The geometry is specified in the interface with the
surfaceAreas[n], the roughnesses[n] and
the lengths[n] along the flow path. Moreover the fluid flow is
characterized for different types of devices by the characteristic
dimensions[n+1] and the average velocities
vs[n+1] of fluid flow. See 
Pipes.BaseClasses.CharacteristicNumbers.ReynoldsNumber for
example definitions.