Aerodynamic Design Testing Method

Aerodynamic Design Testing Method

INTRODUCTION:

The performance, handling, safety and comfort of an automobile are significantly affected by its aerodynamic properties. Low drag is important for good fuel economy and low emission. Increasing fuel prices and stringent regulations have made this long established relationship now widely acknowledged. But the other aspects of vehicle aerodynamics are no less important for the quality of an automobile, directional stability, wind noise stability, soiling of lights, windows and body, cooling of an engine, gearbox and brakes and finally heating, ventilating and air-conditioning of the passenger compartment. These all depend on the flow filled around and through the vehicle.
In terms of fluid dynamics, road vehicles are bluff bodies in very close proximity to the ground. Their external geometry is extremely complex. There are also internal ducts and recessed cavities and they communicate with the external flow. Rotating wheels add to the complexity. The flow over a vehicle is fully three-dimensional. Boundary layers are turbulent. Flow separation is common and may be followed by the reattachment. Large turbulent wakes are formed at the rear, and in many cases they interact with longitudinal vortices shade from the after body. As is typical for bluff bodies, drag is primarily pressure drag. Accordingly, the avoidance of separation or, if this is not possible, its control among the main objective of vehicle aerodynamics.
The uniqueness of vehicle aerodynamics becomes evident when it is compared to aerodynamics of ‘Aeronautics and Turbo-machinery’. In latter theoretical and experimental methods are based on the facts that “machine can be broken down into components”. These components are then optimized individually, and put them together to study interference aspects.
Vehicle aerodynamics has to be prepared in a different manner. From the very beginning, the entire body has to be considered since breakdown into components is not meaningful. Nevertheless, the influence of many individual shape parameters on over all flow fields has been elaborated. However, because of strong interference effects it is difficult to predict whether in any given case, a desired effect will be achieved. Hence up to now, no analytical design procedure is available for road vehicle. The aerodynamicist must refer to a large amount of detailed information resulting from previous development works conducted using various testing methods. His success depends and on the accuracy of results from these testing and his own ability to transfer them to his own problem. A solution to vehicle aerodynamics problem still requires much of intuition as experimental data. It is still an empirical science, some say even an art!


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