ILES for mechanism of ramp-type MVG reducing shock induced flow separation

A high order implicit large eddy simulation (ILES) is carried out to study the mechanism of shock induced flow separation reduction under ramp-type MVG control. The mechanism was originally considered as that MVG can generate streamwise vortices which strongly mix boundary layer and the boundary layer becomes more capable to resist strong adverse pressure gradient caused by shock and to keep the boundary layer attached. However, according to our ILES, a chain of ring-like vortices is generated behind the ramp-type MVG and goes further to interact with the shock. When the ring-like vortices pass through the shock, the shock wave is weakened and altered while the vortex structures are quite stable. The instantaneous simulation shows that the spanwise ring-like vortex, not the streamwise vortex, plays a key role to weaken the shock and reduce the shock-induced separation. Detailed investigation on ring-like vortices and shock interaction will be given in this paper.     

扰流板作用下湍流/非湍流界面特性

在零压梯度平板湍流边界层流动中安装垂直流向高度为h的扰流板,诱导流场产生横向涡,研究横向涡影响下湍流/非湍流界面特性沿流向的发展。结果表明,在本实验条件下,整个流动经历了从湍流边界层到流动分离和再附,再向湍流边界层恢复的过程。在扰流板下游约18h距离后,扰流板尾迹的影响逐渐衰退,壁面剪切对湍流强度的贡献开始逐渐恢复,在扰流板下游约55h距离后,湍流边界层再次充分发展。与此同时,由于扰流板后流场流动结构拟序性的增强,湍流/非湍流界面的分形维度受扰流板影响而减小,表明脱落涡有使界面多尺度特性、三维性减弱的趋势。此外,界面高度的概率密度分布受扰流板影响呈现显著的右偏,主要与扰流板增强喷射运动强度,使得界面更容易抬升相关。流动结构及界面特性受扰流板影响后的流向演化有同步变化的模式,扰流板对界面特性影响主要集中于（-5~18）h的流向范围。      

确定分布的展向Lorentz力调制下的槽道湍流涡结构

采用直接数值模拟方法,对槽道湍流中确定分布的Lorentz力的流动控制与减阻问题进行研究.讨论了Lorentz力作用于槽道湍流后,流场的特性和涡结构的特性,并对此类Lorentz力对槽道湍流的控制与减阻机理进行了讨论.研究发现:1)Lorentz力诱导的层流流场壁面附近存在梯度极大的展向速度剪切层,该剪切层容易形成流向涡结构;2)在给定合适参数的确定分布的Lorentz力作用下,湍流流场仅剩周期分布的准流向涡;3)与未控制流场相比,控制后的流场中,准流向涡的抬升高度大大降低,从而减小猝发强度,使壁面阻力下降.     

Secondary instability of a swept-wing boundary layer disturbed by controlled roughness elements

Abstract  

Wind-tunnel data on velocity perturbations evolving in a laminar swept-wing flow under low subsonic conditions are reported. The focus of the present experiments are secondary disturbances of the boundary layer which is modulated by stationary streamwise vortices. Both the stationary vortices and the secondary oscillations of interest are generated in a controlled manner. The experimental data are obtained through hot-wire measurements. Thus, evolution of the vortices, either isolated or interacting with each other, is reconstructed in detail. As is found, the secondary disturbances, initiating the laminar-flow breakdown, are strongly affected by configuration of the stationary boundary-layer perturbation that may have an implication to laminar–turbulent transition control.     

Visualizing shear stress in Görtler vortex flow

Shear stress distributions were obtained from velocity measurements in a concave surface boundary layer flow in the presence of Görtler vortices by means of a single hot-wire probe for several streamwise (x) locations. A set of vertical wires of 0.20 mm diameter were positioned at a distance of 10 mm upstream from the leading edge of a concave surface of radius of curvature R=1.0 m to pre-set the wavelength of the vortices so to obtain the most amplified wavelength Görtler vortices. Consequently, the wavelength of the vortices was set equal to the wire spacing and preserved downstream. In addition to the high shear regions near the wall, one positive peak at the head of the mushroom-like structures and two relatively weak negative peaks at the vicinity of the low-speed streaks are found in the iso-?u/?y contours. They are believed to be related to the formation of the inflectional point in the velocity profile across boundary layer. The occurrence of the inflection points in the spanwise distributions of streamwise velocity component u is associated with the appearance of the second peak of the ?u/?z shear near the boundary layer edge. The nonlinear effect of Görtler instability is to increase the wall shear stress, and further enhancement beyond the turbulent values is due to the presence of secondary instability.     

基于三角波瓣混合器的超声速流场精细结构和掺混特性

在超声速吸气式混合层风洞中,采用基于纳米粒子的平面激光散射(NPLS)技术对平板混合层和三角波瓣混合器诱导的混合层流场精细结构进行了对比实验研究.上下两层来流的实测马赫数分别为1.98和2.84,对流马赫数为0.2.NPLS图像清晰地展示了Kelvin-Helmholtz涡、流向涡、波系结构以及大尺度涡结构的配对合并过程.通过对比分析时间相关的NPLS流场图像,发现了大尺度拟序结构随时间发展演化的非定常特性.基于流动显示结果,采用分形维数和间歇因子指标对流场结构和混合特性进行了定量分析.实验研究表明,三角波瓣混合器诱导的流向涡结构显著提高了上下两层来流的掺混效率,其流动远场的分形维数突破了平板混合层中完全湍流区的分形维数值,达到了1.88,流场结构表现出明显的破碎性,有利于流动在标量层面的扩散和掺混.流动间歇性分析表明,流向涡与展向涡的相互剪切作用主导着混合层的掺混特性,同时由于流向涡的卷吸作用,三角波瓣混合器诱导的混合层混合区域更大,更多的流质被卷入混合区完成混合.     

超声速湍流边界层湍流统计与湍流结构分析

可压缩边界层转捩问题与湍流问题一直是制约高超声速飞行器发展的关键基础问题,也是近年来流体力学领域研究的热点问题.采用直接数值模拟方法,获得了空间发展的Ma=2.25超声速湍流边界层流场,通过对湍流边界层的发展状态进行评估,得出有效的Reynolds数Reθ 范围约为2600～4600.对壁面摩阻系数开展了分解,获得了各...     

湍流边界层内流向涡的数值模拟研究

采用准二维共振三波作为湍流边界层近壁区相干结构初值,用直接数值模拟方法计算了流动从二维结构发展到三维结构并且伴随流向涡生成的整个过程,分析结果显示流向涡对湍流动能和质量传输有着重要作用,是湍流边界层相干结构的重要特征和运动形式.     

Convection and correlation of coherent structure in turbulent boundary layer using tomographic particle image velocimetry

The present experimental work focuses on a new model for space–time correlation and the scale-dependencies of convection velocity and sweep velocity in turbulent boundary layer over a flat wall. A turbulent boundary layer flow at Reθ= 2460 is measured by tomographic particle image velocimetry(tomographic PIV). It is demonstrated that arch, cane,and hairpin vortices are dominant in the logarithmic layer. Hairpins and hairpin packets are responsible for the elongated low-momentum zones observed in the instantaneous flow field. The conditionally-averaged coherent structures systemically illustrate the key roles of hairpin vortice in the turbulence dynamic events, such as ejection and sweep events and energy transport. The space–time correlations of instantaneous streamwise fluctuation velocity are calculated and confirm the new elliptic model for the space–time correlation instead of Taylor hypothesis. The convection velocities derived from the space–time correlation and conditionally-averaged method both suggest the scaling with the local mean velocity in the logarithmic layer. Convection velocity result based on Fourier decomposition(FD) shows stronger scale- dependency in the spanwise direction than in streamwise direction. Compared with FD, the proper orthogonal decomposition(POD) has a distinct distribution of convection velocity for the large- and small-scales which are separated in light of their contributions of turbulent kinetic energy.     

超声速平板圆台突起物绕流实验和数值模拟研究

高速飞行器表面不可避免的存在突起物并形成复杂流场, 从而引起飞行器气动特性和热载荷的变化; 同时, 突起物是流动控制的重要方法之一, 合适的突起物形状及安装位置对于改善冲压发动机进气道性能有重要意义. 本文采用基于纳米粒子的平面激光散射技术(NPLS)研究了马赫3.0来流边界层为层流的平板上三个不同高度圆台突起物绕流流场, 主要关注了突起物后方的尾迹边界层, 并采用高精度的显式五阶精度加权紧致非线性格式(WCNS-E-5)离散求解Navier-Stokes方程模拟了该流场. 获得了超声速圆台绕流精细流场结构, 观察到突起物后方尾迹区域边界层发展的过程. 结合实验和数值模拟结果可以发现, 当圆台高度接近或者小于当地边界层厚度时, 突起物对边界层的扰动非常弱, 圆台后方尾迹边界层能够维持较长距离的层流状态, 在边界层转捩阶段也有清晰的发卡涡结构出现; 反之, 边界层受到的扰动明显增大, 在突起物后方很快发展为湍流; 风洞噪声对本文研究圆台引起的边界层扰动有一定影响, 实验获得的边界层转捩位置要比数值结果靠前. 基于NPLS流场图像, 采用间歇性方法分析了圆台突起物后方边界层的特性, 对于高度大于边界层厚度的圆台其间歇性曲线较为接近并且更加饱满, 边界层的脉动也更为强烈.     

三维边界层内定常横流涡的感受性研究

层流向湍流转捩的预测与控制一直是研究的前沿热点问题之一,其中感受性阶段是转捩过程中的初始阶段,它决定着湍流产生或形成的物理过程.但是有关三维边界层内感受性问题的数值和理论研究都比较少;实际工程问题中大部分转捩过程都是发生在三维边界层流中,所以研究三维边界层中的感受性问题显得尤为重要.本文以典型的后掠角45?无限长平板为例,数值研究了在三维壁面局部粗糙作用下的三维边界层感受性问题,探讨了三维边界层感受性问题与三维壁面局部粗糙长、宽和高之间的关系;然后,考虑在后掠平板上设计不同的三维壁面局部粗糙的分布状态、几何形状、距离后掠平板前缘的位置以及流向和展向设计多个三维壁面局部粗糙对三维边界层感受性问题有何影响;最后,讨论两两三维壁面局部粗糙中心点之间的距离以及后掠角的改变对三维边界层感受性的物理过程将会发生何种影响等.这一问题的深入研究将为三维边界层流中层流向湍流转捩过程的认识和理解提供理论依据.     

Decaying two-dimensional turbulence in a circular container

We present direct numerical simulations of two-dimensional decaying turbulence at initial Reynolds number 5 x 10(4) in a circular container with no-slip boundary conditions. Starting with random initial conditions the flow rapidly exhibits self-organization into coherent vortices. We study their formation and the role of the viscous boundary layer on the production and decay of integral quantities. The no-slip wall produces vortices which are injected into the bulk flow and tend to compensate the enstrophy dissipation. The self-organization of the flow is reflected by the transition of the initially Gaussian vorticity probability density function (PDF) towards a distribution with exponential tails. Because of the presence of coherent vortices the pressure PDF become strongly skewed with exponential tails for negative values.     

湍流边界层中下扫流与“反发卡涡”

用氢气泡法观测湍流边界层的下扫流和有关的流动结构.实验中发现一种新型涡结构，它的特征与典型的发卡涡正好相反.发卡涡的头部指向下游，而它的头部指向上游； 发卡涡的两腿之间，由于涡的诱导产生上升流，而它则在其两腿之间，由于涡的诱导产生下扫流. 关键词： 湍流边界层 流动显示 流动结构 发卡涡     

Visualization of the vortical structure of a circular jet excited by axial and azimuthal perturbations

The vortical structure of a circular water jet was investigated by a flow visualization technique. The jet was excited by axial and azimuthal perturbations to stabilize and enhance the large-scale axisymmetric and streamwise vortices. A laser fluorescent dye and a laser light sheet were used to visualize the vortical structure, and the whole view of the structure was captured by applying the Taylor hypothesis to the cross-sectional images and by scanning a laser light sheet in the streamwise direction. The visualized images reveal the details of the complicated structure of axisymmetric and streamwise vortices, and it is confirmed that the streamwise vortices have fundamental effect on the entrainment of ambient fluid. From the images, the length of jet boundary was calculated to estimate the mixing effect. The result suggests that the jet mixing is significantly increased by the break-down of the vortices enhanced by axial and azimuthal perturbations. We also discussed the jet diffusion effect in consideration of the jet widths obtained by velocity measurement. The result indicates that the vortical structure including streamwise vortices plays an important role to enhance diffusion.     

关于转捩边界层中流向涡的产生

给出了转捩边界中流向涡产生的物理过程,这个过程是A－涡和二次涡环相互作用产生旋涡轴向失稳断裂造成的。     

基于温敏漆技术的圆锥高超声速大攻角绕流背风面流动结构实验研究

高超声速流动中, 大攻角下圆锥背风面边界层会存在流动分离与再附、边界层转捩等多种流动现象, 进而对圆锥表面温度分布产生显著的影响。为了对这一复杂流动规律及其对表面温升分布的影响进行讨论, 研究基于温敏漆技术, 得到了在Mach数为6的低湍流度来流条件下, 攻角为10°的圆锥背风面温升分布结果。通过对不同位置、不同方位角处温升分布曲线的分析, 对大攻角下圆锥背风面边界层流动发展过程及不同发展阶段的流动特征进行了讨论。同时, 通过对来流总压的调节, 得到了不同Reynolds数下的圆锥背风面温升分布结果, 总结了Reynolds数对流动的影响规律。研究发现, 高超声速大攻角圆锥背风面边界层流动发展过程中会依次出现层流分离、定常横流涡影响、转捩以及湍流分离与再附等流动特征, 而在不同的Reynolds数下, 各个流动特征产生影响的范围不同, 随着Reynolds数的降低, 层流范围和定常横流涡影响范围均有所增加, 而从观察到横流影响到转捩开始发生的范围基本相同。      

Evolution of three-dimensional coherent structures in compressible axisymmetric jet

A high order difference scheme is used to simulate the spatially developing compressible axisymmetric jet. The results show that the Kelvin-Helmholtz instability appears first when the jet loses its stability, and then with development of jet the increase in nonlinear effects leads to the secondary instability and the formation of the streamwise vortices. The evolution of the three-dimensional coherent structure is presented. The computed results verify that in axisymmetric jet the secondary instability and formation of the streamwise vortices are the important physical mechanism of enhancing the flow mixing and transition occurring.     

Mean velocity behaviour in the near wake of long slender cylinder with a sharp trailing edge at a high Reynolds number

A simple eddy viscosity model is applied to the governing equations to establish the behaviour of the mean velocity in the turbulent axisymmetric near wake. The near wake develops from a long slender cylinder which is kept parallel to the flow and is developing under zero streamwise pressure gradient. The upstream turbulent boundary layer on the body of revolution is fully developed. In the inner layer of the flow downstream of the trailing edge, the turbulent inner layer of the upstream boundary layer grows into the initial logarithmic layer, and as a consequence, the centreline velocity in the near wake is shown to increase logarithmically with streamwise distances for large streamwise distances. The analysis further leads to two regions of the near wake flow (the inner near wake and the outer near wake), similar to that of a fully developed turbulent boundary layer, for which the governing equations have been derived. The matching between these two regions leads to a logarithmic variation in the normal direction. Also shown is the variation of the square of the wake width which varies logarithmically with streamwise distance in the near wake. These features are validated by comparison with available experimental data.     

后掠机翼的横流不稳定性分析及转捩预测

后掠机翼边界层的流动稳定性及转捩对翼型的设计及优化有着重要的参考价值,而横流失稳是引起后掠机翼边界层转捩的关键因素之一.以NLF（2）-0415翼型为研究对象,采用三维可压缩Navier-Stokes方程并结合γ-Re_θt转捩模式计算了展向无限长后掠机翼的基本流场.由于原始γ-Re_θt模式只能预测流向边界层转捩,因此在原始转捩模式中添加横流间歇因子项,进而对复杂构型进行横流不稳定性转捩预测.计算结果显示,利用改进后γ-Re_θt转捩模式预测得到的后掠翼型的转捩位置与实验数据吻合较好,证明了修正的转换模式的合理性和实用性.      

DNS of compressible turbulent boundary layer around a sharp cone

Direct numerical simulation of the turbulent boundary layer over a sharp cone with 20° cone angle (or 10° half-cone angle) is performed by using the mixed seventh-order up-wind biased finite difference scheme and sixth-order central difference scheme. The free stream Mach number is 0.7 and free stream unit Reynolds number is 250000/inch. The characteristics of transition and turbulence of the sharp cone boundary layer are compared with those of the flat plate boundary layer. Statistics of fully developed turbulent flow agree well with the experimental and theoretical data for the turbulent flat-plate boundary layer flow. The near wall streak-like structure is shown and the average space between streaks (normalized by the local wall unit) keeps approximately invariable at different streamwise locations. The turbulent energy equation in the cylindrical coordinate is given and turbulent energy budget is studied. The computed results show that the effect of circumferential curvature on turbulence characteristics is not obvious.