Reynolds Number (Re = ρvd/η)
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Re = ρvd/η
What each symbol means
| Symbol | What it stands for |
|---|---|
| d | Characteristic length, e.g., pipe diameter (m) |
| v | Average flow velocity (m/s) |
| Re | Reynolds number (dimensionless) |
| η | Coefficient of viscosity (Pa·s) |
| ρ | Density of fluid (kg/m³) |
When to use this
Predicts flow regime: Re < 2000 → laminar, Re > 4000 → turbulent, 2000–4000 → transitional. NCERT mentions critical speed concept; Reynolds number quantifies this. Higher viscosity or smaller pipe favors laminar flow. Dimensionless number.