MULTI-MODE STABILITY OF BOUNDARY LAYER OVER A FLAT PLATE CONTROLLED BY MICROGROOVES-SUCTION METHOD

Boundary layer transition can significantly increase the wall friction and heat flow of hypersonic vehicles, which often plays an important role in the design process of hypersonic vehicles. A coupled control method of microgrooves and boundary layer suction was proposed for the control of the hypersonic transition in this paper. The stability of a Ma 4.5 flat-plate boundary layer and the control effects of transition were studied using direct numerical simulation and linear stability theory. The results show that there exist both of the unstable first and second mode without the coupled control, between which the two-dimensional second mode is the most unstable one. Sole microgrooves lead to a significant reduction in the second-mode growth rate and a weak excitation of the first-mode. In contrast, the "microgrooves-suction" method not only enhances the suppression effects of the second-mode wave but also weakens the excitation degree of the first-mode. For the microgrooves located upstream of the synchronization point, the growth of the disturbance wave can still be suppressed in the control region but may be promoted in the downstream; while the "microgrooves-suction" method can effectively avoid the disturbance growth. In addition, with the increase of suction intensity, the unstable region of the second-mode shrinks obviously and the frequency becomes higher; while the first-mode only change slightly. Due to the enhancement of "microgroove absorption" and "acoustic wave scattering" effects caused by the suction, the “microgrooves-suction” coupled method reduces the thickness of boundary layer and enhances the expansion and compression waves system respectively, which achieves an inhibition effect of 12.63% and 28.02% for the growth rate of the first and second mode, respectively. The results show that the "microgrooves-suction" coupled method has the advantages of wide frequency application and flexible location, and also states the achievement of multi-mode control effects in a certain degree.