Water in pipes: flow, pressure & viscosity
Before you start
Why does water speed up in a narrow pipe, and why is a viscous liquid harder to push? Separate these questions using pressure columns and moving dye.
The setupSwitch between an ideal horizontal Venturi tube and viscous flow through a straight pipe of radius 6 mm and length 1 m. Geometry is enlarged for teaching; use the pressure readings for quantitative comparisons.
What to do and noticeNarrow the pipe: the same volume per second must cross a smaller area faster. · Watch the pressure column fall at the neck; total energy has not vanished. · Switch to viscous flow and raise viscosity: the same pressure difference produces less flow.
Predict firstDoes less water pass the neck each second?
Distinguish flow rate, speed, static pressure and viscous pressure loss.
Prerequisites: Flow rate Q is volume per second, not speed v.The same amount of water
Predict firstDoes less water pass the neck each second?
In steady flow, water does not continually pile up inside the pipe. Every cross-section has the same Q = Av, so smaller area A requires greater speed v.
How to observe
- Narrow the pipe: the same volume per second must cross a smaller area faster.
- Watch the pressure column fall at the neck; total energy has not vanished.
- Switch to viscous flow and raise viscosity: the same pressure difference produces less flow.
Model notes
Ideal mode assumes incompressible, horizontal, lossless steady flow. Viscous mode assumes fully developed laminar flow of a Newtonian liquid in a straight circular pipe, with no slip: zero speed at the wall and maximum at the centre. Do not combine the assumptions of the two modes. Dye sizes, geometry and playback speed are enlarged for observation, not molecular scales.