Analysis of non-axisymmetric underexpanded jet flow fields

Abstract

A study has been performed that involved the experimental testing and numerical analysis of the underexpanded free jet °ows issuing from rectangular, elliptical and slot nozzles. Aspect ratios of 1, 1.5, 2, 3 and 4 were tested at pressure ratios (exit plane pressure to ambient pressure) of 2 and 3 for rectangular nozzles. In the case of the slot and the elliptical nozzles the aspect ratios of 1.5, 2, 3 and 4 were tested at the same pressure ratios. There is reasonable qualitative agreement between the experimental observations and the numerical predictions. In the case of rectangular jets, a complex system of shock waves forming the incident shock system was discovered. This shock wave system originated at the corners of the nozzle exits, and proceeded to propagate downstream. Mach re°ections have been discovered on the incident shock wave surface of the underexpanded jet from rectangular nozzles. The incident shock system has also been observed to undergo a Mach re°ection in the direction of the jet °ow. The resulting Mach disk has had a cross-sectional shape of a square, a hexagon and octagon in some of the °ow ¯elds from the nozzles that were tested. For slot and elliptical jets the formation of the incident shock wave was not observed along the minor axis plane of the nozzle for aspect ratios > 2. The incident shock wave was observed to originate downstream of the nozzle exit in the major axis plane. In cross-sectional slices it has been established that the incident shock wave undergoes a transition to Mach re°ection as it propagates downstream of the nozzle exit. In all cases tested, the shape of the jet boundary has been signi¯cantly distorted. In rectangular jets the narrowing of the jet boundary along the diagonal axis of the nozzle exit has been observed, and in the case of the elliptical and slot jets axis switching has been observed on the cross-sectional shape of the jet boundary.