Abstract
The drag of a car has big impact on the customer-relevant fuel consumption and goes hand in hand with the CO2-emission. Especially for electric vehicles, drag has a big influence on maximum speed and range.
The main task of the aerodynamics development is to minimize the drag coefficient. Besides the drag, lift of course is also within the scope of the aerodynamics. The aerodynamics development process splits in two areas. Firstly the external aerodynamics and secondly the internal aerodynamics.
The external aerodynamics is linked directly to styling. The shape of the body shell is essential for both. In the early phase of a project the shape is determined by styling. In this phase it is important that the aerodynamics department supports the styling selection. Later aerodynamics development optimises details like radius, angles and deflectors.
The internal aerodynamics is nothing other than the flow through the engine compartment, including the cooling package and the outlet with the under body. It has a major impact on the overall behaviour of the car. About 10% of the drag is caused by this flow.
The development of the aerodynamics consists of two ways: physical (Aero wind tunnel) and virtual (Computational Fluid Dynamics, CFD) development. The importance of CFD has increased in today’s world to reduce costs of prototype vehicles and expensive test facilities. The challenge however remains how good the predictions of CFD simulation are.
At MAGNA STEYR Fahrzeugtechnik (MSF), a huge importance is given to validation of simulation methods. This is termed as validated virtual development. The aerodynamics department has worked out a method of combining wind-tunnel-tests and CFD-simulations. This improves the efficiency of aerodynamics development in a project process.
The physical simulation of real road conditions is done in an aero-wind-tunnel. Amongst many other parameters, moving belts and rotating wheels are used to reproduce road conditions. By implementing some of these important geometrical and physical characteristics of the wind-tunnel in the CFD-simulation it is possible to improve the correlation between tests & CFD. These characteristics include rotating wheels, moving floor, and boundary layer conditions causing friction on the floor of the test section through the wind tunnel.
This paper points out the challenges of CFD-simulation and aero-wind-tunnel-tests used during the development of a vehicle. The consideration of wind-tunnel characteristics (geometrical measures and physics) in the CFD-simulation is described. The simulation results are validated using tests in a wind-tunnel with a series-production vehicle.
Keywords: aerodynamics, drag, lift, CFD, virtual development