Corewise cross-flow transport in hairpin vortices—The ‘‘tornado effect’’

Abstract

There is increasing awareness that even in fully turbulent boundary layers, large-scale structures in the form of hairpin vortices abound. Although their implications are not all that clear at the present time, they seem to play an important role in turbulent flows. Due to the inherent unpredictability of hairpin vortices in their natural state, in the past effort has been made to generate synthetic hairpin vortices in a laminar boundary layer; from their study considerable insight into the processes underlying various features of turbulent flows has been gained. Contrary to those preceding studies where attention has been directed to the flows external to hairpin vortices, interest here is focused solely upon their interiors: the possible existence of cross-flow transport inside the cores of the hairpin legs. In synthetic hairpin vortices, the presence of such a corewise transport away from a wall surface, or the ‘‘tornado effect,’’ is substantiated in a water tunnel with flow visualization techniques. The effect is also verified using heated fluid injected near the wall surface by measuring the temperature at various points along the hairpin vortex core.