A study of streamwise vortex structure in a stratified shear layer

Abstract

The existence of an organized streamwise vortical structure, which is superimposed on the well known coherent spanwise vorticity in nominally two-dimensional free shear layers, has been studied extensively. In the presence of stratification, however, buoyancy forces contribute to an additional mechanism for the generation of streamwise vorticity. As the spanwise vorticity layer rolls up and pulls high-density fluid above low-density fluid, a local instability results. The purpose of the current investigation is to force the three-dimensional instability in the stratified shear layer. In this manner, we experimentally observe the effect of buoyancy on the streamwise vortex tube evolution, the evolution of the buoyancy-induced instability, and the interaction between these two vortical structures. A simple numerical model is proposed which captures the relevant physics of the flow evolution. It is found that, depending on the location, streamwise vortices resulting from vortex stretching may be weakened or enhanced by the stratification. Buoyancy-induced vortex structures are shown to form where the unstable part of the interface is tilted by the streamwise vortex tubes. These vortices strengthen initially, then weaken downstream, the timescale for this process depending upon the degree of stratification. For initial Richardson numbers larger than about 0.03, the baroclinically weakened vortex tubes eventually disappear as the flow evolves downstream and the baroclinically generated vortices dominate the three-dimensional flow structure.

The effect of Prandtl number on mixing in low Reynolds number Kelvin-Helmholtz billows

“…At P r = 1, the density structure of the billow has been mainly destroyed by diffusion at t * 2 . The highly structured density field of the P r = 700 billow resembles the density field of the billows observed in the experiments of Schowalter et al (1994) with P r ∼ 1500 and Atsavapranee and Gharib (1997) with P r ∼ 600, where the billow consists of layers of rolling fluid with very sharp interfaces. At P r = 700, very fine structures have formed, with very little diffusion.…”

Transition to Turbulence in Shear above the Tropopause

“…Recent investigations of Kelvin‐Helmholtz instability have found that the transition to turbulence involves vortices that are elongated along the direction of shear and wrap around the overturning billows. This was originally anticipated by Klaassen and Peltier [1985b], and has since been demonstrated in both the laboratory [ Thorpe , 1985; Showalter et al , 1994] and numerical simulations [e.g., Palmer et al , 1996]. We would expect to see this process manifested in the airborne measurements as three‐dimensional motion developing along the direction of shear, initially as coherent oscillations of the cross‐shear component of the wind.…”

The Mechanics of Marine Sediment Gravity Flows

“…parallel to the front). Using a stratified-mixing-layer analogue, Parsons (1998) noticed a similarity to the Klaasen-Peltier instability observed in stratifiedmixing layers (Schowalter et al, 1994). The KlaasenPeltier instability occurs when dense fluid is forced over light fluid by the Kelvin-Helmholtz instability in a stratified flow (Klaasen & Peltier, 1985).…”