Determination of the liquid permeability of bordered pits in wood with Computational Fluid Dynamics

Rain is the main source of moisture on the building envelope. Wood cladding, window wood frame and, in the case of water infiltration, even the wood frame structure can be in contact with water. In the context of the development of a multi-scale moisture transfer model for wood components of the building envelope, it is found that one element, namely the pit of the wood structure plays a critical role in water transfer. Hundreds of pits, small porous openings between adjacent wood cells, allow for water to move along the cells, across and along growth rings. It seems that the permeability of wood would depend largely on the permeability of these pits. Liquid transfer models for wood require the knowledge of the liquid permeability of the pits. Information about pit geometry in literature is rather limited, and relatively little information about pit permeability has been provided. This is at least partly due to the fact that experimental data for this small and fragile microcellular structure are very difficult to obtain. Pits are characterised by a rather complex geometry. They consist of a central impermeable torus, which is held in place by the margo, which is a loose and complex mesh of microfibrillar strands through which water can pass. As a result of this complex geometry, the dynamics of water flow is also quite complex. This paper will present Computational Fluid Dynamics (CFD) models for a range of generic pit geometries. The models take into account the complex network structure of the margo by the detailed implementation of each of the individual microfibrillar strands. In this paper, the construction of the CFD models will be presented, the calculation results in terms of liquid permeability will be provided and their applicability for different wood species will be discussed. This paper also discusses issues with modelling porous materials with CFD.Presenters: name: Bert Blocken affiliation: (Technische Universiteit Eindhoven, The Netherlands) email: b.j.e.blocken@tue.n