EXPERIMENTAL STUDY OF FLOW AROUND A ROTATING SPHERE AT A MODERATE REYNOLDS NUMBER

Aerodynamic forces on a rotating golf ball were measured in a water tunnel for a Reynolds number of Re-D=8000 at a spinning ratio Vs I U0 (the surface velocity over the freestream velocity) between 0 and 6.0. The flow fields downstream of a spinning sphere were also measured using a time-resolved stereoscopic Ply system with field-of-views perpendicular to the incoming flow to capture the three velocity components. There appeared to be several critical spinning ratios at V-S/U-0 approximate to 0.75, 2.0 and 3.0. Both lift and drag increased quickly for V-S/U0 less than or similar to 0.75; there was a sudden drop in lift at V-s/U-0 less than or similar to 0.75; lift increased quickly while drag increased slowly for 1.0 less than or similar to V-S/U-0 less than or similar to 2.0; both lift and drag plateaued for 2.0 less than or similar to V-S/U-0 less than or similar to 3.0; lift increased while drag decreased for large spinning ratios 3.0 less than or similar to V-s/U-o less than or similar to 6.0. Flow measurements suggested the lift increase was associated with a downwash downstream of spinning ball. Down wash induced a pair of counter-rotating vortices that caused increases in drag, similar to the induced drag on a finite-span wing. Boundary layer transition occurred on retreating side at a large spinning ratio 3.0 < Vs/U0 < 6.0, the downward appeared to be weaker in this situation and drag increased thus became smaller again.