The "rotating annulus" is a classic experiment in geophysical fluid dynamics. The inner metal cylinder is cooled and the outer metal cylinder is heated.  This sets up a radial temperature field in the fluid the drives a single meridional overturning convection cell.  When rotating, Coriolis forces deflect the horizontal branches of the cell leading a simple Hadley regime.  When rotation increases the Hadley regime becomes unstable to azimuthal waves.  The instability is baroclinic.  By changing rotation and heating one can view:

  1.  Stable Hadley flow
  2.  Instability onset
  3.  Instability equilibration 
  4.  Steady waves 
  5.  Vacillating waves
  6.  Shape vacillations
  7.  Turbulence

Motions are visualized by suspending aluminum flakes in oil.  The upper surface is imaged by a co-rotating camera, so one can think of this as looking at motions in the upper troposphere, for example. The filled and ice-charged apparatus, or rotating annulus cell, is placed on the Rotating Table Facility.   Various rotating and heating rates are set.  Spin up time between changes in external parameters is about 5 minutes - so it is best to talk about this as you go along.

Image of the cell placed on the rotating table (above).

prepare the experiment:

  1. Set the cell on the rotating table.  Connect the heater wires (red and green above) to the slip ring terminals that are connected to the heater supply transformer and Variac.  THE VARIAC CONNECTS TO THE GREY POWER BOX LABELED HEATER (NOT THERMOELECTRIC).  DO NOT CONNECT THE VARIAC DIRECTLY TO THE ROTATING TABLE SLIP RINGS (TO MUCH POWER)!
  2. Fill the center cylinder and square plastic container below with ice and water.  Insert the ice carefully through the top with a funnel.  Clean up any water spills in the annulus section.
  3. Fill the annulus section with 5.5 centistoke silicone oil laced with aluminum flakes.  After filling clean off surface bubbles with the small paint brush.
  4. Plug in the ac cords for the magnetic stirrer and the ring light.
  5. When ready to run, turn on the light, and the stirrer.  The stirrer should be set to a relatively low speed.
  6. Focus the set the exposure of the overhead camera.


The AV Library has a tape that shows many of the possible states in time-lapse.  This can be used to summarize the results of the experiment if you don't want to run all the cases in class.

Possible Settings:

65 non rotating radial motion, from hot outside rim to inner cold rim, on top surface
65 13 weak spiral Hadley cell
65 8 stronger spiral Hadley cell
65 3 as a transient, notice spin up, wavy instability, equilibration to amplitude vacillation state
20 3 weak amplitude vacillation
75 2 structural or shape vacillation
35 4 wave number 5