COREWISE CROSS-FLOW TRANSPORT IN HAIRPIN VORTICES - THE TORNADO EFFECT

There is increasing awareness that even in fully turbulent boundary la yers, large-scale structures in the form of hairpin vortices abound. A lthough their implications are not all that clear at the present time, they seem to play an important role in turbulent flows. Due to the in herent 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 int o the processes underlying various features of turbulent flows has bee n 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-f low transport inside the cores of the hairpin legs. In synthetic hairp in vortices, the presence of such a corewise transport away from a wal l surface, or the ''tornado effect,'' is substantiated in a water tunn el with flow visualization techniques. The effect is also verified usi ng heated fluid injected near the wall surface by measuring the temper ature at various points along the hairpin vortex core.