<p>The inherent complexity of the aerodynamics of Vertical-Axis Wind Turbines (VAWTs), characterized by a continuous variation of the angle of attack during the cycloidal motion of the airfoils and the corresponding onset of many related unsteady phenomena such as dynamic stall, requires investigation methods ensuring a level of fidelity higher than that of the commonly adopted Blade Element Momentum theory (BEM). Among them, the Actuator Line Method (ALM), combining a lumped-parameter representation of the rotating blades with the CFD resolution of the turbine flow field, stands out in terms of accuracy and computational cost. In the frame of the ALM approach nonetheless, the implementation of a dynamic stall model, necessary to accurately predict the blade loads during turbine unstable operation, is extremely challenging. Existing models were indeed conceived for BEM methods, thus reducing the flow oncoming on the airfoil to a single velocity vector. In case of a solved flow field, like in the case of ALM models, however, two main issues arise: first, it is important to extrapolate a single value of the angle of attack (AoA) from the CFD solved three-dimensional flow field at each solution timestep; second, the AoA history required as an input to calculate the rate of dynamic variation of the angle itself is characterized by a low signal-to-noise ratio, leading to severe numerical oscillations of the solution and, in extreme cases, to numerical divergence. &nbsp;</p><p>Moving from this background, in this paper a robust procedure to improve the quality of the AoA signal extracted from an ALM simulation is introduced. The procedure combines a novel method for the sampling of the inflow velocity from the numerical flow field with a low-pass filtering of the corresponding angle of attack signal based on Cubic Spline Smoothing (CSS). Such procedure has been implemented in the Actuator Line module developed by the authors for the commercial ANSYS® FLUENT® solver. In order to verity the reliability of the proposed methodology, two-dimensional unsteady RANS simulations of a test 2-blade Darrieus H-rotor, for which high-fidelity experimental and numerical blade loading data are available, have been eventually performed for a selected turbine unstable operation point. A relevant improvement in the solution stability and accuracy is observed and discussed.</p>

A Robust Procedure to Implement Dynamic Stall Models Into Actuator Line Methods for the Simulation of Vertical-Axis Wind Turbines

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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