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Steady State Vs Transient answers

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Old   June 19, 2008, 21:11
Default Steady State Vs Transient answers
  #1
Kushagra
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Hello, It may not sound a serious question, but I still need to clear my doubt.

If I run the same simulation in steady state solver and transient solver, and somehow I manage the simulation to converge in steady state solver.

Will the result from both the simulations almost same? I could only manage the steady state simulation to converge by playing around the false time step.

Thanks so much, Kushagra
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Old   June 20, 2008, 20:06
Default Re: Steady State Vs Transient answers
  #2
cfd.newbie
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No, we expect more accuracy from transient simulation because it is time marching solution. I am very new to CFD though, it will be interesting if more experienced CFD user can give us insight into this.

Regards
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Old   June 22, 2008, 18:51
Default Re: Steady State Vs Transient answers
  #3
Glenn Horrocks
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Hi,

The difference between a steady state simulation and marching a transient solution to steady state is that the SS simulation ignores many of the cross terms and higher order terms dealing with time. These terms all go to zero in steady state so they don't affect the steady state result. The transient simulation includes all these terms. Usually this means the steady state model has an easier convergence as there are less terms to model and some transient non-linearities are removed, but in a few models these non-linearities help convergence (but this is infrequent).

Glenn Horrocks
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Old   July 2, 2008, 23:25
Default Re: Steady State Vs Transient answers
  #4
Kushagra
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Thanks Glenn for providing useful knowledge about convergence in both type of simulations. But what about quality of results from two type of simulations? Some time steady state simulations are difficult to converge but reducing the false time step by 1 or 2 order, they converge. Does it affect the quality of results?
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Old   July 3, 2008, 01:55
Default Re: Steady State Vs Transient answers
  #5
Glenn Horrocks
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Hi,

A fully converged simulation, run to steady state by either steady state or transient approaches should be the same. The only exception is when "local timescale factor" is used in a steady state run as it can accelerate convergence nicely but as different timescales are used across the domain can cause accuracy problems. As long as a steady state run is run to final convergence with a physical timescale (including auto timescale) then it should be fine.

The timescale is a steady state simulation is like under-relaxation from SIMPLE based solvers. Too high a URF and the simulation diverges, too low and convergence is slow, so you try to fiddle until you get the optimum in the middle somewhere.

Glenn Horrocks
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Old   July 3, 2008, 22:50
Default Re: Steady State Vs Transient answers
  #6
Kushagra
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Thanks so much Glenn.

1) So reducing the 'false time step' is similar to reducing the under-relaxation factor. (Or is it just opposite?)

2) Suppose the residual for Volume Fractions are not getting low and fluctuating around mean values, what should the user try first? Reducing or increasing the 'false time step'?

3) The residence time (volume of domain / flow rate) is 13 second for my multiphase case. what might be a good time scale to start with steady state problem.

Meanwhile, I found some of your, Robin's, Cyclone's replies on this forum about how the steady state solver works. They were really helpful.

Thanks, Kushagra

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Old   July 6, 2008, 22:41
Default Re: Steady State Vs Transient answers
  #7
Glenn Horrocks
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Hi,

1) Be careful thinking of the physical timestep size as a false timestep. False timestepping is a different technique used in SIMPLE based solvers (I think). In CFX, a steady state solver is similar to the transient solver, just with some higher order transient stuff and cross terms removed, and a different residual calculation.

But in basic idea, yes, tuning the URF of a SIMPLE run is similar to tuning the timestep size of a CFX run. There will be an optimum value somewhere between too slow and unstable.

2) http://www.cfd-online.com/Wiki/Ansys...gence_criteria

3) A good starting point is 13 seconds.

Glenn Horrocks
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