Turbulent convection in a fluid heated from below occurs naturally in Earth's atmosphere and oceans where it influences climate and weather, in Earth's mantle where it contributes to the motion of continental plates, in Earth's outer core where it determines the magnetic field, in the Sun where it influences the temperature on Earth, and in many industrial processes where it may have significant economic consequences. This grant will support experiments under highly controlled laboratory conditions and in samples of idealized shapes where some of the central physical components of this process can be studied quantitatively. These components include relatively quiet fluid layers just above the bottom and below the top plate (the "boundary layers"), and a turbulent interior with highly fluctuating temperature and fluid-flow. A large convection cell, known as the "wind of turbulence", is superimposed on these interior fluctuations. Quantitative measurements will be made of the turbulent enhancement of the heat transport, of the temperature distribution in the interior, and of the wind dynamics. The highly quantitative experiments are of modest complexity and thus afford an exceptional diverse learning experience for both graduate and undergraduate students who participate in the work.
Turbulent convection in a fluid heated from below occurs in many geophysical, astrophysical, and industrial situations. This grant will support experiments under highly controlled laboratory conditions and in samples of idealized, mostly cylindrical, shapes where some of the central physical components of this process can be studied quantitatively. These components include top and bottom boundary layers, and a turbulent interior with highly fluctuating temperature and velocity fields. A large-scale flow (LSC) is superimposed on the interior fluctuations. Quantitative measurements of heat transport will be made at the largest possible Rayleigh numbers (dimensionless temperature differences) in a search for an ultimate regime where extrapolation to astrophysical conditions may become possible. Studies of the temperature distribution in the interior and of the LSC dynamics will be undertaken. A theoretical model of the LSC dynamics will be developed. The highly quantitative experiments are of modest complexity and thus afford an exceptional diverse learning experience for both graduate and undergraduate students who participate in the work.
Turbulent convection in a fluid heated from below is an important process in numerous natural and industrial processes. In the core of the Earth it creates the magnetic field which in turn protects us from harmful radiation from space, in the outer part of the Sun it affects the heat radiated to the Earth and thus the climate in which we live, in the oceans it creates large currents like the Golf Stream which makes northern Europe inhabitable, and in industry it plays important roles in many processes. The work we did with support by the NSF advanced the understanding of fundamental physical components of turbulent convection by carrying out experiments under idealized laboratory conditions. For instance, we measured the amount of heat carried by the turbulent flow from the bottom to the top of containers of various shapes filled with a variety of fluids, including water and compressed gases. We learned about the general laws of physics that govern this process. We found that the heat transport by the turbulent system could be enhanced by as much a 30% by gently rotating the sample about its vertical axis. The reason for this effect was found to be the formation of vortices, or little tornadoes, which would suck hot liquid away from the bottom end and cold liquid away from the top end of the sample, thereby transporting hot fluid to the top and cold fluid to the bottom. Another problem that we pursued was turbulent convection in a gas that could condense at the top cold surface of the sample. The physics of this condensation, or droplet formation, is important in engineering applications where the heat transport can be enhanced by the droplet formation by a factor of ten or more. It also plays a major role in the formation of clouds and thus is central to understanding our climate. We found that the formation of droplets in our system was "homogeneous", meaning that it is spontaneous and not provoked by nucleating particles of aerosol for instance. A major advance of our research was the finding that the turbulent convection state undergoes a transition to a new state, known as the "ultimate" state, when the temperature difference is, in some sense, large enough. This new state is the one relevant to most of the processes outlines above in the first paragraph, but it is extremely difficult to establish and study in the laboratory. In order to find it, we built a very large convection sample, holding about 2000 liters (550 gallons) of an especially suited fluid (sulfur hexafluoride at a pressure of 20 atmospheres) in a collaboration with scientists at the Max-Planck-Institute for Dynamics and Self-Organization in Goettingen, Germany. We now are studying the properties of this new turbulent state, and look forward to learning much more about how it behaves and what the fundamental physical reasons are for its characteristics.
