A Mechanical Energy Control Volume Approach Applied to CFD Simulations of Road Vehicles

This paper presents a mechanical energy control volume analysis for incompressible flow around road vehicles using results from Detached Eddy Simulation Computational Fluid Dynamics calculations. The control volume approach equates the rate of work done by surface forces of the vehicle to (i) the rate of work and kinetic energy flux at the control volume boundaries (particularly in the vehicle wake) and (ii) the rate of energy loss in the domain. At the downstream control volume boundary, the wake terms can be divided into lift-induced and profile drag terms. The rate of energy loss in the domain can be used as a volumetric analog for drag (drag counts/m³, when normalized). This allows for a quantitative break down of the contributions of different flow features/regions to the overall drag force. The formulation presented here includes terms for common features in automotive CFD simulations including shaft power (also known as ventilation drag) from rotating wheels, static ground patches, moving belts, and porous media. The Windsor body at 2.5 degrees yaw from the AutoCFD-3 workshop is used for a proof of concept on a simple bluff body. The effect of changing mesh resolution, time step, numerical scheme, and CFD solver are considered. Simulations of the closed-cooling DrivAer and Generic Truck Utility (GTU) are used to evaluate the method on more realistic representations of road vehicles, where it is shown that the control volume approach provides a different but complimentary perspective from the surface-based drag.