The basics of fluid mechanics



Sitting yesterday watching water flow I just wanted to know more. I know this is advanced stuff but hey, it’s worth a shot!

In: Physics

I’ll do my best to explain as a student.

A fluid is a state of matter where it aims to occupy the surrounding volume. This can be a liquid or gas.

A liquid still has a bond between its particles i.e. it doesn’t break away or fall apart from itself, but the only structure it has is the volume it occupies. A gas has neither a bond or a structure.

Liquids cannot be compressed, meaning the pressure you put onto it, it will apply that pressure onto its surrounding volume without it changing size, but a gas can be compressed (compressing gases leads into thermodynamics)

A fluid will have set properties that determine how one can use it. Viscosity determines how easy it is for the fluid to flow and density is how heavy the fluid is per unit of volume.

Hydrostatic forces are forces that a body will experience due to the fluid, so submarines, dam’s, bridge structures etc. As you go deeper in a fluid, you will experience greater pressure acting on you, since you have more mass of the fluid to provide more force due to weight over the area of your body.

Buoyancy is essentially the difference in mass between the volume of the body, and the volume of the *fluid* that is displaced by the body. A lot of times this is overlooked and many are easily confused by this concept. This is why a pebble will sink but a ship will float.

There’s 2 kinds of flow for a liquid, external and internal, one describing ships and the other describing pipes, essentially.

It is often important to know how much fluid is travelling at any one time, and this is known as the volumetric flow rate Q. When a fluid flows in a pipe, it goes at a certain speed u, that travels a distance x in time t. The area of the pipe, if it were cut in half and looked at face on, would have an area a of πr^2 (area of the walls is irrelevant, since the fluid doesn’t flow there) assuming the pipe is circular. To find the volume, it is simply the distance x times the area, and the flow rate is then the volume over time: Q=au.

One thing to remember, is that fluid has a mass, since it is matter. The amount of mass flowing is simply m’ = ρQ (usually denoted by m dot) where ρ is density of the fluid. This is important to determine an equilibrium, a state of balance. If mass was lost, it would mean the system has unpredictable energy loss that isn’t accounted for, since matter cannot be created nor destroyed; if mass is lost, it is converted into energy (E=mc^2)

I have more to talk about but this is beyond a reddit comment and would require charts, diagrams and equations.