Table of Contents
What Is Kinematic Viscosity?
Kinematic viscosity is the ratio of a fluid's dynamic (absolute) viscosity to its density. It represents the fluid's resistance to flow under the influence of gravity, combining the effects of internal friction (dynamic viscosity) and inertia (density). The SI unit is m^2/s, though the CGS unit Stokes (1 St = 10^-4 m^2/s) is also commonly used.
For air, kinematic viscosity increases strongly with temperature because dynamic viscosity increases with temperature while density decreases. At standard conditions (20 degrees C, 1 atm), the kinematic viscosity of air is approximately 1.516 x 10^-5 m^2/s. This value is critical for computing the Reynolds number in aerodynamics, HVAC engineering, and fluid mechanics.
Sutherland's Law
Where μ0 = 1.716 x 10^-5 Pa·s, T0 = 273.15 K, S = 110.4 K (Sutherland constant for air), and Rair = 287.058 J/(kg·K).
Air Viscosity at 1 atm
| Temp (°C) | μ (Pa·s ×10-5) | ν (m²/s ×10-5) | ρ (kg/m³) |
|---|---|---|---|
| -20 | 1.630 | 1.169 | 1.395 |
| 0 | 1.716 | 1.328 | 1.293 |
| 20 | 1.813 | 1.516 | 1.205 |
| 40 | 1.907 | 1.702 | 1.127 |
| 60 | 1.999 | 1.896 | 1.059 |
| 100 | 2.174 | 2.306 | 0.946 |
Applications
- Reynolds number: Re = V × L / ν, the most fundamental dimensionless number in fluid mechanics.
- HVAC design: Duct sizing and pressure drop calculations require accurate viscosity values.
- Aerodynamics: Boundary layer thickness, drag, and lift coefficients depend on kinematic viscosity.
- Pollution dispersion: Atmospheric transport models use viscosity to calculate turbulent mixing rates.
Frequently Asked Questions
Why does air viscosity increase with temperature?
Unlike liquids, gas viscosity increases with temperature because gas molecules move faster and transfer momentum between layers more effectively. There are no intermolecular bonds to break as in liquids, so the dominant mechanism is molecular collision, which increases with temperature.
How does pressure affect kinematic viscosity?
Dynamic viscosity of gases is nearly independent of pressure at moderate pressures. However, density is directly proportional to pressure (ideal gas law), so kinematic viscosity (nu = mu / rho) decreases with increasing pressure. At 2 atm, kinematic viscosity is approximately half the value at 1 atm.
What is the difference between kinematic and dynamic viscosity?
Dynamic viscosity (mu) measures a fluid's internal resistance to shearing flow and has units of Pa*s. Kinematic viscosity (nu) equals dynamic viscosity divided by density and has units of m^2/s. Kinematic viscosity is more practical for many engineering calculations because it naturally accounts for the fluid's inertia.