Effect of Turbulence Models and Spatial Resolution on Resolved Velocity Structure and Momentum Fluxes in Large-Eddy Simulations of Neutral Boundary Layer Flow

2009 ◽  
Vol 48 (6) ◽  
pp. 1161-1180 ◽  
Author(s):  
Francis L. Ludwig ◽  
Fotini Katopodes Chow ◽  
Robert L. Street
Keyword(s):  
Dynamic Models ◽  
Turbulence Models ◽  
Coarse Grid ◽  
Large Eddy Simulations ◽  
Near Surface ◽  
Velocity Statistics ◽  
Neutral Boundary Layer ◽  
Regional Prediction ◽  
Large Eddy ◽  
Flow Features

Abstract This paper demonstrates the importance of high-quality subfilter-scale turbulence models in large-eddy simulations by evaluating the resolved-scale flow features that result from various closure models. The Advanced Regional Prediction System (ARPS) model was used to simulate neutral flow over a 1.2-km square, flat, rough surface with seven subfilter turbulence models [Smagorinsky, turbulent kinetic energy (TKE)-1.5, and five dynamic reconstruction combinations]. These turbulence models were previously compared with similarity theory. Here, the differences are evaluated using mean velocity statistics and the spatial structure of the flow field. Streamwise velocity averages generally differ among models by less than 0.5 m s−1, but those differences are often significant at a 95% confidence level. Flow features vary considerably among models. As measured by spatial correlation, resolved flow features grow larger and less elongated with height for a given model and resolution. The largest differences are between dynamic models that allow energy backscatter from small to large scales and the simple eddy-viscosity closures. At low altitudes, the linear extent of Smagorinsky and TKE-1.5 structures exceeds those of dynamic models, but the relationship reverses at higher altitudes. Ejection, sweep, and upward momentum flux features differ among models and from observed neutral atmospheric flows, especially for Smagorinsky and TKE-1.5 coarse-grid simulations. Near-surface isopleths separating upward fluxes from downward are shortest for the Smagorinsky and TKE-1.5 coarse-grid simulations, indicating less convoluted turbulent interfaces; at higher altitudes they are longest. Large-eddy simulation (LES) is a powerful simulation tool, but choices of grid resolution and subfilter model can affect results significantly. Physically realistic dynamic mixed models, such as those presented here, are essential when using LES to study atmospheric processes such as transport and dispersion—in particular at coarse resolutions.

High-Resolution Large-Eddy Simulations of Flow in a Steep Alpine Valley. Part I: Methodology, Verification, and Sensitivity Experiments

2006 ◽  
Vol 45 (1) ◽  
pp. 63-86 ◽  
Author(s):  
Fotini Katopodes Chow ◽  
Andreas P. Weigel ◽  
Robert L. Street ◽  
Mathias W. Rotach ◽  
Ming Xue
Keyword(s):  
Land Use ◽  
Soil Moisture ◽  
High Resolution ◽  
Turbulence Models ◽  
Large Eddy Simulations ◽  
Swiss Alps ◽  
Advanced Regional Prediction System ◽  
Alpine Valley ◽  
Regional Prediction ◽  
Large Eddy

Abstract This paper investigates the steps necessary to achieve accurate simulations of flow over steep, mountainous terrain. Large-eddy simulations of flow in the Riviera Valley in the southern Swiss Alps are performed at horizontal resolutions as fine as 150 m using the Advanced Regional Prediction System. Comparisons are made with surface station and radiosonde measurements from the Mesoscale Alpine Programme (MAP)-Riviera project field campaign of 1999. Excellent agreement between simulations and observations is obtained, but only when high-resolution surface datasets are used and the nested grid configurations are carefully chosen. Simply increasing spatial resolution without incorporating improved surface data gives unsatisfactory results. The sensitivity of the results to initial soil moisture, land use data, grid resolution, topographic shading, and turbulence models is explored. Even with strong thermal forcing, the onset and magnitude of the upvalley winds are highly sensitive to surface processes in areas that are well outside the high-resolution domain. In particular, the soil moisture initialization on the 1-km grid is found to be crucial to the success of the finer-resolution predictions. High-resolution soil moisture and land use data on the 350-m-resolution grid also improve results. The use of topographic shading improves radiation curves during sunrise and sunset, but the effects on the overall flow are limited because of the strong lateral boundary forcing from the 1-km grid where terrain slopes are not well resolved. The influence of the turbulence closure is also limited because of strong lateral forcing and hence limited residence time of air inside the valley and because of the stable stratification, which limits turbulent stress to the lowest few hundred meters near the surface.

scholarly journals Pressure-Gradient Forcing Methods for Large-Eddy Simulations of Flows in the Lower Atmospheric Boundary Layer

Atmosphere ◽  
2020 ◽  
Vol 11 (12) ◽  
pp. 1343
Author(s):  
François Pimont ◽  
Jean-Luc Dupuy ◽  
Rodman R. Linn ◽  
Jeremy A. Sauer ◽  
Domingo Muñoz-Esparza
Keyword(s):  
Pressure Gradient ◽  
Turbulent Flows ◽  
Large Eddy Simulations ◽  
Gradient Force ◽  
Pressure Gradient Force ◽  
Computational Domain ◽  
Near Surface ◽  
Large Eddy ◽  
Flow Features ◽  
The Stability

Turbulent flows over forest canopies have been successfully modeled using Large-Eddy Simulations (LES). Simulated winds result from the balance between a simplified pressure gradient forcing (e.g., a constant pressure-gradient or a canonical Ekman balance) and the dissipation of momentum, due to vegetation drag. Little attention has been paid to the impacts of these forcing methods on flow features, despite practical challenges and unrealistic features, such as establishing stationary velocity or streak locking. This study presents a technique for capturing the effects of a pressure-gradient force (PGF), associated with atmospheric patterns much larger than the computational domain for idealized simulations of near-surface phenomena. Four variants of this new PGF are compared to existing forcings, for turbulence statistics, spectra, and temporal averages of flow fields. Results demonstrate that most features of the turbulent flow are captured. The variants can either enable modelers to prescribe a wind speed and direction at a reference height close to the ground as required in wildfire simulations, and/or mitigate streaks locking by reproducing the stability of the Ekman balance. Conditions of use, benefits, and drawbacks are discussed. PGF approaches, therefore, provide a viable solution for precursor inflows, including for the specific domains used in fire simulations.

Estimating the Risk of Extreme Wind Gusts in Tropical Cyclones Using Idealized Large-Eddy Simulations and a Statistical-Dynamical Model

2021 ◽  
Author(s):  
Daniel P. Stern ◽  
George H. Bryan ◽  
Chia-Ying Lee ◽  
James D. Doyle
Keyword(s):  
Tropical Cyclones ◽  
The United States ◽  
Large Eddy Simulations ◽  
Dynamical Model ◽  
Near Surface ◽  
Southeastern Us ◽  
Wind Gusts ◽  
Extreme Wind ◽  
Large Eddy

AbstractRecent studies have shown that extreme wind gusts are ubiquitous within the eyewall of intense tropical cyclones (TCs). These gusts pose a substantial hazard to human life and property, but both the short-term (i.e., during the passage of a single TC) and long-term (over many years) risk of encountering such a gust at a given location is poorly understood. Here, simulated tower data from large-eddy simulations of idealized TCs in a quiescent (i.e., no mean flow or vertical wind shear) environment are used to estimate these risks for the offshore region of the United States. For both a category 5 and category 3 TC, there is a radial region where nearly all simulated towers experience near-surface (the lowest 200 m) 3-s gusts exceeding 70 m s−1 within a 10-minute period; on average, these towers respectively sample peak 3-s gusts of 110 and 80 m s−1. Analysis of an observational dropsonde database supports the idealized simulations, and indicates that offshore structures (such as wind turbines) in the eyewall of a major hurricane are likely to encounter damaging wind speeds. This result is then incorporated into an estimate of the long-term risk, using analyses of the return period for major hurricanes from both a best-track database and a statistical-dynamical model forced by reanalysis. For much of the nearshore region of the Gulf of Mexico and southeastern US coasts, this analysis yields an estimate of a 30-60% probability of any given point experiencing at least one 70 m s−1 gust within a 30-year period.

scholarly journals Study of near-surface models for large-eddy simulations of a neutrally stratified atmospheric boundary layer

2007 ◽  
Vol 124 (3) ◽  
pp. 405-424 ◽  
Author(s):  
Inanc Senocak ◽  
Andrew S. Ackerman ◽  
Michael P. Kirkpatrick ◽  
David E. Stevens ◽  
Nagi N. Mansour
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Large Eddy Simulations ◽  
Near Surface ◽  
Large Eddy ◽  
Surface Models ◽  
Neutrally Stratified

Turbulence Models and Large Eddy Simulations Applied to Ascending Mixed Convection Flows

2012 ◽  
Vol 89 (3) ◽  
pp. 407-434 ◽  
Author(s):  
Amir Keshmiri ◽  
Mark A. Cotton ◽  
Yacine Addad ◽  
Dominique Laurence
Keyword(s):  
Mixed Convection ◽  
Turbulence Models ◽  
Large Eddy Simulations ◽  
Large Eddy

scholarly journals Near-Surface Intensification of Tornado Vortices

2007 ◽  
Vol 64 (7) ◽  
pp. 2176-2194 ◽  
Author(s):  
D. C. Lewellen ◽  
W. S. Lewellen
Keyword(s):  
Analytical Model ◽  
Broad Class ◽  
Surface Flow ◽  
Large Eddy Simulations ◽  
Fine Tuning ◽  
Near Surface ◽  
Corner Flow ◽  
Large Eddy ◽  
End Wall ◽  
Corner Flows

Abstract An idealized analytical model and numerical large-eddy simulations are used to explore fluid-dynamic mechanisms by which tornadoes may be intensified near the surface relative to conditions aloft. The analytical model generalizes a simple model of Barcilon and Fiedler and Rotunno for a steady supercritical end-wall vortex to more general vortex corner flows, angular momentum distributions, and time dependence. The model illustrates the role played by the corner flow swirl ratio in determining corner flow structure and intensification; predicts an intensification of near-surface swirl velocities relative to conditions aloft of Iυ ∼ 2 for supercritical end-wall vortices in agreement with earlier analytical, numerical, and laboratory results; and suggests how larger intensification factors might be achieved in some more general corner flows. Examples of the latter are presented using large-eddy simulations. By tuning the lateral inflow boundary conditions near the surface, quasi-steady vortices exhibiting nested inner and outer corner flows and Iυ ∼ 4 are produced. More significantly, these features can be produced without fine tuning, along with an additional doubling (or more) of the intensification, in a broad class of unsteady evolutions producing a dynamic corner flow collapse. These scenarios, triggered purely by changes in the far-field near-surface flow, provide an attractive mechanism for naturally achieving an intense near-surface vortex from a much larger-scale less-intense swirling flow. It is argued that, applied on different scales, this may sometimes play a role in tornadogenesis and/or tornado variability. This phenomenon of corner flow collapse is considered further in a companion paper.

Stochastic backscatter in large-eddy simulations of boundary layers

1992 ◽  
Vol 242 ◽  
pp. 51-78 ◽  
Author(s):  
P. J. Mason ◽  
D. J. Thomson
Keyword(s):  
Large Scale ◽  
Flow Simulation ◽  
Surface Region ◽  
Large Eddy Simulations ◽  
Flow Profile ◽  
Mean Velocity ◽  
Near Surface ◽  
Eddy Simulation ◽  
Subgrid Model ◽  
Large Eddy

The ability of a large-eddy simulation to represent the large-scale motions in the interior of a turbulent flow is well established. However, concerns remain for the behaviour close to rigid surfaces where, with the exception of low-Reynolds-number flows, the large-eddy description must be matched to some description of the flow in which all except the larger-scale ‘inactive’ motions are averaged. The performance of large-eddy simulations in this near-surface region is investigated and it is pointed out that in previous simulations the mean velocity profile in the matching region has not had a logarithmic form. A number of new simulations are conducted with the Smagorinsky (1963) subgrid model. These also show departures from the logarithmic profile and suggest that it may not be possible to eliminate the error by adjustments of the subgrid lengthscale. An obvious defect of the Smagorinsky model is its failure to represent stochastic subgrid stress variations. It is shown that inclusion of these variations leads to a marked improvement in the near-wall flow simulation. The constant of proportionality between the magnitude of the fluctuations in stress and the Smagorinsky stresses has been empirically determined to give an accurate logarithmic flow profile. This value provides an energy backscatter rate slightly larger than the dissipation rate and equal to idealized theoretical predictions (Chasnov 1991).

Effects of Parallel Processing on Large Eddy Simulations in ANSYS Fluent

2016 ◽  
Author(s):  
Puxuan Li ◽  
Steve J. Eckels ◽  
Ning Zhang ◽  
Garrett W. Mann
Keyword(s):  
Parallel Processing ◽  
Data Transfer ◽  
Turbulence Models ◽  
Large Eddy Simulations ◽  
Grid Turbulence ◽  
Ansys Fluent ◽  
Square Duct ◽  
Eddy Simulation ◽  
Eddy Viscosity Model ◽  
Large Eddy

Parallel processing is an effective computation in which many calculations are carried out simultaneously. In this paper, effects of shared-memory parallel processing on Large Eddy Simulations (LES) in ANSYS Fluent are presented. Fluent provides parallel processing to improve the speed of running programs. LES is one of the most popular viscosity models for turbulence used in computational fluid dynamics (CFD). Three kinds of LES with different sub-grid turbulence models were evaluated: Smagorinsky-Lilly Model (Lilly model), Wall-Adapting Local Eddy-viscosity Model (WALE model) and Wall Modeled Large Eddy Simulation (WMLES model). The running speed of the different models simulating a square duct on a single computer are compared. The relationship between wall-clock time and number of processors reveals the performances of different LES models. The part of the time that is not parallelizable such as file IO and data transfer is also considered based on Amdahl’s law.

Sign in / Sign up

Export Citation Format

Share Document