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November 2, 2017, 09:34 |
Energy residual changing between two values
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#1 |
New Member
Join Date: Oct 2017
Posts: 26
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Hello all,
I am simulating combustion in a flare. For this I am using the transient solver, the solution has a hard time converging in the steady state solver. But here is the problem: My energy residual keeps switching between two values. If I change the timestep the values change, but it still switches between two values(so for a timestep of 1e-5s it switches between 1,311e-4 to 3,843e-4 and with a timestep of 1e-6 it switches between 1,423e-5 to 4,562e-5). If I reduce the residual far enough (0,5 or lower) it gets stuck on one value. How do I solve this? Thanks! |
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November 2, 2017, 11:45 |
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#2 |
Super Moderator
Alex
Join Date: Jun 2012
Location: Germany
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Are you sure that you are not just seeing the normal convergence behavior of a transient simulation? High residuals at the beginning of a time step that are then reduced to lower values during the inner iterations...
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November 3, 2017, 01:13 |
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#3 |
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Maybe... I'm just wierded out because it starts jumping between two values after some iterations. For each timestep the same values!
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November 3, 2017, 03:52 |
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#4 |
Super Moderator
Alex
Join Date: Jun 2012
Location: Germany
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You can upload a screenshot if you want.
Until then I suspect the residuals look like this. |
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November 3, 2017, 04:01 |
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#5 | |
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Quote:
It converges a little bit, and then it starts changing between the same two values in one timestep. So basically it starts to oscillate between two set values untill the maximum iterations/timestep is reached. And then it does it again for the next timestep. |
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November 3, 2017, 04:33 |
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#6 |
Super Moderator
Alex
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A picture is worth a thousand words...
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November 3, 2017, 05:09 |
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#7 |
New Member
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It looks worse than it is. The first few timesteps look like the image you uploaded, it became so irregular after the spark ignition kicked in. I could not replicate the "jumping between two set values", but this is definitly how it looked. What do you think? |
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November 3, 2017, 05:26 |
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#8 |
Super Moderator
Alex
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Looks fine to me. The time step size might be too high which causes a high level for the continuity residual and poor convergence in general.
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November 3, 2017, 05:28 |
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#9 |
New Member
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I tried decreasing the time step to 1e-8... Even then it does not converge good. I also changed the relaxation factors, no joy there either. Maybe it's so unstable that normal convergence cannot be achieved?
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April 21, 2019, 05:00 |
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#10 |
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D.Sarangi
Join Date: Apr 2019
Location: Chennai
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Hi
I am also facing same type of problem.Did u get the solution!!! Thanks |
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