Length of Flow Separation over Dunes

Dimensional analysis was used to define the length of the flow saparation for the case of two-dimensional, uniform flow over dunes, which was expressed in terms of bed-form geometry and the sand size of the bed material. Experiments were conducted in a rectangular flume using artificial dunes and a flow visualization method to measure the flow separation length. When dunes are flat, length of the flow separation is affected by the sand-grain roughness. When dunes are steep, the length of the flow separation is due mainly to the bed-form geometry and is relatively constant. The reverse rectangular steps can be considered as dunes of infinite length, and the length of the flow separation for such steps will always be greater than that obtained for dunes of finite length.