Dynamic model of coherent structure in the wall region of a turbulent boundary layer

Based on the theoretical model for a single coherent structure in the wall region of a turbulent boundary layer, we studied the interaction of two coherent structures by direct numerical simulation in order to explain the mechanism for the formation of low-speed streaks.     

PSE在边界层中预测转捩位置的应用

层流到湍流的转捩是自然界和各项工程实践中广泛存在的现象,层流和湍流的性质大不相同．因此,预测转捩位置是流体力学中的重要理论和实际问题．针对不可压缩边界层,入口加入展向等幅值型和展向波包型两类扰动,展向等幅值型扰动是由一个二维波(2-D)和两个三维波(3-D)组成,使用抛物化稳定性方程(PSE)的方法来研究扰动的演化和预测转捩位置,并且与数值模拟的结果相比较．结果表明,PSE可以研究扰动的演化和预测转捩位置,同时其计算比数值模拟快得多．     

用高精度有限差分法模拟烧蚀瑞利-泰勒不稳定性

求解烧蚀面附近流场的定常解,并以此作为基本流实现用高精度的WENO格式对烧蚀瑞利-泰勒不稳定性的数值模拟.线性增长率与Lindl公式以及线性稳定性分析给出的结果相符合,证明该数值模拟方法的准确性与精度,该方法还具有较好的界面变形捕捉能力.     

壁面定常波纹状吹吸槽道流中湍流特性的研究

在非平衡湍流中,如具有周期性边界条件的流动,由于雷诺应力与平均流速的变形率有着 不同的性质,当周期性边界条件发生变化时,雷诺应力和平均流速变形率的相位对边界条件 的响应也不同,但是二者的相位差在相当大的范围内是稳定的. 这一特性加深了对雷诺应力 的认识,并对非平衡湍流中的模式理论及大涡模拟中亚格子雷诺应力模式的建立提出了许多 需要注意的问题. 利用层流模型,把空间周期性边界条件作为某种扰动,研究了扰动 及其非线性项的分布以及相位间的关系,得到了一些有益的结果.     

A theoretical model of the coherent structure in the wall region of a turbulent boundary layer

In this paper,the formation of the coherent structures in the wall region of aturbulent boundary layer was studied,using the nonlinear theory of the hydrodynamicstability.The spanwise and streamwise wavelengths of the most amplified unstablewave obtained by this study were found in good agreement with the experiments,whichmakes the distinct feature of this study in the present paper,as the basis of thestability analysis,a more rational velocity profile has been used,which is different fromthat of the turbulent mean flow.And also,the new nonlinear theory was used.Theresult is useful in understanding of the quasi-periodicity of the coherent structure in theturbulent boundary layer.     

湍流边界层底层相干结构的一个理论模型*

本文采用非线性稳定性分析方法,研究了湍流边界层底层相干结干结构的成因.计算得到的增长最快的不稳定波的展向尺度与纵向尺度都与实验相符.这一分析的特点是采用了不同于湍流平均速度剖面的更合理的速度剖面作为稳定性分析的基础,并采用了新的非线性理论.文中结果有助于理解湍流边界层底层相干结构的拟有序现象.     

Improved algorithm for solving nonlinear parabolized stability equations

Due to its high computational efficiency and ability to consider nonparallel and nonlinear effects, nonlinear parabolized stability equations(NPSE) approach has been widely used to study the stability and transition mechanisms. However,it often diverges in hypersonic boundary layers when the amplitude of disturbance reaches a certain level. In this study, an improved algorithm for solving NPSE is developed. In this algorithm, the mean flow distortion is included into the linear operator instead of into the nonlinear forcing terms in NPSE. An under-relaxation factor for computing the nonlinear terms is introduced during the iteration process to guarantee the robustness of the algorithm. Two case studies, the nonlinear development of stationary crossflow vortices and the fundamental resonance of the second mode disturbance in hypersonic boundary layers, are presented to validate the proposed algorithm for NPSE. Results from direct numerical simulation(DNS) are regarded as the baseline for comparison. Good agreement can be found between the proposed algorithm and DNS, which indicates the great potential of the proposed method on studying the crossflow and streamwise instability in hypersonic boundary layers.     

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.     

A mechanism for excitation of coherent structures in wall region of a turbulent boundary layer

Introduction Themechanismforthegenerationofcoherentstructuresinthewallregionofaturbulent boundarylayerhasalwaysbeeninconcernandinvestigated.AccordingtoTsujimotoand Miyake[1],thecharacteristicsofturbulenceinthewallregionweremainlydeterminedbythe generationandevolutionofcoherentstructures,notbythesmall_scaleturbulence.However, excitationsfromregionofy >60werefoundtobenecessary,otherwisethewallregionwould degeneratetolaminarflow.Therefore,theinvestigationofthemechanismthathowcoherent structuresi…     

超音速混合层稳定性分析及增强混合的研究

利用流动稳定性提高超音速混合层的混合效率,对于提高超音速流的高效混合是一个有效途径。研究结果表明,有展向曲率的三维混合层中,三维扰动的增长率很大,且法向的掺混能力也较强,可以有效地增强混合。对于高马赫数来流的超音速混合层,这一特性依然存在,这将有利于提高高超音速混合层的混合能力。