The mound as a gas exchange device. 3

Mound and sky

The largest problem to crack is how air actually flows within the mound. Because the mound air and nest air are poorly mixed, this is a somewhat separate question from respiratory gas exchange for the colony. That question cannot be answered without a complete understanding of how the mound interacts with wind.

To crack this problem, we need to assess the complex pressure field that arises when the mound encounters wind. There are two main parameters that control this.

Wind speed varies vertically in the surface boundary layer. The upper parts of the mound intercept more energetic winds than the lower.

Wind direction varies constantly, so the relative orientation of mound and wind velocity change.

Pressure distribution over the mound surface

induced pressures

Schematic of distribution of wind-induced pressure over a mound


Normalized pressures measured at two vertical points on the mound surface

mound pressure

Wind-induced pressures at various points on mound surface.

Structural studies show the mound surface is porous over nearly its entire extent, with porosity greatest whereever there is active building going on.

Because the mound spans the surface boundary layer, there is a vertical distribution of wind speed over the mound, slow at the base, and increasing toward the top, and a commensurate vertical distribution of wind-induced dynamic pressures.

Layered over this is a circumferential distribution of Bernoulli-induced pressures: strong positive pressures at the mound's leading surface, declining to strong suction pressures at the lateral surfaces, and smaller suction pressures and a turbulent wake at the trailing surface. This is readily seen in the plot below. Emissions of combustible gas (CG) tracer are strongly biased to the mound's lateral surfaces with respect to wind direction.

surface emissions of tracer

Two points are essential. Because winds here are dominated by local convective instabilities (which can arise anywhere), these pressures can shift radially ove the surface. They also vary strongly with time. In other words, it is transient winds that are the dominant generator of pressures at the mound surface.


Expected patterns of wind-induced flow through the mound

internal flow

The complex pressure field over the mound surface will generate complex flows within the mound.

First, the positive pressures at the mound's leading surface will force air into the mound, most strongly at the top. This will impart an overall downward vector of pressure within the mound, driving air downward.

Along the lateral surfaces, the opposite field of pressure appplies: strong negative pressures toward the top, and weak toward the bottom. This draws air upward. The same applies to the trailing surface, although more weakly.

This all translates into a complex pattern of flow within the mound. It is worth reiterating that these are transient and will continually shift radially within the mound and in intensity.

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Observed patterns of wind-induced flow in the mound

The actual patterns of flow are observable using tracer pulse-chase experiments. The patterns are extraordinarily complex, but they show three broad patterns.

First, air within the mound tends to be drawn up and out of the mound in its upper reaches. In the left graph below, tracer injected into the nest appears most strongly in the upper part of the mound, but more weakly in the lower. When wind is gusty (right below), the same pattern pertains, but is more "choppy."

up and outup and out

Second, air flows within the surface conduits appear to be strongly driven by wind. Injecting tracer into an upwind surface conduit appears almost instantaneously in downwind surface conduits (left below). We also see that when wind speed changes direction (right below), a bolus of tracer in the formerly downwind conduit then shifts with the wind to the new downwind conduit.

well mixedwell mixed air

Third, the mixing between the superficial air spaces and interior air spaces of the mound is anisotropic. This means that mixing of mound air from the superficial to the central air spaces is slow (left below), while mixing in the other direction, from central to superficial air spaces, is easier (right below).

filteringwell mixed


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