Numerical investigation of the flow over a golf ball in the subcritical and supercritical regimes

In order to understand the role of surface dimpling on the flow over a golf ball, direct numerical simulations (DNS) are conducted within the framework of an immersed boundary approach for two physical regimes. Computations of the flow over a non-rotating golf ball are reported for a subcritical flow at a Reynolds number of 2.5×10⁴ and a supercritical case at a Reynolds number of 1.1×10⁵. Grid refinement studies for both Reynolds numbers indicated that characteristics of the subcritical flow could be captured using a mesh of 337×10⁶ points, and for the supercritical case using a grid with 1.2×10⁹ points. Flow visualizations reveal the differences in separation characteristics between the two Reynolds numbers. Profiles of the mean velocity indicate that the flow detaches completely at approximately 84° in the subcritical case (measured from the stagnation point at the front of the ball), while in the supercritical regime there are alternating regions of reattachment and separation within dimples with complete detachment around 110°. Energy spectra highlight frequencies associated with vortex formation over the dimples prior to complete detachment in the supercritical regime. Reynolds stresses quantify momentum transport in the near-wall region, showing that the axial stress increases around 90° for the subcritical case. In the supercritical regime these stress components alternately increase and decrease, corresponding to local separation and reattachment. Prediction of the drag coefficient for both Reynolds numbers is in reasonable agreement with measurements.