Three-dimensionality of oscillatory flow in a two-dimensional symmetric sinusoidal wavy-walled channel

Oscillatory flow in a two-dimensional symmetric sinusoidal wavy-walled channel is investigated experimentally in a wide range of flow parameters. Attention is focused on the transition from a two-dimensional to a threedimensional flow. The critical Reynolds number for the transition to three-dimensional flow varies with the Strouhal number for the forced oscillation, and this value is lowest in the 0.02-0.03 range of Strouhal numbers. Two distinct three-dimensional flows are formed; time-symmetric flow and time-asymmetric flow, distinguished from each other by whether or not the Strouhal number exceeds about 0.02.