Airfoil vibration due to upstream alternating vortices generated by a circular cylinder

An experimental investigation of airfoil vibration due to upstream alternating vortices was carried out in a re-circulating wind tunnel. A circular cylinder with a diameter D = 102mm was positioned upstream of an airfoil (NACA0012), with a chord length c = 200mm and a zero angle of attack placed at a gap distance S, to generate the vortex street. The circular cylinder and airfoil were arranged in tandem and the spacing ratio S/D was varied from 0.5 to 6.5 to investigate the effect of the vortices generated upstream on the vibration of the airfoil. The experiment was carried out in a free stream Re range of 1.6×105 to 2.3×105. The vortex formation region behind a single circular cylinder was measured using a hot wire anemometer and the airfoil dynamic responses were examined using a laser vibrometer. It is found that when S/D is reduced beyond a critical value, there is a rapid drop in vortex shedding frequency and a suppression in airfoil vibration. This critical S/D is found to be the normalized length of the vortex formation region behind the single cylinder. It is hypothesized that the vortex could not be formed at this location within the gap distance in the presence of the airfoil, but instead is formed behind the airfoil. Consequently, as vortex shedding is switched from upstream to downstream of the airfoil, the flow-induced vibration of the airfoil is suppressed at the same time.