Guinness and the Physics of the Bubbly Flow

This month's lead paper in the American Journal of Physics[8] regards a subject that is dear to most grad students hearts, beer.  A team of scientists Limerick, Ireland report on research they did to determine why the bubbles near the wall of a pint glass of Guinness flow down instead of up.  The preprint[1] of the article can be found on arXiv.  The article inoduces fluid mechanics of the physics of the 'bubbly flow'.  In addition to the bubbly flow, the article introduces the 'anti-pint' (picture 1).  One wonders if the  phrase was coined while the research team[4] performed the experimental portion of their work, measuring the settling time of a Guinness Stout.

If you have a fear of fluid mechanics like I did until today, then this article is an excellent gentle introduction to the subject.  In terms of their relation to your Guinness, the article introduces the Reynolds number[2], the ratio of inertial to viscous forces in a fluid flow, and the Bond number[3], a parameter that defines the weight (pun intended), of gravity on the shape of a bubble, and Stokes Formula[6] to determine the drag force on bubbles in the beer.

The article surmises that the downward flow of bubbles is due to the Boycott Effect[5] discovered by Dr. Boycott in 1920.  If you've ever wondered why the doctor in medical dramas holds the test tube of blood at an angle, there's actually a reason.  The tilt influences the flow of sediment, (blood corpuscles in this case), and the settling time is reduced.  An example of this is shown in the video of Guinness in a tube below[7].

References:

1.  The article in question on arXiv:
http://arxiv.org/abs/1205.5233

3.  Bond Number:
http://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_number

4.  About the research team:
http://www3.ul.ie/wlee/sinking_bubbles.html

5.  Boycott Effect:
http://www.nature.com/nature/journal/v104/n2621/abs/104532b0.html

7.  The tilted Guiness video:

8.  The AJP article
http://ajp.aapt.org/resource/1/ajpias/v81/i2/p88_s1

On waves in Guiness:
http://www.chem.ed.ac.uk/guinness/waves.pdf

Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems, there seem to be fewer explanations of the procedure for deriving the conversion, so here goes!

What do we actually want?

To convert the Cartesian nabla

to the nabla for another coordinate system, say… cylindrical coordinates.

What we’ll need:

1. The Cartesian Nabla:

2. A set of equations relating the Cartesian coordinates to cylindrical coordinates:

3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system:

How to do it:

Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables.

The chain rule can be used to convert a differential operator in terms of one variable into a series of differential operators in terms of othe…

The Valentine's Day Magnetic Monopole

There's an assymetry to the form of the two Maxwell's equations shown in picture 1.  While the divergence of the electric field is proportional to the electric charge density at a given point, the divergence of the magnetic field is equal to zero.  This is typically explained in the following way.  While we know that electrons, the fundamental electric charge carriers exist, evidence seems to indicate that magnetic monopoles, the particles that would carry magnetic 'charge', either don't exist, or, the energies required to create them are so high that they are exceedingly rare.  That doesn't stop us from looking for them though!

Keeping with the theme of Fairbank[1] and his academic progeny over the semester break, today's post is about the discovery of a magnetic monopole candidate event by one of the Fairbank's graduate students, Blas Cabrera[2].  Cabrera was utilizing a loop type of magnetic monopole detector.  Its operation is in concept very simpl…

Unschooling Math Jams: Squaring Numbers in their own Base

Some of the most fun I have working on math with seven year-old No. 1 is discovering new things about math myself.  Last week, we discovered that square of any number in its own base is 100!  Pretty cool!  As usual we figured it out by talking rather than by writing things down, and as usual it was sheer happenstance that we figured it out at all.  Here’s how it went.

I've really been looking forward to working through multiplication ala binary numbers with seven year-old No. 1.  She kind of beat me to the punch though: in the last few weeks she's been learning her multiplication tables in base 10 on her own.  This became apparent when five year-old No. 2 decided he wanted to do some 'schoolwork' a few days back.

"I can sing that song... about the letters? all by myself now!"  2 meant the alphabet song.  His attitude towards academics is the ultimate in not retaining unnecessary facts, not even the name of the song :)

After 2 had worked his way through the so…