At large Reynolds numbers, the inertial forces, which are proportional to
the fluid density and the square of the fluid velocity, are large relative to the
viscous forces, and thus the viscous forces cannot prevent the random and
rapid fluctuations of the fluid. At small or moderate Reynolds numbers,
however, the viscous forces are large enough to suppress these fluctuations
and to keep the fluid “in line.” Thus the flow is turbulent in the first case
and laminar in the second