Structure of a d-type Rough Wall Turbulent Boundary Layer at a Low Reynolds Number : Higher order moments and violent events

Some properties associated with the large eddy structures of a d-type rough wall turbulent boundary layer at a momentum thickness Reynolds number R_θ≒800 have been examined experimentally. The higher order moments and the contribution of violent events to the Reynolds shear stress are measured and compared with the results of both smooth and k-type rough wall flow as well as a d-type rough wall flow at a high Reynolds number R_θ=5000. The low Reynolds number effect in the outer layer, which in general enlarges the contorted surface of the viscous super-layer, is found even in the present rough wall flow. The examination about the triple velocity products shows that the Reynolds shear stress flux due to the ejection event has a large portion of turbulent energy transport in the outer layer of the present flow. From the comparison among results of three different types of measuring flat plate surface, significant differences are found in the distribution of the ratio of fractional contributions of the violent ejection to the stress to those of the violent sweep and also in the distribution of the ratio of mean frequency of those events.