Fluids and their properties

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1.What is Fluid?

Fluid is a substance that is capable of flowing.It has no definite shape of its own. It assumes the shape of its container. Both liquids and gases are fluids.Examples of fluids are : i. water ii. milk iii. kerosene iv. petrol v. emulsions etc.

2. Types of FluidsFluids can be classified into five basic types. They are:Ideal FluidReal FluidPseudo-plastic FluidNewtonian FluidNon-Newtonian Fluid

2.1 Ideal FluidAn Ideal Fluid is a fluid that has noviscosity. It is incompressible in nature. Practically, no ideal fluid exists.

2.2 Real FluidReal fluids are compressible in nature. They have some viscosity.Real fluids implies friction effects.Examples: Kerosene, Petrol, Castor oil

2.3 Pseudo-plastic FluidAfluidwhoseapparentviscosityorconsistency decreasesinstantaneouslywithanincreasein shearrate.Examples are: i. quick sand ii. ketch-up etc.

2.4 Newtonian fluidFluids that obey Newtons law of viscosity are known as Newtonian Fluids. For a Newtonian fluid, viscosity is entirely dependent upon thetemperatureandpressureof the fluid.Examples: water, air, emulsions

2.5 Non-Newtonian FluidsFluids that do not obey Newtons law of viscosity are non-Newtonian fluids.Examples: Flubber, Oobleck (suspension of starch in water), Pastes, Gels & Polymer solutions.

3. Properties of FluidsProperties offluidsdetermine how fluids can be used in engineering and technology.They also determine the behaviour of fluids in fluid mechanics. They are:DensityViscositySurface TensionCapillary ActionSpecific WeightSpecific Gravity

3.1 densityDensity is the mass per unit volume of a fluid. In other words, it is the ratio between mass (m) and volume (V) of a fluid.Density is denoted by the symbol . Its unit is kg/m3.

3.2 ViscosityViscosity is the fluid property that determines the amount of resistance of the fluid to shear stress. It is the property of the fluid due to which the fluid offers resistance to flow of one layer of the fluid over another adjacent layer.

3.2.1 Dynamic ViscosityThe Dynamic (shear) viscosity of a fluid expresses its resistance to shearing flows, where adjacent layers move parallel to each other with different speeds.

3.2.2 Kynematic ViscosityThe kinematic viscosity (also called "momentum diffusivity") is the ratio of the dynamic viscosityto thedensity of the fluid .

3.3 Surface TensionThe property of fluids to resist tensile stresses on their surface is called as Surface Tension.

3.4 Capillary ActionCapillary action is the property of fluid to flow in a narrow spaces without assistance of and in opposition to external forces like gravity.The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper and plaster, in some non-porous materials such as sand or in a cell.It occurs because ofintermolecular forces between the liquid and surrounding solid surfaces.

3.5 Specific WeightSpecific weight is the weight possessed by unit volume of a fluid. It is denoted by w. Its unit is N/m3.Specific weight varies from place to place due to the change of acceleration due to gravity (g).

3.6 Specific GravitySpecific gravity is the ratio of specific weight of the given fluid to the specific weight of standard fluid. It is denoted by the letter S. It has no unit.