Split Function/Accessing components

General Discussion
1

Hi all,

I am working on a code to solve the Cahn-Hilliard equation, and I am having trouble trying to split the solution function u into its phi and mu components for updating the Newton loop.

I have the mesh and functions declared as this:

// Mesh and function space

int h = 64;

mesh Th = square(h, h);

fespace Vh(Th, P1);

fespace ME = Vh*Vh;

// Define functions

Vh c, c0, dc, dmu;

Vh psi, v;

And the inside of the Newton loop as this:

// Assemble

matrix A = Lhs(ME, ME);

real[int] b = Rhs(0, ME);

real[int] sol = A^-1 * b;

dc[] = sol(0:Vh.ndof - 1);

dmu[] = sol(Vh.ndof:2*Vh.ndof - 1);

c[] -= dc[];

mu[] -= dmu[];

I have also tried:

//Extract solution blocks: ucur = [dmu; dc]

int nd = Vh.ndof;

for (int i=0; i<nd; ++i) dmu[i] = ucur[i];

for (int i=0; i<nd; ++i) dc[i] = ucur[nd+i];

c = c - dc;

mu = mu - dmu;

However, something seems to go wrong with the updates in subsequent Newton Iterations. Particularly, c (phi) does not appear to be updating at all. Thanks in advance.

2

To get the correct components you have to declare an unknown in the space ME

ME [csol,musol];
csol[]=sol;// csol[] represents all the components of the solution [csol,musol]
dc=csol;
dmu=musol;