Viscous flow through three-dimensional models of fibrous porous media is analysed. The periodic models are based on ordered networks of cylindrical fibres on regular cubic lattices. Numerical solutions are obtained using the spectral boundary element method introduced by Muldowney & Higdon (1995). Results are presented for solid volume fractions ranging from extreme dilution to near the maximum volume fraction for permeable media. Theoretical estimates are derived using slender-body theory and lubrication approximations in the appropriate asymptotic regimes. Comparisons are made with model predictions based on properties of two-dimensional media (Jackson & James 1986), and with results for disordered dispersions of prolate spheroids (Claeys & Brady 1993 b).

The problem of cottage cheese sludge was investigated by attempting to recreate sludge in experimental vats and by retrospective examination of sludge as well as samples of starter and skimmed milk used in cases of commercial vat failures. Two independent types of sludge problem were identified. Minor sludge occurred regularly on the bottom of vats and was unconnected with major sludge which leads to total loss of vat contents. Major sludge was unrelated to composition of the skimmed milk and was probably caused by phage which infected the vat after development of acidity had begun.