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HS2 or gigabit internet?

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Should we fund HS2 or free gigabit internet connections to every household and business in the UK
HS2
12%
 12%  [ 1 ]
Free gigabit internet connections
62%
 62%  [ 5 ]
Something else
25%
 25%  [ 2 ]
Total Votes : 8

cnb
1385151.  Wed Jul 14, 2021 3:44 am Reply with quote

suze wrote:
Now, it may be that building the line to accommodate 400 km/h isn't actually much more expensive than building it to accommodate 300 km/h and so they might as well. That goes well beyond Andy's knowledge let alone mine, but the guys he was speaking to didn't appear to think so.


I'm not a railway expert, but there are at least two things that make a 400km/h railway more expensive to build than a 350km/h railway.

1) Track curvature - you can have a smaller curve radius with slower trains, which makes it easier to avoid expensive land or ecologically important sites (which apart from the direct environmental impact generally require expensive mitigation measures), and the track can be better fitted to the natural ground contours requiring less cutting and filling (and potentially less bridging and tunnelling too)

2) Tunnels - a slower train can operate at the same efficiency in a narrower tunnel than a faster train, so all the tunnels could be slightly smaller. There are also complex engineering works required to prevent trains entering and leaving tunnels from causing sonic booms, and those get easier the slower the train goes (detail of this is something I'm sure PDR can explain).

 
suze
1385170.  Wed Jul 14, 2021 11:05 am Reply with quote

ali wrote:
That's Arthur Cayley.


Camnit all, yes it is!

Still, all in all it's probably better that I call that mathematician Andrew in here than that I ever call the good husband Arthur in a private moment!

 
CB27
1385180.  Wed Jul 14, 2021 12:03 pm Reply with quote

From the top, I'm not an engineer, so could be thinking absolute rubbish, but if I understand it the Rayleigh wave is generated from the impact the train wheels make on the track and therefore the ground the tracks are on.

Presumably the level of the speed as it increases the wave is affected by the weight and pressure of the train on the tracks?

If so, could we not have a system similar to that used in Maglev trains, but less powerful, to alleviate some of the weight of the train as it passes through a tunnel? It would not have to be as powerful as a maglev train because it won't need to lift and propel the train, only to reduce the pressure on the ground, and would only need to be used in tunnel sections.

As I said, I have no knowledge of these things, so it's simply my imagination thinking here.

 
Leith
1385189.  Wed Jul 14, 2021 1:42 pm Reply with quote

I don't know if that would help with the Rayleigh wave issue, but I suspect it would be difficult to reduce down force on a non-maglev train without seriously compromising the braking performance.

Friction between wheels and rails needs to be maximised for a quick and safe emergency stop. It's sufficiently critical that the emergency stop algorithm for automated train braking systems sometimes involves locking the front wheels so that they will scrub the track clean for the rest of the train.

 
PDR
1385196.  Wed Jul 14, 2021 2:35 pm Reply with quote

Leith wrote:
I don't know if that would help with the Rayleigh wave issue, but I suspect it would be difficult to reduce down force on a non-maglev train without seriously compromising the braking performance.


I guess I'm being thick, but I can't see how you could significantly reduce the rail loading on even a maglev train (let alone a wheeled one). The rail reacts the weight of the train. This weight could conceivably be reduced aerodynamically, but the amount of reduction available (especially with the upper surfaces bestrewn with overhead power cable tack) is going to be pretty negligible. It's an extreme case of the "slender body" (long, thin - aspect ratio barely making it into whole numbers of per cent) so even if it had a clean form and maybe a smooth undertray riding close to a smooth track you're still only looking at a few hundred newtons of downforce on an object with a weight in the hundreds of tonnes region. And it doesn't even meet that description because of the clutter on the upper surfaces and the chaotic underside & track surface.

What am I missing here?

PDR

 
Alexander Howard
1385197.  Wed Jul 14, 2021 2:53 pm Reply with quote

No - you're right. We non-engineers are just hoping maglev is magic.

 
Leith
1385200.  Wed Jul 14, 2021 4:56 pm Reply with quote

PDR wrote:
Leith wrote:
I don't know if that would help with the Rayleigh wave issue, but I suspect it would be difficult to reduce down force on a non-maglev train without seriously compromising the braking performance.


I guess I'm being thick, but I can't see how you could significantly reduce the rail loading on even a maglev train (let alone a wheeled one).

I don't suggest that you can reduce the rail loading for a maglev train - only that they are not dependent on rail loading to provide friction for braking (as far as I know).

Conversely, taking CB's suggestion of some sort mechanism to reduce rail loading of a conventional train in tunnels as a hypothetical, it wouldn't necessarily need to be done through the rails, since the goal isn't to reduce friction. You might, for the sake of argument, imagine an electro-magnetic "third rail", designed to take a partial load off the primary rails, or some such approach.

The fundamental problem, though, appears to be that reducing friction then seems like an unavoidable consequence, and one that would appear to have significant negative impacts on braking and traction, which would require additional complexities to compensate for.

Given that, at their simplest, the requirements for a train boil down to 1. Go forward, and 2. Stop going forward, I think this notional approach to mitigating Rayleigh wave impact is sufficiently at odds with the way conventional trains achieve those two things as to be unlikely an improvement on whatever approach the HS2 engineers are currently looking at.

While I have worked as a railway engineer in a specialised capacity (mainly braking software, signalling and track to train comms), I'm not a mechanical engineer or physicist, so happy to defer to the more experienced in those subjects.

 
PDR
1385201.  Wed Jul 14, 2021 5:41 pm Reply with quote

Leith wrote:
PDR wrote:
Leith wrote:
I don't know if that would help with the Rayleigh wave issue, but I suspect it would be difficult to reduce down force on a non-maglev train without seriously compromising the braking performance.


I guess I'm being thick, but I can't see how you could significantly reduce the rail loading on even a maglev train (let alone a wheeled one).

I don't suggest that you can reduce the rail loading for a maglev train - only that they are not dependent on rail loading to provide friction for braking (as far as I know).

Conversely, taking CB's suggestion of some sort mechanism to reduce rail loading of a conventional train in tunnels as a hypothetical, it wouldn't necessarily need to be done through the rails, since the goal isn't to reduce friction. You might, for the sake of argument, imagine an electro-magnetic "third rail", designed to take a partial load off the primary rails, or some such approach.


Yes, but that would still be putting a live (moving) load into the surface just like the rolling wheels would, so as far as I can see it would have pretty well the same effect in respect to Rayleigh waves. Which ever way you do it (assuming aerodynamics are not going to achieve anything) a 1,000tonne train has to be reacted by a 1,000tonne(f) pressure from the surface it's on - otherwise it would start accelerating downwards. If the train is moving then the pressure has to propagate horizontally, and the Rayleigh wave concept is a way of describing the fundamental limit on how fast it can propagate just like sonic shocks are a way of understanding the speeds that pressure waves can propagate in fluids.

But whichever way you look at it (assuming aero is out) you remain stuck with the fact that the weight of the train is reacted by the surface the tracks/maglev rails or WHY are sitting on.

PDR

 
crissdee
1385220.  Thu Jul 15, 2021 4:51 am Reply with quote

Same principle as the truck full of birds then, they may be flying, but they're pushing the floor of the truck with the same force anyway...

 
PDR
1385231.  Thu Jul 15, 2021 6:58 am Reply with quote

Depends on whether it's an open or closed truck, but yes.

PDR

 
CB27
1385246.  Thu Jul 15, 2021 11:23 am Reply with quote

In that case, if you're saying the force is the same, how do maglev trains go at such high speeds through tunnels without apparently causing this issue?

Again, I might be talking rubbish here, but I thought it's the difference between two sets of pressure.

The force of the wheels on the ground, the weight they carry, and their speed will determine the level of the wave. However, a strong static force that simply lifts the train a little without moving it forward is not contributing to the wave? I'm suggesting magnetic levitation, but same applies for any other force that can be used?

The mass of the train and the force lifting it are the same when static, but the force applied to the ground affecting the wave would then be mass of train less mass reduced by a force lifting it?

 
PDR
1385250.  Thu Jul 15, 2021 12:08 pm Reply with quote

A maglev train is lifted by the interaction between the magnetic field and the train/rail in various different ways depending on the specific configuration. By the weight of the train still ends up being carried by the rail regardless of whether the force is transmitter by physical contact or a magnetic field.

The Rayleigh wave is produced by the pressure on the ground moving forwards - picture a steamroller running over a toothpaste tube. When the steamroller moves slowly the toothpaste just squeezes out adding a messy mintiness to the road. If you increase the speed of the steam roller the the toothpaste has to propagate (move from inside to outside) faster. At a certain speed the toothpaste cannot squeeze out of the tube fast enough, so the steam roller climbs over it while it is still in the tube. The weight of the steamroller produces a large pressure inside the toothpaste because it cannot be relieved by escaping through the nozzle. The faster the steam roller moves the more quickly the pressure rises and the higher the resulting pressure in the toothpaste.

Now you're average toothpaste tube can take some pressure, but it's unlikely to be able to stand the several tonnes per square inch produced by standing a steamroller on it, so what happens is the toothpaste tube bursts [stop me if I'm getting too technical].

Now this is an analogy, so it only works at a simplistic level, but you can consider the tube of toothpaste to be the concrete rail bed. and the pressure in the toothpaste is the Rayleigh pressure wave. Concrete is a solid, so the concrete deforms elastically rather than squirting out the front, but there speed with which it can respond is limited. So if the pressure source is moving too quickly it cannot pass the deformation along to the next bit of concrete before more pressure is applied just like the toothpaste can't get out of the tube before the steamroller bursts it. So in the case of the concrete it reaches its elastic limit (the amount it can deform before permanently changes shape or ruptures) because the pressure can't be passed on in time to stop it.

This is not a good analogy so please don't take it too literally, but alternatives are more abstract (like filling a bath with water faster than it can escape from the plughole).

But note that nowhere in the above does it matter whether the load was applied to the concrete via a wheel or a magnetic field. It would only be different if the lifting force was aerodynamic because in that case the forces can dissipate much more quickly, and appear as general air pressure rather than a localised load.

But the aerodynamic load has exactly the same problem at the speed of sound - the pressure can't be passed on quickly enough so it all adds up into one big pressure wave called a "shock". This is a picture of Thrust SSC travelling at the speed of sound on the desert:



What may not be clear is that the shockwave pressure is digging a 2" deep swathe of desert sand just as effectively as a bulldozer (but at over 750mph) because the pressure couldn't get away fast enough.

PDR

 
PDR
1385251.  Thu Jul 15, 2021 12:34 pm Reply with quote

CB27 wrote:

The force of the wheels on the ground, the weight they carry, and their speed will determine the level of the wave. However, a strong static force that simply lifts the train a little without moving it forward is not contributing to the wave? I'm suggesting magnetic levitation, but same applies for any other force that can be used?

The mass of the train and the force lifting it are the same when static, but the force applied to the ground affecting the wave would then be mass of train less mass reduced by a force lifting it?


Firstly forget mass, as we are talking about weight here. Secondly a magnetic filed repels or attracts two objects - in the typical Maglev configuration it produces a repulsion force between the train and the rail (there are cases where you use an attraction force between the train and the underside of a rail, but for obvious reasons that's inherently unstable and much more difficult to control). The rail then pushes down onto the concrete bed [in a tunnel] and the planet pushes the ground up in an equal and opposite manner to maintain equilibrium and brings balance to the Force.

Then the train moves forward, so the force applied to the rail also moves forward in precisely the same way that it does with a wheel. So now we have a moving pressure wave, and all the baggage that goes with it.

I could draw a different analogy using ships, bow waves and hull speeds - but I'd rather keep that for later in case we need it.

PDR

 
Leith
1385252.  Thu Jul 15, 2021 12:44 pm Reply with quote

PDR wrote:
But whichever way you look at it (assuming aero is out) you remain stuck with the fact that the weight of the train is reacted by the surface the tracks/maglev rails or WHY are sitting on.

If everything is attached to the same bit of concrete, yes, of course, but perhaps using something other than the two primary rails to take the load of the train could conceivably open up the possibility of creative approaches to distributing the load differently.

That wasn't really the point of my original post though - I was just observing that any approach based on reducing the load of the train through the rails would likely also reduce mechanical traction, which is quite important to conventional trains.

 
PDR
1385256.  Thu Jul 15, 2021 2:12 pm Reply with quote

Yes, but whatever the load-bearing member is it will have to be within the standard rail footprint, so it's going to be putting the load into a similar area of the concrete which (in wave propagation speed terms) will be pretty-well the same place where the rest of the pressure is reacted.

I was thinking about describing two toothpaste tubes joined by a pipe, but this is getting too weird.

PDR

 

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