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Old   May 7, 2008, 00:35
Default help needed with transient problem
  #1
Tim
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Hello,

I have been working with ansys cfx awhile now, and I still have no idea It is very frustrating. I am running a series of multiphase isothermal transient problems. Typically I have air as my continuous phase with solid dispersed particles as my second phase. I use the kinetic theory, which I understand is very unstable in the solver. Depending on my geometry, sometimes I obtain a solution and sometimes the solver crashes. When it does crash, I get the same error EVERY time;

ERROR #001100279 has occurred in subroutine ErrAction. | | Message: | | Floating point exception: Overflow

When I track back through the solver out file, there doesn't seem to be any reason why it crashes. The u,v,w velocities for both phases display OK although sometimes P-vol only has ok. I have had some success with decreasing the timestep, but it is just trial and error. I don't understand why I am picking which value. This method is also time consuming as I decrease timestep, let simulation run until it crashes, decrease timestep again, etc etc.

Also, when I open the last outputted transient results for a crashed simulation, I don't really know what to look for. The velocity and pressure distributions seem ok.

Does anyone have any general advice?

Thanks, Tim.
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Old   May 7, 2008, 07:44
Default Re: help needed with transient problem
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Andrew
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Depending on the models you use, the behavior can be very unstable. One of the current simulations I'm running has taken several attempts to get it working. Best advice I have:

- Use a high quality, reasonably fine hexahedral mesh. - Be sure y+ is good enough to adequately resolve boundary flow - Start from a very well-converged initial state, if applicable. I use RMS 1E-5 as my convergence criteria. - At the beginning of the transient, use very small time steps with very tight convergence criteria. I use RMS Courant number < 0.5 with 1E-7 convergence criteria, max 10 coefficient loops - Increase time step manually and loosen convergence criteria a little after 20 or so time steps. - Make sure your Courant numbers aren't too high. Large time step size can lead to unstable behavior. - Select the option in Pre to output residuals. Regions with high residuals can indicate mesh quality problems.
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Old   May 7, 2008, 20:12
Default Re: help needed with transient problem
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zhiyuan
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First steady,then transient,i think.
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Old   May 8, 2008, 01:17
Default Re: help needed with transient problem
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Tim
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Hi Andrew,

Thanks for your response, you have some good points in there I have taken on board. A method I have trialled today is that I have solved the problem using the upwind scheme (1st order) and then I have used the solution for this as initial conditions for solving the high resolution scheme. This has worked today for a couple of problems where trying to solve the problem using the high resolution scheme straight away (with zero velocities in domain for initialisation) has not worked. I guess this leads to the conclusion that initial guesses are very important.

Tim.
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