Unsteady Numerical Simulations of Flow around Heavy Vehicles, Trains and Passenger Cars

Licentiatavhandling, 2012

In this thesis several simulations have been carried out aimed at improving the knowledge of vehicle aerodynamics and improving the flow around vehicles with respect to the aerodynamic drag. Four flows around different simplified vehicles have been considered. One model of a tractor-trailer, one passenger car model, one freight wagon model and one regional train model. The simplified tractor-trailer model consists of a front box representing a tractor and a rear box representing the trailer. The two boxes are separated by a gap width which is varied in the simulations. In the simulations on this geometry, the focus is on the flow in the gap between the tractor and the trailer and how this flow affects the global drag on the model. Large Eddy Simulation is used to simulate the flow around four variants of the model with different geometrical configurations. The behavior of the drag coefficient of the tractor-trailer model when varying the gap width and the shape of the front edges on the tractor is explained by identifying and analysing the large vortices around and in the the gap. The focus in the study on the passenger vehicle model is on the so-called A-pillar vortex. This is a swirling longitudinal vortex formed along the side windows on passenger vehicles due to the separation of the flow from the side edges of the front window and the engine hood. Flow control, both blowing and suction, from actuators located on the side of the front of the model is applied in the LES simulations. Steady blowing into the vortex causes expedited breakdown of the vortex, which in turn influence the pressure distribution on the side windows and the overall drag of the model. Steady suction causes the vortex to not form at all, thereby removing the vortex entirely. Simulations aimed at improving the knowledge on the flow around a generic freight wagon model using LES is also reported. The model is smoothed in comparison to a real container wagon, but the overall geometrical features such as wheels and underhood are included. The simulations of the flow around the regional train model is done using Partially Averaged Navier Stokes (PANS). PANS is a recently proposed hybrid turbulence model for engineering types of flow. The regional train model consists of a bluff body with a length to height/width ration of 7:1. The model poses several challenging flow situations to simulate such as separation from the leading curved front edges, an attached boundary layer flow and separation from the curved rear edges at the moderate Reynolds number of 400 000 based on the models’ width. An open cavity is placed on the model at the base and the drag is thereby decreased by some 10%