DNS Study on Vorticity Structures in Late Flow Transition

TL;DR

This study investigates the formation and development of vorticity structures, especially vorticity rings, during late flow transition, revealing their origins, growth mechanisms, and relation to vortex formation.

Contribution

It introduces the concept of self-closed vorticity rings within flow transition and explains their generation and role in vortex development.

Findings

Vorticity rings are generated inside the domain during flow transition.

Vorticity rings are found around hairpin vortex heads and legs.

Vorticity line stretching, tilting, and twisting drive vortex buildup.

Abstract

Vorticity and vortex are two different but related concepts. This paper focuses on the investigation of vorticity generation and development, and vorticity structure inside/ outside the vortex. Vortex is a region where the vorticity overtakes deformation. Vortex cannot be directly represented by the vorticity. Except for those vorticity lines which come from and end at side boundaries, another type of vorticity, self-closed vorticity lines named vorticity rings, is numerously generated inside the domain during flow transition. These new vorticity rings are found around the hairpin vortex heads and legs. The generation and growth of vorticity rings are produced by the buildup of the vortices according to the vorticity transport equation. On the other hand, vortex buildup is a consequence of vorticity line stretching, tilting and twisting. Both new vorticity and new vortices are generated during the flow transition. According to the Helmholtz vorticity flux conservation law, vorticity line cannot be interrupted, started, or ended inside the flow field, the newly produced vorticity has only one form which is the vorticity rings. In addition, an interesting finding is that a single hairpin vortex consists of several types of vorticity lines which could come from the side boundaries, whole vorticity rings and part of vorticity rings.