Description:

Title: Reversals of the Large-scale Circulation in Thermal Convection

Speaker: Nick Moore, Colgate University

Abstract: Thermal convection, or the tendency of heat to rise and cool material to descend, often gives rise to a large-scale circulatory flow structure. It is known that the large-scale circulation (LSC) can undergo spontaneous reversals. In the atmosphere, reversals can result in a sudden change in wind direction, while in Earth’s liquid core, reversals may play a role in magnetic dipole shifts. I will discuss LSC reversals in the context of 2D annular thermal convection. Through comparison with direct numerical simulations, I’ll show that a low-dimensional dynamical system derived systematically from Galerkin truncation of the governing equations accurately describes a sequence of parameter bifurcations, including the onset of circulatory flow, the appearance of chaotic LSC reversals, and finally a high-Rayleigh-number state of periodic LSC reversals with small-scale turbulence. When cast in terms of the fluid’s angular momentum and center of mass, the model reveals equivalence to a pendulum system with driving term that raises the center of mass above the fulcrum. It is the competition between driving, restoring, and damping that leads to the range of convective states. This physical picture yields accurate predictions for the frequency of regular LSC reversals in the high Rayleigh-number limit and offers a transparent mechanism for reversals. I will briefly discuss extensions of the model, including one that accurately recovers the gross heat transport.

Zoom: https://northwestern.zoom.us/j/98171601500

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