Collapsible Tube Filling is a phenomenon that occurs in stesdy flow in tubes with significant distensibility.
Such conditions occur rarely in industrial applications but are very common in biological studies.
Understanding the physical origin and nature of these phenomena remains a significant experimental, analytical, and computational challenge, involving unsteady flows at low or high reynolds numbers, large-amplitude fluid-structure interactions, free-surface flows, and intrinsically 2D or 3D motion.
Whereas frequently the internal flow involves a single fluid phase in many instances.
The presence of two or more distinct flowing phases is of primary importance.