| Offshore structures such as Spar Platforms and Tension Leg Platforms may experience resonant oscillations in heave under first and more likely second-order wave forces; thus damping becomes a critical factor in limiting the response amplitude of the structures. Usually, these offshore structures are lightly damped; hence, drag-augmenting devices may be required to limit the response amplitude to a safe range. In the present research, thin plates are studied as one method to enhance the hydrodynamic damping of offshore structures. The problem is investigated from both the hydrodynamic force and underlying flow physics viewpoints through force measurement and qualitative/quantitative flow visualization experiments. The force measurements show geometric dependence by examining thickness-to-diameter ratio, reinforcing structure, and edge radius. Additionally, parametric dependence such as oscillation amplitude and frequency are included. The flow physics includes a comparison of the Keulegan-Carpenter number and thickness-to-diameter ratio dependence through a series of flow visualization and Digital Particle Image Velocimetry experiments. It is found that damping coefficients behave differently in three ranges of KC number. The transitional KC number is thickness-to-diameter ratio dependent, and the transition occurs at larger KC number for the thicker disk. The plate thickness-to-diameter ratio affects damping coefficients dramatically at small KC numbers (0 to 1.1). The flow around a 1/87.5 plate experiences four distinct vortex formation modes as the KC number increases from 0 to 1.1. A quantitative analysis of the vortex formation mode at large KC (unidirectional shedding) is conducted. A new turbulent decomposition method is applied to calculate Reynolds stresses; it is concluded that the cycle-to-cycle variation, due to vortex instability and azimuthal structures resulting from vortex-vortex interaction and turbulence-vortex interaction, is not negligible. |
