The uniform distortion of a turbulent wake

Abstract

The effect of a uniform and constant rate of strain imposed upon a turbulent wake which is generated behind a circular cylinder has been examined experimentally. The strain field, produced by passing the flow through a distorting duct of constant cross-sectional area, simultaneously compresses the wake in the plane of the cylinder and extends it in the other lateral direction but applies no strain in the direction of the main stream. The investigation shows the existence of a large-scale, periodic eddy structure within the strained wake, similar to, but more intense than, that which was observed for the undistorted wake. These large lateral convective motions appear to be the primary mechanism controlling the development of the flow, and are responsible for the failure of the wake to attain the theoretically predicted self-preservation state. It is shown that the strain rate, which acts favourably upon these large eddies, is sufficient to produce a net increase in vorticity in spite of the ordinary processes of turbulent viscous decay. At the same time, however, the rate of viscous dissipation of the smallest eddies in the flow is augmented by the strain, and the resulting wake structure is almost entirely dominated by the large-eddy motions. A possible means by which the large eddies are initiated is discussed.