Designing an Airtrack for Mechanical Experiments: How to Get Started

[sophiecentaur]

Hi
No I didn't read it thoroughly after I came to your 'little gaff' haha. You're not the first to fail to make a reality check. Me too, frequently.

Can you take me through this jet equation? Even there, you end up with a velocity of 300m/s. Is that realistic? Wouldn't a real system with those speeds involved make a huge amount of noise? 150holes! But, if the volume of air in equals the volume of air out, it should end up moving somewhat faster through the holes.

I think that vsquared equation must be too simplified for finding the air flow. It assumes laminar flow and also that pressure is the same all over the bearing surface. This won't be true. There will be more pressure away from the edge (hence, less pressure where you are applying that volume flow calculation). The air in the centre will stay there because its pressure is only a bit higher than the air next to it. (That earlier practical comment of someones, about needing no holes up there confirms what I say. There will be a steep gradient of pressure, I think, between holes and edge. As a reality check, we wouldn't dream of putting the holes right near the edge, would we? So the pressure must be different all over the underside.

I have no idea what the right solution is but I can't even believe an exit speed of 20m/s is right, now I think of it. That's 72km/hr! It would blow your hat off - and, as for the air coming out of the holes at 300m/s, that would drill holes in your skin I think.
This is turning out to be quite hard ain't it?

btw, If you are shifting 0.012m³ at 3.42kPa, doesn't that represent over 4kW? Sorry but I think that's right.

 

[cjl]

Hi
No I didn't read it thoroughly after I came to your 'little gaff' haha. You're not the first to fail to make a reality check. Me too, frequently.

Can you take me through this jet equation? Even there, you end up with a velocity of 300m/s. Is that realistic? Wouldn't a real system with those speeds involved make a huge amount of noise? 150holes! But, if the volume of air in equals the volume of air out, it should end up moving somewhat faster through the holes.

I think that vsquared equation must be too simplified for finding the air flow. It assumes laminar flow and also that pressure is the same all over the bearing surface. This won't be true. There will be more pressure away from the edge (hence, less pressure where you are applying that volume flow calculation). The air in the centre will stay there because its pressure is only a bit higher than the air next to it. (That earlier practical comment of someones, about needing no holes up there confirms what I say. There will be a steep gradient of pressure, I think, between holes and edge. As a reality check, we wouldn't dream of putting the holes right near the edge, would we? So the pressure must be different all over the underside.

I have no idea what the right solution is but I can't even believe an exit speed of 20m/s is right, now I think of it. That's 72km/hr! It would blow your hat off - and, as for the air coming out of the holes at 300m/s, that would drill holes in your skin I think.
This is turning out to be quite hard ain't it?

btw, If you are shifting 0.012m³ at 3.42kPa, doesn't that represent over 4kW? Sorry but I think that's right.

20 m/s sounds believable to me. That's not that fast, and it will dissipate fairly quickly coming out of the holes (especially due to the small jet diameter). As for the power required? The change in volume during compression is only going to be about 3% (assuming standard sea level conditions), so as a really quick estimate, the change in volume will be 3% of 0.012m³/s, and the average gauge pressure will be about 1.8 kPa. This gives a power required of less than 1 watt. Admittedly, this is a very quick and dirty estimate, but it should at least give a decent order-of-magnitude guess.

(300 m/s definitely sounds too high though - I definitely agree about that)

 

[sophiecentaur]

It isn't the change in volume that counts, surely. You are pushing all the air out with that pressure difference. Your argument would imply that an incompressible fluid would take no power to pump across a pressure difference.

 

[cjl]

True enough. I was in a hurry, and accounted for the compression work but not the flow work. Compression work, as I said before, is less than a watt, so that can safely be neglected. Flow work is still only 41 watts though (0.012 m³/s * 3.42 kPa).

 

[adamxrt]

I have taken everybodys concerns and considerations into mind and have kept them there for now.

this is a nasty old problem, and I am feeling my way though it as best as possible.
Sophiecentaur, youre so right about the crappy v squared equation, but its the best i can muster right now! lol I am literally just trying to get a raw feel for what's going on.


I have rejigged the whole lot and delved a bit deeper. I am still assuming laminar flow like you said, we'll tackle the matter of it probably actually not being that simple in due course. For now, I am looking at 13Kpa blower and 0.019m^3/s flow rate. I need to add in losses through an orifice to this somehow (not a clue how to, but will have to look) so i anticipate this pressure value possibly rising? not sure.

V=20 at glider edge @ Q=
V=15 at hole exit
v=6inside track
V=26 inside blower air inlet.

I also did a conversion for hydraulic diameter in ther somehwere, for the square airtrack tube. Does anyone know if this was the correct thing to do?
Anyhow, for anyone that has MATHCAD 15 i will upload the mathcad file, and for anyone that hasent, you will have to settle for a very very long RTF! lol


All feedback welcome, as usual point out silly mistakes and other crap! Its 5am here and I am exhausted, have an exam in 2 days and generally worn thin of this! But i think I am making progress.
thanks

 

[adamxrt]

Hey guys.i want to put a pendulum impact rig on one end of my track. I.E, have assembly attached to one of the endstops which can be deployed for use or retracted if its not needed.

the idea of this is to use the pendulum as a start mechanism for the gliders, so that we can control the speed we send them down the track at if it was to be in a horizontal position, i.e no acceleration down the track due to gravity.I need to design one pretty much for the end of my track (a small one) , and the impact needs to measurable. i have seen "impact pendulum" testers in the lab and online, and on these you are able to measure the impact though the use of a circular read out at the centre of the pendulums swing.

You think i should make a new thread for this bit?

Im not sure how to start.