Nonsteady Supercritical Discharge Through an Orifice

Previous studies of shock reflection from open-ended duct configurations indicate that a steady discharge is not instantaneously formed and that the effects of this lag may occasionally be important. A theory is available which satisfactorily describes the lag effects in subcritical flow, but its validity for supercritical flow has not previously been verified. Shock-tube experiments are therefore carried out to study the lag effects in supercritical flow from a sharp-edged orifice. The incident shock wave either modifies an initial supercritical discharge, or establishes such a discharge with the gas initially being at rest. Schlieren photographs show a violent transition of the flow downstream of the orifice that lasts several milliseconds. Pressure records taken inside the duct indicate a small, but distinct, pressure rise that also lasts for several milliseconds following the passage of the reflected shock wave. It is shown that this apparent agreement of the transition times is accidental. A method is described to evaluate the effect of boundary-layer growth on the pressure behind the reflected shock wave, and the results indicate that the entire observed pressure rise is accounted for by this effect. Consequently, flow adjustment in the orifice may be considered as instantaneous for all practical purposes.