Re: [deal.II] Function apply_boundary_values()

2020-09-28 Thread Xuefeng Li 
On Sun, Sep 27, 2020 at 10:33 PM Wolfgang Bangerth 
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!

-- 
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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Re: [deal.II] Function apply_boundary_values()

2020-09-27 Thread Wolfgang Bangerth 

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.


Best
 W.


--

Wolfgang Bangerth  email: bange...@colostate.edu
   www: http://www.math.colostate.edu/~bangerth/

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Re: [deal.II] Function apply_boundary_values()

2020-09-25 Thread Xuefeng Li 
On Fri, Sep 25, 2020 at 4:29 PM Wolfgang Bangerth 
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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Re: [deal.II] Function apply_boundary_values()

2020-09-25 Thread Jean-Paul Pelteret 
To add to what Wolfgang already said, there’s also the 
constrained_linear_operator() 

 that may be of use in this scenario.

Best,
Jean-Paul

> On 25 Sep 2020, at 23:29, Wolfgang Bangerth  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...)
> 
> Cheers
> W.
> -- 
> 
> Wolfgang Bangerth  email: bange...@colostate.edu
>   www: http://www.math.colostate.edu/~bangerth/
> 
> -- 
> The deal.II project is located at http://www.dealii.org/
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Re: [deal.II] Function apply_boundary_values()

2020-09-25 Thread Wolfgang Bangerth 

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...)


Cheers
 W.
--

Wolfgang Bangerth  email: bange...@colostate.edu
   www: http://www.math.colostate.edu/~bangerth/

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[deal.II] Function apply_boundary_values()

2020-09-25 Thread Xuefeng Li 
Hi, there!

I have a question concerning the use of the function 
apply_boundary_values():

   MatrixTools::apply_boundary_values(boundary_values,
 system_matrix,
 solution,
 system_rhs);

Iteration is used to find a numerical solution. We know that
  system_matrix is independent of the iterate and remains unchanged during 
the iteration
  system_rhs depends on the iterate so it changes during the iteration 

Below is the pseudo-code of the program.

  initialize iterate u;
  assemble system_matrix; //Just once
  assemble system_rhs; //1st time
  call apply_boundary_values(); //Apply bdry 1st time
  while ( not converged ) {
solve FEM to get new iterate u;
assemble system_rhs; // repeated 
call apply_boundary_values(); // repeated
  }

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)?

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.

Thanks for any assistance.

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