Calculate the dimensionless relative roughness or roughness number for pipes and rectangular ducts.
`rr = (ir) / (ID) `
where :
rr = relative roughness or roughness number
ir = surface roughness
ID = pipe inside diameter
For rectangular ducts the hydraulic diameter is used
`ID = 2 w h / (w + h) `
where :
ID = hydraulic diameter
w = duct width
h = duct height
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