The Separation Characteristics of the Flow Past a Volleyball at Critical Reynolds Number

This report attempts to answer the question that why a flying volleyball may swing, serve or curve. It has been pointed by many researchers that the swing, swere and curve motion of a sports ball is predicated by a Magnus Effect or that it has been attributed to asymmetric boundary layer separation caused by a spinning ball or by a ball seam which is placed asymmetrically about the velocity direction of the ball. The problem is whether, if the ball were not spinning and the ball seam were symmetric about the velocity direction of the ball, there would be some aerodynamic loads under which the ball may swing, swerve or curve. In this experiment a non-spinning volleyball whose seam was placed symmetrically about the wind direction was tested in a wind tunnel. The aerodynamic loads were measured by use of a strain gauge balance. The separation on the volleyball was measured with hot-film. The experimental results suggest that at the region of critical Reynolds number the separation on a nonspinning volleyball is unstable. There is not only alternation between laminar and turbulent separation but also change of the turbulent separation region on the surface of the volleyball. It causes the fluctuations of drag and lateral force, which is the same order of magnitude as the mean drag, and is essential reason causing swing, swerve or curve motion of a volleyball.