Transitions in the style of mantle convection at high Rayleigh numbers

Weeraratne and Manga, Earth and Planet. Science Lett., 160,563-568, 1998. pdf

We do convection experiments with a glass box of corn syrup placed between 2 alum plates that circulate cold and hot water and maintain a constant temperature difference across the fluid layer. Thermocouples suspended from the top measure temperature at any location in the box over time. Experiment duration ranges from a few hours to .....

Temperature as a function of depth within the fluid layer along a vertical line. We see indications of a conductive lid, isothermal convecting interior and what is called an "active convecting layer" at the bottom and below the conducting lid (unstable region).

Shadow graphs of fluid motion within the convecting tank. Rising and sinking mushroom shaped plumes are visible at the bottom and top.

From looking at the temperature time-series obtained from thermocouple measurements (below), we are able to identify 3 regimes of convective behavior for a range of Rayleigh numbers and viscosity ratios (lambda). Steady convection, unsteady convection where convection cells may migrate within the fluid, and plume dominated behavior is identified in the regime diagriam to the right.

Three suites of experiments are shown here covering a range of Rayleigh numbers approaching that expected for planetary mantles. At highest Rayleigh numbers the Nusselt number falls off from the expected relationship. Black squares shows numberical experiments performed with infinite Prandtl number to test our results. We find the low Nusselt numbers at high Rayleigh numbers is do to approach high Reynold numbers where inertial forces may become important.

Convection experiments with Internal heating

Weeraratne, D. S., Convective Heat Transport in High Prandtl Number Fluids and Planetary Mantles, masters thesis, UOregon, 1999.