We study natural thermal convection of a fluid (corn syrup) with a large Prandtl
number (103–107) and temperature-dependent viscosity. The experimental tank (1 × 1 × 0.3m) is heated from below with insulating top and side boundaries, so that the
fluid experiences secular heating as experiments proceed. This setup allows a focused
study of thermal plumes from the bottom boundary layer over a range of Rayleigh
numbers relevant to convective plumes in the deep interior of the Earth's mantle. The
effective value of Ra, based on the viscosity of the fluid at the interior temperature,
varies from 105 at the beginning to almost 108 toward the end of the experiments.
Thermals (plumes) from the lower boundary layer are trailed by continuous conduits
with long residence times. Plumes dominate flow in the tank, although there is a
weaker large-scale circulation induced by material cooling at the imperfectly insulating
top and sidewalls. At large Ra convection is extremely time-dependent and exhibits
episodic bursts of plumes, separated by periods of quiescence. This bursting behaviour
probably results from the inability of the structure of the thermal boundary layer
and its instabilities to keep pace with the rate of secular change in the value of Ra.
The frequency of plumes increases and their size decreases with increasing Ra, and
we characterize these changes via in situ thermocouple measurements, shadowgraph
videos, and videos of liquid crystal films recorded during several experiments. A scaling
analysis predicts observed changes in plume head and tail radii with increasing Ra.
Since inertial effects are largely absent no transition to ‘hard’ thermal turbulence is
observed, in contrast to a previous conclusion from numerical calculations at similar
Rayleigh numbers. We suggest that bursting behaviour similar to that observed may
occur in the Earth's mantle as it undergoes secular cooling on the billion-year time
scale.
Rotating turbulent thermal convection at very large Rayleigh numbers