Wind tunnel measurement of boundary-layer response to sediment transport

This paper presents an empirical analysis of the boundary-layer response to sediment transport in a wind tunnel with working section 0.7 x 0.76 x 12.5 m. With equilibrium transport, both the boundary-layer depth and displacement thickness increase relative to clean air. The logarithmic expression of the Law of the Wall provides a good description of the velocity profile within the lower 20% of the boundary-layer depth, at least. Shear velocity magnitudes (u infinity) are similar to and scale with those measured in the equivalent clean air flow, though this relation is dependent to some extent on particle shape and size. There is no empirical evidence to suggest that u* drops either to or below threshold, as hypothesized in some deterministic models of saltation. Departure from the Law of the Wall is evident in the flow above the saltation curtain at high velocities (u* > 9 m s(-1)). Though similar in appearance to an outer wake effect, direct solution of Coles' wake parameter indicates that this is not a wake phenomenon. This departure is most likely ascribed to a wind tunnel constraint on the downwind adjustment of a relatively thin boundary layer to the effective roughness associated with the saltation cloud. Froude numbers, computed for the high incident velocities where this departure is most evident, exceed the conservative limit of 10 suggested by White and Mounla (1991) for an equilibrium boundary layer.