Three-Dimensional Compressible Wakes of Cargo Aircraft Afterbodies*

The swept afterbody of cargo aircraft promotes a highly three-dimensional wake that generates additional drag and can result in hazardous conditions for parajumpers exiting the aircraft. These challenges have prompted the exploration of cargo aircraft wakes using a sharp-edged simplified bluff body known as the slanted-cylinder. This study expands current knowledge of the slanted-cylinder and further relates the geometry to real-world cargo aircraft through the addition of an edge radius. The three-dimensional flow features of the sharp-edge and rounded-edge afterbodies with a 45deg slant angle are explored under mildly compressible conditions (M=0.30) using Scanning-Stereoscopic PIV (S-SPIV). S-SPIV is a novel technique that utilizes a continuously traversing SPIV system to conduct spatio-temporal averaging of the flowfield, thereby reconstructing the mean (3D/3C) velocity field in a volume. The flow morphology derived from these reconstructed whole-field measurements provides valuable insight into the afterbody wake dynamics and parameters that influence its principal features such as the separating shear layer and the separation bubble. We also explore the impact of vortex stretching and compressible effects through the vorticity transport equation.