Basic Fluid Properties Definitions
- A fluid is said to be real if it has a viscosity, finite compressibility, and surface tension.
- A fluid is said to be ideal if it is assumed to be both incompressible and non-viscous.
- Its Bulk Modulus is infinite.
- Ideal fluids do not have surface tension. In fact, ideal fluids do not exist in nature and are imaginary.
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Definitions of Important Terms Related to Fluid
The density of a fluid is defined as the mass of the fluid over an infinitesimal volume or mass per unit volume. It’s
The unit in the SI system is- kg/m³.
ρ = m/V
where, ρ = density
m = mass
V = Volume
Specific weight (ω) or weight density
- It is a ratio of weight to volume.
ω = weight/volume
ω = mg / V = ρg
where ρ = Density
g = Acc. Due to gravity
Remember- Specific weight of water is 9810 N/m³
It is a ratio of volume to the mass, i.e. Volume per unit mass.
Specific Volume = 1/ density = 1/ρ
Specific Gravity (S) or Relative density
It is the ratio of the density of the fluid to the density of standard fluid or Ratio of Specific weight of a fluid to the Specific weight of the standard fluid.
- The specific gravity of water at 4 °C is ‘1’.
- For Mercury Specific gravity is 13.6.
- Specific gravity varies with temperature, therefore, it must be determined at a specified temperature (4 °C or 27 degrees Celsius).
Newton’s Law of Viscosity
It says that the shear stress between adjacent fluid layers is proportional to the negative value of the velocity gradient between the two layers.
- It is Viscosity that offers resistance to the fluid flow.
- This is basically can be defined in two ways, namely- Dynamic Viscosity (µ) and Kinematic Viscosity (ν).
Dynamic Viscosity (µ)
- Its SI unit is pascal-second or N-sec/m²
- Its cgs unit, most commonly used, is Poise = Dyne-sec/cm²
- 1 Poise = 0.1 N-sec/m²
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Kinematic Viscosity (ν)
ν = µ/ρ
- Its SI unit is m²/s
- Its cgs unit is stoke = cm²/s
- 1 stoke = 10-4 m²/s
- The viscosity of liquids decreases with temperature whereas the viscosity of gases increases with an increase in temperature.
- The viscosity of liquids is due to cohesion and molecular momentum transfer.
- Liquids with increasing order of viscosity are gasoline, water, crude oil, castor oil.
- The viscosity of water at 1 degree Celsius is 1 centipoise.
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