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rans6andrew
28th Oct 2016, 22:17
We went on a short rail trip last Sunday. The line through here is normally 4 lines, an express and a stopping in each direction but there was signalling problems closing the two express lines. All of the traffic was using the two stopping lines, including freight trains. Some services were cancelled and we had to wait a while on the platform. While we were waiting several HS125 trains went through at a good speed. Thanks to their streamlining (faired in gaps between coaches, faired in around the lower coach edges and bogeys, pointed engines at each end etc) they did little to disturb the air around us. Then a train of empty ballast wagons went through. The engine was flat front and back and the trucks behind it were lower, slab ended and open topped. The whole lot was probably not going as fast as the express trains but the turbulence it created was massive.

I wonder what the relative fuel consumption and weight of the two trains is.

G0ULI
29th Oct 2016, 04:46
To a reasonable approximation air resistance increases by a factor of four each time speed is doubled. An HS125 moving at twice the speed of a freight train would experience four times the air resistance, hence the streamlining.

Freight trains can be made up of many different types of wagons carrying loads that are constantly being switched around to reach different destinations. It simply isn't possible or practical to develop a streamlining system that wouldn't introduce considerable delays in making up a string of cargo wagons and increase transport costs.

It would seem reasonable to reckon that the fuel consumption of the two types is probably similar per unit distance covered despite the additional weight of a freight train and lack of streamlining. In the case of the freight train, the additional weight and inertia will make it considerably less susceptible to the effects of wind drag, especially at lower speeds.

lomapaseo
29th Oct 2016, 06:03
I wonder what the relative fuel consumption and weight of the two trains is.

Is this one of those trick questions where we have to keep track of who got off and how many got on at a station?

core_dump
29th Oct 2016, 06:09
If they streamlined freight cars, it would be far too difficult to hop on and hitch a ride.

Krystal n chips
29th Oct 2016, 06:21
Streamlining.....and turbulence.

You get the impression the OP doesn't actually travel by train very much.

There are rather prominent yellow lines painted on platforms.....for one very good reason...... the safety of pax.

"Freight trains can be made up of many different types of wagons carrying loads that are constantly being switched around to reach different destinations"

Alas, another classic from the myopic paradise of Norfolk.

Freight train rolling stock is almost invariably now dedicated to one type of freight at a time.

The days of mixed stock have all but vanished, although some dedicated engineering work / test and calibration stock may still comprise such and they tend to be very short in length. These trains are the exception, rather than the rule.

2 sheds
29th Oct 2016, 08:28
If they streamlined freight cars, it would be far too difficult to hop on and hitch a ride.
But you would avoid that terrible affliction of hitching a ride on a freight car - Boxcar Willy.

2 s

Allan Lupton
29th Oct 2016, 09:02
You will find that Sighard F. Hoerner's 1965 book "Fluid-Dynamic Drag" has a good deal about the aerodynamics of railway trains, if you have access to a copy.
As we aeroplane people know, aerodynamic drag is proportional to the square of the speed and therefore the power required to overcome it the cube of the speed.

What we have little experience of is bodies of the fineness ratio of a train and the importance of sealing what would otherwise be gaps between carriages. Some railway engineers, such as those at LNER who designed the "Silver Jubilee" trains, understood it and I think I remember that Hoerner covers it well. What I can't remember is what importance should be given to the nose shape of the train - I'd say that the A4 steamers' shape was more important as smoke deflection than streamlining.

radeng
29th Oct 2016, 13:06
In his book 'La Locomotive a Vapeur', Andre Chapelon devotes some 13 pages to streamlining. He states that the A4 shape provided smoke lifting as well as reducing air resistance. In 1884, Ricour of the Etat railway in France, by filling in the spaces round the spokes of the driving wheels obtained an increase of 10% in drawbar power at 100km/h. He also obtained an improvement by filling in the gaps between vehicles and extending bodysides downwards. In Germany, the Reichsbahn partially streamlined an 03 Pacific and obtained gains of 33, 85, 140 and 172 hp at speeds of 80,100, 120 and 140km/h respectively.

Meanwhile, the PLM in France found that streamlined rolling stock requires a streamlined locomotive to get the maximum improvement, while if modifications are limited in scope, it is more worthwhile to streamline the locomotive than the rolling stock. Chapelon considers that streamlining is really only justified for trains running consistently at 75 or 80 mph.

Stanwell
29th Oct 2016, 15:59
Penrith station, UK..
I remember seeing a pic of a prominent sign on a platform there.
It warned intending passengers not to stand close to the edge - otherwise they may be 'sucked off'.
Ooer!

ricardian
29th Oct 2016, 16:04
You will find that Sighard F. Hoerner's 1965 book "Fluid-Dynamic Drag" has a good deal about the aerodynamics of railway trains, if you have access to a copy.
As we aeroplane people know, aerodynamic drag is proportional to the square of the speed and therefore the power required to overcome it the cube of the speed.

What we have little experience of is bodies of the fineness ratio of a train and the importance of sealing what would otherwise be gaps between carriages. Some railway engineers, such as those at LNER who designed the "Silver Jubilee" trains, understood it and I think I remember that Hoerner covers it well. What I can't remember is what importance should be given to the nose shape of the train - I'd say that the A4 steamers' shape was more important as smoke deflection than streamlining.
A4 streamlining and an inadvertent thumb-print on a Plasticine wind tunnel model (http://www.lner.info/locos/A/a4.php)

lomapaseo
29th Oct 2016, 19:06
I remember seeing a pic of a prominent sign on a platform there.
It warned intending passengers not to stand close to the edge - otherwise they may be 'sucked off'.
Ooer!

I've never had much luck. Which way should you stand ... facing the engine or away? Does loose fitting garments help or hinder?

rans6andrew
29th Oct 2016, 21:56
Quote " In the case of the freight train, the additional weight and inertia will make it considerably less susceptible to the effects of wind drag, especially at lower speeds."

What!!!!????

How can weight or inertia have any relevance to wind drag? Wind drag must be related to shape, size and surface texture, weight or density don't enter into it.

meadowrun
30th Oct 2016, 04:03
I understood that as once you get a heavy weight moving at speed, it will just punch through the air resistance. Resistance is still there but overcome more easily by a more powerful force that has little inclination for stopping or slowing.


Little streamlining on train below, but wind resistance will have essentially nil effect in slowing it down.
http://cl.jroo.me/z3/V/_/u/e/a.baa-most-long-freight-train.jpg

radeng
30th Oct 2016, 10:21
Apparently cross winds make a considerable difference, too, at least with 'conventional' rolling stock. Plenty of references in the literature to cross winds increasing coal consumption.

PDR1
30th Oct 2016, 11:41
To a reasonable approximation air resistance increases by a factor of four each time speed is doubled.


A reasonable approximation, ignoring reynolds number effects and assuming the train remains subsonic, yes.


An HS125 moving at twice the speed of a freight train would experience four times the air resistance, hence the streamlining.


That's an approximation too far - the HS125 has a much lower drag coefficient, so the actual drag force experienced by the HS125 would be much less than four times that of the freight train.

It would seem reasonable to reckon that the fuel consumption of the two types is probably similar per unit distance covered despite the additional weight of a freight train and lack of streamlining. In the case of the freight train, the additional weight and inertia will make it considerably less susceptible to the effects of wind drag, especially at lower speeds.

The additional mass won't effect the steady-state drag (strange idea!). The steady-state power (and hence fuel consumption) of two trains will still be largely proportional to the square of their airspeeds. What the mass DOES affect is the transient responses - the heavier train will take more fuel to accelerate to a given speed and (for the same reasons) more fuel if the destination is at a greater height than the starting point.

Also the effect of cutting the power will be different. The deceleration when the power is cut will be a function of the mass/drag ratio, so a heavier train with similar drag will decelerate more slowly. But it won't make the heavier train "immune" to drag.

PDR

ExRAFRadar
30th Oct 2016, 12:53
That is a great photo meadowrun.

wings folded
30th Oct 2016, 13:58
How long is that bugger?

lomapaseo
30th Oct 2016, 14:06
Nice summary by PDR1

What also needs to be considered in this conversation is rolling friction. I suspect that rolling friction in trains is many times greater than aero drag but I would be happy for somebody to post some crossover graphs at two different weight levels.

Of course my interest is more along the lines of my car and its max speed :)

abgd
30th Oct 2016, 15:28
I'd be willing to bet that rolling resistance is fairly negligible compared to aerodynamic drag. Rails have a very low rolling resistance but even at lowish speeds aerodynamics becomes very important. As an example, the world record for faired bicycles on a reasonably level surface is now over 85 mph.

Allan Lupton
30th Oct 2016, 15:41
Nice summary by PDR1

What also needs to be considered in this conversation is rolling friction. I suspect that rolling friction in trains is many times greater than aero drag but I would be happy for somebody to post some crossover graphs at two different weight levels.

Of course my interest is more along the lines of my car and its max speed :)
At service speeds rolling resistance, trains or cars, is quite low cf. aerodynamic drag (see Hoerner op. cit.). Steel wheel on steel rail resistance is very low by any standard.
As for cars, I did some work on horsepower required for a Subaru I had in 1999 and calculated Hoerner's suggested value for rolling resistance power at 100 m.p.h. at just over 4 h.p. - aerodynamic drag was some 73 h.p. (the latter measured using downhill coasting).

ETA abgd answered while I was looking up my work of 1999! I bet those bicycles have tyre pressures high enough to have pretty low rolling resistance.

PDR1
30th Oct 2016, 16:29
Your Hoerner reference allows me to boast about one of my more treasured possessions, found in a 2nd-hand bookshop for 50p:

http://i925.photobucket.com/albums/ad93/sir_pdr/WP_20161030_14_56_01_Pro.jpg

As you can see, this is one of the original self-published edition, with it's typewritten pages and hand drawn diagrams:

http://i925.photobucket.com/albums/ad93/sir_pdr/WP_20161030_14_56_15_Pro.jpg

There's never a "smug" emoticon when you want one...

;)

PDR