Variation of enstrophy production and strain rotation relation in a turbulent boundary layer

Abstract

The production of enstrophy is strongly coupled to the vortex stretching process that is of inherent importance to the cascading process, one of the driving processes of turbulence in a flow. In this work the enstrophy production mechanism is investigated to identify its variation in the wall-normal direction for the case of a turbulent boundary layer. Production is decomposed into its core quantities including the ratio of the principal strains and the alignment of vorticity with the eigenvectors of the strain rate tensor. The strong variations of these quantities with the wall distance are presented and explained. A self-similar shape of the probability distribution of the enstrophy production is found for regions above the buffer layer. Based on these findings we propose a modification to an existing vortex stretch model that accounts for the wall-normal variation in enstrophy production in a boundary layer. A characteristic decomposition is applied on the turbulence field that allows for the study of the individual production mechanisms of the separate structure types. This analysis reveals a potential backscatter mechanism that transfers kinetic energy from smaller scales towards larger ones, for a structure type described as unstable vortices.