Fluid Dynamics of Drag A hundred thousand particles stream past a cylinder as the Reynolds number climbs. Slow: the flow slips around cleanly. Faster: it can’t follow the back, separates, and trails a fat wake of vortices — now most of the drag is the low pressure in that wake. Near Re ≈ 300,000 the thin surface layer trips to turbulent, clings on further round, the wake snaps narrow, and the drag drops — why a golf ball has dimples. The journey: Reynolds (1883) showed one number, ρUD/μ, sorts every regime. The curve at bottom right is measured drag vs Reynolds number; the notch is the “drag crisis.”