scholarly journals Large Eddy Simulation of Unstably Stratified Turbulent Flow over Urban-Like Building Arrays

2013 ◽  
Vol 2013 ◽  
pp. 1-10 ◽  
Author(s):  
Bobin Wang ◽  
Guixiang Cui
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Buoyancy Force ◽  
Thermal Instability ◽  
Urban Atmosphere ◽  
Scale Model ◽  
Mean Flow ◽  
Mean Velocity ◽  
Eddy Simulation ◽  
Large Eddy

Thermal instability induced by solar radiation is the most common condition of urban atmosphere in daytime. Compared to researches under neutral conditions, only a few numerical works studied the unstable urban boundary layer and the effect of buoyancy force is unclear. In this paper, unstably stratified turbulent boundary layer flow over three-dimensional urban-like building arrays with ground heating is simulated. Large eddy simulation is applied to capture main turbulence structures and the effect of buoyancy force on turbulence can be investigated. Lagrangian dynamic subgrid scale model is used for complex flow together with a wall function, taking into account the large pressure gradient near buildings. The numerical model and method are verified with the results measured in wind tunnel experiment. The simulated results satisfy well with the experiment in mean velocity and temperature, as well as turbulent intensities. Mean flow structure inside canopy layer varies with thermal instability, while no large secondary vortex is observed. Turbulent intensities are enhanced, as buoyancy force contributes to the production of turbulent kinetic energy.

Large eddy simulation study of turbulent flow around smooth and rough domes

2013 ◽  
Vol 227 (12) ◽  
pp. 2686-2700 ◽  
Author(s):  
N Kharoua ◽  
L Khezzar
Keyword(s):  
Large Eddy Simulation ◽  
Turbulent Flow ◽  
Flow Behavior ◽  
Pressure Coefficient ◽  
Horseshoe Vortex ◽  
Scale Model ◽  
Stream Velocity ◽  
Mean Velocity ◽  
Eddy Simulation ◽  
Large Eddy

Large eddy simulation of turbulent flow around smooth and rough hemispherical domes was conducted. The roughness of the rough dome was generated by a special approach using quadrilateral solid blocks placed alternately on the dome surface. It was shown that this approach is capable of generating the roughness effect with a relative success. The subgrid-scale model based on the transport of the subgrid turbulent kinetic energy was used to account for the small scales effect not resolved by large eddy simulation. The turbulent flow was simulated at a subcritical Reynolds number based on the approach free stream velocity, air properties, and dome diameter of 1.4 × 105. Profiles of mean pressure coefficient, mean velocity, and its root mean square were predicted with good accuracy. The comparison between the two domes showed different flow behavior around them. A flattened horseshoe vortex was observed to develop around the rough dome at larger distance compared with the smooth dome. The separation phenomenon occurs before the apex of the rough dome while for the smooth dome it is shifted forward. The turbulence-affected region in the wake was larger for the rough dome.

High-resolution atmospheric boundary layer simulations using WRF-LES

2020 ◽  
Author(s):  
Gokhan Kirkil
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Cross Sections ◽  
Wrf Model ◽  
Field Measurements ◽  
Mean Flow ◽  
Spatial And Temporal Scales ◽  
Eddy Simulation ◽  
Wide Range ◽  
Large Eddy

<p>WRF model provides a potentially powerful framework for coupled simulations of flow covering a wide range of<br>spatial and temporal scales via a successive grid nesting capability. Nesting can be repeated down to turbulence<br>solving large eddy simulation (LES) scales, providing a means for significant improvements of simulation of<br>turbulent atmospheric boundary layers. We will present the recent progress on our WRF-LES simulations of<br>the Perdigao Experiment performed over mountainous terrain. We performed multi-scale simulations using<br>WRF’s different Planetary Boundary Layer (PBL) parameterizations as well as Large Eddy Simulation (LES)<br>and compared the results with the detailed field measurements. WRF-LES model improved the mean flow field<br>as well as second-order flow statistics. Mean fluctuations and turbulent kinetic energy fields from WRF-LES<br>solution are investigated in several cross-sections around the hill which shows good agreement with measurements.</p>

Large Eddy Simulation of Constant Heat Flux Turbulent Channel Flow With Property Variations: Quasi-Developed Model and Mean Flow Results

2003 ◽  
Vol 125 (1) ◽  
pp. 27-38 ◽  
Author(s):  
Lyle D. Dailey ◽  
Ning Meng ◽  
Richard H. Pletcher
Keyword(s):  
Large Eddy Simulation ◽  
Channel Flow ◽  
Scale Model ◽  
Bulk Temperature ◽  
Finite Volume Scheme ◽  
Subgrid Scale ◽  
Mean Flow ◽  
Eddy Simulation ◽  
Heating And Cooling ◽  
Large Eddy

Turbulent planar channel flow has been computed for uniform wall heating and cooling fluxes strong enough to cause significant property variations using large eddy simulation. Channels with both walls either heated or cooled were considered, with wall-to-bulk temperature ratios as high as 1.5 for the heated case, and as low as 0.56 for the cooled case. An implicit, second order accurate finite volume scheme was used to solve the time dependent filtered set of equations to determine the large eddy motion, while a dynamic subgrid-scale model was used to account for the subgrid scale effects. Step-periodicity was used based on a quasi-developed assumption. The effects of strong heating and cooling on the flow were investigated and compared with the results obtained under low heating conditions.

Large eddy simulation of a stratified boundary layer under an oscillatory current

2009 ◽  
Vol 643 ◽  
pp. 233-266 ◽  
Author(s):  
BISHAKHDATTA GAYEN ◽  
SUTANU SARKAR ◽  
JOHN R. TAYLOR
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Mixed Layer ◽  
Numerical Study ◽  
Bottom Boundary Layer ◽  
Mean Velocity ◽  
Bottom Boundary ◽  
Eddy Simulation ◽  
Large Eddy ◽  
The Mean

A numerical study based on large eddy simulation is performed to investigate a bottom boundary layer under an oscillating tidal current. The focus is on the boundary layer response to an external stratification. The thermal field shows a mixed layer that is separated from the external stratified fluid by a thermocline. The mixed layer grows slowly in time with an oscillatory modulation by the tidal flow. Stratification strongly affects the mean velocity profiles, boundary layer thickness and turbulence levels in the outer region although the effect on the near-bottom unstratified fluid is relatively mild. The turbulence is asymmetric between the accelerating and decelerating stages. The asymmetry is more pronounced with increasing stratification. There is an overshoot of the mean velocity in the outer layer; this jet is linked to the phase asymmetry of the Reynolds shear stress gradient by using the simulation data to examine the mean momentum equation. Depending on the height above the bottom, there is a lag of the maximum turbulent kinetic energy, dissipation and production with respect to the peak external velocity and the value of the lag is found to be influenced by the stratification. Flow instabilities and turbulence in the bottom boundary layer excite internal gravity waves that propagate away into the ambient. Unlike the steady case, the phase lines of the internal waves change direction during the tidal cycle and also from near to far field. The frequency spectrum of the propagating wave field is analysed and found to span a narrow band of frequencies clustered around 45°.

Study of flow in a planar asymmetric diffuser using large-eddy simulation

1999 ◽  
Vol 390 ◽  
pp. 151-185 ◽  
Author(s):  
H.-J. KALTENBACH ◽  
M. FATICA ◽  
R. MITTAL ◽  
T. S. LUND ◽  
P. MOIN
Keyword(s):  
Kinetic Energy ◽  
Large Eddy Simulation ◽  
Scale Model ◽  
Subgrid Scale ◽  
Mean Flow ◽  
Eddy Simulation ◽  
Unsteady Separation ◽  
Large Eddy ◽  
Rear Part ◽  
Subgrid Scale Model

Large-eddy simulation (LES) has been used to study the flow in a planar asymmetric diffuser. The wide range of spatial and temporal scales, the presence of an adverse pressure gradient, and the formation of an unsteady separation bubble in the rear part of the diffuser make this flow a challenging test case for assessing the predictive capability of LES. Simulation results for mean flow, pressure recovery and skin friction are in excellent agreement with data from two recent experiments. The inflow consists of a fully developed turbulent channel flow at a Reynolds number based on shear velocity, Reτ=500. It is found that accurate representation of the in flow velocity field is critical for accurate prediction of the flow in the diffuser. Although the simulation in the diffuser is well resolved, the subgrid-scale model plays a significant role for both mean momentum and turbulent kinetic energy balances. Subgrid-scale stresses contribute a maximum of 8% to the local value of the total shear stress with the maximum values found in the inlet duct and along the flat wall where the flow remains attached. The subgrid-scale model adapts to the enhanced turbulence levels in the rear part of the diffuser by providing more than 80% of the dissipation rate for turbulent kinetic energy. The unsteady separation excites large scales of motion which extend over the major part of the duct cross-section and penetrate deeply into the core of the flow. Instantaneous flow reversal is observed along both walls immediately behind the diffuser throat which is far upstream of the location of main separation. While the mean flow profile changes gradually as the flow enters the expansion, turbulent stresses undergo rapid changes over a short streamwise distance along the deflected wall. An explanation is offered which considers the strain field as well as the influence of geometry changes. The effect of grid resolution and spanwise domain size on the flow field prediction has been documented and this allows an assessment of the computational requirements for carrying out such simulations.

Large-eddy simulation of flow over a cylinder with from to : a skin-friction perspective

2017 ◽  
Vol 820 ◽  
pp. 121-158 ◽  
Author(s):  
W. Cheng ◽  
D. I. Pullin ◽  
R. Samtaney ◽  
W. Zhang ◽  
W. Gao
Keyword(s):  
Boundary Layer ◽  
Friction Coefficient ◽  
Large Eddy Simulation ◽  
Skin Friction ◽  
Flow Separation ◽  
Skin Friction Coefficient ◽  
Cylinder Surface ◽  
Mean Flow ◽  
Eddy Simulation ◽  
Large Eddy

We present wall-resolved large-eddy simulations (LES) of flow over a smooth-wall circular cylinder up to$Re_{D}=8.5\times 10^{5}$, where$Re_{D}$is Reynolds number based on the cylinder diameter$D$and the free-stream speed$U_{\infty }$. The stretched-vortex subgrid-scale (SGS) model is used in the entire simulation domain. For the sub-critical regime, six cases are implemented with$3.9\times 10^{3}\leqslant Re_{D}\leqslant 10^{5}$. Results are compared with experimental data for both the wall-pressure-coefficient distribution on the cylinder surface, which dominates the drag coefficient, and the skin-friction coefficient, which clearly correlates with the separation behaviour. In the super-critical regime, LES for three values of$Re_{D}$are carried out at different resolutions. The drag-crisis phenomenon is well captured. For lower resolution, numerical discretization fluctuations are sufficient to stimulate transition, while for higher resolution, an applied boundary-layer perturbation is found to be necessary to stimulate transition. Large-eddy simulation results at$Re_{D}=8.5\times 10^{5}$, with a mesh of$8192\times 1024\times 256$, agree well with the classic experimental measurements of Achenbach (J. Fluid Mech., vol. 34, 1968, pp. 625–639) especially for the skin-friction coefficient, where a spike is produced by the laminar–turbulent transition on the top of a prior separation bubble. We document the properties of the attached-flow boundary layer on the cylinder surface as these vary with$Re_{D}$. Within the separated portion of the flow, mean-flow separation–reattachment bubbles are observed at some values of$Re_{D}$, with separation characteristics that are consistent with experimental observations. Time sequences of instantaneous surface portraits of vector skin-friction trajectory fields indicate that the unsteady counterpart of a mean-flow separation–reattachment bubble corresponds to the formation of local flow-reattachment cells, visible as coherent bundles of diverging surface streamlines.

Turbulence Modeling Using Z-F, RSM, LES and WMLES for Flow Analysis in Z-Shape Ducts

2020 ◽  
Author(s):  
Mohammed Karbon ◽  
Ahmad K. Sleiti
Keyword(s):  
Experimental Data ◽  
Steady State ◽  
Large Eddy Simulation ◽  
Flow Separation ◽  
Temporal Variations ◽  
Mean Flow ◽  
Mean Velocity ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Rsm Model

Abstract Turbulent flow in Z-shape duct configuration is investigated and analyzed using Reynolds Stress Model (RSM), Large Eddy Simulation (LES), ζ-f Model, and Wall-Modeled Large Eddy Simulation (WMLES). The results are validated and compared to experimental data. Both RSM and ζ-f models are based on steady-state RANS solutions, while LES and WMLES models account for temporal variations transient behavior of the flow turbulence. The focus was on regions where RSM has over or under predicted the flow and regions where there are flow separations and high turbulence. LES simulation results have shown under-prediction and over-prediction in the flow separation and re-attachment regions. It is found that the turbulent kinetic energy production in ζ equation is much easier to reproduce accurately than other models. Both mean velocity gradient and local turbulent stress terms are also much easier to resolve properly. The current research has found that ζ-f model not only takes less time to complete the simulation but also the mean flow velocity profile results are in better agreement with experimental data than RSM model despite both are coupled steady-state RANS. ζ-f model numerically resolved both the flow separation and re-attachment regions better than RSM model. WMLES model is employed to investigate the SGS model impact on the small eddies dissipated from the large eddies. Such WMLES model produces much better results than the LES model, however the SGS viscosity damps the energy of the flow.

scholarly journals Large-eddy simulation of twin impinging jets in cross-flow

2007 ◽  
Vol 111 (1117) ◽  
pp. 195-206 ◽  
Author(s):  
Q. Li ◽  
G. J. Page ◽  
J. J. McGuirk
Keyword(s):  
Large Eddy Simulation ◽  
Cross Flow ◽  
Jet Impingement ◽  
Impinging Jets ◽  
Scale Model ◽  
Mean Flow ◽  
Eddy Simulation ◽  
Suitable Tool ◽  
Large Eddy ◽  
Good Agreement

The flow-field beneath a jet-borne vertical landing aircraft is highly complex and unsteady. large-eddy simulation is a suitable tool to predict both the mean flow and unsteady fluctuations. This work aims to evaluate the suitability of LES by applying it to two multiple jet impingement problems: the first is a simple twin impinging jet in cross-flow, while the second includes a circular intake. The numerical method uses a compressible solver on a mixed element unstructured mesh. The smoothing terms in the spatial flux are kept small by the use of a monitor function sensitive to vorticity and divergence. The WALE subgrid scale model is utilised. The simpler jet impingement case shows good agreement with experiment for mean velocity and normal stresses. Analysis of time histories in the jet shear layer and near impingement gives a dominant frequency at a Strouhal number of 0·1, somewhat lower than normally observed in free jets. The jet impingement case with an intake also gives good agreement with experimental velocity measurements, although the expansion of the grid ahead of the jets does reduce the accuracy in this region. Turbulent eddies are observed entering the intake with significant swirl. This is in qualitative agreement with experimental visualisation. The results show that LES could be a suitable tool when applied to multiple jet impingement with realistic aircraft geometry.

A scale-dependent dynamic model for large-eddy simulation: application to a neutral atmospheric boundary layer

2000 ◽  
Vol 415 ◽  
pp. 261-284 ◽  
Author(s):  
FERNANDO PORTÉ-AGEL ◽  
CHARLES MENEVEAU ◽  
MARC B. PARLANGE
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Atmospheric Boundary Layer ◽  
Dynamic Model ◽  
Scale Invariance ◽  
Scale Model ◽  
Near Surface ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Dependent Model

A scale-dependent dynamic subgrid-scale model for large-eddy simulation of turbulent flows is proposed. Unlike the traditional dynamic model, it does not rely on the assumption that the model coefficient is scale invariant. The model is based on a second test-filtering operation which allows us to determine from the simulation how the coefficient varies with scale. The scale-dependent model is tested in simulations of a neutral atmospheric boundary layer. In this application, near the ground the grid scale is by necessity comparable to the local integral scale (of the order of the distance to the wall). With the grid scale and/or the test-filter scale being outside the inertial range, scale invariance is broken. The results are compared with those from (a) the traditional Smagorinsky model that requires specification of the coefficient and of a wall damping function, and (b) the standard dynamic model that assumes scale invariance of the coefficient. In the near-surface region the traditional Smagorinsky and standard dynamic models are too dissipative and not dissipative enough, respectively. Simulations with the scale-dependent dynamic model yield the expected trends of the coefficient as a function of scale and give improved predictions of velocity spectra at different heights from the ground. Consistent with the improved dissipation characteristics, the scale-dependent model also yields improved mean velocity profiles.

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