The modification of the fluid model to describe the turbulent flow is
considered. We study the hydrodynamic instability, methods of description of
turbulence, the problem of averaging and the energy balance of the turbulent fluid.
Finally, we derive the Reynolds equation of motion.
Keywords: Eddy viscosity, Hydrodynamic instability, Laminar mode, Reynolds
conditions, Reynolds equation, Reynolds number, Reynolds stress tensor, The
averaging problem, Turbulence, Turbulent energy, Turbulent mode.
