Re: [Gfs-devel] scaling issue with GfsVariablePoisson

[Vladimir K. <vi...@cf...>]

a fix has been found:

in variable_poisson_event, we need to reduce the units of the RHS (aka 
div) by 2 with the reference to the variable to be solved for:

gfs_function_set_units (p.f, v->units - 2.);

The new call comes between

     DivData p = { GFS_VARIABLE_FUNCTION (event)->f, div };

--> new call to gfs_function_set_units (p.f, v->units - 2.);

and

     gfs_domain_traverse_leaves (domain, (FttCellTraverseFunc) 
rescale_div, &p);

Cheers,

Vladimir.

On 10/30/2014 11:13 AM, Vladimir Kolobov wrote:
> Dear Gerris Developers,
>
> There is an issue with GfsVariablePoisson with non-unity domain scaling.
>
> If L is set to something different from 1 in PhysicalParams, the
> solutions of the Poisson equations in the attached script differ by a
> factor of L*L.
>
> This is because gfs_function_value in calculating RHS is used in
> GfsVariablePoisson which returns adimensional_value.
>
> If one sets
>
> VariablePoisson { istep = 1 } Phi Div*0.1*0.1
>
> the solutions coincide.
>
> This just a note to those using GfsVariablePoisson that the divergence
> (RHS of the Poisson equation) needs to be scaled with L*L.
>
> Cheers,
>
> Vladimir
>
> ====================================
>
> 1 0 GfsPoisson GfsBox GfsGEdge {} {
>     Time { iend = 10 }
>
>     Refine 5
>
>     PhysicalParams { L = 0.1 }
>
>     Init {} {
>       Div = {
>         return 1.;
>       }
>     }
>
>     VariablePoisson { istep = 1 } Phi Div
>
>     OutputSimulation { start = end } end-SOLVER.gfs
> }
> GfsBox {
>      left =   Boundary { BcDirichlet P 0. BcDirichlet Phi 0. }
>      right =  Boundary { BcDirichlet P 0. BcDirichlet Phi 0. }
>      top =    Boundary { BcNeumann P 0. BcNeumann Phi 0. }
>      bottom = Boundary { BcNeumann P 0. BcNeumann Phi 0. }
> }
>
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