On Fri, Sep 25, 2020 at 4:29 PM Wolfgang Bangerth <bange...@colostate.edu>
wrote:
> On 9/25/20 3:07 PM, Xuefeng Li wrote:
> >
> > My question is: will the repeated call to function
> apply_boundary_values()
> > inside the loop
> > alter the system_matrix that is meant to be constant (independent of
> iterate u)?
>
> It doesn't change it any further than it was changed the first time
> around.
> But it doesn't do the appropriate modifications: you need the original
> matrix
> for this to work.
>
>
>
> > If the answer is no, then the above code will be fine, I guess.
> >
> > If the answer is yes, I guess I'll need to assemble system_matrix inside
> the
> > loop repeatedly
> > even though it is independent of the iterate u.
>
> Right. Or do what step-26 does -- gosh darn, there seems to be a tutorial
> program for everything ;-) (It's Friday afternoon and I'm already having a
> drink -- the joster in me is shining through...)
>
> Yes, step-26 assembles system_matrix and system_rhs inside the loop
exactly because they depend on time and time_step.
Time and time_step change during the iteration.
For step-26, because system_rhs depends on time and time_step, we need to
assemble system_rhs in the loop. We therefore
need to call apply_boundary_values() inside the loop. Here is my question. *If
system_matrix were independent of time and time_step*
*(such as in the case of uniform time_step), **do we still need to assemble
system_matrix inside the loop? *
--
Stay put, practice social distancing, and be safe!
Best,
--Xuefeng Li, (504)865-3340(phone)
Like floating clouds, the heart rests easy
Like flowing water, the spirit stays free
Loyola University New Orleans
New Orleans, Louisiana (504)865-2051(fax)
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