Experimental investigation of flexible rotors in water

Rotor blades often exhibit some degree of flexibility that can potentially be used to increase the mechanical strength of the rotor or expand its operational range. Our interest is to explore experimentally the fundamental fluid-structure interactions of a flexible rotor in water. We consider a single-bladed small-scale rotor of 88 mm radius and 20 mm chord. The rotor blade is rectangular, without twist or taper, and has a flat plate profile. The effect of varying the pitch angle, the freestream velocity, and the rotation frequency is analyzed through optical measurements of the flapwise bending and twist. The flow field is obtained through Particle Image Velocimetry. Interesting behaviors are observed, including extreme bending states and the reversal of bending direction for high pitch angles. In addition, for negative pitch angles, large-amplitude, low-frequency bending fluctuations are observed, accompanied by large-scale recirculation zones. These zones are formed and shed behind the rotor, resembling the dynamics of the Vortex Ring State known from helicopter aerodynamics. The blade deformation depends mainly on the tip speed ratio at low pitch and on the tip Reynolds number at high pitch. Comparisons with a rigid blade and two-bladed flexible rotors will also be presented.