The near wake development of a wind turbine operating in stalled conditions – Part I: Assessment of numerical models

Abstract. This study comprehensively investigates the near wake development of a model wind turbine operating at a low tip speed ratio in stalled conditions. In the present paper, part I, different ways of representing the turbine, that is the full geometrical representation and the modeling by means of the actuator line method, but also different approaches for the modeling of turbulence are assessed. The simulation results are compared with PIV measurements from the MEXICO and NewMexico experiments. A highly-resolved numerical setup was created and a higher-order numerical scheme was applied targeting an optimal resolution of the tip vortex development and the wakes of the blades. Besides the classical unsteady Reynolds-averaged methodology, a recently developed variant of the Detached Eddy Simulation (DES) was employed, which features robust shielding capabilities of the boundary layers and enhanced transition to a fully developed LES state. An actuator line setup featuring the same turbulence modeling as for the DES simulation was created, where the aerodynamic forces were either evaluated by means of tabulated data or imposed from the averaged blade loads of the simulations with full blade geometry. The purpose is to distinguish between the effects of the force projection and the force calculation in the underlying blade-element method on the blade wake development. The averaged properties of the near wake flow field were accurately captured by all methods. Unsteady flow features of the separated shear layer were very well reproduced by the scale-resolving simulation with full geometric turbine representation. This method was also capable to predict the tip vortex development, in terms of vortex strength, position and size over the entire simulation domain. The classical URANS method performed very poorly. In the actuator line simulations it was found that very similar results are obtained compared to the fully resolved simulations, when inheriting their forces. However, when operating the actuator line with forces from blade-element momentum theory, the wake topology is not predicted correctly, in particular, in the inner part of the rotor. In the separated flow conditions of this case, it demands for correction models to take in to account the effect of rotational augmentation. In part II of the study the development and the dynamics of the early tip vortex formation is detailed in another paper.