An insight into the rotational augmentation on HAWTs by means of CFD simulations - Part I: State of the art and numerical results

The scientific debate about the rotational augmentation is currently quite open as there is no consensus on the rise and development of this phenomenon. Centrifugal forces, Coriolis effects and spanwise pressure distribution are recognized as being responsible for the rise of the rotational augmentation but, at present, their specific relevance has not been physically demonstrated. Tens of empirical equations were in fact proposed by several authors in order to take into account the aforementioned phenomena inside simplified 1D design codes. The results are quite different amongst each other and seem to be related to particular conditions instead of giving a generalized vision of the problem. The present paper starts from the aforementioned assumptions and attempts to provide a new global physical view of the phenomenon. Through the use of validated CFD models, an experimental micro HAWT was extensively analyzed. The model was based on a calibrated RANS transition turbulence model, particularly suitable for low Reynolds applications. In this first part, a review of the CFD modeling strategy and validation is presented. Sectional data of 3D lift and drag coefficients were extrapolated depending on radial position, wind speed, rotational speed and finally compared to 2D data. Furthermore, 3D CFD simulations of a single fixed wing, with a velocity profile equal to the one generated by rotation and a momentum equal to that due to rotation was implemented. This was done in order to try to separate the effect of centrifugal and Coriolis forces from that caused by the spanwise pressure distribution.