PCNN-preconditioner and "floating domains"

[Barry Smith]

    As we have said before the BNN in PETSc is ONLY implemented for a  
scalar PDE with
a null space of the constant functions. If you shove in linear  
elasticity it isn't really going to work.

    Barry

On Jun 23, 2009, at 8:56 AM, Andreas Grassl wrote:

> Hello again,
>
> the issues from my last request regarding VecView and data  
> distribution among
> the different processors are solved, but I'm experiencing still  
> great problems
> on the performance of the actual preconditioner.
>
> I have an implementation of the BNN-algorithm in Matlab from a  
> previous project
> which is performing very well (about 5 iterations vs. 200 for plain- 
> cg) for a
> long linear elastic beam fixed at one end and loaded at the other end,
> discretized with solid cubic bricks (8 nodes, 24 DOF's). condition  
> of the
> Matrix: 1.5e7
>
> I now modeled a similar beam in DIANA (a bit shorter, less elements  
> due to
> restrictions of DIANA-preprocessor) and tried to solve with PETSc- 
> solver.
>
> The condition of the Matrix is of the same magnitude: ~3e7 (smallest  
> singular
> value: ~1e-3, largest sv: ~4e4), number of iterations for plain-cg  
> seems
> reasonable (437), but for the preconditioned system I get completely  
> unexpected
> values:
>
> condition: ~7e12 (smallest sv: ~1, largest sv: ~7e12) and therefore  
> 612
> iterations for cg.
>
> The beam is divided in 4 subdomains. For more subdomains ksp ran out  
> of
> iterations (Converged_Reason -3). I can imagine this is a problem of  
> properly
> setting the null space, because only the first subdomain is touching  
> the
> boundary, but I have no idea how to specify the null space. So far I  
> didn't
> regard this issue at all.
>
> Do I have to define a function which calculates the weights of the  
> interface
> DOF's and applies in a some way to create an orthonormal basis? How  
> do I realize
> that? Is there anywhere an example?
>
> Cheers,
>
> ando
>
> -- 
> /"\                               Grassl Andreas
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