I would like to know what variables to consider when I experiment with pouring from bottles and the "glug" effect? I am a middle school science teacher and want to model a science fair project for my students. Specifically I want to investigate using a vent in the top of the handle of a gallon milk jug to alleviate the air glugging when pouring and achieve faster pouring rates. So far I believe the following factors are in play (or would at least seem to be worthy of consideration as possible variables):
Initial angle of tilt.
Opening size, both the pouring opening and the vent opening.
Stiffness of the container.
Viscosity of the liquid.
Atmospheric pressure.
At this point in time I intend to make assumptions that the last three are relatively constant given that I will be using standard plastic milk jugs and water from the tap. I do not believe the changes in atmospheric pressure will be significant but I do think that will make for a good thinking/discussion point for my students. 
I am particularly interested in the variables aspect of experimentation as I know from experience this is something my 6th graders are not good with. I am mostly looking for the obvious things, factors that even with their limited experimental and life experiences they can relate to.
Any input would be much appreciated!
 A: I'll just throw some ideas out.

Initial angle of tilt.

Higher the angle the higher the back pressure has to get before air will hold the fluid from flowing until it equalizes (glugs).  It will have longer time between "glugs" but this will also tend to slow the flow down.
At a very low angle air can enter while fluid exists at the same time even without a vent.

Opening size, both the pouring opening and the vent opening.

At some point the vent will be large enough to allow all the air to flow through at a high enough rate to prevent the back pressure from building and stopping the fluid flow.  Perhaps the experiment might start with a large vent that you partially cover.  The challenge is for angle X, how do you get the bottle to drain fast (adjust vent size so laminar flow occurs).  But this could be tedious and depending on the shape of the bottle it could he hard to get an obvious laminar flow.  Also the idea of turbulence and friction might be hard to convey.
The opening will help larger amounts of fluid to exit faster but equalization between air pressures will still have to occur.

Viscosity of the liquid.

A thicker fluid will take longer to pour but the same principles apply.
The other aspect is the shape of the opening because as fluid flows it will either create a laminar flow or a turbulent flow.  The ideal rate will balance the air vent inflow with the fluid outflow so that the fluid remains laminar because that will have the least friction and the fastest smoothest flow.

I would suggest looking into another experiment that shows the exchange between air and fluid in the form of a tornado.  This is a classic demonstration.  You can find many examples and construction ideas but here's one:

If you just flip the bottles upside down and let one drain into the other it is very slow.  However if you circulate the fluid with a little swirl a "tornado" forms and the air flows upwards rapidly and the fluid will spin and fall into the lower bottle.
There are many things you could learn from this and tease the kids with. Challenge them to find a fast way to move the fluid down (don't show them the swirl trick).  Then discuss why it works and what doesn't work, and perhaps prepare a little side material about how this happens in nature (low pressure/high pressure etc.).
