# Effect of Surface Roughness on Aerodynamic Loads of Bluff Body in Vicinity of Smoothed Moving Wall
## 3 Results and Discussion
Figure 10: Complete configuration of the wake formed downstream of the isolated cylinder with different rough surfaces—\(\rm{Re}=1\times 10^{5}\): (**a**) \(\varepsilon/D=0.00110\), (**b**) \(\varepsilon/D=0.004\), and (**c**) \(\varepsilon/D=0.009\) (\(\rm{Nc}=48;\ \sigma_{o}=0.001\); \(\rm{t_{S}}=50\)). In this investigative path, the induction circumstances of vortex shedding suppression were verified for the hybrid approach by adding surface roughness variation as a passive method. Under these new flow conditions, we observed a considerable drop in the drag coefficient at moderate roughness, a severe drop at intermediate roughness, and a recovery at higher roughness (Table 1). The behavior of the drag in the two conditions, for the variation in the magnitude of the surface roughness, is explained in Figure 12. Interestingly, intermediate roughness values induce a greater reduction in the drag coefficient in the presence of the moving wall--which follows a similar behavior to that observed for the isolated rough cylinders. Therefore, the Venturi effect is not predominant on the drag force over the roughness at this juncture. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Method** & **Turb.** & \(\mathbf{C_{D}}\) & \(\mathbf{C_{L}}\) & **St** & **Source** \\ \hline Present simulation & LES & 1.41 & 0.23 & 0.20 & - \\ Numeric (vortex method) & LES & 1.474 & 0.104 & 0.204 & [37] \\ Numeric (vortex method) & LES & 1.448 & 0.140 & 0.205 & [11] \\ Experimental with end & - & 1.323 & 0.090 & - & [12] \\ plates (\(\mathrm{y_{e}/D=0.40}\)) & & & & & \\ Experimental with end & - & 1.282 & 0.034 & - & [12] \\ plates (\(\mathrm{y_{e}/D=0.00}\)) & & & & & \\ Experimental without end & - & 0.924 & 0.045 & - & [12] \\ plates & - & & & & \\ \hline \end{tabular}
\end{table} 
Table 2: Global quantities resulting from turbulent flow around the cylinder with a smooth surface near the moving wall—\(\mathrm{G/D=0.5}\) and \(\mathrm{Re=1\times 10^{5}\;(Nc=104;\;Np=100;\sigma_{o}=0.001;\;t_{S}=100)}\). Figure 11: Distribution of the circumferential pressure coefficient for the cylinder with a smooth surface in isolation and under the effect of the wall—\(\mathrm{Re=1\times 10^{5}\;(Nc=104;\;Np=100;\sigma_{o}=0.001;\;t_{S}=100)}\). The Strouhal number increased for rough surfaces compared to that observed for smooth surfaces (\(\mathrm{St}=0.20\)) in the condition of proximity to the moving wall, but it tended to be insensitive to increasing roughness values, alternating between 0.21 and 0.22. This is consistent with the experiments of [PERSON] and [PERSON] [63] for \(\mathrm{G/D}\leq 0.6\) (small and intermediate gap-to-diameter ratios [13]): on the one hand, the wake flow develops a distinct asymmetry about the cylinder centerline; on the other hand the Strouhal number and the convection velocity of the shed vortex are kept approximately constant and virtually independent of \(\mathrm{G/D}\). It was also found that the lift coefficient declined sharply from a positive value for a smooth surface, \(\mathrm{C_{L}}=0.23\), to a negative value for a rough surface \(\varepsilon/\mathrm{D}=0.00110\), \(\mathrm{C_{L}}=-0.15\), with considerable recovery for \(\varepsilon/\mathrm{D}=0.002\) and \(\mathrm{C_{L}}=-0.04\). Subsequently, a moderately increasing drop was observed until the roughness \(\varepsilon/\mathrm{D}=0.004\) and \(\mathrm{C_{L}}=-0.11\) and a sudden recovery for \(\varepsilon/\mathrm{D}=0.007\) and \(\mathrm{C_{L}}=0.03\). Finally, the lift coefficient showed a tendency to stabilize for the larger roughness values, \(\varepsilon/\mathrm{D}=0.009\) and \(\varepsilon/\mathrm{D}=0.02\), which culminated in the value \(\mathrm{C_{L}}=-0.05\). To the best of the present authors' knowledge, there are no experimental data related to Strouhal number for flows around rough cylinders near a moving horizontal plane wall at higher subcritical Reynolds numbers. In Figure 13, changes in the cylinder wake can be seen for increasing values of surface roughness, starting at \(\varepsilon/\mathrm{D}=0.00\) to \(\varepsilon/\mathrm{D}=0.003\).