Effect of airfoil thickness on onset of dynamic stall is investigated using large eddy simulations at chord-based Reynolds number of 200 000. Four symmetric NACA airfoils of thickness-to-chord ratios of 9 %, 12 %, 15 % and 18 % are studied. The three-dimensional Navier–Stokes solver, FDL3DI is used with a sixth-order compact finite difference scheme for spatial discretization, second-order implicit time integration and discriminating filters to remove unresolved wavenumbers. A constant-rate pitch-up manoeuver is studied with the pitching axis located at the airfoil quarter chord. Simulations are performed in two steps. In the first step, the airfoil is kept static at a prescribed angle of attack ( $=4^{\circ }$ ). In the second step, a ramp function is used to smoothly increase the pitch rate from zero to the selected value and then the pitch rate is held constant until the angle of attack goes past the lift-stall point. The solver is verified against experiments for flow over a static NACA 0012 airfoil. Static simulation results of all airfoil geometries are also compared against XFOIL predictions with a generally favourable agreement. FDL3DI predicts two-stage transition for thin airfoils (9 % and 12 %), which is not observed in the XFOIL results. The dynamic simulations show that the onset of dynamic stall is marked by the bursting of the laminar separation bubble (LSB) in all the cases. However, for the thickest airfoil tested, the reverse flow region spreads over most of the airfoil and reaches the LSB location immediately before the LSB bursts and dynamic stall begins, suggesting that the stall could be triggered by the separated turbulent boundary layer. The results suggest that the boundary between different classifications of dynamic stall, particularly leading edge stall versus trailing edge stall, is blurred. The dynamic-stall onset mechanism changes gradually from one to the other with a gradual change in some parameters, in this case, airfoil thickness.
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Low Reynolds number wake modification using a Gurney flap
We numerically examine the use of Gurney flap to modify the two-dimensional wake dynamics for lift enhancement on NACA 0000 (flat plate), 0006, 0012 and 0018 airfoils. Incompressible flows over the airfoils at different angles of attack are considered at Re = 1000. It is observed that the attachment of the Gurney flap at the trailing edge is able to enhance the lift force experienced by the airfoil appreciably with increase in Gurney flap height. The lift-to-drag ratio of the airfoils is also observed to increase at lower angles of attack. The lift spectra and airfoil wake are examined to reveal the effect of the Gurney flap on the formation of different characteristic wake modes and the associated change in the aerodynamic forces exerted on the airfoils. Based on the observations, we classify the resulting wakes into four distinct modes. The emergence of these modes (steady, 2S, P and 2P) are mapped over a wide range of angles of attack and Gurney flap heights for all four airfoils in consideration.
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- Award ID(s):
- 1632003
- NSF-PAR ID:
- 10064540
- Date Published:
- Journal Name:
- 55th AIAA Aerospace Sciences Meeting
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.2514/6.2017-0543
