turbulent mixing conditions in a co-axial jet mixer

    NASA Astrophysics Data System (ADS)

    Chorny, A.; Kornev, N.; Hassel, E.

    2010-12-01

    This paper considers the numerical results on the interaction between a turbulent co-axial jet and a co-flow of incompressible fluid (Schmidt number Sc≈1000) when competitive-consecutive reactions occur in a co-axial jet mixer. Firstly, RANS modeling was performed to predict flow phenomena. Two different mixing regimes were analyzed with and without a recirculation zone near a mixer wall. To describe the problem mathematically, the two-parameter turbulence k-ɛ model and various models for the computation of the averaged mixture fraction \\skew3\\bar f and its variance σ2 were used and verified by comparing them with the experimental and large eddy simulation (LES) data. The results revealed that the decay of \\skew5\\bar f and σ2 obtained by the developed RANS mixing model with the low-Reynolds-number effects (mechanical-to-scalar time ratio and turbulent Schmidt number in the transfer equation for σ2 as a function of Ret) was similar to the one found by LES and experiment. Second, the behavior of the competitive-consecutive reactions (A+B→P, B+R→S) in the co-axial mixer was considered. To calculate averaged chemical reaction rates, the transfer equations for concentrations adopted two approaches: a model with no regard to concentration fluctuations and the Li-Toor model with the Gaussian PDF of the mixture fraction. The yield of a desired product R was found to depend strongly on the mixing regime. The regime without recirculation zone appeared to be preferable as the reaction selectivity was smaller within the whole range of Reynolds number and initial reactant concentration ratio. This means that the amount of an undesired by-product S to be formed is minimal.

  7. LES of a Jet Excited by the Localized Arc Filament Plasma Actuators

    NASA Technical Reports Server (NTRS)

    Brown, Clifford A.

    2011-01-01

    The fluid dynamics of a high-speed jet are governed by the instability waves that form in the free-shear boundary layer of the jet. Jet excitation manipulates the growth and saturation of particular instability waves to control the unsteady flow structures that characterize the energy cascade in the jet.The results may include jet noise mitigation or a reduction in the infrared signature of the jet. The Localized Arc Filament Plasma Actuators (LAFPA) have demonstrated the ability to excite a high-speed jets in laboratory experiments. Extending and optimizing this excitation technology, however, is a complex process that will require many tests and trials. Computational simulations can play an important role in understanding and optimizing this actuator technology for real-world applications. Previous research has focused on developing a suitable actuator model and coupling it with the appropriate computational fluid dynamics (CFD) methods using two-dimensional spatial flow approximations. This work is now extended to three-dimensions (3-D) in space. The actuator model is adapted to a series of discrete actuators and a 3-D LES simulation of an excited jet is run. The results are used to study the fluid dynamics near the actuator and in the jet plume.

  8. Hourly observations of the jet stream - Wind shear, Richardson number and pilot reports of turbulence

    NASA Technical Reports Server (NTRS)

    Syrett, William J.

    1991-01-01

    Results are presented of observations of the jet stream made on the basis of over 400 hr of wind and temperature data taken during two prolonged jet stream passages above western and central Pennsylvania during mid-November 1986 and mid-January 1987. Wind profilers are found to be far better suited for the detailed examination of jet stream structure than are weather balloons. The combination of good vertical resolution with not previously obtained temporal resolution reveals structural details not seen before. Development of probability forecasts of turbulence based on wind profiler-derived shear values appears possible. A good correlation between pilot reports and turbulence and wind shear is found.

  9. Distorted turbulence submitted to frame rotation: RDT and LES results

    NASA Technical Reports Server (NTRS)

    Godeferd, Fabien S.

    1995-01-01

    The objective of this effort is to carry the analysis of Lee et al. (1990) to the case of shear with rotation. We apply the RDT approximation to turbulence submitted to frame rotation for the case of a uniformly sheared flow and compare its mean statistics to results of high resolution DNS of a rotating plane channel flow. In the latter, the mean velocity profile is modified by the Coriolis force, and accordingly, different regions in the channel can be identified. The properties of the plane pure strain turbulence submitted to frame rotation are, in addition, investigated in spectral space, which shows the usefulness of the spectral RDT approach. This latter case is investigated here. Among the general class of quadratic flows, this case does not follow the same stability properties as the others since the related mean vorticity is zero.

  10. Comprehensive Approaches to Multiphase Flows in Geophysics - Application to nonisothermal, nonhomogenous, unsteady, large-scale, turbulent dusty clouds I. Hydrodynamic and Thermodynamic RANS and LES Models

    SciTech Connect

    S. Dartevelle

    2005-09-05

    The objective of this manuscript is to fully derive a geophysical multiphase model able to ''accommodate'' different multiphase turbulence approaches; viz., the Reynolds Averaged Navier-Stokes (RANS), the Large Eddy Simulation (LES), or hybrid RANSLES. This manuscript is the first part of a larger geophysical multiphase project--lead by LANL--that aims to develop comprehensive modeling tools for large-scale, atmospheric, transient-buoyancy dusty jets and plume (e.g., plinian clouds, nuclear ''mushrooms'', ''supercell'' forest fire plumes) and for boundary-dominated geophysical multiphase gravity currents (e.g., dusty surges, diluted pyroclastic flows, dusty gravity currents in street canyons). LES is a partially deterministic approach constructed on either a spatial- or a temporal-separation between the large and small scales of the flow, whereas RANS is an entirely probabilistic approach constructed on a statistical separation between an ensemble-averaged mean and higher-order statistical moments (the so-called ''fluctuating parts''). Within this specific multiphase context, both turbulence approaches are built up upon the same phasic binary-valued ''function of presence''. This function of presence formally describes the occurrence--or not--of any phase at a given position and time and, therefore, allows to derive the same basic multiphase Navier-Stokes model for either the RANS or the LES frameworks. The only differences between these turbulence frameworks are the closures for the various ''turbulence'' terms involving the unknown variables from the fluctuating (RANS) or from the subgrid (LES) parts. Even though the hydrodynamic and thermodynamic models for RANS and LES have the same set of Partial Differential Equations, the physical interpretations of these PDEs cannot be the same, i.e., RANS models an averaged field, while LES simulates a filtered field. In this manuscript, we also demonstrate that this multiphase model fully fulfills the second law of

  11. Direct numerical simulation of transitional and turbulent buoyant planar jet flames

    NASA Astrophysics Data System (ADS)

    Mehravaran, K.; Jaberi, F. A.

    2004-12-01

    The effects of gravity on the physical and compositional structures of transitional and turbulent diffusion flames are studied via analysis of the data generated by direct numerical simulation of a planar jet flame at various gravity conditions. A fully compressible, high-order compact, finite-difference computational scheme is used together with a global kinetics model for chemical reaction. The results of our nonreacting turbulent jet simulations are in good agreement with the available experimental data. The results of our reacting jet simulations are also consistent with previous findings and indicate that in the absence of gravity, combustion damps the flow instability, and hence reduces "turbulence production" and jet growth. However, in the "finite-gravity" conditions, combustion generated density variations may promote turbulence and enhance both the mixing and the combustion through buoyancy effects. Our results also indicate that the gravity effects on a transitional/turbulent jet flame is not limited to large-scale flame flickering, and there is a significant impact on small-scale turbulence and mixing as well. Furthermore, the analysis of compositional flame structures suggests that the finite-rate chemistry effects are more significant in finite-gravity conditions than in zero-gravity.

  12. LES, DNS and RANS for the analysis of high-speed turbulent reacting flows

    NASA Technical Reports Server (NTRS)

    Adumitroaie, V.; Colucci, P. J.; Taulbee, D. B.; Givi, P.

    1995-01-01

    The purpose of this research is to continue our efforts in advancing the state of knowledge in large eddy simulation (LES), direct numerical simulation (DNS), and Reynolds averaged Navier Stokes (RANS) methods for the computational analysis of high-speed reacting turbulent flows. In the second phase of this work, covering the period 1 Aug. 1994 - 31 Jul. 1995, we have focused our efforts on two programs: (1) developments of explicit algebraic moment closures for statistical descriptions of compressible reacting flows and (2) development of Monte Carlo numerical methods for LES of chemically reacting flows.

  13. Statistical state dynamics of jet/wave coexistence in beta-plane turbulence

    NASA Astrophysics Data System (ADS)

    Constantinou, Navid; Farrell, Brian; Ioannou, Petros

    Jets are commonly observed to coexist in the turbulence of planetary atmospheres with planetary scale waves and embedded vortices. These large-scale coherent structures arise and are maintained in the turbulence on time scales long compared to dissipation or advective time scales. The emergence, equilibration at finite amplitude, maintenance and stability of these structures pose fundamental theoretical problems. The emergence of jets and vortices from turbulence is not associated with an instability of the mean flow and their equilibration and stability at finite amplitude does not arise solely from the linear or nonlinear dynamics of these structures in isolation from the turbulence surrounding them. Rather the dynamics of these large-scale structures arises essentially from their cooperative interaction with the small-scale turbulence in which they are embedded. It follows that fundamental theoretical understanding of the dynamics of jets and vortices in turbulence requires adopting the perspective of the statistical state dynamics (SSD) of the entire turbulent state. In this work a theory for the jet/wave coexistence regime is developed using the SSD perspective.

  14. Effect of nozzle length-to-diameter ratio on atomization of turbulent liquid jets

    NASA Astrophysics Data System (ADS)

    Osta, Anu Ranjan

    Breakup of liquid jets is of considerable interest motivated by its applicability in combustion and propulsion systems (CI and SI engines), and agricultural fertilizer/pesticide sprays, among others. Almost all of the practical liquid injectors introduce some degree of turbulence in the liquid jet leaving the injector passage and an intriguing question is the relative importance of the liquid turbulence, cavitation, and the aerodynamic forces in the breakup processes of fuel injectors. A better design of liquid fuel injector would reduce pollutants and increase the efficiency of liquid fuel combustion processes. An experimental study to investigate the effect of nozzle length to diameter ratio on the surface properties of turbulent liquid jets in gaseous crossflow and still air was carried out. Straight cavitation-free nozzles with length/diameter ratios of 10, 20 and 40 were used to generate turbulent liquid jets in gaseous crossflow. The present study was limited to small Ohnesorge number liquid jets (Oh < 0.01) injected in crossflow within the shear breakup regime (WeG > 110). The diagnostics consisted of pulsed shadowgraphy, pulsed digital holographic microscopy and x-ray diagnostics. The x-ray tests were conducted at the Advanced Photon Source (APS) facility of Argonne National Laboratory. The test matrix was designed to maintain the same aerodynamic forces in order to isolate the effects of jet turbulence on the breakup process. The measurements included liquid jet surface properties, breakup location of the liquid column as a whole, the breakup regime transitions, bubble size inside the jet and seeding particle displacement inside the jet structures. The results include the jet surface characteristics, the liquid column breakup lengths, bubble growth, and phenomenological analysis to explain the observed results. It is observed that for a jet breakup in crossflow the injector passage length does play a role in determining the breakup length as well as

  15. Equilibration of the jet forming instability in barotropic beta-plane turbulence

    NASA Astrophysics Data System (ADS)

    Constantinou, Navid; Bakas, Nikolaos; Ioannou, Petros

    2016-04-01

    Planetary turbulent flows are observed to self-organize into large scale structures such as zonal jets and coherent vortices. Recently, it was shown that a comprehensive understanding of the properties of these large scale structures and of the dynamics underlying their emergence and maintenance is gained through the study of the dynamics of the statistical state of the flow. Previous studies ad-dressed the emergence of the coherent structures in barotropic turbulence and showed the zonal jets emerge as an instability of the Statistical State Dynamics (SSD). In this work, the equilibration of the incipient instabilities and the stability of the equilibrated jets near onset is investigated. It is shown through a weakly nonlinear analysis of the SSD that the amplitude of the jet evolves according to a Ginzburg-Landau equation. The equilibrated jets were found to have a harmonic structure and an amplitude that is an increasing function of the planetary vorticity gradient. It is also shown that most of the equilibrated jets are secondarily unstable and will evolve through jet merging and branching to the stable jets that have a scale close to the most unstable (refering to the jet forming primary instability) emerging jet.

  16. An experimental investigation of confined, multiple, turbulent jet mixing with recirculation

    NASA Astrophysics Data System (ADS)

    Ghosh, A.

    1984-12-01

    An axisymmetric duct that simulated a combustor configuration, was used to examine the flow fields in combustors that utilize injector plates to establish turbulent jet mixing of fuels and oxidizers. Five different injector plates were tested. Flow fields downstream of the plates were investigated using flow visualization and laser velocimetry. Qualitative data for flows created by rings of discrete jets were obtained with dye injection and hydrogen bubble generation flow visualization techniques. Wall static pressure distributions were measured with a sensitive pressure transducer. The velocity and turbulence fields were quantified using a Bragg-diffracted, two-component laser doppler velocimeter (LV). The locations and extents of recirculation zones and jet expansion rates were deduced from the LV data. Results show that the flow due to an annular jet is two dimensional, but those due to discrete jets are fully three dimensional.

  17. Random Vortex-Street Model for a Self-Similar Plane Turbulent Jet

    NASA Astrophysics Data System (ADS)

    L'Vov, Victor S.; Pomyalov, Anna; Procaccia, Itamar; Govindarajan, Rama

    2008-08-01

    We ask what determines the (small) angle of turbulent jets. To answer this question we first construct a deterministic vortex-street model representing the large-scale structure in a self-similar plane turbulent jet. Without adjustable parameters the model reproduces the mean velocity profiles and the transverse positions of the large-scale structures, including their mean sweeping velocities, in a quantitative agreement with experiments. Nevertheless, the exact self-similar arrangement of the vortices (or any other deterministic model) necessarily leads to a collapse of the jet angle. The observed (small) angle results from a competition between vortex sweeping tending to strongly collapse the jet and randomness in the vortex structure, with the latter resulting in a weak spreading of the jet.

  18. A theoretical and experimental study of turbulent particle-laden jets

    NASA Technical Reports Server (NTRS)

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

    1983-01-01

    Mean and fluctuating velocities of both phases, particle mass fluxes, particle size distributions in turbulent particle-laden jets were measured. The following models are considered: (1) a locally homogeneous flow (LHF) model, where slip between the phases was neglected; (2) a deterministic separated flow (DSF) model, where slip was considered but effects of particle dispersion by turbulence were ignored; and (3) a stochastic separated flow (SSF) model. The SSF model performed reasonably well with no modifications in the prescriptions for eddy properties from its original calibration. A modified k- model, incorporating direct contributions of interphase transport on turbulence properties (turbulence modulation), was developed within the framework of the SSF model.

  19. Effect of initial tangential velocity distribution on the mean evolution of a swirling turbulent free jet

    NASA Technical Reports Server (NTRS)

    Farokhi, S.; Taghavi, R.; Rice, E. J.

    1988-01-01

    An existing cold jet facility at NASA-Lewis was modified to produce swirling flows with controllable initial tangential velocity distribution. Distinctly different swirl velocity profiles were produced, and their effects on jet mixing characteristics were measured downstream of an 11.43 cm diameter convergent nozzle. It was experimentally shown that in the near field of a swirling turbulent jet, the mean velocity field strongly depends on the initial swirl profile. Two extreme tangential velocity distributions were produced. The two jets shared approximately the same initial mass flow rate of 5.9 kg/s, mass averaged axial Mach number and swirl number. Mean centerline velocity decay characteristics of the solid body rotation jet flow exhibited classical decay features of a swirling jet with S = 0.48 reported in the literature. It is concluded that the integrated swirl effect, reflected in the swirl number, is inadequate in describing the mean swirling jet behavior in the near field.

  20. Characteristics of a two-dimensional turbulent jet in a bounded slipstream

    NASA Astrophysics Data System (ADS)

    Voronov, S. K.; Girshovich, T. A.; Grishin, A. N.

    1985-06-01

    A method for computing a two-dimensional turbulent jet injected at a right angle into a channeled flow is presented, taking into account the rarefaction behind the jet. The conditions at the front jet boundary are approximated using a solution to a displacement flow problem and data available on the flow around elliptical bodies. The boundedness of the flow and the jet velocity to stream velocity ratio are found to have a considerable effect on the jet trajectory; the axial velocity distribution in the jet, the nondimensional velocity profiles, and the back boundary of the stream, however, are not affected noticeably by these parameters. Analytical results are corroborated by experimental data for a two-dimensional jet in free and bounded flows, exhibiting complete qualitative and partial quantitative agreement.

  1. An Experimental Study of Turbulent Nonpremixed Jet Flames in Crossflow Under Low-Gravity Conditions

    NASA Astrophysics Data System (ADS)

    Boxx, Isaac G.; Idicheria, Cherian A.; Clemens, Noel T.

    2002-11-01

    We will present results of a study of turbulent nonpremixed jet flames in crossflow under normal and low gravity conditions. This enables us to experimentally separate the competing influences of initial jet-to-crossflow momentum ratio and buoyancy effects on the flame structure. The low gravity conditions (10-30 milli-g) are achieved by dropping a self-contained jet flame rig in the University of Texas 1.25-second drop tower facility. This rig uses a small blow-through wind tunnel to create the crossflow. The jet flames issue from an orifice that is flush with the wall. High-speed CCD imaging of jet flame luminosity is the primary diagnostic. We present results for hydrocarbon jet flames with initial jet-to-crossflow momentum ratios of 10-20. Results such as flame trajectory, flame length, large scale structure and flame tip dynamics will be presented.

  2. Theoretical study of the effects of refraction on the noise produced by turbulence in jets

    NASA Technical Reports Server (NTRS)

    Graham, E. W.; Graham, B. B.

    1974-01-01

    The production of noise by turbulence in jets is an extremely complex problem. One aspect of that problem, the transmission of acoustic disturbances from the interior of the jet through the mean velocity profile and into the far field is studied. The jet (two-dimensional or circular cylindrical) is assumed infinitely long with mean velocity profile independent of streamwise location. The noise generator is a sequence of transient sources drifting with the surrounding fluid and confined to a short length of the jet.

  3. Investigation of the interaction of a turbulent impinging jet and a heated, rotating disk

    NASA Astrophysics Data System (ADS)

    Manceau, R.; Perrin, R.; Hadžiabdić, M.; Benhamadouche, S.

    2014-03-01

    The case of a turbulent round jet impinging perpendicularly onto a rotating, heated disc is investigated, in order to understand the mechanisms at the origin of the influence of rotation on the radial wall jet and associated heat transfer. The present study is based on the complementary use of an analysis of the orders of magnitude of the terms of the mean momentum and Reynolds stress transport equations, available experiments, and dedicated Reynolds-averaged Navier-Stokes computations with refined turbulence models. The Reynolds number Rej = 14 500, the orifice-to-plate distance H = 5D, where D is the jet-orifice diameter, and the four rotation rates were chosen to match the experiments of Minagawa and Obi ["Development of turbulent impinging jet on a rotating disk," Int. J. Heat Fluid Flow 25, 759-766 (2004)] and comparisons are made with the Nusselt number distribution measured by Popiel and Boguslawski ["Local heat transfer from a rotating disk in an impinging round jet," J. Heat Transfer 108, 357-364 (1986)], at a higher Reynolds number. The overestimation of turbulent mixing in the free-jet before the impact on the disk is detrimental to the prediction of the impingement region, in particular of the Nusselt number close to the symmetry axis, but the self-similar wall jet developing along the disk is correctly reproduced by the models. The analysis, experiments, and computations show that the rotational effect do not directly affect the outer layer, but only the inner layer of the wall jet. A noteworthy consequence is that entrainment at the outer edge of the wall jet is insensitive to rotation, which explains the dependence of the wall-jet thickness on the inverse of the non-dimensional rotation rate, observed in the experiments and the Reynolds stress model computations, but not reproduced by the eddy-viscosity models, due to the algebraic dependence to the mean flow. The analysis makes moreover possible the identification of a scenario for the appearance of

  4. Prediction of Turbulent Jet Mixing Noise Reduction by Water Injection

    NASA Technical Reports Server (NTRS)

    Kandula, Max

    2008-01-01

    A one-dimensional control volume formulation is developed for the determination of jet mixing noise reduction due to water injection. The analysis starts from the conservation of mass, momentum and energy for the confrol volume, and introduces the concept of effective jet parameters (jet temperature, jet velocity and jet Mach number). It is shown that the water to jet mass flow rate ratio is an important parameter characterizing the jet noise reduction on account of gas-to-droplet momentum and heat transfer. Two independent dimensionless invariant groups are postulated, and provide the necessary relations for the droplet size and droplet Reynolds number. Results are presented illustrating the effect of mass flow rate ratio on the jet mixing noise reduction for a range of jet Mach number and jet Reynolds number. Predictions from the model show satisfactory comparison with available test data on perfectly expanded hot supersonic jets. The results suggest that significant noise reductions can be achieved at increased flow rate ratios.

  5. Comparison of Turbulence Models for Nozzle-Afterbody Flows with Propulsive Jets

    NASA Technical Reports Server (NTRS)

    Compton, William B., III

    1996-01-01

    A numerical investigation was conducted to assess the accuracy of two turbulence models when computing non-axisymmetric nozzle-afterbody flows with propulsive jets. Navier-Stokes solutions were obtained for a Convergent-divergent non-axisymmetric nozzle-afterbody and its associated jet exhaust plume at free-stream Mach numbers of 0.600 and 0.938 at an angle of attack of 0 deg. The Reynolds number based on model length was approximately 20 x 10(exp 6). Turbulent dissipation was modeled by the algebraic Baldwin-Lomax turbulence model with the Degani-Schiff modification and by the standard Jones-Launder kappa-epsilon turbulence model. At flow conditions without strong shocks and with little or no separation, both turbulence models predicted the pressures on the surfaces of the nozzle very well. When strong shocks and massive separation existed, both turbulence models were unable to predict the flow accurately. Mixing of the jet exhaust plume and the external flow was underpredicted. The differences in drag coefficients for the two turbulence models illustrate that substantial development is still required for computing very complex flows before nozzle performance can be predicted accurately for all external flow conditions.

  6. High-fidelity Simulation of Jet Noise from Rectangular Nozzles . [Large Eddy Simulation (LES) Model for Noise Reduction in Advanced Jet Engines and Automobiles

    NASA Technical Reports Server (NTRS)

    Sinha, Neeraj

    2014-01-01

    This Phase II project validated a state-of-the-art LES model, coupled with a Ffowcs Williams-Hawkings (FW-H) far-field acoustic solver, to support the development of advanced engine concepts. These concepts include innovative flow control strategies to attenuate jet noise emissions. The end-to-end LES/ FW-H noise prediction model was demonstrated and validated by applying it to rectangular nozzle designs with a high aspect ratio. The model also was validated against acoustic and flow-field data from a realistic jet-pylon experiment, thereby significantly advancing the state of the art for LES.

  7. Effects of differential diffusion on the flame structure of oxygen enhanced turbulent non-premixed jet flames

    NASA Astrophysics Data System (ADS)

    Dietzsch, Felix; Gauding, Michael; Hasse, Christian

    2014-11-01

    By means of Direct Numerical Simulation we have investigated the influence of differential diffusion for non-premixed oxygen-enhanced turbulent flames. Oxygen-enhanced conversion usually yields higher amounts of H2 as compared to conventional air combustion. It is well known that H2 as a very diffusive species leads to differential diffusion effects. In addition to the diffusive transport mixing processes are also often controlled by turbulent transport. Previous investigations of a turbulent CH4/H2 oxygen-enhanced jet flame have shown that in mixture fraction space it is important to distinguish between regions of equal diffusivities and detailed transport. These findings are of particular interest when performing Large-Eddy simulations applying a flamelet approach. Using this approach a LES study was performed of the aforementioned flame considering differential diffusion. Therefore, flamelet equations including differential diffusion via non-unity constant Lewis numbers were solved. However, this study showed that keeping the non-unity Lewis numbers constant, is not sufficient to capture the diffusion phenomena in this particular flame. Direct Numerical Simulations have been conducted in order to investigate how Lewis numbers are affected in mixture fraction space. Computer resources for this project have been provided by the Gauss Centre for Supercomputing/Leibniz Supercomputing Centre under Grant: pr83xa.

  8. A dynamic hybrid RANS/LES modeling methodology for turbulent/transitional flow field prediction

    NASA Astrophysics Data System (ADS)

    Alam, Mohammad Faridul

    A dynamic hybrid Reynolds-averaged Navier-Stokes (RANS)-Large Eddy Simulation (LES) modeling framework has been investigated and further developed to improve the Computational Fluid Dynamics (CFD) prediction of turbulent flow features along with laminar-to-turbulent transitional phenomena. In recent years, the use of hybrid RANS/LES (HRL) models has become more common in CFD simulations, since HRL models offer more accuracy than RANS in regions of flow separation at a reduced cost relative to LES in attached boundary layers. The first part of this research includes evaluation and validation of a dynamic HRL (DHRL) model that aims to address issues regarding the RANS-to-LES zonal transition and explicit grid dependence, both of which are inherent to most current HRL models. Simulations of two test cases---flow over a backward facing step and flow over a wing with leading-edge ice accretion---were performed to assess the potential of the DHRL model for predicting turbulent features involved in mainly unsteady separated flow. The DHRL simulation results are compared with experimental data, along with the computational results for other HRL and RANS models. In summary, these comparisons demonstrate that the DHRL framework does address many of the weaknesses inherent in most current HRL models. Although HRL models are widely used in turbulent flow simulations, they have limitations for transitional flow predictions. Most HRL models include a fully turbulent RANS component for attached boundary layer regions. The small number of HRL models that do include transition-sensitive RANS models have issues related to the RANS model itself and to the zonal transition between RANS and LES. In order to address those issues, a new transition-sensitive HRL modeling methodology has been developed that includes the DHRL methodology and a physics-based transition-sensitive RANS model. The feasibility of the transition-sensitive dynamic HRL (TDHRL) model has been investigated by

  9. Deflected jet experiments in a turbulent combustor flowfield. Ph.D. Thesis Final Report

    NASA Technical Reports Server (NTRS)

    Ferrell, G. B.; Lilley, D. G.

    1985-01-01

    Experiments were conducted to characterize the time-mean and turbulent flow field of a deflected turbulent jet in a confining cylindrical crossflow. Jet-to-crossflow velocity ratios of 2, 4, and 6 were investigated, under crossflow inlet swirler vane angles of 0 (swirler removed), 45 and 70 degrees. Smoke, neutrally buoyant helium-filled soap bubbles, and multi-spark flow visualization were employed to highlight interesting features of the deflected jet, as well as the tracjectory and spread pattern of the jet. A six-position single hot-wire technique was used to measure the velocities and turbulent stresses in nonswirling crossflow cases. In these cases, measurements confirmed that the deflected jet is symmetrical about the vertical plan passing through the crossflow axis, and the jet penetration was found to be reduced from that of comparable velocity ratio infinite crossflow cases. In the swirling crossflow cases, the flow visualization techniques enabled gross flow field characterization to be obtained for a range of lateral jet-to-crossflow velocity ratios and a range of inlet swirl strengths in the main flow.

  10. The effect of exit conditions on the development of an axisymmetric turbulent free jet

    NASA Technical Reports Server (NTRS)

    Kleis, S. J.; Foss, J. F.

    1974-01-01

    The mean flow in the near field of a submerged axisymmetric jet emitting from a plane wall is presented. An experimental configuration to provide a nearly uniform mean velocity profile with a core of homogeneous turbulence of variable intensity and scale was developed. Eight cases with intensity values of 0.004 less than or equal to U prime less than or equal to 0.035 and integral scales up to l sub x/R = 0.28 were investigated using conditional sampling techniques. It was found that the jet exhibits an increasing momentum flux in the near field. Contrary to expectation and the accepted assumption of ambient static pressure in a turbulent jet, results seem to be conclusive and borne out by comparison with published data. Both integral measures, mass and momentum flux ratios, are insensitive to exit turbulence variations, but, the detailed structure (including centerline velocity) variations with exit conditions are systematic and explainable.

  11. A turbulent nonisothermal jet in an Archimedean force field

    NASA Astrophysics Data System (ADS)

    Elemasov, V. E.; Glebov, G. A.; Kozlov, A. P.

    An integral method for calculating a vertical nonisothermal jet is presented which allows for the effects of Archimedean forces and nonisothermality. The method can be extended to the calculation of axisymmetric and plane jets in a slipstream and also to the case of jets issuing into a medium of a different concentration. It is shown that the consideration of Archimedean forces and nonisothermality results in a better agreement between calculations and experimental data.

  12. Impact pressures of turbulent high-velocity jets plunging in pools with flat bottom

    NASA Astrophysics Data System (ADS)

    Manso, P. A.; Bollaert, E. F. R.; Schleiss, A. J.

    2007-01-01

    Dynamic pressures created by the impact of high-velocity turbulent jets plunging in a water pool with flat bottom were investigated. Pressure fluctuations were sampled at 1 kHz at the jet outlet and at the pool bottom using piezo-resistive pressure transducers, jet velocities of up to 30 m/s and pool depth to jet diameter ratios from 2.8 to 11.4. The high-velocity jets entrain air in the pool in conditions similar to prototype applications at water release structures of dams. The intermittent character of plunge pool flows was investigated for shallow and deep pools, based on high order moments and time correlations. Maximum intermittency was observed for pool depths at 5.6 jet diameters, which approximate the core development length. Wall pressure skewness was shown to allow identifying the zone of influence of downward and upward moving currents.

  13. Turbulent jet flow generated downstream of a low temperature dielectric barrier atmospheric pressure plasma device.

    PubMed

    Whalley, Richard D; Walsh, James L

    2016-01-01

    Flowing low temperature atmospheric pressure plasma devices have been used in many technological applications ranging from energy efficient combustion through to wound healing and cancer therapy. The generation of the plasma causes a sudden onset of turbulence in the inhomogeneous axisymmetric jet flow downstream of the plasma plume. The mean turbulent velocity fields are shown to be self-similar and independent of the applied voltage used to generate the plasma. It is proposed that the production of turbulence is related to a combination of the small-amplitude plasma induced body forces and gas heating causing perturbations in the unstable shear layers at the jet exit which grow as they move downstream, creating turbulence. PMID:27561246

  14. Turbulent jet flow generated downstream of a low temperature dielectric barrier atmospheric pressure plasma device

    PubMed Central

    Whalley, Richard D.; Walsh, James L.

    2016-01-01

    Flowing low temperature atmospheric pressure plasma devices have been used in many technological applications ranging from energy efficient combustion through to wound healing and cancer therapy. The generation of the plasma causes a sudden onset of turbulence in the inhomogeneous axisymmetric jet flow downstream of the plasma plume. The mean turbulent velocity fields are shown to be self-similar and independent of the applied voltage used to generate the plasma. It is proposed that the production of turbulence is related to a combination of the small-amplitude plasma induced body forces and gas heating causing perturbations in the unstable shear layers at the jet exit which grow as they move downstream, creating turbulence. PMID:27561246

  15. Evolution of a confined turbulent jet in a long cylindrical cavity: Homogeneous fluids

    NASA Astrophysics Data System (ADS)

    Voropayev, S. I.; Sanchez, X.; Nath, C.; Webb, S.; Fernando, H. J. S.

    2011-11-01

    The flow induced in a long cylinder by an axially discharging round turbulent jet was investigated experimentally with applications to crude oil storage in the U.S. strategic petroleum reserves (SPR). It was found that the flow does not reach a true steady state, but vacillates periodically. Digital video recordings and particle image velocimetry were used to map the flow structures and velocity/vorticity fields, from which the frequency of jet switching, jet stopping distance, mean flow, turbulence characteristics, and the influence of end-wall boundary conditions were inferred. The results were parameterized using the characteristic length D and velocity J1/2/D scales based on the jet kinematic momentum flux J and cylinder width D. The scaling laws so developed could be used to extrapolate laboratory observations to SPR flows.

  16. The effects of external and self excitations on axisymmetric jet turbulence and noise

    NASA Astrophysics Data System (ADS)

    Hasan, M. A. Z.

    The effects of external- and self-excitations on axisymmetric jet turbulence and noise as well as the self-sustained oscillation phenomena (SSOP) were experimentally studied. A high subsonic Mach number (Maximum Mach number = 0.7) air jet was used for most investigations while the SSOP study has involved ring tone, hole tone and 'whistler nozzle' excitations in low velocity air jets. Based on the data ring and hole tones and the 'whistler nozzle', it is shown that such tones are strongly dependent on the initial condition. For the initially turbulent jet, the tone occurs only when there is a resonator in the system. The data collapse when nondimensonalized by the exit momentum thickness and the distance between separation and impingment points rather than the diameter as suggested previously.

  17. Similarity solutions of jet flows using a multiple-scale turbulence model

    NASA Technical Reports Server (NTRS)

    Chen, C. P.; Guo, K. L.

    1989-01-01

    An accurate finite difference scheme is used to study the performance of a newly developed multiple-scale turbulence model in the similarity region of plane/radial/round jet flows. Under the assumption that molecular viscosity can be neglected, the governing equations contain a hidden eigenvalue which corresponds to the sharp boundary of the flowfield. It is found that this eigenvalue is the locus of the free boundary separating the turbulent shear flow and the irrotational ambient fluid.

  18. On similarity solutions for turbulent and heated round jets

    NASA Technical Reports Server (NTRS)

    So, R. M. C.; Hwang, B. C.

    1986-01-01

    Commonly used empirical correlations for incompressible, heated round jets are shown to represent similarity solutions of the governing jet equations. These solutions give rise to self-similar eddy viscosities. Not all the similarity solutions are physically valid because some lead to zero eddy viscosities at the jet centerline. One physically valid solution is found to correlate best with round jet measurements and it gives a Gaussian error function description for the normalized mean velocity and temperature. Heat and momentum fluxes thus calculated are also in good agreement with measurements. Therefore, in addition to the classical similarity solution obtained by assuming constant eddy viscosity, another similarity solution to the jet equations is found where the eddy viscosity is self-similar.

  19. Synthetic Jet Interaction With A Turbulent Boundary Layer Flow

    NASA Technical Reports Server (NTRS)

    Smith, Douglas R.

    2002-01-01

    Perhaps one of the more notable advances to have occurred in flow control technology in the last fifteen years is the application of surface-issuing jets for separation control on aerodynamic surfaces. The concept was introduced by Johnston and Night (1990) who proposed using circular jets, skewed and inclined to the wall, to generate streamwise vortices for the purpose of mitigating boundary layer separation. The skew and inclination angles have subsequently been shown to affect the strength and sign of the ensuing vortices. With a non-circular orifice, in addition to skew and inclination, the yaw angle of the major axis of the orifice can influence the flow control effectiveness of the jet. In particular, a study by Chang arid Collins (1997) revealed that a non-circular orifice, yawed relative to the freestream, can be used to control the size and strength of the vortices produced by the control jet. This early work used jets with only a steady injection of mass. Seifert et al. revealed that an unsteady blowing jet, could be as effective at separation control as a steady jet but with less mass flow. Seifert et al. showed that small amplitude blowing oscillations superimposed on a low momentum steady jet Was the most effective approach to delaying separation on a NACA 0015 airfoil at post-stall angles of attack. More recent work suggests that perhaps the most efficient jet control effect comes from a synthetic (oscillatory) jet where the time-averaged mass flux through the orifice is zero, but the net wall normal momentum is non-zero. The control effectiveness of synthetic jets has been demonstrated for several internal and external flow fields used synthetic jet control on a thick, blunt-nosed airfoil to delay stall well beyond the stall angles for the uncontrolled airfoil and with a dramatic increase in the lift-to-drag performance. Amitay et al. used an array of synthetic jets to mitigate flow separation in curved and diffusing ducts. While the control

  20. DNS of aerosol evolution in a turbulent jet

    NASA Astrophysics Data System (ADS)

    Zhou, Kun; Attili, Antonio; Bisetti, Fabrizio

    2011-11-01

    The effects of turbulence on the evolution of aerosols are not well understood. In this work, the interaction of aerosol dynamics and turbulence are studied in a canonical flow configuration by numerical means. The configuration consists of a hot nitrogen stream saturated with dibutyl phthalate (DBP) vapor mixing with cool air in a shear layer. A direct numerical simulation (DNS) for the momentum and scalar fields is coupled with the direct quadrature method of moments (DQMOM) for the condensing liquid phase. The effects of turbulent mixing on aerosol processes (nucleation, condensation, and coagulation) are quantified by analyzing the statistics of number density and droplet sizes.

  1. A method for calculating a weakly nonisobaric supersonic turbulent jet in a subsonic slipstream

    NASA Astrophysics Data System (ADS)

    Kozlov, V. E.

    A novel approach for computing weakly nonisobaric turbulent jets in a subsonic slipstream is proposed whereby the flow is divided into a subsonic and a supersonic zone and two different evolutionary systems of equations are used in each of the two zones. The solutions are then joined at the Mach 1 line. Closure of the systems of equations is achieved by using a known one-parameter turbulence model that has been modified to allow for the effect of the Mach number on turbulent mixing. The results obtained are compared against experimental data.

  2. Pulsed laser measurements of particle and vapour concentrations in a turbulent jet

    NASA Astrophysics Data System (ADS)

    Kennedy, I. M.

    1989-10-01

    A turbulent jet of air has been seeded with both particles and a vapor. An excimer pumped dye laser excited visible fluorescence from the biacetyl vapor and Mie scattering from the micron-size particles. It was possible to measure the simultaneous scatteirng from both phases by using interference filters to separate the signals. It has been found that the biacetyl vapor provides an adequate tracer for measurements of the concentration field in a turbulent flow. Furthermore, the feasibility of making simultaneous concentration measurements of two phases in a turbulent flow has been demonstrated.

  3. Investigation with an Interferometer of the Turbulent Mixing of a Free Supersonic Jet

    NASA Technical Reports Server (NTRS)

    Gooderum, Paul B; Wood, George P; Brevoort, Maurice J

    1950-01-01

    The free turbulent mixing of a supersonic jet of Mach number 1.6 has been experimentally investigated. An interferometer, of which a description is given, was used for the investigation. Density and velocity distributions through the mixing zone have been obtained. It was found that there was similarity in distribution at the cross sections investigated and that, in the subsonic portion of the mixing zone, the velocity distribution fitted the theoretical distribution for incompressible flow. It was found that the rates of spread of the mixing zone both into the jet and into the ambient air were less than those of subsonic jets.

  4. Turbulent amplification of magnetic fields in colliding laboratory jets

    NASA Astrophysics Data System (ADS)

    Tzeferacos, P.; Meinecke, J.; Bell, A. R.; Doyle, H.; Bingham, R.; Churazov, E. M.; Crowston, R.; Murphy, C. D.; Woolsey, N. C.; Drake, R. P.; Kuranz, C. C.; MacDonald, M. J.; Wan, W. C.; Koenig, M.; Pelka, A.; Ravasio, A.; Yurchak, R.; Kuramitsu, Y.; Sakawa, Y.; Park, H.-S.; Reville, B.; Miniati, F.; Schekochihin, A. A.; Lamb, D. Q.; Gregori, G.

    2015-11-01

    Turbulence and magnetic fields are ubiquitous in the universe. In galaxy clusters, turbulence is believed to amplify seed magnetic fields to values of a few μG, as observed through diffuse radio-synchrotron emission and Faraday rotation measurements. In this study we present experiments that emulate such a process in a controlled laboratory environment. Two laser-driven plasma flows collide to mimic the dynamics of a cluster merger. From the measured density fluctuations we infer the development of Kolmogorov-like turbulence. Measurements of the magnetic field show it is amplified by turbulent motions, reaching a non-linear regime that is a precursor to turbulent dynamo. We also present numerical simulations with the FLASH code that model these experiments. The simulations reproduce the measured plasma properties and enable us to disentangle and characterize the complex physical processes that occur in the experiment. This study provides a promising experimental platform to probe magnetic field amplification by turbulence in plasmas, a process thought to occur in many astrophysical phenomena.

  5. Kinetic energy budgets near the turbulent/nonturbulent interface in jets

    NASA Astrophysics Data System (ADS)

    Taveira, Rodrigo R.; da Silva, Carlos B.

    2013-01-01

    The dynamics of the kinetic energy near the turbulent/nonturbulent (T/NT) interface separating the turbulent from the irrotational flow regions is analysed using three direct numerical simulations of turbulent planar jets, with Reynolds numbers based on the Taylor micro-scale across the jet shear layer in the range Reλ ≈ 120-160. Important levels of kinetic energy are already present in the irrotational region near the T/NT interface. The mean pressure and kinetic energy are well described by the Bernoulli equation in this region and agree with recent results obtained from rapid distortion theory in the turbulent region [M. A. C. Teixeira and C. B. da Silva, "Turbulence dynamics near a turbulent/non-turbulent interface," J. Fluid Mech. 695, 257-287 (2012)], 10.1017/jfm.2012.17 while the normal Reynolds stresses agree with the theoretical predictions from Phillips ["The irrotational motion outside a free turbulent boundary," Proc. Cambridge Philos. Soc. 51, 220 (1955)], 10.1017/S0305004100030073. The use of conditional statistics in relation to the distance from the T/NT interface allow a detailed study of the build up of kinetic energy across the T/NT interface, pointing to a very different picture than using classical statistics. Conditional kinetic energy budgets show that apart from the viscous dissipation of kinetic energy, the maximum of all the mechanisms governing the kinetic energy are concentrated in a very narrow region distancing about one to two Taylor micro-scales from the T/NT interface. The (total and fluctuating) kinetic energy starts increasing in the irrotational region by pressure-velocity interactions - a mechanism that can act at distance, and continue to grow by advection (for the total kinetic energy) and turbulent diffusion (for the turbulent kinetic energy) inside the turbulent region. These mechanisms tend to occur preferentially around the core of the large-scale vortices existing near T/NT interface. The production of turbulent

  6. Broadband Shock Noise Reduction in Turbulent Jets by Water Injection

    NASA Technical Reports Server (NTRS)

    Kandula, Max

    2008-01-01

    The concept of effective jet properties introduced by the author (AIAA-2007-3 645) has been extended to the estimation of broadband shock noise reduction by water injection in supersonic jets. Comparison of the predictions with the test data for cold underexpanded supersonic nozzles shows a satisfactory agreement. The results also reveal the range of water mass flow rates over which saturation of mixing noise reduction and existence of parasitic noise are manifest.

  7. EXPERIMENTS WITH HEAVY GAS JETS IN LAMINAR AND TURBULENT CROSS-FLOWS

    EPA Science Inventory

    A wind tunnel study was performed to determine the dispersion characteristics of gas jets with densities heavier than that of air. he experiments were done in a laminar cross-flow and then repeated in a turbulent boundary layer. ll major boundary-layer characteristics were measur...

  8. Turbulent transport and length scale measurement experiments with comfined coaxial jets

    NASA Technical Reports Server (NTRS)

    Johnson, B. V.; Roback, R.

    1984-01-01

    A three phase experimental study of mixing downstream of swirling and nonswirling confined coaxial jets was conducted to obtain data for the evaluation and improvement of turbulent transport models currently employed in a variety of computational procedures. The present effort was directed toward the acquisition of length scale and dissipation rate data that provide more accurate inlet boundary conditions for the computational procedures and a data base to evaluate the turbulent transport models in the near jet region where recirculation does not occur, and the acquisition of mass and momentum turbulent transport data for a nonswirling flow condition with a blunt inner jet inlet configuration rather than the tapered inner jet inlet. A measurement technique, generally used to obtain approximate integral length and microscales of turbulence and dissipation rates, was computerized. Results showed the dissipation rate varied by 2 1/2 orders of magnitude across the inlet plane, by 2 orders of magnitude 51 mm from the inlet plane, and by 1 order of magnitude at 102 mm from the inlet plane for a nonswirling flow test conditions.

  9. LES, DNS and RANS for the analysis of high-speed turbulent reacting flows

    NASA Technical Reports Server (NTRS)

    Givi, Peyman; Taulbee, Dale B.; Adumitroaie, Virgil; Sabini, George J.; Shieh, Geoffrey S.

    1994-01-01

    The purpose of this research is to continue our efforts in advancing the state of knowledge in large eddy simulation (LES), direct numerical simulation (DNS), and Reynolds averaged Navier Stokes (RANS) methods for the computational analysis of high-speed reacting turbulent flows. In the second phase of this work, covering the period 1 Sep. 1993 - 1 Sep. 1994, we have focused our efforts on two research problems: (1) developments of 'algebraic' moment closures for statistical descriptions of nonpremixed reacting systems, and (2) assessments of the Dirichlet frequency in presumed scalar probability density function (PDF) methods in stochastic description of turbulent reacting flows. This report provides a complete description of our efforts during this past year as supported by the NASA Langley Research Center under Grant NAG1-1122.

  10. Investigation of the pressure and velocity fields in a turbulent separated flow using the LES technique

    NASA Astrophysics Data System (ADS)

    Arnal, M.; Friedrich, R.

    1991-01-01

    The large eddy simulation (LES) technique is utilized to investigate the turbulent separating and reattaching flow over a rearward-facing step. Simulations on a series of successively refined grids were performed (maximum resolution: 320 x 64 x 48). Statistical results are compared with experimental data and show good agreement. An examination of the simulated flow fields reveals the instantaneous structure of the separating shear layer, the reattachment zone and the recirculation region. Large departures from the mean in both the velocity and pressure fields are found to occur in all three regions. The shape and size of structures in the velocity and pressure fields varies with the proximity of solid walls and the region of the flow domain. Awareness of the instantaneous flow field structure is shown to be instrumental to having a complete understanding of the unsteady turbulent flow.

  11. a Second-Order Closure Prediction of Premixed Turbulent Combustion in Jets

    NASA Astrophysics Data System (ADS)

    Dave, Nikhil

    1985-12-01

    This thesis is a report on work carried out and results obtained in the prediction of a turbulent flow of premixed combustible gases discharging from a pipe and developing into a turbulent, combusting roundjet. The expressions for the chemical reaction rate term and other unclosed terms in the Favre averaged turbulent transport equations at the level of second-order closure are based on the Bray-Moss-Libby aerothermochemistry for premixed turbulent combustion, extended to variable enthalpy systems as in Bray, Champion, Dave, Libby (referenced herein). The numerical technique used is a parabolic solver developed by Kollmann from the GENMIX program due to Patankar and Spalding. Various test cases such as constant density and variable density jets are calculated using the program and the results are compared herein with experimentally observed values. Results for premixed turbulent combusting jets are compared with experimental data of Yoshida and of Shepherd and Moss. Buoyancy is found to play an important role in the behavior of these primixed combusting jets. Reasonable numerical agreement is obtained with the results of Yoshida, and good qualitative agreement is obtained with the data of Shepherd and Moss. Reasons for the discrepancies and limitations of the numerical simulation are discussed.

  12. Evolution of a pulsed vortex generator jet in a turbulent boundary layer

    NASA Astrophysics Data System (ADS)

    Rixon, Gregory S.; Johari, Hamid

    2001-11-01

    The development of a pulsed vortex generator jet in a turbulent boundary layer was studied experimentally. The jet was pitched 45-deg and skewed 90-deg with respect to the surface and flow direction, respectively. The jet-to-freestream velocity ratio was 3, and duty cycles of 0.5 and 0.25 were utilized. Phase-averaged data indicate that the circulation of the primary streamwise vortex averaged over the pulse duration is less than that in a steady jet. The circulation and peak vorticity decrease exponentially with the distance from the jet source at a rate comparable to that in a steady jet. The core of the primary vortex penetrates approximately 50boundary layer than the steady jet. The larger penetration takes place during the initial portion of the pulse and is due to the jet starting vortex ring. The superior performance of pulsed vortex generator jets observed in the past appears to stem from the placement of the streamwise vortices in the outer portions of the boundary layer.

  13. The interaction of jet/front systems and mountain waves: Implications for lower stratospheric aviation turbulence

    NASA Astrophysics Data System (ADS)

    Vollmer, David Russell

    The role of jet streaks and their associated upper-level structures (fronts, troughs, thermal fields, etc.) in enhancing orographically-induced aviation turbulence near and above the tropopause is investigated. The primary hypothesis for this research suggests that there is an optimal configuration for the positioning of upper-level circulations leading to vertically confluent flow and differential thermal advection, forming an intense inversion. Such a configuration may be associated with vertically-intersecting ageostrophic jet circulations or trough-induced differential vertical motions leading to cold air undercutting a warm layer aloft, and compression of the warm layer in the presence of jet-induced shear. This structure is then perturbed by mountain waves, leading to a downscale cascade of kinetic energy, eventually leading to potential aviation turbulence. Two cases of clear-air turbulence (CAT) are examined using mesoscale numerical simulations. The first case involved a DC-8 attempting to cross the Colorado Front Range when it encountered extreme CAT resulting in loss of part of one wing and an engine. In this case the superposition of two distinct jet features was hypothesized to have established an unusually strong tropopause which allowed strong buoyancy-driven motions to enhance the horizontal shear and turbulent eddies, eventually leading to the turbulent downburst hypothesized to have played a role in damaging the aircraft. The second study used data from the Terrain-Induced Rotor Experiment (T-REX) and examined a turbulent wave-breaking event recorded by a research aircraft in the lower stratosphere. A different jet regime was found in this case, with a strong upstream trough and decreasing cyclonic curvature with height above the tropopause and a strong lower stratospheric inversion. The vertical variation of static stability in the lower stratosphere was found to create a favorable environment for amplification and breaking of the mountain wave

  14. Analysis of the effect of initial conditions on the initial development of a turbulent jet

    NASA Technical Reports Server (NTRS)

    Kim, Soong KI; Chung, Myung Kyoon; Cho, Ji Ryong

    1992-01-01

    The effect of the initial condition at the jet exit on the downstream evolution, particularly within the potential core length, were numerically investigated as well as with available experimental data. In order to select the most dependable computational model for the present numerical experiment, a comparative study has been performed with different turbulence models at k-epsilon level, and it was found that the k-epsilon-gammma model yields superior prediction accuracy over other conventional models. The calculated results show that the potential core length and the spreading rate the initial mixing layer are dependent on the initial length scale as well as the turbulent kinetic energy at the jet exit. Such effect of the initial length scale increases with higher initial turbulence level. An empirical parameter has been devised to collapse the calculated data of the potential core length and the spreading rate with various initial conditions onto a single curve.

  15. Turbulent Statistics from Time-Resolved PIV Measurements of a Jet Using Empirical Mode Decomposition

    NASA Technical Reports Server (NTRS)

    Dahl, Milo D.

    2012-01-01

    Empirical mode decomposition is an adaptive signal processing method that when applied to a broadband signal, such as that generated by turbulence, acts as a set of band-pass filters. This process was applied to data from time-resolved, particle image velocimetry measurements of subsonic jets prior to computing the second-order, two-point, space-time correlations from which turbulent phase velocities and length and time scales could be determined. The application of this method to large sets of simultaneous time histories is new. In this initial study, the results are relevant to acoustic analogy source models for jet noise prediction. The high frequency portion of the results could provide the turbulent values for subgrid scale models for noise that is missed in large-eddy simulations. The results are also used to infer that the cross-correlations between different components of the decomposed signals at two points in space, neglected in this initial study, are important.

  16. Turbulent Statistics From Time-Resolved PIV Measurements of a Jet Using Empirical Mode Decomposition

    NASA Technical Reports Server (NTRS)

    Dahl, Milo D.

    2013-01-01

    Empirical mode decomposition is an adaptive signal processing method that when applied to a broadband signal, such as that generated by turbulence, acts as a set of band-pass filters. This process was applied to data from time-resolved, particle image velocimetry measurements of subsonic jets prior to computing the second-order, two-point, space-time correlations from which turbulent phase velocities and length and time scales could be determined. The application of this method to large sets of simultaneous time histories is new. In this initial study, the results are relevant to acoustic analogy source models for jet noise prediction. The high frequency portion of the results could provide the turbulent values for subgrid scale models for noise that is missed in large-eddy simulations. The results are also used to infer that the cross-correlations between different components of the decomposed signals at two points in space, neglected in this initial study, are important.

  17. Computation of a Synthetic Jet in a Turbulent Cross-Flow Boundary Layer

    NASA Technical Reports Server (NTRS)

    Rumsey, Christopher L.

    2004-01-01

    A series of unsteady Reynolds-averaged Navier-Stokes computations are performed for the flow of a synthetic jet issuing into a turbulent boundary layer through a circular orifice. This is one of the validation test cases from a synthetic jet validation workshop held in March 2004. Several numerical parameters are investigated, and the effects of three different turbulence models are explored. Both long-time-averaged and time-dependent phase-averaged results are compared to experiment. On the whole, qualitative comparisons of the mean flow quantities are fairly good. There are many differences evident in the quantitative comparisons. The calculations do not exhibit a strong dependence on the type of turbulence model employed.

  18. Reynolds number influence on statistical behaviors of turbulence in a circular free jet

    NASA Astrophysics Data System (ADS)

    Mi, J.; Xu, M.; Zhou, T.

    2013-07-01

    The present paper examines the effect of Reynolds number on turbulence properties in the transition region of a circular jet issuing from a smoothly contracting nozzle. Hot-wire measurements were performed for this investigation through varying the jet-exit Reynolds number Red (≡ Ujd/ν, where Uj, d, and ν are the jet-exit mean velocity, nozzle diameter, and kinematic viscosity) approximately from Red ≈ 4 × 103 to Red ≈ 2 × 104. Results reveal that the rates of the mean flow decay and spread vary with Reynolds number for Red < 104 and tend to become Reynolds-number independent at Red ≥ 104. Even more importantly, the small-scale turbulence properties, e.g., the mean rate of dissipation of kinetic energy (ɛ), the Kolmogorov and Taylor microscales, are found to vary in different forms over the Red ranges of Red > 104 and Red < 104. Namely, the critical Reynolds number appears to occur at Red,cr ≈ 104 across which the jet turbulence behaves distinctly. Two turbulence regimes are therefore identified: (i) developing or partially developed turbulence at Red < Red,cr and (ii) fully developed turbulence at Red ≥ Red,cr. It is suggested that the energy dissipation rate (DR) can be expressed as \\varepsilon ˜ ν U_c^2 /R^2 in regime (i) and \\varepsilon ˜ U_c^3 /R in regime (ii), where Uc and R are the centerline (or maximum) mean velocity and half-radius at which the mean velocity is 0.5Uc. In addition, the critical Reynolds number appears to vary from flow to flow.

  19. Direct and Large Eddy Simulations of Three Dimensional Round Turbulent Jets

    NASA Astrophysics Data System (ADS)

    Lienau, Jeffrey Jay

    1994-01-01

    The turbulent flow in a spatially developing three dimensional incompressible round jet is simulated numerically. High order accurate finite difference methods are developed using compact differencing with formal accuracy varying from second to seventh order in the spatial directions. A large eddy simulation model is used to simulate the effect of the unresolved part of the motion. The spatially developing flow in a round jet is computed starting with the inviscid solution and structural and statistical properties of the flow are established. The Kelvin-Helmholtz instability is induced with small disturbances at the jet pipe. The vortex sheet rolls up into rings that interact and produce a variety of structures. The structures emerging in the development of the flow are analyzed using the local and global properties of kinetic energy and vorticity. It is found that the dynamics of breakdown in the round turbulent jet consists of four mechanisms. The first is the evolution of the vortex rings via the Kelvin Helmholtz instability. The second is the instability of the vorticity between the rings which develops into the braid region. The third is the interaction of the braids with the rings and the final is the transition to turbulent flow. The transport of particles is simulated numerically in the round turbulent jet. The particles investigated were the fluid particles, hexadecane particles ranging in size from 35 to 160 μm and vaporizing pentane particles in both heated and non-heated environments. For the heated environments, limiting cases of zero and infinite heat conductivity within the droplet were investigated. Particle dispersion, time of flight, average velocity and droplet diameter are presented for the first sixty diameters of the jet and compared to experimental data. The comparisons are good which provides support for the computational results.

  20. Direct and large eddy simulations of three dimensional round turbulent jets

    NASA Astrophysics Data System (ADS)

    Lienau, Jeffrey Jay

    1994-01-01

    The turbulent flow in a spatially developing three dimensional incompressible round jet is simulated numerically. High order accurate finite difference methods are developed using compact differencing with formal accuracy varying from second to seventh order in the spatial directions. A large eddy simulation model is used to simulate the effect of the unresolved part of the motion. The spatially developing flow in a round jet is computed starting with the inviscid solution and structural and statistical properties of the flow are established. The Kelvin-Helmholtz instability is induced with small disturbances at the jet pipe. The vortex sheet rolls up into rings that interact and produce a variety of structures. The structures emerging in the development of the flow are analyzed using the local and global properties of kinetic energy and vorticity. It is found that the dynamics of breakdown in the round turbulent jet consists of four mechanisms. The first is the evolution of the vortex rings via the Kelvin Helmholtz instability. The second is the instability of the vorticity between the rings which develops into the braid region. The third is the interaction of the braids with the rings and the final is the transition to turbulent flow. The transport of particles is simulated numerically in the round turbulent jet. The particles investigated were the fluid particles, hexadecane particles ranging in size from 35 to 160 micrometers and vaporizing pentane particles in both heated and nonheated environments. For the heated environments, limiting cases of zero and infinite heat conductivity within the droplet were investigated. Particle dispersion time of flight, average velocity and droplet diameter are presented for the first sixty diameters of the jet and compared to experimental data. The comparisons are good which provides support for the computational results.

  1. Dynamics of the gas flow turbulent front in atmospheric pressure plasma jets

    NASA Astrophysics Data System (ADS)

    Pei, X.; Ghasemi, M.; Xu, H.; Hasnain, Q.; Wu, S.; Tu, Y.; Lu, X.

    2016-06-01

    In this paper, dynamic characterizations of the turbulent flow field in atmospheric pressure plasma jets (APPJs) are investigated by focusing on the effect of different APPJ parameters, such as gas flow rate, applied voltage, pulse repetition frequency, and time duration of the pulse. We utilize Schlieren photography and photomultiplier tubes (PMT) as a signal triggering of an intensified charge coupled device (ICCD) and also a high speed camera to examine the formation of the turbulent front and its dynamics. The results reveal that the turbulent front will appear earlier and closer to the tube nozzle by increasing the gas flow rate or the applied voltage amplitude. However, the pulse time duration and repetition frequency cannot change the dynamics and formation of the turbulent front. Further investigation shows that every pulse can excite one turbulent front which is created in a specific position in a laminar region and propagates downstream. It seems that the dominating mechanisms responsible for the formation of turbulent fronts in plasma jets might not be ion momentum transfer.

  2. Effect of liquid droplets on turbulence in a round gaseous jet

    NASA Technical Reports Server (NTRS)

    Mostafa, A. A.; Elghobashi, S. E.

    1986-01-01

    The main objective of this investigation is to develop a two-equation turbulence model for dilute vaporizing sprays or in general for dispersed two-phase flows including the effects of phase changes. The model that accounts for the interaction between the two phases is based on rigorously derived equations for turbulence kinetic energy (K) and its dissipation rate epsilon of the carrier phase using the momentum equation of that phase. Closure is achieved by modeling the turbulent correlations, up to third order, in the equations of the mean motion, concentration of the vapor in the carrier phase, and the kinetic energy of turbulence and its dissipation rate for the carrier phase. The governing equations are presented in both the exact and the modeled formes. The governing equations are solved numerically using a finite-difference procedure to test the presented model for the flow of a turbulent axisymmetric gaseous jet laden with either evaporating liquid droplets or solid particles. The predictions include the distribution of the mean velocity, volume fractions of the different phases, concentration of the evaporated material in the carrier phase, turbulence intensity and shear stress of the carrier phase, droplet diameter distribution, and the jet spreading rate. The predictions are in good agreement with the experimental data.

  3. Radiation from Relativistic Jets in Turbulent Magnetic Fields

    NASA Technical Reports Server (NTRS)

    Nishikawa, K.-I.; Mizuno, Y.; Medvedev, M.; Zhang, B.; Hardee, P.; Niemiec, J.; Nordlund, A.; Frederiksen, J.; Mizuno, Y.; Sol, H.; Fishman, G. J.

    2008-01-01

    Using our new 3-D relativistic electromagnetic particle (REMP) code parallelized with MPI, we have investigated long-term particle acceleration associated with an relativistic electron-positron jet propagating in an unmagnetized ambient electron-positron plasma. The simulations have been performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks. The acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value. Behind the bow shock in the jet shock strong electromagnetic fields are generated. These fields may lead to the afterglow emission. We have calculated the time evolution of the spectrum from two electrons propagating in a uniform parallel magnetic field to verify the technique.

  4. Composition PDF/photon Monte Carlo modeling of moderately sooting turbulent jet flames

    SciTech Connect

    Mehta, R.S.; Haworth, D.C.; Modest, M.F.

    2010-05-15

    A comprehensive model for luminous turbulent flames is presented. The model features detailed chemistry, radiation and soot models and state-of-the-art closures for turbulence-chemistry interactions and turbulence-radiation interactions. A transported probability density function (PDF) method is used to capture the effects of turbulent fluctuations in composition and temperature. The PDF method is extended to include soot formation. Spectral gas and soot radiation is modeled using a (particle-based) photon Monte Carlo method coupled with the PDF method, thereby capturing both emission and absorption turbulence-radiation interactions. An important element of this work is that the gas-phase chemistry and soot models that have been thoroughly validated across a wide range of laminar flames are used in turbulent flame simulations without modification. Six turbulent jet flames are simulated with Reynolds numbers varying from 6700 to 15,000, two fuel types (pure ethylene, 90% methane-10% ethylene blend) and different oxygen concentrations in the oxidizer stream (from 21% O{sub 2} to 55% O{sub 2}). All simulations are carried out with a single set of physical and numerical parameters (model constants). Uniformly good agreement between measured and computed mean temperatures, mean soot volume fractions and (where available) radiative fluxes is found across all flames. This demonstrates that with the combination of a systematic approach and state-of-the-art physical models and numerical algorithms, it is possible to simulate a broad range of luminous turbulent flames with a single model. (author)

  5. Three-dimensional Particle Tracking Velocimetry for Turbulence Applications: Case of a Jet Flow.

    PubMed

    Kim, Jin-Tae; Kim, David; Liberzon, Alex; Chamorro, Leonardo P

    2016-01-01

    3D-PTV is a quantitative flow measurement technique that aims to track the Lagrangian paths of a set of particles in three dimensions using stereoscopic recording of image sequences. The basic components, features, constraints and optimization tips of a 3D-PTV topology consisting of a high-speed camera with a four-view splitter are described and discussed in this article. The technique is applied to the intermediate flow field (5 jet at Re ≈ 7,000. Lagrangian flow features and turbulence quantities in an Eulerian frame are estimated around ten diameters downstream of the jet origin and at various radial distances from the jet core. Lagrangian properties include trajectory, velocity and acceleration of selected particles as well as curvature of the flow path, which are obtained from the Frenet-Serret equation. Estimation of the 3D velocity and turbulence fields around the jet core axis at a cross-plane located at ten diameters downstream of the jet is compared with literature, and the power spectrum of the large-scale streamwise velocity motions is obtained at various radial distances from the jet core. PMID:26967544

  6. Dynamic formulation of thickened flame model for LES of premixed turbulent combustion.

    NASA Astrophysics Data System (ADS)

    Meneveau, C.; Nottin, C.; Veynante, D.

    2000-11-01

    As demonstrated in Colin et al. (Phys. Fluids 12, p. 1843, 2000) the thickened flame model for LES of premixed combustion (TFLES) has a number of attractive features such as correct asymptotics in the limit of DNS, in the case of a thickened laminar, steady flame, etc.. For the general case of turbulent, unsteady and curved, premixed flames, the model requires empirical parameters to be specified. With the aim of decreasing the dependence on empirical parameters, the dynamic procedure is applied to this problem. We find that the traditional application of the Germano identity, which seeks undetermined multiplicative model coefficients, fails because of a trivial cancellation of the coefficients when inserted in the Germano identity. We suggest that this is a general problem when applying the dynamic model to phenomena that occur at very disparate length-scales (here the true reaction occurs in a region which is typically much thinner than the LES grid-size). On the other hand, we find that the dynamic procedure is well-posed when searching for unknown scaling exponents (instead of coefficients). A new power-law formulation of dynamic TFLES is developed, and tested using a fully compressible, sixth-order finite-difference code (NTMIX). Applications to several cases are discussed: (a) 1-D laminar flame, (b) laminar flame-vortex interaction, and (c) flame propagation through 2-D decaying isotropic turbulence.

  7. Turbulent Heat Transfer From a Slot Jet Impinging on a Flat Plate.

    PubMed

    Benmouhoub, Dahbia; Mataoui, Amina

    2013-10-01

    The flow field and heat transfer of a plane impinging jet on a hot moving wall were investigated using one point closure turbulence model. Computations were carried out by means of a finite volume method. The evolutions of mean velocity components, vorticity, skin friction coefficient, Nusselt number and pressure coefficient are examined in this paper. Two parameters of this type of interaction are considered for a given impinging distance of 8 times the nozzle thickness (H/e = 8): the jet-surface velocity ratio and the jet exit Reynolds number. The flow field structure at a given surface-to-jet velocity ratio is practically independent to the jet exit Reynolds number. A slight modification of the flow field is observed for weak surface-to-jet velocity ratios while the jet is strongly driven for higher velocity ratio. The present results satisfactorily compare to the experimental data available in the literature for Rsj ≤ 1.The purpose of this paper is to investigate this phenomenon for higher Rsj values (0 ≤ Rsj ≤ 4). It follows that the variation of the mean skin friction and the Nusselt number can be correlated according to the surface-to-jet velocity ratios and the Reynolds numbers. PMID:24895466

  8. A second-order closure prediction of premixed turbulent combustion in jets

    NASA Astrophysics Data System (ADS)

    Davé, N.; Kollmann, W.

    1987-02-01

    In this paper, a numerical prediction is reported involving second-order closure of a turbulent flow of a vertically burning, lean mixture of premixed combustible gases discharging from a pipe and developing into a turbulent combusting roundjet. Classical closures are used where available. Expressions for the chemical reaction rate term and other unclosed terms related to variable density flow in the Favre-averaged turbulent transport equations are based on the Bray-Moss-Libby aerothermochemistry for premixed turbulent combustion, extended to variable enthalpy systems. Mixing of hot burned and cool ambient gases and the attendant buoyancy effects are found to be significant physical phenomena in the behavior of such lean premixed combusting jets. Results of the simulation are compared with experimental data of Yoshida [Proceedings of the Eighteenth International Symposium on Combustion (The Combustion Institute, Pittsburgh, 1981), p. 931] with which reasonable numerical agreement is obtained. Reasons for discrepancies and possible lines for future research are discussed.

  9. On the effect of fractal generated turbulence on the heat transfer of circular impinging jets

    NASA Astrophysics Data System (ADS)

    Astarita, Tommaso; Cafiero, Gioacchino; Discetti, Stefano

    2013-11-01

    The intense local heat transfer achieved by circular impinging jets is exploited in countless industrial applications (cooling of turbine blades, paper drying, tempering of glass and metals, etc). The heat transfer rate depends mainly on the Reynolds number, the nozzle-to-plate distance and the upstream turbulence. It is possible to enhance the heat transfer by exciting/altering the large scale structures embedded within the jet. In this work turbulent energy is injected by using a fractal grid at the nozzle exit. Fractal grids can generate more intense turbulence with respect to regular grids with the same blockage ratio by enhancing the jet turbulence over different scales. Consequently, they are expected to improve the convective heat transfer. The results outline that a significant improvement is achieved (for small nozzle-to-plate distances up to 100% at the stagnation point and more than 10% on the integral heat transfer over a circular area of 3 nozzle diameters) under the same power input.

  10. Specific features of a stopped pipe blown by a turbulent jet: Aeroacoustics of the panpipes.

    PubMed

    Auvray, Roman; Fabre, Benoît; Meneses, Felipe; de la Cuadra, Patricio; Lagrée, Pierre-Yves

    2016-06-01

    Flute-like instruments with a stopped pipe were widely used in ancient cultures and continue to be used in many musical expressions throughout the globe. They offer great flexibility in the input control parameters, allowing for large excursions in the flux and in the geometrical configuration for the lips of the instrumentalist. For instance, the transverse offset of the jet axis relative to the labium can be shifted beyond the operational limits found in open-open pipes, and the total jet flux can be increased up to values that produce highly turbulent jets while remaining on the first oscillating regime. Some of the fundamental aspects of the acoustics and hydrodynamics of this kind of instrument are studied, like the instability of the jet wave and the static aerodynamic balance in the resonator. A replica of an Andean siku has been created to observe, through the Schlieren flow visualization, the behavior of both excitation and resonator of the instrument. PMID:27369145

  11. Hourly wind profiler observations of the jet stream - Wind shear and pilot reports of turbulence

    NASA Technical Reports Server (NTRS)

    Syrett, William J.

    1991-01-01

    Hourly wind profiler observations of the jet stream are reported on the basis of over 400 hr of wind and temperature data taken during two prolonged jet stream passages over western and central Pennsylvania during mid-November 1986 and mid-January 1987. The mean wind speed profile with error bars for the 79 hr that the Crown radar was determined to be 'under' the jet stream is shown. A mean speed of 83 m/s for the period was found. A plot of wind shear for the hours of interest is given. Typically, the shear was at a maximum from 3 to 4 km below the level of maximum wind. Thus, an aircraft would have to fly through potentially rough air to reach the fuel savings and relative smoothness of flight at the jet stream level. A good correlation between pilot reports of turbulence and wind shear was found.

  12. Computation of Supersonic Jet Mixing Noise Using PARC Code With a kappa-epsilon Turbulence Model

    NASA Technical Reports Server (NTRS)

    Khavaran, A.; Kim, C. M.

    1999-01-01

    A number of modifications have been proposed in order to improve the jet noise prediction capabilities of the MGB code. This code which was developed at General Electric, employees the concept of acoustic analogy for the prediction of turbulent mixing noise. The source convection and also refraction of sound due to the shrouding effect of the mean flow are accounted for by incorporating the high frequency solution to Lilley's equation for cylindrical jets (Balsa and Mani). The broadband shock-associated noise is estimated using Harper-Bourne and Fisher's shock noise theory. The proposed modifications are aimed at improving the aerodynamic predictions (source/spectrum computations) and allowing for the non- axisymmetric effects in the jet plume and nozzle geometry (sound/flow interaction). In addition, recent advances in shock noise prediction as proposed by Tam can be employed to predict the shock-associated noise as an addition to the jet mixing noise when the flow is not perfectly expanded. Here we concentrate on the aerodynamic predictions using the PARC code with a k-E turbulence model and the ensuing turbulent mixing noise. The geometry under consideration is an axisymmetric convergent-divergent nozzle at its design operating conditions. Aerodynamic and acoustic computations are compared with data as well as predictions due to the original MGB model using Reichardt's aerodynamic theory.

  13. Numerical Study of Turbulent Laryngeal Jet in the MDCT-based Human Lung Model.

    NASA Astrophysics Data System (ADS)

    Lin, Ching-Long; Tawhai, Merryn H.; McLennan, Geoffrey; Hoffman, Eric A.

    2006-11-01

    The geometry of the human upper respiratory tract is constructed from x-ray-based multidetector computed tomography (MDCT: Sensation 64) images using in house developed segmentation software. The geometry consists of a mouth piece, the mouth, the oropharynx, the larynx, and up to 6 generations of the intra-thoracic airway tree. We applied a custom-developed Characteristic-Galerkin finite element method, which solves the three-dimensional incompressible Navier-Stokes equations, to study the effect of turbulence on air flow structures in the MDCT-based lung model. In order to gather sufficient data for analysis of turbulence statistics, a constant flow rate of about 320 ml/s at the peak inspiratory phase is imposed at the terminal branches to draw air into the upper respiratory tract. The flow rate yields an average speed of about 2 m/s and a Reynolds number of 1,700 in the trachea. The characteristics of mean velocity and turbulent kinetic energy are analyzed. A curved sheet-like high-speed laryngeal jet with high turbulence intensity is formed in the trachea. Some peak frequencies associated with the jet flow are detected. Their association with turbulent coherent structures is examined. The work is sponsored by NIH Grants R01-EB-005823 and R01-HL-064368.

  14. Scattering from condensates in turbulent jets. [for crossed beam instruments

    NASA Technical Reports Server (NTRS)

    Wilson, L. N.; Dennen, R. S.

    1970-01-01

    An analysis is made of the scattering signal levels to be expected from condensed water vapor droplets for crossed-beam instruments operating in the wavelength region. 18 to 4.3 microns. The results show that scattering should not present a problem for the infrared system operating under conditions typical of the IITRI jet facility. Actual measurements made for comparison indicate that scattering levels are appreciable, and presumably result from oil mist added by the facility air compressors.

  15. Large Eddy Simulation of Gravitational Effects on Transitional and Turbulent Gas-Jet Diffusion Flames

    NASA Technical Reports Server (NTRS)

    Givi, Peyman; Jaberi, Farhad A.

    2001-01-01

    The basic objective of this work is to assess the influence of gravity on "the compositional and the spatial structures" of transitional and turbulent diffusion flames via large eddy simulation (LES), and direct numerical simulation (DNS). The DNS is conducted for appraisal of the various closures employed in LES, and to study the effect of buoyancy on the small scale flow features. The LES is based on our "filtered mass density function"' (FMDF) model. The novelty of the methodology is that it allows for reliable simulations with inclusion of "realistic physics." It also allows for detailed analysis of the unsteady large scale flow evolution and compositional flame structure which is not usually possible via Reynolds averaged simulations.

  16. Vector control of two-dimensional turbulent free jet by both-side flaps

    NASA Astrophysics Data System (ADS)

    Enokida, K.; Okamoto, T.

    1985-05-01

    This paper presents an experimental investigation of the characteristics of a two-dimensional turbulent jet deflected by the flaps attached at both sides of the nozzle. It was found that the deflection angle was nearly equal to the flap angle when the flap angle was less than the critical angle and it approached the deflection angle for one-side flap. And the length of potential core, decay of maximum velocity and spread of jet were varied with the flap angle, but were unchanged by the flap width.

  17. Coherent structures of a supersonic turbulent jet generated by natural oscillations

    NASA Astrophysics Data System (ADS)

    Zheltukhin, N. A.; Terekhova, N. M.

    The paper is concerned with the mathematical modeling of coherent structures of the longitudinal vortex type generated in supersonic turbulent jets by natural oscillations of finite intensity. Secondary regimes induced by Reynolds stresses are investigated by integrating numerically averaged equations of motion of a nonviscous non-heat-conducting compressible gas. It is found that additional radial-tangential gas flows (which lead to additional large-scale mixing) may be formed under the effect of a pair of spiral oscillations. Nonzero defects formed in the velocity profile produce additional gradients of mean motion which lead to the deformation of the jet and may affect noise generation processes.

  18. Validation Analysis for the Calculation of a Turbulent Free Jet in Water Using CFDS-FLOW 3-D and FLUENT

    SciTech Connect

    Dimenna, R.A.; Lee, S.Y.

    1995-05-01

    The application of computational fluid dynamics methods to the analysis of mixing in the high level waste tanks at the Savannah River Site requires a demonstration that the computer codes can properly represent the behavior of fluids in the tanks. The motive force for mixing the tanks is a set of jet pumps taking suction from the tank fluid and discharging turbulent jets near the bottom of the tank. The work described here focuses on the free turbulent jet in water as the simplest case of jet behavior for which data could be found in the open literature. Calculations performed with both CFDS-FLOW3D and FLUENT were compared with data as well as classical jet theory. Results showed both codes agreed reasonably well with each other and with the data, but that results were sensitive to the computational mesh and, to a lesser degree, the selection of turbulence models.

  19. Proceedings of the 2004 Workshop on CFD Validation of Synthetic Jets and Turbulent Separation Control

    NASA Technical Reports Server (NTRS)

    Rumsey, Christopher L. (Compiler)

    2007-01-01

    The papers presented here are from the Langley Research Center Workshop on Computational Fluid Dynamics (CFD) Validation of Synthetic Jets and Turbulent Separation Control (nicknamed "CFDVAL2004"), held March 2004 in Williamsburg, Virginia. The goal of the workshop was to bring together an international group of CFD practitioners to assess the current capabilities of different classes of turbulent flow solution methodologies to predict flow fields induced by synthetic jets and separation control geometries. The workshop consisted of three flow-control test cases of varying complexity, and participants could contribute to any number of the cases. Along with their workshop submissions, each participant included a short write-up describing their method for computing the particular case(s). These write-ups are presented as received from the authors with no editing. Descriptions of each of the test cases and experiments are also included.

  20. A Model for Jet-Surface Interaction Noise Using Physically Realizable Upstream Turbulence Conditions

    NASA Technical Reports Server (NTRS)

    Afsar, Mohammed Z.; Leib, Stewart J.; Bozak, Richard F.

    2016-01-01

    This paper is a continuation of previous work in which a generalized Rapid Distortion Theory (RDT) formulation was used to model low-frequency trailing-edge noise. The research was motivated by proposed next-generation aircraft configurations where the exhaust system is tightly integrated with the airframe. Data from recent experiments at NASA on the interaction between high-Reynolds-number subsonic jet flows and an external flat plate showed that the power spectral density (PSD) of the far-field pressure underwent considerable amplification at low frequencies. For example, at the 90deg observation angle, the low-frequency noise could be as much as 10 dB greater than the jet noise itself. In this paper, we present predictions of the noise generated by the interaction of a rectangular jet with the trailing edge of a semi-infinite flat plate. The calculations are based on a formula for the acoustic spectrum of this noise source derived from an exact formal solution of the linearized Euler equations involving (in this case) one arbitrary convected scalar quantity and a Rayleigh equation Green's function. A low-frequency asymptotic approximation for the Green's function based on a two-dimensional mean flow is used in the calculations along with a physically realizable upstream turbulence spectrum, which includes a finite decorrelation region. Numerical predictions of the sound field, based on three-dimensional RANS solutions to determine the mean flow, turbulent kinetic energy and turbulence length and time scales, for a range of subsonic acoustic Mach number jets and nozzle aspect ratios are compared with experimental data. Comparisons of the RANS results with flow data are also presented for selected cases. We find that a finite decorrelation region in the turbulence spectrum increases the low-frequency algebraic decay (the low frequency "roll-off") of the acoustic spectrum with angular frequency thereby producing much closer agreement with noise data for Strouhal

  1. Self-similar solution of the problem of a turbulent flow in a round submerged jet

    NASA Astrophysics Data System (ADS)

    Shmidt, A. V.

    2015-05-01

    A mathematical model of the flow in a round submerged turbulent jet is considered. The model includes differential transport equations for the normal components of the Reynolds stress tensor and Rodi's algebraic approximations for shear stresses. A theoretical-group analysis of the examined model is performed, and a reduced self-similar system of ordinary differential equations is derived and solved numerically. It is shown that the calculated results agree with available experimental data.

  2. Simulations of Turbulent Momentum and Scalar Transport in Confined Swirling Coaxial Jets

    NASA Technical Reports Server (NTRS)

    Shih, Tsan-Hsing; Liu, Nan-Suey; Moder, Jeffrey P.

    2015-01-01

    This paper presents the numerical simulations of confined three-dimensional coaxial water jets. The objectives are to validate the newly proposed nonlinear turbulence models of momentum and scalar transport, and to evaluate the newly introduced scalar APDF and DWFDF equation along with its Eulerian implementation in the National Combustion Code(NCC). Simulations conducted include the steady RANS, the unsteady RANS (URANS), and the time-filtered Navier-Stokes (TFNS); both without and with invoking the APDF or DWFDF equation.

  3. Influence of Turbulence on the Restraint of Liquid Jets by Surface Tension in Microgravity Investigated

    NASA Technical Reports Server (NTRS)

    Chato, David J.

    2002-01-01

    Microgravity poses many challenges to the designer of spacecraft tanks. Chief among these are the lack of phase separation and the need to supply vapor-free liquid or liquidfree vapor to the spacecraft processes that require fluid. One of the principal problems of phase separation is the creation of liquid jets. A jet can be created by liquid filling, settling of the fluid to one end of the tank, or even closing a valve to stop the liquid flow. Anyone who has seen a fountain knows that jets occur in normal gravity also. However, in normal gravity, the gravity controls and restricts the jet flow. In microgravity, with gravity largely absent, surface tension forces must contain jets. To model this phenomenon, a numerical method that tracks the fluid motion and the surface tension forces is required. Jacqmin has developed a phase model that converts the discrete surface tension force into a barrier function that peaks at the free surface and decays rapidly away. Previous attempts at this formulation were criticized for smearing the interface. This can be overcome by sharpening the phase function, double gridding the fluid function, and using a higher order solution for the fluid function. The solution of this equation can be rewritten as two coupled Poisson equations that also include the velocity. After the code was implemented in axisymmetric form and verified by several test cases at the NASA Glenn Research Center, the drop tower runs of Aydelott were modeled. Work last year with a laminar model was found to overpredict Aydelott's results, except at the lowest Reynolds number conditions of 400. This year, a simple turbulence model was implemented by adding a turbulent viscosity based on the mixing-length hypothesis and empirical measurements of previous works. Predictions made after this change was implemented have been much closer to experimentally observed flow patterns and geyser heights. Two model runs is shown. The first, without any turbulence correction

  4. Mass and momentum turbulent transport experiments with confined swirling coaxial jets

    NASA Technical Reports Server (NTRS)

    Roback, R.; Johnson, B. V.

    1983-01-01

    Swirling coaxial jets mixing downstream, discharging into an expanded duct was conducted to obtain data for the evaluation and improvement of turbulent transport models currently used in a variety of computational procedures throughout the combustion community. A combination of laser velocimeter (LV) and laser induced fluorescence (LIF) techniques was employed to obtain mean and fluctuating velocity and concentration distributions which were used to derive mass and momentum turbulent transport parameters currently incorporated into various combustor flow models. Flow visualization techniques were also employed to determine qualitatively the time dependent characteristics of the flow and the scale of turbulence. The results of these measurements indicated that the largest momentum turbulent transport was in the r-z plane. Peak momentum turbulent transport rates were approximately the same as those for the nonswirling flow condition. The mass turbulent transport process for swirling flow was complicated. Mixing occurred in several steps of axial and radial mass transport and was coupled with a large radial mean convective flux. Mixing for swirling flow was completed in one-third the length required for nonswirling flow.

  5. Direct numerical simulation of ignition in turbulent n-heptane liquid-fuel spray jets

    SciTech Connect

    Wang, Y.; Rutland, C.J.

    2007-06-15

    Direct numerical simulation was used for fundamental studies of the ignition of turbulent n-heptane liquid-fuel spray jets. A chemistry mechanism with 33 species and 64 reactions was adopted to describe the chemical reactions. The Eulerian method is employed to solve the carrier-gas flow field and the Lagrangian method is used to track the liquid-fuel droplets. Two-way coupling interaction is considered through the exchange of mass, momentum, and energy between the carrier-gas fluid and the liquid-fuel spray. The initial carrier-gas temperature was 1500 K. Six cases were simulated with different droplet radii (from 10 to 30 {mu}m) and two initial velocities (100 and 150 m/s). From the simulations, it was found that evaporative cooling and turbulence mixing play important roles in the ignition of liquid-fuel spray jets. Ignition first occurs at the edges of the jets where the fuel mixture is lean, and the scalar dissipation rate and the vorticity magnitude are very low. For smaller droplets, ignition occurs later than for larger droplets due to increased evaporative cooling. Higher initial droplet velocity enhances turbulence mixing and evaporative cooling. For smaller droplets, higher initial droplet velocity causes the ignition to occur earlier, whereas for larger droplets, higher initial droplet velocity delays the ignition time. (author)

  6. Numerical modeling of laser-driven experiments of colliding jets: Turbulent amplification of seed magnetic fields

    NASA Astrophysics Data System (ADS)

    Tzeferacos, Petros; Fatenejad, Milad; Flocke, Norbert; Graziani, Carlo; Gregori, Gianluca; Lamb, Donald; Lee, Dongwook; Meinecke, Jena; Scopatz, Anthony; Weide, Klaus

    2014-10-01

    In this study we present high-resolution numerical simulations of laboratory experiments that study the turbulent amplification of magnetic fields generated by laser-driven colliding jets. The radiative magneto-hydrodynamic (MHD) simulations discussed here were performed with the FLASH code and have assisted in the analysis of the experimental results obtained from the Vulcan laser facility. In these experiments, a pair of thin Carbon foils is placed in an Argon-filled chamber and is illuminated to create counter-propagating jets. The jets carry magnetic fields generated by the Biermann battery mechanism and collide to form a highly turbulent region. The interaction is probed using a wealth of diagnostics, including induction coils that are capable of providing the field strength and directionality at a specific point in space. The latter have revealed a significant increase in the field's strength due to turbulent amplification. Our FLASH simulations have allowed us to reproduce the experimental findings and to disentangle the complex processes and dynamics involved in the colliding flows. This work was supported in part at the University of Chicago by DOE NNSA ASC.

  7. Large eddy simulation study of turbulent kinetic energy and scalar variance budgets and turbulent/non-turbulent interface in planar jets

    NASA Astrophysics Data System (ADS)

    Watanabe, Tomoaki; Sakai, Yasuhiko; Nagata, Koji; Ito, Yasumasa

    2016-04-01

    Spatially developing planar jets with passive scalar transports are simulated for various Reynolds (Re = 2200, 7000, and 22 000) and Schmidt numbers (Sc = 1, 4, 16, 64, and 128) by the implicit large eddy simulation (ILES) using low-pass filtering as an implicit subgrid-scale model. The budgets of resolved turbulent kinetic energy k and scalar variance < {φ }\\prime 2> are explicitly evaluated from the ILES data except for the dissipation terms, which are obtained from the balance in the transport equations. The budgets of k and < {φ }\\prime 2> in the ILES agree well with the DNS and experiments for both high and low Re cases. The streamwise decay of the mean turbulent kinetic energy dissipation rate obeys the power low obtained by the scaling argument. The mechanical-to-scalar timescale ratio C ϕ is evaluated in the self-similar region. For the high Re case, C ϕ is close to the isotropic value (C ϕ = 2) near the jet centerline. However, when Re is not large, C ϕ is smaller than 2 and depends on the Schmidt number. The T/NT interface is also investigated by using the scalar isosurface. The velocity and scalar fields near the interface depend on the interface orientation for all Re. The velocity toward the interface is observed near the interface facing in the streamwise, cross-streamwise, and spanwise directions in the planar jet in the resolved velocity field.

  8. Deposition of micron liquid droplets on wall in impinging turbulent air jet

    NASA Astrophysics Data System (ADS)

    Liu, Tianshu; Nink, Jacob; Merati, Parviz; Tian, Tian; Li, Yong; Shieh, Tom

    2010-06-01

    The fluid mechanics of the deposition of micron liquid (olive oil) droplets on a glass wall in an impinging turbulent air jet is studied experimentally. The spatial patterns of droplets deposited on a wall are measured by using luminescent oil visualization technique, and the statistical data of deposited droplets are obtained through microscopic imagery. Two distinct rings of droplets deposited on a wall are found, and the mechanisms of the formation of the inner and outer rings are investigated based on global diagnostics of velocity and skin friction fields. In particular, the intriguing effects of turbulence, including large-scale coherent vortices and small-scale random turbulence, on micron droplet deposition on a wall and coalescence in the air are explored.

  9. Effect of Pulsed Plasma Jets on Reflected Shock-Turbulent Boundary Layer Interaction

    NASA Astrophysics Data System (ADS)

    Greene, Benton R.; Clemens, Noel T.; Magari, Patrick; Micka, Daniel

    2013-11-01

    Shock-induced turbulent boundary layer separation can have many detrimental effects in supersonic inlets including flow instability, fatigue of structural panels, poor pressure recovery, and unstart. Pulsed plasma jets (or ``spark jets''), zero net mass flow jets characterized by high bandwidth and the ability to direct momentum into the flow, are one promising method of reducing shock-induced separation and boundary layer distortion. The current study is focused on investigating the efficacy of pulsed plasma jets to reduce the boundary layer distortion induced by a reflected shock interaction in a Mach 3 flow. A 7° shock generator placed outside the tunnel ceiling boundary layer produces an incident shock on the floor of the tunnel of sufficient strength to induce separation. An array of pulsed plasma jets are placed approximately 2 boundary layer thicknesses upstream of the interaction and pulsed at between 1 kHz and 4 kHz. PIV is used to investigate the effect of the jets on the nature of the separation as well as the boundary layer distortion and pressure recovery downstream of the interaction. Funded through AFRL in collaboration with Creare, Inc.

  10. The Interaction of a Vortex Generator Jet with a Turbulent Boundary Layer

    NASA Astrophysics Data System (ADS)

    Rixon, Gregory S.; Johari, Hamid

    2000-11-01

    The interaction between a steady vortex generator jet and a turbulent boundary layer has been experimentally investigated using the Digital Particle Image Velocimetry technique. Experiments were conducted over a flat plate in a re-circulating water tunnel. The jet velocity vector had a fixed pitch angle of 45° with respect to the flat plate surface, and a fixed skew angle of 90° with respect to the streamwise direction. Nominal jet-to-freestream velocity ratios of 1, 2, and 3 were examined at four different locations, 5, 10, 20, and 30 d downstream of the jet exit, corresponding to 0.7, 1.4, 2.8, and 4.2 δ. Peak vorticity, circulation, and vortex core trajectory were found to vary linearly with the jet velocity. Peak vorticity and circulation decay exponentially with distance along the plate. This decay rate was found to be much higher than that of solid vortex generators. In the near field, however, the peak vorticity of the high velocity jet was comparable to that of a solid vortex generator.

  11. Large Eddy Simulation of a cooling impinging jet to a turbulent crossflow

    NASA Astrophysics Data System (ADS)

    Georgiou, Michail; Papalexandris, Miltiadis

    2015-11-01

    In this talk we report on Large Eddy Simulations of a cooling impinging jet to a turbulent channel flow. The impinging jet enters the turbulent stream in an oblique direction. This type of flow is relevant to the so-called ``Pressurized Thermal Shock'' phenomenon that can occur in pressurized water reactors. First we elaborate on issues related to the set-up of the simulations of the flow of interest such as, imposition of turbulent inflows, choice of subgrid-scale model and others. Also, the issue of the commutator error due to the anisotropy of the spatial cut-off filter induced by non-uniform grids is being discussed. In the second part of the talk we present results of our simulations. In particular, we focus on the high-shear and recirculation zones that are developed and on the characteristics of the temperature field. The budget for the mean kinetic energy of the resolved-scale turbulent velocity fluctuations is also discussed and analyzed. Financial support has been provided by Bel V, a subsidiary of the Federal Agency for Nuclear Control of Belgium.

  12. Scalar dissipation rate statistics in turbulent swirling jets

    NASA Astrophysics Data System (ADS)

    Stetsyuk, V.; Soulopoulos, N.; Hardalupas, Y.; Taylor, A. M. K. P.

    2016-07-01

    The scalar dissipation rate statistics were measured in an isothermal flow formed by discharging a central jet in an annular stream of swirling air flow. This is a typical geometry used in swirl-stabilised burners, where the central jet is the fuel. The flow Reynolds number was 29 000, based on the area-averaged velocity of 8.46 m/s at the exit and the diameter of 50.8 mm. The scalar dissipation rate and its statistics were computed from two-dimensional imaging of the mixture fraction fields obtained with planar laser induced fluorescence of acetone. Three swirl numbers, S, of 0.3, 0.58, and 1.07 of the annular swirling stream were considered. The influence of the swirl number on scalar mixing, unconditional, and conditional scalar dissipation rate statistics were quantified. A procedure, based on a Wiener filter approach, was used to de-noise the raw mixture fraction images. The filtering errors on the scalar dissipation rate measurements were up to 15%, depending on downstream positions from the burner exit. The maximum of instantaneous scalar dissipation rate was found to be up to 35 s-1, while the mean dissipation rate was 10 times smaller. The probability density functions of the logarithm of the scalar dissipation rate fluctuations were found to be slightly negatively skewed at low swirl numbers and almost symmetrical when the swirl number increased. The assumption of statistical independence between the scalar and its dissipation rate was valid for higher swirl numbers at locations with low scalar fluctuations and less valid for low swirl numbers. The deviations from the assumption of statistical independence were quantified. The conditional mean of the scalar dissipation rate, the standard deviation of the scalar dissipation rate fluctuations, the weighted probability of occurrence of the mean conditional scalar dissipation rate, and the conditional probability are reported.

  13. Summary of the 2004 CFD Validation Workshop on Synthetic Jets and Turbulent Separation Control

    NASA Technical Reports Server (NTRS)

    Rumsey, C. L.; Gatski, T. B.; Sellers, W. L., III; Vatsa, V. N.; Viken, S. A.

    2004-01-01

    A CFD validation workshop for synthetic jets and turbulent separation control (CFDVAL2004) was held in Williamsburg, Virginia in March 2004. Three cases were investigated: synthetic jet into quiescent air, synthetic jet into a turbulent boundary layer crossflow, and flow over a hump model with no-flow-control, steady suction, and oscillatory control. This paper is a summary of the CFD results from the workshop. Although some detailed results are shown, mostly a broad viewpoint is taken, and the CFD state-of-the-art for predicting these types of flows is evaluated from a general point of view. Overall, for synthetic jets, CFD can only qualitatively predict the flow physics, but there is some uncertainty regarding how to best model the unsteady boundary conditions from the experiment consistently. As a result. there is wide variation among CFD results. For the hump flow, CFD as a whole is capable of predicting many of the particulars of this flow provided that tunnel blockage is accounted for, but the length of the separated region compared to experimental results is consistently overpredicted.

  14. Summary of the 2004 CFD Validation Workshop on Synthetic Jets and Turbulent Separation Control

    NASA Technical Reports Server (NTRS)

    Rumsey, C. L.; Gatski, T. B.; Sellers, W. L., III; Vatsa, V. N.; Viken, S. A.

    2006-01-01

    A computational fluid dynamics (CFD) validation workshop for synthetic jets and turbulent separation control (CFDVAL2004) was held in Williamsburg, Virginia in March 2004. Three cases were investigated: synthetic jet into quiescent air, synthetic jet into a turbulent boundary layer crossflow, and flow over a hump model with no-flow-control, steady suction, and oscillatory control. This paper is a summary of the CFD results from the workshop. Although some detailed results are shown, mostly a broad viewpoint is taken, and the CFD state-of-the-art for predicting these types of flows is evaluated from a general point of view. Overall, for synthetic jets, CFD can only qualitatively predict the flow physics, but there is some uncertainty regarding how to best model the unsteady boundary conditions from the experiment consistently. As a result, there is wide variation among CFD results. For the hump flow, CFD as a whole is capable of predicting many of the particulars of this flow provided that tunnel blockage is accounted for, but the length of the separated region compared to experimental results is consistently overpredicted.

  15. A comparison of turbulence models for uses in calculations of free jets and flames

    NASA Astrophysics Data System (ADS)

    Chang, K. C.; Lin, R. S.

    1989-01-01

    A comparison of commonly used turbulence models in jet flows is made. The selected turbulence models include: Prandtl mixing length model and the k-epsilon model. Among many modified k-epsilon models, five models which were reported to work well are selected for test. It is found that the mixing length model and the modified k-epsilon model of Launder, et al., are able to give quite satisfactory predictions in cold free jets. The application of Launder's model to the prediction of jet flame reveals that it totally fails to work. This result demonstrates that the empirical modification of k-epsilon model made according to the case of cold flows should be carefully examined before applicable to the case of reacting flows. The prediction of jet flame using mixing length model are comparable to that using the standard k-epsilon model incorporated with the battlement-shape PDF and the eddy-break-up model. However, since the k-epsilon models have more potential to integrate with the more sophisticated combustion model, they are worthy to be subjected to further study.

  16. On the origin of jets and vortices in turbulent planetary atmospheres.

    NASA Astrophysics Data System (ADS)

    Jougla, Thibault; Dritschel, David G.

    2016-04-01

    Stratified rotating fluids tend to form large scale coherent structures. These structures are present in many different geophysical fluids, for example jet streams in the Earth's atmosphere, the famous and conspicuous jets in the Jovian atmosphere, and oceanic jets like the latent jets and the well-known main currents including the Gulf stream and Kuroshio. Observations, numerical models, and laboratory experiments have sought to explain their origins and their evolutions. To investigate the coexistence, evolution and vertical structure of jets and vortices in turbulent planetary atmospheres, we make use of the widely studied two-layer quasi-geostrophic shallow water model on the β-plane. Numerical simulations at ultra-high resolution are carried out with the Combined Lagrangian Advection Method [1]. Following Panetta 1988 [2], to characterise the pole to equator heating variation on a planet, a vertical shear is imposed and maintained by thermal damping. To crudely represent convection from the bottom layer to the top layer, hetons are constantly added to the flow. Many numerical simulations covering a large range of parameters have been run. The thermal damping and vertical shear dependence has been widely studied and analysed. The baroclinicity of the flow is clearly evident in all cases studied. Moreover, the flow is strongly dependent on thermal damping. There is a competition between baroclinic instabilities trying to reduce the imposed vertical shear and thermal damping trying to maintain the vertical shear. Without any thermal damping, the imposed vertical shear quickly erodes. On the other hand if the thermal damping is very high, the flow is mainly dominated by incoherent, small-scale turbulence. For weaker thermal damping, the competition between baroclinic instability and thermal damping may lead to oscillations between stable and turbulent phases. However, thermal damping does not have a significant impact on the number of homogeneous regions and jets

  17. Effect of the driving algorithm on the turbulence generated by a random jet array

    NASA Astrophysics Data System (ADS)

    Pérez-Alvarado, Alejandro; Mydlarski, Laurent; Gaskin, Susan

    2016-02-01

    Different driving algorithms for a large random jet array (RJA) were tested and their performance characterized by comparing the statistics of the turbulence generated downstream of the RJA. Of particular interest was the spatial configuration of the jets operating at any given instant (an aspect that has not been documented in previous RJAs studies), as well as the statistics of their respective on/off times. All algorithms generated flows with nonzero skewnesses of the velocity fluctuation normal to the plane of the RJA (identified as an inherent limitation of the system resulting from the unidirectional forcing imposed from only one side of the RJA), and slightly super-Gaussian kurtoses of the velocity fluctuations in all directions. It was observed that algorithms imposing spatial configurations generated the most isotropic flows; however, they suffered from high mean flows and low turbulent kinetic energies. The algorithm identified as RANDOM (also referred to as the "sunbathing algorithm") generated the flow that, on an overall basis, most closely approximated zero-mean-flow homogeneous isotropic turbulence, with variations in horizontal and vertical homogeneities of RMS velocities of no more than ±6 %, deviations from isotropy ( w RMS/ u RMS) in the range of 0.62-0.77, and mean flows on the order of 7 % of the RMS velocities (determined by averaging their absolute values over the three velocity components and three downstream distances). A relatively high turbulent Reynolds number ( Re T = u T ℓ/ ν = 2360, where ℓ is the integral length scale of the flow and u T is a characteristic RMS velocity) was achieved using the RANDOM algorithm and the integral length scale ( ℓ = 11.5 cm) is the largest reported to date. The quality of the turbulence in our large facility demonstrates the ability of RJAs to be scaled-up and to be the laboratory system most capable of generating the largest quasi-homogeneous isotropic turbulent regions with zero mean flow.

  18. "Hypothetical" Heavy Particles Dynamics in LES of Turbulent Dispersed Two-Phase Channel Flow

    NASA Technical Reports Server (NTRS)

    Gorokhovski, M.; Chtab, A.

    2003-01-01

    The extensive experimental study of dispersed two-phase turbulent flow in a vertical channel has been performed in Eaton's research group in the Mechanical Engineering Department at Stanford University. In Wang & Squires (1996), this study motivated the validation of LES approach with Lagrangian tracking of round particles governed by drag forces. While the computed velocity of the flow have been predicted relatively well, the computed particle velocity differed strongly from the measured one. Using Monte Carlo simulation of inter-particle collisions, the computation of Yamamoto et al. (2001) was specifically performed to model Eaton's experiment. The results of Yamamoto et al. (2001) improved the particle velocity distribution. At the same time, Vance & Squires (2002) mentioned that the stochastic simualtion of inter-particle collisions is too expensive, requiring significantly more CPU resources than one needs for the gas flow computation. Therefore, the need comes to account for the inter-particle collisions in a simpler and still effective way. To present such a model in the framework of LES/Lagrangian particle approach, and to compare the calculated results with Eaton's measurement and modeling of Yamamoto is the main objective of the present paper.

  19. Efficiency prediction for a low head bulb turbine with SAS SST and zonal LES turbulence models

    NASA Astrophysics Data System (ADS)

    Jošt, D.; Škerlavaj, A.

    2014-03-01

    A comparison between results of numerical simulations and measurements for a 3-blade bulb turbine is presented in order to determine an appropriate numerical setup for accurate and reliable simulations of flow in low head turbines. Numerical analysis was done for three angles of runner blades at two values of head. For the smallest blade angle the efficiency was quite accurately predicted, but for the optimal and maximal blade angles steady state analysis entirely failed to predict the efficiency due to underestimated torque on the shaft and incorrect results in the draft tube. Transient simulation with SST did not give satisfactory results, but with SAS and zonal LES models the prediction of efficiency was significantly improved. From the results obtained by SAS and zonal LES the interdependence between turbulence models, vortex structures in the flow, values of eddy viscosity and flow energy losses in the draft tube can be seen. Also the effect of using the bounded central differential scheme instead of the high resolution scheme was evident. To test the effect of grid density, simulations were performed on four grids. While a difference between results obtained on the basic grid and on the fine grid was small, the results obtained on the coarse grids were not satisfactory.

  20. Response of flame thickness and propagation speed under intense turbulence in spatially developing lean premixed methane–air jet flames

    SciTech Connect

    Sankaran, Ramanan; Hawkes, Evatt R.; Yoo, Chun Sang; Chen, Jacqueline H.

    2015-06-22

    Direct numerical simulations of three-dimensional spatially-developing turbulent Bunsen flames were performed at three different turbulence intensities. We performed these simulations using a reduced methane–air chemical mechanism which was specifically tailored for the lean premixed conditions simulated here. A planar-jet turbulent Bunsen flame configuration was used in which turbulent preheated methane–air mixture at 0.7 equivalence ratio issued through a central jet and was surrounded by a hot laminar coflow of burned products. The turbulence characteristics at the jet inflow were selected such that combustion occured in the thin reaction zones (TRZ) regime. At the lowest turbulence intensity, the conditions fall on the boundary between the TRZ regime and the corrugated flamelet regime, and progressively moved further into the TRZ regime by increasing the turbulent intensity. The data from the three simulations was analyzed to understand the effect of turbulent stirring on the flame structure and thickness. Furthermore, statistical analysis of the data showed that the thermal preheat layer of the flame was thickened due to the action of turbulence, but the reaction zone was not significantly affected. A global and local analysis of the burning velocity of the flame was performed to compare the different flames. Detailed statistical averages of the flame speed were also obtained to study the spatial dependence of displacement speed and its correlation to strain rate and curvature.

  1. Response of flame thickness and propagation speed under intense turbulence in spatially developing lean premixed methane–air jet flames

    DOE PAGESBeta

    Sankaran, Ramanan; Hawkes, Evatt R.; Yoo, Chun Sang; Chen, Jacqueline H.

    2015-06-22

    Direct numerical simulations of three-dimensional spatially-developing turbulent Bunsen flames were performed at three different turbulence intensities. We performed these simulations using a reduced methane–air chemical mechanism which was specifically tailored for the lean premixed conditions simulated here. A planar-jet turbulent Bunsen flame configuration was used in which turbulent preheated methane–air mixture at 0.7 equivalence ratio issued through a central jet and was surrounded by a hot laminar coflow of burned products. The turbulence characteristics at the jet inflow were selected such that combustion occured in the thin reaction zones (TRZ) regime. At the lowest turbulence intensity, the conditions fall onmore » the boundary between the TRZ regime and the corrugated flamelet regime, and progressively moved further into the TRZ regime by increasing the turbulent intensity. The data from the three simulations was analyzed to understand the effect of turbulent stirring on the flame structure and thickness. Furthermore, statistical analysis of the data showed that the thermal preheat layer of the flame was thickened due to the action of turbulence, but the reaction zone was not significantly affected. A global and local analysis of the burning velocity of the flame was performed to compare the different flames. Detailed statistical averages of the flame speed were also obtained to study the spatial dependence of displacement speed and its correlation to strain rate and curvature.« less

  2. Dynamic Models for LES of Turbulent Front Propagation With a Spectral Method

    NASA Technical Reports Server (NTRS)

    Im, H. G.; Lund, T. S.; Ferziger, J. H.

    1996-01-01

    Direct numerical simulation of turbulent reacting flows places extreme demands on computational resources. At the present time, simulations can be performed only for greatly simplified reaction systems and for very low Reynolds numbers. Direct simulation of more realistic cases occurring at higher Reynolds number and including multiple species and numerous chemical reactions will exceed available computational resources far into the future. Because of this, there is a clear need to develop the technique of large eddy simulation for reacting flows. Unfortunately this task is complicated by the fact that combustion arises from chemical reactions that occur at the smallest scales of the flow. Capturing the large-scale behavior without resolving the small-scale details is extremely difficult in combustion problems. Thus LES modeling for turbulent combustion encounters difficulties not present in modeling momentum transport, in which the main effect of the small scales is to provide dissipation. The difficulty is more pronounced in premixed combustion, where detailed chemistry plays an essential role in determining the flame speed (or overall burning rate); in nonpremixed combustion infinite rate chemistry can be assumed, eliminating the small scale features to a first approximation.

  3. A simulation of a bluff-body stabilized turbulent premixed flame using LES-PDF

    NASA Astrophysics Data System (ADS)

    Kim, Jeonglae; Pope, Stephen

    2013-11-01

    A turbulent premixed flame stabilized by a triangular cylinder as a flame-holder is simulated. The computational condition matches the Volvo experiments (Sjunnesson et al. 1992). Propane is premixed at a fuel lean condition of ϕ = 0 . 65 . For this reactive simulation, LES-PDF formulation is used, similar to Yang et al. (2012). The evolution of Lagrangian particles is simulated by solving stochastic differential equations modeling transport of the composition PDF. Mixing is modeled by the modified IEM model (Viswanathan et al. 2011). Chemical reactions are calculated by ISAT and for the good load balancing, PURAN distribution of ISAT tables is applied (Hiremath et al. 2012). To calculate resolved density, the two-way coupling (Popov & Pope 2013) is applied, solving a transport equation of resolved specific volume to reduce statistical noise. A baseline calculation shows a good agreement with the experimental measurements in turbulence statistics, temperature, and minor species mass fractions. Chemical reaction does not significantly contribute to the overall computational cost, in contrast to non-premixed flame simulations (Hiremath et al. 2013), presumably due to the restricted manifold of the purely premixed flame in the composition space.

  4. Turbulence Analysis Upstream of a Wind Turbine: a LES Approach to Improve Wind LIDAR Technology

    NASA Astrophysics Data System (ADS)

    Calaf, M.

    2015-12-01

    Traditionally wind turbines learn about the incoming wind conditions by means of a wind vane and a cup anemometer. This approach presents two major limitations: 1) because the measurements are done at the nacelle, behind the rotor blades, the wind observations are perturbed inducing potential missalignement and power losses; 2) no direct information of the incoming turbulence is extracted, limiting the capacity to timely adjust the wind turbine against strong turbulent intensity events. Recent studies have explored the possibility of using wind LIDAR (Light Detection and Ranging) to overcome these limitations (Angelou et al. 2010 and Mikelsen et al., 2013). By installing a wind LIDAR at the nacelle of a wind turbine one can learn about the incoming wind and turbulent conditions ahead of time to timely readjust the turbine settings. Yet several questions remain to be answered such as how far upstream one should measure and what is the appropriate averaging time to extract valuable information. In light of recent results showing the relevance of atmospheric stratification in wind energy applications, it is expected that different averaging times and upstream scanning distances are advised for wind LIDAR measurements. A Large Eddy Simulation (LES) study exploring the use of wind LIDAR technology within a wind farm has been developed. The wind farm consists of an infinite array of horizontal axis wind turbines modeled using the actuator disk with rotation. The model also allows the turbines to dynamically adjust their yaw with the incoming wind vector. The flow is forced with a constant geostrophic wind and a time varying surface temperature reproducing a realistic diurnal cycle. Results will be presented showing the relevance of the averaging time for the different flow characteristics as well as the effect of different upstream scanning distances. While it is observed that within a large wind farm there are no-significant gains in power output by scanning further

  5. Numerical simulation of the interaction of a transverse jet with a supersonic flow using different turbulence models

    NASA Astrophysics Data System (ADS)

    Volkov, K. N.; Emelyanov, V. N.; Yakovchuk, M. S.

    2015-09-01

    This paper presents a numerical simulation of the flow resulting from transverse jet injection into a supersonic flow through a slot nozzle at different pressures in the injected jet and the crossflow. Calculations on grids with different resolutions use the Spalart-Allmaras turbulence model, the k- ɛ model, the k- ω model, and the SST model. Based on a comparison of the calculated and experimental data on the wall pressure distribution, the length of the recirculation area, and the depth of jet penetration into the supersonic flow, conclusions are made on the accuracy of the calculation results for the different turbulence models and the applicability of these models to similar problems.

  6. A Study of Strain Rate Effects for Turbulent Premixed Flames with Application to LES of a Gas Turbine Combustor Model

    DOE PAGESBeta

    Kemenov, Konstantin A.; Calhoon, William H.

    2015-03-24

    Large-scale strain rate field, a resolved quantity which is easily computable in large-eddy simulations (LES), could have profound effects on the premixed flame properties by altering the turbulent flame speed and inducing local extinction. The role of the resolved strain rate has been investigated in a posterior LES study of GE lean premixed dry low NOx emissions LM6000 gas turbine combustor model. A novel approach which is based on the coupling of the lineareddy model with a one-dimensional counter-flow solver has been applied to obtain the parameterizations of the resolved premixed flame properties in terms of the reactive progress variable,more » the local strain rate measure, and local Reynolds and Karlovitz numbers. The strain rate effects have been analyzed by comparing LES statistics for several models of the turbulent flame speed, i.e, with and without accounting for the local strain rate effects, with available experimental data. The sensitivity of the simulation results to the inflow velocity conditions as well as the grid resolution have been also studied. Overall, the results suggest the necessity to represent the strain rate effects accurately in order to improve LES modeling of the turbulent flame speed.« less

  7. A Study of Strain Rate Effects for Turbulent Premixed Flames with Application to LES of a Gas Turbine Combustor Model

    SciTech Connect

    Kemenov, Konstantin A.; Calhoon, William H.

    2015-03-24

    Large-scale strain rate field, a resolved quantity which is easily computable in large-eddy simulations (LES), could have profound effects on the premixed flame properties by altering the turbulent flame speed and inducing local extinction. The role of the resolved strain rate has been investigated in a posterior LES study of GE lean premixed dry low NOx emissions LM6000 gas turbine combustor model. A novel approach which is based on the coupling of the lineareddy model with a one-dimensional counter-flow solver has been applied to obtain the parameterizations of the resolved premixed flame properties in terms of the reactive progress variable, the local strain rate measure, and local Reynolds and Karlovitz numbers. The strain rate effects have been analyzed by comparing LES statistics for several models of the turbulent flame speed, i.e, with and without accounting for the local strain rate effects, with available experimental data. The sensitivity of the simulation results to the inflow velocity conditions as well as the grid resolution have been also studied. Overall, the results suggest the necessity to represent the strain rate effects accurately in order to improve LES modeling of the turbulent flame speed.

  8. Analysis of turbulent free jet hydrogen-air diffusion flames with finite chemical reaction rates

    NASA Technical Reports Server (NTRS)

    Sislian, J. P.

    1978-01-01

    The nonequilibrium flow field resulting from the turbulent mixing and combustion of a supersonic axisymmetric hydrogen jet in a supersonic parallel coflowing air stream is analyzed. Effective turbulent transport properties are determined using the (K-epsilon) model. The finite-rate chemistry model considers eight reactions between six chemical species, H, O, H2O, OH, O2, and H2. The governing set of nonlinear partial differential equations is solved by an implicit finite-difference procedure. Radial distributions are obtained at two downstream locations of variables such as turbulent kinetic energy, turbulent dissipation rate, turbulent scale length, and viscosity. The results show that these variables attain peak values at the axis of symmetry. Computed distributions of velocity, temperature, and mass fraction are also given. A direct analytical approach to account for the effect of species concentration fluctuations on the mean production rate of species (the phenomenon of unmixedness) is also presented. However, the use of the method does not seem justified in view of the excessive computer time required to solve the resulting system of equations.

  9. Numerical Implementation of Molecular Transport and Mixing in LES/FDF of Turbulent Flows

    NASA Astrophysics Data System (ADS)

    Viswanathan, Sharadha; Pope, Stephen

    2009-11-01

    In large-eddy simulations of turbulent flames, the effect of molecular diffusion on scalar transport is significant and therefore needs to be modeled correctly in Lagrangian Filtered Density Function (FDF) methods. McDermott et al. (2007) show that in FDF methods, modeling molecular diffusion as a mean drift term in the scalar equation avoids the spurious production of scalar variance. Following McDermott et al., we incorporate the effects of molecular diffusion in the Interaction by Exchange with the Mean (IEM) mixing model. In this study, we evaluate two second-order-accurate numerical methods viz. Particle-In-Cell (PIC) and Cloud-In-Cell (CIC) for implementing mixing. Given the nominal particle number density and problem geometry in a typical FDF calculation of a jet flame, the estimated mean field needs to be smoothed both for variance reduction and for proper treatment of empty cells near the jet axis. Our numerical studies show that while both implementations achieve detailed conservation and guarantee boundedness of the scalar field, CIC is computationally expensive and PIC is dependent on the scalar bounds. But for a given accuracy, PIC with smoothing incurs typically half the cost of CIC, for an appropriate choice of the smoothing parameter. This research is supported by the Department of Energy under Grant No. DE-FG02-90ER.

  10. Mean Velocity, Turbulence Intensity and Turbulence Convection Velocity Measurements for a Convergent Nozzle in a Free Jet Wind Tunnel. Comprehensive Data Report

    NASA Technical Reports Server (NTRS)

    Mccolgan, C. J.; Larson, R. S.

    1977-01-01

    The effect of flight on the mean flow and turbulence properties of a 0.056m circular jet were determined in a free jet wind tunnel. The nozzle exit velocity was 122 m/sec, and the wind tunnel velocity was set at 0, 12, 37, and 61 m/sec. Measurements of flow properties including mean velocity, turbulence intensity and spectra, and eddy convection velocity were carried out using two linearized hot wire anemometers. This report contains the raw data and graphical presentations. The final technical report includes a description of the test facilities, test hardware, along with significant test results and conclusions.

  11. Conjugate heat transfer study of a turbulent slot jet impinging on a moving plate

    NASA Astrophysics Data System (ADS)

    Achari, A. Madhusudana; Das, Manab Kumar

    2016-07-01

    Numerical simulation of the flow field and conjugate heat transfer in an impinging jet with moving impingement plate is one of the important problems as it mimics closely with practical applications in industries. The Yang-Shih version of low Reynolds number k-ɛ model has been used to resolve the flow field and the temperature field in a two-dimensional, steady, incompressible, confined, turbulent slot jet impinging normally on a moving flat plate of finite thickness. The turbulence intensity and the Reynolds number considered at the inlet are 2 % and 15,000, respectively. The bottom face of the impingement plate has been maintained at a constant temperature higher than the nozzle exit temperature. The confinement plate has been considered to be adiabatic. The nozzle-to-surface spacing for the above study has been taken to be 6 and the surface-to-jet velocity ratios have been taken over a range of 0.25-1. The effects of impingement plate motion on the flow field and temperature field have been discussed elaborately with reference to stationary impingement plate. The dependence of flow field and fluid temperature field on impingement plate motion has been analyzed by plotting streamlines, isotherms for different plate speeds. A thorough study of flow characteristics for different surface-to-jet velocity ratios has been carried out by plotting profiles of mean vertical and horizontal components of velocity, pressure distribution, local shear stress distribution. The isotherms in the impingement plate of finite thickness, the distributions of solid-fluid interface temperature, the local Nusselt number, and the local heat flux for different surface-to-jet velocity ratios added to the understanding of conjugate heat transfer phenomenon.

  12. DNS and LES of Turbulent Backward-Facing Step Flow Using 2ND-and 4TH-Order Discretization

    NASA Astrophysics Data System (ADS)

    Meri, Adnan; Wengle, Hans

    Results are presented from a Direct Numerical Simulation (DNS) and Large-Eddy Simulations (LES) of turbulent flow over a backward-facing step (Reh=3300) with a fully developed channel flow (Rcτ=180) utilized asatime-dependent inflow condition. Numerical solutions using a fourth-order compact (Hermitian) scheme, which was formulated directly for anon-equidistant and staggered grid in [1] are compared with numerical solutions using the classical second-order central scheme. There sults from LES (using the dynamic subgrid scale model) are evaluated against a corresponding DNS reference data set (fourth-order solution).

  13. Turbulent Jet Flames Into a Vitiated Coflow. PhD Thesis awarded Spring 2003

    NASA Technical Reports Server (NTRS)

    Holdeman, James D. (Technical Monitor); Cabra, Ricardo

    2004-01-01

    Examined is the vitiated coflow flame, an experimental condition that decouples the combustion processes of flows found in practical combustors from the associated recirculating fluid mechanics. The configuration consists of a 4.57 mm diameter fuel jet into a coaxial flow of hot combustion products from a lean premixed flame. The 210 mm diameter coflow isolates the jet flame from the cool ambient, providing a hot environment similar to the operating conditions of advanced combustors; this important high temperature element is lacking in the traditional laboratory experiments of jet flames into cool (room) air. A family of flows of increasing complexity is presented: 1) nonreacting flow, 2) all hydrogen flame (fuel jet and premixed coflow), and 3) set of methane flames. This sequence of experiments provides a convenient ordering of validation data for combustion models. Laser Raman-Rayleigh-LIF diagnostics at the Turbulent Diffusion Flame laboratory of Sandia National Laboratories produced instantaneous multiscalar point measurements. These results attest to the attractive features of the vitiated coflow burner and the well-defined boundary conditions provided by the coflow. The coflow is uniform and steady, isolating the jet flame from the laboratory air for a downstream distance ranging from z/d = 50-70. The statistical results show that differential diffusion effects in this highly turbulent flow are negligible. Complementing the comprehensive set of multiscalar measurements is a parametric study of lifted methane flames that was conducted to analyze flame sensitivity to jet and coflow velocity, as well as coflow temperature. The linear relationship found between the lift-off height and the jet velocity is consistent with previous experiments. New linear sensitivities were found correlating the lift-off height to coflow velocity and temperature. A blow-off study revealed that the methane flame blows off at a common coflow temperature (1260 K), regardless of

  14. Wall jet analysis for circulation control aerodynamics. Part 1: Fundamental CFD and turbulence modeling concepts

    NASA Technical Reports Server (NTRS)

    Dash, S. M.; York, B. J.; Sinha, N.; Dvorak, F. A.

    1987-01-01

    An overview of parabolic and PNS (Parabolized Navier-Stokes) methodology developed to treat highly curved sub and supersonic wall jets is presented. The fundamental data base to which these models were applied is discussed in detail. The analysis of strong curvature effects was found to require a semi-elliptic extension of the parabolic modeling to account for turbulent contributions to the normal pressure variations, as well as an extension to the turbulence models utilized, to account for the highly enhanced mixing rates observed in situations with large convex curvature. A noniterative, pressure split procedure is shown to extend parabolic models to account for such normal pressure variations in an efficient manner, requiring minimal additional run time over a standard parabolic approach. A new PNS methodology is presented to solve this problem which extends parabolic methodology via the addition of a characteristic base wave solver. Applications of this approach to analyze the interaction of wave and turbulence processes in wall jets is presented.

  15. Numerical investigation on the jet pump performance based on different turbulence models

    NASA Astrophysics Data System (ADS)

    Yang, X. L.; Long, X. P.

    2012-11-01

    This paper aims to figure out the influence of turbulence model and wall boundary condition on the simulation of performance and flow field of jet pumps. And then try to find out one combination of turbulence model and wall treatment method that gives out more accurate performance prediction and reasonable internal flow details. Six turbulence models, (namely the three k-epsilon, the standard and SST k-omega, and Reynolds stress models) and two wall treatment methods (standard wall functions and enhanced wall treatment) were involved. A jet pump model used in an experiment was chosen as the simulation prototype. The static pressure distribution along the wall and the performance data from the experiment were used as the reference data for validating with those from the simulation results. It is found that all the ten combinations agree well with the experiment data when the volumetric flow ratio is low, however, none of them could give a performance prediction with errors less than 10% under the lager flow ratio work conditions. The errors between predicted results by several combinations and the experiment data were lowered to be less than 5% under all the working conditions by adjusting the model constants.

  16. Effect of inlet conditions on the turbulent statistics in a buoyant jet

    NASA Astrophysics Data System (ADS)

    Kumar, Rajesh; Dewan, Anupam

    2015-11-01

    Buoyant jets have been the subject of research due to their technological and environmental importance in many physical processes, such as, spread of smoke and toxic gases from fires, release of gases form volcanic eruptions and industrial stacks. The nature of the flow near the source is initially laminar which quickly changes into turbulent flow. We present large eddy simulation of a buoyant jet. In the present study a careful investigation has been done to study the influence of inlet conditions at the source on the turbulent statistics far from the source. It has been observed that the influence of the initial conditions on the second-order buoyancy terms extends further in the axial direction from the source than their influence on the time-averaged flow and second-order velocity statistics. We have studied the evolution of vortical structures in the buoyant jet. It has been shown that the generation of helical vortex rings in the vicinity of the source around a laminar core could be the reason for the larger influence of the inlet conditions on the second-order buoyancy terms as compared to the second-order velocity statistics.

  17. Turbulent eddies in a compressible jet in crossflow measured using pulse-burst particle image velocimetry

    NASA Astrophysics Data System (ADS)

    Beresh, Steven J.; Wagner, Justin L.; Henfling, John F.; Spillers, Russell W.; Pruett, Brian O. M.

    2016-02-01

    Pulse-burst Particle Image Velocimetry (PIV) has been employed to acquire time-resolved data at 25 kHz of a supersonic jet exhausting into a subsonic compressible crossflow. Data were acquired along the windward boundary of the jet shear layer and used to identify turbulent eddies as they convect downstream in the far-field of the interaction. Eddies were found to have a tendency to occur in closely spaced counter-rotating pairs and are routinely observed in the PIV movies, but the variable orientation of these pairs makes them difficult to detect statistically. Correlated counter-rotating vortices are more strongly observed to pass by at a larger spacing, both leading and trailing the reference eddy. This indicates the paired nature of the turbulent eddies and the tendency for these pairs to recur at repeatable spacing. Velocity spectra reveal a peak at a frequency consistent with this larger spacing between shear-layer vortices rotating with identical sign. The spatial scale of these vortices appears similar to previous observations of compressible jets in crossflow. Super-sampled velocity spectra to 150 kHz reveal a power-law dependency of -5/3 in the inertial subrange as well as a -1 dependency at lower frequencies attributed to the scales of the dominant shear-layer eddies.

  18. Noise Sources in a Low-Reynolds-Number Turbulent Jet at Mach 0.9

    NASA Technical Reports Server (NTRS)

    Freund, Jonathan B.

    2001-01-01

    The mechanisms of sound generation in a Mach 0.9, Reynolds number 3600 turbulent jet are investigated by direct numerical simulation. Details of the numerical method are briefly outlined and results are validated against an experiment at the same flow conditions. Lighthill's theory is used to define a nominal acoustic source in the jet, and a numerical solution of Lighthill's equation is compared to the simulation to verify the computational procedures. The acoustic source is Fourier transformed in the axial coordinate and time and then filtered in order to identify and separate components capable of radiating to the far field. This procedure indicates that the peak radiating component of the source is coincident with neither the peak of the full unfiltered source nor that of the turbulent kinetic energy. The phase velocities of significant components range from approximately 5% to 50% of the ambient sound speed which calls into question the commonly made assumption that the noise sources convect at a single velocity. Space-time correlations demonstrate that the sources are not acoustically compact in the streamwise direction and that the portion of the source that radiates at angles greater than 45 deg. is stationary. Filtering non-radiating wavenumber components of the source at single frequencies reveals that a simple modulated wave forms for the source, as might be predicted by linear stability analysis. At small angles from the jet axis the noise from these modes is highly directional, better described by an exponential than a standard Doppler factor.

  19. Global NOx Measurements in Turbulent Nitrogen-Diluted Hydrogen Jet Flames

    SciTech Connect

    Weiland, N.T.; Strakey, P.A.

    2007-03-01

    Turbulent hydrogen diffusion flames diluted with nitrogen are currently being studied to assess their ability to achieve the DOE Turbine Program’s aggressive emissions goal of 2 ppm NOx in a hydrogen-fueled IGCC gas turbine combustor. Since the unstrained adiabatic flame temperatures of these diluted flames are not low enough to eliminate thermal NOx formation the focus of the current work is to study how the effects of flame residence time and global flame strain can be used to help achieve the stated NOx emissions goal. Dry NOx measurements are presented as a function of jet diameter nitrogen dilution and jet velocity for a turbulent hydrogen/nitrogen jet issuing from a thin-lipped tube in an atmospheric pressure combustor. The NOx emission indices from these experiments are normalized by the flame residence time to ascertain the effects of global flame strain and fuel Lewis Number on the NOx emissions. In addition dilute hydrogen diffusion flame experiments were performed in a high-pressure combustor at 2 4 and 8 atm. The NOx emission data from these experiments are discussed as well as the results from a Computational Fluid Dynamics modeling effort currently underway to help explain the experimental data.

  20. Driving Solar Spicules and Jets with Magnetohydrodynamic Turbulence: Testing a Persistent Idea

    NASA Astrophysics Data System (ADS)

    Cranmer, Steven R.; Woolsey, Lauren N.

    2015-10-01

    The solar chromosphere contains thin, highly dynamic strands of plasma known as spicules. Recently, it has been suggested that the smallest and fastest (Type II) spicules are identical to intermittent jets observed by the Interface Region Imaging Spectrograph. These jets appear to expand out along open magnetic field lines rooted in unipolar network regions of coronal holes. In this paper we revisit a thirty-year-old idea that spicules may be caused by upward forces associated with Alfvén waves. These forces involve the conversion of transverse Alfvén waves into compressive acoustic-like waves that steepen into shocks. The repeated buffeting due to upward shock propagation causes nonthermal expansion of the chromosphere and a transient levitation of the transition region (TR). Some older models of wave-driven spicules assumed sinusoidal wave inputs, but the solar atmosphere is highly turbulent and stochastic. Thus, we model this process using the output of a time-dependent simulation of reduced magnetohydrodynamic turbulence. The resulting mode-converted compressive waves are strongly variable in time, with a higher TR occurring when the amplitudes are large and a lower TR when the amplitudes are small. In this picture, the TR bobs up and down by several Mm on timescales less than a minute. These motions produce narrow, intermittent extensions of the chromosphere that have similar properties as the observed jets and Type II spicules.

  1. Video Image Analysis of Turbulent Buoyant Jets Using a Novel Laboratory Apparatus

    NASA Astrophysics Data System (ADS)

    Crone, T. J.; Colgan, R. E.; Ferencevych, P. G.

    2012-12-01

    Turbulent buoyant jets play an important role in the transport of heat and mass in a variety of environmental settings on Earth. Naturally occurring examples include the discharges from high-temperature seafloor hydrothermal vents and from some types of subaerial volcanic eruptions. Anthropogenic examples include flows from industrial smokestacks and the flow from the damaged well after the Deepwater Horizon oil leak of 2010. Motivated by a desire to find non-invasive methods for measuring the volumetric flow rates of turbulent buoyant jets, we have constructed a laboratory apparatus that can generate these types of flows with easily adjustable nozzle velocities and fluid densities. The jet fluid comprises a variable mixture of nitrogen and carbon dioxide gas, which can be injected at any angle with respect to the vertical into the quiescent surrounding air. To make the flow visible we seed the jet fluid with a water fog generated by an array of piezoelectric diaphragms oscillating at ultrasonic frequencies. The system can generate jets that have initial densities ranging from approximately 2-48% greater than the ambient air. We obtain independent estimates of the volumetric flow rates using well-calibrated rotameters, and collect video image sequences for analysis at frame rates up to 120 frames per second using a machine vision camera. We are using this apparatus to investigate several outstanding problems related to the physics of these flows and their analysis using video imagery. First, we are working to better constrain several theoretical parameters that describe the trajectory of these flows when their initial velocities are not parallel to the buoyancy force. The ultimate goal of this effort is to develop well-calibrated methods for establishing volumetric flow rates using trajectory analysis. Second, we are working to refine optical plume velocimetry (OPV), a non-invasive technique for estimating flow rates using temporal cross-correlation of image

  2. Application of the k-epsilon turbulence model to the simulation of a fully pulsed free air jet

    NASA Astrophysics Data System (ADS)

    Graham, L. J. W.; Bremhorst, K.

    1993-03-01

    The work describes application of the k-epsilon turbulence model to a fully pulsed air jet. The standard model failed to predict the change in slope of the velocity decay where the jet changes from pulsed to steady jet behavior. A change in one of the constants of the k-epsilon model based on the behavior of the periodic velocity component relative to the intrinsic component yielded satisfactory results. Features of the pulsed jet which were successfully simulated included the flow reversal near the edge of the jet, increased entrainment when compared to steady jets and large radial outflow near the leading edge of the pulse and large radial inflow near the outer edge of the jet for the remainder of the pulse.

  3. Assessment of Hybrid RANS/LES Turbulence Models for Aeroacoustics Applications

    NASA Technical Reports Server (NTRS)

    Vatsa, Veer N.; Lockhard, David P.

    2010-01-01

    Predicting the noise from aircraft with exposed landing gear remains a challenging problem for the aeroacoustics community. Although computational fluid dynamics (CFD) has shown promise as a technique that could produce high-fidelity flow solutions, generating grids that can resolve the pertinent physics around complex configurations can be very challenging. Structured grids are often impractical for such configurations. Unstructured grids offer a path forward for simulating complex configurations. However, few unstructured grid codes have been thoroughly tested for unsteady flow problems in the manner needed for aeroacoustic prediction. A widely used unstructured grid code, FUN3D, is examined for resolving the near field in unsteady flow problems. Although the ultimate goal is to compute the flow around complex geometries such as the landing gear, simpler problems that include some of the relevant physics, and are easily amenable to the structured grid approaches are used for testing the unstructured grid approach. The test cases chosen for this study correspond to the experimental work on single and tandem cylinders conducted in the Basic Aerodynamic Research Tunnel (BART) and the Quiet Flow Facility (QFF) at NASA Langley Research Center. These configurations offer an excellent opportunity to assess the performance of hybrid RANS/LES turbulence models that transition from RANS in unresolved regions near solid bodies to LES in the outer flow field. Several of these models have been implemented and tested in both structured and unstructured grid codes to evaluate their dependence on the solver and mesh type. Comparison of FUN3D solutions with experimental data and numerical solutions from a structured grid flow solver are found to be encouraging.

  4. Impact of Nocturnal Low-Level Jets on Near-Surface Turbulence Kinetic Energy

    NASA Astrophysics Data System (ADS)

    Duarte, Henrique F.; Leclerc, Monique Y.; Zhang, Gengsheng; Durden, David; Kurzeja, Robert; Parker, Matthew; Werth, David

    2015-09-01

    We report on the role of low-level jets (LLJs) on the modulation of near-surface turbulence in the stable boundary layer, focusing on the behaviour of the transport terms of the turbulence kinetic energy (TKE) budget. We also examine the applicability of Monin-Obukhov similarity theory (MOST) in light of these terms. Using coincident near-surface turbulence and LLJ data collected over a three-month period in South Carolina, USA, we found that turbulence during LLJ periods was typically stronger and more well-developed in comparison with periods without a LLJ. We found a local imbalance in the near-surface TKE budget, in which the imbalance (residual) term was typically positive (i.e., energy gain) and nearly in equilibrium with buoyant consumption. Based on a comparison with previous studies, we assume that this residual term represents mostly pressure transport. We found the behaviour of the residual term to be better delineated in the presence of LLJs. We found shear production to adhere to MOST remarkably well during LLJs, except under very stable conditions. Gain of non-local TKE via pressure transport, likely consisting of large-scale fluctuations, could be the cause of the observed deviation from the MOST -less prediction. The fact that this deviation was observed for periods with well-developed turbulence with an inertial subrange slope close to indicates that such Kolmogorov turbulence is not a sufficient condition to guarantee the applicability of the MOST -less concept, as recently suggested in the literature. The implications of these results are discussed.

  5. Turbulence measurements in a swirling confined jet flowfield using a triple hot-wire probe

    NASA Technical Reports Server (NTRS)

    Janjua, S. I.; Mclaughlin, D. K.

    1982-01-01

    An axisymmetric swirling confined jet flowfield, similar to that encountered in gas turbine combustors was investigated using a triple hot-wire probe. The raw data from the three sensors were digitized using ADC's and stored on a Tektronix 4051 computer. The data were further reduced on the computer to obtain time-series for the three instantaneous velocity components in the flowfield. The time-mean velocities and the turbulence quantities were deduced. Qualification experiments were performed and where possible results compared with independent measurements. The major qualification experiments involved measurements performed in a non-swirling flow compared with conventional X-wire measurements. In the swirling flowfield, advantages of the triple wire technique over the previously used multi-position single hot-wire method are noted. The measurements obtained provide a data base with which the predictions of turbulence models in a recirculating swirling flowfield can be evaluated.

  6. Simulations of Turbulent Momentum and Scalar Transport in Non-Reacting Confined Swirling Coaxial Jets

    NASA Technical Reports Server (NTRS)

    Shih, Tsan-Hsing; Liu, Nan-Suey; Moder, Jeffrey P.

    2015-01-01

    This paper presents the numerical simulations of confined three-dimensional coaxial water jets. The objectives are to validate the newly proposed nonlinear turbulence models of momentum and scalar transport, and to evaluate the newly introduced scalar APDF and DWFDF equation along with its Eulerian implementation in the National Combustion Code (NCC). Simulations conducted include the steady RANS, the unsteady RANS (URANS), and the time-filtered Navier-Stokes (TFNS); both without and with invoking the APDF or DWFDF equation. When the APDF (ensemble averaged probability density function) or DWFDF (density weighted filtered density function) equation is invoked, the simulations are of a hybrid nature, i.e., the transport equations of energy and species are replaced by the APDF or DWFDF equation. Results of simulations are compared with the available experimental data. Some positive impacts of the nonlinear turbulence models and the Eulerian scalar APDF and DWFDF approach are observed.

  7. Simulations of Turbulent Momentum and Scalar Transport in Confined Swirling Coaxial Jets

    NASA Technical Reports Server (NTRS)

    Shih, Tsan-Hsing; Liu, Nan-Suey

    2014-01-01

    This paper presents the numerical simulations of confined three dimensional coaxial water jets. The objectives are to validate the newly proposed nonlinear turbulence models of momentum and scalar transport, and to evaluate the newly introduced scalar APDF and DWFDF equation along with its Eulerian implementation in the National Combustion Code (NCC). Simulations conducted include the steady RANS, the unsteady RANS (URANS), and the time-filtered Navier-Stokes (TFNS) with and without invoking the APDF or DWFDF equation. When the APDF or DWFDF equation is invoked, the simulations are of a hybrid nature, i.e., the transport equations of energy and species are replaced by the APDF or DWFDF equation. Results of simulations are compared with the available experimental data. Some positive impacts of the nonlinear turbulence models and the Eulerian scalar APDF and DWFDF approach are observed.

  8. On investigating wall shear stress in two-dimensional plane turbulent wall jets

    NASA Astrophysics Data System (ADS)

    Mehdi, Faraz; Johansson, Gunnar; White, Christopher; Naughton, Jonathan

    2012-11-01

    Mehdi & White [Exp Fluids 50:43-51(2011)] presented a full momentum integral based method for determining wall shear stress in zero pressure gradient turbulent boundary layers. They utilized the boundary conditions at the wall and at the outer edge of the boundary layer. A more generalized expression is presented here that uses just one boundary condition at the wall. The method is mathematically exact and has an advantage of having no explicit streamwise gradient terms. It is successfully applied to two different experimental plane turbulent wall jet datasets for which independent estimates of wall shear stress were known. Complications owing to experimental inaccuracies in determining wall shear stress from the proposed method are also discussed.

  9. Proper Orthogonal Decomposition Analysis of Turbulent Jet Impingement on Rib-roughened Surface

    NASA Astrophysics Data System (ADS)

    Lam, Prasanth Anand Kumar; Karaiyan, Arul Prakash; Thermo-Fluid Dynamics Laboratory Team

    2015-11-01

    A Proper Orthogonal Decomposition (POD) analysis on turbulent flow dynamics of confined slot jet impinging on rib-roughened surface is numerically investigated. The data for POD analysis has been obtained by solving mass, momentum and energy equations in Cartesian framework using Streamline Upwind/Petrov-Galerkin Finite element method. Further, turbulent kinetic energy (k) and its dissipation rate (ɛ) are modeled using standard k- ɛ turbulence model with standard wall functions. POD is applied to computational data for a wide range of Reynolds number (Re) = 5000 - 30000 and non-dimensional channel height (H/L) = 0.5 - 4.0 to reveal large scale vortical structures in the flow field. The simulated results demonstrate a better understanding on effect of turbulence and its influence on individual vortical structures for enhancement of heat transfer. The enhancement of heat transfer in stagnation region due to combined effect of oscillation in impingement position caused by large vortical structures and strong acceleration of fluid during impingement is quantified. Furthermore, non-dimensional correlations have been derived for pressure drop and Surface averaged Nusselt number.

  10. Analysis of turbulent free-jet hydrogen-air diffusion flames with finite chemical reaction rates

    NASA Technical Reports Server (NTRS)

    Sislian, J. P.; Glass, I. I.; Evans, J. S.

    1979-01-01

    A numerical analysis is presented of the nonequilibrium flow field resulting from the turbulent mixing and combustion of an axisymmetric hydrogen jet in a supersonic parallel ambient air stream. The effective turbulent transport properties are determined by means of a two-equation model of turbulence. The finite-rate chemistry model considers eight elementary reactions among six chemical species: H, O, H2O, OH, O2 and H2. The governing set of nonlinear partial differential equations was solved by using an implicit finite-difference procedure. Radial distributions were obtained at two downstream locations for some important variables affecting the flow development, such as the turbulent kinetic energy and its dissipation rate. The results show that these variables attain their peak values on the axis of symmetry. The computed distribution of velocity, temperature, and mass fractions of the chemical species gives a complete description of the flow field. The numerical predictions were compared with two sets of experimental data. Good qualitative agreement was obtained.

  11. Liquid helium inertial jet for comparative study of classical and quantum turbulence

    SciTech Connect

    Duri, D.; Charvin, P.; Rousset, B.; Poncet, J.-M.; Diribarne, P.

    2011-11-15

    We present a new cryogenic wind tunnel facility developed to study the high Reynolds number developed classical or quantum turbulence in liquid {sup 4}He. A stable inertial round jet flow with a Reynolds number of 4 x 10{sup 6} can be sustained in both He I and He II down to a minimum temperature of 1.7 K. The circuit can be pressurized up to 3.5 x 10{sup 5} Pa. The system has been designed to exploit the self-similar properties of the jet far field in order to adapt to the spatial resolution of the existing probes. Multiple and complementary sensors can be simultaneously installed to obtain spatial and time resolved measurements. The technical difficulties and design details are described and the system performance is presented.

  12. Direct numerical simulation of temporally evolving turbulent luminous jet flames with detailed fuel and soot chemistry

    NASA Astrophysics Data System (ADS)

    Lecoustre, Vivien; Arias, Paul; Roy, Somesh; Wang, Wei; Luo, Zhaoyu; Haworth, Dan; Im, Hong; Lu, Tianfeng; Ma, Kwan-Liu; Sankaran, Ramanan; Trouve, Arnaud

    2011-11-01

    Direct numerical simulations of 2D temporally-evolving luminous turbulent ethylene-air jet diffusion flames are performed using a high-order compressible Navier-Stokes solver. The simulations use a reduced mechanism derived from a detailed ethylene-air chemical kinetic mechanism that includes the reaction pathways for the formation of polycyclic aromatic hydrocarbons. The gas-phase chemistry is coupled with a detailed soot particle model based on the method of moments with interpolative closure that accounts for soot nucleation, coagulation, surface growth through HACA mechanism, and oxidation. Radiative heat transfer of CO2, H2O, and soot is treated by solving the radiative transfer equation using the discrete transfer method. This work presents preliminary results of radiation effects on soot dynamics at the tip of a jet diffusion flame with a particular focus on soot formation/oxidation.

  13. Effect of collector configuration on test section turbulence levels in an open-jet wind tunnel

    NASA Technical Reports Server (NTRS)

    Manuel, G. S.; Molloy, John K.; Barna, P. Stephen

    1992-01-01

    Flow quality studies in the Langley 14- by 22-Foot Subsonic Tunnel indicated periodic flow pulsation at discrete frequencies in the test section when the tunnel operated in an open-jet configuration. To alleviate this problem, experiments were conducted in a 1/24-scale model of the full-scale tunnel to evaluate the turbulence reduction potential of six collector configurations. As a result of these studies, the original bell-mouth collector of the 14- by 22-Foot Subsonic Tunnel was replaced by a collector with straight walls, and a slot was incorporated between the trailing edge of the collector and the entrance of the diffuser.

  14. A theoretical and numerical investigation of turbulent steam jets in BWR steam blowdown.

    SciTech Connect

    NguyenLe, Q.

    1998-06-26

    The preliminary results of PHOENICS and RELAP5 show that the current numerical models are adequate in predicting steam flow and stratification patterns in the upper Drywell of a BWR containment subsequent to a blow-down event. However, additional modeling is required in order to study detailed local phenomena such as condensation with non-condensables, natural convection, and stratification effects. Analytically, the intermittence modified similarity solutions show great promise. Once {gamma} is accounted for, the jet's turbulent shear stress can be determined with excellent accuracy.

  15. Application of crossed beam technology to direct measurements of sound sources in turbulent jets, part 1

    NASA Technical Reports Server (NTRS)

    Wilson, L. N.

    1970-01-01

    The mathematical bases for the direct measurement of sound source intensities in turbulent jets using the crossed-beam technique are discussed in detail. It is found that the problems associated with such measurements lie in three main areas: (1) measurement of the correct flow covariance, (2) accounting for retarded time effects in the measurements, and (3) transformation of measurements to a moving frame of reference. The determination of the particular conditions under which these problems can be circumvented is the main goal of the study.

  16. Triple Cascade Behavior in Quasigeostrophic and Drift Turbulence and Generation of Zonal Jets

    SciTech Connect

    Nazarenko, Sergey; Quinn, Brenda

    2009-09-11

    We study quasigeostrophic (QG) and plasma drift turbulence within the Charney-Hasegawa-Mima (CHM) model. We focus on the zonostrophy, an extra invariant in the CHM model, and on its role in the formation of zonal jets. We use a generalized Fjoertoft argument for the energy, enstrophy, and zonostrophy and show that they cascade anisotropically into nonintersecting sectors in k space with the energy cascading towards large zonal scales. Using direct numerical simulations of the CHM equation, we show that zonostrophy is well conserved, and the three invariants cascade as predicted by the Fjoertoft argument.

  17. Effect of Heating on Turbulent Density Fluctuations and Noise Generation From High Speed Jets

    NASA Technical Reports Server (NTRS)

    Panda, Jayanta; Seasholtz, Richard G.; Elam, Kristie A.; Mielke, Amy F.; Eck, Dennis G.

    2004-01-01

    Heated jets in a wide range of temperature ratios (TR), and acoustic Mach numbers (Ma) were investigated experimentally using far field microphones and a molecular Rayleigh scattering technique. The latter provided density fluctuations measurements. Two sets of operating conditions were considered: (1) TR was varied between 0.84 and 2.7 while Ma was fixed at 0.9; (2) Ma was varied between 0.6 and 1.48, while TR was fixed at 2.27. The implementation of the molecular Rayleigh scattering technique required dust removal and usage of a hydrogen combustor to avoid soot particles. Time averaged density measurements in the first set of data showed differences in the peripheral density shear layers between the unheated and heated jets. The nozzle exit shear layer showed increased turbulence level with increased plume temperature. Nevertheless, further downstream the density fluctuations spectra are found to be nearly identical for all Mach number and temperature ratio conditions. To determine noise sources a correlation study between plume density fluctuations and far field sound pressure fluctuations was conducted. For all jets the core region beyond the end of the potential flow was found to be the strongest noise source. Except for an isothermal jet, the correlations did not differ significantly with increasing temperature ratio. The isothermal jet created little density fluctuations. Although the far field noise from this jet did not show any exceptional trend, the flow-sound correlations were very low. This indicated that the density fluctuations only acted as a "tracer parameter" for the noise sources.

  18. Lagrangian model simulation of the turbulent transport of evaporating jet droplets

    NASA Astrophysics Data System (ADS)

    Edson, James Bearer

    The HEXOS (Humidity EXchange Over the Sea) subprograms HEXIST (HEXOS EXperiment in Simulation Tunnel) and CLUSE (Couche Limite Unidimensionnelle Stationaire d'Embruns) were designed to investigate the generation, turbulent transport, and evaporation of droplets ejected by bursting bubbles within the air-sea simulation tunnel at Institute de Mecanique Statistique de la Turbulence. Droplet concentration size spectra and turbulence characteristics were measured under various conditions to study the effects of wind speed and humidity on the above processes. The bubbles were produced by aeration devices to insure repeatability when necessary. The Lagrangian model developed as part of this investigation simulates the generation and advection of evaporating jet drops. The technique used to advect the droplets is similar to Langevin simulations of vertical dispersion. However, the approach differs significantly in that the particles (i.e., jet drops) are not constrained to follow the turbulent motion of the wind field exactly. This was accomplished by deriving an expression for the particle's vertical velocity which includes a mean fall velocity, reduced velocity variance, and a particle integral time scale which includes parameters to account for inertial effects and nonzero fall velocity. The source function is derived from an assumed bubble spectrum of the form dN/dr=Cr(-beta), where C and beta are chosen to best fit the measured data. Once chosen, these parameters are fixed so that true comparisons can be between modeled and measured data. The droplets are released at their ejection heights so that droplet production is treated as an elevated source. Profiles of droplet size spectra are then calculated downwind of the source by keeping track of the particle's size and position. These profiles show excellent agreement with those measured during HEXIST and CLUSE.

  19. Large-eddy simulations of turbulent plane and radial wall-jets

    NASA Astrophysics Data System (ADS)

    Banyassady, Rayhaneh; Piomelli, Ugo

    2014-11-01

    Large-eddy simulations of turbulent plane and radial wall-jets were conducted at different Reynolds numbers. The results were validated with the available experimental data. The radial wall-jets decay faster compared to the plane ones, due to the extra expansion in the azimuthal direction. This causes the pressure-gradient distributions to be different in radial and plane wall-jets (e.g. the inner layer in the plane case is under a favorable pressure-gradient, while in the radial case it subjected to an adverse pressure-gradient). However, these pressure gradients are not strong enough to cause any structural difference between plane and radial wall-jets. In both cases, the local Reynolds number (based on the local maximum velocity and local boundary-layer thickness) is an important determining factor in characterization of the flow. The joint probability-density function analysis shows that the local Reynolds number determines the level of intrusion of the outer layer into the inner layer: the lower the local Reynolds number the stronger is the interaction of inner and outer layers. These results were used to clarify some of the observations reported in literature; as an example the scatter of the reported log-law constants can be explained using the above-mentioned results.

  20. 3D outflow jets originating from turbulence in the reconnection current layer

    NASA Astrophysics Data System (ADS)

    Fujimoto, Keizo

    2016-07-01

    Satellite observations in the Earth's magnetosphere and in solar flares have suggested that the reconnection outflow jets are fully three dimensional, consisting of a series of narrow channels. The jet structure is important in evaluating the energy and flux transport in the reconnection process. Previous theoretical models based on fluid simulations have relied on patchy reconnection where reconnection takes place predominantly in patchy portions of the current layer. The problem of the previous models is that the gross reconnection rate is much smaller than that in the 2D reconnection case. The present study shows a large-scale 3D PIC simulation revealing that the 3D outflow jets are generated through the 3D flux ropes formed in the turbulent electron current layer around the x-line. Reconnection proceeds almost uniformly along the x-line, so that the gross reconnection rate is comparable to that in the 2D reconnection case. The flux ropes and resultant outflow channels have a typical current-aligned scale provided by the wavelength of an electron shear mode that is much larger than the typical kinetic scales. It is found that the structure of the 3D outflow jets obtained in the simulation is consistent with the bursty bulk flow observed in the Earth's magnetotail.

  1. The application of complex network time series analysis in turbulent heated jets

    SciTech Connect

    Charakopoulos, A. K.; Karakasidis, T. E. Liakopoulos, A.; Papanicolaou, P. N.

    2014-06-15

    In the present study, we applied the methodology of the complex network-based time series analysis to experimental temperature time series from a vertical turbulent heated jet. More specifically, we approach the hydrodynamic problem of discriminating time series corresponding to various regions relative to the jet axis, i.e., time series corresponding to regions that are close to the jet axis from time series originating at regions with a different dynamical regime based on the constructed network properties. Applying the transformation phase space method (k nearest neighbors) and also the visibility algorithm, we transformed time series into networks and evaluated the topological properties of the networks such as degree distribution, average path length, diameter, modularity, and clustering coefficient. The results show that the complex network approach allows distinguishing, identifying, and exploring in detail various dynamical regions of the jet flow, and associate it to the corresponding physical behavior. In addition, in order to reject the hypothesis that the studied networks originate from a stochastic process, we generated random network and we compared their statistical properties with that originating from the experimental data. As far as the efficiency of the two methods for network construction is concerned, we conclude that both methodologies lead to network properties that present almost the same qualitative behavior and allow us to reveal the underlying system dynamics.

  2. An Experimental Study of the Structure of Turbulent Non-Premixed Jet Flames in Microgravity

    NASA Astrophysics Data System (ADS)

    Boxx, Isaac; Idicheria, Cherian; Clemens, Noel

    2000-11-01

    The aim of this work is to investigate the structure of transitional and turbulent non-premixed jet flames under microgravity conditions. The microgravity experiments are being conducted using a newly developed drop rig and the University of Texas 1.5 second drop tower. The rig itself measures 16”x33”x38” and contains a co-flowing round jet flame facility, flow control system, CCD camera, and data/image acquisition computer. These experiments are the first phase of a larger study being conducted at the NASA Glenn Research Center 2.2 second drop tower facility. The flames being studied include methane and propane round jet flames at jet exit Reynolds numbers as high as 10,000. The primary diagnostic technique employed is emission imaging of flame luminosity using a relatively high-speed (350 fps) CCD camera. The high-speed images are used to study flame height, flame tip dynamics and burnout characteristics. Results are compared to normal gravity experimental results obtained in the same apparatus.

  3. Separation control using plasma actuators: application to a free turbulent jet

    NASA Astrophysics Data System (ADS)

    Labergue, A.; Moreau, E.; Zouzou, N.; Touchard, G.

    2007-02-01

    This experimental work deals with active airflow control using non-thermal surface plasma actuators in the case of a rectangular cross section turbulent jet. A wide-angle diffuser composed of two adjustable hinged baseplates is linked at the jet exit. Two types of actuators are considered: the DC corona discharge and the dielectric barrier discharge (DBD). In both cases, an ionic wind with a velocity of several m s-1 is generated tangentially to the wall surface. Thus, this induced aerodynamic effect is applied in order to create the separation along the lower hinged baseplate. The effects of both actuators on the flow separation are measured by means of particle image velocimetry for velocity up to 30 m s-1. The main results show that the DBD seems more efficient than the DC corona discharge but the effect decreases with the jet velocity. However, in increasing the discharge frequency up to 1500 Hz, it is possible to separate a 30 m s-1 jet. Finally, by reducing the actuators' length in the spanwise direction, results lead to a visualization of the three-dimensional effects on the separation along the lower hinged baseplate.

  4. The application of complex network time series analysis in turbulent heated jets

    NASA Astrophysics Data System (ADS)

    Charakopoulos, A. K.; Karakasidis, T. E.; Papanicolaou, P. N.; Liakopoulos, A.

    2014-06-01

    In the present study, we applied the methodology of the complex network-based time series analysis to experimental temperature time series from a vertical turbulent heated jet. More specifically, we approach the hydrodynamic problem of discriminating time series corresponding to various regions relative to the jet axis, i.e., time series corresponding to regions that are close to the jet axis from time series originating at regions with a different dynamical regime based on the constructed network properties. Applying the transformation phase space method (k nearest neighbors) and also the visibility algorithm, we transformed time series into networks and evaluated the topological properties of the networks such as degree distribution, average path length, diameter, modularity, and clustering coefficient. The results show that the complex network approach allows distinguishing, identifying, and exploring in detail various dynamical regions of the jet flow, and associate it to the corresponding physical behavior. In addition, in order to reject the hypothesis that the studied networks originate from a stochastic process, we generated random network and we compared their statistical properties with that originating from the experimental data. As far as the efficiency of the two methods for network construction is concerned, we conclude that both methodologies lead to network properties that present almost the same qualitative behavior and allow us to reveal the underlying system dynamics.

  5. Hybrid RANS/LES of turbulent flow in a rotating rib-roughened channel

    NASA Astrophysics Data System (ADS)

    Xun, Qian-Qiu; Wang, Bing-Chen

    2016-07-01

    In this paper, we investigate the effect of the Coriolis force on the flow field in a rib-roughened channel subjected to either clockwise or counter-clockwise system rotation using hybrid RANS/LES based on wall modelling. A simplified dynamic forcing scheme incorporating backscatter is proposed for the hybrid simulation approach. The flow is characterized by a Reynolds number of Re = 1.5 × 104 and a rotation number Ro ranging from -0.6 to 0.6. The mean flow speed and turbulence level near the roughened wall are enhanced under counter-clockwise rotation and suppressed under clockwise rotation. The Coriolis force significantly influences the stability of the wall shear layer and the free shear layers generated by the ribs. Consequently, it is interesting to observe that the classification of the roughness type relies not only on the pitch ratio, but also on the rotation number in the context of rotating rib-roughened flows. In order to validate the present hybrid approach, the first- and second-order statistical moments of the velocity field obtained from the simulations are thoroughly compared with the available laboratory measurement data.

  6. Assessment of the t-model as a SGS model for LES of high-Re turbulent flows

    NASA Astrophysics Data System (ADS)

    Chandy, Abhilash; Frankel, Steven

    2008-11-01

    The recently developed optimal prediction-based t-model (PNAS, 2007) is quantitatively assessed as a SGS turbulence model for LES of decaying homogeneous turbulence (DHT) and transition to turbulence for the Taylor-Green vortex (TGV) through comparisons to laboratory measurements and DNS. The t-model is based on the idea the motion of a vortex at one scale is influenced by the past history of motion of vortices in other scales (``long memory'' effects). t-model predictions are compared to the classic non-dynamic Smagorinsky model. Regarding the t-model, this work represents its first application to decaying turbulence with comparison to active-grid-generated decaying turbulence measurements of Kang et al. (J. Fluid Mech., 2003) at Reλ 720 and Re=3000 DNS of transition to turbulence in the TGV of Drikakis et al. (J. Turb., 2007). For DHT non-dynamic Smagorinsky is in excellent agreement with measurements for t.k.e. but higher-order moments show slight discrepancies and for TGV, energy decay rates agree reasonably well with DNS. Regarding the t-model, predictions are worse than Smagorinsky at the same grid resolution due to the insufficient resolution of small scales. Improved results are obtained at higher resolutions, but are still not as good as Smagorinsky.

  7. Quantitative saltwater modeling for validation of sub-grid scale LES turbulent mixing and transport models for fire

    NASA Astrophysics Data System (ADS)

    Maisto, Pietro; Marshall, Andre; Gollner, Michael

    2015-11-01

    A quantitative understanding of turbulent mixing and transport in buoyant flows is indispensable for accurate modeling of combustion, fire dynamics and smoke transport used in both fire safety design and investigation. This study describes the turbulent mixing behavior of scaled, unconfined plumes using a quantitative saltwater modeling technique. An analysis of density difference turbulent fluctuations, captured as the collected images scale down in resolution, allows for the determination of the largest dimension over which LES averaging should be performed. This is important as LES models must assume a distribution for sub-grid scale mixing, such as the ?-PDF distribution. We showed that there is a loss of fidelity in resolving the flow for a cell size above 0 . 54D* ; where D* is a characteristic length scale for the plume. Such a point represents the threshold above which the fluctuations start to monotonically grow. Turbulence statistics were also analyzed in terms of span-wise intermittency and time and space correlation coefficients. An unexpected condition for the core of the plume, where a substantial amount of ambient fluid (fresh water) is found, and the mixing process under buoyant conditions were found depending on the resolution of measurements used.

  8. A Model for Jet-Surface Interaction Noise Using Physically Realizable Upstream Turbulence Conditions

    NASA Technical Reports Server (NTRS)

    Afsar, Mohammed Z.; Leib, Stewart J.; Bozak, Richard F.

    2015-01-01

    This paper is a continuation of previous work in which a generalized Rapid Distortion Theory (RDT) formulation was used to model low-frequency trailing-edge noise. The research was motivated by proposed next-generation aircraft configurations where the exhaust system is tightly integrated with the airframe. Data from recent experiments at NASA on the interaction between high-Reynolds-number subsonic jet flows and an external flat plate showed that the power spectral density (PSD) of the far-field pressure underwent considerable amplification at low frequencies. For example, at the 900 observation angle, the low-frequency noise could be as much as 10dB greater than the jet noise itself. In this paper, we present predictions of the noise generated by the interaction of a rectangular jet with the trailing edge of a semi-infinite flat plate. The calculations are based on a formula for the acoustic spectrum of this noise source derived from an exact formal solution of the linearized Euler equations involving (in this case) one arbitrary convected scalar quantity and a Rayleigh equation Green's function. A low-frequency asymptotic approximation for the Green's function based on a two-dimensional mean flow is used in the calculations along with a physically realizable upstream turbulence spectrum, which includes a finite de-correlation region. Numerical predictions, based on three-dimensional RANS solutions for a range of subsonic acoustic Mach number jets and nozzle aspect ratios are compared with experimental data. Comparisons of the RANS results with flow data are also presented for selected cases. We find that a finite decorrelation region increases the low-frequency algebraic decay (the low frequency "rolloff") of the acoustic spectrum with angular frequency thereby producing much closer agreement with noise data for Strouhal numbers less than 0.1. Secondly, the large-aspectratio theory is able to predict the low-frequency amplification due to the jet

  9. A pulsed jet for generation of turbulent spots in a mach 6 boundary layer

    NASA Astrophysics Data System (ADS)

    Abney, Andrew D.

    Hypersonic vehicles operate in an environment with a high level of boundary-layer pressure fluctuations. The largest of these fluctuations are due to the intermittent passage of turbulent spots within the transition region, and can be sufficiently large to affect internal components of the vehicle. In order to better predict these flow variations, a turbulent spot model could be used. The boundary layer on the Boeing/ AFOSR Mach 6 Quiet Tunnel nozzle-wall provided a convenient location for studying the statistical properties of turbulent spots in a hypersonic boundary layer, including growth rates, convection velocities, and pressure-fluctuation intensities. A valve system was developed to inject a transverse jet of air into the nozzle-wall boundary layer in an attempt to generate controlled spots. The resulting perturbations were characterized. The pressure fluctuations were measured on the tunnel centerline downstream of the perturber. Pressure traces and spectra were examined to determine the effectiveness of the perturber system. The initial perturber system produced disturbances with a duration that was too long. A reduction in perturbation duration was achieved through modification of the perturber electronics. Physical modifications were also made to the valve, with mixed success. The perturbations were of a longer duration and higher initial amplitude than perturbations generated using a pulsed glow perturber in previous studies in the Boeing/AFOSR Mach-6 Quiet Tunnel, including fully turbulent perturbations at the maximum quiet freestream Reynolds number. Inconsistent alignment of the perturber became the largest difficulty in using the device. A new method of aligning the valve was developed. A more effective means of maintaining the alignment during transfer from the alignment tool to the nozzle wall is necessary. Additional reductions to the perturbation duration could be possible by optimizing the geometry of the passage through the nozzle wall

  10. Stabilization of turbulent lifted jet flames assisted by pulsed high voltage discharge

    SciTech Connect

    Criner, K.; Cessou, A.; Louiche, J.; Vervisch, P.

    2006-01-01

    To reduce fuel consumption or the pollutant emissions of combustion (furnaces, aircraft engines, turbo-reactors, etc.), attempts are made to obtain lean mixture combustion regimes. These lead to poor stability of the flame. Thus, it is particularly interesting to find new systems providing more flexibility in aiding flame stabilization than the usual processes (bluff-body, stabilizer, quarl, swirl, etc.). The objective is to enlarge the stability domain of flames while offering flexibility at a low energy cost. Evidence is presented that the stabilization of a turbulent partially premixed flame of more than 10 kW can be enhanced by pulsed high-voltage discharges with power consumption less than 0.1% of the power of the flame. The originality of this work is to demonstrate that very effective stabilization of turbulent flames is obtained when high-voltage pulses with very short rise times are used (a decrease by 300% in terms of liftoff height for a given exit jet velocity can be reached) and to provide measurements of minimum liftoff height obtained with discharge over a large range of the stability domain of the lifted jet flame.

  11. Imaging Fourier-transform spectrometer measurements of a turbulent nonpremixed jet flame.

    PubMed

    Harley, Jacob L; Rankin, Brent A; Blunck, David L; Gore, Jay P; Gross, Kevin C

    2014-04-15

    This work presents recent measurements of a CH4/H2/N2 turbulent nonpremixed jet flame using an imaging Fourier-transform spectrometer (IFTS). Spatially resolved (128×192 pixels, 0.72  mm/pixel) mean radiance spectra were collected between 1800  cm(-1)≤ν˜≤4500  cm(-1) (2.22  μm≤λ≤5.55  μm) at moderate spectral resolution (δν=16  cm(-1), δλ=20  nm) spanning the visible flame. Higher spectral-resolution measurements (δν=0.25  cm(-1), δλ=0.3  nm) were also captured on a smaller window (8×192) at 20, 40, and 60 diameters above the jet exit and reveal the rotational fine structure associated with various vibrational transitions in CH4, CO2, CO, and H2O. These new imaging measurements compare favorably with existing spectra acquired at select flame locations, demonstrating the capability of IFTS for turbulent combustion studies. PMID:24978990

  12. A study on large coherent structures and noise emission in a turbulent round jet

    NASA Astrophysics Data System (ADS)

    Wan, ZhenHua; Zhou, Lin; Sun, DeJun

    2014-08-01

    A moderate Reynolds number, and high subsonic turbulent round jet is investigated by large eddy simulation. The detailed results (e.g. mean flow properties, turbulence intensities, etc.) are validated against the experimental data, and special attention is paid to study motions of coherent structures and their contributions to far-field noise. Eulerian methods (e.g. Q-criteria and λ2 criteria) are utilized for visualizing coherent structures directly for instantaneous flow fields, and Lagrangian coherent structures accounting for integral effect are shown via calculating fields of finite time Lyapunov exponents based on bidimensional velocity fields. All visualizations demonstrate that intrusion of three-dimensional vortical structures into jet core occurs intermittently at the end of the potential core, resulting from the breakdown of helical vortex rings in the shear layer. Intermittencies in the shear layer and on the centerline are studied quantitatively, and distinctively different distributions of probability density function are observed. Moreover, the physical sound sources are obtained through a filtering operation of defined sources in Lighthill's analogy, and their distributions verify that intrusion of vortical structures into the core region serves as important sound sources, in particular for noise at aft angles. The facts that intermittent behaviors are caused by motions of coherent structures and correlated with noise generation imply that to establish reasonable sound sources in active noise production region based on intermittent coherent structures is one of the key issues for far-field noise prediction.

  13. Turbulent Eddies in a Compressible Jet in Crossflow Measured using Pulse-Burst PIV

    NASA Astrophysics Data System (ADS)

    Beresh, Steven; Wagner, Justin; Henfling, John; Spillers, Russell; Pruett, Brian

    2015-11-01

    Pulse-burst Particle Image Velocimetry (PIV) has been employed to acquire time-resolved data at 25 kHz of a supersonic jet exhausting into a subsonic compressible crossflow. Data were acquired along the windward boundary of the jet shear layer and used to identify turbulent eddies as they convect downstream in the far-field of the interaction. Eddies were found to have a tendency to occur in closely-spaced counter-rotating pairs and are routinely observed in the PIV movies, but the variable orientation of these pairs makes them difficult to detect statistically. Correlated counter-rotating vortices are more strongly observed to pass by at a larger spacing, both leading and trailing the reference eddy. This indicates the paired nature of the turbulent eddies and the tendency for these pairs to convect through the field of view at repeatable spacings. Velocity spectra reveal a peak at a frequency consistent with this larger spacing between shear-layer vortices rotating with identical sign. Super-sampled velocity spectra to 150 kHz reveal a power-law dependency of -5/3 in the inertial subrange as well as a -1 dependency at lower frequencies attributed to the scales of the dominant shear-layer eddies.

  14. Structure of hydrogen-rich transverse jets in a vitiated turbulent flow

    SciTech Connect

    Lyra, Sgouria; Wilde, Benjamin; Kolla, Hemanth; Seitzman, Jerry M.; Lieuwen, Timothy C.; Chen, Jacqueline H.

    2014-11-24

    Our paper reports the results of a joint experimental and numerical study of the flow characteristics and flame structure of a hydrogen rich jet injected normal to a turbulent, vitiated crossflow of lean methane combustion products. Simultaneous high-speed stereoscopic PIV and OH PLIF measurements were obtained and analyzed alongside three-dimensional direct numerical simulations of inert and reacting JICF with detailed H2/COH2/CO chemistry. Both the experiment and the simulation reveal that, contrary to most previous studies of reacting JICF stabilized in low-to-moderate temperature air crossflow, the present conditions lead to a burner-attached flame that initiates uniformly around the burner edge. Significant asymmetry is observed, however, between the reaction zones located on the windward and leeward sides of the jet, due to the substantially different scalar dissipation rates. The windward reaction zone is much thinner in the near field, while also exhibiting significantly higher local and global heat release than the much broader reaction zone found on the leeward side of the jet. The unsteady dynamics of the windward shear layer, which largely control the important jet/crossflow mixing processes in that region, are explored in order to elucidate the important flow stability implications arising in the inert and reacting JICF. The paper concludes with an analysis of the ignition, flame characteristics, and global structure of the burner-attached flame. FurthermoreChemical explosive mode analysis (CEMA) shows that the entire windward shear layer, and a large region on the leeward side of the jet, are highly explosive prior to ignition and are dominated by non-premixed flame structures after ignition. The predominantly mixing limited nature of the flow after ignition is examined by computing the Takeno flame index, which shows that ~70% of the heat release occurs in non-premixed regions.

  15. Structure of hydrogen-rich transverse jets in a vitiated turbulent flow

    DOE PAGESBeta

    Lyra, Sgouria; Wilde, Benjamin; Kolla, Hemanth; Seitzman, Jerry M.; Lieuwen, Timothy C.; Chen, Jacqueline H.

    2014-11-24

    Our paper reports the results of a joint experimental and numerical study of the flow characteristics and flame structure of a hydrogen rich jet injected normal to a turbulent, vitiated crossflow of lean methane combustion products. Simultaneous high-speed stereoscopic PIV and OH PLIF measurements were obtained and analyzed alongside three-dimensional direct numerical simulations of inert and reacting JICF with detailed H2/COH2/CO chemistry. Both the experiment and the simulation reveal that, contrary to most previous studies of reacting JICF stabilized in low-to-moderate temperature air crossflow, the present conditions lead to a burner-attached flame that initiates uniformly around the burner edge. Significantmore » asymmetry is observed, however, between the reaction zones located on the windward and leeward sides of the jet, due to the substantially different scalar dissipation rates. The windward reaction zone is much thinner in the near field, while also exhibiting significantly higher local and global heat release than the much broader reaction zone found on the leeward side of the jet. The unsteady dynamics of the windward shear layer, which largely control the important jet/crossflow mixing processes in that region, are explored in order to elucidate the important flow stability implications arising in the inert and reacting JICF. The paper concludes with an analysis of the ignition, flame characteristics, and global structure of the burner-attached flame. FurthermoreChemical explosive mode analysis (CEMA) shows that the entire windward shear layer, and a large region on the leeward side of the jet, are highly explosive prior to ignition and are dominated by non-premixed flame structures after ignition. The predominantly mixing limited nature of the flow after ignition is examined by computing the Takeno flame index, which shows that ~70% of the heat release occurs in non-premixed regions.« less

  16. Coherent structure in the turbulent planar jet. Part 2. Structural topology via POD eigenmode projection

    NASA Astrophysics Data System (ADS)

    Gordeyev, S. V.; Thomas, F. O.

    2002-06-01

    The topology of the large-scale structure in the similarity region of a turbulent planar jet is investigated experimentally. The large-scale structure is reconstructed in physical space by projection of measured proper orthogonal decomposition eigenmodes onto instantaneous flow-field realizations. The instantaneous flow-field realizations are obtained by a spanwise aligned triple X-wire rake arrangement which is used in conjunction with the linear stochastic estimation technique. Instantaneous realizations are also acquired via a second triple rake arrangement which provides an assessment of the effect of spatial aliasing on the resulting structural topology. Results indicate that the self-similar large-scale structure in the planar jet consists of a dominant planar component consisting of two lines of large-scale spanwise vortices arranged approximately asymmetrically with respect to the jet centreline. This planar component of the structure resembles the classic Kármán vortex street. There is a strong interaction between structures on opposite sides of the jet in the form of nearly two-dimensional lateral streaming motions that extend well across the flow. In addition, results indicate that the effect of the nonplanar spanwise modes is to both tilt and bend the primary spanwise vortex tubes and thereby redistribute large-scale vorticity. The bending occurs primarily in the streamwise direction. The degree to which the spanwise vortices are distorted varies greatly; in some cases they are nearly streamwise oriented and in others only slight distortion of a spanwise vortex is noted. Based upon the experimental results, prospects for low-order modelling of the jet large-scale structure are discussed.

  17. Investigation of Turbulent Tip Leakage Vortex in an Axial Water Jet Pump with Large Eddy Simulation

    NASA Technical Reports Server (NTRS)

    Hah, Chunill; Katz, Joseph

    2012-01-01

    Detailed steady and unsteady numerical studies were performed to investigate tip clearance flow in an axial water jet pump. The primary objective is to understand physics of unsteady tip clearance flow, unsteady tip leakage vortex, and cavitation inception in an axial water jet pump. Steady pressure field and resulting steady tip leakage vortex from a steady flow analysis do not seem to explain measured cavitation inception correctly. The measured flow field near the tip is unsteady and measured cavitation inception is highly transient. Flow visualization with cavitation bubbles shows that the leakage vortex is oscillating significantly and many intermittent vortex ropes are present between the suction side of the blade and the tip leakage core vortex. Although the flow field is highly transient, the overall flow structure is stable and a characteristic frequency seems to exist. To capture relevant flow physics as much as possible, a Reynolds-averaged Navier-Stokes (RANS) calculation and a Large Eddy Simulation (LES) were applied for the current investigation. The present study reveals that several vortices from the tip leakage vortex system cross the tip gap of the adjacent blade periodically. Sudden changes in local pressure field inside tip gap due to these vortices create vortex ropes. The instantaneous pressure filed inside the tip gap is drastically different from that of the steady flow simulation. Unsteady flow simulation which can calculate unsteady vortex motion is necessary to calculate cavitation inception accurately even at design flow condition in such a water jet pump.

  18. Eulerian dispersion modeling with WRF-LES of plume impingement in neutrally and stably stratified turbulent boundary layers

    NASA Astrophysics Data System (ADS)

    Nunalee, Christopher G.; Kosović, Branko; Bieringer, Paul E.

    2014-12-01

    The vast range of space-time scales associated with turbulent flow adjacent to rugged terrain is especially problematic to predictive dispersion modeling in atmospheric boundary layers (ABLs) partly due to the presence of non-linear flow features (e.g., recirculation zones, diffusion enhancement, etc.). It has been suggested that in such ABLs, explicitly modeling large turbulent eddies, through large-eddy simulation (LES), may help to curtail predicted concentration errors. In this work, passive scalars were introduced into the Weather Research and Forecasting (WRF) LES model for the purpose of simulating scalar plume interaction with an isolated terrain feature. Using measurements from the Cinder Cone Butte (CCB) field campaign, we evaluate the ability of WRF-LES to realistically simulate the impingement of Sulfur Hexafluoride (SF6) plumes onto CCB in both neutrally and stably stratified environments. Simulations reveal relatively accurate scalar trajectories with respect to thermal stability, including complex patterns such as plume splitting below the hill dividing streamline. Statistical accuracy varied with case study, but for the neutral case we recorded greater than 50% of predicted 1 h averaged surface concentrations within a factor of 2 of the observations. This metric, along with several others, indicates a performance accuracy similar to, or slightly better than, alternative Reynolds Averaged Navier-Stokes models. For the stably stratified case, the spatial distribution of surface concentrations was captured well; however, a positive concentration bias was observed which degraded quantitative accuracy scores. The variable accuracy of the WRF-LES model with respect to thermal stability is similar to what has been observed in regulatory analytical models (i.e., concentration under predictions in neutral environments and concentration over predictions in stable environments). Possible sources of error and uncertainty included the omission of mesoscale wind

  19. Effects of Buoyancy on Laminar, Transitional, and Turbulent Gas Jet Diffusion Flames

    NASA Technical Reports Server (NTRS)

    Bahadori, M. Yousef; Stocker, Dennis P.; Vaughan, David F.; Zhou, Liming; Edelman, Raymond B.

    1993-01-01

    Gas jet diffusion flames have been a subject of research for many years. However, a better understanding of the physical and chemical phenomena occurring in these flames is still needed, and, while the effects of gravity on the burning process have been observed, the basic mechanisms responsible for these changes have yet to be determined. The fundamental mechanisms that control the combustion process are in general coupled and quite complicated. These include mixing, radiation, kinetics, soot formation and disposition, inertia, diffusion, and viscous effects. In order to understand the mechanisms controlling a fire, laboratory-scale laminar and turbulent gas-jet diffusion flames have been extensively studied, which have provided important information in relation to the physico-chemical processes occurring in flames. However, turbulent flames are not fully understood and their understanding requires more fundamental studies of laminar diffusion flames in which the interplay of transport phenomena and chemical kinetics is more tractable. But even this basic, relatively simple flame is not completely characterized in relation to soot formation, radiation, diffusion, and kinetics. Therefore, gaining an understanding of laminar flames is essential to the understanding of turbulent flames, and particularly fires, in which the same basic phenomena occur. In order to improve and verify the theoretical models essential to the interpretation of data, the complexity and degree of coupling of the controlling mechanisms must be reduced. If gravity is isolated, the complication of buoyancy-induced convection would be removed from the problem. In addition, buoyant convection in normal gravity masks the effects of other controlling parameters on the flame. Therefore, the combination of normal-gravity and microgravity data would provide the information, both theoretical and experimental, to improve our understanding of diffusion flames in general, and the effects of gravity on the

  20. Computational Study of the Effect of Compositionally Inhomogeneous Fuel Streams on Turbulent Jet Flames

    NASA Astrophysics Data System (ADS)

    Mueller, Michael E.; Perry, Bruce A.; Masri, Assaad R.

    2015-11-01

    A new piloted turbulent jet burner has been developed at The University of Sydney to investigate how inhomogeneous partially premixed inlet conditions affect flame structure and stability characteristics. Compositional inhomogeneity at the inlet is achieved by recessing a central tube that separates the fuel stream and a surrounding annular air flow to allow for a controlled amount of mixing before the gases reach the nozzle exit. In this work, Large Eddy Simulation of the burner is performed using a conventional nonpremixed flamelet/progress variable model. The geometry is divided into three separately computed domains: fully developed pipe/annulus flow, pipe flow in the region of fuel/air mixing upstream of the nozzle, and the turbulent flame. The results for two recess distances of the central tube (inhomogeneous fuel inlet and effectively homogeneous fuel inlet) are compared to recent experimental measurements. Discrepancies between the simulation and experiment show that premixed combustion is dominant only for the inhomogeneous case at the base of the flame. Sensitivitiese to grid resolution in both the upstream mixing domain and the turbulent flame domain as well as pilot conditions are assessed.

  1. On waves in gases. Part I: Acoustics of jets, turbulence, and ducts

    NASA Astrophysics Data System (ADS)

    Campos, L. M. B. C.

    1986-01-01

    This review on some aspects of waves in gases concentrates first (Part I) on modern research in the acoustics of fluids at rest or in steady or turbulent motion, in free space, in the presence of obstacles, or in ducts. The study of sound, for which the sole restoring force is pressure, will be extended in a later paper (Part II) to include the other three restoring forces, namely, gravity, electromagnetic, and Coriolis forces, leading to current research on internal, magnetic, and inertial waves and their couplings. The Introduction at the beginning of Part I, and the discussion at the end of Part II, concern all four types of waves in gases, and their relevance in physics and engineering. In Part I, the following areas of acoustics are addressed: the generation of noise by turbulence, inhomogeneities or bubbles, in natural and engineering flows, e.g., wind or jets; the scattering of sound by interfaces and diffraction by turbulence, and their effects on spectral and directional redistribution of energy; propagation in ducts, without or with mean flow, e.g., the horns of musical instruments and loudspeakers, and inlets and exhausts of engines; the effects of dissipation and nonlinearity on waves, e.g., in laboratory and engineering shock tubes, and in geophysical and astrophysical conditions. Underlying these topics is the interaction of acoustics with manking, ranging from the processes of human hearing and speech to the reproduction of desirable sounds (music) and reduction of undesirable sounds (noise).

  2. Turbulence Measurements on a 2D NACA 0036 with Synthetic Jet Flow Control

    NASA Technical Reports Server (NTRS)

    Wilson, J. S.

    2006-01-01

    An active flow control experiment was conducted on a 2-ft chord NACA 0036 airfoil in a 3-ft by 4-ft Wind Tunnel at Re = 1 x 10(exp 6). The model was equipped with synthetic jet actuators at x/c = 0.30 and 0.65 that provided 120 Hz periodic excitation at a C(sub mu) 0.86% through 0.06-in wide slots. Three different slot con gurations were tested, including a baseline with no slots. Surface pressure data was collected to compare to previous tests and to combine with turbulence data to aid future CFD modeling efforts. Turbulence data, measured by hot-wire, was compared with and without flow control. Pressure data corroborates previous test data and provides more points for CFD validation. Hot-wire results showed ow control reduced the separated wake size and brought the high Reynolds stress shear layer closer to the airfoil surface. The position of this layer to the surface was altered more significantly than the magnitude of the peak stresses. Flow control was shown to increase turbulent energy in the attached boundary layer downstream of the slot but to have little effect upstream. These results provide further justification to continue assessing the potential of active flow control to reduce drag of helicopter airframe components.

  3. Calculation of 3D turbulent jets in crossflow with a multigrid method and a second-moment closure model

    NASA Technical Reports Server (NTRS)

    Demuren, A. O.

    1990-01-01

    A multigrid method is presented for calculating turbulent jets in crossflow. Fairly rapid convergence is obtained with the k-epsilon turbulence model, but computations with a full Reynolds stress turbulence model (RSM) are not yet very efficient. Grid dependency tests show that there are slight differences between results obtained on the two finest grid levels. Computations using the RSM are significantly different from those with k-epsilon model and compare better to experimental data. Some work is still required to improve the efficiency of the computations with the RSM.

  4. Characteristics of the turbulent laryngeal jet and its effect on airflow in the human intra-thoracic airways

    PubMed Central

    Lin, Ching-Long; Tawhai, Merryn H.; McLennan, Geoffrey; Hoffman, Eric A.

    2007-01-01

    A computational fluid dynamics technique is applied to understand the relative importance of the upper and intra-thoracic airways and their role in determining central airflow patterns with particular attention paid to the importance of turbulence. The geometry of the human upper respiratory tract is derived from volumetric scans of a volunteer imaged via multidetector-row computed tomography. Geometry 1 consists of a mouth piece, the mouth, the oropharynx, the larynx, and the intra-thoracic airways of up to 6 generations. Geometry 2 comprises only the intra-thoracic airways. The results show that a curved sheet-like turbulent laryngeal jet is observed only in geometry 1 with turbulence intensity in the trachea varying from 10% to 20%, whereas the turbulence in geometry 2 is negligible. The presence of turbulence is found to increase the maximum localised wall shear stress by three folds. The proper orthogonal decomposition analysis reveals that the regions of high turbulence intensity are associated with Taylor-Görtler-like vortices. We conclude that turbulence induced by the laryngeal jet could significantly affect airway flow patterns as well as tracheal wall shear stress. Thus airflow modeling, particularly subject specific evaluations, should consider upper as well as intra-thoracic airway geometry. PMID:17360247

  5. Pdf modeling of finite-rate chemistry effects in turbulent nonpremixed jet flames

    SciTech Connect

    Jones, W.P.; Kakhi, M.

    1998-10-01

    The Eulerian, joint-scalar probability density function (pdf) approach, at the single point and time level, was applied to predict the evolution of a piloted methane-air turbulent jet diffusion flame. At a certain fuel inlet velocity, the measurements indicate that the flame exhibits local extinction at 20 diameters from the fuel nozzle and subsequent downstream relighting. The objective was to investigate whether the current pdf approach can treat extinction/reignition phenomena and accurately predict the concentrations of species such as CO and H{sub 2}. Two different mixing models--coalescence-dispersion and linear mean square estimation (LMSE)--were tested, together with two turbulence models ({kappa}-{epsilon} and full Reynolds stress). The solution method involved a coupled finite-volume solution of the velocity field and a Monte Carlo simulation of the pdf transport equation. A global hydrocarbon scheme (no radical species present) was employed to represent the chemistry. The predictions demonstrated that in general the mixing and fuel consumption rates were well represented. Other scalars such as H{sub 2}O and temperature were also satisfactorily predicted for the flame not subjected to local extinction; furthermore, in this particular case, the results obtained were very similar regardless of the mixing models and turbulence closures applied. However, with regard to the intermediate species, H{sub 2} and CO, some overprediction was observed. For the flame where extinction was observed experimentally, the predictions showed that with LMSE, extinction with no subsequent relighting was predicted whereas with the coalescence-dispersion model stable burning was reproduced though the low temperatures observed at x/D -= 20 were not predicted. In addition the results were sensitive to the predicted spreading rate and hence to the turbulence model.

  6. The ground vortex flow field associated with a jet in a cross flow impinging on a ground plane for uniform and annular turbulent axisymmetric jets. M.S. Thesis

    NASA Technical Reports Server (NTRS)

    Cavage, William M.; Kuhlman, John M.

    1993-01-01

    An experimental study was conducted of the impingement of a single circular jet on a ground plane in a cross flow. This geometry is a simplified model of the interaction of propulsive jet exhaust from a V/STOL aircraft with the ground in forward flight. Jets were oriented normal to the cross flow and ground plane. Jet size, cross flow-to-jet velocity ratio, ground plane-to-jet board spacing, and jet exit turbulence level and mean velocity profile shape were all varied to determine their effects on the size of the ground vortex interaction region which forms on the ground plane, using smoke injection into the jet. Three component laser Doppler velocimeter measurements were made with a commercial three color system for the case of a uniform jet with exit spacing equal to 5.5 diameters and cross flow-to-jet velocity ratio equal to 0.11. The flow visualization data compared well for equivalent runs of the same nondimensional jet exit spacing and the same velocity ratio for different diameter nozzles, except at very low velocity ratios and for the larger nozzle, where tunnel blockage became significant. Variation of observed ground vortex size with cross flow-to-jet velocity ratio was consistent with previous studies. Observed effects of jet size and ground plane-to-jet board spacing were relatively small. Jet exit turbulence level effects were also small. However, an annular jet with a low velocity central core was found to have a significantly smaller ground vortex than an equivalent uniform jet at the same values of cross flow-to-jet velocity ratio and jet exit-to-ground plane spacing. This may suggest a means of altering ground vortex behavior somewhat, and points out the importance of proper simulation of jet exit velocity conditions. LV data indicated unsteady turbulence levels in the ground vortex in excess of 70 percent.

  7. Experimental investigation of stabilization mechanisms in turbulent, lifted jet diffusion flames

    SciTech Connect

    Su, L.K.; Sun, O.S.; Mungal, M.G.

    2006-02-01

    Simultaneous planar-laser induced fluorescence (PLIF) and particle image velocimetry (PIV) provide a comprehensive view of the molecular mixing and velocity fields in the stabilization region of turbulent, lifted jet diffusion flames. The Mie scattering medium for PIV is a glycerol-water fog, which evaporates at elevated temperatures and allows inference of the location of the high-temperature interface at the flame base. The jet Reynolds numbers vary from 4400 to 10,700. The mixing and velocity fields upstream of the flame base evolve consistently with nonreacting jet scaling. Conditional statistics of the fuel mole fraction at the instantaneous high-temperature interface show that the flame stabilization point does not generally correspond to the most upstream point on the interface (called here the leading point), because the mixture there is typically too lean to support combustion. Instead, the flame stabilization point lies toward the jet centerline relative to the leading point. Conditional axial velocity statistics indicate that the mean axial velocity at the flame front is {approx}1.8S{sub L}, where S{sub L} is the stoichiometric laminar flame speed. The data also permit determination of the scalar dissipation rates, {chi}, with the results indicating that {chi} values near the high-temperature interfaces do not typically exceed the quenching value. Thus, the flame stabilization process is more consistent with theories based on partial fuel-air premixing than with those dependent on diffusion flame quenching. We propose a description of flame stabilization that depends on the large-scale organization of the mixing field. (author)

  8. Mechanisms of Flame Stabilization and Blowout in a Reacting Turbulent Hydrogen Jet in Cross-Flow

    SciTech Connect

    Kolla, H.; Grout, R. W.; Gruber, A.; Chen, J. H.

    2012-08-01

    The mechanisms contributing to flame stabilization and blowout in a nitrogen-diluted hydrogen transverse jet in a turbulent boundary layer cross-flow (JICF) are investigated using three-dimensional direct numerical simulation (DNS) with detailed chemistry. Non-reacting JICF DNS were performed to understand the relative magnitude and physical location of low velocity regions on the leeward side of the fuel jet where a flame can potentially anchor. As the injection angle is reduced from 90{sup o} to 70{sup o}, the low velocity region was found to diminish significantly, both in terms of physical extent and magnitude, and hence, its ability to provide favorable conditions for flame anchoring and stabilization are greatly reduced. In the reacting JICF DNS a stable flame is observed for 90{sup o} injection angle and, on average, the flame root is in the vicinity of low velocity magnitude and stoichiometric mixture. When the injection angle is smoothly transitioned to 75{sup o} a transient flame blowout is observed. Ensemble averaged quantities on the flame base reveal two phases of the blowout characterized by a kinematic imbalance between flame propagation speed and flow normal velocity. In the first phase dominant flow structures repeatedly draw the flame base closer to the jet centerline resulting in richer-than-stoichiometric mixtures and high velocity magnitudes. In the second phase, in spite of low velocity magnitudes and a return to stoichiometry, due to jet bending and flame alignment normal to the cross-flow, the flow velocity normal to the flame base increases dramatically perpetuating the blowout.

  9. Large eddy simulations of a Mach 0.9 jet with fully-turbulent nozzle-exit boundary layer

    NASA Astrophysics Data System (ADS)

    Bres, Guillaume; Ham, Frank; Jordan, Peter

    2014-11-01

    From past studies, it is well known that the state of the nozzle-exit boundary layer is a key parameter for the flow development and noise characteristics of a jet. However, because of the computational cost of simulating high Reynolds number wall-driven turbulence, the nozzle boundary layer is typically assumed to be laminar or weakly disturbed in most jet simulations. This approach often leads to enhanced laminar to turbulent shear-layer transition and increased noise due to vortex pairing. In the present work, large eddy simulations of an isothermal Mach 0.9 jet (Re = 1E6) issued from a convergent-straight nozzle are performed using the compressible flow solver ``Charles'' developed at Cascade Technologies. Localized adaptive mesh refinement, synthetic turbulence and wall modeling are used inside the nozzle to ensure fully turbulent profiles at the nozzle exit. This resulted in significant improvements for the flowfield and sound predictions, compared to the typical approach based on laminar flow assumption in the nozzle. The far-field noise spectra now remarkably match the measurements from the companion experiment conducted at Pprime Institute, within 0.5 dB for most angles and relevant frequencies. As a next step toward better understanding of jet noise, the large transient database collected during the simulation is currently being mined using reduced order modeling and wavepacket analysis. Work supported in part by NAVAIR. Computer allocation provided by DoD HPC centers at ERDC and AFRL.

  10. A Conceptual Physical Model of Jet Current Turbulence in the Mouth of a River Inflowing Into a Nontidal Enclosed Sea

    NASA Astrophysics Data System (ADS)

    Khanbilvardi, R. M.; Shteinman, B.; Ozkurt, O.

    2005-05-01

    The results of measurements performed in multiple river mouths, as well as theoretical analysis, have led to revealing a number of special features of mouth currents. This made it possible to formulate a conceptual physical model of free jet current turbulence in the mouth of a river inflowing into a nontidal enclosed sea. Its main principles are as follows: The structure of turbulence is characterized by the presence of a hierarchy of energy supply zones in the velocity spectra. The energy spectra have two basic discrete zones of energy supply, which are related: the first to the hydrodynamic instability of the averaged flow, and the second to the increased friction on the river jet boundaries. These discrete zones are separated by the inertial interval. Moving away from the mouth into the sea (to the bar), the ordinates of the spectral density are decreasing and the zones of additional energy supply move into the domain of higher frequencies. In all sections of the jet flow within the river mouth, the generation of turbulent energy exceeds its dissipation. This excess increases along the jet flow. In all sections, dissipation and generation of turbulent energy increase from the surface to the bottom. The dissipation of energy decreases in the section before the bar, while beyond the bar it rises along the flow, due to the increasing hydraulic resistance at the bar. The longitudinal dimensions of turbulent eddies get smaller with the distance from the mouth gauge, while their orientation is changing from the predominantly vertical-longitudinal rotation over a horizontal axis to the predominantly horizontal-transversal rotation over a vertical axis. With the advance of the river jet from the mouth into the sea, the predominance of the longitudinal dimensions of the eddies changes to the predominance of their transversal dimensions. The farther from the mouth, in the spectra of the river jet current there appear frequency intervals that can be described not by the

  11. The effects of grids on the near-field evolution of turbulence in a low-Reynolds number jet

    NASA Astrophysics Data System (ADS)

    Senthil Kumar, P.; Raghavan, V.; Sundararajan, T.

    2015-11-01

    The near field evolution of a low Reynolds number free jet and associated turbulent fluctuations are investigated experimentally, in the presence of a grid placed at nozzle exit. The jet spreading characteristics without grid are validated first with available data in the literature and the well known features of vortex roll-up and pairing phenomena in the shear layer region are confirmed. The inclusion of a grid at the nozzle exit appears to reduce velocity fluctuations in the near field, but it accelerates the transition to a full-grown turbulent flow field with respect to axial distance, by the introduction of finer scales. Grids also cause mean velocity overshoot in the radial profile by distorting the approach flow in the boundary layer region ahead of the grid. Distinct vortex roll-up and pairing phenomena in the shear layer region could not be observed with grids. However, weak shear mode instabilities originating in the shear layer for 0 ≤ z/D ≤ 3 appear to interact with the potential core, leading to the formation of a "unified shear/preferred mode" instability. These features are dominantly seen for grid-mounted jet flows having large displacement thickness to grid opening ratio. Power spectra indicate a progressive evolution of turbulent flow features with respect to axial distance, starting with the development of inertial sub-range into a -5/3 slope exponent; followed by the establishment of full range of turbulent scales, and the eventual attainment of isotropy. The establishment of inertial sub-range is attributable to vortex-pairing and turbulent fragmentation in free jets and to the introduction of fine scales at the jet inlet in the cases with grids.

  12. Direct numerical simulations of temporally developing turbulent reacting liquid-fueled jets

    NASA Astrophysics Data System (ADS)

    Shashank, Shashank; Pitsch, Heinz

    2012-11-01

    Liquid fueled engines are ubiquitous in the transportation industry because liquid fuel minimizes the weight and volume of propulsion systems. The combustion that occurs in these engines is an inherently multi-physics process, involving fuel evaporation, reaction kinetics, and high levels of turbulence. A desire for high fidelity data that explains complex interaction between different physical mechanisms motivates the consideration of direct numerical simulation (DNS) as an investigation tool. In this study three-dimensional DNS of a reacting n-heptane liquid fueled temporal jet have been performed to study auto-ignition and subsequent burning in conditions that are representative of a diesel engine environment. In these simulations the continuous phase is described using an Eulerian representation whereas Lagrangian particle tracking is used to model the dispersed phase. The results of this study will demonstrate the importance of unsteady effects, and of accounting for the interaction between different modes of combustion, when simulating spray combustion.

  13. Prediction of extinction and reignition in nonpremixed turbulent flames using a flamelet/progress variable model. 2. Application in LES of Sandia flames D and E

    SciTech Connect

    Ihme, Matthias; Pitsch, Heinz

    2008-10-15

    An extension of the flamelet/progress variable (FPV) model for the prediction of extinction and reignition is applied in large-eddy simulation (LES) of flames D and E of the Sandia piloted turbulent jet flame series. This model employs a presumed probability density function (PDF), in which the marginal PDF of a reactive scalar is modeled by a statistically most likely distribution. This provides two advantages. First of all, the shape of the distribution depends on chemical and mixing time-scale information, and second, an arbitrary number of moments can be enforced. This model was analyzed in an a priori study in the first part of this work. In the present LES application, the first two moments of mixture fraction and reaction progress variable are used to constrain the shape of the presumed PDF. Transport equations for these quantities are solved, and models for the residual scalar dissipation rates, which appear as unclosed terms in the equations for the scalar variances, are provided. Statistical flow field quantities for axial velocity, mixture fraction, and temperature, obtained from the extended FPV model, are in good agreement with experimental data. Mixture-fraction-conditioned data, conditional PDFs, and burning indices are computed and compared with the delta-function flamelet closure model, which employs a Dirac distribution as a model for the marginal PDF of the reaction progress parameter. The latter model considerably underpredicts the amount of local extinction, which shows that the consideration of second-moment information in the presumed PDF of the reaction progress parameter is important for the accurate prediction of extinction and reignition. Mixture-fraction-conditioned results obtained from the extended FPV model are in good agreement with experimental data; however, the overprediction of the consumption of fuel and oxidizer on the fuel-rich side results in an overprediction of minor species. The predictions for the conditional PDFs and

  14. Turbulent structure and emissions of strongly-pulsed jet diffusion flames

    NASA Astrophysics Data System (ADS)

    Fregeau, Mathieu

    This current research project studied the turbulent flame structure, the fuel/air mixing, the combustion characteristics of a nonpremixed pulsed (unsteady) and unpulsed (steady) flame configuration for both normal- and microgravity conditions, as well as the flame emissions in normal gravity. The unsteady flames were fully-modulated, with the fuel flow completely shut off between injection pulses using an externally controlled valve, resulting in the generation of compact puff-like flame structures. Conducting experiments in normal and microgravity environments enabled separate control over the relevant Richardson and Reynolds numbers to clarify the influence of buoyancy on the flame behavior, mixing, and structure. Experiments were performed in normal gravity in the laboratory at the University of Washington and in microgravity using the NASA GRC 2.2-second Drop Tower facility. High-speed imaging, as well as temperature and emissions probes were used to determine the large-scale structure dynamics, the details of the flame structure and oxidizer entrainment, the combustion temperatures, and the exhaust emissions of the pulsed and steady flames. Of particular interest was the impact of changes in flame structure due to pulsing on the combustion characteristics of this system. The turbulent flame puff celerity (i.e., the bulk velocity of the puffs) was strongly impacted by the jet-off time, increasing markedly as the time between pulses was decreased, which caused the degree of puff interaction to increase and the strongly-pulsed flame to more closely resemble a steady flame. This increase occurred for all values of injection time as well as for constant fuelling rate and in both the presence and absence of buoyancy. The removal of positive buoyancy in microgravity resulted in a decrease in the flame puff celerity in all cases, amounting to as much as 40%, for both constant jet injection velocity and constant fuelling rate. The mean flame length of the strongly

  15. Comparison of 10kHz TR-PIV and LES near-field data in high speed jets

    NASA Astrophysics Data System (ADS)

    Lewalle, Jacques; Kan, Pinqing

    2012-11-01

    The identification of the sources of noise in high-speed jets may help formulate control strategies, an important unsolved problem. We report on the existence of large intermittent and localized relative phase velocities for near-jet fluctuations, and on the flow patterns that are associated with them (see companion abstract by P. Kan). Here we analyze two data sets. Experimentally, 10 kHz TR-PIV in a Ma = 0 . 6 cold jet (Re = 700,000) yielded two components of velocity, from which we calculate the phase velocities for various indicators (see related abstracts by Z.P. Berger and by M.G. Berry; data provided by Spectral Energies LLC). Similar results are obtained for Ma=0.9 LES results (Re = 400,000, sampling at 80 kHz). The comparison of algorithms and flow patterns vindicates our approach. Correlations with far-field events will also be attempted. Thanks to Guillaume Daviller (Institut PPrime, France) for the LES data, and to the Glauser group at Syracuse University. Thanks for partial support from Spectral Energies LLC (under SBIR grant from AFOSR), Syracuse University and the LCS College.

  16. On using large-eddy simulation for the prediction of noise from cold and heated turbulent jets

    NASA Astrophysics Data System (ADS)

    Bodony, Daniel J.; Lele, Sanjiva K.

    2005-08-01

    The results of a series of large-eddy simulations of heated and unheated jets using approximately 106 grid points are presented. The computations were performed on jets at operating conditions originally investigated by Tanna in the late 1970s [H. K. Tanna, "An experimental study of jet noise Part I: Turbulent mixing noise," J. Sound Vib., 50, 405 (1977)]. Three acoustic Mach numbers are investigated (Uj/a∞=0.5, 0.9, and 1.5) at cold (constant stagnation temperature) and heated conditions (Tj/T∞=1.8, 2.7, and 2.3, respectively). The jets' initial annular shear layers are thick relative to experimental jets and are quasi-laminar with superimposed disturbances from linear instability theory. It is observed that qualitative changes in the jets' mean- and turbulent field structure with Uj and Tj are consistent with previous experimental data. However, the jets exhibit a faster centerline mean velocity decay rate relative to the existing data, with a corresponding 3-4 % over-prediction of the peak root-mean-square level. The acoustic pressure fluctuations in the far field are analyzed in detail. The accuracy of the overall sound pressure level predictions is found to be a strong function of the jet Mach number, with the lowest speed jets being the least accurate. At all conditions the peak acoustic frequency occurs at approximately St =fDj/Uj=0.25. The limited resolution of the computations is shown to impact the radiated sound by yielding effectively low-pass filtered versions of the experimental spectra, with a maximum frequency of St ≈1.2.

  17. Assessment of subgrid-scale models with a large-eddy simulation-dedicated experimental database: The pulsatile impinging jet in turbulent cross-flow

    NASA Astrophysics Data System (ADS)

    Baya Toda, Hubert; Cabrit, Olivier; Truffin, Karine; Bruneaux, Gilles; Nicoud, Franck

    2014-07-01

    Large-Eddy Simulation (LES) in complex geometries and industrial applications like piston engines, gas turbines, or aircraft engines requires the use of advanced subgrid-scale (SGS) models able to take into account the main flow features and the turbulence anisotropy. Keeping this goal in mind, this paper reports a LES-dedicated experiment of a pulsatile hot-jet impinging a flat-plate in the presence of a cold turbulent cross-flow. Unlike commonly used academic test cases, this configuration involves different flow features encountered in complex configurations: shear/rotating regions, stagnation point, wall-turbulence, and the propagation of a vortex ring along the wall. This experiment was also designed with the aim to use quantitative and nonintrusive optical diagnostics such as Particle Image Velocimetry, and to easily perform a LES involving a relatively simple geometry and well-controlled boundary conditions. Hence, two eddy-viscosity-based SGS models are investigated: the dynamic Smagorinsky model [M. Germano, U. Piomelli, P. Moin, and W. Cabot, "A dynamic subgrid-scale eddy viscosity model," Phys. Fluids A 3(7), 1760-1765 (1991)] and the σ-model [F. Nicoud, H. B. Toda, O. Cabrit, S. Bose, and J. Lee, "Using singular values to build a subgrid-scale model for large eddy simulations," Phys. Fluids 23(8), 085106 (2011)]. Both models give similar results during the first phase of the experiment. However, it was found that the dynamic Smagorinsky model could not accurately predict the vortex-ring propagation, while the σ-model provides a better agreement with the experimental measurements. Setting aside the implementation of the dynamic procedure (implemented here in its simplest form, i.e., without averaging over homogeneous directions and with clipping of negative values to ensure numerical stability), it is suggested that the mitigated predictions of the dynamic Smagorinsky model are due to the dynamic constant, which strongly depends on the mesh resolution

  18. Simulations of sooting turbulent jet flames using a hybrid flamelet/stochastic Eulerian field method

    NASA Astrophysics Data System (ADS)

    Consalvi, Jean-Louis; Nmira, Fatiha; Burot, Daria

    2016-03-01

    The stochastic Eulerian field method is applied to simulate 12 turbulent C1-C3 hydrocarbon jet diffusion flames covering a wide range of Reynolds numbers and fuel sooting propensities. The joint scalar probability density function (PDF) is a function of the mixture fraction, enthalpy defect, scalar dissipation rate and representative soot properties. Soot production is modelled by a semi-empirical acetylene/benzene-based soot model. Spectral gas and soot radiation is modelled using a wide-band correlated-k model. Emission turbulent radiation interactions (TRIs) are taken into account by means of the PDF method, whereas absorption TRIs are modelled using the optically thin fluctuation approximation. Model predictions are found to be in reasonable agreement with experimental data in terms of flame structure, soot quantities and radiative loss. Mean soot volume fractions are predicted within a factor of two of the experiments whereas radiant fractions and peaks of wall radiative fluxes are within 20%. The study also aims to assess approximate radiative models, namely the optically thin approximation (OTA) and grey medium approximation. These approximations affect significantly the radiative loss and should be avoided if accurate predictions of the radiative flux are desired. At atmospheric pressure, the relative errors that they produced on the peaks of temperature and soot volume fraction are within both experimental and model uncertainties. However, these discrepancies are found to increase with pressure, suggesting that spectral models describing properly the self-absorption should be considered at over-atmospheric pressure.

  19. The Effects of Buoyancy on Characteristics of Turbulent Nonpremixed Jet Flames

    NASA Astrophysics Data System (ADS)

    Idicheria, Cherian; Boxx, Isaac; Clemens, Noel

    2002-11-01

    This work addresses the influence of buoyant forces on the underlying structure of turbulent nonpremixed jet flames. Buoyancy effects are investigated by studying transitional and turbulent propane and ethylene flames (Re_D=2500-10500) at normal, low and microgravity conditions. The reduced gravity experiments are conducted by dropping a combustion rig in the University of Texas 1.25-second drop tower and the NASA Glenn 2.2-second drop tower. The diagnostic employed is high-speed luminosity imaging using a CCD camera. The images obtained are used to compare flame length, mean, RMS and flame tip oscillation characteristics The results showed that, in contrast to previous studies, the high Reynolds number flames at all gravity levels were essentially identical. Furthermore, the parameter ξL (Becker and Yamazaki, 1978) is sufficient for quantifying the effects of buoyancy on the flame characteristics. The large-scale structure and flame tip dynamics are essentially identical to those of purely momentum driven flames provided ξL is less than approximately 3.

  20. Turbulent crude oil jets in crossflow: holographic measurements of droplet size distributions

    NASA Astrophysics Data System (ADS)

    Xue, Xinzhi; Murphy, David; Katz, Joseph

    2015-11-01

    Buoyant, immiscible jets and plumes are created by subsurface oil well blowouts. In this experimental study, high speed visualizations and digital holography follow vertical crude oil turbulent jets of varying Reynolds and Ohnesorge numbers, all falling in the atomization range, while being towed in a towing tank generating `crossflows' at varying crossflow-to-exit speed velocity ratios. The droplet size distributions are measured using a submerged miniature holographic microscopy system, enabling comparison between the plume behavior and the droplet size distributions. Due to variations in rise-velocity with droplet size, the shape and dispersion rate of the plume depends on the interfacial tension. Hence, the crude oil plume rises faster than a `control' miscible oil analog with the same density and viscosity. Premixing the oil with dispersant (Corexit 9500A) at dispersant to oil (DOR) ratios of 1:100 and 1:25 reduces the oil-seawater interfacial tension by up to two orders of magnitude, promoting formation of micro-droplets. Hence, the plume rises at a slower rate, with the large droplets rapidly escaping, leaving smaller ones behind. Furthermore, for the DOR 1:25 case, some of the microdroplets are entrained into the vortices prominent in the wake region under the plume. Funding provided by the Gulf of Mexico Research Initiative.

  1. An Evaluation of Linear Instability Waves as Sources of Sound in a Supersonic Turbulent Jet

    NASA Technical Reports Server (NTRS)

    Mohseni, Kamran; Colonius, Tim; Freund, Jonathan B.

    2002-01-01

    Mach wave radiation from supersonic jets is revisited to better quantify the extent to which linearized equations represent the details of the actual mechanism. To this end, we solve the linearized Navier-Stokes equations (LNS) with precisely the same mean flow and inflow disturbances as a previous direct numerical simulation (DNS) of a perfectly expanded turbulent M = 1.92 jet. We restrict our attention to the first two azimuthal modes, n = 0 and n = 1, which constitute most of the acoustic field. The direction of peak radiation and the peak Strouhal number matches the DNS reasonably well, which is in accord with previous experimental justification of the linear theory. However, it is found that the sound pressure level predicted by LNS is significantly lower than that from DNS. In order to investigate the discrepancy, individual frequency components of the solution are examined. These confirm that near the peak Strouhal number, particularly for the first helical mode n = 1, the amplification of disturbances in the LNS closely matches the DNS. However, away from the peak frequency (and generally for the azimuthal mode n = 0), modes in the LNS are damped while those in the DNS grow at rates comparable to those at the peak Strouhal number.

  2. Stable Boundary Layers with Low-Level Jets: What did we Learn from the LES Intercomparison within GABLS?

    NASA Astrophysics Data System (ADS)

    Basu, S.; Bosveld, F. C.; Holtslag, B.

    2009-12-01

    Recently, the third intercomparison of single column and large-eddy models for atmospheric stable boundary layer simulations was organized under the auspices of the GEWEX atmospheric boundary layer study (GABLS). More than ten LES modeling groups from around the world participated in this intercomparison study to model a moderately stratified, baroclinic, mid-latitude nighttime boundary layer utilizing several LES subgrid-scale (SGS) models. The boundary layer was observed over Cabauw, Netherlands on July 1st, 2006. The initial condition for the LES intercomparison was created by synthetically merging the observed 200 m Cabauw tower data and a high-resolution 00 UTC sounding from DeBilt. Time-height-dependent geostrophic wind forcings were derived from a network of surface pressure stations in Netherlands combined with analysis of a 3D weather-forecast model. In this presentation, we discuss the performances of several dynamic and static (stability corrected) SGS models in capturing the characteristics of this moderately stable boundary layer. We will put a strong emphasis on the evolution of low-level jets (LLJs) and on the morning transition. A comparison with the first GABLS-LES study will also be given as well as a discussion on the important lessons for representing boundary layers in large scale models.

  3. Sound generated aerodynamically revisited - Large-scale structures in a turbulent jet as a source of sound

    NASA Technical Reports Server (NTRS)

    Mankbadi, R.; Liu, J. T. C.

    1984-01-01

    The theoretical capability of identifying the source of turbulent jet noise is assessed in comparison with experimental data. Account is taken of axisymmetric and spiral turbulence modes in Lighthill's (1952) formulation of turbulent noise. Coherent structures interacting with the mean flow and the fine-grained turbulence are the primary noise sources, modeled as an oscillating streamwise distribution. Low-frequencies arise farther downstream while high-frequencies congregate close to the nozzle lip. Previous measurements at various exit velocities, angles with respect to the nozzle axis, the Strouhal number and downstream distance are discussed. The model successfully predicted the angular distribution of noise frequency due to coherent structures. Further work is indicated on compressibility effects.

  4. Optimization and Modeling of Noise Reduction for Turbulent Jets with Induced Asymmetry

    NASA Astrophysics Data System (ADS)

    Rostamimonjezi, Sara

    This project relates to the development of next-generation high-speed aircraft that are efficient and environmentally compliant. The emphasis of the research is on reducing noise from high-performance engines that will power these aircraft. A strong component of engine noise is jet mixing noise that comes from the turbulent mixing process between the high-speed exhaust flow of the engine and the atmosphere. The fan flow deflection method (FFD) suppresses jet noise by deflecting the fan stream downward, by a few degrees, with respect to the core stream. This reduces the convective Mach number of the primary shear layer and turbulent kinetic energy in the downward direction and therefore reduces the noise emitted towards the ground. The redistribution of the fan stream is achieved with inserting airfoil-shaped vanes inside the fan duct. Aerodynamic optimization of FFD has been done by Dr. Juntao Xiong using a computational fluid dynamics code to maximize reduction of noise perceived by the community while minimizing aerodynamic losses. The optimal vane airfoils are used in a parametric experimental study of 50 4-vane deflector configurations. The vane chord length, angle of attack, and azimuthal location are the parameters studied in acoustic optimization. The best vane configuration yields a reduction in cumulative (downward + sideline) effective perceived noise level (EPNL) of 5.3 dB. The optimization study underscores the sensitivity of FFD to deflector parameters and the need for careful design in the practical implementation of this noise reduction approach. An analytical model based on Reynolds Averaged Navier Stokes (RANS) and acoustic analogy is developed to predict the spectral changes from a known baseline in the direction of peak emission. A generalized form for space-time correlation is introduced that allows shapes beyond the traditional exponential forms. Azimuthal directivity based on the wavepacket model of jet noise is integrated with the acoustic

  5. Experimental validation of potential and turbulent flow models for a two-dimensional jet enhanced exhaust hood.

    PubMed

    Kulmala, I

    2000-01-01

    A two-dimensional jet-reinforced slot exhaust hood was modeled using a mathematical model based on potential flow theory and with a computational fluid dynamics (CFD) model using the standard k-epsilon model for turbulence closure. The accuracy of the calculations was verified by air velocity and capture efficiency measurements. The comparisons show that, for normal operating conditions, both the models predicted the mean airflows in front of the hood well. However, the CFD model gave more realistic results in the jet flow region and also of the short-circuiting flow. Both models became increasingly inaccurate when the ratio of the supply jet momentum to the exhaust flow rate increased. The jet enhancement proved to be a very efficient way to increase the effective control range of exhaust hoods. Controlled air movements can be created at distances that are two to three times larger than with conventional suction alone without increasing the exhaust flow rate. PMID:10782190

  6. Experimental study of vorticity-strain rate interaction in turbulent partially-premixed jet flames using tomographic particle image velocimetry

    DOE PAGESBeta

    Coriton, Bruno; Frank, Jonathan H.

    2016-02-16

    In turbulent flows, the interaction between vorticity, ω, and strain rate, s, is considered a primary mechanism for the transfer of energy from large to small scales through vortex stretching. The ω-s coupling in turbulent jet flames is investigated using tomographic particle image velocimetry (TPIV). TPIV provides a direct measurement of the three-dimensional velocity field from which ω and s are determined. The effects of combustion and mean shear on the ω-s interaction are investigated in turbulent partially premixed methane/air jet flames with high and low probabilities of localized extinction as well as in a non-reacting isothermal air jet withmore » Reynolds number of approximately 13,000. Results show that combustion causes structures of high vorticity and strain rate to agglomerate in highly correlated, elongated layers that span the height of the probe volume. In the non-reacting jet, these structures have a more varied morphology, greater fragmentation, and are not as well correlated. The enhanced spatiotemporal correlation of vorticity and strain rate in the stable flame results in stronger ω-s interaction characterized by increased enstrophy and strain-rate production rates via vortex stretching and straining, respectively. The probability of preferential local alignment between ω and the eigenvector of the intermediate principal strain rate, s2, which is intrinsic to the ω-s coupling in turbulent flows, is larger in the flames and increases with the flame stability. The larger mean shear in the flame imposes a preferential orientation of ω and s2 tangential to the shear layer. The extensive and compressive principal strain rates, s1 and s3, respectively, are preferentially oriented at approximately 45° with respect to the jet axis. As a result, the production rates of strain and vorticity tend to be dominated by instances in which ω is parallel to the s1¯-s2¯ plane and orthogonal to s3¯.« less

  7. Experimental study of vorticity-strain rate interaction in turbulent partially premixed jet flames using tomographic particle image velocimetry

    NASA Astrophysics Data System (ADS)

    Coriton, Bruno; Frank, Jonathan H.

    2016-02-01

    In turbulent flows, the interaction between vorticity, ω, and strain rate, s, is considered a primary mechanism for the transfer of energy from large to small scales through vortex stretching. The ω-s coupling in turbulent jet flames is investigated using tomographic particle image velocimetry (TPIV). TPIV provides a direct measurement of the three-dimensional velocity field from which ω and s are determined. The effects of combustion and mean shear on the ω-s interaction are investigated in turbulent partially premixed methane/air jet flames with high and low probabilities of localized extinction as well as in a non-reacting isothermal air jet with Reynolds number of approximately 13 000. Results show that combustion causes structures of high vorticity and strain rate to agglomerate in highly correlated, elongated layers that span the height of the probe volume. In the non-reacting jet, these structures have a more varied morphology, greater fragmentation, and are not as well correlated. The enhanced spatiotemporal correlation of vorticity and strain rate in the stable flame results in stronger ω-s interaction characterized by increased enstrophy and strain-rate production rates via vortex stretching and straining, respectively. The probability of preferential local alignment between ω and the eigenvector of the intermediate principal strain rate, s2, which is intrinsic to the ω-s coupling in turbulent flows, is larger in the flames and increases with the flame stability. The larger mean shear in the flame imposes a preferential orientation of ω and s2 tangential to the shear layer. The extensive and compressive principal strain rates, s1 and s3, respectively, are preferentially oriented at approximately 45° with respect to the jet axis. The production rates of strain and vorticity tend to be dominated by instances in which ω is parallel to the s1 ¯-s2 ¯ plane and orthogonal to s3 ¯.

  8. The effects of buoyancy on turbulent nonpremixed jet flames in crossflow

    NASA Astrophysics Data System (ADS)

    Boxx, Isaac G.

    An experimental research study was conducted to investigate what effect buoyancy had on the mean and instantaneous flow-field characteristics of turbulent jet-flames in crossflow (JFICF). The study used an experimental technique wherein a series of normal-gravity, hydrogen-diluted propane JFICF were compared with otherwise identical ones in low-gravity. Experiments were conducted at the University of Texas Drop Tower Facility, a new microgravity science laboratory built for this study at the University of Texas at Austin. Two different diagnostic techniques were employed, high frame-rate digital cinematographic imaging and planar laser Mie scattering (PLMS). The flame-luminosity imaging revealed significant elongation and distortion of the large-scale luminous structure of the JFICF. This was seen to affect the flametip oscillation and burnout characteristics. Mean and root-mean-square (RMS) images of flame-luminosity were computed from the flame-luminosity image sequences. These were used to compare visible flame-shapes, flame chord-lengths and jet centerline-trajectories of the normal- and low-gravity flames. In all cases the jet-centerline penetration and mean luminous flame-width were seen to increase with decreasing buoyancy. The jet-centerline trajectories for the normal-gravity flames were seen to behave differently to those of the low-gravity flames. This difference led to the conclusion that the jet transitions from a momentum-dominated forced convection limit to a buoyancy-influenced regime when it reaches xiC ≈ 3, where xiC is the Becker and Yamazaki (1978) buoyancy parameter based on local flame chord-length. The mean luminous flame-lengths showed little sensitivity to buoyancy or momentum flux ratio. Consistent with the flame-luminosity imaging experiments, comparison of the instantaneous PLMS flow-visualization images revealed substantial buoyancy-induced elongation and distortion of the large-scale shear-layer vortices in the flow. This effect

  9. Effects of external intermittency and mean shear on the spectral inertial-range exponent in a turbulent square jet

    NASA Astrophysics Data System (ADS)

    Zhang, J.; Xu, M.; Pollard, A.; Mi, J.

    2013-05-01

    This study investigates by experiment the dependence of the inertial-range exponent m of the streamwise velocity spectrum on the external intermittency factor γ (≡ the fraction of time the flow is fully turbulent) and the mean shear S in a turbulent square jet. Velocity measurements were made using hot-wire anemometry in the jet at 15 < x/De < 40, where De denotes the exit equivalent diameter, and for an exit Reynolds number of Re = 50 000. The Taylor microscale Reynolds number Rλ varies from about 70 to 450 in the present study. The TERA (turbulent energy recognition algorithm) method proposed by Falco and Gendrich [in Near-Wall Turbulence: 1988 Zoran Zariç Memorial Conference, edited by S. J. Kline and N. H. Afgan (Hemisphere Publishing Corp., Washington, DC, 1990), pp. 911-931] is discussed and applied to estimate the intermittency factor from velocity signals. It is shown that m depends strongly on γ but negligibly on S. More specifically, m varies with γ following m=mt+(lnγ-0.0173)1/2, where mt denotes the spectral exponent found in fully turbulent regions.

  10. Effects of external intermittency and mean shear on the spectral inertial-range exponent in a turbulent square jet.

    PubMed

    Zhang, J; Xu, M; Pollard, A; Mi, J

    2013-05-01

    This study investigates by experiment the dependence of the inertial-range exponent m of the streamwise velocity spectrum on the external intermittency factor γ (≡ the fraction of time the flow is fully turbulent) and the mean shear S in a turbulent square jet. Velocity measurements were made using hot-wire anemometry in the jet at 15 < x/D(e) < 40, where D(e) denotes the exit equivalent diameter, and for an exit Reynolds number of Re = 50,000. The Taylor microscale Reynolds number R(λ) varies from about 70 to 450 in the present study. The TERA (turbulent energy recognition algorithm) method proposed by Falco and Gendrich [in Near-Wall Turbulence: 1988 Zoran Zariç Memorial Conference, edited by S. J. Kline and N. H. Afgan (Hemisphere Publishing Corp., Washington, DC, 1990), pp. 911-931] is discussed and applied to estimate the intermittency factor from velocity signals. It is shown that m depends strongly on γ but negligibly on S. More specifically, m varies with γ following m=m(t)+(lnγ(-0.0173))(1/2), where m(t) denotes the spectral exponent found in fully turbulent regions. PMID:23767622

  11. Using a wall-normal jet to modify the large-scale structures in a turbulent boundary layer

    NASA Astrophysics Data System (ADS)

    Talluru, Murali Krishna; Bishop, Brett; Hutchins, Nicholas; Manzie, Chris; Marusic, Ivan; The University of Melbourne Team

    2012-11-01

    We report on attempts to use a wall-normal jet to modify the large-scale structures (``super structures'') that are known to populate the logarithmic regions of high Reynolds number turbulent boundary layers. An upstream spanwise array of surface mounted shear-stress sensors detects the passage of the large-scale events. A rectangular wall-normal jet, located downstream of this array targets the identified event and a second spanwise array downstream of the jet monitors any alterations to the large-scale structure. A traversing hot wire probe is mounted above the downstream array to look for modifications across the depth of the boundary layer. As a first step, an off-line control strategy is investigated. In this case, there is no active controller, the jet is periodically fired with fixed parameters and during post-processing, the ``control'' strategy is emulated in a conditional sense to understand the interactions of an actuated jet with the larger turbulent structures. The results from off-line control scheme are used to develop a real-time control scheme to systematically target the large-scale high skin friction events. The outcome of this control approach on both the instantaneous coherent structures and also the time-averaged quantities is investigated.

  12. Simultaneous planar measurements of soot structure and velocity fields in a turbulent lifted jet flame at 3 kHz

    NASA Astrophysics Data System (ADS)

    Köhler, M.; Boxx, I.; Geigle, K. P.; Meier, W.

    2011-05-01

    We describe a newly developed combustion diagnostic for the simultaneous planar imaging of soot structure and velocity fields in a highly sooting, lifted turbulent jet flame at 3000 frames per second, or two orders of magnitude faster than "conventional" laser imaging systems. This diagnostic uses short pulse duration (8 ns), frequency-doubled, diode-pumped solid state (DPSS) lasers to excite laser-induced incandescence (LII) at 3 kHz, which is then imaged onto a high framerate CMOS camera. A second (dual-cavity) DPSS laser and CMOS camera form the basis of a particle image velocity (PIV) system used to acquire 2-component velocity field in the flame. The LII response curve (measured in a laminar propane diffusion flame) is presented and the combined diagnostics then applied in a heavily sooting lifted turbulent jet flame. The potential challenges and rewards of application of this combined imaging technique at high speeds are discussed.

  13. Ultra-High Throughput Synthesis of Nanoparticles with Homogeneous Size Distribution Using a Coaxial Turbulent Jet Mixer

    PubMed Central

    2015-01-01

    High-throughput production of nanoparticles (NPs) with controlled quality is critical for their clinical translation into effective nanomedicines for diagnostics and therapeutics. Here we report a simple and versatile coaxial turbulent jet mixer that can synthesize a variety of NPs at high throughput up to 3 kg/d, while maintaining the advantages of homogeneity, reproducibility, and tunability that are normally accessible only in specialized microscale mixing devices. The device fabrication does not require specialized machining and is easy to operate. As one example, we show reproducible, high-throughput formulation of siRNA-polyelectrolyte polyplex NPs that exhibit effective gene knockdown but exhibit significant dependence on batch size when formulated using conventional methods. The coaxial turbulent jet mixer can accelerate the development of nanomedicines by providing a robust and versatile platform for preparation of NPs at throughputs suitable for in vivo studies, clinical trials, and industrial-scale production. PMID:24824296

  14. Turbulent convection in rapidly rotating spherical shells: A model for equatorial and high latitude jets on Jupiter and Saturn

    NASA Astrophysics Data System (ADS)

    Heimpel, Moritz; Aurnou, Jonathan

    2007-04-01

    The origin of zonal jets on the jovian planets has long been a topic of scientific debate. In this paper we show that deep convection in a spherical shell can generate zonal flow comparable to that observed on Jupiter and Saturn, including a broad prograde equatorial jet and multiple alternating jets at higher latitudes. We present fully turbulent, 3D spherical numerical simulations of rapidly rotating convection with different spherical shell geometries. The resulting global flow fields tend to be segregated into three regions (north, equatorial, and south), bounded by the tangent cylinder that circumscribes the inner boundary equator. In all of our simulations a strong prograde equatorial jet forms outside the tangent cylinder, whereas multiple jets form in the northern and southern hemispheres, inside the tangent cylinder. The jet scaling of our numerical models and of Jupiter and Saturn is consistent with the theory of geostrophic turbulence, which we extend to include the effect of spherical shell geometry. Zonal flow in a spherical shell is distinguished from that in a full sphere or a shallow layer by the effect of the tangent cylinder, which marks a reversal in the sign of the planetary β-parameter and a jump in the Rhines length. This jump is manifest in the numerical simulations as a sharp equatorward increase in jet widths—a transition that is also observed on Jupiter and Saturn. The location of this transition gives an estimate of the depth of zonal flow, which seems to be consistent with current models of the jovian and saturnian interiors.

  15. Mechanisms of flame stabilisation at low lifted height in a turbulent lifted slot-jet flame

    DOE PAGESBeta

    Karami, Shahram; Hawkes, Evatt R.; Talei, Mohsen; Chen, Jacqueline H.

    2015-07-23

    A turbulent lifted slot-jet flame is studied using direct numerical simulation (DNS). A one-step chemistry model is employed with a mixture-fraction-dependent activation energy which can reproduce qualitatively the dependence of the laminar burning rate on the equivalence ratio that is typical of hydrocarbon fuels. The basic structure of the flame base is first examined and discussed in the context of earlier experimental studies of lifted flames. Several features previously observed in experiments are noted and clarified. Some other unobserved features are also noted. Comparison with previous DNS modelling of hydrogen flames reveals significant structural differences. The statistics of flow andmore » relative edge-flame propagation velocity components conditioned on the leading edge locations are then examined. The results show that, on average, the streamwise flame propagation and streamwise flow balance, thus demonstrating that edge-flame propagation is the basic stabilisation mechanism. Fluctuations of the edge locations and net edge velocities are, however, significant. It is demonstrated that the edges tend to move in an essentially two-dimensional (2D) elliptical pattern (laterally outwards towards the oxidiser, then upstream, then inwards towards the fuel, then downstream again). It is proposed that this is due to the passage of large eddies, as outlined in Suet al.(Combust. Flame, vol. 144 (3), 2006, pp. 494–512). However, the mechanism is not entirely 2D, and out-of-plane motion is needed to explain how flames escape the high-velocity inner region of the jet. Finally, the time-averaged structure is examined. A budget of terms in the transport equation for the product mass fraction is used to understand the stabilisation from a time-averaged perspective. The result of this analysis is found to be consistent with the instantaneous perspective. The budget reveals a fundamentally 2D structure, involving transport in both the streamwise and transverse

  16. Mechanisms of flame stabilisation at low lifted height in a turbulent lifted slot-jet flame

    SciTech Connect

    Karami, Shahram; Hawkes, Evatt R.; Talei, Mohsen; Chen, Jacqueline H.

    2015-07-23

    A turbulent lifted slot-jet flame is studied using direct numerical simulation (DNS). A one-step chemistry model is employed with a mixture-fraction-dependent activation energy which can reproduce qualitatively the dependence of the laminar burning rate on the equivalence ratio that is typical of hydrocarbon fuels. The basic structure of the flame base is first examined and discussed in the context of earlier experimental studies of lifted flames. Several features previously observed in experiments are noted and clarified. Some other unobserved features are also noted. Comparison with previous DNS modelling of hydrogen flames reveals significant structural differences. The statistics of flow and relative edge-flame propagation velocity components conditioned on the leading edge locations are then examined. The results show that, on average, the streamwise flame propagation and streamwise flow balance, thus demonstrating that edge-flame propagation is the basic stabilisation mechanism. Fluctuations of the edge locations and net edge velocities are, however, significant. It is demonstrated that the edges tend to move in an essentially two-dimensional (2D) elliptical pattern (laterally outwards towards the oxidiser, then upstream, then inwards towards the fuel, then downstream again). It is proposed that this is due to the passage of large eddies, as outlined in Suet al.(Combust. Flame, vol. 144 (3), 2006, pp. 494–512). However, the mechanism is not entirely 2D, and out-of-plane motion is needed to explain how flames escape the high-velocity inner region of the jet. Finally, the time-averaged structure is examined. A budget of terms in the transport equation for the product mass fraction is used to understand the stabilisation from a time-averaged perspective. The result of this analysis is found to be consistent with the instantaneous perspective. The budget reveals a fundamentally 2D structure, involving transport in both

  17. Centerline Depletion in Direct-Chill Cast Aluminum Alloys: The Avalanche Effect and Its Consequence for Turbulent Jet Casting

    NASA Astrophysics Data System (ADS)

    Wagstaff, Samuel R.; Allanore, Antoine

    2016-07-01

    Avalanche dynamics of sedimenting grains in direct-chill casting of aluminum ingots is investigated as a primary driving force for centerline segregation. An analytical model predicting the importance of avalanche events as a function of casting parameters is proposed and validated with prior art results. New experimental results investigating the transient and steady-state centerline segregation of DC casting with a turbulent jet are reported.

  18. Prediction of Turbulence-Generated Noise in Unheated Jets. Part 2; JeNo Users' Manual (Version 1.0)

    NASA Technical Reports Server (NTRS)

    Khavaran, Abbas; Wolter, John D.; Koch, L. Danielle

    2009-01-01

    JeNo (Version 1.0) is a Fortran90 computer code that calculates the far-field sound spectral density produced by axisymmetric, unheated jets at a user specified observer location and frequency range. The user must provide a structured computational grid and a mean flow solution from a Reynolds-Averaged Navier Stokes (RANS) code as input. Turbulence kinetic energy and its dissipation rate from a k-epsilon or k-omega turbulence model must also be provided. JeNo is a research code, and as such, its development is ongoing. The goal is to create a code that is able to accurately compute far-field sound pressure levels for jets at all observer angles and all operating conditions. In order to achieve this goal, current theories must be combined with the best practices in numerical modeling, all of which must be validated by experiment. Since the acoustic predictions from JeNo are based on the mean flow solutions from a RANS code, quality predictions depend on accurate aerodynamic input.This is why acoustic source modeling, turbulence modeling, together with the development of advanced measurement systems are the leading areas of research in jet noise research at NASA Glenn Research Center.

  19. Measurements of soot, OH, and PAH concentrations in turbulent ethylene/air jet flames

    SciTech Connect

    Lee, Seong-Young; Turns, Stephen R.; Santoro, Robert J.

    2009-12-15

    This paper presents results from an investigation of soot formation in turbulent, non-premixed, C{sub 2}H{sub 4}/air jet flames. Tests were conducted using a H{sub 2}-piloted burner with fuel issuing from a 2.18 mm i.d. tube into quiescent ambient air. A range of test conditions was studied using the initial jet velocity (16.2-94.1 m/s) as a parameter. Fuel-jet Reynolds numbers ranged from 4000 to 23,200. Planar laser-induced incandescence (LII) was employed to determine soot volume fractions, and laser-induced fluorescence (LIF) was used to measure relative hydroxyl radical (OH) concentrations and polycyclic aromatic hydrocarbons (PAHs) concentrations. Extensive information on the structure of the soot and OH fields was obtained from two-dimensional imaging experiments. Quantitative measurements were obtained by employing the LII and LIF techniques independently. Imaging results for soot, OH, and PAH show the existence of three soot formation/oxidation regions: a rapid soot growth region, in which OH and soot particles lie in distinctly different radial locations; a mixing-dominated region controlled by large-scale motion; and a soot-oxidation region in which the OH and soot fields overlap spatially, resulting in the rapid oxidation of soot particles. Detailed quantitative analyzes of soot volume fractions and OH and soot zone thicknesses were performed along with the temperature measurement using the N{sub 2}-CARS system. Measurements of OH and soot zone thicknesses show that the soot zone thickness increases linearly with axial distance in the soot formation region, whereas the OH zone thickness is nearly constant in this region. The OH zone thickness then rapidly increases with downstream distance and approximately doubles in the soot-oxidation region. Probability density functions also were obtained for soot volume fractions and OH concentrations. These probability density functions clearly define the spatial relationships among the OH, PAH concentrations, the

  20. Effective Jet Properties for the Prediction of Turbulent Mixing Noise Reduction by Water Injection

    NASA Technical Reports Server (NTRS)

    Kandula, Max; Lonergan, Michael J.

    2007-01-01

    A one-dimensional control volume formulation is developed for the determination of jet mixing noise reduction due to water injection. The analysis starts from the conservation of mass, momentum and energy for the control volume, and introduces the concept of effective jet parameters (jet temperature, jet velocity and jet Mach number). It is shown that the water to jet mass flow rate ratio is an important parameter characterizing the jet noise reduction on account of gas-to-droplet momentum and heat transfer. Two independent dimensionless invariant groups are postulated, and provide the necessary relations for the droplet size and droplet Reynolds number. Results are presented illustrating the effect of mass flow rate ratio on the jet mixing noise reduction for a range of jet Mach number and jet Reynolds number. Predictions from the model show satisfactory comparison with available test data on supersonic jets. The results suggest that significant noise reductions can be achieved at increased flow rate ratios.

  1. Prediction of Turbulence-Generated Noise in Unheated Jets. Part 1; JeNo Technical Manual (Version 1.0)

    NASA Technical Reports Server (NTRS)

    Khavaran, Abbas; Bridges, James; Georgiadis, Nicholas

    2005-01-01

    The model-based approach, used by the JeNo code to predict jet noise spectral directivity, is described. A linearized form of Lilley's equation governs the non-causal Green s function of interest, with the non-linear terms on the right hand side identified as the source. A Reynolds-averaged Navier-Stokes (RANS) solution yields the required mean flow for the solution of the propagation Green s function in a locally parallel flow. The RANS solution also produces time- and length-scales needed to model the non-compact source, the turbulent velocity correlation tensor, with exponential temporal and spatial functions. It is shown that while an exact non-causal Green s function accurately predicts the observed shift in the location of the spectrum peak with angle as well as the angularity of sound at low to moderate Mach numbers, the polar directivity of radiated sound is not entirely captured by this Green s function at high subsonic and supersonic acoustic Mach numbers. Results presented for unheated jets in the Mach number range of 0.51 to 1.8 suggest that near the peak radiation angle of high-speed jets, a different source/Green s function convolution integral may be required in order to capture the peak observed directivity of jet noise. A sample Mach 0.90 heated jet is also discussed that highlights the requirements for a comprehensive jet noise prediction model.

  2. Large eddy simulation of soot formation in a turbulent non-premixed jet flame

    SciTech Connect

    El-Asrag, Hossam; Menon, Suresh

    2009-02-15

    A recently developed subgrid model for soot dynamics [H. El-Asrag, T. Lu, C.K. Law, S. Menon, Combust. Flame 150 (2007) 108-126] is used to study the soot formation in a non-premixed turbulent flame. The model allows coupling between reaction, diffusion and soot (including soot diffusion and thermophoretic forces) processes in the subgrid domain without requiring ad hoc filtering or model parameter adjustments. The combined model includes the entire process, from the initial phase, when the soot nucleus diameter is much smaller than the mean free path, to the final phase, after coagulation and aggregation, where it can be considered in the continuum regime. A relatively detailed but reduced kinetics for ethylene-air is used to simulate an experimentally studied non-premixed ethylene/air jet diffusion flame. Acetylene is used as a soot precursor species. The soot volume fraction order of magnitude, the location of its maxima, and the soot particle size distribution are all captured reasonably. Along the centerline, an initial region dominated by nucleation and surface growth is established followed by an oxidation region. The diffusion effect is found to be most important in the nucleation regime, while the thermophoretic forces become more influential downstream of the potential core in the oxidation zone. The particle size distribution shows a log-normal distribution in the nucleation region, and a more Gaussian like distribution further downstream. Limitations of the current approach and possible solution strategies are also discussed. (author)

  3. Shear layer structure of a low speed jet. Ph.D. Thesis. Final Report, 28 Jun. 1974 - 31 Dec. 1975; [measurements of field pressure and turbulent velocity functions

    NASA Technical Reports Server (NTRS)

    Petersen, R. A.

    1976-01-01

    A series of measurements of near field pressures and turbulent velocity fluctuations were made in a low speed jet with a Reynolds number near 50,000 in order to investigate more quantitatively the character and behavior of the large scale structures and their interactions with each other. The near field measurements were modelled according to the vortex pairing hypothesis to deduce the distribution of pairings along the jet axis and the variances about the mean locations. The hodograph plane description of turbulence was explored in some detail, and a complex correlation quantity was synthesized which has useful properties for turbulence in the presence of mean shear.

  4. A Hybrid URANS/LES Approach Used for Simulations of Turbulent Flows

    NASA Astrophysics Data System (ADS)

    Fraňa, Karel; Stiller, Jörg

    A hybrid model based on the unsteady Reynolds averaged Navier-Stokes approach represented by the one-equation Spalart-Allmaras model and the Large Eddy Simulation called Detached Eddy Simulation (DES) was applied for turbulent flow simulations. This turbulent approach was implemented into the flow solver based on the Finite-Element Method with pressure stabilized and streamlines upwind Petrov-Galerkin stabilization techniques. The effectiveness and robustness of this updated solver is successfully demonstrated at benchmark calculation represented by an unsteady turbulent flow past a cylinder at Reynolds number 3900. Results such as velocity fields and the flow periodicity, Reynolds stress tensor and eddy viscosity and pressure coefficient distributions are discussed and relatively good agreement was found to direct numerical simulations and experiments.

  5. LES, DNS, and RANS for the Analysis of High-Speed Turbulent Reacting Flows

    NASA Technical Reports Server (NTRS)

    Colucci, P. J.; Jaberi, F. A.; Givi, P.

    1996-01-01

    A filtered density function (FDF) method suitable for chemically reactive flows is developed in the context of large eddy simulation. The advantage of the FDF methodology is its inherent ability to resolve subgrid scales (SGS) scalar correlations that otherwise have to be modeled. Because of the lack of robust models to accurately predict these correlations in turbulent reactive flows, simulations involving turbulent combustion are often met with a degree of skepticism. The FDF methodology avoids the closure problem associated with these terms and treats the reaction in an exact manner. The scalar FDF approach is particularly attractive since it can be coupled with existing hydrodynamic computational fluid dynamics (CFD) codes.

  6. The entrainment rate for a row of turbulent jets. M.S. Thesis Final Report

    NASA Technical Reports Server (NTRS)

    Gordon, Eliott B.; Greber, Isaac

    1990-01-01

    Entrainment rates for a row of isothermal circular air jets issuing into a quiescent environment are found by integrating velocity distributions measured by a linearized hot-wire anemometer. Jet spacing to jet diameter ratios of 2.5, 5, 10, and 20 are studied at jet Reynold's numbers ranging from 5110 to 12070. Velocity distributions are determined at regular downstream intervals at axial distances equal to 16.4 to 164 jet diameters from the jet source. The entrainment rates for the four spacing configurations vary monotonically with increasing spacing/diameter between the limiting case of the slot jet entrainment rate (where the jet spacing to diameter ratio is zero) and the circular jet entrainment rate (in which the spacing to diameter ratio is infinity).

  7. A DNS study on the stabilization mechanism of a turbulent lifted ethylene jet flame in highly-heated coflow

    SciTech Connect

    Yoo, Chun S

    2011-01-01

    Direct numerical simulation (DNS) of the near-field of a three-dimensional spatially-developing turbulent ethylene jet flame in highly-heated coflow is performed with a reduced mechanism to determine the stabilization mechanism. The DNS was performed at a jet Reynolds number of 10,000 with over 1.29 billion grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. The Damkoehler number and chemical explosive mode (CEM) analysis also verify that auto-ignition occurs at the flame base. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base similar to a previous study (Yoo et al., J. Fluid Mech. 640 (2009) 453-481) with hydrogen/air jet flames. It is also observed that the present lifted flame base exhibits a cyclic 'saw-tooth' shaped movement marked by rapid movement upstream and slower movement downstream. This is a consequence of the lifted flame being stabilized by a balance between consecutive auto-ignition events in hot fuel-lean mixtures and convection induced by the high-speed jet and coflow velocities. This is confirmed by Lagrangian tracking of key variables including the flame-normal velocity, displacement speed, scalar dissipation rate, and mixture fraction at the stabilization point.

  8. A DNS study on the stabilization mechanism of a turbulent lifted ethylene jet flame in highly-heated coflow

    SciTech Connect

    Yoo, C. S.; Richardson, E.; Sankaran, R.; Chen, J. H.

    2011-01-01

    Direct numerical simulation (DNS) of the near-field of a three-dimensional spatially-developing turbulent ethylene jet flame in highly-heated coflow is performed with a reduced mechanism to determine the stabilization mechanism. The DNS was performed at a jet Reynolds number of 10,000 with over 1.29 billion grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. The Damköhler number and chemical explosive mode (CEM) analysis also verify that auto-ignition occurs at the flame base. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base similar to a previous study (Yoo et al., J. Fluid Mech. 640 (2009) 453–481) with hydrogen/air jet flames. It is also observed that the present lifted flame base exhibits a cyclic ‘saw-tooth’ shaped movement marked by rapid movement upstream and slower movement downstream. This is a consequence of the lifted flame being stabilized by a balance between consecutive auto-ignition events in hot fuel-lean mixtures and convection induced by the high-speed jet and coflow velocities. This is confirmed by Lagrangian tracking of key variables including the flame-normal velocity, displacement speed, scalar dissipation rate, and mixture fraction at the stabilization point.

  9. Operation in the turbulent jet field of a linear array of multiple rectangular jets using a two-dimensional jet (Variation of mean velocity field)

    NASA Astrophysics Data System (ADS)

    Fujita, Shigetaka; Harima, Takashi

    2016-03-01

    The mean flowfield of a linear array of multiple rectangular jets run through transversely with a two-dimensional jet, has been investigated, experimentally. The object of this experiment is to operate both the velocity scale and the length scale of the multiple rectangular jets using a two-dimensional jet. The reason of the adoption of this nozzle exit shape was caused by the reports of authors in which the cruciform nozzle promoted the inward secondary flows strongly on both the two jet axes. Aspect ratio of the rectangular nozzle used in this experiment was 12.5. Reynolds number based on the nozzle width d and the exit mean velocity Ue (≅ 39 m / s) was kept constant 25000. Longitudinal mean velocity was measured using an X-array Hot-Wire Probe (lh = 3.1 μm in diameter, dh = 0.6 mm effective length : dh / lh = 194) operated by the linearized constant temperature anemometers (DANTEC), and the spanwise and the lateral mean velocities were measured using a yaw meter. The signals from the anemometers were passed through the low-pass filters and sampled using A.D. converter. The processing of the signals was made by a personal computer. Acquisition time of the signals was usually 60 seconds. From this experiment, it was revealed that the magnitude of the inward secondary flows on both the y and z axes in the upstream region of the present jet was promoted by a two-dimensional jet which run through transversely perpendicular to the multiple rectangular jets, therefore the potential core length on the x axis of the present jet extended 2.3 times longer than that of the multiple rectangular jets, and the half-velocity width on the rectangular jet axis of the present jet was suppressed 41% shorter compared with that of the multiple rectangular jets.

  10. On Laminar to Turbulent Transition of Arc-Jet Flow in the NASA Ames Panel Test Facility

    NASA Technical Reports Server (NTRS)

    Gokcen, Tahir; Alunni, Antonella I.

    2012-01-01

    This paper provides experimental evidence and supporting computational analysis to characterize the laminar to turbulent flow transition in a high enthalpy arc-jet facility at NASA Ames Research Center. The arc-jet test data obtained in the 20 MW Panel Test Facility include measurements of surface pressure and heat flux on a water-cooled calibration plate, and measurements of surface temperature on a reaction-cured glass coated tile plate. Computational fluid dynamics simulations are performed to characterize the arc-jet test environment and estimate its parameters consistent with the facility and calibration measurements. The present analysis comprises simulations of the nonequilibrium flowfield in the facility nozzle, test box, and flowfield over test articles. Both laminar and turbulent simulations are performed, and the computed results are compared with the experimental measurements, including Stanton number dependence on Reynolds number. Comparisons of computed and measured surface heat fluxes (and temperatures), along with the accompanying analysis, confirm that that the boundary layer in the Panel Test Facility flow is transitional at certain archeater conditions.

  11. High Order Numerical Methods for LES of Turbulent Flows with Shocks

    NASA Technical Reports Server (NTRS)

    Kotov, D. V.; Yee, H. C.; Hadjadj, A.; Wray, A.; Sjögreen, B.

    2014-01-01

    Simulation of turbulent flows with shocks employing explicit subgrid-scale (SGS) filtering may encounter a loss of accuracy in the vicinity of a shock. In this work we perform a comparative study of different approaches to reduce this loss of accuracy within the framework of the dynamic Germano SGS model. One of the possible approaches is to apply Harten's subcell resolution procedure to locate and sharpen the shock, and to use a one-sided test filter at the grid points adjacent to the exact shock location. The other considered approach is local disabling of the SGS terms in the vicinity of the shock location. In this study we use a canonical shock-turbulence interaction problem for comparison of the considered modifications of the SGS filtering procedure. For the considered test case both approaches show a similar improvement in the accuracy near the shock.

  12. Implicit LES of Turbulent, Separated Flow: Wall-Mounted Hump Configuration

    NASA Technical Reports Server (NTRS)

    Sekhar, Susheel; Mansour, Nagi N.; Caubilla, David Higuera

    2015-01-01

    Direct simulations (ILES) of turbulent, separated flow over the wall-mounted hump configuration is conducted to investigate the physics of separated flows. A chord-based Reynolds number of Re(sub c) = 47,500 is set up, with a turbulent in flow of Re(sub theta) = 1,400 (theta/c = 3%). FDL3DI, a code that solves the compressible Navier-Stokes equations using high- order compact-difference scheme and filter, with the standard recycling/rescaling method of turbulence generation, is used. Two different configurations of the upper-wall are analyzed, and results are compared with both a higher Re(sub c) (= 936,000, Re(sub theta) = 7,200, theta/c = 0.77%) experiment for major flow features, and RANS (k-omega SST) results. A lower Rec allows for DNS-like mesh resolution, and an adequately wide span. Both ILES and RANS show delayed reattachment compared to experiment, and significantly higher skin friction in the forebody of the hump, as expected. The upper-wall shape influences the C(sub p) distribution only. Results from this study are being used to setup higher Rec (lower theta/c) ILES.

  13. Turbulence in Compressible Flows

    NASA Technical Reports Server (NTRS)

    1997-01-01

    Lecture notes for the AGARD Fluid Dynamics Panel (FDP) Special Course on 'Turbulence in Compressible Flows' have been assembled in this report. The following topics were covered: Compressible Turbulent Boundary Layers, Compressible Turbulent Free Shear Layers, Turbulent Combustion, DNS/LES and RANS Simulations of Compressible Turbulent Flows, and Case Studies of Applications of Turbulence Models in Aerospace.

  14. Experimental investigation of a free-surface turbulent jet with Coanda effect

    NASA Astrophysics Data System (ADS)

    Miozzi, M.; Lalli, F.; Romano, G. P.

    2010-07-01

    The deviation of a jet from the straight direction due to the presence of a lateral wall is investigated from the experimental point of view. This flow condition is known as Coanda jet (from the Romanian aerodynamicist Henry Marie Coanda who discovered and applied it at the beginning of XXth century) or offset jet. The objective of the work is to detail the underlying mechanisms of such a phenomenon aiming to use it as a flow control method at polluted river flows mouth. To do this, a large laboratory free-surface tank with an incoming channel has been set up and velocity field measurements are performed by Optical Flow methods (namely Feature Tracking). Preliminary tests on the well-known free jet configuration without any marine structure ( i.e. lateral wall) are performed to allow comparison with free jet scaling and self-similar solutions. The presence of the free-surface gives rise to centerline velocity decay which is lower than in free unbounded plane or circular jets due to the vertically limited ambient fluid entrainment. In the second part of the paper, the effect of a lateral wall on the jet configuration is examined by placing it at different lateral distances from the jet outlet. The resulting velocity fields clearly show an inclined Coanda jet with details which seems to depend on the lateral wall distance itself. The analysis of self-similarity along the inclined jet direction reveals that for wall distances larger than 5 jet widths this dependence almost disappears.

  15. Computation of supersonic jet mixing noise for an axisymmetric CD nozzle using k-epsilon turbulence model

    NASA Technical Reports Server (NTRS)

    Khavaran, Abbas; Krejsa, Eugene A.; Kim, Chan M.

    1991-01-01

    The turbulent mixing noise of a supersonic jet is calculated for a round convergent-divergent nozzle at the design pressure ratio. Aerodynamic computations are performed using the PARC code with a k-epsilon turbulence model. Lighthill's acoustic analogy combined with Ribner's assumption is adopted. The acoustics solution is based upon the methodology followed by GE in the MGB code. The source correlation function is expressed as a linear combination of second-order tensors. Assuming separable second-order correlations and incorporating Batchelor's isotropic turbulence model, the source term was calculated from the kinetic energy of turbulence. A Gaussian distribution for the time-delay of correlation was introduced. The computational fluid dynamics (CFD) solution was used to obtain the source strength as well as the characteristic time-delay of correlation. The effect of sound/flow interaction was incorporated using the high frequency asymptotic solution to Lilley's equation for axisymmetric geometries. Acoustic results include sound pressure level directivity and spectra at different polar angles. The aerodynamic and acoustic results demonstrate favorable agreement with experimental data.

  16. LES of turbulent heat transfer: proper convection numerical schemes for temperature transport

    NASA Astrophysics Data System (ADS)

    Châtelain, A.; Ducros, F.; Métais, O.

    2004-03-01

    Large eddy simulations of two basic configurations (decay of isotropic turbulence, and the academic plane channel flow) with heat transfer have been performed comparing several convection numerical schemes, in order to discuss their ability to evaluate temperature fluctuations properly. Results are compared with the available incompressible heat transfer direct numerical simulation data. It is shown that the use of regularizing schemes (such as high order upwind type schemes) for the temperature transport equation in combination with centered schemes for momentum transport equation gives better results than the use of centred schemes for both equations.

  17. Efficient implicit LES method for the simulation of turbulent cavitating flows

    NASA Astrophysics Data System (ADS)

    Egerer, Christian P.; Schmidt, Steffen J.; Hickel, Stefan; Adams, Nikolaus A.

    2016-07-01

    We present a numerical method for efficient large-eddy simulation of compressible liquid flows with cavitation based on an implicit subgrid-scale model. Phase change and subgrid-scale interface structures are modeled by a homogeneous mixture model that assumes local thermodynamic equilibrium. Unlike previous approaches, emphasis is placed on operating on a small stencil (at most four cells). The truncation error of the discretization is designed to function as a physically consistent subgrid-scale model for turbulence. We formulate a sensor functional that detects shock waves or pseudo-phase boundaries within the homogeneous mixture model for localizing numerical dissipation. In smooth regions of the flow field, a formally non-dissipative central discretization scheme is used in combination with a regularization term to model the effect of unresolved subgrid scales. The new method is validated by computing standard single- and two-phase test-cases. Comparison of results for a turbulent cavitating mixing layer obtained with the new method demonstrates its suitability for the target applications.

  18. Simulating flame lift-off characteristics of diesel and biodiesel fuels using detailed chemical-kinetic mechanisms and LES turbulence model.

    SciTech Connect

    Som, S; Longman, D. E.; Luo, Z; Plomer, M; Lu, T; Senecal, P.K.; Pomraning, E

    2012-01-01

    Combustion in direct-injection diesel engines occurs in a lifted, turbulent diffusion flame mode. Numerous studies indicate that the combustion and emissions in such engines are strongly influenced by the lifted flame characteristics, which are in turn determined by fuel and air mixing in the upstream region of the lifted flame, and consequently by the liquid breakup and spray development processes. From a numerical standpoint, these spray combustion processes depend heavily on the choice of underlying spray, combustion, and turbulence models. The present numerical study investigates the influence of different chemical kinetic mechanisms for diesel and biodiesel fuels, as well as Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) turbulence models on predicting flame lift-off lengths (LOLs) and ignition delays. Specifically, two chemical kinetic mechanisms for n-heptane (NHPT) and three for biodiesel surrogates are investigated. In addition, the RNG k-{epsilon} (RANS) model is compared to the Smagorinsky based LES turbulence model. Using adaptive grid resolution, minimum grid sizes of 250 {micro}m and 125 {micro}m were obtained for the RANS and LES cases respectively. Validations of these models were performed against experimental data from Sandia National Laboratories in a constant volume combustion chamber. Ignition delay and flame lift-off validations were performed at different ambient temperature conditions. The LES model predicts lower ignition delays and qualitatively better flame structures compared to the RNG k-{epsilon} model. The use of realistic chemistry and a ternary surrogate mixture, which consists of methyl decanoate, methyl 9-decenoate, and NHPT, results in better predicted LOLs and ignition delays. For diesel fuel though, only marginal improvements are observed by using larger size mechanisms. However, these improved predictions come at a significant increase in computational cost.

  19. An experimental and numerical study of confined non-reacting and reacting turbulent jets to facilitate homogeneous combustion in industrial furnaces

    NASA Astrophysics Data System (ADS)

    Lee, Insu

    Confined non-reacting turbulent jets are ideal for recirculating the hot flue gas back into the furnace from an external exhaust duct. Such jets are also used inside the furnace to internally entrain and recirculate the hot flue gas to preheat and dilute the reactants. Both internal and external implementation of confined turbulent jets increase the furnace thermal efficiency. For external implementation, depending on the circumstances, the exhaust gas flow may be co- or counter-flow relative to the jet flow. Inside the furnaces, fuel and air jets are injected separately. To create a condition which can facilitate near homogeneous combustion, these jets have to first mix with the burned gas inside the furnace and simultaneously being heated and diluted prior to combustion. Clearly, the combustion pattern and emissions from reacting confined turbulent jets are affected by jet interactions, mixing and entrainment of hot flue gas. In this work, the flow and mixing characteristics of a non-reacting and reacting confined turbulent jet are investigated experimentally and numerically. This work consists of two parts: (i) A study of flow and mixing characteristics of non-reacting confined turbulent jets with co- or counter-flowing exhaust/flue gas. Here the axial and radial distributions of temperature, velocity and NO concentration (used as a tracer gas) were measured. FLUENT was used to numerically simulate the experimental results. This work provides the basic understanding of the flow and mixing characteristics of confined turbulent jets and develops some design considerations for recirculating flue gas back into the furnace as expressed by the recirculation zone and the stagnation locations. (ii) Numerical calculations of near homogeneous combustion are performed for the existing furnace. The exact geometry of the furnace in the lab is used and the real dimensional boundary conditions are considered. The parameters such as air nozzle diameter (dair), fuel nozzle

  20. Investigation of buoyancy effects on turbulent nonpremixed jet flames by using normal and low-gravity conditions

    NASA Astrophysics Data System (ADS)

    Idicheria, Cherian Alex

    An experimental study was performed with the aim of investigating the structure of transitional and turbulent nonpremixed jet flames under different gravity conditions. In particular, the focus was to determine the effect of buoyancy on the mean and fluctuating characteristics of the jet flames. Experiments were conducted under three gravity levels, viz. 1 g, 20 mg and 100 mug. The milligravity and microgravity conditions were achieved by dropping a jet-flame rig in the UT-Austin 1.25-second and the NASA-Glenn Research Center 2.2-second drop towers, respectively. The principal diagnostics employed were time-resolved, cinematographic imaging of the visible soot luminosity and planar laser Mie scattering (PLMS). For the cinematographic flame luminosity imaging experiments, the flames studied were piloted nonpremixed propane, ethylene and methane jet flames at source Reynolds numbers ranging from 2000 to 10500. From the soot luminosity images, mean and root-mean square (RMS) images were computed, and volume rendering of the image sequences was used to investigate the large-scale structure evolution and flame tip dynamics. The relative importance of buoyancy was quantified with the parameter, xL , as defined by Becker and Yamazaki [1978]. The results show, in contrast to previous microgravity studies, that the high Reynolds number flames have the same flame length irrespective of the gravity level. The RMS fluctuations and volume renderings indicate that the large-scale structure and flame tip dynamics are essentially identical to those of purely momentum driven flames provided xL is approximately less than 2. The volume-renderings show that the luminous structure celerities (normalized by jet exit velocity) are approximately constant for xL < 6, but are substantially larger for xL > 8. The celerity values for xL > 8 are seen to follow a x3/2L scaling, which can be predicted with a simplified momentum equation analysis for the buoyancy-dominated regime. The underlying

  1. 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.

  2. On the appearance of a system of ring vortices in the mixing layer of axially symmetric turbulent jets under acoustic action

    NASA Astrophysics Data System (ADS)

    Pimshtein, V. G.

    2016-07-01

    The shadow visualization method is applied to study the process of loss of stability of the mixing layer of a subsonic axially symmetric turbulent jet under longitudinal internal action of saw-tooth sound waves of finite amplitude. Such action leads to the formation of a system of ring vortices in the mixing layer at the frequency of its intrinsic instability. The interaction of the vortices can be accompanied by sound emission. A similar phenomenon is also observed in turbulent jets for small supercritical pressure fluctuations on a nozzle.

  3. Ultrahigh-energy cosmic ray production by turbulence in gamma-ray burst jets and cosmogenic neutrinos

    NASA Astrophysics Data System (ADS)

    Asano, Katsuaki; Mészáros, Peter

    2016-07-01

    We propose a novel model to produce ultrahigh-energy cosmic rays (UHECRs) in gamma-ray burst jets. After the prompt gamma-ray emission, hydrodynamical turbulence is excited in the GRB jets at or before the afterglow phase. The mildly relativistic turbulence stochastically accelerates protons. The acceleration rate is much slower than the usual first-order shock acceleration rate, but in this case it can be energy independent. The resultant UHECR spectrum is so hard that the bulk energy is concentrated in the highest energy range, resulting in a moderate requirement for the typical cosmic-ray luminosity of ˜1 053.5 erg s-1 . In this model, the secondary gamma-ray and neutrino emissions initiated by photopion production are significantly suppressed. Although the UHECR spectrum at injection shows a curved feature, this does not conflict with the observed UHECR spectral shape. The cosmogenic neutrino spectrum in the 1017- 1018 eV range becomes distinctively hard in this model, which may be verified by future observations.

  4. Effect of fuel composition and differential diffusion on flame stabilization in reacting syngas jets in turbulent cross-flow

    DOE PAGESBeta

    Minamoto, Yuki; Kolla, Hemanth; Grout, Ray W.; Gruber, Andrea; Chen, Jacqueline H.

    2015-07-24

    Here, three-dimensional direct numerical simulation results of a transverse syngas fuel jet in turbulent cross-flow of air are analyzed to study the influence of varying volume fractions of CO relative to H2 in the fuel composition on the near field flame stabilization. The mean flame stabilizes at a similar location for CO-lean and CO-rich cases despite the trend suggested by their laminar flame speed, which is higher for the CO-lean condition. To identify local mixtures having favorable mixture conditions for flame stabilization, explosive zones are defined using a chemical explosive mode timescale. The explosive zones related to flame stabilization aremore » located in relatively low velocity regions. The explosive zones are characterized by excess hydrogen transported solely by differential diffusion, in the absence of intense turbulent mixing or scalar dissipation rate. The conditional averages show that differential diffusion is negatively correlated with turbulent mixing. Moreover, the local turbulent Reynolds number is insufficient to estimate the magnitude of the differential diffusion effect. Alternatively, the Karlovitz number provides a better indicator of the importance of differential diffusion. A comparison of the variations of differential diffusion, turbulent mixing, heat release rate and probability of encountering explosive zones demonstrates that differential diffusion predominantly plays an important role for mixture preparation and initiation of chemical reactions, closely followed by intense chemical reactions sustained by sufficient downstream turbulent mixing. The mechanism by which differential diffusion contributes to mixture preparation is investigated using the Takeno Flame Index. The mean Flame Index, based on the combined fuel species, shows that the overall extent of premixing is not intense in the upstream regions. However, the Flame Index computed based on individual contribution of H2 or CO species reveals that hydrogen

  5. Effect of fuel composition and differential diffusion on flame stabilization in reacting syngas jets in turbulent cross-flow

    SciTech Connect

    Minamoto, Yuki; Kolla, Hemanth; Grout, Ray W.; Gruber, Andrea; Chen, Jacqueline H.

    2015-07-24

    Here, three-dimensional direct numerical simulation results of a transverse syngas fuel jet in turbulent cross-flow of air are analyzed to study the influence of varying volume fractions of CO relative to H2 in the fuel composition on the near field flame stabilization. The mean flame stabilizes at a similar location for CO-lean and CO-rich cases despite the trend suggested by their laminar flame speed, which is higher for the CO-lean condition. To identify local mixtures having favorable mixture conditions for flame stabilization, explosive zones are defined using a chemical explosive mode timescale. The explosive zones related to flame stabilization are located in relatively low velocity regions. The explosive zones are characterized by excess hydrogen transported solely by differential diffusion, in the absence of intense turbulent mixing or scalar dissipation rate. The conditional averages show that differential diffusion is negatively correlated with turbulent mixing. Moreover, the local turbulent Reynolds number is insufficient to estimate the magnitude of the differential diffusion effect. Alternatively, the Karlovitz number provides a better indicator of the importance of differential diffusion. A comparison of the variations of differential diffusion, turbulent mixing, heat release rate and probability of encountering explosive zones demonstrates that differential diffusion predominantly plays an important role for mixture preparation and initiation of chemical reactions, closely followed by intense chemical reactions sustained by sufficient downstream turbulent mixing. The mechanism by which differential diffusion contributes to mixture preparation is investigated using the Takeno Flame Index. The mean Flame Index, based on the combined fuel species, shows that the overall extent of premixing is not intense in the upstream regions. However, the Flame Index computed based on individual contribution of H2 or CO species reveals that

  6. Local Velocity Field Measurements towards Understanding Flame Stabilization of Turbulent Non-premixed Jet Flames in Vitiated Coflow

    NASA Astrophysics Data System (ADS)

    Ramachandran, Aravind; Mothe, Anirudh Reddy; Narayanaswamy, Venkateswaran

    2015-11-01

    Turbulent combustion of a non-premixed methane jet issuing into a vitiated coflow is being studied in our lab. Flame luminosity studies demonstrated three dominant characteristic flame motions - a stable flame base (Mode A), complete blowout (Mode B), and partial blowout followed by re-anchoring of the flame by autoignition kernels (Mode C). The experiments presented in this work focused on Mode A, and were carried out over a range of oxidizer temperatures, oxygen molefractions, and fuel jet Reynolds numbers. Measurements of 2-D velocity fields near the base of the lifted jet flame were obtained using Particle Image Velocimetry (PIV) with the objective to delineate the dominant mechanisms involved in the flame stabilization. Statistical analysis of these instantaneous velocity fields will be presented, which shows non-trivial contributions from autoignition kernels as well as edge flame propagation towards flame stabilization. The effect of vortices and high local strain rates was observed to produce local extinctions and destabilize the flame, indicating their role as precursors to (unstable) Mode B and Mode C motions. NSF Grant CBET-1511216.

  7. DNS/LES of turbulent flow in a square duct: A priori evaluation of subgrid models

    NASA Astrophysics Data System (ADS)

    O'Sullivan, Peter L.; Biringen, Sedat; Huser, Asmund

    We have performed a priori tests of two dynamic subgrid-scale (SGS) turbulence models using a highly resolved direct numerical simulation (DNS) data-base for the case of incompressible flow in a straight duct of square cross-section. The model testing is applied only to the homogeneous flow direction where grid filtering can be applied without the introduction of commutation errors. The first model is the dynamic (Smagorinsky/eddy viscosity) SGS model (DSM) developed by Germano et al. [5] while the second is the dynamic two-parameter (mixed) model (DTM) developed by Salvetti and Banerjee [2]. As found in prior studies of this sort there is a very poor correlation of the modelled and exact subgrid-scale dissipation in the case of the DSM. The DSM over-predicts subgrid-scale dissipation on average. Instantaneously, the model provides an inaccurate representation of subgrid-scale dissipation, in general underestimating the magnitude by approximately one order of magnitude. On the other hand, the DTM shows excellent agreement with the exact SGS dissipation over most of the duct cross-section with a correlation coefficient of approximately 0.9.

  8. Experimental study of turbulence in isothermal jet impingement at intermediate plate spacings

    NASA Astrophysics Data System (ADS)

    Landfried, D. Tyler; Valentino, Alex; Mazumdar, Sagnik; Jana, Anirban; Kimber, Mark

    2013-11-01

    One fundamental problem in fluid dynamics is that of the axisymmetric round flow impinging on a plate placed some distance downstream of the jet. Impinging jets have a rich history of applications including small plate spacings, H/D ~ 1, such as encountered in electronics cooling, or large plate spacings, H/D ~ 102, such as vertical takeoff aircrafts and rocket engines. However, intermediate plate spacings, such as the lower plenum of the next generation nuclear reactors, are not typically studied. In this paper, an experimental study is conducted investigating the effect of the impingement plate on the flow behavior compared to the near free jet behavior when the plate is removed. Using air as the working fluid, a single jet is considered at jet Reynolds numbers of 10000, 20000, and 30000. A three-wire anemometer probe is used to quantify the mean components of velocities as well as the Reynolds stress and the third-order moments in the flow field at various distances between the jet outlet and the impingement plate. When present, the impingement plate is placed a distance of 8, 11, 14, and 17 diameters downstream of the jet. Additionally trends in the kinetic energy and dissipation are investigated for validation with numerical models.

  9. Large Eddy Simulations and Turbulence Modeling for Film Cooling

    NASA Technical Reports Server (NTRS)

    Acharya, Sumanta

    1999-01-01

    The objective of the research is to perform Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) for film cooling process, and to evaluate and improve advanced forms of the two equation turbulence models for turbine blade surface flow analysis. The DNS/LES were used to resolve the large eddies within the flow field near the coolant jet location. The work involved code development and applications of the codes developed to the film cooling problems. Five different codes were developed and utilized to perform this research. This report presented a summary of the development of the codes and their applications to analyze the turbulence properties at locations near coolant injection holes.

  10. Numerical simulation of jet noise

    NASA Astrophysics Data System (ADS)

    Paliath, Umesh

    In the present work, computational aeroacoustics and parallel computers are used to conduct a study of flow-induced noise from different jet nozzle geometries. The nozzle is included as part of the computational domain. This is important to predict jet noise from nozzles associated with military aircraft engines. The Detached Eddy Simulation (DES) approach is used to simulate both the jet nozzle internal and external flows as well as the jet plume. This methodology allows the turbulence model to transition from an unsteady Reynolds Averaged Navier-Stokes (URANS) method for attached boundary layers to a Large Eddy Simulation (LES) in separated regions. Thus, it is ideally suited to jet flow simulations where the nozzle is included. Both cylindrical polar and Cartesian coordinate systems are used. A spectral method is used to avoid the centerline singularity when using the cylindrical coordinate system. The one equation Spalart-Allmaras turbulence model, in DES mode, is used to describe the evolution of the turbulent eddy viscosity. An explicit 4th order Runge-Kutta time marching scheme is used. For spatial discritization the Dispersion Relation Preserving scheme(DRP) is used. The farfield sound is evaluated using the Ffowcs Williams-Hawkings permeable surface wave extrapolation method. This permits the noise to be predicted at large distances from the jet based on fluctuations in the jets near field. The present work includes a study of the effect of different nozzle geometries such as axisymmetric/non-axisymmetric and planar/non-planar exits on the far field noise predictions. Also the effect of operating conditions such as a heated/unheated jet, the effect of forward flight, a jet flow at an angle of attack, and the effect of a supersonic exit Mach number, are included in the study.

  11. Simultaneous measurement of Raman scattering and laser-induced OH fluorescence in nonpremixed turbulent jet flames.

    PubMed

    Barlow, R S; Dibble, R W; Lucht, R P

    1989-03-01

    Spontaneous Raman scattering and laser-induced fluorescence are combined to perform simultaneous point measurements of major species concentrations, temperature, and hydroxyl radical concentration in turbulent nonpremixed flames. The Raman-scattering data for major species concentrations and temperature characterize the instantaneous, local, collisional quenching environment of the OH molecule. Collisional quenching corrections are applied for each laser shot so that absolute hydroxyl concentrations are obtained in turbulent flames using linear laser-induced fluorescence. PMID:19749889

  12. A laboratory investigation into the influence of a rigid vegetation on the evolution of a round turbulent jet discharged within a cross flow.

    PubMed

    Malcangio, Daniela; Mossa, Michele

    2016-05-15

    The study of buoyant jets, those between pure jets and plumes, has been carried out with ever greater frequency over recent years due to its application in different practical engineering fields, i.e. appropriate design of outfalls for the disposal of municipal and industrial waste waters. The dispersion of waste and the related dilution of pollutants are governed by the mean-flow and turbulence characteristics of the resulting jets, which themselves depend on environmental conditions. The present study deals with how a uniform cross-stream with a channel bed surface covered by rigid emergent stems affects the behaviour of a circular turbulent buoyant jet. The time-averaged temperature and velocity fields are investigated in order to understand jet diffusion and penetration within the ambient fluid. The examination and comparison of the measured scalar and vector quantities show that the presence of emergent vegetation in the receiving environment affects both the average flow field and the jet structure, reducing the mean channel velocity, with a notable increase in jet penetration height and dilution compared to the test case without vegetation. This result is confirmed by the several vertical profiles of the mean scalar concentration and the normalized vertical velocity component along the channel centre plane. Moreover, the rigid emergent vegetation and its driven instabilities promote a distortion of the mean concentration and normalized axial velocity component profiles in the trajectory-based coordinate system. PMID:26978732

  13. Turbulence

    NASA Astrophysics Data System (ADS)

    Frisch, Uriel

    1996-01-01

    Written five centuries after the first studies of Leonardo da Vinci and half a century after A.N. Kolmogorov's first attempt to predict the properties of flow, this textbook presents a modern account of turbulence, one of the greatest challenges in physics. "Fully developed turbulence" is ubiquitous in both cosmic and natural environments, in engineering applications and in everyday life. Elementary presentations of dynamical systems ideas, probabilistic methods (including the theory of large deviations) and fractal geometry make this a self-contained textbook. This is the first book on turbulence to use modern ideas from chaos and symmetry breaking. The book will appeal to first-year graduate students in mathematics, physics, astrophysics, geosciences and engineering, as well as professional scientists and engineers.

  14. Estimation of Broadband Shock Noise Reduction in Turbulent Jets by Water Injection

    NASA Technical Reports Server (NTRS)

    Kandula, Max; Lonerjan, Michael J.

    2008-01-01

    The concept of effective jet properties introduced by the authors (AIAA-2007-3645) has been extended to the estimation of broadband shock noise reduction by water injection in supersonic jets. Comparison of the predictions with the test data for cold underexpanded supersonic nozzles shows a satisfactory agreement. The results also reveal the range of water mass flow rates over which saturation of mixing noise reduction and existence of parasitic noise are manifest.

  15. Low-level jets and above-canopy drainage as causes of turbulent exchange in the nocturnal boundary layer

    NASA Astrophysics Data System (ADS)

    El-Madany, T. S.; Duarte, H. F.; Durden, D. J.; Paas, B.; Deventer, M. J.; Juang, J.-Y.; Leclerc, M. Y.; Klemm, O.

    2014-08-01

    Sodar (SOund Detection And Ranging), eddy-covariance, and tower profile measurements of wind speed and carbon dioxide were performed during 17 consecutive nights in complex terrain in northern Taiwan. The scope of the study was to identify the causes for intermittent turbulence events and to analyze their importance in nocturnal atmosphere-biosphere exchange as quantified with eddy-covariance measurements. If intermittency occurs frequently at a measurement site, then this process needs to be quantified in order to achieve reliable values for ecosystem characteristics such as net ecosystem exchange or net primary production. Fourteen events of intermittent turbulence were identified and classified into above-canopy drainage flows (ACDFs) and low-level jets (LLJs) according to the height of the wind speed maximum. Intermittent turbulence periods lasted between 30 and 110 min. Towards the end of LLJ or ACDF events, positive vertical wind velocities and, in some cases, upslope flows occurred, counteracting the general flow regime at nighttime. The observations suggest that the LLJs and ACDFs penetrate deep into the cold air pool in the valley, where they experience strong buoyancy due to density differences, resulting in either upslope flows or upward vertical winds. Turbulence was found to be stronger and better developed during LLJs and ACDFs, with eddy-covariance data presenting higher quality. This was particularly indicated by spectral analysis of the vertical wind velocity and the steady-state test for the time series of the vertical wind velocity in combination with the horizontal wind component, the temperature, and carbon dioxide. Significantly higher fluxes of sensible heat, latent heat, and shear stress occurred during these periods. During LLJs and ACDFs, fluxes of sensible heat, latent heat, and CO2 were mostly one-directional. For example, exclusively negative sensible heat fluxes occurred while intermittent turbulence was present. Latent heat fluxes

  16. A Theory for Emergence of Equatorial Deep Jets from Turbulence based on Statistical Mean State Dynamics

    NASA Astrophysics Data System (ADS)

    Fitzgerald, J.; Farrell, B.

    2013-12-01

    Equatorial deep jets (EDJs) are persistent, zonally-coherent jets found within one degree of the equator in all ocean basins (Luyten and Swallow, 1976). The jets are characterized by a vertically oscillating ('stacked') structure between ~500-2000m depth, with jet amplitudes on the order of 10 cm/s superimposed upon a large-scale background shear flow. EDJs are a striking feature of the equatorial climate system and play an important role in equatorial ocean transport. However, the physical mechanism responsible for the presence of EDJs remains uncertain. Previous theoretical models for EDJs have suggested mechanisms involving the reflection and constructive interference of equatorially trapped waves (Wunsch 1977, McCreary 1984) and the instability of mixed Rossby-gravity waves with EDJs as the fastest-growing eigenfunction (Hua et al. 2008, Eden et al. 2008). In this work we explore the jet formation mechanism and the parameter dependence of EDJ structure in the idealized theoretical model of the stochastically-driven equatorial beta plane. The model is formulated in three ways: 1) Fully nonlinear equations of motion 2) Quasilinear (or mean-field) dynamics 3) Statistical state dynamics employing a second order closure method (stochastic structural stability theory). Results from the three models are compared, and the implications for both the jet formation and equilibration mechanisms, as well as the role of eddy-eddy nonlinearity in the EDJ system, are discussed.

  17. Theoretical study of refraction effects on noise produced by turbulent jets

    NASA Technical Reports Server (NTRS)

    Graham, E. W.; Graham, B. B.

    1975-01-01

    The transmission of acoustic disturbances from the interior of a jet into the ambient air is studied. The jet is assumed infinitely long with mean velocity profile independent of streamwise location. The noise generator is a sequence of transient sources drifting with the local fluid and confined to a short length of the jet. In Part 1, supersonic jets are considered. Numerical results for mean-square pressure versus angle in the far-field show unexpected peaks which are very sharp. Analysis of simplified models indicates that these are complex quasi-resonant effects which appear to the stationary observer in a high frequency range. The peaks are real for the idealized model, but would be smoothed by mathematical integration over source position, velocity, and frequency. Subsonic jets were considered in part 2, and a preliminary study of the near-field was attempted. Mean-square radial displacements (or mean radial energy flow or space-time correlations of radial pressure gradient) are first found for very simple cases. The most difficult case studied is a sequence of transient sources at the center of a uniform-velocity circular cylindrical jet. Here a numerical triple integration is required and seems feasible although only preliminary results for mean square radial displacement are now available. These preliminary results show disturbances decreasing with increasing radial distance, and with increasing distance upstream and downstream from the source. A trend towards greater downstream disturbances appears even in the near field.

  18. MODELING AND EXPERIMENTAL EVALUATION OF AN AEROSOL GENERATOR FOR VERY HIGH NUMBER CURRENTS BASED ON A FREE TURBULENT JET. (R827354C008)

    EPA Science Inventory

    In this paper we report on theoretical and experimental work on aerosol formation in a free turbulent jet. A hot DEHS vapor issues through a circular nozzle into slowly moving cold air. Vapor concentration and temperatures are such that particles are formed via homogeneous nuc...

  19. The effects of upstream mass injection by vortex generator jets on shock-induced turbulent boundary layer separation

    NASA Astrophysics Data System (ADS)

    Bueno, Pablo Cesar

    An experimental study was conducted to investigate the effects of upstream mass injection, by means of vortex generator jets (VGJs), on the shock wave/turbulent boundary layer interaction (SWTBLI) generated by cylinder in a Mach 2 flow. The objectives of the study were: (i) to characterize the changes to the global structure of the flowfield introduced by the VGJs, and (ii) to study how the injection affected the dynamics of the separation shock foot. The injection was provided by three high-speed valves, placed across the span of the test section, which generated underexpanded jets. Two jet orientations were studied: normal to the wall, and pitched at 60° and skewed at 90° with respect to the freestream. In addition, the effects of both continuous and pulsed injection were investigated. Velocity measurements of the upstream boundary layer, VGJs, and -shock system were made using a wide-field particle image velocimetry (PIV) system, and an array of fast-response pressure transducers was used to monitor the wall pressure under the intermittent region of the interaction. The velocity measurements of the baseline case captured nearly all the relevant flow structures including the -shock and primary and secondary vortices. However, neither mean supersonic reversed flow nor the supersonic jet that presumably exists downstream of the triple point were observed. When injection was applied, the scale of the separated flow changed considerably. For normal injection, the primary vortex was nearly suppressed, while for pitched/skewed injection, the secondary vortex increased in size. Mean streamwise and transverse velocity profiles upstream of the separation shock showed that, for both jet orientations, the VGJs caused the flow in the boundary layer to accelerate. However, for normal injection, the acceleration did not extend down to the wall. Phase-averaged velocity measurements of the opening phase of the valves indicated that there is a phase lag between the jets and the

  20. Effects of buoyancy and forcing on transitioning and turbulent circular jet flames

    NASA Astrophysics Data System (ADS)

    Nichols, Joseph W.; Riley, James J.; Schmid, Peter J.

    2003-11-01

    The effects of buoyancy and forcing on circular jet flames are first studied by comparing direct numerical simulations (DNS) of the jet flames in zero-gravity environments to simulations of similar jet flames in normal gravity. A low Mach number approximation is applied to the fully compressible Navier-Stokes equations which filters out acoustic waves but still captures variable density effects. It is found that buoyancy tends to make the flow much more unstable, so that strong instabilities appear even in the absence of externally applied forcing. In order to understand the effects of the enhancement of the instability by buoyancy, a spatial stability analysis of the linearized low Mach number equations is performed for a variable density jet by means of a primitive variable formulation. Using spectral expansion methods, a spectrum of eigenvalues and corresponding eigenfunctions is obtained. Additionally, the stability analysis determines the optimal forcing needed to enhance the instability growth, which is thought to play a key role in the control of the flow development in the near field of a jet.

  1. On the correlation of plume centerline velocity decay of turbulent acoustically excited jets

    NASA Technical Reports Server (NTRS)

    Vonglahn, Uwe H.

    1987-01-01

    Acoustic excitation was shown to alter the velocity decay and spreading characteristics of jet plumes by modifying the large-scale structures in the plume shear layer. The present work consists of reviewing and analyzing available published and unpublished experimental data in order to determine the importance and magnitude of the several variables that contribute to plume modification by acoustic excitation. Included in the study were consideration of the effects of internal and external acoustic excitation, excitation Strouhal number, acoustic excitation level, nozzle size, and flow conditions. The last include jet Mach number and jet temperature. The effects of these factors on the plume centerline velocity decay are then summarized in an overall empirical correlation.

  2. Quasi 1-D Analysis of a Circular, Compressible, Turbulent Jet Laden with Water Droplets. Appendix C

    NASA Technical Reports Server (NTRS)

    2001-01-01

    Recent experimental studies indicate that presence of small amount of liquid droplets reduces the Overall Sound Pressure Level (OASPL) of a jet. Present study is aimed at numerically investigating the effect of liquid particles on the overall flow quantities of a heated, compressible round jet. The jet is assumed perfectly expanded. A quasi-1D model was developed for this purpose which uses area-averaged quantities that satisfy integral conservation equations. Special attention is given to represent the early development region since it is acoustically important. Approximate velocity and temperature profiles were assumed in this region to evaluate entrainment rate. Experimental correlations were used to obtain spreading rate of shear layer. The base flow thus obtained is then laden with water droplets at the exit of the nozzle. Mass, momentum and energy coupling between the two phases is represented using empirical relations. Droplet size and mass loading are varied to observe their effect on flow variables.

  3. Turbulent two-dimensional jet flow and its effect on laser beam degradation

    NASA Technical Reports Server (NTRS)

    Catalano, G. D.; Cudahy, G. F.; Vankuren, J. T.; Wright, H. E.

    1980-01-01

    An experiment in which visible wavelength lasers traversed a well-documented two dimensional jet was conducted. Temperature perturbations varied from 0.25 to 1.80 K and velocity fluctuations ranged from 9.2 to 30.8 m/sec. Measured central spot intensities were as low as 18% of the undisturbed beam, depending on jet Mach number, beam position theory and experiment was two percent in terms of far field intensity. To supplement the flow field information, a laser Doppler velocimeter was developed to measure both mean and fluctuating velocities and a photo correlator was used as a signal processor.

  4. Transported PDF Modeling of Nonpremixed Turbulent CO/H-2/N-2 Jet Flames

    SciTech Connect

    Zhao, xinyu; Haworth, D. C.; Huckaby, E. David

    2012-01-01

    Turbulent CO/H{sub 2}/N{sub 2} (“syngas”) flames are simulated using a transported composition probability density function (PDF) method. A consistent hybrid Lagrangian particle/Eulerian mesh algorithm is used to solve the modeled PDF transport equation. The model includes standard k–ϵ turbulence, gradient transport for scalars, and Euclidean minimum spanning tree (EMST) mixing. Sensitivities of model results to variations in the turbulence model, the treatment of radiation heat transfer, the choice of chemical mechanism, and the PDF mixing model are explored. A baseline model reproduces the measured mean and rms temperature, major species, and minor species profiles reasonably well, and captures the scaling that is observed in the experiments. Both our results and the literature suggest that further improvements can be realized with adjustments in the turbulence model, the radiation heat transfer model, and the chemical mechanism. Although radiation effects are relatively small in these flames, consideration of radiation is important for accurate NO prediction. Chemical mechanisms that have been developed specifically for fuels with high concentrations of CO and H{sub 2} perform better than a methane mechanism that was not designed for this purpose. It is important to account explicitly for turbulence–chemistry interactions, although the details of the mixing model do not make a large difference in the results, within reasonable limits.

  5. Flame-vortex interaction and mixing behaviors of turbulent non-premixed jet flames under acoustic forcing

    SciTech Connect

    Kim, Munki; Choi, Youngil; Oh, Jeongseog; Yoon, Youngbin

    2009-12-15

    This study examines the effect of acoustic excitation using forced coaxial air on the flame characteristics of turbulent hydrogen non-premixed flames. A resonance frequency was selected to acoustically excite the coaxial air jet due to its ability to effectively amplify the acoustic amplitude and reduce flame length and NO{sub x} emissions. Acoustic excitation causes the flame length to decrease by 15% and consequently, a 25% reduction in EINO{sub x} is achieved, compared to coaxial air flames without acoustic excitation at the same coaxial air to fuel velocity ratio. Moreover, acoustic excitation induces periodical fluctuation of the coaxial air velocity, thus resulting in slight fluctuation of the fuel velocity. From phase-lock PIV and OH PLIF measurement, the local flow properties at the flame surface were investigated under acoustic forcing. During flame-vortex interaction in the near field region, the entrainment velocity and the flame surface area increased locally near the vortex. This increase in flame surface area and entrainment velocity is believed to be a crucial factor in reducing flame length and NO{sub x} emission in coaxial jet flames with acoustic excitation. Local flame extinction occurred frequently when subjected to an excessive strain rate, indicating that intense mass transfer of fuel and air occurs radially inward at the flame surface. (author)

  6. The Prediction and Analysis of Jet Flows and Scattered Turbulent Mixing Noise About Flight Vehicle Airframes

    NASA Technical Reports Server (NTRS)

    Miller, Steven A.

    2014-01-01

    Jet flows interacting with nearby surfaces exhibit a complex behavior in which acoustic and aerodynamic characteristics are altered. The physical understanding and prediction of these characteristics are essential to designing future low noise aircraft. A new approach is created for predicting scattered jet mixing noise that utilizes an acoustic analogy and steady Reynolds-averaged Navier-Stokes solutions. A tailored Green's function accounts for the propagation of mixing noise about the air-frame and is calculated numerically using a newly developed ray tracing method. The steady aerodynamic statistics, associated unsteady sound source, and acoustic intensity are examined as jet conditions are varied about a large at plate. A non-dimensional number is proposed to estimate the effect of the aerodynamic noise source relative to jet operating condition and airframe position. The steady Reynolds-averaged Navier-Stokes solutions, acoustic analogy, tailored Green's function, non- dimensional number, and predicted noise are validated with a wide variety of measurements. The combination of the developed theory, ray tracing method, and careful implementation in a stand-alone computer program result in an approach that is more first principles oriented than alternatives, computationally efficient, and captures the relevant physics of fluid-structure interaction.

  7. The Prediction and Analysis of Jet Flows and Scattered Turbulent Mixing Noise about Flight Vehicle Airframes

    NASA Technical Reports Server (NTRS)

    Miller, Steven A. E.

    2014-01-01

    Jet flows interacting with nearby surfaces exhibit a complex behavior in which acoustic and aerodynamic characteristics are altered. The physical understanding and prediction of these characteristics are essential to designing future low noise aircraft. A new approach is created for predicting scattered jet mixing noise that utilizes an acoustic analogy and steady Reynolds-averaged Navier-Stokes solutions. A tailored Green's function accounts for the propagation of mixing noise about the airframe and is calculated numerically using a newly developed ray tracing method. The steady aerodynamic statistics, associated unsteady sound source, and acoustic intensity are examined as jet conditions are varied about a large flat plate. A non-dimensional number is proposed to estimate the effect of the aerodynamic noise source relative to jet operating condition and airframe position.The steady Reynolds-averaged Navier-Stokes solutions, acoustic analogy, tailored Green's function, non-dimensional number, and predicted noise are validated with a wide variety of measurements. The combination of the developed theory, ray tracing method, and careful implementation in a stand-alone computer program result in an approach that is more first principles oriented than alternatives, computationally efficient, and captures the relevant physics of fluid-structure interaction.

  8. TRAJECTORIES OF PLUMES AND JETS IN TURBULENT SHEAR FLOW UNDER NEAR-NEUTRAL CONDITIONS

    EPA Science Inventory

    A new method of determination of trajectories of plumes and jets using measured vertical profiles of tracer concentration is proposed. odel calculations show that this method can reduce significantly the scatter in the data and eliminate the appearance of an artificial final rise...

  9. Turbulence Suppression by ExB Shear in JET Optimized Shear Pulses

    SciTech Connect

    M.A. Beer; R.V. Budny; C.D. Challis; G. Conway; C. Gomezano; et al

    1999-07-01

    We calculate microinstability growth rates in JET optimized shear plasmas with a comprehensive gyrofluid model, including sheared E x B flows, trapped electrons, and all dominant ion species in realistic magnetic geometry. We find good correlation between E x B shear suppression of microinstabilities and both the formation and collapse of the internal transport barrier.

  10. Turbulence suppression by E x B shear in JET optimized shear pulses

    SciTech Connect

    Beer, M.A.; Budny, R.V.; Challis, C.D.; Conway, G.

    2000-01-06

    The authors calculate microinstability growth rates in JET optimized shear plasmas with a comprehensive gyrofluid model, including sheared E x B flows, trapped electrons, and all dominant ion species in realistic magnetic geometry. They find good correlation between E x B shear suppression of microinstabilities and both the formation and collapse of the internal transport barrier.

  11. Stable, high-order SBP-SAT finite difference operators to enable accurate simulation of compressible turbulent flows on curvilinear grids, with application to predicting turbulent jet noise

    NASA Astrophysics Data System (ADS)

    Byun, Jaeseung; Bodony, Daniel; Pantano, Carlos

    2014-11-01

    Improved order-of-accuracy discretizations often require careful consideration of their numerical stability. We report on new high-order finite difference schemes using Summation-By-Parts (SBP) operators along with the Simultaneous-Approximation-Terms (SAT) boundary condition treatment for first and second-order spatial derivatives with variable coefficients. In particular, we present a highly accurate operator for SBP-SAT-based approximations of second-order derivatives with variable coefficients for Dirichlet and Neumann boundary conditions. These terms are responsible for approximating the physical dissipation of kinetic and thermal energy in a simulation, and contain grid metrics when the grid is curvilinear. Analysis using the Laplace transform method shows that strong stability is ensured with Dirichlet boundary conditions while weaker stability is obtained for Neumann boundary conditions. Furthermore, the benefits of the scheme is shown in the direct numerical simulation (DNS) of a Mach 1.5 compressible turbulent supersonic jet using curvilinear grids and skew-symmetric discretization. Particularly, we show that the improved methods allow minimization of the numerical filter often employed in these simulations and we discuss the qualities of the simulation.

  12. Laser Raman Diagnostics in Subsonic and Supersonic Turbulent Jet Diffusion Flames.

    NASA Astrophysics Data System (ADS)

    Cheng, Tsarng-Sheng

    1991-02-01

    UV spontaneous vibrational Raman scattering combined with laser-induced predissociative fluorescence (LIPF) is developed for temperature and multi-species concentration measurements. For the first time, simultaneous measurements of temperature, major species (H_2, O_2, N_2, H_2O), and minor species (OH) concentrations are made with a "single" narrowband KrF excimer laser in subsonic and supersonic lifted turbulent hydrogen-air diffusion flames. The UV Raman system is calibrated with a flat -flame diffusion burner operated at several known equivalence ratios from fuel-lean to fuel-rich. Temperature measurements made by the ratio of Stokes/anti-Stokes signal and by the ideal gas law are compared. Single-shot uncertainties for temperature and concentration measurements are analyzed with photon statistics. Calibration constants and bandwidth factors are used in the data reduction program to arrive at temperature and species concentration measurements. UV Raman measurements in the subsonic lifted turbulent diffusion flame indicate that fuel and oxidizer are in rich, premixed, and unignited conditions in the center core of the lifted flame base. The unignited mixtures are due to rapid turbulent mixing that affects chemical reaction. Combustion occurs in an intermittent annular turbulent flame brush with strong finite-rate chemistry effects. The OH radical exists in sub-equilibrium and super-equilibrium concentrations. Major species and temperature are found with non-equilibrium values. Further downstream the super-equilibrium OH radicals decay toward equilibrium through slow three-body recombination reactions. In the supersonic lifted flame, a little reaction occurs upstream of the flame base, due to shock wave interactions and mixing with hot vitiated air. The strong turbulent mixing and total enthalpy fluctuations lead to temperature, major, and minor species concentrations with non-equilibrium values. Combustion occurs farther downstream of the lifted region. Slow

  13. The effect of a jet stream on the generation of mountain wave-induced mean flows and turbulence near the tropopause

    NASA Astrophysics Data System (ADS)

    Dörnbrack, Andreas; Sharman, Robert

    2015-04-01

    Observational evidence indicates a higher incidence of turbulence near the tropopause, especially over mountainous terrain. Previous work by McHugh and Sharman (2013) indicate this may be due to nonlinear amplification of topographically-induced gravity waves as they impinge on the tropopause. However, that study did not consider nonlinear topography amplification effects, nor did it consider the more realistic case of a jet stream in the vicinity of the tropopause. This study extends the McHugh and Sharman study by considering these effects using fully nonlinear simulations with the jet modeled as a sech**2 profile. Sensitivity studies are performed to study such effects as the location of the nose of the jet relative to the tropopause height, the jet width, the height of the tropopause, and the size and shape of the obstacle. Momentum and energy flux profiles are used to deduce those configurations most conducive to gravity wave amplification, breakdown and turbulence near the tropopause. McHugh J., Sharman R., 2013: Generation of mountain wave-induced mean flows and turbulence near the tropopause. Q. J. R. Meteorol. Soc. 139: 1632-1642. DOI:10.1002/qj.2035

  14. Influence of large-scale motion on turbulent transport for confined coaxial jets. Volume 2: Navier-Stokes calculations of swirling and nonswirling confined coaxial jets

    NASA Technical Reports Server (NTRS)

    Weinberg, B. C.; Mcdonald, H.

    1986-01-01

    The existence of large scale coherent structures in turbulent shear flows has been well documented. Discrepancies between experimental and computational data suggest a necessity to understand the roles they play in mass and momentum transport. Using conditional sampling and averaging on coincident two-component velocity and concentration velocity experimental data for swirling and nonswirling coaxial jets, triggers for identifying the structures were examined. Concentration fluctuation was found to be an adequate trigger or indicator for the concentration-velocity data, but no suitable detector was located for the two-component velocity data. The large scale structures are found in the region where the largest discrepancies exist between model and experiment. The traditional gradient transport model does not fit in this region as a result of these structures. The large scale motion was found to be responsible for a large percentage of the axial mass transport. The large scale structures were found to convect downstream at approximately the mean velocity of the overall flow in the axial direction. The radial mean velocity of the structures was found to be substantially greater than that of the overall flow.

  15. LES of High-Reynolds-Number Coanda Flow Separating from a Rounded Trailing Edge of a Circulation Control Airfoil

    NASA Technical Reports Server (NTRS)

    Nichino, Takafumi; Hahn, Seonghyeon; Shariff, Karim

    2010-01-01

    This slide presentation reviews the Large Eddy Simulation of a high reynolds number Coanda flow that is separated from a round trailing edge of a ciruclation control airfoil. The objectives of the study are: (1) To investigate detailed physics (flow structures and statistics) of the fully turbulent Coanda jet applied to a CC airfoil, by using LES (2) To compare LES and RANS results to figure out how to improve the performance of existing RANS models for this type of flow.

  16. Ensemble Diffraction Measurements of Spray Combustion in a Novel Vitiated Coflow Turbulent Jet Flame Burner

    NASA Technical Reports Server (NTRS)

    Cabra, R.; Hamano, Y.; Chen, J. Y.; Dibble, R. W.; Acosta, F.; Holve, D.

    2000-01-01

    An experimental investigation is presented of a novel vitiated coflow spray flame burner. The vitiated coflow emulates the recirculation region of most combustors, such as gas turbines or furnaces; additionally, since the vitiated gases are coflowing, the burner allows exploration of the chemistry of recirculation without the corresponding fluid mechanics of recirculation. As such, this burner allows for chemical kinetic model development without obscurations caused by fluid mechanics. The burner consists of a central fuel jet (droplet or gaseous) surrounded by the oxygen rich combustion products of a lean premixed flame that is stabilized on a perforated, brass plate. The design presented allows for the reacting coflow to span a large range of temperatures and oxygen concentrations. Several experiments measuring the relationships between mixture stoichiometry and flame temperature are used to map out the operating ranges of the coflow burner. These include temperatures as low 300 C to stoichiometric and oxygen concentrations from 18 percent to zero. This is achieved by stabilizing hydrogen-air premixed flames on a perforated plate. Furthermore, all of the CO2 generated is from the jet combustion. Thus, a probe sample of NO(sub X) and CO2 yields uniquely an emission index, as is commonly done in gas turbine engine exhaust research. The ability to adjust the oxygen content of the coflow allows us to steadily increase the coflow temperature surrounding the jet. At some temperature, the jet ignites far downstream from the injector tube. Further increases in the coflow temperature results in autoignition occurring closer to the nozzle. Examples are given of methane jetting into a coflow that is lean, stoichiometric, and even rich. Furthermore, an air jet with a rich coflow produced a normal looking flame that is actually 'inverted' (air on the inside, surrounded by fuel). In the special case of spray injection, we demonstrate the efficacy of this novel burner with a

  17. Soot formation and radiation in turbulent jet diffusion flames under normal and reduced gravity conditions

    NASA Technical Reports Server (NTRS)

    Ku, Jerry C.; Tong, LI; Sun, Jun; Greenberg, Paul S.; Griffin, Devon W.

    1993-01-01

    Most practical combustion processes, as well as fires and explosions, exhibit some characteristics of turbulent diffusion flames. For hydrocarbon fuels, the presence of soot particles significantly increases the level of radiative heat transfer from flames. In some cases, flame radiation can reach up to 75 percent of the heat release by combustion. Laminar diffusion flame results show that radiation becomes stronger under reduced gravity conditions. Therefore, detailed soot formation and radiation must be included in the flame structure analysis. A study of sooting turbulent diffusion flames under reduced-gravity conditions will not only provide necessary information for such practical issues as spacecraft fire safety, but also develop better understanding of fundamentals for diffusion combustion. In this paper, a summary of the work to date and of future plans is reported.

  18. Unlocking the Keys to Vortex/Flame Interactions in Turbulent Gas-Jet Diffusion Flames--Dynamic Behavior Explored on the Space Shuttle

    NASA Technical Reports Server (NTRS)

    Stocker, Dennis P.

    1999-01-01

    Most combustion processes in industrial applications (e.g., furnaces and engines) and in nature (e.g., forest fires) are turbulent. A better understanding of turbulent combustion could lead to improved combustor design, with enhanced efficiency and reduced emissions. Despite its importance, turbulent combustion is poorly understood because of its complexity. The rapidly changing and random behavior of such flames currently prevents detailed analysis, whether experimentally or computationally. However, it is possible to learn about the fundamental behavior of turbulent flames by exploring the controlled interaction of steady laminar flames and artificially induced flow vortices. These interactions are an inherent part of turbulent flames, and understanding them is essential to the characterization of turbulent combustion. Well-controlled and defined experiments of vortex interaction with laminar flames are not possible in normal gravity because of the interference of buoyancy- (i.e., gravity) induced vortices. Therefore, a joint microgravity study was established by researchers from the Science and Technology Development Corp. and the NASA Lewis Research Center. The experimental study culminated in the conduct of the Turbulent Gas-Jet Diffusion Flames (TGDF) Experiment on the STS-87 space shuttle mission in November 1997. The fully automated hardware, shown in photo, was designed and built at Lewis. During the mission, the experiment was housed in a Get Away Special (GAS) canister in the cargo bay.

  19. Turbulent density fluctuations in a subsonic and transonic free jet using crossed-beam schlieren techniques

    NASA Technical Reports Server (NTRS)

    Martin, R. A.

    1979-01-01

    Scalar density fluctuations were measured nonintrusively in the shear layer of a 5.08-cm (2-in.) cold air jet using a crossed-beam schlieren method. Two statistics, covariance and three-dimensional spectrum function, were estimated for an exit Mach number range of 0.3 to 0.97. The density fluctuation intensity, integral scale, and eddy convection speed were calculated and compared to available data where possible. Spectra were found to change significantly in shape becoming less peaked between 3 and 9 jet diameters downstream from the orifice, but they consistently exhibited a -5/3 power law decay at 3, 6, and 9 diameters for frequencies above the peak.

  20. The use of digital techniques to examine the intermittent region of a turbulent jet

    NASA Technical Reports Server (NTRS)

    Foss, J. F.; Merrow, T. E.

    1974-01-01

    Voltage signals, sampled at a high rate in the intermittent region of a round jet, are analyzed to provide instantaneous velocity vector information and measures of the vorticity and dissipation scales. A clustering routine to assess the feasibility of using the voltage readings to define the vortical, nonvortical state of the flow is also utilized. The results indicate that the clustering routine is partially successful; more sophisticated discrimination techniques will be required for a complete specification.

  1. Turbulent eddies in a compressible jet in crossflow measured using pulse-burst particle image velocimetry

    DOE PAGESBeta

    Beresh, Steven J.; Wagner, Justin L.; Henfling, John F.; Spillers, Russell Wayne; Pruett, Brian Owen Matthew

    2016-01-01

    Pulse-burst Particle Image Velocimetry(PIV) has been employed to acquire time-resolved data at 25 kHz of a supersonic jet exhausting into a subsonic compressible crossflow. Data were acquired along the windward boundary of the jet shear layer and used to identify turbulenteddies as they convect downstream in the far-field of the interaction. Eddies were found to have a tendency to occur in closely spaced counter-rotating pairs and are routinely observed in the PIV movies, but the variable orientation of these pairs makes them difficult to detect statistically. Correlated counter-rotating vortices are more strongly observed to pass by at a larger spacing,more » both leading and trailing the reference eddy. This indicates the paired nature of the turbulenteddies and the tendency for these pairs to recur at repeatable spacing. Velocity spectra reveal a peak at a frequency consistent with this larger spacing between shear-layer vortices rotating with identical sign. The spatial scale of these vortices appears similar to previous observations of compressible jets in crossflow. Furthermore,super-sampled velocity spectra to 150 kHz reveal a power-law dependency of –5/3 in the inertial subrange as well as a –1 dependency at lower frequencies attributed to the scales of the dominant shear-layer eddies.« less

  2. Turbulent eddies in a compressible jet in crossflow measured using pulse-burst particle image velocimetry

    SciTech Connect

    Beresh, Steven J.; Wagner, Justin L.; Henfling, John F.; Spillers, Russell Wayne; Pruett, Brian Owen Matthew

    2016-01-01

    Pulse-burst Particle Image Velocimetry(PIV) has been employed to acquire time-resolved data at 25 kHz of a supersonic jet exhausting into a subsonic compressible crossflow. Data were acquired along the windward boundary of the jet shear layer and used to identify turbulenteddies as they convect downstream in the far-field of the interaction. Eddies were found to have a tendency to occur in closely spaced counter-rotating pairs and are routinely observed in the PIV movies, but the variable orientation of these pairs makes them difficult to detect statistically. Correlated counter-rotating vortices are more strongly observed to pass by at a larger spacing, both leading and trailing the reference eddy. This indicates the paired nature of the turbulenteddies and the tendency for these pairs to recur at repeatable spacing. Velocity spectra reveal a peak at a frequency consistent with this larger spacing between shear-layer vortices rotating with identical sign. The spatial scale of these vortices appears similar to previous observations of compressible jets in crossflow. Furthermore,super-sampled velocity spectra to 150 kHz reveal a power-law dependency of –5/3 in the inertial subrange as well as a –1 dependency at lower frequencies attributed to the scales of the dominant shear-layer eddies.

  3. Influence of swirl on the initial merging zone of a turbulent annular jet

    NASA Astrophysics Data System (ADS)

    Vanierschot, M.; Van den Bulck, E.

    2008-10-01

    This paper presents an extensive study of the influence of swirl on the initial region of an annular jet. A total of five different swirl numbers S are investigated: one at zero swirl, one at low swirl (S=0.18), two at intermediate swirl (S =0.37 and 0.57), and one at high swirl (S=0.74). The flow fields are measured using the stereoscopic particle image velocimetry (PIV) technique. A detailed study on the accuracy of the PIV measurements is presented, including a validation with laser Doppler anemometry data. In this way a detailed set of accurate data is presented of the three components of velocity and the root-mean square value of their fluctuations in a plane through the central axis of the geometry. Despite its simple geometry, the immediate flow field of an annular jet is very complex. The concentric central tube of the nozzle acts as a bluff body to the flow, thus creating a central recirculation zone (CRZ) behind it. At low swirl numbers the swirl induced pressure gradients alter the structure of the CRZ significantly, increasing its complexity. The CRZ becomes toroidal and the jet fluid is entrained near the apex. At intermediate swirl numbers a vortex breakdown bubble appears downstream which moves upstream with increasing swirl. At high swirl, the CRZ and breakdown bubble merge which creates a complex and highly anisotropic flow field.

  4. 2D numerical simulation of impinging jet onto the concave surface by k - w - overline{{v2 }} - f turbulence model

    NASA Astrophysics Data System (ADS)

    Seifi, Zeinab; Nazari, Mohammad Reza; Khalaji, Erfan

    2016-03-01

    In the present article, the characteristics of turbulent jet impinging onto a concave surface is studied using k - w - overline{{v2 }} - f turbulence model. Dependent parameters such as inlet Reynolds number (2960 < Re < 12,000), nozzle-plate distance (4 < H/B < 10), concavity (D/B = 30, 60) of confined and unconfined impinging jet are scrutinized to find out whether this approach would bring any privileges compared to other investigations or not. The obtained results indicate better performance in low nozzle-plate distance in comparison with those mentioned in other literatures. Furthermore, the average Nusselt number of confined impinging jet overtakes unconfined one (similar circumstances) while this trend will decline as relative concavity increases. Moreover, local heat transfer of stagnation area and wall jet goes up and down through nozzle-plate distance enhancement respectively. Finally, the effects of sinusoidal pulsed inlet profile on heat transfer of unconfined impinging jet indicate direct affiliation of amplitude and neutral impact of frequency on Nusselt number distribution.

  5. Numerical aspects and implementation of a two-layer zonal wall model for LES of compressible turbulent flows on unstructured meshes

    NASA Astrophysics Data System (ADS)

    Park, George Ilhwan; Moin, Parviz

    2016-01-01

    This paper focuses on numerical and practical aspects associated with a parallel implementation of a two-layer zonal wall model for large-eddy simulation (LES) of compressible wall-bounded turbulent flows on unstructured meshes. A zonal wall model based on the solution of unsteady three-dimensional Reynolds-averaged Navier-Stokes (RANS) equations on a separate near-wall grid is implemented in an unstructured, cell-centered finite-volume LES solver. The main challenge in its implementation is to couple two parallel, unstructured flow solvers for efficient boundary data communication and simultaneous time integrations. A coupling strategy with good load balancing and low processors underutilization is identified. Face mapping and interpolation procedures at the coupling interface are explained in detail. The method of manufactured solution is used for verifying the correct implementation of solver coupling, and parallel performance of the combined wall-modeled LES (WMLES) solver is investigated. The method has successfully been applied to several attached and separated flows, including a transitional flow over a flat plate and a separated flow over an airfoil at an angle of attack.

  6. Supersonic jet noise prediction and noise source investigation for realistic baseline and chevron nozzles based on hybrid RANS/LES simulations

    NASA Astrophysics Data System (ADS)

    Du, Yongle

    Jet noise simulations have been performed for a military-style baseline nozzle and a chevron nozzle with design Mach numbers of Md = 1:5 operating at several off-design conditions. The objective of the current numerical study is to provide insight into the noise generation mechanisms of shock-containing supersonic hot jets and the noise reduction mechanisms of chevron nozzles. A hybrid methodology combining advanced CFD technologies and the acoustic analogy is used for supersonic jet noise simulations. Unsteady Reynolds-averaged Navier-Stokes (URANS) equations are solved to predict the turbulent noise sources in the jet flows. A modified version of the Detached Eddy Simulation (DES) approach is used to avoid excessive damping of fine scale turbulent fluctuations. A multiblock structured mesh topology is used to represent complex nozzle geometries, including the faceted inner contours and finite nozzle thickness. A block interface condition is optimized for the complex multiblock mesh topology to avoid the centerline singularity. A fourth-order Dispersion-Relation-Preserving (DRP) scheme is used for spatial discretization. To enable efficient calculations, a dual time-stepping method is used in addition to parallel computation using MPI. Both multigrid and implicit residual smoothing are used to accelerate the convergence rate of sub-iterations in the fictitious time domain. Noise predictions are made with the permeable surface Ffowcs Williams and Hawkings (FWH) solution. All the numerical methods have been implemented in the jet flow simulation code "CHOPA" and the noise prediction code "PSJFWH". The computer codes have been validated with several benchmark cases. A preliminary study has been performed for an under-expanded baseline nozzle jet with Mj = 1:56 to validate the accuracy of the jet noise simulations. The results show that grid refinement around the jet potential core and the use of a lower artificial dissipation improve the resolution of the predicted

  7. Effect of wind turbulence and shear on landing performance of jet transports

    NASA Technical Reports Server (NTRS)

    Blick, E. F.; Mccarthy, J.; Bensch, R. R.; Sarabudla, N. R.

    1978-01-01

    Computer simulations of a Boeing 727 class aircraft landing in turbulence were developed by programming the longitudinal aircraft equations of motion into a digital computer with various input values of vertical and horizontal wind speeds. Turbulent wind data was fed to the computer in one-second intervals. The computer computed in one-second intervals the aircraft speed, altitude, horizontal distance traveled, rate-of-descent, pitch attitude, glide path angle (from edge of runway) and elevator angle. All computer runs were made in the 'stick-fixed' mode. The RMS values of altitude and velocity perturbations (from equilibrium) were found to be large when horizontal wind gusts had sinusoidal components at or near the phugoid (long period) frequency. Maximum RMS altitude deviations occurred when the vertical wind had sinusoidal components which were 1/10 to 1/5 of the phugoid frequency. When real wind data (obtained from NCAR Queen Air) were used as input winds good correlations were found to exist between RMS velocity perturbations and both horizontal and vertical wind shears.

  8. Laser Raman diagnostics in subsonic and supersonic turbulent jet diffusion flames

    NASA Technical Reports Server (NTRS)

    Cheng, T. S.; Wehrmeyer, J. A.; Pitz, R. W.

    1991-01-01

    Ultraviolet (UV) spontaneous vibrational Raman scattering combined with laser-induced predissociative fluorescence (LIPF) is developed for temperature and multi-species concentration measurements. Simultaneous measurements of temperature, major species (H2, O2, N2, H2O), and minor species (OH) concentrations are made with a 'single' narrow band KrF excimer laser in subsonic and supersonic lifted turbulent hydrogen-air diffusion flames. The UV Raman system is calibrated with a flat-flame diffusion burner operated at several known equivalence ratios from fuel-lean to fuel-rich. Temperature measurements made by the ratio of Stokes/anti-Stokes signal and by the ideal gas law are compared. The single shot measurement precision for concentration and temperature measurement is 5 to 10 pct. Calibration constants and bandwidth factors are determined from the flat burner measurements and used in a data reduction program to arrive at temperature and species concentration measurements. These simultaneous measurements of temperature and multi-species concentrations allow a better understanding of the complex turbulence-chemistry interactions and provide information for the input and validation of CFD models.

  9. Three dimensional analysis of turbulent steam jets in enclosed structures : a CFD approach.

    SciTech Connect

    Ishii, M.; NguyenLe, Q.

    1999-04-20

    This paper compares the three-dimensional numerical simulation with the experimental data of a steam blowdown event in a light water reactor containment building. The temperature and pressure data of a steam blowdown event was measured at the Purdue University Multi-Dimensional Integrated Test Assembly (PUMA), a scaled model of the General Electric simplified Boiling Water Reactor. A three step approach was used to analyze the steam jet behavior. First, a 1-Dimensional, system level RELAP5/Mod3.2 model of the steam blowdown event was created and the results used to set the initial conditions for the PUMA blowdown experiments. Second, 2-Dimensional CFD models of the discharged steam jets were computed using PHOENICS, a commercially available CFD package. Finally, 3-Dimensional model of the PUMA drywell was created with the boundary conditions based on experimental measurements. The results of the 1-D and 2-D models were reported in the previous meeting. This paper discusses in detail the formulation and the results of the 3-Dimensional PHOENICS model of the PUMA drywell. It is found that the 3-D CFD solutions compared extremely well with the measured data.

  10. Temperature, Oxygen, and Soot-Volume-Fraction Measurements in a Turbulent C2H4-Fueled Jet Flame

    SciTech Connect

    Kearney, Sean P.; Guildenbecher, Daniel Robert; Winters, Caroline; Farias, Paul Abraham; Grasser, Thomas W.; Hewson, John C.

    2015-09-01

    We present a detailed set of measurements from a piloted, sooting, turbulent C 2 H 4 - fueled diffusion flame. Hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (CARS) is used to monitor temperature and oxygen, while laser-induced incandescence (LII) is applied for imaging of the soot volume fraction in the challenging jet-flame environment at Reynolds number, Re = 20,000. Single-laser shot results are used to map the mean and rms statistics, as well as probability densities. LII data from the soot-growth region of the flame are used to benchmark the soot source term for one-dimensional turbulence (ODT) modeling of this turbulent flame. The ODT code is then used to predict temperature and oxygen fluctuations higher in the soot oxidation region higher in the flame.

  11. Analysis of the Injection of a Heated, Turbulent Jet into a Moving Mainstream, with Emphasis on a Thermal Discharge in a Waterway. Ph.D. Thesis - Virginia Polytechnic Inst. and State Univ., Blacksburg

    NASA Technical Reports Server (NTRS)

    Campbell, J. F.

    1972-01-01

    An experimental and theoretical investigation was undertaken to study the trajectory and growth of thermal effluents having a range of discharge velocities and temperatures. The discharge of a turbulent effluent into a waterway was mathematically modeled as a submerged jet injection process by using an integral method which accounts for natural fluid mechanisms such as turbulence, entrainment, buoyancy, and heat transfer. The analytical results are supported by experimental data and demonstrate the usefulness of the theory for estimating the location and size of the effluent with respect to the discharge point. The capability of predicting jet flow properties, as well as two- and three-dimensional jet paths, was enhanced by obtaining the jet cross-sectional area during the solution of the conservation equations. Realistic estimates of temperature in the effluent were acquired by accounting for heat losses in the jet flow due to forced convection and to entrainment of free-stream fluid into the jet.

  12. Simultaneous Laser Raman-rayleigh-lif Measurements and Numerical Modeling Results of a Lifted Turbulent H2/N2 Jet Flame in a Vitiated Coflow

    NASA Technical Reports Server (NTRS)

    Cabra, R.; Chen, J. Y.; Dibble, R. W.; Myhrvold, T.; Karpetis, A. N.; Barlow, R. S.

    2002-01-01

    An experiment and numerical investigation is presented of a lifted turbulent H2/N2 jet flame in a coflow of hot, vitiated gases. The vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics exemplary of the reacting flow in the recirculation region of advanced combustors. It also simplifies numerical investigation of this coupled problem by removing the complexity of recirculating flow. Scalar measurements are reported for a lifted turbulent jet flame of H2/N2 (Re = 23,600, H/d = 10) in a coflow of hot combustion products from a lean H2/Air flame ((empty set) = 0.25, T = 1,045 K). The combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of the major species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated coflow throughout the entire test region. Two combustion models (PDF: joint scalar Probability Density Function and EDC: Eddy Dissipation Concept) are used in conjunction with various turbulence models to predict the lift-off height (H(sub PDF)/d = 7,H(sub EDC)/d = 8.5). Kalghatgi's classic phenomenological theory, which is based on scaling arguments, yields a reasonably accurate prediction (H(sub K)/d = 11.4) of the lift-off height for the present flame. The vitiated coflow admits the possibility of auto-ignition of mixed fluid, and the success of the present parabolic implementation of the PDF model in predicting a stable lifted flame is attributable to such ignition. The measurements indicate a thickened turbulent reaction zone at the flame base. Experimental results and numerical investigations support the plausibility of turbulent premixed flame propagation by small scale (on the order of the flame thickness) recirculation and mixing of hot products into reactants and subsequent rapid ignition of the mixture.

  13. Transport de particules massives dans un fluide turbulent: Application a l'erosion due au sable sur les parois d'une turbine hydraulique

    NASA Astrophysics Data System (ADS)

    Bergeron, Stephen

    Le transport de particules massives par un champ turbulent est un vaste domaine de la mécanique des fluides. Il possède de nombreuses applications comme par exemple le transport de sable dans une turbine hydraulique. En raison de la dureté des grains de quartz et des grandes vitesses de collision avec les parois métalliques, un phénomène d'érosion intensif se produit. Les dommages résultants peuvent diminuer le rendement de la turbine au cours des quelques mois suivant la mise en opération. L'objectif de cette thèse est de mettre au point un outil permettant de prédire ces zones d'érosion. Ce projet de recherche en contexte industriel a été réalisé en collaboration avec la compagnie General Electric Hydro du Canada. Dans un régime hautement turbulent, il est possible d'obtenir une expression suffisamment générale en utilisant une formulation partiellement empirique: l'équation de Basset- Boussinesq-Oseen modifiée. Ce choix de modèle tient compte du niveau de précision recherché et de la méthode numérique employée afin de résoudre la phase fluide. Il permet aussi d'éliminer plusieurs ambiguïtés fréquemment rencontrées dans la littérature et implementées dans certains codes commerciaux courants. La formulation mathématique du problème est effectuée dans un espace mixte Euler-Lagrange. Les paramètres dynamiques sont relies au type de particules et à l'intensité de la turbulence. Le code numérique résultant est le plus performant développé à ce jour (août 1998). Les trajectoires de plusieurs centaines de milliers de particules peuvent être simulées et visualisées de manière interactive sur une station de travail (SGI R4K, R8K et R10K). L'utilisateur du logiciel est libre de se déplacer dans l'espace à l'aide d'un environnement similaire a un ``simulateur de vol''. Il peut ainsi analyser les détails du processus d'érosion de même que l'écoulement du fluide dans la turbine. Les zones d'érosion obtenues à l

  14. Investigation of Francis Turbine Part Load Instabilities using Flow Simulations with a Hybrid RANS-LES Turbulence Model

    NASA Astrophysics Data System (ADS)

    Krappel, Timo; Ruprecht, Albert; Riedelbauch, Stefan; Jester-Zuerker, Roland; Jung, Alexander

    2014-03-01

    The operation of Francis turbines in part load condition causes high pressure fluctuations and dynamic loads in the turbine as well as high flow losses in the draft tube. Owing to the co-rotating velocity distribution at the runner blade trailing edge a low pressure zone arises in the hub region finally leading to a rotating vortex rope in the draft tube. A better understanding and a more accurate prediction of this phenomenon can help in the design process of a Francis turbine. The goal of this study is to reach a quantitatively better numerical prediction of the flow at part load and to evaluate the necessary numerical depth with respect to effort and benefit. As standard practice, simulation results are obtained for the steady state approach with SST turbulence modelling. Those results are contrasted with transient simulations applying a SST as well as a SAS (Scale Adaptive Simulation) turbulence model. The structure of the SAS model is such, that it is able to resolve the turbulent flow behaviour in more detail. The investigations contain a comparison of the flow losses in different turbine components. A detailed flow evaluation is done in the cone and the diffuser of the draft tube. The different numerical approaches show a different representation of the vortex rope phenomenon indicating differences in pressure pulsations at different geometric positions in the entire turbine. Finally, the turbulent flow structures in the draft tube are illustrated with several evaluation methods, such as turbulent eddy viscosity, velocity invariant and turbulent kinetic energy spectra.

  15. Soot volume fraction in a piloted turbulent jet non-premixed flame of natural gas

    SciTech Connect

    Qamar, N.H.; Alwahabi, Z.T.; King, K.D.; Chan, Q.N.; Nathan, G.J.; Roekaerts, D.

    2009-07-15

    Planar laser-induced incandescence (LII) has been used to measure soot volume fraction in a well-characterised, piloted, turbulent non-premixed flame known as the ''Delft Flame III''. Simulated Dutch natural gas was used as the fuel to produce a flame closely matching those in which a wide range of previous investigations, both experimental and modelling, have been performed. The LII method was calibrated using a Santoro-style burner with ethylene as the fuel. Instantaneous and time-averaged data of the axial and radial soot volume fraction distributions of the flame are presented here along with the Probability Density Functions (PDFs) and intermittency. The PDFs were found to be well-characterised by a single exponential distribution function. The distribution of soot was found to be highly intermittent, with intermittency typically exceeding 97%, which increases measurement uncertainty. The instantaneous values of volume fraction are everywhere less than the values in strained laminar flames. This is consistent with the soot being found locally in strained flame sheets that are convected and distorted by the flow. (author)

  16. Extreme events in a vortex gas simulation of a turbulent half-jet

    NASA Astrophysics Data System (ADS)

    Suryanarayanan, Saikishan; Pathikonda, Gokul; Narasimha, Roddam

    2012-11-01

    Extensive simulations [arXiv:1008.2876v1 [physics.flu-dyn], BAPS.2010.DFD.LE.4] have shown that the temporally evolving vortex gas mixing layer has 3 regimes, including one which has a universal spreading rate. The present study explores the development of spatially evolving mixing layers, using a vortex gas model based on Basu et al. (1995 Appl. Math. Modelling). The effects of the velocity ratio (r) are analyzed via the most extensive simulations of this kind till date, involving up to 10000 vortices and averaging over up to 1000 convective times. While the temporal limit is approached as r approaches unity, striking features such as extreme events involving coherent structures, bending, deviation of the convection velocity from mean velocity, spatial feedback and greater sensitivity to downstream and free stream boundary conditions are observed in the half-jet (r = 0) limit. A detailed statistical analysis reveals possible causes for the large scatter across experiments, as opposed to the commonly adopted explanation of asymptotic dependence on initial conditions. Supported in part by contract no. Intel/RN/4288.

  17. Large-eddy simulations of contrails in a turbulent atmosphere

    NASA Astrophysics Data System (ADS)

    Picot, J.; Paoli, R.; Thouron, O.; Cariolle, D.

    2014-11-01

    In this work, the evolution of contrails in the vortex and dissipation regimes is studied by means of fully three-dimensional large-eddy simulation (LES) coupled to a Lagrangian particle tracking method to treat the ice phase. This is the first paper where fine-scale atmospheric turbulence is generated and sustained by means of a stochastic forcing that mimics the properties of stably stratified turbulent flows as those occurring in the upper troposphere lower stratosphere. The initial flow-field is composed by the turbulent background flow and a wake flow obtained from separate LES of the jet regime. Atmospheric turbulence is the main driver of the wake instability and the structure of the resulting wake is sensitive to the intensity of the perturbations, primarily in the vertical direction. A stronger turbulence accelerates the onset of the instability, which results in shorter contrail decent and more effective mixing in the interior of the plume. However, the self-induced turbulence that is produced in the wake after the vortex break-up dominates over background turbulence at the end of the vortex regime and dominates the mixing with ambient air. This results in global microphysical characteristics such as ice mass and optical depth that are be slightly affected by the intensity of atmospheric turbulence. On the other hand, the background humidity and temperature have a first order effect on the survival of ice crystals and particle size distribution, which is in line with recent and ongoing studies in the literature.

  18. LES investigation of the transport and decay of various-strengths wake vortices in ground effect and subjected to a turbulent crosswind

    NASA Astrophysics Data System (ADS)

    Bricteux, L.; Duponcheel, M.; De Visscher, I.; Winckelmans, G.

    2016-06-01

    This study is concerned with the investigation of two-vortex systems (2VS) of various strengths that are released near the ground and evolve in the presence of a turbulent crosswind. We analyze the physics of the vortices interactions with the turbulent wind and with the ground during the rebound phase, and that lead to the fully developed turbulent flow and interactions. The transport and decay of the vortices are also analyzed. The turbulent wind itself is obtained by direct numerical simulation using a half channel flow. The flow is then supplemented with the 2VS, using vortices with a circulation distribution that is representative of vortices after roll-up of a near wake. The vortex strengths, Γ0, are such that ReΓ = Γ0/ν = 2.0 × 104 for the baseline; there is then a case with twice weaker vortices, and a case with twice stronger vortices. The simulations are run in wall-resolved Large Eddy Simulation (LES) mode. The baseline is in line with the wall-resolved LES study of a similar case [A. Stephan et al., "Aircraft wake-vortex decay in ground proximity - Physical mechanisms and artificial enhancement," J. Aircr. 50(4), 1250-1260 (2013)]. They highlighted the significant effect that the near-wall streaks of the wind have on the development of instabilities in the secondary vortices, and the ensuing turbulence. Our analysis complements theirs by also showing the significant effect that the wind turbulent structures, away from the ground and that are stretched by the primary vortices, also have on the destabilization of the secondary vortices. Comparisons are also made with the most recent study [F. N. Holzäpfel et al., "Wind impact on single vortices and counter-rotating vortex pairs in ground proximity," in 7th AIAA Atmospheric and Space Environments Conference, AIAA Aviation (American Institute of Aeronautics and Astronautics, 2015)], where ReΓ = 2.0 × 104 for all cases and where it is the wind intensity that is varied. Diagnostics on the vortex

  19. Generation of Alfvénic waves and turbulence in reconnection jets

    NASA Astrophysics Data System (ADS)

    Hoshino, Masahiro; Higashimori, Katsuaki

    2015-05-01

    The magnetohydrodynamic linear stability with the localized bulk flow oriented parallel to the neutral sheet is investigated, by including the Hall effect and the guide magnetic field. We observe three different unstable modes: a "streaming tearing" mode at a slow flow speed, a "streaming sausage" mode at a medium flow speed, and a "streaming kink" mode at a fast flow speed. The streaming tearing and sausage modes have a standard tearing mode-like structure with symmetric density fluctuations in the neutral sheet, while the kink mode has an asymmetric fluctuation. The growth rate of the streaming tearing mode decreases with increasing magnetic Reynolds number, while the growth rates of the sausage and kink modes do not depend strongly on the Reynolds number. The sausage and kink modes can be unstable for not only super-Alfvénic flow but also sub-Alfvénic flow when the lobe density is low. The wavelengths of these unstable modes are of the same order of magnitude as the thickness of the plasma sheet. Their maximum growth rates are higher than that of a standard tearing mode, and under a strong guide magnetic field, the growth rates of the sausage and kink modes are enhanced, while under a weak guide magnetic field, they are suppressed. For a thin plasma sheet with the Hall effect, the fluctuations of the streaming modes can exist over the plasma sheet. These unstable modes may be regarded as being one of the processes generating Alfvénic turbulence in the plasma sheet during magnetic reconnection.

  20. Simulation of a Wall-Bounded Flow using a Hybrid LES/RAS Approach with Turbulence Recycling

    NASA Technical Reports Server (NTRS)

    Quinlan, Jesse R.; Mcdaniel, James; Baurle, Robert A.

    2012-01-01

    Simulations of a supersonic recessed-cavity flow are performed using a hybrid large-eddy/ Reynolds-averaged simulation approach utilizing an inflow turbulence recycling procedure and hybridized inviscid flux scheme. Calorically perfect air enters the three-dimensional domain at a free stream Mach number of 2.92. Simulations are performed to assess grid sensitivity of the solution, efficacy of the turbulence recycling, and effect of the shock sensor used with the hybridized inviscid flux scheme. Analysis of the turbulent boundary layer upstream of the rearward-facing step for each case indicates excellent agreement with theoretical predictions. Mean velocity and pressure results are compared to Reynolds-averaged simulations and experimental data for each case, and these comparisons indicate good agreement on the finest grid. Simulations are repeated on a coarsened grid, and results indicate strong grid density sensitivity. The effect of turbulence recycling on the solution is illustrated by performing coarse grid simulations with and without inflow turbulence recycling. Two shock sensors, one of Ducros and one of Larsson, are assessed for use with the hybridized inviscid flux reconstruction scheme.

  1. A 2D domain decomposition, a customized immersed boundary method and a zest of numerical dissipation: a successful cocktail to tackle turbulence on HPC systems

    NASA Astrophysics Data System (ADS)

    Laizet, Sylvain; Lamballais, Eric; Vassilicos, J. Christos

    2015-11-01

    Incompact3d is a high-order flow solver dedicated to Direct and Large Eddy Simulations (DNS/LES) using High Performance Computing (HPC) systems which isdevoted to turbulent flows at the interface between academic research and upstream industrial R&D. It is originating from the University of Poitiers (France) and was developed there as well as, more recently, in the Turbulence, Mixing and Flow Control Group at Imperial College London (UK). This high-order flow solver can reconcile accuracy, efficiency, versatility and scalability using a simple Cartesian mesh and up to one million computational cores. The three key ingredients of this successful cocktail to tackle turbulence on HPC systemswill be given in this talkfollowed by various applications such as fractal-generated turbulence, gravity currents in an open basin, impinging jets on a heated plate and a micro-jet device to control a turbulent jet.

  2. Laser Raman scattering measurements of differential molecular diffusion in turbulent nonpremixed jet flames of H{sub 2}/CO{sub 2} fuel

    SciTech Connect

    Smith, L.L.; Dibble, R.W.; Talbot, L.; Barlow, R.S.; Carter, C.D.

    1994-01-01

    This paper explores effects of differential diffusion in nonpremixed turbulent jet flames. Pulsed Raman scattering spectroscopy is used to measure temperature and species concentrations in chemically reacting jets of H{sub 2}/CO{sub 2} into air, over a range of jet Reynolds numbers from 1,000 to 30,000 based on cold jet fluid properties. Results show significant effects of differential diffusion at all jet Reynolds numbers considered. Differential diffusion between H{sub 2} and C0{sub 2} produces differences between the hydrogen element mixture fraction ({xi}{sub H}) and the carbon element mixture fraction ({xi}{sub c}). The greatest effects occur on the rich side of stoichiometric, where {xi}{sub H} is observed to be smaller than {xi}{sub C} at all Reynolds numbers. Differential diffusion between H{sub 2} and H{sub 2}O creates a net flux of hydrogen element toward the stoichiometric contour and causes a local maximum in {xi}H that occurs near the stoichiometric condition. A differential diffusion variable {sup Z}H is defined as the difference between {xi}{sub H} and {xi}{sub C}. The variance Of {sup Z}H conditional on {xi}{sub C} also shows that differential diffusion effects are greatest on the rich side of the flame. Conditional variances of {sup Z}H are largest at intermediate Reynolds numbers.

  3. 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

  4. A method of measuring the peak flow rate and the regurgitant volume of regurgitation based on the characteristics of turbulent free jets.

    PubMed

    Sugawara, M; Hirai, A; Seo, Y; Miyajima, Y; Uchibori, T

    1991-01-01

    The issuing flow rate of a jet, Q0, is given by Q0 = U0A, where U0 is the issuing velocity of the jet and A is the cross-sectional area of the orifice. However, measurement of A of a regurgitant jet in the cardiovascular system is difficult. On the assumption that the jet is 'free' and turbulent, the following relations apply between the diameter of the orifice D, the length of the core region of the jet L, the centerline velocity of the jet Uc(chi) at an arbitrary distance chi(greater than L) from the orifice, and U0:L = 6.8D and Uc(chi)/U0 = L/chi. From these equations we obtain Q0 = 0.017 (chi Uc(chi))2/U0. Pulsatile jets issuing into an aqueous glycerol bath through orifices with various diameters were studied. U0, chi and Uc(chi) were measured with a color Doppler system. There was a good linear correlation between the peak flow rates estimated from the above equation and those measured with a flowmeter irrespective of the diameter of orifices (y = 0.93 chi + 3.8, r = 0.95, SEE = 6.6 ml/s). Assuming that the flow rate waveform is similar to the issuing velocity waveform, we estimated the issuing volume per stroke by integrating the issuing velocity. There was a good linear correlation between the estimated issuing volume and the known stroke volume of the pump (y = 1.0 chi + 2.6, r = 0.92, SEE = 2.8 ml). PMID:1854673

  5. Modeling of turbulent chemical reaction

    NASA Technical Reports Server (NTRS)

    Chen, J.-Y.

    1995-01-01

    Viewgraphs are presented on modeling turbulent reacting flows, regimes of turbulent combustion, regimes of premixed and regimes of non-premixed turbulent combustion, chemical closure models, flamelet model, conditional moment closure (CMC), NO(x) emissions from turbulent H2 jet flames, probability density function (PDF), departures from chemical equilibrium, mixing models for PDF methods, comparison of predicted and measured H2O mass fractions in turbulent nonpremixed jet flames, experimental evidence of preferential diffusion in turbulent jet flames, and computation of turbulent reacting flows.

  6. Turbulence Characteristics of Swirling Flowfields. Ph.D. Thesis

    NASA Technical Reports Server (NTRS)

    Jackson, T. W.

    1983-01-01

    Combustor design phenomena; recirculating flows research; single-wire, six-orientation, eddy dissipation rate, and turbulence modeling measurement; directional sensitivity (DS); calibration equipment, confined jet facility, and hot-wire instrumentation; effects of swirl, strong contraction nozzle, and expansion ratio; and turbulence parameters; uncertain; and DS in laminar jets; turbulent nonswirling jets, and turbulent swirling jets are discussed.

  7. Explosive turbulent magnetic reconnection.

    PubMed

    Higashimori, K; Yokoi, N; Hoshino, M

    2013-06-21

    We report simulation results for turbulent magnetic reconnection obtained using a newly developed Reynolds-averaged magnetohydrodynamics model. We find that the initial Harris current sheet develops in three ways, depending on the strength of turbulence: laminar reconnection, turbulent reconnection, and turbulent diffusion. The turbulent reconnection explosively converts the magnetic field energy into both kinetic and thermal energy of plasmas, and generates open fast reconnection jets. This fast turbulent reconnection is achieved by the localization of turbulent diffusion. Additionally, localized structure forms through the interaction of the mean field and turbulence. PMID:23829741

  8. CAA for Jet Noise Physics

    NASA Technical Reports Server (NTRS)

    Mankbadi, Reda

    2001-01-01

    Dr. Mankbadi summarized recent CAA results. Examples of the effect of various boundary condition schemes on the computed acoustic field, for a point source in a uniform flow, were shown. Solutions showing the impact of inflow excitations on the result were also shown. Results from a large eddy simulation, using a fourth-order MacCormack scheme with a Smagorinsky sub-grid turbulence model, were shown for a Mach 2.1 unheated jet. The results showed that the results were free from spurious modes. Results were shown for a Mach 1.4 jet using LES in the near field and the Kirchhoff method for the far field. Predicted flow field characteristics were shown to be in good agreement with data and predicted far field directivities were shown to be in qualitative agree with experimental measurements.

  9. 2D numerical simulation of impinging jet to the flat surface by k - ω - overline{{v2 }} - f turbulence model

    NASA Astrophysics Data System (ADS)

    Khalaji, E.; Nazari, M. R.; Seifi, Z.

    2016-01-01

    In this article, the effects of dependent parameters such as inlet Reynolds number (4000 ≤ Re ≤ 20,000), nozzle-plate distance (4 ≤ H/D ≤ 10), plate diameter (18 < D/B < 40 based on other approaches), inlet flow type (fully developed, uniform and pulsed) and thermal boundary condition (constant heat flux and temperature) in confined and unconfined turbulent impinging jet are investigated considering constant inlet fluid (air). The obtained results are compared with other experimental and numerical results found in the literature. This investigation indicates that the k - ω - overline{{v2 }} - f model acquires appropriate performance in terms of thermal and dynamical fluid analysis compared to other turbulence models and experimental results.

  10. Turbulent heat flux measurement in a non-reacting round jet, using BAM:Eu2+ phosphor thermography and particle image velocimetry

    NASA Astrophysics Data System (ADS)

    Lee, Hyunchang; Böhm, Benjamin; Sadiki, Amsini; Dreizler, Andreas

    2016-07-01

    Turbulent mixing is highly important in flows that involve heat and mass transfer. Information on turbulent heat flux is needed to validate the mixing models implemented in numerical simulations. The calculation of turbulent heat fluxes requires instantaneous information on temperature and velocity. Even using minimally intrusive laser optical methods, simultaneous measurement of temperature and velocity is still a challenge. In this study, thermographic phosphor particles are used for simultaneous thermometry and velocimetry: conventional particle image velocimetry is combined with temperature-dependent spectral shifts of BAM:Eu2+ phosphor particles upon UV excitation. The novelty of this approach is the analysis of systematic errors and verification using the well-known properties of a heated turbulent jet issuing into a low velocity, cold coflow. The analysis showed that systematic errors caused by laser fluence, multiple scattering, or preferential signal absorption can be reduced such that reliable measurement of scalar fluxes becomes feasible, which is a prerequisite for applying the method to more complex heat transfer problems.

  11. Large eddy simulations of a forced semiconfined circular impinging jet

    NASA Astrophysics Data System (ADS)

    Olsson, M.; Fuchs, L.

    1998-02-01

    Large eddy simulations (LES) of a forced semiconfined circular impinging jet were carried out. The Reynolds number was 104 and the inflow was forced at a Strouhal number of 0.27. The separation between the jet inlet and the opposing wall was four jet inlet diameters. Four different simulations were made. Two simulations were performed without any explicit sub-grid-scale (SGS) model using 1283 and 963 grid points, respectively. Two simulations were performed with two different SGS-models using 963 grid points; one with a dynamic Smagorinsky based model and one with a stress-similarity model. The simulations were performed to study the mean velocity, the turbulence statistics, the SGS-model effects, the dynamic behavior of the jet with a focus on the near wall region. The existence of separation vortices in the wall jet region was confirmed. These secondary vortices were found to be related to the radially deflected primary vortices generated by the circular shear layer of the jet. It was also shown that the primary vortex structures that reach the wall were helical and not axisymmetric. A quantitative gain was found in the simulations with SGS-models. The stress-similarity model simulation correlated slightly better with the higher resolution simulation than the other coarse grid simulations. The variations in the results predicted by the different simulations were larger for the turbulence statistics than for the mean velocity. However, the variation among the different simulations in terms of the turbulence intensity was less than 10%.

  12. The effects of actuation frequency on the separation control over an airfoil using a synthetic jet

    NASA Astrophysics Data System (ADS)

    Abe, Y.; Okada, K.; Nonomura, T.; Fujii, K.

    2015-06-01

    The simulation of separation control using a synthetic jet (SJ) is conducted around an NACA (National Advisory Committee for Aeronautics) 0015 airfoil by large-eddy simulation (LES) with a compact difference scheme. The synthetic jet is installed at the leading edge of the airfoil and the effects of an actuation frequency F+ (normalized by chord length and velocity of freestream) are observed. The lift-drag coefficient is recovered the most for F+ = 6. The relationship between momentum addition by turbulent mixing and large vortex structures is investigated using a phase-averaging procedure based on F+. The Reynolds shear stress is decomposed into periodic and turbulent components where the turbulent components are found to be dominant on the airfoil. The strong turbulent components appear near the large vortex structures that are observed in phase- and span-averaged flow fields.

  13. Simultaneous Raman-Rayleigh-LIF Measurements and Numerical Modeling Results of a Lifted H2/N2 Turbulent Jet Flame in a Vitiated Coflow

    NASA Technical Reports Server (NTRS)

    Cabra, R.; Chen, J. Y.; Dibble, R. W.; Hamano, Y.; Karpetis, A. N.; Barlow, R. S.

    2002-01-01

    An experimental and numerical investigation is presented of a H2/N2 turbulent jet flame burner that has a novel vitiated coflow. The vitiated coflow emulates the recirculation region of most combustors, such as gas turbines or furnaces. Additionally, since the vitiated gases are coflowing, the burner allows for exploration of recirculation chemistry without the corresponding fluid mechanics of recirculation. Thus the vitiated coflow burner design facilitates the development of chemical kinetic combustion models without the added complexity of recirculation fluid mechanics. Scalar measurements are reported for a turbulent jet flame of H2/N2 in a coflow of combustion products from a lean ((empty set) = 0.25) H2/Air flame. The combination of laser-induced fluorescence, Rayleigh scattering, and Raman scattering is used to obtain simultaneous measurements of the temperature, major species, as well as OH and NO. Laminar flame calculation with equal diffusivity do agree when the premixing and preheating that occurs prior to flame stabilization is accounted for in the boundary conditions. Also presented is an exploratory pdf model that predicts the flame's axial profiles fairly well, but does not accurately predict the lift-off height.

  14. Input-output analysis of high-speed axisymmetric isothermal jet noise

    NASA Astrophysics Data System (ADS)

    Jeun, Jinah; Nichols, Joseph W.; Jovanović, Mihailo R.

    2016-04-01

    We use input-output analysis to predict and understand the aeroacoustics of high-speed isothermal turbulent jets. We consider axisymmetric linear perturbations about Reynolds-averaged Navier-Stokes solutions of ideally expanded turbulent jets with jet Mach numbers 0.6 < Mj < 1.8. For each base flow, we compute the optimal harmonic forcing function and the corresponding linear response using singular value decomposition of the resolvent operator. In addition to the optimal mode, input-output analysis also yields sub-optimal modes associated with smaller singular values. For supersonic jets, the optimal response closely resembles a wavepacket in both the near-field and the far-field such as those obtained by the parabolized stability equations (PSE), and this mode dominates the response. For subsonic jets, however, the singular values indicate that the contributions of sub-optimal modes to noise generation are nearly equal to that of the optimal mode, explaining why the PSE do not fully capture the far-field sound in this case. Furthermore, high-fidelity large eddy simulation (LES) is used to assess the prevalence of sub-optimal modes in the unsteady data. By projecting LES source term data onto input modes and the LES acoustic far-field onto output modes, we demonstrate that sub-optimal modes of both types are physically relevant.

  15. Experimental Investigation of Three-Dimensional Turbulent Jets Issuing from a Nozzle with a Rectangular Output Section

    NASA Astrophysics Data System (ADS)

    Isataev, S. I.; Toleuov, G.; Isataev, M. S.; Bolysbekova, Sh. A.

    2016-03-01

    A detailed analysis of average dynamic characteristics of flow in a nozzle with a rectangular output section in relation to the nozzle aspect ratio parameter and Reynolds number has been carried out and its results have been generalized. It has been established that the flow velocity profile on the jet axis in the nozzle is determined by the parameter of its aspect ratio and by the jet initial velocity. A semiempirical formula that describes well the change in the maximum flow velocity in the main section of a three-dimensional jet is suggested.

  16. Large-scale turbulent structures in jets and in flows over cavities and their relationship to entrainment and mixing

    NASA Technical Reports Server (NTRS)

    Sarohia, V.; Massier, P. F.

    1979-01-01

    Large scale structures in jets and in flows over cavities were investigated experimentally to determine their role in entrainment, mixing, and noise production. The presence of these structures resulted in growth of the shear layer and entrainment. Merging of adjacent large scale structures caused the near field pressure signal in excited flows. It is believed that both the entrained fluid as well as its eventual mixing with the jet flow can be controlled by introducing pulsation in the jet flow at a frequency for which the flow is most unstable.

  17. Investigation of blown boundary layers with an improved wall jet system. Ph.D. Thesis. Final Technical Report, 1 Jul. 1978 - Dec. 1979; [to prevent turbulent boundary layer separation

    NASA Technical Reports Server (NTRS)

    Saripalli, K. R.; Simpson, R. L.

    1979-01-01

    The behavior of two dimensional incompressible turbulent wall jets submerged in a boundary layer when they are used to prevent boundary layer separation on plane surfaces is investigated. The experimental set-up and instrumentation are described. Experimental results of zero pressure gradient flow and adverse pressure gradient flow are presented. Conclusions are given and discussed.

  18. Strain rate effects on soot evolution in turbulent nonpremixed flames

    NASA Astrophysics Data System (ADS)

    Lew, Jeffry K.; Mueller, Michael E.; Mahmoud, Saleh; Alwahabi, Zeyad T.; Dally, Bassam B.; Nathan, Graham J.

    2015-11-01

    Large Eddy Simulations (LES) of turbulent nonpremixed ethylene/hydrogen/nitrogen (2/2/1 by volume) jet flames are conducted to investigate the effects of global strain rate on soot evolution. The exit strain rate is varied by fixing the Reynolds number as the burner diameter and exit velocity are altered. A detailed integrated LES approach is employed that includes a nonpremixed flamelet model that accounts for heat losses from radiation, a transport equation model to account for unsteadiness in polycyclic aromatic hydrocarbon (PAH) evolution, a detailed soot model based on the Hybrid Method of Moments, and a novel presumed subfilter PDF model for soot-turbulence interactions. As the strain rate increases, the maximum soot volume fraction decreases due to the suppression of PAH formation. This trend with increasing strain rate is validated against experimental measurements conducted at The University of Adelaide.

  19. Supersonic gas jets

    NASA Astrophysics Data System (ADS)

    Dulov, V. G.

    The papers presented in this volume provide an overview of the current state of research in the gas dynamics of jet flows. In particular, attention is given to free supersonic jets and to the interaction of supersonic jets with one another and with obstacles under stationary and nonstationary flow conditions. Papers are presented on a method for calculating a weakly anisotropic supersonic turbulent jet in a subsonic slipstream; composite supersonic jets; the principal gas-dynamic characteristics of the processes occurring in gas-jet-driven shock-wave generators; and the construction of models for supersonic jet flows. For individual items see A84-16902 to A84-16918

  20. Quantification of Mixing of a Sonic Jet in Supersonic Crossflow due to Thick Turbulent Boundary Layer Interaction

    NASA Astrophysics Data System (ADS)

    Rossmann, Tobias; Pizzaia, Adam

    2013-11-01

    The upstream injection surface boundary layer is shown to have a significant effect on the mixing characteristics of a sonic jet in supersonic cross flow. A circular, high-pressure, sonic jet is injected into a M =3.5 supersonic crossflow through different boundary layer thickness (δ/D = 7.5 and 1), with variable injection angles (-20 to +20 degrees), and variable momentum ratios (J = 2, 5, and 10). Planar Laser Mie Scattering of condensed ethanol droplets is used to quantitatively image the injected fluid concentration in both the side and end views. Jet fluid concentrations PDFs are constructed to better understand the mixing dynamics. These PDFs are integrated to create mixed fluid fraction profiles that are then reduced to mixing efficiency. Mixing efficiency values are computed from different two-dimensional planes to determine if centerline mixing efficiencies are characteristic of the entire three-dimensional flow. Through these analyses, it is seen that thick boundary layers tend to marginally alter jet penetration and spread, but significantly worsen jet mixing capabilities, regardless of momentum ratio or injection angle.

  1. Interphase Momentum and Heat Exchange in Turbulent Dust-Laden Plasma Jet under Continuous Radial Powder Injection

    SciTech Connect

    Solonenko, Oleg P.; Smirnov, Audrey V.

    2006-05-05

    Potential possibilities of an advanced approach based on the usage of DC cascade torch providing an axially symmetric plasma jet outflow, and continuous radial injection of powder into a plasma flow are discussed. Comparison is made of the results, obtained using two models of interphase heat and momentum exchange between polydisperse alumina particles and air plasma jet, other factors being the same. The widely used model of gradientless particles' heating was applied for computing the two-phase plasma jets' temperature and velocity fields. The model is compared with corresponding model of gradient particle heating computed by using an efficient numerical method developed. Calculations were conducted under different scales of dense loading conditions to estimate the maximum productivity of plasma spray process.

  2. Heat transfer measurements and CFD simulations of an impinging jet

    NASA Astrophysics Data System (ADS)

    Petera, Karel; Dostál, Martin

    2016-03-01

    Heat transport in impinging jets makes a part of many experimental and numerical studies because some similarities can be identified between a pure impingement jet and industrial processes like, for example, the heat transfer at the bottom of an agitated vessel. In this paper, experimental results based on measuring the response to heat flux oscillations applied to the heat transfer surface are compared with CFD simulations. The computational cost of a LES-based approach is usually too high therefore a comparison with less computationally expensive RANS-based turbulence models is made in this paper and a possible improvement of implementing an anisotropic explicit algebraic model for the turbulent heat flux model is evaluated.

  3. Vortical structures and heat transfer in a round impinging jet

    NASA Astrophysics Data System (ADS)

    Had?Iabdi?, M.; Hanjali?, K.

    In order to gain a better insight into flow, vortical and turbulence structure and their correlation with the local heat transfer in impinging flows, we performed large-eddy simulations (LES) of a round normally impinging jet issuing from a long pipe at Reynolds number Re = 20000 at the orifice-to-plate distance H = 2D, where D is the jet-nozzle diameter. This configuration was chosen to match previous experiments in which several phenomena have been detected, but the underlying physics remained obscure because of limitations in the measuring techniques applied. The instantaneous velocity and temperature fields, generated by the LES, revealed interesting time and spatial dynamics of the vorticity and eddy structures and their imprints on the target wall, characterized by tilting and breaking of the edge ring vortices before impingement, flapping, precessing, splitting and pairing of the stagnation point/line, local unsteady separation and flow reversal at the onset of radial jet spreading, streaks pairing and branching in the near-wall region of the radial jets, and others. The LES data provided also a basis for plausible explanations of some of the experimentally detected statistically-averaged flow features such as double peaks in the Nusselt number and the negative production of turbulence energy in the stagnation region. The simulations, performed with an in-house unstructured finite-volume code T-FlowS, using second-order-accuracy discretization schemes for space and time and the dynamic subgrid-scale stress/flux model for unresolved motion, showed large sensitivity of the results to the grid resolution especially in the wall vicinity, suggesting care must be taken in interpreting LES results in impinging flows.

  4. Three-dimensional direct numerical simulation of a turbulent lifted hydrogen jet flame in a heated coflow: flame stabilization and structure

    SciTech Connect

    Chen, Jackie; Sankaran, Ramanan; Yoo, Chun S

    2009-01-01

    Direct numerical simulation (DNS) of the near field of a three-dimensional spatially developing turbulent lifted hydrogen jet flame in heated coflow is performed with a detailed mechanism to determine the stabilization mechanism and the flame structure. The DNS was performed at a jet Reynolds number of 11,000 with over 940 million grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. A chemical flux analysis shows the occurrence of near-isothermal chemical chain branching preceding thermal runaway upstream of the stabilization point, indicative of hydrogen auto-ignition in the second limit. The Damkoehler number and key intermediate-species behaviour near the leading edge of the lifted flame also verify that auto-ignition occurs at the flame base. At the lifted-flame base, it is found that heat release occurs predominantly through ignition in which the gradients of reactants are opposed. Downstream of the flame base, both rich-premixed and non-premixed flames develop and coexist with auto-ignition. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base. In particular, the relative position of the flame base and the coherent flow structure induces a cyclic motion of the flame base in the transverse and axial directions about a mean lift-off height. This is confirmed by Lagrangian tracking of key scalars, heat release rate and velocity at the stabilization point.

  5. Experimental investigation of turbulent wall-jets in the presence of adverse pressure gradients in a rectangular diffuser

    NASA Technical Reports Server (NTRS)

    Back, L.; Cuffel, R.

    1982-01-01

    An experimental study of wall static pressure distributions and mean velocity profiles along a duct and diffuser downstream of wall-jet injection was conducted over a range of diffuser total angles from 15 to 40 deg at injection to core flow mass flux ratios from 0 to 6. Pressure recovery in the diffuser increased with injection ratio and decreased with diffuser total angle. Peak velocities in the wall-jet decayed along the flow and the inner shear layer and outer mixing region grew in thickness along the flow. The inner layer was near similarity condition, but non-similar variations were found in the outer layer. Estimated wall shear stresses depended upon injection mass fluxes, downstream distance and diffuser total angle. Greater decay of peak velocity and larger friction coefficients were found in the diffuser than indicated by correlations from data for a wall-jet without a pressure gradient. At the largest diffuser total angle and the highest injection ratio flow reversal occurred in the core region.

  6. Perspectives on jet noise

    NASA Technical Reports Server (NTRS)

    Ribner, H. S.

    1981-01-01

    Jet noise is a byproduct of turbulence. Until recently turbulence was assumed to be known statistically, and jet noise was computed therefrom. As a result of new findings though on the behavior of vortices and instability waves, a more integrated view of the problem has been accepted lately. After presenting a simple view of jet noise, the paper attempts to resolve the apparent differences between Lighthill's and Lilley's interpretations of mean-flow shear, and examines a number of ad hoc approaches to jet noise suppression.

  7. Large-eddy simulation of cavitating nozzle and jet flows

    NASA Astrophysics Data System (ADS)

    Örley, F.; Trummler, T.; Hickel, S.; Mihatsch, M. S.; Schmidt, S. J.; Adams, N. A.

    2015-12-01

    We present implicit large-eddy simulations (LES) to study the primary breakup of cavitating liquid jets. The considered configuration, which consists of a rectangular nozzle geometry, adopts the setup of a reference experiment for validation. The setup is a generic reproduction of a scaled-up automotive fuel injector. Modelling of all components (i.e. gas, liquid, and vapor) is based on a barotropic two-fluid two-phase model and employs a homogenous mixture approach. The cavitating liquid model assumes thermodynamic- equilibrium. Compressibility of all phases is considered in order to capture pressure wave dynamics of collapse events. Since development of cavitation significantly affects jet break-up characteristics, we study three different operating points. We identify three main mechanisms which induce primary jet break-up: amplification of turbulent fluctuations, gas entrainment, and collapse events near the liquid-gas interface.

  8. Large-eddy simulation of contrail evolution in the vortex phase and its interaction with atmospheric turbulence

    NASA Astrophysics Data System (ADS)

    Picot, J.; Paoli, R.; Thouron, O.; Cariolle, D.

    2015-07-01

    In this work, the evolution of contrails in the vortex and dissipation regimes is studied by means of fully three-dimensional large-eddy simulation (LES) coupled to a Lagrangian particle tracking method to treat the ice phase. In this paper, fine-scale atmospheric turbulence is generated and sustained by means of a stochastic forcing that mimics the properties of stably stratified turbulent flows as those occurring in the upper troposphere and lower stratosphere. The initial flow field is composed of the turbulent background flow and a wake flow obtained from separate LES of the jet regime. Atmospheric turbulence is the main driver of the wake instability and the structure of the resulting wake is sensitive to the intensity of the perturbations, primarily in the vertical direction. A stronger turbulence accelerates the onset of the instability, which results in shorter contrail descent and more effective mixing in the interior of the plume. However, the self-induced turbulence that is produced in the wake after the vortex breakup dominates over background turbulence until the end of the vortex regime and controls the mixing with ambient air. This results in mean microphysical characteristics such as ice mass and optical depth that are slightly affected by the intensity of atmospheric turbulence. However, the background humidity and temperature have a first-order effect on the survival of ice crystals and particle size distribution, which is in line with recent studies.

  9. Hybrid LES/RANS Simulation of Transverse Sonic Injection into a Mach 2 Flow

    NASA Technical Reports Server (NTRS)

    Boles, John A.; Edwards, Jack R.; Baurle, Robert A.

    2008-01-01

    A computational study of transverse sonic injection of air and helium into a Mach 1.98 cross-flow is presented. A hybrid large-eddy simulation / Reynolds-averaged Navier-Stokes (LES/RANS) turbulence model is used, with the two-equation Menter baseline (Menter-BSL) closure for the RANS part of the flow and a Smagorinsky-type model for the LES part of the flow. A time-dependent blending function, dependent on modeled turbulence variables, is used to shift the closure from RANS to LES. Turbulent structures are initiated and sustained through the use of a recycling / rescaling technique. Two higher-order discretizations, the Piecewise Parabolic Method (PPM) of Colella and Woodward, and the SONIC-A ENO scheme of Suresh and Huyhn are used in the study. The results using the hybrid model show reasonably good agreement with time-averaged Mie scattering data and with experimental surface pressure distributions, even though the penetration of the jet into the cross-flow is slightly over-predicted. The LES/RANS results are used to examine the validity of commonly-used assumptions of constant Schmidt and Prandtl numbers in the intense mixing zone downstream of the injection location.

  10. Experimental and numerical characterization of turbulent slot film cooling

    NASA Astrophysics Data System (ADS)

    Cruz, Carlos A.

    This study presents an experimental and numerical characterization of the turbulent mixing in two-dimensional slot film cooling flows. Three different flows are considered by varying the coolant to mainstream velocity ratio (VR): a wall jet case (VR ≈ 2.0), a boundary layer case (VR ≈ 1.0) and a wall-wake case (VR ≈ 0.5). For each flow, detailed measurements of the film cooling effectiveness, the heat flux, and the heat transfer coefficient are obtained for adiabatic and backside cooled wall conditions. Additionally, detailed flow velocity and temperature are measured under hot conditions using Particle Image Velocimetry (PIV) and a micro-thermocouple probe, respectively. These comprehensive measurements provide a unique data set for characterizing the momentum and thermal mixing of the turbulent flows, and for validating turbulence models in Reynolds averaged Navier-Stokes (RANS) simulations and large-eddy simulations (LES). The three flow families display different performances. The mixing of the film is strongly influenced by the mean shear between the coolant and the hot mainstream, thus explaining that the boundary layer case provides the best effectiveness. Initially governed by the film kinematics at the injection point, the convective heat transfer is influence by the mainstream when the film mixes. Additionally, measurements indicate that semi-empirical correlations largely overpredict the mixing of the film. The results obtained with the Spalart-Allmaras RANS model compare favorably with the measurements, thereby proving that this model is a viable alternative to using correlations for the film cooling effectiveness. A Large-Eddy Simulation (LES) with the dynamic models is performed for the wall jet case under adiabatic wall conditions with inflow conditions prescribed from precursor simulations. The LES results show good agreement with measured adiabatic wall temperatures and provide unique insight into the turbulent transport mechanism and

  11. Numerical Investigation of the Control of Separation from Curved and Blunt Trailing Edges Using DNS and LES

    NASA Astrophysics Data System (ADS)

    Fasel, Hermann F.

    2002-07-01

    Wall jets over a curved wall geometry (Coanda flows) are investigated using DNS and turbulence modeling. In experiments large coherent structures have enhanced the effectiveness of wall jets in delaying or preventing flow separation on airfoils. Understanding the behavior of these structures is essential for utilizing wall jets for separation control. The research objective is to investigate curvature effects on large coherent structures, in particular the development of longitudinal (Goertler-type) vortices and their interaction with 2D vortices. The focus is on Coanda cylinders using two computational approaches. With the Flow Simulation Methodology (FSM), a turbulent wall jet is computed over a cylinder segment on a body-fitted grid. In FSM, the contribution of the turbulence model depends on the grid resolution relative to a local turbulent length scale. For a flat-plate reference case, FSM is employed as DNS, LES, and URANS. In all cases the large 2D vortices are captured. For the curved-wall geometry, FSM is employed as a DNS. Goertler-type vortices emerge in the simulation but remain weak due to the narrow computational domain. In the second approach, Coanda flows including nozzle and separated region are computed using immersed boundary techniques (IBT). The feasibility of IBT for Coanda Flows is established.

  12. A parametric study of NO{sub 2} emission from turbulent H{sub 2} and CH{sub 4} jet diffusion flames

    SciTech Connect

    Chen, R.H.

    1998-01-01

    Nitrogen dioxide (NO{sub 2}) emission levels of fuel jets were experimentally studied for H{sub 2}, H{sub 2}/He mixtures, a H{sub 2}/He/CH{sub 4} mixture, and CH{sub 4}. The study was undertaken to understand the dependence of NO{sub 2} emission in turbulent diffusion flames on parameters other than the widely known effects of rapid mixing. These parameters are fuel types (CH{sub 4} vs H{sub 2}), the initial NO level, and the flame temperature. The fuel mixtures were chosen such that these factors could be investigated independently. In all the flames studied, NO{sub 2}/NO{sub x} increases with decreasing NO concentration in the flame and with decreasing adiabatic flame temperature. The CH{sub 4} fuel demonstrates a qualitatively different influence on the NO{sub 2}/NO{sub x} ratio than H{sub 2}. Its effects are most pronounced when the flame blowout limit is approached. Adding a small amount of CH{sub 4} to the H{sub 2} flames also qualitatively affected the NO{sub 2}/NO{sub x} ratio.

  13. Influence of large-scale motion on turbulent transport for confined coaxial jets. Volume 1: Analytical analysis of the experimental data using conditional sampling

    NASA Technical Reports Server (NTRS)

    Brondum, D. C.; Bennett, J. C.

    1986-01-01

    The existence of large scale coherent structures in turbulent shear flows has been well documented. Discrepancies between experimental and computational data suggest a necessity to understand the roles they play in mass and momentum transport. Using conditional sampling and averaging on coincident two component velocity and concentration velocity experimental data for swirling and nonswirling coaxial jets, triggers for identifying the structures were examined. Concentration fluctuation was found to be an adequate trigger or indicator for the concentration-velocity data, but no suitable detector was located for the two component velocity data. The large scale structures are found in the region where the largest discrepancies exist between model and experiment. The traditional gradient transport model does not fit in this region as a result of these structures. The large scale motion was found to be responsible for a large percentage downstream at approximately the mean velocity of the overall flow in the axial direction. The radial mean velocity of the structures was found to be substantially greater than that of the overall flow.

  14. Airframe-Jet Engine Integration Noise

    NASA Technical Reports Server (NTRS)

    Tam, Christopher; Antcliff, Richard R. (Technical Monitor)

    2003-01-01

    It has been found experimentally that the noise radiated by a jet mounted under the wing of an aircraft exceeds that of the same jet in a stand-alone environment. The increase in noise is referred to as jet engine airframe integration noise. The objectives of the present investigation are, (1) To obtain a better understanding of the physical mechanisms responsible for jet engine airframe integration noise or installation noise. (2) To develop a prediction model for jet engine airframe integration noise. It is known that jet mixing noise consists of two principal components. They are the noise from the large turbulence structures of the jet flow and the noise from the fine scale turbulence. In this investigation, only the effect of jet engine airframe interaction on the fine scale turbulence noise of a jet is studied. The fine scale turbulence noise is the dominant noise component in the sideline direction. Thus we limit out consideration primarily to the sideline.

  15. The influence of geometry on jet plume development

    NASA Astrophysics Data System (ADS)

    Xia, H.; Tucker, P. G.; Eastwood, S.; Mahak, M.

    2012-07-01

    Our recent efforts of using large-eddy simulation (LES) type methods to study complex and realistic geometry single stream and co-flow nozzle jets and acoustics are summarized in this paper. For the LES, since the solver being used tends towards having dissipative qualities, the subgrid scale (SGS) model is omitted, giving a numerical type LES (NLES). To overcome near wall streak resolution problems a near wall RANS (Reynolds averaged Navier-Stokes) model is smoothly blended in the LES making a hybrid RANS-NLES approach. Several complex nozzle geometries including the serrated (chevron) nozzle, realistic co-axial nozzles with eccentricity, pylon and wing-flap are discussed. The hybrid RANS-NLES simulations show encouraging predictions for the chevron jets. The chevrons are known to increase the high frequency noise at high polar angles, but decrease the low frequency noise at lower angles. The deflection effect of the potential core has an important mechanism of noise reduction. As for co-axial nozzles, the eccentricity, the pylon and the deployed wing-flap are shown to influence the flow development, especially the former to the length of potential core and the latter two having a significant impact on peak turbulence levels and spreading rates. The studies suggest that complex and real geometry effects are influential and should be taken into count when moving towards real engine simulations.

  16. Calibration, Data Acquisition, and Post Analysis of Turbulent Fluid Flow in a Calibration Jet Using Hot-wire Anemometry

    NASA Technical Reports Server (NTRS)

    Moreno, Michelle

    2004-01-01

    The Turbine Branch concentrates on the following areas: Computational Fluid Dynamics (CFD), and implementing experimental procedures to obtain physical modeling data. Hot-wire Anemometry is a valuable tool for obtaining physical modeling data. Hot-wire Anemometry is likely to remain the principal research tool for most turbulent air/gas flow studies. The Hot-wire anemometer consists of a fine wire heated by electric current. When placed in a fluid stream, the hot-wire loses heat to the fluid by forced convection. In forced convection, energy transfer is due to molecular motion imposed by an extraneous force moving fluid parcels. When the hot-wire is in "equilibrium", the rate of heat input to the wire is equal to the rate of heat loss at the wire ends. The equality between heat input and heat loss is the basis for King s equation, which relates the electrical parameters of the hot-wire to the flow parameters of the fluid. Hot-wire anemometry is based on convective heat transfer from a heated wire element placed in a fluid flow. Any change in the fluid flow condition that affects the heat transfer from the heated element will be detected virtually instantaneously by a constant-temperature Hot-wire anemometry system. The system implemented for this research is the IFA 300. The system is a fully-integrated, thermal anemometer-based system that measures mean and fluctuating velocity components in air, water, and other fluids. It also measures turbulence and makes localized temperature measurements. A constant-temperature anemometer is a bridge and amplifier circuit that controls a tiny wire at constant temperature. As a fluid flow passes over the heated sensor, the amplifier senses the bridge off-balance and adjusts the voltage to the top of the bridge, keeping the bridge in balance. The voltage on top of the bridge can then be related to the velocity of the flow. The bridge voltage is sensitive to temperature as well as velocity and so the built-in thermocouple

  17. Synthetic Jets

    NASA Technical Reports Server (NTRS)

    Milanovic, Ivana M.

    2003-01-01

    Current investigation of synthetic jets and synthetic jets in cross-flow examined the effects of orifice geometry and dimensions, momentum-flux ratio, cluster of orifices, pitch and yaw angles as well as streamwise development of the flow field. This comprehensive study provided much needed experimental information related to the various control strategies. The results of the current investigation on isolated and clustered synthetic jets with and without cross-flow will be further analyzed and documented in detail. Presentations at national conferences and publication of peer- reviewed journal articles are also expected. Projected publications will present both the mean and turbulent properties of the flow field, comparisons made with the data available in an open literature, as well as recommendations for the future work.

  18. Large-eddy simulation of the flow and acoustic fields of a Reynolds number 105 subsonic jet with tripped exit boundary layers

    NASA Astrophysics Data System (ADS)

    Bogey, Christophe; Marsden, Olivier; Bailly, Christophe

    2011-03-01

    Large-eddy simulations (LESs) of isothermal round jets at a Mach number of 0.9 and a diameter-based Reynolds number ReD of 105 originating from a pipe are performed using low-dissipation schemes in combination with relaxation filtering. The aim is to carefully examine the capability of LES to compute the flow and acoustic fields of initially nominally turbulent jets. As in experiments on laboratory-scale jets, the boundary layers inside the pipe are tripped in order to obtain laminar mean exit velocity profiles with high perturbation levels. At the pipe outlet, their momentum thickness is δθ(0)=0.018 times the jet radius, yielding a Reynolds number Reθ=900, and peak turbulence intensities are around 9% of the jet velocity. Two methods of boundary-layer tripping and five grids are considered. The results are found to vary negligibly with the tripping procedure but appreciably with the grid resolution. Based on analyses of the LES quality and on comparisons with measurements at high