Flows around two airfoils performing fling and subsequent translation and translation and subsequent clap |
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Authors: | Sun Mao Yu Xin |
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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 structures of two airfoils performing “fling and subsequent translation” and “translation
and subsequent clap” are studied by numerically solving the Navier-Stokes equations in moving overset grids. These motions
are relevant to the flight of very small insects. The Reynolds number, based on the airfoil chord lengthc and the translation velocityU, is 17. It is shown that: (1) For two airfoils performing fling and subsequent translation, a large lift is generated both
in the fling phase and in the early part of the translation phase. During the fling phase, a pair of leading edge vortices
of large strength is generated; the generation of the vortex pair in a short period results in a large time rate of change
of fluid impulse, which explains the large lift in this period. During the early part of the translation, the two leading
edge vortices move with the airfoils; the relative movement of the vortices also results in a large time rate of change of
fluid impulse, which explains the large lift in this part of motion. (In the later part of the translation, the vorticity
in the vortices is diffused and convected into the wake.) The time averaged lift coefficient is approximately 2.4 times as
large as that of a single airfoil performing a similar motion. (2) For two airfoils performing translation and subsequent
clap, a large lift is generated in the clap phase. During the clap, a pair of trailing edge vortices of large strength are
generated; again, the generation of the vortex pair in a short period (which results in a large time rate of change of fluid
impulse) is responsible for the large lift in this period. The time averaged lift coefficient is approximately 1.6 times as
large as that of a single airfoil performing a similar motion. (3) When the initial distance between the airfoils (in the
case of clap, the final distance between the airfoils) varies from 0.1 to 0.2c, the lift on an airfoil decreases only slightly but the torque decreases greatly. When the distance is about 1c, the interference effects between the two airfoils become very small. |
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Keywords: | two airfoils fling translation clap Navier-Stokes simulation |
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