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The influence of a large truck on the aerodynamics of a small passenger car in an overtaking manoeuvre on the motorway was considered, many years ago, during the 1970's, to be a potential problem for the vehicle aerodynamicist. The concern never became significant as vehicle architecture evolved and car weights increased. The current drive for improved fuel economy is advocating that a considerable reduction in vehicle mass is desirable and therefore it may be time to readdress the significance of the truck passing manoeuvre. A quasi-steady experiment has been undertaken at small model scale to examine the aerodynamic characteristics of a small car in proximity to a large truck. Measurements at yaw were included to crudely simulate the effects of a crosswind. The wind tunnel data is presented and the limitations of the experimental procedure are discussed.

In the wind tunnel the effect of a wind input on the aerodynamic characteristics of any road vehicle is simulated by yawing the vehicle. This represents a wind input where the wind velocity is constant with height above the ground. In reality the natural wind is a boundary layer flow and is sheared so that the wind velocity will vary with height. A CFD simulation has been conducted to compare the aerodynamic characteristics of a DrivAer model, in fastback and squareback form, subject to a crosswind flow, with and without shear. The yaw simulation has been carried out at a yaw angle of 10° and with one shear flow exponent. It is shown that the car experiences almost identical forces and moments in the two cases when the mass flow in the crosswind over the height of the car is similar. Load distributions are presented for the two cases. The implications for wind averaged drag are discussed.

Aerodynamic aids, such as spoilers, applied to the rear of cars can provide drag reduction to improve performance, or can enhance high speed stability by reducing lift at the rear axle. In some cases these can be conflicting demands. It has been noted, however, that when rear axle lift is reduced there is often a reduction in yawing moment which has a beneficial effect on crosswind sensitivity. Wind tunnel results from real road vehicles are presented to illustrate this effect. This beneficial relationship is further explored in a wind tunnel experiment using simple models to represent road vehicles. Force and moment coefficients as a function of yaw angle are measured for a range of vehicle geometries which generate a substantial variation in lift. It is shown that as lift is reduced, yawing moment is also reduced, while side force and rolling moment are increased.

Side force has an influence on the behaviour of passenger cars in windy conditions. It increases approximately linearly with yaw angle over a significant range of yaw for almost all cars and the side force derivative, (the gradient of side force coefficient with yaw angle), is similar for vehicles of a given category and size. The shape factors and components which affect side force for different vehicle types are discussed. The dominant influence on side force, for most cars, however, is shown to be the vehicle height which is consistent with slender wing theory if the car and its mirror image are considered. This simple theory is shown to apply to 1-box and 2- box shapes, covering most MPVs, hatchbacks and SUVs, but does not adequately represent the side forces on notchback and fastback car shapes. Data from simple bodies is used to develop a modification to the basic theory, which is applied to these vehicle types.

With increasing speeds and the anticipated reduction in weight of modern cars, the issue of crosswind sensitivity is becoming increasingly important. In a previous paper by the same authors, the normal method of testing such aerodynamic characteristics at model scale, using static models at yaw to the freestream, was compared with dynamic testing, in which the model is propelled across a ‘gust’ simulated by a wind tunnel. A direct comparison using a similar gust profile for both static and dynamic tests was made with the conclusion that the simple static test technique was underestimating the true transient loads. Further tests have been carried out, on a generic squareback (or estate) model, during which the effect of varying both the vertical velocity profile and the turbulence intensity within the gust was considered.

Transient force and surface pressure data has been measured on a range of simple geometric shapes in order to gain an understanding of the complex time dependent and separated flow around a vehicle when subjected to a crosswind. The experiments were carried out using the Cranfield University model crosswind facility. It is found that the leeward face is the dominant area of transient activity. Maximum and minimum peak yawing moments at gust entry and exit are compared

The effect of aerodynamic lift on both straight line stability and lane change manoeuvrability of several small and medium sized European passenger cars has been determined from subjective track tests. The straight line and manoeuvring performance degrades with increasing lift and decreasing pitching moment. Increasing speed exacerbates the problem.

The aerodynamic drag characteristics of a passenger car are typically defined by a single parameter, the drag coefficient at zero yaw angle. While this has been acceptable in the past, it may not allow a true comparison between vehicles with regard to the impact of drag on performance, especially fuel economy. An alternative measure of aerodynamic drag should take into account the effect of non-zero yaw angles and some proposals have been made in the past, including variations of wind-averaged drag coefficient. For almost all cars the drag increases with yaw, but the increase can vary significantly between vehicles. In this paper the effect of various parameters on the drag rise with yaw are considered for a range of different vehicle types. The increase of drag with yaw is shown to be an essentially induced drag, which is strongly dependent on both side force and lift. Shape factors which influence the sensitivity of drag with yaw are discussed.