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Infinite cylinder to cylinder

Function f2d_4(r, s) provides the view factor from an infinitely long cylinder to a parallel infinitely long cylinder of identical diameter.
Inputs:
r is the radius of the cylinder [m or ft]
s is the distance between the cylinders, measured from the closest points on the cylinders' surfaces [m or ft]  
Example:  
$UnitSystem SI K Pa J deg
r=0.2 [m]
S=0.5 [m]
F=f2d_4(r, s)
{Solution: 
F=0.07198}
Reference:  Siegel and Howell, Thermal radiation heat transfer, 4th edition, p.847.
View Factor Index