Flow-induced vibration of an elastically mounted airfoil under the influence of the wake of a circular cylinder

Abstract

The effect of vortices generated by a rigidly mounted cylinder on the dynamic response of an airfoil is investigated in this study. This work extends previous investigation of vortex interaction with a second cylinder to capture the wake energy (Derakhshandeh et al. 2014). Accordingly, the Flow-Induced Vibration (FIV) of a symmetric NACA 0012 airfoil positioned in the wake of the rigidly-mounted upstream cylinder is studied. The airfoil mount allows it to move in two degrees-of-freedom; pitch and heave. The pitching axis is kinematically driven using a brushless permanent-magnet DC servo motor. The heave axis of the airfoil is coupled with a Virtual Elastic Mechanism (VEM), which is an electro-mechanical device designed to create any desired impedance. The VEM system replaces the physical damper and spring systems and allows the airfoil to oscillate due to the lift force in the normal direction of the flow. The airfoil is set at different positions in the wake of the upstream cylinder in order to characterise the impact of the arrangement of a coupled cylinder–airfoil on energy extraction. Special attention was paid to the angle of attack of the airfoil to explore the optimal performance of the system. Force and displacement measurements of the airfoil were conducted in a closed loop water channel. The experimental work was complimented by a Computational Fluid Dynamics (CFD) modelling study which was aimed at calculating the vortex frequency shedding and visualize the vortex structure. It is observed that the vortex shedding frequency, the length scale and transverse spacing of the vortices are function of the longitudinal and lateral distances between the cylinder the airfoil. The results also demonstrate that there is a correlation between the configuration of cylinder and airfoil and vortex structure. Due to this correlation, the shear forces acting on the airfoil alters the fluttering response of the airfoil depending on the angle of attack, which in turn influences the obtained power coefficient of the device. The maximum power coefficient of FIV is obtained for cases with 3.5⩽x0/D⩽4.5 and 1⩽y0/D⩽1.5 arrangements, which is limited to the narrower lateral distances as compared to the previous study (1⩽y0/D⩽2) by the authors (Derakhshandeh et al. 2014) employing a pair of cylinders.