Dear Floriane,

> On 5 Nov 2018, at 09:18, Floriane Gidel [RPG] <mmfg@leeds.ac.uk> wrote:
>
> Dear all,
>
> I went back to the problem with one disk because I realized it only converges when initialized with the exact solution (in which case I made a test of convergence that confirmed convergence to the exact solution).
> When I initialise it with u=0 both in the plane and disk, then the solver diverges (DIVERGED_MAX_IT).
> My solver is defined as follows:
>
> V = V_p*V_d
> u = Function(V)
> u_d, u_p = split(u)
> v_d, v_p = TestFunction(V)
>
> a = dot(grad(u_p),grad(v_p))*I_p*dx + dot(grad(u_d),grad(v_d))*I_d*dx + (u_d('+')-u_p('+'))*(v_d('+')-v_p('+'))*4*avg(I_d)*avg(I_p)*dS
> solve(a==0, u, bcs=[bc_left, bc_right, bc_disk_only, bc_plane_only])
>
> where bc_disk_only and bc_plane_only force nodes of V.sub(0) to be zeros in the plane and nodes of V.sub(1)  to be zeros in the disk, respectively, and are defined as in Tomasz fluid/structure code, using continuous indicators I_cg_d and I_cg_p (while I_d and I_p are discontinuous).
> The solver above only converges when u_d and u_p are initialized with the exact analytical solution, or when I replace I_d and I_p in the first two integrals by their continuous forms I_cg_d and I_cg_p (but then the obtained solution is wrong).
>
> This is quite a simple equation so I don't know what I'm doing wrong. Does anyone understand what's wrong ?

Let's start by confirming that you can solve this equation with an exact (direct) solver.

If you say:

parameters = {"snes_type": "ksponly",
              "ksp_type": "preonly",
              "pc_type": "lu",
              "pc_factor_mat_solver_type": "mumps",
              "ksp_converged_reason": True,
              "snes_converged_reason": True}

and run with:

solve(..., solver_parameters=parameters)

can you obtain a solution?

Cheers,

Lawrence


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