On the Aerodynamics of Tractor-Trailers

Abstract

Wind tunnel experiments on the aerodynamics of tractor-trailer models show that the drag on the model is sensitive to the width of the tractor-trailer gap (G) and to the angle of yaw with respect to wind direction. At zero-yaw, relatively low drag is measured up to a critical gap width G/√A ≈ 0.5, where A is the cross-sectional area. At the critical width the drag experiences a sharp and large increase; most of the drag contribution is attributed to the trailer alone. As the gap is widened further, tractor and trailer become increasingly decoupled from each other and the drag reaches a near-plateau, rising much more gradually.

DPIV measurements in horizontal planes in the gap show that the flow is steady and consists of a relatively stable, symmetric toroidal vortex when the width is below critical. The symmetry breaks down at the critical gap, as evidenced by intermittent ejections of flow from the cavity to either side of the model. These ejections are believed to be at the origin of the sharp increase in trailer drag. As the gap width is increased further, the nature of the flow transitions from cavity-like to wake-like.

These observations can be qualitatively extended to moderate yaw angles (up to ~4 degrees), but the size of the critical gap width diminishes with yaw angle. At higher angles, the drag rises much faster with gap width.

The second part of this paper discusses the drag savings that can be realized by arranging two truck-like models in a tandem. Four tandems were formed by combining two models; each of the models was either “rounded” (i.e. lower drag) or “blunt” (higher drag). The drag of any tandem is generally lower than the sum of the drags of the models in isolation. However, the drag savings also depends on the choice of models (rounded vs. blunt) and on which model is placed in front. A rounded model followed by a blunt model achieves the most relative drag savings, while reversing the order produces the tandem with the least savings.