It has been proposed, as a rough rule of thumb, that for a given pressure gradient and fluid viscosity the total flow rate through a pipe of given cross-section is proportional to A3/P2 where A is the area and P is the perimeter length of a pipe section normal to the pipe axis. This dependence on perimeter length as well as on area allows for the additional friction which comes from more contact of fluid with the walls of pipes which are corrugated or highly deformed from a circular cross-section. Comparison is made with exactly calculated flows for a variety of different pipe shapes.

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