Reynolds number
The Reynolds number is a dimensionless quantity that is used to help predict similar flow patterns in different fluid flow situations. The concept was introduced by George Gabriel Stokes in 1851, but the Reynolds number is named after Osborne Reynolds (1842–1912), who popularized its use in the 1880s.
The Reynolds number is defined as the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions.

(1) 
Where:
 is the internal diameter of the pipe
 is the average velocity of the fluid
 is the density of the fluid
 is the viscosity of the fluid
The Reynolds number is used to characterize different flow regimes within a similar fluid, such as laminar or turbulent flow. Laminar flow occurs at low Reynolds numbers, where viscous forces are dominant. Laminar flow is characterized by smooth, constant fluid motion. Turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities.
By defining the following nondimensional variables

(2) 
and substituting into the Navierstokes equation, we get

(3) 
or

(4) 
This limit permits solutions in which backward flow is possible, which gives rise to the instabilities that lead to turbulent flow.
The Reynolds number is commonly used for fluid flow in a pipe. This can be represented by:

(5) 
Where
 is the internal diameter of the pipe
 is the average velocity of the fluid
 is the density of the fluid
 is the viscosity of the fluid
 is the kinematic viscosity of the fluid
 Q is the volumetric flow rate of the fluid
 A is the the crosssectional area of the fluid
This could be used typically for a round pipe but also could be adjusted for different types of pipes by adjusting the diameter or This could be used for AC units in which you could adjust the to be defined as Where
 P is the wetted parameter