I would like to ask experienced colleagues: what is the difference between a Venturi tube and a Laval tube?

Literal interpretation of the last sentence: When the flow reaches the speed of sound, the venturi tube will be called a Laval nozzle (converging-diverging nozzle). What does it mean, and how should it be understood? The Laval nozzle is a type of venturi tube

The Haichuan Forum has a fairly detailed discussion on Laval nozzles and Venturi tubes: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=276554 (Source: Haichuan Chemical Industry Forum website)

This post was last edited by Desert Fish on 2018-2-3 at 17:45. Recently, students at Purdue University used a simple device to make a table tennis ball exceed the speed of sound. However, a table tennis ball moving at supersonic speed is not the key point; what’s important is the device they use, called a \"de Laval nozzle\". This type of device has a history of nearly 100 years. And it is no exaggeration to say that such devices made the Industrial Revolution possible—and also a revolution in the history of space exploration. The students pushed a table tennis ball into a tube, blocked both ends of the tube, and then removed the air from inside. Then, a section of tube is connected to the other end; a nozzle is placed between the two tubes. Air is then forced into this setup until the barrier between the two tubes is broken down by the air pressure, causing the table tennis ball to be propelled at the speed of sound, thus breaking through the sound barrier in the air. This experiment demonstrates the destructive power that a table tennis ball can exert. With just a little compressed air, a little vacuum, and a nozzle of the right shape, a table tennis ball can break the sound barrier, easily penetrate 4 aluminum cans to reach the other side of the room, and even punch through a table tennis racket without any problem. When calculating the speed of the table tennis ball, the thrust provided by the air pushed into the first section of the tube and the suction effect created by the vacuum in the second section play a role, as do the diameters of these two sections, all of which contribute to giving the ball a certain acceleration. But what really makes the difference is the nozzle located between the two sections of the tube.

The Laval nozzle, also known as a \"convergent-divergent nozzle\" or \"thick-waisted nozzle\", is this simple device that is not only crucial for \"ordinary table tennis balls\" and \"supersonic table tennis balls\", but it also played a role in shaping the vastly different landscape of the Industrial Revolution and the Space Age. You may have noticed that even in cartoon rockets, the nozzles of the thrusters at the bottom are not straight tubes; instead, they are shaped with a narrower part at the front and a wider part at the back, resembling an hourglass. This wasn’t something thought up casually. During the Industrial Revolution, people wanted to accelerate gases, such as steam, in order to power engines and turbines. At that time, it was already common knowledge to allow gas to enter the pipeline, thereby reducing its volume and accelerating it. We have all seen that when a river flows from a wide bed to a narrower channel, its flow speed increases. But this is not a proportional relationship; although the gas accelerates within the pipe, when the pipe becomes narrow enough, the flow is blocked. No matter how thin the pipe is or how high the pressure, the effect of acceleration is weak, or non-existent. Engineers tried to fix it by changing the texture and size and making one end of the pipe narrower, but nothing worked. It was not until 1888 that the Swede Raffael designed a sharp nozzle, similar to those created by others, but he added a trumpet-shaped opening at the back of the nozzle. Thus, the gas was ejected at an astonishing rate never seen before. Engineers during the Industrial Revolution eventually discovered that the same material could exhibit completely different properties under different conditions. The airflow is completely different at supersonic speeds compared to normal conditions. At subsonic speeds, it is similar to water flow in narrow channels. When the river channel narrows, the flow of water speeds up. If you sit by the riverbank and calculate the amount of water flowing past, and then go to a faster-flowing area downstream to measure the flow again, you will find that the amount of water flowing per unit of time is the same. When the river channel narrows, the flow velocity increases but the mass flow rate of water remains unchanged. This is the same as with gas flowing through a thin tube: once the flow velocity of the gas reaches the speed of sound, a critical point is reached – no matter how thin the tube becomes, the gas can no longer accelerate. The Laval nozzle also causes the pipe to narrow toward the front, with the gas continuously accelerating to the speed of sound. After reaching the speed of sound, common-sense-breaking wisdom comes into play. To keep the gas that has reached the speed of sound accelerating, instead of making the pipe narrower, the trumpet-shaped opening widens the pipe again, allowing the pressure exerted on the gas to be released instantly; as a result, the gas’s cross-sectional area increases and it continues to accelerate beyond the speed of sound. It’s like a spring that is compressed to the bottom and then suddenly released; the pressure built up inside the gas is converted into velocity, and all the released energy is used to push the airflow in one direction, causing it to be ejected at supersonic speeds. Supersonic gas can make table tennis balls move at high speeds, allowing them to penetrate aluminum and wood. It could also turn Germany’s V-2 rockets into weapons, and enable America’s early exploration spacecraft to travel into space. Modern jet engines still use this structural design, and it’s all thanks to this hourglass-shaped nozzle.