An experimental study of Hydrodynamic Forces on Cylinders and Cables in Near Axial Flow

Abstract

The thesis addresses the hydrodynamic forces on cylinders where the angle between incoming flow and the cylinder axis, the angle of attack, is low. Measured results for a rigid cylinder with length to diameter ratio of 40 towed at both constant angle of attack and oscilllating in the transverse direction are used to discuss the applicability of suggested methods like the cross flow or the 2D+t principle. It is found that the longitudional flow do influence the transverse forces. The importance of the flow pattern initiated at the nose of the cylinder is clearly illustrated.

A combination of linear and quadratic dependence on the sine of the angle is used to model the response of a flexible cylinder with forced oscillation of the tow point. The result is compared to experimental result for a flexible cylinder with length to diameter ratio of 1100 and Reynolds numbers in and above the critical range. The cylinder is simulated in time domain with a Finite Element Method with second order elements. As an example of practical application of the model, the response of a part of a full scale streamer subject to irregular waves and a control device is investigated. In realistic sea states the response is found to be rather small, but not damped by the control device.