Numerical investigation of flow over a variable length wind turbine blade

Telescopic (variable length) blades on a small wind turbine allow higher power generation without increasing the generator rating. Such a blade can adapt its length by an extension and retraction mechanism such that it extends at wind speed below the rated and retracts at wind speed greater than the rated speed. An inherent feature of a variable length blade is a discontinuity in the chord between the fixed root blade section, and the movable tip blade. This paper investigates the effect of sudden step change in the chord of a variable length wind turbine blade, using a 3D numerical analysis. The ratio of average chord in the root blade (C1) and tip blade (C2) governs flow stability as well as the mechanical strength of the blade. Numerical analysis for blade with chord ratio of 0.5 shows an abrupt drop in pressure close to the step region. This reduces the lift to drag (L/D) ratio by about 65% in the blade with chord ratio 0.5, where as in the case of blade with chord ratio 0.8, flow remained streamlined at all the elemental sections with an estimated 21% reduction in L/D.