尺度自适应模拟和大涡模拟的关联性分析

采用理论分析和数值模拟相结合的方法,系统研究了尺度自适应模拟（scale-adaptive simulation,SAS）和大涡模拟（large-eddy simulation,LES）的关联性问题.在理论分析方面,对比分析了系综平均和滤波的定义、Spalart-Allmaras（SA）湍流模型和动态亚格子（subgrid-scale,SGS）模型关于湍流黏性系数的求解方式.理论分析结果表明,系综平均等价于盒式直接滤波,SAS和LES的控制方程在数学形式上具有一致性;SAS存在过多的湍流耗散,主要来自于SA输运方程中的扩散项.在数值模拟方面,选取来流Mach数0.55,Reynolds数2×105的圆柱可压缩绕流为分析算例.计算结果表明,SAS和LES预测的大尺度平均流场信息几乎一致,SAS预测的湍流脉动信息略低于LES.SAS在圆柱近尾迹区的湍流耗散过大,而在稍远的尾迹区几乎能够完全等效于LES.      

A new mixed subgrid-scale model for large eddy simulation of turbulent drag-reducing flows of viscoelastic fluids

A mixed subgrid-scale(SGS) model based on coherent structures and temporal approximate deconvolution(MCT) is proposed for turbulent drag-reducing flows of viscoelastic fluids. The main idea of the MCT SGS model is to perform spatial filtering for the momentum equation and temporal filtering for the conformation tensor transport equation of turbulent flow of viscoelastic fluid, respectively. The MCT model is suitable for large eddy simulation(LES) of turbulent dragreducing flows of viscoelastic fluids in engineering applications since the model parameters can be easily obtained. The LES of forced homogeneous isotropic turbulence(FHIT) with polymer additives and turbulent channel flow with surfactant additives based on MCT SGS model shows excellent agreements with direct numerical simulation(DNS) results. Compared with the LES results using the temporal approximate deconvolution model(TADM) for FHIT with polymer additives, this mixed SGS model MCT behaves better, regarding the enhancement of calculating parameters such as the Reynolds number.For scientific and engineering research, turbulent flows at high Reynolds numbers are expected, so the MCT model can be a more suitable model for the LES of turbulent drag-reducing flows of viscoelastic fluid with polymer or surfactant additives.     

圆形射流湍流场的大涡模拟研究

使用发展的大涡模拟并行程序,数值模拟了圆形射流湍流场,与实验值进行详细比较,对大涡模拟程序进行检验,并分析网格尺度对计算结果的影响。结果表明,大涡模拟计算的平均速度场和雷诺应力场与实验值符合得非常好,验证了该大涡模拟计算程序的可靠性和精确度。同时发现一阶统计矩受网格尺度影响不大,二阶及高阶矩与网格尺度有较大的关系,因此使用大涡模拟方法计算湍流及湍流燃烧问题应对网格依赖性进行必要分析,至少保证二阶统计矩具有较好的精度。     

An improved dynamic subgrid-scale model and its application to large eddy simulation of rotating channel flows

Rotating turbulence occurs extensively in nature and engineering circumstances. Meanwhile, understanding physical mechanisms of the rotating turbulence is important to the fundamental research of turbulence. The turbulent flow in rotating frames undergoes two kinds of Coriolis force effects. First, a secondary flow is induced in the case that there is a mean vorticity component perpendicular to the rotating axis. Second, there are augmenting or suppressing effects on the turbulence if there i…     

Constrained large-eddy simulation of supersonic turbulent boundary layer over a compression ramp

A supersonic turbulent boundary layer over a compression ramp is numerically investigated using the constrained large-eddy simulation (CLES) method. The compression corner is characterised by a deflection angle of 24°. The free-stream Mach number is Ma_∞ = 2.9, and the Reynolds number based on the momentum thickness of inlet boundary layer is Re_θ = 2300. The mean and statistical quantities, such as mean velocity, wall pressure and Reynolds stresses, are thoroughly analysed and compared with those from traditional large-eddy simulation (LES), experimental measurement and direct numerical simulation (DNS). It turns out that CLES can predict the friction coefficient, wall-pressure distribution, size of separation bubble, Reynolds stresses, etc. more accurately than traditional LES, and the results are in reasonable agreement with the experimental and/or DNS data. Also discussed are the effects of specific parameterisations of the Reynolds constraint and interfacial positions separating the constrained and unconstrained regions on the performance of the CLES method.     

Investigating asymptotic suction boundary layers using a one-dimensional stochastic turbulence model

The turbulent asymptotic suction boundary layer is studied using a one-dimensional turbulence (ODT) model. ODT is a fully resolved, unsteady stochastic simulation technique. While flow properties reside on a one-dimensional domain, turbulent advection is represented using mapping events whose occurrences are governed by a random process. Due to its reduced spatial dimensionality, ODT achieves major cost reductions compared to three-dimensional (3D) simulations. A comparison to recent direct numerical simulation (DNS) data at moderate Reynolds number (Re = u_∞ / v₀ = 333, where u_∞ and v₀ are the free stream and suction velocity, respectively) suggests that the ODT model is capable of reproducing several velocity statistics, i.e. mean velocity and turbulent kinetic energy budgets, while peak turbulent stresses are under-estimated by ODT. Variation of the Reynolds number in the range Re ∈ [333,400,500,1000] shows that ODT can reproduce various trends observed as a result of increased suction in turbulent asymptotic suction boundary layers, i.e. the reduction of Reynolds stresses and enhanced skin friction. While up to Re = 500 our results can be directly compared to recent LES data, the simulation at Re = 1000 is currently not feasible through full 3D simulations, hence ODT may assist the design of future DNS or LES simulations at larger Reynolds numbers.     

On the effect of an anisotropy-resolving subgrid-scale model on large eddy simulation predictions of turbulent open channel flow with wall roughness

A large eddy simulation (LES) was conducted of turbulent flow in a channel with a rough wall on one side and a free surface on the other by adopting an anisotropy-resolving subgrid-scale (SGS) model. A shear Reynolds number of Re_τ = 395 was used based on the mean friction velocity and channel height. To investigate the grid dependency of the LES results caused by the SGS model, three grid resolutions were tested under the same definition of a roughness shape by using the immersed boundary method. The results obtained were compared with direct numerical simulation data with and without the wall roughness and those without the extra anisotropic term. The primary focus was on how the present anisotropic SGS model with coarser grid resolutions can properly provide the effects of roughness on the mean velocity and turbulent stresses, leading to a considerable reduction of the computational cost of LES.     

Three-dimensional turbulent flow over cube-obstacles

In order to investigate the influence of surface roughness on turbulent flow and examine the wall-similarity hypothesis of Townsend, three-dimensional numerical study of turbulent channel flow over smooth and cube-rough walls with different roughness height has been carried out by using large eddy simulation(LES) coupled with immersed boundary method(IBM). The effects of surface roughness array on mean and fluctuating velocity profiles, Reynolds shear stress, and typical coherent structures such as quasi-streamwise vortices(QSV) in turbulent channel flow are obtained. The significant influences on turbulent fluctuations and structures are observed in roughness sub-layer(five times of roughness height).However, no dramatic modification of the log-law of the mean flow velocity and turbulence fluctuations can be found by surface cube roughness in the outer layer. Therefore, the results support the wall-similarity hypothesis. Moreover, the von Karman constant decreases with the increase of roughness height in the present simulation results. Besides, the larger size of QSV and more intense ejections are induced by the roughness elements, which is crucial for heat and mass transfer enhancement.     

Suitability of artificial bulk viscosity for large-eddy simulation of turbulent flows with shocks

The artificial bulk viscosity method to numerically capture shocks is investigated for large-eddy simulation (LES). Different variations of this method are tested on a turbulent flow over a cylinder at Reynolds number of 10,000 and free-stream Mach number of 0.85. The artificial bulk viscosity model by Cook and Cabot, which is parameterized by the strain rate magnitude, is found to provide unnecessary bulk viscosity in turbulent regions away from shocks. While developed turbulent structures are found unaffected, this extra bulk viscosity is shown to significantly damp the sound field. An alternative formulation of the model which is parameterized by the rate of dilatation is proposed. This formulation is shown to avoid the unnecessary bulk viscosity and enhance the sound-prediction capability of the model. It was found that standard LES combined with artificial bulk viscosity is a promising approach for simulation of turbulent flows with shocks. The formulation of the model on curvilinear coordinates is presented in the appendix.     

Constrained large-eddy simulation of separated flow in a channel with streamwise-periodic constrictions

Constrained large-eddy simulation (CLES) method has been recently developed by Chen and his colleagues for simulating attached and detached wall-bounded turbulent flows. In CLES, the whole domain is simulated using large-eddy simulation (LES) while a Reynolds stress constraint is enforced on the subgrid-scale (SGS) stress model for near wall regions. In this paper, CLES is used to simulate the separated flow in a channel with streamwise-periodic constrictions at Re = 10,595. The results of CLES are compared with those of Reynolds-averaged Navier-Stokes (RANS) method, LES, detached eddy simulation (DES) and previous LES results by Breuer et al. and Ziefle et al. Although a coarse grid is used, our results from the present LES, DES and CLES do not show large deviations from the reference results using much finer grid resolution. The comparison also shows that CLES performs the best among different turbulence models tested, demonstrating that the CLES provides an excellent alternative model for separated flows. Furthermore, the cross-comparisons among different CLES implementations have been carried out. Our simulation results are in favor of using the constraint from algebraic RANS model or solving the RANS model equations in the whole domain with a length scale modification according to the idea from DES.     

瞬变流中气泡动力过程研究

微气泡的生成等基础问题制约着鼓泡脱气技术的发展。本文基于对文丘里鼓泡器内流场的研究,对不同雷诺数下的气泡动力特性进行研究,采用大涡模拟和流体体积法相结合的数值模拟方法,对气泡在不同瞬时动态变化流场下的动力特性进行研究,通过分析气泡在不同湍流下的表面积,气泡球度,以及气泡表面所处的湍流应力,计算气泡所受的破坏应力与聚合应力之比,得到气泡的临界韦伯数随气泡在瞬变湍流中的变化过程,分析气泡破碎的临界条件。     

A Eulerian PDF scheme for LES of nonpremixed turbulent combustion with second-order accurate mixture fraction

Monte Carlo simulations of joint probability density function (PDF) approaches have been developed in the past largely with Reynolds averaged Navier Stokes (RANS) applications. Current interests are in the extension of PDF approaches to large eddy simulation (LES). As LES resolves accurately the large scales of turbulence in time, the Monte Carlo simulation and the flow field need to be tightly coupled. A tight coupling can be achieved if the consistency between the scalar field solution obtained via finite-volume (FV) methods and that from the stochastic solution of the PDF is ensured. For nonpremixed turbulent flames with two distinct streams, the local reactive mixture is described by the mixture fraction. A Eulerian Monte Carlo method is developed to achieve a second-order accuracy in the instantaneous filtered mixture fraction that is consistent with the corresponding FV. The performances of the proposed scheme are extensively evaluated using a one-dimensional model. Then, the scheme is applied to two cases with LES. The first one is a non-reacting mixing flow of two different fluids. The second case is the Sandia piloted turbulent flame D with a steady state flamelet model. Both results confirm the consistency of the proposed method to the level of filtered mixture fraction.     

逆压梯度下层流分离泡转捩的大涡模拟

本文采用有限体积法,发展了适合复杂几何边界条件的可压缩流动大涡模拟程序,通过对槽道湍流的计算,验证了程序的可靠性。对低雷诺数、逆压梯度环境中的层流分离泡转捩现象进行了数值模拟,分析了其统计特性及大尺度相干结构的演化规律．初步的结果表明:大涡模拟可以很好地处理层流分离泡转捩问题,既能给出满意的统计平均结果,又可以通过相干结构的演化细致描述分离泡转捩的动力学过程。     

Predictions of Conjugate Heat Transfer in Turbulent Channel Flow Using Advanced Wall-Modeled Large Eddy Simulation Techniques

In this paper, advanced wall-modeled large eddy simulation (LES) techniques are used to predict conjugate heat transfer processes in turbulent channel flow. Thereby, the thermal energy transfer process involves an interaction of conduction within a solid body and convection from the solid surface by fluid motion. The approaches comprise a two-layer RANS–LES approach (zonal LES), a hybrid RANS–LES representative, the so-called improved delayed detached eddy simulation method (IDDES) and a non-equilibrium wall function model (WFLES), respectively. The results obtained are evaluated in comparison with direct numerical simulation (DNS) data and wall-resolved LES including thermal cases of large Reynolds numbers where DNS data are not available in the literature. It turns out that zonal LES, IDDES and WFLES are able to predict heat and fluid flow statistics along with wall shear stresses and Nusselt numbers accurately and that are physically consistent. Furthermore, it is found that IDDES, WFLES and zonal LES exhibit significantly lower computational costs than wall-resolved LES. Since IDDES and especially zonal LES require considerable extra work to generate numerical grids, this study indicates in particular that WFLES offers a promising near-wall modeling strategy for LES of conjugated heat transfer problems. Finally, an entropy generation analysis using the various models showed that the viscous entropy production is zero inside the solid region, peaks at the solid–fluid interface and decreases rapidly with increasing wall distance within the fluid region. Except inside the solid region, where steep temperature gradients lead to high (thermal) entropy generation rates, a similar behavior is monitored for the entropy generation by heat transfer process.     

Towards a classification of models for the numerical simulation of premixed combustion based on a generalized regime diagram

The inner structure, and the physical behaviour of turbulent premixed flames are usually described, and classified by means of the regime diagram introduced by Borghi and Peters. Thereby properties related to both the flame and the (turbulent) flow are considered. In this work a diagram valid for all physical regimes, comprising suitable requirements for laminar simulations, direct numerical simulation (DNS), large-eddy simulation (LES), and Reynolds averaging based numerical simulation (RANS) is proposed. In particular the diagram describes essential situations within the validity limits of the “Borghi, Peters diagram” which physical phenomena are resolved by the simulation, and which have to be modelled. This information is used for systematic classification of various models by suggesting specific models that are appropriate depending on the regime and numerical resolution, and may provide guidance for numerical simulation methods and model development in turbulent premixed combustion. This might help users as a guideline in choosing appropriate models for a given device, and numerical effort available. The regime diagram suggested by Pitsch and Duchamp de Lageneste, which includes DNS and LES by explicitely accounting for the numerical related variable filterwidth, emerges here as one of the special two-dimensional cases possible. In contrast to the generalized regime diagram, their diagram does not include laminar simulations, and RANS based considerations, while transition between wrinkled and corrugated flamelets is not clearly established.     

Vorticity,backscatter and counter-gradient transport predictions using two-level simulation of turbulent flows

The two-level simulation (TLS) method evolves both the large-and the small-scale fields in a two-scale approach and has shown good predictive capabilities in both isotropic and wall-bounded high Reynolds number (Re) turbulent flows in the past. Sensitivity and ability of this modelling approach to predict fundamental features (such as backscatter, counter-gradient turbulent transport, small-scale vorticity, etc.) seen in high Re turbulent flows is assessed here by using two direct numerical simulation (DNS) datasets corresponding to a forced isotropic turbulence at Taylor’s microscale-based Reynolds number Re_λ ≈ 433 and a fully developed turbulent flow in a periodic channel at friction Reynolds number Re_τ ≈ 1000. It is shown that TLS captures the dynamics of local co-/counter-gradient transport and backscatter at the requisite scales of interest. These observations are further confirmed through a posteriori investigation of the flow in a periodic channel at Re_τ = 2000. The results reveal that the TLS method can capture both the large- and the small-scale flow physics in a consistent manner, and at a reduced overall cost when compared to the estimated DNS or wall-resolved LES cost.     

Large eddy simulation of a light gas stratification break-up by an entraining turbulent fountain

The erosion process of a stably stratified light gas layer by a vertical turbulent fountain of denser fluid inside a generic containment – for which experimental reference data are available – is studied computationally using large eddy simulation (LES). In addition, various Reynolds averaged Navier–Stokes (RANS) models are applied aiming at a comparative assessment of different computational approaches for the considered case. With the LES methodology included into the present modelling study, a novelty to date is established for fountain-stratification interaction inside generic containments. The high Reynolds number RANS models applied in the framework of this study include both the realisable k–? eddy viscosity model (EVM) as well as the basic Reynolds stress model (RSM). Furthermore, we show that certain regimes of the present configuration can be predicted using an analytically derived scaling approach. Various data beyond the experimentally obtained ones are computationally provided in order to facilitate the calibration of less costly statistical turbulence models and lumped parameter codes, since the presently considered configuration is regarded to be a valuable small-scale equivalent for containment flow applications.     

Proposed stochastic parameterisations of subgrid turbulence in large eddy simulations of turbulent channel flow

Stochastic and deterministic subgrid parameterisations are developed for the large eddy simulation (LES) of a turbulent channel flow with friction-velocity-based Reynolds number of Re_τ = 950 and centreline-based Reynolds number of Re₀ = 20,580. The subgrid model coefficients (eddy viscosities) are determined from the statistics of truncated reference direct numerical simulations (DNSs). The stochastic subgrid model consists of a mean-field shift, a drain eddy viscosity acting on the resolved field and a stochastic backscatter force of variance proportional to the backscatter eddy viscosity. The deterministic variant consists of a net eddy viscosity acting on the resolved field, which represents the net effect of the drain and backscatter. LES adopting the stochastic and deterministic models is shown to reproduce the time-averaged kinetic energy spectra of the DNS within the resolved scales.     

Rotation effect on near-wall turbulence statistics and flow structures

Turbulent flow in an axially rotating pipe, involving complicated physical mecha- nism of turbulence, is a typical problem for the study of rotating turbulent flow. The pipe rotation induces two effects on the flow. One is the stabilizing effect due to the centrifu- gal and Coriolis forces, which accounts for the relaminarization of the turbulence[1—3] and the reduction of the friction coefficient at the pipe wall. The behavior is also related to the wall streaks inclining to the azimuthal di…     

流体机械中高雷诺数流动的大涡模拟

本文应用大涡模拟方法建立了管道流在高雷诺数条件下的数值求解方法，推导了用求和下标所表示的三维控制方程在贴体坐标下的张量形式．应用有限差分法对控制方程进行离散求解，用编制的三维湍流场LES计算程序对方弯管内的流动进行了计算验证．