可压缩边界层与混合层失稳结构的研究进展及其工程应用

本文回顾了可压缩边界层与混合层中失稳结构及其应用的研究进展. 这些工作包括人们对高超声速平板边界层失稳特性、高超声速圆锥边界层转捩攻角效应的产生机制和可压缩混合层失稳特性的研究认识,以及这些相关认识的3 个应用方向. 这些相关工作丰富了人们对高速流动转捩和湍流拟序结构的认识.     

基于HLNS方法对高超声速边界层中非模态扰动演化的研究

高超声速边界层转捩是航天飞行器设计中的基础难题,发生在线性失稳区上游的亚临界转捩是常规风洞实验中常见的现象.亚临界转捩一般是由非模态扰动的演化及二次失稳触发的,为了揭示局部突变对高超声速边界层亚临界转捩的影响机理,发展了基于谐波型线性化Navier-Stokes (HLNS)方程及其伴随系统的描述非模态扰动演化的求解框架.该框架的优点是不改变原始系统的椭圆型特性,因而可以处理非模态扰动(条带)在局部突变附近的快速畸变.针对马赫数为5.96、攻角为-4?的高超声速钝平板边界层,研究了不同深度凹槽对条带幅值的影响.数值结果表明凹槽对条带有促进作用,这与实验中发现的规律定性相符,且存在使促进作用最大的最优凹槽深度.     

可压缩横流失稳及其控制

边界层流动转捩的预测与控制一直是流体力学研究中的一个重要问题. 三维边界层流动工程中十分常见, 而横流失稳是导致三维边界层流动转捩的主要原因. 本文综述了近些年来三维边界层失稳和转捩方面的研究概况. 从机理上讨论了横流扰动的感受性、首次失稳、二次失稳和转捩控制等方面的研究进展. 在数值计算方面, 简要概述了线性稳定性理论、非线性稳定性理论和直接数值模拟方法在横流失稳和转捩方面的应用.本文对横流失稳研究当前存在的问题进行了讨论, 对今后研究的发展趋势作了相应展望.     

高超声速流场激光测速技术研究进展

高超声速气流条件下飞行器内/外部流动中存在强湍流及脉动、边界层转捩、激波-边界层干扰和高温真实气体效应等耦合效应,表征该非定常流动现象对飞行器气动力、气动热以及目标光电特性等产生的影响是高超声速流动研究中的前沿课题.速度作为表征流动过程最重要的参数之一,准确的速度测量对于深入理解上述复杂流动-传输机理以及高超声速飞行器设计具有重要指导意义.文章针对高超声速流场速度测量中几种常用的非接触式激光测试技术进行了综述,主要包括基于空间法的粒子图像测速,基于激光吸收光谱、激光诱导荧光和瑞利散射的多普勒测速,基于飞行时间法的分子标记测速,以及基于流场折射率的聚焦激光差分干涉测速技术.首先简要介绍每种激光测速技术的基本原理,然后进一步介绍该技术在高超声速自由流、层/湍流边界层、激波/边界层干扰、尾流或其他复杂流动区域的速度及其脉动度测量等方面的典型应用,分析各种技术环境适用性及面临的局限性和挑战.最后对基于激光技术的高超声速流场速度测量进行了总结及发展趋势展望.     

触摸高温气体动力学

回顾了高温气体动力学与高超声速科技相关的一些重要研究进展,探讨几个具有基础性研究意义的方向：即高超声速流动模拟;高温气体热化学反应机制;高超声速流动滞止区预测;高超声速边界层转捩和激波/激波相互作用诱导的气动热问题．这些研究方向与高温气体效应和强激波密切相关,对高超声速科技关键技术的突破起着重要作用．     

k-ω-γ模式对转捩影响因素的预测性能研究

高超声速边界层转捩的准确预测对飞行器的防热、减阻至关重要,而影响高超声速边界层转捩的因素众多.从模式角度出发研究边界层转捩的影响因素,采用k-ω-γ 转捩模式对5°圆锥的边界层转捩进行了数值分析,计算了不同头部钝度、来流雷诺数和湍流度情况下的边界层转捩,并与实验结果进行了对比. 研究结果表明:k-ω-γ 转捩模式基本能够反映头部钝度、来流雷诺数、来流湍流度对高超声速圆锥边界层转捩的影响规律,但对转捩后的热流峰值预测不准;从模式构造角度分析发现,雷诺数越高或头部钝度越小,层流区边界层越薄,k-ω-γ 转捩模式中第一、第二模态时间尺度增大,因此转捩起始位置提前;来流湍流度越大,等效脉动动能初值越大,导致层流区发展过程中等效脉动动能越大,因此转捩易于发生.      

微槽-吸气组合控制平板边界层多模态稳定性研究

边界层转捩会使高超声速飞行器壁面摩阻和热流显著增加,因此在高超声速飞行器设计过程中往往占据重要地位.针对高超声速飞行器多模态转捩控制问题,提出了微槽道(1 mm)与边界层吸气的组合控制方法,并通过直接数值模拟和线性稳定性理论研究了Ma=4.5平板边界层的稳定性及组合控制效果.边界层在无控状态时,同时存在失稳的第一、二模态波,且二维第二模态波最不稳定;单纯施加微槽道控制时,边界层第二模态波会被抑制但第一模态波会被略微激发.对比而言,采用“微槽-吸气”组合控制后,不仅增强了对第二模态波的抑制效果,而且减弱了第一模态波的激发程度;同时随着吸气强度的增加,第二模态波不稳定区域明显收缩、频率显著增高,而第一模态波则变化不明显.相较于单纯的微槽道,吸气增强了“微槽吸收”与“声波散射”作用,因此中等吸气强度下该组合控制方法对第一和第二模态波的增长率分别实现了12.63%和28.02%的抑制效果.以上结果表明“微槽-吸气”组合控制手段具有适用宽频、布置区域灵活的优点,展现出了一定的多模态控制效果.     

高超声速高焓边界层稳定性与转捩研究进展

边界层由层流向湍流的转捩是高超声速飞行器设计面临的重大空气动力学问题. 随着飞行速域与空域的不断拓展, 高超声速高焓边界层中的高温气体效应会使得量热完全气体假设失效, 从而深刻影响流动转捩过程. 相关研究涉及多个学科, 是典型的多物理场耦合问题. 近年来, 随着相关飞行器技术的快速发展, 高超声速高焓边界层转捩问题的重要性越来越得到体现, 相关研究已成为国际上的热点领域. 本文综述相关研究进展, 首先介绍目前常用的高温气体物理模型, 尤其关注热化学非平衡模型, 并介绍激波捕捉、激波装配和边界层方程解等常用的高焓流动求解方法, 以及相关风洞和飞行试验技术的进展. 然后综述高温气体效应对转捩过程中的感受性、模态增长、瞬态增长和非线性作用等的影响的相关研究, 其中流向不稳定性中出现较大增长率的第三模态和超声速模态引起了广泛的研究兴趣. 最后进行总结, 并对未来发展略作展望.      

高超声速边界层中模态转化的数值研究

在高超声速边界层中,第一模态和第二模态是与转捩有关的两个主要不稳定模态.除了不稳定模态,还存在一类稳定模态,其相速度在前缘接近快声波的相速度称为快模态.在感受性过程中,这类模态对激发边界层中不稳定模态起着很重要的作用.前缘感受性理论解释了边界层外扰动激发边界层中第一模态波的机理.针对高超声速平板边界层,利用相似性解剖面作为基本流,采用线性稳定性理论和直接数值模拟的方法研究了快模态和慢模态的稳定性行为.研究发现模态转化的位置与马赫数有关.根据线性稳定性理论的结果定义了临界频率.当扰动频率高于临界频率,第一模态与第二模态同支;而当扰动频率低于临界频率,第一模态与第二模态的共轭模态同支.借助稳定性方程的伴随方程分析了直接数值模拟的结果.直接数值模拟结果表明不论上游是快模态还是慢模态,当它们经过第二模态的不稳定区,它们都会演化成第二模态. 这可用模态在非平行流中传播的特征来解释.      

高超声速边界层中模态转化的数值研究

在高超声速边界层中,第一模态和第二模态是与转捩有关的两个主要不稳定模态.除了不稳定模态,还存在一类稳定模态,其相速度在前缘接近快声波的相速度称为快模态.在感受性过程中,这类模态对激发边界层中不稳定模态起着很重要的作用.前缘感受性理论解释了边界层外扰动激发边界层中第一模态波的机理.针对高超声速平板边界层,利用相似性解剖面作为基本流,采用线性稳定性理论和直接数值模拟的方法研究了快模态和慢模态的稳定性行为.研究发现模态转化的位置与马赫数有关.根据线性稳定性理论的结果定义了临界频率.当扰动频率高于临界频率,第一模态与第二模态同支;而当扰动频率低于临界频率,第一模态与第二模态的共轭模态同支.借助稳定性方程的伴随方程分析了直接数值模拟的结果.直接数值模拟结果表明不论上游是快模态还是慢模态,当它们经过第二模态的不稳定区,它们都会演化成第二模态.这可用模态在非平行流中传播的特征来解释.     

波纹壁对高超声速平板边界层稳定性的影响

高超声速边界层转捩会使飞行器表面热流和摩阻增加3～5倍,极大影响高超声速飞行器的性能.波纹壁作为一种可能的推迟边界层转捩的被动控制方法,具有较强的工程应用前景.文章研究了不同高度和安装位置的波纹壁对来流马赫数6.5的平板边界层稳定性的影响.采用直接数值模拟(DNS)得到层流场,并在上游分别引入不同频率的吹吸扰动以研究波纹壁对扰动演化的作用.对于不同位置的波纹壁,探究了其与同步点相对位置对其作用效果的影响,与相同工况下光滑平板的扰动演化结果进行了对比,发现当快慢模态同步点位于波纹壁上游时,波纹壁会对该频率的第二模态扰动起到抑制作用.当同步点位于波纹壁之中或者下游时,波纹壁对扰动的作用可能因为存在两种不同的机制而使得结果较为复杂.对于不同高度波纹壁,发现高度较低的波纹壁,其作用效果强弱与波纹壁高度成正相关,而更高的波纹壁则会减弱其作用效果.与DNS结果相比,线性稳定性理论可以定性预测波纹壁对高频吹吸扰动的作用,但在波纹壁附近的强非平行性区域误差较大.     

Second mode unstable disturbance measurement of hypersonic boundary layer on cone by wavelet transform

Experimental investigation of hypersonic boundary layer instability on a cone is performed at Mach number 6 in a hypersonic wind tunnel.Time series signals of instantaneous fluctuating surface-thermal-flux are measured by Pt-thin-film thermocouple temperature sensors mounted at 28 stations on the cone surface in the streamwise direction to investigate the development of the unstable disturbance.Wavelet transform is employed as a mathematical tool to obtain the multi-scale characteristics of fluctuating surfacethermal-flux both in the temporal and spectrum space.The conditional sampling algorithm using wavelet coefficient as an index is put forward to extract the unstable disturbanceThe generic waveform for the second mode unstable disturbance is obtained by a phase-averaging technique.The development of the unstable disturbance in the streamwise direction is assessed both in the temporal and spectrum space.Our study shows that the local unstable disturbance detection method based on wavelet transformation offers an alternative powerful tool in studying the hypersonic unstable mode of laminar-turbulent transition.It is demonstrated that,at hypersonic speeds,the dominant flow instability is the second mode,which governs the course of laminar-turbulent transition of sharp cone boundary layer.     

Effect of disturbances at inlet on hypersonic boundary layer transition on a blunt cone at small angle of attack

To investigate the effect of different disturbances in the upstream, we present numerical simulation of transition for a hypersonic boundary layer on a 5-degree half-angle blunt cone in a freestream with Mach number 6 at 1-degree angle of attack. Evolution of small disturbances is simulated to compare with the linear stability theory （LST）, indicating that LST can provide a good prediction on the growth rate of the disturbance. The effect of different disturbances on transition is investigated. Transition onset distributions along the azimuthal direction are obtained with two groups of disturbances of different frequencies. It shows that transition onset is relevant to frequencies and amplitudes of the disturbances at the inlet, and is decided by the amplitudes of most unstable waves at the inlet. According to the characteristics of environmental disturbances in most wind tunnels, we explain why transition occurs leeside-forward and windside-aft over a circular cone at an angle of attack. Moreover, the indentation phenomenon in the transition curve on the leeward is also revealed.     

Prehistory of Instability in a Hypersonic Boundary Layer

The initial phase of hypersonic boundary-layer transition comprising excitation of boundary-layer modes and their downstream evolution from receptivity regions to the unstable region (instability prehistory problem) is considered. The disturbance spectrum reveals the following features: (1) the first and second modes are synchronized with acoustic waves near the leading edge; (2) further downstream, the first mode is synchronized with entropy and vorticity waves; (3) near the lower neutral branch of the Mack second mode, the first mode is synchronized with the second mode. Disturbance behavior in Regions (2) and (3) is studied using the multiple-mode method accounting for interaction between modes due to mean-flow nonparallel effects. Analysis of the disturbance behavior in Region 3) provides the intermodal exchange rule coupling input and output amplitudes of the first and second modes. It is shown that Region (3) includes branch points at which disturbance group velocity and amplitude are singular. These singularities can cause difficulties in stability analyses. In Region (2), vorticity/entropy waves are partially swallowed by the boundary layer. They may effectively generate the Mack second mode near its lower neutral branch. Received 17 July 2000 and accepted 23 March 2001     

On the onset of instability in the wake of super-hydrophobic spheres

We report an experimental investigation of free falling super-hydrophobic (SH) spheres in glycerine-water mixtures over a wide range of Reynolds number. SH coatings have the ability to reduce the contact area between the surrounding liquid and the solid surface by entrapping an air layer in the surface roughness. We investigate the effect of this air plastron on the hydrodynamic performance of spheres, focusing our attention on the onset of wake instabilities. Our results emphasise the key role of the surface roughness properties on the triggering of wake instabilities. It is shown that, unlike what was reported in previous numerical studies on SH bluff-bodies, local deformation of the interface may act as a by-pass mechanism promoting earlier transition, yielding a decrease of the critical Reynolds number at which the wake becomes unstable. A scenario coupling the hydrodynamic instabilities (scaling with the Reynolds number) and the interface deformation (scaling with the roughness-based Weber number) is proposed to describe the different transition mechanisms in presence of SH surfaces. It is found that the promotion of wake instabilities over SH surfaces occurs when the roughness-based Weber number is larger than a critical threshold. These findings are of primary importance for guiding the design of resilient and efficient SH surfaces.     

Hypersonic viscous flow over large roughness elements

Viscous flow over discrete or distributed surface roughness has great implications for hypersonic flight due to aerothermodynamic considerations related to laminar?Cturbulent transition. Current prediction capability is greatly hampered by the limited knowledge base for such flows. To help fill that gap, numerical computations are used to investigate the intricate flow physics involved. An unstructured mesh, compressible Navier?CStokes code based on the space?Ctime conservation element, solution element (CESE) method is used to perform time-accurate Navier?CStokes calculations for two roughness shapes investigated in wind tunnel experiments at NASA Langley Research Center. It was found through 2D parametric study that at subcritical Reynolds numbers, spontaneous absolute instability accompanying by sustained vortex shedding downstream of the roughness is likely to take place at subsonic free-stream conditions. On the other hand, convective instability may be the dominant mechanism for supersonic boundary layers. Three-dimensional calculations for both a rectangular and a cylindrical roughness element at post-shock Mach numbers of 4.1 and 6.5 also confirm that no self-sustained vortex generation from the top face of the roughness is observed, despite the presence of flow unsteadiness for the smaller post-shock Mach number case.     

Stabilizing effect of diffusion in enhanced oil recovery and three-layer Hele-Shaw flows with viscosity gradient

In the presence of diffusion, stability of three-layer Hele-Shaw flows which models enhanced oil recovery processes by polymer flooding is studied for the case of variable viscosity in the middle layer. This leads to the coupling of the momentum equation and the species advection-diffusion equation the hydrodynamic stability study of which is presented in this paper. Linear stability analysis of a potentially unstable three-layer rectilinear Hele-Shaw flow is used to examine the effects of species diffusion on the stability of the flow. Using a weak formulation of the disturbance equations, upper bounds on the growth rate of individual disturbances and on the maximal growth rate over all possible disturbances are found. Analytically, it is shown that a short-wave disturbance if unstable can be stabilized by mild diffusion of species, where as an unstable long-wave disturbance can always be stabilized by strong diffusion of species. Thus, an otherwise unstable three-layer Hele-Shaw flow can be completely stabilized by a large enough diffusion, i.e., by increasing enough the magnitude of the species diffusion coefficient. The magnitude of this diffusion coefficient required to completely stabilize the flow will depend on the magnitude of interfacial viscosity jumps and the viscosity gradient of the basic viscous profile of the middle layer.     

Local receptivity in non-parallel boundary layer

The research on boundary-layer receptivity is the key issue for the laminarturbulent transition prediction in fluid mechanics. Many of the previous studies for local receptivity are on the basis of the parallel flow assumption which cannot accurately reflect the real physics. To overcome this disadvantage, local receptivity in the non-parallel boundary layer is studied in this paper by the direct numerical simulation (DNS). The difference between the non-parallel and parallel boundary layers on local receptivity is investigated. In addition, the effects of the disturbance frequency, the roughness location, and the multiple roughness elements on receptivity are also determined. Besides, the relations of receptivity with the amplitude of free-stream turbulence (FST), with the roughness height, and with the roughness length are ascertained as well. The Tollmien- Schlichting (T-S) wave packets are excited in the non-parallel boundary layer under the interaction of the FST and the localized wall roughness. A group of T-S waves are separated by the fast Fourier transform. The obtained results are in accordance with Dietz’s measurements, Wu’s theoretical calculations, and the linear stability theory (LST).