MY SIMULATOR

Prairie Dog Burrow

Two mounds, one breeze: how the wind pulls air through a burrow.

INTERACTIVE 3D

Wind 0 m/sDrive 0 PaFlow 0 L/sAir changes 0/minCO₂ 0%FPS 0

Prairie Dog Burrow Ventilation Lab: Mound Heights, Bernoulli Pressure and the Chamber CO₂

A prairie dog town has openings of different heights. Wind is faster higher up, so the taller mound sits at lower pressure and the burrow air is drawn through the tunnel from the low mound to the high one. Change the wind, the mounds, the tunnel and the number of animals, and watch the flow, the air exchange and the carbon dioxide in the nest chamber settle.

Scenario

Right now fresh

Wind speed above the ground

Log law. The two dashed lines are the openings; the speed difference between them is what drives the flow.

Chamber carbon dioxide (%)

Flow through the burrow against wind (L/s)

1 · Wind and ground

2 · Mounds

Heights of the openings above the surface. The low mound is on the left, the high one on the right; the flow is positive when it enters on the left.

3 · Tunnel

The cutaway draws the tunnel and the animals larger than scale so that you can see them; the numbers use the sliders.

4 · Chamber and animals

Experiments (measured by solving the model)

Run an experiment: each point is a full solution of the pipe-flow model with your other settings.

Independent numerical checks

Also run outside the browser: node src/check.js (log profile, Hagen-Poiseuille laminar limit, energy balance, symmetry, RK4 against the exponential CO₂ solution, square-root scaling, Kleiber).

Educational model, not a field-calibrated one. Wind comes from the neutral log law; each opening feels the static pressure drop of the speed at its crest (no mound-shape pressure coefficient); the tunnel is one straight-equivalent pipe with loss coefficients; soil-gas exchange (1.5e-5 m³/s), the animals’ activity factor and the comfort thresholds are [assumptions]. CO₂ above 1% is drawn as “stuffy” and above 3% as “poor” only as illustrative markers.

Controls

Drag / one finger: orbit; wheel / pinch: zoom; Section, Mounds and Chamber buttons.
Sliders change the physics immediately; the 3D mounds change height with the mound sliders.
The particles inside the tunnel move at a speed proportional to the computed mean air speed; their colour shows how much CO₂ they have picked up in the chamber.
“Fresh air in chamber” resets the CO₂ to the outside value; “Swap mound heights” reverses which opening is the inlet.

The science

Wind profile. Over short grass the neutral log law gives v(z) = (u_*/κ) ln(z/z₀) with κ = 0.4, roughness length z₀, and friction velocity fixed by the speed at 10 m. Air near the ground is slow, so even a few decimetres of mound height matter.

Bernoulli pressure. Outside the burrow the air moves almost frictionlessly, so the static pressure at an opening is p = p_∞ - ½ ρ v². The pressure difference between the two openings is Δp = ½ ρ (v_B² - v_A²). This is the Venturi-type suction that Vogel and co-workers described for prairie dog mounds (the low-mound inlet and high-mound outlet).

Stack effect. Warm air in the burrow is lighter: Δp_stack = ρ g Δh (T_soil - T_air)/T, pushing air out of the higher opening when the soil is warmer than the air and the other way round on a hot day. It is small, but it is the only drive on a calm day.

Pipe flow. Mean speed u solves Δp = (K + f L/D) ½ ρ u² by bisection, with Darcy friction f = 64/Re in the laminar regime and the Haaland approximation for a rough wall otherwise (the larger of the two), and loss coefficients for the entrance, exit and two bends. The flow rate is Q = u π D²/4. In the fully rough, fast regime Q ∝ Δp^1/2; for very narrow tunnels Q = π D⁴ Δp/(128 μ L) (checked in the page).

Animals and gas. Each animal makes 1.5 × 3.4 m^0.75 watts (Kleiber basal rate times an assumed activity factor); with 20.1 kJ per litre of oxygen and a respiratory quotient of 0.85, CO₂ production is 0.85 × VO₂.

Chamber balance. V dC/dt = Q_tot (C_out - C) + V̇_CO2 gives C(t) = C_ss + (C₀ - C_ss) e^{-Q t/V} with C_ss = C_out + V̇_CO2/Q_tot. Q_tot adds molecular diffusion along the tunnel and a fixed soil-gas conductance.

Not modelled: real turbulence and gusts, wind direction and mound shape (pressure coefficients), several branches and other openings, rain, burrow flooding, behaviour such as digging or plugging entrances, and the heat balance of the soil.