Newton's law of viscosity and Newtonian fluids

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Newton’s law of viscosity is given by

Ƭ = µ du/dy


Where,

Ƭ = shear stress.
µ = absolute or dynamic viscosity.
du/dy = velocity gradient.
Newtonian fluids: Fluids which follow this law are known as Newtonian fluids.
Example of Newtonian fluids are Air, water etc.
But non Newtonian fluids are tar, paste, blood, glycerin etc. 
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Fluid and some of its properties.

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A fluid may be defined as a substance which deforms continuously when subjected to a shear stress or in other words a fluid is a substance which cannot withstand shear stress. Fluid mechanics deals with the behavior of fluids at rest and in motion. Fluids may be of liquid or gaseous forms. Liquid is practically incompressible and occupy definite volume and have free surfaces. Whereas, gaseous fluids are compressible. Gases can expand until it occupies all portion of any containing vessel. 


Properties of fluid:


Important fluid properties are mentioned below

Mass Density, ρ:

It is the mass per unit volume. The density of water at standard temperature and pressure is 1000 kg/m3 .

Specific volume Vs :

It is the volume occupied by unit mass of fluid i.e. specific volume is the reciprocal of density. Therefore, Vs = 1/ρ.

Specific weight, ɣ : 

Specific weight may be defined as the weight of a unit volume of a substance. For water at standard conditions specific weight is equal to 9.81 kN/m3 . The relation between specific weight and density can be written as ɣ = ρg.

Viscosity, µ:

Viscosity may be defined as the property of a fluid which determines the amount of its resistance to a shearing stress.
The viscosity of a liquid decreases with the increase of temperature due to decrease of molecular cohesion between molecules and this is responsible for reduction in liquid viscosity. But is case of gas viscosity increases with the increase of temperature due to greater molecular activity as the temperature increases.  

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What is system, boundary and surrounding is thermodynamics ?

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A system is defined as a region in space containing a specific amount of matter whose behavior is being observed.
The system is separated from its surrounding by a boundary. The boundary may be a real one or some imaginary surface covering  the region. The boundary may be at rest or in motion and may change its size or shape.
The term surrounding is restricted to those portions of matter external to the system which is thermodynamically affected by the changes occurring within the system.
Any thermodynamically analysis begins with the selection of the system, its boundary and the surroundings.  

Open and closed systems

A closed system (sometimes termed as a control mass) is a system for which no masses cross the boundary i.e. quantity of matter within the system remains fixed throughout the investigation. But energy is allowed to cross the boundary (in the form of heat and work).

An open system (sometimes termed as a control volume) on the other hand is a region in the space defined by a boundary across (in/out/through) which matter may flow in addition to energy (in the form of heat and work).
A system is termed as isolated system if neither matter nor energy is allowed to transfer across the boundary. A truly isolated system can only be obtained ideally.

Examples of closed systems


  • Mixtures of water and steam in a closed vessel
  • Gas expanding in a piston- cylinder

 Examples of open systems


  • Water entering boiler and leaving as steam
  • Gases flowing through turbine
  • Gas expanding from pressurized container, through a nozzle.

Example of isolated system: 


Open and Closed systems


Open system, closed system and isolated system
  • A thermo flask may be considered as approaching an isolated system.




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What is substance in thermodynamics ?

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In thermodynamics a working substance is defined as a fluid (liquid or gas) in which energy can be stored, removed or transferred through.
Example Air in IC engine, steam in boiler, water in hydraulic turbine.


Pure substance

A pure substance needs to be –
1.  Homogeneous in composition (i.e. single chemical species)
      2.  Homogeneous and invariable in chemical aggregation (i.e. chemically stable)


What is substance in thermodynamics
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What is Thermodynamics and what are objectives of studying thermodynamics ?

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Thermodynamics is the study of energy and its transformation. Most studies of thermodynamics are primarily concerned with two forms of energy – heat and work. Thermodynamics study includes quantitative analysis of machine and processes for transformation of energy and between work and heat. In classical thermodynamics a macroscopic viewpoint is taken regarding such matters. The term thermodynamics was first introduced by Lord Kelvin in 1849. The term comes from the Greek words therme (heat) and dynamics (power).

Objectives of studying thermodynamics


  •           Improvement of efficiency processes.
  •           Making the processes more non-polluting and environmental friendly.
  •           Study and research regarding alternative energy sources or transformation methods.


What is Thermodynamics
There is a difference between Thermodynamics and Heat Transfer. Heat Transfers deal with the rate of transformation of energy from one form to another. But Thermodynamics studies the equilibrium condition and change of that equilibrium condition to another equilibrium condition. 








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