Fluid Properties

Fluid Properties | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Definition of Fluid

A fluid is a substance which flows continuously under the action of shear force (however small the force may be), as long as shear force is there the fluid moves or deforms.
E.g: - Liquids, gases, vapour etc.
For a static fluid shear force = 0

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/m3
-Density of water is maximum at 4ºC.
-Density of water at 4ºC = 1000 kg/m3

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/m3 in SI unit.

Question for Fluid Properties
Try yourself:What is the unit of mass density?

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

-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  co-efficient 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:

Question for Fluid Properties
Try yourself:
What is the definition of specific gravity?

Newton’s Law of Viscosity

-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 co-efficient of viscosity.

-Mathematically,

Types of fluids

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. Non-Newtonian 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 non-Newtonian fluid.
-Example : Tooth Paste

5.Ideal-Plastic 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
swater = 0.0736 N/m at 20ºC
smercury = 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

The document Fluid Properties | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course Civil Engineering SSC JE (Technical).
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FAQs on Fluid Properties - Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

 1. What is the definition of fluid?
Ans. A fluid is a substance that can flow and take the shape of its container. It includes both liquids and gases.
 2. What is viscosity?
Ans. Viscosity is a measure of a fluid's resistance to flow. It determines how thick or thin a fluid is. High viscosity fluids are thick and flow slowly, while low viscosity fluids are thin and flow easily.
 3. What is Newton's Law of Viscosity?
Ans. Newton's Law of Viscosity states that the shear stress between adjacent layers of a fluid is directly proportional to the velocity gradient between them. It can be mathematically expressed as τ = μ(dv/dy), where τ is the shear stress, μ is the dynamic viscosity, dv/dy is the velocity gradient.
 4. What are the different types of fluids?
Ans. Fluids can be classified into two main types - Newtonian fluids and non-Newtonian fluids. Newtonian fluids have a constant viscosity that does not change with shear rate, while non-Newtonian fluids have a viscosity that varies with shear rate.
 5. What are some common properties of fluids in civil engineering?
Ans. Some common properties of fluids in civil engineering include density, specific gravity, compressibility, surface tension, and capillarity. These properties are important in various applications such as fluid mechanics, hydrology, and geotechnical engineering.

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