合成冷/热射流控制超声速边界层流动稳定性

为了探究超声速边界层流动稳定性及其转捩控制机理,提出基于合成冷/热射流的边界层速度-温度耦合控制方法,并通过数值模拟研究了Ma=4.5超声速平板边界层不稳定波的传播,采用线性稳定性理论中的时间模式分析了壁面吹吸、射流温度、扰动频率、扰动振幅等对不稳定波控制效果的影响.结果表明:无射流控制时,边界层内同时存在不稳定的第一模态扰动波和第二模态扰动波,且二维波形式的第二模态占主导地位;壁面吹吸作用下,仅出现更加不稳定的第二模态,第一模态被抑制;速度-温度耦合控制下,射流温度对扰动模态的不稳定区域大小及扰动增长率影响显著,射流温度与来流温度不同时,温度的脉动使得流动转捩为湍流的速度加快,边界层速度型更加饱满,抗干扰能力增强,流动稳定性提高;高频的吹吸扰动对流场的控制效果优于低频扰动,扰动频率超过400 Hz时,第二模态扰动波时间增长率降低,扰动分量对边界层速度剖面和温度剖面的修正加快,第二模态更加稳定;扰动振幅减小为主流速度的1%时,仅出现时间增长率较小的第二模态,控制效果较好,进一步减小时,第一模态重新出现,并且波数范围与第二模态先重合后分离,对应的时间增长率先增加后减小.研究结果为边界层转捩控制技术提供了新的思路.

Linear stability of supersonic boundary layer with synthetic cold/hot jet control

To investigate the stability and transition control mechanism of supersonic boundary layer, a coupled method of velocity/temperature control based on synthetic cold/hot jet is proposed. Based on the prior dual-synthetic jet actuator, a high performance synthetic cold/hot jet is achieved by adding a cooling/heating module. By placing the actuator under the flat-plate, periodic blow-suction is produced and low momentum jets are injected into the boundary layer to control the transition. Numerical simulations are conducted to study the propagation and evolvement of the unstable waves in the supersonic flat-plate boundary layer with Ma=4.5. Influences of wall blow-suction, synthetic jet temperature, perturbation frequency, and perturbation amplitude on control effect of the unstable wave are mainly studied. The flow field and control effect are analyzed using the temporal mode of linear stability theory. The results show that without jet control, the first and second mode perturbation wave coexist simultaneously with the second mode dominant in the two-dimensional wave. In the effect of the wall blow-suction, the second mode appears to be more unstable while the first mode is suppressed. Under the control of the coupled speed-temperature, the jet temperature has significant influences on the area of the unstable region and the growth rate of the perturbation mode. When the jet temperature is different from the inlet fluid temperature, the fluctuation of temperature accelerates the transition of laminar flow to turbulent flow, and the velocity profile becomes more full, which leads to a more stable flow field. The control effect of high frequency blow-suction disturbance on flow field are better than that of low frequency. When the control frequency is higher than 400 Hz, the imaginary part of the eigenvalue ω _i of the second mode disturbance wave decreases, and the disturbance component accelerates the correction between velocity profile and temperature profile of supersonic boundary layer, thus making a more stable second mode. When the disturbance amplitude decreases to 1% of the main flow speed, only the second mode is detected of low time growth rate, which results in a better control effect. However, as the disturbance amplitude further decreases, the first mode reemerges, and its wave number overlaps with that of the second mode at first, and then, separates from each other. The research results provide a new idea for supersonic boundary layer transition control from laminar flow to turbulent flow.