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On-highway tractor-trailer vehicles operate in a complex aerodynamic environment that includes influences of surrounding vehicles. Typical aerodynamic analyses and testing of single vehicles on test track, in wind tunnel or in computational fluid dynamics (CFD) do not account for these real world effects. However, it is possible with simulation and on-road testing to evaluate these aerodynamic interactions. CFD and physical testing of multiple vehicle interactions show that traffic interactions can impact the overall drag of leading and trailing vehicles. This paper will discuss results found in evaluating the effects of separation distances on tractor-trailer aerodynamics in on-road and CFD evaluations using a time-accurate Lattice Boltzmann Method based approach and the ramifications for improving real world prediction versus controlled single vehicle testing.

Comparison of computation fluid dynamics (CFD) analysis of physical testing on curved test tracks highlights that simplifications in both methods can lead to significant error in predicting real world fuel economy performance for heavy duty tractor/trailer vehicles. The importance of understanding the differences in and the accuracy of these test methods have increased recently due to North American greenhouse gas (GHG) regulations and similar developments in other regions requiring manufacturers to assess aerodynamic performance of nearly every tractor sold. This paper discusses lessons learned in enhancing the accuracy of CFD and track predictions of real world aero performance by quantifying the effects of test track curves and challenging simplifying assumptions.