- How Big Is Skin Friction
- The Mystery of Skin Friction from Tripping Resolved
- Breaking the Prandtl Spell: Do Not Trip!
The New Theory of Flight is based on the following features of slightly viscous incompressible flow, which go back to Euler 1755:
- slip boundary condition on wall as model of small skin friction.
In 1904 Prandtl introduced the concept of boundary layer connecting free stream flow velocity to zero velocity on a fixed wall corresponding to a no-slip boundary condition, which became the leading principle of modern fluid mechanics. A no-slip boundary condition generates a turbulent boundary layer with substantial skin friction, and so modern fluid mechanics was led to seek the origin of drag of a body moving through a fluid as skin friction from turbulent thin boundary layers.
This blog shows that this was a mistake: The main drag of a body is form/pressure drag with skin friction drag much smaller.
Even more serious: without small skin friction, flight is not affordable, because also lift will suffer.
Prandtl introduced the dictate of no-slip to resolve d’Alembert’s paradox of zero drag in potential flow with slip. By no-slip the potential solution with slip could be discriminated as non-physical, but that was a resolution based on formality. The true resolution of d’Alembert’s paradox is based on the understanding that the potential solution is unstable and this the true physical reason why zero drag flow cannot be observed. What can be observed is a turbulent solution with slip and form/pressure drag.