Mean Flow Properties of the Boundary Layer over a d-type Rough Surface at a Low Reynolds Number

Mean flow properties of the boundary layer over a d-type rough surface at a low Reynolds number for which the skin friction coefficient does agree well with that of smooth wall boundary layer have been investigated experimentally. The data obtained are non-dimensionalized using both inner variables and outer one, and primarily examined comparing with that of the smooth wall flow at a low Reynolds number. The results show that the universal logarithmic velocity profile for d-type rough wall flow is valid at value of momentum-deficit Reynolds number R_θ of about 800,and that the additive constant, however, varies. The extent of the logarithmic region has not been found to disappear with decreasing Reynolds number, but the ratio of the extent of the logarithmic region to the boundary layer thickness slightly increases with decreasing Reynolds number. The wake strength has been found to vary with Reynolds number in a manner different to that described in Coles.