Flow characteristics of supersonic mixing layer under forced vibration

Abstract

Supersonic mixing layer is one of the most important phenomenon of turbulent flow, which occurs in scramjet, ejector and supersonic airfoil. With the development of efficient propulsion systems, rapid mixing of two high-speed streams in a short distance always receives a great deal of attention all over the world. There are a lot of literatures that investigated flow characteristics and mixing efficiency of supersonic mixing layer. To evaluate the compressibility level of mixing layer, convective Mach number ( Mc ) was proposed by Papamoschou, their experimental study revealed that Mc had an important effect on the growth rate of mixing layer. By employing Schlieren and PLIF techniques, Rossmann found that with the value of Mc risen, the thickness of mixing layer had strong reduction. Considering the low mixing efficiency of supersonic mixing layer, many methods were proposed to control supersonic flow and increase mixing efficiency. By introducing chevrons to supersonic flow, Callende et al. found that the mixing efficiency was strongly enhanced, while the pilot loss could not be ignored. The experiments done by McLaughlin et al. revealed that an effective way to promote mixing was to use glow discharge excitation system in a supersonic mixing layer. In their experimental study, a high oscillating signal was passed through the copper electrode insulted from the aluminum trailing edge, which produced a glow between the copper and the aluminum when a threshold voltage was reached. This glow brought about a very high temperature disturbance locally, slightly perturbing the flow adjacent to the electrodes. However, flow fields of supersonic mixing layer under conditions of vibration have not been thoroughly researched, partly because of the complexity of fluid-structure interaction (FSI). Flutter of airfoil is one example of vortex induced vibration, and can cause severe damage. By employing PIV technique, Kim et al. investigated the flow structures induced by the vibration of cantilever, the contours of velocity field were displayed with the Reynolds numbers of 101, 126 and 146 respectively. However, due to the equipment limitations, they could not get the characteristics of turbulent flow. In the present study, forced vibration which decouples the FSI is applied in the supersonic mixing layer through a vibration shaker, and flow characteristics under different vibration conditions are experimentally and numerically investigated. Besides, through employing edge detection technology, the thickness of supersonic mixing layer along the flow direction is displayed, which demonstrates that forced vibration is responsible for the increase of growth rate.