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Aerodynamics: This cut in the Ford GT is a cool balcony – see the technical reasons for this

A few days ago – and already entering the climate of Le Mans 2016 – the official channel of Ford Racing Performance published a video called “Aero and Design” on the Ford GT LM GTE, which was reproduced on a number of sites around the world And left many people salivating. At first. After all, with this title, you get the impression that the answers would finally come to those side cuts cut to a degree that has never been seen in a street car and which have been faithfully preserved in the competition model.

And then came words like Bernie Marcus, Ford Performance aerodynamicist: “If you look at the fairing, you’ll see that highlight in the area of ​​the rear mudguards. It’s a rather quirky aerodynamic feature, but it’s a style item too. ” Or Garen Nicoghosian, Ford GT’s External Design Manager: “In addition to joining the body panels, that component also houses the intercoolers. It is a beautiful piece of sculpture, but at the same time, it performs a series of functions. In a way it is representative of what the car is about. There is nothing accidentally present in the car. ”

I do not know about you, but the FlatOut team was a bit disappointed with the breach of expectation.

However, in the midst of this Anglo-American soapy, three CFD (Computational Fluid Dynamics) images of the Ford GT appear. With only 15 blades running around the body and no comment, they add little … but left me pensive.

A car of this caliber – and with the mission of extreme responsibility to represent Ford’s return to Le Mans in 2016 – can not have elements of this nature at random, for mere aesthetics. And then I began to study the photographs I took myself at the Detroit Motor Show this year and look for images from other angles of the GT. And then I realized the obvious: it was right there in our face, the whole time.

Before we point to the cat’s leap, however, we must go through some concepts related to aerodynamic drag and Venturi effect of acceleration of flow.

Dragging the mass of air

When we think of aerodynamic dragging in cars, common sense leads us to think of a low, wedge-shaped front – as opposed to a flat, high, trucklike front – as the solution to all problems. If this reasoning is correct, on the other hand it is far from complete.

Can you drag something forward? No, you push. Then the name already gives the indicative: in aerodynamic drag, the biggest problem is not in front of the body, but behind it. It is a force that pulls you from behind, not one that resists you from the front.

To understand why this happens, first of all, one must regard air as viscous fluid, not as an empty space. When the body of a car at speed penetrates this fluid, some things happen: the fluid imposes its resistance to movement, it flows around it and, because it is viscous, a thin layer adheres to the body – like an oil film . Or, to visualize more dramatically, honey.

Because of the viscosity, this fluid flowing well glued onto the body is much slower than the fluid flowing a few millimeters above – and so on. After all, if it’s viscous, it sticks. So, not only visualize the air as fluid, but visualize this fluid flowing in the body as several overlapping layers in gradient, all in contact with one another as if they were pages of a book. The closer to the body, the slower the air velocity, then there are shear forces between these “pages”. Friction. Of course, from a certain point in relation to the body the influence of the body in relation to the speed of the fluid becomes minimal.

Limit layer is the name of this region where there is adhesion of the fluid to the body, its consequent deceleration by the viscous adhesion and the notorious friction between the layers of air by difference of speed. It is absolutely fundamental to understand the aerodynamic drag, which results in the chaos that occurs in the rear of the vehicles illustrated above.

Can you see that in the example on the right the air behaves in a seemingly chaotic way, forming little vortex swirls – the vortices? This is what happens when the boundary layer loses sufficient adhesion to the body. Keep in mind that there is a difference in speed between the air blades within the boundary layer. It is the friction caused by this difference in speed that causes the blades to “stumble” from top to bottom and wind up with each other, losing even more speed and, at worst, forming a large zone of cumulative aerodynamic disturbance that generates a whirlwind, Entanglement that hangs far behind the body in the area known as “aerodynamic mat,” if dragging (remember that the fluid is viscous and there is adhesion and interaction even between its layers) and generating resistance to movement. Here is the drag force – see the pictures below.

There are positive ways of using small vortices to help prevent the formation of this whirlwind. It sounds contradictory and we will address these vortex generators in a future post so that we do not deviate from the main topic.

Among the several causes that cause the boundary layer to take off from the surface, the main one is the very sudden transition from a high-velocity (and therefore low-pressure) region to a low-velocity (and therefore high pressure). Like the end of the roof or the back of a car.

And the larger the flattened area of ​​this rear, the greater will be the whirlwind and the drag. Therefore, we already see an advantage in this solution of the Ford GT: note how the rear panel is just the central section and the support of the lanterns. Such a “aerodynamic track” is very narrow. This is a great advantage, but the hole is deeper!

The video below is quite didactic: it shows the drag generated by a ball and modifications made to reduce the aerodynamic disturbance behind the body. Watch it thinking about the issues of air viscosity, adhesion and detachment of the boundary layer that we talked about above.

The video is also important to understand the concept of a drop form. As aerodynamic flow encounters difficulties in maintaining a steady transition from high to low speed, “bidding off” the body gradually is the ideal solution to avoid abrupt detachment of the boundary layer and thus reduce drag.

For this reason, long tails and with this silhouette were used for a long time in motorsport in an era in which the great concern was only drag. Downforce and even wings were relative novelty.

But as the motor sport evolved, they found that this had a few penalties, the greater being the fact that this rabbit moved the center of aerodynamic pressure back. It is as if at high speeds an invisible hand pressed the rear end down, leaving the front light and therefore virtually unguided. The inertia also made the long tails quite dangerous cars in medium-high curves.

Accelerating the air

The point is that the flow of air in a body can be accelerated with some tricks, the main one being the Venturi tube concept. If there is no energy loss or forced compression of the fluid, the sum of the energies of pressure (potential) and velocity (kinetics) must be constant. So if in a tube we make this flow accelerate – for example, by reducing the diameter of the channel -, necessarily the pressure needs to fall, because energy can not be created from nothing or disappear. This is the crude explanation of the Venturi tube, the flow accelerator essential for the operation of the carburetors

From the late 1960s onwards, motorsport aerodynamists realized that the wing principle – which was already used in single-seaters and passenger cars – could be extended to the shape of the vehicle itself: that of the wing-car, Of the main ones the Lotus 78, of Peter Wright and Colin Chapman.

The wing principle is similar to the Venturi Tube concept, but instead of the flow velocity gain rolling through the body / tube narrowing, it happens by an asymmetrical drawing between the upper and lower portion of the wing profile. The longest profile is one in which the air will need to accelerate further, generating pressure drop. See below.

In the case of wing cars, the effect becomes very intense because there is a combination of concepts: the silhouette of the fairing profile is wing, but the volume between the asphalt and the floor forms a duct narrowing similar to the Venturi Tube. That’s why the Formula 1 teams used skirts and brushes around the sides and were dragging on the ground: dramatizing the tube effect even more.

Nowadays names like “car-wing” and “ground effect” are old nostalgic things like me, but the diffusers present in single-seaters and in touring cars (including street cars) are exactly the same thing. In order not to leave the absurd downforce, regulators increasingly limit the length of the diffusers, which reduces their ability to accelerate airflow.

A new perspective

You’d even forgotten that the post was about the Ford GT, but we had to go through the concepts of drag and acceleration of aerodynamic flow to explain the big cheap of this guy and why those huge cuts on the back sides. In the photo below, starting at the front, we can see that there are two large air outlets. The flow enters through the small front grille, passes through the inclined radiator and exits through these huge ducts. The idea is to reduce the flow pressure and, as we already know, to increase the velocity of the flow.

As you may have noticed, almost all conversations, demonstrations and aerodynamic studies prioritize the lateral profile, mainly because the sacred chalice is to increase downforce and reduce drag. To increase the downforce, in fact you need to reduce the pressure of the airflow under the vehicle, so the profile view is the most appropriate angle. But are there other ways and perspectives to reduce trawling ….

Do you see the white arrows? They represent the flow of exhaust gases. The pipes are not around for free: they free up space on the floor for colossal diffusers, like those in the competition Ford GT. And by pouring hot gases, that is, low pressure, there is a tendency to accelerate the air flow in that region, preventing abrupt detachment of boundary layer in this central region, favoring the termination of the drop shape of the image above. These benefits add to the reduction of the back panel area, as we have seen above.

Yes. The space between the mudguards wall and the cockpit wall is an aerodynamic flow accelerator. Two Venturi channels, combined with good aerodynamic flow of the drop shape. This duo not only knocks down the Ford GT’s aerodynamic drag, but also breakage can increase the vehicle’s directional stability at high speeds – acting as a kind of rudder. These are two very important benefits for long-term and ultra-high-speed events such as the Le Mans Series. With little drag, the final speed becomes larger and less fuel is consumed. With more directional stability, the car is more reliable and easy to adjust, without sacrificing or dynamic patching.

What Ford guys did was to use the same concepts already embodied in motorsport, but turned the miniature 90 degrees on the horizontal axis. It’s not a revolution, not least because the Nissan GT-R LM Nismo has already employed similar concept (read more here). But the Ford GT is not only a street car, but has managed to solve a packaging problem that the Japanese either failed to achieve or failed because of the radicalism of the mechanical concept: the Ford has a central-rear engine and rear-wheel drive.

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