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Research

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Publication details for Dr David Sims-Williams

Sims-Williams, DB, Marwood, D & Sprot, AJ (2011). Links between Notchback Geometry, Aerodynamic Drag, Flow Asymmetry and Unsteady Wake Structure. SAE International Journal of Passenger Cars - Mechanical Systems 4(1): 156-165.

Author(s) from Durham

Abstract

The rear end geometry of road vehicles has a significant
impact on aerodynamic drag and hence on energy
consumption. Notchback (sedan) geometries can produce a
particularly complex flow structure which can include
substantial flow asymmetry. However, the interrelation
between rear end geometry, flow asymmetry and
aerodynamic drag has lacked previous published systematic
investigation.
This work examines notchback flows using a family of 16
parametric idealized models. A range of techniques are
employed including surface flow visualization, force
measurement, multi-hole probe measurements in the wake,
PIV over the backlight and trunk deck and CFD.
It is shown that, for the range of notchback geometries
investigated here, a simple offset applied to the effective
backlight angle can collapse the drag coefficient onto the
drag vs backlight angle curve of fastback geometries. This is
because even small notch depth angles are important for a
sharp-edged body but substantially increasing the notch depth
had little further impact on drag.
This work shows that asymmetry originates in the region on
the backlight and trunk deck and occurs progressively with
increasing notch depth, provided that the flow reattaches on
the trunk deck and that the effective backlight angle is several
degrees below its crucial value for non-reattachment. A
tentative mapping of the flow structures to be expected for
different geometries is presented.
CFD made it possible to identify a link between flow
asymmetry and unsteadiness. Unsteadiness levels and
principal frequencies in the wake were found to be similar to
those for high-drag fastback geometries. The shedding of
unsteady transverse vortices from the backlight recirculation
region has been observed.