INTRODUCTION TO FLUID

Course :- Diploma Mechanical EngineeringSub :- Hydraulics & Hydraulic Machines

Unit :- 1

Introduction of fluid• A gas is a fluid that is easily compressed. It fills any vessel in which it is contained.

•A liquid is a fluid which is hard to compress. A given mass of liquid will occupy a fixed volume, irrespective of the size of the container.

•A fluid is a substance that flows under the action of shearing forces. If a fluid is at rest, we know that the forces on it are in balance.

•A free surface is formed as a boundary between a liquid and a gas above it.

• Solid :When external forces stress and deform solids, the solid will regain their original shape when these external forces are removed [Elasticity].

• Fluid : it will continue to change shape in time even after the removal of the external forces causing the deformation.

• Ideal fluid: is a substance that is unable to resist internal shear and tensile forces.

• Fluids can be classified into 2 forms of matter• 1) Liquids• 2) Gases

PROPERTIES OF FLUID

Adhesion is the property of different molecules or surfaces to cling to each other. For example, solids have high cohesive properties so they do not stick to the surfaces they come in contact.

Cohesion is the property of like molecules (of the same substance) to stick to each other due to mutual attraction. 

a. Adhesive Forces = polar liquid has intermolecular forces with polar surface.

b. Cohesive Forces = intermolecular forces of the liquid for itself

Applications Adhesion is used for the proper functioning of glue, paint, tar, cement, ink etc.

Adhesive and cohesive forces together induce capillary action which is the principle used in lamp wicks. Synthetic fibers use wicking to remove sweat from the skin.

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Ideal Fluid :-is unable to resist F-the relative velocity would remain constant, even F is removed

Real Fluid :-is able to resist F due to 1) internal molecular activity 2) friction between the layers

VISCOSITY Viscosity: It is the property of the fluid by virtue of

which it offers resistance to shear or angular deformation. Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion.

Mollases,tar, glycerine are highly viscous fluids. Water,air,petrol have very small viscosity and are called

thin fluids

Newton’s Law of Viscosity

a liquid’s resistance to flow

= (du/dy)

Where, du/dy =velocity gradient

= coefficient of viscosity, absolute viscosity or dynamic viscosity.

Viscous fluids tend to cling to a solid surface.

• Syrup and honey are more viscous than water.

• Grease is more viscous than engine oils.

• Liquids are more viscous than gases.

• Lava is an example of a very viscous material

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Viscosity is the measure of a fluid’s resistance to internal shear stresses.

Dynamic Viscosity ()

= Dynamic viscosity, N.s/m2 or Pa.s = Internal shear stress, N ( =F/A)v = Velocity, m/s

= hv

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Let -A = the horizontal area of each layer-d = the vertical distance between their centerlines-F = internal shear force

A thin layer of fluid

The top layer is acted upon by F

The top layer will move with a velocity, v relative to the bottom

layer

Fig.:-1

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The kinematic viscosity of a fluid is the ratio of its dynamic viscosity to its density.

Kinematic Viscosity ()

n = kinematic viscosity, m2/s = Dynamic viscosity, N.s/m2 or Pa.s = Density of fluid, kg/m3

=

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The specific weight is the weight of the fluid per unit volume

= Specific weight, N/m3

W = Weight of fluid, N (W=mg)V = Unit volume, m3

= WV

The specific weight of water at 40C = 9.81 kN/m3

Specific Weight ()

SPECIFIC VOLUME

Specific Volume is the inverse of the mass density.

v= V/m or v=1/ The concept of specific volume is found to be more useful in the

study of flow of compressible fluids i.e gases

Specific Volume: Represents the volume per unit mass of fluid.

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The specific gravity is the ratio of the density or specific weight of the fluid to the density or specific weight of water, at a temperature of 4oC

Specific Gravity/Relative Gravity (S)

S = w

=

w

Dimensionless

Specific Gravity The ratio of the density of any substance to the density of some other substance taken as standard, waterbeing the standard for liquids and solids, and hydrogen or air being the standard for gases.

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Capillary action

Capillary action is the ability of a fluid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity.

Fig.:-2

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Capillary action

• Capillary Action = spontaneous rising of a liquid up a narrow tube

For a contact angle less than 90o, the liquid will rise in the tube, but the liquid can also fall if the contact angle is greater than 90o. If the liquid is water, solids with a contact angle less than 90o are called hydrophilic, the opposite is hydrophobic.

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APPLICATION OF CAPILLARY ACTION

• Capillary action is found in thermometer where fluid used in it automatically rises when comes in contact with higher temperature or falls down with lower ones.

• Capillary action can be performed to transfer fluid from one vessel to another on its own.

• Capillary action can be experienced in the half dipped cloth as well as on lantern

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Compressibility• : to compress something into a small space.

Liquids• Liquids are considered to be incompressible.• Liquids only change in volume even when

subjected to very high pressure.

Gases• Gases are very compressible. • Their volume can increase/decrease when

subjected to slight variation in pressure.

IMAGE REFERENCES

Sr. No. Source/Links

1. http://www.oocities.org/venkatej/mech/fluid_mechanics/viscosity.gif2. http://chemwiki.ucdavis.edu/@api/deki/files/8632/hg_h20_meniscus.png?revision=

1

CONTENT REFERENCES

A TEXT BOOK OF FLUID MECHANICS AND HYDRAULIC MACHINES , Dr. R.K.BANSAL , LAXMI PUBLICATIONFLUID MECHANICS AND FLUID POWER ENGINEERING , Dr. D.S.KUMAR , S.K.KATARIA & SONSA TEXT BOOK OF HYDRAULICS,FLUID MECHANICS AND HYDRAULIC MACHINES , R.S.KHURMI , S.CHAND PUBLISHING

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