[petsc-users] Convergence Criteria?

[Alan Wei]

Oh, Thanks, Matt,
   I got a little bit confused, since in the code, it described:
         div \rho grad u = f,  0 < x,y < 1,
   But you said, the solver solves -\Delta u = f  (Eq.1), which means:
   for example, to solve a equation like Delta p = 1, I should put rhs = -1
= f in (Eq.1) in the code, therefore -\Delta u = -1, which, then, will give
me a good result for Delta p = 1, is that true?

thanks in advance,
Alan

On Wed, Sep 21, 2011 at 11:19 AM, Matthew Knepley <knepley at gmail.com> wrote:

> On Wed, Sep 21, 2011 at 4:16 PM, Alan Wei <zhenglun.wei at gmail.com> wrote:
>
>> However, why signs for v[] in ComputeMatrix, which contains the values of
>> a row of the matrix. They all have a negative signs. Therefore, I got
>> confused which equation does this program solve for:
>> 1)  u[j][i] = (u[j+1][i] + u[j-1][i] + u[j][i+1] + u[j][i-1] - rhs *
>> dx*dy)/4
>> or
>> 2)  4u[j][i] - u[j+1][i] - u[j-1][i] - u[j][i+1] - u[j][i-1] + rhs * dx*dy
>> = 0
>>
>
> The Laplacian is a negative definite operator, so we usually solver -\Delta
> u = f since that
> is a positive definite problem.
>
>   Thanks,
>
>      Matt
>
>
>> thanks,
>> Alan
>>
>>
>> On Wed, Sep 21, 2011 at 8:22 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>
>>>
>>> On Sep 20, 2011, at 10:47 PM, Alan Wei wrote:
>>>
>>> > Dear Dr. Smith,
>>> >     I figure out this problem. Actually, I made my own RHS, but I did
>>> not multiply them by the volume (dx*dy).
>>> >     However, I met another problem. All values calculated from this
>>> example are exactly opposite to values from my own code. I wonder if the RHS
>>> I input by ComputeRHS are the really RHS or -1.*RHS?
>>>
>>>    We do not change the sign of the right hand side.
>>>
>>>   Barry
>>>
>>> >
>>> > thanks in advance,
>>> > Alan
>>> >
>>> > On Mon, Sep 19, 2011 at 8:43 PM, Barry Smith <bsmith at mcs.anl.gov>
>>> wrote:
>>> >
>>> > On Sep 19, 2011, at 6:25 PM, Alan Wei wrote:
>>> >
>>> > > Dear folks,
>>> > >     I hope you guys are having a nice day.
>>> > >     I'm reading the /src/ksp/ksp/examples/tutorials/ex29.c.html and
>>> wonder how to set up the convergence criteria?
>>> >
>>> >   -ksp_rtol 1.e-10 for example
>>> >
>>> >   Run with -ksp_monitor_true_residual -ksp_converged_reason
>>> >
>>> >
>>> > > Currently I use it as a poisson solver to solve a Poisson Equation
>>> with three direction Neumann BC's and one direction Diriechlet BC's. It
>>> seems very bad on it.
>>> >
>>> >    Hmm, multigrid should likely converge well. Are you sure you've set
>>> the BC's correctly?
>>> >
>>> >   Barry
>>> >
>>> > >
>>> > > thanks in advance,
>>> > > Alan
>>> >
>>> >
>>>
>>>
>>
>
>
> --
> 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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