Self-Preservation for a Turbulent Boundary Layer over a d-type Rough Surface

This paper describes an analytical and a detailed experimental study for self-preservation of a turbulent boundary layer with zero pressure gradient over a d-type rough surface. It is not confirmed experimentally so far that a zero pressure gradient boundary layer considered here closely satisfies the requirement of exact self-preservation, because of the difficulty for measurement of the local wall shear stress accurately. In the present paper, the wall shear stress is determined exactly from the direct measurement by the use of a floating element device. Result shows that the skin friction coefficient is almost constant. The mean velocity and turbulence quantities characteristics are also investigated experimentally, and the distributions indicate the similarity. From the results mentioned above, it is found that a d-type rough wall turbulent boundary layer confirms to Rotta's condition of precise self-preserving flow.