Figure 1 A Car in a Wind tunnel With Luminous Flow Lines for Visualization
Aerodynamics is the study of how air or gasses flow and behave when they move through an object (i.e. air flow down a tunnel) or when an object moves through it (car or airplane moving through air). Our understanding of aerodynamics has increased tremendously over the last century and a lot of principles and thinking that were pioneered in the aircraft industry have now made the transition over to the automotive industry.
Figure 2 Basic Diagram of Aerodynamic Forces on a body
In figure 2 the lift would be the tendency for the car to lift as speed increases, the opposite of this is the weight of the car. If the force that tends to lift the car is reversed, it is called downforce and acts in the same direction as the weight. The Drag is what is colloquially known as air resistance and the thrust would be the propulsion force provided by the engine.
Drag is the resistance a body would feel when trying to move through the air at speed. In the case of a car, we can break it down into three main causes.
- Frontal pressure, or the effect created by a vehicle body trying to push air out of the way.
- Rear vacuum, or the effect created by air not being able to fill the hole left by the vehicle body fast enough
- Skin friction of the air moving on or very near the surface of the vehicle
These three forces make up the aerodynamic forces a car will experience
Frontal pressure is caused by the air attempting to flow around the front of the vehicle as shown in diagram D1 below.
Figure 3 Front Pressure Buildup In Front Of Car ( Pic Courtesy of http://www.buildyourownracecar.com/)
As the car moves faster and faster the air coming in from the front cannot move out of the way fast enough and the air pressure begins to build on the front of the car. You can see this for your self if you put your hand outside of a moving vehicle. If you have your palm open towards the direction of travel, you will feel the pressure of the air on your palm, which will increase as the speed increases.
Flow detachment occurs when the object (in this case a car) is moving forward and the air being pushed aside has to fill the void that the car leaves behind it, like the wake of a ship.
Figure 4 Flow Detachment Behind Car ( Pic Courtesy of http://www.buildyourownracecar.com/)
If the air cannot fill the voids fast enough, this will cause a vacuum effect to form that will create a suction force on the car that will try and pull it back in the opposite direction of travel.
Flow detachment can be a serious issue as the speed rises and this is why many sports cars and race cars have smooth wedge shaped bodywork. This has transitioned over to passenger cars as well with cars like the Toyota Prius having a sloped hatchback shape to help smooth and channel the flow of air and prevent detachment. By reducing this flow detachment, aerodynamic efficiency can be increased significantly, reducing fuel consumption.
Figure 5 Diagram of Turbulent Flow Around Mirror (Pic Courtesy of http://www.buildyourownracecar.com/)
When airflow detaches from the contours it is following, the flow becomes very chaotic and non-uniform. This type of flow is called turbulent flow. This is the same type of turbulence you may encounter on an aircraft where the captain may tell you to fasten your seat belts and prepare for a bumpy ride. This bumpiness is caused by turbulent flow around the aircraft.
In figure 4, you can see the flow beginning to detach around the wing mirrors. This will increase as the speed increases. This is why some sports cars have aerodynamic mirrors. Completely preventing turbulent flow is impossible in a vehicle, but reducing and controlling it is very much a key area of vehicle design.
How do you measure and quantify drag? The old-fashioned way used to be with a wind tunnel and meticulous testing, but now modern computer simulations known as CFD (Computational Fluid Dynamics) allow the testing to be done virtually. To compare the drag produced by one object to another a quantity known as Coefficient of Drag is used. Drag coefficient or Cd for short, is a measure of how aerodynamic an object is and can be used in drag equations to calculate the amount of drag at a given speed. The lower the Cd, the more aerodynamically efficient it is.
Intuitively you will find that to minimize Cd you would want a car with a small frontal area, low ground clearance, smooth lines, a long tail that slopes down gently instead of suddenly and a steeply raked windscreen. This is will give you the classic sports/race car wedge/dagger shape.
This should be a good primer for getting into automotive aerodynamics and we will go into it more in our next installment of T&T Tech.
Figure 6 The Classic Sports-Car Wedge Shape As Seen in This Nissan Le Mans Racer
Words: Kanchana Gunasekera