Car design course

Free car aerodynamics course

Automotive Design & Marketing Management

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Previous week we already explained the basics of aerodynamics. Now we're going to see the main applications that are carried out in the car. Next week we will see the famous crash tests and how they work, revealing some secrets.

AERODYNAMIC SOLUTIONS FOR CARS DESIGNERS

Aerodynamic solutions for cars designers

The aerodynamic study is a really complex area. The concept designer will have to use his notions in aerodynamics and his intuition to make sketches keeping a certain coherence. Even at the engineering level it's also important to have these notions to have a reference base with which to start.

It must be clear that, in the aerodynamics field, we will always have to carry out a study using CFDs to optimize the aerodynamics of the vehicle. Intuition cannot be used for this, since it's a complex area of study. That is to say, under no circumstances can these notions replace an aerodynamic study.

It should be noted that sketching an aerodynamic-looking car doesn't imply that it's aerodynamic. We saw an example with the Lamborghini Countach. Still, there are always some general keys that we can apply to try to have an aerodynamic car, but always, it will have to be studied in hindsight if we're talking about a real design.

We're going to deal with the aerodynamics in conventional vehicles, since if we delve into the aerodynamics of a Formula One or Formula Indy, we would need, at least, another post.

Currently there is a tendency to create an “invisible” aerodynamics for the consumer, creating harmonic shapes and discreet solutions. For example, the Mercedes CLA has an incredible Cx of 0.22.

Mercedes CLA Cx = 0,22

HOW TO MODIFY THE AERODYNAMICS OF ANY CAR?

How to modify the aerodynamics of our car?

Factors that modify the front surface of the vehicle:

We saw that one of the factors that determines the aerodynamic drag of a vehicle is the front surface.

Mirrors: Smaller mirrors reduce this surface area and significantly improve the coefficient of penetration, and therefore the aerodynamic resistance of the car. The front surface of a vehicle oscillates between 2m^2 and 2.5m^2, the more we reduce it the better. The front surface of a Formula One is around 1.4 m^2. This is one of the reasons why some manufacturers are replacing mirrors with thin cameras. In addition, these mirrors tend to generate minor turbulence that is not beneficial at all.

Proportion: In the same way, a narrower car will present a smaller frontal surface, therefore it will improve the coefficient of penetration by reducing its value. But this, as we already know, this also influences the vehicle dynamics, which we will see in its corresponding post. Here's a key to car styling, and a little secret: << The width-to-length ratio is usually around one-third for minimal strength. >>

Shoulders: A pronounced shoulders in the car can help us to achieve a smaller frontal surface, since we will obtain a narrower windshield. This is one of the strategies of Volvo's design language, its distinctive shoulders give a greater sense of security, but at the same time, reduce fuel consumption. ​The latter will not be told by the brand in a public way, since its claim is < safety > and it's where the marketing team will focus. Of course, it is also reflected in the consumption of the vehicle; but they will not tell you their tricks to obtain that figure.

Factors that modify the Drag coefficient (Cx) of the car:

We also know that the Drag coefficient is also a factor that determines the aerodynamic resistance of the vehicle.

Rear: The rear is even more important than the front of the vehicle. When a vehicle moves forward, it displaces a certain volume of air leaving a vacuum behind it, we can see it in the image below. When this speed is low, the air it displaces has enough time to fill the free space that remains as it moves forward. So there is not much air gap left behind the car at low speeds.

Source: Altair

 That is, there is less time to fill the air gap, as the car moves faster. Then, when the air gap cannot be filled, a depression zone in the form of turbulence is created behind the car. Low pressure at the rear of the vehicle and the generation of turbulence or vortexes is one of the most negative effects on a car's aerodynamics.
But this situation changes as speed increases, the mass of air that needs to move in the same period of time will be much greater when going faster.

To help the flow remain laminar, and the air gap can be quickly filled, one of the possible solutions is the following: Lower the rear window as much as possible through a smooth transition, in addition to narrowing the rear of the car to facilitate that transition.

Another radically different option is to use a smooth descent of the rear glass with a practically vertical rear. This type of bodywork is called Kammback. In summary, the approach is based on vehicles with aerodynamic shapes like the one shown below, but "sectioning" the tail. This is done because it's not useful to create the rear long-tail, as it would excessively increase the length of the car, so the shape is sectioned.
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Smooth the lines: Any protrusion of the car tends to generate small turbulence, such as the windshield wipers. One of the keys is trying to achieve as smooth a transition as possible between the several components of the car. For example, traditional door handles "disrupt" the harmony of the car, which is why they are increasingly embedded in the vehicle rather than protruding. Regarding the windshields, one option is to keep them as hidden as possible under the hood of the car. An aerodynamic shape is useless if the airflow encounters interruptions at the junction of the windshields or other elements.


Air intake: The front of the vehicle is an important cooling area, but also a dangerous area from an aerodynamic point of view. Electric cars have a great advantage here, since they don't need to cool an internal combustion engine, the front is practically covered. As a direct consequence, the Cx of the vehicle is noticeably improved. 

This practice is also taking place in internal combustion vehicles, reducing the surface area of the natural air intake. In more advanced cars, active aerodynamic elements are used, for example, regulating the opening or closing of the air intake electronically. This keeps the air inlet closed and opens only when it's necessary to cool the engine components.

Although it seems counterproductive, a car with a smooth front and without air intakes usually has a better Cx than one with an aggressive air intake. But, in a large air intake, not everything is negative, currently this air flow is also used to generate downforce (negative lift). In this way, the front is taken advantage of by channeling the air.
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Vehicle underfloor: Anything that implies breaking the "harmony" of the air flow will imply a worse Drag Coefficient, and may even generate turbulence. Therefore, a flat bottom is sought. Cars like the Porsche Taycan also take this to the suspensions, covering them in such a way that they don't create turbulences.
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Porsche Taycan , Cx = 0,22

Factors that prevent the boundary layer separation:

In the previous post we saw what is the boundary layer, here we will see some of the solutions that are applied to avoid the Flow separation. We will give solutions that usually work, but its isolated application doesn't necessarily lead to a solution or an aerodynamic improvement, it must be studied and analyzed. In addition, we will give simple explanations, however to understand the mathematical  explanations a deeper knowledge is necessary. 

Two-plane ailerons: As a general rule, the aerodynamic elements that are incorporated in a vehicle must work to hold the laminar regime of the vehicle as much as possible. ​In large ailerons, two pieces are used to avoid its total curvature, in this way we will prevent the aileron from stalling, that is, the boundary layer is detached (Flow separation). If this happens, there would be no airflow putting pressure on the aileron,so it will not do the negative lift effect we are looking for. This can also be applied to other vehicles that have spoilers with a longer travel.
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Vortex generators: You will almost always be interested in laminar flow in aerodynamic elements, but as always, there are exceptions. In very specific situations, we may be interested in generating a turbulent flow, when this is precisely what we generally try to avoid. That is, sometimes we will try to do just the opposite of what we have been trying to avoid all the time. This is because turbulent flow increases drag (Cx increases it), but it also increases vehicle lift (negative Cz).

But in addition to this, a boundary layer separation is more likely to occur when we are in a laminar regime than when we are in a turbulent regime. So, as we have already seen, it's essential to avoid that layer separation.

A common solution is to modify the geometry of the vehicle by making the rear window lower more smoothly, but if you don't want to modify the geometry of the vehicle, you can opt for vortex generators. ​This means that, in specific areas, it enters the turbulent regime by creating small vortices that generate greater lift and prevent boundary layer separation. Something that usually produces a negative effect, with a good aerodynamics study we can obtain a positive effect. Vortex generators are also used to modify the behavior of the air, redirecting it to an area of our interest.

Let's look at the vortex generators that the Mitsubishi Lancer EVO 8 carries in the transition between the roof and the rear windshield.
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ANALYSIS OF THE AERODYNAMIC COMPONENTS OF A DTM

Analysis of the aerodynamic components of a DTM

We hope you are learning in this transportation design course. Now, we're going to see the application of the above in the aerodynamic additions of a DTM. In this way, we can extend the aerodynamic solutions discussed above a bit, with a practical case. Names are not important as there is no standard and they tend to vary widely.

Source: BMW Motorsport

1 - Splitter / Front Skirt: If we look at the image above, the number one is the Splitter, which is a leading edge that divides the air, sending the high pressure towards the upper or to the interior of the vehicle. It also sends a part of the air flow through the underside of the vehicle, towards the diffuser.


2 - Dive Planes: Also called sidewings, flicks or canards, they are placed on the edges of the front bumpers to generate greater downforce and redirect the air flow.  They are also used to avoid the entry of the air to the underside of the vehicle. Like any aerodynamic component, it requires a previous study of Cfd to incorporate it effectively. As a general rule, we find a greater angle of attack on the dive planes than on the rear wings.

3 - Side Vents: These are the vents that are placed near the wheel arches. It's increasingly applied in street cars. It allows the flow to go out of the wheels, and at times, that flow can be re-channeled. It can also be used to allow hot gases to escape from engine components.

4 - Side Skirt: Point 4 is the side skirt, it serves to avoid the air from escaping through the sides by controlling the flow in the lower part of the car. Similarly, it prevents outside air from entering the underside of the car from the side.

5 - Flap Gurney: Normally we don't go into explaining these small aerodynamic details due to the large aerodynamics solutions that exist. But this one is so simple and ingenious in turn that it's worth it. Point five is not pointing to the wing, but to a small addition on the edge of the wing. Flap Gurney is a small tab that forms about 90º with respect to the edge of the rear wing. This helps preserve the suction effect, increases downforce and reduces the possibility of the boundary layer separation.
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HOW DO REAR WINGS WORKS?

How do rear wings work?

A spoiler is, in short, an inverted airplane wing. In airplanes, when the air hits the profile of the plane it bifurcates in two, an air current that goes above and another that will go below the wing. In the lower part of the wing profile the particles are slowed down and the pressure increases. This pressure difference causes a lift effect. That is, it tends to raise the wing of the plane.

We encourage you to search the internet for this question: "How does an airplane wing work?" "How do the ailerons work?" In practically every site you will find the following:

<< The flow that moves up and the one that moves below the wing reaches the end of the wing at the same time. As the upper flow travels a greater path than the lower one, the speed of the upper flow will be greater so that both flows finish traveling the wing at the same time…>>.

If you have searched the internet for the question, you will see this explanation in many places. This explanation is fake, is not true. 

The air flow moving above and below the aircraft wing don't reach the end of the wing profile at the same time.

This invalidates a widely held incorrect theory that gives an erroneous explanation for the speed difference between both air flows. This widespread theory (and erroneous in turn) considers that the air moves faster in the upper zone because it has a larger path; therefore, if they must reach the end of the wing profile at the same time, they will have to go faster.

So why do they have it? Because they improve the negative lift of the vehicle, achieving greater grip, better vehicle dynamics and greater cornering.Another common mistake: If we talk about top speed, a rear wing doesn't help a car run more, but it does just the opposite. Rear wings increase the frontal area of the vehicle thereby increasing its aerodynamic resistance. That is, it increases the Cx (Drag coefficient), which makes them slower and makes them consume more gasoline. 

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THE CAR UNDERBODY AND AERODYNAMICS

The car underbody and aerodynamics

If the underbody is fully flat, the air will pass without encountering obstacles, and it will not generate turbulence, so, it's good enough. But this can be further improved.

Source: Siemens

 
Ground Effect: It is achieved by using the inverted wing shape at the bottom of the vehicle, due to a low pressure area between the bottom of the car and the ground. 

With the consequent pressure difference the suction effect is created (we will see why later). This creates a much higher negative lift without affecting aerodynamic resistance. Its use is not allowed or strongly regulated in most race competitions. In IndyCar its use is much less limited than in Formula One.

With the following video we will understand why its prohibition.
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In designing the Mercedes CLR the engineers tried to create a car that was as long as possible but with a short wheelbase. With this, larger overhangs were achieved to take advantage of the floor effect and have a diffuser as large as possible.

Therefore, all the negative lift (downforce) was generated practically with the underside of the car, and not so much with the ailerons, in this way the aerodynamic resistance of the vehicle was hardly affected.

In addition, most racing cars are tipping forwards, in this way, the same bodywork can generate some negative lift, but in the Mercedes CLR it doesn't happen. This option was rejected to minimize aerodynamic resistance. In the previous video we see what happens when passing a slope change and that the air enters under the car.  That makes it very clear the dangers of overusing the ground effect.

Here we have a similar situation with the Porsche 911GT1:
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Venturi Flume:  The Venturi effect is based on Bernoulli's Principle. The Venturi effect says that if a flow circulates through a closed conduit and its section is reduced, the velocity of this fluid will increase and then the pressure at that point will decrease.

The important thing about this part is, without going into a full explanation here: As the speed of the fluid increases, then the pressure decreases.

But lets go one step at a time.  If a section is reduced, then the fluid goes faster. Let's take a hose as an example: If I press on the edge of the hose the water will come out much faster than usual.

So, now applying the Venturi effect, inside a closed duct, we will have the following:

• Greater surface / section: Lower is the speed of the fluid and the pressure increases
• Smaller surface / section: The greater the speed of the fluid and the pressure decreases

Let's simplify this: Where there is a higher air speed, a depression happens, if the difference is considerable, this leads to a suction effect.

In summary and applying it to the venturi flume:
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• By significantly increasing the air speed we will create a depression and therefore we will be able to generate a suction. Therefore, if I have two fluids, the one with higher speed or lower pressure will attract the other by suction. As long as the difference is noticeable and in closed ducts.

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The definition is not academic, but it helps us to understand the operation of the Venturi flume. 

So, if I have two air flows, one that goes underneath and the other that goes above the vehicle, you are interested in that the one below has less pressure than the one above. This creates the ground effect on the vehicle. Mostly part of ground effect performance comes from taking advantage of viscosity, but this falls under advanced aerodynamics.

Therefore, How do we decrease the pressure of the air that circulates below? Very easy, increasing the speed.

Let's go to the next step then: How do we increase the speed of the air under the vehicle to create suction? This part is even easier: Channeling the air and reducing the section, as we do with a hose and the water jet. This is, in essence, the operation of the venturi flume.

Therefore, we will seek to increase the air speed to create that depression and the desired ground effect. Because the resulting depression creates a suction effect (ground effect), it is used on the underbody to increase the downforce.

The air is channeled through the lower part of the vehicle thanks to some of the aerodynamic additions that we have seen previously, such as the splitter. That airflow will be channeled into the bottom of the car and out through the diffuser.
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Diffuser: Usually it's located at the rear of the vehicle, although there are also front diffusers. It considerably increases the speed of the flow that goes under the car creating a greater ground effect by creating a depression in the lower part of the vehicle.
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Source: 1016 Industries

With this we conclude the last part on aerodynamics of this transport design course, we hope we have delved into the subject in a reasonable manner. This part may be a bit heavier for some, as the aerodynamics are complex. As will happen in the following posts, there will always be subjects that we cannot see in depth due to time and extension constraints.

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14 - Aerodynamics tests (II)

16 - Crash tests