Vortex pairs and rings interacting with shear-layer vortices

Abstract

In order to understand how free-stream turbulence affects the vortex-pairing mechanism in free-mixing layers, water-flume experiments have been performed on the interaction between single vortex pairs (with circulation Γp) and also vortex rings propelled towards the rolling-up vortices (of circulation Γh) in a two-dimensional separated shear layer. The Reynolds number of an obstacle generating the shear layer is about 103 and of the vortex pairs and rings about 10–100. Our observations show that, if the ratio Γp/Γh exceeds about 1.4, the vortex pairs break away any shear-layer vortices in their path and if Γp/Γh is less than about 0.5 the shear-layer vortices destroy the vortex pair. But if Γp/Γh = 1.0 ± 0.4 the vortex pairs and shed vortices interact strongly, for example by a pairing between one vortex of the pair and the shed vortex, by the vortex pair eliminating coalescence of vortices in the shear layer, or by combining with the rolling-up vortices to generate large vortices. Qualitatively similar effects are observed with the vortex rings.Numerical calculations of the interaction between an array of vortices on a line and a vortex pair are described. The results are similar to those of the physical experiments, in particular displaying the same sensitivity to Γp/Γh at Γp/Γh ≈ 1.0. It is suggested that these results demonstrate one important way in which free-stream turbulence interacts with shear layers and how shear layers interact with other vortices.