Comments on: The cow’s tubing

By: Corn

Corn — Tue, 16 May 2006 23:15:10 +0000

the skinny tubes worked well last year. you had to suck pretty hard to get it going but then the steady stream was easy to maintain. with the fat tubes maybe you get drunker faster which may or may not be a good thing considering how drunk i got last year. (probably a bad thing).

i think we need to do some testing..

By: christine

christine — Wed, 17 May 2006 01:31:39 +0000

I think this depends on too many factors. Assuming you held constant the length of the straw (which I’d argue is the biggest limiting factor), and that both straws were vertical — with flow going against gravity, — I feel like diameter is irrelevant for any system where: the diameter is not so small that head loss and adhesion forces cannot be overcome by the pressure differential human lungs can create, nor is the diameter so large that the gravitational forces (weight of liquid) cannot be accounted for either. So you either have less volume but more head loss, or more volume but less interference with flow. I’m pretty sure head loss is inversely proportional to straw diameter. So I say (and I am likey very, very wrong, in which case I blame the stroke) that for diameters inbetween the parameters I mentioned, it won’t be any easier or harder from one D to the next. Drinking a box of Hi-C is just as easy as drinking bubble tea. As long as the weight and head loss don’t exceed atmospheric pressure, liquid will flow into your mouth. Unless the liquid is super dense or super viscous, I feel like straw length is your main concern. And the magma straws were really long. I also feel like I did not earn my passing grade in fluid mechanics (I never went and copied most of the homework), so I’d ignore everything I just said. I’ll probably delete this comment later.

By: christine

christine — Wed, 17 May 2006 02:36:45 +0000

So, I need to correct myself. After thinking about this for the entire drive home, I realized what I said about “diameter so large that the gravitational forces… cannot be accounted for either” is completely wrong. The same diameter that increases weight/volume also allows for atmospheric pressure to act on a larger surface area, so it nulls itself out. Assuming identical liquids, identical atmospheres, and equal straw length, I say diameter doesn’t matter. Now I can finally stop thinking about this and go back to thinking about how I haven’t gotten any action in ages…

By: some dude

some dude — Fri, 19 May 2006 02:46:54 +0000

Steven J emailed me his answer, complete with his equations and spreadsheet.

“Bigger is better, with regard to suction. Of course, bigger tubing will be heavier when its full, and is harder to form a seal around. I recommend 1/2″ tubing– it requires 300 times less suction than 1/4″ tubing.

“Of course, it takes 1.4 psi to raise water 1 meter, so even additional .3 psi required to overcome the viscous forces in a 1/4″ tube isn’t that bad. Just don’t use a coffee stirrer.

“Now I need to determine the maximum possible size of a self-supporting, cardboard rooster tail.”

By: some dude

some dude — Fri, 19 May 2006 07:30:47 +0000

Steven J, your conclusion is what mine was last year, so I’d have to agree with you. Except I never actually calculated it. Nice spreadsheet.

More explanation for the non believers:
When you suck on the tube, you’re creating a pressure differential between the end of the tube and the bucket, which is what moves fluid. For a steady flow and if you neglect friction, the Bernoulli equation says that the pressure required to move the fluid depends on the height—where you’re sucking relative to the bucket in this case—and the velocity of the liquid.

The size of the tube makes no difference in the pressure needed to move the liquid a to a certain height and velocity.

Now you throw friction into the pina colada mix (this is what Steven J’s calcs are). You have to account for the energy (pressure) loss due to the friction from the wall of the tube. That first equation in his document says the pressure drop (delta p) is inversely proportional to the diameter (D). In other words, pressure drop increases as diameter decreases. That means the smaller the tube, the greater the pressure differential you need to apply (i.e. the harder you need to suck) to overcome that pressure drop and move the liquid to the same height and velocity.

So, ignore friction and size doesn’t matter, but include friction and bigger is easier to suck. (What exactly are we talking about again?)

Well actually, even without friction, the same pressure differential will give you the same velocity no matter the size of the tube, but a bigger tube will give you a higher volume flow rate, so (assuming the point is to drink) bigger is STILL better. … I think.

Christine, I think you pretty much had it right too. Length does seem like the biggie, but actually that’s more due to the fact that the diameter is changed in increments of 1/4 or 1/8 inch, whereas the length is changed in increments of feet. And of course, ~~Big Bird~~ Corn has it the rightest in that we should just do a drink test instead of talking about it.

But all those people that keep telling me “smaller is easier, duh,” yet didn’t bother responding here: Ha ha, I was right (…kind of… to be continued…)

By: some dude

some dude — Sat, 20 May 2006 17:42:45 +0000

So maybe “easier” could have been defined better. You don’t have to suck as hard for a bigger tube. But for a bigger tube, there is more air you have to suck out of the way before you get to the drink. That’s more cycles of suck, put tongue over tube, blow, remove tongue from tube, repeat, but the sucking is still easier. And the “smaller is better, duh,” crowd never brought this point up as an argument, so I’m still gonna claim, Ha ha I was right.

I guess someone could calculate the work (energy) required for this all and really figure out which is “easier.” But that someone is not gonna be me.