[ff3d-users] Problem in Solving Laplace with Neumann BC's

[Juzar Thingna]

Hello Everyone,
I'm trying to solve the laplace equation on a rectangular cube. On y min I set the Dirichlet BC's whereas on y max I want to set the value of the co-normal derivative. Since the equation is a laplace equation the solution along the y-direction should be a straight line. Since on the other surfaces the normal derivative = 0 automatically. Here is a copy of my input file: 

vector n=(50,70,50);
vector a=(0,-37,0);
vector b=(25,-7,25);
mesh M=structured(n,a,b);
scene S=pov("ferromagnet_1.pov");
domain O =domain(S,inside(<1,0,0>));
function dv=0.130;
solve(v) in O by M
{
pde(v)
div(grad(v))=0;
v=10.0 on M ymin;
dnu(v)= dv on M ymax;
};
mesh Z = tetrahedrize (O,M);
save(medit,"v_fm_1",v,Z);
save(medit,"v_fm_1",Z);
save(raw,"v_fm_1.dat",v,M);
vector n=(50,70,50);
vector a=(0,-37,0);
vector b=(25,-7,25);
mesh M=structured(n,a,b);
scene S=pov("ferromagnet_1.pov");
domain O =domain(S,inside(<1,0,0>));
function dv=0.130;
solve(v) in O by M
{
pde(v)
div(grad(v))=0;
v=10.0 on M ymin;
dnu(v)= dv on M ymax;
};
mesh Z = tetrahedrize (O,M);
save(medit,"v_fm_1",v,Z);
save(medit,"v_fm_1",Z);
save(raw,"v_fm_1.dat",v,M);

But when I plot my solution as a function of y I notice that there are many discrepancies. I have attaced my plot here. In the vicinity of ymax I get the derivative to be = dv but the solution is not a straight line as expected. I earlier thought it could be because of the roughness of my grid, but even when my grid is finer I have the same problems.
Is there a way to get around this?

Regards,
Juzar Thingna
Center for Computation Science and Engineering
National University of Singapore

v_vs_y.pdf
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