Accuracy Of Large Eddy Simulation Research Articles

A Statistical Evaluation of WRF-LES Trace Gas Dispersion Using Project Prairie Grass Measurements

AbstractIn recent years, new measurement systems have been deployed to monitor and quantify methane emissions from the natural gas sector. Large-eddy simulation (LES) has complemented measurement campaigns by serving as a controlled environment in which to study plume dynamics and sampling strategies. However, with few comparisons to controlled-release experiments, the accuracy of LES for modeling natural gas emissions is poorly characterized. In this paper, we evaluate LES from the Weather Research and Forecasting (WRF) model against Project Prairie Grass campaign measurements and surface layer similarity theory. Using WRF-LES, we simulate continuous emissions from 30 near-surface trace gas sources in two stability regimes: strong and weak convection. We examine the impact of grid resolutions ranging from 6.25 m to 52 m in the horizontal dimension on model results. We evaluate performance in a statistical framework, calculating fractional bias and conducting Welch’s t-tests. WRF-LES accurately simulates observed surface concentrations at 100 m and beyond under strong convection; simulated concentrations pass t-tests in this region irrespective of grid resolution. However, in weakly convective conditions with strong winds, WRF-LES substantially overpredicts concentrations – the magnitude of fractional bias often exceeds 30%, and all but one C-test fails. The good performance of WRF-LES under strong convection correlates with agreement with local free convection theory and a minimal amount of parameterized turbulent kinetic energy. The poor performance under weak convection corresponds to misalignment with Monin-Obukhov similarity theory and a significant amount of parameterized turbulent kinetic energy.

Large-eddy simulation of pollutant dispersion in generic urban street canyons: Guidelines for domain size

Pollutant dispersion in urban street canyons has been widely investigated by large-eddy simulation (LES). Many LES studies focused on generic street canyons under a wind direction perpendicular to the street axis. Accurate LES simulations require a sufficiently large domain size to minimize the effects of the artificial boundary conditions at the domain faces on the results. As opposed to RANS simulations, there is a lack of guidelines for an appropriate domain size for LES simulations of wind flow and pollutant dispersion in street canyons. The present study systematically investigates the effect of the domain width, domain height and upstream and downstream domain lengths on the wind flow and pollutant dispersion within a generic 2.5D street canyon with spanwise periodic boundary conditions. Following a validation study, 16 LES simulations are performed for different domain sizes. The results show that the minimum requirement for the domain width is 2.5H, where H is the roof height of the street canyon. For the domain height, upstream domain length and downstream domain length, 7.5H, 5H and 10H are recommended, respectively. These guidelines should help to reduce the computational costs of this type of simulation without significantly compromising the accuracy.

Role of Inflow Turbulence and Surrounding Buildings on Large Eddy Simulations of Urban Wind Energy

Predicting flow patterns that develop on the roof of high-rise buildings is critical for the development of urban wind energy. In particular, the performance and reliability of devices largely depends on the positioning strategy, a major unresolved challenge. This work aims at investigating the effect of variations in the turbulent inflow and the geometric model on the flow patterns that develop on the roof of tall buildings in the realistic configuration of the University of Birmingham’s campus in the United Kingdom (UK). Results confirm that the accuracy of Large Eddy Simulation (LES) predictions is only marginally affected by differences in the inflow mean wind speed and turbulence intensity, provided that turbulence is not absent. The effect of the presence of surrounding buildings is also investigated and found to be marginal to the results if the inflow is turbulent. The integral length scale is the parameter most affected by the turbulence characteristics of the inflow, while gustiness is only marginally influenced. This work will contribute to LES applications on the urban wind resource and their computational setup simplification.

Some notes on numerical simulation of the turbulent cavitating flow with a dynamic cubic nonlinear sub-grid scale model in OpenFOAM

The accuracy of large eddy simulation (LES) is highly dependent on the performance of sub-grid scale (SGS) model. In the present paper, a dynamic cubic nonlinear sub-grid scale model (DCNM) proposed by Huang et al. is implemented for the simulation of unsteady cavitating flow around a 3-D Clark-Y hydrofoil in OpenFOAM. Its performance in predicting the evolution of cloud cavitation is discussed in detail. The simulation with a linear model, the dynamic Smagorinsky model (DSM), is also conducted as a comparison. The results with DCNM show a better agreement with the available experimental observation. The comparison between DCNM and DSM further suggests that the DCNM is able to predict the backscatter more precisely, which is an important feature in LES. The characteristics of DCNM is analyzed to account for its advantages in the prediction of unsteady cloud cavitation as well. The results reveal that it is the nonlinear terms of DCNM that makes DCNM capture sub-grid scale vortices better and more suitable for studying the transient behaviors of cloud cavitation than DSM.

Assessment of Grid Anisotropy Effects on Large-Eddy-Simulation Models with Different Length Scales

The effect of the eddy viscosity length scale on the accuracy of large-eddy simulation (LES) is investigated for several commonly used subgrid models. Multiple types of length scales are considered, both purely geometric length scales (including the cube root of the cell volume and the maximum cell dimension) and length scales that depend on both the grid and the flow. The assessment focuses on how the LES prediction changes for increasingly anisotropic grids in forced isotropic turbulence and channel flow at high Reynolds number. Although some length scales perform better than others, the main conclusion is that no single length scale produces LES results that are completely independent of the grid anisotropy. Because every model and length scale tested produces accurate results on at least one grid and inaccurate results on at least one other grid, the study shows clearly the need to assess LES models on multiple grids with multiple different anisotropy ratios.

Characteristics of conical vortices and their effects on wind pressures on flat-roof-mounted solar arrays by LES

Large eddy simulations (LES) are performed to examine the flow characteristics around solar arrays mounted on a flat-roof building for two oblique wind directions, 45° and 135°. The main purpose of this study is to clarify the mechanisms of some special features of wind pressure characteristics acting on solar panels obtained by wind tunnel experiments. Numerical simulation is first conducted for a flat-roof building without solar arrays to validate the accuracy of LES for oblique wind directions. Numerical simulations are then made for a flat-roof building with solar arrays. For 45° wind direction, local separations and reattachments of the downflows of the left-side conical vortex are found to play important roles in large negative pressures near the higher edges of the upper surfaces and positive pressures on some parts of the lower surfaces of solar panels. For 135° wind direction, local vortices generated from the higher edges of the solar panels form local wake regions downwind of the panels and cause downward net pressures on most of the panels. The effects of the gaps on pressures on the upper surfaces of solar arrays are not significant, but those on the lower surfaces are significant because of the flows passing under the panels.

LES study of wind pressure and flow characteristics of flat-roof-mounted solar arrays

Wind loading is a major concern for solar array systems. To clarify the relations between flow field and wind pressure distributions on solar panels, large eddy simulations (LES) are performed to examine the flow characteristics around solar arrays mounted on a flat roof building for two normal wind directions, 0° and 180°. The mean, standard deviation and peak value of pressure and force coefficients obtained from LES and wind tunnel experiment are firstly compared to validate the accuracy of LES. Then the mean and unsteady flow fields around solar arrays are discussed to explain the corresponding wind pressure distributions. The local separation at the higher edges of solar panels located inside of both the separation bubble and the reattached flow are detected. The wind pressures on upper and lower surfaces of solar panels resulted by the local flow field are discussed accordingly for different scenarios. Ensemble-averaged flow fields on the condition of the occurrence of peak pressures at the higher edge of solar panels are also investigated. It is found that besides the large-scale separating and reattaching flow induced by the building, small-scale local separations at the higher edges of the solar panels are also responsible for the large peak of uplift.

Large eddy simulations of wind loads on an external floating-roof tank

Wind load of an external floating-roof tank is analyzed using large eddy simulation (LES). First, the governing equations and a LES solution procedure employing FLUENT software are presented. Subsequently, using two benchmark tank models, the accuracy of LES is examined and the advantages of LES over wind Reynolds-averaged Navier-Stokes (RANS) method in computing the unsteady wind loads and vortex structures around a tank are validated. Finally, using LES, the turbulent flows and wind loads around an external floating-roof tank at varying Reynolds numbers and liquid levels are analyzed. The DSM SGS model was found to have better accuracy when LES is applied to the tank flow. For an external floating-roof tank, the Reynolds number has little influence on the wind load at Re > 106, while the liquid level affects significantly both the wind loads on the tank and the vortex structures in the flow, and the wind pressure on the internal wall distributes more evenly than that on the external wall, due to the recirculation of the flow in the cavity above the roof. In all cases, the pressure on the external floating-roof tank oscillates apparently with time, which indicates the necessity of applying LES to the wind load analysis of tanks.

Large-Eddy Simulation and RANS Analysis of the End-Wall Flow in a Linear Low-Pressure Turbine Cascade, Part I: Flow and Secondary Vorticity Fields Under Varying Inlet Condition

Abstract In low-pressure turbines (LPTs), around 60–70% of losses are generated away from end-walls, while the remaining 30–40% is controlled by the interaction of the blade profile with the end-wall boundary layer. Experimental and numerical studies have shown how the strength and penetration of the secondary flow depends on the characteristics of the incoming end-wall boundary layer. Experimental techniques did shed light on the mechanism that controls the growth of the secondary vortices, and scale-resolving computational fluid dynamics (CFD) allowed to dive deep into the details of the vorticity generation. Along these lines, this paper discusses the end-wall flow characteristics of the T106 LPT profile at Re = 120 K and M = 0.59 by benchmarking with experiments and investigating the impact of the incoming boundary layer state. The simulations are carried out with proven Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) solvers to determine if Reynolds-averaged models can capture the relevant flow details with enough accuracy to drive the design of this flow region. Part I of the paper focuses on the critical grid needs to ensure accurate LES and on the analysis of the overall time-averaged flow field and comparison between RANS, LES, and measurements when available. In particular, the growth of secondary flow features, the trace and strength of the secondary vortex system, and its impact on the blade load variation along the span and end-wall flow visualizations are analyzed. The ability of LES and RANS to accurately predict the secondary flows is discussed together with the implications this has on design.

Extension of the subgrid-scale gradient model for compressible magnetohydrodynamics turbulent instabilities

Performing accurate large eddy simulations in compressible, turbulent magnetohydrodynamics (MHDs) is more challenging than in nonmagnetized fluids due to the complex interplay between kinetic, magnetic, and internal energy at different scales. Here, we extend the subgrid-scale gradient model, so far used in the momentum and induction equations, to also account for the unresolved scales in the energy evolution equation of a compressible ideal MHD fluid with a generic equation of state. We assess the model by considering box simulations of the turbulence triggered across a shear layer by the Kelvin-Helmholtz instability, testing cases where the small-scale dynamics cannot be fully captured by the resolution considered, such that the efficiency of the simulated dynamo effect depends on the resolution employed. This lack of numerical convergence is actually a currently common issue in several astrophysical problems, where the integral and fastest-growing-instability scales are too far apart to be fully covered numerically. We perform a priori and a posteriori tests of the extended gradient model. In the former, we find that, for many different initial conditions and resolutions, the gradient model outperforms other commonly used models in terms of correlation with the residuals coming from the filtering of a high-resolution run. In the second test, we show how a low-resolution run with the gradient model is able to quantitatively reproduce the evolution of the magnetic energy (the integrated value and the spectral distribution) coming from higher-resolution runs. This extension is the first step toward the implementation in relativistic MHDs.

High-order accurate large-eddy simulations of compressible viscous flow in cylindrical coordinates

Dynamic interactions of shock waves and turbulent structures occur in a wide range of applications. The accurate simulation of turbulence in these flows requires a numerical scheme with minimal dissipation whereas the shock-capturing requires a dissipative scheme to stabilise the solution in the shock-wave discontinuous regions. These contradictory requirements make simulations of these flows extremely challenging. A compatible implementation of the solid boundary conditions at the point of incepting the acoustic waves and their internalisation into hydrodynamic instabilities is also challenging.Here we present, a high-fidelity, massively parallel and accurate solver for both direct numerical simulations and large-eddy simulations of supersonic turbulent flows in cylindrical coordinates. A hybrid WENO/ high order central difference scheme is used for the spatial discretisation with a fourth-order five-stage 2N-storage Runge–Kutta for the time integration. The least square contraction of Lilly [1] is utilised for large-eddy subgrid-scale modelling. The solid surface boundary condition is implemented using the offset wall and ghost cell technique.The numerical implementation is validated through a wide range of test cases from simple to more complex cases. The numerical results show good agreement with analytical solutions and available experimental results. In the large-eddy simulation of a supersonic under-expanded impinging jet, it is observed that the conventional Ducros sensor leads to a more dissipative hybrid scheme. In this paper, a new shock sensor based on a WENO smoothness indicator is proposed and it is demonstrated to perform better especially in the simulation of the supersonic under-expanded impinging jet by limiting the expensive WENO scheme to the discontinuous regions and reducing the computational cost by approximately 10%.

Large-Eddy Simulations of the Steady Wintertime Antarctic Boundary Layer

Observations of two typical contrasting weakly stable and very stable boundary layers from the winter at Dome C station, Antarctica, are used as a benchmark for two centimetre-scale-resolution large-eddy simulations. By taking the Antarctic winter, the effects of the diurnal cycle are eliminated, enabling the study of the long-lived steady stable boundary layer. With its homogeneous, flat snow surface, and extreme stabilities, the location is a natural laboratory for studies on the long-lived stable boundary layer. The two simulations differ only in the imposed geostrophic wind speed, which is identified as the main deciding factor for the resulting regime. In general, a good correspondence is found between the observed and simulated profiles of mean wind speed and temperature. Discrepancies in the temperature profiles are likely due to the exclusion of radiative transfer in the current simulations. The extreme stabilities result in a considerable contrast between the stable boundary layer at the Dome C site and that found at typical mid-latitudes. The boundary-layer height is found to range from approximately 50,mathrm {m} to just 5,mathrm {m} in the most extreme case. Remarkably, heating of the boundary layer by subsidence may result in thermal equilibrium of the boundary layer in which the associated heating is balanced by the turbulent cooling towards the surface. Using centimetre-scale resolutions, accurate large-eddy simulations of the extreme stabilities encountered in Antarctica appear to be possible. However, future simulations should aim to include radiative transfer and sub-surface heat transport to increase the degree of realism of these types of simulations.

Large-eddy simulation of compressible turbulent flow in convergent-divergent nozzles with isothermal wall

We present results from high-order accurate large-eddy simulations of compressible flow through convergent-divergent nozzles with and without bell-shaped divergent portions. The nozzles have isothermal walls and circular cross-sections. The incoming flow is a turbulent, subsonic fully-developed pipe flow at M=0.37 and friction Reynolds number 214. A smooth transition from subsonic to supersonic flow occurs in the nozzles when the back pressure is kept small, accompanied by a reduction of turbulence intensities in the diverging portion of the nozzles. Strong reductions in production and pressure-strain correlations are observed as the flow undergoes acceleration and expansion. When the back pressure is high, the flow in the nozzle remains subsonic, accompanied by a reduction in turbulence in the convergent portion and an amplification in the divergent portion.

Assessment of artificial fluid properties for high-order accurate large-eddy simulations of shock-free compressible turbulent flows with strong temperature gradients

The capability of artificial fluid properties (AFP) to serve as a sub-grid scale (SGS) model and to provide stability in high-order accurate large-eddy simulations (LES) of shock-free compressible turbulent flows with strong temperature gradients is assessed. Predictive LES of such flows, which are directly relevant to applications such as engines, require an accurate description of turbulence, which typically makes them susceptible to issues of accuracy and numerical instabilities. In the present implementation, AFP schemes along with a high-order compact filter ensure robustness of otherwise non-dissipative high-order central difference spatial discretisation schemes. It is shown that a judicious choice of both filter and AFP parameters is required in order for AFP to be used as an accurate SGS model. Optimal parameter values for the AFP and the corresponding filter parameter are identified and validated in the framework of the current LES solver. First, an inviscid Taylor-Green vortex configuration is examined to evaluate the ideal combination of the filter parameter and the coefficients in the AFP framework. It is verified that AFP works as an alternative model for the unclosed subgrid-scale (SGS) dissipation terms. The AFP results are also compared to those from the dynamic Smagorinsky model (DSM), with good agreement. Second, the optimal parameter values of AFP obtained from the inviscid Taylor-Green vortex tests are validated on a series of test configurations with increasing complexity, that include an inviscid 1D advection problem involving a thermal contact discontinuity, an inviscid 2D Kelvin-Helmholtz instability test and 3D experimental unheated and heated spatially developing turbulent round jets. Robust and accurate predictions are shown with the LES solver employing the optimal model parameters in each of these tests. In particular, good agreement between the LES predictions and experimental measurements for both the flow and temperature statistics is achieved for the laboratory jets, demonstrating the accuracy of the LES/AFP simulations applied to practical turbulent configurations. The robustness of the solver in the presence of strong temperature gradients is also demonstrated by simulating a round jet case having very high temperature gradients similar to those occurring in reacting flows. In contrast, the LES/DSM simulation of this case is noted to be unstable, and failed to provide converged results.

Pedestrian-level wind and gust around buildings with a ‘lift-up’ design: Assessment of influence from surrounding buildings by adopting LES

An architectural feature, ‘lift-up’ (or elevated) design, has been reported as an effective design to improve weak pedestrian-level wind (PLW) conditions in a subtropical high-density city. This design provides a semi-outdoor space under the elevated building which allows wind to penetrate through, but the influence of surrounding buildings on its performance has been rarely reported. This study aims to assess the influence of surrounding buildings on the PLW around an elevated building. LES (large eddy simulation) approach is applied after validation against the previous wind tunnel tests. Validation studies have confirmed and proven the accuracy of LES for the simulations, with the correlation coefficients R above 0.89 between the predictions and the experiments. Furthermore, the differences of flow fields are assessed around a single elevated building and a building array with an open space, a building and an elevated building respectively in the centre. Results indicate that wind amplification exists within the semi-outdoor space under the elevated building in all conditions although this effect is partially lowered by the surrounding buildings. The fields of turbulence intensity and gust wind speed around the surrounded elevated building are investigated by LES. These findings can be helpful for using the ‘lift-up’ design to improve wind and thermal comfort at pedestrian level.

A novel method for expressing anisotropy of subgrid-scale components for thermal and scalar fields

Some types of mixed subgrid-scale (SGS) models combining an isotropic eddy-viscosity model and a scale-similarity model can be used to effectively increase the accuracy of large eddy simulation (LES). For example, Abe (2013) recently proposed a stabilized mixed model, which can successfully express the anisotropy of the SGS stress, and remarkably improves the predictive performance for wall turbulence at coarse grid resolutions without sacrificing computational stability. In the present work, this approach is extended for thermal and scalar field modeling to express the anisotropy of SGS heat (scalar) flux. A priori tests using direct numerical simulation data demonstrated that the proposed model can accurately predict the anisotropic SGS heat flux in channel flow and its variation with Prandtl number in the range 0.1–2. Its effectiveness is also confirmed by a posteriori tests under flow conditions corresponding to those in the a priori tests.

Large-eddy simulation of Sandia Flame F using structural subgrid-scale models and partially-stirred-reactor approach

Owing to the strong interaction between turbulence and combustion, it is particularly challenging to accurately predict local flame extinctions in a turbulent flame at high Reynolds numbers. Subgrid-scale (SGS) parameterization and model for calculating the filtered reaction rates are the main determinants of an accurate large-eddy simulation (LES) of turbulent flow. This study integrates the recently introduced gradient-type structural SGS models with a simplified partially-stirred-reactor approach to simulate a piloted partially premixed jet flame, Sandia Flame F. An advantage of using the nonlinear SGS models is that they can provide reverse energy transfer from subgrid to resolved scales. To quantitatively understand the performance of the LES framework, we have comprehensively compared temperature and mass fractions of major and minor species with experimental data. The statistics of the simulated field show good agreement with measurements and a notable improvement over previous simulations. Results support the assertion that the proposed nonlinear LES framework can capture extinction and re-ignition in turbulent flames with reasonable computational cost.

On the accuracy of Large Eddy Simulation of multiple impinging jets

Large Eddy Simulation results are often claimed to have sufficient accuracy to be used for development and validation of statistical turbulence models. This requires not only a verification of LES results against laboratory or numerical experiments, but also a quantification of modeling uncertainty, numerical errors, etc. A good agreement of our LES results with the experiment was already demonstrated in our previous work (Draksler et al., 2014). Thus, in the present work a sensitivity study regarding the grid refinement and the choice of SGS model coefficient is reported. Discussion is based on time-averaged simulation results; therefore, the sampling error is evaluated as well. The experimental test case with 13 air jets at Reynolds number around 20,000 impinging on the heated plate is simulated. The explicit Wall-Adaptive Local Eddy-viscosity (WALE) subgrid-scale (SGS) model is used to model the subgrid-scale turbulence.

Large-eddy simulation of MILD combustion using partially stirred reactor approach

Subgrid-scale (SGS) parameterization and method for calculating filtered reaction rate are critical components of an accurate large-eddy simulation (LES) of turbulent flames. In this study, we integrate gradient-type structural SGS models with a partially stirred reactor approach by using detailed chemical kinetics to simulate a turbulent methane/hydrogen jet flame under moderate or intense low-oxygen dilution (MILD) conditions. The study examines two oxygen dilution levels. The framework is assessed through a systematic and comprehensive comparison of temperature, and mass fractions of major and minor species with experimental data and other reference simulation results. Overall, the statistics of the combustion field show excellent agreement with measurements at different axial locations, and a significant improvement compared to some previous simulations. It suggests that the proposed nonlinear LES framework is able to accurately model MILD combustion with reasonable computational cost.

Large-eddy simulations of stratified plane Couette flow using the anisotropic minimum-dissipation model

The anisotropic minimum-dissipation (AMD) model for large-eddy simulation (LES) has been recently developed, and here the model performance is examined in stratified plane Couette flow. To our knowledge, this is the first use of the AMD model for resolved LES of stratified wall-bounded flow. A comparison with previously published direct numerical simulations (DNS) provides insight into model and grid requirements. Prandtl numbers of Pr = 0.7–70 and a range of Richardson numbers show that the AMD LES performs well even with a strong stabilising buoyancy flux. We identify three new requirements for accurate LES of stratified wall-bounded flow. First, the LES must resolve the turbulent structures at the edge of the viscous sublayer in order to satisfy the Obukov length scale condition, Ls+>200. Otherwise the LES solution may laminarise where the DNS solution remains turbulent. Second, the LES must have enough vertical grid resolution within the viscous and diffusive sublayers to resolve the wall fluxes. Third, the grid must be reasonably isotropic (vertical-to-horizontal grid aspect ratio >0.25) at the edge of the sublayer and through the turbulent interior for the AMD LES to correctly simulate the scalar flux. When these model requirements are fulfilled, the AMD LES performs very well, producing vertical mean profiles, friction Reynolds numbers, and Nusselt numbers consistent with DNS solutions at significantly higher grid resolution.