High-fidelity simulations of moving and flexible airfoils at low Reynolds numbers |
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Authors: | Miguel R Visbal Raymond E Gordnier Marshall C Galbraith |
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Institution: | (1) Computational Sciences Branch, Air Vehicles Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433, USA |
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Abstract: | The present paper highlights results derived from the application of a high-fidelity simulation technique to the analysis
of low-Reynolds-number transitional flows over moving and flexible canonical configurations motivated by small natural and
man-made flyers. This effort addresses three separate fluid dynamic phenomena relevant to small fliers, including: laminar
separation and transition over a stationary airfoil, transition effects on the dynamic stall vortex generated by a plunging
airfoil, and the effect of flexibility on the flow structure above a membrane airfoil. The specific cases were also selected
to permit comparison with available experimental measurements. First, the process of transition on a stationary SD7003 airfoil
section over a range of Reynolds numbers and angles of attack is considered. Prior to stall, the flow exhibits a separated
shear layer which rolls up into spanwise vortices. These vortices subsequently undergo spanwise instabilities, and ultimately
breakdown into fine-scale turbulent structures as the boundary layer reattaches to the airfoil surface. In a time-averaged
sense, the flow displays a closed laminar separation bubble which moves upstream and contracts in size with increasing angle
of attack for a fixed Reynolds number. For a fixed angle of attack, as the Reynolds number decreases, the laminar separation
bubble grows in vertical extent producing a significant increase in drag. For the lowest Reynolds number considered (Re
c
= 104), transition does not occur over the airfoil at moderate angles of attack prior to stall. Next, the impact of a prescribed
high-frequency small-amplitude plunging motion on the transitional flow over the SD7003 airfoil is investigated. The motion-induced
high angle of attack results in unsteady separation in the leading edge and in the formation of dynamic-stall-like vortices
which convect downstream close to the airfoil. At the lowest value of Reynolds number (Re
c
= 104), transition effects are observed to be minor and the dynamic stall vortex system remains fairly coherent. For Re
c
= 4 × 104, the dynamic-stall vortex system is laminar at is inception, however shortly afterwards, it experiences an abrupt breakdown
associated with the onset of spanwise instability effects. The computed phased-averaged structures for both values of Reynolds
number are found to be in good agreement with the experimental data. Finally, the effect of structural compliance on the unsteady
flow past a membrane airfoil is investigated. The membrane deformation results in mean camber and large fluctuations which
improve aerodynamic performance. Larger values of lift and a delay in stall are achieved relative to a rigid airfoil configuration.
For Re
c
= 4.85 × 104, it is shown that correct prediction of the transitional process is critical to capturing the proper membrane structural
response. |
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Keywords: | |
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