The LES model's role in jet noise

Computation of unsteady turbomachinery flows: Part 2—LES and hybrids

of thesis entitled Large-Eddy Simulation of Wind Flow and Air Pollutant Transport inside Urban Street Canyons of Different Aspect Ratios submitted by Li, Xianxiang for the degree of Doctor of Philosophy at the University of Hong Kong in June 2008 The characteristics of the wind flow and air pollutant transport inside urban street canyons are of fundamental importance to the air quality monitoring and improvement. An investigation of these characteristics was performed in this study using both experimental and numerical techniques. The focus is on the mechanisms of pollutant transport and removal inside urban street canyons of high aspect ratios (AR, ratio of the building height to the street width). A physical model in water channel was first developed to study the wind flow in street canyons of different ARs of 0.5, 1.0, and 2.0. The velocity and turbulent fluctuation were measured by a Laser-Doppler Anemometer (LDA). The measured velocity and turbulent fluctuation at various locations were validated with several experimental datasets available in literature. The measured results in most locations were also in good agreement with previous numerical results. The comprehensive measurement data can provide a validation database for the numerical model development. To take into account the detailed transient turbulent processes, a large-eddy simulation (LES) model was developed based on a one-equation subgrid-scale (SGS) model and finite element method (FEM). This model was validated and fine-tuned by applying to an open channel flow at Reτ = 180. By comparing the calculated velocity and fluctuations with those obtained from experiment and direct numerical simulation (DNS), a set of model constants was determined for the LES model. A 1/7th wall model was further incorporated into this LES model to mitigate the strict near-wall resolution requirement. To validate the newly developed LES model for street canyons, the LES results for the street canyons of AR 1 and 2 were compared extensively with the waterchannel experimental data and previous LES results. The good agreement showed that the newly developed LES model was capable of predicting the complicated flow patterns and pollutant dispersion in street canyons. The validated LES model was then employed to simulate the street canyons of AR 3, 5, and 10. Three, five, and eight vertically aligned primary recirculations were found for the three cases, respectively, which showed decreasing strength with decreasing height. The very small ground-level wind speeds made the ground-level pollutants extremely difficult to disperse. Local maxima of the turbulence intensities were found at the interfaces between the primary recirculations and the free surface layer. The pollutant followed the trajectories of the primary recirculations. High pollutant concentration and variance were found near the buildings where wind flowed upward. Large gradients of pollutant concentration and variance were also observed at the interfaces between the primary recirculations and the free surface layer. Detailed analyses of concentration budget terms showed that the advection terms were responsible for pollutant redistribution within primary recirculations, while the turbulent transport terms were responsible for pollutant penetration between primary recirculations and pollutant removal from the street canyon. Based on the LES results, several quantities were introduced to compare the pollutant removal capability of different street canyon configurations. It was found that these quantities were all non-linear functions of the street canyon AR. Large-Eddy Simulation of Wind Flow and Air Pollutant Transport inside Urban Street Canyons of Different Aspect Ratios

Present-day demands on combustion equipment are increasing the need for improved understanding and prediction of turbulent combustion. Large eddy simulation (LES), in which the large-scale flow is resolved on the grid, leaving only the small-scale flow to be modeled, provides a natural framework for combustion simulations, as the transient nature of the flow is resolved. In most situations, however, the flame is thinner than the LES grid, and subgrid modeling is required to also handle the turbulence-chemistry interactions. Here, we examine the predictive capabilities of the flamelet LES models, such as the Flamelet Progress Variable LES (LES-FPV) models, and the finite rate chemistry LES models, such as the LES-Thickened Flame Model (LES-TFM), the partially stirred reactor model (LES-PaSR) and the Eddy Dissipation Concept (LES-EDC) model. These different combustion LES models are used here to study the reacting flow in an axisymmetric dump combustor at a Reynolds number of 55,800, the Damköhler number of 167 and a Karlowitz number of 0.15, placing the flame in the corrugated flame regime. The computational results are compared to experimental data of velocity and temperature to examine predictive capabilities of the different models.

A large eddy simulation (LES) model, with ice phase included, has been used to study the marine convective boundary layer filled with snow. Extensions to Moeng's LES model include the diagnosis of cloud ice mixing ratio, snow precipitation, and the parameterization of detailed microphysical processes. Model simulations are compared with cold air outbreak field observations over Lake Michigan, as well as with the liquid phase LES results for the same atmospheric conditions. The buoyancy flux and vertical velocity variance profiles generated by the ice phase LES are found to be more consistent with the observations than those generated by the liquid phase LES results. The incorporation of the ice phase into the LES model has also improved the agreement of vertical velocity skewness (Sw) between observations and LES model results. It has also been found that the presence of precipitation, and the associated microphysical processes, has a significant effect on the structure of the convective boundary...

LES model of large scale hydrogen–air planar detonations: Verification by the ZND theory

Comparison of RANS and LES turbulent flow models in a real stenosis

Large-eddy simulation coupled to mesoscale meteorological model for gas dispersion in an urban district

Application of modified eddy dissipation concept with large eddy simulation for numerical investigation of internal combustion engines

<div class="section abstract"><div class="htmlview paragraph">This study simulates soot formation processes in diesel combustion using a large eddy simulation (LES) model, based on a one-equation subgrid turbulent kinetic energy model. This approach was implemented in the KIVA4 code, and used to model diesel spray combustion within a constant volume chamber. The combustion model uses a direct integration approach with a fast explicit ordinary differential equation (ODE) solver, and is additionally parallelized using OpenMP. The soot mass production within each computation cell was determined using a phenomenological soot formation model developed by Waseda University. This model was combined with the LES code mentioned above, and included the following important steps: particle inception during which acenaphthylene (A<sub>2</sub>R<sub>5</sub>) grows irreversibly to form soot; surface growth with driven by reactions with C<sub>2</sub>H<sub>2</sub>; surface oxidation by OH radical and O<sub>2</sub> attack; and particle coagulation.</div><div class="htmlview paragraph">The results obtained using our new model are compared to those generated using a RANS (RNG k-epsilon) model, and also to experimental data from the engine combustion network (ECN) of Sandia National Laboratories. The sensitivity of the LES results to mesh resolution is also discussed. The results show that both RANS and LES simulations predict the dispersion and vapor penetration of the injected fuel fairly well. LES generally provides flow and spray characteristics in better agreement with experimental data than RANS. It is also shown that the phenomenological soot model is useful for investigating soot particle production and distribution. The LES model was better than the RANS model at describing instantaneous soot concentration contour.</div></div>

As part of the European Project on Cloud Systems in Climate Models, the diurnal cycle of stratocumulus has been simulated with Large‐Eddy Simulation (LES) models and Single Column Models (SCMs). The models were initialized and compared with observations collected in marine stratocumulus in July 1987 during the First International Satellite Cloud Climatology Project Regional Experiment. The results of the six LES models are found to be in a fair agreement with the observations. They all capture the distinct diurnal variation in the cloud liquid‐water path, the turbulence profiles and clearly show a decoupled boundary layer during daytime and a vertically well‐mixed boundary layer during the night. Entrainment of relatively dry and warm air from just above the inversion into the boundary layer is the major process modifying the thermodynamic structure of the boundary layer during the night. The differences that arise in the liquid‐water path evolution can therefore be attributed mainly to differences in the entrainment rate. The mean entrainment rates computed from the LES model results are 0.58±0.08 cm s−1 and 0.36±0.03 cm s−1 for the night‐time and daytime periods, respectively. If the horizontal domain size in a LES model is enlarged, mesoscale fluctuations develop. This leads to a broader liquid‐water path distribution and a reduction of the cloud albedo.To assess the quality of the representation of stratocumulus in general‐circulation models, results from ten SCMs are compared with observations and LES results. The SCM latent and sensible heat fluxes at the surface agree fairly well with the LES results. Many of the SCMs predict a liquid‐water path which is much too low, a cloud cover smaller than unity, and cloud tops that are lower than the observations and the LES results. This results in a much larger amount of downwelling short‐wave radiation absorbed at the sea surface. Improvement of entrainment parametrizations is needed for a better representation of stratocumulus in SCMs.Observations and LES results of entrainment rates for different stratocumulus cases are compared. The observed entrainment rates in Atlantic stratocumulus clouds during the Atlantic Stratocumulus Transition Experiment (ASTEX) are larger than for the ones over the Pacific Ocean off the coast of California. Results from LES models corroborate these findings. The differences in the entrainment rate can likely be attributed to the smaller inversion jumps of the liquid‐water potential temperature for the ASTEX stratocumulus cases. © Royal Meteorological Society, 2004. A. P. Lock's contribution is Crown copyright

Flows around maneuvering ships, submarines and underwater vehicles are usually quite complicated, and often experience three-dimensional open flow separation resulting in unsteady forces and moments that may be detrimental to ship performance. The objective of this investigation is to support more applied maneuvering studies by investigating the computational model performance with respect to approaches to flow simulation methodologies, turbulence modeling, grid resolution and the effects of tripping devices often used in experimental studies. To study these issues simulations are performed of the flow past a 6:1 prolate spheroid, tripped at the nose, and experimentally studied in a series of papers by Simpson et al. Here, we compare predictions from Reynolds Averaged Navier Stokes (RANS), Detached Eddy Simulation (DES) and Large Eddy Simulation (LES) models with experimental data. For the LES computations, different subgrid models are utilized and since the experimental study is carried out using a tripped model, a simple trip model is also developed and tested together with the LES models. Large scatter between the predictions is found, with the DES model and one of the trip-ped LES model showing very good agreement with the data. For LES, the modeling of the trip, which is usually not considered, appears as important as that of the near wall handling and modeling in wall-modeled LES.

Fluid flow plays a significant role in the continuous casting of molten steel. In this paper, two Reynolds Averaged Naiver-Stokes (RANS) models and a Large Eddy Simulation (LES) model were comparatively employed to characterize the fluid flow inside a dissipative ladle shroud and a tundish. LES model was proved to be powerful to characterize the turbulence structure inside the ladle shroud. The effect of meshing density on the computational accuracy was considered using the LES model. The fine-mesh model can capture multiscale vortices inside the ladle shroud; while, the coarse-mesh model disables the LES to obtain the detailed flow information. The experiment of particle image velocimetry (PIV) was used to verify the flow field obtained by the LES model inside the tundish. The PIV and the LES results agree well in terms of flow pattern and velocity vector.

Based on a pseudo-spectral large eddy simulation (LES) model, an LES model with an anisotropy turbulent kinetic energy (TKE) closure model and an explicit multi-stage third-order Runge-Kutta scheme is established. The modeling and analysis show that the LES model can simulate the planetary boundary layer (PBL) with a uniform underlying surface under various stratifications very well. Then, similar to the description of a forest canopy, the drag term on momentum and the production term of TKE by subgrid city buildings are introduced into the LES equations to account for the area-averaged effect of the subgrid urban canopy elements and to simulate the meteorological fields of the urban boundary layer (UBL). Numerical experiments and comparison analysis show that: (1) the result from the LES of the UBL with a proposed formula for the drag coefficient is consistent and comparable with that from wind tunnel experiments and an urban subdomain scale model; (2) due to the effect of urban buildings, the wind velocity near the canopy is decreased, turbulence is intensified, TKE, variance, and momentum flux are increased, the momentum and heat flux at the top of the PBL are increased, and the development of the PBL is quickened; (3) the height of the roughness sublayer (RS) of the actual city buildings is the maximum building height (1.5–3 times the mean building height), and a constant flux layer (CFL) exists in the lower part of the UBL.

Quantifying sub-grid scale (SGS) turbulent dispersion force and its effect using one-equation SGS large eddy simulation (LES) model in a gas–liquid and a liquid–liquid system

Cities can offer an extraordinarily high or low urban level of climatic stressors depending on their location and topographical setting, infrastructural geometry and anthropogenic activities. To protect human well-being today and in the future, it is crucial to better understand how to mitigate temperature extremes in cities. Since cities are constantly growing and transforming in response to their residents’ needs, planning a foresighted sustainable climate-friendly infrastructure is critical. This need creates a niche for research to assess local climate effects that effect the lower atmosphere ground layer where human activity takes place. Large Eddy Simulation (LES) models can simulate heat transport and mixing processes by directly resolving large-scale turbulence and are often used to simulate urban development activities potentially mitigating the adverse effects of heatwaves in cities. Despite their growing use in forming recommendations, these models are inherently difficult to validate which leads to ‘simply believing them’. We evaluate the performance of an urban LES model against a reference multi-station observational network focusing how well the space-time dynamics of distinct urban microclimate including densely-built hot spots, peri urban and park-cool islands agree. We selected a 72-hour extreme heatwave period in July 2019 in a mid-sized city in Germany which suffers from a similarly large urban heat island effect as larger cities. We investigated air temperature, air humidity, wind speed and direction as key elements impacting the perceived heat stress or relief by humans. Observations were compared to the PALM-4U LES model with a nested domain dynamically driven by the mesoscale COSMO-D2 output by the German Meteorological Service at spatial resolutions of 20 m and 5 m domain. We employed the stochastic multiresolution decomposition (MRD) technique applied to two-point correlation statistics for characterizing the space-time behavior. Absolute air temperatures differences amounted to +5 K overestimation of modeled nocturnal air temperatures. A key finding from the MRD analysis is that correlation between stations does not follow separation distance (as expected for homogeneous domains) but rather the distinct urban microclimatic for air temperature and specific humidity in both observations and model at both resolutions. Separating the results into day and night shows distinct differences for air temperature and specific humidities for both model resolutions compared to the observations, but only small differences for near-surface winds. The model performance varies with its resolution and climate element: while winds are better represented in the finer 5 m resolution, specific humidity cannot be simulated properly by the model at night. Air temperature during day is better represented by the 20 m resolution, while the match between observations and the 5 m-prediction is better at night. We show that the LES model can simulate the statistical space-time behavior of urban microclimates but performs poorly when absolute targets are modeled. Simulated air temperature and specific humidity follow mostly the implemented synoptic advective forcing large scale model which does not recognize local microclimatic effects. For near-surface winds, this model performs better with finer resolution as the larger eddies resolved depend on the geometry of the city.