A study of the wake structure behind bluff rings

Abstract

The structure of the wakes of the ring models with various cross-sections has been studied. They are compared with the wakes of a solid circular disk and two flat plates arranged side-by-side. Measurements were carried out in the wind tunnel at Reynolds numbers of the order of 10^. Periodic components were extracted from the turbulent velocity fluctuations measured by hot-wires in the wake by employing a conditional averaging technique. Also, surface pressure distributions together with correlation and coherence of the velocity fluctuations in the wake were measured. The wake structure of the ring model was found to be highly dependent on the geometry of model. A regular and coherent axisymmetric vortex ring shedding alternately from the inside and outside of ring models with d/w _> 5.0 was observed, where d is the mean diameter and w is the width of the ring model. There exists a sudden change of the correlation curves for the ring model between d/w = 4.5 and d/w = 5.0 and the base pressure coefficient rapidly decreases with increasing d/w between d/w = 4.0 and d/w = 6.0. Velocity fluctuations in the wake of the ring model with d/w £ 4.0 have a fairly weak predominate frequency and the velocity fluctuation components at the peak frequency are about 180° out of phase about the axis of symmetry. The counterrotating inside and outside vortex rings shed from the thin sharp-edged cross-section ring model with d/w = 6.0 are nearly 180° out of phase with each other at 3.4 w downstream from the ring model. However at further distances downstream they get closer to each other because of the opposite signed self-induced velocities of both sets of vortex rings. A remarkable regular periodic velocity fluctuation was recognised on the axis of symmetry in the wake of a rectangular cross-section ring model. It was also observed that vortex shedding from two flat plates arranged side-by-side with fairly wide spacing is symmetric about the axis of the gap and the arrangement of vortices is almost constant along the wake. The vortex wakes of a ring and two flat plates were numerically modelled by using infinite rows of circular line vortices and two-dimensional point vortices respectively. The arrangements of vortices and their variation with downstream distance observed in the wind tunnel experiment were well predicted in spite of the simple numerical simulations used. The streaklines and particle paths in the wakes of ring models and two flat plates were visualized in a water tank at Reynolds numbers just below 10^. The results from flow visualization agree with the results of the wind tunnel experiments.