International Journal of Automotive Technology

, Volume 20, Issue 1, pp 11–23 | Cite as

On the Near-Wake of a Ground-Effect Diffuser with Passive Flow Control

  • Obinna Ehirim
  • Kevin Knowles
  • Alistair SaddingtonEmail author
  • Mark Finnis
  • Nicholas Lawson


A ground-effect diffuser is an upwardly-inclined section of an automobile’s underbody which increases aerodynamic performance by generating downforce. To understand the diffuser flow physics (force behaviour, surface and offsurface flow features), we established the near-wake (within one vehicle width of the base) velocity profiles and flow structures of an automotive ground-effect diffuser using a bluff body with a 17 degree slanted section forming the plane diffuser ramp surface (baseline geometry), and endplates extending along both sides of the ramp. Wind tunnel experiments were conducted at a Reynolds number of 1.8 million based on the bluff body length, and laser Doppler velocimetry was used to measure two-dimensional velocity components on three planes of the diffuser near-wake. We also measured the velocity field in the near-wake of diffusers with modified geometry (with an inverted wing or a convex bump) as passive flow control devices. The near-wake velocity profiles indicated that the passive flow control methods increased the diffuser flow velocity and that the longitudinal vortices along the diffuser determined the shape of the flow structures in the near-wake of the diffuser bluff body.

Key words

Laser doppler velocimtery Ground effect Diffuser Flow control 



bluff body frontal area, m2


drag coefficient, D/(qAS)


lift coefficient, L/(qAS)


diffuser half width, m


aerodynamic drag, N


seeding particle diameter, m


focal length of the LDV probe lens, m


bluff body model ride height, m


bluff body length, m


aerodynamic lift (positive upwards), N


laser doppler velocimetry


freestream dynamic pressure (ρU2/2), Pa


Reynolds number based on l and U


flow characteristic timescale, s


freestream velocity, ms−1


downstream flow velocity, ms−1


total velocity in x-y plane √u2+v2, ms−1


root mean square of turbulent velocity fluctuations, ms−1


plane diffuser U - modified diffuser U, ms−1


plane diffuser u′ - modified diffuser u′, ms−1


velocity component in x direction, ms−1


velocity component in y direction, ms−1


cartesian coordinate: x is positive downstream of origin at the start of underbody flat section (Figure 4 (a))


cartesian coordinate: y is positive upwards of origin on the ground plane (Figure 4 (a))


cartesian coordinate: z is positive portside of origin on the centreline of the body (Figure 4 (a))


circulation, m2s−1


dynamic viscosity kgm−1s−1


air density, kgm−3


seeding particle density, kgm−3


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Copyright information

© The Korean Society of Automotive Engineers and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  • Obinna Ehirim
    • 1
  • Kevin Knowles
    • 1
  • Alistair Saddington
    • 1
    Email author
  • Mark Finnis
    • 1
  • Nicholas Lawson
    • 2
  1. 1.Aeromechanical Systems Group, Centre for Defence EngineeringCranfield UniversityShrivenham, SwindonUK
  2. 2.National Flying Laboratory CentreCranfield UniversityCranfield, BedfordUK

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