Abstract
As a tidal turbine blade rotates and deflects under transient loading, it effectively accelerates ambient water (the “added mass”). This leads to a force proportional to and in phase with the relative acceleration, analogous to an additional mass. Hence, the blade will have a greater effective mass than the blade mass alone. This inertial force is treated as an additional load on the blade that resists acceleration. Furthermore, damping forces on the blade have contributions from friction and pressure fluctuations due to wake formation and vortex shedding. From previous studies, it was found that the added mass and damping variations are highly dependent on the amplitude and frequency of the motion. These forces contribute important physical effects to marine turbines that need to be considered in device design. However, the added mass contribution is numerically neglected by many engineering design codes developed for floating wind turbines.
Recently, the OpenFAST code, originally developed by the National Renewable Energy Laboratory for wind turbines, has been extended for designing and modeling both fixed and floating hydrokinetic turbines. Several important physical effects caused by the relatively higher density of water must be accounted for to accurately model marine turbines. These effects have been implemented in OpenFAST and include buoyancy, added mass, and inflow accelerations. Specifically, added mass has been included in OpenFAST’s AeroDyn module using a Morison equation type strip theory inertia force term with user-defined added mass coefficients. These coefficients can be defined for the normal-to-chord, tangential-to-chord, and pitch directions along the span of each blade. Transverse added mass coefficients can also be defined along the length of the support structure.
In this study, we use computational fluid dynamics (CFD) simulations to estimate added mass coefficients along the span of the blade of an axial-flow hydrokinetic turbine. The Reference Model 1 marine turbine attached to a floating platform, details of which were presented at the UMERC 2023 conference, will be used for this study. A prescribed harmonic motion will be imposed on the floating RM1 turbine. Total unsteady hydrodynamic forces exerted on whole system, as well as selected blade sections, will be recorded. Subsequently, these force components will be decomposed to derive added mass coefficients that can be used in future work as inputs for OpenFAST simulations.
OpenFAST development for marine turbines is supported by the U.S. Department of Energy Water Power Technologies Office and Advanced Research Projects Agency-Energy.