Viscous damping analysis of WEC’s hull in yaw motion - methodology and viscous damping parameters identification

A comprehensive characterization of a Wave Energy Converter (WEC) requires an overall analysis of the hull, which is identified by inertial terms like mass, added mass and inertia. Being the considered WEC a floating body, from pre-design phases until final stages such as optimization and executive design, equation of motion, both in time and frequency domain, must be solved accurately. All the terms of the equation must be identified a priori via experimental campaigns or numerical simulation like Boundary Element Methods (BEMs) or Computational Fluid Dynamics (CFD) techniques. On the one hand, parameters like added mass, inertial terms, or even roll and pitch damping coefficients can be identified via well-known methodologies, for instance, free decay; on the other hand, the damping related to yaw degree of freedom (DOF) can be difficult to handle, since the absence of an equilibrium condition. The work proposes a methodology, similar to the radiation tests, to identify the viscous damping along the rotational degree of freedom (DOF) of yaw which involves the imposition of different rotation rates about yaw-axis to compute moments and fluid forces via CFD, then the identification of damping terms. The outcomes consist of all those terms that can be used to tune and refine lower-fidelity models like BEMs, which require damping parameters of every DOF, included the non-linear ones, for instance, proportional to the square of velocity about the corresponding axis. The methodology shown in this work tries to give a tool of computation of damping coefficient when classical free decay tests can not be performed.