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Old   November 21, 2008, 10:41
Default not getting convergence
  #1
NVSD BABU
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HI

iam working on simple natural convection problem. i have taken a copper plate of 1250*80*5mm and placed vertically in a steel enclosure of size 1630*510*150 and 2mm steel thickness.steel enclosure opened at bottom and closed at top.

for analysis i have considered only copper plate and air inside the steel encloser,i have given inflation mesh to air,which is having around 105000 elements.i have given copper-air interfaces. I have given opening boundary condition to bottom portion of air and all other outer faces declared as adiabatic walls.my model's vertical direction is y , i have given density as -9.81 in Y direction.tarbulence model used is k-e.I have tried both idealgas and air at 25 degC.i have tried different time steps for air and copper.

Plese help me

Thanks a lot

NVSD BABU

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Old   November 22, 2008, 07:19
Default Re: not getting convergence
  #2
NVSD BABU
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hi plz find the geometry and give the comments that i have to follow

http://img153.imageshack.us/img153/4...mainlr5.th.png

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Old   November 24, 2008, 12:35
Default Re: not getting convergence
  #3
pratik mehta
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Hi,

Could you please tell what kind of convergence issue are you facing, is your problem diverging or you simply not achieving the residual lower limit?.

thanks,,

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Old   November 25, 2008, 01:14
Default Re: not getting convergence
  #4
NVSD BABU
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thank you pratik

i have given convergence as 1e-4, v-momentum not getting converged, it is oscillating and all other u,w and p getting 1e-4 convergence,but they are also oscillating.

Thank you NVSD BABU
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Old   November 25, 2008, 04:55
Default Re: not getting convergence
  #5
pratik mehta
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Is it is high resolution scheme that you are running with or blend factor, kindly switch on the double precision tab as well, what is your global Y plus range and let me know more about the moniter points if at all you have

best of luck
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Old   November 25, 2008, 08:08
Default Re: not getting convergence
  #6
NVSD BABU
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Thanks pratik, i will try with double precision.

I have tried high resolution,upwind and blend factor1 schemes.

Maximum mesh expansion factor is 70.1

moniter point is temperature.

Thanks

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Old   November 25, 2008, 08:22
Default Re: not getting convergence
  #7
NVSD BABU
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hi prathik

global Y+ range is 0 to 3.412

Thanks
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Old   November 25, 2008, 16:59
Default Re: not getting convergence
  #8
Pratik Mehta
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Sounds all basic settings seems good, are you solving it steady state or unsteady state . Try running with smaller time steps . First use K epsilon model and first order trasient case. Let me know what happens then
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Old   November 26, 2008, 05:04
Default Re: not getting convergence
  #9
NVSD BABU
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thanks prathik

I am solving the problem as steady state.If i do in trasient state,please tell me how to take total duration and time steps. practically, to get convergence our model took 4hrs. do i give 4hrs as total duration.

Thanks

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Old   November 26, 2008, 22:49
Default Re: not getting convergence
  #10
Glenn Horrocks
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http://www.cfd-online.com/Wiki/Ansys...gence_criteria
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Old   November 27, 2008, 09:36
Default Re: not getting convergence
  #11
pratik mehta
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You need to find out the suitable timescale of your problem in way from the velocity lenght scale and the integral lenght scale , but usually it is so straight forward for complex geometries , in that case you can iterate your time scale factor starting from the time scale factor that CFX uses during the steady state calculation , we can get this number from the output file.

So run trasient with Kepsilon n first order ... i hope your fluid properties are reasonably quoted , anyways try out this and lets see what you get
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