Open-ended shock tube flows: Influence of pressure ratio and diaphragm position

The influence of the pressure ratio and the diaphragm location on the flow from open-ended shock tubes is investigated. In contrast to previous studies, in which attention was focused on the discharge of the shock wave from the shock tube, we consider also the influence of the contact discontinuity and the expansion fan. It is found that if the pressure ratio is large enough to lead to supersonic flow behind the contact discontinuity, the flow at the open end relaxes from the conditions behind the contact discontinuity to sonic conditions once the tail of the expansion fan arrives at the open end. Theory indicates that the time scale over which the flow relaxes to sonic conditions is nearly independent of the initial Mach number. Also, the time scale is much longer than that required by the acceleration of subsonic conditions behind the contact discontinuity to sonic conditions. The relaxation process is shown to influence the evolution of the Mach-disk shock, the barrel shock, and the reflected shock wave in an underexpanded jet.