Search Results

This paper presents a series of observations of time-averaged wake asymmetry for a range of “notchback” vehicle geometries. The primary focus is on a reduced scale experiment using full-sized saloon geometry. Substantial flow asymmetry was observed in the vehicle “notch”. Similar asymmetries are reported for a full scale experiment on the same geometry along with others as diverse as production models of a luxury and mid-sized saloon; basic car shapes and a simple body. In one case a physical explanation is proposed, based on the degeneration of an unstable symmetric wake structure.

There are many aspects of the unsteady flow around fastback passenger cars that remain to be understood. These include the source and nature of unsteady flow structures, the relevant time-scales, the effect of geometric parameters and the impact of the unsteadiness in terms of steady and unsteady forces on the vehicle. This paper investigates large scale unsteady structures in the wake of the Ahmed form and of a scale model of a real car shape using two wind tunnels and model scales between 12.5% and 40%. The unsteadiness demonstrated only low coherence and weak periodicity and the Strouhal number of a given structure varied from tunnel to tunnel indicating a high sensitivity to external influences. Nevertheless, a novel visualization technique, used to display the results of time-accurate pressure probe measurements, was able to reveal structures involving both symmetric and anti-symmetric oscillations in the strength of the rear-pillar vortices.

MIRA and Rover Group Ltd have undertaken a systematic study of the ability of CFD methods to predict the aerodynamic characteristics of simplified car-like shapes. This paper reports the latest stage of this investigation, which examines the use of a commercial Reynolds-averaged Navier-Stokes code (STAR-CD) to predict the aerodynamic characteristics of a series of simplified car shapes. Comparable experimental data were obtained by testing in the MIRA Full Scale Wind Tunnel (FSWT). This paper shows that CFD techniques are improving in their ability to predict flow separation from curved surfaces accurately. Further, encouraging results for vehicle drag (coefficients to within 2% of experiment) and the effect of limited geometric modifications on drag (within 7% of full-scale experiment) were obtained. However these latter results should be viewed with some caution as the results for lift were considerably poorer.

The principal development tool for the vehicle aerodynamicist continues to be the full-scale wind tunnel. It is expected that this will continue for many years in the absence of a reliable alternative. As a true simulation of conditions on the road, the conventional full-scale wind tunnel has limitations. For example, the ground is fixed relative to the vehicle, allowing an unrepresentative boundary layer to develop, and the wheels of the test vehicle do not rotate. These limitations are known to influence measured aerodynamic data. In order to improve the representation of road conditions in the wind tunnel, most of the techniques used have attempted to control the ground plane boundary layer. Only at model scale has the introduction of a moving ground plane and rotating wheels been widely adopted. The Pininfarina full-scale wind tunnel now incorporates the Ground Effect Simulation System which allows testing with a moving belt and rotating wheels.

An accurate simulation of a ground vehicle interacting with a crosswind gust can be achieved by using a moving model mounted on a track such that it can traverse the working section of a conventional atmospheric boundary layer wind tunnel. This paper will briefly describe the facility that is being developed at Cranfield University and detail comparisons between static and dynamic data from tests on three basic model configurations. Under the same nominal wind input, data from static tests compares well with that from dynamic tests at yaw angles below 15°. At higher yaw angles, after the onset of “large scale” separation, the dynamic values of the forces and moments become larger than the static values.

Ever since aerodynamics became an essential element of the automobile design process, the principal development tool for the vehicle aerodynamicist has been the full-scale wind tunnel. In the absence of a reliable alternative, it is expected that this will continue for many years. As a true simulation of the conditions on the road the conventional full-scale wind tunnel has limitations. The ground is fixed relative to the vehicle allowing an unrepresentative boundary layer to develop, the wheels of the test vehicle do not rotate and there is some uncertainty over the influences imposed by the tunnel walls. In addition, the aerodynamic data obtained from different wind tunnels shows a degree of scatter and even configuration changes do not necessarily produce consistent effects. With particular regard for aerodynamic drag, the aerodynamicist should ensure that gains obtained in the wind tunnel generate real benefits on the road.