Basing on the generalized variational principle for dissipative continuum mechanics it is shown that the term with shear viscosity in Navier-Stokes equation can be interpreted as a result of relaxation of angular momentum of material points consisting continuum. The rotation degree of freedom as an independent variable appears in addition to the mean displacement field. The independent equation of motion for the micro rotation field corresponds to this additional degree of freedom. In the absence of dissipation, this approach leads to the well known Cosserat continuum. When dissipation dominates over inertia, this approach describes local relaxation of angular momentum that appears as a shear viscosity term in Navier - Stokes equation.
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