A PIV Flow Field Investigation of Chevron Nozzle Mechanisms

Abstract

Particle Imaging Velocimetry (PIV) measurements were performed to provide insight into the effect of core chevron nozzles on the flow field of a separate flow exhaust system. This study served as the follow on to previous investigations focusing on the chevron effect on the nozzle near and far field acoustics. Mean flow results showed that the chevron effectively redistributes energy from the high velocity primary stream outward to the lower velocity secondary stream by creating a series of high velocity lobes or secondary lateral jet structures. This leads to a more rapid decay of the peak jet velocity and a consequent reduction in the length of the jet potential core. Local increases of up to 65% in the fan stream velocity were also noted as a result of this increased mixing. The interaction of the high velocity secondary jets with the lower velocity fan stream also produces dramatic increases in turbulent kinetic energy (TKE) near the primary nozzle lip. At an axial distance of 2.5 equivalent diameters, TKE increases of nearly 50% were documented. Comparison of these flow field effects to the previously obtained acoustic results showed clear correlations and identified two primary physical mechanisms of the chevron nozzle, namely, reduced far field low frequency noise due to potential core shortening, and increased high frequency noise due to increased near field turbulence.