# Two-dimensional explosion

We solve the Euler equations for a compressible gas.

``````#include "compressible.h"
``````

We make boundary conditions free outflow.

``````w.y[top]    = neumann(0);
w.y[bottom] = neumann(0);
w.x[left]   = neumann(0);
w.x[right]  = neumann(0);``````

The domain spans $\left[-1:1\right]×\left[-1:1\right]$.

``````#define LEVEL 7

int main() {
origin (-1, -1);
size (2.);
init_grid (1 << LEVEL);
run();
}``````

Initial conditions come from Toro’s book (Riemann Solvers and Numerical Methods for Fluid Dynamics, 3rd Edition, Springer Ed.) Chapter 17 section 17.1.1 are given in terms of density ($\rho$), pression ($p$), velocity ($u$) both at the left and right side of the discontinuity placed at $R=0.4$.

``````event init (t = 0)
{
double R = 0.4 ;
double rhoL = 1., rhoR = 0.125 ;
double VmL = 0.0, VmR = 0.0 ;
double pL = 1.0,  pR = 0.1 ;``````

Left and right initial states for $\rho$, $\mathbf{\text{w}}$ and energy $E=\rho {\mathbf{\text{u}}}^{2}/2+p/\left(\gamma -1\right)$.

``````  foreach() {
double r = sqrt(sq(x) + sq(y));
double p;
if (r <= R) {
ρ[] = rhoL;
w.x[] = w.y[] = VmL;
p = pL;
}
else {
ρ[] = rhoR;
w.x[] = w.y[] = VmR;
p = pR;
}
E[] = ρ[]*sq(w.x[])/2. + p/(gammao - 1.);
w.x[] *= x*ρ[]/r;
w.y[] *= y*ρ[]/r;
}
}

event print (t = 0.25)
{``````

At $t=0.25$ we output the values of $\rho$ and the normal velocity ${\mathbf{\text{u}}}_{n}$ as functions of the radial coordinate.

``````  foreach() {
double r = sqrt(sq(x) + sq(y));
double wn = (w.x[]*x + w.y[]*y)/r;
printf ("%g %g %g\n", r, ρ[], wn/ρ[]);
}``````

For reference we also output a cross-section at $y=0$.

``````  for (double x = 0; x <= 1; x += 1e-2)
fprintf (stderr, "%g %.4f %.4f\n", x,
interpolate (rho, x, 0.),
interpolate (w.x, x, 0.));
}
web trial``````

On quadtrees, we adapt the mesh by controlling the error on the density field.

``````#if QUADTREE
Results are presented in terms of $\rho$ and normal velocity ${u}_{n}$ for Cartesian (7 levels) and adaptive (8 levels) computations. The numerical results compare very well with Toro’s numerical experiments.