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Publication details

Sims-Williams, D.B. (2011). Cross Winds and Transients: Reality, Simulation and Effects. SAE International Journal of Passenger Cars - Mechanical Systems 4(1): 172-183.

Author(s) from Durham

Abstract

This paper provides a published counterpart to the address of
the same title at the 2010 SAE World Congress.
A vehicle on the road encounters an unsteady flow due to
turbulence in the natural wind, due to the unsteady wakes of
other vehicles and as a result of traversing through the
stationary wakes of road side obstacles. This last term is of
greatest significance.
Various works related to the characterization, simulation and
effects of on-road turbulence are compared together on the
turbulence spectrum to highlight differences and similarities.
The different works involve different geometries and
different approaches to simulating cross wind transients but
together these works provide guidance on the most important
aspects of the unsteadiness.
On-road transients include a range of length scales spanning
several orders of magnitude but the most important scales are
in the in the 2-20 vehicle length range. There are significant
levels of unsteadiness experienced on-road in this region and
the corresponding frequencies are high enough that a
dynamic test is required to correctly determine the vehicle
response. Fluctuations at these scales generate significant
unsteady loads (aerodynamic admittance typically 0.6-1.4)
and the corresponding frequencies can adversely affect
vehicle dynamics.
The generation of scales larger than the scale of the vehicle is
impractical with passive grids and so active turbulence
generation systems are preferred. These can be classified into
lift and drag-based devices. Lift-based devices provide better
control of the turbulence but can only just reproduce the
smaller scales in the 2-20 vehicle length range. Different
moving model approaches are also discussed. CFD offers real
advantages through its ability to allow arbitrary time-varying
boundary conditions.