Acoustic Characteristics of Supersonic Jets from Grooved Nozzles

Abstract

The effect of internal grooves cut along the inner surface of the diverging portion of a Mach 1.8 convergingdiverging nozzle on the characteristics of an axisymmetric jet was investigated experimentally. Decay, growth, and noise suppression characteristics of supersonic jets from plain nozzle, nozzle with two semicircular grooves, and nozzle with two square grooves are presented. The grooves act as effective passive controls, resulting in significant enhancement of jet mixing. The shock cell structure from grooved nozzle is weaker than that of plain nozzle. Acoustic measurement was taken in the nozzle exit plane and in the far field. In the grooved plane grooves show a definite advantage in terms of jet noise attenuation. However, in the plane normal to the grooved plane they are not effective in screech suppression. Further, the present results authenticate that nozzle pressure ratio plays an important role in the case where an adverse pressure gradient exists near the nozzle exit. Nomenclature M = nozzle-exit Mach number Pa = ambient pressure Pc = jet centerline pitot pressure P0 = stagnation pressure in the settling chamber X = coordinate perpendicular to the nozzle-exit plane Y = coordinate parallel to the grooved plane Z = coordinate normal to the grooved plane I. Introduction T HE passive control scheme investigated in this study is based on the modification of the boundary layer, growing along the nozzle inner walls achieved through partial notches. Streamwise vortices generated by the notches cut at the nozzle exit have been demonstrated to be effective in jet noise reduction. 1 These vortices provide the necessary secondary instabilities that aid the faster amplification of the primary instabilities and hence the growth of the coherent structures. In effect, the streamwise vortices bring in threedimensionality to the otherwise, basically, two-dimensional spanwise organized vortical structures (coherent structures). Thus the evolution of the large-scale structures gets altered by the notches, which, in turn, alter the mixing and acoustic characteristics of the jet. In view of the effectiveness and simplicity of the notched axisymmetric nozzles, they were investigated in the present study. Similar studies have been conducted by many researchers. 1−3 Pannu and Johannesen 1 investigated underexpanded jets issuing from notched nozzles. The centerline pitot-pressure data indicated that the shock cell structure was modified and the jet decayed faster than the unnotched nozzle flow beyond the core region. They demonstrated that the dominant feature of the flow which determined the structure far downstream was the trailing vortices shed from the swept edges of the notches. They concluded that the notches were effective silencers, mainly because they caused the noise sources to be surrounded by a broad region of low-speed turbulent flow. Smith and Hughes 2 presented experimental results obtained for jets from notched nozzles in a coflowing freestream. The results showed