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July 21, 2016, 03:55 |
using RNG k-e
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#1 |
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I used k-e for free surface model the result was converged, but for the same model (same mesh size time scale .... ) I used RNG k-e (for comparison)the simulation does not reach to convergence after 1200 iteration. can anyone tell me the reasons
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July 22, 2016, 01:48 |
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#2 |
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have anyone any suggestion for above problem ??????, please
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July 22, 2016, 05:42 |
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#3 |
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Glenn Horrocks
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K-e and RNG k-e turbulence models are very similar. If this change in your model makes a difference then you model is obviously on the verge of being numerically unstable.
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July 22, 2016, 06:34 |
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#4 |
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but I do not change anything in my model, only turbulence model from k-e to RNG k-e, what should I do to the model in order become convergence with RNG K-e??? I reduced time scale but It does not reach convergence
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July 22, 2016, 06:42 |
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#5 |
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The standard things to improve numerical stability: Double precision numerics, better mesh quality, smaller time step, better initial condition.
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July 22, 2016, 07:28 |
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#6 |
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but I did all above parameters for simulation by k-e and successfully reach convergence but for RNG k-e does not reach
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July 22, 2016, 07:57 |
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#7 | |
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Let me repeat post #3:
Quote:
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July 22, 2016, 15:13 |
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#8 |
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July 22, 2016, 18:36 |
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#9 |
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Glenn Horrocks
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Yes, correct.
But the point I am really trying to say is that your simulation is very unstable anyway. So anything you do to make the RNG simulation converge will help the k-e simulation as well. |
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July 23, 2016, 10:25 |
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#10 | |
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Quote:
https://drive.google.com/open?id=0B3...0pNRmtnemtQR00 https://drive.google.com/open?id=0B3...W5IUmp1dkJHQTg while previous residual for fine mesh with lower time step are https://drive.google.com/open?id=0B3...HctaFA1TkwxeUE |
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July 24, 2016, 07:38 |
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#11 |
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Glenn Horrocks
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This is normal. But note that your results have probably reduced in accuracy significantly by coarsening the mesh.
In my experience many free surface flow simulations require a transient solution to obtain convergence. This is because surface waves have very low dissipation and they can be resolved in a transient simulation but cause havoc with convergence in steady state simulations. So have you tried a transient simulation? You should be able to get that to converge even with the fine mesh. But it will take significantly longer to run. |
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July 24, 2016, 13:32 |
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#12 | |
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Quote:
note: 1-adaptive time step need higher computational time, than specifying time step value, 2-I saw many researches in free surface flow performed by CFX using steady state analysis type !! |
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July 24, 2016, 20:45 |
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#13 |
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For transient simulations I recommend adaptive with 3-5 coeff loops. You can use time step formula you like, but you need to check the time step sensitivity.
If other researchers have got this model to converge steady state then have a look at what they have done and try to figure out why they get it working and you can't. |
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July 28, 2016, 04:36 |
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#14 | |
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Quote:
If I used time step according to equation, how can I check my result does reach convergence |
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July 28, 2016, 07:06 |
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#15 |
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By defining a convergence criteria and seeing if it is achieved.
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July 28, 2016, 15:46 |
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#16 |
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ok, I defined residual (10-4), and imbalances 0.01, and a monitor point at outlet to monitor mass flow rate (or any interest variable), if the a above criterion are met, so the convergence was achieved. is it right?
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July 28, 2016, 18:35 |
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#17 |
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Yes, the convergence tolerance you defined has been achieved. But is the tolerance you defined sufficient to give an accurate enough result? That is an important question as well.
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August 4, 2016, 07:14 |
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#18 |
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according to residuals and imbalances the results appear that reached convergence, but Courant No. is high (RMS Courant no. = 15 , MAX Courant no. = 286) is the a problem?
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August 4, 2016, 07:31 |
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#19 |
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Do a sensitivity analysis on your time step size and find out.
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August 5, 2016, 03:10 |
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#20 |
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