Numerical Study of Mechanism of U-shaped Vortex Formation

TL;DR

This study uses experiments and direct numerical simulation to reveal that U-shaped vortices are part of coherent vortex structures, formed by tertiary streamwise vortices induced by secondary vortices, challenging previous theories.

Contribution

The paper introduces a new understanding of U-shaped vortex formation as part of coherent vortex structures, differing from existing theories that link them to turbulence spot development.

Findings

U-shaped vortices are tertiary streamwise vortices induced by secondary vortices.

They serve as a second neck supplying vorticity to ringlike vortices.

The formation mechanism differs from the theory linking them to turbulence spot breakdown.

Abstract

This paper illustrates the mechanism of U-shaped vortex formation which is found both by experiment and DNS. The main goal of this paper is to explain how the U-shaped vortex is formed and further develops. According to the results obtained by our direct numerical simulation with high order accuracy, the U-shaped vortex is part of the coherent vortex structure and is actually the tertiary streamwise vortices induced by the secondary vortices. The new finding is quite different from existing theories which describe that the U-shaped vortex is newly formed as the head of young turbulence spot and finally break down to small pieces. In addition, we find that the U-shaped vortex has the same vorticity sign as the original {\lambda}-shaped vortex tube legs and serves as a second neck to supply vorticity to the ringlike vortex when the original vortex tube is stretched and multiple rings are generated.