d’Alembert’s Paradox states that in an ideal, inviscid, and incompressible fluid (potential flow with ), the drag force on a body moving at a constant velocity is exactly zero.
This is paradoxical, as in real-world scenarios, we always observe drag. Even in seemingly smooth flows, there is resistance. The paradox arises because potential flow theory ignores viscous effects; in reality, boundary layers form, creating friction and separation, which generates drag.
- The drag is heavily dependent on Reynolds Number, which characterizes the flow regime (laminar or turbulent)