Hysteresis in the aerodynamic forces and moments of a discus governed by laminar separation bubbles

This study investigates the aerodynamic forces and moments acting on a women's discus as functions of angle of attack, freestream velocity, and spin rate about its axis of symmetry. Wind tunnel experiments were conducted to measure these aerodynamic forces and moments and to visualize the flow field using oil-flow visualization and particle image velocimetry (PIV). The results show that no significant dependence of the aerodynamic coefficients on the spin parameter was observed within the tested ranges (20-30 m/s; 0-7 rev/s). The drag, lift, and pitching moment coefficients depend strongly on the angle of attack, whereas the side force and the rolling and yawing moment coefficients remain approximately zero. The drag, lift, and pitching moment coefficients increase with angle of attack until stall occurs at approximately 28-30°, after which the lift and pitching moment decrease sharply. Stall recovery occurs at a lower angle of attack, around 25°, corresponding to a hysteresis window of approximately 4-5°. Surface static pressure measurements and flow visualizations suggest that this hysteresis is associated with the history-dependent formation and collapse of a leading-edge laminar separation bubble, which appears during increasing-angle-of-attack process with flow reattachment and disappears during the decreasing-angle-of-attack process.