On streamwise vortices and their role in the development of a curved wall jet

Abstract

The streamwise growth of longitudinal vortices in a curved wall jet was investigated experimentally, mostly through Particle Image Velocimetry (PIV). This growth was partially achieved through amalgamation of neighboring vortices having the same sense of rotation and partially due to centrifugal instability, thus the average spacing of the vortices far downstream was independent of the protuberances (micro vortex generators) that initiated them at the slot lip. The spacing of these protuberances along the span established only the preferred distance from the nozzle where vortex amalgamations occurred. The streamwise vortices initiated spanwise undulations in the mean flow creating a radial component of vorticity that when examined at a given distance from the surface was found to be inviscidly unstable, and could enhance further this component of vorticity. A strong coupling was observed between plane periodic excitation of the nozzle-flow and the small stationary protuberances that were placed at the nozzle in the presence of curvature. This coupling enhanced the coherence and strength of the longitudinal vortices far downstream from the nozzle, while in the absence of curvature these structures simply decayed.