When in a train going through a longish tunnel, am I right in thinking it is the train pushing all the air out of the tunnel that causes a partial vacuum and hence makes my ears “pop”?
If so, why isn’t this offset by the air returning from the “rear” of the tunnel? Will the people sitting at the back of the train experience popping at all? Does it depend on the speed of the train, and tunnel dimensions? What is the speed of air returning into a vacuum in any case?
Actually, my WAG is that the air pressure goes up, not down, the reason being that the air in front of the train can’t ‘move out of the way’ as easily as it could if the train was travelling in the open air. Hence the air gets compressed.
Don’t know about front of train vs back, but I’d think that both the speed of the train (faster = more effect) and the tunnel dimensions (bigger = smaller effect) would play their part.
Think of it like a bicycle pump - the cylinder is the tunnel and the piston is the train. The faster you push the piston, the higher pressure you’ll generate, and if the piston’s not a good fit you won’t get as much compression.
There has been surprisingly much research. There’s even an “Aerodynamics and Ventilation of Vehicle Tunnels Symposium”.
I don’t think there’s really much air pushed through the tunnel. It’s rather because of the abrupt change in the way the air in front of the train moves, causing an initial pressure wave. Like the ‘fwoomp’ you hear when you hit the end of a pipe with your flat hand. After the train entered the tunnel, the pressure in front of it isn’t much greater than it would be outside. The train and the changed pressure are moving relative to and through each other, so there shouldn’t be a big difference in the changes between back and front of the train.
http://www.vki.ac.be/research/themes/ground99/1.html
This here http://shed.snu.ac.kr/~flowsys/CFD/applications/noma/CFD_noma_application.htm has color images of a simulation visualizing the pressure distribution over time.