Experimental investigation of shock train oscillation suppression by a plasma jet in a supersonic isolator

Abstract

In a real ramjet engine, combustion instability gives rise to an unstable combustion pressure, which in turn disrupts the shock train in the inlet isolator, thereby introducing operational risks. This paper investigates the potential of the plasma jet (PJ) in supersonic flow control through a series of cold flow tests conducted at constant and incremental back pressure at Mach 2.5 freestream flow, respectively. As the pressure of the working material increases, the stable core region of the PJ expands, thereby demonstrating enhanced control capabilities. In the supersonic inlet isolator, the vertically injected PJ is compressed and induces a strong shock wave that dominates the downstream flow field. The shock wave induced by the PJ interacts with the leading-edge shock wave and results in a slight upstream movement of the shock train. The self-excited oscillation is suppressed with the implementation of the plasma actuator. The spatial FFT analysis indicates a reduction in both the oscillation frequency and the oscillation range of the leading-edge shock wave, with a suppression rate of 37.44% for the range. The PJ facilitates the momentum exchange near the shear layer and redistributes the oscillation energy. The dominant shock wave propagates the oscillation energy to the lower wall, thereby equalizing the energy distribution. Further studies on forced moving features demonstrate that the PJ effectively reduces low-frequency oscillation energy by at least 43.63% under incremental back pressure and achieves better control efficiency closer to the shock train. The high-frequency environment generated by the PJ and the shock wave induced by the PJ are considered the important factors in suppressing both self-excited and forced-excited oscillation of the shock train.