Rayleigh–Bénard_convection

# Rayleigh–Bénard convection Heat a shallow layer of fluid from below and nothing happens — until it does. Below a threshold the layer conducts [[Heat_transfer|heat]] quietly; above it, the fluid organises into a regular array of rolls or hexagonal cells, warm fluid rising in one place and cool sinking in another. It is the cleanest laboratory demonstration of [[Pattern_formation]] there is, and the standard exhibit for [[Self-organization]] appearing at a sharp [[Bifurcation_theory|bifurcation]] rather than gradually. ## The threshold The control parameter is the Rayleigh number, a [[Dimensionless_quantity]] weighing buoyancy against [[Viscosity]] and thermal [[Diffusion]]. Below roughly 1,700 for a layer between rigid plates, any disturbance dies; above it, one wavelength grows faster than the rest and sets the cell size. The pattern is not imposed — it is selected, which is exactly why the system is cited so often in [[Complex_system|complex systems]] and [[Emergence|emergence]] arguments. Push further and the cells oscillate, then wander, then break down into [[Turbulence]], a route to [[Chaos_theory|chaos]] mapped in detail. ## Where it runs the world The [[Sun]]'s granulation is convection cells the size of countries. [[Earth]]'s mantle convects on a hundred-million-year clock and moves continents. The [[Atmosphere_of_Earth|atmosphere]] does it daily, and the cellular cloud decks over cold ocean are the pattern seen from orbit — a [[Weather_forecasting]] signal and an [[Atmospheric_model]] test case. [[Edward_Norton_Lorenz]] built the [[Lorenz_system]] by truncating this exact problem to three variables, which is how a convecting layer launched [[Chaos_theory]]. **Microsim brief (next pass):** Heated layer seen edge-on; one slider for the temperature difference. Below threshold, flat colour; above, rolls nucleate and lock to a wavelength. Push further and they wobble, then break up. **On the spine:** [[Pattern_formation]] · [[Self-organization]] · [[Turbulence]] · [[Lorenz_system]] · [[Heat_transfer]] · [[WT!Thury_Hydrodynamics_Compendium]].  *Linked from the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]], section 5, When flow becomes unstable.*   **Microsim — three.js (Wikitube framework):** *Rayleigh–Bénard convection* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/thury/Rayleigh–Bénard_convection.html" data-title="Rayleigh–Bénard convection"></div> *Built from `MICROSIM_GUIDE/specs/variants/Rayleigh–Bénard_convection.json`; part of the [[WT!Thury_Hydrodynamics_Compendium|Thury Hydrodynamics Compendium]] set.*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Rayleigh–Bénard_convection) : [Wikitube](https://en.wikitube.io/wiki/Rayleigh–Bénard_convection) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_WT!Thury_Hydrodynamics_Compendium]], [[PORTAL_Physics]], [[PORTAL_Self-organization]], [[PORTAL_Emergence]]. --- *Fluid seed wave · 2026-09-10 · seed register · microsim layer pending.*