请教tingeMM老师下面这两句话怎么理解，谢谢！

wangdingxi

[这个贴子最后由wangdingxi在 2004/07/24 06:01pm 第 5 次编辑]

Molecules migrating across y=0 are typical of where they come from.(Page 24,Turbulence Modeling for CFD by Wilcox)
The turbulence is continually trying to adjust to its environment,without ever succeeding.(P29)
其中标记处在wilcox书中多次引用，但是本人愚钝，不能尽解，望tingeMM老师指点，谢谢！

tingeMM

Based on the context before and after the sentence, my understanding to the first sentence is as follows: [Here, “typical” = “characteristic”.]
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The original text:
We consider the flux of momentum across the plane y=0, noting that molecular motion is random in both magnitude and direction. Molecules migrating across y=0 are typical of where they come from. That is, molecules moving up bring a momentum deficit and vice versa. This gives rise to a shear stress t_xy.
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The explicitly explained text:
We consider the flux of momentum across the plane y=0, noting that molecular motion is random in both magnitude and direction. Molecules migrating across y=0 have the same characteristic velocity or momentum as those molecules left at the original place where they come from. Therefore, molecules moving up bring a momentum deficit to their new surrounding environment whereas molecules moving down bring a momentum surplus to the new environment. This process of momentum exchange produces a shear stress t_xy at the plane or the surface y=0.
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Without the context before and after the sentence, my understanding to the second sentence is as follows:
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The original text:
The turbulence is continually trying to adjust to its environment, without ever succeeding.
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The explicitly explained text:
Through turbulent motion, the fluid parcel is continually trying to adjust its physical properties to the same values as those of its environment. But, this has never been successful.

wangdingxi

[这个贴子最后由wangdingxi在 2004/07/25 09:46am 第 2 次编辑]

At first thanks a lot！
does the first sentence indicate that when molecules migrating across y=0  experiece a free journey as long as a mean free path?Otherwise molecules will collide with others which will change their characteristic properties.
When mrigrating across y=0 without collision,molecules preserve their original properties.
Does the second sentence tells us that turbulence is trying to ajust itself to fit the environment,but it never succeed,so turbulence is changing itself all the time？

tingeMM

First sentence: Yes. The thickness of a “surface” is zero, which should be less than the length of any mean free path.
Second sentence: Yes. Therefore, the word “stationary” in turbulence always refers to stationary processes in statistical sense, i.e., the probability distribution function does not change with time. “Stationary” in turbulence does not mean a constant field.

wangdingxi

why can't turbulence succeed in adjusting itself to the environment?
Is the environment changing itself when turbulence is adjusting itself?
What is the environment? I think the so-called environment consists of molecules.Then I can say that every parcel of fluid are trying to adjust itself according to their own understanding of the other parcels which results turbulence?

wangdingxi

下面引用由tingeMM在 2004/07/25 10:14am 发表的内容：
First sentence: Yes. The thickness of a “surface” is zero, which should be less than the length of any mean free path.
Second sentence: Yes. Therefore, the word “stationary” in turbulence alwa ...

Thanks,I got it!