What car can be sucked on the top of the tunnel and speed? You have to learn aerodynamics!

Recently, a kid sent me a video: a sports car entered a tunnel, found a traffic jam ahead, and decisively drove onto the tunnel roof to overtake. He asked me: Is this video real or fake? Can a sports car drive backwards on the top of a tunnel?

Video circulating online: Lamborghini climbing a wall

Let me first state the conclusion: this video is definitely fake. In reality, only the racing king Schumacher has ever driven backwards on the top of a tunnel in a Mercedes-Benz advertisement, but there are two conditions: **1. The top of the tunnel is curved, so that centrifugal force can be used to counteract gravity. ** 2. The tunnel is empty, so that there is no danger.

Advertisement: Schumacher does a 360-degree spin in the tunnel

In this video, firstly, the top of the tunnel is flat, so there is no circular motion and no centrifugal force to counteract gravity. Secondly, the tunnel is packed with cars, so it is impossible to do juggling without accidents in this situation. Therefore, the video is 99.9% fake.

However, theoretically, it is still possible for a sports car to drive upside down on the roof of a tunnel. However, the car must be light enough and have good enough aerodynamic design. Today, I will talk to you about aerodynamics and its application in life, and finally explain what kind of car can hang upside down on the roof of a tunnel.

1. Aerodynamics

Our lives are full of air and water, both of which are fluids. People have been conducting various studies on fluids for thousands of years, which gave rise to fluid mechanics. Aerodynamics is a branch of fluid mechanics.

For example, we are all familiar with the story: Archimedes shouted "Eureka" and ran out of the bathtub because he discovered the law of buoyancy: the buoyancy of an object placed in water is equal to the weight of the liquid it displaces.

Archimedes

This is called Archimedes' principle of buoyancy, which is the conclusion of fluid mechanics. Using buoyancy, we can make steel ships float on the water.

Another example is the scientist Pascal, who discovered Pascal's law: liquids transmit pressure.

Pascal

If we have a pipe with openings of different sizes at both ends, assuming their areas are S1 and S2 respectively, when we apply pressure F1 on one side, pressure F2 will be generated on the other side. According to Pascal's law, the pressure of the liquid on both sides is equal, that is,

Because S1<<S2, so F1<<F2, that is to say: a relatively small force can generate a large force, this is the principle of the hydraulic press.

Pascal also did an experiment: insert a very high tube into a wooden barrel. Pour a cup of water into it through the tube, and the barrel cracks. This is because although the mass of a cup of water is very small, it generates a very high pressure in the very thin tube, which turns into a very large force on the barrel, which is similar to the principle of a hydraulic press.

Pascal's barrel experiment

However, the person who truly turned fluid mechanics11 into an independent discipline was the famous scientist Daniel Bernoulli.

Daniel Bernoulli

We have previously told the story of his father, Johann Bernoulli, who was a famous mathematician and the teacher of Euler, the master of mathematics. Under the training of his father, Daniel Bernoulli proposed the famous Bernoulli equation.

What can this formula tell us? Let me give you a few examples:

The first one is a tube named after the Italian Venturi, called a Venturi tube.

Venturi tube

You see, if the venturi tube is placed horizontally, the heights at A1 and A2 are the same: h₁=h₂

When a liquid flows from a thick section to a thin section, the liquid volume remains unchanged, the cross-sectional area becomes smaller, and the flow rate must increase. Therefore: v₁＜v₂

Substituting this into the Bernoulli equation, you will find that: Ρ₁＞Ρ₂

That is, in the narrow place, the flow rate is large and the pressure is small. This conclusion is also the Bernoulli principle we learned in junior high school. If we open a hole in a narrow place, water will not spray out, but will suck air in. People use this principle to make a venturi fertilizer absorber, which can be used to absorb fertilizer. Water flows through the main pipe, and the fertilizer is sucked in from the small tube.

Venturi fertilizer suction device

Another example: French engineer Pitot invented the Pitot tube. It has two openings, A facing the wind and B facing the side wind. In the place facing the wind, the air is blocked and the flow rate becomes 0, the air flow rate is low and the pressure is high; in the place facing the side wind, the air moves without obstruction, the air flow rate is high and the pressure is low.

If we can measure the pressure difference between two places, we can know the air flow rate.

A Pitot tube must be installed on an airplane to measure the surrounding wind speed. If the Pitot tube is blocked, it will cause measurement abnormalities, which can have very serious consequences. Many air crashes are related to this.

Aircraft anemometer

You see, whether it is a ship, a hydraulic press, a fertilizer extractor or an anemometer, all require knowledge of fluid mechanics. The part of fluid mechanics that studies air is called aerodynamics. Its greatest use is to send airplanes into the sky.

2. Why can airplanes fly into the sky?

Why can airplanes fly? A long time ago, I made a video to explain this question, but it was not detailed enough. This time I can explain it in more detail. The two sides of the airplane wing have different shapes. The lower surface is concave and the upper surface is convex. When the airplane moves forward, due to the asymmetry of the upper and lower surfaces and the influence of viscosity, a circulation will be formed around the wing: the upper surface is backward and the lower surface is forward.

Don't forget that the plane is still flying forward, so there is also a backward airflow. You will find that the circulation and airflow above the wing are in the same direction, so the air flow is fast, and the faster the flow, the lower the pressure. Under the wing, the circulation and airflow are in opposite directions, and they collide, so the flow is slow, and the slower the flow, the higher the pressure. There is a pressure difference between the two, and this pressure difference provides the lift of the plane. Of course, it also generates a certain amount of backward resistance.

Zhukovsky, the father of Russian aviation, gave a formula for calculating the lift of an aircraft, which we can simplify as follows:

Where P represents air density, s represents wing area, v represents relative air velocity in the distance, and u represents circulation velocity. In fact, a soccer ball can be kicked into a banana ball because the circulation u generated when the soccer ball rotates is superimposed on the external air velocity v.

Also, sometimes, when a plane is landing, it feels as if there is a force pulling the plane upward when it is very close to the ground, as if the lift has increased. What is going on? This is called ground effect.

According to the Bernoulli principle we just mentioned, the pressure below the wing is high and the pressure above is low, so the air below the wing will flow upward and form vortices. This is especially true at the wing tip, so the so-called wingtip vortex is formed. When the plane flies over, if there are colored particles in the air, this vortex is more obvious. If there is a lot of water vapor in the air, the effect of the plane pulling the line will also appear.

Wing tip vortex

This phenomenon will reduce the originally large pressure difference. However, when the aircraft is very close to the ground, due to the obstruction of the ground, the wingtip vortex is weakened, and the gas below cannot move upward smoothly, so the pressure difference is increased compared to in the air, which will make the aircraft encounter some difficulties when landing. We call it ground effect.

Air flow caused by wingtip vortices

In fact, people have designed many new means of transportation based on ground effect. For example, aircraft that can fly close to the water surface. The former Soviet Union was very advanced in this regard and developed a series of large-capacity and high-speed seaplanes for military equipment. People called it the Caspian Monster. However, it was not mass-produced and assembled in the end. Finally, with the disintegration of the Soviet Union, the project was completely terminated.

Caspian Monster

However, many countries are now beginning to study near-water aircraft. my country has also conducted research in this area, hoping to use this effect to create missiles that can fly close to the water surface.

Modern seaplanes using ground effect

In fact, there are many things to say about airplanes. For example, the wing must form a certain elevation angle with the air. If the elevation angle is too small, the lift will be insufficient; if the elevation angle is too large, the air will quickly separate after passing through the wing, forming a low-pressure area, and the wing will have great resistance. What kind of wing shape can maximize the lift and minimize the resistance? This is what aerodynamics studies.

Compared to Daniel Bernoulli's time, the research methods of aerodynamics have greatly improved. We not only have computer simulation software, but also wind tunnels, which can adjust the wind speed without moving the wings of the aircraft. Many of the problems we just mentioned can be tested using wind tunnels.

Wind Tunnel Testing

3. Aerodynamics of racing cars

In fact, cars, especially sports cars, require exactly the opposite force to airplanes: airplanes require lift, while sports cars require downward pressure.

The sports car is very fast and needs a lot of centripetal force when turning. The centripetal force can only be provided by the friction force f.

The friction force has an upper limit, which is related to the roughness μ of the contact surface and the pressure N. The greater the pressure N between the car and the ground, the greater the friction force.

But we absolutely cannot rely on increasing the weight of the car to increase the pressure, because increasing the weight will not only make the car slower and less flexible, but also increase the centripetal force required when turning as the car's mass increases, which is not worth the loss. Is there a way to increase the pressure between the car and the ground without increasing its weight?

People think of aerodynamics.

In the 1950s and 1960s, there was an automobile inventor and entrepreneur in Britain, Colin Chapman.

Colin Chapman

He studied structural engineering at university and later became a British Air Force pilot. After retiring, he founded a car company, Lotus. Due to his study and military service experience, he was very persistent in pursuing speed, lightness and aerodynamics. Soon, the car he designed won the F1 Grand Prix and became a famous team.

Lotus logo

Compared to Ferrari's pursuit of high-power engines, the Lotus team pursued lightness and the use of air. In 1968, Chapman thought: since airplanes can use wings to gain lift, why can't cars use inverted wings to gain downforce? So he designed the Lotus Type 49B, fixing the rear wing high on the car.

Lotus Type 49B

In this way, when the air flows through the car from the front, it will pass through the upper and lower sides respectively, producing an effect similar to that of an airplane wing. Of course, the front wing is also indispensable. The front and rear wings firmly press the car to the ground.

Although the Type 49B's engine power was not as good as Ferrari's, its firm grip on the road and relatively light body helped the Lotus team win the 1968 F1 championship.

Soon, everyone discovered this new invention. Ferrari's engine technology is difficult to learn, but Lotus' aerodynamic equipment is all exposed and easy to learn. Almost all teams began to install tail wings. However, the tail wing is actually a double-edged sword. Although it can increase grip, it will also increase wind resistance, which is the same principle as airplanes.

When everyone made front and rear wings standard, Chapman started to think of ways to improve his car. He wondered how to get more downforce without increasing drag too much. He thought of the ground effect. The distance between the chassis and the ground is very close. If the chassis is designed to protrude, wouldn't a venturi be formed between the chassis and the ground?

Lotus Type 79 and Type 80

In 1977, Chapman invented the Lotus Type 79 racing car. The chassis of this racing car has a convex structure. When air flows into this part, the air flow rate increases due to the compression of the space under the car, forming a venturi effect. The air above will press the entire car to the ground.

The TYPE79 car uses ground effect to gain downforce

In order to prevent the air from escaping, Chapman also designed aprons on both sides of the car chassis, which can block the air and prevent it from escaping. This car helped the Lotus team win the F1 championship again.

TYPE79 racing car adds air fence

When Chapman was in charge of Lotus, the team won a total of 7 F1 constructors' championships in 15 years. Unfortunately, Colin Chapman died at the age of 54 due to a heart attack.

Colin Chapman and Lotus

He has a famous saying: If you haven't won yet, it means you haven't worked hard enough .

In the F1 arena, British Lotus is a racing car brand on par with German Porsche and Italian Ferrari. But if you think Lotus only has racing cars, you are wrong.

In 1957, Lotus launched the first civilian car, the Lotus Elite. Because there was no wind tunnel, Chapman pasted cashmere on the car body. He and his friend drove the car, and one of them tied the cashmere to the hood to observe the movement direction of the cashmere at a speed of 160km/h. Finally, the Elite with a drag coefficient of 0.29 was created. Even today's sports cars have a drag coefficient of more than 0.3.

Lotus Elite

In 2019, Lotus launched the Evija supercar, which is limited to 130 units worldwide. This sports car fully embodies Chapman's spirit and takes aerodynamics to the extreme. The front and rear hollow design can greatly reduce wind resistance.

Lotus Evija

The diffuser is installed on the chassis, which has the same function as the venturi formed by the undercarriage in a racing car, which compresses the air, speeds up the flow and generates downforce. It is estimated that this car can generate 1.8 tons of downforce.

Evija's aerodynamic design

However, due to its carbon fiber structure, it weighs only 1.68 tons, which means that theoretically, it can run upside down on the top of a flat tunnel.

Of course, this is only theoretical, so don't try it. Another reason is that the price of this car is about 20 million, and most people can't afford to try it.

From studying a glass of water, a breath, to an airplane, a sports car. Technology is changing the world faster and faster. Confucius said: "Isn't it a pleasure to learn and practice it from time to time?" Many people understand "practice" as review. I think it is better to understand it as practice. Colin Chapman applied the aerodynamics knowledge he learned to cars and created the top sports car brand Lotus. What a happy thing it is!

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