Experimental investigation on the connection between boundary layer thickness and base drag of a bluff body.

In this work the results are described of a wind tunnel investigation carried out to study the influence of the characteristics of the boundary layer developing over the lateral surface of an axisymmetric bluff body upon its base pressure and near-wake flow features. The tests were carried out at Re = Ul/ν = 5.5 × 10^5, for which the boundary layer over the lateral surface of the smooth model becomes turbulent before reaching the base contour. Different strips of emery cloth were wrapped in various positions around the body circumference in order to modify the thickness and characteristics of the boundary layer. With increasing thickness of the boundary layer measured at 0.1d from the base contour the results show that the base suctions decrease, and thus the pressure drag of the body. This variation is found to be connected with an increase of the length of the mean recirculation region that is present behind the body, and with a corresponding reduction of the curvature of the streamlines at separation. Finally, a dominating peak at a frequency that decreases with increasing boundary layer thickness is found