On the suppression of vortex induced vibrations of circular cylinders by radial water jets
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- Institutt for marin teknikk 
For oilfield developments in deep water, it is of utmost importance to accurately predict Vortex Induced Vibrations (VIV) of risers and free span pipelines subjected to ocean current. In order to prolong operational life of such structures, VIV suppression devices such as helical strakes or shrouds are often employed. Such devices will, however, imply certain disadvantages such as handling problems during installation and drag amplification for non-vibrating conditions. The quest for effective suppresion devices with a minimum of such drawbacks is therefore still ongoing. The present thesis presents an approach for VIV suppression based on radial water jets from prescibed patterns of circular openings in the cylinder wall. The initial intension of the jet flow was to disrupt correlation of vortex shedding along the cylinder. However, several other effects are important when jet flow is introduced, such as: alteration of hydrodynamic pressure around the cylinder, and increased turbulence in the flow surrounding the cylinder. Suppression of VIV is investigated using both hydrodynamic experiments as well as Direct Numerical Simulations. Experiments are preformed with elastically mounted rigid circular cylinders of length L = 2m and diameter D = 10cm. The total amount of experiments conducted is in excess of 4700. Apart from the smooth cylinder, 15 different jet configurations are tested for 4-7 different jet flow rates, most of which are subjected to 20-50 different towing velocities. Experimental results presented show that steady blowing of radial water jets at discrete locations along the cylinder can effectively reduce VIV amplitudes as well as drag forces. While other means of VIV suppression may give rise to an increase in drag forces, the method described in this thesis shows effective reduction also for the drag coefficient. Changes in oscillation amplitudes and drag coefficents are dependent upon the jet pattern on the cylinder surface, orientation with respect to the ambient flow, as well as jet flow velocity. Direct Numerical Simulations have been performed using the Spectral/hp element code Nεκταr. Smooth cylinder results and grid resolution studies show good comparison with previously published findings. Jet flow through openings in the cylinder wall is modeled in 2-D, and a parameter study is performed where number of jets as well as jet location on the cylinder circumference and jet flow rate is varied. 3-D simulations are also performed, however not with jet flow from the cylinder.