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:
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:
INVESTIGATE VARIOUS STATES AND PARAMETER SETTINGS (Explanations and Movies).
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.
|SETTING ON VARIAC||ROTATION PERIOD (sec)||STATE|
|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|