Supersonic jet noise reduction by a porous single expansion ramp nozzle

A shock noise reduction concept for supersonic rectangular jets is investigated for high speed civil transport (HSCT) applications. The noise reduction efficiency of a porous single external expansion ramp nozzle (SERN) with a 5:1 aspect ratio is experimentally investigated at both design and off-design pressure ratios. Porosity is implemented as a regular array of discrete holes in the ramp surface. Near- and far-field acoustic measurements, axial static pressure profiles, and Schlieren flow visualization are used to characterize the noise reduction mechanisms of the porous ramp. Solid ramp configurations are shown to dramatically reduce screech tones relative to a free rectangular jet. Surface porosity produces additional noise reduction by decreasing shock cell strength and consequently the magnitude of shock-associated noise. At a typical takeoff condition, approximately three EPNL dB reduction is estimated with 8 percent porosity ramp relative to no-ramp configuration.