Re: Walk-through trains

On 14 Aug, 01:49, Bruce <docnews2...@xxxxxxxxx> wrote:
On Thu, 13 Aug 2009 22:51:17 GMT, "Richard J."





<rjnews0...@xxxxxxxxxxxxxxxx> wrote:
Tom Anderson <t...@xxxxxxxxxxxxxxx> wrote on 13 August 2009 18:32:19  ....
On Thu, 13 Aug 2009, Basil Jet wrote:

Bruce wrote:
On Thu, 13 Aug 2009 12:22:13 +0100, "Recliner"
<recliner2-n...@xxxxxxxxxxx> wrote:
The ever-reliable Wiki source says that the 2009 stock is 2.68m wide
and the 1973 stock 2.629, so the 2009 stock is apparently 5cm or 2"
wider. It also says that, "Unlike the 1967 Tube Stock, the trains
are built 40 millimetres (1.6 in) wider to take advantage of the
Victoria line's slightly larger than normal loading gauge compared
to the other deep level tube lines."

Ironically, one of the reasons why the Victoria Line tunnel was built
to a larger diameter was to reduce air resistance.  ;-)

It's not unreasonable to build the first stock for the line small to
reduce air resistance, and then build subsequent stock large to push the
hot air along.

If the air's hot, then building the train bigger means there's less of it
surrounding the train, so the train won't get heated up by it so much.

On the contrary, the bigger train has more surface area so is in contact
with a greater area of hot air. Also, the smaller gap between train and
tunnel will increase the frictional heating effect.

The main heating effect comes partly from friction, but mainly from
turbulence.  The smaller gap significantly increases the turbulence
and therefore the heating.


But turbulence does not heat the air. Turbulent air generally stays at
the same temperature as the still air that was in position before.
Heat is all in the internal vibrations of the air molecules, not in
the bulk movement.
Heating still air will lead to turbulence (hot air rises), but
turbulence doesn't directly lead to heating; however for gases, the
frictional energy will be proportional to the velocity, so turbulence
will lead to greater friction and heating when the air interacts with
the tunnel / train.

The increased turbulence is caused because the ratio of the cross
sectional area of the tunnel to the area of the annulus (the gap
between train and tunnel) is much higher than before, so the train
will force the same amount of air through a much smaller gap.  The
result is higher flow velocities, which mean increased turbulence.

As I stated before, the Victoria Line tunnels were originally built to
a larger diameter in order to *reduce* air resistance.  While the
trains were larger than previous Tube stock, they only used up a small
proportion of the increased cross sectional area of the tunnel
compared with previous Tube lines.

.

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