Large-Eddy Simulation for Computing the Flow Around Vehicles

Doktorsavhandling, 2002

The feasibility of the use of large-eddy simulation (LES) in external vehicle aerodynamics is investigated. The computational requirement for LES of the full size car at road conditions is beyond the capability of computers in the near future. Since LES cannot be used for quantitative prediction of this flow, i.e. obtaining the aerodynamic forces and moments, an alternative use of LES is suggested than can be useful in increasing our understanding of the flow around a car. It is found that making LES of the flow around simplified car-like shapes at lower Reynolds numbers can increase our knowledge of the flow around a car. Two simulations are made, one of the flow around a cube and the other of the flow around a simplified bus. The former simulation proved that LES with a relatively coarse resolution and simple inlet boundary condition can provide accurate results. The latter simulation resulted in a flow that agrees with experimental observations and displayed some flow features that were not observed in the experiments or steady simulations of such flows. This simulation afforded us the possibility to study the transient mechanisms that are responsible for the aerodynamic properties of a car. The knowledge gained from this simulation can be used by the stylist to tune the aerodynamics of the car's design as well as by CFD specialists to improve turbulence models. A mixed one-equation subgrid-scale model for LES is proposed. A new model is constructed following the observation that the chief transport of turbulent energy is a local process that occurs between the scales closest to the cut-off. This transport of energy is modeled with the scale-similarity part and the remaining non-local energy transport is represented with the eddy viscosity part.