Report on the flow phenomena at supersonic speed in the neighbourhood of the entry of a propulsive duct

Abstract

The present work continued that reported in Ref. 1 and extended some of the results there described to lower entry Mach numbers (1.3 to 1.9). It was found, as in Ref. 1, that with a parallel entry duct followed by a straight divergent diffuser of 10 deg total angle the flow inside the parallel tube was supersonic provided the outlet pressure of the diffuser was less than a certain critical value (about 0.93 of the upstream pitot pressure). In this case the mass flow of air through the tube was equal to that calculated, assuming that all the air incident on the internal section of the tube entry passed through it. For pressures higher than the critical value the flow became subsonic at the duct entry, a shock-wave was formed at the entrance lip and the rate of airflow through the tube decreased. Similar results were obtained for a uniformly divergent tube of 7 deg total angle; in this case, however, an outlet pressure equal to 0.97 of the upstream pitot pressure was attained before the shock-wave left the lip. For outlet pressures less than the critical the flow was supersonic for a distance inside the duct entry depending on the outlet pressure, the flow becoming subsonic further along the duct. The assumption of unidimensional flow in the duct led to results which showed considerable disagreement with the observed pressure distribution more especially in the subsonic flow region. This could be explained by assuming that the flow in this region separated from the duct wall. The results of tests on models of two forms of annular entry (the Q1 and E24/43 entries respectively) showed that two types of flow rdgime were possible depending on the outlet pressure. For low outlet pressures a shock-wave was formed at the lip of the entry and the flow passed into the entry but for higher outlet diffuser pressures there was no distinct shock-wave from the annular lip and the flow through the annulus was reversed. The outlet diffuser pressure at which the flow changed direction for each entry tested was about 0.5 of the free-stream pitot pressure and was thus considerably lower than for the unobstructed type of duct. With the airflow passing into the annulus the flow through the Q1 entry was independent of the outlet pressure over the range of Mach numbers tested and was about 0.88 of the flow through an area equal to the intake area in the free stream. For the E24/43 entry the airflow decreased as the outlet diffuser pressure increased probably due to changes in the boundary-layer thictmess at the annulus.