Design Parameter Impact of Wind-Averaged Drag Optimization

With the increasing prevalence of electric vehicles (EVs), decreasing vehicle drag is increasingly important, as range is a primary consideration for customers and has a direct bearing on the cost of the vehicle. While the relationship between drag and range is well understood, there still exists a discrepancy between the label range and the real-world range experienced by customers. One of the factors influencing the difference is the ambient wind condition that modifies the resultant air speed and yaw angle, which is typically minimized during SAE coastdown testing. Previous work has put forward a methodology to derive this wind-averaged drag (WAD) from either a full yaw sweep or a more efficient 3-point yaw curve while also showing the impact to drag coefficient as compared to the zero-yaw condition. This leads to an interesting dilemma for the vehicle aerodynamicist: whether to optimize the vehicle's exterior shape for low wind (zero yaw) conditions or for real-world conditions where the ambient wind generally produces a few degrees of yaw. This paper uses a generic truck and SUV model (eGTU) evaluated in CFD simulation to demonstrate the difference in common aerodynamic features when optimizing for zero-yaw drag vs wind-averaged drag. Features examined include tire deflectors, front corner plan view curvature, A-pillar, diffuser, rear separation edge radius, and roofline taper. Further, the implications in vehicle label range vs. in-service range with average wind conditions are calculated for a typical EV of the same body style, demonstrating the importance of deciding which condition to optimize for early in the design process.