- What is fluid: A fluid is defined as a substance which deforms continuously when a shear force is applied to it.
- In other words, “a fluid is a substance that is capable to flow if allowed. The Flowing fluid does not have any shape but it defines or decides the shape of containing vessel”.
- Liquid, gases, or vapour show such properties, hence referred to as fluid.
- Ideal fluid: Ideal which does not offer resistance (No deformation). Ideal fluid show characteristics such as Incompressible, frictionless, and no viscosity. Ideal fluids are also referred to as imaginary fluids and they do not exist in nature.
- Real fluid: A real fluid that deforms continuously when subjected to shear force. All fluids which are occurring naturally called real fluids.
- Classification of fluids: Classification of fluid is described below:
- According to the behavior of fluid based on the externally applied pressure and temperature: the fluid is categorized into two types.
- Compressible fluids
- Incompressible fluids
“Density is a property of the fluid which is dependent upon the pressure and temperature. Variation in density to pressure and temperature may be small or large”.
- Compressible fluids: If the density of the fluid changes with respect to the pressure and temperature in appreciable terms, it is referred to as compressible fluids.
- Incompressible fluids: If there are no significant changes in the density of fluid with respect to moderate pressure and temperature, fluid is said to be incompressible fluids.
Liquids are considered incompressible, whereas gases are referred to as compressible fluids.
Based on the fluid behavior under the action of shear stress: fluid is also categorised into two types:
- Newtonian fluids
- Non-Newtonian fluids
The difference between these can be understood based on newton’s law of viscosity which is a plot between shear stress and velocity gradient (Shear rate).
- Newtonian fluid: Fluids that obey newton’s law of viscosity (i.e. fluids for which the ratio of shear stress to shear rate is constant). For All gases and mostly pure liquid, this is true.
- Non-Newtonian fluids: Fluid that does not obey the newton law of viscosity (i.e. fluids for which the ratio of shear stress to shear rate is not constant).
Examples of Non – Newtonian fluids: Toothpaste, gels, paints, and polymer solution.
- Properties of fluids: The properties of fluids are:
- Mass density (Specific Mass)
- Weight density (Specific weight)
- Specific volume
- Specific gravity
- Vapor pressure
- Surface tension
- Viscosity
- Mass density (ρ): Mass density of the fluid is defined as the ratio of the mass of fluid to the per unit volume. In the SI system, the unit of density is Kg/ m3. The density of pure water at 4° C (277 K) is 1000 Kg/ m3.
- Specific Weight: Specific weight is the weight of fluid per unit volume of the fluid. The unit of measurement of specific weight in the SI system is N/m3. The specific weight of pure water at 4° C (277 K) is 9810 N/m3.
- The relationship between the mass density and weight density of the fluid can be understood by the below equation:
- Specific weight = acceleration due to gravity x Mass density of the fluid
Where, g is the acceleration due to gravity 9.8 m/s2.
w = ρ g
- Specific Volume: Specific volume of the fluid is the fluid volume per unit mass. Specific volume is the reciprocal of the Mass density of the fluid. Unit measurement of specific volume in S.I system is m3/Kg.
- Specific Gravity: Specific gravity of the fluid is the ratio of the density of the fluid to the density of the standard fluid.
- For liquid, water at condition 4 °C (277 K) is used as the standard fluid. For gas gases air is considered as standard fluid at NTP conditions 0 °C (273 K) and 760 torr (101.325) KPA. Specific gravity is dimensionless, hence no unit of measurement.
- Vapor pressure: Vapor pressure of the liquid is the absolute pressure at which the liquid and its vapor are in equilibrium (equal state) or pressure exerted by the vapor over the liquid surface in the equilibrium state.
- Surface tension: The property of liquid surface film to exert tension is called surface tension. The S.I unit of surface tension is N/m. Surface tension is denoted by the sign (σ) sigma.
- Viscosity: The viscosity of a fluid is a measure of the resistance offered by the fluid to flow at a given temperature.
- Types of Viscosity: Kinematic and dynamic viscosity.
- Dynamic viscosity: Dynamic viscosity may be defined as the shear stress required to produce a unit rate of shear deformation. Dynamic viscosity is indicated by (µ). The unit of measurement for dynamic viscosity in the S.I system is Pa.s or Kg/(m.s).
- Kinematic viscosity: Kinematic viscosity is defined as the ratio of dynamic viscosity to the density of the fluid. Kinematic viscosity is represented by (υ). The unit of kinematic viscosity in the S.I system is m2/s.
Note: The viscosity of the fluid (gas and liquid) is independent of the pressure, whereas directly depends upon the temperature of the fluid. In the case of gases, viscosity increases with an increase in temperature. In the case of liquid visco
Image Credit: https://byjus.com/physics/fluid-flow/