# Linear and weakly nonlinear global instability of a fluid flow through a collapsible channel

## TL;DR

This paper investigates the linear and weakly nonlinear global instability of fluid flow in a collapsible channel, revealing bifurcation behaviors and the effects of membrane tension and Reynolds number through analytical models.

## Contribution

It introduces a variant of an existing one-dimensional model using a weighted residual approach and analyzes bifurcation phenomena with weakly nonlinear theory.

## Key findings

- Steady bifurcating flows occur at low membrane tension and Reynolds numbers.
- Flow becomes time-periodic at high tension and Reynolds numbers.
- Bifurcation types include transcritical saddle node and either subcritical or supercritical Hopf.

## Abstract

Interactions between an internal flow and wall deformation occur in many biological systems. Such interactions can involve a complex and rich dynamical behavior and a number of peculiarities which depend on the flow parameter range. The aim of this paper is to present a variant (obtained via a weighted residual approach) of the averaged one-dimensional model derived by Stewart et al. "Local and global instabilities of flow in a flexible-walled channel," Eur. J. Mech. B/Fluids 28, 541-557 (2009)]. The asymptotic expansions for small Reynolds numbers of these two models, compared to the exact solution obtained from the lubrication approach, reveal some quantitative difference, even at higher Reynolds numbers. Qualitatively, the two models give similar results at least at a linear level. It is shown that for relatively low membrane tension (T), there are distinct regions in the (T,R) parameter space where steady bifurcating flows may occur. These flows can also be observed at vanishingly small Reynolds numbers combined with relatively high membrane tension. At sufficiently high T and R, the bifurcating flow is rather time periodic. A weakly nonlinear analysis is then performed in both cases leading to the derivation of evolution equations for the amplitudes of the bifurcating flows. The amplitude equations show that the saddle node bifurcation has a transcritical character while the Hopf bifurcation is either subcritical or supercritical, depending both on the mode number and membrane tension.