The slow viscous flow around a general rectangular doubly-periodic arrays of infinite slender cylinders

The slow viscous flow through a doubly-periodic array of cylinders does not have an analytical solution. However, as a reduced model for the flow within fibrous porous media and microfluidic arrays, this solution is important for many real-world systems. We asymptotically determine the flow around a general rectangular doubly-periodic array of infinite slender cylinders, extending the existing asymptotic solution for square arrays. The flow in the cell is represented by a collection of doubly-periodic, rapidly-convergent two-dimensional singularity solutions, and the boundary condition on the surface of the cylinder is solved asymptotically in powers of the cylinder radius. The asymptotic solution provides an easily computed closed-form estimate for the flow and forces as a function of the radius and the dimensions of the cell. The force is compared to results from lattice-Boltzmann simulations of low-Reynolds-number flows in the same geometry, and the accuracy of the no-slip condition on the surface of the cylinder, predicted by the asymptotic theory, is assessed. Finally, the behaviour of the flow, flux, force and effective permeability of the cell is investigated as a function of the geometric parameters. The structure of the asymptotic permeability is consistent with previous single-geometry predictions but provides a closed-form estimate for how the aspect ratio of the cell changes the leading-order behaviour. These models could be used to help understand the flows within porous systems composed of fibres and systems involving periodic arrays such as systems based on deterministic lateral displacement.