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Turn off step size tuning in parametric continuation?
Posted Jun 4, 2011, 9:21 p.m. EDT Studies & Solvers Version 4.3b 4 Replies
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Instead, it changes the parameter by 0.2, and if it can't converge very very easily it changes the parameter back to take smaller steps of 0.05, etc. I want to turn this behavior off.
There's an option called "Tuning of step size". But if the box is unchecked, COMSOL doesn't care, it tunes the step size anyway.
Then I tried something else: I checked the box for "Tuning of step size", and typed in the parameters: "Initial step is 0.2, Minimum step size is 0.2, Maximum step size is 0.2". Did that work? Nope, COMSOL doesn't care, it still changes the step size to 0.05, etc.
The annoying thing is that COMSOL is so eager to change the step size that it does it prematurely! I'm using Newton's method and I set it to give up only after 25 iterations. But COMSOL doesn't care, it does only 5 or 6 iterations, and then just gives up and change the step size instead.
My model, it turns out, converges more reliably by using more iterations, than by using a small step size, but COMSOL does not seem to give me that choice!
Is there a trick or option I'm missing? Has anyone else had this issue??
Thanks in advance!
Hello David964
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this seems to be a very old posting, but I am having the observations
with the 4.3 versions. Did anyone find a good way to the CMP *not* do these
intermedieate steps.
Thanks in advance
Regards
Jens
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I'm not quite sure if I understand your question...
but wouldn't a Parametric Solver (right click on the Study node) solve your problem?
try playing with the Parametric Solver option (turn it Off and Automatic and see which option suits you).
If you want something fancier, like going back and trying a smaller step in case the model dont converge, then I suggest you try using Livelink for Matlab. This module will let you write scripts that can handle those tasks.
--
cheers,
Felipe BM
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The Minimum step size is respected as long as the solver converges well. The automatic step-size algorithm detects poor convergence speed and other "problems" and reduces the step size. For a problem that never reaches quadratic convergence speed (for example, due to an incomplete Jacobian or too crude tolerance) this algorithm is not optimal.
We are looking into improving this behavior for future versions. In 4.3b, you can add a "dummy parameter" to the parametric solver; doing so will turn off the continuation.
Best regards,
Magnus Ringh, COMSOL
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thanks for replies.
Felipe, to better understand the problem: I want fewer paramtric step. Say I want to
solve only for two paramenter values. When set the parameters and use a simple stationary solve it converges nicely for any parameter. But when I start a parametric solve (be it "parametrc" or "continuation") the algorithm decides in some models to do many intermediate steps, which I believe to be unnecessary
And Magnus' trick avoids thes intermediate steps.
For the records and the help of others: I added a parameter, that is never used in the model, to the list of "continuation parameters" with one value and use the sweep type "all combination".
Thanks
Jens