On the inherent bias of swirling strength in defining vortical structure

Abstract

The traditional practice of using rotational motion as the principal attribute of coherent vortical structures in the buffer region of near-wall turbulent flow is shown to create a biased accounting of the role of vorticity within the structures. Vorticity associated with rotation is given a favored consideration against vorticity that is equally strong but not associated with rotation. Using data from a highly resolved direct numerical simulation of channel flow, it is shown that describing the structures based on the properties of the rotational field leads to a distorted view of the actual structures that are present. As a practical matter, this means that where hairpins are typically considered to be the flow structures, a more accurate description of the coherent events is that they are elongated mushroom-shaped vortical objects ejecting over low speed streaks. In this, hairpin-shaped rotational regions are embedded in the lobes of the mushrooms.