“…Many studies have examined the complex flow patterns in the pulmonary region with increasingly more sophisticated geometries: a rigid axisymmetric circular model (Tsuda et al, 1995), a torus on a tube (Tippe and Tsuda, 2000), an expanding threedimensional (3D) single alveoli (Haber et al, 2003), square channels (Karl et al, 2004), square channels on a bend (van Ertbruggen et al, 2008), 3D terminal alveolar sac with spherical alveoli (Oakes et al, 2010), 3D terminal alveolar sac with 14-hedron alveoli (Sznitman et al, 2007b(Sznitman et al, , 2009, and terminal sac with octahedron shaped alveoli (Kumar et al, 2009). It is clear from these studies that flow in the pulmonary region involves recirculation to varying degrees depending on geometry, flow rate, and cyclic wall motion.…”