[petsc-users] nonzero prescribed boundary condition

[Tian(ICT)]

First, thanks again, the issue was gone.

I just followed up with some test results.
I have tested SNES using one finite element for a geometric large 
deformation problem.
Those are just the very early test results so they may be not telling what 
happened exactly.
For the displacement controlled load, I found that convergence is much 
slower than that of force loading.
Even worse, linear search is so sensitive to the displacement increment and 
diverged no matter what the increment size was used (too small incremnt also 
led to diverged soloution (-6 reason), trust region works well in the sense 
of not sensitive to the displacement increment, but during each load step, 
it requires around ten to several tens of Newton interations whereas for the 
force loading case and the almost same amount of deformation, this is 
normally 3. This is against my expectation. Any hint?

Rong

----- Original Message ----- 
From: "Barry Smith" <bsmith at mcs.anl.gov>
To: "PETSc users list" <petsc-users at mcs.anl.gov>
Sent: Tuesday, May 10, 2011 10:22 AM
Subject: Re: [petsc-users] nonzero prescribed boundary condition



On May 9, 2011, at 9:15 PM, Tian(ICT) wrote:

> Dear Barry, Thanks a lot for quick answering.
> I checked the development documents and found the new version of 
> MatZeroRows() does support the nonzero prescribed boundary conditions.
>
> I followed up with more details.
> I am using Petasc 2.3.3. to solve a nonlinear problem, e.g. using SNES 
> solvers.
> I used a displacement-controlled load (as this type of loading works well 
> for all cases).
> This is the reason the nonzero prescribed boundary came up.
>
> In FormJacobian, I modified Jacobian and residual to satisfy the nonzero 
> prescribed boundary.
> In FormFunction, I modified the solution to the known solution(this should 
> not be necessary as the modified Jacobian and rhs should give the 
> prescribed solution also)

   You should not do it this way. See below.
>
> Now I found another issue, no matter if I prescried the solution or not in 
> FormFunction,
> SNES solver always call FormFunction and never call FormJacobian.

   The only reason it would not call FormJacobian is if decided that the 
residual norm was small enough before any Newton steps; for example if the 
FormFunction() computed exactly the zero function initially. When you run 
with -snes_monitor -ksp_monitor what does it print for residual norms.

> Of course the solver finally diverged or converged to a zero solution.
>
> So my quick follow up question is How a displacement-controled load is 
> done corrently in Petsc 2.3.3?

   To do it in 2.3.3 simply have for those components of F() the formula 
F_i = x_i - givenvalue_i and in your Jacobian just use MatZeroRows() for 
those rows

   We strongly urge you to upgrade to the latest PETSc before doing anything 
further.


    Barry

>
> Rong
>
> ----- Original Message ----- From: "Barry Smith" <bsmith at mcs.anl.gov>
> To: "PETSc users list" <petsc-users at mcs.anl.gov>
> Sent: Tuesday, May 10, 2011 9:31 AM
> Subject: Re: [petsc-users] nonzero prescribed boundary condition
>
>
>
>  In petsc-dev http://www.mcs.anl.gov/petsc/petsc-as/developers/index.html 
> we have modified the calling sequence for MatZeroRows() so that it can 
> automatically adjust the appropriate right hand side values for the zeroed 
> rows to support zero or non-zero prescribed boundary conditions easily.
>
>   Barry
>
> On May 9, 2011, at 8:18 PM, Tian(ICT) wrote:
>
>> Dear all,
>>
>> I got this question long ago and searched the prior posting but did not 
>> find the solution.
>> The question is about nonzero prescribed boundary condition.
>> My understanding is that MatZeroRows() works only for zero prescribed 
>> value, not non-zero value.
>> For the non-zero values, we have to remove the rows associated with the 
>> boundary, but this
>> will lead to a zero dignal and accordingly the rows in r.h.s should also 
>> be removed.
>> My question is that does MatZeroRows() also works for nonzero prescribed 
>> boundary and if so how to do it simply?
>>
>> Rong
>
>