English: Drag coefficient C_d for a sphere as a function of Reynolds number Re, as obtained from laboratory experiments. The solid line is for a sphere with a smooth surface, while the dashed line is for the case of a rough surface. The numbers along the line indicate several flow regimes and associated changes in the drag coefficient:

• 2: attached flow (Stokes flow) and steady separated flow,
• 3: separated unsteady flow, having a laminar boundary layer upstream of the separation, and producing a vortex street,
• 4: separated unsteady flow with a laminar boundary layer at upstream side, before flow separation, with downstream of the sphere a chaotic turbulent wake,
• 5: post-critical separated flow, with a turbulent boundary layer.