You can add text in between [b][[/b]code[b]][/b] and [b][[/b]/code[b]][/b]  to keep space in the text. (I have already modified your text for you. :-)

Bernoulli's equation is valid for [u]steady fluid[/u] which can be represented by [b]stream lines[/b].

It is a different situation you have pistons on both side of the tube: this is a closed system.

We can use fluid model to simulate gas flow when the mean free path is much smaller than the size of the tube. (fluid dynamics)

The molecular model can be used when the mean free path is smaller than the size of the tube, and this is a good time to use particles to simulate the situation.  (However, a lot of computational power is needed: to simulate large number of particles!)