English: Lattice-Boltzmann simulation (in 2D) of flow round a disc (radius 20 lattice sites) at a Reynolds number of 100 (using diameter as lengthscale). Around a reduced time of 10, the instability studied by von Karman and others sets in and the wake becomes unstable and starts to oscillate up and down every few reduced time units. It sheds vortices downstream, with alternate vortices shed at the top and bottom of the obstacle to flow. This pattern of vortex shedding forms a "von Karman street" of vortices seen in the second half of this movie.

A von Karman street is characteristic of intermediate, around 100, values of Reynolds numbers. The curves shown are streamlines, which are tangent curves to the flowfield at each point. Vortices show up as loops in the streamlines. Note that the loops/vortices that detach from the top and bottom halves of the disc rotate in opposite directions.

Technical details: Simulation box is nx=420 (flow direction) by ny=140 lattice sites, Zou-He BC at left (entrance) sets flow speed to 0.04 in LB units, kinematic viscosity 0.012. Lattice Boltzmann code modified version of Palabos (U Geneva) Python LB code. PBCs along vertical axis and note that for convenience simulation box is quite small so there will be finite size effects.