Single Expansion Ramp Nozzles: Impact of Ramp Length on Flow and Acoustics

This paper investigates the effect of external expansion ramps on supersonic flow emitted from high-aspect-ratio rectangular nozzles with a focus on additional sources of noise due to ramps as well as their effectiveness as noise reduction devices. Schlieren videos showed that the extended ramps stabilized the jet, reduced spreading, and elongated the shock cell systems. Far-field acoustics revealed that the most severe noise penalties arose from the impingement of the shear layer on the ramps and the expansion of the jet at the trailing edge of the ramp. For transonic jets, which contain only broadband sources of noise, noise due to ramps was louder than turbulence mixing noise. The penalty increased with the ramp length. On the contrary, the ramps were effective at reducing noise at higher jet Mach numbers, where the shock-associated components of noise were strong and turbulence mixing noise dominated jet scraping noise. The extent of reduction was dependent on the relative length between the ramp and the jet potential core. A ramp approximately as long as, but no longer than, the potential core minimizes peak mixing noise, shock-associated noise, and noise arising from the interaction between flow and the ramp.