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As we saw in the previous post, we begin to see the tests that are carried out on the vehicle. There are some known tests such as aerodynamics and crash tests, but there are others that are almost a mystery. In this week we will see aerodynamics, it will be the technical field that we will see most in depth due to the high interest that exists in aerodynamics.
We will see what tests are carried out and we will explain the logic of aerodynamics, avoiding performing mathematical demonstrations. This post and the next will be an overview of all the basics of vehicle aerodynamics.
AERODYNAMICS AND AEROACOUSTICS
The main part of this type of testing is carried out in wind tunnels. They are really expensive facilities, so the construction details are usually a secret, but here we will reveal some of their main characteristics.
The building is built on springs and platforms that absorb possible vibrations from the ground so that they aren't transmitted to the interior of the building. The entire installation is designed to minimize any vibration and any possible change in temperature.
In the design of the building, even the smallest detail is calculated, such as the velocity gradient that happens in the air ducts or the impact that the air has when it collides with the walls and with possible corners within the wind tunnel. For that, wind deflectors are placed to re-circulate it, all in order to keep a laminar flow within the installation.
The only obstacle the air must encounter is the test vehicle itself, no more. On the contrary, if the canals and walls are too wide, the energy loss would be enormous. So the sizing of the installation is key. The walls of the test area are usually mobile to be able to adapt it to different tests.
HOW DOES THE WIND TUNNEL WORK?
How does the wind tunnel work?
Inside the wind tunnel, we find huge turbines that blow or intake the air through ducts with stabilizing grids to channel the air. This is to achieve a laminar flow before reaching the vehicle. Without these anti-rotation elements, the air would come out of the fan rotating in the direction of the movement of the blades. The larger the blades, the slower they must rotate to be able to move the same volume of air, therefore, the less vibrations they will have.
There is also the scale wind tunnel. In this case, if the object is at 1: 8 scale, the wind will have to move eight times faster than it would at full scale to conduct an effective test. This entails a series of constructive limitations, but in turn, the risk of vibrations occurring in the installation is greater because the motor and the blades have to move faster. The advantage of the scale wind tunnel is that it has a much lower cost, because a scale model of the vehicle is tested in it.
The car is mounted on a platform called a balance, it's equipped with a multitude of sensors and rollers to simulate the vibrations that would occur when the vehicle is running in real conditions.
All this can be calculated by a computer, using CFD (computational fluid mechanics), so, What are wind tunnels for?
what are wind tunnels for?
The computer calculation is effective when we are talking about a laminar regime, but when the flow is turbulent the CFD is not able to obtain reliable results. So there are certain areas of the vehicle that cannot be calculated with the computer and we will have to calculate them in a practical way. In fact, in the Cfd calculations carried out with the computer, there are usually discrepancies with respect to those obtained in the wind tunnel. Still, the CFD calculation is very useful to approximate the final solution and save a lot of money in wind tunnel testing. However, CFD is not only used in aerodynamics, but in other aspects such as cooling and overheating of components, cylinder combustion, exhaust gas flow, etc.
WHAT IS AERODYNAMICS?
What is aerodynamics?
Aerodynamics in a vehicle is the study of the influence that the wind has on the vehicle. With a correct study of the aerodynamics, we will have a vehicle with greater cornering stability and better behavior against side winds. This study is also crucial in order to properly cool different elements of the vehicle. In conventional vehicles, the main reason a manufacturer has to improve the aerodynamics of a vehicle is to reduce its consumption. Since if the aerodynamic resistance is reduced, the vehicle needs to make less “effort” to overcome the opposition of the air and therefore, the fuel consumption will be less. At 90km / h, aerodynamic resistance accounts for 30% of the vehicle's consumption, while at 120km / h this figure rises to 50%.
AERODYNAMIC RESISTANCE FACTORS
Aerodynamic resistance factors
Aerodynamic resistance or aerodynamic drag can be understood as a force that opposes the movement of the vehicle when it's moving through the air. It depends on four elements: air density, speed, the vehicle's front surface and the drag coefficient, which depends on the shape of the vehicle itself. All these elements are important, but we will see how one of them has a greater influence than the rest.
1. The density of the air cannot be varied as it depends on external conditions, so it is generally a relatively constant value.
2. The front surface of the vehicle defines the amount of air mass that will need to be removed. In other words, it would be practically the initial surface against which the air hits.
A bullet offers little aerodynamic resistance , among other things, because the first surface of impact with the air is minimal. On the contrary it happens with trucks, they have a very large initial impact surface; something that doesn't help at all.
3. Speed: All the above factors multiply, but speed squares. This means that if the speed is doubled, the aerodynamic resistance multiplies by four. At the same time, the power required to overcome that aerodynamic resistance increases with the cube, requiring eight times more power to double the speed.
For example, considering only the aerodynamic resistance, without considering the mechanical or rolling resistance.
We consider as an example that to go 50km / h we need a power of 2CV.
If we double the speed (100km / h), the resistance increases to the square and the necessary power to the cube (2CV x 2 ^ 3 = 16CV). If we double the speed again (200km / h) we need a power of 128CV. This is also reflected in the car's consumption.
4. The vehicle's own shape: The fourth and last factor influencing aerodynamic resistance. It's one of the aspects that any car designer is most interested in knowing, since it's a factor that we can control.
This factor is solely tied to the car's shape and, to a certain extent, is not tied to its size. They are dimensionless.
There are three different factors, one for each axis, we can see them in the previous image. Among the factors on which the aerodynamic resistance depends, we only will consider the Cx, which is the Penetration Coefficient, this is because we are evaluating the aerodynamic resistance of the vehicle when moving forward. Remember, we only consider the penetration coefficient Cx to evaluate the aerodynamic resistance, that doesn't mean that we discard the rest of the coefficients (Cz, Cy).
4.1 Coefficient of penetration (Cx): It's known as Cx in European terminology, or Cd in British terminology. The Cx is also known as the front drag coefficient, aerodynamics coefficient of drag, or "drag".
The Drag Coefficient defines the resistance of the object as it moves through the air. The lower the number, the better, since the lower the aerodynamic resistance offered by the vehicle when moving forward. In other words, the less effort the vehicle will have to make to overcome the air barrier, this will make it consume much less gasoline.
This is of great interest to the largest companies. If they manage to reduce aerodynamic resistance, they will put vehicles with much lower fuel consumption and emissions on the market, and this is something that consumers are very interested in. In addition to facilitating compliance with emissions regulations.
As we discussed earlier, the numerical value of this factor depends solely on the shape and to a certain extent, it doesn't depend on the size of the object. We have seen that the four factors that determine the aerodynamic capabilities of a vehicle are the following:
1 - Air density, 2 - Speed, 3 - Car's front surface 4 - Coefficient of aerodynamic resistance (Cx).
In practice, the SCx is used to relate the front surface of the vehicle to the numerical value of the coefficient. It's the product resulting from multiplying the frontal area with the penetration coefficient (Cx), in this way we dimension the vehicle correctly, since this way we take into account the size of the vehicle.
4.2 Aerodynamic side coefficient / Side force coefficient: In the lateral axis of the vehicle, in a symmetrical vehicle, the value must be zero, since the values of both sides would have the same magnitude with opposite sign. It doesn't affect aerodynamic resistance, since when we talk about aerodynamic resistance it's forward, affecting only the X axis, not the Y axis. (Remember the previous image to see the directions of the axes)
4.3 Lift coefficient: Positive lift makes airplanes fly, which is why cars are interested in negative lift. This is known as DownForce, it's the force generated perpendicular to the direction the car is moving. With a positive lift the vehicle tends to lift off the ground, so we are interested in the opposite, in negative lift to obtain a better grip on the road.
The Lift coefficient (Cz / Cl) doesn't affect the aerodynamic resistance. Since when we talk about aerodynamic resistance it's to the advance, affecting only the X axis, not the Z axis. Even so, as we will see, it's a key factor in the design of a vehicle.
With a higher negative lift coefficient, the manufacturer seeks to increase the grip in the curves of the vehicle at notable speeds. On the other hand, a greater effort is necessary for the advance of the car so consumption increases. Note what we have discussed above: Higher negative lift, the term can seem confusing. We are not talking about a lower lift, but a higher negative lift. That is, on the horizontal axis, from top to bottom.
It seems confusing right? We're going to explain it without being strict academically, just with the intention of having a final image.
These last definitions are not exact at all, but they help us to associate the concepts.
Which of these cars has a higher Drag coefficient (Cd)?
Let's look at the cars above, and then try to guess the car with the highest and lowest Drag Coefficient? The lower the penetration coefficient, the lower the resistance to advance it will have. Let's look at how aggressive that Lamborghini Countach is, or the sophisticated lines of the Ferrari F12 based on thousands of hours in the wind tunnel. Even though, the Volkswagen Beetle is quite rounded
This is the result:
Audi A2 - 0.24
Renault 25 TS - 0.28
Ferrari F12 Berlinetta - 0.3
Fiat Ducato 2014 - 0.31
Lamborghini Countach - 0.42
Volkswagen Beetle 1938 - 0.48
The van, the Fiat Ducato, has almost the same drag coefficient as the Ferrari F12.
The Renault 25 TS is much more (frontal) aerodynamic than the Lamborghini Countach, and the Ferrari F12in fact.
With this we draw a clear conclusion: The aerodynamics is not intuitive, it must be tested by CFD. Even so, there are some key points that we will see in the next post, but it will always have to be studied either by computer or in a wind tunnel. By the way, the previous images were ordered from lowest to highest drag coefficient, you can look at it again and check it.
What has a higher drag coefficient (Cx)? A Formula One or a Truck ?
This part is important, and interesting in turn. Remember that Penetration Coefficient and Drag Coefficient are the same thing.
We have already seen the previous example exercise. But then, What happens if we compare a Formula One to a truck? Aerodynamics is key in Formula One, or Formula Indy, as we already know.
Indeed, a truck has a lower Drag Coefficient (Cx or Cd) than a Formula One or a Formula Indy. The Drag Coefficient (Cx) of conventional trucks is usually around 0.6 and 0.7. A Formula One, depending on the circuit, ranges between 0.7 and 1.2.
This is due to the fact that in trucks we're interested in reducing gasoline consumption, so the designers work to reduce the (Cx), that is, they have the least possible drag. In Formula One this doesn't happen, what matters most is the high corner speed, so we work to obtain the greatest possible adherence. This is achieved by raising the negative Lift Coefficient (Cz) thanks to the ailerons and aerodynamic add-ons of the vehicle. So we sacrifice the Cx to get more adherence by improving our negative Cz.
So, Does a truck have less aerodynamic resistance than a Formula One?
Now we're going to learn about aerodynamics. Looking at the above, Does a Truck cut the air better than a Formula One?
Let's think about the question a bit before looking at the solution. We know that a lower Drag coefficient implies less aerodynamic resistance. We also know that a truck has a lower Drag Coefficient than a Formula One.
Knowing this, what solution would we give? The solution is clear then, right?
No, a Formula One has less aerodynamic resistance than a truck. But weren't we saying that a truck had a lower Drag Coefficient?
We must not confuse terms. On the one hand we have the Drag coefficient (Cx / Cd ) that is given only by the car's shape. Also called Coefficient of penetration or aerodynamics coefficient of drag.
On the other hand we have the aerodynamic resistance that is given by the four factors that we saw previously. Two of those factors are the front surface and drag coefficient (Cx). That is, the Drag Coefficient (Cx) is a factor of the Aerodynamic resistance. They are not synonymous.
Aerodynamic resistance is also called Aerodynamic drag.
The truck has a better Drag coefficient, but the front surface of a Formula One is much smaller than that of a truck.
As we saw earlier, sometimes we work directly with the SCx, which simply consists of multiplying the car's surface with its respective drag coefficient (Cx). The SCx of a truck is approximately four times that of a Formula One.
In summary, there is an aerodynamic coefficient that is the Drag coefficient(Cx), this is one more factor to determine the aerodynamic resistance, but they are not synonymous.
For the conceptual designer, it's interesting to have some notions of aerodynamics, in this way they will design vehicles that are closer to the final solution. But as we have seen in the previous exercises, the vehicle will always go through aerodynamic tests, since we cannot rely on intuition. Although having knowledge can help to get closer to the solution directly in the initial phases.
One of the challenges designers currently face is finding a vehicle with good aerodynamics but good habitability. A solar vehicle has very low aerodynamic resistance, but as we can see, it isn't feasible to put it on the market. On the other hand, a van is not very aerodynamic, but really practical and spacious, which is why thousands of units are sold every year.
So we have to play with a shape that is as aerodynamic as possible, but without losing interior space. A square box would be the most optimal space, but it has very poor aerodynamic resistance due to the high coefficient of penetration and the large front surface, therefore it would have a very high fuel consumption.
We must also consider that at low speeds the effect of aerodynamics is less, remember what we mentioned earlier:
At 90km / h, aerodynamic resistance accounts for 30% of the vehicle's consumption, while at 120km / h this figure rises to 50%.
Therefore, in the design of a delivery van, it would be wise for a designer to know beforehand if the van will circulate mainly in or around the city, to estimate the average speed. In this way, the designer will give more or less importance to the aerodynamics versus the interior space of the vehicle.
What is the boundary layer?
Understanding the boundary layer can be complex, but the important thing is to stay with the general idea. In fact, it will be possibly the most complex concept of the entire course.
The boundary layer is a key concept in aerodynamics, we will not see it in depth since its full explanation entails a much more technical explanation behind it. If your interest is aerodynamics, we recommend that you expand the information with some books on car aerodynamics, with them you will be able to understand much better key aspects of the car aerodynamics. In the meantime, we will give a very brief explanation.
If we look at the CFD test of a car, the layers we see correspond to a laminar flow.
When a car moves forward, if we consider a laminar fluid, we can divide the air into different layers as in the previous image.
For this explanation we consider that the car is moving at 120km / h and the wind at 80km / h, linearly and constantly.
Then the layer of air that is stuck just to the surface of the car will go at the same speed as the car. This always happens, to simplify, we can say that the adjoining layer is "attached" to the surface. This is due to viscosity effects.
Then the next contiguous layer will go a little slower, and so on getting a velocity gradient (or strain-rate tensor or rate-of-strain tensor). What is a velocity gradient? It's a measure of how the velocity of the air changes between different points within the fluid. In this case, it would mean that each layer will go a little slower than the previous one, gradually decreasing the speed until it reaches 80km / h.
So we can say that when the air goes at 80km / h, it will no longer be under the influence of the surface of the car passing through it. The boundary layer is the one that divides that transition moment, between the layers that are affected by the car and the layers that are not.
Let's explain it in another way, even if it isn't an academic explanation.
Let's change the scene for a moment and go to a swimming pool. When we were little we all tried to push the water into a pool creating a kind of wave with the hand or the arm.
We are going to focus now on the moment in which we push the water, and not on the waves that occur in the pool afterwards, since that is a very different issue. We only focus on the surface of the pool water.
When pushing the water with the hand, it is evident that we produce a change: Before the water was calm, but when pushing the water with the hand, the surrounding water moved.
But at the moment in which we push with the hand we don't move all the water from the surface of the entire swimming pool, but we move only what we have around us. The boundary layer would be that imaginary boundary that divides the water that we have moved from the swimming pool, and the water that has not been affected by our movement.
Returning to the example of the car going through the air. From the boundary layer to the outside, the air will always go at 80km / h because it is not influenced. From the boundary layer towards the interior it will go from 80km / h to 120km / h, which is the speed that the car takes.
In the previous example we only talked about the boundary layer in the laminar regime, but there is also the boundary layer of turbulent flow and they can even coexist. This topic is already more advanced, so we recommend seeing some aerodynamics books, for those who want to dig deeper into this subject.
Engineers must avoid the boundary layer separation (or Flow separation) at the beginning of the "air path", from a surface into a wake. Separation occurs for example, in flow that is slowing down with pressure increasing.
For experts in the field, sometimes, they are interested in looking for the intentional detachment of the boundary layer in specific situations making use of variable aerodynamics, but this already goes into advanced aerodynamics.
To really understand the boundary layer, it's necessary to delve into specific aerodynamics books.
All this explanation has been necessary because in the next post we will see in a practical way the application of these concepts through the different ailerons and aerodynamic solutions that are used in cars. That is, we will see how to apply aerodynamics to car design.
In the image we have the CFD analysis of a Ferrari 488 GTE, we can see how the lines advance in laminar flow. We can also identify the plane of the maximum section of the car. In this case it's located shortly after the top of the windshield.
At that point will be when the flow begins to separate from the body and to decrease the pressure.
That maximum section plane is key. Without being strict academically, we can say that: From the section plane to the car's front the air is pushing the vehicle, and from that section plane backwards it's pulling the vehicle.
Next week we will see the aerodynamic solutions and how the factors that we have seen previously are modified. We will also see the operation of a spoiler and a diffuser, and many other things.
For the next post, after aerodynamics we will see the crash tests. The most interesting thing is that we will see how to interpret a crash test, something about which there is no information.
If you have missed any post, here you can see the complete index.
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