Interaction of a shock train with inherent isentropic waves in a curved isolator

Abstract

Shock-train transitions in simplified curved isolators are carefully studied by simulation. The results show the shock-train behavior is subject to the complex pressure field created by the duct deflection, eventually presenting five modes during a backpressure-varying process. Of them, the most special one is the abrupt shock-train leap. It appears as the leading shocks interact with an adverse pressure gradient and follows a different path after a reversal of the direction the backpressure takes, which causes a shock-train hysteresis. If the curvature increases, the leap phenomenon, together with the related hysteresis, grows in number and intensity. Analysis indicates the background pressure gradients stem from the inherent left-running expansion waves and right-running compression waves. They control alternately the near-wall flow state, provoking the cyclic changes in the pressure gradient sign. Unlike the former, the latter can enhance separation through a positive feedback mechanism, rendering the shock train highly sensitive to backpressure. This is why the leap occurs. Comparing with the previously reported shock-induced leap indicates that there is a marked similarity in their behaviors, suggesting the irrelevance of the occurrence of a leap to the category of incident waves. Nevertheless, a delay in the onset usually follows a compression-wave-typed leap, which reflects that there is a triggering threshold for an incident wave. Given the fact that no local separation is provoked by the compression waves, it is speculated that the threshold should lie below the criterion for causing a separation, as opposed to the impression from the previous research.