Vortex structures in the wakes of two- and three-dimensional bodies

This thesis reports on an experimental study into the flow structure of two different bluff bodies: an inclined flat plate with a quasi-two-dimensional wake and the Ahmed body with a highly three-dimensional wake. Each wake is measured using particle image velocimetry with high spatial and temporal resolution. Time-average results are presented showing the effect of angle-of-attack and Reynolds number on the flow around the flat plate. Results showing the formation of large scale von Kármán vortices from the merging of smaller shear layer vortices is presented and the ratio of these frequencies is quantified. The nature of the major wake structures of the Ahmed body is presented and the strength of each structure is quantified through measuring the circulation. An analysis of the circulation variation with downstream location reveals the tilting of vorticity and the subsequent merging of structures. The dynamics of the vortex motion is illustrated using two-dimensional space-time plots revealing large, periodic motions in the wake. The influence of the longitudinal structures on the wake is investigated by systematically varying the lateral spacing of the c-pillars. By increasing the rear-slant aspect ratio, the induced downwash between the two longitudinal structures is reduced. The critical aspect ratio between reattaching and completely separated flow is presented. The effect of aspect ratio on the dynamics of these structures is shown to be minimal.