As human-built ships get bigger and bigger, the propellers needed are getting bigger and bigger, spinning faster, and the material of the propellers is getting harder and harder, such as the four propellers of the American aircraft carrier Ford, which are 6.4 meters in diameter and weigh a staggering 30 tons.
For many marine life, they are simply a nightmare existence, not only can the small fish and shrimp instantly crushed to pieces, even if dozens of tons of hundreds of tons of whales encounter, it is heavy death, light or scaly. Because some large marine life has a habit of following the wake of the ship, and some predatory fish are attracted by the smell of blood on the propeller, it is not uncommon for ships to harm marine life.
However, these propellers that easily kill all kinds of marine overlords also have their natural enemies, and this natural enemy is actually a small "bubble".
In the past, because human ships were slower, people did not notice this problem, but in 1984, British engineers found that the main engine power, speed and speed of the "Brave" destroyer were much slower than the design indicators, and they repeatedly modified the design and still failed to solve this problem, until the disc ratio of the blade was increased. In hindsight, people finally found out that the culprit behind it was the "bubble" generated when the propeller rotated at high speed.
More than a hundred years later, the same thing happened on another famous ship, the charles de Gaulle. As the only active aircraft carrier in France, although the "Charles de Gaulle" aircraft carrier has made great achievements for France, it has always had problems since it left the factory, including several famous "broken oar" incidents. First found that the sound of the propeller was too loud, so that the officers and soldiers could not sleep, and then not long after found that the propeller blade became potholed, was seriously corroded, came back to replace the new one and went out to the sea test, a propeller that walked well suddenly fell off, and the other three also turned off, making a lot of jokes.
To know that although the "Charles de Gaulle" aircraft carrier is not a big aircraft carrier, it also has tens of thousands of tons, which belongs to the ranks of 10,000-ton giant ships, why is it overwhelmed by small bubbles?
Before talking about the propeller of a ship, let's talk about a creature called the "pistol shrimp", which is a small shrimp that lives in shallow tropical seas. This creature comes with a secret weapon, its huge "claw", which is only five centimeters long, but this claw is 2.5 centimeters long. Not only is it big in appearance, but it is special in that it is not used to clip people like other shrimp and crabs, but to fire a "water bullet" to knock the opponent down. It turns out that the shrimp's large claw can close in an instant, and shoot a water projectile with a speed of 100 km/h to kill the opponent or even directly penetrate.
Don't be surprised, its giant claw, when closed, is able to make a loud noise of up to 210 decibels, stirring up a high-speed water flow, and the water flow is accompanied by a large number of millimeter-sized bubbles. When these millimeter bubbles burst, they can release a huge amount of energy, and the impact of the water flow is enough to hurt its opponents.
So why is the running water bubble bomb released by a small pistol shrimp so lethal? This is actually the same principle as the damage ship blade, acting on the ship blade, it is called "cavitation". "Cavitation" is divided into two processes, one is cavitation and the other is cavitation.
Let's start with the phenomenon of "hollowing out". In our impression, water seems to boil and vaporize only when it is heated and boiled, but we also know that the vaporization temperature of water is affected by pressure. When the pressure is very low, the temperature required for water vaporization is very low, which is why the water on the plateau is more likely to boil than on the plain. And compared to the boiling of water, cavitation is a more rapid and violent process, as short as hundreds to thousandths of a second.
This cavitation constant has a formula, more complex, I will not write here, you can roughly look at the above figure, you can see that when the pressure is very low, the vaporization temperature of water also becomes very low. This means that if the sea temperature in this area is 20 ° C, then the pressure only needs to be less than or equal to 238 Pa, and the water can vaporize.
Then we'll talk about cavitation. According to the most famous inference of the bernoulli principle of well-known fluid mechanics, we can know that when flowing at contour, the greater the flow rate of the liquid, the smaller the pressure. In our middle school textbooks, we were told that the cross-section of an airplane could form a pressure difference that would lift the plane up (a theory that doesn't seem comprehensive today), and that the cross-section of a ship's propeller was similar to the wings of an airplane.
When the propeller of the ship rotates at high speed, the pressure somewhere on the back of the blade is rapidly reduced, so that the pressure required for water vaporization at this temperature is reduced, and the sea water here will quickly become gas, forming an "airspace" here. This resulting bubble will follow the propeller's water flow rapidly to the high pressure area, and is squeezed by the high pressure instantaneously, this crushing process is very short, only one hundredth to one thousandth of a second time, so it can produce jets and shock waves with speeds of up to several thousand kilometers per hour, generating an impact force of up to 4000 kilograms per cubic centimeter on the blade, accompanied by more than two hundred decibels of noise and a local water temperature of several thousand degrees Celsius.
Just like the pistol shrimp we said at the beginning, the pistol shrimp also accelerates the speed of the liquid, reduces the local pressure of the liquid, resulting in water vaporization, and then is quickly squeezed by high pressure, resulting in huge energy to kill each other. This high-speed impact is not a one-time, but continuous, as if there are hundreds of pistol shrimp constantly bombarding the propeller, if the propeller material is not hard enough, for a long time to withstand this impact, it will slowly appear "corrosion phenomenon".
Depending on the cause, shape and location of the "cavitation", these cavitations can have different negative effects on the propeller, some can quickly "corrode and peel" the propeller, some can make the propeller make a huge noise, and some can have both. For example, the vortex bubbles generated downstream of the blades will significantly increase the noise of the blades, so it is only natural that the officers and men on the "Charles de Gaulle" cannot sleep.
In fact, it is not only the propellers of ships, even the dams that block the water, when they are discharged, they will be troubled by them. Since the harm of the phenomenon of "cavitation" is so serious, is there any countermeasure? There is no doubt that there are countermeasures, otherwise today's high-speed giant ships will have to change their propellers frequently.
The first measure we can think of is to optimize the material and production accuracy of the propeller, or to coat the surface with anti-ablation paint and special materials, such as Russia, which uses titanium alloys and carbon fiber composite materials to make propellers.
Secondly, we can also reduce the thrust per unit area, such as increasing the area of the propeller disc or increasing the number of propeller blades (in the past many submarines are the seven-blade propeller used), or change the shape of the blade profile, reduce the maximum pressure reduction coefficient of the blade, delay the generation of cavitation, and also add a diversion cover, pump spray, punch holes at the root of the blade to form a cavitation pad, etc.
Of course, these cavitations that destroy the paddles are not useless, and they are now also used in weapons to reduce underwater drag, the most famous application case being the Russian "super-cavitation torpedo". This torpedo is able to actively or passively generate a layer of bubbles and attach to the surface of the torpedo, reducing drag (passive generation is high-speed production).
The Soviet V-111 Storm torpedo can reach an astonishing speed of 200 knots or even 250 knots, six times that of ordinary torpedoes, and is very difficult to intercept, and the United States and Germany are also jointly developing such weapons, but Russia has always been the aggregator of such weapons. In the civilian field, the advantages of cavitation resistance reduction have also received more and more attention, and corresponding means of transportation may appear in the future.
It can only be said that the defense and utilization of empty bubbles by human beings perfectly confirms the saying that "water can carry boats and can also overturn boats".