International audienceA century ago, Joukowsky proposed an aerodynamic rotor model utilizing an actuator disc with constant circulation. Since then, this model has been the subject of much controversy, as it predicts a power performance that for all tip speed ratios exceeds the Betz limit and that goes to infinity when the tip speed ratio approaches zero. Recently, it was demonstrated that the Joukowsky model is fully compatible with the inviscid Euler equations and that the apparent inconsistency can partly be explained by the fact that the model neglects lateral forces due to pressure or friction [1]. Furthermore, CFD computations using an actuator line model have shown that the excessive swirl of the hub vortex at small tip speed ratios generates vortex breakdown on the wake axis, causing a recirculating zone that limits the power yield [2]. In the past years we have studied in detail the flow behavior of Joukowsky rotors at small tip speed ratios (TSR). It is shown that the excessive swirl appearing towards the rotor center at small tip speed ratios generates vortex breakdown, causing a recirculating zone in the wake that limits the power yield of the rotor. The appearance of vortex breakdown has a similar effect on the flow behavior as the vortex ring state that usually appears at higher tip speed ratios. Limits to where vortex breakdown might occur with tip speed ratio and rotor loading as parameter are investigated. The limits found correspond to well-known criterion for vortex breakdown onset for swirling flows in general. By applying a criterion for vortex breakdown in combination with the general momentum theory, the power performance always stays below the Betz limit
