Natural Convection — Buoyancy-Driven Flow & Rayleigh Number
Boussinesq approximation, Rayleigh number regimes, vertical plate correlations, enclosure convection, and stability of buoyancy-driven flows.
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Quick Explainer
What drives natural convection and how does the Rayleigh number characterize it?
Natural convection is driven by density differences from temperature gradients — hot fluid rises, cold sinks. Rayleigh number Ra = Gr * Pr combines buoyancy (Grashof number) and fluid properties. At low Ra, conduction dominates. Above Ra~10^9 for vertical plates, flow becomes turbulent.
How is natural convection different in CFD compared to forced convection?
Natural convection is fundamentally a coupled problem — you must solve fluid flow and heat transfer simultaneously because the density difference driving flow comes from the temperature field. The Boussinesq approximation treats density as constant except in the buoyancy term: rho = rho0*(1 - beta*dT), avoiding full compressibility while capturing buoyancy.