Reynolds Number

What is Reynolds Number?

Re = ρVL/μ — the ratio of inertial to viscous forces, predicting laminar versus turbulent flow. A low value means viscosity dominates and the flow stays smooth; a high value means inertia wins and the flow tends toward chaotic mixing.

The five clues

Engineerdle #41 described Reynolds Number with five clues, revealed one at a time from hardest to most direct. Each clue points at a different property of the concept.

1. 01A dimensionless group — no units at all.
2. 02It compares two competing forces inside a flow.
3. 03Below roughly 2300 in a pipe you get one regime; above 4000, another entirely.
4. 04It is the ratio of inertial to viscous forces.
5. 05Named after the British engineer who ran the famous dye-in-a-pipe experiment.

More Fluid Mechanics concepts

• Venturi EffectPuzzle #50
• ViscosityPuzzle #46
• Newtonian FluidPuzzle #36
• Non-Newtonian FluidPuzzle #31
• Pascal's LawPuzzle #13
• Archimedes' PrinciplePuzzle #8