Experimental Investigation of Base-Drag Reduction via Boundary-Layer Modification

Abstract

This investigation identifies and characterizes the changes in the near wake of a bluff body produced by boundary-layer thickening, which, in turn, result in an overall reduction in the base drag. For this study, wedge and flat-plate models with both smooth and rough forebody surfaces were used. The forebody surface roughness on both models was increased to modify/thicken the forebody boundary layer. Hot-wire surveys were carried out to characterize the boundary-layer state ahead of the separation point, and base-pressure measurements were used to quantify base-drag reduction associated with boundary-layer modification. In addition, particle image velocimetry measurements in the near wake were performed for both models, and proper orthogonal decomposition analysis was used to analyze the velocity data. Low-order near-wake velocity reconstructions obtained from proper orthogonal decomposition analysis accurately represented the near-wake behavior for the studied cases. It was observed from these low-order reconstructions that the shed vortices became more organized, convected faster, and decreased in strength when the boundary layer was thickened/modified. These changes in the wake behavior resulted in an increase in base pressure and an overall decrease in the base drag.