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 (zeta = 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 m(x)*= 2.87 and m(y)* = 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 theta between the horizontal and vertical displacements. The most obvious is the appearance of a local maximum of A(y)* 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.