These long train noses are because of civil engineering short-sightedness, ironically enough... #RailwaysExplained
The only reason they are this shape is to reduce the sonic boom resulting from tunnels that are too narrow and have too tight a portal 🚇💥
The only reason they are this shape is to reduce the sonic boom resulting from tunnels that are too narrow and have too tight a portal 🚇💥
How does this happen? #RailwaysExplained
As the train enters the tunnel, it creates a pressure wave. This pressure wave reflects off the mass of air at the other end of the tunnel and then hits the train (which is still travelling through the tunnel) at the speed of sound. Boom!
As the train enters the tunnel, it creates a pressure wave. This pressure wave reflects off the mass of air at the other end of the tunnel and then hits the train (which is still travelling through the tunnel) at the speed of sound. Boom!
This isn't just a noisy nuisance... The sonic boom can injure passengers and staff, as well as damaging the train and tunnel structure. #RailwaysExplained
There are three options to avoid this:
🎯 bigger tunnels
🕳️ better portals
🚅 silly train noses
There are three options to avoid this:
🎯 bigger tunnels
🕳️ better portals
🚅 silly train noses
Bigger cross cross sections allow the reflected pressure wave to pass around the train without causing a sonic boom.
Doing this is easy enough for short tunnels, but things very quickly get expensive, particularly if you have to use a tunnel boring machine... #RailwaysExplained
Doing this is easy enough for short tunnels, but things very quickly get expensive, particularly if you have to use a tunnel boring machine... #RailwaysExplained
Designing the tunnel portal to dissipate some or all of the energy of the pressure wave so that little or no reflection occurs is the other option.
Here are two German high speed rail tunnel portals with and without a dissipative design... #RailwaysExplained
Here are two German high speed rail tunnel portals with and without a dissipative design... #RailwaysExplained
The design for High Speed 2 (the UK's new high speed line) has undertaken significant research into this topic, meaning that the proposed tunnel portals are probably the most advanced in the world (which is really cool)! #RailwaysExplained
Read more here: railengineer.co.uk/2019/10/02/hs2…
Read more here: railengineer.co.uk/2019/10/02/hs2…
@therailengineer @HS2ltd @HSRailGroup @HSRUniOfLeeds @bcrre The last option is having a really silly long train nose, like the newer Japanese bullet trains... These gradually spread the incoming compression wave, reducing the pressure gradient more gradually and mitigating or avoiding the sonic boom. #RailwaysExplained
So if ever someone asks "why the long nose", it isn't necessarily the punchline to a dad joke... Someone might well require you to launch this #RailwaysExplained thread at them!
If any of you found that interesting, feel free to drop me a coffee in the usual way... Also, please do keep asking questions for new #RailwaysExplained threads 🤓
ko-fi.com/garethdennis
ko-fi.com/garethdennis
In hindsight, I should have probably slotted one of these in this thread somewhere... #RailwaysExplained
