Large Scale Motion in Turbulent Boundary Layers

Abstract

Experiments reported in the last five years confirmed more and more the existence of large scale coherent motion in turbulent boundary layers as well as in other turbulent shear flows. The principal experimental tool was the hot-wire anemometer followed by novel signal processing techniques. Conditional sampling and averaging, as well as the mapping out of double space-time correlations for different flow variables (velocity components, inter-mittency function, temperature, etc.) (Ref. 1), have indicated that the large scale flow structures (eddies) are relatively long lasting entities that may extend all across the layer’s thickness in the direction of the highest gradient, namely perpendicular to the wall. They may be quite elongated in the flow direction and their inferred average life-times correspond to time intervals that allows them to travel downstream with the free stream velocity as much as 10–20 boundary thicknesses.