激波与转捩边界层干扰非定常特性数值分析

激波与边界层干扰的非定常问题是高速飞行器气动设计中基础研究内容之一.以往研究主要针对层流和湍流干扰,在分离激波低频振荡及其内在机理方面存在着上游机制和下游机制两类截然不同的理论解释.分析激波与转捩边界层干扰下非定常运动现象有助于进一步加深理解边界层状态以及分离泡结构对低频振荡特性的影响规律,为揭示其产生机理指出新的方向.采用直接数值模拟方法对来流马赫数2.9,24?压缩拐角内激波与转捩边界层干扰下激波的非定常运动特性进行了数值分析.通过在拐角上游平板特定的流向位置添加吹吸扰动激发流动转捩,使得进入拐角的边界层处于转捩初期阶段.在验证了计算程序可靠性的基础上,详细分析了转捩干扰下激波运动的间歇性和振荡特征,着重研究了分离泡展向三维结构对激波振荡特性的影响规律,最后还初步探索了转捩干扰下激波低频振荡产生的物理机制.研究结果表明:分离激波的非定常运动仍存在强间歇性和低频振荡特征,其时间尺度约为上游无干扰区内脉动信号特征尺度的10倍量级;分离泡展向三维结构不会对分离激波的低频振荡特征产生实质影响.依据瞬态脉动流场的低通滤波结果,转捩干扰下激波低频振荡的诱因来源于拐角干扰区下游,与流场中分离泡的收缩/膨胀运动存在一定的关联.

NUMERICAL ANALYSIS OF UNSTEADY MOTION IN SHOCK WAVE/TRANSITIONAL BOUNDARY LAYER INTERACTION 1)

The unsteadiness in shock wave and boundary layer interactions is one of foundation problems in the aerody-namic design of high-speed vehicles. Most previous researches have focused on laminar and turbulent interaction. The intrinsic physical origin of separation shock low-frequency oscillation is still under debate. There exist two utterly op-posite theoretical explanations, upstream influence and downstream influence. The analysis of unsteady motion in shock wave and transitional boundary layer interactions are helpful to aware of the effects of boundary layer state and sepa-ration bubble structures on low-frequency oscillation, which providing an insight to point out new direction for forcing mechanism. A numerical analysis of unsteady motion in shock wave and transitional boundary layer interaction for a 24 deg compression ramp at Mach 2.9 is performed by the mean of direct numerical simulation. The blowing and suction disturbances are added upstream at specified stream wise locations to induce the interaction of shock wave with early stage of transitional boundary layer in compression ramp. Firstly, the reliability of the used program is verified. Secondly, the intermittency and oscillation of shock motion are then analyzed in detail. Through analysis of power spectral den-sity of wall pressure signals, effects of separation bubble structure on unsteady motion are studied. Finally, the physical mechanisms of low-frequency oscillation are initially discussed. Results indicate that the unsteady shock motion is highly intermittent, the characteristic of shock oscillation is low-frequency. The time scale is about 10 times the magnitude of fluctuating signals in the incoming boundary layer. Three dimensional structure of separation bubble has little effect on the low-frequency unsteadiness. Based on the low-pass filtered instantaneous flow fields, evidence is found of a correlation between the low-frequency oscillation of shock and the contraction/dilation of separation bubble in the downstream.