Spatial organization of large structures in the turbulent far wake of a cylinder

Abstract

Using an array of ×-probes aligned in the plane of mean shear in the turbulent far wake of a circular cylinder, instantaneous velocity vector patterns are obtained from which stream-function approximations and sectional streamlines are derived. Conditional patterns obtained using different methods for detecting the organized motion are essentially independent of the particular method used. The spatial arrangement of the organized motion about the flow centreline varies in a continuous manner between opposing and alternating modes, the latter being nearly twice as common as the former. Results presented include conditional patterns for the opposing and alternating modes and the relative contributions made by each mode to the Reynolds stresses. A modified Rankine vortex kinematic model, based as much as possible on experimental data and incorporating both modes, yields mean velocity and Reynolds stress distributions which agree well with experiment. A quasi-three-dimensional version of the model implies that large spanwise vortices and shear-aligned double rollers represent the same three-dimensional organized motion from two different viewpoints.