time derivatives in terms

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The topic is discussed for Stokes-like terms, the same holds, however, for
Biot-like terms, piezo-coupling, and other terms too.

As dw_grad, dw_div, d_div are now all treated by dw_stokes, what to do with
dw_grad_dt? It is still the same term, just the pressure variable is
differentiated w.r.t. time. There are several possibilities:
1. dw_stokes_dpdt, dw_stokes_dudt terms
2. enrich the equation parse to allow:
   dw_stokes( v, dp/dt ), dw_stokes( du/dt, q ), possibly with one more
argument saying what kind of numerical derivative is used (backward
(default) or forward difference).
3. ?

Note that variables already support history, so option 2. is feasible.

Opinions?

Original issue reported on code.google.com by robert.c...@gmail.com on 14 Oct 2008 at 2:44

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Do what you think is the best. I don't have an opinion here, as I don't use 
those terms.

Original comment by ondrej.c...@gmail.com on 14 Oct 2008 at 2:57

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Option 2. is in, see http://hg.sympy.org/sfepy/rev/186603f3c0aa

Test it, if you can, please, by running '$ ./simple.py input/time_poisson.py'

Original comment by robert.c...@gmail.com on 15 Oct 2008 at 2:58

• Changed state: Fixed

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Migrated to http://github.com/sfepy/sfepy/issues/63

Original comment by robert.c...@gmail.com on 30 Jan 2012 at 10:25

• Changed state: Migrated