Relation between Sound Radiated from Underexpanded Impinging Jet and Disturbance Convection Velocity

Abstract

An underexpanded jet perpendicularly impinging on a flat plate was experimentally studied in this study. The flow field radiates noise including screech, and its frequency and sound pressure are varied against the nozzle pressure ratio and the nozzle-plate spacing. A feedback loop in the flow field is known to describe the screech phenomena well, and disturbances moving along the jet boundary play an important role in the feedback loop. This study focused on contribution of the disturbance velocity to the sound pressure variation of the screech. The jet issued from a circular convergent nozzle with a diameter D. Sound pressure measurement and visualization were carried out at different nozzle-plate spacings l/D. The sound pressure measurement showed the screech due to the jet impingement was very weak in a certain range of l/D. The Schlieren photographs described the disturbance did not develop in such range. The disturbance velocity is one of the parameters for the screech phenomena. The disturbance velocity was estimated from frequency characteristics of the sound pressure. Characteristics of the disturbance velocity changed in the weak screech range of l/D, where there are disturbances with different velocities. This denotes the change of the disturbance velocity characteristics attenuated the sound pressure of the screech.