Courtesy of Freightliner Corp.

Next time you see semi-articulated tractor/trailer combinations parked at a roadside truck stop, compare the shapes of the tractors. You will likely see traditional, squarish tractor shapes mingling in with modern, streamlined designs equipped with devices such as air-shields, air-dams, and side extenders. The drag coefficients of modern tractor/trailers range from 0.4 to 0.5, substantially lower than the values of 0.6-0.7 characterizing the older designs. The reduced drag results in improved fuel economy and reduced fuel costs, important factors given the reliance on trucking for the transportation of goods.

The timeline for the calculation left the audience impressed.

How much further can the aerodynamic drag of heavy vehicles be reduced? This was considered at a US Department of Energy Workshop on Heavy Vehicle Aerodynamic Drag Reduction, held in Phoenix, Arizona in January 1997. Fluent was invited to represent the interests of commercial CFD. FLUENT/UNS, with features such as hybrid mesh, solution-adaptive mesh refinement, non-conformal mesh interfaces, and an efficient parallel capability, is very well-suited to vehicle external aerodynamics. This was underscored by a case study of a truck simulation, undertaken specifically for the workshop.

Figure 1. The case study of truck aerodynamics was completed in less than three weeks; parallel processing allowed the 1.5 million cell model to be solved in under 48 hours.

From Geometry to Solution in Three Weeks

The timeline for the calculation left the audience impressed. Freightliner Corporation provided the geometry of a non-production tractor/trailer combination in early January. Fluent's automotive team created a 1.5 million cell, viscous hybrid mesh model, solved the flow, and postprocessed the results in time for the workshop -- representing a turnaround time of less than 3 weeks.

The flow solution was obtained within a 48-hour time period, using parallel processing on a 2GB 4-processor HP K-series machine. The flow prediction highlighted a mismatch in the tractor/trailer height, intentionally created to illustrate how CFD might be applied to gauge the effect of a poorly adjusted air-shield.

How much drag reduction can practically be realized for tractor/trailers is a very complex question and no consensus was reached during the DOE workshop. However, the FLUENT/UNS case study left no doubt that CFD can make a significant contribution.

Figure 2. Flow visualization highlights the mismatch in tractor/trailer heights; color on the vehicle surface indicates pressure.

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