Definition Of A
Fluid: A fluid is a substance that deforms continuously when subjected
to a shear stress, no matter how small the stress is. Mathematically,
Shear
Stress, τ = F/A
Where F = Force component tangent to a surface
And A = Area of the said surface.
Consider
a substance placed between 2 closely spaced parallel plates so large that
conditions at their edges may be neglected (Fig. 1). The bottom plate is fixed,
and a force F is applied to the upper plate which exerts a shear stress F/A on any substance between the
plates. A is the area of the upper
plate.
Newton’s
Law of Viscosity states that the shear stress varies proportionally with the
rate of angular deformation for one-dimensional flow of a fluid.
Mathematically,
Shear Stress, τ = F/A = µu/t = µdu/dy
Where u = Velocity of fluid adjacent to the upper plate,
T = Thickness of the
fluid layer, and
µ = Fluid absolute
viscosity (proportionality constant)
du/dy = rate of angular deformation
Types Of Fluids:
1.
Ideal Fluids: These are fluids that have zero
absolute viscosity. This fluid does not exist in nature.
2.
Newtonian Fluids: These fluids show a
liner relationship between the applied shear stress and the rate of fluid deformation.
Examples of Newtonian fluids are water, oil, petrol, alcohol, and glycerin.
3.
Non-Newtonian Fluids: These fluids show a
non-linear deformation when a shear stress is applied. Examples of non-newtonian
fluids are printer’s ink, honey, and tomato sauce.
a.
Thixotropic Fluids: These non- Newtonian fluids
have absolute viscosities that depend on the immediate prior angular
deformation. They reduce in viscosity when subjected to a shear stress over
time and also tend to set when at rest. Honey is an example of a thixotrpic
fluid.
b.
Rheopectic Fluids: These non-newtonian
fluids increase in absolute viscosity when subjected to a shear stress over
time. Printer’s ink is an example of a rheopectic fluid.
c.
Shear Thinning Fluids: These
non-newtonian fluids decrease in absolute viscosity when subjected to increased
shear stress. Tomato sauce is an example of a shear thinning fluid.
d.
Shear Thickening or Dilatant Fluids:
These non-newtonian fluids increase in absolute viscosity when subjected to
increased shear stress. This kind of fluids are very rare.
References
1. Victor L. Streeter and E. Benjamin Wylie, Fluid Mechanics. McGraw-Hill, 1983.
5. https://www.pumpsandsystems.com/topics/pumps/centrifugal-pumps/newtonian-and-non-newtonian-fluids-0
No comments:
Post a Comment