Vorticity dissipation associated with vortex pairing within a curved shear layer

Abstract

In this study, vorticity dissipation phenomenon within a curved shear layer while Karman vortices are formed behind a body was investigated experimentally based on time variation of energy dissipation function distribution. The vorticity dissipation within a separated shear layer would be caused by viscous effect associated with shear strain in the process of pairing between two vortices formed by Kelvin-Helmholtz instability. Therefore, in this experiment, the vortex pairing phenomenon within a separated shear layer was reproduced within a centrifugally stable curved free shear layer. Time variation of velocity distribution in one period of vortex pairing process was obtained by means of L.D.V. and phase ensemble averaging technique. By differentiating the velocity distribution for each phase of pairing process, time varying distributions of vorticity, shear strain rate and energy dissipation function were obtained and investigated. As the results, it was found that the strong shear strain occurs and energy is dissipated significantly when the vortices are started to starch in the process of pairing. Also, it was found that the diffusion of vorticity from the vortices to surrounding irrotational fluid becomes significant after the vortices are fully stretched, instead of in the process of stretching. Moreover, it was found that the energy dissipation by shear strain is almost ended when the pairing vortices align vertically, that is defined as the "merging location."

Keywords

• vorticity dissipation
• vortex pairing
• curved shear layer
• vorticity
• shear strain rate
• energy dissipation function
• ldv
• phase ensemble averaging technique