# Question of this paper.

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 February 18, 2022, 08:12 #2 Senior Member   Sayan Bhattacharjee Join Date: Mar 2020 Posts: 495 Rep Power: 7 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. FluidKo likes this.

February 18, 2022, 08:30
#3
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Sangho Ko
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Quote:
 Originally Posted by aerosayan 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.
Actually I've added 2 more questions.(1 and 2)
If you know that, can you answer those questions?
Thanks

February 18, 2022, 09:11
#4
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Sayan Bhattacharjee
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Quote:
 Originally Posted by FluidKo 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.

February 18, 2022, 10:12
Thanks a lot
#5
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Sangho Ko
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Quote:
 Originally Posted by aerosayan "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.

 February 18, 2022, 12:56 #6 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,631 Rep Power: 65 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. aerosayan and FluidKo like this.

February 18, 2022, 13:04
#7
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Sangho Ko
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Quote:
 Originally Posted by LuckyTran 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.

 Tags jet impingment, turbulence