Aerodynamic Optimization for Freight Trucks Using a Genetic Algorithm and CFD

Abstract

A computational fluid dynamics (CFD) code is being combined with a genetic algorithm (GA) to perform a two-dimensional drag minimization study on tractor trailers. This paper involves the minimization of drag on two different parts of the tractor trailer: the rearview mirrors and the base region. Both optimization problems were simplified to two dimensional analyses due to the complexity of a CFD driven GA problem.

Few studies have been conducted with CFD driven by a GA due to extensive run times the general non autonomous characteristics of meshing a suitable CFD geometry. This study solves the Reynolds-averaged Navier-Stokes (RANS) equations to obtain a drag calculation used by the GA. Two Fortran codes were written from scratch to handle the meshing of the variable geometry mirror and base region for the entire design space. The mesh generators were used as subroutines 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 mirror shape optimization involved a mirror concept consisting of transparent flat plates extending beyond the mirror and tapering to a point similar to an airfoil. Four geometric parameters were varied to determine a minimum drag geometry within the design space. The resulting optimized mirror geometry was somewhat trivial due to the impractical length of the GA chosen mirror design.

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 that required 4 variables plus one additional variable to set the length of the flaps. Federal regulations limit length of base protuberances. 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.