Large aerodynamic forces on a sweeping wing at low Reynolds number |
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Authors: | Sun Mao Wu Jianghao |
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Institution: | (1) Institute of Fluid Mechanics, Beijing University of Aeronautics & Astronautics, 100083 Beijing, China |
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Abstract: | The aerodynamic forces and flow structure of a model insect wing is studied by solving the Navier-Stokes equations numerically.
After an initial start from rest, the wing is made to execute an azimuthal rotation (sweeping) at a large angle of attack
and constant angular velocity. The Reynolds number (Re) considered in the present note is 480 (Re is based on the mean chord length of the wing and the speed at 60% wing length from the wing root). During the constant-speed
sweeping motion, the stall is absent and large and approximately constant lift and drag coefficients can be maintained. The
mechanism for the absence of the stall or the maintenance of large aerodynamic force coefficients is as follows. Soon after
the initial start, a vortex ring, which consists of the leading-edge vortex (LEV), the starting vortex, and the two wing-tip
vortices, is formed in the wake of the wing. During the subsequent motion of the wing, a base-to-tip spanwise flow converts
the vorticity in the LEV to the wing tip and the LEV keeps an approximately constant strength. This prevents the LEV from
shedding. As a result, the size of the vortex ring increases approximately linearly with time, resulting in an approximately
constant time rate of the first moment of vorticity, or approximately constant lift and drag coefficients. The variation of
the relative velocity along the wing span causes a pressure gradient along the wingspan. The base-to-tip spanwise flow is
mainly maintained by the pressure-gradient force.
The project supported by the National Natural Science Foundation of China (10232010) |
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Keywords: | model insect wing sweeping motion high lift leading-edge-vortex |
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