Kinematic viscosity

Kinematic viscosity is dynamic viscosity divided by fluid density. It measures how quickly momentum diffuses through a fluid relative to its inertia.

Dynamic viscosity: A fluid’s resistance to shear deformation, quantified as shear stress divided by velocity gradient. Dividing this quantity by density produces kinematic viscosity.
Fluid density: Mass per unit volume of a fluid. Density is the divisor that converts dynamic viscosity into kinematic viscosity.
Cinematograph cup: A calibrated vessel that estimates kinematic viscosity from the time required for a fluid to drain through an orifice. It measures the property directly through a standardized flow time.
Apparent viscosity: The effective viscosity assigned to a non-Newtonian fluid at a specified shear rate or flow condition. It can vary with the measurement condition, whereas kinematic viscosity is a ratio of material properties at a stated state.
Momentum diffusivity: The rate at which momentum spreads through a material, analogous to thermal diffusivity for heat. Kinematic viscosity is momentum diffusivity in a Newtonian fluid.
Shear stress: Tangential force per unit area acting across a surface within a material. For a Newtonian fluid, shear stress and velocity gradient define dynamic viscosity.
Saybolt viscometer: An instrument that measures the time for a fixed volume of petroleum fluid to flow through a standardized orifice. Its efflux-time readings are used to determine kinematic viscosity in petroleum testing.
Bulk viscosity: A transport coefficient describing a fluid’s resistance to uniform compression or expansion. It concerns volumetric deformation rather than momentum diffusion from shear.
Navier–Stokes equations: The partial differential equations governing fluid motion through conservation of momentum and mass. Kinematic viscosity sets the coefficient of the momentum-diffusion term in their standard form.
Velocity gradient: The spatial rate of change of fluid velocity. It links shear stress to dynamic viscosity in the Newtonian constitutive relation.

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