Flowfield of a Rotating-Wing Micro Air Vehicle

Experiments were conducted to measure the performance of a rotor, typical of that used on a rotating-wing micro air vehicle, which was shown to develop relatively low hovering efficiency. This inefficiency can be traced to high profile drag losses on the blades and also to the relatively large turbulent and rotational aerodynamic losses that are associated with the structure of the rotor wake. High-resolution flow visualization images have divulged several interesting flow features that appear unique to rotors operating at low Reynolds numbers. The wake sheets trailing from the rotor blades were found to be much thicker and also more turbulent than their higher chord Reynolds number counterparts. Similarly, the viscous core sizes of the tip vortices were relatively large as a fraction of blade chord. However, the flows in the tip vortices themselves were found to be similar, with laminar flow near their core axis and an outer turbulent region. Particle image velocimetry measurements were made to quantify the structure and strength of the wake flow, including the tip vortices. An analysis of the vortex aging process was conducted. A new nondimensional equivalent time scaling parameter is proposed to normalize the rate of growth of the vortex cores that are generated at substantially different vortex Reynolds numbers.