[petsc-users] Cannot iterate well when using Newton iteration of SNES

[David Jiawei LUO LIANG]

Coincidentally, Matt. I am gonna write the 2d heat dynamic problem. 


Now I still not that much understand how DM mesh works.


It is a good chance to learn DM by studying your example. 


Hope you the best!










David Jiawei LUO LIANG



南方科技大学/学生/研究生/2024



广东省深圳市南山区学苑大道1088号




 
 
 
------------------ Original ------------------
From:  "Matthew Knepley"<knepley at gmail.com>;
Date:  Thu, Nov 21, 2024 11:37 PM
To:  "Jed Brown"<jed at jedbrown.org>; 
Cc:  "David Jiawei LUO LIANG"<12431140 at mail.sustech.edu.cn>; "petsc-users"<petsc-users at mcs.anl.gov>; 
Subject:  Re: [petsc-users] Cannot iterate well when using Newton iteration of SNES

 

One more suggestion email. I solve the linear version myself in ts/tutorials/ex45.c

  Thanks,


     Matt


On Thu, Nov 21, 2024 at 10:35 AM Jed Brown <jed at jedbrown.org> wrote:

You should add VecZeroEntries(f) near the top of your FormFunction (it's currently accumulating into whatever was there last) and MatZeroEntries(B) to FormJacobian.
 
 I reduced to nElem = 5 for ease of viewing. With these changes, I see quadratic convergence but the problem is still nonlinear. To explore further, consider using these diagnostics
 
 ./SNES_heat -{snes,ksp}_monitor -{snes,ksp}_converged_reason -snes_linesearch_monitor -ksp_view_mat
 
 with and without -snes_fd.
 
 For readability, I would suggest consistency in "u" vs "x".
 
 "David Jiawei LUO LIANG"        <12431140 at mail.sustech.edu.cn> writes:
 
 > I am using the Newton iteration to solve a nonlinear 1D heat equation problem by using FEM.
 >
 >
 > I attached my source code named "SNES_heat.cpp"&nbsp;
 >
 >
 > when I run the code
 >
 > &nbsp; 0 SNES Function norm 1.206289245288e+01
 >  
 > &nbsp; 1 SNES Function norm 7.128802192789e+00
 >  
 > &nbsp; 2 SNES Function norm 6.608812909525e+00
 >
 >
 >
 > you can find that it only iterate 3 steps, and then do all the function evaluation and finally just stop the program.&nbsp;
 >
 >
 > I think it is not reasonble. I check my code, it is correct if I set it as a linear problem. it means my Jacobian and Residual function is correct.
 >
 >
 > But when I set it as a nonlinear, the residual seems reduces as not expected.&nbsp;
 >
 >
 > I doubt that whether my understanding of the newton iteration is different from SNES's newton iteration process.
 >
 >
 >
 >
 >
 >
 >
 >
 > David Jiawei LUO LIANG
 >
 >
 >
 > 南方科技大学/学生/研究生/2024
 >
 >
 >
 > 广东省深圳市南山区学苑大道1088号
 >
 >
 >
 >
 > &nbsp;
 



-- 
What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
-- Norbert Wiener


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