Shock train behavior affected by continuous Mach number variation of incoming flow

Experimental and numerical investigations have been performed to study the influence of continuous Mach number variation on the shock train. An adjustable Laval nozzle which could continuously change the throat area was developed to change the Mach number from 2.4 to 1.8. As the incoming flow Mach number decreased, the background waves moved upstream. The movement of positive and negative pressure gradient regions caused by background waves resulted in different pressure histories at different wall positions. The pressure history at a position could be used to determine whether shock reflection points passed through this position and how many shock reflection points passed through this position. During the Mach number decreasing, the shock train first moved downstream and then upstream, always accompanied by oscillations. Two types of oscillations and one type of jump movement were observed during the movement of shock train, and the first type of oscillation occurred interlaced with the second type of oscillation or jump movement. The first shock of shock train changed from X-shaped to 2-shaped due to the decreasing of Mach number. The structure of shock train head became more complicated when the first shock was A-shaped, because the background shock would interfere with different parts of the 2-shaped shock. In addition, the mechanism of shock train movement affected by Mach number variation was discussed. It was found that the Mach number decreasing of incoming flow had two opposite effects on the shock train movement, including the decreasing of pressure boosting capacity of a single shock, which caused the shock train to move upstream, and the upstream movement of background waves, which caused the shock train to jump downstream. Whether the shock train moved upstream or downstream depended on which effect dominated.