Nonlinear turbulence models for predicting strong curvature effects

Prediction of the characteristics of turbulent flows with strong streamline curvature, such as flows in turbomachines, curved channel flows, flows around airfoils and buildings, is of great importance in engineering applications and poses a very practical challenge for turbulence modeling. In this paper, we analyze qualitatively the curvature effects on the structure of turbulence and conduct numerical simulations of a turbulent Uduct flow with a number of turbulence models in order to assess their overall performance. The models evaluated in this work are some typical linear eddy viscosity turbulence models, nonlinear eddy viscosity turbulence models （NLEVM） （quadratic and cubic）, a quadratic explicit algebraic stress model （EASM） and a Reynolds stress model （RSM） developed based on the second-moment closure. Our numerical results show that a cubic NLEVM that performs considerably well in other benchmark turbulent flows, such as the Craft, Launder and Suga model and the Huang and Ma model, is able to capture the major features of the highly curved turbulent U-duct flow, including the damping of turbulence near the convex wall, the enhancement of turbulence near the concave wall, and the subsequent turbulent flow separation. The predictions of the cubic models are quite close to that of the RSM, in relatively good agreement with the experimental data, which suggests that these models may be employed to simulate the turbulent curved flows in engineering applications.

Study of New Eddy Viscosity Modelling for Turbulence

1 The Eddy Viscosity Is a Fourth-order Tensor In 1877 Boussinesq proposed that turbulence stress be proportional to the average velocity gradient.He introduced the concept of eddy viscosity coefficient and described the turbulence stress

New eddy viscosity modelling of turbulent boundary layer with adverse pressure gradient

A new eddy viscosity modelling of turbulence is proposed in ref. [1], whose characteristicsexpress the anisotropy of the turbulence stress. In ref. [1], the way of proposing somedistributional laws of the eddy viscosity tensor components is found from experimental re-sults for elementary flow of each kind of shearing flow, and then we can forecast thecomplicated shearing flows more accurately. In this way, the distributional laws of the ed-

基于修正RNG k-ε模型的叶片泵非设计工况数值模拟

对于非设计工况下流体机械的流场计算,由于其偏离设计工况,分离流动严重等特点,使得用普通的湍流模型数值模拟精度不高。本文采用一种将涡粘性系数Cμ与湍流脉动动能和湍流耗散率的变化相关联的方法,对RNG k-ε湍流模型进行修正。为验证其对非设计工况下叶片泵的预测精度,分别采用修正模型和原始RNG模型,数值模拟了一比转速为439的叶片泵的内部流场,得到了其扬程、效率曲线及在典型工况下的内部流场流线分布。通过比较改进前后RNG湍流模型的数值计算结果,并与实验结果对比,得出修正RNG模型更能准确预测出非设计工况下叶片泵的流场特性。

一种EASM k-ω两方程湍流模型的应用研究

显式代数应力 ( EASM)模型是一种比线性涡粘模型 ( LEVM)更高级的非线性构成模型。这类构成模型用平均速度梯度、平均应变率和旋转应变率张量 ,给出了更高级的湍流剪应力表达式。通过把 EASM构成模型和 k- ω两方程模型耦合到一起 ,可得到一种更高级的 EASM k- ω两方程湍流模型。采用这种非线性的构成模型后 ,可以极大地拓展湍流模型的应用范围 ,提高数值计算精度。在求解湍流流场时 ,Navier- Stokes( N- S)方程和 EASM k-ω两方程湍流模型的控制方程采用非耦合方法求解 ,通过对平板、RAE 2 82 2超临界翼型、ONERA M6机翼等标准算例的计算 ,验证了此种湍流模型对湍流流场的预测能力 。

可压缩k-ξ湍流模式理论研究

基于Boussinesq涡黏性假设,选取新的第二输运变量,推导出k-ξ湍流模式的可压缩形式。针对可压缩流动的特点以及分离流动的复杂性,在可压缩k-ξ湍流模式的基础上添加应力限制器,通过限制涡黏系数的方式来达到限制湍流剪切应力以及减小湍能增幅的效果。将添加剪切应力限制器的k-ξ可压缩湍流模式用于RAE2822翼型流场数值模拟,结果表明:压力系数及不同截面速度与实验值符合较好,且在分离与掺混的复杂流动区域,修正后的k-ξ湍流模式数值模拟结果优于k-ε模式及k-ω模式。

圆柱绕流的三大类型湍流模型及稳定性数值模拟研究

海港桩基码头下方桩柱周围的绕流特性对港口水动力与泥沙输运具有重大影响。为探究湍流数值模拟方法对三维桩柱绕流精度的影响,基于CFD方法建立OpenFOAM湍流模型,在雷诺数Re=3900条件下,研究不同近壁取值范围y+=0.95~5.00内的大涡模拟LES、原始雷诺时均法(RANS)湍流模型和增加涡流黏度限制公式的RANS湍流模型的阻力系数、斯特劳哈尔数、平均压力系数、平均速度和涡流结构。研究结果表明,与前人RANS各湍流模型及试验计算结果比较,RNGκ-ε湍流模型对阻力系数和斯特劳哈尔数模拟较准确,对涡捕捉精细程度最好;κ-ω、κ-ωSST湍流模型对斯特劳哈尔数模拟较准确,在y+=0.95~2.00时对平均压力系数模拟较准确;各RANS湍流模型对平均速度的模拟均存在回流区内速度偏差较大的问题;大涡模拟LES湍流模型在近壁网格精度较高时,所有水动力模拟结果相对准确。研究还发现增加涡流黏度限制后的RANS湍流模型,对涡结构捕捉的精细程度更高,更符合真实流动。通过对不同湍流模型下单桩水动力特性的分析研究,可为海港码头下圆形桩群的水动力计算提供参考。

汽车外流场数值仿真k-ε模型适用性研究

由于目前的湍流模型并非针对汽车空气动力学进行数值仿真,因而造成潜在的计算误差。因此,为探寻适合的数值计算模型,以MIRA阶梯背汽车1/3比例模型为研究对象,对常用的Standard k-ε、RNG k-ε和Realizablek-ε3种k-ε涡黏湍流模型进行数值仿真,以计算所得的气动力系数、尾部流场及表面压力系数为适用性评价指标,并与HD-2风洞试验数据对标。研究结果表明:3种模型中,Standard k-ε模型收敛速度和效率最优,但气动阻力的计算精准度最差;RNG k-ε模型的表现一般,而Realizable k-ε模型能获得最高的气动阻力计算精度,与风洞试验结果对比,误差在5%以内,但收敛速度和效率相对较差;同时,3种湍流模型对气动升力的计算结果与风洞试验对比存在较大误差。

A study on turbulence transportation and modification of Spalart–Allmaras model for shock-wave/turbulent boundary layer interaction flow

It is of great significance to improve the accuracy of turbulence models in shock-wave/ boundary layer interaction flow. The relationship between the pressure gradient, as well as the shear layer, and the development of turbulent kinetic energy in impinging shock-wave/turbulent bound- ary layer interaction flow at Mach 2.25 is analyzed based on the data of direct numerical simulation （DNS）. It is found that the turbulent kinetic energy is amplified by strong shear in the separation zone and the adverse pressure gradient near the separation point. The pressure gradient was non-dimensionalised with local density, velocity, and viscosity. Spalart Allmaras （S A） model is modified by introducing the non-dimensional pressure gradient into the production term of the eddy viscosity transportation equation. Simulation results show that the production and dissipation of eddy viscosity are strongly enhanced by the modification of S-A model. Compared with DNS and experimental data, the wall pressure and the wall skin friction coefficient as well as the velocity profile of the modified S-A model are obviously improved. Thus it can be concluded that the mod- ification of S-A model with the pressure gradient can improve the predictive accuracy for simulat- ing the shock-wave/turbulent boundary laver interaction.

Experimental investigation of Reynolds stress complex eddy viscosity model for coherent structure dynamics

Time sequence signals of streamwise and normal velocity components,as well as velocity strain rate,at different vertical locations in the turbulent boundary layer over a smooth flat plate in a wind tunnel have been finely examined by the use of double-sensor hot-wire anemometry.The local module maximum for wavelet coefficient of longitudinal velocity component,as a detecting index,is employed to educe the ejection and sweep process of the coherent structure burst in the turbulent boundary layer from the random fluctuating background.The coherent waveforms of Reynolds stress residual contribution term for random fluctuations to coherent structure,as well as the velocity strain rate of coherent structure,are extracted by the conditional phase average technique.Based on the theoretical analysis of eddy viscosity coefficient in complex eddy viscosity model for coherent structure,the macro-relaxation effect between Reynolds stress residual contribution term of random fluctuations to coherent structure and the velocity strain rate of coherent structure is studied and the variations of the phase difference between them across the turbulent boundary layer are investigated experimentally.The rationality of complex eddy viscosity model for coherent structure is confirmed through the investigation.

Numerical study of transonic separation flow using an anisotropic turbulence model

A transonic turbulent separation flow in a converging-diverging transonic diffuser was studied,when there existed a separation bubble on the top wall of the diffuser triggered by strong shock-wave-boundary-layer-interaction(SWBLI).To capture the essential behavior of this complex flow,the current study utilized an anisotropic turbulence model developed on the basis of a statistical partial average scheme.The first order moment of turbulent fluctuations,retained by a novel average scheme,and the turbulent length scale,can be determined from the momentum equations and mechanical energy equation of the fluctuation flow,respectively.The two physical quantities were readily used to construct the nonlinear anisotropic eddy viscosity tensor and to significantly improve the computational results.Comparisons between the computational results and experimental data were carried out for velocity profiles,pressure distribution,skin friction coefficient,Reynolds stress as well as streamline vectors distribution.Without using any empirical coefficients and wall functions,the numerical results were in good agreement with the available experimental data,further confirming that the nonlinear anisotropic eddy viscosity tensor is the decisive factor for the success of the computational results.

Turbulence and mixing in a freshwater-influenced tidal bay: Observations and numerical modeling

In situ observations and numerical simulations of turbulence are essential to understanding vertical mixing processes and their dynamical controls on both physical and biogeochemical processes in coastal embayments. Using in situ data collected by bottom-mounted acoustic Doppler current profilers(ADCPs) and a free-falling microstructure profiler, as well as numerical simulations with a second-moment turbulence closure model, we studied turbulence and mixing in the Xiamen Bay, a freshwater-influenced tidal bay located at the west coast of the Taiwan Strait. Dynamically, the bay is driven predominantly by the M2 tide, and it is under a significant influence of the freshwater discharged from the Jiulong River. It is found that turbulence quantities such as the production and dissipation rates of the turbulent kinetic energy(TKE) were all subject to significant tidal variations, with a pronounced ebb-flood asymmetry. Turbulence was stronger during flood than ebb. During the flooding period, the whole water column was nearly well mixed with the depth-averaged TKE production rate and vertical eddy viscosity being up to 5?10?6 W kg?1 and 2?10?2 m2 s?1, respectively. In contrast, during the ebb strong turbulence was confined only to a 5?8 m thick bottom boundary layer, where turbulence intensity generally decreases with distance from the seafloor. Diagnosis of the potential energy anomaly showed that the ebb-flood asymmetry in turbulent dissipation and mixing was due mainly to tidal straining process as a result of the interaction between vertically shared tidal currents and horizontal density gradients. The role of vertical mixing in generating the asymmetry was secondary. A direct comparison of the modeled and observed turbulence quantities confirmed the applicability of the second-moment turbulence closure scheme in modeling turbulent processes in this weakly stratified tidally energetic environment, but also pointed out the necessity of further refinements of the model.

内锥流量计流出系数预测方法研究

采用标准k-ε模型、RNG(Renormalization Group)k-ε模型、Realizablek-ε模型和Reynolds应力方程模型RSM(Reynolds Stress Model)对100 mm口径6种结构的内锥流量计内流场进行了数值模拟。在等效直径比β值为0.65的三种结构内锥流量计流出系数的仿真计算中,四种湍流模型计算结果与物理实验结果误差的平均值分别为4.19%,2.84%,2.88%和-0.822%;对β值为0.85的情况,各模型计算误差的平均值分别为11.8%,9.62%,9.30%和4.76%。研究结果表明,RSM模型在6种结构内锥流量计流出系数的预测中,计算精度较高,表现出了较好的性能,优于三种k-ε涡粘模型,更适于内锥流量计流场数值模拟与流出系数的预测。

FBM湍流模型在非定常通气超空化流动计算中的评价与应用

为了建立通气超空化流动计算的流动模型,应用二次开发技术将FBM湍流模型嵌入商业软件,分别采用FBM湍流模型以及商业软件中的标准k-ε湍流模型模拟了绕圆盘空化器的通气空化流场,并从空泡形态、流动结构和阻力特性等方面与试验结果进行了对比。结果表明,标准k-ε湍流模型过高预测了流场的湍流粘性,预测的空泡形态和实验观测结果有较大的差距;采用滤波器湍流模型计算,可以明显地减小通气空泡尾端流场的湍流黏性,精确地捕捉通气空化区域空泡脱落的非定常细节,更加准确地描述通气空化的过程,与试验结果更加接近。

新概念车外流场数值仿真研究

使用Yakhot等的新版RNGk-ε湍流方程,首次对菱形新概念车车体气动特性进行了数值仿真。仿真中由于考虑了地面移动边界条件和轮胎转动对流场的影响(这与现实的物理环境基本上一致),因而具有较高的数值精度。对新概念车外流场进行数值仿真研究,揭示了新概念车车身的低阻气动特性的原因,为今后进行具有良好气动特性的车身设计提供参考思路。

三方程在汽车外流场仿真计算中的应用

汽车外流场仿真计算有多种模型,通过仿真和试验研究找到计算精度和效率高的模型对汽车外流场的仿真计算具有十分重要的意义。在使用三方程模型进行汽车外流场的计算中,由于涡粘系数μt考虑了湍流空间尺度和湍流时间尺度对湍流输运的影响,k-ε-v2湍流模型更多地反映了湍流输运的物理机理,有着比切应力传输、k-ε湍流模型更高的计算精度,是汽车外流场较为理想的数值仿真模型。运用涡粘型k-ε-v2湍流模型、切应力传输(Shear stress transport,SST)模型、k-ε湍流模型对汽车绕流进行数值仿真,将计算的阻力系数和试验数据进行对比,结果证实k-ε-v2湍流模型的优越性。给出汽车纵对称面上的压力系数与试验值的对比曲线,使用湍流k-ε-v2模型所计算的距车尾0.1m处的湍动能分布、速度矢量线及外部流线图。

汽车外流场典型湍流模型比较研究(英文)

在三方程(k-ε-v2)涡粘系数(μt)的计算中,不仅考虑了湍流空间尺度对涡粘系数的影响,而且考虑了湍流时间尺度对涡粘系数的影响,即在模型中更多的湍流输运的物理机理得到了描述,因此,三方程(k-ε-v2)有着比二方程:SST和标准的k-ε等模型更高的计算精度,它是汽车外流场计算中较为理想的湍流模型。通过Ahmed类车体和汽车两个算例的阻力系数计算值与风洞试验数据的对比证实了这点。另外使用k-ε-v2、SST和标准的k-ε模型计算的汽车的纵对称面的压力分布曲线与风洞试验值相比,三方程k-ε-v2模型更接近试验值。

内燃机缸内几种湍流流动涡粘度模型的比较

将几种湍流涡粘度模型 ,即压缩修正的 k-ε模型、RNG- k-ε模型和修正的非线性三方程模型用于计算内燃机缸内的湍流流动 ;对计算结果进行了分析 ,并与实验数据作了对比。结果表明 :RNG- k-ε湍流模型的计算结果较 k-ε模型有所改进 ,而修正的非线性三方程模型则克服了涡粘度模型不能反映平顶活塞压缩上止点处雷诺应力各向异性的缺点 ,在定量和定性上明显优于各种 k-ε模型。

关于RNG代数湍流模式的研究

研究了ＲＮＧ代数湍流模型，提出的涡黏性方程根的识别方法对涡黏性的选择给予了合理的物理意义，它减少了模型使用中的经验性，缩短了计算时间。另外关于用外部涡函数的双峰平均法来确定分离区附近边界层的厚度也是有意义的，它继承了ＢａｌｄｗｉｎＬｏｍａｘ代数模型的长处，改进了其不足，可用于分离流动的工程计算．

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．