Evaluating aerodynamic drag of class 8 trucks is critical for improving fuel economy, meeting EPA targets and maintaining a competitive edge in the industry. For a standard tractor trailer configuration, aerodynamic drag is responsible for approximately 30% of total fuel consumption. Reducing drag by 3% would improve fuel economy by 1%, which demonstrates the need for accurate methods to predict aerodynamic drag. Depending on available resources, truck companies may use one or more tools to evaluate drag, including wind tunnels and CFD software.
This presentation focuses on the application of STAR-CCM+ to evaluate the aerodynamic drag of tractor-trailer configurations. In particular, we identify differences between steady-state and transient simulations. Steady-state models are attractive for economical reasons, but the wake flow phenomena are not accurately captured and drag predictions are not sufficiently accurate. In fact, in our studies steady-state solutions under-predicted drag by as much as 20% when compared with wind tunnel tests.
To accurately predict aerodynamic drag of a tractor-trailer configuration, careful consideration must be given to the trailer (bluff body) wake and interaction of wakes between tractor and trailer. Bluff body wakes are inherently unsteady and therefore require a transient approach when using CFD. In our transient CFD simulations we predict drag to within 2% of wind tunnel values.
Steady simulations do not accurately predict wake flow phenomena. We will present requirements for transient CFD simulation of flow around a class 8 truck and detail the benefits of using transient over steady-state analysis.