Lava Lamp Fluid Dynamics: Rayleigh–Bénard Convection (2D)

Heat Source & Sink

Heater power (base) 70%

Cooling (cap) 15%

Fluid

Viscosity 5

Wax blobs 12

Live Readout

Elapsed time

0.0 s

Mean fluid temp

Rayleigh number

Regime

—

Rising / sinking

0 / 0

Mean blob speed

0.0

How it works

A coarse temperature grid fills the tube. Every physics step it diffuses heat explicitly (finite differences): T += α·Δt·∇²T, with the bottom row clamped to the heater temperature and the top row clamped to the cooling temperature — a real (if simplified) 1-D conduction model, not a scripted gradient.

Each wax blob samples the grid at its own position, its internal temperature relaxes toward that local value, and its buoyant acceleration follows the Boussinesq approximation: a = g·β·(T_blob − T̄). Hot blobs near the base rise; as they climb into cooler fluid they lose heat, buoyancy fades, and gravity pulls them back down — the same heat-driven cycle a real lava lamp runs on, not a scripted up/down loop.

Ra = g·β·ΔT·H³ / (ν·α)

ΔT — heater minus cooling temperature, from the two sliders.
ν — kinematic viscosity, mapped from the viscosity slider.
Ra > ~1708 — the classic Rayleigh–Bénard threshold: below it, heat only conducts; above it, buoyancy overcomes viscous damping and organized convection currents form. The regime readout compares the live value against this threshold.

Hot / rising blob Cool / sinking blob Heat-diffusion field