"Fluid particle is an abstraction" (ANSYS simulations)

[mech-eng]

What does "fluid particle is an abstraction" mean?

https://confluence.cornell.edu/display/SIMULATION/Big+Ideas:+Fluid+Dynamics

In the second vid.

Thank you.

 

[DrClaude]

A real fluid is a collection of molecules. At the macroscopic level, you describe the fluid as a continuous substance, emerging from the collective behavior of the molecules, but without treating it as made up of molecules. In other words, you zoom out and don't see individual molecules, just a continuous fluid.

When you do fluid dynamics, you need to consider an infinitesimally small volume of that fluid. But as you zoom back in, you don't come back to a molecule (you never consider the behavior of individual molecules). You simply take smaller and smaller amounts of the fluid, keeping the same properties as the bulk of the fluid.

In that sense, the fluid particle is not a real particle (i.e., a molecule), but an abstract particle that has the properties of the fluid.

 

[mech-eng]

A real fluid is a collection of molecules. At the macroscopic level, you describe the fluid as a continuous substance, emerging from the collective behavior of the molecules, but without treating it as made up of molecules. In other words, you zoom out and don't see individual molecules, just a continuous fluid.

When you do fluid dynamics, you need to consider an infinitesimally small volume of that fluid. But as you zoom back in, you don't come back to a molecule (you never consider the behavior of individual molecules). You simply take smaller and smaller amounts of the fluid, keeping the same properties as the bulk of the fluid.

In that sense, the fluid particle is not a real particle (i.e., a molecule), but an abstract particle that has the properties of the fluid.

Then, does abstract mean here imaginary?

Thank you.

 

[DrClaude]

Then, does abstract mean here imaginary?

If you mean "imaginary" in the sense "doesn't really exist," then yeah, sort of.

But "abstract" is really the correct word:

2 : expressing a quality apart from an object

 

[mech-eng]

If you mean "imaginary" in the sense "doesn't really exist," then yeah, sort of.

But "abstract" is really the correct word:

For years ago, I encountered with "hypothetical planet". Does abstract here also mean "hypothetical"?
Thank you.

 

[DrClaude]

For years ago, I encountered with "hypothetical planet". Does abstract here also mean "hypothetical"?

Not at all. "Hypothetical" means that someone made a hypothesis that it might exist. Nobody is postulating that the fluid particle really exists, we know it doesn't. But we still use the concept of a fluid particle.