Effects of nose bluntness, roughness and surface perturbations on the asymmetric flow past slender bodies at large angles of attack
An experimental investigation of the effects of geometric perturbations on the asymmetric flow past slender bodies has been conducted. The perturbations included variations in model tip sharpness and roughness as well as discrete surface perturbations represented by cylindrical beads of varying height and diameter attached to the model surface. A cone/cylinder model having a 10 deg semi-apex angle and a 3. 0 caliber tangent ogive model, both having base diameters of 3.5 in were tested at laminar flow conditions at angles of attack from 30 to 60 deg. Data were taken with each model from 3 circumferential rows of pressure taps at a Reynolds number of 84, 000 based on cylinder diameter and by a helium bubble flow visualization technique at a Reynolds number of 24, 000. The beads were used to represent single, discrete roughness elements and were tested with a variety of tips by placing the model at a roll position having either a symmetric or a highly asymmetric flowfield, and then attaching a bead to the model surface at varying axial and circumferential positions. Bead effectiveness was judged by the extent to which the flowfield was affected. Changes in the flow due to the beads matched that of earlier tests using a different cone/cylinder model in a different wind tunnel and using beads of a different shabe. Both tests showed the beads to be most effective when placed about 140 deg from the windward ray, and to decrease in effectiveness with decreasing bead size and distance from the tip. The data showed no significant change in this behavior with variations in angle of attack and tip bluntness. Tests without beads conducted on sets of tips manufactured to identical specifications showed the behavior of sectional side-force with roll position to be a function of machining imperfections. Maximum side-force and the nature of the side-force variations changed within a given set of tips, whether sharp or blunt. The variations of side force with model roll position had the largest amplitudes and was most erratic for the sharp tips, and had the lowest amplitude and was least erratic for the most blunt tip, indicating that the blunter tips were more accurately machined.
