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Before we dive more into details about downforce, drag and their effects on a car, we should probably take a step back and try to understand why these forces exist at all. We will dig into more details about this when we will talk about cars “aero components”.
It is useful to briefly describe how a car produces aerodynamic actions, when travelling at a certain speed, because these basic concepts will help us to go through some other topics more easily.
The first fundamental principle to keep in mind is that any object will experience aerodynamic actions if it is in contact with the air (if we talk about races on the earth this happens always) and if it moves with respect to it.
In general, a body moving inside a fluid (as a car moving with respect to the air) experiences aerodynamic forces because of two main reasons: friction between the fluid and the body itself and pressure distribution around the body, connected with its shape and dimensions.
Friction exists anytime a body moves relative to a fluid (always assuming the two are in contact with each other) because of fluid viscosity.
Viscosity is a measurement of fluid’s resistance to deformation. In other terms, it is a physical property that tells us how much friction is generated when the molecules of a fluid move with respect to each other. For example, we can get a feeling about a liquid’s viscosity when we pour it into a container. If a liquid moves more freely than another, it means its viscosity is lower, because its molecules have a lower resistance to relative motion. For example, water has a much lower viscosity than honey.
Despite it being much lower than most of the liquids, also gasses (like air) have their own viscosity and this is one of the most important sources of all the aerodynamic forces a race car experiences and the reason why certain elements work as they actually do, as we will see.
A body moving inside a fluid will experience friction forces because the fluid will have to deform in order to allow the body itself to go through. Because of fluid’s viscosity, a part of its molecules (the ones seating in direct contact with the body surface) will not move with respect to the body; particles seating at a bigger and bigger distance from body’s surface will move faster and faster with respect to the body. Because of the speed difference between contiguous particles, we will see friction forces raising inside the fluid itself.
Aerodynamicists often refer to the area close to a body’s surface, where air’s speed grows from zero (in contact with the body) to its free stream magnitude as the boundary layer. How thick the boundary layer is depends on many factors, including, of course, fluid viscosity and the relative speed of the body with respect to the fluid.
The picture above shows a body (the thick black line on the lower part of the picture) in contact with a fluid, with a relative motion between the two taking place. The arrows identify the relative speed of the fluid with respect to the body: as we see, the portion of the fluid in contact with the body has no velocity; anyway, as we move away from the surface, fluid’s speed increases until it reaches its free stream values (so the values it has before meeting the body). The distance required for the speed to increase from zero to its free stream condition is the boundary layer’s thickness.
If we ideally had a fluid without viscosity, a body moving into it would not experience any resistance to its motion, hence no drag.
As we said, friction and viscosity are only a part of our picture, as another source of any aerodynamic action is pressure and its distribution around a body.
From a mathematical perspective, a pressure is a force divided by an area and can be defined using the following equation:
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- P is pressure, measured in Newton per square meters (N/m2).
- F is a force, measured in Newton (N).
- A is an area, measured in square meters (m2).
The above equation tells us that, if we talk about pressure, a surface must be involved. The reader probably remembers that, when we wrote the aerodynamic forces equations, an element in our formula was actually car’s front surface, which is an area. This immediately suggests a connection between aerodynamic forces and pressure.
What actually happens when a body moves with respect to a fluid is that fluid’s pressure around the body changes. As we will see in more details in one of the next articles of this series, pressure changes also depending on body shape and dimensions.
We know that pressure and forces are strictly related and, as the above equation tells us, anytime there is a pressure, there is a force acting on a surface.
The reality of things is that, anytime a body, no matters its shape, is immersed in a fluid, a pressure acts on every portion of body’s surface in contact with the fluid. If the body is symmetric and the pressure acting on both side of it is the same, it will experience two forces with the same magnitude and direction but pointing against each other, that will produce no net effect.
In the above picture, the grey parallelogram is our body and the orange portions are its surface in contact with the fluid, with an area equal to A. P is the pressure acting on both side of the body and F is the force exerted by the fluid on the body, according to the equation:
Since P is the same on both side of the body and the body is in contact with the fluid with two surfaces of the same dimensions (A), the forces acting on both side will have the same magnitude and direction, but pointing against each other; consequently, they will cancel off and we will have no net force acting.
If for any reason, the pressure acting on one side of the body is different than the one acting on the other side, we will have two different forces that will not cancel off and the fluid will exert a net force on the body itself.
As shown in the previous picture, we now have a pressure P1 on the left side of the body and producing a force F1 acting on it. On the right side, we now have a pressure P2 and the fluid applies a force F2 on the body. If we suppose that
P1 > P2
It follows that:
F1 > F2
And we will have a net force F acting on the body and given by the difference between F1 and F2.
If a body, because of its motion through air, is able to create a situation similar to what is shown in the picture above (pressure on one side is bigger than the one on the other side), air will apply a net force on the body (F in our picture).
Although this was an extremely simplified explanation, this is exactly what happens in a race car, with some components designed specifically in a way to bend the air flow around them in order to create net forces. For this to happen, we will see how important is air’s viscosity.