Drag of a turbulent boundary layer with transverse 2D circular rods on the wall

In this paper, we present the results of a turbulent boundary layer developing over a rod-roughened wall with a spacing of lambda/k = 8 (lambda is the spacing between two adjacent roughness elements, and k is the rod diameter). Static pressure measurements are taken around a single roughness element to accurately determine the friction velocity, U-tau and the error in the origin, d(0), which are the two prominent issues that surround rough-wall boundary layers. In addition, velocity measurements are taken at several streamwise locations using hot-wire anemometry to obtain C-f from the momentum integral equation. Results showed that both methods give consistent values for U-sigma, indicating that the contribution of the viscous drag over this rough wall is negligible. This supports the results of Perry et al. (J Fluid Mech 177: 437-466, 1969) and Antonia and Luxton (J Fluid Mech 48(04): 721-761, 1971) in a boundary layer and of Leonardi et al. (2003) in a channel flow but does not agree with those of Furuya et al. (J Fluids Eng 98(4): 635-643, 1976). The results show that both U-tau and d(0) can be unambiguously measured on this particular rough wall. This paves the way for a proper comparison between the boundary layer developing over this wall and the smooth-wall turbulent boundary layer.