Some aerodynamic aspects of coin-like cylinders

Abstract

The aspect ratio of short coin-like cylinders is defined as L/D, where L is the length and D is the diameter of the cylinder. Force and pressure measurements are extended down to L/D=0.025. The force measurements indicate an unexpected increase in drag coefficient with decreasing aspect ratio. The basic equation used to define the drag coefficient is inapplicable for very low aspect ratio and the projected area should be replaced by the side area. Surface flow visualization tests in a wind tunnel reveal the variation in both shape and size of separation bubbles, which form on the flat sides of the model. A crescent-shaped area is observed between the primary and secondary separation, followed by an unsteady re-attachment region. A strong hysteresis effect is observed in the development of the separation bubbles. The separation bubbles can be suppressed by rounding the sharp edges of the model, with considerable reduction in the drag coefficient. Finally, a flow topology is suggested consisting of two horseshoe vortices attached onto the flat sides and detached in a streamwise direction, thus forming two counter-rotating vortex pairs.