Simple three-dimensional vortex motions in coflowing jets and wakes

Abstract

The vortex patterns which occur in coflowing jets and wakes at moderate Reynolds numbers (of order 500) are examined in detail. Flow visualization is used in conjunction with a flying hot-wire system which allows instantaneous velocity vector fields to be rapidly measured and related to the smoke patterns. The structures were made perfectly periodic in time by artificial stimulation. The experiments were therefore completely deterministic. This newly developed data-acquisition technique does not require the use of Taylor's hypothesis for inferring patterns from a fixed streamwise position. It therefore allows the vector fields of rapidly evolving patterns to be produced. It also allows the phenomenon of three-dimensional vortex pairing to be studied. The classification of patterns and conjectured topologies made by Perry & Lim (1978a, b) and the interpretations of Perry, Lim & Chong (1980) and Perry & Watmuff (1981) are examined. In the light of more-detailed measurements, it is found that some of these interpretations require modification.Vortex ‘skeleton’ models of the patterns are constructed and it is found that the Biot-Savart law gives vector fields which are in reasonable agreement with the experiments. It is also found that the power-spectral density of a periodic array of eddies can be explained in terms of potential flow vortices.