Mechanical Properties of Fluids 3 Flashcards
what is viscosity
the property of a fluid by virtue of which an opposing force comes into play between the different layres of a fluid when there is relative motion between different layers is called as viscosity
what is coefficient viscosity
it is the ratio between the shearing stress and strain rate
eta= Fl/vA
The SI unit of viscosity is poiseiulle (Pl). Its
other units are N s m-2 or Pa s.
It can be defined as force required to maintain unit velocity gradient between 2 layers of fluid each of unit area.
what does viscosity depend upon temperature
The viscosity of liquids decreases with temperature (because cohesive force decreases )while it increases in the case of gases.
what is stokes law
It states that the viscous force acting on a spherical body of radius r falling freely in a viscous medium of coeff of viscosity eta is
F= 6pi eta rV
what is terminal velocity
It is defined as the constant velocity that is attained by a spherical body falling freely in a viscous medium.
what is reynold’s number
it is a dimensionless constant which determines whether the flow of a liquid is turbulent or streamline
Re= ρvd/η
where ρ is the density of the fluid flowing with
a speed v, d stands for the diameter n of the
pipe, and η is the viscosity of the fluid.
how is reynolds’ number used
It is found that
flow is streamline or laminar for Re
less than
1000. The flow is turbulent for Re
> 2000. The
flow becomes unsteady for Re
between 1000 and
2000. The critical value of Re
(known as critical
Reynolds number), at which turbulence sets, is
found to be the same for the geometrically
similar flows.
turbulence is a necessary evil.
Turbulence dissipates kinetic energy usually
in the form of heat. Racing cars and planes are
engineered to precision in order to minimise
turbulence. The design of such vehicles involves experimentation and trial and error. On the
other hand turbulence (like friction) is
sometimes desirable. Turbulence promotes
mixing and increases the rates of transfer of
mass, momentum and energy. The blades of a
kitchen mixer induce turbulent flow and provide
thick milk shakes as well as beat eggs into a
uniform texture.
define surface tension
It is a property of a liquid by a virtue of which the free surface of the liquid tends to occupy minimum surface area and behaves like a streched membrane.
Surface tension is a force per unit length
(or surface energy per unit area) acting in
the plane of the interface between the plane
of the liquid and any other substance;
T= F/l
define:
i) force of adhesion
ii) force of cohesion
iii) molecular range
i)force of attraction between molecules of different substances
ii) force of attraction between molecules of same substance
iii) The maximum distance upto which the force of attraction between two molecules is appreciable is called as molecular range. the sphere with the molecule at the centre and the molecular range as the radius is called the sphere of influence.
define surface energy
the work done to increase unit area of the free surface of the liquid at constant temperature and pressure is called surface energy.
E= W/A
= T dA
surface tension x change in area
define angle of contact
the angle which the tangent to the liquid at the point of contact makes with the solid surface inside the liquid is called angle of contact.
explain the function of detergent nd waterproofing agent
Soaps,
detergents and dying substances are wetting
agents. When they are added the angle of
contact becomes small so that these may
penetrate well and become effective. Water
proofing agents on the other hand are added to
create a large angle of contact between the water
and fibres.
relation between angle of contact and shape of drop
i) when angle of contact is 90*. the meniscus is plane. the water wets the surface. hemispherical drop
ii) when angle of contact is acute, the meniscus is concave, the water wetsthe surface, flattened drop
iii) when angle of contact is obtuse, the meniscus is convex, the water does not wet the surface, spherical drop
why does concave side of free surface have more pressure
due to resultant of force due to durface tension
