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Research Papers: Flows in Complex Systems

An Experimental Study of the Laminar Flow Separation on a Low-Reynolds-Number Airfoil

[+] Author and Article Information
Hui Hu

Department of Aerospace Engineering, Iowa State University, Ames, IA 50011huhui@iastate.edu

Zifeng Yang

Department of Aerospace Engineering, Iowa State University, Ames, IA 50011

J. Fluids Eng. 130(5), 051101 (Apr 25, 2008) (11 pages) doi:10.1115/1.2907416 History: Received April 07, 2007; Revised January 31, 2008; Published April 25, 2008

An experimental study was conducted to characterize the transient behavior of laminar flow separation on a NASA low-speed GA (W)-1 airfoil at the chord Reynolds number of 70,000. In addition to measuring the surface pressure distribution around the airfoil, a high-resolution particle image velocimetry (PIV) system was used to make detailed flow field measurements to quantify the evolution of unsteady flow structures around the airfoil at various angles of attack (AOAs). The surface pressure and PIV measurements clearly revealed that the laminar boundary layer would separate from the airfoil surface, as the adverse pressure gradient over the airfoil upper surface became severe at AOA8.0deg. The separated laminar boundary layer was found to rapidly transit to turbulence by generating unsteady Kelvin–Helmholtz vortex structures. After turbulence transition, the separated boundary layer was found to reattach to the airfoil surface as a turbulent boundary layer when the adverse pressure gradient was adequate at AOA<12.0deg, resulting in the formation of a laminar separation bubble on the airfoil. The turbulence transition process of the separated laminar boundary layer was found to be accompanied by a significant increase of Reynolds stress in the flow field. The reattached turbulent boundary layer was much more energetic, thus more capable of advancing against an adverse pressure gradient without flow separation, compared to the laminar boundary layer upstream of the laminar separation bubble. The laminar separation bubble formed on the airfoil upper surface was found to move upstream, approaching the airfoil leading edge as the AOA increased. While the total length of the laminar separation bubble was found to be almost unchanged (20% of the airfoil chord length), the laminar portion of the separation bubble was found to be slightly stretched, and the turbulent portion became slightly shorter with the increasing AOA. After the formation of the separation bubble on the airfoil, the increase rate of the airfoil lift coefficient was found to considerably degrade, and the airfoil drag coefficient increased much faster with increasing AOA. The separation bubble was found to burst suddenly, causing airfoil stall, when the adverse pressure gradient became too significant at AOA>12.0deg.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of a laminar separation bubble formed on a low-Reynolds-number airfoil

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Figure 2

GA(W)-1 airfoil geometry and pressure tap locations

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Figure 3

Schematic of the experimental setup for the PIV measurements

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Figure 4

Surface pressure distribution profiles around the airfoil

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Figure 5

Pressure distribution on an airfoil with laminar separation bubble (Russell (22))

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Figure 6

The estimated locations of the separation points, transition points, and reattachment points at various AOAs

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Figure 7

PIV measurement results at various AOAs

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Figure 8

PIV measurements near the airfoil leading edge with AOA=6.0deg; (a) instantaneous velocity vectors; (b) instantaneous vorticity distribution; (c) ensemble-averaged velocity vectors; and (d) streamlines of the mean flow

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Figure 9

PIV measurements near the airfoil leading edge with AOA=10.0deg; (a) instantaneous velocity vectors; (b) instantaneous vorticity distribution; (c) ensemble-averaged velocity vectors; and (d) streamlines of the mean flow

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Figure 10

PIV measurement results at the rear portion of the separation bubble with AOA=10.0deg; (a) instantaneous velocity field; (b) instantaneous vorticity distribution; (c) ensemble-averaged velocity field; (d) streamlines of the mean flow; (e) normalized Reynolds stress distribution; and (f) normalized turbulent kinetic energy distribution

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Figure 11

PIV measurements near the airfoil leading edge with AOA=12.0deg; (a) instantaneous velocity vectors; (b) instantaneous vorticity distribution; (c) ensemble-averaged velocity vectors; and (d) streamlines of the mean flow

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Figure 12

The measured airfoil lift and drag coefficients; (a) airfoil lift and drag coefficients vs. angle of attack; and (b) lift-drag polar dot

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