Equations · equation
Reynolds Number
Fluid Dynamics · 1883
- Equation
- Re = ρ v L / μ
- Field
- Fluid Dynamics
- Named After
- Osborne Reynolds
- Year
- 1883
Symbols
Re a dimensionless ratio of inertial to viscous forces · ρ density · v flow speed · L a characteristic length — pipe diameter, blade chord · μ dynamic viscosity
Significance
One number decides whether a flow is smooth or turbulent: below about 2,000 in a pipe it is laminar and predictable, above about 4,000 it breaks into eddies. Reynolds watched a thread of dye in a glass tube in 1883 and saw where it happened. It is why a model in a wind tunnel tells you about the full-size aircraft only if Re matches, and why bacteria swim in a world that feels like treacle.
Referenced by
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