# The limit of validity of the Newtonian fluid friction law in flows around vehicles and the limit of validity of the Navier-Stokes equations

## Abstract

Known that the occurrence of shear stress is the result of the momentum exchange of particles (molecules in the case of gases), which takes place between the superimposed layers due to the thermal movement. This means that the forces corresponding to shear stress are arising actually in a very thin layer whose thickness is about the order of magnitude of the mean free path of the particles, so they are no surface forces in the strict sense, which is neglected in most cases.

We will show that there exists such a macroscopic flow at which this neglect is not allowed.

## Keywords:

flows around vehicles; non-stationary parallel planar; parabolic partial differential equation; Newtonian fluid friction law; Navier-Stokes equations## Published Online

2013-08-08

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