GMS, SMS, and WMS User Forum

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Dear John:

Is there any clue to choose the time step, based on Courant number or other for ADCIRC? When I run a "normal" ADCIRC case I have not problems, usually; but when I use drying and wetting, in general I do. Time step should be smaller in that case, right?, But how much?.

Best regards,

Jose M.

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Jose,

SMS currently doesn't have any tools to assist in the selection of a time step, although it would be an interesting area to investigate in the future. You may be interested in reading the paper "Assessment of ADCIRC's Wetting and Drying Algorithm" by J.C. Dietric, R.L. Kolar, and R.A. Luettich available at http://aos.princeton.edu/WWWPUBLIC/PROFS/adcirc_report.pdf. It includes several examples and the maximum stable time step for a given domain with and without wetting and drying.

Linear Problem 1

1. No wetting / drying - 60 second time step
2. Wetting / drying - 55 second time step (decrease of 8.3%)

Linear Problem 2

1. No wetting / drying - 50 second time step
2. Wetting / drying - 15 second time step (decrease of 70%)

I will see if anyone on the ADCIRC list has ideas.

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Jose,

The following is from http://seahorsecoastal.com/wiki/doku.php?i...irc_users_guide

Q: I have made an ADCIRC mesh for my area of interest, but the resulting ADCIRC simulation is unstable, even with a very small time step. What should I do?

A: Here are some places to start:

1. Can you compute the CFL parameter for each of your elements and ensure that it is less that 1 globally?
2. Can you compute the element quality (a measure of how 'equilateral' the element is) for each element? Low quality elements (even just one or two; I learned the hard way) can give you problems.
3. Try running a test simulation with the nonlinearity / wetting-drying features turned off and then bring them back in (presuming you get a stable run) one at a time.
4. You may be able to get a more stable run by adjusting tau0 (the weighting between the primitive equation and the wave equation in the overall continuity equation). If you are running version 47, you can use the built-in time varying tau0, or for earlier versions, you can manually adjust tau0. For detailed information on how to do this, please see the documentation of tau0 (in the fort.15 file) in the Official ADCIRC Users Manual at http://www.adcirc.org.

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Thanks, John. Those suggestion are very useful for me.

Jose

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