Two Degrees of Freedom Vortex-Induced Vibrations of a Rigid Circular Cylinder With Varying Natural Frequencies in the X and Y Directions

VIV experiments in two degrees of freedom are performed with a new apparatus designed to achieve a very low mass ratio and structural damping (ζ = 0.01). We investigated the influence of the ratio between the natural frequencies in the horizontal (f_x) and vertical (f_y) directions on the system response. Experiments were conducted at f_x/f_y = 0.42, 0.87, 1.16, 1.36 and 1.44 with $mx*$ = 2.87 and $my*$ = 1.65. For f_x/f_y < 1, the amplitude and frequency response were found to be similar to the classical case where f_x/f_y = 1, except in the transition zone between the upper and lower branches. For f_x/f_y > 1 however, radical changes were observed in the system response in amplitude, frequency and phase θ between the horizontal and vertical displacements. The most obvious is the appearance of a local maximum of $Ay*$ in the middle of the upper branch. Secondly, the nature of the transition between the upper and lower branches changes from intermittent switching to a hysteretic one. The shape of the figure-of-eights describing the cylinder trajectory is also affected so that the cylinder is moving upstream at the top of its trajectory instead of downstream, indicating a profound modification of the interaction between the two degrees of freedom. Lastly, the range of reduced velocities over which stable, two-degrees-of-freedom oscillations were recorded is greatly increased up to 3.8 < U* < 8.4.