A simple lecture demonstration has been developed which demonstrates how viscosity dramatically changes fluid flow in a pipe. Water in an open reservoir is first allowed to escape through circular apertures near the bottom of the reservoir, which shows volume flow rates proportional to the area of the aperture for a low viscosity fluid. Then, a viscous 60% glycerol–40% water solution escapes from the same open container but through long horizontal tubes, with the flow rate proportional to the square of the area. This is direct visual confirmation of the Hagen–Poiseuille prediction that volume flow rates scale as the fourth power of the tube radius for laminar flow. Overcoming experimental complications involving vena contracta and entry length issues is discussed, along with detailed measurements and fits to theory.

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