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3 International Journal of Applied Mechanics and Engineering

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Prediction of the three-dimensional separation on a rotating blade

100%

Dumitrescu H. , Cardos V.

2007

tom Vol. 12, no 4

941-950

Blade rotation routinely and significantly augments aerodynam c forces during zero yaw horizontal axis wind turbine operation. To understand better the flow physics underlying this phenomenon, three-dimensional and rotational viscous effects on wind turbine blades are investigated by means of a 3-D boundary-Iayer model. The governing equations of the model are derived from 3-D primitive variable boundary-Iayer equations written in cylindrical coordinates in the rotating fratne of reference. The latter are integrated along the peripheral direction with the radial distance as parameter for a particular external flow. The skin friction coefficient is used to identify boundary layer separation and shear layer reattachment locations. Separation and reattachment kinematics shows at inboard locations that while the separation point location is not realIy atTected and remains near the leading edge, the reattachment point advances forward rapidly on the blade chord from the trailing edge as radial distance decreases. It is concluded that the rotational augmentation is linked to specific separation and reattachment state strictly determined by the Coriolis forces.

Analysis of inboard flow around the wind turbine blades

100%

Dumitrescu H. , Cardos V.

2011

tom Vol. 16, no 2

345-358

The complex flow in the rotor root area is analyzed by means of the boundary layer approach. The approach involves a radial angular velocity (Ekman) boundary-layer on the rotor disk rotating with an angular velocity smaller than that of the fluid, and the circumferential velocity boundary-layer developing on blades. The first explains the smaller adverse pressure gradient at the leading-edge of blades by a vortex-induced sucking effect, and the other shows the contribution of Coriolis force to the closed separation behavior on the suction side of the inboard blade sections, that explains the stall-delay phenomenon. Three-dimensional incompressible steady momentum integral boundary layer equations are used to analyze the leading-edge separation bubble on a rotating blade, including the effect of enhanced rotation at strong winds. The stall-delay phenomenon is described as a three contribution process: vortex-induced sucking effect by an Ekman layer type boundary-layer followed by Coriolis force, which acts in the chordwise direction as a favorable pressure gradient, and centrifugal forces producing a spanwise pumping effect. It appears that the first two contributions play the primary role for the rise of the inboard stall-delay and the centrifugal pumping effect is much less important than generally was supposed before.

On the inboard stall delay due to rotation

86%

Dumitrescu H. , Cardos V.

2008

tom Vol. 13, no 2

359-372

This paper investigates the boundary-layer characteristics of a wind turbine blade of small chord length. The three-dimensional form of momentum integral equations is derived and used to predict the boundary-layer growth and limiting streamline angles on the blade surface for both attached and separating flow. The chordwise skin friction coefficient is used to identify boundary layer separation and shear layer reattachment locations. The nature of flow near the axis of rotation is discussed and the physical mechanism associated with 3-D and rotational effects is identified. A semi-empirical correction law for the lift coefficient based on 2-D airfoil data is established. Comparing calculated and measured lift curves of a stall controlled wind turbine, it is shown that the proposed correction law may improve significantly the accuracy of the predictions.