Pressure change between passing trains

[mp01]

Hello everyone

When reviewing this https://www.flickr.com/photos/mikeplonk/31373141691/in/dateposted-public/ image that I took of light trails of two trains passing, I noticed that there is a kink in the trails of both trains at the point at which they pass, near the top of the image. I assume this is because of the low pressure area between the trains as they pass (Bernoulli principle?) but I was a bit surprised that the deviation of the trails looked so large. So, am I right to assume that this is the effect that's taking place, or does something else explain it?

Thanks in advance!

 

[Peter Mugridge]

First... test the theory.

Do other pictures of this nature taken at the same location show the same effect? If the effect is reproducible, then you may well be right.

Does a picture of a single train at this location show this effect? If so, then there is another factor in this somewhere.

 

[gimmea50anyday]

Close your mouth, pinch your nose and blow. Pressure differential eliminated!

 

[Abpj17]

I did something possibly related to this at uni - but I'm very fuzzy on it now.

Wikipedia attempt (I googled boundary layer - the math I did basically looked at what air did around a plane wing)

https://en.wikipedia.org/wiki/Flow_separation in particular:

All solid objects [train] traveling through a fluid [air] ...acquire a boundary layer of fluid around them [which] can be either laminar or turbulent.

...

When the boundary layer separates, its displacement thickness increases sharply, which modifies the outside potential flow and pressure field....

Another effect of boundary layer separation is shedding vortices [which could] cause vibrations in the structure that they are shedding off.

I note the kinks in the pic are most around where the track curves and suspect that this is where the air flow becomes most turbulent because of the combination of the curve and the shapes of the carriages. Aerodynamically it's quite different to two trains passing each other on a straight (ditto F1 cars etc.)

(And is it low pressure, or high pressure?)

Edit: more googling.

http://www.birmingham.ac.uk/Documents/college-eps/railway/RSSB-project.pdf < the guy that wrote these seems to be the/a national expert. While the slide pack is about trains passing structures, not trains passing trains, there is a nice graph on the pressure on one of the early slides.

and I've only skimmed the pics but fig 12 of http://pure-oai.bham.ac.uk/ws/files/16022306/Baker_2014a_Aeronautical_Journal.pdf seems relevant

 

[edwin_m]

If you sit on the correct side of a Mk3 coach, then when passing another train at speed you can feel the side moving with the pressure wave. It's entirely possible that such a shock would cause both trains to roll on their suspensions away from either other, and then to roll back and possibly overshoot slightly.

 

[mp01]

Thanks everyone.

Peter Mugridge - here (https://www.flickr.com/photos/mikeplonk/31117961390) is a similar image where the trains didn't pass in the frame and there are no kinks.

I didn't note down whether these were sprinters or pacers, but there's every likelihood that they were pacers which I think collective wisdom would regard as bouncy, so the theory of pressure causing additional bobbing, particularly on the curve, seems well-founded.

 

[Peter Mugridge]

If you sit on the correct side of a Mk3 coach, then when passing another train at speed you can feel the side moving with the pressure wave.

As it happens, I always try to sit on the right hand side on a Mk3 especially if it's a line like the GWML where there is a certainty of passing another Mk3 rake at high speed because I like to feel the "bang" as they pass.

here (https://www.flickr.com/photos/mikeplonk/31117961390) is a similar image where the trains didn't pass in the frame and there are no kinks.

Thanks; interesting - and it pretty much proves the other picture does capture an effect of the trains passing - whether or not direct air pressure or a result of the trains reacting or a bit of both.

 

[wingsoveryorks]

Pendolino's seem to create a tremendous pressure wave. Far more than any other train set I have encountered. If you are on a slam door stock, (Yes we still do have some up north. Crew doors) You have to lock them because the risk of them flying open is very real on the WCML. when passing a 'Pongo' at speed.

 

[edwin_m]

Possibly the curvy nose throws more of the air out to the side, rather than upwards with the more wedge-shaped ends of the HST and Voyager.

 

[TheManBehind]

An at-speed Desiro passing a 455 in the opposite direction will have the rather fun effect of slamming all the hopper windows shut on the 455 as it passes by. Makes the carriage duck!

 

[gimmea50anyday]

HST or mk4 passing a 185 causes the doors to implode slightly. When first introduced this caused an unsolicited emergency brake on the 185s bringing them to a sudden and almighty halt, and boy can they stop!

 

[Bletchleyite]

That's interesting as I haven't heard of the same effect on a 350 - the doors seem very solid compared with Electrostars which sound like they are coming off every time you pass a Pendolino.


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[damo44]

Hello everyone

When reviewing this image that I took of light trails of two trains passing

Can't add anything to the technical discussion, but I'd like to say how much I like the photograph.

 

[sk688]

That's interesting as I haven't heard of the same effect on a 350 - the doors seem very solid compared with Electrostars which sound like they are coming off every time you pass a Pendolino.

Electrostar doors seem solid after the pleasure of 319 and 321 doors every time you pass a train . LM's 319s are horrible at this , particularly when its a 350 that passes by

 

[mp01]

Can't add anything to the technical discussion, but I'd like to say how much I like the photograph.

Thank you very much!

 

[fairysdad]

A 444 or 450 overtaking a 455 can often cause the 455's windows to close.

(Whoops, missed - until Peter's post below - that this had already been mentioned. I shall go and hang my head in shame, just a good distance away from any overtaking Desiros!)

 

[Peter Mugridge]

An at-speed Desiro passing a 455 in the opposite direction will have the rather fun effect of slamming all the hopper windows shut on the 455 as it passes by. Makes the carriage duck!

A 444 or 450 overtaking a 455 can often cause the 455's windows to close.

It works in the same direction between Wimbledon and Raynes Park when a 455 is being overtaken by a 450 or 444. Quite a fun effect to watch.

 

[Envoy]

I can certainly feel a pressure wave when going through tunnels at speed. It seems to be worse on single bore tunnels such as Dinemore on the Marches line. This is because the displaced air has no escape space.

 

[j.crocker355]

I've noticed the same on the Jubilee line you seem to get a pressure wave that slams against the doors occasionally at speed, not so much on other lines

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