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Cross Winds and Transients: Reality, Simulation and Effects

Sims-Williams, D.B.

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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.


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

Journal Article Type Article
Publication Date Jun 1, 2011
Deposit Date Dec 29, 2011
Publicly Available Date Mar 27, 2014
Journal SAE International Journal of Passenger Cars - Mechanical Systems
Print ISSN 1946-3995
Electronic ISSN 1946-4002
Publisher SAE International
Peer Reviewed Peer Reviewed
Volume 4
Issue 1
Pages 172-183


Published Journal Article (3 Mb)

Copyright Statement
Copyright © 2011 SAE International

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