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February 18, 2022, 07:50 |
Question of this paper.
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#1 |
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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?
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. B. What is pipe rim? I guess pipe rom is same meaning with viscous sublayer. C. I don't understand why mixing layer springs from pipe rim I don't understand what does it mean. 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. 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? B. What is 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. Last edited by FluidKo; February 18, 2022 at 10:13. |
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February 18, 2022, 08:12 |
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#2 |
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Sayan Bhattacharjee
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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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February 18, 2022, 08:30 |
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#3 | |
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Actually I've added 2 more questions.(1 and 2) If you know that, can you answer those questions? Thanks |
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February 18, 2022, 09:11 |
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#4 | |
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Sayan Bhattacharjee
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Quote:
"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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February 18, 2022, 10:12 |
Thanks a lot
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February 18, 2022, 12:56 |
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Lucky
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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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February 18, 2022, 13:04 |
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#7 | |
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Thanks for your sincere answer. |
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jet impingment, turbulence |
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