Siheyuan: I eat melons while doing scientific research

Chapter 1181 Troublesome Bubbles (4k, 5k more)

Cavitation is almost always harmful to ships, and it mainly attacks one place - the propeller.

Moreover, the means of attack are varied, including causing degradation of propeller performance, cavitation that directly damages the propeller, and huge noise.

Moreover, cavitation comes in many forms. It is not just the small round bubbles you imagine. There are even sheet-like ones that are generated close to the propeller.

The Gauls' nuclear-powered aircraft carrier, which is not even as big as the 076, had major problems with its propellers due to cavitation. During the sea trial in January 99, it was found that the propeller noise was so loud that it affected the sailors' sleep. By November 1, all four propellers had suffered serious cavitation, and three of them were unusable. A new propeller was replaced in March 2000, but it broke again due to cavitation shortly afterwards.

Even the Gauls still stumbled over this issue in the 21st century. At this time, our country has only been established for a short time, so our research in this area is naturally not in-depth enough.

At this time, Engineer Zhu came over to ask Gao Zhendong. Actually, he just wanted to try his luck. After all, from what he knew, Gao Zhendong was very familiar with materials and fluid mechanics, and he had just made outstanding performance in supercavitation. So asking him was a very appropriate approach.

Mr. Zhu was very happy to find that he had chosen the right path.

Even Engineer Zhu himself did not expect it to be so comprehensive. After all, the most obvious harm of cavitation is cavitation erosion, but propeller performance and noise have not been taken into consideration. To put it bluntly, submarines at that time were so noisy that our comrades probably did not give much thought to cavitation.

In fact, the noise problem caused by propeller cavitation has a huge impact on the quietness performance of submarines. The famous Japanese violated the "Battalion Agreement" and sold machine tools to the Russians to solve the Russians' propeller noise problem.

At the same time, the performance of propellers has not been studied much. The most difficult and most obvious hazard is cavitation.

"Ah, it seems that Commissioner Gao has done some research. To be honest, we just want to solve the problem of cavitation. We haven't really considered the other two items." Engineer Zhu did not hide it.

Let's be frank. The more thoroughly the problem is exposed, the clearer Commissioner Gao will have a grasp of the situation, and the greater the possibility of getting help, and the more help he may get.

Gao Zhendong was silent for a while. Zhu Gong thought he was thinking, but in fact he was looking for something in the system. With his knowledge, he only understood a little bit, but to solve the problem, he still had to cheat.

After a while, the meeting was announced to start again, and Gao Zhendong pulled Zhu Gong: "Come on, come on, sit next to me, let's talk about this."

Anyway, other comrades also have to speak and discuss today's topic, so it's no big deal if I slack off.

Gao Zhendong blatantly pulled Zhu Gong to sit next to him, and the two of them held a small meeting in whispers.

Other comrades turned a blind eye and a deaf ear to his behavior. It was no big deal. Comrade Zhendong and Comrade Zhu had just come up with some inspiration through their conversation. If necessary, they could even stop for a while to let them discuss.

But judging from their performance, the meeting here would not interfere with their conversation at all, so there was no need to stop.

While they were discussing it over there, Gao Zhendong was talking to Zhu Gong about cavitation.

"From the cavitation pressure of water, we know that when the water is at 15 degrees Celsius, as long as the water pressure is less than 174 Pascals, cavitation will be generated, and the movement and shape of the propeller create the conditions for the generation of this low pressure..."

Under normal circumstances, the water pressure will not be so low. To put it simply, even the water on the surface is already at the lowest pressure, but it is still under at least 10 Pascals, which is one atmosphere of pressure. The deeper the water, the greater the pressure. Water itself has pressure.

However, the moving propeller gives the water an opportunity to reduce pressure, which naturally produces cavitation.

"The generation of cavitation mainly goes through two stages. The first stage is the generation of local cavitation..."

"In the second stage, local cavitation expands and gathers on the back of the blade..."

Gao Zhendong followed suit and gave a detailed introduction to the causes of cavitation, part of which was roughly understood by Zhu Gong, while the other part was something that Zhu Gong and his colleagues had never considered before.

"...After studying foreign literature and my own thinking and research, I think cavitation can be divided into four main types..."

When Mr. Zhu heard this, he was delighted. The fact that he could speak so clearly showed that he had done in-depth research. It seemed that he would gain something today.

"...spherical cavitation, this is because..." Gao Zhendong introduced the general cause of this cavitation, and then analyzed: "...this cavitation usually occurs at the maximum thickness of the cross-section on the back of the propeller blade. In terms of harm, this cavitation has little effect on the performance of the propeller, but it will produce strong noise and serious erosion."

Listening to Gao Zhendong's explanation of the causes and hazards of cavitation, Zhu Gong was so excited that his hands were shaking while taking notes.

"... As for lamellar cavitation, the fluid separates at the leading edge of the blade, which will produce lamellar cavitation attached to the surface of the blade. The harm of this kind of cavitation is very interesting. When it extends beyond the trailing edge of the blade and forms a 'full cavitation' covering the entire blade, it will cause the propulsion performance of the propeller to decline. You should be able to imagine this, right?"

Engineer Zhu nodded as he listened. When the entire blade is wrapped in cavitation, the reason for the decline in its propulsion performance is actually similar to the "supercavitating torpedo" mentioned by Commissioner Gao just now. They are all changes caused by complete bubbles.

"...When it does not form a complete cavitation, but only partially covers the surface of the blade, its harm is erosion and noise."

It is too detailed. Regardless of whether it can solve the problem, at least we have figured out the cause and harm, which will make it possible for the next step of work.

"Cloud-like cavitation is a periodic phenomenon in which large-scale cavitation bubbles are shed when the propeller rotates in an uneven flow field..."

"This cavitation has no obvious effect on the performance of the propeller, but the noise is very strong and the erosion is serious, which is very harmful."

Even the severity of the harm has been studied, so it's done~~~~~~
"Vortex cavitation mainly occurs at the blade tip and the hub, and is caused by..."

"... eventually a vortex tube-shaped cavitation bubble is formed. This cavitation bubble does not cause any damage to the blades and will not affect the propulsion performance, but it may erode the hub. However, its main harm is that it will significantly increase the noise of the propeller."

After listening to Gao Zhendong's classification, Mr. Zhu felt that he had gained a lot.

"Well...Commissioner Gao, it seems that the easiest problem to solve here is the degradation of propeller propulsion performance." This is easy to judge. Noise and erosion are problems that almost every type of cavitation will cause, while propulsion performance problems almost only occur in the "full cavitation" state of lamellar cavitation. It seems that solving a single problem is much simpler.

This is also the reason why the reduction in propulsion performance is rarely mentioned in propeller cavitation problems - it is relatively easy to solve.

After all, there are many ways to destroy the lamellar full cavitation state. Even Zhu Gong himself could think of more than one way after listening to Gao Zhendong's words.

Gao Zhendong fully affirmed Zhu Gong's feelings: "Yes, in terms of the main hazards, it is true. The main hazards of erosion are spherical cavitation, local lamellar cavitation, and cloud cavitation, while cavitation vibration and noise are mainly cloud cavitation and vortex cavitation."

Gao Zhendong was very considerate and even classified the types of cavitations that each hazard targeted.

"Thank you so much, Commissioner Gao. Thank you so much. With your contribution, we can be more targeted in our work."

Compared with the almost blank state before, what they have learned from Gao Zhendong now can support them in doing targeted work. They are no longer in the state of being unable to find where to start as before.

"Well, as for the erosion phenomenon, I do have a suggestion." Gao Zhendong said with a smile.

To be honest, Mr. Zhu did not expect that he could find a solution from Gao Zhendong. After all, this was not his major, and the fact that he could summarize the previous theoretical knowledge was already a great support. As for the specific methods, under normal circumstances, who would think so much and so deeply about things that are not their own major.

But he obviously didn't expect that Gao Zhendong was not a normal person.

Regardless of whether it is normal or not, it is even possible to find a solution or idea. Such a good thing is really hard to find even with a lantern. Zhu Gong nodded repeatedly: "Please tell me, please tell me."

"For the issue of erosion, we should mainly solve the problem from the perspective of materials. Because the generation of cavitation is an objective existence and it is impossible not to have cavitation, so no matter how the propeller is optimized, we must directly face the existence of erosion. It is better to solve this problem directly from the material."

In fact, to some extent, as long as you are willing to spend money, erosion is not the most troublesome thing, because you can replace the propeller. Of course, severe erosion will cause serious damage to the blades, and it may even happen during the mission, which is a lot of fun, and replacing the propeller of a ship is not a small job.

Therefore, the problem of erosion should be solved as much as possible, at least to extend the life of the propeller long enough.

After hearing what Gao Zhendong said, Engineer Zhu suddenly realized, well, isn't this back to Commissioner Gao's old job, materials!

"Do you have this material?" He looked at Gao Zhendong hopefully.

Gao Zhendong nodded: "No, I don't have it, actually the Third Line has it. This material is titanium alloy! You have two choices, one is to coat the surface of the propeller with titanium, the other is to make the propeller directly with titanium alloy."

"Titanium alloy? Is this okay?" When it comes to materials, Engineer Zhu has to listen to Gao Zhendong.

"Well, yes. Titanium alloy has good corrosion resistance, sufficient mechanical properties, good strength, and is light enough. Although not all of them are top-notch, it is one of the most suitable materials at present for this matter." Gao Zhendong explained the benefits of titanium alloy in this matter one by one.

“But there is a shortage of titanium materials in our country, right? And I heard that it is difficult to process?” Mr. Zhu had some knowledge about titanium.

"You don't have to worry about this. The production and processing technology of titanium materials is being developed by the Hongmian Steel Plant in Shu Province over on the Third Front, and it has already achieved results." Gao Zhendong was well prepared and confident.

"Really? That's great, I'll go look for it at the Hongmian Factory later." Mr. Zhu didn't expect that after searching for a long time, he would find it without any effort.

To be honest, the overall cost of titanium alloy is still very high. It is indeed unaffordable to use it to make an entire submarine like the Russians did, but it is much easier to deal with a single propeller.

Gao Zhendong's next words made Zhu Gong deeply realize that this man was not only a materials expert, but also a top researcher in fluid mechanics.

"Of course, using materials to solve the erosion problem is only a temporary solution. We still have to start with the propeller itself and try to reduce the generation of cavitation, so as to solve the problem from multiple angles. After all, if we only solve the erosion but not the noise, it will not be of much significance to you."

You're overthinking it, Commissioner Gao. We actually just wanted to solve the erosion problem at the beginning.

"Then what suggestions do you have?"

"My suggestion is to start from several angles. One is to increase the number of propellers. This can reduce the thrust per unit area of ​​the propellers and delay the formation of cavitation. Oh, I forgot. This suggestion doesn't make much sense to you."

With the development of submarines, most of them are single-propeller submarines, so this suggestion does not make much sense to Mr. Zhu.

Mr. Zhu smiled bitterly and nodded, "I'm glad you knew it."

"Then you can change your thinking. When the diameter is constant, increase the number of blades to increase the disk ratio and reduce the thrust per unit area of ​​the propeller. Or conversely, when the disk ratio is constant, increase the diameter of the propeller to achieve the same effect."

Zhu Gong nodded. He had given this aspect some thought just now. It seemed that Commissioner Gao meant more than just "understanding" as he said.

"Secondly, you can choose a suitable blade profile, reduce the maximum pressure drop coefficient of the profile, and use a smaller relative thickness to make the pressure drop distribution on the back of the blade more uniform."

If the maximum pressure drop coefficient is smaller, the probability of the pressure at a certain point on the back of the blade being reduced to the point where cavitation occurs will also be smaller, which can reduce the generation of cavitation. One of the primary hazards of cavitation is cavitation caused by pressure reduction.

"Well, we will try our best to meet your request in this regard."

"Third, try to avoid uneven wake and oblique flow. This uneven flow field is the primary cause of cloud cavitation, which is very important for reducing noise and erosion."

After that, Gao Zhendong smiled and said, "So, I do have a paddle type I can recommend to you."

(End of this chapter)