I've seen many questions and answers about this topic, which keep saying that pressure decrease is the cause of acceleration of flow, and don't explain where that pressure drop comes from in the first place, while others say that pressure drop is just due to energy conservation as the flow speeds up.
My question is regarding ideal gases at low speeds. So, what is the actual reason that causes a drop in pressure (maybe explain it at molecular level), because if pressure is calculated from this relation: $PV=nRT$, and we don't observe a decrease in any of those factors, that would cause a drop in pressure. Also, given that pressure of a gas is due to collision of molecules with the walls and themselves, how does pressure decrease in terms of collision between surface and themselves?
Also, why don't we observe a drop in temperature if, in free gas expansion, the temperature drops momentarily as it gets turned to kinetic energy of flow, and then temperature returns back to normal as the kinetic energy of flow converts back. Why doesn't the same happen in this case?