A linearization coefficient for Morison force considering the intermittent effect due to free surface fluctuation

Abstract

For members located near the free surface, an intermittent effect caused by the free surface fluctuation makes statistical properties of fluid particle kinematics different. Therefore, a linearization coefficient of the drag force derived under the assumption that the particle velocity and acceleration follow the Gaussian process are not valid above the still water level. The present paper derives the linearization coefficient for the drag force considering the intermittent effect. Force spectra and mean values of force maxima at each location are compared between nonlinear and linearized drag force to verify the validity of the proposed coefficient. The linearization coefficients derived from fully submerged section are constant irrespective of the observation position, and it tends to underestimate both the mean values of peak forces and force spectra as the observation height increases near the free surface. The proposed linearization coefficient reflects the effect of the free surface fluctuation and improves the error between the nonlinear and linear drag with increasing observation position.