On the stability of parallel shear flows with embedded swirl

Abstract

The temporal stability of inviscid parallel shear flows with embedded swirl is investigated. The base flow is divided into three regions: an inner irrotational jet flow, a thin swirl layer on the outer rim of the nozzle, and an external potential flow outside the nozzle. The swirl distribution in the thin shear layer is of solid-body rotation and of Rankine type outside the nozzle. The axial velocity profile in the jet base flow is assumed to be top-hat. The linear dispersion relation was derived and solved numerically. A parametric study on the effect of swirl parameter, S, and the swirl layer thickness fraction, d, on the amplification rates of the unstable modes is performed. A helical instability wave with azimuthal wave number, m = –3, is the most unstable mode with S = 0.8 and  = 0.08. Our results indicate that the embedded thin layer swirl is capable of triggering the centrifugal instability waves to promote jet mixing. This powerful technique may be employed to mitigate jet noise without incurring significant thrust loss.