Time-averaged flow field behind a transversely spinning sphere: An experimental study

Abstract

The aerodynamic forces on a sphere with a rough surface were measured in a water tunnel at a Reynolds number of 7930 and for a range of spinning ratios (α) from 0 to 6.0. The time-averaged flow fields were also measured using particle image velocimetry. The effect of the spinning ratio α on the flow was found to show distinct trends in different regimes, including α≤0.25; 0.25<α≤0.75; 0.75<α≤2.0; 2.0<α≤3.0; and 3.0<α≤6.0. The study identified two critical spinning ratios, where the flow underwent significant changes. The first change occurred in regime II, where the boundary layer over one side of the sphere transitioned from laminar to turbulent, leading to a significant modification in the lift force on the sphere. The second significant change took place across regimes II and III, where the boundary flow in the vicinity of the entire sphere became turbulent. Beyond this range, with α≥3.0, the high spinning rate disturbed the incoming flow, resulting in less-efficient downwash production. The lift increased with α at a slower rate compared to other regimes, and the less-efficient downwash production caused a decrease in drag as more momentum was directed downstream in the horizontal direction.