Unsteady Low Reynolds Number Aerodynamics of a Rotating Wing

Abstract

Micro air vehicles (MAVs) are small, unmanned aircraft useful for reconnaissance. The small size of MAVs presents unique challenges as they operate at low Reynolds numbers O(10^4), and they share a flight regime with insects rather than conventional aircraft. The low Reynolds number regime is dominated by poor aerodynamic characteristics such as low lift-to-drag ratios. To overcome this, birds and insects utilize unsteady high lift mechanisms to generate sufficient lift. A leading edge vortex (LEV), one of these unsteady lift mechanism, is thought to be responsible for the high lift generated by these natural fliers, but the factors which contribute to the formation, stability, and persistence of LEVs are still unclear.

The objectives of this study are to: 1) qualitatively understand the formation of the LEV by evaluating the effect of wing acceleration profiles, wing root geometry, Reynolds number, and unsteady variations of pitch, 2) quantify whether high lift can be sustained at low Reynolds numbers on a rotary wing in continuous revolution, and 3) determine the effect of wing flexibility on the unsteady lift coefficient.

Experiments were performed on a rotating wing setup designed to model the translational phase of the insect wing stroke during hover. Experiments were performed in a water tank at Reynolds numbers between 5,000 and 25,000, and the flow was investigated using dye flow visualization, as well as lift and drag force measurements. A rigid wing and a simple one degree-of-freedom flexible wing were tested.

Dye flow visualization on a rotating wing showed the formation of a coherent LEV near the wing root. The LEV became less coherent further outboard, and eventually burst. As the wing continued to rotate, the location where the LEV burst moved inboard. Dye injection within the burst vortex showed the formation of multiple small scale shedding vortices that traveled downstream and formed a region of recirculating flow (i.e., a burst vortex). Parameter variations in this experiment included velocity profiles, acceleration profiles, and Reynolds numbers.

High lift and drag coefficient peaks were measured during the acceleration phase of the wing stroke at Reynolds numbers of 15,000 and 25,000. After the initial peak, the coefficients dropped, increased, and eventually attained a ``steady-state" intermediate value after 5 chord-lengths of travel, which they maintained for the remainder of the first revolution. When the wing began the second revolution, both the lift and drag coefficients decreased, and leveled out at a second intermediate value. Force measurements on a chordwise flexible wing revealed lower lift coefficients. For all of the cases tested, high lift was achieved during the acceleration phase and first revolution of the wing stroke, though values dropped during the second revolution.