Extracting the angle of attack on rotor blades from CFD simulations

In today's wind energy research, comparisons of low-fidelity aerodynamic models with CFD simulations are common practice. While the approach for loads with respect to the rotor-plane coordinate system such as distributed driving force or thrust is straightforward, a comparison of aerodynamic characteristics is more challenging. The radial distributions of lift, drag, and moment coefficient depend on the local angle of attack and inflow velocity, which cannot be directly determined from the flow field. It requires the elimination of the influence of the rotor blade's bound circulation and also involves the step from a three-dimensional flow field to quantities, which depend on the blade radius. The present investigation analyzes 4 different approaches to determine the angle of attack and inflow velocity from three-dimensional rotor simulations. In addition to 3 existing methods, a new line average technique is presented. It eliminates the effect of bound circulation by averaging along a closed shape, which is symmetric to the quarter-chord point. All methods are assessed in cases with successively increasing complexity. The observed discrepancies are assigned to a different consideration of trailing and shed vortices. The line average approach was found to be a valuable alternative to existing approaches especially in unsteady cases and regarding the outer or tip sections of the rotor blade.