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We present a two-dimensional (2D) mathematical model of a highly concentrated suspension or a thin film of the rigid inclusions in an incompressible Newtonian fluid. Our objectives are two-fold: (i) to obtain all singular terms in the asymptotics of the overall viscous dissipation rate as the interparticle distance parameter δ tends to zero, (ii) to obtain a qualitative description of a microflow between neighboring inclusions in the suspension.Due to reduced analytical and computational complexity, 2D models are often used for a description of 3D suspensions. Our analysis provides the limits of validity of 2D models for 3D problems and highlights novel features of 2D physical problems (e.g. thin films). It also shows that the Poiseuille type microflow contributes to a singular behavior of the dissipation rate. We present examples in which this flow results in anomalous rate of blow up of the dissipation rate in 2D. We show that this anomalous blow up has no analog in 3D.While previously developed techniques allowed to derive and justify the leading singular term only for special symmetric boundary conditions, a fictitious fluid approach, developed in this paper, captures all singular terms in the asymptotics of the dissipation rate for generic boundary conditions. This approach seems to be an appropriate tool for rigorous analysis of 3D models of suspensions as well as various other models of highly packed composites.

A heterogeneous medium of constituents with vastly different mechanical properties, whose inhomogeneities are in close proximity to each other, is considered. The gradient of the solution to the corresponding problem exhibits singular behavior (blow up) with respect to the distance between inhomogeneities. This paper introduces a concise procedure for capturing the leading term of gradient's asymptotics precisely. This procedure is based on a thorough study of the system's energy. The developed methodology allows for straightforward generalization to heterogeneous media with a nonlinear constitutive description.

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Summary. We introduce a discrete network approximation to the problem of the effective conductivity of a high contrast, densely packed composite in three dimensions. The inclusions are irregularly (randomly) distributed in a host medium. For this class of arrays of inclusions we derive a discrete network approximation for effective conductivity and obtain a priori error estimates. We use a variational duality approach to provide rigorous mathematical justification for the approximation and its error estimate.