Dynamic stall model modifications to improve the modeling of vertical axis wind turbines

The Beddoes-Leishman type dynamic stall model was originally implemented in HAWC2 with a focus on horizontal axis wind turbines. In case of HAWTs, some terms in the unsteady airfoil lift and drag are very small and can be neglected, which are very important for VAWTs. Furthermore, the angle of attack variations during normal operation of VAWTs are by far larger than those occuring on HAWTs. This posed a challenge to the Beddoes-Leishman-type dynamic stall model, which had previously been validated for small variations in angle of attack against CFD and measurements. This report contains some necessary modifications of the Beddoes-Leishman type dynamic stall model in HAWC2 to enable unsteady aerodynamic
computations on VAWTs. A short validation against measurements of the NREL/NASA Ames Phase VI rotor in standstill is included. There, it is shown that the model changes have only a small, but beneficial effect at small angle of attack variations.