Reynolds Number

Where:
	= density
	v = velocity (m/s)
	d = distance (m)
	µ = dynamic viscosity

It is used to describe the type of fluid flow. If a flow has a Reynolds number < 2000 it is considered to be Laminar flow. On the other hand a Reynolds number > 4000 would be said to describe a turbulent flow.

Laminar flow is comparatively easy to model mathematically, unfortunately there is relatively little in the real world that has laminar flow. Lubrication, High viscosity fluids and low leakage flows can be said to have laminar flows.

Turbulent flow describes virtually all fluid flow unfortunately. Turbulent flow is disorderly, disturbed and well. turbulent. Turbulent flow is difficult to model mathematically and so high-powered computers are generally used, this becomes the realm of Computational Fluid Dynamics (CFD).