Lung parenchyma is a foam-like material consisting of millions of alveoli. Sound transmission through parenchyma plays an important role in the non-invasive diagnosis of many lung diseases. We model the parenchyma as a porous solid with air-filled pores and consider the Biot equations as a model for its acoustic properties. The Biot equations govern small-amplitude wave propagation in fluid-saturated porous solids, and include the effects of relative motion between the fluid and the solid frame. The Biot equations can be derived from a micro-structure model of the porous material, and the material parameters in the equations can be obtained from the solution of two independent microstructure problems, a fluid-cell problem (governed by the unsteady Stokes equations) and a solid-cell problem. We review the homogenization approach for media with periodic micro-structure, and solve a fluid-cell problem numerically for an idealized two-dimensional micro-scale geometry for a wide range of frequencies. We also discuss sound speeds in lung tissue.

In this paper, we present a computer-aided method (based on [Ya98]) that establishes the existence and local uniqueness of a stationary solution to the viscous Burgers' equation. The problem formulation involves a left boundary condition and one integral boundary condition, which is a variation of the approach taken in [Si04].

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