What is Dynamic Viscosity?
Dynamic viscosity, also known as absolute viscosity, is a measure of a fluid's resistance to flow when an external force is applied. It quantifies the internal friction between adjacent layers of fluid as they move past each other. Imagine pushing a spoon through honey versus water; honey has a much higher dynamic viscosity because it offers more resistance to the spoon's movement due to its internal friction.
What is Kinematic Viscosity?
Kinematic viscosity, on the other hand, describes a fluid's resistance to flow under the influence of gravity, without an external force directly applied. It is the ratio of a fluid's dynamic viscosity to its density. Essentially, it tells us how fast momentum diffuses through the fluid. For example, a heavy oil might have a high dynamic viscosity but flow relatively quickly if it's very dense, illustrating that kinematic viscosity accounts for both internal friction and inertial resistance.
Key Differences and Relationship
The primary difference lies in the influence of density. Dynamic viscosity (μ) focuses solely on the internal resistance to shear flow, measured in Pascal-seconds (Pa·s) or Poise (P). Kinematic viscosity (ν) considers this internal resistance relative to the fluid's inertia, by dividing dynamic viscosity by mass density (ρ), thus measured in square meters per second (m²/s) or Stokes (St). The relationship is expressed as ν = μ / ρ, making kinematic viscosity more relevant when gravity or inertia are dominant factors in fluid motion.
Importance and Applications
Understanding both types of viscosity is crucial in various fields. Dynamic viscosity is vital for designing lubrication systems, pipelines for viscous fluids, and predicting drag on submerged objects. Kinematic viscosity is particularly important in fluid flow under gravity, such as in open channels, pouring liquids, or determining the flow behavior of oils and fuels where density significantly impacts how easily they spread or drain. Both are fundamental for material selection, process optimization, and fluid machinery design in industries from automotive to pharmaceuticals.