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Old   February 18, 2022, 07:50
Question Question of this paper.
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Sangho Ko
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Hello everyone.
I'm reading paper and I don't understand some part.
So I want to ask you.

Link of this paper is below.
https://www.researchgate.net/publica...ld_Experiments

I'm not good at English so there can be difficulty to communicate.
Also cause of this, there is a confusion of nomenclature.
So please understand me.



1. In page 1,
"the local turbulent length scales near the wall are strongly affected by the length scales of the jet turbulence (in a parallel flow, length scales are usually determined by the distance from the wall alone) "

A. What jet is this?
Does it mean shooting jet before impingement or jet on the wall after impingement?


2. In page 1,
"convective transport of turbulence energy towards the stagnation point is important (in a parallel flow, convective effects are usually negligible, an approximate balance existing between generation and dissipation processes). "

A. What is convection of turbulence energy of jet towarding stagnation point?

What is the convection of turbulence energy?
Does it mean convection of scalar(Turbulence energy) by mixing?
or What?
B. Neglecting convective effect in parallel flow
Why convective effect is negligible in parallel flow? Is there any special reason?


3. In page 6,
"These differences are mainly due to the fact that, for H/D = 4.0 and below, the mixing layer that springs from the pipe rim will not have spread to the jet axis. "

A. What is mixing layer?
I guess mixing layer is same with outer region because in outer region, there is a active mixing by turbulence.
But I'm not sure what is the correct meaning of mixing layer.
B. What is pipe rim?
I guess pipe rom is same meaning with viscous sublayer.
There are 2 reasons why I guess like that.
First reason, meaning of rim what I know is the outer edge of round object.
And in turbulence, viscous sublayer is very thin and enclosed with solid boundary.
Cause of this I think rim is same meaning with viscous sublayer.
But I'm not sure what is the correct meaning of pipe rim.
C. I don't understand why mixing layer springs from pipe rim
I don't understand what does it mean.
In my opinion, it means turbulence is caused by mean velocity gradient.
So just above the viscous sublayer, there should be source that produces much turbulence because there is high gradient of mean velocity.
It means mixing layer(region that produces much turbulence) springs from viscous sublayer(pipe rim).
But I'm not sure what does it mean correctly.
Upper is just my guess.
D. I don't understand what does 'mixing layer not have spread to the jet axis'.
I don't understand what situation does it describe.
I've guessed like 'It means there is insufficient mixing(turbulence) before impingement.'
But I think this experiment is set by fully developed condition because of below sentence.
'In both experiments, before discharge, the air passed along a smooth pipe sufficiently long to give fully developed flow at the exit plane of the jet-a feature that is helpful in using the data for turbulence-model evaluation.'
So I can't understand what does this sentence mean.
I think it is caused by my deficient English level. Sorry.


4. In page 6,
"The other item deserving comment is the fact that the indicated mean velocity does not vanish at the wall, the apparent slip velocity being particularly high for H/D = 6 and 10."

A. What is apparent velocity.
What it the correct physical meaning of apparent velocity?
I guess it means velocity at that moment.
What I mean is 'mean velocity+fluctuating velocity'.
But I'm not sure what is the correct meaning of apparent velocity.
B. What is apparent slip velocity.

I don't understand the meaning of slip.
So I've searched the meaning of slip velocity from Google but what I found is velocity difference between jet and solid particle.
But there is No solid particle in this experiment.
And also I guessed it means there is a measured velocity on the wall neglecting No slip condition cause of measurement error.
But I'm not sure whtat is the correct meaning of apparent slip velocity.




Thanks for reading thess long questions.
I'm not good at English so please excuse my bad English.
I'm not professional about turbulence.
But I'm interested in that field.
So help me please
Thanks

Edit: I've solved 1 and 3. Please help me for 2 and 4.
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Last edited by FluidKo; February 18, 2022 at 10:13.
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Old   February 18, 2022, 08:12
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i haven't read the paper, but i think this is what they meant. if i'm wrong, maybe someone will fix it.

"mixing layer that springs from the pipe rim":

in simple english, the sentence could be restructured as "mixing layer that comes out of the pipe rim".

the pipe rim, is the circumference of the the pipe. as the fluid comes out of the pipe, the shearing forces of the fluid will also drag the air around it and after it hits the wall, it will start to mix. i think those are what the authors refer as mixing layers.

since the pipe is wide, the column of air flow / air jet in the center will not be disturbed by disturbances at the circumference of the air jet.

the authors are referring to the airflow at the center of this jet, and saying that it will remain undisturbed.
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Old   February 18, 2022, 08:30
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Quote:
Originally Posted by aerosayan View Post
i haven't read the paper, but i think this is what they meant. if i'm wrong, maybe someone will fix it.

"mixing layer that springs from the pipe rim":

in simple english, the sentence could be restructured as "mixing layer that comes out of the pipe rim".

the pipe rim, is the circumference of the the pipe. as the fluid comes out of the pipe, the shearing forces of the fluid will also drag the air around it and after it hits the wall, it will start to mix. i think those are what the authors refer as mixing layers.

since the pipe is wide, the column of air flow / air jet in the center will not be disturbed by disturbances at the circumference of the air jet.

the authors are referring to the airflow at the center of this jet, and saying that it will remain undisturbed.
Thanks for your answer.
Actually I've added 2 more questions.(1 and 2)
If you know that, can you answer those questions?
Thanks
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Old   February 18, 2022, 09:11
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Quote:
Originally Posted by FluidKo View Post
Thanks for your answer.
Actually I've added 2 more questions.(1 and 2)
If you know that, can you answer those questions?
Thanks

"the local turbulent length scales near the wall are strongly affected by the length scales of the jet turbulence (in a parallel flow, length scales are usually determined by the distance from the wall alone) "

i think this means that the turbulence in the final mixed flow after impingement will be affected by the turbulence in the jet stream. in simple words, if the jet is high speed and very turbulent, the turbulence after hitting the wall will also be high. or if the jet is slow and laminar, the turbulence after hitting the wall will also be low.



edit : i don't understand the second one.

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Old   February 18, 2022, 10:12
Thumbs up Thanks a lot
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Quote:
Originally Posted by aerosayan View Post
"the local turbulent length scales near the wall are strongly affected by the length scales of the jet turbulence (in a parallel flow, length scales are usually determined by the distance from the wall alone) "

i think this means that the turbulence in the final mixed flow after impingement will be affected by the turbulence in the jet stream. in simple words, if the jet is high speed and very turbulent, the turbulence after hitting the wall will also be high. or if the jet is slow and laminar, the turbulence after hitting the wall will also be low.



edit : i don't understand the second one.
Thanks for your sincere answer
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Old   February 18, 2022, 12:56
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4. The velocity at the wall should be zero due to no-slip but the hot-wire results don't show this. If the velocity next to the wall is different than the wall then we call it a slip velocity. We know the no-slip condition should apply so the slip is not real, it's an artifact of the measurement. The slip isn't real, they're just explaining why the data makes no sense. You can ignore this for the purpose of understanding turbulence of impinging jets unless you want a lesson on how hot-wire anemometry works.



2. For simple shear layers like boundary layers in pipes and such there is only 1 length scale present because there is only one boundary layer. For parallel flows, indeed the local turbulent length scale is determined by distance from the wall (because the wall generates the boundary layer and all the turbulence). Impinging jets bring turbulence from the jet and throws (it impinges) it at the wall. In pipe flows, there is no external source of turbulent kinetic energy like there is in an impinging jet.
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Old   February 18, 2022, 13:04
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Quote:
Originally Posted by LuckyTran View Post
4. The velocity at the wall should be zero due to no-slip but the hot-wire results don't show this. If the velocity next to the wall is different than the wall then we call it a slip velocity. We know the no-slip condition should apply so the slip is not real, it's an artifact of the measurement. The slip isn't real, they're just explaining why the data makes no sense. You can ignore this for the purpose of understanding turbulence of impinging jets unless you want a lesson on how hot-wire anemometry works.



2. For simple shear layers like boundary layers in pipes and such there is only 1 length scale present because there is only one boundary layer. For parallel flows, indeed the local turbulent length scale is determined by distance from the wall (because the wall generates the boundary layer and all the turbulence). Impinging jets bring turbulence from the jet and throws (it impinges) it at the wall. In pipe flows, there is no external source of turbulent kinetic energy like there is in an impinging jet.
Now I understand.
Thanks for your sincere answer.
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