Penetration Characteristics of Pulsed Injection into Supersonic Crossflow

This report experimentally and numerically investigated the penetration characteristics of the pulsed injection into a supersonic crossflow. In the experiment, we injected helium gas with the pulse rise time of 250 μs and the pulse width of 750 μs into Mach 2 supersonic crossflow-air. One hundred time-series schlieren images captured the pulsed jet motion and investigated its penetration performance. The images revealed the pulsed jet penetration in a rising phase of injection pressure was higher than that in a declining phase of injection pressure at a certain value of injection pressure. Two-dimensional unsteady RANS simulations injecting air well captured the trends on the jet penetration observed in the experiment. They revealed the hysteresis of pulsed jet penetration was owing to the upward flow produced by the pulsed jet like a slug which inclined toward the injection wall. A counter-rotating vortex pair was generated in front of the slug and induced the upward flow. The upward flow deformed the jet shapes as it traveled downstream. As the result, the hysteresis of pulsed jet penetration occurred in a low regime of pulsation frequency below 5 kHz. The size and strength of vortex pair in the pulsed jet were greatly depended on the pulsation frequency. The vortex rings was generated in each pulse cycle with the pulsation frequency above 10 kHz. They remarkably increased the jet penetration and promoted mixing of the injectant with crossflow-air, compared with the steady mode at the peak injection pressure of the pulse mode. For further higher pulsation frequency, the jet penetration rapidly decreased with increasing the pulsation frequency. The penetration height approached to that in the steady mode of the average injection pressure in a pulse cycle, because an interval between the neighboring vortex rings became close to weaken the upward flow.