The military-industrial scientific research system of the academic master

In the next two days, in addition to preparing some relevant materials, Chang Haonan moved some of his belongings from the dormitory to his new residence.

There are certain differences between interviews and defenses. For example, you don’t need to prepare a lot of relevant materials in advance. You just need to answer whatever the other person asks you.

The postgraduate interview is scheduled for an afternoon a week later.

Since it is an interview, naturally everyone goes in one by one.

According to the order of the strokes of his surname, Chang Haonan ranked quite low, so he was not in a hurry. By the time he arrived at the scene, more than half of the people in front of him had already finished.

Before long, the defense secretary came out and called his name.

"Next, Chang Haonan."

When Chang Haonan walked into the interview room, the seven interviewing teachers adjusted their sitting postures almost simultaneously.

Especially the two people sitting in the middle, Liu Zhenxiang and Du Yishan.

One put on the glasses that had been placed casually on the table, and the other opened a thick notebook in front of him.

From Chang Haonan's perspective, you can see that there are a lot of small words written densely in the book.

Obviously, the purpose of the two people coming here today is very clear. The dozen or so people in front of them cannot arouse the interest of these two big guys.

This posture is quite impressive to say the least.

Many students performed abnormally due to nervousness after entering.

But it's not a problem at all for Chang Haonan.

Apart from anything else, the "review meeting" he experienced before going to Institute 601 more than a month ago was not bad at all.

So after standing in front of the table, Chang Haonan looked as usual.

"Please sit down, classmate Chang Haonan."

Tang Lintian was the first to speak.

Originally, this kind of job should be done by Liu Hongbo and a few others, but this time due to the presence of two academicians, the previous F4 postgraduate interview may not even have a chance to speak.

According to the interview process, there is first an examination of some professional knowledge, followed by the reading and translation of professional English fragments.

These two parts were naturally no problem for Chang Haonan, and Liu Zhenxiang and Tang Lintian were not interested in this, so they ended hastily in less than ten minutes.

The third and final link is free questioning.

For most people's postgraduate interviews, this part is basically equivalent to a chat session. The interviewer just asks two questions, such as future research interests, and then it's over.

But Chang Haonan's situation is obviously different from that of most people.

This time, Liu Zhenxiang, who had been silent just now, finally spoke:

"Classmate Chang Haonan, I have read the relevant technical documents submitted by the 603 Institute on 'Flutter Simulation and Active Control of Large Aspect Ratio Straight Wings', so I have a few questions that I am interested in, and I would like to take advantage of them. I’ll take this opportunity to ask you.”

"I don't believe you should read it, please tell me."

"When conducting aero-servo-elastic modeling of the research target wing, you introduced two approximation methods in the Laplace domain for the aerodynamic matrix. The second one is based on the known reduced frequency point in the frequency domain. The corresponding aerodynamic coefficient matrix is ​​used to calculate the undetermined coefficients, but in subsequent technical documents, this one does not seem to be used?”

Liu Zhenxiang said that he had read the technical documents, and it was obviously not as simple as just flipping through them.

Not only did I see it, but I also understood it.

Although he is not an expert in aeronautical engineering or structural mechanics.

This is what a great boss is like, he knows everything about the relevant fields.

"This question...I need a piece of chalk."

The defense secretary quickly brought it in from outside.

In fact, Chang Haonan himself has never seen what the specific documents reported by 603 Office look like.

However, relying on his understanding of the model he "created", he quickly gave the answer to the question:

"That's right. The aerodynamic influence coefficient matrix Q(k, M) obtained based on the dipole grid method is a function of the reduction frequency k and the Mach number M. It belongs to the frequency domain aerodynamic force, but we want to obtain an aerodynamic servo elastic system. The state space model requires converting the aerodynamic expression from the frequency domain form to the time domain form."

"At any point in this conversion process, the two methods I introduced are substitution relationships. Among them, the second one you mentioned, with the smallest error, the expression of the aerodynamic force F is F=q [Q1X(t)+Q2X'(t)b/v+Q3X''(t)b^2/v^2+E1Y1(t)+E2Y2(t)+E3Y3(t)+E4Y4(t)]. "

"The premise for minimizing the error is that the relationship between the undetermined coefficients must satisfy J/Q=∑kε=0..."

Chang Haonan wrote a series of formulas on the blackboard, then turned back and said:

"For computers, the second approximation method will have higher fitting accuracy, but since the order of the space equation will also increase, the computational burden will be heavy. Therefore, the 603 Institute chose to use the first method throughout the process. , should take into account the practical factors of calculation amount.”

After hearing this answer, Liu Zhenxiang nodded.

To be honest, until the beginning of this interview, he was dubious about the authenticity of Chang Haonan's abilities.

The main reason is that someone in his twenties who has not yet graduated from a bachelor's degree can preside over an advanced achievement in the aviation industry system as the first person to complete it. It is really...

A bit outrageous.

But now the facts are before us.

Chang Haonan's explanation is very clear. He is equivalent to giving 603 institutes two choices, one is slightly less accurate but requires less calculations, and the other is more accurate but requires more calculations.

It’s just that in the end Party B chose the former one.

There is no problem with the theoretical calculation process.

And there are no reference materials in the whole process, just think for a moment and write quickly on the blackboard.

It can definitely be done by someone who knows the model very well.

"Then, there is another question." Liu Zhenxiang turned the notebook to another page:

"Have you ever considered applying similar models to the rotor blades of aeroengines? Many advanced foreign engines have begun to use small aspect ratio rotor blades to reduce the number of compressor stages and reduce the failure rate, but then The chattering problem became serious again.”

"Well..." Chang Haonan lowered his head and pondered for a moment.

Logically speaking, this issue has nothing to do with his previous research.

But it was related to the design issue of turbine blade sweep that he planned to study later.

So happen to have some ideas.

"good……"

Of course, Liu Zhenxiang also knew that his question was unreasonable, so after seeing Chang Haonan's expression and posture, he closed his book and prepared to end the interview.

But as soon as he said the first word, the latter raised his head again:

“The model constructed using the dipole grid method has poor versatility and can only be used to describe straight wings with large aspect ratios, so it certainly cannot be directly used in the field of compressor design.”

"But I think we can try to use the energy method to perform a full three-dimensional unsteady flow calculation, obtain the unsteady average accumulated work on the blade, and judge whether stall flutter has occurred in the blade based on the positive or negative of the total accumulated work..."

The first order data can only be seen at noon, so the update will be in the afternoon as usual.