Characterization of Aerodynamic Design Spaces for Adjustable Tractor Surfaces

Justin J. Novacek (Volvo Group North America), Bhargav Sowmianarayanan (Exa Corporation)

CHALLENGE - Trailer positioning plays a significant role in the overall aerodynamics of a tractor-trailer combination and varies widely depending on configuration and intended use. In order to minimize aerodynamic drag over a range of trailer positions, adjustable aerodynamic devices may be utilized. For maximum benefit, it is necessary to determine the optimal position of the aerodynamic device for each trailer position. 

SOLUTION - Optimization may be achieved by characterizing a two-dimensional design space consisting of trailer height and tractor-trailer gap length, with aerodynamic drag as the response. The parameterization method was applied to a specific case on two different class 8 tractor-trailer configurations.  CFD simulations were carried out using a Lattice-Boltzmann based method, and coupled with modeFRONTIER for the creation of multiple Kriging Response Surfaces. The modeFRONTIER optimization environment was employed to automate the simulation process by setting the trailer height, tractor-trailer gap length and trim tab position as input variables with front drag as the desired response variable. Simulations were carried out in multiple phases, allowing for the generation of intermediate response surfaces to estimate predictive error and track response surface convergence.  This method was then utilized to obtain aerodynamic benefits maps for two different tractor models.

BENEFITS - The first model demonstrates that for small tractor-trailer gap lengths and large trailer heights the trim tab must be set at its maximum setting in order to yield the greatest drag reduction. As the tractor trailer gap increases and/or the trailer height decreases the optimal trim tab position lowers. The results presented indicate that adjustable aerodynamic devices can offer improved fuel economy for a range of tractor-trailer gap lengths and heights.