Permeability estimation with the method of cells

The Method of Cells (MC) is used to predict the permeability of preforms re gardless of their architecture, and to study the effect of disturbances dur ing preform manufacturing on the permeability of the preform. Preforms are made up of bundles of fibers called fiber tows. Different types of preforms can be produced from the same fiber tow. In the MC, a repetitive unit cell is subdivided into subcells in such a way that there is only fiber tow or air in a subcell. The pressure field in each subcell is modeled as a second order polynomial of local coordinates. The coefficients of the polynomial are estimated using a set of equations that include governing equations, bo undary conditions, continuity conditions and constitutive equations. Subseq uently, the flow field in the subcell is determined. The pressure is averag ed at the faces of the unit cell and the pressure gradient is calculated as suming a linear pressure profile across the unit cell in all three dimensio ns. The velocity is averaged over the unit cell volume. Finally, overall pe rmeability values are estimated using Darcy's Law. The geometry of the pref orm varies to some extent due to several effects during production or proce ssing; or there might be some imperfections in the preform, such as imprope r stitches. The permeability is influenced by both of these disturbances. B y varying the properties of the subcells in the model, the effect of these disturbances on the permeability can be evaluated.