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Old   April 10, 2008, 12:16
Default transient calculations!
  #1
lelia
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hy, what can be changed or improved in mesh generation or boundaries conditions in order to increase the time step for the transient calculations? the biggest time step for which the calculations are not divergent after few iterations is 0.001s... thanx! lelia
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Old   April 10, 2008, 22:42
Default Re: transient calculations!
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Sachin
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Can you please try to decrease the mesh size and see whether it works or else you have to check the boundary conditions again.. that is the best if you have to increase the time step.

Sachin
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Old   April 11, 2008, 04:52
Default Re: transient calculations!
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lelia
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hy! thankx for the advice...but can u be more explicit...by decreasing the mesh size u mean the number of cells or the cells dimensions? because i have around 200.000 cells but the smallest one has 0.0006m. then, for the boundaries conditions, i have an inlet and an outlet...i'am confused a bit...i appreciate if u give a more detalied explanation. thankx! regards,lelia
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Old   April 11, 2008, 11:15
Default Re: transient calculations!
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Sachin Patil
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You have to decrease the mesh size. Refine your mesh. But this will cause increase in number of cells which will take more PC run time. Regarding boundary conditions, do check them again by plotting each and every boundary conditions. Even if you miss small thing it will diverge. Good luck.

Sachin

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Old   April 11, 2008, 12:00
Default Re: transient calculations!
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lelia
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thankx, just that i can't go bellow 0.0006m which is the smallest cell dimension, because i already have 270.000 cells...and for the boundaries conditions, i cant find the problem, if there any...im trying to solve the fluid flow in steady state and then just the particles ( because i have a 2 phase problem) in transient hopping that it will take less time and that the errors induced are acceptable.. regards, lelia
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Old   April 11, 2008, 12:50
Default Re: transient calculations!
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Sachin
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It is ok if you can not reduce the mesh size for some of the cells. Try to refine the mesh whereever you can. I hope this helps.

Regards, Sachin
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Old   April 11, 2008, 14:50
Default Re: transient calculations!
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tom
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If u are using ccm+ turn off volume sources, and hope for the best by refining your surface mesh
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Old   April 11, 2008, 15:36
Default Re: transient calculations!
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lelia
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no, i'am working under Star-Cd 3.26...but is there any chance to simulate in the same time the fluid flow in steady state and particles motion in transient?
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Old   April 13, 2008, 11:17
Default Re: transient calculations!
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Anton Lyaskin
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No, it should be done in two steps. If you do uncoupled simulation (i.e. particle do not affect the flow) then you can first get a steady state solution for the flow without particles, afterwards add particles, turn off flow field solution and make a restart. In this case at the second step you'll have to solve only particle motion equations.
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Old   April 14, 2008, 08:33
Default Re: transient calculations!
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lelia
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hi Anton! you always have answers for my problems?! thankx a lot! in fact that's what i have done. first i considered just the fluid flow in steady state, then i have restarted the simulation using the steady state results and adding the particles and this time in transient state. i also turned off the solvers for fluid ( equation behavior - primary variables - u-mom,v-mom...). that's the idea, isn't it? thankx! regards, lelia
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