Hydraulic Diameter

 a         D  
 b    d  
 Cross section A    A
 Wet perimeter S    S
 Dh    Dh

The laminar flow Q' [m3/s] through a pipe with circular cross section is calculated using:
pipeflow calculator >>
Where Dh is the hydraulic diameter, which is the inner diameter of the pipe. For different cross sections the hydraulic diameter Dh is calculated using:

where A is the area of the cross section and S the wetted perimeter. For a rectangular groove with sides a and b the diameter Dh becomes:

and the space formed between two concentric pipes:

The flow through a thin film is calculated using:  
filmflow calculator >>