Turbulent and Laminar Flow
Friction appears in relative motion between solid bodies, between fluids in a pipe and its wall, but also far from boundaries inside flowing fluids between layers of same velocities (lamina). This has deep implications on the way the fluid behaves. The flow through a pipe or channel can be discriminated in turbulent and lam- inar flow, which depends on the size of the channel, the flow speed and the properties of the liquid. That means: The larger the size of the channel, the higher the flow speed and the lower the viscosity, the more likely the flow develops turbulence which is described by the Reynolds number Re:
Re = ū ρ L / η
with the (average) flow speed ū, the channel length L, the density ρ and the dynamic viscosity η of the fluid.