Prairie dogs, Cynomys ludovicianus, are burrow makers found across North America. Vogel performed a suit of experiments investigating the airflow through these burrows. Using geometric measurements he mathematically determined that were atmospheric diffusion the sole process of oxygen exchange occurring, the amounts of oxygen predicted by Fick’s Law, that to satisfy the O2 requirements of a single resting Prairie Dog, the tunnel could be at most 4cm long, i.e. half the length of the rodent itself.
N.B. This work is modified from my submitted MSc dissertation. Plagiarisim is not encouraged.
Domes and Craters
Instead it was proposed that the earth structures at either end of the tunnel induced a low flow rate, bulk exchange of air. Through experimental investigation and use of wind tunnels it was determined that the inclusion of these structures did indeed accelerate flow through the subterranean structure. Futher, that only the ‘crater mound’ was required, the ‘dome’ did not seem to impact ventilation rate and obviously has a different function (lookout tower and flood defence have both been proposed).
Bernoulli flow and viscous entrainment were identified by the researchers as the processes acting on the crater mound. These create a higher velocity flow at v1, and so negative pressure gradient from one end, which draws fresher external air right through the structure.
Wind Velocity Profile
Further, if we consider an idealised wind speed profile against the ground, just the small difference in height also has an effect. The wind directly adjacent to the ground can generally be assumed to be stationary. As you rise from the ground however, the wind speed increases in a curve, similar to the one displayed below.
Therefore the wind 20cm above the ground, at the raised mouth of the crater, is already moving faster, and being accelerated by the shape of the mound, leading to even higher wind velocities, stronger pressure gradients and faster flow rates.
‘Crater’ – Crater structure at one end of the tunnel exploits the wind velocity profile close to the ground, where the speed difference is greatest
–> ‘Entrainment’ – The static air mass at the mouth of the crater is viscously entrained by the faster moving air, independently of wind direction
–> ‘Mound’ – The negative pressure induced by the viscous entrainment and Bernoulli Effect at the crater mouth induces flow linearly through the burrow, resulting in intake at the opposite ‘mound’ structure.
Vogel, S., Ellington, C. & Kilgore, D., 1973. Wind-Induced Ventilation of the Burrow of the Prairie-Dog, Cynomys ludovicianus. Journal of Comparative Physiology A, Volume 85, pp. 1-14.
Biomimicry Template: Prarie Dog Burrow Ventilation by Biomimicron: DaveRGP is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Based on a work at http://www.asknature.org/strategy/e27b89ebcdec8c9b5b2cd9ac84b8f8a0.