Chapter 74 Postgraduate Interview
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 stroke order of his surname, Chang Haonan ranked quite low, so he was not in a hurry. By the time he arrived at the scene, most of the people in front of him had already finished.
Not long after, 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 at the same time.
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 coming in.
But it's not a problem at all for Chang Haonan.
If nothing 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 take a seat, classmate Chang Haonan."
The first person to speak was Tang Lintian.
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 interview teacher just asks two questions, such as future research interests, and 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 paper submitted by the 603 Institute about the 'large aspect ratio straight wing' "Flutter simulation and active control" related technical documents, so I have a few questions that I am interested in, and I would like to take this opportunity to ask you."
"If you don't dare to read it, please tell me."
“When performing 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 what is known in the frequency domain. The aerodynamic coefficient matrix corresponding to the frequency point is used to calculate the undetermined coefficient, but this method does not seem to be used in subsequent technical documents? ”
Liu Zhenxiang said that he had read the technical documents, and it was obviously not as simple as just flipping through them.
Not only saw it, but also understood it.
Although he is not an expert in aerospace engineering and 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 looked like.
However, relying on his understanding of the model he "created", he quickly gave the answer to the question:
"That's it, the gas obtained based on the dipole grid method The dynamic influence coefficient matrix Q (k, M) is a function of the reduction frequency k and the Mach number M. It belongs to the frequency domain aerodynamic force. However, if you want to obtain the state space model of the aeroservoelastic system, you need to change the aerodynamic force expression from the frequency domain. The form is converted into the time domain form. "
"At any point in this conversion process, there is a substitution relationship between the two methods I introduced. Among them, the second one you mentioned has the smallest error. In the case of , the expression of 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 prerequisite 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, and then turned back 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. 603 The choice of using the first method throughout the whole process should be due to 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 in his twenties, before he graduated from a bachelor's degree, he could preside over an advanced achievement in the aviation industry system as the first person to complete it. It is really...
It's 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 required in the whole process. You can write quickly on the blackboard after just thinking for a moment.
It can only be done by someone who knows this model very well.
"Then, there is another question." Liu Zhenxiang turned to another page in his notebook:
"Have you ever considered applying a similar model to the field of aeroengine rotor blades? Abroad? Many advanced engines have begun to use small aspect ratio rotor blades to reduce the number of compressor stages and reduce the failure rate, but the resulting flutter problem has become serious. ”
"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 I happen to have some ideas.
“Okay…”
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 generality and can only be used to describe A straight wing with a large aspect ratio cannot be directly used in the field of compressor design. ”
“But I think we can try to use the energy method to perform full three-dimensional unsteady flow calculations on the blades. Unsteady average accumulated work, based on the positive or negative of the total accumulated work, it is judged whether the blade has stalled or fluttered..."
The first order data can only be seen at noon, so the update will be in the afternoon as usual.
(End of this chapter)