Although considerable efforts have been made with respect to the
reduction of fuel consumption of trucks during the last decades,
the diminishing natural resources as well as the evolution of the
truck traffic require continuous improvements in the field of
aerodynamics. Indeed, the forces generated by the air on the trucks
may originate, depending on weather, road type, truck type,
dimension, etc., up to 50% of the fuel consumption.
In order to analyze the influence of proportion variations
(mainly related to the length) and add-on devices on the
aerodynamic performance of a truck, a representative model was
first generated. This simplified geometry of a tractor-trailer was
based on the geometrical data of six European OEMs: Daimler, Iveco,
and MAN (tractors), Kögel, Krone and Schmitz Cargobull (trailers).
The model included a reduced level of details (exterior mirrors,
wheels, simplified underbody and engine block).
The following processing chain was used for the numerical
investigations. The grid generation tool, Spider, is an
octree-based preprocessor producing boundary fitted, locally
refined, conformal meshes, including hexahedral cell layers close
to walls. The computational grid obtained was then used for the
spatial discretization of the Reynolds-Averaged Navier-Stokes
equations that are solved in parallel using the open-source
software OpenFOAM®. For comparison purposes the generic truck was
built on a scale of 1:2.5 and investigated in Daimler's wind
tunnel in Stuttgart parallel to the numerical study. Both
experimental and numerical results were found to match in terms of
flow topology and drag coefficient.
Furthermore, an optimization analysis based on the generic body
was carried out, where geometric variations or fittings for the
front and the rear of the tractor-trailer were considered. The wind
tunnel models of the trucks were made in modules to enable the
experimental testing of almost all of the optimized configurations
that were simulated. The numerical results showed very good
agreement with the experimental data. In particular, the drag
improvements matched satisfactorily. Consequently, this study will
be extended with numerical investigations focusing on the
25.25-meter-long truck combinations (European Eurocombis).
