Acoustics from a rectangular supersonic nozzle exhausting over a flat surface.

A jet exhausting over a plate at different distances away from the nozzle was investigated to simulate jets exhausting over airframe surfaces and the jet-ground interaction during take-off and landing operations. A supersonic rectangular nozzle of 2:1 aspect ratio and 1.5 design Mach number was tested with and without the plate. Far-field acoustics from the cold and heated jets at over-expanded, design, and under-expanded conditions were measured at the reflected, sideline, and shielded azimuthal directions. When the plate starts at the nozzle exit, the "scrubbing" and "scattering" noise from the surface-jet interaction was observed at the low-end frequencies of the reflected and shielded measurements and increased as the plate approached the jet. In the sideline, the plate attached to the nozzle exit diminished the broadband shock-associated noise while the shadowgraph results showed a connection to weakening of the shock-cell structures. In the cold jet, screech was mitigated with the plate attached at the nozzle exit. When the plate was moved away from the nozzle, screech tones were intensified at the under-expanded condition. Crackle levels were significantly intensified in the sideline within a range of plate positions. Noise levels in the shielded region were considerably lower due to the shielding effect.