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The current generation of sports racing cars such as those competing under the Le Mans “LM”P and “LM”GTP regulations are particularly sensitive to the pitch of the vehicle. This is a consequence of the low ground clearances that must be adopted to maximise the benefits that can be gained from ground effect and of the very large floor plan area of these cars. To achieve optimum cornering and straight line performance the suspension characteristics are often tuned to the aerodynamic forces in order to reduce the pitch and hence the drag of the vehicle at high speeds whilst retaining relatively high downforce when cornering. A series of accidents at the 1999 Le Mans 24-hour race have highlighted the potential instability of these vehicles which resulted in the catastrophic ‘take-off’ of one of the “LM”GTP cars during the race and others during qualifying and the pre-race ‘warm-up’.

Transient wake surveys have been conducted on a generic three dimensional vehicle shape. The flow conditions were those generated by the unique crosswind facility at Durham University, which imitates a vehicle passing through a sharp-edged, finite length cross-wind gust. Each survey consisted of some 7000 cross-wind gusts, with each point in the wake being phase-averaged over 20 gusts. The surveys clearly show the development of the wake structure from the familiar axial flow conditions, through the transient to a nominally steady yawed flow. Although both the structure and total pressure loss that develop in the wake flow are comparable to those found for quasi-steady flow conditions, the developing flow reveals characteristics that are not found in the quasi-steady measurements. New data are also presented with regard to the character of the gust that develops in this wind tunnel and their impact upon the reported wake measurements is discussed.

An improved technique is described for the experimental modeling of transient cross wind gust influences on passenger vehicles. The new configuration uses a set of vertical axis shutters which open and close in a ‘Mexican wave’ fashion to scan the cross wind jet along the working section of the wind tunnel. The new arrangement dramatically increases the rate at which experiments can be performed and offers the opportunity to apply phase-averaging techniques to multiple data sets in order to reduce noise. This is a significant development as most previous test methods have suffered from poor signal to noise ratios. Experimental results are presented for transient surface pressure measurements on a simplified vehicle model which clearly demonstrate the benefits of the new technique.

The aerodynamic forces induced on a generic ‘hatchback’ model have been measured as it passes through a perpendicular cross-wind jet generating a relative yaw angle of 30°. This has been done in the unique University of Durham automotive wind tunnel, which utilises the stationary model approach, with the cross-wind being introduced by means of a second jet which is separated from the main jet by a moving belt and aperture assembly. Data acquisition was by means of an array of surface pressure tappings. Transient pressure force and moment coefficients have been measured and it is shown that the side and lift forces experienced in the transient situation exceed the steady state values at corresponding yaw angles by between 10% and 20%.