Effect of Reynolds Number on Centerline Separation of a Vortex Dominated Bluff Body Wake

A generalized bluff body intended to mimic the wake produced by cargo aircraft was examined to evaluate the impact of Reynolds number on centerline separation. The bluff body, known as the ‘slanted afterbody’, consists of an elliptic nosecone, a cylindrical main body, and an afterbody with a slant angle, φ. In this study, the traditional slanted afterbody geometry is modified by adding an edge-radius, making it distinct from the oft-studied sharp-edged models and closer to the configuration of cargo aircraft afterbodies. This model was examined over a range of Reynolds numbers from ReD = 40K to ReD = 200K and two different slant angles, φ = 32° and φ = 45°. Drag measurements from an in-house designed low-drag instrument revealed a Reynolds number dependent transition between a centerline separated (CS) vortex state, observed to have higher drag, and a centerline attached (CA) vortex state, observed to have lower drag. Planar PIV identified the transition mechanism to be related to upstream boundary layer transition. Specifically, with increasing Reynolds number, the onset of instabilities within the boundary layer cause earlier transition of the separated shear layer, thereby reducing the size of the recirculation region until the boundary layer remains attached. Additionally, the influence of the centerline flow state on the downstream vortices was examined through stereo PIV. Vorticity and standard deviation measurements showed that vortices within the CS vortex state were diffuse and highly unsteady whereas the CA vortex state produced well defined, largely steady vortices.