Assessment of Vertical Mesh Refinement in Concurrently Nested Large-Eddy Simulations Using the Weather Research and Forecasting Model

2017 ◽  
Vol 145 (8) ◽  
pp. 3025-3048 ◽  
Author(s):  
Jeffrey D. Mirocha ◽  
Katherine A. Lundquist
Keyword(s):  
Mesh Refinement ◽  
Large Eddy Simulations ◽  
Weather Research And Forecasting ◽  
Forecasting Model ◽  
Grid Refinement ◽  
Near Surface ◽  
Aspect Ratios ◽  
Large Eddy ◽  
Vertical Grid ◽  
Parameter Values

To facilitate multiscale simulation using the Weather Research and Forecasting Model, vertical mesh refinement for one-way concurrent nested simulation was recently introduced. Grid refinement in the vertical dimension removes the requirement of different grid aspect ratios on the bounding versus the nested domain, such that results from refinement are in the horizontal directions only, and thereby can also reduce numerical errors on the bounding domain arising from large aspect ratios in the presence of complex terrain. Herein, the impacts of vertical grid refinement on the evolving downstream flow in nested large-eddy simulations are evaluated in relation to other model configuration choices, including turbulence subfilter-scale (SFS) stress models, mesh configuration, and alternative methods for calculating several near-surface flow parameters. Although vertical nesting requires coarsening of the vertical grid on the bounding domain, leading to a smaller range of resolved turbulence scales in the nest’s lateral boundary conditions, parameter values within the nested domains are generally only minimally impacted, relative to nesting using the same vertical grid on each domain. Two dynamic SFS models examined herein generally improved the simulated mean wind speed, turbulence kinetic energy, stresses and spectra, on both domains, and accelerated equilibration rates within nested domains, relative to two constant coefficient models. A new method of extrapolating horizontal velocity components to near-surface locations at nested domain lateral boundaries, and a correction to the calculation of deformation elements near the surface, are each shown to slightly alter the mean parameter values, yet only minimally impact equilibration rates within the nested domain.

scholarly journals Transition and Equilibration of Neutral Atmospheric Boundary Layer Flow in One-Way Nested Large-Eddy Simulations Using the Weather Research and Forecasting Model

2013 ◽  
Vol 141 (3) ◽  
pp. 918-940 ◽  
Author(s):  
Jeff Mirocha ◽  
Gokhan Kirkil ◽  
Elie Bou-Zeid ◽  
Fotini Katopodes Chow ◽  
Branko Kosović
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Large Eddy Simulations ◽  
Single Domain ◽  
Surface Velocity ◽  
Weather Research And Forecasting ◽  
Forecasting Model ◽  
Near Surface ◽  
Large Eddy ◽  
Weather Research

Abstract The Weather Research and Forecasting Model permits finescale large-eddy simulations (LES) to be nested within coarser simulations, an approach that can generate more accurate turbulence statistics and improve other aspects of simulated flows. However, errors are introduced into the finer domain from the nesting methodology. Comparing nested domain, flat-terrain simulations of the neutral atmospheric boundary layer with single-domain simulations using the same mesh, but instead using periodic lateral boundary conditions, reveals the errors contributed to the nested solution from the parent domain and nest interfaces. Comparison of velocity spectra shows good agreement among higher frequencies, but greater power predicted on the nested domain at lower frequencies. Profiles of mean wind speed show significant near-surface deficits near the inflow boundaries, but equilibrate to improved values with distance. Profiles of the vertical flux of x momentum show significant underprediction by the nested domain close to the surface and near the inlet boundaries. While these underpredictions of the stresses, which cause the near-surface velocity deficits, attenuate with distance within the nested domains, significant errors remain throughout. Profiles of the resolved turbulence kinetic energy show considerable deviations from their single-domain values throughout the nested domains. The authors examine the accuracy of these parameters and their sensitivities to the turbulence subfilter stress model, mesh resolution, and grid aspect ratio, and provide guidance to practitioners of nested LES.

scholarly journals Resolved Turbulence Characteristics in Large-Eddy Simulations Nested within Mesoscale Simulations Using the Weather Research and Forecasting Model

2014 ◽  
Vol 142 (2) ◽  
pp. 806-831 ◽  
Author(s):  
Jeff Mirocha ◽  
Branko Kosović ◽  
Gokhan Kirkil
Keyword(s):  
Heat Fluxes ◽  
Large Eddy Simulations ◽  
Forced Flow ◽  
Weather Research And Forecasting ◽  
Forecasting Model ◽  
Surface Heat Fluxes ◽  
Large Eddy ◽  
Stress Models ◽  
Mesoscale Simulations ◽  
Weather Research

Abstract One-way concurrent nesting within the Weather Research and Forecasting Model (WRF) is examined for conducting large-eddy simulations (LES) nested within mesoscale simulations. Wind speed, spectra, and resolved turbulent stresses and turbulence kinetic energy from the nested LES are compared with data from nonnested simulations using periodic lateral boundary conditions. Six different subfilter-scale (SFS) stress models are evaluated using two different nesting strategies under geostrophically forced flow over both flat and hilly terrain. Neutral and weakly convective conditions are examined. For neutral flow over flat terrain, turbulence appears on the nested LES domains only when using the two dynamic SFS stress models. The addition of small hills and valleys (wavelengths of 2.4 km and maximum slopes of ± 10°) yields small improvements, with all six models producing some turbulence on nested domains. Weak convection (surface heat fluxes of 10 W m−2) further accelerates the development of turbulence on all nested domains. However, considerable differences in key parameters are observed between the nested LES domains and their nonnested counterparts. Nesting of a finer LES within a coarser LES provides superior results to using only one nested LES domain. Adding temperature and velocity perturbations near the inlet planes of nested domains shows promise as an easy-to-implement method to accelerate turbulence generation and improve its accuracy on nested domains.

scholarly journals A New Vertical Grid Nesting Capability in the Weather Research and Forecasting (WRF) Model

2016 ◽  
Vol 144 (10) ◽  
pp. 3725-3747 ◽  
Author(s):  
Megan H. Daniels ◽  
Katherine A. Lundquist ◽  
Jeffrey D. Mirocha ◽  
David J. Wiersema ◽  
Fotini K. Chow
Keyword(s):  
Mesh Refinement ◽  
Wrf Model ◽  
Weather Research And Forecasting ◽  
Numerical Errors ◽  
Aspect Ratios ◽  
Neutral Boundary Layer ◽  
Vertical Grid ◽  
Concurrent Simulation ◽  
Grid Nesting ◽  
Weather Research

Mesoscale atmospheric models are increasingly used for high-resolution (<3 km) simulations to better resolve smaller-scale flow details. Increased resolution is achieved using mesh refinement via grid nesting, a procedure where multiple computational domains are integrated either concurrently or in series. A constraint in the concurrent nesting framework offered by the Weather Research and Forecasting (WRF) Model is that mesh refinement is restricted to the horizontal dimensions. This limitation prevents control of the grid aspect ratio, leading to numerical errors due to poor grid quality and preventing grid optimization. Herein, a procedure permitting vertical nesting for one-way concurrent simulation is developed and validated through idealized cases. The benefits of vertical nesting are demonstrated using both mesoscale and large-eddy simulations (LES). Mesoscale simulations of the Terrain-Induced Rotor Experiment (T-REX) show that vertical grid nesting can alleviate numerical errors due to large aspect ratios on coarse grids, while allowing for higher vertical resolution on fine grids. Furthermore, the coarsening of the parent domain does not result in a significant loss of accuracy on the nested domain. LES of neutral boundary layer flow shows that, by permitting optimal grid aspect ratios on both parent and nested domains, use of vertical nesting yields improved agreement with the theoretical logarithmic velocity profile on both domains. Vertical grid nesting in WRF opens the path forward for multiscale simulations, allowing more accurate simulations spanning a wider range of scales than previously possible.

Using a Canopy Model Framework to Improve Large-Eddy Simulations of the Neutral Atmospheric Boundary Layer in the Weather Research and Forecasting Model

2019 ◽  
Vol 147 (1) ◽  
pp. 31-52 ◽  
Author(s):  
Robert S. Arthur ◽  
Jeffrey D. Mirocha ◽  
Katherine A. Lundquist ◽  
Robert L. Street
Keyword(s):  
Boundary Layer ◽  
Boundary Condition ◽  
Large Eddy Simulations ◽  
Weather Research And Forecasting ◽  
Forecasting Model ◽  
Model Framework ◽  
Roughness Elements ◽  
Large Eddy ◽  
Log Law ◽  
Canopy Model

A canopy model framework is implemented in the Weather Research and Forecasting Model to improve the accuracy of large-eddy simulations (LES) of the atmospheric boundary layer (ABL). The model includes two options that depend on the scale of surface roughness elements. A resolved canopy model, typically used to model flow through vegetation canopies, is employed when roughness elements are resolved by the vertical LES grid. In the case of unresolved roughness, a modified “pseudocanopy model” is developed to distribute drag over a shallow layer above the surface. Both canopy model options are validated against idealized test cases in neutral stability conditions and are shown to improve surface layer velocity profiles relative to simulations employing Monin–Obukhov similarity theory (MOST), which is commonly used as a surface boundary condition in ABL models. Use of the canopy model framework also leads to increased levels of resolved turbulence kinetic energy and turbulent stresses. Because LES of the ABL has a well-known difficulty recovering the expected logarithmic velocity profile (log law) in the surface layer, particular focus is placed on using the pseudocanopy model to alleviate this issue over a range of model configurations. Tests with varying surface roughness values, LES closures, and grid aspect ratios confirm that the pseudocanopy model generally improves log-law agreement relative to simulations that employ a standard MOST boundary condition. The canopy model framework thus represents a low-cost, easy-to-implement method for improving LES of the ABL.

High Resolution Large Eddy Simulations to Evaluate Turbulence Properties Within a Real Helicopter Engine Combustor

2018 ◽  
Author(s):  
Charlie Koupper ◽  
Jean Lamouroux ◽  
Stephane Richard ◽  
Gabriel Staffelbach
Keyword(s):  
Gas Turbine ◽  
Turbulence Intensity ◽  
Numerical Scheme ◽  
Mesh Refinement ◽  
Large Eddy Simulations ◽  
Scale Model ◽  
Large Eddy ◽  
Set Up ◽  
High Level ◽  
The Impact

In a gas turbine, the combustor is feeding the turbine with hot gases at a high level of turbulence which in turns strongly enhances the heat transfer in the turbine. It is thus of primary importance to properly characterize the turbulence properties found at the exit of a combustor to design the turbine at its real thermal constraint. This being said, real engine measurements of turbulence are extremely rare if not inexistent because of the harsh environment and difficulty to implement experimental techniques that usually operate at isothermal conditions (e.g. hot wire anemometry). As a counterpart, high fidelity unsteady numerical simulations using Large Eddy Simulations (LES) are now mature enough to simulate combustion processes and turbulence within gas turbine combustors. It is thus proposed here to assess the LES methodology to qualify turbulence within a real helicopter engine combustor operating at take-off conditions. In LES, the development of turbulence is primarily driven by the level of real viscosity in the calculation, which is the sum of three contributions: laminar (temperature linked), turbulent (generated by the sub-grid scale model) and artificial (numerics dependent). In this study, the impact of the two main sources of un-desired viscosity is investigated: the mesh refinement and numerical scheme. To do so, three grids containing 11, 33 and 220 million cells for a periodic sector of the combustor are tested as well as centred second (Lax-Wendroff) and third order (TTGC) in space schemes. The turbulence properties (intensity and integral scales) are evaluated based on highly sampled instantaneous solutions and compared between the available simulations. Results show first that the duration of the simulation is important to properly capture the level of turbulence. If short simulations (a few combustor through-times) may be sufficient to evaluate the turbulence intensity, a bias up to 14% is introduced for the turbulence length scales. In terms of calculation set-up, the mesh refinement is found to have a limited influence on the turbulence properties. The numerical scheme influence on the quantities studied here is small, highlighting that the employed schemes dissipation properties are already sufficient for turbulence characterization. Finally, spatially averaged values of turbulence intensity and lengthscale at the combustor exit are almost identically predicted in all cases. However, significant variations from hub to tip are reported, which questions the pertinence to use 0-D turbulence boundary conditions for turbines. Based on the set of simulations discussed in the paper, guidelines can be derived to adequately set-up (mesh, scheme) and run (duration, acquisition frequency) a LES when turbulence evaluation is concerned. As no experimental counterpart to this study is available, the conclusions mainly aim at knowing the possible numerical bias rather than commenting on the predictivity of the approach.

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 Lattice Boltzmann simulation of turbulent natural convection in tall enclosures

2015 ◽  
Vol 19 (1) ◽  
pp. 155-166 ◽  
Author(s):  
Hasan Sajjadi ◽  
Reza Kefayati
Keyword(s):  
Nusselt Number ◽  
Natural Convection ◽  
Lattice Boltzmann ◽  
Average Nusselt Number ◽  
Large Eddy Simulations ◽  
Turbulent Natural Convection ◽  
Aspect Ratios ◽  
Lattice Boltzmann Simulation ◽  
Large Eddy ◽  
Rayleigh Numbers

In this paper Lattice Boltzmann simulation of turbulent natural convection with large-eddy simulations (LES) in tall enclosures which is filled by air with Pr=0.71 has been studied. Calculations were performed for high Rayleigh numbers (Ra=107-109) and aspect ratios change between 0.5 to 2 (0.5<AR<2). The present results are validated by finds of an experimental research at Ra=1.58x109. Effects of the aspect ratios in different Rayleigh numbers are displayed on streamlines, isotherm counters, vertical velocity and temperature at the middle of the cavity, local Nusselt number and average Nusselt number. The average Nusselt number increases with the augmentation of Rayleigh numbers. The increment of the aspect ratio causes heat transfer to decline in different Rayleigh numbers.

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.

Sign in / Sign up

Export Citation Format

Share Document