[petsc-users] Scaling GMRES residual

[Colin McAuliffe]

Ok thanks, I can see how introducing units to the residual is  
necessary, but when would it be necessary to introduce units to the  
state? I would imagine this would only be needed if a numerically  
differentiated jacobian was being used.

Thanks
Colin

Quoting Jed Brown <jedbrown at mcs.anl.gov>:

> On Tue, Jun 26, 2012 at 2:31 PM, Colin McAuliffe <cjm2176 at columbia.edu>wrote:
>
>> Hello,
>>
>> I would like to use GMRES for a monolithic solution of a multiphysics
>> problem. There are drastically different units in the problem and so the
>> residual norm is dominated by the terms with the largest units. Is there a
>> way to tell petsc to scale the residual before taking the norm to avoid
>> this?
>>
>
> You can "fix" it with preconditioning, but then you can only use the
> preconditioned residual. You should really fix the units when formulating
> the problem. (This does not require formal nondimensionalization, just
> introduced units so that state and residuals are of order 1.)
>



-- 
Colin McAuliffe
PhD Candidate
Columbia University
Department of Civil Engineering and Engineering Mechanics