Tractor Trailer Base Region Drag Optimization by a Genetic Algorithm with CFD

A computational fluid dynamics (CFD) code has been combined with a genetic algorithm (GA) to perform a two-dimensional drag minimization on the base region of tractor trailers. Few studies have been conducted with CFD driven by a GA due to extensive run times and the general non autonomous characteristics of meshing a suitable CFD geometry. This study solves the two-dimensional Reynolds-averaged Navier-Stokes (RANS) equations to obtain a drag calculation used by the GA. Custom FORTRAN subroutines were written to handle the structured meshing of the tractor trailer with a variable geometry base slat for the entire design space. The mesh generator was used as a subroutine in the GA with the only input being the geometric variables. The objective function for the GA consists primarily of the mesh generator code and the CFD solver. The base optimization involved placing flaps of varying size, position, and curvature in the base region to determine a minimum drag configuration. The front of the tractor trailer remained unchanged, but was included in the CFD analysis. The base flaps were defined by a cubic function requiring 4 variables plus one additional variable to set the length of the flaps. For this study the flap length was limited to a maximum of four feet aft of the trailer base. The resulting minimum drag configuration was shown to reduce drag by over 50 percent when compared to the CFD run with no base flaps.