Swing and reverse swing of a cricket ball: laminar separation bubble, secondary vortex and wing-tip-like vortices

Large eddy simulation of flow past a cricket ball with its seam at 30^∘ to the free stream is carried out for 5 × 10⁴ ≤ Re ≤ 4.5 × 10⁵ . Three regimes of flow are identified on the basis of the time-averaged swing force coefficient ( C̄Z ) – no swing (NS), conventional swing (CS, C̄Z>0 ) and reverse swing (RS, C̄Z<0 ). The effect of seam on the boundary layer is investigated. Contrary to the popular belief, the boundary layer does not transition to a turbulent state in the initial stages of CS. The seam energizes the laminar boundary layer and delays its separation. The delay is significantly larger in a region near the poles, whose extent increases with an increase in $Re$ causing C̄Z to increase. Here C̄Z assumes a near constant value in the later stage of CS. The boundary layer transitions to a turbulent state via formation of a laminar separation bubble (LSB) in the equatorial region and directly, without a LSB, in the polar region. The extent of the LSB shrinks while the region of direct transition near the poles increases with an increase in $Re$ . A LSB forms on the non-seam side of the ball in the RS regime. A secondary vortex is observed in the wake bubble. While it exists on the non-seam side for the entire range of $Re$ considered, the mixing in the flow introduced by the seam causes it to disappear beyond a certain $Re$ on the seam side. The pressure difference between the seam and non-seam sides sets up wing-tip-like vortices. Their polarity reverses with the switch from the CS to RS regime.