On Sun, Sep 27, 2020 at 10:33 PM Wolfgang Bangerth <bange...@colostate.edu>
wrote:
> On 9/25/20 4:15 PM, Xuefeng Li wrote:
> >
> > 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? *
>
> Yes. The problem is that apply_b_v() modifies the system matrix during its
> operation, but it needs the original matrix to do its operations. That
> means
> that once you've run apply_b_v(), you can't run it again because the
> original
> matrix is no longer available.
>
> Whether you get the original matrix through assembly (expensive) or by
> saving
> the unmodified state from before you called apply_b_v() the first time
> around
> (cheap) does not matter.
>
> Thanks a lot. The above answered my original question!!! Have a great week!
--
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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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