VORTEX-INDUCED VIBRATIONS OF A TETHERED SPHERE WITH NEUTRAL BUOYANCY

Summary Recent preliminary experiments have indicated that a vertically-tethered sphere develops a large diameter quasi-circular tra-jectory, unlike the oscillations observed for buoyant horizontally-tethered spheres. The previous study of the same authors numerically identified that there exist six different flow regimes within the range of the Reynolds number (Re) = [50, 800], which is based on the mean flow velocity (U) and sphere diameter (D), in the case of a tethered sphere with neutral buoyancy. A series of experiments are carried out in a water channel to verify and support the numerical findings. The experiments covered the range of the Reynolds number = [700, 4000] which is higher than those of the numerical study, and the position of the sphere was recorded during the experiments. Within the Reynolds range = [700, 800], which covers simulations and experiments, and corresponds to the Regime VI, it is observed that the response of the sphere is irregular. This verifies the existence of Regime VI which has been found in the numerical study. It is also observed that the sphere shows quasi-circular motion in the plane normal to incoming flow as the Reynolds number is increased further. BACKGROUND The practical significance of vortex-induced-vibration (VIV) has led to a large number of fundamental studies, especially regarding a rather simple form of bluff body like cylinder. Some comprehensive reviews in the field are [1] and [2]. The majority of early work on tethered spheres was concerned with the action of surface waves on tethered buoyant structures ([3], [4]). The tethered sphere was found to vibrate vigorously due to the waves as expected. However, th