An experimental benchmark study on shape effects of structures under forced vertical oscillation: Flow fields, wave radiation, and hydrodynamic coefficients

Abstract

Floating structures play a crucial role in transitioning from fossil fuels to renewable energies, either in floating offshore wind, floating solar, or wave energy applications. However, an accurate and efficient determination of the floating structures’ motion in waves remains a challenging task, particularly in mid-fidelity applications or applications with the aim of real-time capabilities: In particular, evaluating viscous effects and their importance for hydrodynamic damping is crucial. The wave radiation damping is often deemed dominant for vertically oscillating structures, but viscous effects cannot be neglected. The underlying physical processes of such viscous effects are not fully understood due to a lack of high-quality three-dimensional experimental flow field data. The present study investigates structures under forced vertical oscillation and aims at enhancing the physical understanding of the relation between structural shapes and features, the hydrodynamic parameters and the flow field using 3D time-resolved flow field measurements from a particle tracking velocimetry system. These investigations lead to a benchmark dataset for the development and validation of sophisticated numerical models which is published alongside this study. Three different generic box-type shapes are investigated: (i) a sharp-edged box, (ii) a round-edged box, and (iii) a box with a heave plate at the bottom. The experimental results show strong shape effects on the hydrodynamic coefficients, wave radiation, and the flow field. The radiated wave height can differ up to 50% between the three structures. Viscous effects from vortex structures and separation lead to a $KC$-related increase of damping values for structures with sharp edges or heave plates. Significant shape effects on the vorticity are reported with a factor of up to 500% between the structures. Additionally to structural shape and features, the vorticity depends on $KC$ and oscillation frequency, while the vortex size solely depends on $KC$ and much less on structural shape (with a factor of up to 70%) and frequency. A comparison with potential flow simulations yields qualitatively good agreement to predict shape effects in added mass and radiation damping; however, a comparison between radiated wave heights from experiments and potential flow simulations indicates the necessity to validate potential flow results for quantitatively correct results.