共查询到20条相似文献,搜索用时 15 毫秒
1.
Large eddy simulation (LES) models for flamelet combustion are analyzed by simulating premixed flames in turbulent stagnation
zones. ALES approach based on subgrid implementation of the linear eddy model(LEM) is compared with a more conventional approach
based on the estimation of the turbulent burning rate. The effects of subgrid turbulence are modeled within the subgrid domain
in the LEM-LES approach and the advection (transport between LES cells) of scalars is modeled using a volume-of-fluid (VOF)
Lagrangian front tracking scheme. The ability of the VOF scheme to track the flame as a thin front on the LES grid is demonstrated.
The combined LEM-LES methodology is shown to be well suited for modeling premixed flamelet combustion. The geometric characteristics
of the flame surfaces, their effects on resolved fluid motion and flame-turbulence interactions are well predicted by the
LEM-LES approach. It is established here that local laminar propagation of the flamelets needs to be resolved in addition
to the accurate estimation of the turbulent reaction rate. Some key differences between LEM-LES and the conventional approach(es)
are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
This paper presents the formulation of a mixed dynamic subgrid-scale model in non-Cartesian geometries suitable for the study
of complex flows. Following the approach developed by Jordan [ J. Comput. Phys.
148, 322 (1999)], the variables are first transformed into a contravariant form and then filtered in the computational space.
A dynamic localized mixed model, previously developed within the Cartesian framework has been entirely re-formulated for non-orthogonal
meshes. The model performance was evaluated by carrying out two tests. First, a plane channel flow at Re τ = 395 was simulated using both Cartesian and curvilinear grids; the results show that the model formulation is consistent
and insensitive to grid distortion, and compares well with the reference data. Then, computations of the turbulent flow over
a two-dimensional channel with a wavy wall were performed. Accurate first- and second-order statistics were obtained using
relatively coarse grids.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
The purpose of this paper, dedicated to the memory of our friend Hieu Ha Minh, is to wander through the turbulence universe
using deterministic computational tools based on large-eddy simulations (LES) in physical and spectral space. We first briefly
recall the subgrid models used, then apply them to mixing layers, jets, separated flows (the backstep in particular, on which
Hieu worked a lot) and boundary layers. The influence of compressibility will be also considered. These fine-grain LES allow
us to decipher very nicely the intimate vortical structure of turbulence, and predict their statistics. Finally, we discuss
of the applicability of these methods to industrial flows.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
4.
A Lagrangian dynamic formulation of the mixed similarity subgrid (SGS) model for large-eddy simulation (LES) of turbulence is proposed. In this model, averaging is performed over fluid trajectories, which makes the model applicable to complex flows without directions of statistical homogeneity. An alternative version based on a Taylor series expansion (nonlinear mixed model) is also examined. The Lagrangian models are implemented in a finite difference code and tested in forced and decaying isotropic turbulence. As comparison, the dynamic Smagorinsky model and volume-averaged formulations of the mixed models are also tested. Good results are obtained, except in the case of low-resolution LES (32 3) of decaying turbulence, where the similarity coefficient becomes negative due to the fact that the test-filter scale exceeds the integral scale of turbulence. At a higher resolution (64 3), the dynamic similarity coefficient is positive and good agreement is found between predicted and measured kinetic energy evolution. Compared to the eddy viscosity term, the similarity or the nonlinear terms contribute significantly to both SGS dissipation of kinetic energy and SGS force. In order to dynamically test the accuracy of the modeling, the error incurred in satisfying the Germano identity is evaluated. It is found that the dynamic Smagorinksy model generates a very large error, only 3% lower than the worst-case scenario without model. Addition of the similarity or nonlinear term decreases the error by up to about 50%, confirming that it represents a more realistic parameterization than the Smagorinsky model alone. 相似文献
6.
This paper follows the evolution in understanding of the multiple mapping conditioning (MMC) approach for turbulent combustion
and reviews different implementations of MMC models. As the MMC name suggests, the original version represents a consistent
combination of CMC-type conditional equations (conditional moment closure) and generalised mapping closure. It seems that
the strength of the MMC model, and especially that of its stochastic version, lies in a more general (and much more transparent)
interpretation. In this new generalised interpretation, we can replace complicated derivations by physical reasoning and the
model appears to be a natural extension of modelling approaches developed in recent decades. MMC can be seen as a methodology
for enforcing certain known characteristics of turbulence on a conventional mixing model. This is achieved by localising the
mixing operation in a reference space. The reference space variables are selected to emulate the properties of a turbulent
flow which have a strong effect on reactive quantities. The best and simplest example is an MMC model which has a single reference
variable emulating the mixture fraction. In diffusion flames turbulent fluctuations of reacting quantities are strongly correlated
with fluctuations of the mixture fraction. By making mixing local in the reference mixture fraction space a CMC-type mixing
closure is enforced. In the original interpretation of MMC the reference variables are modelled as Markov processes. Since
the reference variables should emulate properties of turbulent flows as realistically as possible the next step, and the basis
of generalised MMC, is to remove the Markovian restriction and set reference variables equal to traced Lagrangian quantities
within DNS or LES flow fields. Indeed, no Markov value can emulate the mixture fraction better than the mixture fraction itself.
(Using a Markov vector process of dimension higher than the number of conditioning variables represents a more economical
alternative for producing reference variables in generalised MMC.) The generalised MMC approach effectively incorporates the
mixture fraction-based models, the PDF methods and LES/DNS techniques into a single methodology with possibility of blending
useful features developed previously for conventional models. The generalised approach to MMC stimulates a more flexible understanding
of simulations using sparsely placed Lagrangian particles as tools that may provide accurate joint distributions of reactive
scalars at relatively low computational cost. The physical reasoning behind the new interpretation of MMC is supported by
example computations for a partially premixed methane/air diffusion flame (Sandia Flame D). The scheme utilises LES for the
dynamic field and a sparse-Lagrangian filtered density function method with MMC mixing for the scalar field. Two different
particle mixing schemes are tested. Simulations are performed using only 35,000 Lagrangian particles (of these only 10,000
are chemically active) on a single workstation. The relatively low computational cost allows the use of realistic chemical
kinetics containing 34 reactive species and 219 reactions.
Intended for publication in the special issue of Flow, Turbulence and Combustion arising from the 2nd ECCOMAS Thematic Conference
on Computational Combustion held at Delft in mid-2007. 相似文献
7.
A recently introduced nonlinear model undergoes evaluations based on two isotropic turbulent cases: a University of Wiscosion-Madison case at a moderate Reynolds number and a Johns Hopkins University case at a high Reynolds number. The model uses an estimation of the subgrid-scale (SGS) kinetic energy to model the magnitude of the SGS stress tensor, and uses the normalized velocity gradient tensor to model the structure of the SGS stress tensor. Testing is performed for the first case through a comparison between direct numerical simulation (DNS) results and large eddy simulation (LES) results regarding resolved kinetic energy and energy spectrum. In the second case, we examine the resolved kinetic energy, the energy spectrum, as well as other key statistics including the probability density functions of velocities and velocity gradients, the skewness factors, and the flatness factors. Simulations using the model are numerically stable, and results are satisfactorily compared with DNS results and consistent with statistical theories of turbulence. 相似文献
8.
The accuracy of large-eddy simulation (LES) of a turbulent premixed Bunsen flame is investigated in this paper. To distinguish
between discretization and modeling errors, multiple LES, using different grid sizes h but the same filterwidth Δ, are compared with the direct numerical simulation (DNS). In addition, LES using various values
of Δ but the same ratio Δ/ h are compared. The chemistry in the LES and DNS is parametrized with the standard steady premixed flamelet for stochiometric
methane-air combustion. The subgrid terms are closed with an eddy-viscosity or eddy-diffusivity approach, with an exception
of the dominant subgrid term, which is the subgrid part of the chemical source term. The latter subgrid contribution is modeled
by a similarity model based upon 2Δ, which is found to be superior to such a model based upon Δ. Using the 2Δ similarity model
for the subgrid chemistry the LES produces good results, certainly in view of the fact that the LES is completely wrong if
the subgrid chemistry model is omitted. The grid refinements of the LES show that the results for Δ = h do depend on the numerical scheme, much more than for h = Δ/2 and h = Δ/4. Nevertheless, modeling errors and discretization error may partially cancel each other; occasionally the Δ = h results were more accurate than the h ≤ Δ results. Finally, for this flame LES results obtained with the present similarity model are shown to be slightly better
than those obtained with standard β-pdf closure for the subgrid chemistry. 相似文献
9.
Buoyant flows often contain regions with unstable and stable thermal stratification from which counter gradient turbulent
fluxes are resulting, e.g. fluxes of heat or of any turbulence quantity. Basing on investigations in meteorology an improvement
in the standard gradient-diffusion model for turbulent diffusion of turbulent kinetic energy is discussed. The two closure
terms of the turbulent diffusion, the velocity-fluctuation triple correlation and the velocity-pressure fluctuation correlation,
are investigated based on Direct Numerical Simulation (DNS) data for an internally heated fluid layer and for Rayleigh–Bénard
convection. As a result it is decided to extend the standard gradient-diffusion model for the turbulent energy diffusion by
modeling its closure terms separately. Coupling of two models leads to an extended RANS model for the turbulent energy diffusion.
The involved closure term, the turbulent diffusion of heat flux, is studied based on its transport equation. This results
in a buoyancy-extended version of the Daly and Harlow model. The models for all closure terms and for the turbulent energy
diffusion are validated with the help of DNS data for internally heated fluid layers with Prandtl number Pr = 7 and for Rayleigh–Bénard convection with Pr = 0.71. It is found that the buoyancy-extended diffusion model which involves also a transport equation for the variance
of the vertical velocity fluctuation gives improved turbulent energy diffusion data for the combined case with local stable
and unstable stratification and that it allows for the required counter gradient energy flux. 相似文献
10.
本文应用二维双色四光束激光多普勒测速仪和压强探针样细地测量了二维非对称曲面扩张通道内的不可压湍流边界层分离流动,得到了时均速度和雷诺剪应力以及正反向间歇流动因子和静压分布。实验结果分析表明;湍流边界层分离时,沿边界层高度方向存在着明显的压强差。压强差的极小值对应于位移厚度曲率的极大值和瞬时间歇分离点。Bardina对数尾迹律可以较好地描述瞬时间歇分离点之前的边界层速度分布,但无法描述分离的边界层速 相似文献
11.
We first recall the EDQNM two-point closure approach of three-dimensional isotropic turbulence. It allows in particular prediction
of the infrared kinetic-energy dynamics (with a k
4 backscatter) and the associated time-decay law of kinetic-energy, useful in particular for one-point closure modelling. Afterwards,
we show how the spectral eddy viscosity concept may be used for large-eddy simulations: we introduce the plateau-peak model
and the spectral-dynamic models. They are applied to decaying isotropic turbulence, and allow recovery of the EDQNM infrared
energy dynamics. Anew infrared k
2 law for the pressure spectrum, predicted by the closure, is also well verified.
Assuming that subgrid scales are not too far from isotropy, the spectral-dynamic model is applied to the channel flow at h
+= 390, with statistics in very good agreement with DNS, while reducing considerably the computational time. We study with
the aid of DNS and LES the case of the channel rotating about an axis of spanwise direction. The calculations allow to recover
the universal linear behaviour of the mean velocity profile, with a local Rossby number equal to −1.
We present also LES (using the Grenoble Filtered Structure-Function Model), of a turbulent boundary layer passing over a cavity.
Finally, we make some remarks on the future of LES for industrial applications.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
An efficient time accurate algorithm is presented for numerical simulations of low-Mach number variable density flows in the
context of non-premixed flames. The algorithm is based on a segregated solution formalism in the class of pressure-correction
methods. It shows good conservation properties and returns stable results, regardless of the difference in density between
neighboring cells. In the illustrative example, a flamesheet model is used to describe the combustion of fuel and oxidizer.
The stability of the method is discussed for a 1D channel flow, containing both fuel and oxidizer. Results for a 2D testcase
of a reacting mixing layer are also shown.
Bart Merci is a postdoctoral fellow of the Fund of Scientific Research - Flanders (Belgium) (FWO-Vlaanderen). 相似文献
13.
The turbulence modeling in probability density function (PDF) methods is studied through applications to turbulent swirling and nonswirling co-axial jets and to the temporal shear layer. The PDF models are formulated at the level of either the joint PDF of velocity and turbulent frequency or the joint PDF of velocity, wave vector, and turbulent frequency. The methodology of wave vector models (WVMs) is based on an exact representation of rapidly distorted homogeneous turbulence, and several models are constructed in a previous paper [1]. A revision to a previously presented conditional-mean turbulent frequency model [2] is constructed to improve the numerical implementation of the model for inhomogeneous turbulent flows. A pressure transport model is also implemented in conjunction with several velocity models. The complete model yields good comparisons with available experimental data for a low swirl case. The individual models are also assessed in terms of their significance to an accurate solution of the co-axial jets, and a comparison is made to a similar assessment for the temporal shear layer. The crucial factor in determining the quality of the co-axial jet simulations is demonstrated to be the proper specification of a parameter ratio in the modeled source of turbulent frequency. The parameter specification is also shown to be significant in the temporal shear layer. 相似文献
14.
The standard k-ε model and three low-Reynolds number k-ε models were used to simulate pipe flow with a ring device installed in the near-wall region. Both developing and fully developed turbulent pipe flows have been investigated. Turbulence suppression for fully developed pipe flows revealed by hot-wire measurements has been predicted with all three low-Reynolds number models, and turbulence enhancement has been predicted by the standard k-ε model. All three low-Reynolds number models have predicted similar distributions of velocities, turbulence kinetic energy, and dissipation rate. For developing pipe flows, the region of turbulence suppression predicted by the three low-Reynolds number models is much more extensive (up to 30 pipe diameters downstream of the device) than for full developed flow; whereas the standard k-ε model has only predicted turbulence enhancement. 相似文献
15.
The modelling of scalar dissipation rate in conditional methods for large-eddy simulations is investigated based on a priori direct numerical simulation analysis using a dataset representing an igniting non-premixed planar jet flame. The main objective is to provide a comprehensive assessment of models typically used for large-eddy simulations of non-premixed turbulent flames with the Conditional Moment Closure combustion model. The linear relaxation model gives a good estimate of the Favre-filtered scalar dissipation rate throughout the ignition with a value of the related constant close to the one deduced from theoretical arguments. Such value of the constant is one order of magnitude higher than typical values used in Reynolds-averaged approaches. The amplitude mapping closure model provides a satisfactory estimate of the conditionally filtered scalar dissipation rate even in flows characterised by shear driven turbulence and strong density variation. 相似文献
16.
This paper proposes a general algebraic formulation able lo unify all the previous developments in (he frame of subgrid-scale modeling using dynamic mixed models. This formulation can serve as a guide for the design of new multi-parameter SGS models. All usual cases of coupling between momentum and energy (or transport) equation are treated formally, and properties of the associated linear or non-linear system are discussed. It is shown thai all the existing models found in the literature can be grouped into a seven-parameter dynamic model, referred to as the Maximal Complexity Dynamic Model (MCDM) A priori lests on this MCDM are carried out for the subsonic plane channel flow problem, which aim at selecting the most important contributions in multi-parameter dynamic models. 相似文献
18.
A mixedness-reactedness flamelet combustion model coupled with a comprehensive radiation heat transfer model based on the
discrete transfer method of solution of the radiative transport equation is applied for the simulation of a 3 MW non-swirling
turbulent non-premixed natural gas flame in the experimental furnace at the International Flame Research Foundation. In the
calculation, turbulence is represented by the standard k − ε and a differential Reynolds-stress model. Predictions are compared with measurements of mean gas velocity, temperature, major
species concentrations and incident radiation wall flux. The radiative mixedness-reactedness flamelet combustion model, irrespective
of the model for turbulence, is able to reproduce the basic structure of the experimental flame, which is stabilised downstream
of the burner nozzle. In the near burner region, encompassing the non-reacting lift-off zone, good quality predictions are
obtained using both the turbulence models, whereas further downstream, within the combusting zone of the jet, the Reynolds-stress
turbulence model generates better predictions at and about the furnace axis. The nitric oxide (NO) formation via the thermal-
and prompt-NO routes was also calculated and compared with in-flame and flue-gas NO data. The measured NO level at the furnace
exit is well reproduced in the calculation, however discrepancies exist near the burner where NO concentrations around the
furnace axis are overpredicted. 相似文献
20.
The exchange of chemical species between the atmospheric boundary layer and the reservoir layer is investigated by means of an analytical solution of the conservation equation of a decaying chemical species. The exchange mechanism is governed by two parameters: the Damköhler number (the ratio of the turbulence time scale to the chemical time scale) and the ratio of a concentration scale in the atmospheric boundary layer to the concentration in the reservoir layer. Depending on the value of these two parameters, the exchange flux between the two layers can vary in sign and by several orders of magnitude. The study demonstrates to what extent chemical transformation determines the transfer of chemical species between the atmospheric boundary layer and the reservoir layer. 相似文献
|