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**FLUID PROPERTIES**

**Definition of Fluids:**- A fluid is a substance which flow continuously under the action of shear force (however small the force may be), as long as shear force is there the fluid moves or deforms.**E.g: -** Liquids, gases, vapour etc.

For a static fluid shear force = 0

**Density or mass density (S):**- Mass per unit volume of a fluid is called its density

s = (m/v)- Density of water is maximum at 4ºC.
- Density of water at 4ºC = 1000 kg/m
^{3}.

**Ideal Fluid (Perfect Fluid)**- Non-viscous, friction less & incompressible.
- Does not offer shear resistance against flow.
- Bulk modulus is infinite
- Used in mathematical analysis and flow problems.
- Ideal fluid has no surface tension

**Real Fluid**- Possess the properties such as viscosity, surface tension and compressibility.
- Offers resistance against flow.

**Specific gravity (G or S)**- G or S = Density of substance / Density of water at 4
^{0}C - Specific gravity for water is 1.0 at 4ºC and that of mercury is 13.6
- Specific gravity varies with temperature therefore it should be determined at specified temperature (4ºC)

- G or S = Density of substance / Density of water at 4
- S
**pecific weight (unit weight) or weight density (g')**

g'=rg

r = mass density

g = due to gravity, acceleration

__Newton's Law of Viscosity__

__Non-Newtonian Fluids __

- These do not follow Newton's law of viscosity. The relation between shear sterss and velocity gradient is

Where A and B are constants depending upon type of fluid and condition of flow.(i) For Dilatant Fluids: n > 1 & B = 0

Ex. Butter, Quick sand

(ii) For Bingham Plastic Fluids: n = 1 & B ¹ 0 Ex. Sewage sludge, Drilling mud These fluids always have certain minimum shear stress before they yield.

(iii) For Pseudoplastic Fluids: n < 1 & B = 0 Ex. Paper pulp, Rubber solution, Lipsticks, Paints, Blood, Polymetric solution etc.

(iv) For Thixotropic Fluids : n < 1 & B ¹ 0 Viscosity increase with time.

Ex. Printers ink and Enamels.

(v) For Rheopectic Fluids: n > 1 & B ¹ 0 Viscosity decreases with time

Ex. Gypsum solution in water & Bentonite solution.

**Viscosity**- Due to viscosity a fluid offers resistance to flow. It is due to cohesion and intermolecular attraction.

(i) Dynamic Viscosity ( m) Its SI unit is Pascal second or N-s/m2 Its CGS unit is Poise = Dyne-sec/cm2 1 poise = 0.1 N–s/m^{2}

(ii) Kinematic Viscosity

- Due to viscosity a fluid offers resistance to flow. It is due to cohesion and intermolecular attraction.

Its SI unit is m^{2}/s.

Its CGS unit is cm^{2}/s or stoke

1 Stoke = 10^{–4} m^{2}/s

- Viscosity of liquids decrease with temperature whereas viscosity of gases increase with temperature.
- fluids with increasing order of viscosity are air, gasoline, water, crude oil, castor oil.
- Viscosity of water at 1ºc is 1 centipoise.
- Viscosity of fluids is due to cohesion and molecular momentum transfer.

**Surface Tension (S)**- Surface tensions is due to cohesion only.
- Surface tension decrease with increase in temperature and it alos act when fluid is at rest
- It is a surface phenomenon
- At critical point liquid and vapour are at same state thus suface tension is zero s
_{water}= 0.0736 N/m at 20ºC

S_{mercury}= 4.51 N/m - (i) Pressure inside a liquid drop, in excess of atmospheric pressure

p= 4s/d, where s is surface tension d is dia. of drop

- (ii)Pressure inside a soap bubble, in excess of atmospheric pressure

p= 8s/d - (iii) Pressure inside jct

p = 2s/d

**Capillary Action**- Capillary action is due to adhesion and cohesion, both.
- Height of water in capillary tube

where,

g = Specific weight,

h = rise in capillary

ρ = density of fluid

S = surface tension of water & glass

r = radius of tube

θ = angle of contact between the liquid and the material

θ — 0º for water and glass

θ — 128º for mercury and glass

For capillary action dia of tube should be less than 3cm.

- When a liquid surface supports another liquid of density " r
_{b}", then rise in capillary is given as

__ Compressibility __

- It refers to change in volume/density due to change in pressure.
- The compressibility reciprocal of bulk modulus of elasticity (K).

**Compressibility = 1/k**

- In compressible fluids the velocity of sound is given by

C = velocity of sound in fluid

K = Bulk modulus of fluid

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