Hydraulic diameter used in place of diameter of non-circular ducts is equal to
(4A)
m
Hydraulic Diameter: In order to find Reynolds number , friction factor etc , we use a parameter called hydraulic diameter which is given by below equation : Hydraulic Diameter (Dh)=
4A
P
A = area of cross Section P = wetted Perimeter (perimeter of cross section in contact with fluid)