边界层流中当地感受性过程的数值研究

边界层流中当地感受性问题的研究对层流向湍流转捩过程的预测与控制起着非常关键的作用,尤其是对边界层内诱导产生三维Tollmien-Schlichting(T-S)波成因过程的探讨具有更加重要的理论意义.采用高精度、高分辨率变间距的紧致有限差分方法,直接数值模拟了在自由来流湍流与二维壁面局部粗糙相互作用下边界层内的当地感受性问题.数值计算发现,在自由来流湍流与二维壁面局部粗糙作用下,边界层内诱导形成的当地感受性过程是真实存在的;且被激发的一组三维T-S波波包沿流向发展的过程中流向涡结构将逐渐形成,其强度将越演越烈.数值结果还显示,边界层内被诱导产生当地感受性过程的波长转换机制仅使流向波数发生改变,而展向波数保持不变;以及自由来流湍流运动方向的改变将决定三维T-S波波包的传播方向,但其传播速度的大小都近似为无穷远来流速度的1/3.另外,还建立了自由来流湍流的强度和运动方向以及二维壁面局部粗糙的长度和高度与边界层内的当地感受性问题之间的关系等.这一课题的深入研究,将在进一步理解和认识层流向湍流转捩的理论机制,以及湍流的形成机理等方面均起到十分重要的作用.     

自由流中涡扰动的边界层感受性的数值研究

用直接数值模拟的方法研究平板二维边界层对自由流中涡扰动的感受性．在自由流涡扰动与壁面凸起物的相互作用下，在边界层内找到了激发出来的Tollmein-Schlichting(T-S)波，证实了感受性现象及其中波长转变机制的存在．数值模拟得到的T-S波幅值与自由流扰动幅值、凸起高度及矩形凸起物长度的关系，与实验测量所得一致．则由此确定的感受性线性关系式的适用范围亦与实验所得相符．     

二维平板可压缩边界层的二次稳定性分析

本文在二维可压缩边界层中应用Floquet分析,建立了控制次谐波稳定性的方程组,研究在二维可压缩边界层转捩过程中二维有限振幅的T-S波对三维线性小扰动的作用,并计算了来流马赫数对次谐波的产生和发展情况的影响,从中可以看出二维和三维扰动波相互作用对二维可压缩流动边界层的发展过程所产生的影响.     

超音速平板边界层转捩中层流突变为湍流的机理研究

采用空间模式,对来流Mach数为4.5的平板边界层转捩过程做了直接数值模拟．对结果进行的分析发现,在层流-湍流转捩的突变(breakdown)过程中,层流剖面得以快速转变为湍流剖面的机理在于平均剖面的修正导致了其稳定性特征的显著变化．虽然在层流下第2模态T-S波更不稳定,但在层流突变为湍流的过程中,第1模态不稳定波也起了重要作用．     

零攻角小钝头钝锥高超音速绕流边界层的稳定性分析和转捩预报

研究了零攻角小钝头圆锥高超音速边界层的稳定性及转捩预测问题．小钝头的球头半径为0.5 mm,锥的半锥角为5°,来流马赫数为6．采用直接数值模拟方法得到了钝锥的基本流场,利用线性稳定性理论分析了等温壁面和绝热壁面条件下的第一、第二模态不稳定波,并用“e-N”方法对转捩位置进行了预测．在没有实验给出N值的情况下,暂取N为10．研究发现,壁面温度条件对于转捩位置有较大影响．绝热边界层的转捩位置比等温边界层的靠后．且尽管高马赫数下第二模态波的最大增长率远大于第一模态波的最大增长率,但绝热边界层的转捩位置是由第一模态不稳定波决定的．研究方法应能推广到有攻角的三维边界层流动的转捩预测．     

近壁区Reynolds应力及湍能耗散率输运特性的理论模型 *

提出在湍流边界层的近壁区采用三维不稳定波来描述湍流相干结构 ,然后根据理论模型对Reynolds应力及湍能耗散率的输运特性进行系统的计算和定量的分析 ,并计算了平均速度分布 .所得理论结果与直接数值模拟结果符合较好 ,表明本文方法正确地体现了湍流边界层近壁区的物理实质 .这不仅有益于对湍流机制的了解 ,而且可能为湍流的近壁模型化开辟一条新的途径 .     

壁面加热湍流耗散率标度指数测量的实验研究

对中等雷诺数下壁面常温和壁面加热的平板湍流边界层中速度和温度粗粒化的耗散率结构函数标度指数进行了实验测量.用热线风速仪测量了风洞中壁面常温和加热的平板湍流边界层中不同法向位置的流向速度分量和温度的时间序列信号,研究了由于湍流边界层近壁区域相干结构的存在而导致的非各向同性、非均匀性对湍流耗散率结构函数标度指数的影响,研究发现,中等雷诺数下壁面加热的边界条件和剪切湍流的平均速度梯度对速度和温度耗散率结构函数的标度指数没有影响,均匀各向同性湍流的耗散率结构函数标度指数的层次结构模型对壁面加热平板湍流边界层的速度和温度耗散率结构函数的标度指数也是适用的.     

湍流相干结构机理研究(Ⅲ)——湍流拟序结构的统计及动力模型及其对传热影响研究

在文献[1]、[2]的启发下,本文建立一个零压梯度下,考虑局部产生以及外来扰动涡旋的壁湍流边界层大涡拟序结构的统计及动力学模型.在此基础上对充分发展的宽明渠流动中壁面附近的热扩散进行了数值模拟,建立了边界层拟序脉动速度和温度的数据库,发现了与速度快慢条相对应的高低温流条及其随时间在展向的摆动.数值模拟结果与前人的计算和实验结果吻合很好.     

PSE在超音速边界层二次失稳问题中的应用

用抛物化稳定性方程（PSE）研究超音速边界层中的二次失稳问题．结果显示无论二维基本扰动是第一模态还是第二模态的T-S波,二次失稳机制都起作用．三维亚谐波的放大率随其展向波数和二维基本波幅值的变化关系与不可压缩边界层中所得类似．但是,即使二维波的幅值大到2％的量级,三维亚谐波的最大放大率仍远小于最不稳定的第二模态二维T-S波的放大率．因此,二次失稳应该不是导致超音速边界层转捩的主要因素．     

湍流边界层近壁区的传热问题

主要研究了湍流边界层近壁区的传热问题 .利用流动稳定性理论中的共振三波模型来求湍流边界层近壁区的相干结构所引起的流场 ,而小尺度湍流则用一简单的模型来表示 .通过这种新的模型 ,求解了三维温度场 ,得到了近壁区的平均温度剖面及表征热通量的Nusselt数 ,后者与实验符合得很好 .瞬时温度场具有条纹结构 ,也说明了数值模拟中所发现的条纹结构产生机理 .     

Three-dimensional Tollmien-Schlichting waves generated by sound in the boundary layer on an elastic surface at transonic free-stream velocities

Within the framework of the triple-deck theory, the effect of surface elasticity on three-dimensional packets of Tollmien-Schlichting waves generated by acoustic disturbances induced near the boundary layer at transonic free-stream velocities is investigated. It is shown that the elasticity of the surface considerably weakens the most unstable oblique waves but does not change the characteristic horseshoe shape of wave packets with two disturbance peaks propagating at an angle to the incoming flow.     

Implementation of near-wall boundary conditions for modeling boundary layers with free-stream turbulence

The paper is devoted to the extension of the near-wall domain decomposition, earlier developed in some previous works by the authors, to modeling flat-plate boundary layers undergoing laminar-to-turbulent bypass transition. The steady-state wall boundary layers at high-intensity free-stream turbulence are studied on the basis of differential turbulence models with the use of non-overlapping domain decomposition. In the approach the near-wall resolution is replaced by the interface boundary conditions of Robin type. In contrast to the previous studies, the main attention is paid to the laminar–turbulent transitional regime. With the use of modified turbulence models we study an effect of free-stream parameters on the development of dynamic processes in the boundary layer including a transitional regime and fully developed turbulent flow. In addition, for the first time a full scale domain decomposition is realized via iterations between the inner and outer subregions until a convergence. The computational profiles of the velocity and intensity of the turbulence kinetic energy are compared with experimental data. A possible range of location of the near-wall interface boundary is found.     

The Nonparallel Evolution of Nonlinear Short Waves in Buoyant Boundary Layers

Buoyant boundary-layer flows, typified by the flow over a heated flat plate, have the curious property that they can exhibit regions of "overshoot" in which the streamwise velocity exceeds its free-stream value. A consequence of this is the streamwise velocity develops a local maximum and is inflectional in nature. It is therefore inviscidly unstable, and the fastest growing wave mode is known to be one whose wavelength is short compared to the boundary-layer thickness. In this work we consider the nonparallel evolution of these short waves and show that they can be described in terms of the solution of a system of ordinary differential equations. Numerical and asymptotic studies enable us to explain the ultimate fate of the wave and show, depending on a key parameter which is a function of the underlying boundary layer, that two possibilities can arise. Nonparallelism may be sufficiently stabilizing so as to extinguish the linearly unstable waves or, in other cases, the mode may intensify but concentrate itself in a very thin zone surrounding the maximum in the streamwise velocity. These findings enable us to give some indication of the part these modes play in the transition to turbulence in buoyant boundary layers.     

Variation of drag coefficient with wind wave development

Turbulent air flows over developing wind waves in the air-sea boundary layer are numerically simulated without considering wave breaking. Influences of wind waves on air flows are considered using a model of significant wave and surface roughness, with a formula proposed for calculating the surface roughness. κ-ε model is adopted to simulate turbulent flows. The results of the drag coefficient and turbulence characteristics agree well with the observations. Project supported by the National Natural Science Foundation of China (Grant No. 19332010).     

Spreading of Linear Disturbances in Boundary Layers at Mach Number Five and the Effect of Wall Cooling

We determine inviscid eigensolutions in zero pressure gradient flat plate boundary layers at Mach number five and evaluate the eigensolutions with the method of steepest descent. The resulting wave packets show that with wall cooling the tail of the packets becomes slower. Although the boundary layers investigated are all convectively unstable we interpret the slow tail of the wave packets as a trend towards an absolute instability. From a comparison of the wave packets with turbulent spot transition, we conclude that for wall cooling the general transition properties are close to those of an absolutely unstable flow. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the dissipation due to wave ringing in nonelliptic elastic materials

Summary Initial-boundary value problems describing the mechanics of nonelliptic elastic materials give rise to solutions that involve phase boundaries, the motion of which can dissipate mechanical energy. We investigate whether this dissipation, acting alone, can drive such a system toward equilibrium. Moving phase boundaries are regarded as a localized dissipative mechanism, and we consider a model which specifically excludes dissipation away from a phase boundary (such as that due to viscoelastic damping). In the problem under consideration, wave packets reverberate between the fixed external boundary and a single internal phase boundary. The phase boundary remains stationary unless it is acted upon by one of these wave packets, and each such interaction dissipates a finite amount of energy while causing the initiating wave packet to split into a reflected wave packet and a transmitted wave packet. Consequently, the number of wave packets increases in a geometric fashion. Each individual interaction of a wave packet with the phase boundary is, in a certain sense, mechanically underdetermined, and we augment the mechanical theory with two alternative energy criteria, each of which determines a different interaction dynamics. These alternative energy criteria are motivated by considerations of maximizing the energy dissipation in the system. We treat a system that is perturbed out of an initial minimum energy equilibrium state by a disturbance at the external boundary. A framework is developed for treating the resulting wave reverberations and calculating the energy dissipation for large time. Numerical computation indicates that the total energy dissipated in both versions of the dynamical problem is that which is necessary to settle into a new energy-minimal equilibrium state. We then establish the same result analytically for a meaningful limit involving a vanishingly small dynamical perturbation.     

Lattice Boltzmann simulation for high-speed compressible viscous flows with a boundary layer

In this study, the lattice Boltzmann method is employed for simulating high-speed compressible viscous flows with a boundary layer. The coupled double-distribution-function lattice Boltzmann method proposed by Li et al. (2007) is employed because of its good numerical stability and non-free-parameter feature. The non-uniform mesh construction near the wall boundary in fine grids is combined with an appropriate wall boundary treatment for the finite difference method in order to obtain accurate spatial resolution in the boundary layer problem. Three typical problems in high-speed viscous flows are solved in the lattice Boltzmann simulation, i.e., the compressible boundary layer problem, shock wave problem, and shock boundary layer interaction problem. In addition, in-depth comparisons are made with the non-oscillatory and non-free-parameter dissipation (NND) scheme and second order upwind scheme in the present lattice Boltzmann model. Our simulation results indicate the great potential of the lattice Boltzmann method for simulating high-speed compressible viscous flows with a boundary layer. Further research is needed (e.g., better numerical models and appropriate finite difference schemes) because the lattice Boltzmann method is still immature for high-speed compressible viscous flow applications.