M 0 = freestream Mach number p t = pitot pressure p 0 = freestream static pressure t 1st = time of first wind-tunnel running cycle t 2nd = time of second wind-tunnel running cycle
The shock-train transitions in simplified curved isolators are carefully studied by simulation. 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. Comparison 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.
Understanding the hypersonic inlet starting characteristics is the prerequisite for avoiding the abnormal unstart state. To make the work close to the actual situation, an experimental study was performed on a scramjet model at a simulated freestream Mach number of 6.0 with pressure and thrust measurements. The inlet working status is determined by the heat release of the injected ethylene with reciprocating variations. The results show that the critical equivalence ratio (ER) of the restart state is lower than that of the unstart state, which means that the combustion weakens the inlet restart capability and raises the unstart/restart hysteresis phenomena. Specifically, two novel unstart/restart hysteresis phenomena are found: one may come from the dual-solution characteristics of the shock-combustion interaction, and the other may come from the historical effect of reverse flow. Compared to the former type, the latter type requires greater downstream heat release and generates a larger hysteresis loop. In addition, the engine thrust characteristics of the whole unstart and restart processes are analyzed. The thrust increment in the shock-combustion interaction type exhibits nearly linearly. However, the thrust increment meets abrupt changes and strong oscillations in the reverse flow type, accompanied by the reverse flow's formation and disappearance, making the engine more difficult to restart.
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