Effect of Inter-Cylinder Gap and Pivot Point Location of a Pitching Elliptical Aerofoil on Wake Control and Drag Reduction of a Square Cylinder

In this study, the wake control and drag reduction of a square cylinder using a pitching elliptical aerofoil have been investigated using the FFIB-OSLBM solver. The non-dimensional gap between the cylinder and aerofoil is varied as G/D from 0.5 to 2.5, where D denotes the square cylinder's side length. The elliptical aerofoil with a major axis length R = D undergoes a sinusoidal pitching oscillation at a fixed pitching amplitude (θ_m) of 15º and Strouhal number (St_f) of 0.3 about three different pivot locations at a distance of 0.25R, 0.50R, and 0.75R from the leading edge of the aerofoil. Here, the Reynolds number is fixed to Re = 100 and the aspect ratio of the ellipse is taken to 0.25. The results revealed that both the gap ratio and pivot location of the aerofoil significantly impact the wake region and the corresponding aerodynamic forces on the cylinder. The wake flow patterns are categorised as Type-I, Type-II, Type-III, and Type-IV. In Type-I, a stable vortex street behind the square cylinder is found. In Type-II, due to a low inter-cylinder gap, the system behaves like an extended body, resulting in elongated and weakened vortices being observed. For Type-III and Type-IV, the increased gap ratio allows the cylinder’s vortices to form in the gap region. The lowest mean drag coefficient (C_d) is achieved for the configuration with G/D = 2.5 and a pivot at 0.25R, which shows a 35.5% reduction in C_d, compared to an isolated square cylinder. It is also observed that the rms value of the lift coefficient (C_l) increases drastically as the gap ratio increases. However, for a particular gap ratio, C_l exhibits an insignificant variation when the pivot location is changed.