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Fluid Mechanics Fundamentals

Physics | 7-14 yrs | Interactive

What is fluid mechanics?

In science, fluids refers to any substance that takes on the shape of its container or continously deforms.

Fluid mechanics is the study of gases and liquids at rest and in motion. This area of physics is divided into fluid statics – the study of the behavior of fluids at rest, and fluid dynamics – the study of moving fluids. Fluid dynamics is further divided into hydrodynamics or the study of water flow, and aerodynamics or the study of airflow.

Most problems in the field of fluid dynamics are too complex to be solved just by mathematical calculations. Such problems are solved by numeric methods using computer simulations. This branch of study is called numerical or computational fluid dynamics (CFD).

Real life applications of fluid mechanics

Understanding how fluids behave helps us in understanding the mechanism of flight and water currents. For example, fluid mechanics can be used to understand how aircrafts fly through the air, how ships sail through water and also changes in the weather patterns.

Main principles of fluid dynamics

  • Fluids are the substances that flow when an external force is applied to them.
  • Liquids and gases are both fluids.
  • Fluids do not have a definite shape and they conform to the shape of containers they are poured in.
  • The total force exerted by a liquid at rest on a given surface is called thrust.
  • The SI unit of thrust is newton.
  • A faster, moving fluid has less pressure than a slower, moving fluid.

Archimedes’ principle

If an object is immersed in a fluid, be it a balloon in air or a ship sailing in the ocean, it displaces the fluid, in which it is immersed. The amount of fluid displaced is equivalent to the weight of the object.

When a body is partially or fully immersed in a liquid, an upward force acts on it which is known as buoyancy.

Bernoulli’s principle

In 1738, a famous scientist called Daniel Bernoulli introduced the world to an interesting and important scientific theory. According to it, a fluid moving at a higher speed has lower pressure than a fluid moving at a lower speed. We can use Bernoulli’s principle to measure the speed of a liquid or gas moving in a pipe or over a given surface.

This principle also explains how aircrafts are able to take off from the ground. Because the wing of an aircraft is flat at the bottom and curved on the top, the air has to travel a greater distance along the top surface than along the bottom surface. To do this, air has to go faster over the top, causing its pressure to decrease. This makes a higher-pressure air pocket at the bottom of the aircraft which gives it a lift and makes it airborne.

What is fluid flow?

A fluid in motion is called a flow. It includes a wide range of fluid movement through the air, through a pipe or a channel, or just along a given surface. The flow of a fluid is classified into different types based on the properties of the flow.

Steady vs. Unsteady flow

  • Steady flow – If the movement of a fluid does not change over time, it is termed as a steady flow.
    For example : Water flow out of a tap. Though the flow is unsteady to start with, but with time it becomes steady.
  • Unsteady flow – If the movement of a fluid keeps changing with respect to time, then it is called an unsteady flow.

Different types of fluid flow

  • Laminar flow – A smooth flow of liquid is said to have a laminar flow.
  • Turbulent flow – An unsteady, irregular and chaotic flow is called turbulent flow. This type of flow may contain eddies and vortices etc.
  • Pipe flow – Such type of flow is in contact with rigid boundaries on all sides. One example would be water moving through a pipe or air moving through an air duct.
  • Open-S channel flow – It is a type of flow where there is a minimum of one free surface, not in contact with a rigid boundary. For example – water moving through a river, rain water on the streets, and irrigation canals.

3 Interesting facts about fluid dynamics

  1. Archimedes discovered hydrostatics in about 250 BC. According to a legend, he rushed out of his bath and ran through the streets of Syracuse shouting ‘Eureka!’
  2. It is believed that Bernoulli started studying fluids because he was interested in studying the pressure and flow of blood in the human body.
  3. Most of the universe is made of fluids, including our atmosphere and oceans, planets such as Jupiter, and stars like the Sun as well. Even rocks and metals can become fluid upon heating.
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