Fluid mechanics

Fluid mechanics

Fluid mechanics

Fluid mechanics is a physics branch that studies fluids at rest (static) and in motion (dynamics). Fluids are substances that do not have their shapes, such as water or air and adapt to the container containing them.

The study of fluid statics and dynamics has many applications in everyday life, from the design of pipes and engines to the calculation of air resistance at the speed of an airplane.

Eulerian and Lagrangian descriptions are two ways to characterize a flow.

This article will introduce you to the basics of fluid mechanics as well as the properties and uses of fluids.

Why do we need to learn about fluid mechanics?

It is an essential field of engineering because it helps us to understand how fluids interact with solid surface areas, how they flow through pipes, and how they are used in many industrial applications.

There are two main types of fluid mechanics: static fluid mechanics and dynamic fluid mechanics. Static fluid mechanics deals with fluids at rest, while dynamic fluid mechanics deals with fluids in motion. Both types of fluid mechanics are essential for engineers to understand.

What is a fluid?

A fluid is a substance that conforms to the shape of the container it is in and flows. Fluid mechanics studies the behavior of fluids in motion and their interaction with objects in contact with them. Fluids are divided into two categories: Newtonian and non-Newtonian fluids.

The most familiar examples of fluids are liquids and gases, but fluid mechanics also includes the study of plasmas, colloids, and complex fluids.

To simplify the study of fluid mechanics, ideal fluids are considered. The main characteristics of an ideal fluid are that it is inviscid, stationary, and irrotational.

There are two types of fluid motion: static and dynamic. In static fluids, the fluid is at rest and has no motion. The only forces acting on the fluid are those due to gravity and other external forces (such as air resistance). In contrast, in a dynamic fluid, the fluid is in motion and experiences both viscous and inertial forces.

What is fluid mechanics?

From a classical mechanics point of view, fluid mechanics is the branch of physics responsible for studying fluids at rest and in motion and their interaction with solid bodies. It is divided into two large sectors: static and dynamic.

What is fluid statics?

Fluid statics is the branch of fluid mechanics that deals with studying fluids at rest. In this type of flow, the forces acting on the fluid particles are balanced, so there is no motion. It concerns the pressure a fluid exerts on a body and the forces that keep fluid in equilibrium.

There are several principles to studying static fluids, such as Arquimedes principle.

The Archimedes principle is a key concept in fluid mechanics. It states that the upward force on an object submerged in a fluid is equal to the weight of the fluid displaced by the object. This principle can be used to calculate the buoyancy force on an object.

What is fluid dynamics?

Fluid dynamics is the branch of mechanics responsible for studying the movement of fluids. It is a vast discipline encompassing various sub-branches, such as hydrodynamics, aerodynamics, or kinetics.

In general, the principles of static fluid mechanics can be applied to all fluids, while the principles of dynamic fluid mechanics are specific to viscous fluids (fluids that resist flow).

The motion of a dynamic fluid can be either laminar or turbulent.

Laminar flow is a type of fluid motion in which the fluid flows in parallel layers without mixing. Turbulent flow, on the other hand, is a type of fluid motion in which the fluid mixes extensively between different layers.

Many principles and equations have been developed for the study of fluid dynamics, such as the Navier-Stokes equations, Bernoulli's principle,  the continuity equation, and the momentum conservation equation.

What states Bernoulli's principle?

Bernoulli's principle is a principle in fluid mechanics that states that an increase in the speed of fluid results in a decrease in the fluid's pressure.

This principle is named after Swiss mathematician Daniel Bernoulli. Bernoulli's principle can be used to explain how objects like airplanes can fly. When an airplane's wings are shaped in a certain way, they cause the air to flow faster over the top of the wing than under the bottom. 

This difference in speed creates a pressure difference, and it is this difference in pressure that ultimately lifts the airplane into the air.

Bernoulli's principle states that, in a steady flow, the sum of all forms of energy in a fluid along a streamline is the same at all points on that streamline. Therefore, the sum of internal, potential, and kinetic energy remains constant.

This principle is a consequence of the principle of conservation of energy.

What are the mechanical properties of fluids?

Fluids are substances that do not have their shape but rather adapt to the shape of the container they are in. They are made up of molecules in constant motion, which gives them certain properties:

  • The fundamental principle of hydrostatics establishes that the pressure at a point inside a fluid (hydrostatic pressure) is directly proportional to its density, to the depth of said point, and to the gravity of the place where the fluid is found.

  • They do not have a shape of their own but adapt to the shape of the container in which they are placed.

  • They are made up of molecules in constant motion.

  • They exert a pressure force on the surfaces they come into contact with.

  • They can flow; consequently, they can deform and expand when an external force is applied to them.

  • Liquid fluids experiments a surface tension. Surface tension is the force per unit length that acts tangentially on the edge of a free surface of a liquid in equilibrium and that tends to contract said surface. The cohesive forces between the molecules of a liquid are responsible for the phenomenon known as surface tension. The surface tension of a fluid can be measured by its ability to hold a shape.

  • Surface energy is the force that keeps fluids in contact with each other. This force is created by the attraction of the molecules in the fluid. The more attraction there is between the molecules, the higher the surface energy will be.

What kinds of forces act on fluids?

In general, fluids are subjected to three types of forces:

  • Gravity is a force that acts on all bodies and is stronger on heavier bodies.

  • Pressure is a force that is exerted on the walls of containers that contain fluids.

  • Viscosity is a property of fluids that makes them more or less sticky. The coefficient of viscosity is a fluid's resistance to flow. It is affected by the fluid's temperature, density, and pressure. The higher the coefficient of viscosity, the greater the fluid's resistance to flow.

How does pressure affect a fluid?

One of the most important concepts in fluid mechanics is that of pressure. Pressure measures the force per unit area exerted by a fluid on a surface. Pressure acts in all directions and is transmitted throughout a fluid. The pressure at any point in a static fluid is equal to the weight of the column of fluid above it. 

An important particular case is the atmospheric pressure exerted by the air’s weight we have over us.

Gauge pressure is a measure of the pressure of a fluid in relation to atmospheric pressure. It is commonly used to measure the pressure of liquids and gases. The SI unit for gauge pressure is the pascal (Pa).

Fluid mechanics applications

Fluid mechanics is used in a wide variety of applications, from aerospace and marine engineering to hydraulics and meteorology.

For example, in engineering, it is used to design airplanes according to the streamlined flow and the optimal angle of contact with the wings, rockets, and ships. In hydraulics, it is used to study fluid flow, optimize fluid transport, and design pumping systems and cross-section pipes. In meteorology, it is used to predict weather and climate.

Publication Date: September 21, 2022
Last Revision: September 21, 2022