Collective secondary instabilities: an application to three-dimensional boundary-layer flow

TL;DR

This paper introduces a new stability analysis method that captures collective secondary instabilities in three-dimensional boundary-layer flows, overcoming limitations of classical Floquet analysis by coupling multiple length scales.

Contribution

The work develops a novel formulation combining 2D stability analysis with Bloch waves to efficiently analyze high-dimensional, multi-unit instabilities in complex flows.

Findings

Identified large-wavelength collective instabilities involving multiple units.

Detected subharmonic instability patterns in swept boundary-layer flow.

Validated the method against classical Floquet analysis results.

Abstract

In some linearly unstable flows, secondary instability is found to have a much larger wavelength than that of the primary unstable modes, so that it cannot be recovered with a classical Floquet analysis. In this work, we apply a new formulation for capturing secondary instabilities coupling multiple length scales of the primary mode. This formulation, based on two-dimensional stability analysis coupled with a Bloch waves formalism originally described in Schmid et al. (2017), allows to consider high-dimensional systems resulting from several repetitions of a periodic unit, by solving an eigenproblem of much smaller size. Collective instabilities coupling multiple periodic units can be thus retrieved. The method is applied on the secondary stability of a swept boundary-layer flow subject to stationary cross-flow vortices, and compared with Floquet analysis. Two multi-modal instabilities are recovered: for streamwise wavenumber $\alpha_v$ close to zero, approximately twelve sub-units are involved in large-wavelength oscillations; whereas a staggered pattern, characteristic of subharmonic instabilities, is observed for $\alpha_v = 0.087$.