Rayleigh–Bénard convection

Rayleigh–Bénard convection is buoyancy-driven circulation that develops when a fluid layer is heated from below and cooled from above. Above a threshold, the motion organizes into convection cells that transport heat.

Rayleigh number: A dimensionless number comparing buoyancy-driven flow with viscous and thermal diffusion effects. It indicates when a heated fluid layer becomes unstable to convection.
Thermal conduction: Heat transfer through microscopic interactions without bulk movement of matter. It is the competing transport mode before buoyant circulation sets in.
Mantle convection: Slow circulation of Earth's solid, deformable mantle driven by internal heat and buoyancy. It is a geophysical analogue, though mantle rock behaves as a solid over short timescales.
Bénard cells: Polygonal or roll-like circulation patterns observed in a fluid layer heated from below. They are the visible cells commonly associated with the instability.
Critical Rayleigh number: The threshold Rayleigh number at which a fluid layer first becomes unstable to convection under specified conditions. It marks the onset of circulation in the classical horizontal layer.
Buoyancy: The net force on an object or fluid parcel caused by pressure differences in a surrounding fluid. Heating makes fluid less dense, supplying the upward driving force.
Solar convection zone: The outer region of the Sun where energy is transported primarily by rising and sinking plasma. Its cellular motions arise from thermal buoyancy in a stratified fluid.
Convection rolls: Long, approximately parallel circulation cells that form in some convecting fluid layers. They are a common planform in horizontally extended convection.
Boussinesq approximation: A fluid-modeling approximation that treats density as constant except where it creates buoyancy. It captures thermal buoyancy while simplifying the governing equations.
Thermal diffusivity: A material property measuring how quickly temperature differences spread by conduction. It determines how rapidly the destabilizing temperature gradient smooths out.

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