Wall Pressure Unsteadiness In An Over-Expanded Single Expansion Ramp Nozzle

To evaluate the unsteady nature of wall pressure in an over-expanded single expansion ramp nozzle, under fixed nozzle pressure ratio (NPR) of 5.92, 6.55 and 7.19, an experimental investigation has been conducted based on focusing Schlieren techniques and dynamic pressure measurement. For all cases, the results show fully formed restricted shock separation (RSS) on the upper wall, which experiences flow reattachment on the wall downstream of separation resulting in the formation of a separation bubble. The separation mode is also RSS on the lower wall at NPR = 5.92. However, the lower wall pressure is randomly larger or lower than ambient pressure near the nozzle exit at NPR = 6.55, the separated shear-layer can intermittently impinge on the lower wall, and the separation mode is partially restricted shock separation (pRSS). At NPR = 7.19, the separation flow does not reattach downstream of the lower wall. That is, it occurs free shock separation (FSS).