MEASUREMENTS OF THE HYDRODYNAMIC DAMPING OF CIRCULAR CYLINDERS

Circular cylinders have been mounted on a pendulum suspension system and the damping measured by recording the amplitude of decaying oscillations. Results have been obtained for a range of Beta values up to 30,000 and for cylinders surfaces from smooth to a finish with the ratio of roughness height to cylinder diameter of 1/50. The results are presented in the form of drag coefficient versus Keulegan Carpenter number and typically the KC number ranges from 0.1 to about 3. They are compared with those obtained by assuming two-dimensional, attached, laminar flow and in all cases the measured values of CD are above the predicted ones. Flow visualisation shows that in all cases studied a three-dimensional instability develops generating a regular array of vortex structures along the cylinders. The wavelength of this instability, which is known as the Honji instability, is found to decrease with increasing Beta.