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.
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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.
kinematic viscosity=
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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