When an ideal gas in an adiabatic cylinder undergoes expansion, adiabatic cooling occurs and the temperature of the gas drops. This occurs because some of the gas's internal energy is used up as the system does work on the surroundings. This much I understand well. But I can't say the same for the Joule-Thomson Effect
When a real gas is passed through a porous plug for example, all the diagrams I have seen indicate that the size of the pipe before the porous plug is the same as the size of the pipe after the porous plug (https://cdn.comsol.com/cyclopedia/joule-thomson-effect/Porous-plug.ru_RU.png). So my first problem is: How does the gas's pressure decrease if it isn't even expanding after it goes through the plug? Many explanations I find say that the gas expands after it passes through a throttling valve and hence its pressure drops but clearly this can't be the case for a porous plug or any other throttling valve with constant volume on either side. So then why does the pressure drop after the gas has passed through the plug?
My second issue pertains to the fact that the Joule-Thomson effect is isenthalpic. I can't understand how this could be if the effect has anything to do with adiabatic cooling because adiabatic cooling is definitely not isenthalpic. The joule Thomson effect is more or less described by Peter Atkins as the process of allowing a real gas to "expand through a throttle (i.e porous plug) causing it to cool" without allowing heat to transfer in or out of the system (i.e. adiabatic). This description sounds almost identical to adiabatic cooling to me with the exception that the porous plug is involved. But I can't understand how a porous plug or any other throttle for that matter would lower the pressure let alone cool the gas if the pipes on either side are the same volume. The same volume on either side of the restriction indicates to me that there is no expansion occurring at all. If no expansion is occuring then the pressure shouldn't drop and neither should the temperature. Yet both these occur. So what is actually going on in the Joule-thomson effect?
Any help on this issue would be most appreciated!