Viscosity

Introduction

When there is relative motion between the fluid layers, that is when viscosity arises. Due to the internal friction between the fluid layers as they slip past one another when fluid flows, Viscosity gauges the resistance. Viscosity is also exhibited from gasses, but to notice it in ordinary circumstances is hard. Furthermore, viscosity is an intensive property because it does not change as matter changes. Therefore, the kinematic viscosity is much more valuable and useful than the dynamic viscosity (also known as absolute viscosity). For example, viscosity can be determined when a sphere is dropped into a fluid.

Viscosity is:

Poiseuille is the SI unit of Viscosity. PI denotes Poiseuille.

Other units of Viscosity are,

• Newton-second per square meter, which is denoted by (N s m^-2)
• The pascal-second, which is denoted by (Pa s.)
• Viscosity’s dimensional formula is [ML^-1T^-1]

When there is an increase in temperature, the viscosity of liquids decreases. Whereas, the viscosity of gasses also increases with the rise in temperature. When liquids are heated, they flow more easily, whereas, the gasses flow slowly when they are heated. Viscosity is an intensive property because it does not change as matter changes.

The formula of Viscosity

Viscosity is measured in terms of a ratio of shearing stress to the velocity gradient in a fluid. Viscosity can be determined when a sphere is dropped into a fluid by,

Formula,

= 2 ∆ρga²/9v

In which,

• So, the density disparity between sphere tested and fluids are denoted by = ∆ρ
•  By “a” sphere’s radius is denoted
•  Due to gravity, the acceleration is denoted by = g
•  The velocity of the sphere is denoted by = v

Different types of Viscosity

Mainly there are two types of viscosity, and they are as follows;

• The first type is the dynamic viscosity

To measure fluid’s resistance to flow when some external force is applied is understood as dynamic viscosity.

• And the second type is the kinematic viscosity

Under the weight of gravity, when the fluid’s resistance to flow is measured is understood as kinematic viscosity.

There is always some confusion between the two types of viscosity measures. Sometimes it’s considered to be the same. But the truth is that there is a crucial difference between dynamic and kinematic viscosity.

• The kinematic viscosity is much more valuable and useful than the dynamic viscosity (also known as absolute viscosity)

The measurement of Viscosity

• The primary manner of measuring Viscosity is by allowing a sphere, like a metal ball, to drop through a fluid and measure the fall of the metal ball

• The viscosity will be greater in the case when the metal ball falls slowly

• With the help of a viscometer, a more accurate measure of viscosity is provided

Kinematic Viscosity

Viscosity shows a struggle against a flowing fluid, and due to shear stress or external stress forces, it is being distorted. The kinematic Viscosity is computed by calculating the ratio of fluid mass density to the dynamic fluid viscosity or absolute fluid viscosity. The momentum diffusivity is the kinematic viscosity from time to time. Based on area and time, units of kinematic viscosity are established.

The formula of kinematic viscosity is conveyed by,

Formula  ν = μ/ρ

• So, in this, the kinematic viscosity is denoted by = ν
• And absolute or dynamic viscosity is denoted by = μ
• By “ρ,” the density is denoted

Dynamic Viscosity

Due to some shearing force, a fluid that interprets its internal resistance to flow is understood as dynamic Viscosity. A tangential force acts when one horizontal plane moves with another horizontal plane. While evaluating the fluid motion near solid boundaries and the liquid behavior, Viscosity plays a crucial fluid property.

Dynamic Viscosity is conveyed as given below:

Formula  η = τ/γ

In which,

• The dynamic Viscosity or the absolute Viscosity is denoted by = η
• The shearing stress is denoted by = τ
• The shear rate is denoted as γ

Dynamic Viscosity’s SI unit is Ns/m².

Conclusion

Due to the internal friction between the fluid layers as they slip past one another when fluid flows, the Viscosity gauges resistance. When there is an increase in temperature, the viscosity of liquids decreases. Whereas, the Viscosity of gasses increases with the rise in temperature. There is always some confusion between the two types of viscosity measures. Sometimes it’s considered to be the same. However, the Viscosity will be greater when the metal ball falls slowly. With the help of a viscometer, a more accurate measure of Viscosity is provided.