SSC JE Mechanical Engineering 2018 Set 5 Paper

Hydraulic diameter used in place of diameter of non-circular ducts is equal to $\frac{\left(4\mathit{A}\right)}{\mathit{m}}$
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 $\left({\mathit{D}}_{\mathit{h}}\right)=\frac{4\mathit{A}}{\mathit{P}}$
A = area of cross Section
P = wetted Perimeter (perimeter of cross section in contact with fluid)