Fluid and Its Properties
What is Fluid ?
A fluid may be
defined as a substance which is capable of flowing. It has no definite shape of
its own, but conforms to the shape of the containing vessel. A small amount of
shear force exerted on a fluid cause it to undergo a deformation which continues
as long as the force continues to be applied.
Types of fluid
Ideal fluid: ideal
fluids are those fluids which have no viscosity and surface tension and they
are incompressible. Ideal fluids encounter no resistance to motion. Ideal
fluids do not exist in nature and therefore, these are imaginary fluids.
Real fluids: The fluids
actually exist in nature are called real or practical fluids. These fluids
possess the properties such as viscosity, surface tension and compressibility
and therefore a certain amount of resistance is always offered by these fluids
when they are set in motion.
Properties of fluids
Mass density: it is
also called specific mass. Mass density is defined as the mass possess by the
fluid per unit volume. It is denoted by symbol ρ (rho).
Specific weight:
specific weight of a fluid is the weight it possesses per unit volume. It is
also called as weight density. It denoted by symbol w or γ (gamma). It
represents the force exerted by gravity on a unit volume. So, it has unit of
force per unit volume.
It can also be written
as,
γ
= ρg
specific volume:
specific volume of a fluid is defined as the reciprocal of specific weight of
fluid. It is defined as the volume of fluid per unit weight.
Sp.
Vol = 1/γ = v/mg
Specific gravity: specific
gravity (sp. gr.) is ratio of specific weight (or mass density) of a fluid to
the specific weight (or mass density) of a standard fluid.
Viscosity:
Viscosity is that
property of fluid by virtue of which it offers resistance to the movement of
one layer of fluid over an adjacent layer. It is primarily due to cohesion and
molecular momentum exchange between fluid layers, and as flow occurs, these
effects appear as shearing stresses between the moving layers of fluid.
If the fluid is moving with
velocity v, over a stationary plate, then the shear stress acting between
the layers is given by
Where τ = shear stress
µ = dynamic viscosity
v = velocity of fluid
y = distance of fluid layer from the stationary plate
Dynamic
viscosity: (coefficient of viscosity): it is defined as shear stress required to
produce unit rate of angular deformation.
From
the expression of shear stress
kinematic viscosity: the ratio of
dynamic viscosity µ and mass density ρ is kinematic viscosity. It is denoted by
ν (nu).
ν=µ/ρ m2/s
Vapour
pressure:
When
the liquid is confined in a closed vessel, the ejected vapour molecules get accumulated
in the space between the free liquid surface and the top of the vessel. This
accumulated vapour of the liquid exerts a partial pressure on the liquid surface
which is known as vapour pressure.
Cavitation:
if in any flow system the pressure at any point in the liquid approaches the vapour
pressure, vaporization of liquid starts, resulting in the pockets of dissolved
gases and vapours. The bubbles of vapour thus formed are carried by the flowing
liquid into a region of high pressure where they collapse, giving rise to high
impact pressure. The pressure developed by the collapsing bubbles is so high
that the material from the adjoining boundaries gets eroded and cavities are
formed on them. This phenomenon is known as cavitation.
