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I have noticed this weird phenomenon. When a tram brakes with the windows open, there is a huge volume of air entering the vehicle. Much, much more air gets in when it brakes than when it accelerates or has a constant speed. Even if it is moving very slowly, 1-2 Km/h, when it stops the volume of air is much greater than in any other situation. The volume of air does depend on the level of deceleration. If the tram brakes from a high speed there will be more air getting inside than if it brakes from a lower speed. Still, even at very low speeds the volume of air is substantial. The air current is always coming from the front to the back of the tram. I was wondering, could this have something to do with the pressure in front/ at the back of the tram? Like, having such a square front and back, maybe it creates more air pressure in front and less in the back, and when it stops, the air moves from the front to the back. But, could this happen even at very low speeds? I noticed that when two metro trains meet in a tunnel, from opposite directions, you can feel a small brake, both when they meet and when they part away. I assume this is caused by the pressure/vacuum they create. I don't think this is caused by a mechanism of the tram (like the brakes themselves) as there is no sound of air being released and the overall senseation is of a breeze sweeping the tram. Is this a known phenomenon ? Every time when I get home from work I notice this and it drives me crazy, so I decided to ask you guys, maybe you can explain to me why is this happening. Here are two pictures of the tram: http://metropotam.ro/mediaserver/v/q/012-19-1.jpg http://jurnalul.ro/thumbs/big/2012/10/26/tramvaie-23-27-40-trasee-modificate-weekend-doua-linii-speciale-18431793.jpg

Thank You

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  • $\begingroup$ Trams in what city? Can you provide a (link to a) photo of what they look like? $\endgroup$ – Han-Kwang Nienhuys May 25 '16 at 15:53
  • $\begingroup$ In Bucharest, Romania. Here is how they look like: metropotam.ro/mediaserver/v/q/012-19-1.jpg jurnalul.ro/thumbs/big/2012/10/26/… $\endgroup$ – Vlad Pomar May 25 '16 at 16:06
  • $\begingroup$ For some reason I can't edit the previous comment again. Anyway, I was wondering, could this have something to do with the pressure in front/ at the back of the tram? Like, having such a square front and back, maybe it creates more air pressure in front and less in the back, and when it stops the air moves from the front to the back. But could this happen even at very low speeds? I noticed that when two metro trains meet in a tunnel, from different directions, you can feel a small brake, both when they meet and when they part away. I assume this is caused by the pressure/vacuum they create. $\endgroup$ – Vlad Pomar May 25 '16 at 16:21
  • $\begingroup$ I doubt that this has anything to do with the aerodynamics of the tram, because of the direction of the flow and the fact that it doesn't depend on the vehicle speed. More likely, it's something like the brake system pulling a lot of air for cooling from the cabin space. Ask the driver and provide his answer here. $\endgroup$ – Han-Kwang Nienhuys May 25 '16 at 16:34
  • $\begingroup$ It does depend on the speed, if the tram is moving slower when braking there is also less air getting inside, I just wanted to point out that even at lower speeds there is a significant amount of "wind" created. I find it unlikely for it to be caused by the brakes, there is no sound of air being released and the overall sensation is of a breeze sweeping the tram. But maybe you are right, I mean, it is the best explanation I got so far. $\endgroup$ – Vlad Pomar May 25 '16 at 16:41
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It's difficult to give a definite answer based on the description of the phenomenon (I've never encountered it where I live), so here are a few hypotheses that you might test.

If the air displacement is directly caused by the acceleration and deceleration, you would expect the air to flow backward during acceleration and forward during deceleration. Moreover, the flow speed would be lower than the speed involved, and air flowing at a speed of 2 km/h would not feel like a breeze.

It could be that the brake system pulls in air for cooling purposes. You commented that this is not likely because you don't hear it. Try to find out where the air leaves the tram vehicle: through the windows on the back or through some venting aperture out of direct view?

It could be that the motor or brakes at the back are not so much actively pumping air, but that there is a column of rising air near the outside, at the back, that pulls out air via the back windows as a side effect. Try standing next to the back of the tram as it arrives and see whether the hot air is there. Try closing the windows in the back.

Maybe something opens in the front of the tram just before it stops, with the explicit purpose of refreshing the air.

And of course, you could ask the driver...

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  • $\begingroup$ Ok, I will do more "tests" and see what I can find. I'll come back with more details. I'll also ask a few friends to pay attention to this so I can get confirmation that it is not just me. Also, I'd like to ask, if some of you use the tram daily, maybe you can pay attention to this, who knows, maybe it just went unnoticed, but it also happens for you, too. $\endgroup$ – Vlad Pomar May 25 '16 at 17:40
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I'd rather guess it is effect of some external factor. Maybe wind around the tram station is typical for some certain direction, or something like so.

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