An Interactively Coupled CFD-Multi-Zone Approach to Model HCCI Combustion

The objective of this work is to present an innovative interactively coupled CFD-multi-zone approach. In a consistent manner, the approach combines detailed flow field information obtained from CFD with detailed chemical kinetics solved in a multi-zone model. Combustion and pollutant formation in an HCCI engine with recompressing VVA strategy are numerically investigated using the interactively coupled CFD-multi-zone approach. A surrogate fuel for gasoline is used in the simulation that consists of n-heptane (18% liquid volume fraction) and isooctane (82% liquid volume fraction). The underlying complete reaction mechanism comprises 482 elementary reactions and 115 chemical species. The interactively coupled CFD-multi-zone approach shows to be accurate enough to describe HCCI chemistry, and is at the same time economical enough to allow application in an industrial environment. For the test case investigated, the simulation results are compared to experimental data that has been obtained using real gasoline. The overall agreement between simulation and experiment is found to be very good. Based on the work presented at the 2^nd ECCOMAS Thematic Conference on Computational Combustion, Delft, The Netherlands, 2007.     

Development of Chemistry Coordinate Mapping Approach for Turbulent Partially Premixed Combustion

Direct numerical simulation (DNS) coupling with chemistry coordinate mapping (CCM) is presented to simulate flame propagation and auto-ignition in a partially premixed syngas/air mixture. In the CCM approach, the physical domain is mapped into a low dimensional phase space with a few thermodynamic variables as the independent variables. The integration of the chemical reaction rates and heat release rate are done on the grid in the phase space. Previously we showed that for premixed mixtures, two variables temperature and specific element mass fraction of H atom, can be sufficient to construct the phase space for a satisfactory mapping. However, for partially premixed combustion mode, a third phase space variable is required to map the physical cell into the phase space. It is shown that scalar dissipation rate of the element mass ratio of H atom can be used as the third dimension of the phase space. An investigation is carried out on the behavior of CCM and the choice over the element on which the local element mass ratio should be based. Mapping error in the CCM is investigated. It is shown that if the element mass ratio is based on the element involved in the most diffusive molecules, the error of the mapping can approach zero when the grid in the phase space is refined. To validate the CCM approach the results of DNS coupled with CCM (DNS-CCM) are compared with full DNS that integrates the chemical reaction rates and heat release rate directly in physical space. Good agreement between the results from DNS and DNS-CCM is obtained while the computational time is reduced at least by 70 %.     

An Approach to Constructing a Rheological Model of a Strain-Hardening Medium

The one-dimensional constitutive equations of strain-hardening materials subject to nonlinear creep are derived. The solution is found using the hypothesis of unified deformation curve based on the similarity of the tensile and isochronic creep curves. A generalized rheological model is constructed which accounts for the instantaneous strain rate, loading rate, and the mode of strain hardening. This model is used to derive one-dimensional constitutive equations for linear viscoelastic, nonlinear viscoelastic, and linear- and nonlinear-hardening viscoelastoplastic materials. It is shown that the creep of linear viscoelastic and linear-hardening viscoelastoplastic materials is transient. For nonlinear viscoelastic and nonlinear-hardening viscoelastoplastic materials, all the characteristic stages of creep are present     

Joint RANS/LES Approach to Premixed Flame Modelling in the Context of the TFC Combustion Model

We present an original timesaving joint RANS/LES approach to simulate turbulent premixed combustion. It is intended mainly for industrial applications where LES may not be practical. It is based on successive RANS/LES numerical modelling, where turbulent characteristics determined from RANS simulations are used in LES equations for estimation of the subgrid chemical source and viscosity. This approach has been developed using our TFC premixed combustion model, which is based on a generalization of the Kolmogorov’s ideas. We assume existence of small-scale statistically equilibrium structures not only of turbulence but also of the reaction zones. At the same time, non-equilibrium large-scale structures of reaction sheets and turbulent eddies are described statistically by model combustion and turbulence equations in RANS simulations or follow directly without modelling in LES. Assumption of small-scale equilibrium gives an opportunity to express the mean combustion rate (controlled by small-scale coupling of turbulence and chemistry) in the RANS and LES sub-problems in terms of integral or subgrid parameters of turbulence and the chemical time, i.e. the definition of the reaction rate is similar to that of the mean dissipation rate in turbulence models where it is expressed in terms of integral or subgrid turbulent parameters. Our approach therefore renders compatible the combustion and turbulent parts of the RANS and LES sub-problems and yields reasonable agreement between the RANS and averaged LES results. Combining RANS simulations of averaged fields with LES method (and especially coupled and acoustic codes) for simulation of corresponding nonstationary process (and unsteady combustion regimes) is a promising strategy for industrial applications. In this work we present results of simulations carried out employing the joint RANS/LES approach for three examples: High velocity premixed combustion in a channel, combustion in the shear flow behind an obstacle and the impinging flame (a premixed flame attached to an obstacle).     

A New Rational Approach to Jet Noise Reduction

This paper offers a new rational approach to jet noise suppression methodology. The tools required to implement such an approach are discussed. These involve the use of global velocimetry to measure those Lighthill turbulent stress tensor terms thought most important to noise generation. The framework of the dynamic systems model as it applies to the development of a noise suppression model is discussed, as well as the latest results on the development of an actuator for control input. The dynamic systems model offers a unique opportunity to study the influence of a concept noise reduction scheme on noise reduction through appropriate algorithms that relate turbulent jet processes to noise. Received 24 January 1997 and accepted 30 May 1997     

离散变量结构优化设计的连续化方法

把离散变量结构优化设计问题转化为一般的0-1规划问题,进一步把该问题转化为一个带有互补约束的优化问题,利用NCP函数,最终得到待以求解的连续优化问题。离散优化到基于NCP函数的连续优化变换在理论上是等价的,可以利用普通的数学规划方法实施求解。数值算例的计算结果验证了该连续化方法的可行性与有效性。     

A Generalised Multiple Mapping Conditioning Approach for Turbulent Combustion

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.     

A Thickened Stochastic Fields Approach for Turbulent Combustion Simulation

The Stochastic Fields approach is an effective way to implement transported Probability Density Function modelling into Large Eddy Simulation of turbulent combustion. In premixed turbulent combustion however, thin flame-like structures arise in the solution of the Stochastic Fields equations that require grid spacing much finer than the filter scale used for the Large Eddy Simulation. The conventional approach of using grid spacing equal to the filter scale yields substantial numerical error, whereas using grid spacing much finer than the filter length scale is computationally-unaffordable for most industrially-relevant combustion systems. A Thickened Stochastic Fields approach is developed in this study in order to provide physically-accurate and numerically-converged solutions of the Stochastic Fields equations with reduced compute time. The Thickened Stochastic Fields formulation bridges between the conventional Stochastic Fields and conventional Thickened-Flame approaches depending on the numerical grid spacing utilised. One-dimensional Stochastic Fields simulations of freely-propagating turbulent premixed flames are used in order to obtain criteria for the thickening factor required, as a function of relevant physical and numerical parameters, and to obtain a model for an efficiency function that accounts for the loss of resolved flame surface area caused by applying the thickening transformation to the Stochastic Fields equations. The Thickened Stochastic Fields formulation is tested by performing LES of a laboratory premixed Bunsen flame. The results demonstrate that the Thickened Stochastic Fields method produces accurate predictions even when using a grid spacing equal to the filter scale. The present development therefore facilitates the accurate application of the Stochastic Fields approach to industrially-relevant combustion systems.     

连续体拓扑优化的一种光滑化方法

利用有限元离散后的水平集函数的节点值作为拓扑设计变量来求解连续体拓扑优化问题。利用阶跃函数把水平集函数转化为0-1变量。引进无穷次连续可微的sigmoid函数对阶跃函数进行光滑化。通过对sigmoid函数中陡度参数的不断调整来达到加快收敛速度的目的。利用拓扑导数的概念分析了这种做法的可行性。数值算例表明采用sigmoid函数作为光滑函数不但可以使算法的选取和实现变得更加容易,而且可以获得相对较快的收敛速度。     

信息熵原理在结构优化中的应用

首先将一些结构优化问题化归一类最小—最大优化问题,然后提出一种求解这类问题的信息熵模型.算例表明这一方法有较好的计算效率.     

Assessment of the Dynamic SGS Wrinkling Combustion Modeling Using the Thickened Flame Approach Coupled with FGM Tabulated Detailed Chemistry

The dynamic power-law wrinkling model proposed by Charlette et al. is coupled with Flamelet Generated Manifolds (FGM) tabulated chemistry combined with an artificially thickened flame model (ATF) for large eddy simulation. The dynamic formulation is similar to the “Germano” procedure and uses Taylor series based Gaussian filter. Thereby, the power-law wrinkling model parameter is considered to have both temporal and spatial dependency. Series of simulations are conducted for a lean premixed turbulent flame, using both dynamic and non-dynamic versions of the wrinkling model under different grid levels. The simulation results applying the non-dynamic wrinkling model show different behavior for each particular flame resolution, where none of the simulations could deliver the correct flame statistics, such as flame height. The dynamic version of the power-law wrinkling model improves the results independently of the flame resolution, as a consequence of the conservation of the total flame surface.     

A Similarity Subgrid Model for Premixed Turbulent Combustion

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.     

基于Huth方法的民机气动弹性改进建模方法

使用高仿真精度和高计算效率的有限元模型进行飞机颤振边界预测具有重要意义．本文提出了基于Huth紧固件柔度计算方法的民机气动弹性模型的改进建模方法,计及了固定前缘的刚度及其与机翼盒段之间的弹性效应,建立了全机气动弹性模型并进行了固有特性和颤振特性分析．分析结果与全机地面共振试验、风洞试验结果的偏差均在±1 %以内,分析时长与现有模型基本保持不变,验证了改进建模方法的有效性．     

A Kinetic Approach to the Plane Poiseuille Flow Over a Porous Matrix

The Poiseuille–Couette gas flow in a channel and the gas flow through an adjacent porous medium are considered when the governing equations are obtained via a molecular kinetic approach based on the Boltzmann equation. The mass continuity, momentum balance and energy conservation are written for the gas in the contiguous regions, whereas the behavior of the solid matrix obeys to the heat diffusion equation. Two different space scalings lead to different forms of the equations for the steady flow through the fully saturated matrix. The boundary conditions at the interface between the two domains are investigated via a matching procedure.     

A Two-Dimensional Tabulated Flamelet Combustion Model for Furnace Applications

A new tabulated chemistry approach for representing turbulent combustion in industrial furnaces is presented. This model is based on the tabulation of two dimensional diffusion flamelets to account for ternary mixtures between fuel, oxidant and burned gases which are integrated over probability density functions. To avoid excessive CPU time for the table generation, the calculation of the two dimensional flamelets is performed using the method proposed in the ADF-PCM (Approximated Diffusion Flame - Presumed Conditional Moment) approach: only the equation for the progress variable is solved, instead of the equations for all species. The progress variable reaction rate is given by a table of homogeneous reactors using the DHR model (Diluted Homogeneous Reactor) proposed by Locci et al. These approximated diffusion flames are first compared to exact diffusion flames computed using the flamelet equations and the chemistry for all species. The resulting model, called A2DF (Approximate 2 Dimensional Flamelet) is then applied to the RANS (Reynolds Averaged Navier-Stokes) simulations of Sandia Flames D and F, showing a good agreement with experimental measurements. Finally, this model is applied to the flameless and conventional combustion cases of the burner of Verissimo et al., showing a correct agreement for temperature and species predictions.     

A Multidimensional Model for the Combustion of Compressible Fluids

We consider a multidimensional model for the combustion of compressible reacting fluids. The flow is governed by the Navier–Stokes in Eulerian coordinates and the chemical reaction is irreversible and is governed by the Arrhenius kinetics. The existence of globally defined weak solutions is established by using weak convergence methods, compactness and interpolation arguments in the spirit of Feireisl [16] and P.L. Lions [24].     

Kinetic Relations for a Lattice Model of Phase Transitions

The aim of this article is to analyse travelling waves for a lattice model of phase transitions, specifically the Fermi–Pasta–Ulam chain with piecewise quadratic interaction potential. First, for fixed, sufficiently large subsonic wave speeds, we rigorously prove the existence of a family of travelling wave solutions. Second, it is shown that this family of solutions gives rise to a kinetic relation which depends on the jump in the oscillatory energy in the solution tails. Third, our constructive approach provides a very good approximate travelling wave solution.     

An Introduction to the Adjoint Approach to Design

Optimal design methods involving the solution of an adjoint system of equations are an active area of research in computational fluid dynamics, particularly for aeronautical applications. This paper presents an introduction to the subject, emphasising the simplicity of the ideas when viewed in the context of linear algebra. Detailed discussions also include the extension to p.d.e.'s, the construction of the adjoint p.d.e. and its boundary conditions, and the physical significance of the adjoint solution. The paper concludes with examples of the use of adjoint methods for optimising the design of business jets. This revised version was published online in July 2006 with corrections to the Cover Date.