The mound as a gas exchange device. 4

Wind-driven movements in the mound are only half the story for the mound's function.

The mound harnesses transient wind energy, not to promote air flow through the nest, but to promote mixing at the boundary between nest air and mound air.

Patterns of gas exchange observed with tracer gas studies point to two likely mechanisms for promoting mixing.

Impedance filtering

wind onlysurface conduitscenter

The complex tunnel network within the mound appears to differentially filter particular frequencies inherent in turbulent wind. We can see this by doing the same kind of Fourier analysis seen here. To reiterate, the mound intercepts at its surface unfiltered turbulent wind, with a broad spectrum distribution of wind velocities.

In the surface conduits, much of this broad spectrum filters through the surface conduits. Some slight filtering of flows at the upper frequency ranges is evident, however.

Deep within the mound, nearly all these high frequency components have been filtered out.

This means the mound acts as a low-pass filter of transient wind energy. The mound, in short, has an impedance. This is at the heart of the AC mechanisms that power the mound's service to respiratory gas exchange.

filtering


Pendelluft mechanisms

"Pendelluft" literally translates as "air pendulum." In the lung, pendelluft effects refer to slight air movements between the penultimate branches of the respiratory tree. These can arise from slight variations of inflation and deflation within the lung. This promotes mixing in the mixed regime phase, which promotes mixing between the diffusive and convective regimes.

pendelluft

Within the mound and nest, pendelluft mechanisms probably are behind the intermittent flows of tracer from the nest to the mound, seen here. The pendelluft probably operates from the AC filtering function of the mound described above.

mound

In this scheme, the central air spaces of the nest are driven by the low frequency components that penetrace to the chimney. The nest's peripheral air spaces are driven by the broad-spectrum transients in the surface conduits that extend down to the paraoecie.

The boundary between nest air and mound air is thus differentially perturbed from periphery to center. Occasionally, this differential driving is vigorous enough to promote the mixing of a bubble across the boundary. This is illustrated schematically in the video below.


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Acoustic mixing

Another interesting mechanism that promotes mixing in the lung depends upon resonance phenomena.

Whenever there is a mass of air enclosed in a space, either a column or a blind chamber, energy can be stored in the inertia and pressure. Oscillatory pressures can resonate at particular frequencies.

This is easily seen in a the plastic bottle with a stratified smoke layer. A sound is imposed on this bottle at three frequencies: 4Hz (left), 8Hz (middle) and 16Hz (right). When the sound is imposed, the 8Hz wave promotes vigorous mixing. Mixing is not so vigorous at other frequencies.

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The tunnels of the surface conduits/paraoecie can be as long as 2-3 m, which means they will resonate at frequencies just below the audible. We believe the broad spectrum transients in turbulent winds can be "tuned" in these tunnels to promote mixing between fresh mound air above, and spent nest air below.

Here is a video that describes all this.

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