Modeling Dynamic Stall for a Free Vortex Wake Model of Floating Offshore Wind Turbines

Abstract

Floating offshore wind turbines in deep waters offer significant advantages to onshore and near-shore wind turbines. However, due to the motion of floating platforms in response to wind and wave loading, the aerodynamics are substantially more complex. Traditional aerodynamic models and design codes do not adequately account for the floating platform dynamics. Turbines must therefore be over designed due to loading uncertainty and are not fully optimized for their operating conditions. Previous research at the University of Massachusetts, Amherst developed the Wake Induced Dynamics Simulator, or WInDS, a free vortex wake model of wind turbines that explicitly includes the velocity components from platform motion. WInDS rigorously accounts for the unsteady interactions between the wind turbine rotor and its wake, however, as a potential flow model, the unsteady viscous response in the blade boundary layer is neglected. This work addressed this concern through the integration of a Leishman-Beddoes dynamic stall model into WInDS. The stand-alone dynamic stall model was validated against two-dimensional unsteady data from the OSU pitch oscillation experiments and the coupled WInDS model was validated against three-dimensional data from NREL's UAE Phase VI campaign. WInDS with dynamic stall shows substantial improvements in load predictions for both steady and unsteady conditions. WInDS with the dynamic stall model should provide the necessary aerodynamic model fidelity for future research and design work on floating offshore wind turbines. Model development and validation will be presented, along with several thrusts of ongoing work. The addition of the dynamic stall model allows more insight into the prevalence and severity of dynamic stall in response to unsteady floating platform motion. Full characterization of these transient loads is an important consideration due to the high reliability necessitated by the offshore environment. Collaborative work is also ongoing to couple WInDS with NREL's computer-aided engineering tool FAST, fully coupled with FAST's existing submodules for controls, hydrodynamic, and structural dynamics. Finally, preliminary work on the use of the coupled FAST and WInDS models for design optimization will be presented. The goal of this research is to further optimize the turbine rotor to specifically account for the unsteady aerodynamic loading brought on by floating platform motion.