Thesis-2019-Varney.pdf (67.21 MB)

Base drag reduction for squareback road vehicles

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posted on 20.02.2020, 08:53 by Max Varney
As we are in the throes of a climate crisis, we need to scrutinise how our everyday activity impacts it. A significant portion of the global CO 2 emissions are a result of passenger vehicles (16%) with a significant portion of these being sports utility vehicle geometries (37.5%). The squareback nature of sports utility vehicles is desired by some manufacturers to maintain brand identity, but these geometries are not typically aerodynamic. On top of this, the emissions regulations do not take into account realistic conditions that a vehicle will operate in, resulting in an underestimation of their impact on the global CO2 emissions.

This thesis implements cavities and small trailing edge side edge tapers on a quarter scale Windsor model (with and without wheels) at more realistic flow conditions to generalise drag reduction techniques. These generalisations focus on the base of the model as this region generates a significant portion of the overall drag. As the geometries are unlikely to be implemented on a full-scale vehicle, the generalisations provide a goal for a lower drag vehicle which can be applied to any baseline geometry.

Irrespective of the device used it was found that a way to reduce drag generally was to force the wake into a balanced, symmetric condition. The lateral symmetry is shown to reduce the instantaneous base drag at yaw on the baseline geometry, but also the mean base drag with a cavity or side edge tapering at yaw. The vertical symmetry is shown to improve the mean base drag on the Windsor model with wheels, both with a cavity and with the side edge tapering at all yaw angles tested.

A symmetric, balanced wake was shown not to be the only route to low drag. The cavities showed a reduction in base drag due to the reduced flow velocity in the wake with flow field measurements inside the cavity, while presenting an asymmetric wake. The tapering generated base drag reductions with slower velocities parallel to the base, higher velocities perpendicular to the base and shorter wakes that were upwash or downwash dominated.


EPSRC industrial CASE award (Voucher no. 15220097)



  • Aeronautical, Automotive, Chemical and Materials Engineering


  • Aeronautical and Automotive Engineering


Loughborough University

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© Max Varney

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A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of the degree of Doctor of Philosophy of Loughborough University.




Martin Passmore

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