# Effect of Surface Roughness on Aerodynamic Loads of Bluff Body in Vicinity of Smoothed Moving Wall
## 3 Results and Discussion
Now, when glimpsing the numerical findings for the condition of a cylinder with rough surfaces close to a moving wall, it is necessary to refer to [PERSON] [12]'s experiments for a smooth cylinder. For the problem now addressed, when considering high Reynolds numbers, there is a lack of experimental data available in the literature. This corroborates the importance of this paper. [PERSON] et al. [13] conducted an extensive experimental investigation with a cylinder near a moving wall under various gap ratios for two upper-subcritical Reynolds numbers (\(\mathrm{Re}=0.4\times 10^{5}\) and \(\mathrm{Re}=1\times 10^{5}\)) and achieved essentially two-dimensional flow patterns for cylinder with end plates. The author classified the flows into three gap regimes: large-gap, \(\mathrm{G/D}>0.5\); intermediate-gap, \(0.35\)\(<\mathrm{G/D}<0.5\); and small-gap, \(\mathrm{G/D}<0.35\). Thus, the transition gap of the large-gap and intermediate-gap regimes, \(\mathrm{G/D}=0.5\), was fixed for investigations in the present work. The Reynolds number was set at \(1\times 10^{5}\) for comparisons with the literature data (Table 2). Experimental results using end plates were obtained for ratios \(\mathrm{y_{e}/D}=0.00\) and \(\mathrm{y_{e}/D}=0.40\) in [PERSON]'s investigations [12]. The length \(\mathrm{y_{e}}\) was defined as the distance from the bottom edge of the cylinder to the bottom edge of the end plates, and the uncertainties of the drag and lift coefficients were equal to \(\pm 0.016\) and \(\pm 0.011\), respectively, with 95% confidence. The numerical responses for the drag coefficient of the smooth cylinder, listed in Table 2, show good agreement with the results of [PERSON] [12] for the configurations with end plates. Compared to the cylinder aided by end plates in the ratio \(\mathrm{y_{e}/d}=0.40\), the difference was only 6.6%. We also verified proximity with the recent numerical results of [PERSON] et al. [37] and [PERSON] [11], both for the drag coefficient and the Strouhal number. For the mentioned numerical evaluations, another roughness model was employed [27]. The mean variables were predicted to be \(\mathrm{C_{D}}=1.22\), \(\mathrm{C_{L}}=0.01\), and \(\mathrm{St}=0.22\) for the isolated cylinder and \(\mathrm{C_{D}}=1.41\), \(\mathrm{C_{L}}=0.23\), and \(\mathrm{St}=0.20\) for the gap-to-diameter ratio \(\mathrm{G/D}=0.5\) (Table 1). Figure 11 shows the average distributions of the circumferential pressure coefficient for the cylinder with a smooth surface under the two conditions mentioned. Physically, there is a movement from the stagnation point to a lower angular position as the cylinder approaches the ground, and from there, a subsequent reduction in pressure is observed until an angle of approximately 300 degrees, when the pressure becomes relatively higher than that calculated for the isolated cylinder, and then it falls  
Figure 9: Behavior of the mean drag coefficient and boundary layer separation angles for different roughness heights—\(\mathrm{Re}=1\times 10^{5}\) (\(\mathrm{Nc}=48;\ \mathrm{\sigma_{o}}=0.001\); \(\mathrm{t_{S}}=100\)). back near the stagnation point. This region of higher pressure in the lower front part of the cylinder in the ground effect is responsible for the appearance of positive lift (\(\rm{C_{L}}=0.23\)). Furthermore, in this regime transition limit, the flow still exhibits characteristics of the large-gap regime due to the generation of large-scale vortices downstream of the cylinder, and these vortices attract fluid from the base region of the cylinder and, consequently, there is a higher drag coefficient. This is convergent with [PERSON]'s reports [13] for a smooth cylinder. The increase in drag coefficient is 15.6%. The resulting Strouhal number is moderately sensitive to the blockage effect and is predicted to be around \(\rm{St}=0.20\) as opposed to the prediction for an isolated cylinder, \(\rm{St}=0.22\) (Table 1).