In order to describe real flows, friction has to be considered. Due to its viscosity, the flowing medium adheres to the wall of the body in the region of direct contact. In the contact zone, the velocity of the medium also amounts to zero in the case of a stationary body. For a distance y between two surfaces that have a relative velocity v against each other, a linear velocity gradient results.
According to Newton's Law of Friction, the shear stress is given by: