Large-eddy simulation (LES) of turbulent combustion is a relatively new research field. Much research has been carried out over the past years, but to realize the full predictive potential of combustion LES, many fundamental questions still have to be addressed, and common practices of LES of nonreacting flows revisited. The focus of the present review is to highlight the fundamental differences between Reynolds-averaged Navier-Stokes (RANS) and LES combustion models for nonpremixed and premixed turbulent combustion, to identify some of the open questions and modeling issues for LES, and to provide future perspectives.

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Large-eddy simulation of turbulent combustion

The Lagrangian Flamelet Model is formulated as a combustion model for large-eddy simulations of turbulent jet diffusion flames. The model is applied in a large-eddy simulation of a piloted partially premixed methane/air diffusion flame (Sandia flame D). The results of the simulation are compared to experimental data of the mean and RMS of the axial velocity and the mixture fraction and the unconditional and conditional averages of temperature and various species mass fractions, including CO and NO. All quantities are in good agreement with the experiments. The results indicate in accordance with experimental findings that regions of high strain appear in layer like structures, which are directed inwards and tend to align with the reaction zone, where the turbulence is fully developed. The analysis of the conditional temperature and mass fractions reveals a strong influence of the partial premixing of the fuel.

Large-eddy simulation of a turbulent piloted methane/air diffusion flame (Sandia flame D)

Direct numerical simulations of stably and strongly stratified turbulent flows with Reynolds number Re >> 1 and horizontal Froude number F-h > 1, viscous forces are unimportant and l(v) scales as l ...

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Scaling analysis and simulation of strongly stratified turbulent flows

▪ Abstract We review the dynamics of stably stratified flows in the regime in which the Froude number is considered small and the Rossby number is of order one or greater. In particular we emphasize the nonpropagating component of the flow field, as opposed to the internal wave component. Examples of such flows range from the later stages of decay of turbulent flows to mesoscale meteorological flows. Results from theoretical analyses, laboratory experiments, and numerical simulations are presented. The limiting form of the equations of motion appears to describe the laboratory experiments and numerical simulations rather well. There are similarities with the dynamics of two-dimensional flows, but three-dimensional effects are clearly important. A number of remaining open issues are discussed.

Fluid Motions in the Presence of Strong Stable Stratification

Direct numerical simulations and a self-similar analysis of the single-fluid Boussinesq Rayleigh-Taylor instability and transition to turbulence are used to investigate Rayleigh-Taylor turbulence. The Schmidt, Atwood and bulk Reynolds numbers are Sc = 1, A = 0.01, Re ≤ 3000. High-Reynolds-number moment self-similarity, consistent with the the energy cascade interpretation of dissipation, is used to analyse the DNS results

Rayleigh Taylor turbulence: self-similar analysis and direct numerical simulations

Massively parallel computers are now large enough to support accurate direct numerical simulations (DNSs) of laboratory experiments on isotropic turbulence, providing researchers with a full description of the flow field as a function of space and time. The high accuracy of the simulations is demonstrated by their agreement with the underlying laboratory experiment and on checks of numerical accuracy. In order to simulate the experiments, requirements for the largest and smallest length scales computed must be met. Furthermore, an iterative technique is developed in order to initialize the larger length scales in the flow. Using these methods, DNS is shown to accurately simulate isotropic turbulence decay experiments such as those of Comte-Bellot and Corrsin [J. Fluid Mech. 48, 273 (1971)].

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Direct numerical simulation of laboratory experiments in isotropic turbulence

A method for predicting filtered chemical species concentrations and filtered reaction rates in Large-Eddy Simulations of non-premixed, non-isothermal, turbulent reacting flows has been demonstrated to be quite accurate for higher Damkohler numbers. This subgrid-scale model is based on flamelet theory and uses presumed forms for both the dissipation rate and subgrid-scale probability density function of a conserved scalar. Inputs to the model are the chemistry rates, the Favre-filtered scalar, and its subgrid-scale variance and filtered dissipation rate. In this paper, models for the filtered dissipation rate and subgrid-scale variance are evaluated by filtering data from 5123-point Direct Numerical Simulations of a single-step, isothermal reaction developing in the isotropic, incompressible, decaying turbulence field of Comte-Bellot and Corrsin. Both the subgrid-scale variance and the filtered dissipation rate models (the ”sub-models”) are found to be reasonably accurate. The effect of the errors introduced by the sub-models on the overall model is found to be small, and the overall model is shown to accurately predict the spatial average of the filtered species concentrations over a wide range of times.

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Investigation of Modeling for Non-Premixed Turbulent Combustion

It is often desirable to study turbulent flows at steady state even if the flow has no inherent source of turbulence kinetic energy. Doing so requires a forcing schema, and various methods applicable to laboratory experiments or numerical simulations have been studied extensively for turbulence that is isotropic and homogeneous in three dimensions. A review of existing schemata for simulations is used to form a framework for more general forcing methods. In this framework, the problem of developing a forcing method is abstracted into the two problems of (1) prescribing the spectrum of the input power and (2) specifying a force that has the desired characteristics and that adds energy to the flow with the correct spectrum. The framework is used to construct three forcing methods for simulating horizontally homogeneous and isotropic, vertically stratified turbulence. They are implemented in a pseudo-spectral large-eddy simulations and their characteristics are analyzed. The framework is then used to characterize existing laboratory experiments. While no exact analogy can be drawn between forcing in esoteric pseudo-spectral simulations and forcing in physical experiments, there are many similarities. It is suggested that the forcing framework can be applied to predict and systematically test the effects of configuration choices made in the design of simulations and laboratory experiments.

A mathematical framework for forcing turbulence applied to horizontally homogeneous stratified flow

Understanding the passive reaction of two chemical species in shear-free turbulence with order unity Schmidt number is important in atmospheric and turbulent combustion research. The canonical configuration considered here is the reacting scalar mixing layer; in this problem two initially separated species mix and react downstream of a turbulence generating grid in a wind tunnel. A conserved scalar in this flow is, with some restrictions, analogous to temperature in a thermal mixing layer, and considerable laboratory data are available on the latter. In this paper, results are reported from high resolution, direct numerical simulations in which the evolution of the conserved scalar field accurately matches that of the temperature field in existing laboratory experiments. Superimposed on the flow are passive, single-step reactions with a wide range of activation energies and stoichiometric ratios (r). The resulting data include species concentrations as a function of three spatial dimensions plus time, and...

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Direct numerical simulation of reacting scalar mixing layers

The question of whether a temperature mixing layer evolves in a self-similar manner is of importance in developing and validating theories about scalar mixing. The simplicity of the flow encourages the thought that it is self-similar, but several laboratory experiments at moderate Peclet numbers have found inconsistencies with self-similar behavior. The experimentalists are limited, however, by the length of the wind tunnels and by difficulties in aligning the virtual origins of the scalar and velocity fields. Direct numerical simulations virtually eliminate both these problems, and large-eddy simulations add the ability to study an approximation to the case of an infinite Peclet number. These two simulation techniques are used in this paper to show that the mixing layer at a moderate Peclet number very nearly evolves with a single length and time scale, and that behavior consistent with self-similarity is observed in the case of an infinite Peclet number. In addition, the results show that direct numerical simulations can accurately reproduce the data from wind tunnel experiments downstream of a turbulence grid, and that large-eddy simulations are a valuable research tool for studying the large-scale characteristics of mixing.

http://people.umass.edu/debk/Papers/debk00a.pdf

S. M. de Bruyn Kops

Papers

Direct numerical simulation of laboratory experiments in isotropic turbulence

Investigation of Modeling for Non-Premixed Turbulent Combustion

A mathematical framework for forcing turbulence applied to horizontally homogeneous stratified flow

Direct numerical simulation of reacting scalar mixing layers

Re-examining the thermal mixing layer with numerical simulations