Aerodynamic investigations of a simplified truck under high yaw wind conditions

Abstract

The Norwegian Public Roads Administration (NPRA) are investigating the impact of side winds on vehicles travelling over a floating bridge that they wish to construct to shorten travel time. To mitigate the movement of the floating bridge due to high crosswinds, the bridge will be designed in a way that it does not obstruct the wind as much; resulting in high wind impacts on the commuting vehicles. The tractor-trailer combination is a large bluff body and experiences large side wind forces. Due to these forces, it may affect its lateral stability and might result in rollover at high crosswind speeds. This automotive engineering project aims to investigate the forces and moments acting on the truck during strong side winds and design a generic modular model for research use in the road vehicle aerodynamics course. The force investigations were performed numerically using STAR-CCM+ and experimentally through the closed-loop wind tunnel test facility at the Chalmers University of Technology. SOLIDWORKS was used to design a simplified model of a truck that can be used for research purposes. To accommodate the option of add-ons and to increase the ease of add-ons interchangeability during the wind tunnel testing, the truck design was made modular. The customization could be, for example, the addition of roof fairing, boat tailing or even an American-nose. The down-scaled model was printed and then sanded down and spray-painted for a smoother surface finish and better aesthetics. Assembly of the parts was done using epoxy, super glue and magnets. This model was tested in the wind tunnel, where a sweeping study was performed at a constant wind speed of 30 m/s for varying yaw angles between 0 and 90 . The modular design made it easy to test different configurations, such as different gap sizes between tractor and trailer, and the addition of a roof fairing. Multiple computational fluid dynamics (CFD) simulations were performed on the 1:1 model to mimic open road conditions and cross-validate the wind tunnel test. STAR-CCM+ software was used to mesh, analyze and post-process the CFD data. A steady-state approach and the k-e turbulence model were used to solve the Reynolds- Averaged Navier-Stokes (RANS) equations. A mesh independence study was performed to ensure the validity of the mesh used for the sweep study. The sweep study tested different relative wind directions between 0 and 90 for the same domain and mesh. A simulation is said to have converged when the force coefficients averaged over 750 iterations lie within 2 drag counts for 750 iterations. A comparison between the CFD and wind tunnel results validate the results as the forces and moments acting on the models follow a similar trend for varying yaw angles. The slight discrepancies in the comparison occurred due to the absence of a moving ground, varying boundary conditions and the mounting struts that act as a flow obstruction