Cricket swing comes from turbulence clinging to one side of the ball
Many fans believe a cricket ball swings because air rushes faster over its polished side. That is a myth. Swing comes from turbulence: on the side where air becomes turbulent, it hugs the ball's surface longer before peeling away, creating a sideways push. Bowl fast enough with a worn ball and the direction reverses.
Fast bowlers, or quicks, can deliver the ball at more than 86 miles per hour, and they combine that speed with movement. Seam bowlers make the ball deviate off the pitch; swing bowlers make it curve through the air. There is no single agreed scale of pace, and labels such as medium, fast medium and express vary by source and level of play.
In conventional swing the raised seam is angled 10 to 20 degrees to one side. Air tripping over the seam turns turbulent on that side, and a turbulent boundary layer, the thin film of air next to the surface, separates later than a smooth one. The imbalance pushes the ball toward the direction the seam points. A new ball with a proud seam swings most, so fielding sides polish one side and let the other roughen to keep the effect alive. Many players believe humid or overcast weather helps, and that the red Test ball swings more than the white one-day ball.
Reverse swing flips the result. At very high speeds, or with a well-worn rough side, both boundary layers become turbulent, and the ball curves toward the shiny side, opposite the seam. It tends to move later and more sharply, which is harder to counter; above roughly 90 miles per hour it always happens. The physics resembles the dimples on a golf ball, which trigger turbulence to reduce drag. Pakistan's Sarfraz Nawaz and Farrukh Ahmed Khan are credited with discovering it.
Batters fear the late outswinger most, since it can catch the outside edge and fly to the slips, while an inswinging yorker threatens the stumps. The effort takes a toll: fast bowlers suffer more injuries than any other players, especially stress fractures in the lower back, though rates had fallen to their lowest in decades by 2019.
Source: Fast bowling
