Free Boundary Oscillations in Thermal Turbulence

 

 
  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) ]

 
 
In this experiment, a Rayleigh-Bénard convection cell is cooled in a laminar hood. The free-moving floater is introduced at the free surface , which covers only a part of  the open cell. A nearly parallel light beam is used to form a shadowgraph of the cell, which shows the position of the floater as well as the flow pattern.
 
 
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. 

 
 
Liquid-crystal beads show the temperature field (as well as the local velocity field) of the whole cell. The blue region indicates a hot, raising flow structure. 
 
 
A graph showing the movement of the free floater in a thermal turbulence: Periodic motion is recovered from thermal turbulence, the feedback mechanism overrides the stochasticity. The Rayleigh number is in this case on the order of 108. The aspect ratio is 3.3 and the floater covers 36% of the free surface of the convection cell.
 
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