Effect of riblets on nonlinear disturbances in the boundary layer

Results of experimental investigations of the nonlinear stage of sinusoidal and varicose instability of a streaky structure, which leads to multiplication of streaky structures and origination of coherent structures (such as Λ-structures), are presented. Riblets suppress the intensity of streaky structures, stabilize the flow against the development of the secondary high-frequency instability of streaky structures, and, for this reason, delay spatial turbulization of the flow. The results of these investigations can be useful for understanding the flow structure in such situations and for possible controlling of the coherent structures aimed at flow stabilization. This work was supported by the President of the Russian Federation (Grant No. NSh-964.2003.1) and by the Russian Foundation for Basic Research (Grant No. 05-01-00034).     

Visualization of sinusoidal and varicose instabilities of streaks in a boundary layer

Nonlinear instabilities of boundary layer streaks are investigated experimentally. Extensive measurements visualizing the sinusoidal and varicose instabilities of streaky structures at nonlinear stage of the breakdown process in boundary layer are presented. The flow behaviour in the course of spatial evolution of the streaky structures with a secondary high-frequency disturbance generated on them is discussed. Various scenarios of origination and development of coherent vortex structures examined in physical experiments are considered. Specific features of the development of sinusoidal and varicose cases of destruction of the steady streamwise streaks are demonstrated, such as transverse and streamwise modulations of the streak by the secondary-disturbance frequency, appearance of new streaky structures in the downstream direction, and emergence and evolution of unsteady Λ-shaped structures localized in space in both cases.     

Modelling of streaky structures and turbulent-spot generation process in wing boundary layer at high free-stream turbulence

Results of an experimental study of turbulent breakdown in gradient boundary layer at high freestream turbulence are reported. For the first time it is shown that, like the flat-plate boundary layer, the wing boundary layer at high freestream turbulence is modulated with streaky structures. One of possible mechanisms underlying the generation of turbulence spots in wing boundary layer is modelled assuming the interaction of streaky structures with high-frequency waves. Qualitative and quantitative data concerning the evolution of streaky structures in swqpt-wing boundary layer and in swept-wing boundary layer are presented. Certain differences between the evolution of streaky structures in wing boundary layer and in flat-plate boundary layer are revealed. This work was supported by the President of the Russian Federation (Grants NSh-454.2008.1, MK- 101.2007.1), by the Russian Foundation for Basic Research (Grant No. 05-01-00034), by the Ministry of Education and Science of the Russian Federation (Grant No. RNP 2.1.2.3370), and by the Russian Government (State Contract No. 02.513.12.0043).     

Influence of an unfavourable pressure gradient on the breakdown of boundary layer streaks

Breakdown of boundary layer streaks is studied experimentally and compared at zero and adverse (positive) streamwise pressure gradients on a wing under fully controlled experimental conditions. The varicose mode of streak breakdown is found to be a dominant mode in the case of the adverse pressure gradient. A strong influence of pressure gradient upon the development of the streak and the secondary instability is revealed. The unfavourable pressure gradient is shown to alter the critical streak amplitude, the dispersion properties of the streak and the secondary disturbance, as well as attained maximum amplitudes for both the streak and the secondary disturbance.     

Regimes and spectra of čerenkov beam instability in a nonlinear plasma

We study the nonlinear evolution of an arbitrary initial disturbance due to the development of Čerenkov beam instability in a magnetized plasma-beam system of finite transverse size. Singleparticle, collective, and aperiodic regimes of this instability are considered. We calculate the nonlinear spatial spectra of the waves excited at different development stages of the beam instability in a plasma for the cases of quasi-monochromatic, pulsed, and noise initial disturbances. We analyze the formation and decay of regular structures in the beam and plasma at the developed nonlinear stage of the process. We find that plasma nonlinearity leads to the transfer of disturbance energy to the short-wave region of the spectrum. We show that, due to the development of beam instability, noise initial disturbance tends to become more monochromatic, whereas the shape of a pulsed one tends to remain unchanged. Transformation of monochromatic spatial disturbances into quasi-monochromatic plasma waves due to the instability development is analyzed. Institute of General Physics of the Russian Academy of Sciences, Moscow, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 10, pp. 958–976, October 1999.     

高超声速边界层中由粗糙元引起强制转捩的机理

由大粗糙元引起的高超声速边界层强制转捩在航天技术中有实际应用, 因而近年来受到人们的广泛关注.虽然目前导致该转捩过程的内在机理尚不完全清楚, 但有一点是明确的, 即粗糙元的尾迹流场中存在强对流不稳定性.文章的出发点是研究这种对流不稳定模态是如何触发转捩的.首先通过CFD方法, 计算出高超声速边界层中粗糙元的尾迹流场, 并对其进行二维稳定性分析.结果发现, 在传统不稳定Tollmien-Schlichting(T-S)模态出现的临界Reynolds数之前, 存在高增长率的无黏不稳定模态, 表现为对称的余弦模态和反对称的正弦模态.然后对该不稳定模态在粗糙元尾迹流中的演化进行了模拟, 验证了二维稳定性分析的结果, 并考察了非平行性效应的影响.最后通过直接数值模拟, 研究由这些不稳定模态触发转捩的全过程.结果表明, 对流不稳定模态确实是导致边界层转捩的关键机制.该转捩过程的特点是, 局部湍斑首先在不稳定模态特征函数的峰值附近出现, 然后向全流场扩散.就文章研究的工况而言, 余弦和正弦模态的相互作用对转捩的影响并不明显, 且后者在转捩过程中起主导作用.      

Influence of initial conditions at the nozzle exit on the structure of round jet

Experimental data concerning the influence of initial conditions at the nozzle exit on the structure and development characteristics of round jets are reported. Features in the development of laminar and turbulent round jets emanating from variously elongated nozzles at identical Reynolds numbers are revealed. Smoke visualization pictures obtained for jets formed under different initial conditions (with different distributions of mean and pulsating flow velocities at the nozzle exit) are discussed. It is shown possible to make the zone of laminar flow in the jet stream more extended, and to delay the jet turbulization process in space, by making the flow-velocity profile more parabolic at the exit of elongated nozzle. Features in the development of vortical structures in a jet under an acoustic action are identified. It is shown that, for a turbulent round jet to be produced right at the nozzle exit, the nozzle length must be increased in excess of a certain value so that to provide for spatial growth of turbulent boundary layer thickness, finally ending in the formation of a fully turbulent flow velocity profile across the channel. This work was supported by the Russian Foundation for Basic Research (Grant No. 08-01-00027), the Ministry of Education and Science of the Russian Federation (project RNP 2.1.2.3370), and by the grants of President of the Russian Federation (NSH-454.2008.1 and. MK-420.2008.1).     

Hot-wire visualization of breakdown to turbulence in complex flows

The nonlinear stage of breakdown to turbulence is a strongly three-dimensional process and represents a difficult task for experimental studies. Investigation of laminar-turbulent transition in aerospace applications additionally involves a complex base-flow with pressure gradients and secondary velocity components resulting in successive increase of necessary measurements. The developed hot-wire visualization technique offers a possibility for an advanced analysis whilst retaining the advantages of traditional visualization methods and is especially suitable for resolving such complex flows. Thanks are due to the Royal Swedish Academy of Sciences, the Swedish Energy Agency (Energimyndigheten). The work was financially supported by the President of the Russian Federation (Grant No. NSh-964.2003.1), the Russian Foundation for Basic Research (Grant No. 05-01-00034) and Ministry of Education and Sciences of the Russian Federation (Grant No. RNP.2.1.2.3370).     

Effects of roughness elements on bypass transition induced by a circular cylinder wake

Abstract  

The bypass transition of flat-plate boundary layer induced by a circular cylinder wake under the influence of roughness elements is experimentally investigated. The hydrogen-bubble visualization results show that the boundary layer separation occurs upstream of the roughness, and the separated shear layer is incised by roughness to different extent, resulting in different kinds of secondary vortices formed immediately downstream of the roughness. During the evolution of the secondary vortex, two types of instabilities are observed, which are denoted as large- and small-scale instabilities, respectively, according to different spatial scale of the hairpin vortices formed afterward. A merging process of hairpin vortices is also observed when the secondary vortices undergo the small-scale instability, and a potential new transition control strategy based on the present observation is proposed.     

Excitation of Convective Cells by Nonlinear Interaction of Interchange Mode

Convective Cell formation due to nonlinear interaction of interchange mode is studied using multiple time and space scales. Basic interchange mode equation is modified to obtain nonlinear localized structure with modulational perturbation in the drift direction and periodic boundary condition for the perturbation in the direction of the density gradient. To study modulational instability we have deduced nonlinear evolution equation for the first order perturbation from third order secularity elimination condition. Modulational instability, which is responsible for localized structure, is studied in two limits of the dispersion relation.     

Development of laminar-separating-flow perturbations on a wavy wing

The development of velocity perturbations at laminar boundary-layer separation from a wing with wavy surface has been examined. Experiments were carried out to identify flow features displayed by the spatially periodic flow structure. An analysis of the linear stability of measured velocity profiles is performed. As a result of the analysis, the influence of surface waviness on the frequency range and growth rates of instability waves was investigated, with a good agreement between calculated and experimental data. This work was supported by the Ministry of Education and Science of the Russian Federation under the Program “Development of Higher-School Scientific Potential” for the years 2006–2008 (Projects RNP 2.1.1.471 and RNP 2.1.2.3370), and also by the Russian Foundation for Basic Research (Grant No. 07-08-00164).     

Generating magnetic field pulses in the development of an explosive Alfvén wave instability in a flow-plasma system

The excitation of Alfvén radiation is studied when an explosive instability is developed in a heated gyrotropic plasma penetrated by a monovelocity flow of ionized gas. It is shown that magnetic field pulses are generated in the plasma under certain conditions. The amplitudes of the Alfvén wave harmonics are found and the conditions for stabilization of the explosion due to the nonlinear absorption associated with weakly damped Alfvén wave harmonics are specified.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 8, pp. 1001–1006, August, 1996.This work was supported in part by the Russian Foundation for Fundamental Research under Grant No. 95-02-05255.     

Spatial structure of the flow at round jet outflow into a narrow channel

Results on visual studies of the flow structure in case of the round submerged jet in a narrow channel are presented. These studies were carried out for the laminar and turbulent flows. The typical large-scale structures and zones of intensive turbulent mixing were identified in the flow. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 08-08-00417-a).     

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.     

Competing mechanisms of plasma transport in inhomogeneous configurations with velocity shear: the solar-wind interaction with earth's magnetosphere

Two-dimensional simulations of the Kelvin-Helmholtz instability in an inhomogeneous compressible plasma with a density gradient show that, in a transverse magnetic field configuration, the vortex pairing process and the Rayleigh-Taylor secondary instability compete during the nonlinear evolution of the vortices. Two different regimes exist depending on the value of the density jump across the velocity shear layer. These regimes have different physical signatures that can be crucial for the interpretation of satellite data of the interaction of the solar wind with the magnetospheric plasma.     

Nonlinear sound waves upon collapse of a vortex dipole

Acoustic emission by a vortex dipole decreases the dipole energy and changes the vortex velocity distribution. A relative shift of the dipole component occurs. Earlier, such a process of acoustic instability of the vortex was considered as applied to the case of weak acoustic emission described by the wave equation. Features of acoustic instability upon emission of a strongly nonlinear sound by a vortex are considered in this work.     

Specific features of unsteady processes in the front regions of streaky structures in the boundary layer on a nonswept wing

Under conditions of a model experiment in nonswept-wing boundary layer, forerunner wave packets were obtained in the flow regions preceding the fronts of streaky structures. A model to describe the formation and development of the fronts is proposed. An external-flow pressure gradient is shown to be a necessary condition for forerunner growth. Besides, the spatial geometry of the forerunners and the evolution of this geometry in the course of downstream development of the wave packet were studied.     

液-液两相液层间传质过程的Rayleigh-Bénard-Marangoni对流特性

传质引发的Rayleigh-Bénard-Marangoni对流(RBM对流)对化工传递过程有着显著影响.但是,已有的相关研究多集中于气-液体系,并且有限的针对液-液体系的相关研究尚缺乏对RBM对流演化及其引发的界面扰动行为的深入分析.因此,本文基于阴影法设计搭建了竖直狭缝内液-液两相液层间传质过程的RBM对流特性可视化实验平台,并实验观测了水-甲苯-丙酮三元体系中丙酮组分扩散传质时出现的RBM对流结构以及其向下层水相主体的发展演变过程,探讨了水相丙酮初始浓度、甲苯相丙酮初始浓度以及甲苯层厚度对RBM对流特性和液-液界面形貌的影响.研究表明:在Rayleigh-Taylor不稳定性作用下,水相上层密度(重力)分层"界面"下凸沉降形成波浪形丘状"界面",并随着"界面"处密度与压力失调的加剧而演变成羽状流;因羽流区"界面"不同浓度梯度引起的传质特性差异,羽状流又可以演变成弱羽状流和强羽状流两种形态;当丙酮浓度梯度增大到一定程度后,近界面处短时间内产生大量RBM对流结构,且结构间相互影响增强而聚并成对流团,并随着传质过程的进行,逐渐演变成独立的强羽状流; RBM对流强度与上下液层丙酮浓度梯度大小呈正相关关系,且液-液界面粗糙度及其非稳态波动随着丙酮浓度梯度的增加而增大.     

高超声速椭圆锥边界层横流转捩特性大涡模拟

横流效应显著影响高超声速飞行器的三维边界层转捩过程, 深化对该流动机制的认识有助于提升和改善飞行器气动性能及热力学环境. 针对HIFiRE5椭圆锥绕流问题, 采用大涡模拟方法计算分析了超声速边界层横流转捩特性, 并揭示其中的流动机理. 参考HIFiRE5风洞模型试验条件, 数值模拟中椭圆锥来流入口处施加人工速度扰动以激发边界层内不稳定扰动波, 进而预测了高超声速边界层流动横流失稳、转捩过程等基本流动特征, 并基于转捩热流分布形态对比, 获得了与试验数据基本吻合的计算结果. 研究发现, 椭圆锥中心线流动汇聚形成的流向涡结构非常容易失稳, 另外在中心线及侧缘之间的中部区域存在较强的横流不稳定性, 两种机制共同作用影响边界层转捩过程. 此外, 分析了来流扰动幅值对边界层横流失稳转捩的影响, 并发现静来流条件下, 横流区域出现两组独立的定常横流涡结构, 而强噪声来流条件下, 中心线主涡和中部横流涡均发生失稳转捩, 且在椭圆锥表面形成多峰状的转捩阵面. 最后, 深入分析流场的压力脉动动力学特性, 揭示了三维边界层发生失稳转捩的非线性演化机制.