NPR Nozzle Pressure Ratio r Radial distance

This paper describes a joint study of the acoustics of conical convergent-divergent (C-D) nozzles such as those found on high-performance military aircraft. In this paper we examine the influence of chevrons on a nozzle with an area ratio corresponding to a design Mach number of 1.5 (design pressure ratio of 3.67). The nozzle is tested at its design condition and at pressure ratios of 2.5, 3.0 and 3.5 representing overexpanded conditions and 4.0, 4.5 and 5.0 representing underexpanded conditions. Each case is compared to a baseline nozzle at the same condition without chevrons. Shadowgraph images show that chevrons reduce the shock cell spacing, and introduce additional weak shock waves. Far-field acoustic measurements show that the application of chevrons reduces screech, broad-band shock-associated noise and mixing noise except for frequencies above the broad-band shock-associated noise peak for over-expanded and perfectly expanded conditions. Near-field acoustic reveal that chevrons produce significant noise near the nozzle exit across the whole range of frequencies, but reduce mixing noise, and broad-band shock-associated noise elsewhere and they all but eliminate screech