Investigation of flow through and around the Macrotermes michaelseni termite mound skin
2011-06-13T09:08:40Z (GMT) by
Macrotermitinae mounds could hold the key for designing a naturally ventilated human habitation using the sustainable Solar and Wind energies. Few attempts were made to mimic these structures into man build structures to achieve natural ventilation. Yet the limited understanding of the ventilation mechanisms applied in these mounds and the lack of fully developed building technologies capable of implementing such complex designs have prevented its further implementation in human habitation. A number of ventilation mechanisms were proposed, however, they are yet to be established. A prerequisite for a thorough understanding of the ventilation mechanism is the comprehension of the role of the mound skin in controlling of ventilation. This thesis focused on studying the flow through the porous mound skin into and out of the mound interior and the flow around the external skin surface. The Macrotermes michaelseni mound structure was visualised and studied in detail by means of field experiments to reveal its plaster-filled internal structure and digital scanning of a plaster-filled mound. The dimensions and interconnectivity of the internal conduits were examined to establish the source of maximum flow resistance. The mound skin and the built-in egress channels were found to be responsible for the bulk of the flow resistance. Computational Fluid dynamics CFD was used to predict the flow rates through the mound skin structure and the internal and external flow patterns. A series of Micro-CFD simulations were conducted to examine the effect of egress channel on the predicted flow rate through the porous mound skin. The effect of the mound conical shape on the external pressure distribution and flow patterns around and within the mound were predicted by Macro-CFD simulation. Wind tunnel experiments were conducted to validate the Macro-CFD analysis. Egress channels are present across the height of the mound stemming from a network of surface conduits that is directly below the mound skin. The surface conduits are highly connected to each other, to the mound central chimney and to the nest structure via peripheral subterranean conduits creating a highly connected network of air conduits. Egress channels keep the mound internal conduits separate from the ambient environment under normal conditions of dry weather. These channels are opened to the skin external surface under rainy conditions to compensate for mound skin diminished air permeability. The flow through the mound skin with closed egress channels is highly sensitive to changes in the egress channel depth from the external skin surface rather than changes in the skin thickness. Closed egress channels within the mound skin doubles the flow rate through the mound. This exceeds the amount necessary for the colony metabolism to allow for part of the inflow to circulate the mound conduits and leave without reaching the subterranean nest structure. Open egress channels increase the flow rate by 1.3 times that of closed egress channels which is necessary during rainy conditions where the mound skin is impermeable. The mound spire is the most efficient in capturing air flow into the mound allowing just under 70% of the total flow rate through the mound skin. Fresh air enters the mound from the upwind conduits with an internal flow velocity ranging from 3 - 12 mm/s. Spent air leaves the mound interior from the lateral and downwind sides. The mound conical shape results in directing the inflow downwards and the outflow upwards.