[Little Demo] 06A – Catching Vortices


There are quite a lot of ways one may get in contact with chaos should there be sufficient interest. In the broadest terms and not really wanting to be helpful, to see some chaotic motion, you can stir your coffee, play snooker on an obtuse triangular billiard table, or just go out (scratch “out” for 2021), living your life normally.

One kind of chaotic system that matters a lot to humanity, though, is the turbulent flow in gaseous media — from that measly stream propelled by the CPU cooling fan as your computer struggles to load FWPhys, to the flow lifting and occasionally shaking an airplane; from the entirety of our planet’s atmosphere, to shocks and jets in any distant nebula.

Introduction and Aim

This upcoming series (Little Demo 06 A,B, and maybe even C) will be my build log of an experimental experiment to be submitted for consideration for the 2nd year physics cohort. In it, I hope to reveal some of the qualitative properties of turbulence, while introducing the able students to the modern suite of tools available to experimental fluid mechanists and researchers in related fields, assuming I can catch up myself:

This is not my PhD topic or vowed direction of research in any near future, but indeed a long-time personal curiosity since Berkeley: not frontier stuff, but a missing piece in my own training. I think I have been successful in motivating such an effort, but do need to admit the time frame may be quite stretched, even for FWPhys.

The project will be reported in three blog posts,

  • A. Introduction, Background, Aim and Preliminary Idea
  • B. Design, Build, and Use
  • C. Additional Matter. Maybe some rambling words on random walks as allegories of progress?

To conclude the intro, I guess, in terms of relevance, a properly constructed vortex visualization chamber can even be used to test the efficacy of facial masks. :p.

Home Made Vortices

It began in the afternoon of Jan 20, 2021. We were cleaning the kitchen after cooking, and the sun was shining directly on me through the kitchen window. Upon feeling this, I pulled down the little window roller, and was treated to the following sight:

Music by Iridium Point

The narrow incoming sunbeams offered what could be described as a little slab of illumination, and when viewed from almost the opposite direction, the motion of dust particles in air could be revealed as they appear bright against a dark background. Incidentally, this reminds me of the Millikan oil drops we already have in place.

I tried recording some of the dust’s dance with my phone camera, which, through its shallow depth of field, made the dust specks more pronounced than they appear to human eye. True randomness (or perhaps not…).

Planar laser-induced fluorescence

For a long time, people have been building controlled observation environments that run on similar principles to my sunset coincidence. The established technique is commonly known as planar laser-induced fluorescence (PLIF).

To begin with, one prepares the light source using a laser. With some simple optics, one may manipulate the beam into the shape of a thin slice (a Laser Sheet?), and if the gas being studied contains certain molecular species that are at resonance with the laser photons, fluorescence can be produced, enabling a camera normal to the laser sheet to study a two-dimensional slice of the dynamics of the gas.

I’ve managed to find an illustration of the set up.

By Mythealias. Wikipedia.

Annnd this is probably what I am going to build, plus some fancy computer vision code that tries to automatically analyze the footage. I will follow up on this thread as soon as time permits.

Cover image credits: Arrival (2016). (c) Paramount Pictures

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