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May 31, 2016, 07:17 |
problem with turbulent model in free surface
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#1 |
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I use k-e and RNGk-e for my free surface flow model, but the result of k-e is better than RNGk-e is the possible? because most papers said RNGk-e is more accurate than k-e in swirl and high curvature flow.
the second question how can I calculate Fr (v/(g y)^0.5) in CFX post in a section where v is the average velocity and y average hight at that section. thanks: Last edited by zryan civil; May 31, 2016 at 11:55. |
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June 1, 2016, 03:18 |
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#2 |
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please can anyone help me in that problem
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June 1, 2016, 08:14 |
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#3 |
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Glenn Horrocks
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There is not much difference between k-e and RNG k-e for most applications. And I would expect them to both have similar problems with swirl and high curvature flow - you need RSM models to handle that.
The equation you quote should be easy enough to process in CFD-Post. What is your problem with it? |
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June 1, 2016, 11:47 |
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#4 | |
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Quote:
the problem in that equation is, how can I express average velocity for example at plane 1, is that right (avgvelocity@plane1) or something like this about y or height of water, how can I express y at VF 0.5. this is my problem. thanks |
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June 1, 2016, 20:08 |
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#5 |
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Glenn Horrocks
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If you have a plane, then the areaInt(water.vf)@plane/area()@plane is the proportion of the plane which has water. There's a starting point for you.
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June 3, 2016, 02:59 |
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#6 | |
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about turbulent model, I think your purpose is, in spite higher computational time of RNGk-e it is result almost similar to k-e, so which kind of turbulent model of Reynolds stress model (RSM) is useful to handle that case, (LRR Reynolds Stress, QI Reynolds Stress, SSG Reynolds Stress, Omega Reynolds Stress, Baseline (BSL) Reynolds Stress model, Explicit Algebraic Reynolds Stress model) are available in CFX |
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June 4, 2016, 10:27 |
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#7 | |
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June 5, 2016, 06:15 |
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#8 |
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Glenn Horrocks
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I do not understand your question.
Are you asking what turbulence models are available in CFX? (the answer is: it is all in the documentation, read it) Or are you asking what turbulence model do you recommend for your application? (the answer is: I always recommend using SST unless you have a good reason not to. You have not stated a good reason not to, so I recommend SST) |
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June 5, 2016, 11:50 |
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#9 | |
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yes, my question about best turbulence model for swirling and high curvature flow like hydraulic jump. but I think SST at the same class like RNGk-e, standard k-e (which is eddy viscosity model), I test RNGk-e, standard k-e almost have the same result. according to your previous post that RSM is the best to handle that high flow curvature, my question is what specific model in RSM in CFX there are many model as I posted previously. |
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June 5, 2016, 20:19 |
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#10 |
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Glenn Horrocks
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No, I do not agree. The curvature I see in the hydraulic jumps I am familiar with is more a turbulent effect, that is it is random in space and time. You do not need things like RSM models to handle this a standard model like SST should work. Well, at least I would try SST first.
The flows which definitely have high curvature which mean 2-equation models like SST are unsuitable (at least without modification for the curvature) are things like industrial cyclone flows. The streamlines have extreme accelerations on them in this case. But at the end of the day if you get good results with RSM and poor results with a 2-equation model then that is convincing to me. |
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June 26, 2016, 10:44 |
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#11 | |
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June 26, 2016, 19:52 |
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#12 |
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Glenn Horrocks
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You suggested RSM models. Have you tried that?
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June 27, 2016, 02:09 |
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#13 |
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June 27, 2016, 07:02 |
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#14 |
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Glenn Horrocks
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All the models have strengths and weaknesses, so none of them are "the best". If you read the documentation the SSG RSM model is probably the one to try. If want to try a omega based model try BSL RSM.
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June 30, 2016, 02:12 |
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#15 | |
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yaseen
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at the end of channel there is 8 cm difference with experiment, which is very high, I dont know why. is it possible to say CFX (RANS, with turbulent model) does not work in hydraulic jump. my mesh and all BC are correct only Iam not sure about outlet B.C I used outlet BC at the end of channel (at drop) with super critical, and at bottom of channel with static pressure = 0 https://drive.google.com/open?id=0B3...0h4VzBLUGJEVlk https://drive.google.com/open?id=0B3...kM3T3QzZUdqaWs https://drive.google.com/open?id=0B3...EtpVWZseGVTdUE |
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June 30, 2016, 06:36 |
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#16 |
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Glenn Horrocks
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CFX can model hydraulic jump. You have not set it up correctly. But the hard bit is finding what you have done wrong.
These results also suggests the turbulence model is not causing the problem, as you say the problem is more likely in you fundamental simulation setup. From your images I can see lots of problems straight away: * Your mesh is way too coarse * Your inlet is too close * Your outlet is too close So they are some things you need to fix before you look at anything else. |
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June 30, 2016, 11:09 |
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#17 | |
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yaseen
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Quote:
https://drive.google.com/open?id=0Bz...DhVYTNjLWZLVnc |
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June 30, 2016, 12:34 |
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#18 |
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Regarding the coarse mesh issue, have you attempted to run a finer mesh and see if the solution is sensitive to the mesh resolution ?
The use of coarse, medium, fine, etc meshes is just relative terminology for a given problem. What is coarse for a case may be just fine for another regime ? There is no generality, and sensitivity studies must always be done to some extent. |
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June 30, 2016, 14:55 |
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#19 | |
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June 30, 2016, 14:58 |
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#20 | |
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yaseen
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