Table of contents  
Definition of Fluid  
Viscosity  
Newton’s Law of Viscosity  
Types of fluids 
Density or mass density (ρ):
Density or mass density of fluid is defined as the ratio of the mass of the fluid to its volume.
The unit of mass density is kg per cubic meter i.e. kg/m^{3}
Density of water is maximum at 4ºC.
Density of water at 4ºC = 1000 kg/m^{3}
Specific Weight or Weight Density:
Specific weight or weight density of a fluid is defined as the ratio of weight of a fluid to its volume.
Thus, weight per unit volume of a fluid is called weight density.
It is denoted by the symbol ‘w’.
Mathematically,
The value of specific weight of water is 9.81 X 1000 N/m^{3} in SI unit.
Specific Gravity or Relative Density:
Specific gravity is defined as the ratio of the density (or weight density) of a fluid to the density (or weight density) of a standard fluid.
For liquids, standard fluid is taken water and for gases, standard fluid is taken air.
Specific gravity is also called relative density.
It is dimensionless quantity and is denoted by symbol S.
Mathematically,
Specific gravity of mercury is 13.6.
Viscosity is defined as the property of fluid which offers resistance to the movement of one layer of fluid over another adjacent layer of fluid.
When two layers of a fluid distance ‘dy’ apart, move one over the another at different velocities, say u and u + du as shown in fig., the viscosity together with relative velocity causes a shear stress acting between the fluid layers.
The top layer causes a shear stress on the adjacent lower layer while the lower layer causes shear stress on the adjacent top layer.
This shear stress is proportional to the rate of change of velocity with respect to y.
It is denoted by symbol Τ (Tau).
Where μ (called mu) is the constant of proportionality and is known as the coefficient of dynamic viscosity or only viscosity
 represents the rate of shear strain or rate of shear deformation or velocity gradient.
Viscosity is also defined as the shear stress required to produce unit rate of shear strain.
Unit:
Its states that the shear stress (Τ) on a fluid element layer is directly proportional to the rate of shear strain.
The constant of proportionality is called the coefficient of viscosity.
Mathematically,
1. Ideal Fluid (Perfect Fluid)
A fluid, which is incompressible and is having no viscosity, is known as ideal fluid.
Ideal fluid is only an imaginary fluid because all the fluids, which exit, have some viscosity.
Used in mathematical analysis and flow problems.
Ideal fluid has no surface tension
2. Real Fluid
Possess the properties such as viscosity, surface tension and compressibility.
Offers resistance against flow.
All the fluids in practice are real fluids.
3.Newtonian fluid
A real fluid,in which the shear stress is directly proportional to the rate of shear strain(or velocity gradient), is known as the Newtonian fluid.
Example:Water, Air, Thin motor oil
4. NonNewtonian Fluid:
A real fluid, in which the shear stress is not proportional to the rate of shear strain(or velocity gradient), is known as the nonNewtonian fluid.
Example : Tooth Paste
5.IdealPlastic Fluid:
A fluid, in which shear stress is more than the yield value and shear stress is proportional to the rate of shear strain(or velocity gradient), is known as ideal plastic fluid.
Example : Sewage sludge
Surface Tension (s or σ):
Surface tensions is due to cohesion only.
Surface tension decrease with increase in temperature and it also act when fluid is at rest
It is a surface phenomenon
At critical point liquid and vapour are at same state thus surface tension is zero
s_{water }= 0.0736 N/m at 20ºC
s_{mercury} = 4.51 N/m
(i) Pressure inside a liquid drop in excess of atmospheric pressure
p=4s/d
p = Pressure intensity inside the droplet (in excess of the outside pressure)
d = Dia. of droplet
s = Surface tension of the liquid
Equation shows that with increase of diameter of the droplet, pressure intensity inside the droplet decreases.
(ii) Soap bubble
p=8s/d
(iii) Pressure inside a water droplet
p=2s/d
Capillary Action:
Capillarity is defined as a phenomenon of rise or fall of a liquid surface in a small tube relative to the adjacent general level of liquid when tube is held vertically in the liquid.
The rise of the liquid surface is known as capillarity rise while the fall of liquid surface is known as capillarity depression or fall.
It is expressed in terms of cm or mm of liquid.
Its value depends upon the specific weight of the liquid, diameter of the tube and surface tension of the liquid.
Expression for Capillarity Rise:
h = rise in capillary
σ = surface tension of water & glass
d = diameter of tube
θ = angle of contact between the liquid and the material
The value of θ between water and clean glass tube is approximately equal to zero and hence cosθ is equal to unity.
Compressibility :
It refers to change in volume/density due to change in pressure.
The compressibility is reciprocal of bulk modulus of elasticity (K).
Compressibility = 1/k
In compressible fluids the velocity of sound is given by
c = (K/ρ)^{1/2}
C = velocity of sound in fluid, K = Bulk modulus,ρ=density
2 videos122 docs55 tests

1. What is the definition of fluid? 
2. What is viscosity? 
3. What is Newton's Law of Viscosity? 
4. What are the different types of fluids? 
5. What are some common properties of fluids in civil engineering? 
2 videos122 docs55 tests


Explore Courses for Civil Engineering (CE) exam
