All the engineering fluids can be classified into two broad classes and then further sub-classes as follows:
(1) Newtonian fluids
(2) Non- Newtonian Fluids
Newtonian Fluids: These are the fluids which follow the Newton's law of viscosity. So shear stress of the fluids is directly proportional to the the velocity gradient, so it varies linearly with it. Newtonian fluids have constant viscosity which does not depend upon the stress. Sometimes it may vary under some specific conditions when the velocity gradient changes with the layer to layer.
Non- Newtonian Fluids: These fluids exhibit non-Newtonian characteristics, i.e. shear stress is not linearly dependent upon the velocity gradient, so doesn't follow the Newton's law of viscosity.
T = A.(du/dy)^n + B ; Where A and B are constants which depend upon type of fluid and conditions imposed on the flow. Value of n is 1 and of B is 0 for the Newtonian fluids. n is also known as power index, and depending upon its value non-Newtonian fluids are further classified as
(a) If n>1, Dilatant (e.g. quick sand, butter, printing ink etc.)
(b) If n = 1, Bingham plastic (e.g. sewage, sludge, drilling mud etc.)
(c) If n<1, Pseodoplastic (e.g. paper pulp, rubber suspension, paints etc.)
Further types of non-Newtonian fluids:
(1) Time independent Fluids : In these fluids rate of deformation or velocity gradient depends only upon the shear stress.
(a) Thixotropic Fluids: These fluids show increase in apparent viscosity with time.
(b) Rheophetic Fluids: These fluids show decrease in apparent viscosity with time.
(2) Time Dependent Fluids: In these fluids fluids rate of deformation and viscosity depends both upon the shear stress and duration of its application.
Bingham plastic fluids: These require a certain minimum shear stress Ty called yield stress before they start flowing.
(1) Newtonian fluids
(2) Non- Newtonian Fluids
Newtonian Fluids: These are the fluids which follow the Newton's law of viscosity. So shear stress of the fluids is directly proportional to the the velocity gradient, so it varies linearly with it. Newtonian fluids have constant viscosity which does not depend upon the stress. Sometimes it may vary under some specific conditions when the velocity gradient changes with the layer to layer.
Non- Newtonian Fluids: These fluids exhibit non-Newtonian characteristics, i.e. shear stress is not linearly dependent upon the velocity gradient, so doesn't follow the Newton's law of viscosity.
T = A.(du/dy)^n + B ; Where A and B are constants which depend upon type of fluid and conditions imposed on the flow. Value of n is 1 and of B is 0 for the Newtonian fluids. n is also known as power index, and depending upon its value non-Newtonian fluids are further classified as
(a) If n>1, Dilatant (e.g. quick sand, butter, printing ink etc.)
(b) If n = 1, Bingham plastic (e.g. sewage, sludge, drilling mud etc.)
(c) If n<1, Pseodoplastic (e.g. paper pulp, rubber suspension, paints etc.)
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| Types of fluids - Newtonian and non-Newtonian. |
(1) Time independent Fluids : In these fluids rate of deformation or velocity gradient depends only upon the shear stress.
(a) Thixotropic Fluids: These fluids show increase in apparent viscosity with time.
(b) Rheophetic Fluids: These fluids show decrease in apparent viscosity with time.
(2) Time Dependent Fluids: In these fluids fluids rate of deformation and viscosity depends both upon the shear stress and duration of its application.
Bingham plastic fluids: These require a certain minimum shear stress Ty called yield stress before they start flowing.
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