On the tip vortex moving inboard before wake expansion drives it outboard

Abstract

Several SPIV experiments on model wind turbine rotors1,2,3 focussing on the flow around the tip have shown that after being released the tip vortex first moves to a lower radius. This is in contrast to what is generally assumed, namely that the wake expansion starts immediately. A detailed analysis of one of these experiments, the 2m Ø TUDelft-B rotor tested in the TUD 3m Ø Open Jet Facility, is combined with CFD calculations and a vortex panel code using the full blade geometry. Figure 1 shows the tip of the rotor, the coordinate system and the contour of the flow analysis. For 6 chordwise positions of this contour the spanwise and chordwise vorticity, the associated circulation and the Kutta-Joukowsky loads acting on the chordwise vorticity have been determined. Figure 2 shows the results as a function of the chordwise position. The squares show the CFD data, the diamond marker indicates the experimental data and the triangles present the results of a momentum balance applied to the experimental data.