Flow Structure in the Wake of a Wall-Mounted Bluff Body

This paper discusses an experimental study of the flow field around three different wall-mounted obstacles in a boundary layer within a relatively shallow flow (d/H ≈ 4, where d is the flow depth and H is the obstacle height). The first two obstacles consist of a semi-ellipsoidal body with the major axis of the base ellipse oriented in the streamwise and the transverse direction. The third obstacle is a low-aspect-ratio cylinder, bearing greater similarity to geometries investigated in other studies. Reynolds numbers of 5500 and 17800, based on obstacle height, were investigated. Digital particle image velocimetry was used to characterize the flow field in each case. At Re_H = 17800, isocontours of streamwise vorticity reveal the presence of a tip vortex pair and a base vortex pair for each geometry; however, the ratio of the relative strengths of the tip and base vortices varies considerably, with the streamwise-oriented ellipsoid having the largest ratio of tip/base vortex circulation and the transverse ellipsoid having the smallest ratio. The presence of a base vortex for such low-aspect-ratio obstacles is unexpected, and therefore we hypothesize that its existence is related to the small d/H. It is anticipated that variation of geometry and d/H will help to elucidate the role of the base vortex in the three-dimensional vortex structure of the wake.