Momentum theory of Joukowsky actuator discs with swirl
G.A.M. Van Kuik (TU Delft - Wind Energy)
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Abstract
Actuator disc theory is the basis for most rotor design methods, be it with many extensions and engineering rules added to make it a well-established method. However, the o_-design condition of a very low rotational speed of the disc is still a topic for scienti_c discussions. Several authors have presented solutions of the associated momentum theory for actuator discs with a constant circulation, the so-called Joukowsky discs, showing the e_ciency Cp ! 1 for ! 0. The momentum balance is very sensitive to the choice of the vortex core radius _ as the pressure and velocity gradients become in_nite for _ ! 0. Viscous vortex cores do not show this singular behaviour so an inviscid core model is sought which removes the momentum balance sensitivity to singular ow. A vortex core with a constant _ does so. Applying this results in Cp ! 0 for ! 0, instead of Cp ! 1. The Joukowsky actuator disc theory is con_rmed by a very good match with the numerically obtained results. It gives higher Cp values than corresponding solutions for discs with a Goldstein-based wake circulation published in literature.