Experimental investigation of mechanism and limits for shock train rapid forward movement

For a scramjet engine, forward motion of a shock train cannot be avoided. A rapid movement of the shock train has been found, which occurs during its forward climbing, but the deep mechanism is still not clear. Few works focus on the conditions and limits that trigger a rapid motion of the shock train. So, mechanism and limits for a shock train rapid forward movement were investigated in this paper. Experiments were carried out in a Mach 3 directly connected wind tunnel with background waves. To generate the background waves similar to a typical hypersonic inlet-isolator, a small wedge was installed at the entrance of the isolator. The rapid forward movements of the shock train were captured in the experiment. The results indicated a local throat-like shape was formed by the separated flow at the head of shock train, and the contraction ratio of the throat varied during the transition of the motion states. The contraction ratio was reduced to a small value, when the shock train went into a rapid forward state. And the rapid forward movement was kept until the contraction ratio turned to a higher value. Based on the experimental data, the mechanisms of these motion states transitions were discussed with flow mechanism. The analysis method for the limits that triggers a shock train rapid motion was discussed based on the fluid physics. The minimum contraction ratio of throat-like shape at shock train head that triggers the rapid movement was found, which has been validated by extra experiments.