The assessment of a fast computational method in predicting the unsteady loads of vertical axis wind turbines undergoing floating motion

Abstract

The low center of gravity and low center of thrust of Vertical Axis Wind Turbines (VAWT) are interesting characteristics when considering Floating-Offshore Wind Turbine (FOWT) application. The motion due to the floating platform adds extra complexity to the unsteady aerodynamics of VAWT. Hence, both numerical and experimental studies become very challenging. This paper focuses on the assessment of TubinesFoam, a fast tool consisting of the actuator line method (ALM) embedded in OpenFoam. Then, instead the typical bladed-solved Navier–Stokes equations, the ALM inserts the blade forces into the field as body forces into the momentum equation. A modified version of the turbinesFoam library that includes surge motion for multiple rotors has been evaluated by comparison with an experimental set of data from a twin-rotor performing surge in a wind tunnel. This paper includes the main equations describing the kinematics implemented in the numerical code, as well as a convergence analysis of mesh size, time step and surge cycles. The numerically predicted thrust forces agreed with the experimental results for both investigated tip speed ratios for the case with no surge motion. The numerical and experiment results including surge motion indicated a minimal influence of the surge motion for the cases evaluated here. The last verification using CFD computations from the literature showed that the turbinesFoam tool properly captures the main features of the surge motion. This verified numerical method is a promising tool to understand the effects of geometrical parameters on the performance and the wake development of VAWTs farms in floating-offshore environments.