Numerical Study of Cylinder Drag Coefficient Reduction Using Passive Flow Control

In this study, the cylinder drag coefficient is reduced by using passive flow control. Installing a flat plate in two heights and different longitudinal distances in upstream flow increases the upstream flow momentum of the cylinder, leading to the higher boundary layer flow resistance against adverse pressure gradient which delays the flow separation. The flow separation delay enhances the pressure on the cylinder downstream. Then, the net pressure on the cylinder in the flow direction and, consequently, the cylinder drag coefficient are decreased. In case that the higher flat plate is utilized, the pressure on the upstream side is reduced more, leading to lower drag coefficient. However, for both heights of the flat plate at specific longitudinal distances from the cylinder due to the cavity flow formation between the cylinder and the flat plat, the vortex shedding is suppressed and the cylinder upstream is changed from the pressure side to suction side, leading to lower net pressure on the cylinder in the flow direction and as a result, less drag coefficient. At the optimal flat plate configuration at and , the minimum cylinder drag coefficient reached 90% reduction in comparison to the single cylinder case in the same flow condition. Results show that the drag coefficient reduction behavior is similar for different sub-critical Reynolds numbers due to the constant flow pattern and no considerable variation of the separation point. The entropy generation for the single cylinder and the case where the flat plate is located in its optimal configuration were investigated. The single cylinder has the highest entropy value, while the entropy of the optimal flat plate configuration with the cylinder reaches the lowest value, the same as the drag coefficient. Then, the drag coefficient is reduced by decreasing entropy generation, indicating the direct relation between drag coefficient and entropy generation.