Salient drag reduction of a heavy vehicle using modified cab-roof fairings

Aerodynamic drag reduction is directly related to fuel consumption and CO2 emission and is thus a main research interest in heavy vehicles. Approximately half of the total aerodynamic drag is attributed to the flow at the forebody of a vehicle and the gap between the tractor and trailer. Therefore, flow control devices that can reduce aerodynamic drag on the forebodies of heavy vehicles offer a considerably practical significance. Cab roof fairing (CRF) is one of the most widely used drag reduction devices installed at the roofs of trucks or tractor-trailers. However, the drag-reducing effect and the three-dimensional flow characteristics around forebodies as a function of the external shape of CRFs have yet to be fully investigated. In this study, the drag reduction effects of typical and modified CRF models are quantitatively examined through wind tunnel tests and numerical simulation (coarse large eddy simulation (LES)). The wind tunnel experiment at a Reynolds number of > 5.5x 10(5) is conducted for a scaled-down model of a 15-tonne truck. The modified CRF significantly changes the flow structure, leading to approximately 19% drag reduction. PIV flow field measurement was conducted to figure out the differences in flow characteristics around the forebody of the vehicle with and without CRFs. Flow characteristics, including vortical structures, turbulent kinetic energy, mean pressure field, and mean velocity field around the forebody of the vehicle model (1/8 scale) with and without CRFs are numerically investigated using coarse LES for further understanding of the mechanism associated with drag reduction. The present results are expected to provide useful information for the design of new CRF models and the improvement of the aerodynamic performance of heavy vehicles, including trucks and tractor-trailers.