Discovering Train Friction: Measuring Aerodynamics on Rails

[konbakas]

Summary:: How can I measure the friction of a train with its rails and the air.

How can I measure the friction of a train with its rails and the air. You can assume that the face of the train is like the picture below.
If you need any specifications just let me know

 

[anorlunda]

Welcome to Physics Forums.

Your question sounds much like homework, so I moved it to this homework forum.

Before our homework helpers help, tell us how you would solve this problem. Clearly, your teacher thinks that you should be able to do it.

 

[jack action]

Does this post help?

Though, I'm not sure if this empirical method is meant to work with trains.

 

[konbakas]

Welcome to Physics Forums.

Your question sounds much like homework, so I moved it to this homework forum.

Before our homework helpers help, tell us how you would solve this problem. Clearly, your teacher thinks that you should be able to do it.

Thank, but that's not an assigned homework, its more like me trying to figure out a thought experiment. But I have nearly no experience in air-friction calculations, so I was hoping someone could enlight me in the mathematic formulas on this field or the thought process behind solving this kind of problem.

 

[anorlunda]

OK. One way to do it experimentally is to start with the train traveling at a known speed on level track. Then cut off the driving power, and observe how rapidly it slows down. The only thing that slows it down is friction and air drag (drag can be considered friction also).

 

[berkeman]

Another way is to find out how much power it takes to keep moving the train at a constant cruising speed. That energy input is what it takes to overcome rolling resistance and air resistance. Look into what the supplied voltage is for the electric train, and what the typical current consumption is at different speeds.

BTW, there is very little rolling resistance for steel wheels on steel rails. Instead, the primary "rolling resistance" will likely be from bearing friction in the axles and wheels, and any other friction losses in the drive train.

 

[256bits]

You can get a really really ball park answer by using some basic formula.
https://www.engineeringtoolbox.com/drag-coefficient-d_627.html
https://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html

Of course, all the the tabulated drag coefficients and rolling resistance coefficients are general.
For the train, they do do not mention what type of train - passenger, freight, loaded, high sped, The Bullet, length, velocity.. so any answer you get is most likely very general in scope if you use the numbers given.
At least you could work through an example just to play around.

Note that while the drag force equation has a v squared term, the power the train needs has a v cubed term ( force times velocity gives the power )

You can look up skin friction, and some of these other terms -Reynolds number - to explore further if you choose.

For the axle bearings, of the journal type, where the axle slides on a layer of grease, you can perhaps investigate that area also. And perhaps find something on how temperature affects the viscosity of the grease and subsequently the bearing friction. You may have noticed your car, if you drive, uses more fuel in the winter, and one reason is that colder oil/grease is more viscous at colder temperatures, and the bearings take longer to warm up in winter.

Engineering wise you won't get anywhere near an exact answer as you do not have all the pertinent information about your train.

 

[Lnewqban]

Thank, but that's not an assigned homework, its more like me trying to figure out a thought experiment. But I have nearly no experience in air-friction calculations, so I was hoping someone could enlight me in the mathematic formulas on this field or the thought process behind solving this kind of problem.

Welcome, konbakas!
Please, see:

https://www.mech.kth.se/courses/5C1211/Orellano_2010.pdf