The dynamic flow behaviour around cylindrical and prismatic pillars of floating wind turbines and offshore platforms is investigated experimentally. Not only the flow conditions, such as the Reynolds number and the turbulence of the ocean current, but also the cross-sectional shape, the angle of attack, and the surface condition dominate the flow phenomena around the pillars. In addition, the relative positions and distances between the pillars play a very important role. They all influence the behaviour of the boundary layer on the surface of a pillar, in particular the positions of the transition from laminar to turbulent flow and the detachment from the surface. In case of a reattachment of the free shear layers on the surface takes, the above mentioned flow and cylinder parameters also have a strong influence on the behaviour of the resulting recirculation bubbles. The interaction of all of these phenomena determines the resulting unsteady hydrodynamic loads, the frequency and strength of both the eddies and vortices, and the flow topology of the wake, and thus the vibration behaviour of the offshore installation. This is a fundamental hydrodynamic bluff-body problem, which is to date still not fully understood.
To study the vortex-induced vibrations of such systems, measurements are carried out in the High Pressure Wind Tunnel Göttingen (DNW-HDG) on two-dimensional circular cylinders and prisms, the latter with a square cross-section with sharp or rounded edges, at realistic Reynolds numbers of the order of 106 to 107. For this purpose, two different test sections are available, one for the investigation of steady models in single and tandem configuration, the other for measurements on axially rotating single cylinders or prisms for the investigation of the dynamic change in angle of attack. By using piezoelectric force and moment balances, pressure transducers in the model and a pressure rake, the unsteady lift and drag forces, the pitching moment, the frequency and strength of the vortices, the pressure distributions on the model's surface, and the wake profile can be determined for Reynolds numbers between 10,000 and 10 million. In case both models are equipped with pressure sensors, the mutual interaction for tandem configurations can also be investigated. As a continuation of these measurements, one test section will be modified in the upcoming years to allow measurements with two models undergoing forced-motions. One of both models will then perform a pure torsional galloping motion, while the other one will move around the first model with a combined pitch and torsional motion.
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