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Essence of Race Car Aerodynamics
Aerodynamics deals with the study of the movement of air and the effects it has on objects moving through air and plays a crucial part in determining the performance, stability and mileage of a racing car. Car bodies are streamlined or designed in a way that it uses airflow to increase its traction and decrease the resistance offered by air.
We shall look at the factors which determine the shape of car bodies.
We are going to understand the Essence of Race Car Aerodynamics broadly through three topics.
1. Pressure
2. Newton’s Third Law of Motion
3. Forces exerted by Air Flow
PRESSURE
Air consists of tiny molecules flying around in the volume it occupies. In an intuitive sense, we can say pressure is the busyness of the air molecules. The air molecules collide with each other and with the objects present in the volume of air. This rate of collision increases if we increase the air density, or raise the temperature.
When we fill a balloon with air, the molecules in the air collide with the surface of the balloon, thus exerting a force and keeping the balloon inflated. Notice that the air outside doesn’t push the balloon to its normal shape even though two forces, external air pressure and elastic force exerted by the surface of the balloon are acting to restore the balloon’s normal shape. Therefore, we conclude that the air inside the balloon exerts more pressure than the air pressure outside the balloon.
However, if we open the mouth of the balloon the air rushes out quickly. This is because the force of the air molecules exiting the balloon is stronger than the force of air molecules trying to enter the balloon. Therefore, we conclude that air flows from a region of high pressure to a region of low pressure. Not only that, but you might have observed that the air flows out really fast from the balloon. This is because the opening of the balloon is small and in order for the mass flowing through all cross-sections to be constant, the velocity of the air in a smaller cross-section must be greater, commonly known as Continuity Equation. Thus, our second conclusion is air flows faster when it is forced to flow from a region of greater volume to a region of smaller volume. Note that in a balloon this effect is greater because of elastic force acting to push out the air.
NEWTON’S THIRD LAW OF MOTION
This law states that every action has an equal and opposite reaction. It means if you push against something, it pushes you back with equal amount of force. When a bird flaps its wings against the wind to push it, the air pushes the bird back and the bird stays afloat. Thus, when we want to move in a particular direction, we need to push the air in the opposite direction.
FORCES EXERTED BY AIR FLOW
From the topics discussed above we are going to look the forces experienced by a Body moving through air. When an object moves through air, two types of forces are experienced by the body, namely DRAG force and LIFT force.
Drag Force:
It is responsible for opposing the motion of the body. Drag forces are caused by two reasons:
1. First one is because of the frictional forces or viscous force acting on the surface of the body, called Friction Drag. This Force increases if the surface area of the body parallel to the direction of air flow is increased .
2. Second one is caused by the pressure difference between the front and back of the moving object, called Pressure Drag. The air near the surface of the object, known as the boundary layer experiences the greatest friction. Eventually the air at the boundary layer stops and reverses its direction due to loss of momentum. This creates a flow separation and an area of low pressure behind the moving object. The air molecules at higher pressure at the front the moving body exerts greater force, pushing the body backwards.
We can minimise the pressure drag if we can delay the flow separation. This is done by generating turbulence which mixes the air from boundary layer with other layers and delays flow separation. A commonly used device is a Vortex Generator used in race cars and aircrafts, which creates spiralling air, mixing the air from boundary layers with other layers so that the net momentum of air at boundary layer doesn’t become zero and avoids flow separation. However, generating turbulence comes at a cost of reducing downforce and increasing Friction Drag.
Thus, keeping in mind the two drag forces, we have to design our object body. Note that a body with minimal pressure drag may not have lowest friction drag, similarly a body with minimal friction drag may not have lowest pressure drag. Thus, we need to modify the shape of our body in such a way that the combined effects of friction and pressure drag is minimal.
Lift Force:
Lift Force acts perpendicular to the flow direction and is responsible for either lifting the body upwards or downwards depending on the shape and orientation of the body.
1. For a Streamlined body, when the angle of attack is positive, the Lift force acts upwards, commonly known as Lift and is used in aircrafts. Let us understand how this works. The air going above the body is observed to be faster than the air going under it, and mostly happens due to the low-pressure region created behind the body due to flow separation which acts as a vacuum and pulls the air moving above it. Now according to Bernoulli’s Principle, fast moving air creates a region of lower pressure. Therefore, the downward force applied by the low-pressure air above the body is smaller than the force applied by the higher-pressure air going under the body.
Furthermore, note that we are pushing the air downwards. According to Newton’s third law of motion, the air will push the body upwards with equal amount of force. These two forces combined applies a net upward force or Lift on the body.
2. For a Streamlined body, when the angle of attack is negative, the Lift force acts downwards, commonly known as Downforce and is essential in race cars to increase traction and performance. This phenomenon is exact opposite of lift force. The air going below the body is observed to be faster than the air going above it, and happens due to the low-pressure region created behind the body due to flow separation which acts as a vacuum and pulls the air moving below it. Again, fast moving air creates a region of lower pressure. Therefore, the upward force applied by the low-pressure air below the body is smaller than the force applied by the higher-pressure air flowing above the body. This applies a net downward force on the body.
In this case we are pushing the air upwards, so the air will push the body downwards with equal amount of force. These two forces combined applies a net downward force or downforce on the body.
From the topics discussed above we shall understand the parts of F1 cars which control the air flow and understand how they function. Let us look at some significant parts of F1 cars which determines the aerodynamic properties of the car.
Streamlined Body:
The body of an F1 car is made in such a way so that it reduces the friction between the air and itself as much as possible, thus decreasing overall drag. It is made streamlined avoiding sharp edges throughout the car to prevent the formation of turbulent air patterns which hamper the performance of the car.
The Cd (or the drag coefficient) is kept as low as possible without compromising the downforce on the car. Most cars have a Cd of around 0.25 to 0.35 but F1 cars have a Cd around 0.7. This is because of the presence of other parts like wings, spoilers, splitters, etc which provide downforce (thus preventing upward lift) but increase drag.
For F1 cars we have to keep upward lift in mind because at very high speeds, reducing lift becomes as important as reducing drag and so, we have to shed as much drag as possible whilst not sacrificing too much downforce.
Front Wing:
The Front wing is suspended from the nose at the front of the car and it runs along the entire length of the car .It is the component of a F1 car which is most regulated by the FIA (Fédération Internationale de l'Automobile).
They carefully design these wings to optimise the downforce on a car without compromising too much drag. It also helps in right airflow throughout the car . The front wing consists of a series of carbon fibre smaller wings (maximum of five mini wings per side of a front wing) arranged one behind another.
The front wing curves upward forcing air to move around it in such a way that a high-pressure area forms above the car and a low pressure beneath. This creates downforce that pushes the car to the track. Where the wing ends and these pressure zones meet, the air crashes into itself which creates a spiralling vortex. Vortices cause drag and slows the car down.
So, endplates are used at the wingtip to interrupt these vortices for a less tense effect. In contrast, the pointed curved end surfaces on the inside edge of the wing direct the vortices around the floor of the car, sealing in the clean air that passes beneath. So, while front wing do cause drag, they are also a useful tool to separate and seal in separate areas of airflow and create huge downforce (about 25% of the total downforce on the car) which improves the car’s grip on the track.
Splitters:
The Splitters(also called T-tray in the F1 cars) form an attachment for the plank running along the length of the flat bottom floor of the car.
As the names suggests, its job is to split the air a car drives into and force the high-pressured slow air upward and the low pressured fast air below the car .
Thus, due to the pressure difference a downforce is created on the splitter which helps in increasing the traction on the road. It also helps preventing understeer by contributing to the downforce at the front of the car along with the front wing thus balancing the huge downforce provided by the rear wings at the end of the car.
Bargeboard and sidepods:
The Bargeboard conditions and clean up the dirty air produced by the front wheels as much as possible and makes the clean air go through the sidepods present at either side of the car body which intakes the clean air and cools the internal components of the car and guide the air over the car towards the rear.
S-Ducts:
As the air flows over the surface of the car it loses energy and slows down, thus becoming turbulent. This air is called dirty air and it heavily impacts the performance of the car and also the cars trailing behind it.
One of the main places this occurs is at the front wing ends. So, we ingest this dirty air from under the nose through two pairs of NACA ducts and release this on top of the chassis rather than letting it travel under the car. This allows the air to be drawn in with high efficiency and minimal drag.
Diffuser:
The key role of diffusers in modern F1 cars is to accelerate the flow of air underneath the car by creating a region of low pressure, so as to increase the downforce of the car. It results in attaining the maximum speed of the car. The diffuser is usually found on each side of the central engine and gearbox fairings and is, by the rules, located behind the rear axle line. Downforce allows the tires to transmit greater thrust without wheel spin, increasing the maximum possible acceleration. In the case of a modern Formula 1 car, the lift-to-drag ratio Cl/Cd has a typical value of, say, 2.5, so downforce dominates performance.
Diffusers eject out air from the underside of the car, which in turn increases the velocity of the air below the car. Using Bernoulli's equation, (which states that where the speed of the fluid is higher, pressure must be lower), we can state the pressure below the car is less than the pressure at the outlets. This results in an increase in downforce, hence increasing the maximum possible acceleration. To prevent the flow separation from its side and roof, it is to be noted that the angle of the diffuser must have a gradual change.
Three main aspects of diffusers are:
1. Ground effect: It creates a low-pressure area underneath the car so that the atmospheric pressure pushes the car to the ground. Asymmetric flow of air was developed in racing cars which reduce the static pressure underneath the car.
2. Underbody Upsweep: It refers to the upsweep of the diffuser at the rear. This is typically cambered and up-curved in shape. Due to the direction of this curving, a resulting downward directed lift force will result during flow interaction.
3. Diffuser Pumping: It refers to the increased cross-section area over the diffuser length, which can be used to increase the flow rate through the diffuser because of the gradual change in pressure. As the ratio of the inlet to outlet area becomes increasingly greater, this generates greater pressure recovery that, since the base pressure which remains constant will increasingly reduce the base pressure at the inlet. The diffuser acts to reduce the underbody pressure due to the expansion resulting in an increased flow rate under the body. This increase results in further decrease in underbody pressure, which produces the "pumping down‟ effect in racing cars.
In F1 racing, the 2010 diffusers increased the value of downforce by a huge value. Red Bull Racing (2010 constructor champions) with their 2010 racer turned out with double diffuser and with exhaust gas blowing inside and over it.
Rear Wing:
The main aim of rear wing is to generate downforce to counterbalance the downforce created by the front wings of the racing car. In F1 racing cars, the rear wing generates about ~10% less downforce than the front wing. The rear wing F1 cars work differently font wings.
The main components of the rear wings:
1. The main plane: the thicker profile of the rear wing assembly. This part remains fixed when DRS is open.
2. Flap: the smaller profile which acts as a slotted flap increasing downforce and preventing flow detachment. The main plane and flap are connected by means of fishbones.
3. On the top part of the endplates (the lateral plates), we have some louvers/gills. These openings can have different shapes (horizontal openings or also curved ones).
4. Middle trim: this part of the endplate, is sometimes trimmed in order to better the flow coming from the wheels and the bodywork.
5. Bottom trim/louvers: These louvers located in the lower part of the endplates are designed to work efficiently with the rear diffuser and exhaust gas.
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