1. SNPIT & RC,UMRAKH

2. FLUID MECHANICS (B.E. SEM-III -2014) TOPIC NAME: VISCOSITY

3. Prepared By:
Guided by:
BANKIM R. JOSHI
SARIKA G. JAVIYA
KARTILA D. UCHDADIYA

4. VISCOCITY: Viscosity is the property of a fluid by virtue of which it offers resistance to the movement of one layer of fluid over another adjacent layer of fluid.

5. Velocity profile and viscosity concept

6. Introduction
Viscosity is a quantitative measure of a fluid’s resistance to flow.
Dynamic (or Absolute) Viscosity:
The dynamic viscosity(η) of a fluid is a measure of the resistance it offers to relative shearing motion.
η= F/ [A×(u/h)]
η= τ /(u/h) N-s/m²
Kinematic Viscosity :
It is defined as the ratio of absolute viscosity to the density of fluid.
ν= η/ρ m²/s ; ρ= density of fluid

7. METHODS OF MEASURMENT OF VISCOCITY:-
METHODS OF MEASURING THE VISCOCITY OF ARE AS FOLLOWING BOLOW:
CAPILLARY TUBE METHOD
2.ROTATING CYLINDER METHOD
3.FALLING SPHERE RESISTANCE METHOD
4.ORIFICE TYPE VISCOMETER OR
EFFLUX VISCOMETER

8. 1)Capillary tube method: -
In this method, the principle of Hagen Poiseuille theory is used. The arrangement of the system used is shown in fig. It consist of a tank containing the liquid whose viscosity is to be measured and a capillary tube of dia “D” is fitted at the bottom of the tank. During the experiment temperature of liquid is maintained constant. The liquid from the constant head tank is allowed to pass through the capillary tube. Then the rate of liquid collected in tank per second is determined. The pressure head ‘h’ is measured at a point far away from tank.  

9. Capillary tube viscometer

10. Let, h= difference of pressure head for length µ= co-efficient of viscosity p=density of fluid µ=πpghD4 /128QL

11. (2) Rotating cylinder method
In this viscometer, the principle of Newton`s law of
viscosity is used to measure the viscosity of the liquid
Two coaxial cylinders of radius R1 and R2 with a clearance
of ( R2 – R1 ) placed one into the other and the clearance
space is filed with oil whose viscosity is to be measured.
The inner cylinder is suspended by a torsion spring and
held stationary when the other cylinder is rotating.
The torque transmitted by the enclosed liquid to the
stationary cylinder is measured by the torsional strain
of the restraining spring attached to the top of the inner
cylinder.
(2) Rotating cylinder method

12. Where,
w = angular speed of outer cylinder
h = clearance at the bottom of cylinders
H = height of liquid in annular space
µ = co-efficient of viscosity to be determined
T = torque measured by the strain of the torsional spring

13. Rotating cylinder viscometer

14. In this viscometer, the principle of Newton`s law of viscosity is used to measure the viscosity of the liquid. Two coaxial cylinders of radius R1 and R2 with a clearance of ( R2 – R1 ) placed one into the other and the clearance space is filed with oil whose viscosity is to be measured. The inner cylinder is suspended by a torsion spring and held stationary when the other cylinder is rotating. The torque transmitted by the enclosed liquid to the stationary cylinder is measured by the torsional strain of the restraining spring attached to the top of the inner cylinder.

15. Where,
w = angular speed of outer cylinder
h = clearance at the bottom of cylinders
H = height of liquid in annular space
µ = co-efficient of viscosity to be determined
T = torque measured by the strain of the torsional spring

16. (3) Falling sphere resistance method
In this method the Stokes's law is used for measuring the viscosity.
The viscous force acting on a sphere passing through a liquid is
given by,
Where, d = diameter of sphere, U = velocity of sphere
Ps = density of sphere
Pf = density of fluid

17. (4) Orifice type Viscometer OR Efflux viscometer
In this method, the viscosity is determined by noting the time
taken by a certain quantity of liquid whose viscosity is to be
determined, to flow through a short capillary tube. The operation
of Saybolt, Redwood and Engler Viscometer is based on this
principle. The specific volume for Saybolt viscometer is 60ml.
the initial height of liquid in the tank is previously adjusted to a
standard height. The kinematic viscosity of liquid is,
Where,
V = kinematic viscosity in strokes
t = time noted in seconds
For saybolt viscometer A = 0.24 and B = 19

18. Effects of temperature
The viscosity of liquids decreases with increase the temperature.
The viscosity of gases increases with the increase the temperature.

19. Viscosity index VI = (L - U)/ (L - H) * 10
An entirely empirical parameter which would accurately describe the viscosity- temperature characteristics of the oils.
The viscosity index is calculated by the following formula:
VI = (L - U)/ (L - H) * 10
where ,
VI is viscosity index
U is the kinematic viscosity
of oil of interest
L and H are the kinematic
viscosity of the reference oils
Fig . Shows the evaluation of viscosity index

20. Viscosity – shear relationship
Pseudoplastic Behavior
Pseudoplastic or shear thinning and is associated with the thinning of the fluid as the shear rate increases.
Thixotropic Behavior
Thixotropic or shear duration thinning, is associated with a loss of consistency of the fluid as the duration of shear increases.
The opposite of this behavior is
known as inverse thyrotrophic.

21. Applications :- Selection of lubricants for various purpose.
- we can choose an optimum range of viscosity for engine oil.
- for high load and also for speed operation high viscous lubricants is required.
In pumping operation
- for high viscous fluid high power will require.
- for low viscous fluid low power will require.
In making of blend fuel
- less viscous fuels easy to mix.
In the operation of coating and printing.

22. THANK YOU…