Measuring the effects of crosswinds on cars
The crosswind sensitivity of a car is described as the sensitivity of a driver-vehicle system to vehicle movements provoked by wind influences. Research has shown that it is a contributory factor in accidents and that it is adversely affected by certain body shapes and by reductions in weight. Future legislation calling for a reduction in the amount of Carbon Dioxide produced by cars may well lead to this reduction in weight. Tests have been carried out on llth scale car models to compare the different results produced by static and dynamic tests. Although static tests are easier to perform, dynamic tests, in which the model is propelled across the wind tunnel, offer the advantage of an improved simulation of the skewed profile that the vehicle encounters at full scale. In these experiments a number of different skew profiles, turbulence profiles and model configurations including estates, hatchbacks and saloons, were tested at Reynolds numbers above 4x10⁵. The results showed that data from the more simple static tests were generally more conservative than those from the dynamic tests in a skewed profile up to approximately 30° of yaw. However, the static tests were unable to predict the peak yawing moment that occurs as a result of the transient flow over the model as it enters the gust. The dynamic tests also indicated oscillations in the flow over the rear end of the models in the hatchbacks with backlight angles of 22° and 36°, although less so in the latter. Other results suggested that there was an improved correlation between lift coefficients from different skew profiles, if the mean local velocity integrated over the height of the model was used to non-dimensionalise the data. Quality Function Deployment is not an appropriate framework to be used in the development of a complete vehicle but would prove useful if applied to the issue of crosswind sensitivity as a sub-component.