Experimental study of acoustic radiation from a boundary layer transition region

Abstract

Wall pressure fluctuations were measured on a rigid axisymmetric body in the CEPRA 19 low‐noise, anechoic wind tunnel, using flush‐mounted microphones placed from the laminar region to the fully turbulent boundary layer. Microphones placed in the laminar flow region are used to detect noise radiated from the transition region, which occurs naturally, without separation, under a slightly positive pressure gradient. Cross‐spectral analyses show upstream acoustic propagation in a very wide frequency band, 4–30 kHz, detected in the laminar region. A method of conditional analysis is then used to establish the sequence of events from the onset of near‐harmonic instability wave packets to the generation, about 10 ms later, of turbulent spots leading to the acoustic emission. This intermittent acoustic radiation is detected in the nearfield for wind velocities ranging from 20–70 ms. Farfield detection was not achieved probably because of instrument limitations and propagation effects. [Work supported by DRET, Direction des Recherches et Etudes Techniques.]