Termite Mound Passive Ventilation Stack

Mound Geometry

Mound height H 4.0 m

Chimney flue radius 0.30 m

Open peripheral conduits 6

Thermal Drive

Core–surface ΔT 8.0 °C

Draft Readouts

Stack (buoyancy) pressure ΔP

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Induced volumetric flow Q

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Nest air changes per hour

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How it works

Metabolic heat from the fungus garden and termites keeps the nest core several degrees warmer than the mound surface. That temperature gap makes the air inside less dense, and gravity does the rest — a "thermosiphon" or stack effect, the same principle that draws smoke up a chimney:

ΔP = ρ·g·H·(ΔT / T)
Q  = Cd·A_eff·√(2ΔP / ρ)
ACH = 3600·Q / V_nest

Warm, CO&sub2;-rich air rises up the central chimney, spreads into the porous ridged conduits near the mound surface where it cools and exchanges gas through the thin outer wall, then sinks back down those peripheral conduits to re-enter the nest — a closed convective loop, not air being pumped in and out. Here H is mound height, ΔT the core–surface temperature difference, ρ ≈ 1.2 kg/m³ air density, g = 9.81 m/s², T ≈ 300 K ambient, and A_eff the narrower of the chimney throat or the combined open peripheral conduits (they sit in series, so whichever is tighter sets the flow). This is a simplified single-loop model of the "induced flow" / thermosiphon hypothesis (Turner, 2001) for fungus-farming Macrotermes-type mounds — real mounds add wind-driven turbulent mixing at the surface and diurnal reversals as the outer wall heats and cools, both left out here to isolate the buoyancy term. Particle speed in the scene is exaggerated roughly 20× from the true centimeters-per-second draft so the loop stays visible.

Mound height and core–surface ΔT set the driving buoyancy pressure.
Chimney flue radius and open peripheral conduits set how much that pressure can push through — a narrow bottleneck throttles the whole loop even with a big ΔP.

Warm air, rising (chimney) Cooled air, sinking (ridge conduits)

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