The airway closure due to capillary instability [1] occurs in lung diseases such as asthma, cystic fibrosis, or emphysema. The reopening process involves displacement of plugs constituted from mucus, a non-Newtonian fluid with a yield stress, in the airways. In this work the transient propagation of mucus plugs in a 2D channel is studied numerically, assuming that the mucus is a Bingham fluid. The governing equations are discretized by a spectral element formulation and the free surface is resolved with an Arbitrary Lagrangian Eulerian (ALE) approach [2]. The constitutive equation for a Bingham fluid is implemented through a regularized constitutive equation. According to the numerical results, the yield stress behavior of the plug modifies the plug shape, the pattern of the streamlines and the distribution of stresses in the plug domain and along the walls in a significant way. The distribution along the walls is a major factor in studying cell injuries.

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