Effect of oscillatory core-flow on a viscoelastic fluid layer coating the inner surface of a tube.

Surface tension on an air-liquid interface induces liquid flows, which may cause the lung's airways to close due to the formation of a liquid plug as a result of drainage of the liquid lining coating the airways. The stability of the liquid layer is also influenced by the air core flow as well as the rheological properties of the liquid. In this study, we develop a computational model of a liquid-lined tube with an oscillatory core flow along the axis of the tube: a Newtonian fluid flows through a cylinder whose inner wall is coated by an upper-convective Maxwell fluid. When no core flow is present, the viscoelastic fluid layer grows faster and closure times are shorter than a Newtonian fluid layer with the same viscosity. When an oscillatory core flow is present, the liquid bulge that develops after the initial growth translates back and forth along the tube axis with an amplitude that increases with elasticity. If this amplitude is large enough, the minimum core radius approaches a non-zero value implying that a liquid plug has not formed. The effects of core flow frequency and amplitude on this instability are discussed.