Quadrupole directivity of jet noise when impinging on a large rigid plate

The aerodynamic sound generated by a subsonic jet normally and obliquely impinging on a large, rigid, flat plate was studied at the Aerosonics Laboratory at UCLA. Correlation techniques were used to focus on source regions and to discriminate against such signals as reflections. Correlations between the signals from both the surface transducers on the plate and hot-wire sensors near the plate, and far-field microphone signals were measured at different jet speeds (Mach number=0.3–0.9), nozzle sizes (1, 11/4, and 11/2 in.), nozzle-to-plate distances (3–22 nozzle-diameters) and impinging angles (0°, 15°, and 30°). For each case, the microphone was placed at several locations around the jet, so that the directivity pattern was obtained. The directivity results showed that the correlation approach resulted in quadrupolelike directivity patterns, suggesting such sources with approximately fixed orientation. The sound intensity had a minimum at around 45° around the plate normal, independent of the impinging angle. This is the only instance of directly observed quadrupole patterns (four-leafed clover) for free jets which the authors know of. Correlation techniques are required, or else the sources overlap the directivity patterns and nearly smear out the maxima and minima.