A comparative study of flamelet and finite rate chemistry LES for an axisymmetric dump combustor

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 calculations 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 handle the turbulence-chemistry interaction. Here, we examine the predictive capabilities and the theoretical links between LES flamelet models, such as the G-equation model (G-LES), and LES finite rate chemistry models, such as the Thickened Flame Model (TFM-LES), the Partially Stirred Reactor model (PaSR-LES), the Eddy Dissipation Concept (EDC-LES) model, a Presumed Probability Density Function (PPDF-LES) model and the Implicit LES (QL-LES) model. The models are described, and theoretical links between these are discussed in terms of the turbulent flame speed and flame thickness. In addition, the different models are used to study a bluff-body stabilized flame and the resulting predictions are compared with experimental data for two operating conditions.

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 handle the turbulence-chemistry interaction. Here we examine the predictive capabilities between LES flamelet models, such as the flamelet progress variable (LES-FPV) model, and LES finite rate chemistry models, such as the thickened flame model (LES-TFM), the eddy dissipation concept (LES-EDC) model, and the partially stirred reactor model (LES-PaSR). The different models are here used to examine a swirl-stabilized premixed flame in a laboratory gas turbine combustor, featuring the triple annular research swirler (TARS), for which high-quality experimental data is available. The comparisons include velocity and temperature profiles as well as combustor dynamics and NO formation.

The LES model's role in jet noise

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

Turbulence and chemistry interaction (TCI) is still an open problem in the combustion science. Models are needed to account for the unclosed chemistry source terms when the averaged or filtered Navier-Stokes governing equations are solved for chemical species in reactive flows. Finite-rate (FR) combustion models attempting to model the low-pass filtered production/consumption rates (in the case of Large Eddy Simulation or LES) directly. The Eddy Dissipation Concept (EDC) and Scale Similarity (SS) models are two types of FR combustion models for LES. The aim of the study is to assess and improve the performance of the two mentioned models using Direct Numerical Simulation (DNS) databases. In DNS all flow and chemistry scales are resolved and no TCI modeling is required. The DNS databases of reactive flows with relatively detailed chemistry, which are now available thanks to massively large parallel computational tools and codes, can be utilized to both gain fundamental insights on the turbulent reactive flows and directly assess TCI models. The assessment can be through a priori and a posterior DNS analyses. In the current work, fundamental analyses are carried out, using DNS databases of non-premixed jet flames, on the spectral behavior (velocity and dissipation of kinetic energy spectra) and the internal intermittency phenomenon in this type of flames. The physical findings are applied to develop new FR combustion models for LES. In particular, Eddy Dissipation Concept (EDC) is improved by the modifications on the coefficients and the intermittency factor of the model. The modification on the coefficients follows a theoretical basis in which the new findings on spectral behavior have been applied to reduce the degree of freedom of the model by relating the two free coefficients of the model. On the other hand, the modification in the intermittency factor of EDC is the direct application of the observations in the scaling of dissipation fluctuations. The new EDC models are then a priori assessed using the DNS databases. Besides, the existing Scale Similarity (SS) models for LES are a priori assessed using the DNS databases and new dynamic SS models are developed based on Germano's identity for LES and further assessed using the a priori DNS analysis.

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...

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

Modeling and discretization errors in large eddy simulations of hydrodynamic and magnetohydrodynamic channel flows

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.

Large eddy simulation of unsteady combustion

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

Dispersion and transfer of passive scalars in and above street canyons—Large-eddy simulations

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

In this study, a hybrid large-eddy simulation (LES) model is developed and applied to simulate the transport of oil droplet aerosols in wind over progressive water waves. The LES model employs a hybrid spectral and finite difference method for simulating the wind turbulence and a bounded finite-volume method for modeling the oil aerosol transport. Using a wave-following coordinate system and computational grid, the LES model captures the turbulent flow and oil aerosol fields in the region adjacent to the unsteady wave surface. A flat-surface case with prescribed roughness (representing a pure wind-sea) and a wavy-surface case with regular plane progressive 100 m long waves (representing long-crest long-wavelength ocean swells) are considered to illustrate the capability of the LES model and study the effects of long progressive waves on the transport of oil droplet aerosols with four different droplet diameters. The simulation results and statistical analysis reveal enhanced suspension of oil droplets in wind turbulence due to strong disturbance from the long progressive waves. The spatial distribution of the aerosol concentration also exhibits considerable streamwise variations that correlate with the phase of the long progressive waves.

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