SEPARATION BUBBLE CONTROL FOR GENTLE STALL ON A WORTMANN AIRFOIL

摘要

Sudden stall and lift hysteresis must be avoided on wind turbine airfoils to achieve steady lift and blade loading conditions. The bursting of the leading edge separation bubble is believed to govern these characteristics as well as the double stall phenomenon observed on certain wind turbine airfoils. Control of the separation bubble was demonstrated on a Wortmann FX 63-137 airfoil with a thin leading edge wire at Reynolds numbers of 200,000 to 600,000. The static wire (diam/chord = 0.25%) was positioned at different locations about the leading edge. Optimum positions of (x/c,y/c)% = (0.0,-3.2) or (-3.2,-8.0), yielded: 1) a gentle rather than precipitous stall, and 2) complete elimination of hysteresis effects in lift and drag without incurring a drag penalty. Velocity profiles showed that the Wortmann airfoil's leading edge separation bubble was eliminated with the static wire, thereby removing the critical triggering mechanism for sudden stall and hysteresis effects. It appears that the static wire induced streamline curvature changes about the leading edge which may have: 1) alleviated the severe adverse pressure gradient of the maximum suction peak and 2) initiated an early or accelerated transition about the leading edge.