Cd of the smooth sphere as a function of Reynolds number (Reynolds based on the diameter). The Cx is defined here as the ratio of the drag by the Dynamic Pressure of the flow and the frontal area of the sphere. The right of the curve highlights "the crisis of sphere's Cd " : there has a sudden drop of Cx for a very small increase in Reynolds. Left, for very low Reynolds, the Cd curve becomes tangential to the blue line representing the Stokes regime. This Cd curve was drawn from the analytical-empirical equations of Clift, Grace and Weber, equations based on experiments on spheres in wind-tunnel, in "free" fall, or during sedimentation in various fluids.