# Pressure, Viscosity, Thermal conduction?

1. Nov 20, 2005

### Raptor01601

I have been reading a number of aerodynamic texts, and it seems that everything in a fluid (gas) is transmitted by molecular collisions.

Pressure is a result of molecules colliding with each other and the walls of a container

Viscosity is caused by collisions between molecules in layers of fluid with different velocity

Thermal energy is "conducted" thru a gas by more energetic molecules colliding with less energetic molecules.

Is there anything that is communicated by some other "transport" mechanism in a fluid besides collisions?
I know that liquids have intermolecular forces because of how close the molecules are.

Since there is only two ways that a fluid can communicate forces to a body immersed in it 1) Normal Forces (pressure) 2) Tangential Forces (shear - friction)

In an "in-viscous " flow, you would still have fluid pressure caused by collisions, I assume that I could make the following argument:

If I was to make a fluid element my "control volume" then all collisions in my control volume would be "pressure communicating collisions, only if a property such as energy or momentum is "transported" outside my control volume would I say that thermal conduction or viscosity has been enabled.

Is this a proper analogy?

Last edited: Nov 21, 2005
2. Nov 25, 2005

### Staff: Mentor

That seems about right. Heat conduction and viscosity are interactions at the CV boundary.

Heat conduction implies a thermal gradient across some boundary, otherwise the fluid would simply transport energy by bulk transport.

Certainly, pressure, heat conduction and viscosity are all related to atomic collisions, and remembering back to elastic collisions, momentum is transferred from higher momentum/energy to lower momentum/energy.

3. Nov 28, 2005

### Rocketa

Thermal fluid problem can be studies in two scales, macro and micro.

If you try to study the problem in micro scale, there is no pressure, temperature, viscosity and so on. There is only kinetic energy of molecules in three directions.

Normally, we study the problem in macro scale, in stead of kinetic energy of single molecule. We use pressure, temperature....to define the properties of a bulk fluid or gas. Therefore, when you use pressure, temperature, and so on properties, even though you know these are results of molecular collision, but you don't need to think about the problem in micro-scale.

Rocketa

4. Dec 2, 2005

### Raptor01601

So when they talk about inviscid flow, are they just talking about the shear stress potion of the collision?

Even in inviscid flow you would still have to have collsions, otherwise how would pressure be comunicated to other parts of the fluid? Sound is also comunicated (propagated) through a fluid as a result of collsions. So, is it correct to say that an "inviscid model" just ignores the shear stress part of the collision? but still allows other properties (sound, pressure) to be comunicated?