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Research Papers: Fundamental Issues and Canonical Flows

Forcing Boundary-Layer Transition on a Single-Element Wing in Ground Effect

[+] Author and Article Information
Luke S. Roberts

Aeromechanical Systems Group,
Centre for Defence Engineering,
Cranfield University,
Defence Academy of the United Kingdom,
Shrivenham SN6 8LA, UK
e-mail: l.roberts@cranfield.ac.uk

Mark V. Finnis

Aeromechanical Systems Group,
Centre for Defence Engineering,
Cranfield University,
Defence Academy of the United Kingdom,
Shrivenham SN6 8LA, UK
e-mail: m.v.finnis@cranfield.ac.uk

Kevin Knowles

Aeromechanical Systems Group,
Centre for Defence Engineering,
Cranfield University,
Defence Academy of the United Kingdom,
Shrivenham SN6 8LA, UK
e-mail: k.knowles@cranfield.ac.uk

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received May 31, 2016; final manuscript received May 2, 2017; published online July 21, 2017. Assoc. Editor: Feng Liu.

J. Fluids Eng 139(10), 101205 (Jul 21, 2017) (12 pages) Paper No: FE-16-1336; doi: 10.1115/1.4037036 History: Received May 31, 2016; Revised May 02, 2017

The transition from a laminar to turbulent boundary layer on a wing operating at low Reynolds numbers can have a large effect on its aerodynamic performance. For a wing operating in ground effect, where very low pressures and large pressure gradients are common, the effect is even greater. A study was conducted into the effect of forcing boundary-layer transition on the suction surface of an inverted GA(W)-1 section single-element wing in ground effect, which is representative of a racing-car front wing. Transition to a turbulent boundary layer was forced at varying chordwise locations and compared to the free-transition case using experimental and computational methods. Forcing transition caused the laminar-separation bubble, which was the unforced transition mechanism, to be eliminated in all cases and trailing-edge separation to occur instead. The aerodynamic forces produced by the wing with trailing-edge separation were shown to be dependent on trip location. As the trip was moved upstream the separation point also moved upstream, this led to an increase in drag and reduction in downforce. In addition to significant changes to the pressure field around the wing, turbulent energy in the wake was considerably reduced by forcing transition. The differences between free- and forced-transition wings were shown to be significant, highlighting the importance of modeling transition for ground-effect wings. Additionally, it has been shown that while it is possible to reproduce the force coefficient of a higher Reynolds-number case by forcing the boundary layer to a turbulent state, the flow features, both on-surface and off-surface, are not recreated.

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Figures

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Fig. 1

Schematic of a laminar-separation bubble after Horton [13]

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Fig. 2

Wind tunnel model: (a) wing profile and coordinate system and (b) assembly

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Fig. 3

Computational domain with boundary conditions indicated. Cell zones colored as laminar (upstream region) and turbulent (downstream region).

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Fig. 4

Downforce and drag curves at varying nondimensional ground clearances at Re = 1.63 × 105, Re = 2.03 × 105, and Re = 2.44 × 105

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Fig. 5

Flow visualization of the suction surface at Re = 1.63 × 105 (top) and Re = 2.44 × 105 (bottom) for (a) free transition and forced transition at (b) x/c = 0.1, (c) x/c = 0.3, and (d) x/c = 0.5 (flow moving top to bottom, h/c = 0.3125)

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Fig. 6

Flow visualization of the suction surface at Re = 1.63 × 105 for (a) free transition and forced transition at (b) x/c = 0.1, (c) x/c = 0.3, and (d) x/c = 0.5 (flow moving top to bottom, h/c = 0.125)

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Fig. 7

MSES results for force coefficients at free transition and varying trip locations (forced transition) at h/c = 0.3125 (Re = 2.44 × 105)

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Fig. 8

Streamlines around the airfoil section with forced transition at (a) 0.1c, (b) 0.3c, and (c) 0.5c and (d) free transition, computed in MSES (h/c = 0.3125 and Re = 2.44 × 105)

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Fig. 9

MSES results for suction surface: (a) pressure, (b) skin friction, (c) boundary-layer displacement thickness, and (d) momentum-thickness Reynolds number for free transition and varying trip location (h/c = 0.3125 and Re = 2.44 × 105)

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Fig. 10

Comparison of experimental (left) and CFD (right) surface streamlines for free-transition (top) and transition forced at x/c = 0.1 (bottom) (h/c = 0.3125 and Re = 2.44 × 105)

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Fig. 11

RANS results for (a) pressure distribution about wing and (b) x-component of shear stress on suction surface of wing at z/s = 0 for free- and forced-transition cases (h/c = 0.3125 and Re = 2.44 × 105)

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Fig. 12

Contours of (a) and (b) static pressure coefficient, (c) and (d) total pressure loss, and (e) and (f) turbulent kinetic energy for free-transition (left) and the difference between the free- and forced-transition cases (right) (h/c = 0.3125 and Re = 2.44 × 105)

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