On the aerodynamic forces of a plunging airfoil |
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Authors: | John Young Joseph C S Lai |
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Affiliation: | 1.School of Aerospace, Civil & Mechanical Engineering,University of New South Wales, Australian Defence Force Academy,Canberra,Australia |
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Abstract: | The generation of aerodynamic forces by a plunging NACA0012 airfoil at a Reynolds number of 20,000 was studied for a range
of non-dimensional plunge frequenciesk and amplitudesh using a 2D unsteady compressible Navier-Stokes solver, an unsteady panel method (UPM) and Garrick’s analysis. Calculations
using these two methods indicate that the forces collapse reasonably well withkh (or equivalents the Strouhal number), but are only weak functions ofk. In contrast results from the NS code indicate that the forces are dependent on bothk andkh independently, with large variations at low frequencies. The frequency dependence was found to be a result of vortex shedding
from the leading edge of the airfoil, and appears to result from two factors. Firstly at high plunge frequenciesk, the leading edge vortex does not have sufficient time to grow, whereas at lowk the vortex can become a sizeable fraction of the airfoil chord before separating. Secondly once the vortex separates, it
is convected downstream over the surface of the airfoil. Due to the low pressure in the vortex core, thrust is maintained
while the vortex is upstream of the airfoil maximum thickness point (where the airfoil surface is tilted upstream and the
vortex low pressure creates an upstream suction force). Once past this point, the airfoil surface is tilted downstream and
the vortex contributes to drag rather than thrust. At high plunge frequencies the vortex cannot be convected far downstream
before the motion cycle creates another leading edge vortex on the opposite side of the airfoil, so the impact is lessened.
At lowk however the vortex travels far downstream over the airfoil surface, causing drag for a larger portion of the flapping cycle,
and therefore lower propulsive efficiency. These results have strong implications on how the thrust and the propulsive efficiency
can be maximised by controlling the relative amplitudes and phases of combined pitching and plunging motions of an airfoil. |
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