Low-Reynolds-number flow around a cylinder — streamlines and drag
Stokes flow (Re → 0) is governed by ∇p = μ∇²u with ∇·u = 0. Around a cylinder the exact solution gives closed, fore-aft symmetric streamlines — no separation, no wake, no drag paradox (Stokes drag F = 4πμUa/ln(1/Re) per unit length in 2D). At higher Re, the symmetry breaks and a recirculating wake forms, eventually shedding vortices (Kármán street).