As these eddies grow, the laminar flow becomes unstable and velocities and pressures in the flowing fluid no longer have constant or smoothly varying values. In pipes, relatively large rotational eddies form in regions of high shear near the pipe wall. These degenerate into smaller eddies as energy is dissipated by action of viscosity. The presence of eddies means that the local velocity is not the same as the bulk velocity and that there are components of velocity in all directions. Fluids with low viscosity and high density tend to support turbulence.
Turbulence makes convective transport dominant.
The transition between laminar and turbulent flow is fuzzy. The behavior depends on entrance conditions and distance from the inlet. Often it is useful to speak of a transition region for flows that are neither laminar nor fully turbulent. The dependence on viscosity means that the Reynolds number only makes sense for Newtonian fluids. Laminar flows have Reynolds numbers below 2100.