Surface Roughness and Reynolds Number Effects on the Aerodynamic Forces and Pressures Acting on a Semicylindrical Roof in Smooth Flow

The effects of surface roughness and Reynolds number on the characteristics of aerodynamic forces and pressures on a semicylindrical roof have been investigated in a wind tunnel. Simultaneous pressure measurements on smooth and rough roofs were obtained under the smooth flow condition. The Reynolds number, based on the cylinder diameter $D$, was varied from $6.90\times {10}^4$ to $8.28\times {10}^5$ for the smooth roof and from $6.90\times {10}^4$ to $3.31\times {10}^5$ for rough roofs. The surface of the rough roof had a mean relative roughness of $k/D=3.0\times {10}^{-4}\mathrm{t}\mathrm{o}7.5\times {10}^{-4}$ ($k$ is the roughness height). The results indicated that, for a smooth roof, the transition of the separated shear layer occurs in-between $\mathsf{R}=6.90\times {10}^4-4.14\times {10}^5$; for rough roofs, the transitional regime is observed at lower Reynolds numbers (in the case of $k/D=7.5\times {10}^{-4}$, $\mathsf{R}<1.24\times {10}^5$). In addition, the mean lift coefficient and the root-mean-square (R.M.S.) pressure coefficient of the rough roofs are considerably lower than those of a smooth roof. It was also found that the surface roughness has a significant influence on the flow around the cylindrical roof. The results of power spectra confirmed that the rough roof presents good organization of vortex shedding; however, no prevailing shedding frequency could be detected for the smooth roof at $\mathsf{R}$ beyond $9.66\times {10}^4$.