Shear-layer-driven transition in a rectangular cavity

Abstract

An experimental study of the flow over a shallow rectangular cavity indicates that, between the states of periodic and fully developed turbulent flow, three different regimes of fluid motion occur as the Reynolds number increases. In the first regime, regime I, the time trace from a pressure transducer, located at the downstream corner of the cavity, varies weakly in amplitude. The frequency spectrum of the trace shows that a single frequency, its’ first harmonic, and a second frequency are selectively amplified. In the second regime, regime II, there is intermittency in the pressure time trace and the two incommensurate frequencies are further apart from each other than in regime I. The primary frequency is the result of a shear-layer instability but the secondary frequency is believed to depend on a transverse wave on the primary cavity vortex. The exchange of fluid between this vortex and the free stream is enhanced when the two waves are constructively interfering and the exchange is attenuated when the two waves are destructively interfering. If the amplitude of the transverse wave is sufficiently large and the two waves are in phase, fluid from the primary vortex is observed to burst through the shear layer giving rise to a period of apparently random motion. In the third regime, regime III, the pressure oscillations vary strongly with time, and include frequent periods of intense irregular behavior. At times, the pressure cycles have double peaks when the vortices that form in the shear layer are partially clipped by the downstream edge of the cavity. This clipping does not, however, coincide with a decay in the shear-layer oscillations, as it does in regime II.