Kinematics and Dynamic Viscosity

The resistance of a liquid to deformation at a given rate is measured by its viscosity. In plainer terms. The “thickness” of a liquid is measured by its viscosity. Some liquids, such as jam, are more viscous/thick, whereas others, such as water, are less viscous. Kinematic and Dynamic viscosity are the two types of viscosity that can be distinguished. Kinematic viscosity relates to the quantity of a fluid’s dynamic viscosity per unit density, while dynamic viscosity refers to the force that a fluid requires to move through its internal molecular friction in order to keep moving.

Viscosity

When analysing the flow of liquid in any application, viscosity is defined as the fundamental attribute. Kinematic and dynamic viscosity are the two types of viscosity. The connection between these two traits is straightforward. At first glance, it appears to be a straightforward notion. However, there are a slew of terms that fall within the umbrella of its meaning. The measurement of it is determined by these terms. Dynamic viscosity, also known as absolute viscosity, measures a fluid’s intrinsic resistance to flow; kinematic viscosity, on the other hand, represents the ratio of dynamic viscosity to density. Based on density, two fluids with the same dynamic thickness can have different kinematic densities, and vice versa.

Kinematic viscosity

Whereas a force is applied to a liquid, its kinematic viscosity tells us how fast it moves. It is the ratio of the liquid’s Dynamic Viscosity (µ) to its Density ( ). Momentum Diffusivity is another name for it. The Greek letter nu (v) stands for it:                

V=  µ    

Kinematic Viscosity has the dimensions (length)2/timeSpecific energy multiplied by time((J/kg) sec) is the SI unit.                                

Dynamic viscosity 

The force required to make a liquid flow at a specific rate is described by dynamic viscosity. The shear stress to shear strain ratio is the shear stress to shear strain ratio. Absolute Viscosity is another name for it. The Greek symbol mu (µ) stands for dynamic viscosity.                                               

(mass / length) / time are the dimensions of Dynamic Viscosity. Pressure multiplied by time equals the SI unit (Pa . sec).

Viscosity Calculation

A ratio of shearing stress to velocity gradient is used to calculate a liquid’s density. We may calculate the density of a spherical resting in a liquid by using the formula below:

Note: Shearing stress occurs when an external force acting on an object is parallel to the plane of the object, causing deformation along the plane and causing pressure to be felt on the object.

The difference in velocity between adjacent liquid layers is called a velocity gradient.

Viscosity is the ratio of shear stress to the velocity gradient in a fluid. When a sphere is dropped into a fluid, then the viscosity is determined by using a formula which is given as

ŋ = 2ga2(∆ρ)9v

 ŋ= viscosity

g = gravitational acceleration

a = radius of sphere

= density difference

v = velocity

The unit of measure for viscosity is Pascal seconds, or Pa s. When a liquid’s Temperature rises, its density decreases. 

CGS Units of Various Viscosities

Students are occasionally quizzed on viscosity units. Because there are multiple types of density, each with its own unit, students can use Poise (P) as the CGS unit of dynamic viscosity and Stokes (St) as the CGS unit of kinematic viscosity to distinguish between them in units. As centipoises, the term poise (P) is used specifically in ASTM standards (cP). The unit centistokes (cST) are used in a variety of fields. It is critical to learn how to compute densities after learning the units of density. The symbols and phrases needed to determine viscosity are given here.

The following are some other types of viscosity:

The link between viscosity and shear rate is referred to as steady shear viscosity. The shear stress applied to a fluid divided by the shear rate is what this term refers to. When measuring the viscosity of Newtonian fluids, this viscosity remains constant, but it is changed when measuring the viscosity of non-Newtonian fluids.

The ratio of the viscosity of a solution to the viscosity of the liquid utilised is known as relative viscosity. 

Extensional viscosity refers to a fluid’s resistance to extensional flow (passing through a fixed region with a sudden change in cross-sectional area). Extensional fluid is used to measure any flow within a cross-sectional area.

To determine viscosity, a viscometer can be used, and there are several ways available on the market to accomplish so. However, only a few tools have the ability to accurately determine viscosity. Some people can accurately measure the viscosity of just Newtonian fluids, yet most fluids are non-Newtonian. Some people can accurately measure the qualities without testing the true viscosity.

Conclusion

The fundamental attribute of viscosity is defined when analyzing the flow of liquid in any application. The two types of viscosity are kinematic and dynamic viscosity. When a force is given to a liquid, the kinematic viscosity of the liquid determines how quickly it travels. It’s the ratio between the liquid’s Dynamic Viscosity  and its Density . Dynamic viscosity is the force necessary to make a liquid flow at a certain rate. The ratio of shear stress to shear strain rate is known as absolute viscosity. The Greek letter mu denotes dynamic viscosity.