A CFD investigation into the transient aerodynamic forces on overtaking road vehicle models

The transient aerodynamic forces occurring when one road vehicle overtakes another were investigated using two-dimensional (2D) computational fluid dynamics. The relative velocity of the vehicles was varied to allow comparison of the quasi-steady and unsteady modelling approaches. The quasi-steady approach captured the pseudo-periodic variation in the aerodynamic forces during the overtaking manoeuvre. However, at vehicle velocities typical of motorway driving conditions, it did not adequately predict the magnitude of these forces. The study also identified significant dynamic flow features occurring during overtaking manoeuvres in a crosswind. The dynamic variation in the aerodynamic forces was up to 400% greater than that predicted using quasi-steady analysis, indicating that the quasi-steady approach is totally unsuitable for modelling overtaking manoeuvres in a crosswind. These dynamic effects are likely to have a considerable impact on the stability of the vehicles involved. With little existing work on passing manoeuvres in a crosswind, these results highlight the importance of dynamic effects and the need for further investigation into the problem.