Abstract
The characterization of the dependence of the base pressure of bluff bodies upon the geometrical and fluid dynamical parameters defining a certain configuration is a complex and still open problem, with great interest from both the scientific and the practical points of view. In particular, several indications exist in the literature that an increase in boundary layer thickness may lead to an increase of the base pressure – and a decrease of the drag – of two-dimensional or axisymmetric blunt-based bodies.
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 surface of an axisymmetric bluff body upon its base pressure and near-wake flow features. The model, whose diameter and overall length are, respectively, d = 70 mm and l = 400 mm, has a forebody with a 3:1 elliptical contour and a sharp-edged base perpendicular to its axis; it is supported above a flat plate by means of a faired strut. The pressure distributions over the lateral and base surfaces of the body were obtained using numerous pressure taps and two pressure scanners directly placed inside the model; furthermore, the boundary layer profiles and the wake velocity field were measured in detail with hot-wire anemometry. Tests were carried out at Re =
Ul/ν = 5.50 x 105, at which the boundary layer over the lateral surface of the body becomes turbulent
before reaching the base contour. A strip of emery cloth was wrapped at 283mm before the base around the body circumference in order to modify the thickness and characteristics of the boundary layer.
The results show that the base suctions decrease with increasing boundary layer thickness, measured 0.1d from the base contour. This variation is found to be connected with a reduction of the curvature of the streamlines at separation, and with a corresponding increase of the length of the mean recirculation region that is present behind the body.
