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Research Papers: Flows in Complex Systems

Aerodynamics of a Convex Bump on a Ground-Effect Diffuser

[+] Author and Article Information
O. H. Ehirim

Aeromechanical Systems Group,
Cranfield University,
Shrivenham SN6 8 LA, UK
e-mail: o.ehirim@cranfield.ac.uk

K. Knowles

Professor
Aeromechanical Systems Group,
Cranfield University,
Shrivenham SN6 8 LA, UK
e-mail: k.knowles@cranfield.ac.uk

A. J. Saddington

Aeromechanical Systems Group,
Cranfield University,
Shrivenham SN6 8 LA, UK
e-mail: a.j.saddington@cranfield.ac.uk

M. V. Finnis

Aeromechanical Systems Group,
Cranfield University,
Shrivenham SN6 8 LA, UK
e-mail: m.v.finnis@cranfield.ac.uk

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received September 28, 2017; final manuscript received February 26, 2018; published online April 19, 2018. Assoc. Editor: Daniel Maynes.

J. Fluids Eng 140(9), 091102 (Apr 19, 2018) (11 pages) Paper No: FE-17-1625; doi: 10.1115/1.4039518 History: Received September 28, 2017; Revised February 26, 2018

A ground-effect diffuser is an upward-sloping section of the underbody of a racing car that enhances aerodynamic performance by increasing the downforce, thus improving tire grip. The downforce generated by a diffuser can be increased by geometric modifications that facilitate passive flow control. Here, we modified a bluff body equipped with a 17deg diffuser ramp surface (the baseline/plane diffuser) to introduce a convex bump near the end of the ramp surface. The flow features, force, and surface pressure measurements determined in wind-tunnel experiments agreed with previous studies but the bump favorably altered the overall diffuser pressure recovery curve by increasing the flow velocity near the diffuser exit. This resulted in a static pressure drop near the diffuser exit followed by an increase to freestream static pressure, thus increasing the downforce across most of the ride heights we tested. We observed a maximum 4.9% increase in downforce when the modified diffuser was compared to the plane diffuser. The downforce increment declined as the ride height was gradually reduced to the low-downforce diffuser flow regime.

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References

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Figures

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Fig. 5

Percentage difference in force coefficients between the modified and plane diffusers across the range of ride heights (h/d=0.764to0.064) investigated for: (a) CL and (b) CD

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Fig. 6

(a) Lift-to-drag ratio measured across the range of ride heights (h/d=0.764to0.064) investigated for the plane and modified diffusers and (b) percentage difference in lift-to-drag ratio between the modified and plane diffusers across the range of ride heights investigated

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Fig. 4

Force coefficients across the range of ride heights investigated for the plane and modified diffusers: (a)CL and (b) CD, showing the different flow regimes A, B, C, and D

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Fig. 3

Distribution of pressure taps on the bluff body lower surface

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Fig. 2

(a) Schematics of midplane cross-sectional and rear views of the baseline diffuser, a 17 deg plane diffuser ramp on a bluff body (dimensions in mm), (b) schematics of midplane cross-sectional and rear views of the modified diffuser, a 17 deg diffuser ramp modified by including a convex bump (dimensions in mm), and (c) the experimental plane-diffuser bluff body model mounted on the overhead strut in the wind tunnel

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Fig. 1

(a) The centerline underbody surface pressure behavior of a diffuser bluff body, highlighting the single-stage pressure recovery at the diffuser section with no flow control [21] and (b) the centerline underbody surface pressure behavior of the diffuser bluff body highlighting the two-stage pressure recovery at the diffuser section using flow control [21]

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Fig. 7

Centerline surface pressure distribution at (a) h/d=0.764, (b) h/d=0.382 (both within the force-enhancement regime), (c) h/d=0.191 (within the maximum-force regime), (d) h/d=0.153 (within the force-reduction regime), and (e) h/d=0.064 (within the low-force regime)

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Fig. 8

Spanwise surface pressure distribution at (a) h/d=0.764 for all x/d locations of the plane diffuser, (b) h/d=0.382 (both within the force-enhancement regime), (c) h/d=0.191 (within the maximum-force regime), (d) h/d=0.153 (within the force-reduction regime), and (e) h/d=0.064 (within the low-force regime)

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Fig. 9

Surface flow patterns at force-enhancement regime ride heights: (a) h/d=0.764 and (b) h/d=0.382 (flow from top to bottom)

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Fig. 10

Surface flow patterns at the ride height of h/d=0.191 in the maximum-force regime (flow from top to bottom)

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Fig. 11

Surface flow patterns at the ride height of h/d=0.153 in the force-reduction regime (flow from top to bottom)

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Fig. 12

Surface flow patterns at the ride height of h/d=0.064 in the low-force regime (flow from top to bottom)

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Fig. 13

Boundary layer velocity profiles at diffuser inlet x/d=3.14 center point z/d=0 for: (a) h/d=0.382, (b) h/d=0.191,(c) h/d=0.153, and (d) h/d=0.064

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