English: This shows the result of a Lattice Boltzmann simulation of a a fluid (air) flowing around obstacles, in two dimensions. The obstacles are discs (shown in brown), and streamlines showing the air flow are in blue; the arrows indicate the direction of flow (from bottom to top). Trajectories of two particles are shown. The orange trajectory is for a Stokes number less than one, and so the particle follows the streamlines, and curves around the obstacles. The green trajectory is for a Stokes number greater than one, and so particle does not follow the streamlines, it tends to go straight ahead. In this case it then collides with an obstacle, collision is shown in yellow.