Fluid Mechanics Fundamentals

Fluid mechanics is an engineering discipline that studies the mechanics of fluids as well as the forces that act on them. And it’s divided into two branches fluid statics, and fluid dynamics.

Fluid statics is the study of fluids at rest. While fluid dynamics is the study of the fluids at motion. 

Thermodynamic Properties of a Fluids 

There are four thermodynamics properties of fluids, which are as follows. 

  • Temperature
  • Density
  • Pressure
  • Specific enthalpy

How do the fluids flow?

Intermolecular forces on fluids are weaker than solids. Therefore, it’s easier to overpower intermolecular forces in fluids. When a force is applied, it overpowers the intermolecular forces in the fluid, and it starts to flow. 

Ideal fluid in fluid mechanics

An ideal fluid is one with no internal resistance to flow (zero viscosity) and is incompressible. Since it has zero viscosity, there is no shear force. And the motion of an ideal fluid is known as ideal or inviscid flow. 

Note: All the real fluids do have viscosities greater than zero. Therefore, such motion is referred to as viscous flow. In some cases of very high-velocity viscous fluid flow away from a solid surface have the qualities of an ideal flow. 

Newton’s law of viscosity in fluid mechanics

No fluid is fully non-newtonian; fluids are classed as Newtonian or non-Newtonian based on their ability to follow Newton’s law of viscosity.

Newtonian fluids

Newtonian fluids are ones with constant viscosity. So, the viscosity does not change when subjected to external stress. Newton developed this law under the assumption that all fluids are incompressible and do not exhibit turbulence. 

No fluid is entirely Newtonian, but certain fluids have extremely low compressibilities and so little turbulence on small scales, therefore we call them Newtonian fluids.

Non-newtonian fluids

The linear relationship between shear stress and deformation rate is incorrect in some fluids. Because the rate of deformation has an impact on the viscosity of these fluids.

Simply, Non-Newtonian fluids are the ones that do not obey Newton’s law of viscosity. 

Non-newtonian fluids are classified into three groups such as, 

  1. Pseudo-plastic
  2. Dilatant
  3. Bingham plastic

Computational fluid dynamics (CFD)

Computational fluid dynamics (CFD) uses computer-based analysis to simulate and solve problems with fluids and the flow. Complex problems involving fluid-fluid, fluid-solid, and fluid-gas interactions can be solved with CFD. 

The Navier-Stokes equations are used in computational fluid dynamics (CFD) to predict the flow of any liquid or gas.

Pascal’s Law

Pascal’s Law explains how any increase in pressure in a contained fluid causes an equal increase elsewhere in the container in an incompressible fluid.


Pascal’s law states that a pressure change in one segment of an incompressible fluid at rest in a closed container is transmitted without a loss to all components of the fluid and the container walls.

Bernoulli Principle

According to Bernoulli’s principle, an increase in the speed of a fluid is happens due to a drop in static pressure or the fluid’s potential energy. In other words, the pressure in a fluid stream decreases as the flow speed increases.

Law of conservation of energy in fluids

The law of conservation of energy ensures that the fluid’s entire energy is conserved, and the energy is neither created nor destroyed. The total energy remains constant, even though energy can change forms. 

Continuity equation in fluid mechanics

According to the continuity equation, the rate at which mass enters a system equals the rate at which mass exits the system. And the continuity equation is nothing more than the law of mass conservation during the flow of liquids or gas. Therefore, the volume of fluid entering any cross-sectional pipe should be equal to the amount of fluid exiting the opposite side of the cross-sectional area.

Mass is always conserved in fluid systems, regardless of the complexities of the pipeline or the direction of flow.


The viscosity of a fluid is its resistance to the flow of neighboring sections relative to each other. In other words, it’s the resistance to flow relative to an adjacent layer. And viscosity results from the interaction of various molecules in a fluid at the molecular level.

The formula for fluid viscosity

The ratio of shearing stress (F/A) to velocity gradient (vx/z or dvx/dz) in a fluid is called viscosity (represented by the symbol “eta”) (represented by the symbol “eta”).

The most common version of this relationship is Newton’s equation, which states that the resultant shear of a fluid is directly proportional to the applied force and inversely proportional to its viscosity. It should be obvious that the parallels to Newton’s second rule of motion (F = ma) are striking.

Harry Williams
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