  The identification of stream in the straight pipe as a flexible rod has
allowed to present the criterion expression for determination of transition of
the laminar flow regime to the turbulent as a loss of stability of the
rectilinear static structure of translational motion of stream in pipe and its
transition to the flexural-vortical dynamic structure of translational motion,
just as a flexible rod buckling. The introduced criterion allows to take into
account an influencing of the inlet geometry, the pipe length, the flow
velocity, and also of any physical factors on stability of the rectilinear flow
structure. It is ascertained, that Reynolds number is the number of local
hydrodynamic similarity and it is displayed that one is constituent part of the
introduced stability criterion. The developed approach to a problem of
stability is applicable for a problem solving on internal flow and external
streamline.
