/**
# Radial/cylindrical coordinates
This implements the radial coordinate mapping illustrated below.
![Radial coordinate mapping](figures/radial.svg)
It works in 1D, 2D and 3D. The 3D version corresponds to cylindrical
coordinates since the $z$-coordinate is unchanged.
The only parameter is $d\theta$, the total angle of the sector. */
double dtheta = pi/3.;
/**
For convenience we add definitions for the radial and angular
coordinates $(r, \theta)$. */
map()
{
double r = x, theta = y*dtheta/L0;
NOT_UNUSED(r); NOT_UNUSED(theta);
}
event metric (i = 0) {
/**
We initialise the scale factors, taking care to first allocate the
fields if they are still constant. */
if (is_constant(cm)) {
scalar * l = list_copy (all);
cm = new scalar;
free (all);
all = list_concat ({cm}, l);
free (l);
}
if (is_constant(fm.x)) {
scalar * l = list_copy (all);
fm = new face vector;
free (all);
all = list_concat ((scalar *){fm}, l);
free (l);
}
/**
The area (in 2D) of a mapped element is the area of an annulus
defined by the two radii $r-\Delta/2$ and $r+\Delta/2$, divided by
the total number of sectors $N=2\pi L0/(d\theta\Delta)$, this gives
$$
\frac{\pi [(r + \Delta / 2)^2 - (r - \Delta / 2)^2]}{2 \pi L 0 / (d \theta
\Delta)} = \frac{2 \pi r \Delta}{2 \pi L 0 / (d \theta \Delta)} = \frac{rd
\theta}{L 0} \Delta^2
$$
By definition, the (area) metric factor `cm` is the mapped area divided
by the unmapped area $\Delta^2$. */
scalar cmv = cm;
foreach()
cmv[] = r*dtheta/L0;
/**
It is important to set proper boundary conditions, in particular
when refining the grid. */
cm[left] = dirichlet (r*dtheta/L0);
cm[right] = dirichlet (r*dtheta/L0);
/**
The (length) metric factor `fm` is the ratio of the mapped length of
a face to its unmapped length $\Delta$. In the present case, it is
unity for all dimensions except for the $x$ coordinates for which it
is the ratio of the arclength by the unmapped length $\Delta$. We
also set a small minimal value to avoid division by zero, in the
case of a vanishing inner radius. */
face vector fmv = fm;
foreach_face()
fmv.x[] = 1.;
foreach_face(x)
fmv.x[] = max(r*dtheta/L0, 1e-20);
}