Experimental study on viscous effect in roll and heave motions of a rectangular structure

Abstract

This study focuses on the interaction between roll and heave motions of a rectangular structure, and the heave influence on the viscous effect on the roll motion experimentally in a two-dimensional wave tank. The dynamic responses were investigated with a range of regular wave periods, including longer, equal to, shorter than the natural roll wave period of the structure with wave steepness in beam sea conditions. The velocity fields were obtained using particle image velocimetry in the vicinity of the structure to elucidate the viscous effect on the roll motion, and its different features between the heave fixed and free conditions. Velocity maps were compared systematically to understand the heave and roll influences on the viscous effect for four motion conditions of all fixed, roll fixed and heave free, roll free and heave fixed, and roll and heave free. It is found that the heave motion has a significant impact on the viscous effect and the roll motion. The roll motion has the nonlinearity and the vortex strength was intensifying with the increase of the wave steepness at the natural roll period. The heave motion should be considered to estimate accurate roll damping and motion of floating structures.