Welcome to Vacuvent’s student series. This series of posts is intended to supplement and help students and qualified persons in their exploration of the physical world.

Fluid mechanics is a vast field of engineering that influences other fields greatly, yet it is a field wherein a lot is still unknown or open for debate.

The field of fluid mechanics describes or influences anything from aero-technologies to breathing and even CGI to some or to a complete extend.

Fluid mechanics will almost always have an inherent thermo-mechanic component as we are dealing with liquids and gasses which are best described when temperature and pressure is considered in the system.

There are a few fields of interest in fluid mechanics that may require further reading, these include:

- Numeric solutions and CFD
- Differential equations
- Laws of conservation
- Eulerian vs Lagrangian descriptions
- Thermodynamic properties of fluids
- Dimensional Homogeneity

These topics should be disregarded for now at least until you know the fundamental concepts of fluid mechanics.

**What is a fluid?**

Before we answer this question, perhaps first we should consider the question that should precede the one asked above:

“What is the difference between fluid and solid states?

Most people will shrug at the question, as of course, we all know that one is… well solid and the other is liquid.

Unfortunately in order to understand something properly it needs to be defined suitably. This forms part of the process of how physics can describe our universe – it might not be the best method that will ever exist, but it works quite well for now.

So let us define the difference between a solid and a liquid, officially the definition goes:

“A solid can resist a shear stress by a static deflection, a fluid cannot.”

…

Let’s break this down.

A shear force- this refers to a force acting on a body in such a way that the body “sheers”. One can, in this definition, substitute shear force with normal force. A normal force is a force that acts on a body in the same direction as the support (or in other words the exact opposite direction as the applied force).

So now we know that a shear force or any force really, acting on a body of matter will somehow tell us whether the body is a solid or a fluid; the answer of how lies in the part of the phrase highlighted below:

“Resist…by static deflection”

The important bit is “static deflection”. When a force is applied to a liquid, the molecules in that liquid will move away from the force, interact, move past and away from each other the give way to the force (*insert star wars joke here*). A solid will deflect under the action of the force, but the molecules will remain in their positions relative to each other providing the force is not so strong as to tear the solid apart. Hence a solid can resist a force by static deflection.

Below is an illustration of the example, a normal force in introduced, and the object deflects. Internal shear forces develop within the body, do some further reading on this subject starting with stress/strain and Mohr’s circle.

Now we can answer our original question- what is a fluid?

Well, fluids are matter. For matter to be considered a fluid, it is defined simply as not being a solid.

Matter is defined as: “anything that has mass that takes up space”.

Remember our original definition defines both solids and fluids, but we need to realize that fluids are, in engineering terms, all gasses and liquids (in simplified terms), or again, any matter that is not solid.

We can comfortably state that all matter is either a solid or a liquid, of which there are two fluids namely- gasses and liquids.

Both fluid types are unable to resist a shear force as they are not solid, but how should we go about differentiating between the two?

We start by looking at the characteristics of both phases. Both phases will flow into a container taking the shape of the container, but liquids will sink to the bottom and gasses will expand.

Again formally:

A liquid is a form of matter with close packed molecules with strong cohesive forces that will form a free surface in a gravitational field while retaining its volume.

This means liquid will flow and form a film or surface when gravity is present. As below:

And formally a gas

Is a state of matter wherein molecules are highly excited and the matter will expand

These three states of matter form the base of fluid dynamics. Understanding the terms used and what these terms mean will assist in ones understanding of more complicated principles.