[ff3d-users] Acousting Scattering and ilufact

[Giovanni Giorgi]

Dear ff3d users,

 I am trying to solve with ff3d a time harmonic scattering problem 
in the case of an impenetrable sound soft obstacle. And I need help...

Being dD the boundary of the obstacle and being S a surface 
posed enough far from it, the problem is stated as follows:

Laplacian(US)+k^2*US=0    (between dD and S)
US=-UI                                  (on dD)
dn(US)=-i*k*US                    (on S)

where k is a constant, UI a known function and US our unknown. 
(In attachment there are the .ff and .pov files I am using). 

My problem is that, when I arrive to solve the linear system, cg and bicgstab 
don't 
converge (the problem could be not coercive). gmres seems to minimize the 
residual but in an extremely slow way and all the other functions (fgmres and 
ilufact) seem not to work... (In ilufact case, ff3d gives me an "unexpected 
error" of 
this type: KrylovSolver.cpp:139: not implemented).

In particular, I would be very interested in being able to use ilufact: before 
passing to the 3D problem, I solved the 2D one using Freefem++. Here, methods 
as ilufact or Crout are available and work very well for this problem (I can't 
use 
Freefem++ also for the 3D problem because of the grid dimension which is too 
big). 

My questions are: is it possible to use ilufact in this case? And, if not, one 
way 
could be to export matrices: how to do it in ff3d?

Thanks a lot in advance. 
Best regards, 

Giovanni Giorgi

sfera.ff
Description: sfera.ff

sferedistinte.pov
Description: sferedistinte.pov