Wakes of Surface-Piercing Cylinders

Abstract The hydrodynamics of surface-piercing cylinders differ from fully submerged cylinders due to the generation of surface wakes and under increasing flow velocities the formation of a ventilated pocket in the lee of the cylinder, both of which grow with increasing velocity, with concomitant effects on the hydrodynamic loading. Laboratory testing of surface-piercing cylinders to date has predominantly been confined to characterising the wakes of a rigid cylinder cantilevered down into the water from a towing tank carriage, which under certain test conditions will exhibit significant Vortex-Induced-Vibration (VIV). The wake characteristics of cylindrical structures such as those protruding through the free surface from a submerged platform do alter substantially depending on the structure, geometrical properties, and operating parameters. The relative length of submergence, or immersed aspect ratio (AR) and end conditions of the cylinder do also impact the wake formation. The wake characteristics can be described by two major regions: above and below the waterline. Where the above waterline wakes are derivatives of the bow wave run-up on the leading face of the cylinder, and the below waterline wakes are derivatives of the ventilation, which is the entrapment of air of the rear face of the cylinder. This paper presents an experimental programme in a towing tank to characterise the wake of cylindrical structures of different diameters, submergence, and forward speeds. Conventional experiments have the model attached from above the surface, however, this method is not representative of some real-world applications such as structures that are bottom mounted. [To the best of our knowledge] The authors developed the novel design of a bottom mount test platform, where the model is cantilevered from a single connection pointed up through the water column. The experiments collected a range of results including the forces, motions and wakes of the models where this paper will focus on presenting and defining the different wakes and insights into these phenomena.