Introduction
Road vehicles are bluff bodies with extremely complex geometry. Engine compartment, wheels, mirrors and other external and internal attachments and recessed cavities creates geometrical and fluid mechanical complexity. The flow over a vehicle is fully three-dimensional. Boundary layers are turbulent. Flow separation is common and may be followed by reattachment. Large turbulent wakes are formed at the rear and in many cases contain longitudinal trailing vortices. Drag in road vehicle is mainly due to pressure drag. In aircraft, the drag is mainly due to the friction drag. The avoidance of separation or its control is the main objectives of vehicle aerodynamics. With regard to their geometry, road vehicles comprise a large variety of configurations. Passenger cars, vans, and buses are closed, single bodies. Trucks and race cars can be of more than one body. Motorcycles and some race cars have open driver compartments. With the race car being the only exception, the shape of a road vehicle is not primarily determined by the need to generate specific aerodynamic effects. Airplane is designed to produce lift. To the contrary, a road vehicle's shape is primarily determined by functional, economic and, last but not least, aesthetic arguments. The aerodynamic characteristics are not usually, generated intentionally; they are the consequences of, but not the reason for, the shape.
Vehicle Attributes Affected by Aerodynamics
(a) Performance and Fuel Economy
Fuel economy and, increasingly, global warming are the current key arguments for low drag worldwide. In Europe, particularly Germany, top speed is still considered an important sales feature despite the rapidly increasing traffic density which largely prohibits fast driving even in the absence of speed limits. Vehicle fuel consumption is a matter of demand and supply. On the demand side are the mechanical energies required for propulsion and by accessories. On the supply side is the efficiency with which this energy can be generated by the power plant and delivered to the points of application. The influence of aerodynamics on this demand-supply relationship is through the drag force, which affects the propulsive part of the demand side.
(b) Handling
While traveling along a road, a vehicle experiences more than just drag. The flow over a vehicle moving through still air is nominally symmetric about the vehicle's plane of symmetry. Lift, pitching moment and, of course, drag are therefore the only aerodynamic components. Unless special measures are taken, the vertical force on a bluff body close to the ground is positive, i.e. it tends to lift the vehicle. The accompanying reduction in load on the tires is, in principle, disadvantageous to handling. This is because the maximum side force that a tire can generate decreases when wheel load is reduced. However, the effect is negligible for most vehicles except race cars-at least at reasonable driving speeds.
(c) Crosswind Sensitivity
In a crosswind, and while passing another vehicle in still air, the flow around a vehicle becomes asymmetric and so a side force, a yawing moment, and a rolling moment are produced. Also, the components of drag, lift, and pitching moment are altered, and normally they are increased. For almost any vehicle, the yawing moment
is unstable, i.e. it tends to twist the vehicle further away from the wind. As a result, the angle of yaw, the yawing moment, and the side force are increased even further. In recent years, the centre of gravity of passenger cars has, on average, moved steadily forward. This is mainly for two reasons. First, rear-engine cars have become rare; second, the move to front-wheel drive has resulted in vertical load being shifted from the rear to the front axle. Consequently, while the yawing moment referred to the centre of the wheelbase has remained relatively constant, that referred to the centre of gravity has decreased. As a result, the matter of crosswind sensitivity seems to have ceased to be of concern to the driving public.
(d) Functional
The flow over a vehicle not only produces aerodynamic forces and moments, but also many other effects that can be summarized under the term functional. During the development of a new vehicle they require at least the same attention as the forces and moments. These functional effects are:
(i) Forces On Body Parts
Vehicle bodies are made up of large and comparatively fiat panels, and these have to withstand considerable aerodynamic loading. Hoods, doors, and frameless windows have to be tight under all conditions. Modern lightweight structures are prone to flutter.
(ii) Wind Noise
The more that the formerly dominant noise sources (i.e. engine and tires) have been attenuated, the more that wind noise has become objectionable inside vehicles. Passenger cars and buses are of particular concern. In 1983, the vehicle speed at which wind noise was 100 mph; in 1992 it is only 60 mph. Open sun roofs and side windows can also cause low-frequency noise (booming).
(iii) Body-Surface Water Flow and Soiling
Water flow on a body's surfaces can impede visibility. Water streaks and droplets accumulating on the forward side windows prevent a clear view into the outside rear view mirrors. Soil diminishes the function of headlights and taillights. The sides of vans and buses are often used for advertising purposes and therefore need to be kept clean.
(iv) Interior Flow Systems
Several interior flow systems pass through a vehicle. For passenger and sports cars the design of engine-cooling ducting has become extremely difficult because of increased engine power and less under-hood space. Race cars are the most demanding. Passenger vehicles require ducting for proper ventilation and heating of the passenger compartment. Buses require high rates of air exchange free of drafts.
Aerodynamic components used in cars
(a) Front Splitter
It is essentially an aerodynamic component that balances out the front and the rear distribution of downforce generated in high speeding car. It is generally found on the front end of a car just fitted under the front bumper. It consists of parallel flaps at the bottom, it creates a high pressure area above the splitter and low pressure below it, and hence this high pressure is drawn to low pressure pushing the front end of the car towards the ground.
(b) Side skirts
Side skirts do not reduce high pressure air on the side of the car however it splits the bottom of the car into two parts one the under body and other the side of the car. Underbody region has low pressure and high air velocity and side of the car has relatively higher pressure. Without the side skirts in place the air from side of the car will rush into the underbody due to pressure diffuser difference reducing downforce. With the presence of side skirts, it acts like a blockage.
(c) Diffusers
These are used to increase volume at the rear section of the car under the body, this increased volume creates a void which had to be filled and therefore increases velocity of air travelling at rear end of the car, and as the resultant of this low pressure will be created to increase downforce.
Conclusion
The importance of aerodynamics in vehicle and car is equated with aerodynamics in the aircraft and is not limited solely to drag reduction. Downforce generation and its effects on lateral stability have important impacts on race car performance, especially in high-speed turns. Due to effects like stream separation, vortex flow or boundary layer change, it is not always possible to predict flow across most types of race vehicles. Due to its competitive nature and the short development period, engineers have to use hybrid track, wind tunnel and CFD tests to assess technology decisions. Modern aerodynamics is a very important science, it is used to save fuel in terms of general vehicles, it is used to make such vehicles stable and finally makes our street safe around large skyscrapers.
[Mr. Yogesh Bhawarker]
HEAD, AERO
Sandip University
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