Aerodynamic Characteristics of Wind Assisted Vehicle Shapes

Abstract

In the presence of ambient winds, almost all present vehicles experience an increase in drag which, in turn, increases energy consumption. However, it is possible to design the vehicle body to passively utilise the wind to cause a drag reduction. With such a vehicle energy used by transportation will be reduced. Several successful research vehicles have been built at the University of Saskatchewan which to some extent demonstrated the use of ambient crosswinds for drag reduction. This thesis represents a wind tunnel investigation of potentially wind assisted vehicle-like shapes, specifically low aspect ratio, truncated, vertical airfoils. Methods of Wind Mean Drag evaluation were reviewed, and a Probabilistic Wind Mean Drag Coefficient was used as a basis for model configuration comparison. An extension to this existing method was proposed which used specific vehicle speed data for Wind Mean Drag calculation. Various geometries of airfoils in ground proximity were tested for aerodynamic forces in the wind tunnel. The vehicle-like models were shown to demonstrate reduced drag at low yaw angles, even for very short, truncated configurations. Also studied was the effect of ground clearance, edge radiusing and plan view shape on the wind assistance performance of these bodies. A surface pressure distribution study of a representative airfoil model at various yaw angles showed that the favourable propulsion effect of crosswinds is dependent on attached flow over the nose and the front, leeward third of the model sides. An ultra-violet mini-tuft surface flow visualisation study indicated strong vortices produced at the top edges of the airfoil shaped model at yaw. These vortices appear to contribute to the observed negative drag by causing the flow to remain attached to the leeward side of the body at very high yaw angles of up to 50°. One of the shapes studied in the thesis research was applied to the design of a new semi-trailer transport cab. This new cab design substantially improved the typically poor yaw performance exhibited by the unmodified transport.