using RNG k-e
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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have anyone any suggestion for above problem ??????, please
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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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The standard things to improve numerical stability: Double precision numerics, better mesh quality, smaller time step, better initial condition.
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Let me repeat post #3:
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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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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 |
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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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 !! |
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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If I used time step according to equation, how can I check my result does reach convergence |
By defining a convergence criteria and seeing if it is achieved.
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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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Do a sensitivity analysis on your time step size and find out.
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