Measurements of transverse forces on circular cylinders undergoing vortex-induced vibration

Experiments have been carried out on a circular cylinder, with and without helical strakes, free to respond in a direction transverse to a water flow. The Reynolds number range was between 3×103 and 2.1×104, the mass ratio was just above 0.8 and the fraction of critical damping was approximately 2×10−4. Measurements are presented of the response, the transverse fluid force and the phase angle between the response and the force, all as a function of reduced velocity. The straked cylinder is observed to respond over a narrow range of reduced velocity and its maximum amplitude is decreased by just over 60%, compared with a plain cylinder. The familiar phase jump that occurs for a plain cylinder did not occur with the straked one, with the phase close to zero over the entire reduced velocity range where response to vortex shedding occurred.