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Please help me out: Linear solver overlow

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Old   October 20, 2008, 01:24
Default Please help me out: Linear solver overlow
  #1
Hao
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I am running the simulations of a turbulent jet flow. The simulition is successfully finished when i use 1e-3 sec as the time step. However, when i change to use 1e-4 sec (with the same setup of the others), there is always a overflow problem. I thought it was due to the mesh quality. So i checked it in Ansys ICEM. It said the quality was good. Does anyone confront this kind of problem? please give me some clue. Cheers!
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Old   October 20, 2008, 11:19
Default Re: Please help me out: Linear solver overlow
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John S.
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This can sometimes happen if you have too fine a mesh as well. Check your domain advection time. A typical time step is usually 1 advection time whereas 1/3 of the advection time would be considered small but can aid convergence.
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Old   October 20, 2008, 21:29
Default Re: Please help me out: Linear solver overlow
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Glenn Horrocks
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Hi,

Using either a smaller timestep or finer mesh often causes the simulation to loose stability. There are a number of reasons but the most common one is that the mesh is now starting to resolve turbulent eddies. This leads to a minimum size allowable for timesteps. That is why it is recommended to increase the timestep agressively in steady state simulations after the initial transient is under control.

Glenn Horrocks
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Old   October 21, 2008, 00:18
Default Re: Please help me out: Linear solver overlow
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Hao
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Thanks, Glenn. When you said 'increase the timestep agressively in steady state simulations after the initial transient is under control', do you mean that i should do a steady-state simulation first to get some results to set as the initial conditions for the later transient simulation?
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Old   October 21, 2008, 00:44
Default Re: Please help me out: Linear solver overlow
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Hao
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Thanks for the response, John. I have a rookie question: how to check the advection time of my domain? cheers.
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Old   October 21, 2008, 18:53
Default Re: Please help me out: Linear solver overlow
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Glenn Horrocks
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Hi,

You should model the physics as they are in the real world. If it starts from a steady state condition then yes, start with a steady state simulation. If it starts from a known condition (ie from rest, or some other known condition) then start straight into the transient simulation.

Glenn Horrocks
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Old   November 12, 2008, 06:08
Default Re: Please help me out: Linear solver overlow
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pmp77
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Hao,

I'm new here but I have found this:

http://www.cfd-online.com/Forum/cfx_...cgi?read=28402
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