# Modulational Instability of Viscous Fluid Conduit Periodic Waves

## TL;DR

This paper rigorously verifies the predictions of Whitham modulation theory regarding the stability of periodic interfacial waves in a viscous fluid conduit, using spectral perturbation methods to connect formal theory with rigorous results.

## Contribution

It provides a rigorous spectral analysis confirming Whitham theory predictions for the modulational stability of conduit waves, advancing the mathematical understanding of viscous fluid interfacial dynamics.

## Key findings

- Confirmed Whitham theory predictions for wave stability
- Connected formal asymptotic methods with rigorous spectral analysis
- Validated recent theoretical results on conduit equation stability

## Abstract

In this paper, we are interested in studying the modulational dynamics of interfacial waves rising buoyantly along a conduit of a viscous liquid. Formally, the behavior of modulated periodic waves on large space and time scales may be described through the use of Whitham modulation theory. The application of Whitham theory, however, is based on formal asymptotic (WKB) methods, thus removing a layer of rigor that would otherwise support their predictions. In this study, we aim at rigorously verifying the predictions of the Whitham theory, as it pertains to the modulational stability of periodic waves, in the context of the so-called conduit equation, a nonlinear dispersive PDE governing the evolution of the circular interface separating a light, viscous fluid rising buoyantly through a heavy, more viscous, miscible fluid at small Reynolds numbers. In particular, using rigorous spectral perturbation theory, we connect the predictions of Whitham theory to the rigorous spectral (in particular, modulational) stability of the underlying wave trains. This makes rigorous recent formal results on the conduit equation obtained by Maiden and Hoefer.