EFFECTS OF INCIDENCE AND AFTERBODY SHAPE ON FLOW PAST BLUFF CYLINDERS

In the present paper the effects of the afterbody shape and the angle of incidence on the structure of the flow past a prismatic body are experimentally investigated, both quantitatively in the form of wind tunnel measurement and qualitatively in the form of water tunnel flow visualization. Four cross-sectional shapes with identical upstream facing side and streamwise dimension were chosen in the present investigation. They were a square, two trapeziums, and a triangle. By studying the structure of the flow associated with the above-mentioned shapes, a systematic investigation in which the effects of the afterbody are gradually reduced can be carried out. From the measured magnitude and frequency of the fluctuating aerodynamic forces, the main effect of differences in afterbody shape is the proximity of the two separated shear layers to each other and to their corresponding side faces. This proximity difference, in turn, results in differences in the normal force, the gradient of its variation with the angle of incidence, and, hence, the susceptibility of the shape (with respect) to flow-induced oscillation. Also, due to the difference in interaction between the separated shear layers and the sides of the prismatic structure, the vortex formation length, the base pressure and, hence, the axial force, the rate at which vortices are shed and, hence, the frequency of the aerodynamic loading on the structure and the longitudinal vortex spacing also vary. Changes in the angle of incidence can be viewed as similar to changes in the afterbody shape and, hence, its effects are similar to those caused by changing the afterbody shape. Experiments show that no cross-sectional shape is absolutely stable to galloping oscillation because a shape that is stable to galloping oscillation at a certain mean angle of incidence may become unstable at a different mean angle of incidence. By idealising the vortex wake as two parallel rows of vortices, various vortex street parameters, including the vortex spacing ratio and the strength of each vortex in the wake, are estimated. They show a dependence on the afterbody shape and the angle of incidence.