Freestream effects on base pressure development of an annular plug nozzle

An experimental investigation has been carried out to study the effect of freestream flow and cowl-length variation on (i) upstream flow interference effects and (ii) the base wake-closure nozzle pressure ratio. It is observed that for supersonic freestream Mach numbers the nozzle exhaust seems to only slightly influence the upstream interference effects for M = 1.2 but shows significant influence for M = 1.6. Increasing the cowl-length further reduces the upstream flow interference effects significantly. Further, the reduced momentum thrust from the inner nozzle in the presence of freestream for similar nozzle pressure ratio (relative to static tests) delays the downstream movement of the system of shocks on the plug surface. In the case of the plug truncated at 40% length, this delays the onset of base-wake closure and hence, increases the base-wake closure nozzle pressure ratio with increasing freestream Mach number. Increasing the cowl-length also helps to increase the base pressure thrust contribution at all operating conditions.