Mini-spoilers for afterbody base drag reduction

Wind tunnel experiments have been carried out in order to study the effect of full-span mini-spoilers on the afterbody vortex drag of a slanted base cylinder. Two different protrusion heights (h/D = 2.5% and 5%) were examined at various chordwise locations along the upsweep. The tests were performed on a slanted base cylindrical model with an afterbody upsweep angle Φ = 28° at a test Reynolds number of 200,000 based on model diameter. Drag measurements, surface pressure measurements and 2D Particle Image Velocimetry measurements were the main experimental tools utilized within this investigation. Placing the spoilers closer to the leading edge of the upsweep caused an increased drag due to the separation induced by the spoiler itself, leading to a more diffused vortex, but with a larger circulation at the trailing-edge. Drag reductions were observed when placing the spoiler closer to the trailing-edge, with the optimum location being x′s /c = 87.5% resulting in drag reductions of 4.5% and 4.8% for h/D = 2.5% and 5% heights respectively. This reduction is due to an increase in surface pressure upstream of the spoiler. For the drag reducing spoiler location, the vortex was found to be displaced away from the surface, with the streamlines of the model trailing-edge deflected downwards into the wake of the spoiler, altering the trailing-edge flow. Unsteady aspects are discussed and underlying flow mechanisms are presented using the Proper Orthogonal Decomposition. Further results are also presented for a vortex generator configuration that was examined which may present a useful starting point for future studies. A co-rotating half delta wing type vortex generator placed at an incidence of β = 20° at x/c = 20% with a leading edge sweep of Λ = 70° was found to show the most promising results.