Towards a Unified Turbulence Simulation Approach for Wall-Bounded Flows

A hybrid Reynolds-averaged Navier–Stokes/Large-Eddy Simulation (RANS/LES) methodology has received considerable attention in recent years, especially in its application to wall-bounded flows at high-Reynolds numbers. In the conventional zonal hybrid approach, eddy-viscosity-type RANS and subgrid scale models are applied in the RANS and LES zones, respectively. In contrast, the non-zonal hybrid approach uses only a generalized turbulence model, which provides a unified simulation approach that spans the continuous spectrum of modeling/simulation schemes from RANS to LES. A particular realization of the non-zonal approach, known as partially resolved numerical simulation (PRNS), uses a generalized turbulence model obtained from a rescaling of a conventional RANS model through the introduction of a resolution control function F _R , where F _R is used to characterize the degree of modeling required to represent the unresolved scales of turbulent motion. A new generalized functional form for F _R in PRNS is proposed in this study, and its performance is compared with unsteady RANS (URANS) and LES computations for attached and separated wall-bounded turbulent flows. It is demonstrated that PRNS behaves similarly to LES, but outperforms URANS in general.     

用子波分析能量最大准则修正检测壁湍流猝发的mu-level法

将壁湍流脉动速度信号子波分析技术引入壁湍流相干结构猝发检测的研究,基于子波分析能量最大准则确定相干结构猝发的时间尺度,对能量最大时间尺度的子波系数进行子波逆变换,获得能量最大时间尺度的脉动速度信号。根据其自相关函数的波长确定相干结构的平均猝发周期;根据能量最大时间尺度的子波系数的正极大值和负极小值检测壁湍流喷射和扫掠事件,获得壁湍流相干结构喷射和扫掠事件的相位平均波形。该方法可以不需要确定门限值,有效排除了小尺度脉动的干扰。     

Anisotropy Invariant Reynolds Stress Model of Turbulence (AIRSM) and its Application to Attached and Separated Wall-Bounded Flows

Numerical predictions with a differential Reynolds stress closure, which in its original formulation explicitly takes into account possible states of turbulence on the anisotropy-invariant map, are presented. Thus the influence of anisotropy of turbulence on the modeled terms in the governing equations for the Reynolds stresses is accounted for directly. The anisotropy invariant Reynolds stress model (AIRSM) is implemented and validated in different finite-volume codes. The standard wall-function approach is employed as initial step in order to predict simple and complex wall-bounded flows undergoing large separation. Despite the use of simple wall functions, the model performed satisfactory in predicting these flows. The predictions of the AIRSM were also compared with existing Reynolds stress models and it was found that the present model results in improved convergence compared with other models. Numerical issues involved in the implementation and application of the model are also addressed.     

Large Eddy Simulations in Turbines: Influence of Roughness and Free-Stream Turbulence

The individual and coupled effects of the incoming free-stream turbulence (FST) and surface roughness on the transition of a separated shear layer over a flat plate is numerically investigated using Large eddy simulation (LES). The upper wall of the test section is inviscid and specifically contoured to impose a streamwise pressure distribution over the flat plate to simulate the suction surface of a low pressure turbine (LPT) blade. The interaction of the streamwise streaks due to FST and roughness with the separated shear layer is captured. The streaks induced by FST are intermittent in nature and the streaks due to roughness are steady for a given topology of the rough surface. Both FST and roughness promoted near-wall mixing. The ‘net effect’ of mixing in the pre-separated region is manifested by a shift the inflection point of the velocity profile towards the wall. This resulted in the upstream shift of the transition point and a significant reduction in the size of the separation bubble. The combined effect of FST and roughness further reduced the size of separation bubble. The streamwise evolution of the boundary layer parameters has been compared for different cases. The potential ‘roughness benefit’ obtained in the case of highly loaded turbine blades in terms of its considerable reduction of profile loss is also shown.     

考虑界面粗糙度动态变化的点接触弹流润滑特性研究

针对点接触弹流润滑的粗糙度效应,建立了考虑表面粗糙度动态变化的点接触弹流润滑模型,实现了油膜厚度和压力分布的快速求解. 对点接触弹流润滑下的粗糙表面弹性变形进行了定性和定量研究,同时分析了表面均方根粗糙度、载荷、相对运动速度和滑滚比对最小膜厚和最大压力的影响,以及表面形貌动态变化对膜厚比的影响. 结果表明:形貌变化改变了弹流油膜和压力分布特性,相对于光滑表面,表面粗糙度总体上提高了最大接触压力、降低了最小膜厚,在轻载工况下表面粗糙度对油膜厚度的削弱更加显著,而不同速度下粗糙度的影响程度基本相同,呈现线性变化趋势,膜厚比随载荷增大呈现先增后减的变化趋势,并在530 MPa左右达到峰值.      

涂层表面粗糙性对流体噪声的影响

在铝合金回转体表面涂上一定粗糙度的低表面能涂层，涂层表面的不平度平均高度在６．３～１０．０μｍ时，涂层模型表面为光滑水力壁，这时表面粗糙所引起的那部分流噪声可忽略不计．在光滑（Ｒｚ＝３．２～６．３μｍ）的铝合金模型表面喷涂涂层后，涂层表面较粗糙（Ｒｚ＝１０．０～２０．０μｍ）时，同样在相应的频率范围内涂层模型仍有降噪的效果，而且对不同涂层材料其表面粗糙度对流噪声的影响也不相同．     

边界层对壁面湍流流动高分子减阻的影响

以“粘性”机制为理论基础,近年来在壁面湍流高分子减阻研究中提出了一种拉伸的高分子会产生自洽的等效粘度模型,这种等效粘度随离开壁面的距离而改变.通过等效粘度模型与Navier-Stokes方程的结合,运用雷诺应力模型计算壁面湍流减阻,并与基于高分子有限拉伸的非线性弹性哑铃模型的直接数值模拟结果进行比较,进一步校验了此等效粘度理论.通过肋条破坏槽道流中的边界层,显示了边界层对高分子减阻的影响,结果表明只有形成稳定的边界层,高分子才能有减阻作用.边界层是高分子减阻的首要条件,边界层中的粘性底层和对数率分布区之间的缓冲层可能是减阻的主要影响区域.     

粗糙度对4种聚氨酯弹性体粘着性能的影响

采用生物摩擦磨损试验台测定了4种聚氨酯的弹性模量，研究了表面粗糙度和法向力对粘着力的影响．结果表明：4种聚氨酯的弹性模量在55～1206kPa之间，粘着力由高到低的排序同其弹性模量排序相反，弹性模量较低的试样的饱和粘着力达到15kPa，且表面粗糙度对粘着力的影响不明显；弹性模量较大的试样的粘着力仅为1．5kPa，随弹性模量增加，表面粗糙度的影响增大；试样之间的接触形式同粘着力密切关系，相当于单个凸峰接触的蓝宝石球对聚氨酯试样的粘着力不受法向力的影响，而在平面接触情况下粘着力先随法向力增加而增大，当法向力超过一定(10～15kPa)值后，粘着力增加趋缓并达到饱和值．     

壁面加热边界条件对湍流扩展的自相似性的影响

用热线风速仪测量了风洞中壁面加热和不加热平板湍流边界层不同法向位置的瞬时流向速度的时间序列,研究了壁面加热的边界条件对流向速度增量的p阶结构函数的扩展的自相似性和层次结构模型的影响。实验结果表明,壁面加热的边界条件对流向速度增量的结构函数扩展的自相似标度律的相对标度指数δ（p,3）和层次结构模型中间歇参数β、最奇异指数γ的影响明显存在。     

Reynolds Number Effect on the Grid Turbulence Degeneration Law

A comprehensive experimental investigation of the effect of the Reynolds number on the degeneration law for turbulence generated by biplanar and wicker grids is carried out over a wide range of the grid geometry parameters and the flow velocity. It is established that an increase in the flow velocity leads to an increase in the turbulence intensity at a given distance from the biplanar grid and a decrease in the turbulence decay rate downstream of the grid. An empirical relation between the turbulence intensity behind the grid, on the one hand, and the relative distance x/M from the grid and the Reynolds number based on the grid rod diameter and the flow velocity at the grid mesh center, on the other hand, is proposed. For the same relative distance x/M from the grid the intensity of wicker-grid turbulence is higher than in the case of the flow past a biplanar grid.     

SUS304钢表面粗糙度对红外图像的影响

为提高红外热像仪图像及测温精度,根据红外热像仪的测温原理和红外辐射原理,通过使用红外热像仪、CCD相机以及激光表面形状测量显微镜,研究了SUS304钢在单轴拉伸塑性变形过程中表面粗糙度与目标发射率的关系;并用热电偶和红外热像仪对塑性变形过程中的温度分布进行了测量。研究结果表明,目标发射率随表面粗糙度成比例增大,造成红外热像仪测定的塑性变形区域比实际变形区域大;通过预先设定材料的表面粗糙度,以提高有效目标发射率,能得到较好的红外图像和测温精度。     

靶材表面粗糙度对GCr15钢表面激光毛化微突体形貌的影响

对不同粗糙度GCr15钢表面激光毛化微突体形貌的几何尺寸、金相组织及各区域表面显微硬度进行了分析.结果表明:粗糙钢表面对激光能量的吸收率高于镜面试样,因此粗糙表面激光毛化微突体的高度比镜面试样的略高;但粗糙表面微突体端部的淬火区组织会因金属蒸发而产生孔洞而降低表面的显微硬度,从而影响表面的耐磨性.为了得到较细化的金相组织而又不产生金属蒸发现象,在控制激光毛化参数条件下,应该尽可能地降低靶材的表面粗糙度,一般应保证Ra≤0.4μm.     

Modeling the Effect of Freestream Turbulence on Unsteady Boundary Layer Flow

We present a new version of the two-equation turbulence model, which makes it possible to calculate continuously the entire flow range from laminar to turbulent, including transition, in the case of a time-periodic, high-turbulence-level freestream. The influence of the parameters characterizing the harmonic fluctuations of the external velocity and the freestream turbulence intensity and scale on the parameters of the flat-plate flow are analyzed. A comparison of the numerical solutions with the experimental and theoretical data indicates the possibility of describing the wall flow properties on the basis of a quasi-stationary turbulence model, as the Reynolds number varies from low to high values.     

Effect of a Short Roughness Strip on a Turbulent Boundary Layer

The response of a turbulent boundary layer to a short roughness strip is investigated using laser Doppler velocimetry (LDV) and laser induced fluorescence (LIF). Skin friction coefficients are inferred from accurate near-wall measurements. There is an undershoot in , where is the undisturbed smooth wall skin friction coefficient, immediately after the strip. Downstream of the strip, overshoots before relaxing back to unity in an oscillatory manner. The roughness strip has a major effect on the turbulent stresses ; these quantities increase, relative to the undisturbed smooth wall, in the region between the two internal layers originating at the upstream and downstream edges of the strip. The increase in the ratio suggests a decrease in near-wall anisotropy. From the flow visualizations, it is inferred that streamwise vortical structures are weakened immediately downstream of the strip. Consistently, streamwise length scales are also reduced; direct support for this is provided by measured two-point velocity correlations.     

Effect of Strong External Turbulence on a Wall Jet Boundary Layer

The initial stage of the development of a wall jet under the influence of strong external turbulence has been studied in a novel shear-flow mixing-box experiment. A fully developed channel flow of depth h (40 mm) enters along the top wall of a cuboidal box of height 11 h in which a combination of oscillatory and turbulent velocity fluctuations are generated by a vertical oscillating grid at the midplane 5 h below the wall. When the ratio of the rms grid-generated velocity fluctuations, , to the local mean velocity inside the wall jet layer, u, is greater than about 0.1, significant changes are observed in the mean shear profile and in the eddy structure of the wall jet. The wall jet thickness increases by approximately 25% but the maximum velocity decreases by less than 10% compared to the case without the external turbulence. Fluctuations of the streamwise velocity component increase as expected in the outer part of the wall jet, but the most significant result is the increase by 70% of the fluctuations in the boundary layer close to the wall. CFD simulations using the k-ɛ RNG of the FLUENT CFD Code do not properly model the effect of the large scale external turbulence in this experiment. However, an artificial method, which introduces a series of small inlet/outlet jets to represent external turbulence, approximately simulates the overall effects of the oscillating grid on the wall jet, but does not simulate the amplification of the near wall turbulence. F. T. M. Nieuwstadt: Rest in peace (1946–2005).     

The Effect of Surface Roughness on Immiscible Displacement Using Pore Scale Simulation

Transport in Porous Media - Pore scale immiscible displacement is crucial in oil industry. The surface roughness of throat is an important factor affecting water–oil interface movement. In...     

The Effect of wake Turbulence Intensity on Transition in a Compressor Cascade

Direct numerical simulations of separating flow along a section at midspan of a low-pressure V103 compressor cascade with periodically incoming wakes were performed. By varying the strength of the wake, its influence on both boundary layer separation and bypass transition were examined. Due to the presence of small-scale three-dimensional fluctuations in the wakes, the flow along the pressure surface undergoes bypass transition. Only in the weak-wake case, the boundary layer reaches a nearly-separated state between impinging wakes. In all simulations, the flow along the suction surface was found to separate. In the simulation with the strong wakes, separation is intermittently suppressed as the periodically passing wakes managed to trigger turbulent spots upstream of the location of separation. As these turbulent spots convect downstream, they locally suppress separation.     

配副金属表面粗糙度对丁腈橡胶O型圈摩擦学特性的影响

UMT-3多功能摩擦磨损试验机上,以丁腈橡胶O型圈/316L不锈钢粗糙表面配副为研究对象,系统研究了配副金属表面粗糙度对丁腈橡胶摩擦学行为的影响,重点探讨了不同表面粗糙度下摩擦系数和磨损体积的变化规律、揭示了不同表面粗糙度下橡胶表面的损伤机制.结果表明:表面粗糙度对丁腈橡胶O型圈的摩擦磨损有重要影响;随着表面粗糙度的增加,稳定阶段的摩擦系数呈先降低后升高的变化趋势.当表面粗糙度在Ra为0.1μm左右时,丁腈橡胶表现出较低的摩擦系数和材料磨耗,磨损表面呈现出典型的花纹磨损特征;而过高或过低的表面粗糙度下丁腈橡胶均表现出较高的摩擦系数和材料磨耗,其磨损机制以黏着磨损和疲劳磨损为主.     

润滑条件下铜锌合金表面粗糙度对磨损率的影响

采用基于原位全息显微技术和放射性核素技术的试验装置对润滑条件下的Cu Zn36和100Cr6进行了摩擦磨损试验,分析了Cu Zn36在试验过程中的摩擦磨损特性,以及摩擦系数、磨痕表面粗糙度和实时磨损率之间的关系,进而建立了关于表面粗糙度的磨损模型,并通过拟合优度方法对磨损模型进行了评价.结果表明:在试验磨合的初始阶段,Cu Zn36表面平整性被迅速破坏而产生了较高的磨损率,随着试验的进行,磨痕表面的强化层逐渐形成,表面粗糙度和实时磨损率逐渐降低并趋于稳定.磨损模型的拟合优度R2的计算结果为90.23%,说明建立的磨损模型能够对给定工况下的实时磨损率进行较为准确的预测.