# Effect of Surface Roughness on Aerodynamic Loads of Bluff Body in Vicinity of Smoothed Moving Wall
## 3 Results and Discussion
It can be seen that the von Karman vortex shedding has become intermittent, and therefore, the effect of roughness appears to be to anticipate features of the intermediate-gap regime, with a correlated drop in drag of greater or lesser magnitude. These viscous wakes take the form of \"mushroom-like\" vortical structures, and the blockage effect transfigures them as they move away from the cylinder. It is encouraging to compare the \(\mathrm{C_{D}}\) values for a smooth cylinder (Table 1) under the conditions of the absence (\(\mathrm{C_{D}}=1.22\)) and presence (\(\mathrm{C_{D}}=1.41\)) of the interaction with the moving wall with those found by [PERSON] et al. [64] for a low subcritical Reynolds number, \(\mathrm{Re}=3.900\), and fixed wall. The researchers arrived at values of \(\mathrm{C_{D}}=1.281\) and \(\mathrm{C_{D}}=1.544\) for the gaps \(\mathrm{G/D}=\infty\) and \(\mathrm{G/D}=0.5\), respectively, using RANS simulations, which reinforces our previous statement that at the eminence of proximity to the wall, in the large-gap regime, the drag coefficient increases before decreasing in the intermediate-gap regime, and this transition point depends on the Reynolds number, which, for the flow configuration studied by the authors, was at \(\mathrm{G/D}=1\) (\(\mathrm{C_{D}}=1.712\)). Figure 12: Drag coefficient variation for different roughness heights—\(\mathrm{Re}=1\times 10^{5}\). The recent computational findings by [PERSON] et al. [11] and [PERSON] et al. [37] are unique references given the lack of experimental work on rough cylinders close to a moving wall. The present simulations for \(\varepsilon/\mathrm{D}=0.007\) revealed the following global values: \(\mathrm{C_{D}}=0.93\), \(\mathrm{C_{L}}=0.03\), and \(\mathrm{St}=0.21\) for \(\mathrm{Re}=1\times 10^{5}\). In this direction, [PERSON] et al. [11] reported \(\mathrm{C_{D}}=1.165\), \(\mathrm{C_{L}}=0.068\), and \(\mathrm{St}=0.2019\). The results are convergent. A difference around \(20.2\%\) in the drag coefficient is credited to the use of different roughness models and other particularities of the different numerical/computational methods, although both used the DVM and LESs. On a similar investigative path, [PERSON] et al. [37] reported \(\mathrm{C_{D}}=1.459\), \(\mathrm{C_{L}}=0.104\), and \(\mathrm{St}=0.216\) by applying a roughness \(\varepsilon/\mathrm{D}=0.001\) to the cylinder surface. The present simulations for roughness \(\varepsilon/\mathrm{D}=0.00110\), which is the closest to that studied by the researchers, returned \(\mathrm{C_{D}}=1.12\), \(\mathrm{C_{L}}=-0.15\), and \(\mathrm{St}=0.21\). Since the roughness of these values is closer to a those of a smooth surface, their results corroborate ours. The time history of the drag and lift coefficients for the cylinder with different roughness values close to the moving wall is presented in Figure 14. In the latter case, the following representative roughness values were chosen: moderate, \(\epsilon/\mathrm{D}=0.00110\); intermediate, \(\epsilon/\mathrm{D}=0.004\); and high, \(\epsilon/\mathrm{D}=0.007\). One of the contributions of the Venturi effect is that it creates two different peaks on the drag coefficient curve. The explanation for this aerodynamic behavior is that while the upper vortical structure finds total freedom to grow at the back of the cylinder until it is incorporated by the viscous wake, bringing the drag coefficient to a higher value, the development of the lower vortical structure is affected by the Venturi effect, and consequently, there is a lower peak on the drag curve. However, in the large-gap regime, this phenomenon is barely noticeable, although it does exhibit a greater or lesser amplitude depending on the magnitude of the roughness.