A Large Eddy Simulation Study for upstream wind energy conditioning
NASA Astrophysics Data System (ADS)
Sharma, V.; Calaf, M.; Parlange, M. B.
2013-12-01
The wind energy industry is increasingly focusing on optimal power extraction strategies based on layout design of wind farms and yaw alignment algorithms. Recent field studies by Mikkelsen et al. (Wind Energy, 2013) have explored the possibility of using wind lidar technology installed at hub height to anticipate incoming wind direction and strength for optimizing yaw alignment. In this work we study the benefits of using remote sensing technology for predicting the incoming flow by using large eddy simulations of a wind farm. The wind turbines are modeled using the classic actuator disk concept with rotation, together with a new algorithm that permits the turbines to adapt to varying flow directions. This allows for simulations of a more realistic atmospheric boundary layer driven by a time-varying geostrophic wind. Various simulations are performed to investigate possible improvement in power generation by utilizing upstream data. Specifically, yaw-correction of the wind-turbine is based on spatio-temporally averaged wind values at selected upstream locations. Velocity and turbulence intensity are also considered at those locations. A base case scenario with the yaw alignment varying according to wind data measured at the wind turbine's hub is also used for comparison. This reproduces the present state of the art where wind vanes and cup anemometers installed behind the rotor blades are used for alignment control.
Large Eddy Simulation Study for Fluid Disintegration and Mixing
NASA Technical Reports Server (NTRS)
Bellan, Josette; Taskinoglu, Ezgi
2011-01-01
A new modeling approach is based on the concept of large eddy simulation (LES) within which the large scales are computed and the small scales are modeled. The new approach is expected to retain the fidelity of the physics while also being computationally efficient. Typically, only models for the small-scale fluxes of momentum, species, and enthalpy are used to reintroduce in the simulation the physics lost because the computation only resolves the large scales. These models are called subgrid (SGS) models because they operate at a scale smaller than the LES grid. In a previous study of thermodynamically supercritical fluid disintegration and mixing, additional small-scale terms, one in the momentum and one in the energy conservation equations, were identified as requiring modeling. These additional terms were due to the tight coupling between dynamics and real-gas thermodynamics. It was inferred that if these terms would not be modeled, the high density-gradient magnitude regions, experimentally identified as a characteristic feature of these flows, would not be accurately predicted without the additional term in the momentum equation; these high density-gradient magnitude regions were experimentally shown to redistribute turbulence in the flow. And it was also inferred that without the additional term in the energy equation, the heat flux magnitude could not be accurately predicted; the heat flux to the wall of combustion devices is a crucial quantity that determined necessary wall material properties. The present work involves situations where only the term in the momentum equation is important. Without this additional term in the momentum equation, neither the SGS-flux constant-coefficient Smagorinsky model nor the SGS-flux constant-coefficient Gradient model could reproduce in LES the pressure field or the high density-gradient magnitude regions; the SGS-flux constant- coefficient Scale-Similarity model was the most successful in this endeavor although not
Study of Hydrokinetic Turbine Arrays with Large Eddy Simulation
NASA Astrophysics Data System (ADS)
Sale, Danny; Aliseda, Alberto
2014-11-01
Marine renewable energy is advancing towards commercialization, including electrical power generation from ocean, river, and tidal currents. The focus of this work is to develop numerical simulations capable of predicting the power generation potential of hydrokinetic turbine arrays-this includes analysis of unsteady and averaged flow fields, turbulence statistics, and unsteady loadings on turbine rotors and support structures due to interaction with rotor wakes and ambient turbulence. The governing equations of large-eddy-simulation (LES) are solved using a finite-volume method, and the presence of turbine blades are approximated by the actuator-line method in which hydrodynamic forces are projected to the flow field as a body force. The actuator-line approach captures helical wake formation including vortex shedding from individual blades, and the effects of drag and vorticity generation from the rough seabed surface are accounted for by wall-models. This LES framework was used to replicate a previous flume experiment consisting of three hydrokinetic turbines tested under various operating conditions and array layouts. Predictions of the power generation, velocity deficit and turbulence statistics in the wakes are compared between the LES and experimental datasets.
Applied large eddy simulation.
Tucker, Paul G; Lardeau, Sylvain
2009-07-28
Large eddy simulation (LES) is now seen more and more as a viable alternative to current industrial practice, usually based on problem-specific Reynolds-averaged Navier-Stokes (RANS) methods. Access to detailed flow physics is attractive to industry, especially in an environment in which computer modelling is bound to play an ever increasing role. However, the improvement in accuracy and flow detail has substantial cost. This has so far prevented wider industrial use of LES. The purpose of the applied LES discussion meeting was to address questions regarding what is achievable and what is not, given the current technology and knowledge, for an industrial practitioner who is interested in using LES. The use of LES was explored in an application-centred context between diverse fields. The general flow-governing equation form was explored along with various LES models. The errors occurring in LES were analysed. Also, the hybridization of RANS and LES was considered. The importance of modelling relative to boundary conditions, problem definition and other more mundane aspects were examined. It was to an extent concluded that for LES to make most rapid industrial impact, pragmatic hybrid use of LES, implicit LES and RANS elements will probably be needed. Added to this further, highly industrial sector model parametrizations will be required with clear thought on the key target design parameter(s). The combination of good numerical modelling expertise, a sound understanding of turbulence, along with artistry, pragmatism and the use of recent developments in computer science should dramatically add impetus to the industrial uptake of LES. In the light of the numerous technical challenges that remain it appears that for some time to come LES will have echoes of the high levels of technical knowledge required for safe use of RANS but with much greater fidelity.
Mesoscale Ocean Large Eddy Simulations
NASA Astrophysics Data System (ADS)
Pearson, Brodie; Fox-Kemper, Baylor; Bachman, Scott; Bryan, Frank
2015-11-01
The highest resolution global climate models (GCMs) can now resolve the largest scales of mesoscale dynamics in the ocean. This has the potential to increase the fidelity of GCMs. However, the effects of the smallest, unresolved, scales of mesoscale dynamics must still be parametrized. One such family of parametrizations are mesoscale ocean large eddy simulations (MOLES), but the effects of including MOLES in a GCM are not well understood. In this presentation, several MOLES schemes are implemented in a mesoscale-resolving GCM (CESM), and the resulting flow is compared with that produced by more traditional sub-grid parametrizations. Large eddy simulation (LES) is used to simulate flows where the largest scales of turbulent motion are resolved, but the smallest scales are not resolved. LES has traditionally been used to study 3D turbulence, but recently it has also been applied to idealized 2D and quasi-geostrophic (QG) turbulence. The MOLES presented here are based on 2D and QG LES schemes.
Large Eddy Simulations in Astrophysics
NASA Astrophysics Data System (ADS)
Schmidt, Wolfram
2015-12-01
In this review, the methodology of large eddy simulations (LES) is introduced and applications in astrophysics are discussed. As theoretical framework, the scale decomposition of the dynamical equations for neutral fluids by means of spatial filtering is explained. For cosmological applications, the filtered equations in comoving coordinates are also presented. To obtain a closed set of equations that can be evolved in LES, several subgrid-scale models for the interactions between numerically resolved and unresolved scales are discussed, in particular the subgrid-scale turbulence energy equation model. It is then shown how model coefficients can be calculated, either by dynamic procedures or, a priori, from high-resolution data. For astrophysical applications, adaptive mesh refinement is often indispensable. It is shown that the subgrid-scale turbulence energy model allows for a particularly elegant and physically well-motivated way of preserving momentum and energy conservation in adaptive mesh refinement (AMR) simulations. Moreover, the notion of shear-improved models for in-homogeneous and non-stationary turbulence is introduced. Finally, applications of LES to turbulent combustion in thermonuclear supernovae, star formation and feedback in galaxies, and cosmological structure formation are reviewed.
Temporal Large-Eddy Simulation
NASA Technical Reports Server (NTRS)
Pruett, C. D.; Thomas, B. C.
2004-01-01
In 1999, Stolz and Adams unveiled a subgrid-scale model for LES based upon approximately inverting (defiltering) the spatial grid-filter operator and termed .the approximate deconvolution model (ADM). Subsequently, the utility and accuracy of the ADM were demonstrated in a posteriori analyses of flows as diverse as incompressible plane-channel flow and supersonic compression-ramp flow. In a prelude to the current paper, a parameterized temporal ADM (TADM) was developed and demonstrated in both a priori and a posteriori analyses for forced, viscous Burger's flow. The development of a time-filtered variant of the ADM was motivated-primarily by the desire for a unifying theoretical and computational context to encompass direct numerical simulation (DNS), large-eddy simulation (LES), and Reynolds averaged Navier-Stokes simulation (RANS). The resultant methodology was termed temporal LES (TLES). To permit exploration of the parameter space, however, previous analyses of the TADM were restricted to Burger's flow, and it has remained to demonstrate the TADM and TLES methodology for three-dimensional flow. For several reasons, plane-channel flow presents an ideal test case for the TADM. Among these reasons, channel flow is anisotropic, yet it lends itself to highly efficient and accurate spectral numerical methods. Moreover, channel-flow has been investigated extensively by DNS, and a highly accurate data base of Moser et.al. exists. In the present paper, we develop a fully anisotropic TADM model and demonstrate its utility in simulating incompressible plane-channel flow at nominal values of Re(sub tau) = 180 and Re(sub tau) = 590 by the TLES method. The TADM model is shown to perform nearly as well as the ADM at equivalent resolution, thereby establishing TLES as a viable alternative to LES. Moreover, as the current model is suboptimal is some respects, there is considerable room to improve TLES.
The effect of atmospheric stability on wind-turbine wakes: A large-eddy simulation study
NASA Astrophysics Data System (ADS)
Abkar, Mahdi; Porté-Agel, Fernando
2014-06-01
In this study, large-eddy simulation is used to investigate the influence of atmospheric stability on wind-turbine wakes. In the simulations, tuning-free Lagrangian scale- dependent dynamic models are used to model the subgrid-scale turbulent fluxes, while the turbine-induced forces are parameterized with an actuator-disk model. Emphasis is placed on studying the structure and characteristics of turbine wake in the cases where the incident flow to the turbine has the same mean velocity at the hub height but different thermal stability condition. The simulation results show that the atmospheric stability has a significant effect on the spatial distribution of the mean velocity deficit and turbulent fluxes in the wake region. In particular, in the convective boundary layer, the wake recovers faster, and the locations of the maximum turbulence intensity and turbulent stresses are closer to the turbine compared with the neutral and stable cases.
A posterirori study of models for large eddy simulations of drop-laden flows
NASA Technical Reports Server (NTRS)
Leboissetier, A.; Okong'o, N. A.; Bellan, J.
2003-01-01
Large Eddy Simulation (LES) is conducted of a three-dimensional temporal mixing layer whose stream is initially laden with liquid drops which may evaporate during the simulation. The gas-phase equations are written in Eulerian frame for two perfect gas species (carrier gas and vapor emanating from the drops), while the liquid-phase equations are written in a Lagrangian frame.
A posterirori study of models for large eddy simulations of drop-laden flows
NASA Technical Reports Server (NTRS)
Leboissetier, A.; Okong'o, N. A.; Bellan, J.
2003-01-01
Large Eddy Simulation (LES) is conducted of a three-dimensional temporal mixing layer whose stream is initially laden with liquid drops which may evaporate during the simulation. The gas-phase equations are written in Eulerian frame for two perfect gas species (carrier gas and vapor emanating from the drops), while the liquid-phase equations are written in a Lagrangian frame.
Parameter studies on the energy balance closure problem using large-eddy simulation
NASA Astrophysics Data System (ADS)
De Roo, Frederik; Banerjee, Tirtha; Mauder, Matthias
2017-04-01
The imbalance of the surface energy budget in eddy-covariance measurements is still a pending problem. A possible cause is the presence of land surface heterogeneity. Heterogeneities of the boundary layer scale or larger are most effective in influencing the boundary layer turbulence, and large-eddy simulations have shown that secondary circulations within the boundary layer can affect the surface energy budget. However, the precise influence of the surface characteristics on the energy imbalance and its partitioning is still unknown. To investigate the influence of surface variables on all the components of the flux budget under convective conditions, we set up a systematic parameter study by means of large-eddy simulation. For the study we use a virtual control volume approach, and we focus on idealized heterogeneity by considering spatially variable surface fluxes. The surface fluxes vary locally in intensity and these patches have different length scales. The main focus lies on heterogeneities of length scales of the kilometer scale and one decade smaller. For each simulation, virtual measurement towers are positioned at functionally different positions. We discriminate between the locally homogeneous towers, located within land use patches, with respect to the more heterogeneous towers, and find, among others, that the flux-divergence and the advection are strongly linearly related within each class. Furthermore, we seek correlators for the energy balance ratio and the energy residual in the simulations. Besides the expected correlation with measurable atmospheric quantities such as the friction velocity, boundary-layer depth and temperature and moisture gradients, we have also found an unexpected correlation with the temperature difference between sonic temperature and surface temperature. In additional simulations with a large number of virtual towers, we investigate higher order correlations, which can be linked to secondary circulations. In a companion
Numerical study of MPS method with large eddy simulation for fluid solid coupling problem
NASA Astrophysics Data System (ADS)
YANG, Chao; ZHANG, Huaixin; YAO, Huilan
2017-02-01
The Moving-Particle Semi-implicit method (MPS) is a kind of meshless Lagrangian calculation method. This method uses particles instead of mesh. In the pretreatment it works simply and conveniently and has high computational efficiency. In practical engineering, many of fluid problems are turbulent flows. Large eddy simulation is a major means of studying turbulence. Fluid-structure coupling is an independent branch of mechanics combined with fluid dynamics and solid mechanics, which is the hot and difficult area of research in many fields at present. In this paper, for the numerical simulation of turbulent flow with interaction of fluid-structure, the modified MPS-LES method is applied in two dimensional dam-break problem. It proves that MPS-LES method can be extended on solving the fluid-solid coupled problem.
Sea salt aerosol deposition in the coastal zone: A large eddy simulation study
NASA Astrophysics Data System (ADS)
Liang, Tinghao; Chamecki, Marcelo; Yu, Xiping
2016-11-01
Inland deposition of sea salt aerosol (SSA) particles emitted over the ocean is studied via numerical and theoretical models. The focus is on the large particles that contribute most to the total mass deposition. Large eddy simulations of idealized sea wind are used to investigate the development of the particle plume over land for different particle sizes and to validate some of the assumptions in the theoretical model. An existing theoretical modeling framework for particle dispersion in the atmospheric boundary layer is adapted to the problem of SSA deposition and it is shown to be adequate for the large particles of interest here. The decay of monodisperse SSA particle deposition flux with distance from the shoreline is shown to have a power-law behavior far from the shoreline. A complete model for predicting mass deposition as a function of distance is formulated and shown to present reasonable agreement with existing data.
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.
Large-eddy simulation of flow past urban-like surfaces: A model validation study
NASA Astrophysics Data System (ADS)
Cheng, Wai Chi; Porté-Agel, Fernando
2013-04-01
Accurate prediction of atmospheric boundary layer (ABL) flow and its interaction with urban surfaces is critical for understanding the transport of momentum and scalars within and above cities. This, in turn, is essential for predicting the local climate and pollutant dispersion patterns in urban areas. Large-eddy simulation (LES) explicitly resolves the large-scale turbulent eddy motions and, therefore, can potentially provide improved understanding and prediction of flows inside and above urban canopies. This study focuses on developing and validating an LES framework to simulate flow past urban-like surfaces. In particular, large-eddy simulations were performed of flow past an infinite long two-dimensional (2D) building and an array of 3D cubic buildings. An immersed boundary (IB) method was employed to simulate both 2D and 3D buildings. Four subgrid-scale (SGS) models, including (i) the traditional Smagorinsky model, (ii) the Lagrangian dynamic model, (iii) the Lagrangian scale-dependent dynamic model, and (iv) the modulated gradient model, were evaluated using the 2D building case. The simulated velocity streamlines and the vertical profiles of the mean velocities and variances were compared with experimental results. The modulated gradient model shows the best overall agreement with the experimental results among the four SGS models. In particular, the flow recirculation, the reattachment position and the vertical profiles are accurately reproduced with a grid resolution of (Nx)x(Ny)x(Nz) =160x40x160 ((nx)x(nz) =13x16 covering the block). After validating the LES framework with the 2D building case, it was further applied to simulate a boundary-layer flow past a 3D building array. A regular aligned building array with seven rows of cubic buildings was simulated. The building spacings in the streamwise and spanwise directions were both equal to the building height. A developed turbulent boundary-layer flow was used as the incoming flow. The results were
Field experimental study of the Smagorinsky model and application to large eddy simulation
NASA Astrophysics Data System (ADS)
Kleissl, Jan
Large-eddy simulation (LES) has become an indispensable tool for prediction of turbulent atmospheric boundary layer (ABL) flow. In LES, a subgrid-scale (SGS) model accounts for the dynamics of the unresolved scales of motion. The most widely used SGS model is an eddy-viscosity closure, the Smagorinsky model, which includes a parameter that must be prescribed in some fashion, the Smagorinsky constant cs. In this dissertation, cs is measured in a specifically designed field experiment. And, the ability of so-called dynamic SGS models to predict c s is studied based on the data obtained, as well as in numerical simulations. In the field study, two vertically separated horizontal arrays of 3d-sonic anemometers are placed in the atmospheric surface layer. Results indicate that cs is reduced when the integral scale of turbulence is small compared to the grid or filter scale, such as near the ground and in stable atmospheric conditions. The field data are processed further to test whether dynamic SGS models can predict the correct coefficient values. In the scale-invariant dynamic model (Germano et al. 1991), the coefficient is derived from various data test-filtered at a larger scale assuming that cs is the same as at scale Delta. The results show that cs is significantly underpredicted whenever Delta is larger than the large-scale limit of the inertial range. The scale-dependent dynamic model (Porte-Agel et al. 2000b) uses a second test-filter to deduce the dependence of cs on filtering scale. This model provides excellent predictions of cs and its dependence upon stability and height. Large eddy simulations of flow over a homogeneous surface with a diurnal heat flux forcing are conducted to study the prediction of c s over a wide range of stabilities in a numerical framework. The scale-invariant and scale-dependent Lagrangian dynamic SGS model are tested and compared to the field data. Consistent with the field studies, the prediction of cs from the scale
Large Eddy Simulation Study on Arctic Marine Clouds: the Effect of Aerosol-Cloud Interactions
NASA Astrophysics Data System (ADS)
Raatikainen, T.; Ahola, J.; Tonttila, J.; Romakkaniemi, S.; Laaksonen, A.; Korhonen, H.
2016-12-01
Dynamics of marine stratocumulus clouds depend on radiative cooling from cloud tops, turbulent transport of moisture and heat from the sea surface, and the availability of atmospheric aerosols to act as cloud condensation nuclei (CCN). These processes and especially aerosol-cloud interactions can be examined with a recently developed Large Eddy Simulation (LES) model UCLALES-SALSA (Tonttila et al., Geosci. Model Dev. Discuss., 2016). Unlike most other LES models, UCLALES-SALSA has fully interactive sectional description for aerosols and liquid and frozen cloud species. UCLALES-SALSA simulations are initialized using atmospheric observations from the Arctic Summer Cloud Ocean Study (ASCOS). First, the model is used to examine the effects of initial total aerosol number concentration on cloud properties. In agreement with several observations, lowering aerosol number concentration decreases cloud lifetime by increasing drizzle and precipitation rates, which further decreases aerosol number concentration. The second test includes comparison between model versions with different microphysics. The new sectional approach seems to produce thicker and more persistent clouds than a two moment model version (Stevens et al., J. Atmos. Sci., 1999) even when the models are tuned to have equal cloud droplet number concentrations. The third part of the study is focused on the effect of ice on cloud properties. Preliminary results indicate that the current cloud case is so warm that the liquid phase dominates, but further studies are ongoing. In general, the results show that cloud evolution depends on aerosol-cloud interactions.
NASA Astrophysics Data System (ADS)
Calaf, Marc; Parlange, Marc B.; Meneveau, Charles
2011-12-01
Wind harvesting is fast becoming an important alternative source of energy. As wind farms become larger, they begin to attain scales at which two-way interactions with the atmospheric boundary layer (ABL) must be taken into account. Several studies have shown that there is a quantifiable effect of wind farms on the local meteorology, mainly through changes in the land-atmosphere fluxes of heat and moisture. In particular, the observed trends suggest that wind farms increase fluxes at the surface and this could be due to increased turbulence in the wakes. Conversely, simulations and laboratory experiments show that underneath wind farms, the friction velocity is decreased due to extraction of momentum by the wind turbines, a factor that could decrease scalar fluxes at the surface. In order to study this issue in more detail, a suite of large eddy simulations of an infinite (fully developed) wind turbine array boundary layer, including scalar transport from the ground surface without stratification, is performed. Results show an overall increase in the scalar fluxes of about 10%-15% when wind turbines are present in the ABL, and that the increase does not strongly depend upon wind farm loading as described by the turbines' thrust coefficient and the wind turbines spacings. A single-column analysis including scalar transport shows that the presence of wind farms can be expected to increase slightly the scalar transport from the bottom surface and that this slight increase is due to a delicate balance between two strong opposing trends.
Xiangyang Zhou; Shankar Mahalingam; David Weise
2007-01-01
This paper presents a combined study of laboratory scale fire spread experiments and a three-dimensional large eddy simulation (LES) to analyze the effect of terrain slope on marginal burning behavior in live chaparral shrub fuel beds. Line fire was initiated in single species fuel beds of four common chaparral plants under various fuel bed configurations and ambient...
Underwater Oil Plume Intrusion from Deepwater Blowouts - A Large-Eddy Simulation Study
NASA Astrophysics Data System (ADS)
Yang, D.; Chen, B.; Chamecki, M.; Meneveau, C. V.
2015-12-01
The interaction of buoyancy-driven hydrocarbon plumes with the stably stratified deep-ocean environment plays a crucial role in the formation of underwater oil intrusions. As gas bubbles and oil droplets are released from an underwater oil well blowout, they induce a strong buoyancy flux that lifts entrained sea water to form an upward plume. Towards higher elevations, the stratification-induced negative buoyancy increases and eventually exceeds the gas/oil-induced buoyancy, causing the plume to decelerate and a large fraction of entrained sea water to peel off from the rising plume to form a fountain-like downward outer plume. During this peeling process, weakly buoyant particles (e.g. small oil droplets) are trapped and fall together with the detrained fluid, and then migrate horizontally at the equilibrium buoyancy depth, forming underwater oil intrusion layers. In this study, the complex plume dynamics and oil intrusion are studied using a large-eddy simulation (LES) model. The LES model captures the essential characteristics of the plume structure and the peeling/intrusion processes, and yields good agreement with prior laboratory experiments. Applying to the Deepwater Horizon oil well blowout condition, the LES model shows considerable underwater trapping and intrusion of oil droplets under various conditions, with the trapping rate significantly affected by the diameter of the oil droplet. This study is supported by Gulf of Mexico Research Initiative RFP-II research grant.
Large eddy simulation study of spanwise spacing effects on secondary flows in turbulent channel flow
NASA Astrophysics Data System (ADS)
Aliakbarimiyanmahaleh, Mohammad; Anderson, William
2015-11-01
The structure of turbulent flow over a complex topography composed of streamwise-aligned rows of cones with varying spanwise spacing, s is studied with large-eddy simulation (LES). Similar to the experimental study of Vanderwel and Ganapathisubramani, 2015: J. Fluid Mech., we investigate the relationship between secondary flow and s, for 0 . 25 <= s / δ <= 5 . For cases with s / δ > 2 , domain-scale rollers freely exist. These had previously been called ``turbulent secondary flows'' (Willingham et al., 2014: Phys. Fluids; Barros and Christensen, 2014: J. Fluid Mech.; Anderson et al., 2015: J. Fluid Mech.), but closer inspection of the statistics indicates these are a turbulent tertiary flow: they only remain ``anchored'' to the conical roughness elements for s / δ > 2 . For s / δ < 2 , turbulent tertiary flows are prevented from occupying the domain by virtue of proximity to adjacent, counter-rotating tertiary flows. Turbulent secondary flows are associated with the conical roughness elements. These turbulent secondary flows emanate from individual conical topographic elements and set the roughness sublayer depth. The turbulent secondary flows remain intact for large and small spacing. For s / δ < 1 , a mean tertiary flow is not present. This work was supported by the Air Force Office of Sci. Research, Young Inv. Program (PM: Dr. R. Ponnoppan and Ms. E. Montomery) under Grant # FA9550-14-1-0394. Computational resources were provided by the Texas Adv. Comp. Center at the Univ. of Texas.
A nested large-eddy simulation study of the Ora del Garda wind in the Alps
NASA Astrophysics Data System (ADS)
Giovannini, Lorenzo; Laiti, Lavinia; Zardi, Dino
2015-04-01
High-resolution numerical simulations performed with the Weather Research and Forecasting (WRF) model are analyzed to investigate the atmospheric boundary layer (ABL) structures associated with the development of a lake-breeze and valley-wind coupled system developing in the southeastern Italian Alps, the so-called "Ora del Garda" wind. Five domains were nested for the simulations: three mesoscale domains, forced by reanalysis data field, are used to drive the finest two domains, in which the large-eddy technique is used, achieving a final horizontal resolution of 80 m. Model results complement an existing dataset composed of a series of measurement flights and surface observations. The flights explored specific valley sections at key locations in the study area, namely over the lake's shore, at half valley and at the end of the valley where the breeze blows. Model results display a good agreement with the experimental dataset. In particular, the surface diurnal cycles of radiation, wind, air temperature and sensible heat flux are satisfactorily reproduced, despite some discrepancies in the timing of thermally-driven circulation onset and offset. The typical structure of the valley ABL, characterized by shallow or even absent mixed layers surmounted by slightly stable layers extending up to the lateral crest level, is also well reproduced in the simulated fields. Moreover, the simulations confirm characteristic local-scale features of the thermally-driven wind field suggested by the analysis of the airborne dataset as well as from previous observations in the area. For example, the model shows the development of inhomogeneities in the cross-valley thermal field, caused by the propagation of the lake breeze and by the different heating between the sidewalls of the valley, as well as the formation of a hydraulic jump in the area where the Ora del Garda flows down into an adjacent valley from an elevated saddle.
Influence of atmospheric stability on wind-turbine wakes: A large-eddy simulation study
NASA Astrophysics Data System (ADS)
Abkar, Mahdi; Porté-Agel, Fernando
2014-05-01
In this study, large-eddy simulation is combined with a turbine model to investigate the influence of atmospheric stability on wind-turbine wakes. In the simulations, subgrid-scale turbulent fluxes are parameterized using tuning-free Lagrangian scale-dependent dynamic models. These models optimize the local value of the model coefficients based on the dynamics of the resolved scales. The turbine-induced forces are parameterized with an actuator-disk model with rotation. In this technique, blade-element theory is used to calculate the lift and drag forces acting on the blades. Emphasis is placed on the structure and characteristics of wind-turbine wakes in the cases where the incident flows to the turbine have the same mean velocity at the hub height but different stability conditions. The simulation results show that atmospheric stability has a significant effect on the spatial distribution of the mean velocity deficit and turbulent fluxes in the wake region. In particular, the magnitude of the velocity deficit increases with increasing stability in the atmosphere. In addition, the locations of the maximum turbulence intensity and turbulent stresses are closer to the turbine in convective boundary layer compared with neutral and stable ones. Detailed analysis of the resolved turbulent kinetic energy (TKE) budget inside the wake reveals also that the thermal stratification of the incoming wind considerably affects the magnitude and spatial distribution of the turbulent production, transport term and dissipation rate (transfer of energy to the subgrid scales). It is also shown that the near-wake region can be extended to a farther distance downstream in stable condition compared with neutral and unstable counterparts. In order to isolate the effect of atmospheric stability, additional simulations of neutrally-stratified atmospheric boundary layers are performed with the same turbulence intensity at hub height as convective and stable ones. The results show that the
A Large Eddy Simulation Study of Heat Entrainment under Sea Ice in the Canadian Arctic Basin
NASA Astrophysics Data System (ADS)
Ramudu, E.; Yang, D.; Gelderloos, R.; Meneveau, C. V.; Gnanadesikan, A.
2016-12-01
Sea ice cover in the Arctic has declined rapidly in recent decades. The much faster than projected retreat suggests that climate models may be missing some key processes, or that these processes are not accurately represented. The entrainment of heat from the mixed layer by small-scale turbulence is one such process. In the Canadian Basin of the Arctic Ocean, relatively warm Pacific Summer Water (PSW) resides at the base of the mixed layer. With an increasing influx of PSW, the upper ocean in the Canadian Basin has been getting warmer and fresher since the early 2000s. While studies show a correlation between sea ice reduction and an increase in PSW temperature, others argue that PSW intrusions in the Canadian Basin cannot affect sea ice thickness because the strongly-stratified halocline prevents heat from the PSW layer from being entrained into the mixed layer and up to the basal ice surface. In this study, we try to resolve this conundrum by simulating the turbulent entrainment of heat from the PSW layer to a moving basal ice surface using large eddy simulation (LES). The LES model is based on a high-fidelity spectral approach on horizontal planes, and includes a Lagrangian dynamic subgrid model that reduces the need for empirical inputs for subgrid-scale viscosities and diffusivities. This LES tool allows us to investigate physical processes in the mixed layer at a very fine scale. We focus our study on summer conditions, when ice is melting, and show for a range of ice-drift velocities, halocline temperatures, and halocline salinity gradients characteristic of the Canadian Basin how much heat can be entrained from the PSW layer to the sea ice. Our results can be used to improve parameterizations of vertical heat flux under sea ice in coarse-grid ocean and climate models.
Influence of atmospheric stability on wind-turbine wakes: A large-eddy simulation study
NASA Astrophysics Data System (ADS)
Abkar, Mahdi; Porté-Agel, Fernando
2015-03-01
In this study, large-eddy simulation is combined with a turbine model to investigate the influence of atmospheric thermal stability on wind-turbine wakes. The simulation results show that atmospheric stability has a significant effect on the spatial distribution of the mean velocity deficit and turbulence statistics in the wake region as well as the wake meandering characteristics downwind of the turbine. In particular, the enhanced turbulence level associated with positive buoyancy under the convective condition leads to a relatively larger flow entrainment and, thus, a faster wake recovery. For the particular cases considered in this study, the growth rate of the wake is about 2.4 times larger for the convective case than for the stable one. Consistent with this result, for a given distance downwind of the turbine, wake meandering is also stronger under the convective condition compared with the neutral and stable cases. It is also shown that, for all the stability cases, the growth rate of the wake and wake meandering in the vertical direction is smaller compared with the ones in the lateral direction. This is mainly related to the different turbulence levels of the incoming wind in the different directions, together with the anisotropy imposed by the presence of the ground. It is also found that the wake velocity deficit is well characterized by a modified version of a recently proposed analytical model that is based on mass and momentum conservation and the assumption of a self-similar Gaussian distribution of the velocity deficit. Specifically, using a two-dimensional elliptical (instead of axisymmetric) Gaussian distribution allows to account for the different lateral and vertical growth rates, particularly in the convective case, where the non-axisymmetry of the wake is stronger. Detailed analysis of the resolved turbulent kinetic energy budget in the wake reveals also that thermal stratification considerably affects the magnitude and spatial distribution
Large-Eddy Simulation and Multigrid Methods
Falgout,R D; Naegle,S; Wittum,G
2001-06-18
A method to simulate turbulent flows with Large-Eddy Simulation on unstructured grids is presented. Two kinds of dynamic models are used to model the unresolved scales of motion and are compared with each other on different grids. Thereby the behavior of the models is shown and additionally the feature of adaptive grid refinement is investigated. Furthermore the parallelization aspect is addressed.
NASA Astrophysics Data System (ADS)
Jones, Sam; Jemcov, Aleksandar; Corke, Thomas
2016-11-01
An Embedded Large Eddy Simulation (ELES) approach is used to simulate the flow path through a high pressure turbine stage that includes the entry duct, stationary inlet and exit guide vanes, and a rotor. The flowfield around the rotor is simulated using LES. A Reynolds Averaged Simulation (RAS) is used for the rest of the flow domain. The interface between RAS and LES domains uses the RAS turbulence quantities as a means of obtaining length scales that are used in computing the vorticity required to trigger a proper energy cascade within the LES part of the flow field. The objective is to resolve the unsteady vortical motions that eminate from the gap between the rotor tip and duct walls that are presumably under-resolved in a RAS approach. A comparative analysis between RAS and ELES approaches for this turbomachinery problem is then presented. APS Fellow.
Regional Bowen ratio controls on afternoon moist convection: A large eddy simulation study
NASA Astrophysics Data System (ADS)
Kang, Song-Lak
2016-12-01
This study examines the effect of the regional Bowen ratio β, the ratio of the domain-averaged surface sensible heat flux (SHF) to latent heat flux (LHF), on afternoon moist convection. With a temporally evolving but spatially uniform surface available energy over a mesoscale domain under a weak capping inversion, we run large eddy simulation of the afternoon convective boundary layer (CBL). We first prescribe a small β of 0.56 (a wet surface) and then the reversed large β of 1.80 (a dry surface) by switching the SHF and LHF fields. The perturbation fields of the fluxes are prescribed with the Fourier spectra of κ- 3 (κ is horizontal wave number; strong mesoscale heterogeneity) and κ0 (homogeneity). The large β cases have strong vertical buoyancy fluxes and produce more vigorous updrafts. In the heterogeneous, large β surface case, with the removal of convective inhibition over a mesoscale subdomain of large SHF, deep convection develops. In the heterogeneous, small β surface case, convective clouds develop but do not progress into precipitating convection. In the homogeneous surface cases, randomly distributed shallow clouds develop with significantly more and thicker clouds in the large β case. (Co)spectral analyses confirm the more vigorous turbulent thermals in the large β cases and reveal that the moisture advection by the surface heterogeneity-induced mesoscale flows makes the correlation between mesoscale temperature and moisture perturbations change from negative to positive, which facilitates the mesoscale pool of high relative humidity air just above the CBL top, a necessary condition for deep convection.
A Numerical Study of Self-Similarity in a Turbulent Plane Wake Using Large-Eddy Simulation
NASA Technical Reports Server (NTRS)
Ghosal, Sandip; Rogers, Michael M.
1996-01-01
Turbulent wakes are known to develop self-similarly sufficiently far downstream from obstacles that generate them. It has long been assumed that the spreading rate of the wake in the self-similar regime is independent of the details of the body generating the wake, being dependent only on the total drag (or momentum deficit). This assumption seems to be in contradiction with some recent experiments. In this study we attempt to complement these experimental investigations through a numerical study of a time-developing wake. A numerical study has the advantage of eliminating many of the uncontrolled factors present in experiments and allowing precise control of initial conditions. Large-eddy simulations employing the recently developed dynamic localization model are used to extend previous results from direct numerical simulations. The large-eddy simulation results are compared to the direct numerical simulation database, wherever such comparisons are feasible, as a check of the method. Like the experiments, the large-eddy simulations suggest that non-unique self-similar states, characterized by different spreading rates and turbulent statistics, are possible and that they can be maintained for significant time periods. The study also demonstrates the predictive capability of the dynamic localization subgrid model.
A Numerical Study of Self-Similarity in a Turbulent Plane Wake Using Large-Eddy Simulation
NASA Technical Reports Server (NTRS)
Ghosal, Sandip; Rogers, Michael M.
1996-01-01
Turbulent wakes are known to develop self-similarly sufficiently far downstream from obstacles that generate them. It has long been assumed that the spreading rate of the wake in the self-similar regime is independent of the details of the body generating the wake, being dependent only on the total drag (or momentum deficit). This assumption seems to be in contradiction with some recent experiments. In this study we attempt to complement these experimental investigations through a numerical study of a time-developing wake. A numerical study has the advantage of eliminating many of the uncontrolled factors present in experiments and allowing precise control of initial conditions. Large-eddy simulations employing the recently developed dynamic localization model are used to extend previous results from direct numerical simulations. The large-eddy simulation results are compared to the direct numerical simulation database, wherever such comparisons are feasible, as a check of the method. Like the experiments, the large-eddy simulations suggest that non-unique self-similar states, characterized by different spreading rates and turbulent statistics, are possible and that they can be maintained for significant time periods. The study also demonstrates the predictive capability of the dynamic localization subgrid model.
Large eddy simulation of cavitating flows
NASA Astrophysics Data System (ADS)
Gnanaskandan, Aswin; Mahesh, Krishnan
2014-11-01
Large eddy simulation on unstructured grids is used to study hydrodynamic cavitation. The multiphase medium is represented using a homogeneous equilibrium model that assumes thermal equilibrium between the liquid and the vapor phase. Surface tension effects are ignored and the governing equations are the compressible Navier Stokes equations for the liquid/vapor mixture along with a transport equation for the vapor mass fraction. A characteristic-based filtering scheme is developed to handle shocks and material discontinuities in non-ideal gases and mixtures. A TVD filter is applied as a corrector step in a predictor-corrector approach with the predictor scheme being non-dissipative and symmetric. The method is validated for canonical one dimensional flows and leading edge cavitation over a hydrofoil, and applied to study sheet to cloud cavitation over a wedge. This work is supported by the Office of Naval Research.
Study of tip clearance flow in a turbomachinery cascade using large eddy simulation
NASA Astrophysics Data System (ADS)
You, Donghyun
In liquid handling systems like pumps and ducted propulsors, low pressure events in the vicinity and downstream of the rotor tip gap can induce tip-leakage cavitation which leads to noise, vibration, performance loss, and erosions of blade and casing wall. In order to analyze the dynamics of the tip-clearance flow and determine the underlying mechanism for the low pressure events, a newly developed large-eddy simulation (LES) solver which combines an immersed-boundary method with a generalized curvilinear structured grid has been employed. An analysis of the LES results has been performed to understand the mean flow field, turbulence characteristics, vortex dynamics, and pressure fluctuations in the turbomachinery cascade with tip gap. In the cascade passage, the tip-leakage jet, which is generated by the pressure difference between the pressure and suction sides of the blade tip, is found to produce highly enhanced vorticity magnitude and significant levels of turbulent kinetic energy. Based on the understanding of the flow field, a guideline for reducing viscous loss in the cascade is provided. Analyses of the energy spectra and space-time correlations of the velocity fluctuations suggest that the tip-leakage vortex is subject to pitchwise wandering motion. The largest pressure drop and most intense pressure fluctuations due to the formation of the tip-leakage vortex are found at the location where the strongest portion of the tip-leakage vortex is found. Present study suggests that the tip-leakage vortex needs to be controlled in its origin to reduce cavitation in the present configuration. The effects of tip-gap size on the end-wall vortical structures and on the velocity and pressure fields have been investigated. The present analysis indicates that the mechanism for the generation of the vorticity and turbulent kinetic energy is mostly unchanged by the tip-gap size variation. However, larger tip-gap sizes are found to be more inductive to tip
Large Eddy Simulation of Turbulent Combustion
2006-03-15
Application to an HCCI Engine . Proceedings of the 4th Joint Meeting of the U.S. Sections of the Combustion Institute, 2005. [34] K. Fieweger...LARGE EDDY SIMULATION OF TURBULENT COMBUSTION Principle Investigator: Heinz Pitsch Flow Physics and Computation Department of Mechanical Engineering ...burners and engines found in modern, industrially relevant equipment. In the course of this transition of LES from a scientifically interesting method
Large eddy simulation - The next five years
NASA Technical Reports Server (NTRS)
Ferziger, J. H.
1984-01-01
The prospect of major improvements in the performance of computers in the next five years means that large eddy simulation (LES), which has until now been strictly a research tool, may become a top-of-the-line engineering tool. In this paper, the historical development and past contributions of LES are reviewed. Then a discussion of the potential for applications of LES in new areas and of the developments needed to make LES a tool for the practicing engineer is given.
Study on wake structure characteristics of a slotted micro-ramp with large-eddy simulation
NASA Astrophysics Data System (ADS)
Dong, Xiangrui; Chen, Yaohui; Dong, Gang; Liu, Yixin
2017-06-01
In this paper, a novel slotted ramp-type micro vortex generator (slotted micro-ramp) for flow separation control is simulated in the supersonic flow of Ma = 1.5, based on large eddy simulation combined with the finite volume method. The wake structure characteristics and control mechanisms of both slotted and standard micro-ramps are presented and discussed. The results show that the wake of standard micro-ramp includes a primary counter-rotating streamwise vortex pair, a train of vortex rings, and secondary vortices. The slotted micro-ramp has more complicated wake structures, which contain a confluent counter-rotating streamwise vortex pair and additional streamwise vortices, with the same rotation generated by slot and the vortex rings enveloping the vortex pair. The additional vortices generated by the slot of the micro-ramp can mix with the primary counter-rotating vortex pair, extend the life time, and strengthen the vortex intensity of primary vortex pair. Moreover, the slot can effectively alleviate, or even eliminate the backflow and decrease the profile drag induced by the standard micro-ramp, therefore improving the efficiency of separation control.
Parametric Study of Decay of Homogeneous Isotropic Turbulence Using Large Eddy Simulation
NASA Technical Reports Server (NTRS)
Swanson, R. C.; Rumsey, Christopher L.; Rubinstein, Robert; Balakumar, Ponnampalam; Zang, Thomas A.
2012-01-01
Numerical simulations of decaying homogeneous isotropic turbulence are performed with both low-order and high-order spatial discretization schemes. The turbulent Mach and Reynolds numbers for the simulations are 0.2 and 250, respectively. For the low-order schemes we use either second-order central or third-order upwind biased differencing. For higher order approximations we apply weighted essentially non-oscillatory (WENO) schemes, both with linear and nonlinear weights. There are two objectives in this preliminary effort to investigate possible schemes for large eddy simulation (LES). One is to explore the capability of a widely used low-order computational fluid dynamics (CFD) code to perform LES computations. The other is to determine the effect of higher order accuracy (fifth, seventh, and ninth order) achieved with high-order upwind biased WENO-based schemes. Turbulence statistics, such as kinetic energy, dissipation, and skewness, along with the energy spectra from simulations of the decaying turbulence problem are used to assess and compare the various numerical schemes. In addition, results from the best performing schemes are compared with those from a spectral scheme. The effects of grid density, ranging from 32 cubed to 192 cubed, on the computations are also examined. The fifth-order WENO-based scheme is found to be too dissipative, especially on the coarser grids. However, with the seventh-order and ninth-order WENO-based schemes we observe a significant improvement in accuracy relative to the lower order LES schemes, as revealed by the computed peak in the energy dissipation and by the energy spectrum.
Large-eddy simulation studies of sea spray in the hurricane atmospheric boundary layer
NASA Astrophysics Data System (ADS)
Kelly, Mark
The growth and maintenance of hurricanes is highly dependent upon the exchange of heat and momentum between the ocean and atmosphere. Because sea spray can significantly affect this ocean-air exchange, accurate hurricane models need to account for spray effects. We incorporate sea spray into large-eddy simulations (LES) to explore its role in the atmospheric boundary layer (ABL) of hurricanes, allowing us to assess the validity of and offer improvements to the simple spray parameterizations currently used in hurricane models. We investigate thermodynamic feedback between spray and surface heat fluxes, and examine the effects of spray upon the dynamics of the hurricane boundary layer. Results of preliminary LES, which use a bulk representation of the dominant range of spray sizes and a simplified diagnostic phase change scheme, indicate an appreciable amount of spray-air heat transfer---consistent with theory---and demonstrate a form of spray-induced thermodynamic feedback. The LES model of the hurricane atmospheric boundary layer (HABL) is adapted to account for variations in spray generation due to wave-breaking, momentum transfer between air and spray in both the vertical (liquid loading and stratification) and horizontal (drag), and dissipative heating in an emulsion-like two-phase environment. These modifications are accompanied by extension of the phase change and spray generation schemes to account for different droplet sizes, and implementation of a moving three-dimensional boundary. Collective inclusion of all these pieces of modeled physics in the LES provides results which offer a better view of the limitations of current spray-flux models, and motivates a simpler and improved alternative model. The refined results of the 'full' LES-HABL model are consistent with early simulations, and underscore the significance of boundary-layer scale thermodynamic balance, spray-induced fluxes, and wind-dependent thermodynamic feedback.
Turbulence topologies predicted using large eddy simulations
NASA Astrophysics Data System (ADS)
Wang, Bing-Chen; Bergstrom, Donald J.; Yin, Jing; Yee, Eugene
In this paper, turbulence topologies related to the invariants of the resolved velocity gradient and strain rate tensors are studied based on large eddy simulation. The numerical results presented in the paper were obtained using two dynamic models, namely, the conventional dynamic model of Lilly and a recently developed dynamic nonlinear subgrid scale (SGS) model. In contrast to most of the previous research investigations which have mainly focused on isotropic turbulence, the present study examines the influence of near-wall anisotropy on the flow topologies. The SGS effect on the so-called SGS dissipation of the discriminant is examined and it is shown that the SGS stress contributes to the deviation of the flow topology of real turbulence from that of the ideal restricted Euler flow. The turbulence kinetic energy (TKE) transfer between the resolved and subgrid scales of motion is studied, and the forward and backward scatters of TKE are quantified in the invariant phase plane. Some interesting phenomenological results have also been obtained, including a wing-shaped contour pattern for the density of the resolved enstrophy generation and the near-wall dissipation shift of the peak location (mode) in the joint probability density function of the invariants of the resolved strain rate tensor. The newly observed turbulence phenomenologies are believed to be important and an effort has been made to explain them on an analytical basis.
Second-moment budgets in cloud topped boundary layers: A large-eddy simulation study
NASA Astrophysics Data System (ADS)
Heinze, Rieke; Mironov, Dmitrii; Raasch, Siegfried
2015-06-01
A detailed analysis of second-order moment budgets for cloud topped boundary layers (CTBLs) is performed using high-resolution large-eddy simulation (LES). Two CTBLs are simulated—one with trade wind shallow cumuli, and the other with nocturnal marine stratocumuli. Approximations to the ensemble-mean budgets of the Reynolds-stress components, of the fluxes of two quasi-conservative scalars, and of the scalar variances and covariance are computed by averaging the LES data over horizontal planes and over several hundred time steps. Importantly, the subgrid scale contributions to the budget terms are accounted for. Analysis of the LES-based second-moment budgets reveals, among other things, a paramount importance of the pressure scrambling terms in the Reynolds-stress and scalar-flux budgets. The pressure-strain correlation tends to evenly redistribute kinetic energy between the components, leading to the growth of horizontal-velocity variances at the expense of the vertical-velocity variance which is produced by buoyancy over most of both CTBLs. The pressure gradient-scalar covariances are the major sink terms in the budgets of scalar fluxes. The third-order transport proves to be of secondary importance in the scalar-flux budgets. However, it plays a key role in maintaining budgets of TKE and of the scalar variances and covariance. Results from the second-moment budget analysis suggest that the accuracy of description of the CTBL structure within the second-order closure framework strongly depends on the fidelity of parameterizations of the pressure scrambling terms in the flux budgets and of the third-order transport terms in the variance budgets. This article was corrected on 26 JUN 2015. See the end of the full text for details.
Large-Eddy Simulation of Subsonic Jets
NASA Astrophysics Data System (ADS)
Vuorinen, Ville; Wehrfritz, Armin; Yu, Jingzhou; Kaario, Ossi; Larmi, Martti; Boersma, Bendiks Jan
2011-12-01
The present study deals with development and validation of a fully explicit, compressible Runge-Kutta-4 (RK4) Navier-Stokes solver in the opensource CFD programming environment OpenFOAM. The background motivation is to shift towards explicit density based solution strategy and thereby avoid using the pressure based algorithms which are currently proposed in the standard OpenFOAM release for Large-Eddy Simulation (LES). This shift is considered necessary in strongly compressible flows when Ma > 0.5. Our application of interest is related to the pre-mixing stage in direct injection gas engines where high injection pressures are typically utilized. First, the developed flow solver is discussed and validated. Then, the implementation of subsonic inflow conditions using a forcing region in combination with a simplified nozzle geometry is discussed and validated. After this, LES of mixing in compressible, round jets at Ma = 0.3, 0.5 and 0.65 are carried out. Respectively, the Reynolds numbers of the jets correspond to Re = 6000, 10000 and 13000. Results for two meshes are presented. The results imply that the present solver produces turbulent structures, resolves a range of turbulent eddy frequencies and gives also mesh independent results within satisfactory limits for mean flow and turbulence statistics.
Large eddy simulation in the ocean
NASA Astrophysics Data System (ADS)
Scotti, Alberto
2010-12-01
Large eddy simulation (LES) is a relative newcomer to oceanography. In this review, both applications of traditional LES to oceanic flows and new oceanic LES still in an early stage of development are discussed. The survey covers LES applied to boundary layer flows, traditionally an area where LES has provided considerable insight into the physics of the flow, as well as more innovative applications, where new SGS closure schemes need to be developed. The merging of LES with large-scale models is also briefly reviewed.
Renormalization group formulation of large eddy simulation
NASA Technical Reports Server (NTRS)
Yakhot, V.; Orszag, S. A.
1985-01-01
Renormalization group (RNG) methods are applied to eliminate small scales and construct a subgrid scale (SSM) transport eddy model for transition phenomena. The RNG and SSM procedures are shown to provide a more accurate description of viscosity near the wall than does the Smagorinski approach and also generate farfield turbulence viscosity values which agree well with those of previous researchers. The elimination of small scales causes the simultaneous appearance of a random force and eddy viscosity. The RNG method permits taking these into account, along with other phenomena (such as rotation) for large-eddy simulations.
Large Eddy Simulation of Powered Fontan Hemodynamics
Delorme, Y.; Anupindi, K.; Kerlo, A.E.; Shetty, D.; Rodefeld, M.; Chen, J.; Frankel, S.
2012-01-01
Children born with univentricular heart disease typically must undergo three open heart surgeries within the first 2–3 years of life to eventually establish the Fontan circulation. In that case the single working ventricle pumps oxygenated blood to the body and blood returns to the lungs flowing passively through the Total Cavopulmonary Connection (TCPC) rather than being actively pumped by a subpulmonary ventricle. The TCPC is a direct surgical connection between the superior and inferior vena cava and the left and right pulmonary arteries. We have postulated that a mechanical pump inserted into this circulation providing a 3–5 mmHg pressure augmentation will reestablish bi-ventricular physiology serving as a bridge-to-recovery, bridge-to-transplant or destination therapy as a “biventricular Fontan” circulation. The Viscous Impeller Pump (VIP) has been proposed by our group as such an assist device. It is situated in the center of the 4-way TCPC intersection and spins pulling blood from the vena cavae and pushing it into the pulmonary arteries. We hypothesized that Large Eddy Simulation (LES) using high-order numerical methods are needed to capture unsteady powered and unpowered Fontan hemodynamics. Inclusion of a mechanical pump into the CFD further complicates matters due to the need to account for rotating machinery. In this study, we focus on predictions from an in-house high-order LES code (WenoHemo™) for unpowered and VIP-powered idealized TCPC hemodynamics with quantitative comparisons to Stereoscopic Particle Imaging Velocimetry (SPIV) measurements. Results are presented for both instantaneous flow structures and statistical data. Simulations show good qualitative and quantitative agreement with measured data. PMID:23177085
Large eddy simulation of powered Fontan hemodynamics.
Delorme, Y; Anupindi, K; Kerlo, A E; Shetty, D; Rodefeld, M; Chen, J; Frankel, S
2013-01-18
Children born with univentricular heart disease typically must undergo three open heart surgeries within the first 2-3 years of life to eventually establish the Fontan circulation. In that case the single working ventricle pumps oxygenated blood to the body and blood returns to the lungs flowing passively through the Total Cavopulmonary Connection (TCPC) rather than being actively pumped by a subpulmonary ventricle. The TCPC is a direct surgical connection between the superior and inferior vena cava and the left and right pulmonary arteries. We have postulated that a mechanical pump inserted into this circulation providing a 3-5 mmHg pressure augmentation will reestablish bi-ventricular physiology serving as a bridge-to-recovery, bridge-to-transplant or destination therapy as a "biventricular Fontan" circulation. The Viscous Impeller Pump (VIP) has been proposed by our group as such an assist device. It is situated in the center of the 4-way TCPC intersection and spins pulling blood from the vena cavae and pushing it into the pulmonary arteries. We hypothesized that Large Eddy Simulation (LES) using high-order numerical methods are needed to capture unsteady powered and unpowered Fontan hemodynamics. Inclusion of a mechanical pump into the CFD further complicates matters due to the need to account for rotating machinery. In this study, we focus on predictions from an in-house high-order LES code (WenoHemo(TM)) for unpowered and VIP-powered idealized TCPC hemodynamics with quantitative comparisons to Stereoscopic Particle Imaging Velocimetry (SPIV) measurements. Results are presented for both instantaneous flow structures and statistical data. Simulations show good qualitative and quantitative agreement with measured data.
Developing large eddy simulation for turbomachinery applications.
Eastwood, Simon J; Tucker, Paul G; Xia, Hao; Klostermeier, Christian
2009-07-28
For jets, large eddy resolving simulations are compared for a range of numerical schemes with no subgrid scale (SGS) model and for a range of SGS models with the same scheme. There is little variation in results for the different SGS models, and it is shown that, for schemes which tend towards having dissipative elements, the SGS model can be abandoned, giving what can be termed numerical large eddy simulation (NLES). More complex geometries are investigated, including coaxial and chevron nozzle jets. A near-wall Reynolds-averaged Navier-Stokes (RANS) model is used to cover over streak-like structures that cannot be resolved. Compressor and turbine flows are also successfully computed using a similar NLES-RANS strategy. Upstream of the compressor leading edge, the RANS layer is helpful in preventing premature separation. Capturing the correct flow over the turbine is particularly challenging, but nonetheless the RANS layer is helpful. In relation to the SGS model, for the flows considered, evidence suggests issues such as inflow conditions, problem definition and transition are more influential.
Scalar excursions in large-eddy simulations
NASA Astrophysics Data System (ADS)
Matheou, Georgios; Dimotakis, Paul E.
2016-12-01
The range of values of scalar fields in turbulent flows is bounded by their boundary values, for passive scalars, and by a combination of boundary values, reaction rates, phase changes, etc., for active scalars. The current investigation focuses on the local conservation of passive scalar concentration fields and the ability of the large-eddy simulation (LES) method to observe the boundedness of passive scalar concentrations. In practice, as a result of numerical artifacts, this fundamental constraint is often violated with scalars exhibiting unphysical excursions. The present study characterizes passive-scalar excursions in LES of a shear flow and examines methods for diagnosis and assesment of the problem. The analysis of scalar-excursion statistics provides support of the main hypothesis of the current study that unphysical scalar excursions in LES result from dispersive errors of the convection-term discretization where the subgrid-scale model (SGS) provides insufficient dissipation to produce a sufficiently smooth scalar field. In the LES runs three parameters are varied: the discretization of the convection terms, the SGS model, and grid resolution. Unphysical scalar excursions decrease as the order of accuracy of non-dissipative schemes is increased, but the improvement rate decreases with increasing order of accuracy. Two SGS models are examined, the stretched-vortex and a constant-coefficient Smagorinsky. Scalar excursions strongly depend on the SGS model. The excursions are significantly reduced when the characteristic SGS scale is set to double the grid spacing in runs with the stretched-vortex model. The maximum excursion and volume fraction of excursions outside boundary values show opposite trends with respect to resolution. The maximum unphysical excursions increase as resolution increases, whereas the volume fraction decreases. The reason for the increase in the maximum excursion is statistical and traceable to the number of grid points (sample size
A Large-eddy Simulation Study of Vertical Axis Wind Turbine Wakes in the Atmospheric Boundary Layer
NASA Astrophysics Data System (ADS)
Shamsoddin, Sina; Porté-Agel, Fernando
2016-04-01
Vertical axis wind turbines (VAWTs) offer some advantages over their horizontal axis counterparts, and are being considered as a viable alternative to conventional horizontal axis wind turbines (HAWTs). Nevertheless, a relative shortage of scientific, academic and technical investigations of VAWTs is observed in the wind energy community with respect to HAWTs. Having this in mind, in this work, we aim to study the wake of a single VAWT, placed in the atmospheric boundary layer, using large-eddy simulation (LES) coupled with actuator line model (ALM). It is noteworthy that this is the first time that such a study is being performed. To do this, for a typical 1 MW VAWT design, first, the variation of power coefficient with both the chord length of the blades and the tip-speed ratio is analyzed using LES-ALM, and an optimum combination of chord length and tip-speed ratio is obtained. Subsequently, the wake of a VAWT with these optimum specifications is thoroughly examined by showing different relevant mean and turbulent wake flow statistics. Keywords: vertical axis wind turbine (VAWT); VAWT wake; Atmospheric Boundary Layer (ABL); large eddy simulation (LES); actuator line model (ALM); turbulence.
Large-eddy simulation of compressible turbulence
NASA Technical Reports Server (NTRS)
Squires, Kyle D.
1991-01-01
The increase in the range of length scales with increasing Reynolds number limits the direct simulation of turbulent flows to relatively simple geometries and low Reynolds numbers. However, since most flows of engineering interest occur at much higher Reynolds number than is currently within the capabilities of full simulation, prediction of these flow fields can only be obtained by solving some suitably-averaged set of governing equations. In the traditional Reynolds-averaged approach, the Navier-Stokes equations are averaged over time. This in turn yields correlations between various turbulence fluctuations. It is these terms, e.g. the Reynolds stresses, for which a turbulence model must be derived. Turbulence modeling of incompressible flows has received a great amount of attention in the literature. An area of research that has received comparatively less attention is the modeling of compressible turbulent flows. An approach to simulating compressible turbulence at high Reynolds numbers is through the use of Large-Eddy Simulation (LES). In LES the dependent variables are decomposed into a large-scale (resolved) component and a sub-grid scale component. It is the small-scale components of the velocity field which are presumably more homogeneous than the large scales and, therefore, more easily modeled. Thus, it seems plausible that simpler models, which should be more universal in character than those employed in second-order closure schemes, may be developed for LES of compressible turbulence. The objective of the present research, therefore, is to explore models for the Large-Eddy Simulation of compressible turbulent flows. Given the recent successes of Zeman in second order closure modeling of compressible turbulence, model development was guided by principals employed in second-order closures.
Large eddy simulations in 2030 and beyond
Piomelli, U
2014-01-01
Since its introduction, in the early 1970s, large eddy simulations (LES) have advanced considerably, and their application is transitioning from the academic environment to industry. Several landmark developments can be identified over the past 40 years, such as the wall-resolved simulations of wall-bounded flows, the development of advanced models for the unresolved scales that adapt to the local flow conditions and the hybridization of LES with the solution of the Reynolds-averaged Navier–Stokes equations. Thanks to these advancements, LES is now in widespread use in the academic community and is an option available in most commercial flow-solvers. This paper will try to predict what algorithmic and modelling advancements are needed to make it even more robust and inexpensive, and which areas show the most promise. PMID:25024415
Large Eddy Simulation of turbulent shear flows
NASA Technical Reports Server (NTRS)
Moin, P.; Mansour, N. N.; Reynolds, W. C.; Ferziger, J. H.
1979-01-01
The conceptual foundation underlying Large Eddy Simulation (LES) is summarized, and the numerical methods developed for simulation of the time-developing turbulent mixing layer and turbulent plane Poiseuille flow are discussed. Computational results show that the average Reynolds stress profile nearly attains the equilibrium shape which balances the downstream mean pressure gradient in the regions away from the walls. In the vicinity of the walls, viscous stresses are shown to be significant; together with the Reynolds stresses, these stresses balance the mean pressure gradient. It is stressed that the subgrid scale contribution to the total Reynolds stress is significant only in the vicinity of the walls. The continued development of LES is urged.
Large eddy simulations in 2030 and beyond.
Piomelli, U
2014-08-13
Since its introduction, in the early 1970s, large eddy simulations (LES) have advanced considerably, and their application is transitioning from the academic environment to industry. Several landmark developments can be identified over the past 40 years, such as the wall-resolved simulations of wall-bounded flows, the development of advanced models for the unresolved scales that adapt to the local flow conditions and the hybridization of LES with the solution of the Reynolds-averaged Navier-Stokes equations. Thanks to these advancements, LES is now in widespread use in the academic community and is an option available in most commercial flow-solvers. This paper will try to predict what algorithmic and modelling advancements are needed to make it even more robust and inexpensive, and which areas show the most promise.
Statistical Ensemble of Large Eddy Simulations
NASA Technical Reports Server (NTRS)
Carati, Daniele; Rogers, Michael M.; Wray, Alan A.; Mansour, Nagi N. (Technical Monitor)
2001-01-01
A statistical ensemble of large eddy simulations (LES) is run simultaneously for the same flow. The information provided by the different large scale velocity fields is used to propose an ensemble averaged version of the dynamic model. This produces local model parameters that only depend on the statistical properties of the flow. An important property of the ensemble averaged dynamic procedure is that it does not require any spatial averaging and can thus be used in fully inhomogeneous flows. Also, the ensemble of LES's provides statistics of the large scale velocity that can be used for building new models for the subgrid-scale stress tensor. The ensemble averaged dynamic procedure has been implemented with various models for three flows: decaying isotropic turbulence, forced isotropic turbulence, and the time developing plane wake. It is found that the results are almost independent of the number of LES's in the statistical ensemble provided that the ensemble contains at least 16 realizations.
Large-eddy simulation of propeller noise
NASA Astrophysics Data System (ADS)
Keller, Jacob; Mahesh, Krishnan
2016-11-01
We will discuss our ongoing work towards developing the capability to predict far field sound from the large-eddy simulation of propellers. A porous surface Ffowcs-Williams and Hawkings (FW-H) acoustic analogy, with a dynamic endcapping method (Nitzkorski and Mahesh, 2014) is developed for unstructured grids in a rotating frame of reference. The FW-H surface is generated automatically using Delaunay triangulation and is representative of the underlying volume mesh. The approach is validated for tonal trailing edge sound from a NACA 0012 airfoil. LES of flow around a propeller at design advance ratio is compared to experiment and good agreement is obtained. Results for the emitted far field sound will be discussed. This work is supported by ONR.
Autonomic Closure for Large Eddy Simulation
NASA Astrophysics Data System (ADS)
King, Ryan; Hamlington, Peter; Dahm, Werner J. A.
2015-11-01
A new autonomic subgrid-scale closure has been developed for large eddy simulation (LES). The approach poses a supervised learning problem that captures nonlinear, nonlocal, and nonequilibrium turbulence effects without specifying a predefined turbulence model. By solving a regularized optimization problem on test filter scale quantities, the autonomic approach identifies a nonparametric function that represents the best local relation between subgrid stresses and resolved state variables. The optimized function is then applied at the grid scale to determine unknown LES subgrid stresses by invoking scale similarity in the inertial range. A priori tests of the autonomic approach on homogeneous isotropic turbulence show that the new approach is amenable to powerful optimization and machine learning methods and is successful for a wide range of filter scales in the inertial range. In these a priori tests, the autonomic closure substantially improves upon the dynamic Smagorinsky model in capturing the instantaneous, statistical, and energy transfer properties of the subgrid stress field.
Large eddy simulation applications in gas turbines.
Menzies, Kevin
2009-07-28
The gas turbine presents significant challenges to any computational fluid dynamics techniques. The combination of a wide range of flow phenomena with complex geometry is difficult to model in the context of Reynolds-averaged Navier-Stokes (RANS) solvers. We review the potential for large eddy simulation (LES) in modelling the flow in the different components of the gas turbine during a practical engineering design cycle. We show that while LES has demonstrated considerable promise for reliable prediction of many flows in the engine that are difficult for RANS it is not a panacea and considerable application challenges remain. However, for many flows, especially those dominated by shear layer mixing such as in combustion chambers and exhausts, LES has demonstrated a clear superiority over RANS for moderately complex geometries although at significantly higher cost which will remain an issue in making the calculations relevant within the design cycle.
Large Eddy Simulation of Cirrus Clouds
NASA Technical Reports Server (NTRS)
Wu, Ting; Cotton, William R.
1999-01-01
The Regional Atmospheric Modeling System (RAMS) with mesoscale interactive nested-grids and a Large-Eddy Simulation (LES) version of RAMS, coupled to two-moment microphysics and a new two-stream radiative code were used to investigate the dynamic, microphysical, and radiative aspects of the November 26, 1991 cirrus event. Wu (1998) describes the results of that research in full detail and is enclosed as Appendix 1. The mesoscale nested grid simulation successfully reproduced the large scale circulation as compared to the Mesoscale Analysis and Prediction System's (MAPS) analyses and other observations. Three cloud bands which match nicely to the three cloud lines identified in an observational study (Mace et al., 1995) are predicted on Grid #2 of the nested grids, even though the mesoscale simulation predicts a larger west-east cloud width than what was observed. Large-eddy simulations (LES) were performed to study the dynamical, microphysical, and radiative processes in the 26 November 1991 FIRE 11 cirrus event. The LES model is based on the RAMS version 3b developed at Colorado State University. It includes a new radiation scheme developed by Harrington (1997) and a new subgrid scale model developed by Kosovic (1996). The LES model simulated a single cloud layer for Case 1 and a two-layer cloud structure for Case 2. The simulations demonstrated that latent heat release can play a significant role in the formation and development of cirrus clouds. For the thin cirrus in Case 1, the latent heat release was insufficient for the cirrus clouds to become positively buoyant. However, in some special cases such as Case 2, positively buoyant cells can be embedded within the cirrus layers. These cells were so active that the rising updraft induced its own pressure perturbations that affected the cloud evolution. Vertical profiles of the total radiative and latent heating rates indicated that for well developed, deep, and active cirrus clouds, radiative cooling and latent
Large eddy simulation of trailing edge noise
NASA Astrophysics Data System (ADS)
Keller, Jacob; Nitzkorski, Zane; Mahesh, Krishnan
2015-11-01
Noise generation is an important engineering constraint to many marine vehicles. A significant portion of the noise comes from propellers and rotors, specifically due to flow interactions at the trailing edge. Large eddy simulation is used to investigate the noise produced by a turbulent 45 degree beveled trailing edge and a NACA 0012 airfoil. A porous surface Ffowcs-Williams and Hawkings acoustic analogy is combined with a dynamic endcapping method to compute the sound. This methodology allows for the impact of incident flow noise versus the total noise to be assessed. LES results for the 45 degree beveled trailing edge are compared to experiment at M = 0 . 1 and Rec = 1 . 9 e 6 . The effect of boundary layer thickness on sound production is investigated by computing using both the experimental boundary layer thickness and a thinner boundary layer. Direct numerical simulation results of the NACA 0012 are compared to available data at M = 0 . 4 and Rec = 5 . 0 e 4 for both the hydrodynamic field and the acoustic field. Sound intensities and directivities are investigated and compared. Finally, some of the physical mechanisms of far-field noise generation, common to the two configurations, are discussed. Supported by Office of Naval research.
Large Eddy Simulation of Transitional Boundary Layer
NASA Astrophysics Data System (ADS)
Sayadi, Taraneh; Moin, Parviz
2009-11-01
A sixth order compact finite difference code is employed to investigate compressible Large Eddy Simulation (LES) of subharmonic transition of a spatially developing zero pressure gradient boundary layer, at Ma = 0.2. The computational domain extends from Rex= 10^5, where laminar blowing and suction excites the most unstable fundamental and sub-harmonic modes, to fully turbulent stage at Rex= 10.1x10^5. Numerical sponges are used in the neighborhood of external boundaries to provide non-reflective conditions. Our interest lies in the performance of the dynamic subgrid scale (SGS) model [1] in the transition process. It is observed that in early stages of transition the eddy viscosity is much smaller than the physical viscosity. As a result the amplitudes of selected harmonics are in very good agreement with the experimental data [2]. The model's contribution gradually increases during the last stages of transition process and the dynamic eddy viscosity becomes fully active and dominant in the turbulent region. Consistent with this trend the skin friction coefficient versus Rex diverges from its laminar profile and converges to the turbulent profile after an overshoot. 1. Moin P. et. al. Phys Fluids A, 3(11), 2746-2757, 1991. 2. Kachanov Yu. S. et. al. JFM, 138, 209-247, 1983.
Large-eddy simulations with wall models
NASA Technical Reports Server (NTRS)
Cabot, W.
1995-01-01
The near-wall viscous and buffer regions of wall-bounded flows generally require a large expenditure of computational resources to be resolved adequately, even in large-eddy simulation (LES). Often as much as 50% of the grid points in a computational domain are devoted to these regions. The dense grids that this implies also generally require small time steps for numerical stability and/or accuracy. It is commonly assumed that the inner wall layers are near equilibrium, so that the standard logarithmic law can be applied as the boundary condition for the wall stress well away from the wall, for example, in the logarithmic region, obviating the need to expend large amounts of grid points and computational time in this region. This approach is commonly employed in LES of planetary boundary layers, and it has also been used for some simple engineering flows. In order to calculate accurately a wall-bounded flow with coarse wall resolution, one requires the wall stress as a boundary condition. The goal of this work is to determine the extent to which equilibrium and boundary layer assumptions are valid in the near-wall regions, to develop models for the inner layer based on such assumptions, and to test these modeling ideas in some relatively simple flows with different pressure gradients, such as channel flow and flow over a backward-facing step. Ultimately, models that perform adequately in these situations will be applied to more complex flow configurations, such as an airfoil.
ENDLESS: a multi-scale large-eddy simulation approach to study oil plumes in the ocean mixed layer
NASA Astrophysics Data System (ADS)
Chen, B.; Yang, D.; Meneveau, C. V.; Chamecki, M.
2015-12-01
Large-eddy simulation (LES) has proven to be a valuable tool in producing high-fidelity simulations of environmental and geophysical turbulent flows. For simulations of the ocean mixed layer (OML), a grid size of a few meters is required and the typical horizontal domain sizes of LES are restricted to hundreds meters or a few kilometers at most. Oil plumes being transported in the OML experience the action of Langmuir turbulence, Ekman transport and submesoscale quasi-geostrophic eddies. As a consequence, oil plumes display complex features on scales from a few meters to tens kilometers. Therefore, accurately reproducing all the relevant scales in computer simulations is a challenging task. In this study, the Extended Nonperiodic Domain LES for Scalar transport (ENDLESS) is proposed as an economic multi-scale approach to tackle this problem with current computing power. The basic idea is to simulate the Langmuir turbulence on a small horizontal domain while simulating the oil plume over an effectively large extended domain. In particular, scalar fields are adaptively added and removed to efficiently enclose the oil plume with minimum computational cost for tracking the evolution of nonhomogeneous plumes. This approach also permits the superposition of larger-scale quasi two-dimensional motions on the oil advection, allowing for coupling with regional circulation models. Validation cases and sample applications are also discussed.
Large eddy simulations of compressible magnetohydrodynamic turbulence
NASA Astrophysics Data System (ADS)
Grete, Philipp
2017-02-01
Supersonic, magnetohydrodynamic (MHD) turbulence is thought to play an important role in many processes - especially in astrophysics, where detailed three-dimensional observations are scarce. Simulations can partially fill this gap and help to understand these processes. However, direct simulations with realistic parameters are often not feasible. Consequently, large eddy simulations (LES) have emerged as a viable alternative. In LES the overall complexity is reduced by simulating only large and intermediate scales directly. The smallest scales, usually referred to as subgrid-scales (SGS), are introduced to the simulation by means of an SGS model. Thus, the overall quality of an LES with respect to properly accounting for small-scale physics crucially depends on the quality of the SGS model. While there has been a lot of successful research on SGS models in the hydrodynamic regime for decades, SGS modeling in MHD is a rather recent topic, in particular, in the compressible regime. In this thesis, we derive and validate a new nonlinear MHD SGS model that explicitly takes compressibility effects into account. A filter is used to separate the large and intermediate scales, and it is thought to mimic finite resolution effects. In the derivation, we use a deconvolution approach on the filter kernel. With this approach, we are able to derive nonlinear closures for all SGS terms in MHD: the turbulent Reynolds and Maxwell stresses, and the turbulent electromotive force (EMF). We validate the new closures both a priori and a posteriori. In the a priori tests, we use high-resolution reference data of stationary, homogeneous, isotropic MHD turbulence to compare exact SGS quantities against predictions by the closures. The comparison includes, for example, correlations of turbulent fluxes, the average dissipative behavior, and alignment of SGS vectors such as the EMF. In order to quantify the performance of the new nonlinear closure, this comparison is conducted from the
Churchfield, Matthew J; Li, Ye; Moriarty, Patrick J
2013-02-28
This paper presents our initial work in performing large-eddy simulations of tidal turbine array flows. First, a horizontally periodic precursor simulation is performed to create turbulent flow data. Then those data are used as inflow into a tidal turbine array two rows deep and infinitely wide. The turbines are modelled using rotating actuator lines, and the finite-volume method is used to solve the governing equations. In studying the wakes created by the turbines, we observed that the vertical shear of the inflow combined with wake rotation causes lateral wake asymmetry. Also, various turbine configurations are simulated, and the total power production relative to isolated turbines is examined. We found that staggering consecutive rows of turbines in the simulated configurations allows the greatest efficiency using the least downstream row spacing. Counter-rotating consecutive downstream turbines in a non-staggered array shows a small benefit. This work has identified areas for improvement. For example, using a larger precursor domain would better capture elongated turbulent structures, and including salinity and temperature equations would account for density stratification and its effect on turbulence. Additionally, the wall shear stress modelling could be improved, and more array configurations could be examined.
Churchfield, M. J.; Li, Y.; Moriarty, P. J.
2012-07-01
This paper presents our initial work in performing large-eddy simulations of tidal turbine array flows. First, a horizontally-periodic precursor simulation is performed to create turbulent flow data. Then that data is used as inflow into a tidal turbine array two rows deep and infinitely wide. The turbines are modeled using rotating actuator lines, and the finite-volume method is used to solve the governing equations. In studying the wakes created by the turbines, we observed that the vertical shear of the inflow combined with wake rotation causes lateral wake asymmetry. Also, various turbine configurations are simulated, and the total power production relative to isolated turbines is examined. Staggering consecutive rows of turbines in the simulated configurations allows the greatest efficiency using the least downstream row spacing. Counter-rotating consecutive downstream turbines in a non-staggered array shows a small benefit. This work has identified areas for improvement, such as the use of a larger precursor domain to better capture elongated turbulent structures, the inclusion of salinity and temperature equations to account for density stratification and its effect on turbulence, improved wall shear stress modelling, and the examination of more array configurations.
Churchfield, M. J.; Li, Y.; Moriarty, P. J.
2011-07-01
This paper presents our initial work in performing large-eddy simulations of tidal turbine array flows. First, a horizontally-periodic precursor simulation is performed to create turbulent flow data. Then that data is used to determine the inflow into a tidal turbine array two rows deep and infinitely wide. The turbines are modeled using rotating actuator lines, and the finite-volume method is used to solve the governing equations. In studying the wakes created by the turbines, we observed that the vertical shear of the inflow combined with wake rotation causes lateral wake asymmetry. Also, various turbine configurations are simulated, and the total power production relative to isolated turbines is examined. Staggering consecutive rows of turbines in the simulated configurations allows the greatest efficiency using the least downstream row spacing. Counter-rotating consecutive downstream turbines in a non-staggered array shows a small benefit. This work has identified areas for improvement, such as the use of a larger precursor domain to better capture elongated turbulent structures, the inclusion of salinity and temperature equations to account for density stratification and its effect on turbulence, improved wall shear stress modeling, and the examination of more array configurations.
Large eddy simulations of laminar separation bubble
NASA Astrophysics Data System (ADS)
Cadieux, Francois
The flow over blades and airfoils at moderate angles of attack and Reynolds numbers ranging from ten thousand to a few hundred thousands undergoes separation due to the adverse pressure gradient generated by surface curvature. In many cases, the separated shear layer then transitions to turbulence and reattaches, closing off a recirculation region -- the laminar separation bubble. To avoid body-fitted mesh generation problems and numerical issues, an equivalent problem for flow over a flat plate is formulated by imposing boundary conditions that lead to a pressure distribution and Reynolds number that are similar to those on airfoils. Spalart & Strelet (2000) tested a number of Reynolds-averaged Navier-Stokes (RANS) turbulence models for a laminar separation bubble flow over a flat plate. Although results with the Spalart-Allmaras turbulence model were encouraging, none of the turbulence models tested reliably recovered time-averaged direct numerical simulation (DNS) results. The purpose of this work is to assess whether large eddy simulation (LES) can more accurately and reliably recover DNS results using drastically reduced resolution -- on the order of 1% of DNS resolution which is commonly achievable for LES of turbulent channel flows. LES of a laminar separation bubble flow over a flat plate are performed using a compressible sixth-order finite-difference code and two incompressible pseudo-spectral Navier-Stokes solvers at resolutions corresponding to approximately 3% and 1% of the chosen DNS benchmark by Spalart & Strelet (2000). The finite-difference solver is found to be dissipative due to the use of a stability-enhancing filter. Its numerical dissipation is quantified and found to be comparable to the average eddy viscosity of the dynamic Smagorinsky model, making it difficult to separate the effects of filtering versus those of explicit subgrid-scale modeling. The negligible numerical dissipation of the pseudo-spectral solvers allows an unambiguous
Study of the flow unsteadiness in the human airway using large eddy simulation
NASA Astrophysics Data System (ADS)
Bernate, Jorge A.; Geisler, Taylor S.; Padhy, Sourav; Shaqfeh, Eric S. G.; Iaccarino, Gianluca
2017-08-01
The unsteady flow in a patient-specific geometry of the airways is studied. The geometry comprises the oral cavity, orophrarynx, larynx, trachea, and the bronchial tree extending to generations 5-8. Simulations are carried out for a constant inspiratory flow rate of 60 liters/min, corresponding to a Reynolds number of 4213 for a nominal tracheal diameter of 2 cm. The computed mean flow field is compared extensively with magnetic resonance velocimetry measurements by Banko et al. [Exp. Fluids 56, 117 (2015), 10.1007/s00348-015-1966-y] carried out in the same computed-tomography-based geometry, showing good agreement. In particular, we focus on the dynamics of the flow in the bronchial tree. After becoming unsteady at a constriction in the oropharynx, the flow is found to be chaotic, exhibiting fluctuations with broad-band spectra even at the most distal airways in which the Reynolds numbers are as low as 300. An inertial range signature is present in the trachea but not in the bronchial tree where a narrower range of scales is observed. The unsteadiness is attributed to the convection of turbulent structures produced at the larynx as well as to local kinetic energy production throughout the bronchial tree. Production occurs predominantly at shear layers bounding geometry-induced separation regions.
Application of large-eddy simulation to the study of pulsatile flow in a modeled arterial stenosis.
Mittal, R; Simmons, S P; Udaykumar, H S
2001-08-01
The technique of large-eddy simulation (LES) has been applied to the study of pulsatile flow through a modeled arterial stenosis. A simple stenosis model has been used that consists of a one-sided 50 percent semicircular constriction in a planar channel. The inlet volume flux is varied sinusoidally in time in a manner similar to the laminar flow simulations of Tutty (1992). LES is used to compute flow at a peak Reynolds number of 2000 and a Strouhal number of 0.024. At this Reynolds number, the flow downstream of the stenosis transitions to turbulence and exhibits all the classic features of post-stenotic flow as described by Khalifa and Giddens (1981) and Lieber and Giddens (1990). These include the periodic shedding of shear layer vortices and transition to turbulence downstream of the stenosis. Computed frequency spectra indicate that the vortex shedding occurs at a distinct high frequency, and the potential implication of this for noninvasive diagnosis of arterial stenoses is discussed. A variety of statistics have been also extracted and a number of other physical features of the flow are described in order to demonstrate the usefulness of LES for the study of post-stenotic flows.
Large-Eddy Simulations of Dust Devils and Convective Vortices
NASA Astrophysics Data System (ADS)
Spiga, Aymeric; Barth, Erika; Gu, Zhaolin; Hoffmann, Fabian; Ito, Junshi; Jemmett-Smith, Bradley; Klose, Martina; Nishizawa, Seiya; Raasch, Siegfried; Rafkin, Scot; Takemi, Tetsuya; Tyler, Daniel; Wei, Wei
2016-11-01
In this review, we address the use of numerical computations called Large-Eddy Simulations (LES) to study dust devils, and the more general class of atmospheric phenomena they belong to (convective vortices). We describe the main elements of the LES methodology. We review the properties, statistics, and variability of dust devils and convective vortices resolved by LES in both terrestrial and Martian environments. The current challenges faced by modelers using LES for dust devils are also discussed in detail.
NASA Astrophysics Data System (ADS)
Chamecki, M.; Pan, Y.; Nepf, H. M.; Follett, E.
2014-12-01
Flexible plants bend in response to fluid motion and this reconfiguration mechanism allows plants to minimize the increase of drag force with increasing velocity, ensuring survival in flow-dominated habitats. The effect of reconfiguration on the flow field can be modeled by introducing a drag coefficient that decreases with increasing velocity. Typically, a power-law decrease of the drag coefficient with increasing velocity is used, and the negative exponent is known as the Vogel number. In practice, the Vogel number is a function of canopy rigidity and flow conditions. In this work we show that accounting for the effect of reconfiguration is required for large-eddy simulation (LES) models to reproduce the skewness of the streamwise and vertical velocity components and the distribution of sweeps and ejections observed in a large cornfield. Additional LES runs are conducted to investigate the structure of turbulence in different reconfiguration regimes, with mean vertical momentum flux constrained by measurements. The change of the Vogel number has negligible effects on LES predictions of the total vertical momentum flux and the components of turbulent kinetic energy, but produces profound changes in the mechanisms of momentum transport. This work demonstrates the necessity to model the effect of reconfiguration in LES studies of canopy flows. It also highlights the impacts of reconfiguration on the structure of turbulence and the dynamics of momentum fluxes, as well as any other process that depends on velocity fluctuations above and within the canopy region.
NASA Astrophysics Data System (ADS)
Zhu, Xiaowei; Anderson, William
2015-11-01
The inherent spatial heterogeneity exhibited by real urban environments complicates a priori estimation of the roughness height needed to parameterize the inertial layer mean streamwise velocity. A large-eddy simulation study of turbulent flow over 3-D random urban-like topographies is conducted to explore the effects of surface geometry on bulk aerodynamic characterization. In a mean sense, we find that statistical attributes including surface height root mean square and skewness can adequately capture the spatial heterogeneities and randomness of real urban geometries. We find, however, that higher-order statistical moments have a negligible affect on aerodynamic drag (i.e. kurtosis may be omitted). The results enable exploration of applicability of some recently-proposed roughness parameterizations that are relevant to complex, urban-like roughness (including the model proposed by Flack and Schultz, 2010: J. Fluids Eng. 132, 041203-1). We evaluate empirical parameters needed in these models for the present urban-like cases. We find that two empirical parameters (relevant to height rms and skewness) can characterize the bulk aerodynamic roughness of topographies with statistical attributes comparable to dense urban environments. This work was supported by the Army Research Office, Atmospheric Sciences Program (PM: Dr. S. Collier) under Grant # W911NF-13-1-0474. Computational resources were provided by the Texas Advanced Computing Center at the University of Texas.
NASA Astrophysics Data System (ADS)
Kang, Hyung Suk; Meneveau, Charles
2008-12-01
A shearless mixing layer characterized by interactions between two regions with different turbulence intensities but without mean shear is investigated experimentally in a wind tunnel. Reynolds numbers higher than those of prior studies [B. Gilbert, "Diffusion mixing in grid turbulence without mean shear," J. Fluid Mech. 100, 349 (1980); S. Veeravalli and Z. Warhaft, "The shearless turbulent mixing layer," J. Fluid Mech. 207, 191 (1989); B. Knaepen, O. Debliquy, and D. Carati, "Direct numerical simulation and large-eddy simulation of a shear-free mixing layer," J. Fluid Mech. 514, 153 (2004); D. Tordella and M. Iovieno, "Numerical experiments on the intermediate asymptotics of shear-free turbulent transport and diffusion," J. Fluid Mech. 549, 429 (2006); D. A. Briggs, J. H. Ferziger, J. R. Koseff, and S. G. Monismith, "Entrainment in a shear-free turbulent mixing layer," J. Fluid Mech. 310, 215 (1996)] are achieved by using an active grid with rotating winglets on one-half of its cross section. Stationary flow-conditioning fine meshes are used to avoid mean velocity gradients. Measurements are performed at five different downstream wind-tunnel locations using an X-type hot-wire probe and a stereoscopic particle image velocimetry system. The Reynolds numbers based on the Taylor microscale in the high- and low-kinetic energy regions are 170 and 88, respectively. The energy and integral length-scale ratios between the two regions are 4.27 and 1.73, respectively. The inlet turbulence in the upper and lower portions of the shearless mixing layer is not fully isotropic, with the streamwise velocity fluctuations being between 6% and 13% higher than the cross-stream ones. Fundamental statistical properties of the flow are documented and analyzed at various scales using band-pass box-filtered velocities. Downstream evolution of variance and half-width of the mixing layer, skewness and flatness factors, as well as the statistics of two-point velocity increments at various
NASA Astrophysics Data System (ADS)
Eriksson, O.; Baltscheffsky, M.; Breton, S.-P.; Söderberg, S.; Ivanell, S.
2017-05-01
The aim of the present paper is to obtain a better understanding of long distance wakes generated by wind farms as a first step towards a better understanding of farm to farm interaction. The Horns Rev I (HR) wind farm is considered for this purpose, where comparisons are performed between microscale Large Eddy Simulations (LES) using an Actuator Disc model (ACD), mesoscale simulations in the Weather Research and Forecasting Model (WRF) using a wind turbine parameterization, production data as well as wind measurements in the wind farm wake. The LES is manually set up according to the wind conditions obtained from the mesoscale simulation as a first step towards a meso/microscale coupling. The LES using an ACD are performed in the EllipSys3D code. A forced boundary layer (FBL) approach is used to introduce the desired wind shear and the atmospheric turbulence field from the Mann model. The WRF uses a wind turbine parameterization based on momentum sink. To make comparisons with the LESs and the site data possible an idealized setup of WRF is used in this study. The case studied here considers a westerly wind direction sector (at hub height) of 270 ± 2.5 degrees and a wind speed of 8 ± 0.5 m/s. For both the simulations and the site data a neutral atmosphere is considered. The simulation results for the relative production as well as the wind speed 2 km and 6 km downstream from the wind farm are compared to site data. Further comparisons between LES and WRF are also performed regarding the wake recovery and expansion. The results are also compared to an earlier study of HR using LES as well as an earlier comparison of LES and WRF. Overall the results in this study show a better agreement between LES and WRF as well as better agreement between simulations and site data. The procedure of using the profile from WRF as inlet to LES can be seen as a simplified coupling of the models that could be developed further to combine the methods for cases of farm to farm
NASA Astrophysics Data System (ADS)
Cheng, Z.; Hsu, T. J.; Chauchat, J.; Revil-Baudard, T.
2016-12-01
Coastal morphological evolution is caused by a wide range of coupled cross-shore and alongshore sediment transport processes associated with short waves, infragravity waves and wave-induced currents. However, the fundamental transport mechanisms occur within the thin bottom boundary layer and are dictated by turbulence-sediment interaction and inter-granular interactions. Recently, a turbulence-averaged two-phase Eulerian sediment transport model, called sedFoam, was developed and validated for U-tube sheet flows (Cheng et al., 2016). With closures of particle stresses and fluid-particle interactions, the model is able to resolve full profiles of sediment transport from the immobile bed, to the concentrated near-bed transport layer and up to dilute transport without conventional bedload/suspended load assumptions. In this study, we further extend this model with a 3D large eddy simulation (LES) approach, where substantial amount of the turbulence-sediment interaction is directly resolved. In the present LES model, a dynamic Smagorinsky sub-grid stress closure is adopted for both fluid and sediment phases, and the sub-grid contribution to the fluid-particle interactions is included by a sub-grid drift velocity in drag model (Ozel et al., 2013). The model is validated with high-resolution measurements in a unidirectional steady sheet flow experiment (Revil-Baudard et al. 2015). We further apply the LES Eulerian two-phase model to medium and fine sand in oscillatory sheet flows (O'Donoghue & Wright, 2004). The LES model performance for the medium sand is similar to the turbulence-averaged model, except that the LES model seems to be superior to the turbulence-averaged model during acceleration and decelerating phases. In addition, the LES model is able to capture the enhanced transport layer thickness for fine sand, which may be related to the burst events near the flow reversal. We further confirm that this phenomenon is absent for medium sand.
NASA Technical Reports Server (NTRS)
Senocak, I.; Ackerman, A. S.; Kirkpatrick, M. P.; Stevens, D. E.; Mansour, N. N.
2004-01-01
Large-eddy simulation (LES) is a widely used technique in armospheric modeling research. In LES, large, unsteady, three dimensional structures are resolved and small structures that are not resolved on the computational grid are modeled. A filtering operation is applied to distinguish between resolved and unresolved scales. We present two near-surface models that have found use in atmospheric modeling. We also suggest a simpler eddy viscosity model that adopts Prandtl's mixing length model (Prandtl 1925) in the vicinity of the surface and blends with the dynamic Smagotinsky model (Germano et al, 1991) away from the surface. We evaluate the performance of these surface models by simulating a neutraly stratified atmospheric boundary layer.
Finecasting for renewable energy with large-eddy simulation
NASA Astrophysics Data System (ADS)
Jonker, Harmen; Verzijlbergh, Remco
2016-04-01
We present results of a single, continuous Large-Eddy Simulation of actual weather conditions during the timespan of a full year, made possible through recent computational developments (Schalkwijk et al, MWR, 2015). The simulation is coupled to a regional weather model in order to provide an LES dataset that is representative of the daily weather of the year 2012 around Cabauw, the Netherlands. This location is chosen such that LES results can be compared with both the regional weather model and observations from the Cabauw observational supersite. The run was made possible by porting our Large-Eddy Simulation program to run completely on the GPU (Schalkwijk et al, BAMS, 2012). GPU adaptation allows us to reach much improved time-to-solution ratios (i.e. simulation speedup versus real time). As a result, one can perform runs with a much longer timespan than previously feasible. The dataset resulting from the LES run provides many avenues for further study. First, it can provide a more statistical approach to boundary-layer turbulence than the more common case-studies by simulating a diverse but representative set of situations, as well as the transition between situations. This has advantages in designing and evaluating parameterizations. In addition, we discuss the opportunities of high-resolution forecasts for the renewable energy sector, e.g. wind and solar energy production.
A Large-Eddy Simulation Study of Vertical Axis Wind Turbine Wakes in the Atmospheric Boundary Layer
NASA Astrophysics Data System (ADS)
Shamsoddin, Sina; Porté-Agel, Fernando
2017-04-01
In a future sustainable energy vision, in which diversified conversion of renewable energies is essential, vertical axis wind turbines (VAWTs) exhibit some potential as a reliable means of wind energy extraction alongside conventional horizontal axis wind turbines (HAWTs). Nevertheless, there is currently a relative shortage of scientific, academic and technical investigations of VAWTs as compared to HAWTs. Having this in mind, in this work, we aim to, for the first time, study the wake of a single VAWT placed in the atmospheric boundary layer using large-eddy simulation (LES). To do this, we use a previously-validated LES framework in which an actuator line model (ALM) is incorporated. First, for a typical three- and straight-bladed 1-MW VAWT design, the variation of the power coefficient with both the chord length of the blades and the tip-speed ratio is analyzed by performing 117 simulations using LES-ALM. The optimum combination of solidity (defined as Nc/R, where N is the number of blades, c is the chord length and R is the rotor radius) and tip-speed ratio is found to be 0.18 and 4.5, respectively. Subsequently, the wake of a VAWT with these optimum specifications is thoroughly examined by showing different relevant mean and turbulence wake flow statistics. It is found that for this case, the maximum velocity deficit at the equator height of the turbine occurs 2.7 rotor diameters downstream of the center of the turbine, and only after that point, the wake starts to recover. Moreover, it is observed that the maximum turbulence intensity (TI) at the equator height of the turbine occurs at a distance of about 3.8 rotor diameters downstream of the turbine. As we move towards the upper and lower edges of the turbine, the maximum TI (at a certain height) increases, and its location moves relatively closer to the turbine. Furthermore, whereas both TI and turbulent momentum flux fields show clear vertical asymmetries (with larger magnitudes at the upper wake edge
On integrating large eddy simulation and laboratory turbulent flow experiments.
Grinstein, Fernando F
2009-07-28
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.
Large-Eddy Simulation of Wind-Plant Aerodynamics: Preprint
Churchfield, M. J.; Lee, S.; Moriarty, P. J.; Martinez, L. A.; Leonardi, S.; Vijayakumar, G.; Brasseur, J. G.
2012-01-01
In this work, we present results of a large-eddy simulation of the 48 multi-megawatt turbines composing the Lillgrund wind plant. Turbulent inflow wind is created by performing an atmospheric boundary layer precursor simulation and turbines are modeled using a rotating, variable-speed actuator line representation. The motivation for this work is that few others have done wind plant large-eddy simulations with a substantial number of turbines, and the methods for carrying out the simulations are varied. We wish to draw upon the strengths of the existing simulations and our growing atmospheric large-eddy simulation capability to create a sound methodology for performing this type of simulation. We have used the OpenFOAM CFD toolbox to create our solver.
Stochastic Large Eddy Simulation of Geostrophic Turbulence
NASA Astrophysics Data System (ADS)
Nadiga, B.; Livescu, D.; McKay, C. Q.
2005-05-01
Results are presented of (fine-scale) eddy-resolving simulations of different instances of turbulent quasi-geostrophic ocean circulation. A stochastic model for the effects of neglected subgrid degrees-of-freedom in coarse-scale simulations is proposed and the results compared to the fine simulations results as well as with existing models. As a precursor to the introduction of the models, we also study various aspects of the nonlinear rectification of stochastic forcing in quasi-geostrophic models of ocean circulation.
NASA Astrophysics Data System (ADS)
Ooi, Seng-Keat
2005-11-01
Lock-exchange gravity current flows produced by the instantaneous release of a heavy fluid are investigated using 3-D well resolved Large Eddy Simulation simulations at Grashof numbers up to 8*10^9. It is found the 3-D simulations correctly predict a constant front velocity over the initial slumping phase and a front speed decrease proportional to t-1/3 (the time t is measured from the release) over the inviscid phase, in agreement with theory. The evolution of the current in the simulations is found to be similar to that observed experimentally by Hacker et al. (1996). The effect of the dynamic LES model on the solutions is discussed. The energy budget of the current is discussed and the contribution of the turbulent dissipation to the total dissipation is analyzed. The limitations of less expensive 2D simulations are discussed; in particular their failure to correctly predict the spatio-temporal distributions of the bed shear stresses which is important in determining the amount of sediment the gravity current can entrain in the case in advances of a loose bed.
NASA Astrophysics Data System (ADS)
Kumar, Vijayant
Large-eddy simulation (LES) studies of the atmospheric boundary layer (ABL) have historically modeled the daytime (convective), nighttime (stable) and dawn/dusk windy (neutral) regimes separately under the assumption of a quasi-steady ABL. The real-world ABL however, continuously transitions between the different stability regimes and development of an LES capable of simulating the entire diurnal evolution of the ABL is needed. We have developed an LES tool (The JHU-LES code) with the new-generation Lagrangian dynamic models capable of dynamic adjustment of the subgrid-scale stresses thereby, making it apt for LES over entire diurnal cycles of the ABL. Preliminary LES studies demonstrate that the JHU-LES code reproduces well-known features of the quasi-steady convective and stable boundary layers, such as the well-known spectral scalings for production and inertial subranges. LES of the entire 24-hour diurnal evolution of the atmospheric boundary layer is then performed and compared successfully to field observations (HATS dataset). Important features of the diurnal ABL such as entrainment-based growth of the CBL, development of the stable boundary layer and evolution of the nocturnal low-level jet are well reproduced. The advantages of using a local Obukhov length-scale to normalize the results are highlighted. To investigate the role of surface boundary conditions and geostrophic wind forcing, LES investigations of multi-day evolution of the ABL flow are then performed with several combinations of surface boundary conditions (imposed temperature and heat flux) and geostrophic forcing (constant, time-varying, time and height varying). The variable geostrophic forcing significantly improves the agreement of LES results with surface flux observations but shows poor agreement with daytime surface fluxes and, daytime and nighttime mean profiles. The LES setup using an imposed surface temperature almost always yields better results than cases where the heat flux is
NASA Astrophysics Data System (ADS)
Cheng, Wai Chi; Porté-Agel, Fernando
2014-05-01
Accurate prediction of atmospheric boundary layer (ABL) flow and its interaction with urban surfaces is critical for understanding the transport of momentum and scalars within and above cities. This, in turn, is essential for predicting the local climate and pollutant dispersion patterns in urban areas. Large-eddy simulation (LES) explicitly resolves the large-scale turbulent eddy motions and, therefore, can potentially provide improved understanding and prediction of flows inside and above urban canopies. This study focuses on the validation and the use of a recently-developed LES framework to simulate a turbulent boundary layer flow through idealized urban canopies represented by uniform arrays of cubes. The LES framework is first validated with wind tunnel experimental data. Good agreement between the simulation results and the experimental data are found in the vertical and spanwise profiles of mean velocities and velocity standard deviations at different streamwise locations. Next, the model is used to simulate ABL flows over surface transitions from a flat homogeneous terrain to aligned and staggered arrays of cubes with height h. For both configurations, five different frontal area densities (Λf), equal to 0.028, 0.063, 0.111, 0.174 and 0.250, are considered. Within the arrays, the flow is found to adjust quickly and shows similar structure of the wake of the cubes after the second row. Above the arrays, an internal boundary layer (IBL) is identified. No significant difference in the depth of the IBL among different cases is observed. The drag exerted by the cubes on the flow (Df) and the drag coefficients of the cubes (Cd) are calculated explicitly using the LES results. For the downstream cubes, Df is found to increases with decreasing density for both configurations, and larger values of Cd are found for the cubes of staggered arrays than those of the aligned arrays with the same Λf. At a downstream location where the flow immediately above the cube
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.
NASA Astrophysics Data System (ADS)
Margairaz, Fabien; Giometto, Marco; Parlange, Marc; Calaf, Marc
2015-11-01
The performance of dealiasing schemes and their computational cost on a pseudo-spectral code are analyzed. Dealiasing is required to limit the error that occurs when two discretized variables are multiplied, polluting the accuracy of the result. In this work three different dealiasing methods are explored: the 2/3 rule, the 3/2 rule, and a high order Fourier smoothing based method. We compare the cost of the traditionally accepted 3/2 rule (Canuto et al., 1988), where an expansion of the computational domain to a larger grid is required, to the cost of the other two techniques that do not require this expansion. This analysis is performed in the framework of Large-Eddy Simulations (LES) of incompressible flows using the constant Smagorinsky sub-grid model with a wall damping function and a wall model based on the log-law. A highly efficient LES code parallelized using a 2D pencil decomposition has been developed. The code employs the traditional pseudo-spectral approach to integrate the incompressible Navier-Stokes equations. Several simulations of a neutral atmospheric boundary layer using different degrees of numerical resolution are considered. Results show a net difference in computational cost between the different techniques without relevant changes in statistics.
Large-Eddy Simulation of Wind-Plant Aerodynamics
Churchfield, M. J.; Lee, S.; Moriarty, P. J.; Martinez, L. A.; Leonardi, S.; Vijayakumar, G.; Brasseur, J. G.
2012-01-01
In this work, we present results of a large-eddy simulation of the 48 multi-megawatt turbines composing the Lillgrund wind plant. Turbulent inflow wind is created by performing an atmospheric boundary layer precursor simulation, and turbines are modeled using a rotating, variable-speed actuator line representation. The motivation for this work is that few others have done large-eddy simulations of wind plants with a substantial number of turbines, and the methods for carrying out the simulations are varied. We wish to draw upon the strengths of the existing simulations and our growing atmospheric large-eddy simulation capability to create a sound methodology for performing this type of simulation. We used the OpenFOAM CFD toolbox to create our solver. The simulated time-averaged power production of the turbines in the plant agrees well with field observations, except with the sixth turbine and beyond in each wind-aligned. The power produced by each of those turbines is overpredicted by 25-40%. A direct comparison between simulated and field data is difficult because we simulate one wind direction with a speed and turbulence intensity characteristic of Lillgrund, but the field observations were taken over a year of varying conditions. The simulation shows the significant 60-70% decrease in the performance of the turbines behind the front row in this plant that has a spacing of 4.3 rotor diameters in this direction. The overall plant efficiency is well predicted. This work shows the importance of using local grid refinement to simultaneously capture the meter-scale details of the turbine wake and the kilometer-scale turbulent atmospheric structures. Although this work illustrates the power of large-eddy simulation in producing a time-accurate solution, it required about one million processor-hours, showing the significant cost of large-eddy simulation.
Large eddy simulation of a wing-body junction flow
NASA Astrophysics Data System (ADS)
Ryu, Sungmin; Emory, Michael; Campos, Alejandro; Duraisamy, Karthik; Iaccarino, Gianluca
2014-11-01
We present numerical simulations of the wing-body junction flow experimentally investigated by Devenport & Simpson (1990). Wall-junction flows are common in engineering applications but relevant flow physics close to the corner region is not well understood. Moreover, performance of turbulence models for the body-junction case is not well characterized. Motivated by the insufficient investigations, we have numerically investigated the case with Reynolds-averaged Naiver-Stokes equation (RANS) and Large Eddy Simulation (LES) approaches. The Vreman model applied for the LES and SST k- ω model for the RANS simulation are validated focusing on the ability to predict turbulence statistics near the junction region. Moreover, a sensitivity study of the form of the Vreman model will also be presented. This work is funded under NASA Cooperative Agreement NNX11AI41A (Technical Monitor Dr. Stephen Woodruff)
NASA Astrophysics Data System (ADS)
Kanani-Sühring, Farah; Raasch, Siegfried
2017-02-01
By means of large-eddy simulation, we investigate the transport of a passive scalar in the lee of forest patches under neutral atmospheric conditions in flat terrain. We found a pronounced local enhancement of scalar concentration and scalar flux in the lee zone of the forest, while further downstream above the unforested surface, the scalar transport adjusted to an equilibrium with the underlying surface conditions. By means of a term-by-term analysis of the scalar transport equation, we determined the local accumulation of the scalar to be caused by the convergence of: (1) mean and turbulent streamwise transport, (2) mean vertical transport. However, the relative importance of each transport mechanism for the accumulation process was found to depend strongly on forest density. Based on systematic parameter changes, we found concentrations to significantly increase with increasing forest density and with decreasing wind speed, while fluxes were invariant to wind speed and showed a similar relation to forest density as for the concentrations. Despite the scalar sources—ground and/or canopy sources—a local flux enhancement was present in the lee zone. Finally, we provide a first step towards localizing enhanced concentrations and fluxes at micrometeorological sites.
Large-eddy simulation using the finite element method
McCallen, R.C.; Gresho, P.M.; Leone, J.M. Jr.; Kollmann, W.
1993-10-01
In a large-eddy simulation (LES) of turbulent flows, the large-scale motion is calculated explicitly (i.e., approximated with semi-empirical relations). Typically, finite difference or spectral numerical schemes are used to generate an LES; the use of finite element methods (FEM) has been far less prominent. In this study, we demonstrate that FEM in combination with LES provides a viable tool for the study of turbulent, separating channel flows, specifically the flow over a two-dimensional backward-facing step. The combination of these methodologies brings together the advantages of each: LES provides a high degree of accuracy with a minimum of empiricism for turbulence modeling and FEM provides a robust way to simulate flow in very complex domains of practical interest. Such a combination should prove very valuable to the engineering community.
Large-eddy simulation of turbulent circular jet flows
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.
Large-eddy simulation of a plane wake
NASA Technical Reports Server (NTRS)
Ghosal, Sandip; Rogers, M. M.
1994-01-01
Previously the theoretical development leading to the dynamic localization model (DLM) for large-eddy simulation (LES) was presented. The method has been successfully applied to isotropic turbulence, channel flow, and the flow over a backward-facing step. Here we apply the model to the computation of the temporally developing place wake. The two main objectives of this project are: (1) Use the model to perform an LES of a time developing plane wake and compare the results with direction numerical simulation (DNS) data to see if important statistical measures can be readily predicted, and to provide a relative evaluation of the several versions of the model in terms of predictive capability and cost; and (2) If the tests in (1) show that the model generates reliable predictions, then use the LES to study various aspects of the physics of turbulent wakes and mixing layers.
Large Eddy Simulation of Vertical Axis Wind Turbine Wakes
NASA Astrophysics Data System (ADS)
Shamsoddin, Sina; Porté-Agel, Fernando
2014-05-01
In this study, large-eddy simulation (LES) is combined with a turbine model to investigate the wake behind a vertical-axis wind turbine (VAWT) in a three dimensional turbulent flow. Two methods are used to model the subgrid-scale (SGS) stresses: (a) the Smagorinsky model, and (b) the modulated gradient model. To parameterize the effects of the VAWT on the flow, two VAWT models are developed: (a) the actuator surface model (ASM), in which the time-averaged turbine-induced forces are distributed on a surface swept by the turbine blades, i.e. the actuator surface, and (b) the actuator line model (ALM), in which the instantaneous blade forces are only spatially distributed on lines representing the blades, i.e. the actuator lines. This is the first time that LES is applied and validated for simulation of VAWT wakes by using either the ASM or the ALM techniques. In both models, blade-element theory is used to calculate the lift and drag forces on the blades. The results are compared with flow measurements in the wake of a model straight-bladed VAWT, carried out in the Institute de Méchanique et Statistique de la Turbulence (IMST) water channel. Different combinations of SGS models with VAWT models are studied and a fairly good overall agreement between simulation results and measurement data is observed. In general, the ALM is found to better capture the unsteady-periodic nature of the wake and shows a better agreement with the experimental data compared with the ASM. The modulated gradient model is also found to be a more reliable SGS stress modeling technique, compared with the Smagorinsky model, and it yields reasonable predictions of the mean flow and turbulence characteristics of a VAWT wake using its theoretically-determined model coefficient. Keywords: Vertical-axis wind turbines (VAWTs); VAWT wake; Large-eddy simulation; Actuator surface model; Actuator line model; Smagorinsky model; Modulated gradient model
NASA's Large-Eddy Simulation Research for Jet Noise Applications
NASA Technical Reports Server (NTRS)
DeBonis, James R.
2009-01-01
Research into large-eddy simulation (LES) for application to jet noise is described. The LES efforts include in-house code development and application at NASA Glenn along with NASA Research Announcement sponsored work at Stanford University and Florida State University. Details of the computational methods used and sample results for jet flows are provided.
Mind the gap: a guideline for large eddy simulation.
George, William K; Tutkun, Murat
2009-07-28
This paper briefly reviews some of the fundamental ideas of turbulence as they relate to large eddy simulation (LES). Of special interest is how our thinking about the so-called 'spectral gap' has evolved over the past decade, and what this evolution implies for LES applications.
Large Eddy Simulation of Supersonic Inlet Flows
1998-04-01
SIMULATION OF SUPERSONIC INLET FLOWS 6. AUTHOR(S) PROF. PARVIZ MOIN PROF. SANJIVA K. LELE 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) STANFORD... Parviz Moin and Sanjiva K. Lele Stanford University Mechanical Engineering, Flow Physics & Computation Division Stanford, CA 94305-3030 Prepared...monitor. I am thankful to Professor Sanjiva Lele and Profes- sor Parviz Moin, and Keith Lucas for useful discussions! I am grateful to Professor Peter
NASA Astrophysics Data System (ADS)
Stap, F. A.; Hasekamp, O. P.; Emde, C.; Röckmann, T.
2016-11-01
We investigate the effect of cloud contamination and 3-D radiative transfer effects on aerosol retrievals from multiangle photopolarimetric measurements in the vicinity of clouds. To this end multiangle, multiwavelength photopolarimetric observations are simulated using a 3-D radiative transfer model for scenes with realistic cloud properties, based on a large eddy simulation. Spatial resolutions of 2 × 2, 4 × 4, and 6 × 6 km2 have been considered. It is found that a goodness-of-fit criterion efficiently filters out cloud contamination. However, it does not filter out all scenes that are affected by 3-D radiative effects, resulting in small biases in the retrieved aerosol optical thickness (AOT) and single-scattering albedo (SSA). We also found that measurements at higher spatial resolution (2 × 2 km2) do not result in retrievals closer to clouds compared to measurements at coarser spatial resolutions (4 × 4 and 6 × 6 km2). If cloud parameters are fitted simultaneously with aerosol parameters using a 1-D radiative transfer model and the Independent Pixel Approximation, more successful retrievals are obtained in partially cloudy scenes and in the vicinity of clouds. This effect is most apparent at 6 × 6 km2 and only marginal at 2 × 2 km2 resolution. The retrieved aerosol AOT and SSA from the simultaneous aerosol and cloud retrievals still have a small bias, like the aerosol-only retrievals. We conclude that in order to substantially improve aerosol retrievals in the vicinity of clouds, a retrieval algorithm is needed that takes into account 3-D radiative transfer effects.
Large-eddy simulation in complex domains using the finite element method
McCallen, R.C.; Kornblum, B.T.; Kollman, W.
1996-11-12
Finite element methods (FEM) are demonstrated in combination with large-eddy simulations (LES) as a valuable tool for the study of turbulent, separating channel flows, specifically the flow over a backward facing step.
NASA Astrophysics Data System (ADS)
Dipankar, A.; Stevens, B. B.; Zängl, G.; Pondkule, M.; Brdar, S.
2014-12-01
The effect of clouds on large scale dynamics is represented in climate models through parameterization of various processes, of which the parameterization of shallow and deep convection are particularly uncertain. The atmospheric boundary layer, which controls the coupling to the surface, and which defines the scale of shallow convection, is typically 1 km in depth. Thus, simulations on a O(100 m) grid largely obviate the need for such parameterizations. By crossing this threshold of O(100m) grid resolution one can begin thinking of large-eddy simulation (LES), wherein the sub-grid scale parameterization have a sounder theoretical foundation. Substantial initiatives have been taken internationally to approach this threshold. For example, Miura et al., 2007 and Mirakawa et al., 2014 approach this threshold by doing global simulations, with (gradually) decreasing grid resolution, to understand the effect of cloud-resolving scales on the general circulation. Our strategy, on the other hand, is to take a big leap forward by fixing the resolution at O(100 m), and gradually increasing the domain size. We believe that breaking this threshold would greatly help in improving the parameterization schemes and reducing the uncertainty in climate predictions. To take this forward, the German Federal Ministry of Education and Research has initiated a project on HD(CP)2 that aims for a limited area LES at resolution O(100 m) using the new unified modeling system ICON (Zängl et al., 2014). In the talk, results from the HD(CP)2 evaluation simulation will be shown that targets high resolution simulation over a small domain around Jülich, Germany. This site is chosen because high resolution HD(CP)2 Observational Prototype Experiment took place in this region from 1.04.2013 to 31.05.2013, in order to critically evaluate the model. Nesting capabilities of ICON is used to gradually increase the resolution from the outermost domain, which is forced from the COSMO-DE data, to the
Large Eddy Simulation of Engineering Flows: A Bill Reynolds Legacy.
NASA Astrophysics Data System (ADS)
Moin, Parviz
2004-11-01
The term, Large eddy simulation, LES, was coined by Bill Reynolds, thirty years ago when he and his colleagues pioneered the introduction of LES in the engineering community. Bill's legacy in LES features his insistence on having a proper mathematical definition of the large scale field independent of the numerical method used, and his vision for using numerical simulation output as data for research in turbulence physics and modeling, just as one would think of using experimental data. However, as an engineer, Bill was pre-dominantly interested in the predictive capability of computational fluid dynamics and in particular LES. In this talk I will present the state of the art in large eddy simulation of complex engineering flows. Most of this technology has been developed in the Department of Energy's ASCI Program at Stanford which was led by Bill in the last years of his distinguished career. At the core of this technology is a fully implicit non-dissipative LES code which uses unstructured grids with arbitrary elements. A hybrid Eulerian/ Largangian approach is used for multi-phase flows, and chemical reactions are introduced through dynamic equations for mixture fraction and reaction progress variable in conjunction with flamelet tables. The predictive capability of LES is demonstrated in several validation studies in flows with complex physics and complex geometry including flow in the combustor of a modern aircraft engine. LES in such a complex application is only possible through efficient utilization of modern parallel super-computers which was recognized and emphasized by Bill from the beginning. The presentation will include a brief mention of computer science efforts for efficient implementation of LES.
Large Eddy Simulation of Homogeneous Rotating Turbulence
NASA Technical Reports Server (NTRS)
Squires, Kyle D.; Mansour, Nagi N.; Cambon, Claude; Chasnov, Jeffrey R.; Kutler, Paul (Technical Monitor)
1994-01-01
Study of turbulent flows in rotating reference frames has proven to be one of the more challenging areas of turbulence research. The large number of theoretical, experimental, and computational studies performed over the years have demonstrated that the effect of solid-body rotation on turbulent flows is subtle and remains exceedingly difficult to predict. Because of the complexities associated with non-homogeneous turbulence, it is worthwhile to examine the effect of steady system rotation on the evolution of an initially isotropic turbulent flow. The assumption of statistical homogeneity considerably simplifies analysis and computation; calculation of homogeneous turbulence is further motivated since it possesses the essential physics found in more complex rotating flows. The principal objectives of the present study have therefore been to increase our fundamental understanding of turbulent flows in rotating reference frames through an examination of the asymptotic state of homogeneous rotating turbulence; particularly as to the existence of an asymptotic state which is self similar. Knowledge of an asymptotic similarity state permits prediction of the ultimate statistical evolution of the flow without requiring detailed knowledge of the complex, and not well understood, non-linear transfer processes. Aside from examination of possible similarity states in rotating turbulence, of further interest in this study has been an examination of the degree to which solid-body rotation induces a two-dimensional state in an initially isotropic flow.
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
Large-Eddy Simulations of Flows in Complex Terrain
NASA Astrophysics Data System (ADS)
Kosovic, B.; Lundquist, K. A.
2011-12-01
Large-eddy simulation as a methodology for numerical simulation of turbulent flows was first developed to study turbulent flows in atmospheric by Lilly (1967). The first LES were carried by Deardorff (1970) who used these simulations to study atmospheric boundary layers. Ever since, LES has been extensively used to study canonical atmospheric boundary layers, in most cases flat plate boundary layers under the assumption of horizontal homogeneity. Carefully designed LES of canonical convective and neutrally stratified and more recently stably stratified atmospheric boundary layers have contributed significantly to development of better understanding of these flows and their parameterizations in large scale models. These simulations were often carried out using codes specifically designed and developed for large-eddy simulations of horizontally homogeneous flows with periodic lateral boundary conditions. Recent developments in multi-scale numerical simulations of atmospheric flows enable numerical weather prediction (NWP) codes such as ARPS (Chow and Street, 2009), COAMPS (Golaz et al., 2009) and Weather Research and Forecasting model, to be used nearly seamlessly across a wide range of atmospheric scales from synoptic down to turbulent scales in atmospheric boundary layers. Before we can with confidence carry out multi-scale simulations of atmospheric flows, NWP codes must be validated for accurate performance in simulating flows over complex or inhomogeneous terrain. We therefore carry out validation of WRF-LES for simulations of flows over complex terrain using data from Askervein Hill (Taylor and Teunissen, 1985, 1987) and METCRAX (Whiteman et al., 2008) field experiments. WRF's nesting capability is employed with a one-way nested inner domain that includes complex terrain representation while the coarser outer nest is used to spin up fully developed atmospheric boundary layer turbulence and thus represent accurately inflow to the inner domain. LES of a
Constrained Large Eddy Simulation of Separated Turbulent Flows
NASA Astrophysics Data System (ADS)
Xia, Zhenhua; Shi, Yipeng; Wang, Jianchun; Xiao, Zuoli; Yang, Yantao; Chen, Shiyi
2011-11-01
Constrained Large-eddy Simulation (CLES) has been recently proposed to simulate turbulent flows with massive separation. Different from traditional large eddy simulation (LES) and hybrid RANS/LES approaches, the CLES simulates the whole flow domain by large eddy simulation while enforcing a RANS Reynolds stress constraint on the subgrid-scale (SGS) stress models in the near-wall region. Algebraic eddy-viscosity models and one-equation Spalart-Allmaras (S-A) model have been used to constrain the Reynolds stress. The CLES approach is validated a posteriori through simulation of flow past a circular cylinder and periodic hill flow at high Reynolds numbers. The simulation results are compared with those from RANS, DES, DDES and other available hybrid RANS/LES methods. It is shown that the capability of the CLES method in predicting separated flows is comparable to that of DES. Detailed discussions are also presented about the effects of the RANS models as constraint in the near-wall layers. Our results demonstrate that the CLES method is a promising alternative towards engineering applications.
Large-Eddy Simulation of turbine wake in complex terrain
NASA Astrophysics Data System (ADS)
Berg, J.; Troldborg, N.; Sørensen, N. N.; Patton, E. G.; Sullivan, P. P.
2017-05-01
We present Large-Eddy Simulation results of a turbine wake in realistic complex terrain with slopes above 0.5. By comparing simulations including and without the wind turbine we can estimate the induction factor, a, and we show how the presence of a strong recirculation zone in the terrain dictates the positioning of the wake. This last finding is in contrast to what would happen in gentle terrain with no substantial increase of turbulent kinetic energy in the terrain induced wakes.
Toward the large-eddy simulations of compressible turbulent flows
NASA Technical Reports Server (NTRS)
Erlebacher, G.; Hussaini, M. Y.; Speziale, C. G.; Zang, T. A.
1987-01-01
New subgrid-scale models for the large-eddy simulation of compressible turbulent flows are developed based on the Favre-filtered equations of motion for an ideal gas. A compressible generalization of the linear combination of the Smagorinsky model and scale-similarity model (in terms of Favre-filtered fields) is obtained for the subgrid-scale stress tensor. An analogous thermal linear combination model is also developed for the subgrid-scale heat flux vector. The three dimensionless constants associated with these subgrid-scale models are obtained by correlating with the results of direct numerical simulations of compressible isotropic turbulence performed on a 96 to the third power grid using Fourier collocation methods. Extensive comparisons between the direct and modeled subgrid-scale fields are provided in order to validate the models. Future applications of these compressible subgrid-scale models to the large-eddy simulation of supersonic aerodynamic flows are discussed briefly.
Toward the large-eddy simulation of compressible turbulent flows
NASA Technical Reports Server (NTRS)
Erlebacher, G.; Hussaini, M. Y.; Speziale, C. G.; Zang, T. A.
1990-01-01
New subgrid-scale models for the large-eddy simulation of compressible turbulent flows are developed and tested based on the Favre-filtered equations of motion for an ideal gas. A compressible generalization of the linear combination of the Smagorinsky model and scale-similarity model, in terms of Favre-filtered fields, is obtained for the subgrid-scale stress tensor. An analogous thermal linear combination model is also developed for the subgrid-scale heat flux vector. The two dimensionless constants associated with these subgrid-scale models are obtained by correlating with the results of direct numerical simulations of compressible isotropic turbulence performed on a 96(exp 3) grid using Fourier collocation methods. Extensive comparisons between the direct and modeled subgrid-scale fields are provided in order to validate the models. A large-eddy simulation of the decay of compressible isotropic turbulence (conducted on a coarse 32(exp 3) grid) is shown to yield results that are in excellent agreement with the fine grid direct simulation. Future applications of these compressible subgrid-scale models to the large-eddy simulation of more complex supersonic flows are discussed briefly.
Large eddy simulation of the atmosphere on various scales.
Cullen, M J P; Brown, A R
2009-07-28
Numerical simulations of the atmosphere are routinely carried out on various scales for purposes ranging from weather forecasts for local areas a few hours ahead to forecasts of climate change over periods of hundreds of years. Almost without exception, these forecasts are made with space/time-averaged versions of the governing Navier-Stokes equations and laws of thermodynamics, together with additional terms representing internal and boundary forcing. The calculations are a form of large eddy modelling, because the subgrid-scale processes have to be modelled. In the global atmospheric models used for long-term predictions, the primary method is implicit large eddy modelling, using discretization to perform the averaging, supplemented by specialized subgrid models, where there is organized small-scale activity, such as in the lower boundary layer and near active convection. Smaller scale models used for local or short-range forecasts can use a much smaller averaging scale. This allows some of the specialized subgrid models to be dropped in favour of direct simulations. In research mode, the same models can be run as a conventional large eddy simulation only a few orders of magnitude away from a direct simulation. These simulations can then be used in the development of the subgrid models for coarser resolution models.
Toward large eddy simulation of turbulent flow over an airfoil
NASA Technical Reports Server (NTRS)
Choi, Haecheon
1993-01-01
The flow field over an airfoil contains several distinct flow characteristics, e.g. laminar, transitional, turbulent boundary layer flow, flow separation, unstable free shear layers, and a wake. This diversity of flow regimes taxes the presently available Reynolds averaged turbulence models. Such models are generally tuned to predict a particular flow regime, and adjustments are necessary for the prediction of a different flow regime. Similar difficulties are likely to emerge when the large eddy simulation technique is applied with the widely used Smagorinsky model. This model has not been successful in correctly representing different turbulent flow fields with a single universal constant and has an incorrect near-wall behavior. Germano et al. (1991) and Ghosal, Lund & Moin have developed a new subgrid-scale model, the dynamic model, which is very promising in alleviating many of the persistent inadequacies of the Smagorinsky model: the model coefficient is computed dynamically as the calculation progresses rather than input a priori. The model has been remarkably successful in prediction of several turbulent and transitional flows. We plan to simulate turbulent flow over a '2D' airfoil using the large eddy simulation technique. Our primary objective is to assess the performance of the newly developed dynamic subgrid-scale model for computation of complex flows about aircraft components and to compare the results with those obtained using the Reynolds average approach and experiments. The present computation represents the first application of large eddy simulation to a flow of aeronautical interest and a key demonstration of the capabilities of the large eddy simulation technique.
Large eddy simulation of unsteady lean stratified premixed combustion
Duwig, C.; Fureby, C.
2007-10-15
Premixed turbulent flame-based technologies are rapidly growing in importance, with applications to modern clean combustion devices for both power generation and aeropropulsion. However, the gain in decreasing harmful emissions might be canceled by rising combustion instabilities. Unwanted unsteady flame phenomena that might even destroy the whole device have been widely reported and are subject to intensive studies. In the present paper, we use unsteady numerical tools for simulating an unsteady and well-documented flame. Computations were performed for nonreacting, perfectly premixed and stratified premixed cases using two different numerical codes and different large-eddy-simulation-based flamelet models. Nonreacting simulations are shown to agree well with experimental data, with the LES results capturing the mean features (symmetry breaking) as well as the fluctuation level of the turbulent flow. For reacting cases, the uncertainty induced by the time-averaging technique limited the comparisons. Given an estimate of the uncertainty, the numerical results were found to reproduce well the experimental data in terms both of mean flow field and of fluctuation levels. In addition, it was found that despite relying on different assumptions/simplifications, both numerical tools lead to similar predictions, giving confidence in the results. Moreover, we studied the flame dynamics and particularly the response to a periodic pulsation. We found that above a certain excitation level, the flame dynamic changes and becomes rather insensitive to the excitation/instability amplitude. Conclusions regarding the self-growth of thermoacoustic waves were drawn. (author)
Large eddy simulation of mechanical mixing in anaerobic digesters.
Wu, Binxin
2012-03-01
A comprehensive study of anaerobic digestion requires an advanced turbulence model technique to accurately predict mixing flow patterns because the digestion process that involves mass transfer between anaerobes and their substrates is primarily dependent on detailed information about the fine structure of turbulence in the digesters. This study presents a large eddy simulation (LES) of mechanical agitation of non-Newtonian fluids in anaerobic digesters, in which the sliding mesh method is used to characterize the impeller rotation. The three subgrid scale (SGS) models investigated are: (i) Smagorinsky-Lilly model, (ii) wall-adapting local eddy-viscosity model, and (iii) kinetic energy transport (KET) model. The simulation results show that the three SGS models produce very similar flow fields. A comparison of the simulated and measured axial velocities indicates that the LES profile shapes are in general agreement with the experimental data but they differ markedly in velocity magnitudes. A check of impeller power and flow numbers demonstrates that all the SGS models give excellent predictions, with the KET model performing the best. Moreover, the performance of six Reynolds-averaged Navier-Stokes turbulence models are assessed and compared with the LES results.
Large eddy simulation of a pumped- storage reservoir
NASA Astrophysics Data System (ADS)
Launay, Marina; Leite Ribeiro, Marcelo; Roman, Federico; Armenio, Vincenzo
2016-04-01
The last decades have seen an increasing number of pumped-storage hydropower projects all over the world. Pumped-storage schemes move water between two reservoirs located at different elevations to store energy and to generate electricity following the electricity demand. Thus the reservoirs can be subject to important water level variations occurring at the daily scale. These new cycles leads to changes in the hydraulic behaviour of the reservoirs. Sediment dynamics and sediment budgets are modified, sometimes inducing problems of erosion and deposition within the reservoirs. With the development of computer performances, the use of numerical techniques has become popular for the study of environmental processes. Among numerical techniques, Large Eddy Simulation (LES) has arisen as an alternative tool for problems characterized by complex physics and geometries. This work uses the LES-COAST Code, a LES model under development in the framework of the Seditrans Project, for the simulation of an Upper Alpine Reservoir of a pumped-storage scheme. Simulations consider the filling (pump mode) and emptying (turbine mode) of the reservoir. The hydraulic results give a better understanding of the processes occurring within the reservoir. They are considered for an assessment of the sediment transport processes and of their consequences.
Large eddy simulations of a turbulent thermal plume
NASA Astrophysics Data System (ADS)
Yan, Zhenghua H.
2007-04-01
Large eddy simulations of a three-dimensional turbulent thermal plume in an open environment have been carried out using a self-developed parallel computational fluid dynamics code SMAFS (smoke movement and flame spread) to study the thermal plume’s dynamics including its puffing, self-preserving and air entrainment. In the simulation, the sub-grid stress was modeled using both the standard Smagorinsky and the buoyancy modified Smagorinsky models, which were compared. The sub-grid scale (SGS) scalar flux in the filtered enthalpy transport equation was modeled based on a simple gradient transport hypothesis with constant SGS Prandtl number. The effect of the Smagorinsky model constant and the SGS Prandtl number were examined. The computation results were compared with experimental measurements, thermal plume theory and empirical correlations, showing good agreement. It is found that both the buoyancy modification and the SGS turbulent Prandtl number have little influence on simulation. However, the SGS model constant C s has a significant effect on the prediction of plume spreading, although it does not affect much the prediction of puffing.
Large-Eddy simulation of pulsatile blood flow.
Paul, Manosh C; Mamun Molla, Md; Roditi, Giles
2009-01-01
Large-Eddy simulation (LES) is performed to study pulsatile blood flow through a 3D model of arterial stenosis. The model is chosen as a simple channel with a biological type stenosis formed on the top wall. A sinusoidal non-additive type pulsation is assumed at the inlet of the model to generate time dependent oscillating flow in the channel and the Reynolds number of 1200, based on the channel height and the bulk velocity, is chosen in the simulations. We investigate in detail the transition-to-turbulent phenomena of the non-additive pulsatile blood flow downstream of the stenosis. Results show that the high level of flow recirculation associated with complex patterns of transient blood flow have a significant contribution to the generation of the turbulent fluctuations found in the post-stenosis region. The importance of using LES in modelling pulsatile blood flow is also assessed in the paper through the prediction of its sub-grid scale contributions. In addition, some important results of the flow physics are achieved from the simulations, these are presented in the paper in terms of blood flow velocity, pressure distribution, vortices, shear stress, turbulent fluctuations and energy spectra, along with their importance to the relevant medical pathophysiology.
NASA Astrophysics Data System (ADS)
Guda, Venkata Subba Sai Satish
There have been several advancements in the aerospace industry in areas of design such as aerodynamics, designs, controls and propulsion; all aimed at one common goal i.e. increasing efficiency --range and scope of operation with lesser fuel consumption. Several methods of flow control have been tried. Some were successful, some failed and many were termed as impractical. The low Reynolds number regime of 104 - 105 is a very interesting range. Flow physics in this range are quite different than those of higher Reynolds number range. Mid and high altitude UAV's, MAV's, sailplanes, jet engine fan blades, inboard helicopter rotor blades and wind turbine rotors are some of the aerodynamic applications that fall in this range. The current study deals with using dynamic roughness as a means of flow control over a NACA 0012 airfoil at low Reynolds numbers. Dynamic 3-D surface roughness elements on an airfoil placed near the leading edge aim at increasing the efficiency by suppressing the effects of leading edge separation like leading edge stall by delaying or totally eliminating flow separation. A numerical study of the above method has been carried out by means of a Large Eddy Simulation, a mathematical model for turbulence in Computational Fluid Dynamics, owing to the highly unsteady nature of the flow. A user defined function has been developed for the 3-D dynamic roughness element motion. Results from simulations have been compared to those from experimental PIV data. Large eddy simulations have relatively well captured the leading edge stall. For the clean cases, i.e. with the DR not actuated, the LES was able to reproduce experimental results in a reasonable fashion. However DR simulation results show that it fails to reattach the flow and suppress flow separation compared to experiments. Several novel techniques of grid design and hump creation are introduced through this study.
Model consistency in large eddy simulation of turbulent channel flows
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Ferziger, Joel H.; Moin, Parviz
1988-01-01
Combinations of filters and subgrid scale stress models for large eddy simulation of the Navier-Stokes equations are examined by a priori tests and numerical simulations. The structure of the subgrid scales is found to depend strongly on the type of filter used, and consistency between model and filter is essential to ensure accurate results. The implementation of consistent combinations of filter and model gives more accurate turbulence statistics than those obtained in previous investigations in which the models were chosen independently from the filter. Results and limitations of the a priori test are discussed. The effect of grid refinement is also examined.
Large-eddy simulation of turbulent sheared convection
NASA Astrophysics Data System (ADS)
Sykes, R. I.; Henn, D. S.
1989-04-01
A series of large-eddy simulations of free and sheared convective flow between moving flat plates is presented. Results for free convection are compared with laboratory data. The ratio of friction velocity to the convective velocity scale is identified as an important parameter in sheared convective flow, determining the formation of longitudinal rolls. Rolls are found for ratios greater than 0.35, with aspect ratio decreasing as this parameter increases. It is shown that, in this regime, two-dimensional simulations with a proper choice of roll orientation and turbulence length-scale can produce correct velocity variances and roll aspect ratio.
Laminar flow transition: A large-eddy simulation approach
NASA Technical Reports Server (NTRS)
Biringen, S.
1982-01-01
A vectorized, semi-implicit code was developed for the solution of the time-dependent, three dimensional equations of motion in plane Poiseuille flow by the large-eddy simulation technique. The code is tested by comparing results with those obtained from the solutions of the Orr-Sommerfeld equation. Comparisons indicate that finite-differences employed along the cross-stream direction act as an implicit filter. This removes the necessity of explicit filtering along this direction (where a nonhomogeneous mesh is used) for the simulation of laminar flow transition into turbulence in which small scale turbulence will be accounted for by a subgrid scale turbulence model.
Large-eddy simulation of trans- and supercritical injection
NASA Astrophysics Data System (ADS)
Müller, H.; Niedermeier, C. A.; Jarczyk, M.; Pfitzner, M.; Hickel, S.; Adams, N. A.
2016-07-01
In a joint effort to develop a robust numerical tool for the simulation of injection, mixing, and combustion in liquid rocket engines at high pressure, a real-gas thermodynamics model has been implemented into two computational fluid dynamics (CFD) codes, the density-based INCA and a pressure-based version of OpenFOAM. As a part of the validation process, both codes have been used to perform large-eddy simulations (LES) of trans- and supercritical nitrogen injection. Despite the different code architecture and the different subgrid scale turbulence modeling strategy, both codes yield similar results. The agreement with the available experimental data is good.
Model consistency in large eddy simulation of turbulent channel flows
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Ferziger, Joel H.; Moin, Parviz
1988-01-01
Combinations of filters and subgrid scale stress models for large eddy simulation of the Navier-Stokes equations are examined by a priori tests and numerical simulations. The structure of the subgrid scales is found to depend strongly on the type of filter used, and consistency between model and filter is essential to ensure accurate results. The implementation of consistent combinations of filter and model gives more accurate turbulence statistics than those obtained in previous investigations in which the models were chosen independently from the filter. Results and limitations of the a priori test are discussed. The effect of grid refinement is also examined.
Applications of large eddy simulation methods to gyrokinetic turbulence
Bañón Navarro, A. Happel, T.; Teaca, B. [Applied Mathematics Research Centre, Coventry University, Coventry CV1 5FB; Max-Planck für Sonnensystemforschung, Max-Planck-Str. 2, D-37191 Katlenburg-Lindau; Max-Planck Jenko, F. [Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-85748 Garching; Max-Planck Hammett, G. W. [Max-Planck Collaboration: ASDEX Upgrade Team
2014-03-15
The large eddy simulation (LES) approach—solving numerically the large scales of a turbulent system and accounting for the small-scale influence through a model—is applied to nonlinear gyrokinetic systems that are driven by a number of different microinstabilities. Comparisons between modeled, lower resolution, and higher resolution simulations are performed for an experimental measurable quantity, the electron density fluctuation spectrum. Moreover, the validation and applicability of LES is demonstrated through a series of diagnostics based on the free energetics of the system.
Large eddy simulations of blood dynamics in abdominal aortic aneurysms.
Vergara, Christian; Le Van, Davide; Quadrio, Maurizio; Formaggia, Luca; Domanin, Maurizio
2017-09-01
We study the effects of transition to turbulence in abdominal aortic aneurysms (AAA). The presence of transitional effects in such districts is related to the heart pulsatility and the sudden change of diameter of the vessels, and has been recorded by means of clinical measures as well as of computational studies. Here we propose, for the first time, the use of a large eddy simulation (LES) model to accurately describe transition to turbulence in realistic scenarios of AAA obtained from radiological images. To this aim, we post-process the obtained numerical solutions to assess significant quantities, such as the ensemble-averaged velocity and wall shear stress, the standard deviation of the fluctuating velocity field, and vortical structures educed via the so-called Q-criterion. The results demonstrate the suitability of the considered LES model and show the presence of significant transitional effects around the impingement region during the mid-deceleration phase. Copyright © 2017 IPEM. Published by Elsevier Ltd. All rights reserved.
On the Computation of Sound by Large-Eddy Simulations
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Streett, Craig L.; Sarkar, Sutanu
1997-01-01
The effect of the small scales on the source term in Lighthill's acoustic analogy is investigated, with the objective of determining the accuracy of large-eddy simulations when applied to studies of flow-generated sound. The distribution of the turbulent quadrupole is predicted accurately, if models that take into account the trace of the SGS stresses are used. Its spatial distribution is also correct, indicating that the low-wave-number (or frequency) part of the sound spectrum can be predicted well by LES. Filtering, however, removes the small-scale fluctuations that contribute significantly to the higher derivatives in space and time of Lighthill's stress tensor T(sub ij). The rms fluctuations of the filtered derivatives are substantially lower than those of the unfiltered quantities. The small scales, however, are not strongly correlated, and are not expected to contribute significantly to the far-field sound; separate modeling of the subgrid-scale density fluctuations might, however, be required in some configurations.
Final Report: "Large-Eddy Simulation of Anisotropic MHD Turbulence"
Zikanov, Oleg
2008-06-23
To acquire better understanding of turbulence in flows of liquid metals and other electrically conducting fluids in the presence of steady magnetic fields and to develop an accurate and physically adequate LES (large-eddy simulation) model for such flows. The scientific objectives formulated in the project proposal have been fully completed. Several new directions were initiated and advanced in the course of work. Particular achievements include a detailed study of transformation of turbulence caused by the imposed magnetic field, development of an LES model that accurately reproduces this transformation, and solution of several fundamental questions of the interaction between the magnetic field and fluid flows. Eight papers have been published in respected peer-reviewed journals, with two more papers currently undergoing review, and one in preparation for submission. A post-doctoral researcher and a graduate student have been trained in the areas of MHD, turbulence research, and computational methods. Close collaboration ties have been established with the MHD research centers in Germany and Belgium.
Large eddy simulations of in-cylinder turbulent flows.
NASA Astrophysics Data System (ADS)
Banaeizadeh, Araz; Afshari, Asghar; Schock, Harold; Jaberi, Farhad
2007-11-01
A high-order numerical model is developed and tested for large eddy simulation (LES) of turbulent flows in internal combustion (IC) engines. In this model, the filtered compressible Navier-Stokes equations in curvilinear coordinate systems are solved via a generalized high-order multi-block compact differencing scheme. The LES model has been applied to three flow configurations: (1) a fixed poppet valve in a sudden expansion, (2) a simple piston-cylinder assembly with a stationary open valve and harmonically moving flat piston, (3) a laboratory single-cylinder engine with three moving intake and exhaust valves. The first flow configuration is considered for studying the flow around the valves in IC engines. The second flow configuration is closer to that in IC engines but is based on a single stationary intake/exhaust valve and relatively simple geometry. It is considered in this work for better understating of the effects of moving piston on the large-scale unsteady vortical fluid motions in the cylinder and for further validation of our LES model. The third flow configuration includes all the complexities involve in a realistic single-cylinder IC engine. The predicted flow statistics by LES show good comparison with the available experimental data.
Unphysical scalar excursions in large-eddy simulations
NASA Astrophysics Data System (ADS)
Matheou, Georgios; Dimotakis, Paul
2016-11-01
The range of physically realizable values of passive scalar fields in any flow is bounded by their boundary values. The current investigation focuses on the local conservation of passive scalar concentration fields in turbulent flows and the ability of the large-eddy simulation (LES) method to observe the boundedness of passive scalar concentrations. In practice, as a result of numerical artifacts, this fundamental constraint is often violated with scalars exhibiting unphysical excursions. The present study characterizes passive-scalar excursions in LES of a turbulent shear flow and examines methods for error diagnosis. Typically, scalar-excursion errors are diagnosed as violations of global boundedness, i.e., detecting scalar-concentration values outside boundary/initial condition bounds. To quantify errors in mixed-fluid regions, a local scalar excursion error metric is defined with respect to the local non-diffusive limit. Analysis of such errors shows that unphysical scalar excursions in LES result from dispersive errors of the convection-term discretization where the subgrid-scale model (SGS) provides insufficient dissipation to produce a sufficiently smooth scalar field. Local scalar excursion errors are found not to be correlated with the local scalar-gradient magnitude. This work is supported by AFOSR, DOE, and Caltech.
NASA Astrophysics Data System (ADS)
Huang, G.; Newchurch, M.; Kuang, S.; Wang, L.; Ouwersloot, H.
2014-12-01
We investigate the diurnal variation of mixed-layer ozone in Huntsville AL, Southeast United States on September, 6, 2013 during the SEAC4RS field campaign. The dynamics and chemistry of the mixed layer are studied with a Large-Eddy Simulation model coupled with a chemical module and Ozone DIAL observations. In this study, we will present calculations of ozone entrainment fluxes using continuous observation by co-located ozone DIAL and Compact Wind Aerosol Lidar (CWAL) at the campus of University of Alabama in Huntsville (UAH). As a part of Tropospheric Ozone Lidar NETwork (TOLNET), UAH ozone DIAL can provide continuous ozone observation in the altitude range from 125 m AGL to 12 km, with 10-min temporal resolution and 150 - 550 m vertical resolution. We also perform an ozone budget study using the Dutch Atmospheric Large-Eddy Simulation (DALES), reasonable approximations of dry deposition, in conjunction with ozone entrainment observations. In this case study, the enhancement of ozone in the mixed layer results from the local emissions of NOx and VOCs. The NOx and VOCs emitted at surface entered into mixed layer by atmospheric turbulence and produced ozone within the whole mixed layer. Simultaneously, non-turbulent air in the residual layer, which is at top of the morning mixed layer, participates in convective mixing through entrainment processes. The clean air in the residual layer decreases the ozone enhancement rate in the mixed layer. After the mixed layer reaches its stable height at 1700m, the large-scale subsidence not only decreases PBL growth but also enhances the entrainment process. The PBL NOx and VOCs mix into the free troposphere through detrainment before producing ozone by photochemical reaction. We have following conclusions from this case study: 1) the relationship between boundary layer height and PBL ozone is complicated. Higher PBL height does not always mean lower PBL ozone. 2) The LES calculation results illuminate the interaction between
Domain nesting for multi-scale large eddy simulation
NASA Astrophysics Data System (ADS)
Fuka, Vladimir; Xie, Zheng-Tong
2016-04-01
The need to simulate city scale areas (O(10 km)) with high resolution within street canyons in certain areas of interests necessitates different grid resolutions in different part of the simulated area. General purpose computational fluid dynamics codes typically employ unstructured refined grids while mesoscale meteorological models more often employ nesting of computational domains. ELMM is a large eddy simulation model for the atmospheric boundary layer. It employs orthogonal uniform grids and for this reason domain nesting was chosen as the approach for simulations in multiple scales. Domains are implemented as sets of MPI processes which communicate with each other as in a normal non-nested run, but also with processes from another (outer/inner) domain. It should stressed that the duration of solution of time-steps in the outer and in the inner domain must be synchronized, so that the processes do not have to wait for the completion of their boundary conditions. This can achieved by assigning an appropriate number of CPUs to each domain, and to gain high efficiency. When nesting is applied for large eddy simulation, the inner domain receives inflow boundary conditions which lack turbulent motions not represented by the outer grid. ELMM remedies this by optional adding of turbulent fluctuations to the inflow using the efficient method of Xie and Castro (2008). The spatial scale of these fluctuations is in the subgrid-scale of the outer grid and their intensity will be estimated from the subgrid turbulent kinetic energy in the outer grid.
Large eddy simulation of the flow in a transpired channel
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Moin, Parviz; Ferziger, Joel
1989-01-01
The flow in a transpired channel has been computed by large eddy simulation. The numerical results compare very well with experimental data. Blowing decreases the wall shear stress and enhances turbulent fluctuations, while suction has the opposite effect. The wall layer thickness normalized by the local wall shear velocity and kinematic viscosity increases on the blowing side of the channel and decreases on the suction side. Suction causes more rapid decay of the spectra, larger mean streak spacing and higher two-point correlations. On the blowing side, the wall layer structures lie at a steeper angle to the wall, whereas on the suction side this angle is shallower.
Contrail Formation in Aircraft Wakes Using Large-Eddy Simulations
NASA Technical Reports Server (NTRS)
Paoli, R.; Helie, J.; Poinsot, T. J.; Ghosal, S.
2002-01-01
In this work we analyze the issue of the formation of condensation trails ("contrails") in the near-field of an aircraft wake. The basic configuration consists in an exhaust engine jet interacting with a wing-tip training vortex. The procedure adopted relies on a mixed Eulerian/Lagrangian two-phase flow approach; a simple micro-physics model for ice growth has been used to couple ice and vapor phases. Large eddy simulations have carried out at a realistic flight Reynolds number to evaluate the effects of turbulent mixing and wake vortex dynamics on ice-growth characteristics and vapor thermodynamic properties.
Large eddy simulation of the flow in a transpired channel
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Moin, Parviz; Ferziger, Joel
1989-01-01
The flow in a transpired channel has been computed by large eddy simulation. The numerical results compare very well with experimental data. Blowing decreases the wall shear stress and enhances turbulent fluctuations, while suction has the opposite effect. The wall layer thickness normalized by the local wall shear velocity and kinematic viscosity increases on the blowing side of the channel and decreases on the suction side. Suction causes more rapid decay of the spectra, larger mean streak spacing and higher two-point correlations. On the blowing side, the wall layer structures lie at a steeper angle to the wall, whereas on the suction side this angle is shallower.
Cosmological fluid mechanics with adaptively refined large eddy simulations
NASA Astrophysics Data System (ADS)
Schmidt, W.; Almgren, A. S.; Braun, H.; Engels, J. F.; Niemeyer, J. C.; Schulz, J.; Mekuria, R. R.; Aspden, A. J.; Bell, J. B.
2014-06-01
We investigate turbulence generated by cosmological structure formation by means of large eddy simulations using adaptive mesh refinement. In contrast to the widely used implicit large eddy simulations, which resolve a limited range of length-scales and treat the effect of turbulent velocity fluctuations below the grid scale solely by numerical dissipation, we apply a subgrid-scale model for the numerically unresolved fraction of the turbulence energy. For simulations with adaptive mesh refinement, we utilize a new methodology that allows us to adjust the scale-dependent energy variables in such a way that the sum of resolved and unresolved energies is globally conserved. We test our approach in simulations of randomly forced turbulence, a gravitationally bound cloud in a wind, and the Santa Barbara cluster. To treat inhomogeneous turbulence, we introduce an adaptive Kalman filtering technique that separates turbulent velocity fluctuations on resolved length-scales from the non-turbulent bulk flow. From the magnitude of the fluctuating component and the subgrid-scale turbulence energy, a total turbulent velocity dispersion of several 100 km s-1 is obtained for the Santa Barbara cluster, while the low-density gas outside the accretion shocks is nearly devoid of turbulence. The energy flux through the turbulent cascade and the dissipation rate predicted by the subgrid-scale model correspond to dynamical time-scales around 5 Gyr, independent of numerical resolution.
Large eddy simulation subgrid model for soot prediction
NASA Astrophysics Data System (ADS)
El-Asrag, Hossam Abd El-Raouf Mostafa
Soot prediction in realistic systems is one of the most challenging problems in theoretical and applied combustion. Soot formation as a chemical process is very complicated and not fully understood. The major difficulty stems from the chemical complexity of the soot formation process as well as its strong coupling with the other thermochemical and fluid processes that occur simultaneously. Soot is a major byproduct of incomplete combustion, having a strong impact on the environment as well as the combustion efficiency. Therefore, innovative methods is needed to predict soot in realistic configurations in an accurate and yet computationally efficient way. In the current study, a new soot formation subgrid model is developed and reported here. The new model is designed to be used within the context of the Large Eddy Simulation (LES) framework, combined with Linear Eddy Mixing (LEM) as a subgrid combustion model. The final model can be applied equally to premixed and non-premixed flames over any required geometry and flow conditions in the free, the transition, and the continuum regimes. The soot dynamics is predicted using a Method of Moments approach with Lagrangian Interpolative Closure (MOMIC) for the fractional moments. Since no prior knowledge of the particles distribution is required, the model is generally applicable. The current model accounts for the basic soot transport phenomena as transport by molecular diffusion and Thermophoretic forces. The model is first validated against experimental results for non-sooting swirling non-premixed and partially premixed flames. Next, a set of canonical premixed sooting flames are simulated, where the effect of turbulence, binary diffusivity and C/O ratio on soot formation are studied. Finally, the model is validated against a non-premixed jet sooting flame. The effect of the flame structure on the different soot formation stages as well as the particle size distribution is described. Good results are predicted with
Measurements and large eddy simulation of propagating premixed flames
Masri, A.R.; Cadwallader, B.J.; Ibrahim, S.S.
2006-07-15
This paper presents an experimental and numerical study of unsteady turbulent premixed flames igniting in an initially stagnant mixture and propagating past solid obstacles. The objective here is to study the outstanding issue of flow-flame interactions in transient premixed combustion environments. Particular emphasis is placed on the burning rate and the structure of the flame front. The experimental configuration consists of a chamber with a square cross-section filled with a combustible mixture of propane-air ignited from rest. An array of baffle plates as well as geometrical obstructions of varying shapes and blockage ratios, are placed in the path of the flame as it propagates from the ignition source to the vented end of the enclosure. A range of flame propagation conditions are studied experimentally. Measurements are presented for pressure-time traces, high-speed images of the flame front, mean velocities obtained from particle imaging velocimetry and laser induced fluorescence images of the hydroxyl radical OH. Three-dimensional large eddy simulations (LES) are also made for a case where a square obstacle and an array of baffle plates are placed in the chamber. The dynamic Germano model and a simple flamelet combustion model are used at the sub-grid scale. The effects of grid size and sub-grid filter width are also discussed. Calculations and measurements are found to be in good agreement with respect to flame structure and peak overpressure. Turbulence levels increase significantly at the leading edge of the flame as it propagates past the array of baffle plates and the obstacle. With reference to the regime diagrams for turbulent premixed combustion, it is noted that the flame continues to lie in the zones of thin reactions or corrugated flamelets regardless of the stage of propagation along the chamber. (author)
Three-dimensional large eddy simulation for jet aeroacoustics
NASA Astrophysics Data System (ADS)
Uzun, Ali
Future design of aircraft engines with low jet noise emissions undoubtedly needs a better understanding of noise generation in turbulent jets. Such an understanding, on the other hand, demands very reliable prediction tools. This study is therefore focused on developing a Computational Aeroacoustics (CAA) methodology that couples the near field unsteady flow field data computed by a 3-D Large Eddy Simulation (LES) code with various integral acoustic formulations for the far field noise prediction of turbulent jets. Turbulent jet simulations were performed at various Reynolds numbers. Comparisons of jet mean flow, turbulence statistics as well as jet aeroacoustics results with experimental data of jets at similar flow conditions were done and reasonable agreement was observed. The actual jet nozzle geometry was not included in the present simulations in order to keep the computational cost at manageable levels, therefore the jet shear layers downstream of the nozzle exit were modelled in an ad hoc fashion. As also observed by other researchers, the results obtained in the simulations were seen to be somewhat sensitive to the way the inflow forcing was done. The study of the effects of the eddy-viscosity based Smagorinsky subgrid-scale (SGS) model on noise predictions shows that the Smagorinsky model suppresses the resolved scale high-frequency noise. Simulations with filtering only suggest that treating the spatial filter as an implicit SGS model might be a good alternative. To our best knowledge, Lighthill's acoustic analogy was applied to a reasonably high Reynolds number jet for the first time in this study. A database greater than 1 Terabytes (TB) in size was post-processed using 1160 processors in parallel on a modern supercomputing platform for this purpose. It is expected that the current CAA methodology will yield better jet noise predictions when improved SGS models for both turbulence and high-frequency noise are incorporated into the LES code and when the
Time-Domain Filtering for Spatial Large-Eddy Simulation
NASA Technical Reports Server (NTRS)
Pruett, C. David
1997-01-01
An approach to large-eddy simulation (LES) is developed whose subgrid-scale model incorporates filtering in the time domain, in contrast to conventional approaches, which exploit spatial filtering. The method is demonstrated in the simulation of a heated, compressible, axisymmetric jet, and results are compared with those obtained from fully resolved direct numerical simulation. The present approach was, in fact, motivated by the jet-flow problem and the desire to manipulate the flow by localized (point) sources for the purposes of noise suppression. Time-domain filtering appears to be more consistent with the modeling of point sources; moreover, time-domain filtering may resolve some fundamental inconsistencies associated with conventional space-filtered LES approaches.
Model consistency in the large eddy simulation of turbulent channel flows
NASA Technical Reports Server (NTRS)
Moin, Parviz; Ferziger, Joel H.; Piomelli, Ugo
1987-01-01
Various combinations of filters and subgrid scale stress models for large eddy simulation of the Navier-Stokes equations are studied by a priori tests and numerical simulations. Consistency between model and filter is found to be essential to ensure accurate results. Results and limitations of the a priori test are discussed. The effect of grid refinement is also examined.
Model consistency in the large eddy simulation of turbulent channel flows
NASA Technical Reports Server (NTRS)
Moin, Parviz; Ferziger, Joel H.; Piomelli, Ugo
1987-01-01
Various combinations of filters and subgrid scale stress models for large eddy simulation of the Navier-Stokes equations are studied by a priori tests and numerical simulations. Consistency between model and filter is found to be essential to ensure accurate results. Results and limitations of the a priori test are discussed. The effect of grid refinement is also examined.
Large Eddy Simulation of Cryogenic Injection Processes at Supercritical Pressure
NASA Technical Reports Server (NTRS)
Oefelein, Joseph C.; Garcia, Roberto (Technical Monitor)
2002-01-01
This paper highlights results from the first of a series of hierarchical simulations aimed at assessing the modeling requirements for application of the large eddy simulation technique to cryogenic injection and combustion processes in liquid rocket engines. The focus is on liquid-oxygen-hydrogen coaxial injectors at a condition where the liquid-oxygen is injected at a subcritical temperature into a supercritical environment. For this situation a diffusion dominated mode of combustion occurs in the presence of exceedingly large thermophysical property gradients. Though continuous, these gradients approach the behavior of a contact discontinuity. Significant real gas effects and transport anomalies coexist locally in colder regions of the flow, with ideal gas and transport characteristics occurring within the flame zone. The current focal point is on the interfacial region between the liquid-oxygen core and the coaxial hydrogen jet where the flame anchors itself.
Large-eddy simulation of transitional channel flow
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Zang, Thomas A.
1990-01-01
A large-eddy simulation (LES) of transition in plane channel flow was carried out. The LES results were compared with those of a fine direct numerical simulation (DNS), and with those of a coarse DNS that uses the same mesh as the LES, but does not use a residual stress model. While at the early stages of transition, LES and coarse DNS give the same results: the presence of the residual stress model was found to be necessary to predict accurately mean velocity and Reynolds stress profiles during the late stages of transition (after the second spike stage). The evolution of single Fourier modes is also predicted more accurately by the LES than by the DNS. As small scales are generated, the dissipative character of the residual stress starts to reproduce correctly the energy cascade. As transition progresses, the flow approaches its fully developed turbulent state, the subgrid scales tend towards equilibrium, and the model becomes more accurate.
Large-Eddy Simulation of Turbulent Wall-Pressure Fluctuations
NASA Technical Reports Server (NTRS)
Singer, Bart A.
1996-01-01
Large-eddy simulations of a turbulent boundary layer with Reynolds number based on displacement thickness equal to 3500 were performed with two grid resolutions. The computations were continued for sufficient time to obtain frequency spectra with resolved frequencies that correspond to the most important structural frequencies on an aircraft fuselage. The turbulent stresses were adequately resolved with both resolutions. Detailed quantitative analysis of a variety of statistical quantities associated with the wall-pressure fluctuations revealed similar behavior for both simulations. The primary differences were associated with the lack of resolution of the high-frequency data in the coarse-grid calculation and the increased jitter (due to the lack of multiple realizations for averaging purposes) in the fine-grid calculation. A new curve fit was introduced to represent the spanwise coherence of the cross-spectral density.
Implicit large eddy simulation of shock-driven material mixing.
Grinstein, F F; Gowardhan, A A; Ristorcelli, J R
2013-11-28
Under-resolved computer simulations are typically unavoidable in practical turbulent flow applications exhibiting extreme geometrical complexity and a broad range of length and time scales. An important unsettled issue is whether filtered-out and subgrid spatial scales can significantly alter the evolution of resolved larger scales of motion and practical flow integral measures. Predictability issues in implicit large eddy simulation of under-resolved mixing of material scalars driven by under-resolved velocity fields and initial conditions are discussed in the context of shock-driven turbulent mixing. The particular focus is on effects of resolved spectral content and interfacial morphology of initial conditions on transitional and late-time turbulent mixing in the fundamental planar shock-tube configuration.
Li, Guoqing; Du, Yang; Wang, Shimao; Qi, Sheng; Zhang, Peili; Chen, Wenzhuo
2017-10-05
In this work, LES simulation coupled with a TFC sub-grid combustion model has been performed in a semi-confined pipe (L/D=10, V=10L) in the presence of four hollow-square obstacles (BR=49.8%) with circular hollow cross-section, in order to study the premixed gasoline-air mixture explosions. The comparisons between simulated results and experimental results have been conducted. It was found that the simulated results were in good agreement with experimental data in terms of flame structures, flame locations and overpressure time histories. Moreover, the interaction between flame propagation process and obstacles, overpressure dynamics were analyzed. In addition, the effects of initial gasoline vapor concentration (lean (ϕ=1.3%), stoichiometric (ϕ=1.7%) and rich (ϕ=2.1%)), and the number of obstacles (from 1 to 4) were also investigated by experiments. Some of the experimental results have been compared with the literature data. It is found that the explosion parameters of gasoline-air mixtures (e.g. the maximum overpressure peaks, average overpressure growth rates, etc.) are different from some other fuels such as hydrogen, methane and LPG, etc. Copyright © 2017. Published by Elsevier B.V.
Large-Eddy Simulation Code Developed for Propulsion Applications
NASA Technical Reports Server (NTRS)
DeBonis, James R.
2003-01-01
A large-eddy simulation (LES) code was developed at the NASA Glenn Research Center to provide more accurate and detailed computational analyses of propulsion flow fields. The accuracy of current computational fluid dynamics (CFD) methods is limited primarily by their inability to properly account for the turbulent motion present in virtually all propulsion flows. Because the efficiency and performance of a propulsion system are highly dependent on the details of this turbulent motion, it is critical for CFD to accurately model it. The LES code promises to give new CFD simulations an advantage over older methods by directly computing the large turbulent eddies, to correctly predict their effect on a propulsion system. Turbulent motion is a random, unsteady process whose behavior is difficult to predict through computer simulations. Current methods are based on Reynolds-Averaged Navier- Stokes (RANS) analyses that rely on models to represent the effect of turbulence within a flow field. The quality of the results depends on the quality of the model and its applicability to the type of flow field being studied. LES promises to be more accurate because it drastically reduces the amount of modeling necessary. It is the logical step toward improving turbulent flow predictions. In LES, the large-scale dominant turbulent motion is computed directly, leaving only the less significant small turbulent scales to be modeled. As part of the prediction, the LES method generates detailed information on the turbulence itself, providing important information for other applications, such as aeroacoustics. The LES code developed at Glenn for propulsion flow fields is being used to both analyze propulsion system components and test improved LES algorithms (subgrid-scale models, filters, and numerical schemes). The code solves the compressible Favre-filtered Navier- Stokes equations using an explicit fourth-order accurate numerical scheme, it incorporates a compressible form of
Smoothed particle hydrodynamics method from a large eddy simulation perspective
NASA Astrophysics Data System (ADS)
Di Mascio, A.; Antuono, M.; Colagrossi, A.; Marrone, S.
2017-03-01
The Smoothed Particle Hydrodynamics (SPH) method, often used for the modelling of the Navier-Stokes equations by a meshless Lagrangian approach, is revisited from the point of view of Large Eddy Simulation (LES). To this aim, the LES filtering procedure is recast in a Lagrangian framework by defining a filter that moves with the positions of the fluid particles at the filtered velocity. It is shown that the SPH smoothing procedure can be reinterpreted as a sort of LES Lagrangian filtering, and that, besides the terms coming from the LES convolution, additional contributions (never accounted for in the SPH literature) appear in the equations when formulated in a filtered fashion. Appropriate closure formulas are derived for the additional terms and a preliminary numerical test is provided to show the main features of the proposed LES-SPH model.
Large Eddy Simulation of FDA’s Idealized Medical Device
Delorme, Yann T.; Anupindi, Kameswararao; Frankel, Steven H.
2013-01-01
A hybrid large eddy simulation (LES) and immersed boundary method (IBM) computational approach is used to make quantitative predictions of flow field statistics within the Food and Drug Administration’s (FDA) idealized medical device. An in-house code is used, hereafter (W enoHemo™), that combines high-order finite-difference schemes on structured staggered Cartesian grids with an IBM to facilitate flow over or through complex stationary or rotating geometries and employs a subgrid-scale (SGS) turbulence model that more naturally handles transitional flows [2]. Predictions of velocity and wall shear stress statistics are compared with previously published experimental measurements from Hariharan et al. [6] for the four Reynolds numbers considered. PMID:24187599
Computing transitional flows using wall-modeled large eddy simulation
NASA Astrophysics Data System (ADS)
Bodart, Julien; Larsson, Johan
2012-11-01
To be applicable to complex aerodynamic flows at realistic Reynolds numbers, large eddy simulation (LES) must be combined with a model for the inner part of the boundary layer. Aerodynamic flows are, in general, sensitive to the location of boundary layer transition. While traditional LES can predict the transition location and process accurately, existing wall-modeled LES approaches can not. In the present work, the behavior of the wall-model is locally adapted using a sensor in the LES-resolved part of boundary layer. This sensor estimates whether the boundary layer is turbulent or not, in a way that does not rely on any homogeneous direction. The proposed method is validated on controlled transition scenarios on a flat plat boundary layer, and finally applied to the flow around a multi-element airfoil at realistic Reynolds number.
Large Eddy Simulation of FDA's Idealized Medical Device.
Delorme, Yann T; Anupindi, Kameswararao; Frankel, Steven H
2013-12-01
A hybrid large eddy simulation (LES) and immersed boundary method (IBM) computational approach is used to make quantitative predictions of flow field statistics within the Food and Drug Administration's (FDA) idealized medical device. An in-house code is used, hereafter (W enoHemo(™) ), that combines high-order finite-difference schemes on structured staggered Cartesian grids with an IBM to facilitate flow over or through complex stationary or rotating geometries and employs a subgrid-scale (SGS) turbulence model that more naturally handles transitional flows [2]. Predictions of velocity and wall shear stress statistics are compared with previously published experimental measurements from Hariharan et al. [6] for the four Reynolds numbers considered.
Synthetic turbulence, fractal interpolation, and large-eddy simulation.
Basu, Sukanta; Foufoula-Georgiou, Efi; Porté-Agel, Fernando
2004-08-01
Fractal interpolation has been proposed in the literature as an efficient way to construct closure models for the numerical solution of coarse-grained Navier-Stokes equations. It is based on synthetically generating a scale-invariant subgrid-scale field and analytically evaluating its effects on large resolved scales. In this paper, we propose an extension of previous work by developing a multiaffine fractal interpolation scheme and demonstrate that it preserves not only the fractal dimension but also the higher-order structure functions and the non-Gaussian probability density function of the velocity increments. Extensive a priori analyses of atmospheric boundary layer measurements further reveal that this multiaffine closure model has the potential for satisfactory performance in large-eddy simulations. The pertinence of this newly proposed methodology in the case of passive scalars is also discussed.
NASA Astrophysics Data System (ADS)
Wolf-Grosse, Tobias; Esau, Igor; Reuder, Joachim
2017-06-01
Street-level urban air pollution is a challenging concern for modern urban societies. Pollution dispersion models assume that the concentrations decrease monotonically with raising wind speed. This convenient assumption breaks down when applied to flows with local recirculations such as those found in topographically complex coastal areas. This study looks at a practically important and sufficiently common case of air pollution in a coastal valley city. Here, the observed concentrations are determined by the interaction between large-scale topographically forced and local-scale breeze-like recirculations. Analysis of a long observational dataset in Bergen, Norway, revealed that the most extreme cases of recurring wintertime air pollution episodes were accompanied by increased large-scale wind speeds above the valley. Contrary to the theoretical assumption and intuitive expectations, the maximum NO2 concentrations were not found for the lowest 10 m ERA-Interim wind speeds but in situations with wind speeds of 3 m s-1. To explain this phenomenon, we investigated empirical relationships between the large-scale forcing and the local wind and air quality parameters. We conducted 16 large-eddy simulation (LES) experiments with the Parallelised Large-Eddy Simulation Model (PALM) for atmospheric and oceanic flows. The LES accounted for the realistic relief and coastal configuration as well as for the large-scale forcing and local surface condition heterogeneity in Bergen. They revealed that emerging local breeze-like circulations strongly enhance the urban ventilation and dispersion of the air pollutants in situations with weak large-scale winds. Slightly stronger large-scale winds, however, can counteract these local recirculations, leading to enhanced surface air stagnation. Furthermore, this study looks at the concrete impact of the relative configuration of warmer water bodies in the city and the major transport corridor. We found that a relatively small local water
Large Eddy Simulation of Vertical Axis Wind Turbines
NASA Astrophysics Data System (ADS)
Hezaveh, Seyed Hossein
Due to several design advantages and operational characteristics, particularly in offshore farms, vertical axis wind turbines (VAWTs) are being reconsidered as a complementary technology to horizontal axial turbines (HAWTs). However, considerable gaps remain in our understanding of VAWT performance since they have been significantly less studied than HAWTs. This thesis examines the performance of isolated VAWTs based on different design parameters and evaluates their characteristics in large wind farms. An actuator line model (ALM) is implemented in an atmospheric boundary layer large eddy simulation (LES) code, with offline coupling to a high-resolution blade-scale unsteady Reynolds-averaged Navier-Stokes (URANS) model. The LES captures the turbine-to-farm scale dynamics, while the URANS captures the blade-to-turbine scale flow. The simulation results are found to be in good agreement with existing experimental datasets. Subsequently, a parametric study of the flow over an isolated VAWT is carried out by varying solidities, height-to-diameter aspect ratios, and tip speed ratios. The analyses of the wake area and power deficits yield an improved understanding of the evolution of VAWT wakes, which in turn enables a more informed selection of turbine designs for wind farms. One of the most important advantages of VAWTs compared to HAWTs is their potential synergistic interactions that increase their performance when placed in close proximity. Field experiments have confirmed that unlike HAWTs, VAWTs can enhance and increase the total power production when placed near each other. Based on these experiments and using ALM-LES, we also present and test new approaches for VAWT farm configuration. We first design clusters with three turbines then configure farms consisting of clusters of VAWTs rather than individual turbines. The results confirm that by using a cluster design, the average power density of wind farms can be increased by as much as 60% relative to regular
Film cooling from inclined cylindrical holes using large eddy simulations
NASA Astrophysics Data System (ADS)
Peet, Yulia V.
2006-12-01
The goal of the present study is to investigate numerically the physics of the flow, which occurs during the film cooling from inclined cylindrical holes, Film cooling is a technique used in gas turbine industry to reduce heat fluxes to the turbine blade surface. Large Eddy Simulation (LES) is performed modeling a realistic film cooling configuration, which consists of a large stagnation-type reservoir, feeding an array of discrete cooling holes (film holes) flowing into a flat plate turbulent boundary layer. Special computational methodology is developed for this problem, involving coupled simulations using multiple computational codes. A fully compressible LES code is used in the area above the flat plate, while a low Mach number LES code is employed in the plenum and film holes. The motivation for using different codes comes from the essential difference in the nature of the flow in these different regions. Flowfield is analyzed inside the plenum, film hole and a crossflow region. Flow inside the plenum is stagnating, except for the region close to the exit, where it accelerates rapidly to turn into the hole. The sharp radius of turning at the trailing edge of the plenum pipe connection causes the flow to separate from the downstream wall of the film hole. After coolant injection occurs, a complex flowfield is formed consisting of coherent vortical structures responsible for bringing hot crossflow fluid in contact with the walls of either the film hole or the blade, thus reducing cooling protection. Mean velocity and turbulent statistics are compared to experimental measurements, yielding good agreement for the mean flowfield and satisfactory agreement for the turbulence quantities. LES results are used to assess the applicability of basic assumptions of conventional eddy viscosity turbulence models used with Reynolds-averaged (RANS) approach, namely the isotropy of an eddy viscosity and thermal diffusivity. It is shown here that these assumptions do not hold
Improved engine wall models for Large Eddy Simulation (LES)
NASA Astrophysics Data System (ADS)
Plengsaard, Chalearmpol
Improved wall models for Large Eddy Simulation (LES) are presented in this research. The classical Werner-Wengle (WW) wall shear stress model is used along with near-wall sub-grid scale viscosity. A sub-grid scale turbulent kinetic energy is employed in a model for the eddy viscosity. To gain better heat flux results, a modified classical variable-density wall heat transfer model is also used. Because no experimental wall shear stress results are available in engines, the fully turbulent developed flow in a square duct is chosen to validate the new wall models. The model constants in the new wall models are set to 0.01 and 0.8, respectively and are kept constant throughout the investigation. The resulting time- and spatially-averaged velocity and temperature wall functions from the new wall models match well with the law-of-the-wall experimental data at Re = 50,000. In order to study the effect of hot air impinging walls, jet impingement on a flat plate is also tested with the new wall models. The jet Reynolds number is equal to 21,000 and a fixed jet-to-plate spacing of H/D = 2.0. As predicted by the new wall models, the time-averaged skin friction coefficient agrees well with experimental data, while the computed Nusselt number agrees fairly well when r/D > 2.0. Additionally, the model is validated using experimental data from a Caterpillar engine operated with conventional diesel combustion. Sixteen different operating engine conditions are simulated. The majority of the predicted heat flux results from each thermocouple location follow similar trends when compared with experimental data. The magnitude of peak heat fluxes as predicted by the new wall models is in the range of typical measured values in diesel combustion, while most heat flux results from previous LES wall models are over-predicted. The new wall models generate more accurate predictions and agree better with experimental data.
Large-eddy simulation of unidirectional turbulent flow over dunes
NASA Astrophysics Data System (ADS)
Omidyeganeh, Mohammad
We performed large eddy simulation of the flow over a series of two- and three-dimensional dune geometries at laboratory scale using the Lagrangian dynamic eddy-viscosity subgrid-scale model. First, we studied the flow over a standard 2D transverse dune geometry, then bedform three-dimensionality was imposed. Finally, we investigated the turbulent flow over barchan dunes. The results are validated by comparison with simulations and experiments for the 2D dune case, while the results of the 3D dunes are validated qualitatively against experiments. The flow over transverse dunes separates at the dune crest, generating a shear layer that plays a crucial role in the transport of momentum and energy, as well as the generation of coherent structures. Spanwise vortices are generated in the separated shear; as they are advected, they undergo lateral instabilities and develop into horseshoe-like structures and finally reach the surface. The ejection that occurs between the legs of the vortex creates the upwelling and downdrafting events on the free surface known as "boils". The three-dimensional separation of flow at the crestline alters the distribution of wall pressure, which may cause secondary flow across the stream. The mean flow is characterized by a pair of counter-rotating streamwise vortices, with core radii of the order of the flow depth. Staggering the crestlines alters the secondary motion; two pairs of streamwise vortices appear (a strong one, centred about the lobe, and a weaker one, coming from the previous dune, centred around the saddle). The flow over barchan dunes presents significant differences to that over transverse dunes. The flow near the bed, upstream of the dune, diverges from the centerline plane; the flow close to the centerline plane separates at the crest and reattaches on the bed. Away from the centerline plane and along the horns, flow separation occurs intermittently. The flow in the separation bubble is routed towards the horns and leaves
Large eddy simulations of flow past a cubic obstacle
NASA Astrophysics Data System (ADS)
Shah, Kishan B.
Turbulent flow over three dimensional obstacles is common in engineering and understanding of them is necessary to engineering design. This work is an effort to provide quantitative data on flows past three-dimensional bodies through large eddy simulations (LES). The flow over a cube mounted on a wall of a plane channel is studied. This flow exhibits characteristics common to this class of flows, including the three dimensionality of the mean flow, separation, and large scale unsteadiness. The goals of our study are to develop subgrid scale (SGS) models suitable for complex flows, to perform LES of these flows, and to use the results to aid in the identification of dynamically significant large scale structures. Included in this investigation are comparison of several SGS models, including a new model, and a study of some of the mean and unsteady characteristics of the flow. Cube flow LES were performed at both low and high Reynolds number (Reb = 3000 and 40000). The quality of results was verified by comparing them to an experiment. All the large scale features seen in the experiment are reproduced and the flow patterns are consistent with kinematic constraints. The mean flow is characterized by a strong horseshoe vortex upstream of the body, an arch vortex behind the body and vortices on the roof and the sides. The recirculation region above the roof has the strongest turbulence kinetic energy while the arch vortex has the largest turbulent shear stresses. Although the unsteady behavior is quite complicated, there are large-scale events that occur roughly periodically such as the motion of the horseshoe vortex and the vortices shed from the roof and the sides. Bimodal PDFs are typical of the region upstream of the obstacle, close to the wall. These are due to the existence of two distinct states of the flow. The dominant characteristic behind the obstacle is the quasi- periodic, alternate vortex shedding from the sides and the roof. The shedding frequency from
Refinement of a mesoscale model for large eddy simulation
NASA Astrophysics Data System (ADS)
Gasset, Nicolas
With the advent of wind energy technology, several methods have become mature and are seen today as standard for predicting and forecasting the wind. However, their results are still site dependent, and the increasing sizes of both modern wind turbines and wind farms tackle limits of existing methods. Some triggered processes extend to the junction between microscales and mesoscales.The main objectives of this thesis are thus to identify, implement and evaluate an approach allowing for microscale and mesoscale ABL flow modelling considering the various challenges of modern wind energy applications. A literature review of ABL flow modelling from microscales to mesoscales first provides an overview of the specificities and abilities of existing methods. The combined mesoscale/large eddy simulation (LES) modelling appears to be the most promising approach, and the Compressible Community Mesoscale Model (MC2) is elected as the basis of the method in which the components required for LES are added and implemented. A detailed description of the mathematical model and the numerical aspects of the various components of the LES-capable MC2 are then presented so that a complete view of the proposed approach along with the specificities of its implementation are provided. This further allows to introduce the enhancements and new components of the method (separation of volumetric and deviatoric Reynolds tensor terms, vertical staggering, subgrid scale models, 3D turbulent diffusion, 3D turbulent kinetic energy equation), as well as the adaptation of its operating mode to allow for LES (initialization, large scale geostrophic forcing, surface and lateral boundaries). Finally, fundamental aspects and new components of the proposed approach are evaluated based on theoretical 1D Ekman boundary layer and 3D unsteady shear and buoyancy driven homogeneous surface full ABL cases. The model behaviour at high resolution as well as the components required for LES in MC2 are all finely
Large-Eddy Simulation of Maritime Deep Tropical Convection
NASA Astrophysics Data System (ADS)
Khairoutdinov, Marat F.; Krueger, Steve K.; Moeng, Chin-Hoh; Bogenschutz, Peter A.; Randall, David A.
2009-04-01
This study represents an attempt to apply Large-Eddy Simulation (LES) resolution to simulate deep tropical convection in near equilibrium for 24 hours over an area of about 205 × 205 km2, which is comparable to that of a typical horizontal grid cell in a global climate model. The simulation is driven by large-scale thermodynamic tendencies derived from mean conditions during the GATE Phase III field experiment. The LES uses 2048 × 2048 × 256 grid points with horizontal grid spacing of 100 m and vertical grid spacing ranging from 50 m in the boundary layer to 100 m in the free troposphere. The simulation reaches a near equilibrium deep convection regime in 12 hours. The simulated vertical cloud distribution exhibits a tri-modal vertical distribution of deep, middle and shallow clouds similar to that often observed in Tropics. A sensitivity experiment in which cold pools are suppressed by switching off the evaporation of precipitation results in much lower amounts of shallow and congestus clouds. Unlike the benchmark LES where the new deep clouds tend to appear along the edges of spreading cold pools, the deep clouds in the no-cold-pool experiment tend to reappear at the sites of the previous deep clouds and tend to be surrounded by extensive areas of sporadic shallow clouds. The vertical velocity statistics of updraft and downdraft cores below 6 km height are compared to aircraft observations made during GATE. The comparison shows generally good agreement, and strongly suggests that the LES simulation can be used as a benchmark to represent the dynamics of tropical deep convection on scales ranging from large turbulent eddies to mesoscale convective systems. The effect of horizontal grid resolution is examined by running the same case with progressively larger grid sizes of 200, 400, 800, and 1600 m. These runs show a reasonable agreement with the benchmark LES in statistics such as convective available potential energy, convective inhibition, cloud fraction
Inviscid Wall-Modeled Large Eddy Simulations for Improved Efficiency
NASA Astrophysics Data System (ADS)
Aikens, Kurt; Craft, Kyle; Redman, Andrew
2015-11-01
The accuracy of an inviscid flow assumption for wall-modeled large eddy simulations (LES) is examined because of its ability to reduce simulation costs. This assumption is not generally applicable for wall-bounded flows due to the high velocity gradients found near walls. In wall-modeled LES, however, neither the viscous near-wall region or the viscous length scales in the outer flow are resolved. Therefore, the viscous terms in the Navier-Stokes equations have little impact on the resolved flowfield. Zero pressure gradient flat plate boundary layer results are presented for both viscous and inviscid simulations using a wall model developed previously. The results are very similar and compare favorably to those from another wall model methodology and experimental data. Furthermore, the inviscid assumption reduces simulation costs by about 25% and 39% for supersonic and subsonic flows, respectively. Future research directions are discussed as are preliminary efforts to extend the wall model to include the effects of unresolved wall roughness. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. Computational resources on TACC Stampede were provided under XSEDE allocation ENG150001.
Properties of shallow convection from Large-Eddie simulations
NASA Astrophysics Data System (ADS)
Denby, Leif; Herzog, Michael
2017-04-01
Utilizing Large-Eddie simulations (LES) of isolated individual convective clouds in an idealised conditionally unstable atmosphere and large-domain LES simulations of radiative-convective equilibrium (RCE) from the RICO measuring campaign (Rauber et al. 2007), vertical profiles of individual clouds and statistical properties of the cloud ensemble have been extracted and compared against predictions by an 1D entraining parcel model and against the cloud-ensemble model of the CCFM (Wagner and Graf 2010) convection scheme (which comprises a solution of a Lotka-Volterra population dynamics system). For the simulations of isolated clouds it was possible to achieve agreement with the entraining parcel model when simulations were carried out with 2D axisymmetry and the entrainment rate was prescribed using an entraining profile estimated from LES simulation using a passive tracer (in place of the traditional Morton- Turner entrainment rate parameterisation), this agreement was not achieved when comparing against 3D simulations. Integrating the entraining parcel model using the horizontal mean environment profile of the RCE simulation (and so the vertical profile as would be predicted by a climate model) it was not possible to achieve the variation in cloud-top height seen in the RCE simulation, even when greatly increasing the entrainment rate. However, if the near-environment of a convective cloud was used as the environmental profile the variation in cloud-top height was achieved (by varying the cloud-base state variables within values extracted from RCE simulation). This indicates that the near-cloud environment is significantly different that the horizontal mean environment and must be taken into account if the effect of entrainment is to be correctly captured in parameterisations for convection. Finally, size-distribution of convective clouds extracted from RCE simulation showed qualitative agreement with predictions of CCFM's spectrum model.
Unsteady RANS and Large Eddy simulations of multiphase diesel injection
NASA Astrophysics Data System (ADS)
Philipp, Jenna; Green, Melissa; Akih-Kumgeh, Benjamin
2015-11-01
Unsteady Reynolds Averaged Navier-Stokes (URANS) and Large Eddy Simulations (LES) of two-phase flow and evaporation of high pressure diesel injection into a quiescent, high temperature environment is investigated. Unsteady RANS and LES are turbulent flow simulation approaches used to determine complex flow fields. The latter allows for more accurate predictions of complex phenomena such as turbulent mixing and physio-chemical processes associated with diesel combustion. In this work we investigate a high pressure diesel injection using the Euler-Lagrange method for multiphase flows as implemented in the Star-CCM+ CFD code. A dispersed liquid phase is represented by Lagrangian particles while the multi-component gas phase is solved using an Eulerian method. Results obtained from the two approaches are compared with respect to spray penetration depth and air entrainment. They are also compared with experimental data taken from the Sandia Engine Combustion Network for ``Spray A''. Characteristics of primary and secondary atomization are qualitatively evaluated for all simulation modes.
Large Eddy Simulations of Colorless Distributed Combustion Systems
NASA Astrophysics Data System (ADS)
Abdulrahman, Husam F.; Jaberi, Farhad; Gupta, Ashwani
2014-11-01
Development of efficient and low-emission colorless distributed combustion (CDC) systems for gas turbine applications require careful examination of the role of various flow and combustion parameters. Numerical simulations of CDC in a laboratory-scale combustor have been conducted to carefully examine the effects of these parameters on the CDC. The computational model is based on a hybrid modeling approach combining large eddy simulation (LES) with the filtered mass density function (FMDF) equations, solved with high order numerical methods and complex chemical kinetics. The simulated combustor operates based on the principle of high temperature air combustion (HiTAC) and has shown to significantly reduce the NOx, and CO emissions while improving the reaction pattern factor and stability without using any flame stabilizer and with low pressure drop and noise. The focus of the current work is to investigate the mixing of air and hydrocarbon fuels and the non-premixed and premixed reactions within the combustor by the LES/FMDF with the reduced chemical kinetic mechanisms for the same flow conditions and configurations investigated experimentally. The main goal is to develop better CDC with higher mixing and efficiency, ultra-low emission levels and optimum residence time. The computational results establish the consistency and the reliability of LES/FMDF and its Lagrangian-Eulerian numerical methodology.
Large Eddy Simulation of High-Speed, Premixed Ethylene Combustion
NASA Technical Reports Server (NTRS)
Ramesh, Kiran; Edwards, Jack R.; Chelliah, Harsha; Goyne, Christopher; McDaniel, James; Rockwell, Robert; Kirik, Justin; Cutler, Andrew; Danehy, Paul
2015-01-01
A large-eddy simulation / Reynolds-averaged Navier-Stokes (LES/RANS) methodology is used to simulate premixed ethylene-air combustion in a model scramjet designed for dual mode operation and equipped with a cavity for flameholding. A 22-species reduced mechanism for ethylene-air combustion is employed, and the calculations are performed on a mesh containing 93 million cells. Fuel plumes injected at the isolator entrance are processed by the isolator shock train, yielding a premixed fuel-air mixture at an equivalence ratio of 0.42 at the cavity entrance plane. A premixed flame is anchored within the cavity and propagates toward the opposite wall. Near complete combustion of ethylene is obtained. The combustor is highly dynamic, exhibiting a large-scale oscillation in global heat release and mass flow rate with a period of about 2.8 ms. Maximum heat release occurs when the flame front reaches its most downstream extent, as the flame surface area is larger. Minimum heat release is associated with flame propagation toward the cavity and occurs through a reduction in core flow velocity that is correlated with an upstream movement of the shock train. Reasonable agreement between simulation results and available wall pressure, particle image velocimetry, and OH-PLIF data is obtained, but it is not yet clear whether the system-level oscillations seen in the calculations are actually present in the experiment.
Large eddy simulation of incompressible turbulent channel flow
NASA Technical Reports Server (NTRS)
Moin, P.; Reynolds, W. C.; Ferziger, J. H.
1978-01-01
The three-dimensional, time-dependent primitive equations of motion were numerically integrated for the case of turbulent channel flow. A partially implicit numerical method was developed. An important feature of this scheme is that the equation of continuity is solved directly. The residual field motions were simulated through an eddy viscosity model, while the large-scale field was obtained directly from the solution of the governing equations. An important portion of the initial velocity field was obtained from the solution of the linearized Navier-Stokes equations. The pseudospectral method was used for numerical differentiation in the horizontal directions, and second-order finite-difference schemes were used in the direction normal to the walls. The large eddy simulation technique is capable of reproducing some of the important features of wall-bounded turbulent flows. The resolvable portions of the root-mean square wall pressure fluctuations, pressure velocity-gradient correlations, and velocity pressure-gradient correlations are documented.
Large Eddy Simulation of a Turbulent Buoyant Plume
NASA Astrophysics Data System (ADS)
Desjardin, Paul E.
1999-11-01
Large Eddy Simulations of a helium-air turbulent plume are conducted in order to investigate the buoyancy induced vorticity production mechanisms of this flow. The inlet condition of the plume consists of a low velocity (0.35 m/sec) 1m diameter helium jet emitting upwards into air. This flow configuration is chosen to best match the experimental conditions of the non-reacting helium plume experiments taken at Sandia's FLAME facility. The compressible form of the Favre filtered Navier Stokes, species and energy equations are closed using localized dynamic Smagorinsky subgrid models . Numerical integration is performed using AUSM+ flux vector splitting that employs fifth order upwind biased interpolating stencils and advanced in time using second order Runge-Kutta along with pressure gradient scaling for improved temporal stability. The code uses MPI domain decomposition and is run on Sandia's ASCI red massively parallel computer. Results from the simulations highlight the buoyancy induced vorticity generation and entrainment properties of these flows and the effect of filter width on subgrid modeling. Comparisons to experimental data will be made whenever possible.
Assessment of dynamic closure for premixed combustion large eddy simulation
NASA Astrophysics Data System (ADS)
Langella, Ivan; Swaminathan, Nedunchezhian; Gao, Yuan; Chakraborty, Nilanjan
2015-09-01
Turbulent piloted Bunsen flames of stoichiometric methane-air mixtures are computed using the large eddy simulation (LES) paradigm involving an algebraic closure for the filtered reaction rate. This closure involves the filtered scalar dissipation rate of a reaction progress variable. The model for this dissipation rate involves a parameter βc representing the flame front curvature effects induced by turbulence, chemical reactions, molecular dissipation, and their interactions at the sub-grid level, suggesting that this parameter may vary with filter width or be a scale-dependent. Thus, it would be ideal to evaluate this parameter dynamically by LES. A procedure for this evaluation is discussed and assessed using direct numerical simulation (DNS) data and LES calculations. The probability density functions of βc obtained from the DNS and LES calculations are very similar when the turbulent Reynolds number is sufficiently large and when the filter width normalised by the laminar flame thermal thickness is larger than unity. Results obtained using a constant (static) value for this parameter are also used for comparative evaluation. Detailed discussion presented in this paper suggests that the dynamic procedure works well and physical insights and reasonings are provided to explain the observed behaviour.
Large-eddy simulation formulation and implementation in HYDRA
McCallen, R.
1995-12-05
This report provides the equation formulation for a large-eddy simulation (LES) approach and Smagorinsky subgrid-scale (SGS) model for incompressible flow using the finite element method (FEM). This report also outlines the model implementation in the computer code HYDRA and the results of a coding check. The check was accomplished by running simple two- and three-element problems for a specified velocity field. The values of the eddy viscosity (the coefficient of proportionality in the SGS eddy diffusion model), the SGS diffusion term, and overall diffusion term (molecular plus SGS plus balancing tensor diffusivity) were compared to known hand-calculated values. Coding checks are best done by comparing the code-calculated solution to known analytical solutions. However, with LES turbulence modeling, these analytical solutions do not exist. It is also impossible to determine that the eddy viscosity is free of coding errors when performing code validation by comparing the LES to direct numerical simulations (DNS) (i.e., fine discretization with no turbulence model) or experimental results. Therefore, the coding checks presented here for a specified velocity field are necessary.
Large eddy simulation for aerodynamics: status and perspectives.
Sagaut, Pierre; Deck, Sébastien
2009-07-28
The present paper provides an up-to-date survey of the use of large eddy simulation (LES) and sequels for engineering applications related to aerodynamics. Most recent landmark achievements are presented. Two categories of problem may be distinguished whether the location of separation is triggered by the geometry or not. In the first case, LES can be considered as a mature technique and recent hybrid Reynolds-averaged Navier-Stokes (RANS)-LES methods do not allow for a significant increase in terms of geometrical complexity and/or Reynolds number with respect to classical LES. When attached boundary layers have a significant impact on the global flow dynamics, the use of hybrid RANS-LES remains the principal strategy to reduce computational cost compared to LES. Another striking observation is that the level of validation is most of the time restricted to time-averaged global quantities, a detailed analysis of the flow unsteadiness being missing. Therefore, a clear need for detailed validation in the near future is identified. To this end, new issues, such as uncertainty and error quantification and modelling, will be of major importance. First results dealing with uncertainty modelling in unsteady turbulent flow simulation are presented.
A Method for Large Eddy Simulation of Acoustic Combustion Instabilities
NASA Astrophysics Data System (ADS)
Wall, Clifton; Pierce, Charles; Moin, Parviz
2002-11-01
A method for performing Large Eddy Simulation of acoustic combustion instabilities is presented. By extending the low Mach number pressure correction method to the case of compressible flow, a numerical method is developed in which the Poisson equation for pressure is replaced by a Helmholtz equation. The method avoids the acoustic CFL condition by using implicit time advancement, leading to large efficiency gains at low Mach number. The method also avoids artificial damping of acoustic waves. The numerical method is attractive for the simulation of acoustic combustion instabilities, since these flows are typically at low Mach number, and the acoustic frequencies of interest are usually low. Both of these characteristics suggest the use of larger time steps than those allowed by an acoustic CFL condition. The turbulent combustion model used is the Combined Conserved Scalar/Level Set Flamelet model of Duchamp de Lageneste and Pitsch for partially premixed combustion. Comparison of LES results to the experiments of Besson et al will be presented.
A Method for Large Eddy Simulation of Acoustic Combustion Instabilities
NASA Astrophysics Data System (ADS)
Wall, Clifton; Moin, Parviz
2003-11-01
A method for performing Large Eddy Simulation of acoustic combustion instabilities is presented. By extending the low Mach number pressure correction method to the case of compressible flow, a numerical method is developed in which the Poisson equation for pressure is replaced by a Helmholtz equation. The method avoids the acoustic CFL condition by using implicit time advancement, leading to large efficiency gains at low Mach number. The method also avoids artificial damping of acoustic waves. The numerical method is attractive for the simulation of acoustics combustion instabilities, since these flows are typically at low Mach number, and the acoustic frequencies of interest are usually low. Additionally, new boundary conditions based on the work of Poinsot and Lele have been developed to model the acoustic effect of a long channel upstream of the computational inlet, thus avoiding the need to include such a channel in the computational domain. The turbulent combustion model used is the Level Set model of Duchamp de Lageneste and Pitsch for premixed combustion. Comparison of LES results to the reacting experiments of Besson et al. will be presented.
Large-eddy simulation of the very stable boundary layer
NASA Astrophysics Data System (ADS)
Chinita, M. J.; Matheou, G.
2016-12-01
The stable boundary layer is ubiquitous and typically forms at night when the ground radiatively cools and in polar regions throughout the day. Stable stratification and the associated reduction in the energetic scales in combination with the large anisotropy of turbulent motions challenge numerical models. This modeling difficulty also affects large-eddy simulation (LES) methods leading to scarce LES results for very stable conditions. In contrast, the NWP of convective flows has greatly benefited from the ample availability of high quality LES data. In order to overcome these limitations, a novel LES model setup is developed to enable the modeling of very stable boundary layers. A series of Ekman layer-type boundary layers at various surface cooling rates, geotropic winds and latitudes (rotation rates) is presented. A temperature surface condition is applied in the LES. The surface heat flux is dynamically computed byresolving the surface layer since the often-used Monin-Obukhov similarity theory cannot represent very stable conditions. Depending on the conditions, the LES gracefully transitions to a direct numerical simulation (DNS) where the flow becomes fully resolved. Two stability regimes can be discerned based on vertical profiles of the Richardson number. Overall, the model predicts that turbulence is very resilient with respect to stability. Temperature and velocity fluctuations persist even at high Richardson numbers. The nature of the fluctuations, i.e., due to turbulence/overturning or waves, is discussed. Scaling relations and spectra are also presented and discussed.
Progress in the Variational Multiscale Formulation of Large Eddy Simulation
NASA Astrophysics Data System (ADS)
Wang, Zhen; Oberai, Assad
2007-11-01
In the variational multiscale (VMS) formulation of large eddy simulation subgrid models are introduced in the variational (or weak) formulation of the Navier Stokes equations and a-priori scale separation is accomplished using projection operators to create coarse and fine scales. This separation also leads to two sets of evolution equations: one for the coarse scales and another for the fine scales. The coarse scale equations are solved numerically while the fine scale equations are solved analytically to obtain an expression for the fine scales in terms of the coarse scales and hence achieve closure. Till date, the VMS formulation has lead to accurate results in the simulation of canonical turbulent flow problems. It has been implemented using spectral, finite element and finite volume methods. In this talk, for the incompressible Navier Stokes equations, we willpresent some new ideas for modeling the fine scales within the context of the VMS formulation and discuss their impact on the coarse scale solution. We will present a simple residual-based approximation for the fine scales that accurately models the cross-stress term and demonstrate that when this term is append with an eddy viscosity model for the Reynolds stress, a new mixed-model is obtained. The application of these ideas will be illustrated through some simple numerical examples.
Analysis of errors occurring in large eddy simulation.
Geurts, Bernard J
2009-07-28
We analyse the effect of second- and fourth-order accurate central finite-volume discretizations on the outcome of large eddy simulations of homogeneous, isotropic, decaying turbulence at an initial Taylor-Reynolds number Re(lambda)=100. We determine the implicit filter that is induced by the spatial discretization and show that a higher order discretization also induces a higher order filter, i.e. a low-pass filter that keeps a wider range of flow scales virtually unchanged. The effectiveness of the implicit filtering is correlated with the optimal refinement strategy as observed in an error-landscape analysis based on Smagorinsky's subfilter model. As a point of reference, a finite-volume method that is second-order accurate for both the convective and the viscous fluxes in the Navier-Stokes equations is used. We observe that changing to a fourth-order accurate convective discretization leads to a higher value of the Smagorinsky coefficient C(S) required to achieve minimal total error at given resolution. Conversely, changing only the viscous flux discretization to fourth-order accuracy implies that optimal simulation results are obtained at lower values of C(S). Finally, a fully fourth-order discretization yields an optimal C(S) that is slightly lower than the reference fully second-order method.
Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Givi, Peyman; Madnia, Cyrus K.; Steinberger, Craig J.
1990-01-01
This research is involved with the implementation of advanced computational schemes based on large eddy simulations (LES) and direct numerical simulations (DNS) to study the phenomenon of mixing and its coupling with chemical reactions in compressible turbulent flows. In the efforts related to LES, a research program to extend the present capabilities of this method was initiated for the treatment of chemically reacting flows. In the DNS efforts, the focus is on detailed investigations of the effects of compressibility, heat release, and non-equilibrium kinetics modelings in high speed reacting flows. Emphasis was on the simulations of simple flows, namely homogeneous compressible flows, and temporally developing high speed mixing layers.
Large eddy simulation modelling of combustion for propulsion applications.
Fureby, C
2009-07-28
Predictive modelling of turbulent combustion is important for the development of air-breathing engines, internal combustion engines, furnaces and for power generation. Significant advances in modelling non-reactive turbulent flows are now possible with the development of large eddy simulation (LES), in which the large energetic scales of the flow are resolved on the grid while modelling the effects of the small scales. Here, we discuss the use of combustion LES in predictive modelling of propulsion applications such as gas turbine, ramjet and scramjet engines. The LES models used are described in some detail and are validated against laboratory data-of which results from two cases are presented. These validated LES models are then applied to an annular multi-burner gas turbine combustor and a simplified scramjet combustor, for which some additional experimental data are available. For these cases, good agreement with the available reference data is obtained, and the LES predictions are used to elucidate the flow physics in such devices to further enhance our knowledge of these propulsion systems. Particular attention is focused on the influence of the combustion chemistry, turbulence-chemistry interaction, self-ignition, flame holding burner-to-burner interactions and combustion oscillations.
Large-eddy simulation of flow past a circular cylinder
NASA Astrophysics Data System (ADS)
Cheng, Wan; Pullin, Dale; Samtaney, Ravi; Zhang, Wei
2015-11-01
Wall-modeled, large-eddy simulations (LES) about a smooth-walled circular cylinder are described. The cylinder is of diameter D and is of extent 3 D in the span-wise direction. The stretched-vortex sub-grid scale model is used away from the cylinder wall, including regions of large-scale separated flow. At the wall this is coupled directly to an extended version of the virtual-wall model (VWM) of Chung & Pullin (2009). Here the wall-adjacent flow is modeled by wall-normal integration of both components of the wall-parallel momentum equation across a thin wall-layer whose thickness is small compared to that of the local boundary layer. This provides a wall-parallel, cell-scale estimate of the surface stress-vector field across the entire cylinder surface, and, with further assumptions, gives a slip-velocity boundary condition for the outer-flow LES. Flow separation is captured. The LES are done with a fourth-order accurate finite-difference method with span-wise periodic boundary conditions. A third-order semi-implicit Runge-Kutta method is used for temporal discretization. The LES methodology is verified by comparison with DNS at ReD = 3 , 900 . LES at larger Reynolds number will be discussed. Supported partially by KAUST OCRF Award No. URF/1/1394-01 and partially by NSF award CBET 1235605.
A family of dynamic models for large-eddy simulation
NASA Technical Reports Server (NTRS)
Carati, D.; Jansen, K.; Lund, T.
1995-01-01
Since its first application, the dynamic procedure has been recognized as an effective means to compute rather than prescribe the unknown coefficients that appear in a subgrid-scale model for Large-Eddy Simulation (LES). The dynamic procedure is usually used to determine the nondimensional coefficient in the Smagorinsky (1963) model. In reality the procedure is quite general and it is not limited to the Smagorinsky model by any theoretical or practical constraints. The purpose of this note is to consider a generalized family of dynamic eddy viscosity models that do not necessarily rely on the local equilibrium assumption built into the Smagorinsky model. By invoking an inertial range assumption, it will be shown that the coefficients in the new models need not be nondimensional. This additional degree of freedom allows the use of models that are scaled on traditionally unknown quantities such as the dissipation rate. In certain cases, the dynamic models with dimensional coefficients are simpler to implement, and allow for a 30% reduction in the number of required filtering operations.
Large Eddy Simulation of Turbulent Flow in a Ribbed Pipe
NASA Astrophysics Data System (ADS)
Kang, Changwoo; Yang, Kyung-Soo
2011-11-01
Turbulent flow in a pipe with periodically wall-mounted ribs has been investigated by large eddy simulation with a dynamic subgrid-scale model. The value of Re considered is 98,000, based on hydraulic diameter and mean bulk velocity. An immersed boundary method was employed to implement the ribs in the computational domain. The spacing of the ribs is the key parameter to produce the d-type, intermediate and k-type roughness flows. The mean velocity profiles and turbulent intensities obtained from the present LES are in good agreement with the experimental measurements currently available. Turbulence statistics, including budgets of the Reynolds stresses, were computed, and analyzed to elucidate turbulence structures, especially around the ribs. In particular, effects of the ribs are identified by comparing the turbulence structures with those of smooth pipe flow. The present investigation is relevant to the erosion/corrosion that often occurs around a protruding roughness in a pipe system. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0008457).
Large eddy simulation of a high aspect ratio combustor
NASA Astrophysics Data System (ADS)
Kirtas, Mehmet
The present research investigates the details of mixture preparation and combustion in a two-stroke, small-scale research engine with a numerical methodology based on large eddy simulation (LES) technique. A major motivation to study such small-scale engines is their potential use in applications requiring portable power sources with high power density. The investigated research engine has a rectangular planform with a thickness very close to quenching limits of typical hydrocarbon fuels. As such, the combustor has a high aspect ratio (defined as the ratio of surface area to volume) that makes it different than the conventional engines which typically have small aspect ratios to avoid intense heat losses from the combustor in the bulk flame propagation period. In most other aspects, this engine involves all the main characteristics of traditional reciprocating engines. A previous experimental work has identified some major design problems and demonstrated the feasibility of cyclic combustion in the high aspect ratio combustor. Because of the difficulty of carrying out experimental studies in such small devices, resolving all flow structures and completely characterizing the flame propagation have been an enormously challenging task. The numerical methodology developed in this work attempts to complement these previous studies by providing a complete evolution of flow variables. Results of the present study demonstrated strengths of the proposed methodology in revealing physical processes occuring in a typical operation of the high aspect ratio combustor. For example, in the scavenging phase, the dominant flow structure is a tumble vortex that forms due to the high velocity reactant jet (premixed) interacting with the walls of the combustor. Since the scavenging phase is a long process (about three quarters of the whole cycle), the impact of the vortex is substantial on mixture preparation for the next combustion phase. LES gives the complete evolution of this flow
Numerical methods for large eddy simulation of acoustic combustion instabilities
NASA Astrophysics Data System (ADS)
Wall, Clifton T.
Acoustic combustion instabilities occur when interaction between the combustion process and acoustic modes in a combustor results in periodic oscillations in pressure, velocity, and heat release. If sufficiently large in amplitude, these instabilities can cause operational difficulties or the failure of combustor hardware. In many situations, the dominant instability is the result of the interaction between a low frequency acoustic mode of the combustor and the large scale hydrodynamics. Large eddy simulation (LES), therefore, is a promising tool for the prediction of these instabilities, since both the low frequency acoustic modes and the large scale hydrodynamics are well resolved in LES. Problems with the tractability of such simulations arise, however, due to the difficulty of solving the compressible Navier-Stokes equations efficiently at low Mach number and due to the large number of acoustic periods that are often required for such instabilities to reach limit cycles. An implicit numerical method for the solution of the compressible Navier-Stokes equations has been developed which avoids the acoustic CFL restriction, allowing for significant efficiency gains at low Mach number, while still resolving the low frequency acoustic modes of interest. In the limit of a uniform grid the numerical method causes no artificial damping of acoustic waves. New, non-reflecting boundary conditions have also been developed for use with the characteristic-based approach of Poinsot and Lele (1992). The new boundary conditions are implemented in a manner which allows for significant reduction of the computational domain of an LES by eliminating the need to perform LES in regions where one-dimensional acoustics significantly affect the instability but details of the hydrodynamics do not. These new numerical techniques have been demonstrated in an LES of an experimental combustor. The new techniques are shown to be an efficient means of performing LES of acoustic combustion
Large-eddy simulation of combustion dynamics in swirling flows
NASA Astrophysics Data System (ADS)
Stone, Christopher Pritchard
The impact of premixer swirl number, S, and overall fuel equivalence ratio, phi, on the stability of a model swirl-stabilized, lean-premixed gas turbine combustor has been numerically investigated using a massively-parallel Large-Eddy Simulations Combustion Dynamics model. Through the use of a premixed combustion model, unsteady vortex-flame and acoustic-flame interactions are captured. It is observed that for flows with swirl intensity high enough to form Vortex-Breakdown (i.e., a phenomena associated with a large region of reverse or recirculating flow along the axis of rotation), the measured rms pressure amplitude (p') are attenuated significantly (over 6.6 dB reduction) compared to flows without this phenomena. The reduced p' amplitudes are accompanied by reduced longitudinal flame-front oscillations and reduced coherence in the shed vortices. Similar p' reduction levels are achieved through changes in the operating equivalence ratio, phi. Compared to the leanest equivalence ratio simulated (phi = 0.52), p' at a stoichiometric mixture is reduced by 6.0 dB. Methodologies for active control based on modulation of the inlet Swirl number (S, a measure of the intensity of swirl) and phi are also investigated. Open-loop control through S variation is demonstrated for a lean mixture with a significant reduction in the fluctuating mass-flow-rate and p' after a convective time-delay. A partially-premixed combustion model, which allows for variations in the local phi, is used to model both temporal and spatial variations in phi. It is found that the response time to changes in phi are much faster than those for changes in S. Also, it is shown that spatial variations in phi (or unmixedness) actually lead to p' attenuation in the current combustor configuration.
NASA Technical Reports Server (NTRS)
Bardino, J.; Ferziger, J. H.; Reynolds, W. C.
1983-01-01
The physical bases of large eddy simulation and subgrid modeling are studied. A subgrid scale similarity model is developed that can account for system rotation. Large eddy simulations of homogeneous shear flows with system rotation were carried out. Apparently contradictory experimental results were explained. The main effect of rotation is to increase the transverse length scales in the rotation direction, and thereby decrease the rates of dissipation. Experimental results are shown to be affected by conditions at the turbulence producing grid, which make the initial states a function of the rotation rate. A two equation model is proposed that accounts for effects of rotation and shows good agreement with experimental results. In addition, a Reynolds stress model is developed that represents the turbulence structure of homogeneous shear flows very well and can account also for the effects of system rotation.
Large eddy simulation of mixing between hot and cold sodium flows - comparison with experiments
Simoneau, J.P.; Noe, H.; Menant, B.
1995-09-01
The large eddy simulation is becoming a potential powerful tool for the calculation of turbulent flows. In nuclear liquid metal cooled fast reactors, the knowledge of the turbulence characteristics is of great interest for the prediction and the analysis of thermal stripping phenomena. The objective of this paper is to give a contribution in the evaluation of the large eddy simulation technique is an individual case. The problem chosen is the case of the mixing between hot and cold sodium flows. The computations are compared with available sodium tests. This study shows acceptable qualitative results but the simple model used is not able to predict the turbulence characteristics. More complex models including larger domains around the fluctuating zone and fluctuating boundary conditions could be necessary. Validation works are continuing.
Large-Eddy Simulation on turbulent flow and plume dispersion over a 2-dimensional hill
NASA Astrophysics Data System (ADS)
Nakayama, H.; Nagai, H.
2010-05-01
The dispersion analysis of airborne contaminants including radioactive substances from industrial or nuclear facilities is an important issue for air quality maintenance and safety assessment. In Japan, many nuclear power plants are located at complex coastal terrains. In these cases, terrain effects on the turbulent flow and plume dispersion should be investigated. In this study, we perform Large-Eddy Simulation (LES) of turbulent flow and plume dispersion over a 2-dimensional hill flow and investigate the characteristics of mean and fluctuating concentrations.
NASA Technical Reports Server (NTRS)
Zhang, Zhibo; Ackerman, Andrew S.; Feingold, Graham; Platnick, Steven; Pincus, Robert; Xue, Huiwen
2012-01-01
This study investigates effects of drizzle and cloud horizontal inhomogeneity on cloud effective radius (re) retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS). In order to identify the relative importance of various factors, we developed a MODIS cloud property retrieval simulator based on the combination of large-eddy simulations (LES) and radiative transfer computations. The case studies based on synthetic LES cloud fields indicate that at high spatial resolution (100 m) 3-D radiative transfer effects, such as illumination and shadowing, can induce significant differences between retrievals ofre based on reflectance at 2.1 m (re,2.1) and 3.7 m (re,3.7). It is also found that 3-D effects tend to have stronger impact onre,2.1 than re,3.7, leading to positive difference between the two (re,3.72.1) from illumination and negative re,3.72.1from shadowing. The cancellation of opposing 3-D effects leads to overall reasonable agreement betweenre,2.1 and re,3.7 at high spatial resolution as far as domain averages are concerned. At resolutions similar to MODIS, however, re,2.1 is systematically larger than re,3.7when averaged over the LES domain, with the difference exhibiting a threshold-like dependence on bothre,2.1and an index of the sub-pixel variability in reflectance (H), consistent with MODIS observations. In the LES cases studied, drizzle does not strongly impact reretrievals at either wavelength. It is also found that opposing 3-D radiative transfer effects partly cancel each other when cloud reflectance is aggregated from high spatial resolution to MODIS resolution, resulting in a weaker net impact of 3-D radiative effects onre retrievals. The large difference at MODIS resolution between re,3.7 and re,2.1 for highly inhomogeneous pixels with H 0.4 can be largely attributed to what we refer to as the plane-parallelrebias, which is attributable to the impact of sub-pixel level horizontal variability of cloud optical thickness onre retrievals
Rayleigh-Taylor mixing: direct numerical simulation and implicit large eddy simulation
NASA Astrophysics Data System (ADS)
Youngs, David L.
2017-07-01
Previous research into three-dimensional numerical simulation of self-similar mixing due to Rayleigh-Taylor instability is summarized. A range of numerical approaches has been used: direct numerical simulation, implicit large eddy simulation and large eddy simulation with an explicit model for sub-grid-scale dissipation. However, few papers have made direct comparisons between the various approaches. The main purpose of the current paper is to give comparisons of direct numerical simulations and implicit large eddy simulations using the same computational framework. Results are shown for four test cases: (i) single-mode Rayleigh-Taylor instability, (ii) self-similar Rayleigh-Taylor mixing, (iii) three-layer mixing and (iv) a tilted-rig Rayleigh-Taylor experiment. It is found that both approaches give similar results for the high-Reynolds number behavior. Direct numerical simulation is needed to assess the influence of finite Reynolds number.
Large eddy simulation of soot evolution in an aircraft combustor
NASA Astrophysics Data System (ADS)
Mueller, Michael E.; Pitsch, Heinz
2013-11-01
An integrated kinetics-based Large Eddy Simulation (LES) approach for soot evolution in turbulent reacting flows is applied to the simulation of a Pratt & Whitney aircraft gas turbine combustor, and the results are analyzed to provide insights into the complex interactions of the hydrodynamics, mixing, chemistry, and soot. The integrated approach includes detailed models for soot, combustion, and the unresolved interactions between soot, chemistry, and turbulence. The soot model is based on the Hybrid Method of Moments and detailed descriptions of soot aggregates and the various physical and chemical processes governing their evolution. The detailed kinetics of jet fuel oxidation and soot precursor formation is described with the Radiation Flamelet/Progress Variable model, which has been modified to account for the removal of soot precursors from the gas-phase. The unclosed filtered quantities in the soot and combustion models, such as source terms, are closed with a novel presumed subfilter PDF approach that accounts for the high subfilter spatial intermittency of soot. For the combustor simulation, the integrated approach is combined with a Lagrangian parcel method for the liquid spray and state-of-the-art unstructured LES technology for complex geometries. Two overall fuel-to-air ratios are simulated to evaluate the ability of the model to make not only absolute predictions but also quantitative predictions of trends. The Pratt & Whitney combustor is a Rich-Quench-Lean combustor in which combustion first occurs in a fuel-rich primary zone characterized by a large recirculation zone. Dilution air is then added downstream of the recirculation zone, and combustion continues in a fuel-lean secondary zone. The simulations show that large quantities of soot are formed in the fuel-rich recirculation zone, and, furthermore, the overall fuel-to-air ratio dictates both the dominant soot growth process and the location of maximum soot volume fraction. At the higher fuel
A perspective on large eddy simulation of problems in the nuclear industry
Hassan, Y.A.; Pruitt, J.M.; Steininger, D.A.
1995-12-01
Because of the complex nature of coolant flow in nuclear reactors, current subchannel methods for light water reactor analysis are insufficient. The large eddy simulation method has been proposed as a computational tool for subchannel analysis. In large eddy simulation, large flow structures are computed while small scales are modeled, thereby decreasing computational time as compared with direct numerical simulation methods. Large eddy simulation has been used in complex geometry calculations providing good results in tube bundle cross-flow situations in steam generators. It is proposed that the large eddy simulation method be extended from single- to two-phase flow calculations to help in the prediction of the thermal diffusion of energy between adjacent subchannels.
Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Givi, Peyman; Madnia, C. K.; Steinberger, C. J.; Tsai, A.
1991-01-01
This research is involved with the implementations of advanced computational schemes based on large eddy simulations (LES) and direct numerical simulations (DNS) to study the phenomenon of mixing and its coupling with chemical reactions in compressible turbulent flows. In the efforts related to LES, a research program was initiated to extend the present capabilities of this method for the treatment of chemically reacting flows, whereas in the DNS efforts, focus was on detailed investigations of the effects of compressibility, heat release, and nonequilibrium kinetics modeling in high speed reacting flows. The efforts to date were primarily focussed on simulations of simple flows, namely, homogeneous compressible flows and temporally developing hign speed mixing layers. A summary of the accomplishments is provided.
Hybrid Reynolds-Averaged/Large-Eddy Simulations of a Coaxial Supersonic Free-Jet Experiment
NASA Technical Reports Server (NTRS)
Baurle, Robert A.; Edwards, Jack R.
2010-01-01
Reynolds-averaged and hybrid Reynolds-averaged/large-eddy simulations have been applied to a supersonic coaxial jet flow experiment. The experiment was designed to study compressible mixing flow phenomenon under conditions that are representative of those encountered in scramjet combustors. The experiment utilized either helium or argon as the inner jet nozzle fluid, and the outer jet nozzle fluid consisted of laboratory air. The inner and outer nozzles were designed and operated to produce nearly pressure-matched Mach 1.8 flow conditions at the jet exit. The purpose of the computational effort was to assess the state-of-the-art for each modeling approach, and to use the hybrid Reynolds-averaged/large-eddy simulations to gather insight into the deficiencies of the Reynolds-averaged closure models. The Reynolds-averaged simulations displayed a strong sensitivity to choice of turbulent Schmidt number. The initial value chosen for this parameter resulted in an over-prediction of the mixing layer spreading rate for the helium case, but the opposite trend was observed when argon was used as the injectant. A larger turbulent Schmidt number greatly improved the comparison of the results with measurements for the helium simulations, but variations in the Schmidt number did not improve the argon comparisons. The hybrid Reynolds-averaged/large-eddy simulations also over-predicted the mixing layer spreading rate for the helium case, while under-predicting the rate of mixing when argon was used as the injectant. The primary reason conjectured for the discrepancy between the hybrid simulation results and the measurements centered around issues related to the transition from a Reynolds-averaged state to one with resolved turbulent content. Improvements to the inflow conditions were suggested as a remedy to this dilemma. Second-order turbulence statistics were also compared to their modeled Reynolds-averaged counterparts to evaluate the effectiveness of common turbulence closure
Coelho, P.J.
2009-05-15
An analysis of the relevance of turbulence-radiation interaction in the numerical simulation of turbulent reactive flows is presented. A semi-causal stochastic model was used to generate a time-series of turbulent scalar fluctuations along optical paths of Sandia flame D, a widely studied piloted turbulent jet nonpremixed flame. The radiative transfer equation was integrated along these paths for every realization using a grid resolution typical of a direct numerical simulation. The correlated k-distribution method was employed to compute the radiative properties of the medium. The results were used to determine the ensemble average, as well as the extreme values, of quantities that indicate the importance of the turbulence-radiation interaction. Several approximate methods are then proposed to solve the filtered radiative transfer equation in the framework of large eddy simulations. The proposed methods are applicable along with combustion models that either assume the filtered probability density function of a conserved scalar or solve a transport equation for a joint scalar or joint scalar/velocity filtered density function. It is concluded that the errors resulting from neglecting the turbulence-radiation interaction in large eddy simulations are much lower than those found in Reynolds-averaged Navier-Stokes calculations. The optically thin fluctuation approximation may be extended to large eddy simulations yielding predictions in excellent agreement with the reference solution. If the turbulence-radiation interaction is accounted for using this approximation, the average relative error of the filtered total radiation intensity is generally below 0.3% for the studied flame. (author)
NASA Astrophysics Data System (ADS)
Lu, Miao-Ling; Seinfeld, John H.
2006-01-01
Through three-dimensional large-eddy simulations of marine stratocumulus we explore the factors that control the cloud spectral relative dispersion (ratio of cloud droplet spectral width to the mean radius of the distribution) as a function of aerosol number concentration and the extent to which the relative dispersion either enhances or mitigates the Twomey effect. We find that relative dispersion decreases with increasing aerosol number concentration (for aerosol number concentrations less than about 1000 cm-3) because smaller droplets resulting from higher aerosol number concentrations inhibit precipitation and lead to (1) less spectral broadening by suppressed collision and coalescence processes and (2) more spectral narrowing by droplet condensational growth at higher updraft velocity because reduced drizzle latent heating at cloud top results in increased boundary layer turbulent kinetic energy production by buoyancy and thereby stronger turbulence. Increased spectral broadening owing to increased cloud-top entrainment mixing, also as a result of increased boundary layer turbulence, is relatively insignificant compared with outcomes 1 and 2. The coefficient k, an important parameter that relates cloud droplet effective radius and volume mean radius in large-scale models, is a function of skewness and relative dispersion of the distribution and is negatively correlated with relative dispersion. Increasing k with increasing aerosol number concentration leads to maximum enhancement of the cloud susceptibility (the change of cloud optical depth due to change of cloud droplet number concentration) over that attributable to the Twomey effect alone by about 4.2% and 39% for simulated FIRE and ASTEX cases, respectively.
Large eddy simulation of Rayleigh-Taylor instability using the arbitrary Lagrangian-Eulerian method
NASA Astrophysics Data System (ADS)
Darlington, Rebecca Mattson
This research addresses the application of a large eddy simulation (LES) to Arbitrary Lagrangian Eulerian (ALE) simulations of Rayleigh-Taylor instability. First, ALE simulations of simplified Rayleigh-Taylor instability are studied. The advantages of ALE over Eulerian simulations are shown. Next, the behavior of the LES is examined in a more complicated ALE simulation of Rayleigh-Taylor instability. The effects of eddy viscosity and stochastic backscatter are examined. The LES is also coupled with ALE to increase grid resolution in areas where it is needed. Finally, the methods studied above are applied to two sets of experimental simulations. In these simulations, ALE allows the mesh to follow expanding experimental targets, while LES can be used to mimic the effect of unresolved instability modes.
Large eddy simulation of Rayleigh-Taylor instability using the arbitrary Lagrangian-Eulerian method
Darlington, Rebecca Mattson
1999-12-01
This research addresses the application of a large eddy simulation (LES) to Arbitrary Lagrangian Eulerian (ALE) simulations of Rayleigh-Taylor instability. First, ALE simulations of simplified Rayleigh-Taylor instability are studied. The advantages of ALE over Eulerian simulations are shown. Next, the behavior of the LES is examined in a more complicated ALE simulation of Rayleigh-Taylor instability. The effects of eddy viscosity and stochastic backscatter are examined. The LES is also coupled with ALE to increase grid resolution in areas where it is needed. Finally, the methods studied above are applied to two sets of experimental simulations. In these simulations, ALE allows the mesh to follow expanding experimental targets, while LES can be used to mimic the effect of unresolved instability modes.
Large-eddy simulation of flow in a plane, asymmetric diffuser
NASA Technical Reports Server (NTRS)
Kaltenbach, Hans-Jakob
1993-01-01
Recent improvements in subgrid-scale modeling as well as increases in computer power make it feasible to investigate flows using large-eddy simulation (LES) which have been traditionally studied with techniques based on Reynolds averaging. However, LES has not yet been applied to many flows of immediate technical interest. Preliminary results from LES of a plane diffuser flow are described. The long term goal of this work is to investigate flow separation as well as separation control in ducts and ramp-like geometries.
Large-eddy simulations of a Salt Lake Valley cold-air pool
NASA Astrophysics Data System (ADS)
Crosman, Erik T.; Horel, John D.
2017-09-01
Persistent cold-air pools are often poorly forecast by mesoscale numerical weather prediction models, in part due to inadequate parameterization of planetary boundary-layer physics in stable atmospheric conditions, and also because of errors in the initialization and treatment of the model surface state. In this study, an improved numerical simulation of the 27-30 January 2011 cold-air pool in Utah's Great Salt Lake Basin is obtained using a large-eddy simulation with more realistic surface state characterization. Compared to a Weather Research and Forecasting model configuration run as a mesoscale model with a planetary boundary-layer scheme where turbulence is highly parameterized, the large-eddy simulation more accurately captured turbulent interactions between the stable boundary-layer and flow aloft. The simulations were also found to be sensitive to variations in the Great Salt Lake temperature and Salt Lake Valley snow cover, illustrating the importance of land surface state in modelling cold-air pools.
Wind turbine wakes in forest and neutral plane wall boundary layer large-eddy simulations
NASA Astrophysics Data System (ADS)
Schröttle, Josef; Piotrowski, Zbigniew; Gerz, Thomas; Englberger, Antonia; Dörnbrack, Andreas
2016-09-01
Wind turbine wake flow characteristics are studied in a strongly sheared and turbulent forest boundary layer and a neutral plane wall boundary layer flow. The reference simulations without wind turbine yield similar results as earlier large-eddy simulations by Shaw and Schumann (1992) and Porte-Agel et al. (2000). To use the fields from the homogeneous turbulent boundary layers on the fly as inflow fields for the wind turbine wake simulations, a new and efficient methodology was developed for the multiscale geophysical flow solver EULAG. With this method fully developed turbulent flow fields can be achieved upstream of the wind turbine which are independent of the wake flow. The large-eddy simulations reproduce known boundary-layer statistics as mean wind profile, momentum flux profile, and eddy dissipation rate of the plane wall and the forest boundary layer. The wake velocity deficit is more asymmetric above the forest and recovers faster downstream compared to the velocity deficit in the plane wall boundary layer. This is due to the inflection point in the mean streamwise velocity profile with corresponding turbulent coherent structures of high turbulence intensity in the strong shear flow above the forest.
Sunset decay of the convective turbulence with Large-Eddy Simulation under realistic conditions
NASA Astrophysics Data System (ADS)
Rizza, U.; Miglietta, M. M.; Degrazia, G. A.; Acevedo, O. C.; Marques Filho, E. P.
2013-10-01
Large-Eddy Simulation is performed for a single day from the Cooperative Atmosphere-Surface Exchange Study (CASES-99) field program. This study investigates an observed case of evening transition boundary layer over land. Parameters of the ambient atmosphere in the LES-decay studies conducted so far were typically prescribed in an idealized form. To provide suitable data under the wide range of the PBL weather conditions, the LES should be able to adequately reproduce the PBL turbulence dynamics including-if possible-baroclinicity, radiation, large scale advection and not only be related to a decreasing surface heating. In addition LES-decay studies usually assume that the sensible heat flux decreases instantaneously or with a very short time scale. The main purpose of this investigation is to study the decay of boundary-layer average turbulent kinetic energy at sunset with Large-Eddy Simulation that is forced with realistic environment conditions. This allows investigating the Turbulent Kinetic Energy decay over the realistic time scale that is observed in the atmosphere. During the intermediate and last stage of decay of the boundary-layer average Turbulent Kinetic Energy the exponents of the decay power law t go from 2 to 6, as evidenced by experimental results and recent analytical modeling in the surface layer.
NASA Astrophysics Data System (ADS)
Zhu, P.
2008-05-01
Hurricane boundary layer (HBL) processes, especially, the structure of the coherent large eddy circulations (LECs) and their induced vertical transport, are not well understood. This paper introduces a large eddy simulation (LES) framework in a weather hindcasting mode developed from a multiple scale nested Weather Research & Forecasting (WRF) model. Using the WRF-LES, this study investigated the structure of the HBL LECs and the associated vertical transport during the landfall of Hurricane Ivan (2004). The simulation shows that the HBL LECs exist in a mean stable environment and consist of well defined updraft and downdraft. Statistically, the HBL LECs are only slightly skewed with the updrafts and downdrafts relatively evenly distributed spatially. The inversion base basically envelopes the upper boundary of LECs. The trough in between two adjacent LECs is where most entrainment takes place, whereas the crest of the LECs is where boundary layer air detrains out of the HBL. In such a way, LECs directly connect the surface, the HBL, and the main body of a hurricane vortex and enhance the exchange of energy, moisture, and momentum between them. It is found that the current boundary layer schemes significantly under-estimate the resolved turbulent fluxes due to the fact that the effects of LECs have not been included in the parameterizations. Based on the statistical structure of LECs simulated by the WRF-LES, this paper proposes a conceptual updraft-downdraft model that can potentially be implemented in weather forecasting models to parameterize the fluxes induced by the HBL LEC transport.
Large Eddy Simulations of Severe Convection Induced Turbulence
NASA Technical Reports Server (NTRS)
Ahmad, Nash'at; Proctor, Fred
2011-01-01
Convective storms can pose a serious risk to aviation operations since they are often accompanied by turbulence, heavy rain, hail, icing, lightning, strong winds, and poor visibility. They can cause major delays in air traffic due to the re-routing of flights, and by disrupting operations at the airports in the vicinity of the storm system. In this study, the Terminal Area Simulation System is used to simulate five different convective events ranging from a mesoscale convective complex to isolated storms. The occurrence of convection induced turbulence is analyzed from these simulations. The validation of model results with the radar data and other observations is reported and an aircraft-centric turbulence hazard metric calculated for each case is discussed. The turbulence analysis showed that large pockets of significant turbulence hazard can be found in regions of low radar reflectivity. Moderate and severe turbulence was often found in building cumulus turrets and overshooting tops.
Discussion of the potential and limitations of direct and large-eddy simulations
NASA Technical Reports Server (NTRS)
Hussaini, M. Y.; Speziale, Charles G.; Zang, Thomas A.
1989-01-01
The full text of the discussion paper presented at the Whither Turbulence Workshop on the potential and limitations of direct and large-eddy simulations is provided. Particular emphasis is placed on discussing the role of numerics and mathematical theory in direct simulations of both compressible and incompressible flows. A variety of unresolved issues with large-eddy simulations such as their implementation in high-order finite difference codes, problems with defiltering, and modifications to accommodate integrations to solid boundaries are elaborated on. These as well as other points are discussed in detail along with the authors' views concerning the prospects for future research.
An Examination of Parameters Affecting Large Eddy Simulations of Flow Past a Square Cylinder
NASA Technical Reports Server (NTRS)
Mankbadi, M. R.; Georgiadis, N. J.
2014-01-01
Separated flow over a bluff body is analyzed via large eddy simulations. The turbulent flow around a square cylinder features a variety of complex flow phenomena such as highly unsteady vortical structures, reverse flow in the near wall region, and wake turbulence. The formation of spanwise vortices is often times artificially suppressed in computations by either insufficient depth or a coarse spanwise resolution. As the resolution is refined and the domain extended, the artificial turbulent energy exchange between spanwise and streamwise turbulence is eliminated within the wake region. A parametric study is performed highlighting the effects of spanwise vortices where the spanwise computational domain's resolution and depth are varied. For Re=22,000, the mean and turbulent statistics computed from the numerical large eddy simulations (NLES) are in good agreement with experimental data. Von-Karman shedding is observed in the wake of the cylinder. Mesh independence is illustrated by comparing a mesh resolution of 2 million to 16 million. Sensitivities to time stepping were minimized and sampling frequency sensitivities were nonpresent. While increasing the spanwise depth and resolution can be costly, this practice was found to be necessary to eliminating the artificial turbulent energy exchange.
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.
Large-eddy simulation of a turbulent flow over a heavy vehicle with drag reduction devices
NASA Astrophysics Data System (ADS)
Lee, Sangseung; Kim, Myeongkyun; You, Donghyun
2015-11-01
Aerodynamic drag contributes to a considerable amount of energy loss of heavy vehicles. To reduce the energy loss, drag reduction devices such as side skirts and boat tails, are often installed to the side and the rear of a heavy vehicle. In the present study, turbulent flow around a heavy vehicle with realistic geometric details is simulated using large-eddy simulation (LES), which is capable of providing unsteady flow physics responsible for aerodynamic in sufficient detail. Flow over a heavy vehicle with and without a boat tail and side skirts as drag reduction devices is simulated. The simulation results are validated against accompanying in-house experimental measurements. Effects of a boat tail and side skirts on drag reduction are discussed in detail. Supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) Grant NTIS 1615007940.
Wall-Resolved Large-Eddy Simulation of Flow Separation Over NASA Wall-Mounted Hump
NASA Technical Reports Server (NTRS)
Uzun, Ali; Malik, Mujeeb R.
2017-01-01
This paper reports the findings from a study that applies wall-resolved large-eddy simulation to investigate flow separation over the NASA wall-mounted hump geometry. Despite its conceptually simple flow configuration, this benchmark problem has proven to be a challenging test case for various turbulence simulation methods that have attempted to predict flow separation arising from the adverse pressure gradient on the aft region of the hump. The momentum-thickness Reynolds number of the incoming boundary layer has a value that is near the upper limit achieved by recent direct numerical simulation and large-eddy simulation of incompressible turbulent boundary layers. The high Reynolds number of the problem necessitates a significant number of grid points for wall-resolved calculations. The present simulations show a significant improvement in the separation-bubble length prediction compared to Reynolds-Averaged Navier-Stokes calculations. The current simulations also provide good overall prediction of the skin-friction distribution, including the relaminarization observed over the front portion of the hump due to the strong favorable pressure gradient. We discuss a number of problems that were encountered during the course of this work and present possible solutions. A systematic study regarding the effect of domain span, subgrid-scale model, tunnel back pressure, upstream boundary layer conditions and grid refinement is performed. The predicted separation-bubble length is found to be sensitive to the span of the domain. Despite the large number of grid points used in the simulations, some differences between the predictions and experimental observations still exist (particularly for Reynolds stresses) in the case of the wide-span simulation, suggesting that additional grid resolution may be required.
The Jefferson Project: Large-eddy simulations of a watershed
NASA Astrophysics Data System (ADS)
Watson, C.; Cipriani, J.; Praino, A. P.; Treinish, L. A.; Tewari, M.; Kolar, H.
2015-12-01
The Jefferson Project is a new endeavor at Lake George, NY by IBM Research, Rensselaer Polytechnic Institute (RPI) and The Fund for Lake George. Lake George is an oligotrophic lake - one of low nutrients - and a 30-year study recently published by RPI's Darrin Fresh Water Institute highlighted the renowned water quality is declining from the injection of salt (from runoff), algae, and invasive species. In response, the Jefferson Project is developing a system to provide extensive data on relevant physical, chemical and biological parameters that drive ecosystem function. The system will be capable of real-time observations and interactive modeling of the atmosphere, watershed hydrology, lake circulation and food web dynamics. In this presentation, we describe the development of the operational forecast system used to simulate the atmosphere in the model stack, Deep ThunderTM (a configuration of the ARW-WRF model). The model performs 48-hr forecasts twice daily in a nested configuration, and in this study we present results from ongoing tests where the innermost domains are dx = 333-m and 111-m. We discuss the model's ability to simulate boundary layer processes, lake surface conditions (an input into the lake model), and precipitation (an input into the hydrology model) during different weather regimes, and the challenges of data assimilation and validation at this scale. We also explore the potential for additional nests over select regions of the watershed to better capture turbulent boundary layer motions.
Large-eddy simulation of supercritical fluid flow and combustion
NASA Astrophysics Data System (ADS)
Huo, Hongfa
The present study focuses on the modeling and simulation of injection, mixing, and combustion of real fluids at supercritical conditions. The objectives of the study are: (1) to establish a unified theoretical framework that can be used to study the turbulent combustion of real fluids; (2) to implement the theoretical framework and conduct numerical studies with the aim of improving the understanding of the flow and combustion dynamics at conditions representative of contemporary liquid-propellant rocket engine operation; (3) to identify the key design parameters and the flow variables which dictate the dynamics characteristics of swirl- and shear- coaxial injectors. The theoretical and numerical framework is validated by simulating the Sandia Flame D. The calculated axial and radial profiles of velocity, temperature, and mass fractions of major species are in reasonably good agreement with the experimental measurements. The conditionally averaged mass fraction profiles agree very well with the experimental results at different axial locations. The validated model is first employed to examine the flow dynamics of liquid oxygen in a pressure swirl injector at supercritical conditions. Emphasis is placed on analyzing the effects of external excitations on the dynamic response of the injector. The high-frequency fluctuations do not significantly affect the flow field as they are dissipated shortly after being introduced into the flow. However, the lower-frequency fluctuations are amplified by the flow. As a result, the film thickness and the spreading angle at the nozzle exit fluctuate strongly for low-frequency external excitations. The combustion of gaseous oxygen/gaseous hydrogen in a high-pressure combustion chamber for a shear coaxial injector is simulated to assess the accuracy and the credibility of the computer program when applied to a sub-scale model of a combustor. The predicted heat flux profile is compared with the experimental and numerical studies. The
Large eddy simulation of combustion instability in a tripropellant air heater
NASA Astrophysics Data System (ADS)
Yuan, Lei; Shen, Chibing
2016-12-01
This research is motivated by the issue associated with high frequency combustion instability. Large eddy simulation was performed to investigate spontaneous combustion instability in an air/LO2/C2H5OH tripropellant air heater. The simulation predicts self-excited transverse oscillations. Overall behavior of combustion instability including pressure time histories, mode shapes, Rayleigh index and unsteady response of the injector were studied in detail. Special emphasis was given to the flame behavior, droplet trajectories, pressure evolutions, and formation of large-scale vortical structures during combustion instability in present air heater. Furthermore, in contrast to previous investigations, a new process is identified in the simulation that may feed energy into the acoustic mode and drive combustion instability.
Large eddy simulation of a particle-laden turbulent plane jet.
Jin, Han-Hui; Luo, Kun; Fan, Jian-Ren; Cen, Ke-Fa
2003-01-01
Gas-solid two-phase turbulent plane jet is applied to many natural situations and in engineering systems. To predict the particle dispersion in the gas jet is of great importance in industrial applications and in the designing of engineering systems. A large eddy simulation of the two-phase plane jet was conducted to investigate the particle dispersion patterns. The particles with Stokes numbers equal to 0.0028, 0.3, 2.5, 28 (corresponding to particle diameter 1 microm, 10 microm, 30 microm, 100 microm, respectively) in Re = 11 300 gas flow were studied. The simulation results of gas phase motion agreed well with previous experimental results. And the simulation results of the solid particles motion showed that particles with different Stokes number have different spatial dispersion; and that particles with intermediate Stokes number have the largest dispersion ratio.
Large Eddy Simulation of complex sidearms subject to solar radiation and surface cooling.
Dittko, Karl A; Kirkpatrick, Michael P; Armfield, Steven W
2013-09-15
Large Eddy Simulation (LES) is used to model two lake sidearms subject to heating from solar radiation and cooling from a surface flux. The sidearms are part of Lake Audrey, NJ, USA and Lake Alexandrina, SA, Australia. The simulation domains are created using bathymetry data and the boundary is modelled with an Immersed Boundary Method. We investigate the cooling and heating phases with separate quasi-steady state simulations. Differential heating occurs in the cavity due to the changing depth. The resulting temperature gradients drive lateral flows. These flows are the dominant transport process in the absence of wind. Study in this area is important in water quality management as the lateral circulation can carry particles and various pollutants, transporting them to and mixing them with the main lake body.
Large eddy simulation of forest canopy flow for wildland fire modeling
Eric Mueller; William Mell; Albert Simeoni
2014-01-01
Large eddy simulation (LES) based computational fluid dynamics (CFD) simulators have obtained increasing attention in the wildland fire research community, as these tools allow the inclusion of important driving physics. However, due to the complexity of the models, individual aspects must be isolated and tested rigorously to ensure meaningful results. As wind is a...
Modelling artificial sea salt emission in large eddy simulations
Maalick, Z.; Korhonen, H.; Kokkola, H.; Kühn, T.; Romakkaniemi, S.
2014-01-01
We study the dispersion of sea salt particles from artificially injected sea spray at a cloud-resolving scale. Understanding of how different aerosol processes affect particle dispersion is crucial when designing emission sources for marine cloud brightening. Compared with previous studies, we include for the first time an explicit treatment of aerosol water, which takes into account condensation, evaporation and their effect on ambient temperature. This enables us to capture the negative buoyancy caused by water evaporation from aerosols. Additionally, we use a higher model resolution to capture aerosol loss through coagulation near the source point. We find that, with a seawater flux of 15 kg s−1, the cooling due to evaporation can be as much as 1.4 K, causing a delay in particle dispersion of 10–20 min. This delay enhances particle scavenging by a factor of 1.14 compared with simulations without aerosol water. We further show that both cooling and particle dispersion depend on the model resolution, with a maximum particle scavenging efficiency of 20% within 5 h after emission at maximum resolution of 50 m. Based on these results, we suggest further regional high-resolution studies which model several injection periods over several weeks. PMID:25404679
Modelling artificial sea salt emission in large eddy simulations.
Maalick, Z; Korhonen, H; Kokkola, H; Kühn, T; Romakkaniemi, S
2014-12-28
We study the dispersion of sea salt particles from artificially injected sea spray at a cloud-resolving scale. Understanding of how different aerosol processes affect particle dispersion is crucial when designing emission sources for marine cloud brightening. Compared with previous studies, we include for the first time an explicit treatment of aerosol water, which takes into account condensation, evaporation and their effect on ambient temperature. This enables us to capture the negative buoyancy caused by water evaporation from aerosols. Additionally, we use a higher model resolution to capture aerosol loss through coagulation near the source point. We find that, with a seawater flux of 15 kg s(-1), the cooling due to evaporation can be as much as 1.4 K, causing a delay in particle dispersion of 10-20 min. This delay enhances particle scavenging by a factor of 1.14 compared with simulations without aerosol water. We further show that both cooling and particle dispersion depend on the model resolution, with a maximum particle scavenging efficiency of 20% within 5 h after emission at maximum resolution of 50 m. Based on these results, we suggest further regional high-resolution studies which model several injection periods over several weeks. © 2014 The Author(s) Published by the Royal Society. All rights reserved.
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.
Characterizing the coherent structures in large eddy simulations of aligned windfarms
NASA Astrophysics Data System (ADS)
Zhang, Mengqi; Stevens, Richard J. A. M.
2017-05-01
The present work studies the large coherent structures in large eddy simulations of windfarms using proper orthogonal decomposition (POD) method. In order to evaluate the effect of wind turbines on coherent structures, we consider three cases. One is a reference flow of a neutral atmospheric boundary layer and the other two are periodic and developing aligned windfarms. The number of wind turbines is large, 16 × 12 for periodic windfarm, and 12 × 12 for developing windfarm. The simulations are run for a long time in order to generate a sufficient database for POD analysis. In all cases, elongated streamwise counter rotating roll structures, covering 1 or 2 turbines in spanwise direction, are identified as the dominant POD mode. Another pattern, varying in streamwise direction, also appears in all the three cases.
Large-eddy simulations of turbulent flow for grid-to-rod fretting in nuclear reactors
Bakosi, J.; Christon, M. A.; Lowrie, R. B.; Pritchett-Sheats, L. A.; Nourgaliev, R. R.
2013-07-12
The grid-to-rod fretting (GTRF) problem in pressurized water reactors is a flow-induced vibration problem that results in wear and failure of the fuel rods in nuclear assemblies. In order to understand the fluid dynamics of GTRF and to build an archival database of turbulence statistics for various configurations, implicit large-eddy simulations of time-dependent single-phase turbulent flow have been performed in 3 × 3 and 5 × 5 rod bundles with a single grid spacer. To assess the computational mesh and resolution requirements, a method for quantitative assessment of unstructured meshes with no-slip walls is described. The calculations have been carried out using Hydra-TH, a thermal-hydraulics code developed at Los Alamos for the Consortium for Advanced Simulation of Light water reactors, a United States Department of Energy Innovation Hub. Hydra-TH uses a second-order implicit incremental projection method to solve the singlephase incompressible Navier-Stokes equations. The simulations explicitly resolve the large scale motions of the turbulent flow field using first principles and rely on a monotonicity-preserving numerical technique to represent the unresolved scales. Each series of simulations for the 3 × 3 and 5 × 5 rod-bundle geometries is an analysis of the flow field statistics combined with a mesh-refinement study and validation with available experimental data. Our primary focus is the time history and statistics of the forces loading the fuel rods. These hydrodynamic forces are believed to be the key player resulting in rod vibration and GTRF wear, one of the leading causes for leaking nuclear fuel which costs power utilities millions of dollars in preventive measures. As a result, we demonstrate that implicit large-eddy simulation of rod-bundle flows is a viable way to calculate the excitation forces for the GTRF problem.
Large-eddy simulations of turbulent flow for grid-to-rod fretting in nuclear reactors
Bakosi, J.; Christon, M. A.; Lowrie, R. B.; ...
2013-07-12
The grid-to-rod fretting (GTRF) problem in pressurized water reactors is a flow-induced vibration problem that results in wear and failure of the fuel rods in nuclear assemblies. In order to understand the fluid dynamics of GTRF and to build an archival database of turbulence statistics for various configurations, implicit large-eddy simulations of time-dependent single-phase turbulent flow have been performed in 3 × 3 and 5 × 5 rod bundles with a single grid spacer. To assess the computational mesh and resolution requirements, a method for quantitative assessment of unstructured meshes with no-slip walls is described. The calculations have been carriedmore » out using Hydra-TH, a thermal-hydraulics code developed at Los Alamos for the Consortium for Advanced Simulation of Light water reactors, a United States Department of Energy Innovation Hub. Hydra-TH uses a second-order implicit incremental projection method to solve the singlephase incompressible Navier-Stokes equations. The simulations explicitly resolve the large scale motions of the turbulent flow field using first principles and rely on a monotonicity-preserving numerical technique to represent the unresolved scales. Each series of simulations for the 3 × 3 and 5 × 5 rod-bundle geometries is an analysis of the flow field statistics combined with a mesh-refinement study and validation with available experimental data. Our primary focus is the time history and statistics of the forces loading the fuel rods. These hydrodynamic forces are believed to be the key player resulting in rod vibration and GTRF wear, one of the leading causes for leaking nuclear fuel which costs power utilities millions of dollars in preventive measures. As a result, we demonstrate that implicit large-eddy simulation of rod-bundle flows is a viable way to calculate the excitation forces for the GTRF problem.« less
Large-Eddy Simulations of Baroclinic Instability and Turbulent Mixing
2013-09-30
lateral mixing of fluid properties across the unstable front, and the transition from strongly horizontal, geostrophic motion on the mesoscale to...three-dimensional, quasi -isotropic, non-hydrostatic motion on turbulent scales. Report Documentation Page Form ApprovedOMB No. 0704-0188 Public reporting...dispersion. RESULTS Baroclinic Wave Experiments Novel numerical simulations of baroclinic instability of a shallow, geostrophically balanced
Large-Eddy Simulations of Baroclinic Instability and Turbulent Mixing
2010-09-30
properties across the unstable front, and the transition from strongly horizontal, geostrophic motion on the mesoscale to three-dimensional, quasi ...Prescribed by ANSI Std Z39-18 RESULTS Geostrophic Upper- Ocean Front Frontal simulations were initialized using density gradients outlined Fox...Turbulent Mixing Eric D. Skyllingstad College of Oceanic and Atmospheric Sciences, Oregon State University 104 COAS Admin. Bldg., Corvallis, OR
Towards large eddy and direct simulation of complex turbulent flows
NASA Technical Reports Server (NTRS)
Moin, Parviz
1991-01-01
Recent advances in the methodology for direct numerical simulation of turbulent flows and some of the current applications are reviewed. It is argued that high-order finite difference schemes yield solutions with comparable accuracy to the spectral methods with the same number of degrees of freedom. The effects of random inflow conditions on the downstream evolution of turbulence are discussed.
Wind Energy-Related Atmospheric Boundary Layer Large-Eddy Simulation Using OpenFOAM: Preprint
Churchfield, M.J.; Vijayakumar, G.; Brasseur, J.G.; Moriarty, P.J.
2010-08-01
This paper develops and evaluates the performance of a large-eddy simulation (LES) solver in computing the atmospheric boundary layer (ABL) over flat terrain under a variety of stability conditions, ranging from shear driven (neutral stratification) to moderately convective (unstable stratification).
Large eddy simulation of wire-wrapped fuel pins I: Hydrodynamics in a periodic array.
Fischer, P.; Lottes, J.; Siegel, A.; Palmiotti, G.
2007-01-01
We present large-eddy simulations of flow in a wire-wrapped fuel assembly at subchannel Reynolds numbers of Re{sub h} = 4684-29184. The domain consists of a single pin in a hexagonally periodic array, corresponding to two interior subchannels. Periodic boundary conditions are also used in the axial direction over a single wire-wrap period.
Scale-Similar Models for Large-Eddy Simulations
NASA Technical Reports Server (NTRS)
Sarghini, F.
1999-01-01
Scale-similar models employ multiple filtering operations to identify the smallest resolved scales, which have been shown to be the most active in the interaction with the unresolved subgrid scales. They do not assume that the principal axes of the strain-rate tensor are aligned with those of the subgrid-scale stress (SGS) tensor, and allow the explicit calculation of the SGS energy. They can provide backscatter in a numerically stable and physically realistic manner, and predict SGS stresses in regions that are well correlated with the locations where large Reynolds stress occurs. In this paper, eddy viscosity and mixed models, which include an eddy-viscosity part as well as a scale-similar contribution, are applied to the simulation of two flows, a high Reynolds number plane channel flow, and a three-dimensional, nonequilibrium flow. The results show that simulations without models or with the Smagorinsky model are unable to predict nonequilibrium effects. Dynamic models provide an improvement of the results: the adjustment of the coefficient results in more accurate prediction of the perturbation from equilibrium. The Lagrangian-ensemble approach [Meneveau et al., J. Fluid Mech. 319, 353 (1996)] is found to be very beneficial. Models that included a scale-similar term and a dissipative one, as well as the Lagrangian ensemble averaging, gave results in the best agreement with the direct simulation and experimental data.
Nesting Large-Eddy Simulations Within Mesoscale Simulations for Wind Energy Applications
NASA Astrophysics Data System (ADS)
Lundquist, J. K.; Mirocha, J. D.; Chow, F. K.; Kosovic, B.; Lundquist, K. A.
2008-12-01
With increasing demand for more accurate atmospheric simulations for wind turbine micrositing, for operational wind power forecasting, and for more reliable turbine design, simulations of atmospheric flow with resolution of tens of meters or higher are required. These time-dependent large-eddy simulations (LES) account for complex terrain and resolve individual atmospheric eddies on length scales smaller than turbine blades. These small-domain high-resolution simulations are possible with a range of commercial and open- source software, including the Weather Research and Forecasting (WRF) model. In addition to "local" sources of turbulence within an LES domain, changing weather conditions outside the domain can also affect flow, suggesting that a mesoscale model provide boundary conditions to the large-eddy simulations. Nesting a large-eddy simulation within a mesoscale model requires nuanced representations of turbulence. Our group has improved the Weather and Research Forecating model's (WRF) LES capability by implementing the Nonlinear Backscatter and Anisotropy (NBA) subfilter stress model following Kosoviæ (1997) and an explicit filtering and reconstruction technique to compute the Resolvable Subfilter-Scale (RSFS) stresses (following Chow et al, 2005). We have also implemented an immersed boundary method (IBM) in WRF to accommodate complex terrain. These new models improve WRF's LES capabilities over complex terrain and in stable atmospheric conditions. We demonstrate approaches to nesting LES within a mesoscale simulation for farms of wind turbines in hilly regions. Results are sensitive to the nesting method, indicating that care must be taken to provide appropriate boundary conditions, and to allow adequate spin-up of turbulence in the LES domain. This work is performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
Large-eddy simulation of a turbulent mixing layer
NASA Technical Reports Server (NTRS)
Mansour, N. N.; Ferziger, J. H.; Reynolds, W. C.
1978-01-01
The three dimensional, time dependent (incompressible) vorticity equations were used to simulate numerically the decay of isotropic box turbulence and time developing mixing layers. The vorticity equations were spatially filtered to define the large scale turbulence field, and the subgrid scale turbulence was modeled. A general method was developed to show numerical conservation of momentum, vorticity, and energy. The terms that arise from filtering the equations were treated (for both periodic boundary conditions and no stress boundary conditions) in a fast and accurate way by using fast Fourier transforms. Use of vorticity as the principal variable is shown to produce results equivalent to those obtained by use of the primitive variable equations.
Statistical Mechanics-based Closures for Large Eddy Simulations
NASA Astrophysics Data System (ADS)
Parish, Eric; Duraisamy, Karthik; Gouasmi, Ayoub; Computational Aerosciences Laboratory Team
2016-11-01
The simulation of high Reynolds-number fluid flows is made challenging by the presence of an enormous range of temporal and spatial scales. The Mori-Zwanzig (MZ) formalism originates from non-equilibrium statistical mechanics and provides a formal backdrop for the construction of coarse-grained models. In this work, a class of models inspired from the Mori-Zwanzig formalism are applied to turbulent flows. The MZ-models are derived directly from the governing equations and require minimal heuristics. The resulting closures are non-Markovian and are akin to modeling the divergence of the sub-grid stress. Non-local temporal effects are captured through a finite memory approximation of the MZ memory kernel. Numerical simulations of rotating homogeneous turbulence and turbulent channel flow are presented. The MZ-based models are shown to accurately characterize the behavior of the unresolved dynamics associated energy transfer mechanisms. This work was supported in part by AFOSR under the project "LES Modeling of Non-local effects using Statistical Coarse-graining" with Dr. Jean-Luc Cambier as the technical monitor.
Nesting large-eddy simulations within mesoscale simulations for wind energy applications
Lundquist, J K; Mirocha, J D; Chow, F K; Kosovic, B; Lundquist, K A
2008-09-08
With increasing demand for more accurate atmospheric simulations for wind turbine micrositing, for operational wind power forecasting, and for more reliable turbine design, simulations of atmospheric flow with resolution of tens of meters or higher are required. These time-dependent large-eddy simulations (LES), which resolve individual atmospheric eddies on length scales smaller than turbine blades and account for complex terrain, are possible with a range of commercial and open-source software, including the Weather Research and Forecasting (WRF) model. In addition to 'local' sources of turbulence within an LES domain, changing weather conditions outside the domain can also affect flow, suggesting that a mesoscale model provide boundary conditions to the large-eddy simulations. Nesting a large-eddy simulation within a mesoscale model requires nuanced representations of turbulence. Our group has improved the Weather and Research Forecasting model's (WRF) LES capability by implementing the Nonlinear Backscatter and Anisotropy (NBA) subfilter stress model following Kosovic (1997) and an explicit filtering and reconstruction technique to compute the Resolvable Subfilter-Scale (RSFS) stresses (following Chow et al, 2005). We have also implemented an immersed boundary method (IBM) in WRF to accommodate complex terrain. These new models improve WRF's LES capabilities over complex terrain and in stable atmospheric conditions. We demonstrate approaches to nesting LES within a mesoscale simulation for farms of wind turbines in hilly regions. Results are sensitive to the nesting method, indicating that care must be taken to provide appropriate boundary conditions, and to allow adequate spin-up of turbulence in the LES domain.
Large Eddy Simulation in the Computation of Jet Noise
NASA Technical Reports Server (NTRS)
Mankbadi, R. R.; Goldstein, M. E.; Povinelli, L. A.; Hayder, M. E.; Turkel, E.
1999-01-01
Noise can be predicted by solving Full (time-dependent) Compressible Navier-Stokes Equation (FCNSE) with computational domain. The fluctuating near field of the jet produces propagating pressure waves that produce far-field sound. The fluctuating flow field as a function of time is needed in order to calculate sound from first principles. Noise can be predicted by solving the full, time-dependent, compressible Navier-Stokes equations with the computational domain extended to far field - but this is not feasible as indicated above. At high Reynolds number of technological interest turbulence has large range of scales. Direct numerical simulations (DNS) can not capture the small scales of turbulence. The large scales are more efficient than the small scales in radiating sound. The emphasize is thus on calculating sound radiated by large scales.
Optimal Numerical Schemes for Compressible Large Eddy Simulations
NASA Astrophysics Data System (ADS)
Edoh, Ayaboe; Karagozian, Ann; Sankaran, Venkateswaran; Merkle, Charles
2014-11-01
The design of optimal numerical schemes for subgrid scale (SGS) models in LES of reactive flows remains an area of continuing challenge. It has been shown that significant differences in solution can arise due to the choice of the SGS model's numerical scheme and its inherent dissipation properties, which can be exacerbated in combustion computations. This presentation considers the individual roles of artificial dissipation, filtering, secondary conservation (Kinetic Energy Preservation), and collocated versus staggered grid arrangements with respect to the dissipation and dispersion characteristics and their overall impact on the robustness and accuracy for time-dependent simulations of relevance to reacting and non-reacting LES. We utilize von Neumann stability analysis in order to quantify these effects and to determine the relative strengths and weaknesses of the different approaches. Distribution A: Approved for public release, distribution unlimited. Supported by AFOSR (PM: Dr. F. Fahroo).
High Speed Jet Noise Prediction Using Large Eddy Simulation
NASA Technical Reports Server (NTRS)
Lele, Sanjiva K.
2002-01-01
Current methods for predicting the noise of high speed jets are largely empirical. These empirical methods are based on the jet noise data gathered by varying primarily the jet flow speed, and jet temperature for a fixed nozzle geometry. Efforts have been made to correlate the noise data of co-annular (multi-stream) jets and for the changes associated with the forward flight within these empirical correlations. But ultimately these emipirical methods fail to provide suitable guidance in the selection of new, low-noise nozzle designs. This motivates the development of a new class of prediction methods which are based on computational simulations, in an attempt to remove the empiricism of the present day noise predictions.
Comparison between experiments and Large-Eddy Simulations of tip spiral structure and geometry
NASA Astrophysics Data System (ADS)
Ivanell, S.; Leweke, T.; Sarmast, S.; Quaranta, H. U.; Mikkelsen, R. F.; Sørensen, J. N.
2015-06-01
Results from Large-Eddy Simulations using the actuator line technique have been validated against experimental results. The experimental rotor wake, which forms the basis for the comparison, was studied in a recirculating free-surface water channel, where a helical vortex was generated by a single-bladed rotor mounted on a shaft. An investigation of how the experimental blade geometry and aerofoil characteristics affect the results was performed. Based on this, an adjustment of the pitch setting was introduced, which is still well within the limits of the experimental uncertainty. Excellent agreement between the experimental and the numerical results was achieved concerning the circulation, wake expansion and pitch of the helical tip vortex. A disagreement was found regarding the root vortex position and the axial velocity along the centre line of the tip vortex. This work establishes a good base for further studies of more fundamental stability parameters of helical rotor wakes.
Large eddy simulation of high frequency oscillating flow in an asymmetric branching airway model.
Nagels, Martin A; Cater, John E
2009-11-01
The implementation of artificial ventilation schemes is necessary when respiration fails. One approach involves the application of high frequency oscillatory ventilation (HFOV) to the respiratory system. Oscillatory airflow in the upper bronchial tree can be characterized by Reynolds numbers as high as 10(4), hence, the flow presents turbulent features. In this study, transitional and turbulent flow within an asymmetric bifurcating model of the upper airway during HFOV are studied using large eddy simulation (LES) methods. The flow, characterized by a peak Reynolds number of 8132, is analysed using a validated LES model of a three-dimensional branching geometry. The pressures, velocities, and vorticity within the flow are presented and compared with prior models for branching flow systems. The results demonstrate how pendelluft occurs at asymmetric branches within the respiratory system. These results may be useful in optimising treatments using HFOV methods.
NASA Technical Reports Server (NTRS)
Baurle, R. A.
2015-01-01
Steady-state and scale-resolving simulations have been performed for flow in and around a model scramjet combustor flameholder. The cases simulated corresponded to those used to examine this flowfield experimentally using particle image velocimetry. A variety of turbulence models were used for the steady-state Reynolds-averaged simulations which included both linear and non-linear eddy viscosity models. The scale-resolving simulations used a hybrid Reynolds-averaged / large eddy simulation strategy that is designed to be a large eddy simulation everywhere except in the inner portion (log layer and below) of the boundary layer. Hence, this formulation can be regarded as a wall-modeled large eddy simulation. This effort was undertaken to formally assess the performance of the hybrid Reynolds-averaged / large eddy simulation modeling approach in a flowfield of interest to the scramjet research community. The numerical errors were quantified for both the steady-state and scale-resolving simulations prior to making any claims of predictive accuracy relative to the measurements. The steady-state Reynolds-averaged results showed a high degree of variability when comparing the predictions obtained from each turbulence model, with the non-linear eddy viscosity model (an explicit algebraic stress model) providing the most accurate prediction of the measured values. The hybrid Reynolds-averaged/large eddy simulation results were carefully scrutinized to ensure that even the coarsest grid had an acceptable level of resolution for large eddy simulation, and that the time-averaged statistics were acceptably accurate. The autocorrelation and its Fourier transform were the primary tools used for this assessment. The statistics extracted from the hybrid simulation strategy proved to be more accurate than the Reynolds-averaged results obtained using the linear eddy viscosity models. However, there was no predictive improvement noted over the results obtained from the explicit
NASA Astrophysics Data System (ADS)
Zhu, Ping
2008-09-01
Hurricane boundary layer (HBL) processes, especially the structure of the coherent large eddy circulations (LECs) and their induced vertical transport, are not well understood. This paper introduces a large eddy simulation (LES) framework in a weather hindcasting mode developed from a multiple scale nested Weather Research and Forecasting (WRF) model. Using the WRF-LES, this study investigated the structure of the HBL LECs and the associated vertical transport during the landfall of Hurricane Ivan (2004). The simulation shows that the HBL LECs exist in a mean stable environment and consist of well-defined updraft and downdraft. Statistically, the HBL LECs are only slightly skewed with the updrafts and downdrafts relatively evenly distributed spatially. The inversion base basically envelopes the upper boundary of LECs. The trough in between two adjacent LECs is where most entrainment takes place, whereas the crest of the LECs is where boundary layer air detrains out of the HBL. In such a way, LECs directly connect the surface, the HBL, and the main body of a hurricane vortex and enhance the exchange of energy, moisture, and momentum between them. It is found that the current boundary layer schemes significantly underestimate the resolved turbulent fluxes due to the fact that the effects of LECs have not been included in the parameterizations. On the basis of the statistical structure of LECs simulated by the WRF-LES, this paper proposes a conceptual updraft-downdraft model that can potentially be implemented in weather forecasting models to parameterize the fluxes induced by the HBL LEC transport.
Large Eddy Simulation for Oscillating Airfoils with Large Pitching and Surging Motions
NASA Astrophysics Data System (ADS)
Kocher, Alexander; Cumming, Reed; Tran, Steven; Sahni, Onkar
2016-11-01
Many applications of interest involve unsteady aerodynamics due to time varying flow conditions (e.g. in the case of flapping wings, rotorcrafts and wind turbines). In this study, we formulate and apply large eddy simulation (LES) to investigate flow over airfoils at a moderate mean angle of attack with large pitching and surging motions. Current LES methodology entails three features: i) a combined subgrid scale model in the context of stabilized finite element methods, ii) local variational Germano identity (VGI) along with Lagrangian averaging, and iii) arbitrary Lagrangian-Eulerian (ALE) description over deforming unstructured meshes. Several cases are considered with different types of motions including surge only, pitch only and a combination of the two. The flow structures from these cases are analyzed and the numerical results are compared to experimental data when available.
Conjugate heat transfer with Large Eddy Simulation for gas turbine components
NASA Astrophysics Data System (ADS)
Duchaine, Florent; Mendez, Simon; Nicoud, Franck; Corpron, Alban; Moureau, Vincent; Poinsot, Thierry
2009-06-01
CHT (Conjugate Heat Transfer) is a main design constraint for GT (gas turbines). Most existing CHT tools are developed for chained, steady phenomena. A fully parallel environment for CHT has been developed and applied to two configurations of interest for the design of GT. A reactive Large Eddy Simulations code and a solid conduction solver exchange data via a supervisor. A flame/wall interaction is used to assess the precision and the order of the coupled solutions. A film-cooled turbine vane is then studied. Thermal conduction in the blade implies lower wall temperature than adiabatic results and CHT reproduces the experimental cooling efficiency. To cite this article: F. Duchaine et al., C. R. Mecanique 337 (2009).
Large Eddy Simulations of Turbulent Reacting Flows in an Opposed-Piston Two Stroke Engine
NASA Astrophysics Data System (ADS)
Srivastava, Shalabh; Schock, Harold; Jaberi, Farhad
2013-11-01
The two-phase filtered mass density function (FMDF) subgrid-scale model has been used for large eddy simulation (LES) of turbulent spray combustion in a generic single cylinder, opposed-piston, two-stroke engine configuration. The LES/FMDF is implemented via an efficient, hybrid numerical method in which the filtered compressible Navier-Stokes equations are solved with a high-order, multi-block, compact differencing scheme, and the spray and FMDF are implemented with stochastic Lagrangian methods. The reliability and consistency of the numerical methods are established for the engine configuration by comparing the Eulerian and Lagrangian components of the LES/FMDF. The effects of various operating conditions like boost pressure, heat transfer model, fuel spray temperature, nozzle diameter, injection pressure, and injector configuration on the flow field, heat loss and the evolution of spray and combustion are studied.
Improving prediction of aerosol deposition in an idealized mouth using large-Eddy simulation.
Matida, Edgar A; Finlay, Warren H; Breuer, Michael; Lange, Carlos F
2006-01-01
Monodisperse aerosol deposition in an idealized mouth geometry with a relatively small inlet diameter (D (in) = 3.0 mm) was studied numerically using a standard Large Eddy Simulation (LES). A steady inhalation flow rate of Q = 32.2 L/min was used. Thousands of particles (2.5, 3.7, and 5.0 microm in diameter and rho (f) = 912.0 kg/m(3) density) were released separately in the computational domain and aerosol deposition was determined. The total aerosol deposition results in this idealized mouth were in relatively good agreement when compared with measured data obtained in separate experiments, showing considerable improvement over the standard RANS/EIM (Reynolds Averaged Navier-Stokes/Eddy Interaction Model) approach.
Using Large Eddy Simulation for understanding vented gas explosions in the presence of obstacles.
Di Sarli, Valeria; Di Benedetto, Almerinda; Russo, Gennaro
2009-09-30
In this work, a validated Large Eddy Simulation model of unsteady premixed flame propagation is used to study the phenomenology underlying vented gas explosions in the presence of obstacles. Computations are run of deflagrating flames in a small-scale combustion chamber closed at the bottom end and open at the opposite face. A single obstacle is centred inside the chamber. Methane-air mixtures of various compositions (ranging from lean to stoichiometric and rich), and obstacles with different area blockage ratios (30, 50 and 70%) and shapes (circular, rectangular and square cross-section in the flow direction) are investigated. All cases are initialized from stagnation. The competition between combustion rate and venting rate allows explaining both number and intensity of the overpressure peaks observed.
Large-Eddy Simulation of Boundary Layer Transition on Swept Wings
NASA Technical Reports Server (NTRS)
Huai, Xiaoli; Joslin, Ronald D.; Piomelli, Ugo
1993-01-01
The large-eddy simulation of the spatial evolution of a stationary crossflow vortex packet in a three-dimensional boundary layer was performed. Although a coarse grid was used (compared to that required by a direct numerical simulation) the essential features of the disturbance evolution, such as the spanwise disturbance spreading and the vortex rollover, were captured accurately. The eddy viscosity became significant only in the late nonlinear stages of the simulation.
Hybrid Reynolds-Averaged/Large Eddy Simulation of the Flow in a Model SCRamjet Cavity Flameholder
NASA Technical Reports Server (NTRS)
Baurle, R. A.
2016-01-01
Steady-state and scale-resolving simulations have been performed for flow in and around a model scramjet combustor flameholder. Experimental data available for this configuration include velocity statistics obtained from particle image velocimetry. Several turbulence models were used for the steady-state Reynolds-averaged simulations which included both linear and non-linear eddy viscosity models. The scale-resolving simulations used a hybrid Reynolds-averaged/large eddy simulation strategy that is designed to be a large eddy simulation everywhere except in the inner portion (log layer and below) of the boundary layer. Hence, this formulation can be regarded as a wall-modeled large eddy simulation. This e ort was undertaken to not only assess the performance of the hybrid Reynolds-averaged / large eddy simulation modeling approach in a flowfield of interest to the scramjet research community, but to also begin to understand how this capability can best be used to augment standard Reynolds-averaged simulations. The numerical errors were quantified for the steady-state simulations, and at least qualitatively assessed for the scale-resolving simulations prior to making any claims of predictive accuracy relative to the measurements. The steady-state Reynolds-averaged results displayed a high degree of variability when comparing the flameholder fuel distributions obtained from each turbulence model. This prompted the consideration of applying the higher-fidelity scale-resolving simulations as a surrogate "truth" model to calibrate the Reynolds-averaged closures in a non-reacting setting prior to their use for the combusting simulations. In general, the Reynolds-averaged velocity profile predictions at the lowest fueling level matched the particle imaging measurements almost as well as was observed for the non-reacting condition. However, the velocity field predictions proved to be more sensitive to the flameholder fueling rate than was indicated in the measurements.
2007-03-30
NOTES 14. ABSTRACT A thorough investigation of leading edge heat transfer on a model geometry has been performed using Large-eddy simulation (LES...investigation of leading edge heat transfer on a model geometry has been performed using Large-eddy simulation (LES), with support from the current and a...and length scale appropriate for turbine blade heat-transfer and study the heat-transfer augmentation in idealized geometry and its interaction with
NASA Astrophysics Data System (ADS)
Silvis, Maurits H.; Remmerswaal, Ronald A.; Verstappen, Roel
2017-01-01
We study the construction of subgrid-scale models for large-eddy simulation of incompressible turbulent flows. In particular, we aim to consolidate a systematic approach of constructing subgrid-scale models, based on the idea that it is desirable that subgrid-scale models are consistent with the mathematical and physical properties of the Navier-Stokes equations and the turbulent stresses. To that end, we first discuss in detail the symmetries of the Navier-Stokes equations, and the near-wall scaling behavior, realizability and dissipation properties of the turbulent stresses. We furthermore summarize the requirements that subgrid-scale models have to satisfy in order to preserve these important mathematical and physical properties. In this fashion, a framework of model constraints arises that we apply to analyze the behavior of a number of existing subgrid-scale models that are based on the local velocity gradient. We show that these subgrid-scale models do not satisfy all the desired properties, after which we explain that this is partly due to incompatibilities between model constraints and limitations of velocity-gradient-based subgrid-scale models. However, we also reason that the current framework shows that there is room for improvement in the properties and, hence, the behavior of existing subgrid-scale models. We furthermore show how compatible model constraints can be combined to construct new subgrid-scale models that have desirable properties built into them. We provide a few examples of such new models, of which a new model of eddy viscosity type, that is based on the vortex stretching magnitude, is successfully tested in large-eddy simulations of decaying homogeneous isotropic turbulence and turbulent plane-channel flow.
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Zang, Thomas A.; Speziale, Charles G.; Lund, Thomas S.
1990-01-01
An eddy viscosity model based on the renormalization group theory of Yakhot and Orszag (1986) is applied to the large-eddy simulation of transition in a flat-plate boundary layer. The simulation predicts with satisfactory accuracy the mean velocity and Reynolds stress profiles, as well as the development of the important scales of motion. The evolution of the structures characteristic of the nonlinear stages of transition is also predicted reasonably well.
Large Eddy Simulations of Transverse Combustion Instability in a Multi-Element Injector
2016-07-27
Briefing Charts 3. DATES COVERED (From - To) 01 July 2016 - 27 July 2016 4. TITLE AND SUBTITLE Large Eddy Simulations of Transverse Combustion...Simulations of Transverse Combustion Instability in a Multi‐Element Injector 2 History Damaged engine injector faceplate caused by combustion...Purdue University Distribution Statement A: Approved for Public Release; Distribution Unlimited. PA Clearance #16346 4 Transverse Instability
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Zang, Thomas A.; Speziale, Charles G.; Lund, Thomas S.
1990-01-01
An eddy viscosity model based on the renormalization group theory of Yakhot and Orszag (1986) is applied to the large-eddy simulation of transition in a flat-plate boundary layer. The simulation predicts with satisfactory accuracy the mean velocity and Reynolds stress profiles, as well as the development of the important scales of motion. The evolution of the structures characteristic of the nonlinear stages of transition is also predicted reasonably well.
Large eddy simulations of coal jet flame ignition using the direct quadrature method of moments
NASA Astrophysics Data System (ADS)
Pedel, Julien
The Direct Quadrature Method of Moments (DQMOM) was implemented in the Large Eddy Simulation (LES) tool ARCHES to model coal particles. LES coupled with DQMOM was first applied to nonreacting particle-laden turbulent jets. Simulation results were compared to experimental data and accurately modeled a wide range of particle behaviors, such as particle jet waviness, spreading, break up, particle clustering and segregation, in different configurations. Simulations also accurately predicted the mean axial velocity along the centerline for both the gas phase and the solid phase, thus demonstrating the validity of the approach to model particles in turbulent flows. LES was then applied to the prediction of pulverized coal flame ignition. The stability of an oxy-coal flame as a function of changing primary gas composition (CO2 and O2) was first investigated. Flame stability was measured using optical measurements of the flame standoff distance in a 40 kW pilot facility. Large Eddy Simulations (LES) of the facility provided valuable insight into the experimentally observed data and the importance of factors such as heterogeneous reactions, radiation or wall temperature. The effects of three parameters on the flame stand-off distance were studied and simulation predictions were compared to experimental data using the data collaboration method. An additional validation study of the ARCHES LES tool was then performed on an air-fired pulverized coal jet flame ignited by a preheated gas flow. The simulation results were compared qualitatively and quantitatively to experimental observations for different inlet stoichiometric ratios. LES simulations were able to capture the various combustion regimes observed during flame ignition and to accurately model the flame stand-off distance sensitivity to the stoichiometric ratio. Gas temperature and coal burnout predictions were also examined and showed good agreement with experimental data. Overall, this research shows that high
Large-eddy simulation of curved-geometry flows using contravariant components of velocity
NASA Astrophysics Data System (ADS)
Yuan, Weixing; Xu, Hongyi; Khalid, Mahmood
2011-01-01
The current large-eddy simulation (LES) research makes use of the contravariant components as the dependent variables on a staggered grid system for the discretisation of the governing equations in curvilinear coordinates. This technology provides a possibility to investigate efficiently turbulent flows in complex geometries. To test and validate the recently developed in-house LES code, LESSGA (Large-Eddy Simulation on a Staggered Grid Arrangement), numerical simulations were performed for turbulent flows in a concentric annular pipe and transitional flows past an airfoil. In this article, the computed results of flows in a concentric annular pipe with a radius ratio of a = R inner/Router = 0.5 at ? and flows past an SD7003 airfoil at Rec = 60,000 and angle of attack α = 4° are compared with available experimental and DNS data. Technical difficulties experienced are also discussed.
Large-eddy simulation of a boundary layer with concave streamwise curvature
NASA Technical Reports Server (NTRS)
Lund, Thomas S.
1994-01-01
Turbulence modeling continues to be one of the most difficult problems in fluid mechanics. Existing prediction methods are well developed for certain classes of simple equilibrium flows, but are still not entirely satisfactory for a large category of complex non-equilibrium flows found in engineering practice. Direct and large-eddy simulation (LES) approaches have long been believed to have great potential for the accurate prediction of difficult turbulent flows, but the associated computational cost has been prohibitive for practical problems. This remains true for direct simulation but is no longer clear for large-eddy simulation. Advances in computer hardware, numerical methods, and subgrid-scale modeling have made it possible to conduct LES for flows or practical interest at Reynolds numbers in the range of laboratory experiments. The objective of this work is to apply ES and the dynamic subgrid-scale model to the flow of a boundary layer over a concave surface.
Large-Eddy Simulations of Strongly Precipitating, Shallow, Stratocumulus-Topped Boundary Layers.
NASA Astrophysics Data System (ADS)
Stevens, Bjorn; Cotton, William R.; Feingold, Graham; Moeng, Chin-Hoh
1998-12-01
Large-eddy simulations that incorporate a size-resolving representation of cloud water are used to study the effect of heavy drizzle on PBL structure. Simulated surface precipitation rates average about 1 mm day1. Heavily drizzling simulations are compared to nondrizzling simulations under two nocturnal PBL regimes-one primarily driven by buoyancy and the other driven equally by buoyancy and shear. Drizzle implies a net latent heating in the cloud that leads to sharp reductions in both entrainment and the production of turbulent kinetic energy by buoyancy (particularly in downdrafts). Drizzle, which evaporates below cloud base, promotes a cooler and moister subcloud layer that further inhibits deep mixing. The cooling and moistening is in quantitative agreement with some observations and is shown to favor the formation of cumuli rising out of the subcloud layer. The cumuli, which are local in space and time, are responsible for most of the heat and moisture transport. They also appear to generate a larger-scale circulation that differs dramatically from the regularity typically found in nonprecipitating stratocumulus. Time-averaged turbulent fluxes of heat and moisture increase in the presence of precipitation, suggesting that drizzle (and drizzle-induced stratification) should not necessarily be taken as a sign of decoupling. Because drizzle primarily affects the vertical distribution of buoyancy, shear production of turbulent kinetic energy mitigates some of the effects described above. Based on large-eddy simulation the authors hypothesize that shallow, well-mixed, radiatively driven stratocumulus cannot persist in the presence of heavy drizzle. In accord with some simpler models, the simulated case with heavy precipitation promotes a reduction in both liquid-water path and entrainment. However, the simulations suggest that time-integrated cloud fraction may increase as a result of drizzle because thinner precipitating clouds may persist longer if the boundary
NASA Astrophysics Data System (ADS)
Schein, David Brian
2004-04-01
This research involves computational study of free, heated jet flow and resultant far-field sound performed using large-eddy simulation (LES) and Lighthill's acoustic analogy. A subgrid-scale model for small-scale compressible turbulence was developed using a combination of the popular Smagorinsky model and a deductive model. An existing software package for compressible flow field computation was substantially modified to perform temporal LES and aerosound simulations. Cases studied extend to large Reynolds number (Re), high subsonic (compressible) flow with realistic geometries more representative of aircraft engine exhausts than typically considered using direct numerical simulation (DNS). Flow-field fluctuations are stored over a period of time and used to calculate rms turbulence within the computational domain. The far-field sound and directivity is computed using the time-derivative form of Lighthill's source-integral result formulated in terms of quadrupole sources from the simulated flow field, which is integrated in time and contains the fluctuations set up by the time-varying stress tensor. A simulation for a WR19-4 turbofan engine exhaust (Re>106 based on exist velocity and diameter) is presented, and propagated jet noise results are compared with experimental acoustic data. Thesis advisor: William C. Meecham Copies of this thesis written in English can be obtained from
NASA Astrophysics Data System (ADS)
Lantz, Jonas; Ebbers, Tino; Karlsson, Matts
2012-11-01
In this study, turbulent kinetic energy (TKE) in an aortic coarctation was studied using both a numerical technique (large eddy simulation, LES) and in vivo measurements using magnetic resonance imaging (MRI). High levels of TKE are undesirable, as kinetic energy is extracted from the mean flow to feed the turbulent fluctuations. The patient underwent surgery to widen the coarctation, and the flow before and after surgery was computed and compared to MRI measurements. The resolution of the MRI was about 7 × 7 voxels in axial cross-section while 50x50 mesh cells with increased resolution near the walls was used in the LES simulation. In general, the numerical simulations and MRI measurements showed that the aortic arch had no or very low levels of TKE, while elevated values were found downstream the coarctation. It was also found that TKE levels after surgery were lowered, indicating that the diameter of the constriction was increased enough to decrease turbulence effects. In conclusion, both the numerical simulation and MRI measurements gave very similar results, thereby validating the simulations and suggesting that MRI measured TKE can be used as an initial estimation in clinical practice, while LES results can be used for detailed quantification and further research of aortic flows.
Large-Eddy Simulations of Plasma Flow Control on a GOE735 Wind Turbine Airfoil
NASA Astrophysics Data System (ADS)
Czulak, Alexander; Franck, Jennifer
2015-11-01
Active flow control using plasma actuation was studied for the GOE735 airfoil and compared to non-actuated baseline cases using numerical simulations. This investigation considers two-dimensional simulations at a Reynolds number of 1,000 using direct numerical simulation (DNS) as well as three-dimensional simulations at a Reynolds number of 50,000 and 100,000 using large-eddy simulation (LES). Plasma actuation is applied in terms of a source term within the boundary layer close to the airfoil surface. Angles of attack of 0°, 5° and 15° were considered, and control is shown to be effective at increasing the lift coefficient, decreasing the drag coefficient and reducing the root mean squared deviation of both lift and drag. An analysis of the flow physics reveals that the actuated cases delay the point of separation, reduce the wake width and diminish the size and strength of the shed vortices. For this particular airfoil, there are significant differences in Reynolds number in terms of the baseline flow, control effectiveness and performance factors such as lift and drag.
High-Accuracy Near-Surface Large-Eddy Simulation with Planar Topography
2015-08-03
Incorrect prediction of vertical transport of CO2 and other greenhouse gases may affect related predictions of upper atmosphere chemistry. The... atmospheric boundary layer, ABL, planetary boundary layer, PBL, turbulent boundary layer REPORT DOCUMENTATION PAGE 11. SPONSOR/MONITOR’S REPORT...in non peer-reviewed journals: Jan 2012, NREL National Wind Technology Center (NWTC) “Designing Large-Eddy Simulation of the Atmospheric Boundary
Turbulence Structure and Implications for Dispersion: Insights from Large-Eddy Simulations
Calhoun, R; Cederwall, R; Street, R
1999-10-04
We have presented two flows where detailed knowledge of the fluid mechanics would appear to be crucial for accurate dispersion modeling. We expect that Large-eddy simulations (LES) will complement traditional dispersion modeling by providing both the ability to discern between cases where traditional models work well and cases where more complicated characterizations are necessary, and a method to investigate potentially unique flow features and turbulence structure for specific flow problems.
Direct-Numerical and Large-Eddy Simulations of a Non-Equilibrium Turbulent Kolmogorov Flow
NASA Technical Reports Server (NTRS)
Woodruff, S. L.; Shebalin, J. V.; Hussaini, M. Y.
1999-01-01
A non-equilibrium form of turbulent Kolmogorov flow is set up by making an instantaneous change in the amplitude of the spatially-periodic forcing. It is found that the response of the flow to this instantaneous change becomes more dramatic as the wavenumber of the forcing is increased, and, at the same time, that the faithfulness with which the large-eddy-simulation results agree with the direct-numerical results decreases.
Mean-state acceleration of cloud-resolving models and large eddy simulations
Jones, C. R.; Bretherton, C. S.; Pritchard, M. S.
2015-10-29
In this study, large eddy simulations and cloud-resolving models (CRMs) are routinely used to simulate boundary layer and deep convective cloud processes, aid in the development of moist physical parameterization for global models, study cloud-climate feedbacks and cloud-aerosol interaction, and as the heart of superparameterized climate models. These models are computationally demanding, placing practical constraints on their use in these applications, especially for long, climate-relevant simulations. In many situations, the horizontal-mean atmospheric structure evolves slowly compared to the turnover time of the most energetic turbulent eddies. We develop a simple scheme to reduce this time scale separation to accelerate themore » evolution of the mean state. Using this approach we are able to accelerate the model evolution by a factor of 2–16 or more in idealized stratocumulus, shallow and deep cumulus convection without substantial loss of accuracy in simulating mean cloud statistics and their sensitivity to climate change perturbations. As a culminating test, we apply this technique to accelerate the embedded CRMs in the Superparameterized Community Atmosphere Model by a factor of 2, thereby showing that the method is robust and stable to realistic perturbations across spatial and temporal scales typical in a GCM.« less
NASA Astrophysics Data System (ADS)
Le, Trung; Khosronejad, Ali; Bartelt, Nicole; Woldeamlak, Solomon; Peterson, Bonnie; Dewall, Petronella; Sotiropoulos, Fotis; Saint Anthony Falls Laboratory, University of Minnesota Team; Minnesota Department of Transportation Team
2015-11-01
We study the dynamics of a river confluence on Mississippi River branch in the city of Minneapolis, Minnesota, United States. Field measurements by Acoustic Doppler Current Profiler using on-board GPS tracking were carried out for five campaigns in the summer of 2014 and 2015 to collect both river bed elevation data and flow fields. Large Eddy Simulation is carried out to simulate the flow field with the total of 100 million grid points for the domain length of 3.2 km. The simulation results agree well with field measurements at measured cross-sections. The results show the existence of wake mode on the mixing interface of two branches near the upstream junction corner. The mutual interaction between the shear layers emanating from the river banks leading to the formation of large scale energetic structures that leads to ``switching'' side of the flow coherent structures. Our result here is a feasibility study for the use of eddy-resolving simulations in predicting complex flow dynamics in medium-size natural rivers. This work is funded by Minnesota Dept. Transportation and Minnesota Institute of Supercomputing.
Mean-state acceleration of cloud-resolving models and large eddy simulations
Jones, C. R.; Bretherton, C. S.; Pritchard, M. S.
2015-10-29
In this study, large eddy simulations and cloud-resolving models (CRMs) are routinely used to simulate boundary layer and deep convective cloud processes, aid in the development of moist physical parameterization for global models, study cloud-climate feedbacks and cloud-aerosol interaction, and as the heart of superparameterized climate models. These models are computationally demanding, placing practical constraints on their use in these applications, especially for long, climate-relevant simulations. In many situations, the horizontal-mean atmospheric structure evolves slowly compared to the turnover time of the most energetic turbulent eddies. We develop a simple scheme to reduce this time scale separation to accelerate the evolution of the mean state. Using this approach we are able to accelerate the model evolution by a factor of 2–16 or more in idealized stratocumulus, shallow and deep cumulus convection without substantial loss of accuracy in simulating mean cloud statistics and their sensitivity to climate change perturbations. As a culminating test, we apply this technique to accelerate the embedded CRMs in the Superparameterized Community Atmosphere Model by a factor of 2, thereby showing that the method is robust and stable to realistic perturbations across spatial and temporal scales typical in a GCM.
Analysis on Turbulent Flows using Large-eddy Simulation on the Seaside Complex Terrain
NASA Astrophysics Data System (ADS)
Kamio, T.; Iida, M.; Arakawa, C.
2014-12-01
The purpose of this study is the Large-eddy Simulation (LES) of the turbulent wind on the complex terrain, and the first results of the simulation are described. The authors tried to apply the LES code, which was developed as an atmospheric simulator in Japan Agency for the Marine-Earth Science and Technology (JAMSTEC), to the wind prediction for the wind energy. On the wind simulation, the highest problem would be the boundary conditions, and the case in this paper was simplified one. The case study in this paper is the west wind on a complex terrain site, which is the wind from sea for the site. The steady flow was employed for the inlet condition, because the wind on the sea is the low turbulent wind, and almost all the turbulence would be generated by the roughness of the ground surface. The wall function was employed as the surface condition on the ground surface. The computational domain size was about 8 × 3 × 2.5 km3, and the minimum cell size was about 10 × 10 × 3 m3. The computational results, the vertical profile of the averaged wind speed and the turbulence intensity, agreed with the measurement by the meteorological masts. Moreover, the authors tried the analysis of the turbulence characteristics. The power spectrum density model, and the cross spectrum analyses gave the knowledge of the turbulent characteristics on the complex terrain and the hints for the domain and grid of the numerical analysis.
Underlying mechanism of numerical instability in large-eddy simulation of turbulent flows
NASA Astrophysics Data System (ADS)
Ida, Masato; Taniguchi, Nobuyuki
2004-04-01
This paper extends our recent theoretical work concerning the feasibility of stable and accurate computation of turbulence using a large eddy simulation [
Large Eddy Simulations and an Analysis of the Flow Field of a Radially Lobed Nozzle
NASA Astrophysics Data System (ADS)
Amini, Noushin; Sekaran, Aarthi
2015-11-01
Lobed nozzles have been a subject of regained interest over the past couple of decades owing to their established mixing capabilities. Despite experimental (Hu et al., 1999 and Hu et al., 2008) and limited numerical studies (Boulenouar et al. 2011 and Cooper et al., 2005), the exact nature of the jet ensuing from this nozzle is yet to be completely understood. The present numerical study is intended to complement prior experimental investigation, involving the analysis of the flow field downstream of a six lobed nozzle (Amini et al., 2012). Preliminary results (presented at DFD 2014, Amin and Sekaran), which involved three dimensional simulations of the full domain via URANS and Large Eddy Simulations (LES) were used to assess the domain extents and simulation technique. Based on these results it was seen that LES were able to capture the region of interest satisfactorily and a qualitative corroboration with previous studies was obtained. The study is thus extended to analyzing the flow originating from within the nozzle, following it downstream in order to confirm the vortical interaction mechanisms inside the lobed nozzle.
Large-eddy simulation of bubble-driven plume in stably stratified flow.
NASA Astrophysics Data System (ADS)
Yang, Di; Chen, Bicheng; Socolofsky, Scott; Chamecki, Marcelo; Meneveau, Charles
2015-11-01
The interaction between a bubble-driven plume and stratified water column plays a vital role in many environmental and engineering applications. As the bubbles are released from a localized source, they induce a positive buoyancy flux that generates an upward plume. As the plume rises, it entrains ambient water, and when the plume rises to a higher elevation where the stratification-induced negative buoyancy is sufficient, a considerable fraction of the entrained fluid detrains, or peels, to form a downward outer plume and a lateral intrusion layer. In the case of multiphase plumes, the intrusion layer may also trap weakly buoyant particles (e.g., oil droplets in the case of a subsea accidental blowout). In this study, the complex plume dynamics is studied using large-eddy simulation (LES), with the flow field simulated by hybrid pseudospectral/finite-difference scheme, and the bubble and dye concentration fields simulated by finite-volume scheme. The spatial and temporal characteristics of the buoyant plume are studied, with a focus on the effects of different bubble buoyancy levels. The LES data provide useful mean plume statistics for evaluating the accuracy of 1-D engineering models for entrainment and peeling fluxes. Based on the insights learned from the LES, a new continuous peeling model is developed and tested. Study supported by the Gulf of Mexico Research Initiative (GoMRI).
Large-eddy and unsteady RANS simulations of a shock-accelerated heavy gas cylinder
Morgan, B. E.; Greenough, J. A.
2015-04-08
Two-dimensional numerical simulations of the Richtmyer–Meshkov unstable “shock-jet” problem are conducted using both large-eddy simulation (LES) and unsteady Reynolds-averaged Navier–Stokes (URANS) approaches in an arbitrary Lagrangian–Eulerian hydrodynamics code. Turbulence statistics are extracted from LES by running an ensemble of simulations with multimode perturbations to the initial conditions. Detailed grid convergence studies are conducted, and LES results are found to agree well with both experiment and high-order simulations conducted by Shankar et al. (Phys Fluids 23, 024102, 2011). URANS results using a k–L approach are found to be highly sensitive to initialization of the turbulence lengthscale L and to the timemore » at which L becomes resolved on the computational mesh. As a result, it is observed that a gradient diffusion closure for turbulent species flux is a poor approximation at early times, and a new closure based on the mass-flux velocity is proposed for low-Reynolds-number mixing.« less
General-relativistic Large-eddy Simulations of Binary Neutron Star Mergers
NASA Astrophysics Data System (ADS)
Radice, David
2017-03-01
The flow inside remnants of binary neutron star (NS) mergers is expected to be turbulent, because of magnetohydrodynamics instability activated at scales too small to be resolved in simulations. To study the large-scale impact of these instabilities, we develop a new formalism, based on the large-eddy simulation technique, for the modeling of subgrid-scale turbulent transport in general relativity. We apply it, for the first time, to the simulation of the late-inspiral and merger of two NSs. We find that turbulence can significantly affect the structure and survival time of the merger remnant, as well as its gravitational-wave (GW) and neutrino emissions. The former will be relevant for GW observation of merging NSs. The latter will affect the composition of the outflow driven by the merger and might influence its nucleosynthetic yields. The accretion rate after black hole formation is also affected. Nevertheless, we find that, for the most likely values of the turbulence mixing efficiency, these effects are relatively small and the GW signal will be affected only weakly by the turbulence. Thus, our simulations provide a first validation of all existing post-merger GW models.
Large-eddy and unsteady RANS simulations of a shock-accelerated heavy gas cylinder
Morgan, B. E.; Greenough, J. A.
2015-04-08
Two-dimensional numerical simulations of the Richtmyer–Meshkov unstable “shock-jet” problem are conducted using both large-eddy simulation (LES) and unsteady Reynolds-averaged Navier–Stokes (URANS) approaches in an arbitrary Lagrangian–Eulerian hydrodynamics code. Turbulence statistics are extracted from LES by running an ensemble of simulations with multimode perturbations to the initial conditions. Detailed grid convergence studies are conducted, and LES results are found to agree well with both experiment and high-order simulations conducted by Shankar et al. (Phys Fluids 23, 024102, 2011). URANS results using a k–L approach are found to be highly sensitive to initialization of the turbulence lengthscale L and to the time at which L becomes resolved on the computational mesh. As a result, it is observed that a gradient diffusion closure for turbulent species flux is a poor approximation at early times, and a new closure based on the mass-flux velocity is proposed for low-Reynolds-number mixing.
Large eddy simulation of a stenosed artery using a femoral artery pulsatile flow profile.
Barber, Tracie J; Simmons, Anne
2011-07-01
Computational fluid dynamics simulation of stenosed arteries allows the analysis of quantities including wall shear stress, velocity, and pressure; detailed in vivo measurement is difficult yet the analysis of the fluid dynamics related to stenosis is important in understanding the likely causes and ongoing effects on the integrity of the vessel. In this study, a three-dimensional Large Eddy Simulation is conducted of a 50% occluded vessel, with a typical femoral artery profile used as the transient inlet conditions. The fluid is assumed to be homogenous, Newtonian and incompressible and the walls are assumed rigid. The stenosis is axisymmetric, however the three-dimensional study allows for a flow field that is not axisymmetric and results show significant three-dimensionality. High values of wall shear stress and oscillatory values of wall shear stress (varying in both space time) are observed. The results of the study give insight into the time-varying flow structures for a mildly stenosed artery and indicate that three-dimensional simulations may be important to gain a complete understanding of the flow field.
Large Eddy/Reynolds-Averaged Navier-Stokes Simulations of CUBRC Base Heating Experiments
NASA Technical Reports Server (NTRS)
Salazar, Giovanni; Edwards, Jack R.; Amar, Adam J.
2012-01-01
ven with great advances in computational techniques and computing power during recent decades, the modeling of unsteady separated flows, such as those encountered in the wake of a re-entry vehicle, continues to be one of the most challenging problems in CFD. Of most interest to the aerothermodynamics community is accurately predicting transient heating loads on the base of a blunt body, which would result in reduced uncertainties and safety margins when designing a re-entry vehicle. However, the prediction of heat transfer can vary widely depending on the turbulence model employed. Therefore, selecting a turbulence model which realistically captures as much of the flow physics as possible will result in improved results. Reynolds Averaged Navier Stokes (RANS) models have become increasingly popular due to their good performance with attached flows, and the relatively quick turnaround time to obtain results. However, RANS methods cannot accurately simulate unsteady separated wake flows, and running direct numerical simulation (DNS) on such complex flows is currently too computationally expensive. Large Eddy Simulation (LES) techniques allow for the computation of the large eddies, which contain most of the Reynolds stress, while modeling the smaller (subgrid) eddies. This results in models which are more computationally expensive than RANS methods, but not as prohibitive as DNS. By complimenting an LES approach with a RANS model, a hybrid LES/RANS method resolves the larger turbulent scales away from surfaces with LES, and switches to a RANS model inside boundary layers. As pointed out by Bertin et al., this type of hybrid approach has shown a lot of promise for predicting turbulent flows, but work is needed to verify that these models work well in hypersonic flows. The very limited amounts of flight and experimental data available presents an additional challenge for researchers. Recently, a joint study by NASA and CUBRC has focused on collecting heat transfer data
Atmospheric stability effects on wind farm performance using large-eddy simulation
NASA Astrophysics Data System (ADS)
Archer, C. L.; Ghaisas, N.; Xie, S.
2014-12-01
Atmospheric stability has been recently found to have significant impacts on wind farm performance, especially since offshore and onshore wind farms are known to operate often under non-neutral conditions. Recent field observations have revealed that changes in stability are accompanied by changes in wind speed, direction, and turbulent kinetic energy (TKE). In order to isolate the effects of stability, large-eddy simulations (LES) are performed under neutral, stable, and unstable conditions, keeping the wind speed and direction unchanged at a fixed height. The Lillgrund wind farm, comprising of 48 turbines, is studied in this research with the Simulator for Offshore/Onshore Wind Farm Applications (SOWFA) developed by the National Renewable Energy Laboratory. Unlike most previous numerical simulations, this study does not impose periodic boundary conditions and therefore is ideal for evaluating the effects of stability in large, but finite, wind farms. Changes in power generation, velocity deficit, rate of wake recovery, TKE, and surface temperature are quantified as a function of atmospheric stability. The sensitivity of these results to wind direction is also discussed.
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.
Dynamic dose assessment by Large Eddy Simulation of the near-range atmospheric dispersion.
Vervecken, Lieven; Camps, Johan; Meyers, Johan
2015-03-01
In order to improve the simulation of the near-range atmospheric dispersion of radionuclides, computational fluid dynamics is becoming increasingly popular. In the current study, Large-Eddy Simulation is used to examine the time-evolution of the turbulent dispersion of radioactive gases in the atmospheric boundary layer, and it is coupled to a gamma dose rate model that is based on the point-kernel method with buildup factors. In this way, the variability of radiological dose rate from cloud shine due to instantaneous turbulent mixing processes can be evaluated. The steady release in an open field of (41)Ar and (133)Xe for 4 different release heights is studied, thus covering radionuclides that decay with a high-energy gamma and a low-energy gamma, respectively. Based on these simulations, the variability of dose rates at ground level for different averaging times in the dose measurements is analyzed. It is observed that turbulent variability in the wind field can lead to dose estimates that are underestimated by up to a factor of four when conventional long-term measurements are used to estimate the dose from short-term exposures.
Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Givi, P.; Madnia, C. K.; Steinberger, C. J.; Frankel, S. H.; Vidoni, T. J.
1991-01-01
The main objective is to extend the boundaries within which large eddy simulations (LES) and direct numerical simulations (DNS) can be applied in computational analyses of high speed reacting flows. In the efforts related to LES, we were concerned with developing reliable subgrid closures for modeling of the fluctuation correlations of scalar quantities in reacting turbulent flows. In the work on DNS, we focused our attention to further investigation of the effects of exothermicity in compressible turbulent flows. In our previous work, in the first year of this research, we have considered only 'simple' flows. Currently, we are in the process of extending our analyses for the purpose of modeling more practical flows of current interest at LaRC. A summary of our accomplishments during the third six months of the research is presented.
Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Givi, P.; Madnia, C. K.; Steinberger, C. J.; Frankel, S. H.
1992-01-01
The basic objective of this research is to extend the capabilities of Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS) for the computational analyses of high speed reacting flows. In the efforts related to LES, we were primarily involved with assessing the performance of the various modern methods based on the Probability Density Function (PDF) methods for providing closures for treating the subgrid fluctuation correlations of scalar quantities in reacting turbulent flows. In the work on DNS, we concentrated on understanding some of the relevant physics of compressible reacting flows by means of statistical analysis of the data generated by DNS of such flows. In the research conducted in the second year of this program, our efforts focused on the modeling of homogeneous compressible turbulent flows by PDF methods, and on DNS of non-equilibrium reacting high speed mixing layers. Some preliminary work is also in progress on PDF modeling of shear flows, and also on LES of such flows.
On the large-eddy simulation of transitional wall-bounded flows
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Zang, Thomas A.; Speziale, Charles G.; Hussaini, M. Y.
1989-01-01
The structure of the subgrid scale fields in plane channel flow has been studied at various stages of the transition process to turbulence. The residual stress and subgrid scale dissipation calculated using velocity fields generated by direct numerical simulations of the Navier-Stokes equations are significantly different from their counterparts in turbulent flows. The subgrid scale dissipation changes sign over extended areas of the channel, indicating energy flow from the small scales to the large scales. This reversed energy cascade becomes less pronounced at the later stages of transition. Standard residual stress models of the Smagorinsky type are excessively dissipative. Rescaling the model constant improves the prediction of the total (integrated) subgrid scale dissipation, but not that of the local one. Despite the somewhat excessive dissipation of the rescaled Smagorinsky model, the results of a large eddy simulation of transition on a flat-plate boundary layer compare quite well with those of a direct simulation, and require only a small fraction of the computational effort. The inclusion of non-dissipative models, which could lead to further improvements, is proposed.
Large Eddy Simulation of Reacting Multiphase Flows in Complex Combustor Geometries
NASA Astrophysics Data System (ADS)
Apte, S.; Mahesh, K.; Iaccarino, G.; Constantinescu, G.; Ham, F.; Moin, P.
2003-11-01
We have developed a massively parallel computational tool (CDP) for large-eddy simulation (LES) of reacting multiphase flows in complex combustor geometries. A co-located, finite-volume scheme on unstructured grids is used to solve the low-Mach number equations for gaseous phase. The liquid phase is modeled by tracking a large number of computational particles in a Lagrangian framework with models for inter-phase mass, momentum, and energy transport. Complex physical phenomena of liquid atomization, droplet deformation, drag, and evaporation are captured using advanced subgrid models. A flamelet/progress variable appraoch by Pierce & Moin (2001) is used to compute non-premixed turbulent combustion. A series of validation studies in coaxial and realistic gas-turbine combustor geometries are performed to test the predictive capability of the solver. Specifically, simulations of non-premixed combustion, particle-laden swirling flows, droplet vaporization in coaxial-jet combustors and spray breakup in realistic injectors are performed and good agreement with avialable experimental data is obtained. This tool is now being used to perform simulations of turbulent spray flames in a realistic Pratt & Whitney gas-turbine combustion chamber using Department of Energy's computational resources under the Accelerated Strategic Computing Initiative (ASCI) project.
Large eddy simulation of a turbulent non-reacting spray jet
Hu, Bing; Banerjee, S; Liu, K; Rajamohan, D; Deur, J M; Xue, Qingluan; Som, Sibendu; Senecal, Peter Kelly; Pomraning, Eric
2015-01-01
We performed Large Eddy Simulation (LES) of a turbulent non-reacting n-Heptane spray jet, referred to as Spray H in the Engine Combustion Network (ECN), and executed a data analysis focused on key LES metrics such as fraction of resolved turbulent kinetic energy and similarity index. In the simulation, we used the dynamic structure model for the sub-grid stress, and the Lagrangian-based spray-parcel models coupled with the blob-injection model. The finest mesh-cell size used was characterized by an Adaptive Mesh Refinement (AMR) cell size of 0.0625 mm. To obtain ensemble statistics, we performed 28 numerical realizations of the simulation. Demonstrated by the comparison with experimental data in a previous study [7], this LES has accurately predicted global quantities, such as liquid and vapor penetrations. The analysis in this work shows that 14 realizations of LES are sufficient to provide a reasonable representation of the average flow behavior that is benchmarked against the 28-realization ensemble. With the current mesh, numerical schemes, and sub-grid scale turbulence model, more than 95% of the turbulent kinetic energy is directly resolved in the flow regions of interest. The large-scale flow structures inferred from a statistical analysis reveal a region of disorganized flow around the peripheral region of the spray jet, which appears to be linked to the entrainment process.
Large-Eddy Simulation of a Shock Train in a Duct with Side Walls
NASA Astrophysics Data System (ADS)
Morgan, Brandon; Duraisamy, Karthik; Lele, Sanjiva
2012-11-01
Large-eddy simulation (LES) is utilized to investigate the three-dimensionality of a shock train in a constant-area isolator model with fully resolved side walls (M∞ = 1.61, Reθ ~ 1660). Flow conditions and geometry are selected to match experimental conditions investigated by Carroll (1988); although Reynolds number is reduced to ensure adequate mesh resolution. Simulations with spanwise periodic boundary conditions are first performed, the results of which are compared to experiment and validated with a three-level grid refinement study. The same shock train interaction is then simulated in a three-dimensional, low-aspect ratio rectangular duct geometry with particular emphasis placed on characterizing secondary corner flows and the effects of these corner flows on the location and structure of the shock train. It is found, for instance, that location of the initial shock is particularly sensitive to the effects of spanwise confinement. Most significantly, it is observed that the same pressure ratio which results in a stable shock train with periodic boundary conditions may result in isolator unstart when side-wall effects are fully resolved.
A minimum dissipation scalar transport model for large-eddy simulation of turbulent flows
NASA Astrophysics Data System (ADS)
Abkar, Mahdi; Bae, Hyun J.; Moin, Parviz
2016-11-01
Minimum-dissipation models are a simple alternative to the Smagorinsky-type approaches to parameterize the sub-filter scale turbulent fluxes in large-eddy simulation. A recently derived minimum-dissipation model for sub-filter stress tensor is the AMD model and has many desirable properties. It is more cost effective than the dynamic Smagorinsky model, it appropriately switches off in laminar and transitional flows, and it is consistent with the theoretic sub-filter stress tensor on both isotropic and anisotropic grids. In this study, an extension of this approach to modeling the sub-filter scalar flux is proposed. The performance of the AMD model is tested in the simulation of a high Reynolds number, rough wall, boundary layer flow with a constant and uniform surface scalar flux. The simulation results obtained from the AMD model show good agreement with well-established empirical correlations and theoretical predictions of the resolved flow statistics. In particular, the AMD model is capable to accurately predict the expected surface-layer similarity profiles and power spectra for both velocity and scalar concentration.
Minimum-dissipation scalar transport model for large-eddy simulation of turbulent flows
NASA Astrophysics Data System (ADS)
Abkar, Mahdi; Bae, Hyun J.; Moin, Parviz
2016-08-01
Minimum-dissipation models are a simple alternative to the Smagorinsky-type approaches to parametrize the subfilter turbulent fluxes in large-eddy simulation. A recently derived model of this type for subfilter stress tensor is the anisotropic minimum-dissipation (AMD) model [Rozema et al., Phys. Fluids 27, 085107 (2015), 10.1063/1.4928700], which has many desirable properties. It is more cost effective than the dynamic Smagorinsky model, it appropriately switches off in laminar and transitional flows, and it is consistent with the exact subfilter stress tensor on both isotropic and anisotropic grids. In this study, an extension of this approach to modeling the subfilter scalar flux is proposed. The performance of the AMD model is tested in the simulation of a high-Reynolds-number rough-wall boundary-layer flow with a constant and uniform surface scalar flux. The simulation results obtained from the AMD model show good agreement with well-established empirical correlations and theoretical predictions of the resolved flow statistics. In particular, the AMD model is capable of accurately predicting the expected surface-layer similarity profiles and power spectra for both velocity and scalar concentration.
Unstructured Large Eddy Simulations of Hot Supersonic Jets from a Chevron Nozzle
NASA Astrophysics Data System (ADS)
Brès, Guillaume; Nichols, Joseph; Lele, Sanjiva; Ham, Frank
2012-11-01
Large eddy simulations (LES) are performed for heated supersonic turbulent jets issued from a converging-diverging round nozzle with chevrons. The unsteady flow processes and shock/turbulence interactions are investigated with the unstructured compressible flow solver ``Charles'' developed at Cascade Technologies. In this study, the complex geometry of the nozzle and chevrons (12 counts, 6° penetration) are explicitly included in the computational domain using unstructured body-fitted mesh and adaptive grid refinement. Sound radiation from the jet is computed using an efficient frequency-domain implementation of the Ffowcs Williams-Hawkings equation. Noise predictions are compared to experimental measurements carried out at the United Technologies Research Center for the same nozzle and operating conditions. The initial blind comparisons show good agreement in terms of spectra shape and levels for both the near-field and far-field noise. The current results show that the simulations accurately capture the main flow and noise features, including the shock cells, broadband shock-associated noise and turbulent mixing noise. Additional analysis of the large database generated by the LES is ongoing, to further investigate jet noise sources and chevron effects. This work is supported by NAVAIR grant N68335-11-C-0026 managed by Dr. John Spyropoulos. The simulations were carried out at DoD supercomputer facilities in ERDC and AFRL as part of the HPC Challenge Project NAWCP30952C5.
Data-Informed Large-Eddy Simulation of Coastal Land-Air-Sea Interactions
NASA Astrophysics Data System (ADS)
Calderer, A.; Hao, X.; Fernando, H. J.; Sotiropoulos, F.; Shen, L.
2016-12-01
The study of atmospheric flows in coastal areas has not been fully addressed due to the complex processes emerging from the land-air-sea interactions, e.g., abrupt change in land topography, strong current shear, wave shoaling, and depth-limited wave breaking. The available computational tools that have been applied to study such littoral regions are mostly based on open-ocean assumptions, which most times do not lead to reliable solutions. The goal of the present study is to better understand some of these near-shore processes, employing the advanced computational tools, developed in our research group. Our computational framework combines a large-eddy simulation (LES) flow solver for atmospheric flows, a sharp-interface immersed boundary method that can deal with real complex topographies (Calderer et al., J. Comp. Physics 2014), and a phase-resolved, depth-dependent, wave model (Yang and Shen, J. Comp. Physics 2011). Using real measured data taken in the FRF station in Duck, North Carolina, we validate and demonstrate the predictive capabilities of the present computational framework, which are shown to be in overall good agreement with the measured data under different wind-wave scenarios. We also analyse the effects of some of the complex processes captured by our simulation tools.
2011-09-01
Isogeometric Variational Multiscale Large-Eddy Simulation of Fully-developed Turbulent Flow over a Wavy Wall K. Chang, T.J.R. Hughes, and V.M. Calo...Reference: K. Chang, T.J.R. Hughes, and V.M. Calo, "Isogeometric Variational Multiscale Large-Eddy Simulation of Fully-developed Turbulent Flow over a...REPORT TYPE 3. DATES COVERED 00-00-2011 to 00-00-2011 4. TITLE AND SUBTITLE Isogeometric Variational Multiscale Large-Eddy Simulation of Fully
Development and Implementation of an Online Chemistry Module to a Large Eddy Simulation Model
NASA Astrophysics Data System (ADS)
Forkel, Renate; Banzhaf, Sabine; Kanani-Sühring, Farah; Ketelsen, Klaus; Khan, Basit; Maronga, Björn; Mauder, Matthias; Raasch, Siegfried
2017-04-01
Large Eddy Simulation (LES) models permit to resolve relevant scales of turbulent motion, so that these models can capture the inherent unsteadiness of atmospheric turbulence and advection. However, LES models are so far hardly applied for urban air quality studies, in particular chemical transformation of pollutants. Within the BMBF (Bundesministerium für Bildung und Forschung) funded joint project MOSAIK (Modellbasierte Stadtplanung und Anwendung im Klimawandel / Model-based city planning and application in climate change) the state of the art LES model PALM (Parallelized LES Model; Maronga et al, 2015, Geosci. Model Dev., 8, doi:10.5194/gmd-8-2515-2015) is extended by an atmospheric chemistry scheme. Due to the high computational demands of a LES based model, compromises in the description of chemical processes are required. Therefore, a reduced chemistry mechanism, which includes only major pollutants namely O3, NO, NO2, CO, a highly simplified VOC chemistry and a small number of products have been implemented. For practical applications, our approach is to go beyond the simulation of single street canyons to chemical transformation, advection and deposition of air pollutants in the larger urban canopy. Tests of chemistry schemes and initial studies of chemistry-turbulence interactions are presented.
ENDLESS: An extended nonperiodic domain large-eddy simulation approach for scalar plumes
NASA Astrophysics Data System (ADS)
Chen, Bicheng; Yang, Di; Meneveau, Charles; Chamecki, Marcelo
2016-05-01
Large-eddy simulation (LES) has proven to be a valuable tool for high-fidelity modeling of environmental and geophysical turbulent flows. An important application of LES is to study the transport of effluents (e.g. oils from a subsea blowout) in the ocean mixed layer (OML). Oil plumes being transported in the OML experience the action of shear-generated turbulence, Langmuir circulations, Ekman transport and submesoscale quasi-geostrophic eddies. To resolve such turbulent processes, grid sizes of a few meters are desirable while horizontal domain sizes of LES are typically restricted from hundreds of meters to a few kilometers, for LES to remain practically affordable. Yet transported oil plumes evolve to large scales extending to tens or even hundreds of kilometers. In this study, the Extended Nonperiodic Domain LES for Scalar transport (ENDLESS) is proposed as a multi-scale approach to tackle this challenge while being computationally affordable. The basic idea is to simulate the shear turbulence and Langmuir circulations on a small horizontal domain with periodic boundary conditions while the resulting transport velocity field is replicated periodically following adaptively the large-scale plume as it evolves spatially towards much larger scales. This approach also permits the superposition of larger-scale quasi two-dimensional flow motions on the oil advection, allowing for coupling with regional circulation models. A validation case and two sample applications to oil plume evolution in the OML are presented in order to demonstrate key features and computational speedup associated with the ENDLESS method.
Large eddy simulation of wind-induced interunit dispersion around multistory buildings.
Ai, Z T; Mak, C M
2016-04-01
Previous studies regarding interunit dispersion used Reynolds-averaged Navier-Stokes (RANS) models and thus obtained only mean dispersion routes and re-entry ratios. Given that the envelope flow around a building is highly fluctuating, mean values could be insufficient to describe interunit dispersion. This study investigates the wind-induced interunit dispersion around multistory buildings using the large eddy simulation (LES) method. This is the first time interunit dispersion has been investigated transiently using a LES model. The quality of the selected LES model is seriously assured through both experimental validation and sensitivity analyses. Two aspects are paid special attention: (i) comparison of dispersion routes with those provided by previous RANS simulations and (ii) comparison of timescales with those of natural ventilation and the survival times of pathogens. The LES results reveal larger dispersion scopes than the RANS results. Such larger scopes could be caused by the fluctuating and stochastic nature of envelope flows, which, however, is canceled out by the inherent Reynolds-averaged treatment of RANS models. The timescales of interunit dispersion are comparable with those of natural ventilation. They are much shorter than the survival time of most pathogens under ordinary physical environments, indicating that interunit dispersion is a valid route for disease transmission. © 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.
Large-eddy simulation of oxygen transport and depletion in waterbodies
NASA Astrophysics Data System (ADS)
Scalo, Carlo; Piomelli, Ugo; Boegman, Leon
2010-11-01
Dissolved oxygen (DO) in water plays an important role in lake and marine ecosystems. Agricultural runoff may spur excessive plant growth on the water surface; when the plants die they sink to the bottom of the water bodies and decompose, consuming oxygen. Significant environmental (and economic) damage may result from the loss of aquatic life caused by the oxygen depletion. The study of DO transport and depletion dynamics in water bodies has, therefore, become increasingly important. We study this phenomenon by large-eddy simulations performed at laboratory scale. The equations governing the transport of momentum and of a scalar (the DO) in the fluid are coupled to a biochemical model for DO depletion in the permeable sediment bed [Higashino et al., Water Res. (38) 1, 2004)], and to an equation for the fluid transpiration in the porous medium. The simulations are in good agreement with previous calculations and experiments. We show that the results are sensitive to the biochemical and fluid dynamical properties of the sediment, which are very difficult to determine experimentally.
DG-FDF solver for large eddy simulation of compressible flows
NASA Astrophysics Data System (ADS)
Sammak, Shervin; Brazell, Michael; Mavriplis, Dimitri; Givi, Peyman
2016-11-01
A new computational scheme is developed for large eddy simulation (LES) of compressible turbulent flows with the filtered density function (FDF) subgrid scale closure. This is a hybrid scheme, combining the discontinuous Galerkin (DG) Eulerian solver with a Lagrangian Monte Carlo FDF simulator. The methodology is shown to be suitable for LES, as a larger portion of the resolved energy is captured as the order of spectral approximation increases. Simulations are conducted of both subsonic and supersonic flows. The consistency and the overall performance of the DG-FDF solver are demonstrated, together with its shock capturing capabilities.
New approximate boundary conditions for large eddy simulations of wall-bounded flows
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Ferziger, Joel; Moin, Parviz; Kim, John
1989-01-01
Two new approximate boundary conditions have been applied to the large eddy simulation of channel flow with and without transpiration. These new boundary conditions give more accurate results than those previously in use, and allow significant reduction of the required CPU time over simulations in which no-slip conditions are applied. Mean velocity profiles and turbulence intensities compare well both with experimental data and with the results of resolved simulations. The influence of the approximate boundary conditions remains confined near the point of application and does not affect the turbulence statistics in the core of the flow.
Near-Wall Models in Large Eddy Simulations of Flow Behind a Backward-Facing Step
NASA Technical Reports Server (NTRS)
Cabot, W.
1996-01-01
Accurate large eddy simulation (LES) of a wall-bounded flow generally requires a near-wall resolution comparable to that in direct numerical simulation (DNS). As much as 50% of the total grid points and computational costs are expended in the near-wall regions in a typical simulation. This limits LES to fairly low Reynolds numbers on current computers. To perform practical flow applications at realistically high Reynolds numbers, such as flow over an airfoil, it is desirable to replace very thin, near-wall regions in the LES with easily and inexpensively computed wall models to specify the near-wall boundary conditions.
New approximate boundary conditions for large eddy simulations of wall-bounded flows
NASA Technical Reports Server (NTRS)
Piomelli, Ugo; Ferziger, Joel; Moin, Parviz; Kim, John
1989-01-01
Two new approximate boundary conditions have been applied to the large eddy simulation of channel flow with and without transpiration. These new boundary conditions give more accurate results than those previously in use, and allow significant reduction of the required CPU time over simulations in which no-slip conditions are applied. Mean velocity profiles and turbulence intensities compare well both with experimental data and with the results of resolved simulations. The influence of the approximate boundary conditions remains confined near the point of application and does not affect the turbulence statistics in the core of the flow.
A survey of modelling methods for high-fidelity wind farm simulations using large eddy simulation.
Breton, S-P; Sumner, J; Sørensen, J N; Hansen, K S; Sarmast, S; Ivanell, S
2017-04-13
Large eddy simulations (LES) of wind farms have the capability to provide valuable and detailed information about the dynamics of wind turbine wakes. For this reason, their use within the wind energy research community is on the rise, spurring the development of new models and methods. This review surveys the most common schemes available to model the rotor, atmospheric conditions and terrain effects within current state-of-the-art LES codes, of which an overview is provided. A summary of the experimental research data available for validation of LES codes within the context of single and multiple wake situations is also supplied. Some typical results for wind turbine and wind farm flows are presented to illustrate best practices for carrying out high-fidelity LES of wind farms under various atmospheric and terrain conditions.This article is part of the themed issue 'Wind energy in complex terrains'.
NASA Astrophysics Data System (ADS)
Maurer, K. D.; Bohrer, G.; Kenny, W. T.; Ivanov, V. Y.
2015-04-01
Surface roughness parameters, namely the roughness length and displacement height, are an integral input used to model surface fluxes. However, most models assume these parameters to be a fixed property of plant functional type and disregard the governing structural heterogeneity and dynamics. In this study, we use large-eddy simulations to explore, in silico, the effects of canopy-structure characteristics on surface roughness parameters. We performed a virtual experiment to test the sensitivity of resolved surface roughness to four axes of canopy structure: (1) leaf area index, (2) the vertical profile of leaf density, (3) canopy height, and (4) canopy gap fraction. We found roughness parameters to be highly variable, but uncovered positive relationships between displacement height and maximum canopy height, aerodynamic canopy height and maximum canopy height and leaf area index, and eddy-penetration depth and gap fraction. We also found negative relationships between aerodynamic canopy height and gap fraction, as well as between eddy-penetration depth and maximum canopy height and leaf area index. We generalized our model results into a virtual "biometric" parameterization that relates roughness length and displacement height to canopy height, leaf area index, and gap fraction. Using a decade of wind and canopy-structure observations in a site in Michigan, we tested the effectiveness of our model-driven biometric parameterization approach in predicting the friction velocity over heterogeneous and disturbed canopies. We compared the accuracy of these predictions with the friction-velocity predictions obtained from the common simple approximation related to canopy height, the values calculated with large-eddy simulations of the explicit canopy structure as measured by airborne and ground-based lidar, two other parameterization approaches that utilize varying canopy-structure inputs, and the annual and decadal means of the surface roughness parameters at the site
Maurer, K. D.; Bohrer, G.; Kenny, W. T.; ...
2015-04-30
Surface roughness parameters, namely the roughness length and displacement height, are an integral input used to model surface fluxes. However, most models assume these parameters to be a fixed property of plant functional type and disregard the governing structural heterogeneity and dynamics. In this study, we use large-eddy simulations to explore, in silico, the effects of canopy-structure characteristics on surface roughness parameters. We performed a virtual experiment to test the sensitivity of resolved surface roughness to four axes of canopy structure: (1) leaf area index, (2) the vertical profile of leaf density, (3) canopy height, and (4) canopy gap fraction.more » We found roughness parameters to be highly variable, but uncovered positive relationships between displacement height and maximum canopy height, aerodynamic canopy height and maximum canopy height and leaf area index, and eddy-penetration depth and gap fraction. We also found negative relationships between aerodynamic canopy height and gap fraction, as well as between eddy-penetration depth and maximum canopy height and leaf area index. We generalized our model results into a virtual "biometric" parameterization that relates roughness length and displacement height to canopy height, leaf area index, and gap fraction. Using a decade of wind and canopy-structure observations in a site in Michigan, we tested the effectiveness of our model-driven biometric parameterization approach in predicting the friction velocity over heterogeneous and disturbed canopies. We compared the accuracy of these predictions with the friction-velocity predictions obtained from the common simple approximation related to canopy height, the values calculated with large-eddy simulations of the explicit canopy structure as measured by airborne and ground-based lidar, two other parameterization approaches that utilize varying canopy-structure inputs, and the annual and decadal means of the surface roughness parameters at
Wall-modeled large-eddy simulation of transonic airfoil buffet at high Reynolds number
NASA Astrophysics Data System (ADS)
Fukushima, Yuma; Kawai, Soshi
2016-11-01
In this study, we conduct the wall-modeled large-eddy simulation (LES) of transonic buffet phenomena over the OAT15A supercritical airfoil at high Reynolds number. The transonic airfoil buffet involves shock-turbulent boundary layer interactions and shock vibration associated with the flow separation downstream of the shock wave. The wall-modeled LES developed by Kawai and Larsson PoF (2012) is tuned on the K supercomputer for high-fidelity simulation. We first show the capability of the present wall-modeled LES on the transonic airfoil buffet phenomena and then investigate the detailed flow physics of unsteadiness of shock waves and separated boundary layer interaction phenomena. We also focus on the sustaining mechanism of the buffet phenomena, including the source of the pressure waves propagated from the trailing edge and the interactions between the shock wave and the generated sound waves. This work was supported in part by MEXT as a social and scientific priority issue to be tackled by using post-K computer. Computer resources of the K computer was provided by the RIKEN Advanced Institute for Computational Science (Project ID: hp150254).
NASA Astrophysics Data System (ADS)
Liu, Zhongqiu; Li, Linmin; Li, Baokuan; Jiang, Maofa
2014-07-01
The current study developed a coupled computational model to simulate the transient fluid flow, solidification, and particle transport processes in a slab continuous-casting mold. Transient flow of molten steel in the mold is calculated using the large eddy simulation. An enthalpy-porosity approach is used for the analysis of solidification processes. The transport of bubble and non-metallic inclusion inside the liquid pool is calculated using the Lagrangian approach based on the transient flow field. A criterion of particle entrapment in the solidified shell is developed using the user-defined functions of FLUENT software (ANSYS, Inc., Canonsburg, PA). The predicted results of this model are compared with the measurements of the ultrasonic testing of the rolled steel plates and the water model experiments. The transient asymmetrical flow pattern inside the liquid pool exhibits quite satisfactory agreement with the corresponding measurements. The predicted complex instantaneous velocity field is composed of various small recirculation zones and multiple vortices. The transport of particles inside the liquid pool and the entrapment of particles in the solidified shell are not symmetric. The Magnus force can reduce the entrapment ratio of particles in the solidified shell, especially for smaller particles, but the effect is not obvious. The Marangoni force can play an important role in controlling the motion of particles, which increases the entrapment ratio of particles in the solidified shell obviously.
Large-eddy simulation of turbulent flow past hydrokinetic turbine arrays in natural waterways
NASA Astrophysics Data System (ADS)
Sotiropoulos, F.; Kang, S.; Yang, X.
2012-12-01
While a considerable amount of work has focused on studying the effects and performance of wind farms, very little is known about the performance of hydrokinetic turbine arrays in open channels such as rivers and oceans. Unlike large wind farms, where the vertical fluxes of momentum and energy from the atmospheric boundary layer comprise the main transport mechanisms, the presence of free surface in hydrokinetic turbine arrays inhibits vertical momentum transport. In addition, the complex bathymetry of natural waterways where turbine arrays are installed impacts the array efficiency and the turbulence-induced loads acting on the turbine blades. To computationally explore these issues we develop a numerical method capable of carrying our large-eddy simulation (LES) of turbulent flows past hydrokinetic turbine arrays mounted in natural waterways. The method employs the LES curvilinear immersed boundary (CURVIB) method of Kang et al. (Adv. in Water Resources, 2010) coupled with the actuator disk model for parameterizing the turbines. Simulations are carried out for the same turbine array placed both in a straight open channel and in a natural meandering stream to systematically investigate the effect of waterway bathymetry on array efficiency and power capture ability. Mean flow quantities and turbulence statistics within and downstream of the arrays will be analyzed and the effect of the turbine arrays as means for increasing the effective roughness of the channel bed will be extensively discussed.
Large Eddy simulation of turbulent hydrogen-fuelled supersonic combustion in an air cross-flow
NASA Astrophysics Data System (ADS)
Ingenito, A.; Cecere, D.; Giacomazzi, E.
2013-09-01
The main aim of this article is to provide a theoretical understanding of the physics of supersonic mixing and combustion. Research in advanced air-breathing propulsion systems able to push vehicles well beyond is of interest around the world. In a scramjet, the air stream flow captured by the inlet is decelerated but still maintains supersonic conditions. As the residence time is very short , the study of an efficient mixing and combustion is a key issue in the ongoing research on compressible flows. Due to experimental difficulties in measuring complex high-speed unsteady flowfields, the most convenient way to understand unsteady features of supersonic mixing and combustion is to use computational fluid dynamics. This work investigates supersonic combustion physics in the Hyshot II combustion chamber within the Large Eddy simulation framework. The resolution of this turbulent compressible reacting flow requires: (1) highly accurate non-dissipative numerical schemes to properly simulate strong gradients near shock waves and turbulent structures away from these discontinuities; (2) proper modelling of the small subgrid scales for supersonic combustion, including effects from compressibility on mixing and combustion; (3) highly detailed kinetic mechanisms (the Warnatz scheme including 9 species and 38 reactions is adopted) accounting for the formation and recombination of radicals to properly predict flame anchoring. Numerical results reveal the complex topology of the flow under investigation. The importance of baroclinic and dilatational effects on mixing and flame anchoring is evidenced. Moreover, their effects on turbulence-scale generation and the scaling law are analysed.
Investigation of particle-laden flow in a straight duct using large eddy simulation
Fairweather, M.; Yao, J.
2007-07-01
A particle-laden turbulent flow in a square duct is predicted using large eddy simulation (LES). The simulation is performed for a Reynolds number of 35,500, and correctly predicts the existence of secondary flows and their effects on the mean flow. The results are also in good qualitative agreement with experimental data obtained at different Reynolds numbers. One-way coupling is assumed between solid particles and the fluid, and a particle equation of motion, including Stokes drag, lift, buoyancy and gravity force terms, solved using a Lagrangian particle tracking technique. Three sizes of particle (1, 50 and 100 {mu}m) are considered, and results demonstrate that size has a significant effect on particle dispersion and deposition in the duct flow. As particle size increases, therefore, they tend to settle on the floor of the duct, with less dispersion in the fluid phase. The study demonstrates the usefulness of LES for nuclear waste processing applications since secondary flows occur in many practically-relevant flows, and since it is desirable that the two-phase waste mixture is kept as homogeneous as possible to prevent, or at least discourage, the settling out of solid particles to form a bed which can promote pipe blockages. (authors)
Scale-adaptive subgrid-scale modelling for large-eddy simulation of turbulent flows
NASA Astrophysics Data System (ADS)
Yu, Changping; Xiao, Zuoli; Li, Xinliang
2017-03-01
The proportionality between the subgrid-scale (SGS) drain rate of kinetic energy and the viscous dissipation rate of the resolved motions is studied a priori by filtering a given fully resolved field and evaluating a generic form of the hypothesized energy spectrum. The ratio of the SGS drain to the resolved dissipation, on which a balance condition for the SGS dissipation across an arbitrary grid scale is founded, is shown to be independent of the turbulence Reynolds number, and can be described by a function in terms of the averaged mesh Reynolds number. Such a balance condition can serve as a physical constraint in the SGS modeling to account for the scale effects of the model coefficient(s). Scale-adaptive dynamic Smagorinsky-Lilly model and mixed nonlinear model are formulated for large-eddy simulation of transitional and/or turbulent flows in such a way that the constraint is satisfied. The newly proposed scale-adaptive dynamic SGS models are validated in simulations of homogeneous isotropic turbulence and turbulent channel flow, and prove to be superior over traditional dynamic SGS models.
Large eddy simulation of tip-leakage flow in an axial flow fan
NASA Astrophysics Data System (ADS)
Park, Keuntae; Choi, Haecheon; Choi, Seokho; Sa, Yongcheol; Kwon, Oh-Kyoung
2016-11-01
An axial flow fan with a shroud generates a complicated tip-leakage flow by the interaction of the axial flow with the fan blades and shroud near the blade tips. In this study, large eddy simulation is performed for tip-leakage flow in a forward-swept axial flow fan inside an outdoor unit of an air-conditioner, operating at the design condition of the Reynolds number of 547,000 based on the radius of blade tip and the tip velocity. A dynamic global model is used for a subgrid-scale model, and an immersed boundary method in a non-inertial reference frame is adopted. The present simulation clearly reveals the generation and evolution of tip-leakage vortex near the blade tip by the leakage flow. At the inception of the leakage vortex near the leading edge of the suction-side of the blade tip, the leakage vortex is composed of unsteady multiple vortices containing high-frequency fluctuations. As the leakage vortex develops downstream along a slant line toward the following blade, large and meandering movements of the leakage vortex are observed. Thus low-frequency broad peaks of velocity and pressure occur near the pressure surface. Supported by the KISTI Supercomputing Center (KSC-2016-C3-0027).
Large eddy simulation and PIV experiments of air-water mixing tanks
NASA Astrophysics Data System (ADS)
Zamankhan, Piroz
2010-06-01
The simulations and experiments of a turbulent bubbly flow are carried out in a cylindrical mixing vessel. Dynamics of the turbulent bubbly flow is visualized using a novel two-phase particle image velocimetry (PIV) with a combination of back lighting, digital masking and fluorescent tracer particles. Using an advanced technique, Mie's scattering at surfaces of bubbles is totally filtered out and, henceforth, images of tracer particles and of bubbles are obtained with high quality. In parallel to the comprehensive experimental studies, numerical results are obtained from large eddy simulations (LES) of the two-phase air-water mixer. The impeller-induced flow at the blade tip radius is modeled by using sliding mesh method. The results demonstrate the existence of large structures such as tip-vortex tips, and also some finer details. In addition, the stability of the jet is found to be connected with the fluctuations of the tip vortices whose dynamics are affected by the presence of bubbles. Numerical results are used to interpret the measurement data and to guide the refinement of consistent theoretical analyses. Such information is invaluable in the development of advanced theories capable of describing bubbly flows in the presence of complex liquid flow. This detailed information is of real significance in facilitating the design and scale-up of practical stirred tanks.
Large-eddy simulation of particle-laden atmospheric boundary layer
NASA Astrophysics Data System (ADS)
Ilie, Marcel; Smith, Stefan Llewellyn
2008-11-01
Pollen dispersion in the atmospheric boundary layer (ABL) is numerically investigated using a hybrid large-eddy simulation (LES) Lagrangian approach. Interest in prediction of pollen dispersion stems from two reasons, the allergens in the pollen grains and increasing genetic manipulation of plants leading to the problem of cross pollination. An efficient Eulerian-Lagrangian particle dispersion algorithm for the prediction of pollen dispersion in the atmospheric boundary layer is outlined. The volume fraction of the dispersed phase is assumed to be small enough such that particle-particle collisions are negligible and properties of the carrier flow are not modified. Only the effect of turbulence on particle motion has to be taken into account (one-way coupling). Hence the continuous phase can be treated separate from the particulate phase. The continuous phase is determined by LES in the Eulerian frame of reference whereas the dispersed phase is simulated in a Lagrangian frame of reference. Numerical investigations are conducted for the convective, neutral and stable boundary layer as well different topographies. The results of the present study indicate that particles with small diameter size follow the flow streamlines, behaving as tracers, while particles with large diameter size tend to follow trajectories which are independent of the flow streamlines. Particles of ellipsoidal shape travel faster than the ones of spherical shape.
Investigations of Subsonic Compressible Boundary Layer Flows using Hybrid Large Eddy Simulations
NASA Astrophysics Data System (ADS)
Taylor, Sara Jo
The objective of this thesis is to investigate the spatially developing turbulent compressible boundary layer on a flat plate using the Spalart-Allmaras Detached Eddy Simulation (SA-DES) model [22] and the Nichols-Nelson hybrid Reynolds-Averaged Navier-Stokes/Large Eddy Simulation (RANS/LES) model [13] which have been implemented into the Wind-US 3.0 computational fluid dynamics code [30]; both of the hybrid approaches involve RANS modeling in the near-wall region and LES treatment in the outer region. Generation of unsteady turbulent inflow data is achieved via the prescribed energy spectrum method. The studies illustrated dependence on Reynolds number based on momentum thickness, Reθ, ranging from 3018 to 19430, and dependence on Mach number,
NASA Astrophysics Data System (ADS)
Fernandez, P.; Nguyen, N. C.; Peraire, J.
2017-05-01
We present a high-order Implicit Large-Eddy Simulation (ILES) approach for transitional aerodynamic flows. The approach encompasses a hybridized Discontinuous Galerkin (DG) method for the discretization of the Navier-Stokes (NS) equations, and a parallel preconditioned Newton-GMRES solver for the resulting nonlinear system of equations. The combination of hybridized DG methods with an efficient solution procedure leads to a high-order accurate NS solver that is competitive to alternative approaches, such as finite volume and finite difference codes, in terms of computational cost. The proposed approach is applied to transitional flows over the NACA 65-(18)10 compressor cascade and the Eppler 387 wing at Reynolds numbers up to 460,000. Grid convergence studies are presented and the required resolution to capture transition at different Reynolds numbers is investigated. Numerical results show rapid convergence and excellent agreement with experimental data. In short, this work aims to demonstrate the potential of high-order ILES for simulating transitional aerodynamic flows. This is illustrated through numerical results and supported by theoretical considerations.
Large-eddy simulation of propeller wake at design operating conditions
NASA Astrophysics Data System (ADS)
Kumar, Praveen; Mahesh, Krishnan
2016-11-01
Understanding the propeller wake is crucial for efficient design and optimized performance. The dynamics of the propeller wake are also central to physical phenomena such as cavitation and acoustics. Large-eddy simulation is used to study the evolution of the wake of a five-bladed marine propeller from near to far field at design operating condition. The computed mean loads and phase-averaged flow field show good agreement with experiments. The propeller wake consisting of tip and hub vortices undergoes streamtube contraction, which is followed by the onset of instabilities as evident from the oscillations of the tip vortices. Simulation results reveal a mutual induction mechanism of instability where instead of the tip vortices interacting among themselves, they interact with the smaller vortices generated by the roll-up of the blade trailing edge wake in the near wake. Phase-averaged and ensemble-averaged flow fields are analyzed to explain the flow physics. This work is supported by ONR.
NASA Astrophysics Data System (ADS)
Gicquel, L. Y. M.; Staffelbach, G.; Sanjose, M.; Boileau, M.
2009-12-01
Being able to ignite or reignite a gas turbine engine in a cold and rarefied atmosphere is a critical issue for many aeronautical gas turbine manufacturers. From a fundamental point of view, the ignition of the first burner and the flame propagation from one burner to another are two phenomena that are usually not studied. The present work presents on-going and past Large Eddy Simulations (LES) on this specific subject and as investigated at CERFACS (European Centre for Research and Advanced Training in Scientific Computation) located in Toulouse, France. Validation steps and potential difficulties are underlined to ensure reliability of LES for such problems. Preliminary LES results on simple burners are then presented, followed by simulations of a complete ignition sequence in an annular helicopter chamber. For all cases and when possible, two-phase or purely gaseous LES have been applied to the experimentally simplified or the full geometries. For the latter, massively parallel computing (700 processors on a Cray XT3 machine) was essential to perform the computation. Results show that liquid fuel injection has a strong influence on the ignition times and the rate at which the flame progresses from burner to burner. The propagation speed characteristic of these phenomena is much higher than the turbulent flame speed. Based on an in-depth analysis of the computational data, the difference in speed is mainly identified as being due to thermal expansion and the flame speed is strongly modified by the main burner aerodynamics issued by the swirled injection.
From large-eddy simulation to multi-UAVs sampling of shallow cumulus clouds
NASA Astrophysics Data System (ADS)
Lamraoui, Fayçal; Roberts, Greg; Burnet, Frédéric
2016-04-01
In-situ sampling of clouds that can provide simultaneous measurements at satisfying spatio-temporal resolutions to capture 3D small scale physical processes continues to present challenges. This project (SKYSCANNER) aims at bringing together cloud sampling strategies using a swarm of unmanned aerial vehicles (UAVs) based on Large-eddy simulation (LES). The multi-UAV-based field campaigns with a personalized sampling strategy for individual clouds and cloud fields will significantly improve the understanding of the unresolved cloud physical processes. An extensive set of LES experiments for case studies from ARM-SGP site have been performed using MesoNH model at high resolutions down to 10 m. The carried out simulations led to establishing a macroscopic model that quantifies the interrelationship between micro- and macrophysical properties of shallow convective clouds. Both the geometry and evolution of individual clouds are critical to multi-UAV cloud sampling and path planning. The preliminary findings of the current project reveal several linear relationships that associate many cloud geometric parameters to cloud related meteorological variables. In addition, the horizontal wind speed indicates a proportional impact on cloud number concentration as well as triggering and prolonging the occurrence of cumulus clouds. In the framework of the joint collaboration that involves a Multidisciplinary Team (including institutes specializing in aviation, robotics and atmospheric science), this model will be a reference point for multi-UAVs sampling strategies and path planning.
Reduction of Cloud Water in Ship Tracks: Observations and Large-Eddy Simulations
NASA Technical Reports Server (NTRS)
Ackerman, A. S.; Stevens, D. E.; Toon, O. B.; Coakley, J. A., Jr.; Gore, Warren J. (Technical Monitor)
2001-01-01
Ship tracks represent a natural laboratory to study the effects of aerosols on clouds. A number of observations and simulations have shown that increased droplet concentrations in ship tracks increase their total cross-sectional area, thereby enhancing cloud albedo and providing a negative radiative forcing at the surface and the top of the atmosphere. In some cases, cloud water has been found to be enhanced in ship tracks, which has been attributed to suppression of drizzle and implies an enhanced susceptibility of cloud albedo to droplet concentrations. However, more recently compiled observations indicate that cloud water is instead reduced in daytime ship tracks on average. Such a response is consistent with cloud-burning due to solar absorption by soot (the semi-direct radiative forcing of aerosols), recently suggested to be suppressing trade cumulus cloud coverage over the Indian Ocean. We will summarize observational evidence and present large-eddy simulations that consider these competing mechanisms in the effects of aerosols on cloud albedo.
Large-eddy simulation of airflow and heat transfer in a general ward of hospital
NASA Astrophysics Data System (ADS)
Hasan, Md. Farhad; Himika, Taasnim Ahmed; Molla, Md. Mamun
2016-07-01
In this paper, a very popular alternative computational technique, the Lattice Boltzmann Method (LBM) has been used for Large-Eddy Simulation (LES) of airflow and heat transfer in general ward of hospital. Different Reynolds numbers have been used to study the airflow pattern. In LES, Smagorinsky turbulence model has been considered and a discussion has been conducted in brief. A code validation has been performed comparing the present results with benchmark results for lid-driven cavity problem and the results are found to agree very well. LBM is demonstrated through simulation in forced convection inside hospital ward with six beds with a partition in the middle, which acted like a wall. Changes in average rate of heat transfer in terms of average Nusselt numbers have also been recorded in tabular format and necessary comparison has been showed. It was found that partition narrowed the path for airflow and once the air overcame this barrier, it got free space and turbulence appeared. For higher turbulence, the average rate of heat transfer increased and patients near the turbulence zone released maximum heat and felt more comfortable.
Lantz, Jonas; Gårdhagen, Roland; Karlsson, Matts
2012-10-01
In this study, large-eddy simulation (LES) is employed to calculate the disturbed flow field and the wall shear stress (WSS) in a subject specific human aorta. Velocity and geometry measurements using magnetic resonance imaging (MRI) are taken as input to the model to provide accurate boundary conditions and to assure the physiological relevance. In total, 50 consecutive cardiac cycles were simulated from which a phase average was computed to get a statistically reliable result. A decomposition similar to Reynolds decomposition is introduced, where the WSS signal is divided into a pulsating part (due to the mass flow rate) and a fluctuating part (originating from the disturbed flow). Oscillatory shear index (OSI) is plotted against time-averaged WSS in a novel way, and locations on the aortic wall where elevated values existed could easily be found. In general, high and oscillating WSS values were found in the vicinity of the branches in the aortic arch, while low and oscillating WSS were present in the inner curvature of the descending aorta. The decomposition of WSS into a pulsating and a fluctuating part increases the understanding of how WSS affects the aortic wall, which enables both qualitative and quantitative comparisons.
On the large eddy simulation of turbulent flows in complex geometry
NASA Technical Reports Server (NTRS)
Ghosal, Sandip
1993-01-01
Application of the method of Large Eddy Simulation (LES) to a turbulent flow consists of three separate steps. First, a filtering operation is performed on the Navier-Stokes equations to remove the small spatial scales. The resulting equations that describe the space time evolution of the 'large eddies' contain the subgrid-scale (sgs) stress tensor that describes the effect of the unresolved small scales on the resolved scales. The second step is the replacement of the sgs stress tensor by some expression involving the large scales - this is the problem of 'subgrid-scale modeling'. The final step is the numerical simulation of the resulting 'closed' equations for the large scale fields on a grid small enough to resolve the smallest of the large eddies, but still much larger than the fine scale structures at the Kolmogorov length. In dividing a turbulent flow field into 'large' and 'small' eddies, one presumes that a cut-off length delta can be sensibly chosen such that all fluctuations on a scale larger than delta are 'large eddies' and the remainder constitute the 'small scale' fluctuations. Typically, delta would be a length scale characterizing the smallest structures of interest in the flow. In an inhomogeneous flow, the 'sensible choice' for delta may vary significantly over the flow domain. For example, in a wall bounded turbulent flow, most statistical averages of interest vary much more rapidly with position near the wall than far away from it. Further, there are dynamically important organized structures near the wall on a scale much smaller than the boundary layer thickness. Therefore, the minimum size of eddies that need to be resolved is smaller near the wall. In general, for the LES of inhomogeneous flows, the width of the filtering kernel delta must be considered to be a function of position. If a filtering operation with a nonuniform filter width is performed on the Navier-Stokes equations, one does not in general get the standard large eddy
Delorme, Yann T; Rodefeld, Mark D; Frankel, Steven H
2017-01-17
Children born with only one functional ventricle must typically undergo a series of three surgeries to obtain the so-called Fontan circulation in which the blood coming from the body passively flows from the Vena Cavae (VCs) to the Pulmonary Arteries (PAs) through the Total Cavopulmonary Connection (TCPC). The circulation is inherently inefficient due to the lack of a subpulmonary ventricle. Survivors face the risk of circulatory sequelae and eventual failure for the duration of their lives. Current efforts are focused on improving the outcomes of Fontan palliation, either passively by optimizing the TCPC, or actively by using mechanical support. We are working on a chronic implant that would be placed at the junction of the TCPC, and would provide the necessary pressure augmentation to re-establish a circulation that recapitulates a normal two-ventricle circulation. This implant is based on the Von Karman viscous pump and consists of a vaned impeller that rotates inside the TCPC. To evaluate the performance of such a device, and to study the flow features induced by the presence of the pump, Computational Fluid Dynamics (CFD) is used. CFD has become an important tool to understand hemodynamics owing to the possibility of simulating quickly a large number of designs and flow conditions without any harm for patients. The transitional and unsteady nature of the flow can make accurate simulations challenging. We developed and in-house high order Large Eddy Simulation (LES) solver coupled to a recent Immersed Boundary Method (IBM) to handle complex geometries. Multiblock capability is added to the solver to allow for efficient simulations of complex patient specific geometries. Blood simulations are performed in a complex patient specific TCPC geometry. In this study, simulations without mechanical assist are performed, as well as after virtual implantation of the temporary and chronic implants being developed. Instantaneous flow structures, hepatic factor distribution
NASA Astrophysics Data System (ADS)
Anupindi, Kameswararao; Delorme, Yann; Shetty, Dinesh A.; Frankel, Steven H.
2013-12-01
Computational fluid dynamics (CFD) simulations are becoming a reliable tool to understand hemodynamics, disease progression in pathological blood vessels and to predict medical device performance. Immersed boundary method (IBM) emerged as an attractive methodology because of its ability to efficiently handle complex moving and rotating geometries on structured grids. However, its application to study blood flow in complex, branching, patient-specific anatomies is scarce. This is because of the dominance of grid nodes in the exterior of the fluid domain over the useful grid nodes in the interior, rendering an inevitable memory and computational overhead. In order to alleviate this problem, we propose a novel multiblock based IBM that preserves the simplicity and effectiveness of the IBM on structured Cartesian meshes and enables handling of complex, anatomical geometries at a reduced memory overhead by minimizing the grid nodes in the exterior of the fluid domain. As pathological and medical device hemodynamics often involve complex, unsteady transitional or turbulent flow fields, a scale resolving turbulence model such as large eddy simulation (LES) is used in the present work. The proposed solver (here after referred as WenoHemo), is developed by enhancing an existing in-house high-order incompressible flow solver that was previously validated for its numerics and several LES models by Shetty et al. (2010) [33]. In the present work, WenoHemo is systematically validated for additional numerics introduced, such as IBM and the multiblock approach, by simulating laminar flow over a sphere and laminar flow over a backward facing step respectively. Then, we validate the entire solver methodology by simulating laminar and transitional flow in abdominal aortic aneurysm (AAA). Finally, we perform blood flow simulations in the challenging clinically relevant thoracic aortic aneurysm (TAA), to gain insights into the type of fluid flow patterns that exist in pathological
Large-Eddy Simulation of the Evolving Stable Boundary Layer Over Flat Terrain
Townsend, R
2002-01-02
The stable boundary layer (SBL) in the atmosphere is of considerable interest because it is often the worse case scenario for air pollution studies and health effect assessments associated with the accidental release of toxic material. Traditional modeling approaches used in such studies do not simulate the non-steady character of the velocity field, and hence often overpredict concentrations while underpredicting spatial coverage of potentially harmful concentrations of airborne material. The challenge for LES is to be able to resolve the rather small energy-containing eddies of the SBL while still maintaining an adequate domain size. This requires that the subgrid-scale (SGS) parameterization of turbulence incorporate an adequate representation of turbulent energy transfer. Recent studies have shown that both upscale and downscale energy transfer can occur simultaneously, but that overall the net transfer is downscale. Including the upscale transfer of turbulent energy (energy backscatter) is particularly important near the ground and under stably-stratified conditions. The goal of this research is to improve the ability to realistically simulate the SBL. The large-eddy simulation (LES) approach with its subgrid-scale (SGS) turbulence model does a better job of capturing the temporally and spatially varying features of the SBL than do Reynolds-averaging models. The scientific objectives of this research are: (1) to characterize features of the evolving SBL structure for a range of meteorological conditions (wind speed and surface cooling), (2) to simulate realistically the transfer of energy between resolved and subgrid scales, and (3) to apply results to improve simulation of dispersion in the SBL.
Large Eddy Simulation and Field Experiments of Pollen Transport in the Atmospheric Boundary Layer
NASA Astrophysics Data System (ADS)
Chamecki, M.; Meneveau, C.; Parlange, M. B.; van Hout, R.
2006-12-01
Dispersion of airborne pollen by the wind has been a subject of interest for botanists and allergists for a long time. More recently, the development of genetically modified crops and questions about cross-pollination and subsequent contamination of natural plant populations has brought even more interest to this field. A critical question is how far from the source field pollen grains will be advected. Clearly the answer depends on the aerodynamic properties of the pollen, geometrical properties of the field, topography, local vegetation, wind conditions, atmospheric stability, etc. As a consequence, field experiments are well suited to provide some information on pollen transport mechanisms but are limited to specific field and weather conditions. Numerical simulations do not have this drawback and can be a useful tool to study pollen dispersal in a variety of configurations. It is well known that the dispersion of particles in turbulent fields is strongly affected by the large scale coherent structures. Large Eddy Simulation (LES) is a technique that allows us to study the typical distances reached by pollen grains and, at the same time, resolve the larger coherent structures present in the atmospheric boundary layer. The main objective of this work is to simulate the dispersal of pollen grains in the atmospheric surface layer using LES. Pollen concentrations are simulated by an advection-diffusion equation including gravitational settling. Of extreme importance is the specification of the bottom boundary conditions characterizing the pollen source over the canopy and the deposition process everywhere else. In both cases we make use of the theoretical profile for suspended particles derived by Kind (1992). Field experiments were performed to study the applicability of the theoretical profile to pollen grains and the results are encouraging. Airborne concentrations as well as ground deposition from the simulations are compared to experimental data to validate the
Large eddy simulation in a turbulent jet exhausting into a submerged space or a cocurrent flow
NASA Astrophysics Data System (ADS)
Volkov, K. N.
2011-01-01
Results of large eddy simulations in a subsonic isothermal turbulent jet exhausting from a circular nozzle into a submerged space or a cocurrent flow are presented. The flow is described by space-averaged Navier-Stokes equations and by the RNG model of subgrid scale viscosity. Results computed for different values of the cocurrency parameter are compared with available results of numerical simulations and experimental data. The results obtained are found to agree well with measured data and to confirm the basic laws of variation of gas-dynamic and fluctuating parameters of submerged and cocurrent jets.
Large-eddy simulations of viscoelastic isotropic turbulence with the FENE-P fluid
NASA Astrophysics Data System (ADS)
Pinho, Fernando T.; Ferreira, Pedro O.; B. da Silva, Carlos; Idmec/Feup Collaboration
2016-11-01
A new subgrid-scale (SGS) model developed for large-eddy simulations (LES) of dilute polymer solutions described by the Finitely Extensible Nonlinear Elastic constitutive equation closed with the Peterlin approximation (FENE-P), is presented. The filtered conformation tensor evolution equation uses the self-similarity of the polymer stretching terms, and the global equilibrium of the trace of the conformation tensor, while the SGS stresses are modelled with the classical Smagorinsky model. The new closure is assessed in direct numerical simulations (DNS) of forced isotropic turbulence using classical a-priori tests, and in a-posteriori (LES) showing excellent agreement with all the exact (filtered DNS) results.
Grid-point requirements for large eddy simulation: Chapman's estimates revisited
NASA Astrophysics Data System (ADS)
Choi, Haecheon; Moin, Parviz
2012-01-01
Resolution requirements for large eddy simulation (LES), estimated by Chapman [AIAA J. 17, 1293 (1979)], are modified using accurate formulae for high Reynolds number boundary layer flow. The new estimates indicate that the number of grid points (N) required for wall-modeled LES is proportional to ReLx , but a wall-resolving LES requires N ˜ReLx 13 /7 , where Lx is the flat-plate length in the streamwise direction. On the other hand, direct numerical simulation, resolving the Kolmogorov length scale, requires N ˜ReLx 37 /14 .
Numerical modelling of odour dispersion around a cubical obstacle using large eddy simulation.
Dourado, Harerton Oliveira; Santos, Jane Meri; Reis, Neyval C; Mavroidis, Ilias
2012-01-01
In the present work two different large eddy simulation (LES) approaches, namely the Dynamic Smagorinsky model and the Wale model, are used to simulate the air flow and pollutant dispersion around a cubical obstacle. Results are compared with wind tunnel data (WT) and with results from the Smagorinsky LES model. Overall agreement was good between the different LES approaches and the WT results, both for the mean and fluctuating flow and concentration patterns. LES models can provide good estimates of concentration fluctuation intensity and enable the calculation of the intermittency factor. The model results indicate that LES is a viable tool for odour impact assessment.
Large eddy simulation using high-resolution and high-order methods.
Drikakis, D; Hahn, M; Mosedale, A; Thornber, B
2009-07-28
Restrictions on computing power make direct numerical simulation too expensive for complex flows; thus, the development of accurate large eddy simulation (LES) methods, which are industrially applicable and efficient, is required. This paper reviews recent findings about the leading order dissipation rate associated with high-resolution methods and improvements to the standard schemes for use in highly turbulent flows. Results from implicit LES are presented for a broad range of flows and numerical schemes, ranging from the second-order monotone upstream-centered schemes for conservation laws to very high-order (up to ninth-order) weighted essentially non-oscillatory schemes.
Large-eddy simulation of flow through a plane, asymmetric diffuser
NASA Technical Reports Server (NTRS)
Kaltenbach, Hans-Jakob
1994-01-01
A challenge for traditional turbulence modeling, based on the Reynolds averaged Navier-Stokes equations, remains the accurate prediction of 'mild', adverse pressure-gradient driven separation from a smooth surface. With this study we want to explore the capability of large-eddy simulation to predict the separation which occurs on the deflected wall of an asymmetric, plane diffuser with opening angle of 10 deg. The flow through the plane diffuser exhibits some additional interesting physical phenomena which make it a challenging test case. In addition to 'mild' separation about halfway down the deflected ramp, the flow is characterized by a small backflow zone with stalled fluid in the rear part of the expanding section. The turbulent flow entering the diffuser is subject to combined adverse and radial pressure gradients stemming from the convex curvature. Finally the flow recovers into a developed, turbulent channel flow in the outlet section. Obi et al. provide measurements of mean flow, Reynolds stresses, and pressure recovery, which were obtained by means of LDV in a wind tunnel. The objective of this study is to investigate whether LES with the standard dynamic model is able to accurately predict the flow in the one-sided diffuser and to explore the resolution requirements and associated costs.
Large eddy simulation of particle-laden flow in a duct with a 90° bend
NASA Astrophysics Data System (ADS)
Njobuenwu, D. O.; Fairweather, M.
2011-12-01
Large eddy simulation (LES) of particle-laden turbulent flow is studied for a square duct with a 90° bend and a radius of curvature of 1.5 times the duct width, and for a Reynolds number based on the bulk flow velocity of 100,000. A Lagrangian particle tracking technique is used to study the motion of particles experiencing drag, shear lift, buoyancy and gravitational forces in the flow. LES predictions capture important physical aspects of these flows known to occur in practice, unlike alternative Reynolds-averaged Navier-Stokes (RANS) approaches, such as flow separation in the boundary layers around the bend entrance on the concave wall of the bend, and around the bend exit on the convex wall. The LES predicted flow and particle statistics are generally in good agreement with both experimental data used for validation purposes and RANS solutions, with r.m.s. fluctuating velocity predictions from the LES in particular being superior to values derived using the RANS technique.
Large-Eddy Simulation of Shock-Wave Boundary Layer Interaction and its Control Using Sparkjet
NASA Astrophysics Data System (ADS)
Yang, Guang; Yao, Yufeng; Fang, Jian; Gan, Tian; Lu, Lipeng
2016-06-01
Large-eddy simulation (LES) of an oblique shock-wave generated by an 8° sharp wedge impinging onto a spatially-developing Mach 2.3 turbulent boundary layer and their interactions has been carried out in this study. The Reynolds number based on the incoming flow property and the boundary layer displacement thickness at the impinging point without shock-wave is 20,000. The detailed numerical approaches are described and the inflow turbulence is generated using the digital filter method to avoid artificial temporal or streamwise periodicity. Numerical results are compared with the available wind tunnel PIV measurements of the same flow conditions. Further LES study on the control of flow separation due to the strong shock-viscous interaction is also conducted by using an active control actuator “SparkJet” concept. The single-pulsed characteristics of the control device are obtained and compared with the experiments. Instantaneous flowfield shows that the “SparkJet” promotes the flow mixing in the boundary layer and enhances its ability to resist the flow separation. The time and spanwise averaged skin friction coefficient distribution demonstrates that the separation bubble length is reduced by maximum 35% with the control exerted.
Large-eddy simulation of cavitating nozzle flow and primary jet break-up
NASA Astrophysics Data System (ADS)
Ã-rley, F.; Trummler, T.; Hickel, S.; Mihatsch, M. S.; Schmidt, S. J.; Adams, N. A.
2015-08-01
We employ a barotropic two-phase/two-fluid model to study the primary break-up of cavitating liquid jets emanating from a rectangular nozzle, which resembles a high aspect-ratio slot flow. All components (i.e., gas, liquid, and vapor) are represented by a homogeneous mixture approach. The cavitating fluid model is based on a thermodynamic-equilibrium assumption. Compressibility of all phases enables full resolution of collapse-induced pressure wave dynamics. The thermodynamic model is embedded into an implicit large-eddy simulation (LES) environment. The considered configuration follows the general setup of a reference experiment and is a generic reproduction of a scaled-up fuel injector or control valve as found in an automotive engine. Due to the experimental conditions, it operates, however, at significantly lower pressures. LES results are compared to the experimental reference for validation. Three different operating points are studied, which differ in terms of the development of cavitation regions and the jet break-up characteristics. Observed differences between experimental and numerical data in some of the investigated cases can be caused by uncertainties in meeting nominal parameters by the experiment. The investigation reveals that three main mechanisms promote primary jet break-up: collapse-induced turbulent fluctuations near the outlet, entrainment of free gas into the nozzle, and collapse events inside the jet near the liquid-gas interface.
Large Eddy Simulation of Motion-Induced Contaminant Transports in Room Compartments
NASA Astrophysics Data System (ADS)
Choi, Jung-Il; Edwards, Jack
2011-11-01
Large eddy simulation (LES) of contaminant transports due to complex human and door motions is conducted for characterizing the effect of the motion-induced wakes on the contaminant transports in room compartments where a contaminated and clean room are connected by a vestibule. We utilize a LES technique with an immersed-boundary method for moving objects (Choi et al., JCP 2007; Choi and Edwards, Indoor Air 2008) and extend the technique to include Eulerian descriptions of gas-phase contaminant transport as well as thermal energy transfer. We demonstrate details of contaminant transport due to human- and door-motion induced wake development during a short-duration event involving the movement of a person (or persons) from a contaminated room, through a vestibule, into a clean room. Parametric studies that capture the effects of human walking pattern, door operation, over-pressure level, and vestibule size are systematically conducted. The results of parameteric studies will be shown in the final presentation. Supported by DARPA/SPO program (HR0011-05-C-0157) and WCU program (R31-10049) of NRF.
Large-eddy simulation of cavitating nozzle flow and primary jet break-up
Örley, F. Trummler, T.; Mihatsch, M. S.; Schmidt, S. J.; Adams, N. A.; Hickel, S.
2015-08-15
We employ a barotropic two-phase/two-fluid model to study the primary break-up of cavitating liquid jets emanating from a rectangular nozzle, which resembles a high aspect-ratio slot flow. All components (i.e., gas, liquid, and vapor) are represented by a homogeneous mixture approach. The cavitating fluid model is based on a thermodynamic-equilibrium assumption. Compressibility of all phases enables full resolution of collapse-induced pressure wave dynamics. The thermodynamic model is embedded into an implicit large-eddy simulation (LES) environment. The considered configuration follows the general setup of a reference experiment and is a generic reproduction of a scaled-up fuel injector or control valve as found in an automotive engine. Due to the experimental conditions, it operates, however, at significantly lower pressures. LES results are compared to the experimental reference for validation. Three different operating points are studied, which differ in terms of the development of cavitation regions and the jet break-up characteristics. Observed differences between experimental and numerical data in some of the investigated cases can be caused by uncertainties in meeting nominal parameters by the experiment. The investigation reveals that three main mechanisms promote primary jet break-up: collapse-induced turbulent fluctuations near the outlet, entrainment of free gas into the nozzle, and collapse events inside the jet near the liquid-gas interface.
Optimization of a Turbine Blade Trailing Edge using Large Eddy Simulations
NASA Astrophysics Data System (ADS)
Blonigan, Patrick; Talnikar, Chaitanya; Bodart, Julien; Pierce, Brian; Bose, Sanjeeb; Wang, Qiqi
2014-11-01
As for many turbomachinery components, heat transfer and pressure loss are the key quantities influencing the design of turbine blades. To compute correct heat transfer and pressure loss data, flow features such as boundary layer transition and flow separation must be captured accurately. While traditional Computation Fluid Dynamics models such as Reynolds Averaged Navier-Stokes (RANS) struggle to capture these features accurately, Large Eddy Simulation (LES) is able to. This talk discusses an optimization study of a turbine blade trailing edge. The design of turbine blades involves two classical competing objectives: minimizing pressure loss and minimizing heat transfer to the blade. This trade-off is especially apparent for the design of the blade's trailing edge. The study was conducted using a novel Bayesian optimization technique developed by the authors. The optimization algorithm is combined with a massively parallel LES solver and the results for a number of trailing edge designs including the optimal geometry will be presented and their implications for turbine blade design will be discussed.
Recent advances in large-eddy simulation of spray and coal combustion
NASA Astrophysics Data System (ADS)
Zhou, L. X.
2013-07-01
Large-eddy simulation (LES) is under its rapid development and is recognized as a possible second generation of CFD methods used in engineering. Spray and coal combustion is widely used in power, transportation, chemical and metallurgical, iron and steel making, aeronautical and astronautical engineering, hence LES of spray and coal two-phase combustion is particularly important for engineering application. LES of two-phase combustion attracts more and more attention; since it can give the detailed instantaneous flow and flame structures and more exact statistical results than those given by the Reynolds averaged modeling (RANS modeling). One of the key problems in LES is to develop sub-grid scale (SGS) models, including SGS stress models and combustion models. Different investigators proposed or adopted various SGS models. In this paper the present author attempts to review the advances in studies on LES of spray and coal combustion, including the studies done by the present author and his colleagues. Different SGS models adopted by different investigators are described, some of their main results are summarized, and finally some research needs are discussed.
Large eddy simulation of LDL surface concentration in a subject specific human aorta.
Lantz, Jonas; Karlsson, Matts
2012-02-02
The development of atherosclerosis is correlated to the accumulation of lipids in the arterial wall, which, in turn, may be caused by the build-up of low-density lipoproteins (LDL) on the arterial surface. The goal of this study was to model blood flow within a subject specific human aorta, and to study how the LDL surface concentration changed during a cardiac cycle. With measured velocity profiles as boundary conditions, a scale-resolving technique (large eddy simulation, LES) was used to compute the pulsatile blood flow that was in the transitional regime. The relationship between wall shear stress (WSS) and LDL surface concentration was investigated, and it was found that the accumulation of LDL correlated well with WSS. In general, regions of low WSS corresponded to regions of increased LDL concentration and vice versa. The instantaneous LDL values changed significantly during a cardiac cycle; during systole the surface concentration was low due to increased convective fluid transport, while in diastole there was an increased accumulation of LDL on the surface. Therefore, the near-wall velocity was investigated at four representative locations, and it was concluded that in regions with disturbed flow the LDL concentration had significant temporal changes, indicating that LDL accumulation is sensitive to not only the WSS but also near-wall flow.
Large-eddy simulation of wind-turbine wakes: Evaluation of turbine parameterizations
NASA Astrophysics Data System (ADS)
Porté-Agel, F.; Wu, Y.-T.; Lu, H.; Chamorro, L.
2010-09-01
Large-eddy simulation (LES) offers a great potential to study the effects of turbulent atmospheric boundary layer flow on the performance of wind turbines and wind farms. The accuracy of the simulations, however, hinges on our ability to parameterize subgrid-scale (SGS) turbulent fluxes as well as turbine-induced forces. In this study, we investigate the performance of LES in simulations of wind-turbine wakes in neutrally stratified boundary layer flows. The subgrid-scale stress tensor is parameterized using the scale-dependent Lagrangian dynamic model (Stoll and Porte-Agel, 2006). This model optimizes the local value of the Smagorinsky coefficient based on the dynamics of the resolved scales. The turbine-induced lift and drag forces are parameterized using two types of models: an actuator disk model (ADM) that distributes the force loading uniformly on the rotor disk; and an actuator line model (ALM) that distributes the forces on lines that follow the position of the blades. Simulation results are compared to wind-tunnel measurements collected with hot-wire anemometry in the wake of a miniature 3-blade wind turbine at the St. Anthony Falls Laboratory atmospheric boundary layer wind tunnel. In general, the characteristics of the wakes simulated with the proposed LES framework are in good agreement with the measurements. The ALM is better able to capture vortical structures such as helicoidal tip vortices, which are induced by the blades in the near-wake region. Our results also show that accounting for rotation in the ADM leads to a more realistic turbine wake structure.
Large-eddy simulation of wind-turbine wakes: Evaluation of turbine parameterizations
NASA Astrophysics Data System (ADS)
Porte-Agel, Fernando; Wu, Yu-Tin; Lu, Hao; Chamorro, Leonardo
2010-05-01
Large-eddy simulation (LES) offers a great potential to study the effects of turbulent atmospheric boundary layer flow on the performance of wind turbines and wind farms. The accuracy of the simulations, however, hinges on our ability to parameterize subgrid-scale (SGS) turbulent fluxes as well as turbine-induced forces. In this study, we investigate the performance of LES in simulations of wind-turbine wakes in neutrally stratified boundary layer flows. The subgrid-scale stress tensor is parameterized using the scale-dependent Lagrangian dynamic model (Stoll and Porte-Agel, 2006). This model optimizes the local value of the Smagorinsky coefficient based on the dynamics of the resolved scales. The turbine-induced lift and drag forces are parameterized using two types of models: an actuator disk model (ADM) that distributes the force loading uniformly on the rotor disk; and an actuator line model (ALM) that distributes the forces on lines that follow the position of the blades. Simulation results are compared to wind-tunnel measurements collected with hot-wire anemometry in the wake of a miniature 3-blade wind turbine at the St. Anthony Falls Laboratory atmospheric boundary layer wind tunnel. In general, the characteristics of the wakes simulated with the proposed LES framework are in good agreement with the measurements. The ALM is better able to capture vortical structures such as helicoidal tip vortices, which are induced by the blades in the near-wake region. Our results also show that accounting for rotation in the ADM leads to a more realistic turbine wake structure.
NASA Astrophysics Data System (ADS)
Xiao, Heng; Endo, Satoshi; Wong, May; Skamarock, William C.; Klemp, Joseph B.; Fast, Jerome D.; Gustafson, William I.; Vogelmann, Andrew M.; Wang, Hailong; Liu, Yangang; Lin, Wuyin
2015-12-01
Yamaguchi and Feingold (2012) note that the cloud fields in their large-eddy simulations (LESs) of marine stratocumulus using the Weather Research and Forecasting (WRF) model exhibit a strong sensitivity to time stepping choices. In this study, we reproduce and analyze this sensitivity issue using two stratocumulus cases, one marine and one continental. Results show that (1) the sensitivity is associated with spurious motions near the moisture jump between the boundary layer and the free atmosphere, and (2) these spurious motions appear to arise from neglecting small variations in water vapor mixing ratio (qv) in the pressure gradient calculation in the acoustic substepping portion of the integration procedure. We show that this issue is remedied in the WRF dynamical core by replacing the prognostic equation for the potential temperature θ with one for the moist potential temperature θm=θ(1 + 1.61qv), which allows consistent treatment of moisture in the calculation of pressure during the acoustic substeps. With this modification, the spurious motions and the sensitivity to the time stepping settings (i.e., the dynamic time step length and number of acoustic sub-steps) are eliminated in both of the example stratocumulus cases. This modification improves the applicability of WRF for LES applications, and possibly other models using similar dynamical core formulations, and also permits the use of longer time steps than in the original code.
Large-eddy simulation of transitional flows using a co-located grid
NASA Astrophysics Data System (ADS)
Langari, Mostafa; Yang, Zhiyin; Page, Gary J.
2013-04-01
A large-eddy simulation (LES) of a transitional separated flow over a plate with a semi-circular leading at low (<0.2%) and high (5.6%) free-stream turbulence (FST) has been performed, using a co-located grid with the Rhie-Chow pressure smoothing. A numerical trip is used to produce a high FST level and a dynamic subgrid-scale model is also employed in the current study. The entire transition process leading to breakdown to turbulence has been shown clearly by the flow visualisations using instantaneous spanwise vorticities, and the differences between the low- and high-FST cases are clearly visible. Coherent structures are also visualised using isosurfaces of the Q-criterion, and for the high-FST case, the spanwise-oriented quasi-two-dimensional rolls, which are clearly present in the low-FST case, are not visible anymore. Detailed quantitative comparisons between the present LES results and experimental data and the previous LES results at low FST using a staggered grid have been done and a good agreement has been obtained, indicating that the current LES using a co-located grid with pressure smoothing can also predict transitional flows accurately.
Modifications to WRFs dynamical core to improve the treatment of moisture for large-eddy simulations
Xiao, Heng; Endo, Satoshi; Wong, May; ...
2015-10-29
Yamaguchi and Feingold (2012) note that the cloud fields in their large-eddy simulations (LESs) of marine stratocumulus using the Weather Research and Forecasting (WRF) model exhibit a strong sensitivity to time stepping choices. In this study, we reproduce and analyze this sensitivity issue using two stratocumulus cases, one marine and one continental. Results show that (1) the sensitivity is associated with spurious motions near the moisture jump between the boundary layer and the free atmosphere, and (2) these spurious motions appear to arise from neglecting small variations in water vapor mixing ratio (qv) in the pressure gradient calculation in themore » acoustic sub-stepping portion of the integration procedure. We show that this issue is remedied in the WRF dynamical core by replacing the prognostic equation for the potential temperature θ with one for the moist potential temperature θm=θ(1+1.61qv), which allows consistent treatment of moisture in the calculation of pressure during the acoustic sub-steps. With this modification, the spurious motions and the sensitivity to the time stepping settings (i.e., the dynamic time step length and number of acoustic sub-steps) are eliminated in both of the example stratocumulus cases. In conclusion, this modification improves the applicability of WRF for LES applications, and possibly other models using similar dynamical core formulations, and also permits the use of longer time steps than in the original code.« less
Properties of young contrails - a parametrisation based on large eddy simulations
NASA Astrophysics Data System (ADS)
Unterstrasser, S.
2015-10-01
Contrail-cirrus is probably the largest climate forcing of aviation. The evolution of contrail-cirrus and their radiative impact depends on a multitude of atmospheric parameters, but also on the geometric and microphysical properties of the young contrails evolving into contrail-cirrus. The early evolution of contrails (t < 5 min) is dominated by an interplay of ice microphysics and wake vortex dynamics. Young contrails may undergo a fast vertical expansion due to a descent of the wake vortices and may lose a substantial fraction of their ice crystals due to adiabatic heating. The geometric depth H and total ice crystal number N of young contrails are highly variable and depend on many environmental and aircraft parameters. Both properties, H and N, affect the later properties of the evolving contrail-cirrus, as they control the extent of shear-induced spreading and sedimentation losses. In this study, we provide parametrisations of H and N after 5 min taking into account the effects of temperature, relative humidity, thermal stratification and aircraft type (mass, wing span, fuel burn). The parametrisations rely on a large data set of recent large-eddy simulations of young contrails. They are suited to be incorporated in larger-scale models in order to refine the present day contrail initialisations by considering the processes that strongly affect the contrail evolution during the vortex phase.
Properties of young contrails - a parametrisation based on large-eddy simulations
NASA Astrophysics Data System (ADS)
Unterstrasser, Simon
2016-02-01
Contrail-cirrus is probably the largest climate forcing from aviation. The evolution of contrail-cirrus and its radiative impact depends not only on a multitude of atmospheric parameters, but also on the geometric and microphysical properties of the young contrails evolving into contrail-cirrus. The early evolution of contrails (t < 5 min) is dominated by an interplay of ice microphysics and wake vortex dynamics. Young contrails may undergo a fast vertical expansion due to a descent of the wake vortices and may lose a substantial fraction of their ice crystals due to adiabatic heating. The geometric depth H and total ice crystal number N of young contrails are highly variable and depend on many environmental and aircraft parameters. Both properties, H and N, affect the later properties of the evolving contrail-cirrus, as they control the extent of shear-induced spreading and sedimentation losses. In this study, we provide parametrisations of H and N after 5 min taking into account the effects of temperature, relative humidity, thermal stratification and aircraft type (mass, wing span, fuel burn). The parametrisations rely on a large data set of recent large-eddy simulations of young contrails. They are suited to be incorporated in larger-scale models in order to refine the present-day contrail initialisations by considering the processes that strongly affect the contrail evolution during the vortex phase.
NASA Astrophysics Data System (ADS)
Drewry, D. T.; Albertson, J. D.
2002-12-01
There are outstanding questions surrounding the measurement and modeling of carbon and water fluxes over complex landscapes. Typically, forest fluxes are measured with the eddy covariance technique from a single tower. A unique study over a loblolly pine stand in the Duke Forest yielded high frequency velocity, temperature, water vapor and carbon dioxide fluxes from a network of six instrumented towers, simultaneously. In this talk we explore the canopy-atmosphere dynamics active during this experiment through the use of a Large Eddy Simulation (LES) code. The LES includes a numerical representation of the plant canopy structure, a biophysical process sub-model, and mixes the sources and sinks through the boundary layer with a filtered form of the Navier-Stokes equations. Through this combination of a spatially distributed dataset and a 3D model of canopy flows and processes we investigate the relative influences of canopy structure and meteorological forcing on observed and modeled fluxes. This work has implications for our understanding of the effects of canopy turbulence on eddy covariance flux measurements.
Large-Eddy Simulation-Based Retrieval of Dissipation from Coherent Doppler Lidar Data
NASA Astrophysics Data System (ADS)
Krishnamurthy, Raghavendra; Calhoun, Ronald; Fernando, Harindra
2010-07-01
Accurate estimation of dissipation rate is important in understanding and analyzing turbulent flows found in environment and engineering processes. Many previous studies have focused on measuring the local dissipation rate at a single point or averaged dissipation rate over a suitable area. Since coherent Doppler lidar is capable of providing multi-point measurements covering a large spatial extent, it is well-suited for examining the distribution of dissipation in the atmosphere. In this paper, an approach is presented that is based on retrieving the dissipation rate from coherent Doppler lidar data using large-eddy simulation. Two Coherent Doppler lidars performed range height indicator (RHI) scans of a vertical/cross-barrier plane during the Terrain-induced Rotor Experiment (T-REX). Two-dimensional velocity vectors were retrieved using a least squares method. The velocity vectors retrieved from co-planar RHI scans are used to estimate subgrid scale (SGS) quantities through a known SGS parameterization. For the T-REX datasets analyzed, the dissipation rate was found to increase in the presence of rotors, subrotors, and, as expected, in regions of high wind shear. Owing to the presence of sharper gradients in subrotors, their dissipation rate is generally larger than that of rotors.
Prediction of wall shear-stress fluctuations in wall-modeled large-eddy simulation
NASA Astrophysics Data System (ADS)
Park, George; Howland, Michael; Lozano-Duran, Adrian; Moin, Parviz
2016-11-01
Wall-modeled large-eddy simulation (WMLES) is emerging as a viable and affordable tool for predicting mean flow statistics in high Reynolds number turbulent boundary layers. Recently, we examined the performance of two RANS-based wall models in prediction of wall pressure and shear stress fluctuations which are important in flow/structure interaction problems. Whereas the pressure statistics were predicted with reasonable accuracy, the magnitude of wall shear stress fluctuations was severely underestimated. The present study expands on this finding to characterize in more detail the capabilities of wall models for predicting τw'. Predictions of several wall models in high Reynolds number channel flows (Reτ = 2000) will be presented. Additionally, a recent empirical inner-outer model for τw' is reconstructed using channel flow DNS database , and it is coupled to WMLES to assess its performance as a predictive model in LES. The majority of this work was carried out during the 16th biannual Center for Turbulence Research (CTR) summer program, 2016. George Park was partially supported through NASA under the Subsonic Fixed-Wing Program (Grant No. NNX11AI60A).
NASA Astrophysics Data System (ADS)
Zhang, Y.; Chandra, A.; Klein, S. A.
2013-12-01
Summertime observations for 13 years at Atmospheric Radiation Measurement Southern Great Plains (SGP) site are used to study air motion in non-precipitating fair-weather shallow cumulus clouds. A composite shallow cumulus case is constructed based on an ensemble of days with observed active shallow cumulus clouds. Large-scale forcing for this composite case is derived accordingly based on observation-constrained variational analysis and is used to drive the large-eddy simulation (LES), whose set-up is most suitable to make an apple-to-apple comparison with radar observation at the site. At the same time, a novel retrieval algorithm, which can remove the insects' contamination on radar reflectivity, is applied to millimeter cloud radar 10s observations to get vertical velocity of air motion in the shallow cumulus cloud ensembles. We then focus on the behavior of cloudy profiles with liquid water path greater than 80 g/m^2. This is done because we believe this portion of cloud makes a major contribution to the total mass flux and by so doing, the uncertainty is minimized in the comparison between observation and LES results. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-641597
Progress-variable approach for large-eddy simulation of turbulent combustion
NASA Astrophysics Data System (ADS)
Pierce, Charles David
A new approach to chemistry modeling for large eddy simulation of turbulent reacting flows is developed. Instead of solving transport equations for all of the numerous species in a typical chemical mechanism and modeling the unclosed chemical source terms, the present study adopts an indirect mapping approach, whereby all of the detailed chemical processes are mapped to a reduced system of tracking scalars. Presently, only two such scalars are considered: a mixture fraction variable, which tracks the mixing of fuel and oxidizer, and a progress variable, which tracks the global extent-of-reaction of the local mixture. The mapping functions, which describe all of the detailed chemical processes with respect to the tracking variables, are determined by solving quasi-steady diffusion-reaction equations with complex chemical kinetics and multicomponent mass diffusion. The performance of the new model is compared to fast chemistry and steady flamelet models for predicting velocity, species concentration, and temperature fields in a methane-fueled coaxial jet combustor for which experimental data are available. The progress-variable approach is able to capture the unsteady, lifted flame dynamics observed in the experiment, and to obtain good agreement with the experimental data and significantly outperform the fast chemistry and steady flamelet models, which both predict an attached flame.
An investigation of the dynamics of marine propeller tip vortices using large-eddy simulations
NASA Astrophysics Data System (ADS)
Schroeder, Seth; Balaras, Elias
2012-11-01
The ability to capture the dynamics of tip vortices, which are generated by marine propellers, is of major interest to naval hydrodynamics designers. The tip vortex of a propeller has a direct impact on performance and acoustics. Additionally, the tip vortex is a major source of erosion damage on downstream components such as rudders and stators. In the present study we utilize large-eddy simulations to compute the flow around a generic, 7-bladed, right-handed submarine propeller in open water testing configuration. We considered three different advance coefficients at Reynolds number (based on the radius and advance speed) of the order of 300,000. The governing equations are discretized on a structured grid in cylindrical coordinates and the boundary conditions on the surface of the propeller, which is not aligned with the grid lines, are introduced using an immersed boundary method. Approximately 1 billion points is used in the computation box. Tip vortices are identified by low pressure areas and the second invariant of the velocity gradient tensor (Q-criterium). In general, the vortex core radius contracts with the acceleration in the wake, and then maintains a constant radius for a certain distance before becoming unstable. Stability is affected by the advance ratio. Work supported by ONR.
Modifications to WRFs dynamical core to improve the treatment of moisture for large-eddy simulations
Xiao, Heng; Endo, Satoshi; Wong, May; Skamarock, William C.; Klemp, Joseph B.; Fast, Jerome D.; Gustafson, Jr., William I.; Vogelmann, Andrew; Wang, Hailong; Liu, Yangang; Lin, Wuyin
2015-10-29
Yamaguchi and Feingold (2012) note that the cloud fields in their large-eddy simulations (LESs) of marine stratocumulus using the Weather Research and Forecasting (WRF) model exhibit a strong sensitivity to time stepping choices. In this study, we reproduce and analyze this sensitivity issue using two stratocumulus cases, one marine and one continental. Results show that (1) the sensitivity is associated with spurious motions near the moisture jump between the boundary layer and the free atmosphere, and (2) these spurious motions appear to arise from neglecting small variations in water vapor mixing ratio (qv) in the pressure gradient calculation in the acoustic sub-stepping portion of the integration procedure. We show that this issue is remedied in the WRF dynamical core by replacing the prognostic equation for the potential temperature θ with one for the moist potential temperature θm=θ(1+1.61qv), which allows consistent treatment of moisture in the calculation of pressure during the acoustic sub-steps. With this modification, the spurious motions and the sensitivity to the time stepping settings (i.e., the dynamic time step length and number of acoustic sub-steps) are eliminated in both of the example stratocumulus cases. In conclusion, this modification improves the applicability of WRF for LES applications, and possibly other models using similar dynamical core formulations, and also permits the use of longer time steps than in the original code.
Effect of stable stratification on dispersion within urban street canyons: A large-eddy simulation
NASA Astrophysics Data System (ADS)
Li, Xian-Xiang; Britter, Rex; Norford, Leslie K.
2016-11-01
This study employs a validated large-eddy simulation (LES) code with high tempo-spatial resolution to investigate the effect of a stably stratified roughness sublayer (RSL) on scalar transport within an urban street canyon. The major effect of stable stratification on the flow and turbulence inside the street canyon is that the flow slows down in both streamwise and vertical directions, a stagnant area near the street level emerges, and the vertical transport of momentum is weakened. Consequently, the transfer of heat between the street canyon and overlying atmosphere also gets weaker. The pollutant emitted from the street level 'pools' within the lower street canyon, and more pollutant accumulates within the street canyon with increasing stability. Under stable stratification, the dominant mechanism for pollutant transport within the street canyon has changed from ejections (flow carries high-concentration pollutant upward) to unorganized motions (flow carries high-concentration pollutant downward), which is responsible for the much lower dispersion efficiency under stable stratifications.
NASA Technical Reports Server (NTRS)
Han, Jongil; Lin, Yuh-Lang; Arya, S. Pal; Proctor, Fred H.
1999-01-01
The effects of ambient turbulence on decay and descent of aircraft wake vortices are studied using a validated, three-dimensional: large-eddy simulation model. Numerical simulations are performed in order to isolate the effect of ambient turbulence on the wake vortex decay rate within a neutrally-stratified atmosphere. Simulations are conducted for a range of turbulence intensities, by injecting wake vortex pairs into an approximately homogeneous and isotropic turbulence field. The decay rate of the vortex circulation increases clearly with increasing ambient turbulence level, which is consistent with field observations. Based on the results from the numerical simulations, simple decay models are proposed as functions of dimensionless ambient turbulence intensity (eta) and dimensionless time (T) for the circulation averaged over a range of radial distances. With good agreement with the numerical results, a Gaussian type of vortex decay model is proposed for weak turbulence: while an exponential type of Tortex decay model can be applied for strong turbulence. A relationship for the vortex descent based on above vortex decay model is also proposed. Although the proposed models are based on simulations assuming neutral stratification, the model predictions are compared to Lidar vortex measurements observed during stable, neutral, and unstable atmospheric conditions. In the neutral and unstable atmosphere, the model predictions appear to be in reasonable agreement with the observational data, while in the stably-stratified atmosphere, they largely underestimate the observed circulation decay with consistent overestimation of the observed vortex descent. The underestimation of vortex decay during stably-stratified conditions suggests that stratification has an important influence on vortex decay when ambient levels of turbulence are weak.
Large eddy simulation of free surface turbulent flow in partly vegetated open channels
NASA Astrophysics Data System (ADS)
Xiaohui, Su; Li, C. W.
2002-08-01
A large eddy simulation (LES) model has been developed to simulate the hydrodynamic behaviour of turbulent flow in an open channel with a domain of vegetation. Vegetation is considered as an internal source of resistant force and turbulence energy. The model is modified from the LES model of Li and Wang (International Journal for Numerical Methods in Fluids 2000; 34), and is distinctive in that the subgrid scale turbulence is parameterized by a k-l model. The length scale of turbulence l is proportional to the grid size and the turbulence energy k is obtained from the solution of the turbulence energy transport equation. An operator splitting method, which splits the solution procedure into advection, diffusion and pressure propagation steps, is employed so that different numerical schemes can be used for the solution of different physical processes. The model has been applied to simulate open channel flow with transverse shear produced by vegetation drag. Some organized large eddies were found in the interface between the vegetated and non-vegetated regions and the organized structure clearly has a life cycle. At the interface the transverse velocity profile exhibits a steep gradient, which induces significant mass and momentum exchange, acts as a source of vorticity, and generates high Reynolds stresses. The logarithmic vertical velocity variation becomes uniform in the vegetated domain. The agreement between the numerical results and the experimental data (Tsujimoto and Kitamura, KHL Progressive Report '92, Hydrology Laboratory, Kanazawa University, Japan, 1992; 21) is satisfactory. The present k-l LES model is proven to be a useful tool for engineering applications, as it can simulate the dynamic development of large eddies and the associated intermittent turbulence. Copyright
Mesoscale Ocean Large Eddy Simulations Using High-resolution Ocean Models
NASA Astrophysics Data System (ADS)
Pearson, B.; Fox-Kemper, B.; Bachman, S.; Bryan, F.; Bailey, D. A.
2016-02-01
Inaccurate parameterization of sub-grid eddies can cause excessive damping and spurious diapycnal mixing, especially in high-resolution [O(10km)] ocean models. The Mesoscale Ocean Large Eddy Simulation (MOLES) approach provides a framework for developing resolution- and flow-adaptive parameterizations of eddy effects. Large eddy simulation techniques are commonly used to simulate 3D turbulence, and MOLES is modified to be appropriate for the more two-dimensional nature of mesoscale ocean turbulence. However, the effect of MOLES in high-resolution ocean models has not been investigated extensively. We will contrast results, and cost, from a suite of idealized simulations of frontal spin-down (MITgcm) and from high-resolution global climate models (0.1o, POP2), under a variety of eddy parameterizations. These include MOLES based upon 2D turbulence theory, MOLES based upon quasi-geostrophic (QG) turbulence theory, and traditional biharmonic schemes. The idealized simulations show that MOLES (particularly QG) improves the spectral slopes of energy and enstrophy near the grid-scale when compared to more traditional eddy parameterizations, across a range of grid resolutions. In the high-resolution global climate model we compare the effect of different parameterizations on the spectral characteristics of the simulated flow, and on the large-scale transport. Using MOLES in a climate model results in greater energy and variability near the grid scale, and this produces a flow, which, spectrally, is more consistent with an inertial turbulent cascade and observations of eddy behavior.
Evaluation of Subgrid-Scale Models for Large Eddy Simulation of Compressible Flows
NASA Technical Reports Server (NTRS)
Blaisdell, Gregory A.
1996-01-01
The objective of this project was to evaluate and develop subgrid-scale (SGS) turbulence models for large eddy simulations (LES) of compressible flows. During the first phase of the project results from LES using the dynamic SGS model were compared to those of direct numerical simulations (DNS) of compressible homogeneous turbulence. The second phase of the project involved implementing the dynamic SGS model in a NASA code for simulating supersonic flow over a flat-plate. The model has been successfully coded and a series of simulations has been completed. One of the major findings of the work is that numerical errors associated with the finite differencing scheme used in the code can overwhelm the SGS model and adversely affect the LES results. Attached to this overview are three submitted papers: 'Evaluation of the Dynamic Model for Simulations of Compressible Decaying Isotropic Turbulence'; 'The effect of the formulation of nonlinear terms on aliasing errors in spectral methods'; and 'Large-Eddy Simulation of a Spatially Evolving Compressible Boundary Layer Flow'.
NASA Astrophysics Data System (ADS)
Shinn, Aaron F.
Computational Fluid Dynamics (CFD) simulations can be very computationally expensive, especially for Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS) of turbulent ows. In LES the large, energy containing eddies are resolved by the computational mesh, but the smaller (sub-grid) scales are modeled. In DNS, all scales of turbulence are resolved, including the smallest dissipative (Kolmogorov) scales. Clusters of CPUs have been the standard approach for such simulations, but an emerging approach is the use of Graphics Processing Units (GPUs), which deliver impressive computing performance compared to CPUs. Recently there has been great interest in the scientific computing community to use GPUs for general-purpose computation (such as the numerical solution of PDEs) rather than graphics rendering. To explore the use of GPUs for CFD simulations, an incompressible Navier-Stokes solver was developed for a GPU. This solver is capable of simulating unsteady laminar flows or performing a LES or DNS of turbulent ows. The Navier-Stokes equations are solved via a fractional-step method and are spatially discretized using the finite volume method on a Cartesian mesh. An immersed boundary method based on a ghost cell treatment was developed to handle flow past complex geometries. The implementation of these numerical methods had to suit the architecture of the GPU, which is designed for massive multithreading. The details of this implementation will be described, along with strategies for performance optimization. Validation of the GPU-based solver was performed for fundamental bench-mark problems, and a performance assessment indicated that the solver was over an order-of-magnitude faster compared to a CPU. The GPU-based Navier-Stokes solver was used to study film-cooling flows via Large Eddy Simulation. In modern gas turbine engines, the film-cooling method is used to protect turbine blades from hot combustion gases. Therefore, understanding the physics of
Shifted periodic boundary conditions for large-eddy simulation of wind farms
NASA Astrophysics Data System (ADS)
Munters, Wim; Meneveau, Charles; Meyers, Johan
2015-11-01
In wall-bounded turbulent flow simulations, periodic boundary conditions combined with insufficiently long domains lead to persistent spanwise locking of large-scale turbulent structures. In the context of wind-farm large-eddy simulations, this effect induces artificial spanwise inhomogeneities in the time-averaged local wind conditions as seen by the wind turbines, leading to spurious differences in power prediction between otherwise equivalent columns of wind turbines in a wind farm (a column is defined here as a set of turbines parallel to the mean flow direction). We propose a shifted periodic boundary condition that eliminates this effect without the need for excessive streamwise domain lengths. Instead of straightforwardly reintroducing the velocity from the outlet plane back at the inlet, as in classic periodic boundary conditions, this plane is first shifted in the spanwise direction by a predefined and constant distance. The method is tested based on a set of direct numerical simulations of a turbulent channel flow, and large-eddy simulations of a high Reynolds number rough-wall half-channel flow. Finally, we apply the method in a precursor simulation, generating inlet conditions for a spatially developing wind-farm boundary layer. WM and JM are supported by the ERC (ActiveWindFarms, grant no: 306471). CM acknowledges support by the NSF (grant IIA-1243482, the WINDINSPIRE project).
Discontinuous Galerkin methodology for Large-Eddy Simulations of wind turbine airfoils
NASA Astrophysics Data System (ADS)
Frére, A.; Sørensen, N. N.; Hillewaert, K.; Winckelmans, G.
2016-09-01
This paper aims at evaluating the potential of the Discontinuous Galerkin (DG) methodology for Large-Eddy Simulation (LES) of wind turbine airfoils. The DG method has shown high accuracy, excellent scalability and capacity to handle unstructured meshes. It is however not used in the wind energy sector yet. The present study aims at evaluating this methodology on an application which is relevant for that sector and focuses on blade section aerodynamics characterization. To be pertinent for large wind turbines, the simulations would need to be at low Mach numbers (M ≤ 0.3) where compressible approaches are often limited and at large Reynolds numbers (Re ≥ 106) where wall-resolved LES is still unaffordable. At these high Re, a wall-modeled LES (WMLES) approach is thus required. In order to first validate the LES methodology, before the WMLES approach, this study presents airfoil flow simulations at low and high Reynolds numbers and compares the results to state-of-the-art models used in industry, namely the panel method (XFOIL with boundary layer modeling) and Reynolds Averaged Navier-Stokes (RANS). At low Reynolds number (Re = 6 x 104), involving laminar boundary layer separation and transition in the detached shear layer, the Eppler 387 airfoil is studied at two angles of attack. The LES results agree slightly better with the experimental chordwise pressure distribution than both XFOIL and RANS results. At high Reynolds number (Re = 1.64 x 106), the NACA4412 airfoil is studied close to stall condition. In this case, although the wall model approach used for the WMLES is very basic and not supposed to handle separation nor adverse pressure gradients, all three methods provide equivalent accuracy on averaged quantities. The present work is hence considered as a strong step forward in the use of LES at high Reynolds numbers.
NASA Technical Reports Server (NTRS)
Kirkpatrick, M. P.; Mansour, N. N.; Ackerman, A. S.; Stevens, D. E.
2003-01-01
The use of large eddy simulation, or LES, to study the atmospheric boundary layer dates back to the early 1970s when Deardor (1972) used a three-dimensional simulation to determine velocity and temperature scales in the convective boundary layer. In 1974 he applied LES to the problem of mixing layer entrainment (Deardor 1974) and in 1980 to the cloud-topped boundary layer (Deardor 1980b). Since that time the LES approach has been applied to atmospheric boundary layer problems by numerous authors. While LES has been shown to be relatively robust for simple cases such as a clear, convective boundary layer (Mason 1989), simulation of the cloud-topped boundary layer has proved more of a challenge. The combination of small length scales and anisotropic turbulence coupled with cloud microphysics and radiation effects places a heavy burden on the turbulence model, especially in the cloud-top region. Consequently, over the past few decades considerable effort has been devoted to developing turbulence models that are better able to parameterize these processes. Much of this work has involved taking parameterizations developed for neutral boundary layers and deriving corrections to account for buoyancy effects associated with the background stratification and local buoyancy sources due to radiative and latent heat transfer within the cloud (see Lilly 1962; Deardor 1980a; Mason 1989; MacVean & Mason 1990, for example). In this paper we hope to contribute to this effort by presenting a number of turbulence models in which the model coefficients are calculated dynamically during the simulation rather than being prescribed a priori.
Large Eddy Simulation of a Cavitating Multiphase Flow for Liquid Injection
NASA Astrophysics Data System (ADS)
Cailloux, M.; Helie, J.; Reveillon, J.; Demoulin, F. X.
2015-12-01
This paper presents a numerical method for modelling a compressible multiphase flow that involves phase transition between liquid and vapour in the context of gasoline injection. A discontinuous compressible two fluid mixture based on the Volume of Fluid (VOF) implementation is employed to represent the phases of liquid, vapour and air. The mass transfer between phases is modelled by standard models such as Kunz or Schnerr-Sauer but including the presence of air in the gas phase. Turbulence is modelled using a Large Eddy Simulation (LES) approach to catch instationnarities and coherent structures. Eventually the modelling approach matches favourably experimental data concerning the effect of cavitation on atomisation process.
Active Control of Combustion Instability in a Ramjet Using Large-Eddy Simulations
1992-09-01
INSTABILITY IN A RAMJET USING LARGE-EDDY SIMULATIONS S. Menon N.TIS CR.A,1i ()TiC TAB September 1992 1 , o -d 6 y ... . ... .. Prepared for t.Cft OFFICE OF...pressure oscillations initially show a large-aznplitude, low- frequency oscillatio that eventually decays o that a high-frequency oscillation at around...injec- tion were recently presented (M o ., 199•1) TP-276/02-91 10 4.1 Acsntlc Fedback Cesto l Active control through acoustic forcing was demonstrated
Mesh refinement in a two-dimensional large eddy simulation of a forced shear layer
NASA Technical Reports Server (NTRS)
Claus, R. W.; Huang, P. G.; Macinnes, J. M.
1989-01-01
A series of large eddy simulations are made of a forced shear layer and compared with experimental data. Several mesh densities were examined to separate the effect of numerical inaccuracy from modeling deficiencies. The turbulence model that was used to represent small scale, 3-D motions correctly predicted some gross features of the flow field, but appears to be structurally incorrect. The main effect of mesh refinement was to act as a filter on the scale of vortices that developed from the inflow boundary conditions.
Nadiga, B T; Livescu, D
2007-04-01
We demonstrate, in the context of implicit-filtering large eddy simulations (LESs) of geostrophic turbulence, that while the attractor of a well-resolved statistically stationary turbulent flow can be reached in a coarsely resolved LES that is forced by the subgrid scale (SGS) terms diagnosed from the well-resolved computation, the attractor is generically unstable: the coarsely resolved LES system forced by the diagnosed SGS eddy terms has multiple attractors. This points to the importance of interpreting the diagnosed SGS forcing terms in a well-resolved computation or experiment from a combined physical-numerical point of view rather than from a purely physical point of view.
A normal stress subgrid-scale eddy viscosity model in large eddy simulation
NASA Technical Reports Server (NTRS)
Horiuti, K.; Mansour, N. N.; Kim, John J.
1993-01-01
The Smagorinsky subgrid-scale eddy viscosity model (SGS-EVM) is commonly used in large eddy simulations (LES) to represent the effects of the unresolved scales on the resolved scales. This model is known to be limited because its constant must be optimized in different flows, and it must be modified with a damping function to account for near-wall effects. The recent dynamic model is designed to overcome these limitations but is compositionally intensive as compared to the traditional SGS-EVM. In a recent study using direct numerical simulation data, Horiuti has shown that these drawbacks are due mainly to the use of an improper velocity scale in the SGS-EVM. He also proposed the use of the subgrid-scale normal stress as a new velocity scale that was inspired by a high-order anisotropic representation model. The testing of Horiuti, however, was conducted using DNS data from a low Reynolds number channel flow simulation. It was felt that further testing at higher Reynolds numbers and also using different flows (other than wall-bounded shear flows) were necessary steps needed to establish the validity of the new model. This is the primary motivation of the present study. The objective is to test the new model using DNS databases of high Reynolds number channel and fully developed turbulent mixing layer flows. The use of both channel (wall-bounded) and mixing layer flows is important for the development of accurate LES models because these two flows encompass many characteristic features of complex turbulent flows.
NASA Astrophysics Data System (ADS)
Cheng, Wan; Samtaney, Ravi
2013-11-01
We present results of large eddy simulation (LES) for a smooth-wall, zero-pressure-gradient turbulent boundary layer. We employ the stretched vortex sub-grid-scale model in the simulations augmented by a wall model. Our wall model is based on the virtual-wall model introduced by Chung & Pullin (J. Fluid Mech 2009). An essential component of their wall model is an ODE governing the local wall-normal velocity gradient obtained using inner-scaling ansatz. We test two variants of the wall model based on different similarity laws: one is based on a log-law and the other on a power-law. The specific form of the power law scaling utilized is that proposed by George & Castillo (Appl. Mech. Rev. 1997), dubbed the ``GC Law''. Turbulent inflow conditions are generated by a recycling method, and applying scaling laws corresponding to the two variants of the wall model, and a uniform way to determine the inlet friction velocity. For Reynolds number based on momentum thickness, Reθ , ranging from 104 to 1012 it is found that the velocity profiles generally follow the log law form rather than the power law. For large Reynolds number asymptotic behavior, LES based on different scaling laws the boundary layer thickness and turbulent intensities do not show much difference. Supported by a KAUST funded project on large eddy simulation of turbulent flows. The IBM Blue Gene P Shaheen at KAUST was utilized for the simulations.
NASA Astrophysics Data System (ADS)
Nichkoohi, Ali Lohrasbi; Tousi, Abolghasem Mesgarpour
2014-10-01
Today, with nonstop improvement in computational power, Large-Eddy Simulation (LES) is a high demanding research tool for predicting engineering flows. Such flows on high pressure condition like diesel engines is extensively employed in ground and marine transportation, oblige the designer to control and predict toxic pollutants, while maintaining or improving their high thermal efficiency. This becomes one of the main challenging issues in decades. In the present work, numerical investigation of diffusion flame dynamics is performed in the near-field of high-Reynolds jet flow on high pressure condition encountered in diesel engine applications. This work discusses the implementation of Partially Stirred Reactor (PaSR) combustion model by the approaches of large eddy simulation (LES). The simulation results show that LES, in comparison with Reynolds-Averaged Navier-Stokes (RANS) simulation predicts and captures transient phenomena very well. These phenomena such as unsteadiness and curvature are inherent in the near-field of high Reynolds diffusion flame. The outcomes of this research are compared and validated by other researchers' results. Detailed comparisons of the statistics show good agreement with the corresponding experiments.
Large-eddy simulation of turbulent flow inside flexible vegetation canopies
NASA Astrophysics Data System (ADS)
Razmi, A.; Nepf, H. M.; Chamecki, M.
2016-12-01
The impact of plant reconfiguration on flow within a seagrass canopy is simulated through large eddy simulation. Transport modeling inside these canopies is among the most challenging topics in fluid mechanics, due to the unsteady plant reconfiguration responding to unsteady periodic coherent flow structures. A Large-Eddy simulation model is developed to simulate the turbulent flow inside the canopy. The model solves the three-dimensional filtered momentum equation using a fully dealiased pseudo-spectral approach in the horizontal directions and a second-order centred finite-difference scheme in the vertical direction. The flow is driven by an imposed mean pressure gradient. The equations are closed using the Lagrangian scale-dependent dynamic Smagorinsky subgrid-scale (SGS) model. The model estimates the canopy drag force reduction in presence of the plant reconfiguration. The unsteady plant reconfiguration is modelled following existing formulations for plant posture as a function of flow velocity. The change in plant posture alters the drag force by altering the frontal area projected into each plane. Available lab measurements (Ghisalberti and Nepf 2006) are used for model calibration and validation for both rigid and flexible canopies. Velocity statistics, i.e., mean flow, Reynolds stress, rms-velocity and skewness of the model compare reasonably well with experimental data.
NASA Technical Reports Server (NTRS)
Givi, Peyman; Madnia, Cyrus K.; Steinberger, C. J.; Frankel, S. H.
1992-01-01
The principal objective is to extend the boundaries within which large eddy simulations (LES) and direct numerical simulations (DNS) can be applied in computational analyses of high speed reacting flows. A summary of work accomplished during the last six months is presented.
Sen, Baris Ali; Menon, Suresh
2010-01-15
A large eddy simulation (LES) sub-grid model is developed based on the artificial neural network (ANN) approach to calculate the species instantaneous reaction rates for multi-step, multi-species chemical kinetics mechanisms. The proposed methodology depends on training the ANNs off-line on a thermo-chemical database representative of the actual composition and turbulence (but not the actual geometrical problem) of interest, and later using them to replace the stiff ODE solver (direct integration (DI)) to calculate the reaction rates in the sub-grid. The thermo-chemical database is tabulated with respect to the thermodynamic state vector without any reduction in the number of state variables. The thermo-chemistry is evolved by stand-alone linear eddy mixing (LEM) model simulations under both premixed and non-premixed conditions, where the unsteady interaction of turbulence with chemical kinetics is included as a part of the training database. The proposed methodology is tested in LES and in stand-alone LEM studies of three distinct test cases with different reduced mechanisms and conditions. LES of premixed flame-turbulence-vortex interaction provides direct comparison of the proposed ANN method against DI and ANNs trained on thermo-chemical database created using another type of tabulation method. It is shown that the ANN trained on the LEM database can capture the correct flame physics with accuracy comparable to DI, which cannot be achieved by ANN trained on a laminar premix flame database. A priori evaluation of the ANN generality within and outside its training domain is carried out using stand-alone LEM simulations as well. Results in general are satisfactory, and it is shown that the ANN provides considerable amount of memory saving and speed-up with reasonable and reliable accuracy. The speed-up is strongly affected by the stiffness of the reduced mechanism used for the computations, whereas the memory saving is considerable regardless. (author)
Large eddy simulations and direct numerical simulations of high speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Givi, P.; Frankel, S. H.; Adumitroaie, V.; Sabini, G.; Madnia, C. K.
1993-01-01
The primary objective of this research is to extend current capabilities of Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS) for the computational analyses of high speed reacting flows. Our efforts in the first two years of this research have been concentrated on a priori investigations of single-point Probability Density Function (PDF) methods for providing subgrid closures in reacting turbulent flows. In the efforts initiated in the third year, our primary focus has been on performing actual LES by means of PDF methods. The approach is based on assumed PDF methods and we have performed extensive analysis of turbulent reacting flows by means of LES. This includes simulations of both three-dimensional (3D) isotropic compressible flows and two-dimensional reacting planar mixing layers. In addition to these LES analyses, some work is in progress to assess the extent of validity of our assumed PDF methods. This assessment is done by making detailed companions with recent laboratory data in predicting the rate of reactant conversion in parallel reacting shear flows. This report provides a summary of our achievements for the first six months of the third year of this program.
A Year-Long Large-Eddy Simulation of the Weather over the Cabauw Site
NASA Astrophysics Data System (ADS)
Siebesma, P.
2015-12-01
Results are presented of two large-eddy simulation (LES) runs of the entire year 2012 centered at the Cabauw observational supersite in the Netherlands. The LES is coupled to a regional weather model that provides the large-scale information. The simulations provide three-dimensional continuous time series of LES-generated turbulence and clouds, which can be compared in detail to the extensive observational dataset of Cabauw. The LES dataset is available from the authors on request. This type of LES setup has a number of advantages. First, it can provide a more statistical approach to the study of turbulent atmospheric flow than the more common case studies, since a diverse but representative set of conditions is covered, including numerous transitions. This has advantages in the design and evaluation of parameterizations. Second, the setup can provide valuable information on the quality of the LES model when applied to such a wide range of conditions. Last, it also provides the possibility to emulate observation techniques. This might help detect limitations and potential problems of a variety of measurement techniques. The LES runs are evaluated through a comparison with observations from the observational supersite and with results from the ''parent'' large-scale model. The long time series that are generated, in combination with information on the spatial structure, provide a novel opportunity to study time scales ranging from seconds to seasons. This facilitates a study of the power spectrum of horizontal and vertical wind speed variance to identify the dominant variance-containing time scales.
NASA Technical Reports Server (NTRS)
Menon, Suresh
2003-01-01
This report summarizes the progress made in the first 8 to 9 months of this research. The Lattice Boltzmann Equation (LBE) methodology for Large-eddy Simulations (LES) of microblowing has been validated using a jet-in-crossflow test configuration. In this study, the flow intake is also simulated to allow the interaction to occur naturally. The Lattice Boltzmann Equation Large-eddy Simulations (LBELES) approach is capable of capturing not only the flow features associated with the flow, such as hairpin vortices and recirculation behind the jet, but also is able to show better agreement with experiments when compared to previous RANS predictions. The LBELES is shown to be computationally very efficient and therefore, a viable method for simulating the injection process. Two strategies have been developed to simulate multi-hole injection process as in the experiment. In order to allow natural interaction between the injected fluid and the primary stream, the flow intakes for all the holes have to be simulated. The LBE method is computationally efficient but is still 3D in nature and therefore, there may be some computational penalty. In order to study a large number or holes, a new 1D subgrid model has been developed that will simulate a reduced form of the Navier-Stokes equation in these holes.
NASA Astrophysics Data System (ADS)
Miller, Daniel J.; Zhang, Zhibo; Ackerman, Andrew S.; Platnick, Steven; Baum, Bryan A.
2016-04-01
Passive optical retrievals of cloud liquid water path (LWP), like those implemented for Moderate Resolution Imaging Spectroradiometer (MODIS), rely on cloud vertical profile assumptions to relate optical thickness (τ) and effective radius (re) retrievals to LWP. These techniques typically assume that shallow clouds are vertically homogeneous; however, an adiabatic cloud model is plausibly more realistic for shallow marine boundary layer cloud regimes. In this study a satellite retrieval simulator is used to perform MODIS-like satellite retrievals, which in turn are compared directly to the large-eddy simulation (LES) output. This satellite simulator creates a framework for rigorous quantification of the impact that vertical profile features have on LWP retrievals, and it accomplishes this while also avoiding sources of bias present in previous observational studies. The cloud vertical profiles from the LES are often more complex than either of the two standard assumptions, and the favored assumption was found to be sensitive to cloud regime (cumuliform/stratiform). Confirming previous studies, drizzle and cloud top entrainment of dry air are identified as physical features that bias LWP retrievals away from adiabatic and toward homogeneous assumptions. The mean bias induced by drizzle-influenced profiles was shown to be on the order of 5-10 g/m2. In contrast, the influence of cloud top entrainment was found to be smaller by about a factor of 2. A theoretical framework is developed to explain variability in LWP retrievals by introducing modifications to the adiabatic re profile. In addition to analyzing bispectral retrievals, we also compare results with the vertical profile sensitivity of passive polarimetric retrieval techniques.
Effects of Eddy Viscosity on Time Correlations in Large Eddy Simulation
NASA Technical Reports Server (NTRS)
He, Guowei; Rubinstein, R.; Wang, Lian-Ping; Bushnell, Dennis M. (Technical Monitor)
2001-01-01
Subgrid-scale (SGS) models for large. eddy simulation (LES) have generally been evaluated by their ability to predict single-time statistics of turbulent flows such as kinetic energy and Reynolds stresses. Recent application- of large eddy simulation to the evaluation of sound sources in turbulent flows, a problem in which time, correlations determine the frequency distribution of acoustic radiation, suggest that subgrid models should also be evaluated by their ability to predict time correlations in turbulent flows. This paper compares the two-point, two-time Eulerian velocity correlation evaluated from direct numerical simulation (DNS) with that evaluated from LES, using a spectral eddy viscosity, for isotropic homogeneous turbulence. It is found that the LES fields are too coherent, in the sense that their time correlations decay more slowly than the corresponding time. correlations in the DNS fields. This observation is confirmed by theoretical estimates of time correlations using the Taylor expansion technique. Tile reason for the slower decay is that the eddy viscosity does not include the random backscatter, which decorrelates fluid motion at large scales. An effective eddy viscosity associated with time correlations is formulated, to which the eddy viscosity associated with energy transfer is a leading order approximation.
Sen, Baris Ali; Menon, Suresh; Hawkes, Evatt R.
2010-03-15
Large eddy simulation (LES) of a non-premixed, temporally evolving, syngas/air flame is performed with special emphasis on speeding-up the sub-grid chemistry computations using an artificial neural networks (ANN) approach. The numerical setup for the LES is identical to a previous direct numerical simulation (DNS) study, which reported considerable local extinction and reignition physics, and hence, offers a challenging test case. The chemical kinetics modeling with ANN is based on a recent approach, and replaces the stiff ODE solver (DI) to predict the species reaction rates in the subgrid linear eddy mixing (LEM) model based LES (LEMLES). In order to provide a comprehensive evaluation of the current approach, additional information on conditional statistics of some of the key species and temperature are extracted from the previous DNS study and are compared with the LEMLES using ANN (ANN-LEMLES, hereafter). The results show that the current approach can detect the correct extinction and reignition physics with reasonable accuracy compared to the DNS. The syngas flame structure and the scalar dissipation rate statistics obtained by the current ANN-LEMLES are provided to further probe the flame physics. It is observed that, in contrast to H{sub 2}, CO exhibits a smooth variation within the region enclosed by the stoichiometric mixture fraction. The probability density functions (PDFs) of the scalar dissipation rates calculated based on the mixture fraction and CO demonstrate that the mean value of the PDF is insensitive to extinction and reignition. However, this is not the case for the scalar dissipation rate calculated by the OH mass fraction. Overall, ANN provides considerable computational speed-up and memory saving compared to DI, and can be used to investigate turbulent flames in a computationally affordable manner. (author)
Modeling flow around bluff bodies and predicting urban dispersion using large eddy simulation.
Tseng, Yu-Heng; Meneveau, Charles; Parlange, Marc B
2006-04-15
Modeling air pollutant transport and dispersion in urban environments is especially challenging due to complex ground topography. In this study, we describe a large eddy simulation (LES) tool including a new dynamic subgrid closure and boundary treatment to model urban dispersion problems. The numerical model is developed, validated, and extended to a realistic urban layout. In such applications fairly coarse grids must be used in which each building can be represented using relatively few grid-points only. By carrying out LES of flow around a square cylinder and of flow over surface-mounted cubes, the coarsest resolution required to resolve the bluff body's cross section while still producing meaningful results is established. Specifically, we perform grid refinement studies showing that at least 6-8 grid points across the bluff body are required for reasonable results. The performance of several subgrid models is also compared. Although effects of the subgrid models on the mean flow are found to be small, dynamic Lagrangian models give a physically more realistic subgrid-scale (SGS) viscosity field. When scale-dependence is taken into consideration, these models lead to more realistic resolved fluctuating velocities and spectra. These results set the minimum grid resolution and subgrid model requirements needed to apply LES in simulations of neutral atmospheric boundary layer flow and scalar transport over a realistic urban geometry. The results also illustrate the advantages of LES over traditional modeling approaches, particularly its ability to take into account the complex boundary details and the unsteady nature of atmospheric boundary layer flow. Thus LES can be used to evaluate probabilities of extreme events (such as probabilities of exceeding threshold pollutant concentrations). Some comments about computer resources required for LES are also included.
A coupled large-eddy simulation sea ice model for simulating Arctic air mass transformation
NASA Astrophysics Data System (ADS)
Dimitrelos, Antonios; Ekman, Annica M. L.; Caballero, Rodrigo
2017-04-01
As warm, moist, maritime air masses are advected north over the high Arctic pack ice, the air mass is transformed with fog and low-level mixed-phase clouds typically forming below the surface temperature inversion. The moist air, and the clouds forming, influence strongly the surface energy fluxes and consequently the formation and melting of sea ice. Further cooling and drying of the air eventually result in cloud dissipation, and the boundary layer transforms into a clear state with strong surface radiative cooling. The processes of air mass transformation, cloud formation and cloud dissipation are challenging to represent in large-scale models, affecting our understanding of their sensitivity and contribution to climate warming. In order to obtain a more detailed understanding of these processes, and their influence on the surface energy balance, we employ atmospheric large-eddy simulation (LES) coupled to a simple sea ice model. In this presentation, we will show results from idealized simulations of winter Arctic air mass transformation for a range of different initial temperature and moisture profiles and discuss the potential impact on sea ice formation.
Large-Eddy Simulation of the Flat-plate Turbulent Boundary Layer at High Reynolds numbers
NASA Astrophysics Data System (ADS)
Inoue, Michio
The near-wall, subgrid-scale (SGS) model [Chung and Pullin, "Large-eddy simulation and wall-modeling of turbulent channel flow'', J. Fluid Mech. 631, 281--309 (2009)] is used to perform large-eddy simulations (LES) of the incompressible developing, smooth-wall, flat-plate turbulent boundary layer. In this model, the stretched-vortex, SGS closure is utilized in conjunction with a tailored, near-wall model designed to incorporate anisotropic vorticity scales in the presence of the wall. The composite SGS-wall model is presently incorporated into a computer code suitable for the LES of developing flat-plate boundary layers. This is then used to study several aspects of zero- and adverse-pressure gradient turbulent boundary layers. First, LES of the zero-pressure gradient turbulent boundary layer are performed at Reynolds numbers Retheta based on the free-stream velocity and the momentum thickness in the range Retheta = 103-1012. Results include the inverse skin friction coefficient, 2/Cf , velocity profiles, the shape factor H, the Karman "constant", and the Coles wake factor as functions of Re theta. Comparisons with some direct numerical simulation (DNS) and experiment are made, including turbulent intensity data from atmospheric-layer measurements at Retheta = O (106). At extremely large Retheta , the empirical Coles-Fernholz relation for skin-friction coefficient provides a reasonable representation of the LES predictions. While the present LES methodology cannot of itself probe the structure of the near-wall region, the present results show turbulence intensities that scale on the wall-friction velocity and on the Clauser length scale over almost all of the outer boundary layer. It is argued that the LES is suggestive of the asymptotic, infinite Reynolds-number limit for the smooth-wall turbulent boundary layer and different ways in which this limit can be approached are discussed. The maximum Retheta of the present simulations appears to be limited by machine
Large eddy simulations of a transcritical round jet submitted to transverse acoustic modulation
Gonzalez-Flesca, M.; Schmitt, T.; Ducruix, S.; Candel, S.
2016-05-15
This article reports numerical computations of a turbulent round jet of transcritical fluid (low temperature nitrogen injected under high pressure conditions) surrounded by the same fluid at rest under supercritical conditions (high temperature and high pressure) and submitted to transverse acoustic modulations. The numerical framework relies on large eddy simulation in combination with a real-gas description of thermodynamics and transport properties. A stationary acoustic field is obtained by modulating the normal acoustic velocity at the lateral boundaries of the computational domain. This study specifically focuses on the interaction of the jet with the acoustic field to investigate how the round transcritical jet changes its shape and mixes with the surrounding fluid. Different modulation amplitudes and frequencies are used to sweep a range of conditions. When the acoustic field is established in the domain, the jet length is notably reduced and the jet is flattened in the spanwise direction. Two regimes of oscillation are identified: for low Strouhal numbers a large amplitude motion is observed, while for higher Strouhal numbers the jet oscillates with a small amplitude around the injector axis. The minimum length is obtained for a Strouhal number of 0.3 and the jet length increases with increasing Strouhal numbers after reaching this minimum value. The mechanism of spanwise deformation is shown to be linked with dynamical effects resulting from reduction of the pressure in the transverse direction in relation with increased velocities on the two sides of the jet. A propagative wave is then introduced in the domain leading to similar effects on the jet, except that a bending is also observed in the acoustic propagation direction. A kinematic model, combining hydrodynamic and acoustic contributions, is derived in a second stage to represent the motion of the jet centerline. This model captures details of the numerical simulations quite well. These various
Cut-cell method based large-eddy simulation of tip-leakage flow
NASA Astrophysics Data System (ADS)
Pogorelov, Alexej; Meinke, Matthias; Schröder, Wolfgang
2015-07-01
The turbulent low Mach number flow through an axial fan at a Reynolds number of 9.36 × 105 based on the outer casing diameter is investigated by large-eddy simulation. A finite-volume flow solver in an unstructured hierarchical Cartesian setup for the compressible Navier-Stokes equations is used. To account for sharp edges, a fully conservative cut-cell approach is applied. A newly developed rotational periodic boundary condition for Cartesian meshes is introduced such that the simulations are performed just for a 72° segment, i.e., the flow field over one out of five axial blades is resolved. The focus of this numerical analysis is on the development of the vortical flow structures in the tip-gap region. A detailed grid convergence study is performed on four computational grids with 50 × 106, 250 × 106, 1 × 109, and 1.6 × 109 cells. Results of the instantaneous and the mean fan flow field are thoroughly analyzed based on the solution with 1 × 109 cells. High levels of turbulent kinetic energy and pressure fluctuations are generated by a tip-gap vortex upstream of the blade, the separating vortices inside the tip gap, and a counter-rotating vortex on the outer casing wall. An intermittent interaction of the turbulent wake, generated by the tip-gap vortex, with the downstream blade, leads to a cyclic transition with high pressure fluctuations on the suction side of the blade and a decay of the tip-gap vortex. The disturbance of the tip-gap vortex results in an unsteady behavior of the turbulent wake causing the intermittent interaction. For this interaction and the cyclic transition, two dominant frequencies are identified which perfectly match with the characteristic frequencies in the experimental sound power level and therefore explain their physical origin.
Numerical errors in the computation of subfilter scalar variance in large eddy simulations
NASA Astrophysics Data System (ADS)
Kaul, C. M.; Raman, V.; Balarac, G.; Pitsch, H.
2009-05-01
Subfilter scalar variance is a key quantity for scalar mixing at the small scales of a turbulent flow and thus plays a crucial role in large eddy simulation of combustion. While prior studies have mainly focused on the physical aspects of modeling subfilter variance, the current work discusses variance models in conjunction with the numerical errors due to their implementation using finite-difference methods. A priori tests on data from direct numerical simulation of homogeneous turbulence are performed to evaluate the numerical implications of specific model forms. Like other subfilter quantities, such as kinetic energy, subfilter variance can be modeled according to one of two general methodologies. In the first of these, an algebraic equation relating the variance to gradients of the filtered scalar field is coupled with a dynamic procedure for coefficient estimation. Although finite-difference methods substantially underpredict the gradient of the filtered scalar field, the dynamic method is shown to mitigate this error through overestimation of the model coefficient. The second group of models utilizes a transport equation for the subfilter variance itself or for the second moment of the scalar. Here, it is shown that the model formulation based on the variance transport equation is consistently biased toward underprediction of the subfilter variance. The numerical issues in the variance transport equation stem from discrete approximations to chain-rule manipulations used to derive convection, diffusion, and production terms associated with the square of the filtered scalar. These approximations can be avoided by solving the equation for the second moment of the scalar, suggesting that model's numerical superiority.
Large-eddy simulation of dense gas dispersion over a simplified urban area
NASA Astrophysics Data System (ADS)
Wingstedt, E. M. M.; Osnes, A. N.; Åkervik, E.; Eriksson, D.; Reif, B. A. Pettersson
2017-03-01
Dispersion of neutral and dense gas over a simplified urban area, comprising four cubes, has been investigated by the means of large-eddy simulations (LES). The results have been compared to wind tunnel experiments and both mean and fluctuating quantities of velocity and concentration are in very good agreement. High-quality inflow profiles are necessary to achieve physically realistic LES results. In this study, profiles matching the atmospheric boundary layer flow in the wind tunnel, are generated by means of a separate precursor simulation. Emission of dense gas dramatically alters the flow in the near source region and introduces an upstream dispersion. The resulting dispersion patterns of neutral and dense gas differ significantly, where the plume in the latter case is wider and shallower. The dense gas is highly affected by the cube array, which seems to act as a barrier, effectively deflecting the plume. This leads to higher concentrations outside of the array than inside. On the contrary, the neutral gas plume has a Gaussian-type shape, with highest concentrations along the centreline. It is found that the dense gas reduces the vertical and spanwise turbulent momentum transport and, as a consequence, the turbulence kinetic energy. The reduction coincides with the area where the gradient Richardson number exceeds its critical value, i.e. where the flow may be characterized as stably stratified. Interestingly, this region does not correspond to where the concentration of dense gas is the highest (close to the ground), as this is also where the largest velocity gradients are to be found. Instead there is a layer in the middle of the dense gas cloud where buoyancy is dynamically dominant.
Large eddy simulations of a transcritical round jet submitted to transverse acoustic modulation
NASA Astrophysics Data System (ADS)
Gonzalez-Flesca, M.; Schmitt, T.; Ducruix, S.; Candel, S.
2016-05-01
This article reports numerical computations of a turbulent round jet of transcritical fluid (low temperature nitrogen injected under high pressure conditions) surrounded by the same fluid at rest under supercritical conditions (high temperature and high pressure) and submitted to transverse acoustic modulations. The numerical framework relies on large eddy simulation in combination with a real-gas description of thermodynamics and transport properties. A stationary acoustic field is obtained by modulating the normal acoustic velocity at the lateral boundaries of the computational domain. This study specifically focuses on the interaction of the jet with the acoustic field to investigate how the round transcritical jet changes its shape and mixes with the surrounding fluid. Different modulation amplitudes and frequencies are used to sweep a range of conditions. When the acoustic field is established in the domain, the jet length is notably reduced and the jet is flattened in the spanwise direction. Two regimes of oscillation are identified: for low Strouhal numbers a large amplitude motion is observed, while for higher Strouhal numbers the jet oscillates with a small amplitude around the injector axis. The minimum length is obtained for a Strouhal number of 0.3 and the jet length increases with increasing Strouhal numbers after reaching this minimum value. The mechanism of spanwise deformation is shown to be linked with dynamical effects resulting from reduction of the pressure in the transverse direction in relation with increased velocities on the two sides of the jet. A propagative wave is then introduced in the domain leading to similar effects on the jet, except that a bending is also observed in the acoustic propagation direction. A kinematic model, combining hydrodynamic and acoustic contributions, is derived in a second stage to represent the motion of the jet centerline. This model captures details of the numerical simulations quite well. These various
NASA Astrophysics Data System (ADS)
El-Asrag, Hossam A.; Braun, Markus; Masri, Assaad R.
2016-07-01
The paper presents Large Eddy Simulations (LESs) for the Sydney ethanol piloted turbulent dilute spray flames ETF2, ETF6, and ETF7. The Flamelet Generated Manifold (FGM) approach is employed to predict mixing and burning of the evaporating fuel droplets. A methodology to match the experimental inflow spray profiles is presented. The spray statistical time-averaged results show reasonable agreement with mean and RMS data. The Particle Size Distribution (PSD) shows a good match downstream of the nozzle exit and up to x/D = 10. At x/D = 20 and 30 the PSD is under-predicted for droplets with mean diameter D10 > 20μm and over-predicted for the smaller size droplets. The simulations reasonably predict the reported mean flame structure and length. The effect of increasing the carrier velocity (ETF2-ETF7) or decreasing the liquid fuel injection mass flow rate (ETF2-ETF6) is found to result in a leaner, shorter flame and stronger spray-flow interactions. Higher tendency to local extinction is observed for ETF7 which is closer to blow-off compared to ETF2 and has higher scalar dissipation rates, higher range of Stokes number, and faster droplet response. The possible sources of LES-FGM deviations from the measurements are discussed and highlighted. In particular, the spray time-averaged statistical error contribution is quantified and the impact of the inflow uncertainty is studied. Sensitivity analysis to the pre-vaporized nozzle fuel mass fraction show that such small inflow perturbations (by ± 2% for the ETF2 flame) have a strong impact on the flame structure, and the droplets' dynamics. Conditional scatter plots show that the flame exhibits wide range of mixing conditions and bimodal mixing lines particularly at upstream locations (x/D < 20), where the injected droplets are still penetrating the centerline. This is relaxed further downstream as droplets gradually evaporate and burn in a diffusion like mode.
VS-FMDF and EPVS-FMDF for large eddy simulation of turbulent flows
NASA Astrophysics Data System (ADS)
Nik, Mehdi B.
The first part of this dissertation is concerned with implementation of the joint “velocity-scalar filtered mass density function” (VS-FMDF) methodology for large eddy simulation (LES) of Sandia Flame D. This is a turbulent piloted non-premixed methane jet flame. In VS-FMDF, the effects of the subgrid scale chemical reaction and convection appear in closed forms. The modeled transport equation for the VS-FMDF is solved by a hybrid finite-difference/Monte Carlo scheme. For this flame (which exhibits little local extinction), a flamelet model is employed to relate the instantaneous composition to the mixture fraction. The LES predictions are compared with experimental data. It is shown that the methodology captures important features of the flame as observed experimentally. In the second part of this dissertation, the joint “energy-pressure-velocity-scalar filtered mass density function” (EPVS-FMDF) is developed as a new subgrid scale (SGS) model for LES of high-speed turbulent flows. In this model, the effects of compressibility are taken into account by including two additional thermodynamic variables: the pressure and the internal energy. The EPVS-FMDF is obtained by solving its modeled transport equation, in which the effect of convection appears in a closed form. The modeled EPVS-FMDF is employed for LES of a temporally developing mixing layer. Keywords: Large eddy simulation, filtered density function, turbulent reacting flows.
Application of large-eddy simulation for trailing-edge noise prediction
NASA Astrophysics Data System (ADS)
Wang, Meng; Moin, Parviz
Turbulent boundary layers near the trailing-edge of a lifting surface are known to generate intense, broadband scattering noise as well as surface pressure fluctuations. To numerically predict the trailing-edge noise requires that the noise-generating eddies over a wide range of length scales be adequately represented. Large-eddy simulation technique provides a promising tool for obtaining the unsteady wall-pressure fields and the acoustic source functions. In the present work, a large-eddy simulation is carried out for turbulent boundary layer flow past an asymmetrically beveled trailing-edge of a flat strut at a chord Reynolds number of 2.15 × 106. The computed velocity and surface pressure statistics compare reasonably well with the experimental measurements of Blake. The far-field acoustic calculation is facilitated by the integral solution to the Lighthill equation derived by Ffowcs Williams & Hall. Computations have been carried out to determine the far-field noise spectra, the source-term characteristics, and the requirement for the integration domain size. It is found that the present LES is adequate for predicting noise radiation over a wide frequency range. At the low frequency end, however, the spanwise source coherence estimated based on surface pressure fluctuations does not decay sufficiently, suggesting the need for a wider computational domain.
A spectral-element dynamic model for the Large-Eddy simulation of turbulent flows
NASA Astrophysics Data System (ADS)
Chapelier, J.-B.; Lodato, G.
2016-09-01
A spectral dynamic modeling procedure for Large-Eddy simulation is introduced in the context of discontinuous finite element methods. The proposed sub-grid scale model depends on a turbulence sensor built from the computation of a polynomial energy spectrum in each of the discretization elements. The evaluation of the energy decay gives an estimation of the quality of the resolution in each element and allows for adapting the intensity of the sub-grid dissipation locally. This approach is simple, robust, efficient and it is shown that the sub-grid model adapts to the amount of numerical dissipation in order to provide an accurate representation of the true sub-grid stresses. The present approach is tested for the large-eddy simulation of transitional, fully-developed and wall-bounded turbulence. In particular, results are reported for the Taylor-Green vortex and periodic turbulent channel flows at moderate Reynolds number. For these configurations, the new model shows an accurate description of turbulent phenomena at relatively coarse resolutions.
NASA Astrophysics Data System (ADS)
Li, Wenhai; Alabi, Ken; Ladeinde, Foluso; Lou, Zhipeng
2016-11-01
In this study, three turbulence-chemistry interaction models: the flamelet, eddy-breakup (EBU), and laminar chemistry models, are compared in the large-eddy simulation (LES) of high speed combustion. It is the case that the simple models still find extensive applications, with fairly acceptable results in many instances. The standard flamelet model developed for low Mach number flows has been modified to account for compressibility effects in supersonic combustion. The comparison exercise has been based on the bluff-body flames that occur under high-speed conditions.
Large-eddy simulation of a backward facing step flow using a least-squares spectral element method
NASA Technical Reports Server (NTRS)
Chan, Daniel C.; Mittal, Rajat
1996-01-01
We report preliminary results obtained from the large eddy simulation of a backward facing step at a Reynolds number of 5100. The numerical platform is based on a high order Legendre spectral element spatial discretization and a least squares time integration scheme. A non-reflective outflow boundary condition is in place to minimize the effect of downstream influence. Smagorinsky model with Van Driest near wall damping is used for sub-grid scale modeling. Comparisons of mean velocity profiles and wall pressure show good agreement with benchmark data. More studies are needed to evaluate the sensitivity of this method on numerical parameters before it is applied to complex engineering problems.
Large-Eddy Simulations and Lidar Measurements of Vortex-Pair Breakup in Aircraft Wakes
NASA Technical Reports Server (NTRS)
Lewellen, D. C.; Lewellen, W. S.; Poole, L. R.; DeCoursey, R. J.; Hansen, G. M.; Hostetler, C. A.; Kent, G. S.
1998-01-01
Results of large-eddy simulations of an aircraft wake are compared with results from ground-based lidar measurements made at NASA Langley Research Center during the Subsonic Assessment Near-Field Interaction Flight Experiment field tests. Brief reviews of the design of the field test for obtaining the evolution of wake dispersion behind a Boeing 737 and of the model developed for simulating such wakes are given. Both the measurements and the simulations concentrate on the period from a few seconds to a few minutes after the wake is generated, during which the essentially two-dimensional vortex pair is broken up into a variety of three-dimensional eddies. The model and experiment show similar distinctive breakup eddies induced by the mutual interactions of the vortices, after perturbation by the atmospheric motions.
Large-Eddy Simulations and Lidar Measurements of Vortex-Pair Breakup in Aircraft Wakes
NASA Technical Reports Server (NTRS)
Lewellen, D. C.; Lewellen, W. S.; Poole, L. R.; DeCoursey, R. J.; Hansen, G. M.; Hostetler, C. A.; Kent, G. S.
1998-01-01
Results of large-eddy simulations of an aircraft wake are compared with results from ground-based lidar measurements made at NASA Langley Research Center during the Subsonic Assessment Near-Field Interaction Flight Experiment field tests. Brief reviews of the design of the field test for obtaining the evolution of wake dispersion behind a Boeing 737 and of the model developed for simulating such wakes are given. Both the measurements and the simulations concentrate on the period from a few seconds to a few minutes after the wake is generated, during which the essentially two-dimensional vortex pair is broken up into a variety of three-dimensional eddies. The model and experiment show similar distinctive breakup eddies induced by the mutual interactions of the vortices, after perturbation by the atmospheric motions.
Large-eddy simulation of flow around an airfoil on a structured mesh
NASA Technical Reports Server (NTRS)
Kaltenbach, Hans-Jakob; Choi, Haecheon
1995-01-01
The diversity of flow characteristics encountered in a flow over an airfoil near maximum lift taxes the presently available statistical turbulence models. This work describes our first attempt to apply the technique of large-eddy simulation to a flow of aeronautical interest. The challenge for this simulation comes from the high Reynolds number of the flow as well as the variety of flow regimes encountered, including a thin laminar boundary layer at the nose, transition, boundary layer growth under adverse pressure gradient, incipient separation near the trailing edge, and merging of two shear layers at the trailing edge. The flow configuration chosen is a NACA 4412 airfoil near maximum lift. The corresponding angle of attack was determined independently by Wadcock (1987) and Hastings & Williams (1984, 1987) to be close to 12 deg. The simulation matches the chord Reynolds number U(sub infinity)c/v = 1.64 x 10(exp 6) of Wadcock's experiment.
Large Eddy Simulation of Pollen Transport in the Atmospheric Boundary Layer
NASA Astrophysics Data System (ADS)
Chamecki, Marcelo; Meneveau, Charles; Parlange, Marc B.
2007-11-01
The development of genetically modified crops and questions about cross-pollination and contamination of natural plant populations enhanced the importance of understanding wind dispersion of airborne pollen. The main objective of this work is to simulate the dispersal of pollen grains in the atmospheric surface layer using large eddy simulation. Pollen concentrations are simulated by an advection-diffusion equation including gravitational settling. Of great importance is the specification of the bottom boundary conditions characterizing the pollen source over the canopy and the deposition process everywhere else. The velocity field is discretized using a pseudospectral approach. However the application of the same discretization scheme to the pollen equation generates unphysical solutions (i.e. negative concentrations). The finite-volume bounded scheme SMART is used for the pollen equation. A conservative interpolation scheme to determine the velocity field on the finite volume surfaces was developed. The implementation is validated against field experiments of point source and area field releases of pollen.
Estimating the effective Reynolds number in implicit large-eddy simulation.
Zhou, Ye; Grinstein, Fernando F; Wachtor, Adam J; Haines, Brian M
2014-01-01
In implicit large-eddy simulation (ILES), energy-containing large scales are resolved, and physics capturing numerics are used to spatially filter out unresolved scales and to implicitly model subgrid scale effects. From an applied perspective, it is highly desirable to estimate a characteristic Reynolds number (Re)-and therefore a relevant effective viscosity-so that the impact of resolution on predicted flow quantities and their macroscopic convergence can usefully be characterized. We argue in favor of obtaining robust Re estimates away from the smallest scales of the simulated flow-where numerically controlled dissipation takes place and propose a theoretical basis and framework to determine such measures. ILES examples include forced turbulence as a steady flow case, the Taylor-Green vortex to address transition and decaying turbulence, and simulations of a laser-driven reshock experiment illustrating a fairly complex turbulence problem of current practical interest.
Shetty, Dinesh A.; Frankel, Steven H.
2013-01-01
Computational fluid dynamics (CFD) simulations are becoming a reliable tool to understand hemodynamics, disease progression in pathological blood vessels and to predict medical device performance. Immersed boundary method (IBM) emerged as an attractive methodology because of its ability to efficiently handle complex moving and rotating geometries on structured grids. However, its application to study blood flow in complex, branching, patient-specific anatomies is scarce. This is because of the dominance of grid nodes in the exterior of the fluid domain over the useful grid nodes in the interior, rendering an inevitable memory and computational overhead. In order to alleviate this problem, we propose a novel multiblock based IBM that preserves the simplicity and effectiveness of the IBM on structured Cartesian meshes and enables handling of complex, anatomical geometries at a reduced memory overhead by minimizing the grid nodes in the exterior of the fluid domain. As pathological and medical device hemodynamics often involve complex, unsteady transitional or turbulent flow fields, a scale resolving turbulence model such as large eddy simulation (LES) is used in the present work. The proposed solver (here after referred as WenoHemo), is developed by enhancing an existing in-house high order incompressible flow solver that was previously validated for its numerics and several LES models by Shetty et al. [Journal of Computational Physics 2010; 229 (23), 8802-8822]. In the present work, WenoHemo is systematically validated for additional numerics introduced, such as IBM and the multiblock approach, by simulating laminar flow over a sphere and laminar flow over a backward facing step respectively. Then, we validate the entire solver methodology by simulating laminar and transitional flow in abdominal aortic aneurysm (AAA). Finally, we perform blood flow simulations in the challenging clinically relevant thoracic aortic aneurysm (TAA), to gain insights into the type of
Anupindi, Kameswararao; Delorme, Yann; Shetty, Dinesh A; Frankel, Steven H
2013-12-01
Computational fluid dynamics (CFD) simulations are becoming a reliable tool to understand hemodynamics, disease progression in pathological blood vessels and to predict medical device performance. Immersed boundary method (IBM) emerged as an attractive methodology because of its ability to efficiently handle complex moving and rotating geometries on structured grids. However, its application to study blood flow in complex, branching, patient-specific anatomies is scarce. This is because of the dominance of grid nodes in the exterior of the fluid domain over the useful grid nodes in the interior, rendering an inevitable memory and computational overhead. In order to alleviate this problem, we propose a novel multiblock based IBM that preserves the simplicity and effectiveness of the IBM on structured Cartesian meshes and enables handling of complex, anatomical geometries at a reduced memory overhead by minimizing the grid nodes in the exterior of the fluid domain. As pathological and medical device hemodynamics often involve complex, unsteady transitional or turbulent flow fields, a scale resolving turbulence model such as large eddy simulation (LES) is used in the present work. The proposed solver (here after referred as WenoHemo), is developed by enhancing an existing in-house high order incompressible flow solver that was previously validated for its numerics and several LES models by Shetty et al. [Journal of Computational Physics 2010; 229 (23), 8802-8822]. In the present work, WenoHemo is systematically validated for additional numerics introduced, such as IBM and the multiblock approach, by simulating laminar flow over a sphere and laminar flow over a backward facing step respectively. Then, we validate the entire solver methodology by simulating laminar and transitional flow in abdominal aortic aneurysm (AAA). Finally, we perform blood flow simulations in the challenging clinically relevant thoracic aortic aneurysm (TAA), to gain insights into the type of
NASA Astrophysics Data System (ADS)
Maurer, K. D.; Bohrer, G.; Ivanov, V. Y.
2014-11-01
Surface roughness parameters are at the core of every model representation of the coupling and interactions between land-surface and atmosphere, and are used in every model of surface fluxes. However, most models assume these parameters to be a fixed property of plant functional type and do not vary them in response to spatial or temporal changes to canopy structure. In part, this is due to the difficulty of reducing the complexity of canopy structure and its spatiotemporal dynamic and heterogeneity to less than a handful of parameters describing its effects of atmosphere-surface interactions. In this study we use large-eddy simulations to explore, in silico, the effects of canopy structure characteristics on surface roughness parameters. We performed a virtual experiment to test the sensitivity of resolved surface roughness to four axes of canopy structure: (1) leaf area index, (2) the vertical profile of leaf density, (3) canopy height, and (4) canopy gap fraction. We found roughness parameters to be highly variable, but were able to find positive relationships between displacement height and maximum canopy height, aerodynamic canopy height and maximum canopy height and leaf area index, and eddy-penetration depth and gap fraction. We also found negative relationships between aerodynamic canopy height and gap fraction, and between eddy-penetration depth and maximum canopy height and leaf area index. Using a decade of wind and canopy structure observations in a site in Michigan, we tested the effectiveness of our model-resolved parameters in predicting the frictional velocity over heterogeneous and disturbed canopies. We compared it with three other semi-empirical models and with a decade of meteorological observations. We found that parameterizations with fixed representations of roughness performed relatively well. Nonetheless, some empirical approaches that incorporate seasonal and inter-annual changes to the canopy structure performed even better than models
Large-Eddy Simulations of Trade Wind Cumuli: Investigation of Aerosol Indirect Effects.
NASA Astrophysics Data System (ADS)
Xue, Huiwen; Feingold, Graham
2006-06-01
The effects of aerosol on warm trade cumulus clouds are investigated using a large-eddy simulation with size-resolved cloud microphysics. It is shown that, as expected, increases in aerosols cause a reduction in precipitation and an increase in the cloud-averaged liquid water path (LWP). However, for the case under study, cloud fraction, cloud size, cloud-top height, and depth decrease in response to increasing aerosol concentration, contrary to accepted hypotheses associated with the second aerosol indirect effect. It is found that the complex responses of clouds to aerosols are determined by competing effects of precipitation and droplet evaporation associated with entrainment. As aerosol concentration increases, precipitation suppression tends to maintain the clouds and lead to higher cloud LWP, whereas cloud droplets become smaller and evaporate more readily, which tends to dissipate the clouds and leads to lower cloud fraction, cloud size, and depth. An additional set of experiments with higher surface latent heat flux, and hence higher LWP and drizzle rate, was also performed. Changes in cloud properties due to aerosols have the same trends as in the base runs, although the magnitudes of the changes are larger. Evidence for significant stabilization (or destabilization) of the subcloud layer due to drizzle is not found, mainly because drizzling clouds cover only a small fraction of the domain. It is suggested that cloud fraction may only increase with increasing aerosol loading for larger clouds that are less susceptible to entrainment and evaporation. Finally, it is noted that at any given aerosol concentration the dynamical variability in bulk cloud parameters such as LWP tends to be larger than the aerosol-induced changes in these parameters, indicating that the second aerosol indirect effect may be hard to measure in this cloud type. The variability in cloud optical depth is, however, dominated by changes in aerosol, rather than dynamics.
Large Eddy Simulation of turbulent flow fields over three- dimensional alluvial dunes
NASA Astrophysics Data System (ADS)
Hardy, R. J.; Parsons, D. R.; Best, J.; Reesink, A. J. H.; Ockelford, A.
2016-12-01
Flow over fluvial dunes has been extensively studied and there is general understanding of the nature of the flow field over two dimensional dunes under equilibrium flow conditions. However, fluvial systems typically experience unsteady flow and therefore the sediment-water interface is constantly reorganizing to form complex three-dimensional morphologies (ripples, dunes and bar forms). Here we report on a numerical experiment which predicts flow over three dimensional dunes using Large Eddy Simulation (LES). Bed topography generated through flume experiments, where fine sand was water worked under a range of unsteady hydraulic conditions to generate quasi-equilibrium three dimensional bed forms, was measured with terrestrial LiDAR to create digital elevation models. This topography was then incorporated into a LES model, with a wall-adapting local eddy-viscosity turbulence model, through a Mass Flux Scaling algorithm, to generate three dimensional, high resolution space time prediction of flow over naturally formed dunes. The numerically predicted flows were analysed by standard Reynolds decomposition approaches, Eulerian and Lagrangian coherent flow structure identification methods and proper orthogonal decomposition. The results show that superimposed bed forms can cause changes in the nature of the classical separated flow regions and turbulence field. In particular, the number of locations where vortices are shed increase which causes coalescence of vortices. This increases the rate of transfer of turbulent kinetic energy into smaller scales. This has significant implications for the time dependent prediction of shear stress and as such for sediment transport dynamics which are required for an improved process understanding of three-dimensional bed form adjustment.
Large eddy simulation of interacting barchan dunes in a steady, unidirectional flow
NASA Astrophysics Data System (ADS)
Omidyeganeh, Mohammad; Piomelli, Ugo; Christensen, Kenneth T.; Best, James L.
2013-12-01
Barchan dunes are bed forms found in many sedimentary environments with a limited supply of sediment, and may occur in isolation or in more complex dune fields. Barchans have a crescentic planform morphology with horns elongated in the downflow direction. To study flow over barchan dunes, we performed large eddy simulations in a channel with different interdune spacings at a flow Reynolds number, Re∞≃26,000 (based on the free stream velocity and channel height). The largest interdune spacing (2.38λ, where λ is the wavelength of the barchan dune) presents similar characteristics to a solitary dune in isolation, indicating that, at this distance, the sheltering effect of the upstream dune is rather weak. Barchan dunes induce two counterrotating streamwise vortices, one along each of the horns, which direct high-momentum fluid toward the symmetry plane and low-momentum fluid near the bed away from the centerline. The flow close to the centerline plane separates at the crest, but away from the centerline plane, and along the horns, flow separation occurs intermittently. The flow in the separation bubble is directed toward the horns and leaves the dune at its tips. The internal boundary layer developing on the bed downstream of the reattachment region develops similarly for various interdune spacings; the development slows down 14.5 dune heights downstream. The turbulent kinetic energy budgets show the importance of pressure transport and mean flow advection in transferring energy from the overlying wake layer to the internal boundary layer over the stoss side. For closely spaced dunes, the bed shear stress is 30% larger than at the largest spacing, and instantaneous coherent high- and low-speed streaks are shorter but stronger. Coherent eddies in the separated shear layer are generated more frequently for smaller interdune spacing, where they move farther away from the bed, toward the free surface, and remain located between the horns.
Sub-grid scale modeling for large eddy simulations in analysis of shock-turbulence interactions
NASA Astrophysics Data System (ADS)
Buckingham, A. C.; Grun, J.
1992-12-01
We continue to study the influence of dynamic shock wave interactions on turbulence. The interactions may significantly increase turbulent energy and Reynolds stress. Strong support for tensor amplification is supplied by the sharp, transiently distorted strain field in the immediate neighborhood of the shock. Beyond this, there develops a gradual decay to a new, more modestly amplified state relative to the pre-shocked level. Practical interest is centered on the significantly altered, albeit shock localized, post-shock turbulent kinetic energy, eddy transport, eddy component mixing and diffusion, wall shear, and heat transfer. In the shock interaction and post-shock region, compressible two dimensional large eddy simulations (LES) are applied. A compressibility modified Smagorinsky model is adapted to represent the non-resolved sub-grid scales. Favre mass-weighted average space and time discretized compressible Navier-Stokes equations are used to represent the explicitly resolved grid scale motions. Predicted amplification levels, modal energy partition, shock translational to turbulence kinetic energy transfer, and viscoelastic response of turbulence to shock interaction are examined in comparison with available experimental evidence. A two-band dynamic eddy viscosity model representing the unresolved subgrid scale field is a possible replacement for the Smagorinsky model. Improvement is sought for predictions in the near wall region, under the influence of stochastic subgrid scale backscatter, and in the neighborhood of the shock. Wall-bounded supersonic compression comer flow experiments and hypersonic cylindrical shock wave turbulence interaction experiments are used as trial cases for test and comparison of the two classes of subgrid scale models.
NASA Astrophysics Data System (ADS)
Liu, S.; Shao, Y.; Hintz, M.
2011-12-01
In this study, we present a fully coupled large-eddy atmosphere and land-surface simulation model (LES-ALM) which integrates a radiation parameterization, a large-eddy flow model for the atmospheric boundary layer with explicit consideration of the canopy drag effect, and a land surface model to investigate the atmosphere and land surface interactions over heterogeneous areas. A 12-hour model simulation is carried out and the model performance is validated with the field measurements collected during the FLUXPAT experiment of the German SFB/TR32 project "Patterns in Soil-Vegetation-Atmosphere System: Monitoring, Modeling and Data Assimilation". The simulated surface fluxes, near-surface atmospheric state variables, soil temperature, and the vertical profiles of atmospheric boundary layer quantities are compared with the data. There is a good agreement between the model simulations and the observation at both footprint- and domain-averaged scales. This suggests that the fully coupled model can be used as a tool for studying the complex atmosphere and land-surface interactions.
NASA Astrophysics Data System (ADS)
Meheut, Heloise; Fromang, Sébastien; Lesur, Geoffroy; Joos, Marc; Longaretti, Pierre-Yves
2015-07-01
Context. Angular momentum transport in accretion discs is often believed to be due to magnetohydrodynamic turbulence mediated by the magnetorotational instability (MRI). Despite an abundant literature on the MRI, the parameters governing the saturation amplitude of the turbulence are poorly understood and the existence of an asymptotic behaviour in the Ohmic diffusion regime has not been clearly established. Aims: We investigate the properties of the turbulent state in the small magnetic Prandtl number limit. Since this is extremely computationally expensive, we also study the relevance and range of applicability of the most common subgrid scale models for this problem. Methods: Unstratified shearing box simulations are performed both in the compressible and incompressible limits, with a resolution up to 800 cells per disc scale height. This is the highest resolution ever attained for a simulation of MRI turbulence. Different magnetic field geometry and a wide range of dimensionless dissipative coefficients are considered. We also systematically investigate the relevance of using large eddy simulations (LES) in place of direct numerical simulations. Results: In the presence of a mean magnetic field threading the domain, angular momentum transport converges to a finite value in the small Pm limit. When the mean vertical field amplitude is such that β (the ratio between the thermal and magnetic pressure) equals 103, we find α ~ 3.2 × 10-2 when Pm approaches zero. In the case of a mean toroidal field for which β = 100, we find α ~ 1.8 × 10-2 in the same limit. Implicit LES and the Chollet-Lesieur closure model both reproduce these results for the α parameter and the power spectra. A reduction in computational cost by a factor of at least 16 (and up to 256) is achieved when using such methods. Conclusions: MRI turbulence operates efficiently in the small Pm limit provided there is a mean magnetic field. Implicit LES offers a practical and efficient means of
Implicit Large Eddy Simulation of a wingtip vortex at Rec =1.2x106
NASA Astrophysics Data System (ADS)
Lombard, Jean-Eloi; Moxey, Dave; Sherwin, Spencer; SherwinLab Team
2015-11-01
We present recent developments in numerical methods for performing a Large Eddy Simulation (LES) of the formation and evolution of a wingtip vortex. The development of these vortices in the near wake, in combination with the large Reynolds numbers present in these cases, make these types of test cases particularly challenging to investigate numerically. To demonstrate the method's viability, we present results from numerical simulations of flow over a NACA 0012 profile wingtip at Rec = 1.2 x106 and compare them against experimental data, which is to date the highest Reynolds number achieved for a LES that has been correlated with experiments for this test case. Our model correlates favorably with experiment, both for the characteristic jetting in the primary vortex and pressure distribution on the wing surface. The proposed method is of general interest for the modeling of transitioning vortex dominated flows over complex geometries. McLaren Racing/Royal Academy of Engineering Research Chair.
Martian dust devil statistics from high-resolution large-eddy simulations
NASA Astrophysics Data System (ADS)
Nishizawa, Seiya; Odaka, Masatsugu; Takahashi, Yoshiyuki O.; Sugiyama, Ko-ichiro; Nakajima, Kensuke; Ishiwatari, Masaki; Takehiro, Shin-ichi; Yashiro, Hisashi; Sato, Yousuke; Tomita, Hirofumi; Hayashi, Yoshi-Yuki
2016-05-01
Dust devils are one of the key elements in the Martian atmospheric circulation. In order to examine their statistics, we conducted high-resolution (up to 5 m) and wide-domain (about 20 × 20 km2) large-eddy simulations of the Martian daytime convective layer. Large numbers of dust devils developed spontaneously in the simulations, which enabled us to represent a quantitative consideration of Martian dust devil frequency distributions. We clarify the distributions of size and intensity, a topic of debate, and conclude that the maximum vertical vorticity of an individual dust devil has an exponential distribution, while the radius and circulation have power law distributions. A grid refinement experiment shows that the rate parameter of the vorticity distribution and the exponent of the circulation distribution are robust. The mode of the size distribution depends on the resolution, and it is suggested that the mode is less than 5 m.
Large Eddy Simulation of an URBAN 2000 Experiment with Various Time-Dependent Forcing
Chan, S T; Leach, M J
2004-06-15
Under the sponsorship of the U.S. DOE and DHS, we have developed a Computational Fluid Dynamics (CFD) model for simulating airflow and dispersion of chemical/biological agents released in the urban environment. Our model, FEM3MP, is based on solving the three-dimensional, time-dependent, incompressible Navier-Stokes equations on massively parallel computer platforms. The numerical algorithm uses the finite element method for accurate representation of complex building shapes and variable terrain, together with a semi-implicit projection method and modern iterative solvers for efficient time integration (Gresho and Chan, 1998). Physical processes treated in our code include turbulence modeling via the RANS (Reynolds Averaged Navier-Stokes) and LES (Large Eddy Simulation) approaches, atmospheric stability, aerosols, UV radiation decay, surface energy budget, and vegetative canopies, etc.
NASA Astrophysics Data System (ADS)
Cerminara, Matteo; Esposti Ongaro, Tomaso; Neri, Augusto
2016-10-01
In the framework of the IAVCEI (International Association of Volcanology and Chemistry of the Earth Interior) intercomparison study on volcanic plume models, we present three-dimensional (3D) numerical simulations carried out with the ASHEE (ASH Equilibrium Eulerian) model. The ASHEE model solves the compressible balance equations of mass, momentum, and enthalpy of a gas-particle mixture and is able to describe the kinematic decoupling for particles characterized by Stokes number (i.e., the ratio between the particle equilibrium time and the flow characteristic time) lower than 0.2 (or particles smaller than about 1 mm). The computational fluid dynamic model is designed to accurately simulate a turbulent flow field using a Large Eddy Simulation approach, and is thus suited to analyze the role of particle non-equilibrium in the dynamics of turbulent volcanic plumes. The two reference scenarios analyzed correspond to a weak (mass eruption rate = 1.5 * 106 kg/s) and a strong volcanic plume (mass eruption rate = 1.5 * 109 kg/s) in absence of wind. For each scenario, we compare the 3D results, averaged in space and time, with theoretical results obtained from integral plume models. Such an approach enables quantitative evaluation of the effects of grid resolution and the subgrid-scale turbulence model, and the influence of gas-particle non-equilibrium processes on the large-scale plume dynamics. We thus demonstrate that the uncertainty on the numerical solution associated with such effects can be significant (of the order of 20%), but still lower than that typically associated with input data and integral model approximations. In the Weak Plume case, 3D results are consistent with the predictions of integral models in the jet and plume regions, with an entrainment coefficient around 0.10 in the plume region. In the Strong Plume case, the self-similarity assumption is less appropriate and the entrainment coefficient in the plume region is more unstable, with an average
Modeling Persistent Contrails in a Large Eddy Simulation and a Global Climate Model
NASA Astrophysics Data System (ADS)
Naiman, A. D.; Lele, S. K.; Wilkerson, J. T.; Jacobson, M. Z.
2009-12-01
Two models of aircraft condensation trail (contrail) evolution have been developed: a high resolution, three-dimensional Large Eddy Simulation (LES) and a simple, low-cost Subgrid Contrail Model (SCM). The LES model was used to simulate contrail development from one second to twenty minutes after emission by the passing aircraft. The LES solves the incompressible Navier-Stokes equations with a Boussinesq approximation for buoyancy forces on an unstructured periodic grid. The numerical scheme uses a second-order finite volume spatial discretization and an implicit fractional-step method for time advancement. Lagrangian contrail particles grow according to a microphysical model of ice deposition and sublimation. The simulation is initialized with the wake of a commercial jet superimposed on a decaying turbulence field. The ambient atmosphere is stable and has a supersaturated relative humidity with respect to ice. Grid resolution is adjusted during the simulation, allowing higher resolution of flow structures than previous studies. We present results of a parametric study in which ambient turbulence levels, vertical wind shear, and aircraft type were varied. We find that higher levels of turbulence and shear promote mixing of aircraft exhaust with supersaturated ambient air, resulting in faster growth of ice and wider dispersion of the exhaust plume. The SCM was developed as a parameterization of contrail dynamics intended for use within a global model that examines the effect of commercial aviation on climate. The SCM provides an analytic solution to the changes in size and shape of a contrail cross-section over time due to global model grid-scale vertical wind shear and turbulence parameters. The model was derived from the physical equations of motion of a plume in a sheared, turbulent environment. Approximations based on physical reasoning and contrail observations allowed these equations to be reduced to simple ordinary differential equations in time with exact
Large-eddy simulation of a three-dimensional compressible tornado vortex
NASA Astrophysics Data System (ADS)
Xia, Jianjun
Large-Eddy simulation (LES) has become a very useful tool for investigating tornadoes, one of the more spectacular and destructive phenomena of nature. A new three-dimensional, unsteady, compressible model is generated to determine how significant the differences between compressible and incompressible LES simulations may be in some extremely violent tornadoes. In particular, this study seeks to determine how high the Mach number within the tornado may become before significant changes occur due to compressibility, and what the major effects of these changes may be expected to be. After developing and verifying the compressible LES model, three different patterns of tornadic corner flows cataloged by local swirl ratio are simulated under quasisteady conditions for different Mach numbers. Simulation comparisons have demonstrated that the compressibility effects are different for different corner flow structures. At peak average Mach numbers less than approximately 0.5, the compressibility effects are not very significant and may be accounted for to leading order by an appropriate isentropic transformation applied to the incompressible results. As the maximum Mach number is increased to more than 1.0, the compressibility effects for low-swirl-ratio corner flows are dramatic, with significant increase in peak vertical velocity and the height of the vortex breakdown above the surface. The effects are much weaker for medium swirl conditions, and expected to be still weaker for high swirl corner flow where the effects are essentially limited to influencing the secondary vortices. In general, compressibility effects would not change the basic dynamics of tornadic corner flows even if Mach numbers greater than one are achieved. This study also shows that during the sharp temporal overshoot in near-surface intensity that can sometimes occur during a tornado's evolution, the maximum pressure drop will tend to be restricted by supersonic velocities, and thus limit the
Evaluation of Smagorinsky variants in large-eddy simulations of wall-resolved plane channel flows
NASA Astrophysics Data System (ADS)
Meyers, Johan; Sagaut, Pierre
2007-09-01
In recent years, variational multiscale (VMS) Smagorinsky models have emerged as new models for large-eddy simulations (LES). A common version is the small-small variant, which uses a small-scale extraction of the LES solution, obtained by high-pass filtering the resolved velocity field, to express a Smagorinsky term. The subsequent small-scale extraction of this term is used as a model. In the current work, three formulations of the small-small VMS model are investigated in large-eddy simulations of the plane channel flow. The basic small-small formulation (Model A) is modified to explicitly incorporate effects of the LES filter and the high-pass filter (Model B). A third modification (Model C) is further inertial-range consistent, allowing the use of constant model coefficients for filters widths which are situated in a finite Reynolds-number inertial subrange. We aim to evaluate the performance of these models in the presence of walls. Therefore, channel-flow simulations are performed for Reτ=110, 300, 400, and 650. Further, the effect of changes in the shape of the high-pass filter used for the three models is investigated. A sharp cutoff filter and a Gaussian high-pass filter are considered. In addition, a range of high-pass-filter widths is included in the analysis. Evaluations of the skin-friction, mean-velocity profiles, Reynolds stresses, and spanwise velocity spectra are presented. We show that Model C is most insensitive to changes in Reynolds number and filter shape, closely followed by Model B. Model A is the most sensitive to the considered variations, and simulation quality depends in particular on variations in the filter shape.
A parallel finite volume algorithm for large-eddy simulation of turbulent flows
NASA Astrophysics Data System (ADS)
Bui, Trong Tri
1998-11-01
A parallel unstructured finite volume algorithm is developed for large-eddy simulation of compressible turbulent flows. Major components of the algorithm include piecewise linear least-square reconstruction of the unknown variables, trilinear finite element interpolation for the spatial coordinates, Roe flux difference splitting, and second-order MacCormack explicit time marching. The computer code is designed from the start to take full advantage of the additional computational capability provided by the current parallel computer systems. Parallel implementation is done using the message passing programming model and message passing libraries such as the Parallel Virtual Machine (PVM) and Message Passing Interface (MPI). The development of the numerical algorithm is presented in detail. The parallel strategy and issues regarding the implementation of a flow simulation code on the current generation of parallel machines are discussed. The results from parallel performance studies show that the algorithm is well suited for parallel computer systems that use the message passing programming model. Nearly perfect parallel speedup is obtained on MPP systems such as the Cray T3D and IBM SP2. Performance comparison with the older supercomputer systems such as the Cray YMP show that the simulations done on the parallel systems are approximately 10 to 30 times faster. The results of the accuracy and performance studies for the current algorithm are reported. To validate the flow simulation code, a number of Euler and Navier-Stokes simulations are done for internal duct flows. Inviscid Euler simulation of a very small amplitude acoustic wave interacting with a shock wave in a quasi-1D convergent-divergent nozzle shows that the algorithm is capable of simultaneously tracking the very small disturbances of the acoustic wave and capturing the shock wave. Navier-Stokes simulations are made for fully developed laminar flow in a square duct, developing laminar flow in a
Large eddy simulations as a parameterization tool for canopy-structure X VOC-flux interactions
NASA Astrophysics Data System (ADS)
Kenny, William; Bohrer, Gil; Chatziefstratiou, Efthalia
2015-04-01
We have been working to develop a new post-processing model - High resolution VOC Atmospheric Chemistry in Canopies (Hi-VACC) - which resolves the dispersion and chemistry of reacting chemical species given their emission rates from the vegetation and soil, driven by high resolution meteorological forcing and wind fields from various high resolution atmospheric regional and large-eddy simulations. Hi-VACC reads in fields of pressure, temperature, humidity, air density, short-wave radiation, wind (3-D u, v and w components) and sub-grid-scale turbulence that were simulated by a high resolution atmospheric model. This meteorological forcing data is provided as snapshots of 3-D fields. We have tested it using a number of RAMS-based Forest Large Eddy Simulation (RAFLES) runs. This can then be used for parameterization of the effects of canopy structure on VOC fluxes. RAFLES represents both drag and volume restriction by the canopy over an explicit 3-D domain. We have used these features to show the effects of canopy structure on fluxes of momentum, heat, and water in heterogeneous environments at the tree-crown scale by modifying the canopy structure representing it as both homogeneous and realistically heterogeneous. We combine this with Hi-VACC's capabilities to model dispersion and chemistry of reactive VOCs to parameterize the fluxes of these reactive species with respect to canopy structure. The high resolution capabilities of Hi-VACC coupled with RAFLES allows for sensitivity analysis to determine important structural considerations in sub-grid-scale parameterization of these phenomena in larger models.
Large Eddy Simulation of Transonic Flow Field in NASA Rotor 37
NASA Technical Reports Server (NTRS)
Hah, Chunill
2009-01-01
The current paper reports on numerical investigations on the flow characteristics in a transonic axial compressor, NASA Rotor 37. The flow field was used previously as a CFD blind test case conducted by American Society of Mechanical Engineers in 1994. Since the CFD blind-test exercise, many numerical studies on the flow field in the NASA Rotor 37 have been reported. Although steady improvements have been reported in both numerical procedure and turbulence closure, it is believed that all the important aspects of the flow field have not been fully explained with numerical studies based on the Reynolds Averaged Navier-Stokes (RANS) solution. Experimental data show large dip in total pressure distribution near the hub at downstream of the rotor at 100% rotor speed. Most original numerical solutions from the blind test exercise did not predict this total pressure deficit correctly. This total pressure deficit at the rotor exit was attributed to a hub corner flow separation by the author. Several subsequent numerical studies with different turbulence closure model also calculated this dip in total pressure rise. Also, several studies attributed this total pressure deficit to a small leakage flow coming from the hub in the test article. As the experimental study cannot be repeated, either explanation cannot be validated. The primary purpose of the current investigation is to investigate the transonic flow field with both RANS and a Large Eddy Simulation (LES). The RANS approach gives similar results presented at the original blind test exercise. Although the RANS calculates higher overall total pressure rise, the total pressure deficit near the hub is calculated correctly. The numerical solution shows that the total pressure deficit is due to a hub corner flow separation. The calculated pressure rise from the LES agrees better with the measured total pressure rise especially near the casing area where the passage shock interacts with the tip clearance vortex and flow
Large-eddy simulations of wind farm production and long distance wakes
NASA Astrophysics Data System (ADS)
Eriksson, O.; Nilsson, K.; Breton, S.-P.; Ivanell, S.
2015-06-01
The future development of offshore wind power will include many wind farms built in the same areas. It is known that wind farms produce long distance wakes, which means that we will see more occasions of farm to farm interaction, namely one wind farm operating in the wake of another wind farm. This study investigates how to perform accurate power predictions on large wind farms and how to assess the long distance wakes generated by these farms. The focus of this paper is the production's and wake's sensitivity to the extension of the grid as well as the turbulence when using Large-eddy simulations (LES) with pregenerated Mann turbulence. The aim is to determine an optimal grid which minimizes blockage effects and ensures constant resolution in the entire wake region at the lowest computational cost. The simulations are first performed in the absence of wind turbines in order to assess how the atmospheric turbulence and wind profile are evolving downstream (up to 12,000 m behind the position where the turbulence is imposed). In the second step, 10 turbines are added in the domain (using an actuator disc method) and their production is analyzed alongside the mean velocities in the domain. The blockage effects are tested using grids with different vertical extents. An equidistant region is used in order to ensure high resolution in the wake region. The importance of covering the entire wake structure inside the equidistant region is analyzed by decreasing the size of this region. In this step, the importance of the lateral size of the Mann turbulence box is also analyzed. In the results it can be seen that the flow is acceptably preserved through the empty domain if a larger turbulence box is used. The relative production is increased (due to blockage effects) for the last turbines using a smaller vertical domain, increased for a lower or narrower equidistant region (due to the smearing of the wake in the stretched area) and decreased when using a smaller turbulence
Large Eddy Simulations of Compositional Density Currents Flowing Over a Mobile Bed
NASA Astrophysics Data System (ADS)
Kyrousi, Foteini; Zordan, Jessica; Leonardi, Alessandro; Juez, Carmelo; Zanello, Francesca; Armenio, Vincenzo; Franca, Mário J.
2017-04-01
Density currents are a ubiquitous phenomenon caused by natural events or anthropogenic activities, and play an important role in the global sediment cycle; they are agents of long distance sediment transport in lakes, seas and oceans. Density gradients induced by salinity, temperature differences, or by the presence of suspended material are all possible triggers of a current. Such flows can travel long distances while eroding or depositing bed materials. This can provoke rapid topological changes, which makes the estimation of their transport capacity of prime interest for environmental engineering. Despite their relevance, field data regarding their dynamics is limited due to density currents scattered and unpredictable occurrence in nature. For this reason, laboratory experiments and numerical simulations have been a preferred way to investigate sediment transport processes associated to density currents. The study of entrainment and deposition processes requires detailed data of velocities spatial and temporal distributions in the boundary layer and bed shear stress, which are troublesome to obtain in laboratory. Motivated by this, we present 3D wall-resolved Large Eddy Simulations (LES) of density currents generated by lock-exchange. The currents travel over a smooth flat bed, which includes a section composed by erodible fine sediment susceptible of eroding. Several sediment sizes and initial density gradients are considered. The grid is set to resolve the velocity field within the boundary layer of the current (a tiny fraction of the total height), which in turn allows to obtain predictions of the bed shear stress. The numerical outcomes are compared with experimental data obtained with an analogous laboratory setting. In laboratory experiments salinity was chosen for generating the initial density gradient in order to facilitate the identification of entrained particles, since salt does not hinder the possibility to track suspended particles. Under these
Large eddy simulations of forest canopies for determination of biological dispersal by wind
NASA Astrophysics Data System (ADS)
Bohrer, Gil
Forest canopies interact with the atmosphere by emitting heat and moisture fluxes, by dragging the flow and by forming obstacles to the flow. Forests are heterogeneous with structural features at a vast range of length scale. The atmospheric effects of micro-scale canopy structures, which describe differences between individual trees, have so far been poorly studied. Changes to turbulence, flow patterns, and fluxes in and above the canopy strongly affect the dispersal of seeds and its ecological consequences because they are strongly dependent on the far "tail" of the dispersal distribution. The Regional Atmospheric Modeling System (RAMS) is further developed to operate as a large-eddy simulation (LES) at high resolution with 3D heterogeneous forest canopies. This RAMS-based Forest LES (RAFLES) represents the canopy through drag, volume restriction by stems, and heat and moisture fluxes in the canopy domain. The model incorporates explicit canopy descriptions, which can be obtained from observations, or from the virtual-canopy generator, which is developed here. RAFLES is used to simulate noontime conditions for two days at the hardwood stand in the Duke Forest, representing two sets of atmospheric and canopy conditions. The results are evaluated against eddy-flux observations from these days. RAFLES compares well to the observed data. Comparison between artificial homogeneous cases and natural heterogeneous cases reveals that small-scale canopy heterogeneity affects the profiles of momentum and scalar fluxes, and modifies the spatial structure of the flow. Low areas in the canopy promote ejection events, which leads to a correlation between the canopy height and flow variables that extends up to four times the canopy height. Seed dispersal kernels simulated with RAFLES closely match those measured in seed release experiments in a temperate forest. It is also used to examine potential biases resulting from simplifications in common dispersal models, such as planar
Idealized gas turbine combustor for performance research and validation of large eddy simulations.
Williams, Timothy C; Schefer, Robert W; Oefelein, Joseph C; Shaddix, Christopher R
2007-03-01
This paper details the design of a premixed, swirl-stabilized combustor that was designed and built for the express purpose of obtaining validation-quality data for the development of large eddy simulations (LES) of gas turbine combustors. The combustor features nonambiguous boundary conditions, a geometrically simple design that retains the essential fluid dynamics and thermochemical processes that occur in actual gas turbine combustors, and unrestrictive access for laser and optical diagnostic measurements. After discussing the design detail, a preliminary investigation of the performance and operating envelope of the combustor is presented. With the combustor operating on premixed methane/air, both the equivalence ratio and the inlet velocity were systematically varied and the flame structure was recorded via digital photography. Interesting lean flame blowout and resonance characteristics were observed. In addition, the combustor exhibited a large region of stable, acoustically clean combustion that is suitable for preliminary validation of LES models.
Influences on the Height of the Stable Boundary Layer as seen in Large-Eddy Simulations
Kosovic, B; Lundquist, J K
2004-03-29
Numerical weather prediction (NWP) models and atmospheric dispersion models rely on parameterizations of planetary boundary layer height. In the case of a stable boundary layer, errors in boundary layer height estimation can result in gross errors in boundary-layer evolution and in prediction of turbulent mixing within the boundary layer. We use large-eddy simulations (LES) of moderately stable boundary layers to characterize the effects of various physical processes on stable boundary layers. The stable boundary layer height is assumed to be a function of surface friction velocity, geostrophic wind, Monin-Obukhov length, and the strength of the temperature inversion atop the stable boundary layer. This temperature inversion induces gravity waves with a frequency determined by the strength of the temperature inversion.
Explaining the convector effect in canopy turbulence by means of large-eddy simulation
Banerjee, Tirtha; De Roo, Frederik; Mauder, Matthias
2017-06-20
Semi-arid forests are found to sustain a massive sensible heat flux in spite of having a low surface to air temperature difference by lowering the aerodynamic resistance to heat transfer (rH) – a property called the canopy convector effect (CCE). In this work large-eddy simulations are used to demonstrate that the CCE appears more generally in canopy turbulence. It is indeed a generic feature of canopy turbulence: rH of a canopy is found to reduce with increasing unstable stratification, which effectively increases the aerodynamic roughness for the same physical roughness of the canopy. This relation offers a sufficient condition to constructmore » a general description of the CCE. In addition, we review existing parameterizations for rH from the evapotranspiration literature and test to what extent they are able to capture the CCE, thereby exploring the possibility of an improved parameterization.« less
Effect of submerged vegetation on solute transport in an open channel using large eddy simulation
NASA Astrophysics Data System (ADS)
Lu, J.; Dai, HC
2016-11-01
Existence of vegetation plays a significant effect on the flow velocity distributions, turbulence structures and solute mixing in an open channel. This paper has implemented a 3D large eddy simulation model for the flow and scalar transport in the open channel with vegetation. The model can produce a typical turbulence characteristics and concentration distribution with vegetation. The scalar transport mechanism is quantitatively explained by the turbulent Schmidt number, Reynolds flux, coherent structures and quadrant conditional analysis. A dominance of ejection-sweeping events occurs in the process of the momentum and scalar flux transport. The spectral analysis is used to identify the Kelvin-Helmholtz frequency. The turbulence characteristics of the length scale of vortexes, Kelvin-Helmholtz frequency and Reynolds stress etc. are analyzed with the vegetation density. The model quantitatively predicts the trend of decreasing in the concentration distribution along the flow direction with the increasing of vegetation density.
NASA Astrophysics Data System (ADS)
Thiesset, Fabien; Maurice, Guillaume; Halter, Fabien; Mazellier, Nicolas; Chauveau, Christian; Gökalp, Iskender
2016-05-01
We propose a model for assessing the unresolved wrinkling factor in the large eddy simulation of turbulent premixed combustion. It relies essentially on a power-law dependence of the wrinkling factor on the filter size and an original expression for the 'active' corrugating strain rate. The latter is written as the turbulent strain multiplied by an efficiency function that accounts for viscous effects and the kinematic constraint of Peters. This yields functional expressions for the fractal dimension and the inner cut-off length scale, the latter being (i) filter-size independent and (ii) consistent with the Damköhler asymptotic behaviours at both large and small Karlovitz numbers. A new expression for the wrinkling factor that incorporates finite Reynolds number effects is further proposed. Finally, the model is successfully assessed on an experimental filtered database.
Application and comparison of two SGS models in large eddy simulation of free turbulent jet flow
NASA Astrophysics Data System (ADS)
Yan, Hong; Su, Mingde
1999-03-01
Large eddy simulations of spatially evolved turbulent round jets were presented. The two SGS models called the standard Smagorinsky's eddy viscosity model and the non-eddy viscosity stimulated small scale (SSS) model developed by Shah & Ferziger were applied. The Reynolds number of the flow was taken as 10000 based on the orifice diameter and the axial velocity in the orifice. The comparison between these two models showed that the standard Smagorinsky's viscosity model with Smagorinsky's constant of 0.1 underestimated the turbulent intensity, while the SSS model showed a better agreement with the experiment. Also the SSS model was used to investigate the development of vortex. The convective boundary condition at the outflow boundary was adopted to ensure less effect of noise on the upstream.
Large-eddy simulation of turbulent flow with a surface-mounted two-dimensional obstacle
NASA Technical Reports Server (NTRS)
Yang, Kyung-Soo; Ferziger, Joel H.
1993-01-01
In this paper, we perform a large eddy simulation (LES) of turbulent flow in a channel containing a two-dimensional obstacle on one wall using a dynamic subgrid-scale model (DSGSM) at Re = 3210, based on bulk velocity above the obstacle and obstacle height; the wall layers are fully resolved. The low Re enables us to perform a DNS (Case 1) against which to validate the LES results. The LES with the DSGSM is designated Case 2. In addition, an LES with the conventional fixed model constant (Case 3) is conducted to allow identification of improvements due to the DSGSM. We also include LES at Re = 82,000 (Case 4) using conventional Smagorinsky subgrid-scale model and a wall-layer model. The results will be compared with the experiment of Dimaczek et al.
Soot prediction by Large-Eddy Simulation of complex geometry combustion chambers
NASA Astrophysics Data System (ADS)
Lecocq, Guillaume; Hernández, Ignacio; Poitou, Damien; Riber, Eléonore; Cuenot, Bénédicte
2013-01-01
This article is dedicated to the modeling of soot production in Large-Eddy Simulations (LES) of complex geometries. Such computations impose a trade-off between accuracy and CPU cost which limits the choice of soot models to semi-empirical ones. As the presence of acetylene is a necessary condition for soot inception, the Leung et al. model that accounts for this feature is chosen and used in this work. However, acetylene concentration is not provided by the reduced chemistries used in LES of complex geometries and a methodology has been developed to predict this key species through a tabulation technique. With this methodology, the model of Leung et al. is first tested and validated against measured laminar premixed flames. Then, the soot prediction method is applied to the LES of the combustion chamber of a helicopter engine.
Development of the Large Eddy Simulation Approach for Modeling Turbulent Flow
NASA Astrophysics Data System (ADS)
Schmidt, R. C.; Smith, T. M.; DesJardin, P. E.; Voth, T. E.; Christon, M. A.
2002-03-01
This report describes research and development of the large eddy simulation (LES) turbulence modeling approach conducted as part of Sandia's laboratory directed research and development (LDRD) program. The emphasis of the work described here has been toward developing the capability to perform accurate and computationally affordable LES calculations of engineering problems using unstructured-grid codes, in wall-bounded geometries and for problems with coupled physics. Specific contributions documented here include (1) the implementation and testing of LES models in Sandia codes, including tests of a new conserved scalar--laminar flamelet SGS combustion model that does not assume statistical independence between the mixture fraction and the scalar dissipation rate, (2) the development and testing of statistical analysis and visualization utility software developed for Exodus II unstructured grid LES, and (3) the development and testing of a novel new LES near-wall subgrid model based on the one-dimensional Turbulence (ODT) model.
Large-Eddy Simulation: Current Capabilities, Recommended Practices, and Future Research
NASA Technical Reports Server (NTRS)
Georgiadis, Nicholas J.; Rizzetta, Donald P.; Fureby, Christer
2009-01-01
This paper presents the results of an activity by the Large Eddy Simulation (LES) Working Group of the AIAA Fluid Dynamics Technical Committee to (1) address the current capabilities of LES, (2) outline recommended practices and key considerations for using LES, and (3) identify future research needs to advance the capabilities and reliability of LES for analysis of turbulent flows. To address the current capabilities and future needs, a survey comprised of eleven questions was posed to LES Working Group members to assemble a broad range of perspectives on important topics related to LES. The responses to these survey questions are summarized with the intent not to be a comprehensive dictate on LES, but rather the perspective of one group on some important issues. A list of recommended practices is also provided, which does not treat all aspects of a LES, but provides guidance on some of the key areas that should be considered.
NASA Astrophysics Data System (ADS)
Park, Hyun Wook; Moon, Kiyoung; Oztekin, Ezgi; McDermott, Randall; Lee, Changhoon; Choi, Jung-Il
2012-11-01
Necessity of the near-wall treatments for the large eddy simulation (LES) without resolving viscous layer is well known for providing a smooth transition from molecular to turbulent transport near wall region. We propose a simple but efficient approach based on modeling of wall shear stress and heat flux that enable accurate predictions of Nusselt number correlations for equilibrium boundary layers. The wall shear stress is directly modeled with Werner and Wengle (1991)'s power law model and wall heat flux is modeled with analogous wall laws between velocity and temperature with Kader (1981)'s empirical correlation. We perform the wall-modeled LES of turbulent convective heat transfer in a channel for various Prandtl numbers. The results show good agreement with the available experimental and numerical data. Supported by WCU (R31-10049) and EDISON (2012-0006663) program of NRF.
Large Eddy Simulation of Wake Vortices in the Convective Boundary Layer
NASA Technical Reports Server (NTRS)
Lin, Yuh-Lang; Han, Jongil; Zhang, Jing; Ding, Feng; Arya, S. Pal; Proctor, Fred H.
2000-01-01
The behavior of wake vortices in a convective boundary layer is investigated using a validated large eddy simulation model. Our results show that the vortices are largely deformed due to strong turbulent eddy motion while a sinusoidal Crow instability develops. Vortex rising is found to be caused by the updrafts (thermals) during daytime convective conditions and increases with increasing nondimensional turbulence intensity eta. In the downdraft region of the convective boundary layer, vortex sinking is found to be accelerated proportional to increasing eta, with faster speed than that in an ideal line vortex pair in an inviscid fluid. Wake vortices are also shown to be laterally transported over a significant distance due to large turbulent eddy motion. On the other hand, the decay rate of the, vortices in the convective boundary layer that increases with increasing eta, is larger in the updraft region than in the downdraft region because of stronger turbulence in the updraft region.
Large-eddy simulation of a three-stream MILD combustion system
NASA Astrophysics Data System (ADS)
Zhang, Jian; Ihme, Matthias; He, Guowei
2011-11-01
Large-eddy simulations (LES) of a three-stream burner system are performed. This burner is operated in the so-called moderate and intense low-oxygen dilution (MILD) combustion regime. An extended flamelet/progress variable (FPV) model is utilized, in which an additional scalar is introduced in order to account for the mixing between the three reactant streams. LES-calculations of three different operating conditions are performed, corresponding to increased levels of oxygen-dilution in the vitiated coflow. The extended FPV model accurately predicts effects of the oxygen-dilution on the flame-structure and heat- release, and model-predictions for temperature and major and minor species are in good agreements with the measurements.
Explaining the convector effect in canopy turbulence by means of large-eddy simulation
NASA Astrophysics Data System (ADS)
Banerjee, Tirtha; De Roo, Frederik; Mauder, Matthias
2017-06-01
Semi-arid forests are found to sustain a massive sensible heat flux in spite of having a low surface to air temperature difference by lowering the aerodynamic resistance to heat transfer (rH) - a property called the canopy convector effect
(CCE). In this work large-eddy simulations are used to demonstrate that the CCE appears more generally in canopy turbulence. It is indeed a generic feature of canopy turbulence: rH of a canopy is found to reduce with increasing unstable stratification, which effectively increases the aerodynamic roughness for the same physical roughness of the canopy. This relation offers a sufficient condition to construct a general description of the CCE. In addition, we review existing parameterizations for rH from the evapotranspiration literature and test to what extent they are able to capture the CCE, thereby exploring the possibility of an improved parameterization.
Large-Eddy Simulation (LES) of a Compressible Mixing Layer and the Significance of Inflow Turbulence
NASA Technical Reports Server (NTRS)
Mankbadi, Mina Reda; Georgiadis, Nicholas J.; Debonis, James R.
2017-01-01
In the context of Large Eddy Simulations (LES), the effects of inflow turbulence are investigated through the Synthetic Eddy Method (SEM). The growth rate of a turbulent compressible mixing layer corresponding to operating conditions of GeobelDutton Case 2 is investigated herein. The effects of spanwise width on the growth rate of the mixing layer is investigated such that spanwise width independence is reached. The error in neglecting inflow turbulence effects is quantified by comparing two methodologies: (1) Hybrid-RANS-LES methodology and (2) SEM-LES methodology. Best practices learned from Case 2 are developed herein and then applied to a higher convective mach number corresponding to Case 4 experiments of GeobelDutton.
Artificial Fluid Properties for Large-Eddy Simulation of Compressible Turbulent Mixing
Cook, A W
2007-01-08
An alternative methodology is described for Large-Eddy Simulation of flows involving shocks, turbulence and mixing. In lieu of filtering the governing equations, it is postulated that the large-scale behavior of an ''LES'' fluid, i.e., a fluid with artificial properties, will be similar to that of a real fluid, provided the artificial properties obey certain constraints. The artificial properties consist of modifications to the shear viscosity, bulk viscosity, thermal conductivity and species diffusivity of a fluid. The modified transport coefficients are designed to damp out high wavenumber modes, close to the resolution limit, without corrupting lower modes. Requisite behavior of the artificial properties is discussed and results are shown for a variety of test problems, each designed to exercise different aspects of the models. When combined with a 10th-order compact scheme, the overall method exhibits excellent resolution characteristics for turbulent mixing, while capturing shocks and material interfaces in crisp fashion.
Explicit filtering and exact reconstruction of the sub-filter stresses in large eddy simulation
NASA Astrophysics Data System (ADS)
Bull, Jonathan R.; Jameson, Antony
2016-02-01
Explicit filtering has the effect of reducing numerical or aliasing errors near the grid scale in large eddy simulation (LES). We use a differential filter, namely the inverse Helmholtz operator, which is readily applied to unstructured meshes. The filter is invertible, which allows the sub-filter scale (SFS) stresses to be exactly reconstructed in terms of the filtered solution. Unlike eddy viscosity models, the method of filtering and reconstruction avoids making any physical assumptions and is therefore valid in any flow regime. The sub-grid scale (SGS) stresses are not recoverable by reconstruction, but the second-order finite element method used here is an adequate source of numerical dissipation in lieu of an SGS model. Results for incompressible turbulent channel flow at Reτ = 180 are presented which show that explicit filtering and exact SFS reconstruction is a significant improvement over the standard LES approach of implicit filtering and eddy-viscosity SGS modelling.
NASA Technical Reports Server (NTRS)
Golubev, Vladimir V.
2003-01-01
The summer fellowship research project focused on further developing an advanced computational technique based on Very Large Eddy Simulation (VLES) for analysis and control of major sources of noise in turbomachinery and propulsion systems, including jet noise and fan noise. Major part of the work during the 10-week tenure dealt with implementing a low-order, implicit A-stable time-stepping scheme in the existing explicit VLES code of Dr. Ray Hixon. The preliminary plan of the work also included application of a new time marching formulation to the problem of viscous gust-airfoil interaction. Other research items selected for implementation (possibly in the future) included investigating a set of new subgrid turbulent models for the code, and code application to a number of test cases, including a supersonic jet and swirling flow downstream of a rotor stage.
A dynamic hybrid subgrid-scale modeling framework for large eddy simulations
NASA Astrophysics Data System (ADS)
Maulik, Romit; San, Omer
2016-11-01
We put forth a dynamic modeling framework for sub-grid parameterization of large eddy simulation of turbulent flows based upon the use of the approximate deconvolution (AD) procedure to compute the eddy viscosity constant self-adaptively from the resolved flow quantities. In our proposed framework, the test filtering process of the standard dynamic model is replaced by the AD procedure and a posteriori error analysis is performed. The robustness of the model has been tested considering the Burgers, Kraichnan, Kolmogorov turbulence problems. Our numerical assessments for solving these canonical decaying turbulence problems show that the proposed approach could be used as a viable tool to address the turbulence closure problem due to its flexibility.
Large eddy simulation of flows around ground vehicles and other bluff bodies.
Krajnovic, Sinisa
2009-07-28
A brief review of large eddy simulation (LES) applications for different bluff-body flows performed by the author and his co-workers is presented. Examples of flows range from simple cube flows characterized by sharp edge separation over a three-dimensional hill where LES relies on good near-wall resolution, to complex flows of a tall, finite cylinder that contains several flow regimes that cause different challenges to LES. The second part of the paper is devoted to flows around ground vehicles at moderate Reynolds numbers. Although the present review proves the applicability of LES for various bluff-body flows, an increase of the Reynolds number towards the operational speeds of ground vehicles requires accurate near-wall modelling for a successful LES.
Preliminary large-eddy simulations of flow around a NACA 4412 airfoil using unstructured grids
NASA Technical Reports Server (NTRS)
Jansen, Kenneth
1995-01-01
Large-eddy simulation (LES) has matured to the point where application to complex flows is desirable. The extension to higher Reynolds numbers leads to an impractical number of grid points with existing structured-grid methods. Furthermore, most real world flows are rather difficult to represent geometrically with structured grids. Unstructured-grid methods offer a release from both of these constraints. However, just as it took many years for structured-grid methods to be well understood and reliable tools for LES, unstructured-grid methods must be carefully studied before we can expect them to attain their full potential. In the past two years, important building blocks have been put into place making possible a careful study of LES on unstructured grids. The first building block was an efficient mesh generator which allowed the placement of points according to smooth variation of physical length scales. This variation of length scales is in all three directions independently, which allows a large reduction in points when compared to structured-grid methods, which can only vary length scales in one direction at a time. The second building block was the development of a dynamic model appropriate for unstructured grids. The principle obstacle was the development of an unstructured-grid filtering operator. In the past year, some of the new filters developed by Jansen have been implemented into a highly parallelized finite element code based on the Galerkin/least-squares finite element method. We have chosen the NACA 4412 airfoil at maximum lift as the first simulation for a variety of reasons. First, it is a problem of significant interest since it would be the first LES of an aircraft component. Second, this flow has been the subject of three experimental studies. The third reason for considering this flow is the variety of flow features which provide an important test of the dynamic model. Only the dynamic model can be expected to perform satisfactorily in this
Large-Eddy Simulation on Turbulent Flow and Plume Dispersion over A 2-Dimensional Hill
NASA Astrophysics Data System (ADS)
Nakayama, H.; Nagai, H.
2009-09-01
The dispersion analysis of airborne contaminant including radioactive substances from industrial or nuclear facilities is the important issue for maintenance of air quality or safety assessment. Many studies on the plume dispersion behavior in the simulated atmospheric boundary layer flow over flat plain have been conducted mainly by wind tunnel experiments. However, many nuclear power plants are located at complex coastal terrains in Japan. In this case, topographical effects on the turbulent flow and plume dispersion should be investigated. Therefore, we perform Large-Eddy Simulations (LES) on turbulent flows and plume dispersions. As the first step of this study, we try it for a 2-dimensional hill flow and investigate the characteristics of mean and fluctuation concentrations. In order to produce a realistic turbulent boundary layer flow, we set up two computational domains. One is the driver region for generating the spatially-developing boundary layer flow and the other is the main computational region for plume dispersion over a 2-dimensional hill. Near the inlet of the driver region, roughness blocks for generating a thick turbulent boundary layer are placed. The inflow turbulence data obtained in the driver region are imposed at the inlet of the main computational domain at each time step. The sizes of driver and main computational regions are 46.25H×6.25H×25.0H and 37.5H×6.25H×25.0H (H: the height of a 2-dimensional hill) in x-, y- and z-directions, respectively. Here, x, y and z indicate streamwise, spanwise and vertical directions, respectively. The number of grid points of driver and main computational regions are 400×100×90 and 400×100×90 in x, y and z directions, respectively. A 2-dimensional hill model has 5.45H length and are placed at a distance of 10.4H downstream from the inlet of the main computational region. A release point of a tracer gas is located at a distance of 9.09H upstream from a 2-dimensional hill top and elevated with the
Large eddy simulation for atmospheric boundary layer flow over flat and complex terrains
NASA Astrophysics Data System (ADS)
Han, Yi; Stoellinger, Michael; Naughton, Jonathan
2016-09-01
In this work, we present Large Eddy Simulation (LES) results of atmospheric boundary layer (ABL) flow over complex terrain with neutral stratification using the OpenFOAM-based simulator for on/offshore wind farm applications (SOWFA). The complete work flow to investigate the LES for the ABL over real complex terrain is described including meteorological-tower data analysis, mesh generation and case set-up. New boundary conditions for the lateral and top boundaries are developed and validated to allow inflow and outflow as required in complex terrain simulations. The turbulent inflow data for the terrain simulation is generated using a precursor simulation of a flat and neutral ABL. Conditionally averaged met-tower data is used to specify the conditions for the flat precursor simulation and is also used for comparison with the simulation results of the terrain LES. A qualitative analysis of the simulation results reveals boundary layer separation and recirculation downstream of a prominent ridge that runs across the simulation domain. Comparisons of mean wind speed, standard deviation and direction between the computed results and the conditionally averaged tower data show a reasonable agreement.
Large-eddy simulation for flow and dispersion in urban streets
NASA Astrophysics Data System (ADS)
Xie, Zheng-Tong; Castro, Ian P.
Large-eddy simulations (LES) with our recently developed inflow approach (Xie, Z.-T., Castro, I.P., 2008a. Efficient generation of inflow conditions for large-eddy simulation of street-scale flows. Flow Turbul. Combust., vol. 81(3), pp. 449-470.) have been used for flow and dispersion within a genuine city area - the DAPPLE site, located at the intersection of Marylebone Rd and Gloucester Pl in Central London. Numerical results up to second-order statistics are reported for a computational domain of 1.2 km (streamwise) × 0.8 km (lateral) × 0.2 km (in full scale), with a resolution down to approximately one meter in space and one second in time. They are in reasonable agreement with the experimental data. Such a comprehensive urban geometry is often, as here, composed of staggered, aligned, square arrays of blocks with non-uniform height and non-uniform base, street canyons and intersections. Both the integrative and local effect of flow and dispersion to these geometrical patterns were investigated. For example, it was found that the peaks of spatially averaged u rms, v rms, w rms and < u' w'> occurred neither at the mean height nor at the maximum height, but at the height of large and tall buildings. It was also found that the mean and fluctuating concentrations in the near-source field is highly dependent on the source location and the local geometry pattern, whereas in the far field (e.g. >0.1 km) they are not. In summary, it is demonstrated that full-scale resolution of around one meter is sufficient to yield accurate prediction of the flow and mean dispersion characteristics and to provide reasonable estimation of concentration fluctuations.
Large-eddy Simulation of the Nighttime Stable Atmospheric Boundary Layer
NASA Astrophysics Data System (ADS)
Zhou, Bowen
A stable atmospheric boundary layer (ABL) develops over land at night due to radiative surface cooling. The state of turbulence in the stable boundary layer (SBL) is determined by the competing forcings of shear production and buoyancy destruction. When both forcings are comparable in strength, the SBL falls into an intermittently turbulent state, where intense turbulent bursts emerge sporadically from an overall quiescent background. This usually occurs on clear nights with weak winds when the SBL is strongly stable. Although turbulent bursts are generally short-lived (half an hour or less), their impact on the SBL is significant since they are responsible for most of the turbulent mixing. The nighttime SBL can be modeled with large-eddy simulation (LES). LES is a turbulence-resolving numerical approach which separates the large-scale energy-containing eddies from the smaller ones based on application of a spatial filter. While the large eddies are explicitly resolved, the small ones are represented by a subfilter-scale (SFS) stress model. Simulation of the SBL is more challenging than the daytime convective boundary layer (CBL) because nighttime turbulent motions are limited by buoyancy stratification, thus requiring fine grid resolution at the cost of immense computational resources. The intermittently turbulent SBL adds additional levels of complexity, requiring the model to not only sustain resolved turbulence during quiescent periods, but also to transition into a turbulent state under appropriate conditions. As a result, LES of the strongly stable SBL potentially requires even finer grid resolution, and has seldom been attempted. This dissertation takes a different approach. By improving the SFS representation of turbulence with a more sophisticated model, intermittently turbulent SBL is simulated, to our knowledge, for the first time in the LES literature. The turbulence closure is the dynamic reconstruction model (DRM), applied under an explicit filtering
Large-eddy simulation of flows over idealized urban areas in thermal stratification
NASA Astrophysics Data System (ADS)
Chan, Mr.; Liu, Dr.
2012-04-01
Large-eddy simulation (LES) equipped with the one-equation subgrid-scale model was employed to investigate the mean wind and turbulence over idealized two-dimensional (2D) street canyons in various thermal stratifications. The prevailing wind is driven by a background pressure gradient above the roof level that is perpendicular to the axis of street canyons. The building-height-to-street-width (aspect) ratio is kept unity so the flows fall into the skimming flow regime. Cyclic boundary conditions are assigned to the domain inlet and outlet, simulating the infinite horizontally homogenous building structures. The buoyancy force is modeled by Boussinesq approximation. Building geometry is the key factor governing the wind flow behaviors aloft. Its effects on the flow structures in isothermal conditions are widely studied. Whereas, thermal stratification, which is caused by the temperature difference between the urban fabrics and the prevailing wind, plays another important role in the wind flow behaviors. The presence of buoyancy force drives/suppresses convective flows that substantially modify the vertical transport processes. In stable stratification, e.g. nighttime, a cooler urban surface favors subsidence that reduces turbulence intensities subsequently. While in unstable stratification, e.g. daytime, a hotter urban surface induces buoyancy and convective current that in turn promotes turbulence. In isothermal conditions, it is well known that the mean flows exhibit a log-law region over an aerodynamically smooth surface where the mean wind profile is in a logarithmic form (law of the wall). The logarithmic wind profile is also observed over homogenously rough surfaces, such as the idealized two-dimensional street canyons used in this study. However, in thermal stratification (both stable and unstable), the mean wind profile deviates from the isothermal one whose extent depends on the intensity of thermal stratification compared with the mean wind (measured by
Large-scale large eddy simulation of nuclear reactor flows: Issues and perspectives
Merzari, Elia; Obabko, Aleks; Fischer, Paul; ...
2016-11-03
Numerical simulation has been an intrinsic part of nuclear engineering research since its inception. In recent years a transition is occurring toward predictive, first-principle-based tools such as computational fluid dynamics. Even with the advent of petascale computing, however, such tools still have significant limitations. In the present work some of these issues, and in particular the presence of massive multiscale separation, are discussed, as well as some of the research conducted to mitigate them. Petascale simulations at high fidelity (large eddy simulation/direct numerical simulation) were conducted with the massively parallel spectral element code Nek5000 on a series of representative problems.more » These simulations shed light on the requirements of several types of simulation: (1) axial flow around fuel rods, with particular attention to wall effects; (2) natural convection in the primary vessel; and (3) flow in a rod bundle in the presence of spacing devices. Finally, the focus of the work presented here is on the lessons learned and the requirements to perform these simulations at exascale. Additional physical insight gained from these simulations is also emphasized.« less
Large-scale large eddy simulation of nuclear reactor flows: Issues and perspectives
Merzari, Elia; Obabko, Aleks; Fischer, Paul; Halford, Noah; Walker, Justin; Siegel, Andrew; Yu, Yiqi
2016-11-03
Numerical simulation has been an intrinsic part of nuclear engineering research since its inception. In recent years a transition is occurring toward predictive, first-principle-based tools such as computational fluid dynamics. Even with the advent of petascale computing, however, such tools still have significant limitations. In the present work some of these issues, and in particular the presence of massive multiscale separation, are discussed, as well as some of the research conducted to mitigate them. Petascale simulations at high fidelity (large eddy simulation/direct numerical simulation) were conducted with the massively parallel spectral element code Nek5000 on a series of representative problems. These simulations shed light on the requirements of several types of simulation: (1) axial flow around fuel rods, with particular attention to wall effects; (2) natural convection in the primary vessel; and (3) flow in a rod bundle in the presence of spacing devices. Finally, the focus of the work presented here is on the lessons learned and the requirements to perform these simulations at exascale. Additional physical insight gained from these simulations is also emphasized.
Large eddy simulation of surface pressure fluctuations generated by elevated gusts
NASA Astrophysics Data System (ADS)
Cain, Jericho E.
Wind gusts cause substantial damage to wind turbines. If these damaging winds could be detected prior to their interaction with the turbine, the turbine rotor can be decoupled from the generator and gearing system to prevent damage during the gust event. This would significantly reduce wind turbine repair costs. Wind gusts can also create unsafe conditions for aircraft landing. A ground based detection system that monitored elevated wind gusts can provide new information for pilots to use when determining whether or not it is safe to land. In addition, the ability to monitor elevated gust events would provide a new probe to study features in the atmospheric boundary layer. Previous research indicates that elevated velocity events, such as gusts, may trigger pressure fluctuations on the ground. If that is true, it should be possible to monitor elevated wind gusts by measuring these pressure fluctuations. The goal of this project is to develop a ground based detector that monitors the behavior of pressure fluctuations on the ground for indicators that a gust event may be taking place at higher altitudes. In order to recognize these indicators from the pressure measurements on the ground, cross-correlation analysis between the time evolution of the frequency structures corresponding to elevated wind gusts and the pressure on the ground below were investigated. The data for these analysis was generated using a large eddy simulation. This numerical approach was chosen because the nature of the cross-correlation analysis demanded full field wind velocities and pressures at several altitudes. Collecting this data outdoors would be impractical. Correlation coefficients between 0.75 - 0.90 were found. These high correlations indicate that the two signals are causally related. Several common features of the pressures caused by elevated gusts were identified. These features were used to develop a tracking program that monitors fast moving high amplitude pressure fluctuations
NASA Astrophysics Data System (ADS)
Canuto, V. M.
1994-06-01
The Reynolds numbers that characterize geophysical and astrophysical turbulence (Re approximately equals 108 for the planetary boundary layer and Re approximately equals 1014 for the Sun's interior) are too large to allow a direct numerical simulation (DNS) of the fundamental Navier-Stokes and temperature equations. In fact, the spatial number of grid points N approximately Re9/4 exceeds the computational capability of today's supercomputers. Alternative treatments are the ensemble-time average approach, and/or the volume average approach. Since the first method (Reynolds stress approach) is largely analytical, the resulting turbulence equations entail manageable computational requirements and can thus be linked to a stellar evolutionary code or, in the geophysical case, to general circulation models. In the volume average approach, one carries out a large eddy simulation (LES) which resolves numerically the largest scales, while the unresolved scales must be treated theoretically with a subgrid scale model (SGS). Contrary to the ensemble average approach, the LES+SGS approach has considerable computational requirements. Even if this prevents (for the time being) a LES+SGS model to be linked to stellar or geophysical codes, it is still of the greatest relevance as an 'experimental tool' to be used, inter alia, to improve the parameterizations needed in the ensemble average approach. Such a methodology has been successfully adopted in studies of the convective planetary boundary layer. Experienc e with the LES+SGS approach from different fields has shown that its reliability depends on the healthiness of the SGS model for numerical stability as well as for physical completeness. At present, the most widely used SGS model, the Smagorinsky model, accounts for the effect of the shear induced by the large resolved scales on the unresolved scales but does not account for the effects of buoyancy, anisotropy, rotation, and stable stratification. The latter phenomenon
NASA Astrophysics Data System (ADS)
Basu, Sukanta; Lacser, Avraham
2017-01-01
In several recent large-eddy simulation studies, the lowest grid level was located well within the roughness sublayer. Monin-Obukhov similarity-based boundary conditions cannot be used under this scenario, and in this note we elaborate on this fundamental problem and suggest potential solutions.
NASA Astrophysics Data System (ADS)
Wang, Haifeng; Popov, Pavel; Hiremath, Varun; Lantz, Steven; Viswanathan, Sharadha; Pope, Stephen
2010-11-01
A large-eddy simulation (LES)/probability density function (PDF) code is developed and applied to the study of local extinction and re-ignition in Sandia Flame E. The modified Curl mixing model is used to account for the sub-filter scalar mixing; the ARM1 mechanism is used for the chemical reaction; and the in- situ adaptive tabulation (ISAT) algorithm is used to accelerate the chemistry calculations. Calculations are performed on different grids to study the resolution requirement for this flame. Then, with sufficient grid resolution, full-scale LES/PDF calculations are performed to study the flame characteristics and the turbulence-chemistry interactions. Sensitivity to the mixing frequency model is explored in order to understand the behavior of sub-filter scalar mixing in the context of LES. The simulation results are compared to the experimental data to demonstrate the capability of the code. Comparison is also made to previous RANS/PDF simulations.
Large eddy simulation model for wind-driven sea circulation in coastal areas
NASA Astrophysics Data System (ADS)
Petronio, A.; Roman, F.; Nasello, C.; Armenio, V.
2013-12-01
In the present paper a state-of-the-art large eddy simulation model (LES-COAST), suited for the analysis of water circulation and mixing in closed or semi-closed areas, is presented and applied to the study of the hydrodynamic characteristics of the Muggia bay, the industrial harbor of the city of Trieste, Italy. The model solves the non-hydrostatic, unsteady Navier-Stokes equations, under the Boussinesq approximation for temperature and salinity buoyancy effects, using a novel, two-eddy viscosity Smagorinsky model for the closure of the subgrid-scale momentum fluxes. The model employs: a simple and effective technique to take into account wind-stress inhomogeneity related to the blocking effect of emerged structures, which, in turn, can drive local-scale, short-term pollutant dispersion; a new nesting procedure to reconstruct instantaneous, turbulent velocity components, temperature and salinity at the open boundaries of the domain using data coming from large-scale circulation models (LCM). Validation tests have shown that the model reproduces field measurement satisfactorily. The analysis of water circulation and mixing in the Muggia bay has been carried out under three typical breeze conditions. Water circulation has been shown to behave as in typical semi-closed basins, with an upper layer moving along the wind direction (apart from the anti-cyclonic veering associated with the Coriolis force) and a bottom layer, thicker and slower than the upper one, moving along the opposite direction. The study has shown that water vertical mixing in the bay is inhibited by a large level of stable stratification, mainly associated with vertical variation in salinity and, to a minor extent, with temperature variation along the water column. More intense mixing, quantified by sub-critical values of the gradient Richardson number, is present in near-coastal regions where upwelling/downwelling phenomena occur. The analysis of instantaneous fields has detected the presence of
Investigation of natural gas plume dispersion using mobile observations and large eddy simulations
NASA Astrophysics Data System (ADS)
Caulton, Dana R.; Li, Qi; Golston, Levi; Pan, Da; Bou-Zeid, Elie; Fitts, Jeff; Lane, Haley; Lu, Jessica; Zondlo, Mark A.
2016-04-01
Recent work suggests the distribution of methane emissions from fracking operations is skewed with a small percentage of emitters contributing a large proportion of the total emissions. These sites are known as 'super-emitters.' The Marcellus shale, the most productive natural gas shale field in the United States, has received less intense focus for well-level emissions and is here used as a test site for targeted analysis between current standard trace-gas advection practices and possible improvements via advanced modeling techniques. The Marcellus shale is topographically complex, making traditional techniques difficult to implement and evaluate. For many ground based mobile studies, the inverse Gaussian plume method (IGM) is used to produce emission rates. This method is best applied to well-mixed plumes from strong point sources and may not currently be well-suited for use with disperse weak sources, short-time frame measurements or data collected in complex terrain. To assess the quality of IGM results and to improve source-strength estimations, a robust study that combines observational data with a hierarchy of models of increasing complexity will be presented. The field test sites were sampled with multiple passes using a mobile lab as well as a stationary tower. This mobile lab includes a Garmin GPS unit, Vaisala weather station (WTX520), LICOR 7700 CH4 open path sensor and LICOR 7500 CO2/H2O open path sensor. The sampling tower was constructed consisting of a Metek uSonic-3 Class A sonic anemometer, and an additional LICOR 7700 and 7500. Data were recorded for at least one hour at these sites. The modeling will focus on large eddy simulations (LES) of the wind and CH4 concentration fields for these test sites. The LES model used 2 m horizontal and 1 m vertical resolution and was integrated in time for 45 min for various test sites under stable, neutral and unstable conditions. It is here considered as the reference to which various IGM approaches can be
Subfilter Scale Combustion Modelling for Large Eddy Simulation of Turbulent Premixed Flames
NASA Astrophysics Data System (ADS)
Shahbazian, Nasim
Large eddy simulation (LES) is a powerful computational tool for modelling turbulent combustion processes. However, for reactive flows, LES is still under significant development. In particular, for turbulent premixed flames, a considerable complication of LES is that the flame thickness is generally much smaller than the LES filter width such that the flame front and chemical reactions cannot be resolved on the grid. Accurate and robust subfilter-scale (SFS) models of the unresolved turbulence-chemistry interactions are therefore required and studies are needed to evaluate and improve them. In this thesis, a detailed comparison and evaluation of five different SFS models for turbulence- chemistry interactions in LES of premixed flames is presented. These approaches include both flamelet- and non-flamelet-based models, coupled with simple or tabulated chemistry. The mod- elling approaches considered herein are: algebraic- and transport-equation variants of the flame surface density (FSD) model, the presumed conditional moment (PCM) with flame prolongation of intrinsic low-dimensional manifold (FPI) tabulated chemistry, or PCM-FPI approach, evaluated with two different presumed probability density function (PDF) models; and conditional source-term estimation (CSE) approach. The predicted LES solutions are compared to the existing laboratory-scale experimental observation of Bunsen-type turbulent premixed methane-air flames, corresponding to lean and stoichiometric conditions lying from the upper limit of the flamelet regime to well within the thin reaction zones regime of the standard regimes diagram. Direct comparison of different SFS approaches allows investigation of stability and performance of the models, while the weaknesses and strengths of each approach are identified. Evaluation of algebraic and transported FSD models highlights the importance of non-equilibrium transport in turbulent premixed flames. The effect of the PDF type for the reaction progress
Subfilter scale combustion modelling for large eddy simulation of turbulent premixed flames
NASA Astrophysics Data System (ADS)
Shahbazian, Nasim
Large eddy simulation (LES) is a powerful computational tool for modelling turbulent combustion processes. However, for reactive flows, LES is still under significant development. In particular, for turbulent premixed flames, a considerable complication of LES is that the flame thickness is generally much smaller than the LES filter width such that the flame front and chemical reactions cannot be resolved on the grid. Accurate and robust subfilter-scale (SFS) models of the unresolved turbulence-chemistry interactions are therefore required and studies are needed to evaluate and improve them. In this thesis, a detailed comparison and evaluation of five different SFS models for turbulence-chemistry interactions in LES of premixed flames is presented. These approaches include both flamelet- and non-flamelet-based models, coupled with simple or tabulated chemistry. The modelling approaches considered herein are: algebraic- and transport-equation variants of the flame surface density (FSD) model, the presumed conditional moment (PCM) with flame prolongation of intrinsic low-dimensional manifold (FPI) tabulated chemistry, or PCM-FPI approach, evaluated with two different presumed probability density function (PDF) models; and conditional source-term estimation (CSE) approach. The predicted LES solutions are compared to the existing laboratory-scale experimental observation of Bunsen-type turbulent premixed methane-air flames, corresponding to lean and stoichiometric conditions lying from the upper limit of the flamelet regime to well within the thin reaction zones regime of the standard regimes diagram. Direct comparison of different SFS approaches allows investigation of stability and performance of the models, while the weaknesses and strengths of each approach are identified. Evaluation of algebraic and transported FSD models highlights the importance of non-equilibrium transport in turbulent premixed flames. The effect of the PDF type for the reaction progress
Hybrid Large-Eddy/Reynolds-Averaged Simulation of a Supersonic Cavity Using VULCAN
NASA Technical Reports Server (NTRS)
Quinlan, Jesse; McDaniel, James; Baurle, Robert A.
2013-01-01
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 a 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 the 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 indicate good agreement on the finest grid. Simulations are repeated on a coarsened grid, and results indicate strong grid density sensitivity. Simulations are performed with and without inflow turbulence recycling on the coarse grid to isolate the effect of the recycling procedure, which is demonstrably critical to capturing the relevant shear layer dynamics. Shock sensor formulations of Ducros and Larsson are found to predict mean flow statistics equally well.
A Parallel, Finite-Volume Algorithm for Large-Eddy Simulation of Turbulent Flows
NASA Technical Reports Server (NTRS)
Bui, Trong T.
1999-01-01
A parallel, finite-volume algorithm has been developed for large-eddy simulation (LES) of compressible turbulent flows. This algorithm includes piecewise linear least-square reconstruction, trilinear finite-element interpolation, Roe flux-difference splitting, and second-order MacCormack time marching. Parallel implementation is done using the message-passing programming model. In this paper, the numerical algorithm is described. To validate the numerical method for turbulence simulation, LES of fully developed turbulent flow in a square duct is performed for a Reynolds number of 320 based on the average friction velocity and the hydraulic diameter of the duct. Direct numerical simulation (DNS) results are available for this test case, and the accuracy of this algorithm for turbulence simulations can be ascertained by comparing the LES solutions with the DNS results. The effects of grid resolution, upwind numerical dissipation, and subgrid-scale dissipation on the accuracy of the LES are examined. Comparison with DNS results shows that the standard Roe flux-difference splitting dissipation adversely affects the accuracy of the turbulence simulation. For accurate turbulence simulations, only 3-5 percent of the standard Roe flux-difference splitting dissipation is needed.
Large eddy simulation and direct numerical simulation of high speed turbulent reacting flows
NASA Technical Reports Server (NTRS)
Adumitroaie, V.; Frankel, S. H.; Madnia, C. K.; Givi, P.
1993-01-01
The objective of this research is to make use of Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) for the computational analyses of high speed reacting flows. Our efforts in the first phase of this research conducted within the past three years have been directed in several issues pertaining to intricate physics of turbulent reacting flows. In our previous 5 semi-annual reports submitted to NASA LaRC, as well as several technical papers in archival journals, the results of our investigations have been fully described. In this progress report which is different in format as compared to our previous documents, we focus only on the issue of LES. The reason for doing so is that LES is the primary issue of interest to our Technical Monitor and that our other findings were needed to support the activities conducted under this prime issue. The outcomes of our related investigations, nevertheless, are included in the appendices accompanying this report. The relevance of the materials in these appendices are, therefore, discussed only briefly within the body of the report. Here, results are presented of a priori and a posterior analyses for validity assessments of assumed Probability Density Function (PDF) methods as potential subgrid scale (SGS) closures for LES of turbulent reacting flows. Simple non-premixed reacting systems involving an isothermal reaction of the type A + B yields Products under both chemical equilibrium and non-equilibrium conditions are considered. A priori analyses are conducted of a homogeneous box flow, and a spatially developing planar mixing layer to investigate the performance of the Pearson Family of PDF's as SGS models. A posteriori analyses are conducted of the mixing layer using a hybrid one-equation Smagorinsky/PDF SGS closure. The Smagorinsky closure augmented by the solution of the subgrid turbulent kinetic energy (TKE) equation is employed to account for hydrodynamic fluctuations, and the PDF is employed for modeling the
Large eddy simulation and direct numerical simulation of high speed turbulent reacting flows
NASA Astrophysics Data System (ADS)
Adumitroaie, V.; Frankel, S. H.; Madnia, C. K.; Givi, P.
The objective of this research is to make use of Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) for the computational analyses of high speed reacting flows. Our efforts in the first phase of this research conducted within the past three years have been directed in several issues pertaining to intricate physics of turbulent reacting flows. In our previous 5 semi-annual reports submitted to NASA LaRC, as well as several technical papers in archival journals, the results of our investigations have been fully described. In this progress report which is different in format as compared to our previous documents, we focus only on the issue of LES. The reason for doing so is that LES is the primary issue of interest to our Technical Monitor and that our other findings were needed to support the activities conducted under this prime issue. The outcomes of our related investigations, nevertheless, are included in the appendices accompanying this report. The relevance of the materials in these appendices are, therefore, discussed only briefly within the body of the report. Here, results are presented of a priori and a posterior analyses for validity assessments of assumed Probability Density Function (PDF) methods as potential subgrid scale (SGS) closures for LES of turbulent reacting flows. Simple non-premixed reacting systems involving an isothermal reaction of the type A + B yields Products under both chemical equilibrium and non-equilibrium conditions are considered. A priori analyses are conducted of a homogeneous box flow, and a spatially developing planar mixing layer to investigate the performance of the Pearson Family of PDF's as SGS models. A posteriori analyses are conducted of the mixing layer using a hybrid one-equation Smagorinsky/PDF SGS closure. The Smagorinsky closure augmented by the solution of the subgrid turbulent kinetic energy (TKE) equation is employed to account for hydrodynamic fluctuations, and the PDF is employed for modeling the
NASA Astrophysics Data System (ADS)
Hernandez Perez, Francisco Emanuel
Hydrogen (H2) enrichment of hydrocarbon fuels in lean premixed systems is desirable since it can lead to a progressive reduction in greenhouse-gas emissions, while paving the way towards pure hydrogen combustion. In recent decades, large-eddy simulation (LES) has emerged as a promising tool to computationally describe and represent turbulent combustion processes. However, a considerable complication of LES for turbulent premixed combustion is that chemical reactions occur in a thin reacting layer at small scales which cannot be entirely resolved on computational grids and need to be modelled. In this thesis, subfilter-scale (SFS) modelling for LES of lean H 2-enriched methane-air turbulent premixed combustion was investigated. Two- and three-dimensional fully-compressible LES solvers for a thermally perfect reactive mixture of gases were developed and systematically validated. Two modelling strategies for the chemistry-turbulence interaction were pursued: the artificially thickened flame model with a power-law SFS wrinkling approach and the presumed conditional moment (PCM) coupled with the flame prolongation of intrinsic low-dimensional manifold (FPI) chemistry tabulation technique. Freely propagating and Bunsen-type flames corresponding to stoichiometric and lean premixed mixtures were considered. Validation of the LES solvers was carried out by comparing predicted solutions with experimental data and other published numerical results. Head-to-head comparisons of different SFS approaches, including a transported flame surface density (FSD) model, allowed to identify weaknesses and strengths of the various models. Based on the predictive capabilities of the models examined, the PCM-FPI model was selected for the study of hydrogen-enrichment of methane. A new progress of reaction variable was proposed to account for NO. The importance of transporting species with different diffusion coefficients was demonstrated, in particular for H2. The proposed approach was
NASA Astrophysics Data System (ADS)
Witte, M.; Chuang, P. Y.; Wang, L. P.; Ayala, O.
2014-12-01
Drizzle occurs frequently in shallow, warm boundary layer clouds. For example, in stratocumulus it occurs approximately 1/3 of the time in full cloud cover conditions (Wood 2012). Drizzle affects moisture and energy budgets, and cloud albedo, morphology and lifetime. At the cloud scale, processes that control drizzle formation include turbulence production via radiative cooling and/or shear, entrainment, and surface moisture fluxes. At the micro-scale, collision-coalescence is the primary process relevant to warm drizzle formation. Differential gravitational sedimentation and turbulent air motions cause cloud droplets to collide, creating drops much larger than can be formed by condensation alone. Other factors, such as preferential concentration and entrainment mixing may also be relevant. The process is typically subdivided into three regimes: autoconversion (small drops self-collide), accretion (large drops collect small drops), and hydrometeor self-collection (large drops self-collide). Of these regimes, autoconversion is the rate-limiting step in existing analytical representations. This study (i) evaluates whether our best theoretical understanding of collision-coalescence in the autoconversion regime can replicate observations, with a broader goal of (ii) exploring which cloud-scale factors are most important for drizzle initiation. A state-of-the-art turbulent collisional growth model is applied to a bin microphysics scheme within a large-eddy simulation such that the full range of cloud drop growth mechanisms are represented (i.e. CCN activation, condensation, collision-coalescence, mixing, etc.) at realistic atmospheric conditions. We compare cloud drop spectra produced by the LES with observations to assess the quality and limits of our theoretical knowledge. The comparison will be performed over a range of observational cases that span a range of drizzle rates. These cases differ in their radiative cooling rates, shear, cloud-top temperature and
Dynamics of Atmospheric Boundary Layers: Large-Eddy Simulations and Reduced Analytical Models
NASA Astrophysics Data System (ADS)
Momen, Mostafa
Real-world atmospheric and oceanic boundary layers (ABL) involve many inherent complexities, the understanding and modeling of which manifestly exceeds our current capabilities. Previous studies largely focused on the "textbook ABL", which is (quasi) steady and barotropic. However, it is evident that the "real-world ABL", even over flat terrain, rarely meets such simplifying assumptions. The present thesis aims to illustrate and model four complicating features of ABLs that have been overlooked thus far despite their ubiquity: 1) unsteady pressure gradients in neutral ABLs (Chapters 2 and 3), 2) interacting effects of unsteady pressure gradients and static stability in diabatic ABLs (Chapter 4), 3) time-variable buoyancy fluxes (Chapter 5) , and 4) impacts of baroclinicity in neutral and diabatic ABLs (Chapter 6). State-of-the-art large-eddy simulations will be used as a tool to explain the underlying physics and to validate analytical models we develop for these features. Chapter 2 focuses on the turbulence equilibrium: when the forcing time scale is comparable to the turbulence time scale, the turbulence is shown to be out of equilibrium, and the velocity profiles depart from the log-law; However, for longer, and surprisingly for shorter forcing times, quasi-equilibrium is maintained. In Chapter 3, a reduced analytical model, based on the Navier-Stokes equations, will be introduced and shown to be analogous to a damped oscillator where inertial, Coriolis, and friction forces mirror the mass, spring, and damper, respectively. When a steady buoyancy (stable or unstable) is superposed on the unsteady pressure gradient, the same model structure can be maintained, but the damping term, corresponding to friction forces and vertical coupling, needs to account for stability. However, for the reverse case with variable buoyancy flux and stability, the model needs to be extended to allow time-variable damper coefficient. These extensions of the analytical model are
Effect of grid-filter width definition on implicitly filtered large eddy simulations using OpenFOAM
NASA Astrophysics Data System (ADS)
Beechook, A.; Medina, H.; Fadhila, H.; Aleksandrova, S.; Benjamin, S.
2017-07-01
Turbulent flow over a 3D backward-facing step at a step-height Reynolds number, Reh, of 5100 is simulated in OpenFOAM using Large Eddy Simulation (LES) to investigate how the definition of the grid-filter width, Δ, influences the computed solutions. As part of this study, two different Δ definitions are tested: Δ = √(Δx Δy Δz3) and Δ = max(Δx, Δy, Δz). The LES results show good agreement when compared with DNS and experimental data. The simulation results are used to further evaluate the effect of grid non-uniformity on the computation of the subgrid-scale eddy viscosity, νSGS.
Large-eddy Simulation of the Near-lip of a Jet
NASA Astrophysics Data System (ADS)
Bohr, Elaine; Yaworski, Michael; Jansen, Kenneth
2003-11-01
A M=0.6, Re=1.0 million cold jet flow with complex geometry is simulated to obtain high-fidelity near-field data and accurate dynamic information on the flow. Large Eddy Simulation (LES) on an unstructured grid is optimal for near-nozzle flow simulation. The modeled problem is a single-stream jet exiting a nozzle which can have tabs. It is too costly to simulate the full problem so the meshed domain is a representative sector of the flow with limited stream-wise extent. The jet flow is simulated using a stable, accurate, finite element method with hierarchic spatial basis, generalized-alpha method and 2nd order time integrator which yields accurate well controlled stabilization. A RANS solution is used as the inflow condition where velocity and temperature are specified for the jet and the entrainment. As RANS only gives averaged quantities the inflow boundary condition needs to be completed by specifying the fluctuations using scaled plane extraction boundary condition (SPEBC). The solution is rescaled from an internal downstream position using self-similarity flow profiles in turbulent boundary layers. This talk will show the need for SPEBC and present preliminary results.
Large eddy simulation for predicting turbulent heat transfer in gas turbines.
Tafti, Danesh K; He, Long; Nagendra, K
2014-08-13
Blade cooling technology will play a critical role in the next generation of propulsion and power generation gas turbines. Accurate prediction of blade metal temperature can avoid the use of excessive compressed bypass air and allow higher turbine inlet temperature, increasing fuel efficiency and decreasing emissions. Large eddy simulation (LES) has been established to predict heat transfer coefficients with good accuracy under various non-canonical flows, but is still limited to relatively simple geometries and low Reynolds numbers. It is envisioned that the projected increase in computational power combined with a drop in price-to-performance ratio will make system-level simulations using LES in complex blade geometries at engine conditions accessible to the design process in the coming one to two decades. In making this possible, two key challenges are addressed in this paper: working with complex intricate blade geometries and simulating high-Reynolds-number (Re) flows. It is proposed to use the immersed boundary method (IBM) combined with LES wall functions. A ribbed duct at Re=20 000 is simulated using the IBM, and a two-pass ribbed duct is simulated at Re=100 000 with and without rotation (rotation number Ro=0.2) using LES with wall functions. The results validate that the IBM is a viable alternative to body-conforming grids and that LES with wall functions reproduces experimental results at a much lower computational cost.
Large eddy simulation of unsteady wind farm behavior using advanced actuator disk models
NASA Astrophysics Data System (ADS)
Moens, Maud; Duponcheel, Matthieu; Winckelmans, Gregoire; Chatelain, Philippe
2014-11-01
The present project aims at improving the level of fidelity of unsteady wind farm scale simulations through an effort on the representation and the modeling of the rotors. The chosen tool for the simulations is a Fourth Order Finite Difference code, developed at Universite catholique de Louvain; this solver implements Large Eddy Simulation (LES) approaches. The wind turbines are modeled as advanced actuator disks: these disks are coupled with the Blade Element Momentum method (BEM method) and also take into account the turbine dynamics and controller. A special effort is made here to reproduce the specific wake behaviors. Wake decay and expansion are indeed initially governed by vortex instabilities. This is an information that cannot be obtained from the BEM calculations. We thus aim at achieving this by matching the large scales of the actuator disk flow to high fidelity wake simulations produced using a Vortex Particle-Mesh method. It is obtained by adding a controlled excitation at the disk. We apply this tool to the investigation of atmospheric turbulence effects on the power production and on the wake behavior at a wind farm level. A turbulent velocity field is then used as inflow boundary condition for the simulations. We gratefully acknowledge the support of GDF Suez for the fellowship of Mrs Maud Moens.
NASA Astrophysics Data System (ADS)
Guo, Huan; Liu, Yangang; Daum, Peter H.; Senum, Gunnar I.; Tao, Wei-Kuo
2008-10-01
We simulated a marine stratus deck sampled during the Marine Stratus/Stratocumulus Experiment (MASE) with a three-dimensional large eddy simulation (LES) model at different model resolutions. Various characteristics of the vertical velocity from the model simulations were evaluated against those derived from the corresponding aircraft in situ observations, focusing on standard deviation, skewness, kurtosis, probability density function (PDF), power spectrum, and structure function. Our results show that although the LES model captures reasonably well the lower-order moments (e.g., horizontal averages and standard deviations), it fails to simulate many aspects of the higher-order moments, such as kurtosis, especially near cloud base and cloud top. Further investigations of the PDFs, power spectra, and structure functions reveal that compared to the observations, the model generally underestimates relatively strong variations on small scales. The results also suggest that increasing the model resolutions improves the agreements between the model results and the observations in virtually all of the properties that we examined. Furthermore, the results indicate that a vertical grid size <10 m is necessary for accurately simulating even the standard-deviation profile, posing new challenges to computer resources.
Large eddy simulation for predicting turbulent heat transfer in gas turbines
Tafti, Danesh K.; He, Long; Nagendra, K.
2014-01-01
Blade cooling technology will play a critical role in the next generation of propulsion and power generation gas turbines. Accurate prediction of blade metal temperature can avoid the use of excessive compressed bypass air and allow higher turbine inlet temperature, increasing fuel efficiency and decreasing emissions. Large eddy simulation (LES) has been established to predict heat transfer coefficients with good accuracy under various non-canonical flows, but is still limited to relatively simple geometries and low Reynolds numbers. It is envisioned that the projected increase in computational power combined with a drop in price-to-performance ratio will make system-level simulations using LES in complex blade geometries at engine conditions accessible to the design process in the coming one to two decades. In making this possible, two key challenges are addressed in this paper: working with complex intricate blade geometries and simulating high-Reynolds-number (Re) flows. It is proposed to use the immersed boundary method (IBM) combined with LES wall functions. A ribbed duct at Re=20 000 is simulated using the IBM, and a two-pass ribbed duct is simulated at Re=100 000 with and without rotation (rotation number Ro=0.2) using LES with wall functions. The results validate that the IBM is a viable alternative to body-conforming grids and that LES with wall functions reproduces experimental results at a much lower computational cost. PMID:25024418
Large eddy simulation of bluff body stabilized premixed and partially premixed combustion
NASA Astrophysics Data System (ADS)
Porumbel, Ionut
Large Eddy Simulation (LES) of bluff body stabilized premixed and partially premixed combustion close to the flammability limit is carried out in this thesis. The main goal of the thesis is the study of the equivalence ratio effect on flame stability and dynamics in premixed and partially premixed flames. An LES numerical algorithm able to handle the entire range of combustion regimes and equivalence ratios is developed for this purpose. The algorithm has no ad-hoc adjustable model parameters and is able to respond automatically to variations in the inflow conditions, without user intervention. Algorithm validation is achieved by conducting LES of reactive and non-reactive flow. Comparison with experimental data shows good agreement for both mean and unsteady flow properties. In the reactive flow, two scalar closure models, Eddy Break-Up (EBULES) and Linear Eddy Mixing (LEMLES), are used and compared. Over important regions, the flame lies in the Broken Reaction Zone regime. Here, the EBU model assumptions fail. In LEMLES, the reaction-diffusion equation is not filtered, but resolved on a linear domain and the model maintains validity. The flame thickness predicted by LEMLES is smaller and the flame is faster to respond to turbulent fluctuations, resulting in a more significant wrinkling of the flame surface when compared to EBULES. As a result, LEMLES captures better the subtle effects of the flame-turbulence interaction, the flame structure shows higher complexity, and the far field spreading of the wake is closer to the experimental observations. Three premixed (φ = 0.6, 0.65, and 0.75) cases are simulated. As expected, for the leaner case (φ = 0.6) the flame temperature is lower, the heat release is reduced and vorticity is stronger. As a result, the flame in this case is found to be unstable. In the rich case (φ = 0.75), the flame temperature is higher, and the spreading rate of the wake is increased due to the higher amount of heat release. The ignition
Requirements for large-eddy simulation of surface wind gusts in a mountain valley
NASA Astrophysics Data System (ADS)
Revell, Michael J.; Purnell, Don; Lauren, Michael K.
1996-09-01
During the passage of a front, data from a light-weight cup anemometer and wind vane, sited in a steep-walled glacial valley of the Mt Cook region of the Southern Alps of New Zealand, were analysed to derive a power spectrum of the wind velocity for periods between 0.5 and 16 min. The energy spectrum roughly followed a -5/3 power law over the range of periods from 0.5 4 min — as might be expected in the case of an inertial subrange of eddies. However, any inertial subrange clearly does not extend to periods longer than this. We suggest that the observed eddies were generated in a turbulent wake associated with flow separation at the ridge crests, and large eddies are shed at periods of 4 8 min or more. A compressible fluid-dynamic model, with a Smagorinsky turbulence closure scheme and a “law of the wall” at the surface, was used to calculate flow over a cross section through this area in neutrally stratified conditions. A range of parameters was explored to assess some of the requirements for simulating surface wind gusts in mountainous terrain in New Zealand. In order to approximate the observed wind spectrum at Tasman aerodrome, Mount Cook, we found the model must be three-dimensional, with a horizontal resolution better than 250 m and with a Reynolds-stress eddy viscosity of less than 5 m2 s-1. In two-dimensional simulations, the eddies were too big in size and in amplitude and at the surface this was associated with reversed flow extending too far downstream. In contrast the three-dimensional simulations gave a realistic gusting effect associated with large scale “cat's paws” (a bigger variety of those commonly seen over water downstream of moderate hills), with reversed flow only at the steep part of the lee slope. The simulations were uniformly improved by better resolution, at all tested resolutions down to 250 m mesh size. The spectra of large eddies simulated in steep terrain were not very sensitive to the details of the eddy stress formulation
Pal, Abhro; Anupindi, Kameswararao; Delorme, Yann; Ghaisas, Niranjan; Shetty, Dinesh A.; Frankel, Steven H.
2014-01-01
In the present study, we performed large eddy simulation (LES) of axisymmetric, and 75% stenosed, eccentric arterial models with steady inflow conditions at a Reynolds number of 1000. The results obtained are compared with the direct numerical simulation (DNS) data (Varghese et al., 2007, “Direct Numerical Simulation of Stenotic Flows. Part 1. Steady Flow,” J. Fluid Mech., 582, pp. 253–280). An inhouse code (WenoHemo) employing high-order numerical methods for spatial and temporal terms, along with a 2nd order accurate ghost point immersed boundary method (IBM) (Mark, and Vanwachem, 2008, “Derivation and Validation of a Novel Implicit Second-Order Accurate Immersed Boundary Method,” J. Comput. Phys., 227(13), pp. 6660–6680) for enforcing boundary conditions on curved geometries is used for simulations. Three subgrid scale (SGS) models, namely, the classical Smagorinsky model (Smagorinsky, 1963, “General Circulation Experiments With the Primitive Equations,” Mon. Weather Rev., 91(10), pp. 99–164), recently developed Vreman model (Vreman, 2004, “An Eddy-Viscosity Subgrid-Scale Model for Turbulent Shear Flow: Algebraic Theory and Applications,” Phys. Fluids, 16(10), pp. 3670–3681), and the Sigma model (Nicoud et al., 2011, “Using Singular Values to Build a Subgrid-Scale Model for Large Eddy Simulations,” Phys. Fluids, 23(8), 085106) are evaluated in the present study. Evaluation of SGS models suggests that the classical constant coefficient Smagorinsky model gives best agreement with the DNS data, whereas the Vreman and Sigma models predict an early transition to turbulence in the poststenotic region. Supplementary simulations are performed using Open source field operation and manipulation (OpenFOAM) (“OpenFOAM,” http://www.openfoam.org/) solver and the results are inline with those obtained with WenoHemo. PMID:24801556
Pal, Abhro; Anupindi, Kameswararao; Delorme, Yann; Ghaisas, Niranjan; Shetty, Dinesh A; Frankel, Steven H
2014-07-01
In the present study, we performed large eddy simulation (LES) of axisymmetric, and 75% stenosed, eccentric arterial models with steady inflow conditions at a Reynolds number of 1000. The results obtained are compared with the direct numerical simulation (DNS) data (Varghese et al., 2007, "Direct Numerical Simulation of Stenotic Flows. Part 1. Steady Flow," J. Fluid Mech., 582, pp. 253-280). An inhouse code (WenoHemo) employing high-order numerical methods for spatial and temporal terms, along with a 2nd order accurate ghost point immersed boundary method (IBM) (Mark, and Vanwachem, 2008, "Derivation and Validation of a Novel Implicit Second-Order Accurate Immersed Boundary Method," J. Comput. Phys., 227(13), pp. 6660-6680) for enforcing boundary conditions on curved geometries is used for simulations. Three subgrid scale (SGS) models, namely, the classical Smagorinsky model (Smagorinsky, 1963, "General Circulation Experiments With the Primitive Equations," Mon. Weather Rev., 91(10), pp. 99-164), recently developed Vreman model (Vreman, 2004, "An Eddy-Viscosity Subgrid-Scale Model for Turbulent Shear Flow: Algebraic Theory and Applications," Phys. Fluids, 16(10), pp. 3670-3681), and the Sigma model (Nicoud et al., 2011, "Using Singular Values to Build a Subgrid-Scale Model for Large Eddy Simulations," Phys. Fluids, 23(8), 085106) are evaluated in the present study. Evaluation of SGS models suggests that the classical constant coefficient Smagorinsky model gives best agreement with the DNS data, whereas the Vreman and Sigma models predict an early transition to turbulence in the poststenotic region. Supplementary simulations are performed using Open source field operation and manipulation (OpenFOAM) ("OpenFOAM," http://www.openfoam.org/) solver and the results are inline with those obtained with WenoHemo.
Large Eddy Simulation of Sound Generation by Turbulent Reacting and Nonreacting Shear Flows
NASA Astrophysics Data System (ADS)
Najafi-Yazdi, Alireza
The objective of the present study was to investigate the mechanisms of sound generation by subsonic jets. Large eddy simulations were performed along with bandpass filtering of the flow and sound in order to gain further insight into the pole of coherent structures in subsonic jet noise generation. A sixth-order compact scheme was used for spatial discretization of the fully compressible Navier-Stokes equations. Time integration was performed through the use of the standard fourth-order, explicit Runge-Kutta scheme. An implicit low dispersion, low dissipation Runge-Kutta (ILDDRK) method was developed and implemented for simulations involving sources of stiffness such as flows near solid boundaries, or combustion. A surface integral acoustic analogy formulation, called Formulation 1C, was developed for farfield sound pressure calculations. Formulation 1C was derived based on the convective wave equation in order to take into account the presence of a mean flow. The formulation was derived to be easy to implement as a numerical post-processing tool for CFD codes. Sound radiation from an unheated, Mach 0.9 jet at Reynolds number 400, 000 was considered. The effect of mesh size on the accuracy of the nearfield flow and farfield sound results was studied. It was observed that insufficient grid resolution in the shear layer results in unphysical laminar vortex pairing, and increased sound pressure levels in the farfield. Careful examination of the bandpass filtered pressure field suggested that there are two mechanisms of sound radiation in unheated subsonic jets that can occur in all scales of turbulence. The first mechanism is the stretching and the distortion of coherent vortical structures, especially close to the termination of the potential core. As eddies are bent or stretched, a portion of their kinetic energy is radiated. This mechanism is quadrupolar in nature, and is responsible for strong sound radiation at aft angles. The second sound generation mechanism
Large Eddy Simulation of wind turbines using the actuator line model and immersed boundary method
NASA Astrophysics Data System (ADS)
Santoni, Christian; Carrasquillo-Solís, Kenneth; Leonardi, Stefano
2014-11-01
Despite the growth of the energy extracted from wind turbines, the flow physics is still not fully understood even under ideal operational conditions. Large Eddy Simulations of the turbulent flow past a wind turbine in a channel have been performed. The numerical setup reproduces the experiment performed in a wind tunnel at the Norwegian University of Science and Technology (NUST). The code is based on a finite difference scheme with a fractional step and Runge-Kutta, which couples the actuator line model (ALM) and the Immersed Boundary Method (IBM). Two simulations were performed, one neglecting the tower and nacelle resulting in the rotating blades only, the other modeling both the rotating blades as well as the tower and nacelle with IBM. Results relative to the simulation with tower and nacelle have a very good agreement with experiments. Profiles of turbulent kinetic energy shows that the effect of the tower and nacelle is not confined to the hub region but extend to the entire rotor. In addition we placed the wind turbine over an undulated topography to understand how it affects the performances and wake of a wind turbine. Comparison with the results obtained for the smooth wall show an interaction between the rough wall and the wake. The numerical simulations were performed on XSEDE TACC under Grant No. CTS070066. The present work is supported by the National Science Foundation (NSF), Grant IIA-1243482 (WINDINSPIRE).
Large eddy simulation of dilute bubbly turbulent flows for aerating hydrofoils
NASA Astrophysics Data System (ADS)
Hajit, Mohammad; Sotiropoulos, Fotis
2014-11-01
We have proposed a formulation for the large eddy simulation of dilute bubbly flows by converting the governing equations to a more loosely-coupled form. This formulation provides an efficient numerical procedure for two-way coupling of bubbly flows at low gas holdups. Subgrid-scale turbulence modeling is based on the dynamic procedure of Germano for the liquid phase and the Jakobson approach for the gas phase. Wall-modeling is implemented using the method of Cabot & Moin. Our approach is employed to simulate flow over aerating hydrofoils at different angles of attack. A structured body-fitted C-grid is employed for domain discretization. Validation of our computational code, for C-grids, is carried out by simulating single-phase flows over a NACA0012 airfoil (20° AOA) with laminar flow and an E387 airfoil (6° AOA) with turbulent flow. Comparisons with available computational and experimental data in terms of time averaged drag coefficient, lift coefficient, separation bubble length, and reattachment point proves the validity of our computational code. The aerating hydrofoil simulation utilizes a NACA0015 hydrofoil, for which experiments were carried out at Saint Anthony Falls Laboratory. Comparisons between computational and experimental datasets show promising results. This work is supported by the U.S. Dept. of Energy and the Hydro Reasearch Foundation.
Sondak, David; Oberai, Assad A.
2012-10-15
Novel large eddy simulation (LES) models are developed for incompressible magnetohydrodynamics (MHD). These models include the application of the variational multiscale formulation of LES to the equations of incompressible MHD. Additionally, a new residual-based eddy viscosity model is introduced for MHD. A mixed LES model that combines the strengths of both of these models is also derived. The new models result in a consistent numerical method that is relatively simple to implement. The need for a dynamic procedure in determining model coefficients is no longer required. The new LES models are tested on a decaying Taylor-Green vortex generalized to MHD and benchmarked against classical LES turbulence models. The LES simulations are run in a periodic box of size [-{pi}, {pi}]{sup 3} with 32 modes in each direction and are compared to a direct numerical simulation (DNS) with 512 modes in each direction. The new models are able to account for the essential MHD physics which is demonstrated via comparisons of energy spectra. We also compare the performance of our models to a DNS simulation by Pouquet et al.['The dynamics of unforced turbulence at high Reynolds number for Taylor-Green vortices generalized to MHD,' Geophys. Astrophys. Fluid Dyn. 104, 115-134 (2010)], for which the ratio of DNS modes to LES modes is 262:144.
Insights into low-latitude cloud feedbacks from large-eddy simulations
NASA Astrophysics Data System (ADS)
Bretherton, Christopher; Blossey, Peter
2017-04-01
Cloud feedbacks are a leading source of uncertainty in the climate sensitivity simulated by global climate models (GCMs). Low-latitude boundary-layer and cumulus cloud regimes are particularly problematic, because they are sustained by tight interactions between clouds and unresolved turbulent circulations. Large-eddy simulations (LES) using sub-100 m grid spacings better simulate such cloud regimes without need for complex models of subgrid variability of cloud and turbulence. Recently, multiday LES over small computational domains have elucidated marine boundary layer cloud response to specified aspects of greenhouse warming and the associated changes in large-scale dynamics and atmospheric radiative heating. The focus will be the CGILS LES intercomparisons and subsequent related work. Four primary contributing mechanisms of subtropical low cloud response are implicated, all with observational support. These are (1) thermodynamic: cloudiness reduction from warming and moistening of the atmosphere-ocean column, (2) radiative: cloudiness reduction from CO2 and H2O-induced increase in atmospheric emissivity aloft, (3) stability-induced: low cloud increase from increased lower-tropospheric stratification, and (4) dynamical: low cloud increase from reduced subsidence. LES as a group robustly suggest that the cloudiness reduction mechanisms typically dominate, giving positive shortwave cloud feedback in the subtropics consistent with the range simulated by conventional global climate models. Finally, a possible approach for better bridging the scale gap between LES and global models will be noted.
Large Eddy Simulation of Airfoil Self-Noise at High Reynolds Number
NASA Astrophysics Data System (ADS)
Kocheemoolayil, Joseph; Lele, Sanjiva
2015-11-01
The trailing edge noise section (Category 1) of the Benchmark Problems for Airframe Noise Computations (BANC) workshop features five canonical problems. No first-principles based approach free of empiricism and tunable coefficients has successfully predicted trailing edge noise for the five configurations to date. Our simulations predict trailing edge noise accurately for all five configurations. The simulation database is described in detail, highlighting efforts undertaken to validate the results through systematic comparison with dedicated experiments and establish insensitivity to grid resolution, domain size, alleatory uncertainties such as the tripping mechanism used to force transition to turbulence and epistemic uncertainties such as models for unresolved near-wall turbulence. Ongoing efforts to extend the predictive capability to non-canonical configurations featuring flow separation are summarized. A novel, large-span calculation that predicts the flow past a wind turbine airfoil in deep stall with unprecedented accuracy is presented. The simulations predict airfoil noise in the near-stall regime accurately. While the post-stall noise predictions leave room for improvement, significant uncertainties in the experiment might preclude a fair comparison in this regime. We thank Cascade Technologies Inc. for providing access to the CharLES toolkit - a massively-parallel, unstructured large eddy simulation framework.
Lorteau, Mathieu Cléro, Franck Vuillot, François
2015-07-15
In the framework of jet noise computation, a numerical simulation of a subsonic turbulent hot jet is performed using large-eddy simulation. A geometrical tripping is used in order to trigger the turbulence at the nozzle exit. In a first part, the validity of the simulation is assessed by comparison with experimental measurements. The mean and rms velocity fields show good agreement, so do the azimuthal composition of the near pressure field and the far field spectra. Discrepancies remain close to the nozzle exit which lead to a limited overestimation of the pressure levels in both near and far fields, especially near the 90{sup ∘} angular sector. Two point correlation analyses are then applied to the data obtained from the simulation. These enable to link the downstream acoustic radiation, which is the main direction of radiation, to pressure waves developing in the shear layer and propagating toward the potential core end. The intermittency of the downstream acoustic radiation is evidenced and related to the coherent structures developing in the shear layer.
Large-eddy simulations of impinging jets at high Reynolds numbers
NASA Astrophysics Data System (ADS)
Wu, Wen; Piomelli, Ugo
2013-11-01
We have performed large-eddy simulations of an impinging jet with embedded azimuthal vortices. We used a hybrid approach in which the near-wall layer is modelled using the RANS equations with the Spalart-Allmaras model, while away from the wall Lagrangian-averaged dynamic eddy-viscosity modelled LES is used. This method allowed us to reach Reynolds numbers that would be prohibitively expensive for wall-resolving LES. First, we compared the results of the hybrid calculation with a wall-resolved one at moderate Reynolds number, Re = 66 , 000 (based on jet diameter and velocity). The mean velocity and Reynolds stresses were in good agreement between the simulations, and, in particular, the generation of secondary vorticity at the wall and its liftup were captured well. The simulation cost was reduced by 86%. We then carried out simulations at Re = 266 , 000 and 1.3 million. The effect of Reynolds number on vortex development will be discussed. Canada Research Chair in Computational Turbulence, HPCVL-Sun Microsystems Chair in Computational Science and Engineering.
Development of an advanced actuator disk model for Large-Eddy Simulation of wind farms
NASA Astrophysics Data System (ADS)
Moens, Maud; Duponcheel, Matthieu; Winckelmans, Gregoire; Chatelain, Philippe
2015-11-01
This work aims at improving the fidelity of the wind turbine modelling for Large-Eddy Simulation (LES) of wind farms, in order to accurately predict the loads, the production, and the wake dynamics. In those simulations, the wind turbines are accounted for through actuator disks. i.e. a body-force term acting over the regularised disk swept by the rotor. These forces are computed using the Blade Element theory to estimate the normal and tangential components (based on the local simulated flow and the blade characteristics). The local velocities are modified using the Glauert tip-loss factor in order to account for the finite number of blades; the computation of this correction is here improved thanks to a local estimation of the effective upstream velocity at every point of the disk. These advanced actuator disks are implemented in a 4th order finite difference LES solver and are compared to a classical Blade Element Momentum method and to high fidelity wake simulations performed using a Vortex Particle-Mesh method in uniform and turbulent flows.
Requirements for Large Eddy Simulation Computations of Variable-Speed Power Turbine Flows
NASA Technical Reports Server (NTRS)
Ameri, Ali A.
2016-01-01
Variable-speed power turbines (VSPTs) operate at low Reynolds numbers and with a wide range of incidence angles. Transition, separation, and the relevant physics leading to them are important to VSPT flow. Higher fidelity tools such as large eddy simulation (LES) may be needed to resolve the flow features necessary for accurate predictive capability and design of such turbines. A survey conducted for this report explores the requirements for such computations. The survey is limited to the simulation of two-dimensional flow cases and endwalls are not included. It suggests that a grid resolution necessary for this type of simulation to accurately represent the physics may be of the order of Delta(x)+=45, Delta(x)+ =2 and Delta(z)+=17. Various subgrid-scale (SGS) models have been used and except for the Smagorinsky model, all seem to perform well and in some instances the simulations worked well without SGS modeling. A method of specifying the inlet conditions such as synthetic eddy modeling (SEM) is necessary to correctly represent the inlet conditions.