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In 1966, Wilson asked an intriguing question:
"Did the Atlantic close and then reopen?" [Nature 211, 676, 1966].
There is considerable evidence that the Atlantic Ocean has closed and reopened
about 4-5 times in the past two billion years, with a period of about 300-500
million years. This period is now called the "Wilson cycle".
Our experiment demonstrates that in a turbulent state,
Ra=10(7-8), a periodic state can develop through a thermal feedback mechanism.
The floating continents behave like "thermal blankets", inducing
dramatic changes in the convection pattern. The modified flow pattern in
turn moves the continents to new positions. This two-way feedback mechanism
is the key to the unforced free oscillation.
This experiment was carried out at Rockefeller University in collaboration
with Albert Libchaber.
[Periodic boundary motion in thermal turbulence,
by Jun Zhang and A. Libchaber, Physical Review Letters, 84, 4361, (2000) ] |
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| A time series of the oscillation in the convection cell,
with Ra ~ 108, Pt ~ 6, Re ~ 50: (1) About 10 minutes after the floater
came to the left side of the cell, a hot raising structure is visible under
the floater. The system now has two large turbulent eddies, with
a large scale flow pattern on the right that tends to move the floater
to the right. (2) Right after the floater arrives at the right side of
the cell, the flow pattern is little changed and the system is temporally
stable. (3) 5 minutes after (2), a new hot structure is induced by the
floater and the flow pattern starts to reorganize. (4) The old flow pattern
is taken over by the new one. The large scale eddy on the left drags the
floater to the left.
The time scales are largely determined by the thermal
boundary thickness and the turnover time around the finite convection cell. |
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