# Effect of Surface Roughness on Aerodynamic Loads of Bluff Body in Vicinity of Smoothed Moving Wall
## 4 Conclusions  
In this paper, the strategy of the statistical control of turbulence in Large Eddy Simulations combined with the Discrete Vortex Method was clarified for the fluid-structure interaction condition at higher subcritical Reynolds numbers--the bluff body chosen was the circular cylinder. For this purpose, the truncated Second-Order Velocity Structure Function was used to model the small scales of the turbulence, while the large scales were solved by considering the singularities distributed on the solid boundaries, the unperturbed incident flow, and the interactions of discrete wake vortices. The additional aid of the technique of varying the cutoff width as a function of the Reynolds number was applied to establish the characteristic distribution of vorticity in the  
Figure 15: Distribution of the circumferential pressure coefficient in the cylinder for some hybrid control configurations—\(\mathrm{G/D=0.5}\), \(\mathrm{Re=1\times 10^{5}}\), and \(\mathrm{t_{S}=100}\). fluid domain. The Lagrangian Dynamic Roughness Model--MLDRVL--was used to add the effect of body roughness, replacing the wall functions. These computational techniques are purely Lagrangian and, as such, dispense with the use of a mesh. The disadvantage of costly processing was overcome by employing Open Multi-Processing  
Numerical investigations on the induction of vortex shedding suppression in a circular cylinder with different rough surfaces, away from and near a moving wall, were performed for subcritical, critical, and supercritical regimes under high subcritical Reynolds numbers. The main results for a rough cylinder isolated from a moving wall are summarized in Figure 8. The roughness that is most efficient for drag reduction was revealed, for a given upper subcritical Reynolds number, to be in the range of \(3\times 10^{4}\leq\text{Re}\leq 2\times 10^{5}\). Intermediate roughness values are most efficient for Reynolds numbers on the order of \(10^{5}\). High roughness values are most efficient for Reynolds numbers on the order of \(10^{4}\). Under the condition of the proximity of the cylinder to the moving wall at the transition gap, \(\text{G}/\text{D}=0.5\), from the large-gap regime (\(\text{G}/\text{D}>0.5\)) to the intermediate-gap regime (\(0.35\text{<G}/\text{D}<0.5\)), as originally defined by [PERSON] [13], our findings indicate that the fit of \(\text{G}/\text{D}\geq 0.5\) can be considered. Furthermore, in the framework of the hybrid vortex shedding control technique, interestingly, the roughness seems to anticipate the characteristics of a lower gap ratio regime (Figure 15). In fixing the Reynolds number, i.e., \(\text{Re}=1\times 10^{5}\), the drag force of rough cylinders in the vicinity of the wall under the condition of the large-gap regime, \(\text{G}/\text{D}=0.5\), was relatively greater than those of isolated rough cylinders, \(\text{G}/\text{D}{\rightarrow}\infty\), due to the action of the Venturi effect. The generation of large-scale vortices downstream of the cylinder attracts fluid from the base region of the cylinder, culminating in a higher drag coefficient. [PERSON] [13] reported similar result for a smooth cylinder. In the scenarios evaluated in this study, cylinder roughness always acted to reduce drag force compared to the smooth surface (Table 1) and was dominant in the transition from a large-gap to intermediate-gap regime. There was also a significant reduction in the drag coefficient induced by the roughness values \(\epsilon/\text{D}=0.003\) and \(\epsilon/\text{D}=0.004\) in the presence of the Venturi effect, which were equivalent to \(45.4\%\) and \(44.7\%\), respectively. In general, the Strouhal number tended to be insensitive to the approach of the cylinder to the moving wall in the same flow configuration as in the isolation condition.