Experimental Flow-Induced Vibration Analysis of the Crossflow Past a Single Cylinder and Pairs of Cylinders in Tandem and Side-by-Side

Flow-induced vibration of a single cylinder and two cylinders in tandem and side-by-side configurations is experimentally investigated in this paper in the subcritical regime. The natural frequency of the system varied from 8.8 Hz to 46.2 Hz. The mass ratio, m*, ranged between 158 and 643 while the damping ratio, ζ, between 0.0005 and 0.009. The pairs of cylinders present a spacing ratio of 1.26 (P/D and L/D). In all cases, one and both cylinders (BV) were free to vibrate. Experiments were performed in an aerodynamic channel with a constant height and a variable width, for the evaluation of the influence of the blockage ratio (BR), using accelerometers and hot wire anemometry. The reference velocity, measured at the entrance of the test section was used to calculate the reduced velocity, Vr = U/f_nD, with values from 4 to 132 and the Reynolds number between 3 × 103 and 8 × 104. The root-mean-square-values of the displacement amplitudes, Y/D, were obtained through the integration of the acceleration signals. Fourier and continuous wavelets were employed in the analysis. For a single cylinder free to vibrate, the higher amplitudes occur at two distinct reduced velocities, associated with the vibration modes of the cylinder. The vibration amplitude of a single cylinder increased as the blockage ratio increased, decreasing for the highest blockage ratio investigated. For the case of cylinders in tandem, the presence of the fixed cylinder in the wake of the cylinder free to vibrate amplifies the vibration response at high reduced velocities. When the blockage ratio is increased, a sudden increase in the vibration amplitude is observed. When both cylinders are free to vibrate, the relation between the natural frequencies of both cylinders influences the response amplitudes. In the case with two cylinders side-by-side, the vibration amplitude remains similar to a single cylinder, but when both cylinders are free to vibrate, the presence and the influence of flow bistability is observed.