Numerical analysis of wall pressure and heat flux fluctuations in shock-turbulent-boundary-layer interaction

The unsteady, compressible, Reynolds-averaged Navier-Stokes equations are solved numerically for an oblique shock-wave-induced turbulent boundary layer sepration. For the freestream Mach number 6 and the freestream Reynolds number 66·1 × 10⁶ m^?1, a time-dependent computation is performed, using MacCormack's explicit-implicit finite difference method with 82 × 42 grid points. A two-layer eddy viscosity turbulence model is employed in conjunction with a relaxation modification. Comparisons of the mean wall pressure and the mean heat transfer coefficient with the available experimental results are made and the evaluation of unsteady data for surface pressure and heat flux fluctuations is presented. It is found that the fluctuations in heat flux have qualitatively the same features as those of wall pressure but are different quantitatively.     

Flow and coupled heat transfer in the cavity between the rotor and stator

The turbulent flow and coupled heat transfer in the cavity between the rotor and stator is numerically simulated. Reynolds-averaged Navier-Stokes equations closed with equations of the k-ɛ turbulence model are used to calculate the viscous compressible gas flow characteristics and heat transfer; the unsteady heat conduction equation is used to calculate the temperature field in the metal. The influence of the mass flow rate of the coolant on the flow structure and efficiency of cooling of the rotor and stator walls is studied. The calculated results are compared with experimental data.     

CFD／CSD紧耦合及新型动网格方法在气动弹性模拟中的应用

研发出一套基于紧耦合的CFD／CSD耦合方法和程序。非定常流场求解采用混合网格有限体积方法,时间离散采用基于LU-SGS隐式格式的双时间步长法。通过求解雷诺平均Navier—Stokes方程模拟了三维机翼的跨音速气动弹性现象。得到了其颤振边界,与风洞实验结果吻合较好,验证了方法和程序的有效性和实用意义。同时将Delau...     

AN ANALYTICAL DESIGN METHODOLOGY FOR AN ANNULAR COMBUSTOR

SUMMARY

This paper describes a computational procedure for the optimization of the performance parameters of a simulated annular combustor. This method has been applied to analyze the influence of the performance parameters and geometries on the annular combustor characteristics and provide a good understanding of combustor internal flow fields, and therefore it can be used for guiding the combustor design process. The approach is based on the solution of governing nonlinear, elliptic partial differential equations for 3-D axisymmetric recirculating turbulent reacting swirling flows and the modelling of turbulence, combustion, thermal radiation and pollutant formation. The turbulence effects are introduced via the modified two-equation κ-ε model. Turbulent combustion is modelled using the κ-ε-g model and a two-step turbulent combustion model is employed for the excess emission of carbon monoxide CO. For the evaluation of the NO pollutant formation rate, the NO pollutant formation model, which takes into account the influence of turbulence, presented here. The radiative heat transfer is handled by the heat flux model. The predictions of the combustor character-istics and performance parameters are made using the present approach.

Predictions of velocity, length of the recirculation zone, combustion efficiency and wall temperature are compared with measurements. Agreement between the predictions and experimental data is very satisfactory.     

Heat release effects on mixing scales of non-premixed turbulent wall-jets: A direct numerical simulation study

The present study concerns the role of heat release effects on characteristics mixing scales of turbulence in reacting wall-jet flows. Direct numerical simulations of exothermic reacting turbulent wall-jets are performed and compared to the isothermal reacting case. An evaluation of the heat-release effects on the structure of turbulence is given by examining the mixture fraction surface characteristics, diagnosing vortices and exploring the dissipation rate of the fuel and passive scalar concentrations, and moreover by illustration of probability density functions of reacting species and scatter plots of the local temperature against the mixture fraction. Primarily, heat release effects delay the transition, enlarge the fluctuation intensities of density and pressure and also enhance the fluctuation level of the species concentrations. However, it has a damping effect on all velocity fluctuation intensities and the Reynolds shear stress. A key result is that the fine-scale structures of turbulence are damped, the surface wrinkling is diminished and the vortices become larger due to heat-release effects. Taking into account the varying density by using semi-local scaling improves the collapse of the turbulence statistics in the inner region, but does not eliminate heat release induced differences in the outer region. Examining the two-dimensional premultiplied spanwise spectra of the streamwise velocity fluctuations indicates a shifting in the positions of the outer peaks, associated with large energetic structures, toward the inner region.     

Large-Eddy and Direct Numerical Simulations of the Bachalo-Johnson Flow with Shock-Induced Separation

The Bachalo-Johnson experiment on an axisymmetric bump has been a primary validation case for turbulence models in shock-boundary-layer interactions since the 1980’s. In the present work, Wall-Modelled Large-Eddy Simulations (WMLES) of this flow were conducted using Improved Delayed Detached-Eddy Simulation (IDDES) as the sub-grid-scale (SGS) and wall model, with a synthetic turbulence generator, expecting close enough agreement with experiment. However, the WMLES results are disappointing, even in terms of the shock position, even though the results from two grids with 4.7 × 10⁸ and 1.6 × 10⁹ cells respectively agree well with each other. This failure of grid refinement to warn of an inaccurate simulation is of great concern, and the reasons for it are explored. We then conducted a Direct Numerical Simulation (DNS) embedded in the LES over a reduced domain, with 8 × 10⁹ grid cells. The DNS has a far more accurate shock position and overall pressure distribution. The skin friction in the favourable pressure gradient is also much higher than in the LES; thus, wide differences appear upstream of the shock wave, most probably caused by the rapid acceleration which leads to atypical shear-stress profiles. Other SGS models were tried, and performed worse than IDDES. The DNS essentially fulfils the initial expectations although in a reduced domain and provides data for turbulence-modelling studies, for instance by extracting an effective eddy viscosity from it. The most noticeable remaining disagreement with experiment is over the Reynolds shear stress.     

基于动态分区概念的高超声速燃烧大涡模拟

本文基于动态分区概念开展了亿级网格的高马赫数全尺寸超燃冲压发动机内外流耦合一体化改进延迟分离涡(IDDES)模拟研究. 研究建立了包括动态分区火焰面湍流燃烧模型(DZFM)、分区自适应化学(Z-DAC)和分区并行自适应建表(Z-ISAT)的完整动态分区燃烧模拟框架, 并通过1.15亿网格的马赫数12 REST标准高超声速燃烧室模型初步验证了分区模拟框架的保真性. DZFM通过分区解耦的思想既准确表征了当地湍流化学交互作用关系, 又有效提升了整场湍流燃烧的计算效率. Z-DAC和Z-ISAT通过在分区框架内对化学反应机理进行动态实时简化和建表查询, 可进一步提升当前分区内化学反应的求解效率. 基于1.25和1.4亿网格动态分区框架对比分析了马赫数10条件下中心支板(strut)和壁面撑挡型(pylon)两类构型氢气高超声速燃烧室特性. 支板或撑挡结构均诱发了明显的边界层分离和头部回流区, 由此两种燃烧室均出现了较长区域的喷注点前部燃烧现象. 基于Borghi图的数值分析表明当前氢气高超声速燃烧室中广泛存在扩散控制为主的火焰面模式, 效率提升的瓶颈在于高效增混. 壁面撑挡燃烧室具有较高的穿透深度和近场混合效率, 因而燃烧效率高于净推力准则80%, 相应的比冲1234 s也远高于中心支板燃烧室的437 s. 分区自适应化学方法在将近一半的计算域上降低了反应求解计算代价, 特别是在无燃料区反应机理的简化幅度更加明显. 相比与传统的有限速率PaSR模型, DZFM模型实现了高达11倍的加速比.      

Finite volume computation of the turbulent flow over a hill employing 2D or 3D non-orthogonal collocated grid systems

A general numerical method for the solution of the complete Reynolds-averaged Navier-Stokes equations for 2D or 3D flows is described. The method uses non-orthogonal co-ordinates, Cartesian velocity components and a pressure-velocity-coupling algorithm adequate for non-staggered grid systems. The capability of the method and the overall performance of the κ–? eddy viscosity model are demonstrated by calculations of 2D and 3D flow over a hill. Solution error estimations based on fine grids, e.g. 320 × 192 control volumes, together with comparisons with standard turbulence model modifications, low-Reynoldsnumber or streamline curvature effects, have allowed the investigation of model drawbacks in predicting turbulent flows over surface-mounted hills.     

Simulation of three-dimensional unsteady viscous free surface flow around a ship model

We present here a numerical method for solving the free surface flow around a ship at forward speed in calm water. The fluid is assumed to be Newtonian and the Reynolds-averaged Navier-Stokes equations are solved by a finite difference method. Modelization of turbulence is achieved by the algebraic model proposed by Baldwin and Lomax. Fully non-linear free surface conditions are satisfied in the model and a method to avoid the incompatibility between free surface conditions and no-slip conditions at the waterline is proposed. Numerical results obtained for a Wigley hull are compared with experimental results.     

Numerical solution of 2D and 3D turbulent internal flow problems

The paper describes a method for solving numerically two-dimensional or axisymmetric, and three-dimensional turbulent internal flow problems. The method is based on an implicit upwinding relaxation scheme with an arbitrarily shaped conservative control volume. The compressible Reynolds-averaged Navier-Stokes equations are solved with a two-equation turbulence model. All these equations are expressed by using a non-orthogonal curvilinear co-ordinate system. The method is applied to study the compressible internal flow in modern power installations. It has been observed that predictions for two-dimensional and three-dimensional channels show very good agreement with experimental results.     

燃烧室两相流场亚网格燃烧模型的研究

在三维任意曲线坐标系下采用不同的亚网格燃烧模型对环形燃烧室火焰筒气液两相湍流瞬态反应流进行大涡模拟．计算中所采用的数学度模型有：k方程亚网格尺度模型估算亚网格湍流黏性;热通量辐射模型估算辐射换热,分别采用亚网格EBU燃烧模型（E-A model）、亚网格二阶矩输运方程模型（SOM）和亚网格二阶矩代数模型（SOM-A）估算化学反应速率．并在非交错网格系统下气相采用SIMPLE算法和混合差分格式求解,液相采用Lagrange处理,并用PSIC算法对其进行求解．通过实验结果和计算结果的比较,表明在三维任意曲线坐标系下对燃烧室火焰简两相湍流油雾燃烧流场进行大涡模拟,3种不同的亚网格燃烧模型都能真实反映两相湍流化学反应流流动及实际燃烧过程,而采用亚网格二阶矩输运方程模型稍优于其他两种亚网格燃烧模型．     

双模态发动机的模态鉴别方法

双模态冲压发动机的不同燃烧模态具有不同的稳焰机制和流态特征,并且在模态转换时伴随着显著的推力变化. 因此,准确判断燃烧模态,对于捕捉发动机的燃烧区位置/范围、释热分布特征,以及为进一步优化燃烧室的设计(流道结构和供油布局) 具有重要意义. 目前尚无鉴别模态的有效试验方法,本文提出了一种模态鉴别的试验方法,并在超燃直连台上开展验证试验. 试验中使用的测量技术包括:壁面静压、高速阴影/纹影、多通道可调谐二极管吸收光谱和高能态碳氢自由基CH* 自发光成像. 利用多种测量方法的组合,可以同时获得燃烧室中气流静温、速度、马赫数分布,释热分布以及燃烧区位置/范围. 这些试验数据能够用于判别模态,并获得不同模态的流动和火焰特征.      

A study of a bluff-body combustor using laser sheet lighting

Laser sheet lighting is used to study reacting flows with and without heat release in an axisymmetric, unducted and vertically mounted bluff-body combustor. The fuel, which is seeded with titanium tetrachloride vapor, is ejected from a jet located in the center of the bluff-body. The TiCl₄ in the dry fuel reacts spontaneously with the water in the annulus air to form titanium dioxide particles. High speed movies and visual observations of vertically and horizontally located sheets of laser light provided remarkably detailed visualization (via Mie scattering) of the vortex dynamics in the near-wake region of the bluff-body.A version of this paper was presented at the ASME Winter Annual Meeting of 1984 and printed in AMD, Vol. 66     

Large-Eddy Simulation for an Axisymmetric Piston-Cylinder Assembly With and Without Swirl

Large-eddy simulation (LES) has been performed for an axisymmetric piston-cylinder assembly with and without swirl. For both cases, the LES mean and rms velocity profiles show better agreement with experimental data than profiles obtained using a Reynolds-averaged Navier–Stokes (RANS) approach with a standard k???ε turbulence model. The sum of the resolved and modeled contributions to turbulence kinetic energy (TKE) approaches grid independence for the meshes used in this study. The sensitivity of LES to key numerical and physical model parameters has been investigated. Results are especially sensitive to mesh and to the subfilter-scale (SFS) turbulence models. Satisfactory results can be obtained using simple viscosity-based SFS turbulence models, although there is room for improvement. No single model gives uniformly best agreement between model and measurements at all spatial locations and at all times. The strong sensitivity of computed mean and rms velocity profiles to variations in the SFS turbulence model suggests that better results might be obtained using more sophisticated models.     

LES of a Multi-burner Annular Gas Turbine Combustor

In this study, Large Eddy Simulation (LES) has been used to predict the flow, mixing and combustion in both a single burner laboratory gas turbine combustor and in an 18 burner annular combustor, having identical cross sections. The LES results for the single burner laboratory combustor are compared with experimental data for a laboratory model of this combustor, and with other LES predictions, with good agreement. An explicit finite volume based LES model, using the mixed subgrid model together with a partially stirred reactor model for the turbulence chemistry interactions, is used. For the annular combustor, with the swirlers parameterized by jet inflow boundary conditions, we have investigated the influence of the a-priori unknown combustor exit impedance, the influence of the swirler characteristics and the fuel type. The combustion chemistry of methane–air and n-decane–air combustion is modeled by a two-step reaction mechanism, whereas NOx is separately modeled with a one-step mechanism. No experimental data exists for the annular combustor, but these results are compared with the single burner LES and experimental results available. The combustor exit impedance, the swirler- and fuel characteristics all seem to influence the combusting flow through the acoustics of the annular combustor. To examine this in greater detail time-series and eigenmodes of the combustor flow fields are analyzed and comparisons are made also with results from conventional thermoacoustic eigenmode analysis, with reasonable agreement. The flow and pressure distributions in the annular combustor are described in some detail and the mechanisms by which the burners interact are outlined.     

Calculation of quasi-three-dimensional incompressible viscous flows by the finite element method

This paper discusses the calculation of quasi-three-dimensional incompressible viscous flow by FEM. The Reynolds-averaged Navier-Stokes equations are solved in curvilinear co-ordinates by the reduced integration and penalty method (RIP). Streamline upwind artificial viscosity (SUAV) and the Baldwin-Lomax algebraic model of turbulence are used. Time discretization is by the general implicit θ-method.     

Large-eddy Simulation of Triangular-stabilized Lean Premixed Turbulent Flames: Quality and Error Assessment

In this numerical study, an algebraic flame surface wrinkling (AFSW) reaction submodel based on the progress variable approach is implemented in the large-eddy simulation (LES) context and validated against the triangular stabilized bluff body flame configuration measurements i.e. in VOLVO test rig. The quantitative predictability of the AFSW model is analyzed in comparison with another well validated turbulent flame speed closure (TFC) combustion model in order to help assess the behaviour of the present model and to further help improve the understanding of the flow and flame dynamics. Characterization of non-reacting (or cold) and reacting flows are performed using various subgrid scale models for consistent grid size variation with 300,000 (coarse), 1.2 million (intermediate) and 2.4 million (fine) grid cells. For non-reacting flows at inlet velocity of 17?m/s and inlet temperature 288?K, coarse grid leads to over prediction of turbulence quantities due to low dissipation at the early stage of flow development behind the bluff body that convects downstream eventually polluting the resulting solution. The simulated results with the intermediate (and fine) grid for mean flow and turbulence quantities, and the vortex shedding frequency (f_s) closely match experimental data. For combusting flows for lean propane/air mixtures at 35?m/s and 600?K, the vortex shedding frequency increase threefold compared with cold scenario. The predicted results of mean, rms velocities and reaction progress variable are generally in good agreement with experimental data. For the coarse grid the combustion predictions show a shorter recirculation region due to higher turbulent burning rate. Finally, both cold and reacting LES data are analyzed for uncertainty in the solution using two quality assessment techniques: two-grid estimator by Celik, and model and grid variation by Klein. For both approaches, the resolved turbulent kinetic energy is used to estimate the grid quality and error assessment. The quality assessment reveals that the cold flows are well resolved even on the intermediate mesh, while for the reacting flows even the fine mesh is locally not sufficient in the flamelet region. The Klein approach estimates that depending on the recirculation region in cold scenario both numerical and model errors rise near the bluff-body region, while in combusting flows these errors are significant behind the stabilizing point due to preheating of unburned mixture and reaction heat release. The total error mainly depends on the numerical error and the influence of model error is low for this configuration.     

A Spalart–allmaras Turbulence Model Implementation for High-order Discontinuous Galerkin Solution of the Reynolds-averaged Navier-stokes Equations

A novel and robust approach has been proposed for the high-order discontinuous Galerkin (DG) discretization of the Reynolds-averaged Navier-Stokes (RANS) equations with the turbulence model of Spalart-Allmaras (SA). The solution polynomials of the SA equation are reconstructed by the Hermite weighted essentially non-oscillatory (HWENO) scheme. Several practical techniques are suggested to simplify and extend a positivity-preserving limiter to further guarantee the positivity of SA working variable. The resulting positivity-preserving HWENO limiting method is compact and easy to implement on arbitrary meshes. Typical turbulent flows are conducted to assess the accuracy and robustness of the present method. Numerical experiments demonstrate that with the increasing grid or order resolution, the limited results of the working variable are getting closer to the unlimited ones. And the most obvious improvement with proposed method is on the computation of the working variable field in wake regions.     

变高度圆柱诱导的激波边界层干扰

采用分区方法及Ｒｏｅ三阶流通量差分分裂格式求解雷诺平均Ｎ－Ｓ方程,湍流附加黏性系数用Ｂａｌｄｗｉｎ－Ｌｏｍａｘ模型计算,数值模拟了高超声速条件下变高度圆柱诱导的激波边界层层干扰,其流场的主要特性均与实验结果一致或规律相同,结果清晰地展示了流场结构以及气动载荷分布随柱高度的变化特征,产说明激波碰撞和旋涡运动都可能导致飞行器表面局部气动载荷的增加。