On Ubuntu 20.04 LTS, when I run file examples/karman.c, I don’t see any problem with the file f.mp4, but the file vort.mp4 looks differently from the video on this website.

    Before reporting this, I updated both Ubuntu and Basilisk.

    Near the top border I see (dark) red and near the bottom border I see (dark) blue. The width of these regions is comparable to the cylinder diameter. In the middle of the geometry, as the flow advances, there might not be any change.

    Bénard–von Kármán Vortex Street for flow around a cylinder at Re=160

    Animation of the vorticity field.

    #include "embed.h"
    #include "navier-stokes/centered.h"
    // #include "navier-stokes/perfs.h"
    #include "tracer.h"
    scalar f[];
    scalar * tracers = {f};
    double Reynolds = 160.;
    int maxlevel = 9;
    face vector muv[];

    The domain is eight units long, centered vertically.

    int main() {
      L0 = 8.;
      origin (-0.5, -L0/2.);
      N = 512;
      mu = muv;

    When using bview we can interactively control the Reynolds number and maximum level of refinement.

      display_control (Reynolds, 10, 1000);
      display_control (maxlevel, 6, 12);

    We set a constant viscosity corresponding to a Reynolds number of 160, based on the cylinder diameter (0.125) and the inflow velocity (1).

    event properties (i++)
        muv.x[] = fm.x[]*0.125/Reynolds;

    The fluid is injected on the left boundary with a unit velocity. The tracer is injected in the lower-half of the left boundary. An outflow condition is used on the right boundary.

    u.n[left]  = dirichlet(1.);
    p[left]    = neumann(0.);
    pf[left]   = neumann(0.);
    f[left]    = dirichlet(y < 0);
    u.n[right] = neumann(0.);
    p[right]   = dirichlet(0.);
    pf[right]  = dirichlet(0.);

    The top and bottom walls are free-slip and the cylinder is no-slip.

    u.n[embed] = fabs(y) > 0.25 ? neumann(0.) : dirichlet(0.);
    u.t[embed] = fabs(y) > 0.25 ? neumann(0.) : dirichlet(0.);
    event init (t = 0)

    The domain is the intersection of a channel of width unity and a circle of diameter 0.125.

      vertex scalar phi[];
      foreach_vertex() {
        phi[] = intersection (0.5 - y, 0.5 + y);
        phi[] = intersection (phi[], sq(x) + sq(y) - sq(0.125/2.));
      boundary ({phi});
      fractions (phi, cs, fs);

    We set the initial velocity field.

        u.x[] = cs[] ? 1. : 0.;

    We check the number of iterations of the Poisson and viscous problems.

    event logfile (i++)
      fprintf (stderr, "%d %g %d %d\n", i, t, mgp.i, mgu.i);

    We produce animations of the vorticity and tracer fields…

    event movies (i += 4; t <= 15.)
      scalar omega[], m[];
      vorticity (u, omega);
        m[] = cs[] - 0.5;
      boundary ({m});
      output_ppm (omega, file = "vort.mp4", box = {{-0.5,-0.5},{7.5,0.5}},
    	      min = -10, max = 10, linear = true, mask = m);
      output_ppm (f, file = "f.mp4", box = {{-0.5,-0.5},{7.5,0.5}},
    	      linear = false, min = 0, max = 1, mask = m);

    We adapt according to the error on the embedded geometry, velocity and tracer fields.

    event adapt (i++) {
      adapt_wavelet ({cs,u,f}, (double[]){1e-2,3e-2,3e-2,3e-2}, maxlevel, 4);