Difference Between Dynamic and Kinematic Viscosity

Main Difference – Dynamic vs. Kinematic Viscosity

Viscosity is very important to any process that depends on a flow of fluid. Usually, two types of viscosity are quoted: dynamic and kinematic viscosity. The main difference between dynamic and kinematic viscosity is that dynamic viscosity is a measurement of how difficult it is for a fluid to flow whereas kinematic viscosity is the dynamic viscosity of a fluid divided by its density.

What is Dynamic Viscosity

Whenever a fluid flows against a surface, the different layers of fluid exert frictional forces between each other, causing them to flow at different speeds. A force needs to be applied to a layer of fluid in order to make it flow at a constant speed relative to any other layer. The force $F$ required to move a layer of fluid in this way is related to the velocity $v$ at which the fluid layer will move by the equation:

$F=\eta A\frac{v}{d}$

where $A$ is the area of the layer and $d$ is the distance between the layers. $\eta$ is a constant of proportionality, and it is known as the dynamic viscosity of the fluid. In this sense viscosity is a measurement of how difficult it is to make a fluid flow. Note that the above equation is only valid for so-called Newtonian fluids. Non-Newtonian fluids cannot be described with one value for viscosity.

Difference Between Dynamic and Kinematic Viscosity - Honey_Drip

Honey is more viscous than water, and so it is more difficult to get it flowing.

The forces between different layers exert a type of shear stress since the forces act parallelly to the layers. For this reason, dynamic viscosity is also called shear viscosity. Dynamic viscosity is also referred to as absolute viscosity at other times. The SI unit of measuring dynamic viscosity is pascal seconds (Pa s). However, the most commonly-used unit for measuring viscosity is the centipose (cP). 1000 cP = 1 Pa s.

What is Kinematic Viscosity

Kinematic viscosity ( $\nu$ ) of a fluid is the ratio of the fluid’s dynamic viscosity $\eta$ to its density $\rho$ :

$\nu=\frac{\eta}{\rho}$

The SI unit of kinematic viscosity is m² s^-1. However, the more commonly-used unit for measuring kinematic viscosity is the centistoke (cSt). 10⁶ cSt = 1 m² s^-1. Note that since viscosity depends on temperature (the viscosity decreases as temperature increases in liquids, while the viscosity decreases when the temperature of a gas increases). When the viscosity of a substance is quoted, the temperature should be specified.

Difference Between Dynamic and Kinematic Viscosity

Involvement of Density

Both dynamic and kinematic viscosities measure how difficult it is for a fluid to flow. Kinematic viscosity measures this in terms of density, whereas dynamic viscosity does not.