Divergence instability in an air-conveying soft tube: Analysis of static zig-zag shapes

This work is devoted to the characterization and modeling of surprising stable states appearing in air-conveying soft tubes. A soft tube differs from a flexible tube by its very thin walls which allows it to fold with a sharp angle. When the upper end of a soft tube is fixed to an air pump while the bottom end is left free, the system exhibits a divergence instability which develops under the form of zig-zag'' shapes constituted by three roughly straight segments. We characterize each stable shape by the lengths and angles of these three segments. We observe, for example, that the intermediate segment is limited to short lengths because its inclined direction with respect to the vertical increases the gravitational torque on the upper fold. In order to theoretically reproduce the zig-zag shapes, we use a model of three articulated straight rigid tubes conveying airflow. The torque exerted by the pressurized tube on a folded part of the pipe is modeled by a nonlinear torsional spring. This model shows that the system can stabilize into zig-zag states qualitatively similar to experimental observations.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This work focuses on characterizing and modeling the stable states that appear in air-conveying soft tubes. By studying the lengths and angles of three segments in the zig-zag shapes, we are able to understand and reproduce these surprising stable states using a model of three articulated straight rigid tubes.