I'm studying fluid dynamics and we just had a lecture about the momentum equation. We started the lecture by talking about pressure in terms of molecules moving across a hypothetical surface element and carrying their momentum with them (in both directions). There are 2 things confusing me about this:
1) we said the pressure is isotropic, but given I'm studying fluid dynamics, with this description it seems pretty clear that the pressure will point in the direction of the fluid velocity, and in a direction normal to that there will be (on average) no molecules moving in that direction so what would cause the pressure?
2) When deriving the momentum equation, we then also included the effect of momentum entering and leaving our arbitrary volume, but this seems to be exactly what we described pressure to be at the start of the lecture. So I think I have misunderstood the root cause of pressure: eg. is it particles colliding or just moving between regions and transferring momentum?
I have been thinking about this in frustration for hours now so any help would be appreciated!