Effect On Vortex Induced Vibrations From RadialWater Jets Along A Circular Cylinder

Abstract

Most of the latest years' research on vortex induced vibrations (VIV) of circular cylinders in water has been motivated by the offshore industry's need for reliable predictions of the behaviour of deepwater risers and free span pipelines subjected to ocean current. An obvious approach to solve VIV problems is to introduce suppression devices such as helical strakes or fearings. Such devices may influence the vortex shedding process in two ways: 1) 2-dimensional (2-D) effects will influence the local forces on the cross section and damping by altering the separation point as well as creating a general disturbance to the flow. 2) 3-D effects include changes in correlation of the vortex shedding process along the span of the cylinder. It is well known that vortex shedding on a long cylinder will lead to significant vibration amplitudes only if the shedding process along the cylinder is correlated, and that the vibration itself will control this correlation. There is hence a positive feedback from oscillation amplitudes to correlation that under ideal conditions will amplify the oscillation amplitude until a maximum value slightly above one cylinder diameter is reached. The present paper presents a novel approach for VIV suppression based on radial water jets from a prescribed pattern of circular openings in the cylinder wall. This flow will introduce a disturbance that is expected to yield reduced VIV amplitudes. Results will be presented from experiments in a towing tank testing a spring-supported cylinder with a helical pattern of radial water jets. The volume flow rate and reduced velocity have been varied in the tests. Oscillation amplitudes, frequencies, added mass and lift and drag force coefficients are presented and compared to results with a cylinder without any water blowing.