Fluid, Characteristic of fluid, Ideal fluid, real fluid, viscosity, cause of viscosity, Newton's law of viscosity, kinematic viscosity ~ MECHTECH GURU

# Definition of fluid

A fluid is a substance which deforms continuously when subjected to external shearing forces.

## Characteristics of fluid

1.  It has no definite shape of its own, but conforms to the shape of the containing vessel.

2.  Even a small amount of shear force exerted on a fluid will cause it to undergo a deformation which continues as long as the force continues to be applied.

3.  It is interesting to note that a solid suffers strain when subjected to shear forces whereas a fluid suffers Rate of Strain i.e. it flows under similar circumstances.

## Ideal and Real Fluids

### 1.  Ideal Fluid

An ideal fluid is one which has no viscosity, no surface tension and incompressible

### 2.  Real Fluid

An Real fluid is one which has viscosity, surface tension and compressible

# Viscosity

Viscosity is the property of a fluid which determines its resistance to shearing stresses.

Definition

### Cause of Viscosity

It is due to cohesion and molecular momentum exchange between fluid layers.

## Newton’s Law of Viscosity:

It states that the shear stress (Ï„) on a fluid element layer is directly proportional to the rate of shear strain.

According to Newton’s lawtm du/dy

 Velocity Variation near a solid boundary

# Units of Viscosity

S.I. Units: Pa.s or N.s/m

C.G.S Unit of viscosity is Poise= dune-sec/cm

1/100 Poise is called centi-poises.

Dynamic viscosity of water at 200 degree centigrade is approx= 1 cP

# Kinematic Viscosity

It is the ratio between the dynamic viscosity and density of fluid.

# Units of Kinematic Viscosity

S.I units: m

C.G.S units: stoke = cm

One stoke = 10-4 m

Thermal diffusivity and molecular diffusivity have same dimension, therefore, by analogy, the kinematic viscosity is also referred to as the

# Effect of Temperature on Viscosity

With increase in temperature:

Temperature response are neglected in case of Mercury.

The lowest viscosity is reached at the critical temperature.

# Effect of Pressure on Viscosity

Pressure has very little effect on viscosity.

But if pressure increases intermolecular gap decreases then cohesion increases so viscosity would be increase.

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