Short Answer
Dynamic viscosity and kinematic viscosity are both measures of a fluid’s resistance to flow, but they differ in how they are defined and measured. Dynamic viscosity refers to the internal friction between fluid layers and is measured in Pascal-seconds (Pa·s) or poise. It indicates the force required to move one layer of fluid over another.
Kinematic viscosity, on the other hand, is the ratio of dynamic viscosity to fluid density. It represents how easily a fluid flows under gravity and is measured in centistokes (cSt) or mm²/s. While dynamic viscosity considers the applied force, kinematic viscosity focuses on how fast the fluid spreads.
Detailed Explanation
Dynamic Viscosity
Dynamic viscosity (also called absolute viscosity) measures the internal resistance of a fluid to flow. It occurs due to molecular interactions within the fluid. It is defined as:
η=τdudy\eta = \frac{\tau}{\frac{du}{dy}}η=dyduτ
where:
- η = Dynamic viscosity
- τ = Shear stress
- du/dy = Rate of change of velocity (velocity gradient)
Higher dynamic viscosity means the fluid resists movement more. For example, honey has high dynamic viscosity, while water has low dynamic viscosity.
Kinematic Viscosity
Kinematic viscosity considers both dynamic viscosity and the fluid’s density. It is defined as:
ν=ηρ\nu = \frac{\eta}{\rho}ν=ρη
where:
- ν = Kinematic viscosity
- η = Dynamic viscosity
- ρ = Fluid density
Kinematic viscosity helps in determining how fast a fluid flows under gravity. It is widely used in hydraulic system designs and lubrication analysis.
Conclusion
The main difference between dynamic and kinematic viscosity is that dynamic viscosity measures fluid resistance due to internal friction, while kinematic viscosity considers both resistance and density. Engineers use these properties in fluid mechanics, lubrication systems, and hydraulic designs to ensure smooth operation and efficiency.