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

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 among the modern simulation methods for Vertical-Axis Wind Turbines (VAWTs) as probably the most interesting compromise between accuracy and computational cost. Being however a method relying on tabulated coefficients for modeling the blade-flow interaction, the correct implementation of the sub-models to account for higher order aerodynamic effects is pivotal. Inter alia, the introduction of a dynamic stall model is extremely challenging. As a matter of fact, two main issues arise: first, it is important to extrapolate a correct value of the angle of attack (AoA) from the CFD solved flow field; 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. In the study, 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 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 verify 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 were available, have been eventually performed for a selected turbine unstable operation point.