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When it comes to supersonic shock waves, we may subconsciously associate nuclear explosions, jet fighters, rocket launches, and so on. The process of these instant bursts of energy, no matter how you look at it, is a far cry from opening champagne. But interestingly, according to the study of Champagne, opening the bottle of champagne is very much like a mini rocket launch, both of which produce supersonic shock waves.

After a violent shake, we stared at the champagne bottle, waiting for the cork to eject the next second. With a bang, wine and foam gushed out, and cheers broke out in the crowd-a common scene in celebrations. But it's not the right way to open champagne. It's dangerous, and people get hurt every year.

Let's start over without shaking the champagne and unraveling the barbed wire over the cork. Press the cork in one hand and rotate the bottle slowly in the other. The cork will naturally be pushed out slowly by the air pressure in the bottle, and then pop out with a "boo" sound, and then a faint white fog appears at the mouth of the bottle.

Perhaps it was the white fog that made people care. In a 2019 article in Science Advances, a physicist turned into a "champagne scholar" and filmed the opening of the champagne bottle with a high-speed camera. They were surprised to find that after the cork popped out, the high-pressure air ejected in the bottle would form a supersonic shock wave.

Supersonic shock wave, in essence, when an object moves at supersonic speed, it will disturb the surrounding medium (such as air), thus constantly forming a compressed air flow in front of the object. These compressed airstreams carry a lot of energy and will impact around in the form of supersonic air waves.

The upper right corner of the ▲ image takes time from 583 microseconds to 1000 microseconds. The high-speed camera captures the Mach ring (the position indicated by the arrow) from the position of figure A closer to the mouth of the bottle, gradually away from the mouth of the bottle to the position of figure E, until figure F dissipates completely. (photo source: original paper)

The air flow in the Mach ring is usually colorless, which means that we cannot see the shock wave directly. So why can the camera capture the supersonic shock wave at the mouth of the champagne bottle? In fact, we see not so much a supersonic shock wave as a phenomenon that only supersonic airflow can be formed.

When you look closely at these photos of the moment the champagne is opened, you will find that a white line is gradually moving away from the mouth of the bottle until it disappears. And if you look down from directly above the mouth of the bottle, you will see that the line is actually a circle-this is the mach disk.

If you've ever noticed supersonic planes taking off or rocket launches, you may notice that they always have a bright halo on their tails, which is also the Mach ring. Both rockets and airplanes need to jet supersonic air to get strong thrust. The pressure of the supersonic flow is very high, so when it is injected into the atmosphere from the nozzle, it will expand directly, but the pressure of the expanded air will be lower than the atmospheric pressure, so it will be compressed again. In this way, the supersonic gas flow will cycle back and forth between expansion and compression, and this process will form expansion wave and compression wave, and the two will meet and superimpose in the process of propagation to form rings, that is, Mach rings.

It is not difficult to see the necessary conditions for the emergence of the Mach ring phenomenon: first, supersonic airflow; second, the difference between airflow pressure and environmental pressure. The former satisfies the conditions for the occurrence of shock waves, while the latter can change the air flow and produce different waves.

The Mach ring at the mouth of the champagne bottle is the same as the Mach ring at the tail of the rocket, but there is one significant difference: the temperature of supersonic air flow. The moment the champagne cork pops up, the airflow in the bottle quickly overflows, resulting in a sudden drop in air pressure and temperature in the bottle, and the mixture of carbon dioxide and water vapor condenses into ice crystals, forming a gray fog. As a result, the Mach ring at the mouth of the champagne bottle will appear in the white fog. The high temperature of the airstream ejected by the rocket will ignite a small amount of fuel mixed with it, making the Mach ring particularly dazzling in it.

The moment the cork pops up, however, although it is known that the air flow from the mouth of the champagne bottle can exceed the speed of sound and produce a Mach ring, the specific process and physical mechanism are not clear. This year, in an article published in the journal Physics of Fluids, scientists used computer simulations to further reveal how the shock wave forms, evolves and eventually dissipates within one millisecond (1000 microseconds) of a champagne cork.

Champagne is rich in carbon dioxide, the pressure in the bottle is about six times the atmospheric pressure, and the compressed carbon dioxide gas in the bottle will constantly push the cork out, trying to push it out. Under stable conditions, the static friction between the cork and the bottle wall will be balanced with the outward thrust. However, once you start twisting the cork, the static friction will quickly turn into dynamic friction, which can no longer compete with the air pressure. The cork is now like a rocket, ready to go.

▲ computer simulation image. From top to bottom, each row corresponds to the first, second and third stages of the evolution of the shock wave. When the cork just pops out in the first row, the air flow can only expand transversely along the gap between the cork and the mouth of the bottle; in the second row, the cork can be ejected directly at a certain distance from the mouth of the bottle, but it will collide with the cork to form a curved shock wave; at 1167 microseconds in the third row, the air pressure difference inside and outside the bottle decreases, unable to support the airflow to escape at supersonic speed. From left to right, each column shows the spatial distribution of velocity, pressure, and temperature. Combined with row and column variables, we can see the state of each stage and the interval distribution of different variables. (photo source: original paper)

According to the computer simulation, the change of supersonic airflow in 1 millisecond after cork ejection can be divided into three stages:

In the first stage of cork ejection (within 600 microseconds), the flow of carbon dioxide in the bottle escapes at supersonic speed, which is very similar to the acceleration process of the airflow launched by a rocket. The nozzle at the tail of the rocket is in the shape of a funnel that is wide on both sides and narrow in the middle, also known as a Laval nozzle nozzle. The high-pressure air flow heated after ignition will continue to compress and accelerate when it gradually narrows through the first half of the nozzle. The narrowed shape at the neck of the champagne bottle has a similar effect, accelerating the airflow to supersonic speed at the mouth of the bottle.

Just as the crowd travels slowly when it is blocked at a narrow intersection, and once it reaches the open space, it will disperse and accelerate, so the air flow will expand and accelerate when it is compressed into the open space through the narrow path. Therefore, when the high-pressure airflow escapes from the bottle mouth and enters the relatively low-pressure external environment, it will be supersonic, while the rocket airflow can reach supersonic speed in the rear part of the nozzle. Unlike rockets, the cork at the mouth of the champagne bottle is too slow compared to the speed of movement, which hinders the direct injection of air. At this stage, the supersonic flow can only expand and escape laterally along the gap between the cork and the mouth of the bottle, forming a coronal shock wave and the phenomenon of Mach ring.

In the second stage when the cork leaves the bottle mouth (600-1000 microseconds), as the gas in the bottle continues to escape, it can finally be ejected straight out like a rocket airstream, and then collide with the farther cork, creating a curved shock wave.

In the third stage (more than 1000 microseconds), the pressure in the bottle gradually equals the atmospheric pressure, unable to maintain the pressure difference at the mouth of the bottle, and the airflow loses power. As a result, the jet will continue to slow down until it is below the speed of sound and the shock wave dissipates completely.

This interesting study, inspired by life, links rocket launch with champagne bottle opening, which not only promotes the progress of champagne research, but also provides a reference for a series of important applications, such as the ballistics of rocket launch and missile launch. The research could also help engineers who develop underwater vehicles and wind turbines to better understand hydrodynamics (the law of motion of flowing matter under force).

In fact, however, champagne is not the only one around us that produces supersonic shock waves. I wonder if you have ever noticed two very penetrating sounds in your life: the "pull" when tearing transparent tape! Sound, and the "bang! bang!" during whip exercise in the park. Sound.

If you tear the tape vigorously, you will find that the tape is always torn apart, which sounds like a "tearing" sound. When you tear the tape off the attached surface, the adhesive stretches like a spring and stores elastic potential energy (so it cannot be torn apart continuously). After the adhesive "spring" can not withstand a greater tensile fracture, the accumulated elastic potential energy will immediately be converted into the kinetic energy of the split edge of the tape (the dividing line between the adhesive tape and the separate tape).

If you film this process with a high-speed camera, you will see that the split edge of the tape will move at a speed of 650 to 900 meters per second, far faster than the speed of a fighter. This means that each time the adhesive tape accumulates potential energy and then peels off, it releases tiny supersonic shock waves. So it's not difficult to understand that, to us, the superposition of mini sonic booms can certainly be harsh.

When ▲ rips transparent tape, he can't avoid the shrill "tearing" sound (photo source: Pixabay)

And the loud "bang! bang!" in the park Whip sound, some people may mistakenly think that it is the sound of whipping on the ground, but in fact these are mini supersonic shock waves erupting in the air. When people shake the whip hard, they will transfer kinetic energy to the whip. Usually, the handle of the whip is thicker and the mass is larger. When the kinetic energy is transferred to the thin and light tip of the whip along the soft body of the whip, in order to ensure the conservation of momentum, the speed of the whip will be much faster than that of the handle and easily exceed the speed of sound, thus forming a local supersonic shock wave.

This phenomenon is also known as the whiplash effect. Like champagne bottle opening, it comes from life, but it also contains complex physical mechanisms.

Now, if someone asks: what do champagne, tape and whip have in common?

Do you know how to answer?

Links to papers:

Https://www.science.org/doi/10.1126/sciadv.aav5528

Https://aip.scitation.org/doi/abs/10.1063/5.0089774

Https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.068005

Reference link:

Https://www.smithsonianmag.com/smart-news/what-really-happens-when-you-pop-champagne-according-to-science-180980218/

Https://www.livescience.com/champagne-bottle-opening-creates-shockwaves.html

Https://www.decanter.com/wine-news/german-scientist-logs-champagne-cork-speed-75173/

Https://kns.cnki.net/kcms/detail/detail.aspx?dbcode=CJFD&dbname=CJFD9495&filename=LXYS505.026

This article comes from the official account of Wechat: global Science (ID:huanqiukexue), written by: not weekly revision: 27

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