Stability analysis of transitional flows based on disturbance magnitude

TL;DR

This paper introduces a new stability criterion for shear flows based on disturbance magnitude, combining input-output analysis and the small-gain theorem, which predicts flow stability or transition to turbulence.

Contribution

It develops a hierarchical stability threshold framework applicable to canonical flows, integrating nonlinear effects and aligning with experimental and simulation data.

Findings

Thresholds match linear stability theory for small perturbations.

Structured models predict transition at realistic Reynolds numbers.

Couette flow stability can be exceeded, leading to transition.

Abstract

We propose a novel stability criterion for incompressible shear flows by combining input-output analysis and the small-gain theorem. The criterion yields an explicit threshold on the magnitude of velocity perturbations about a given base flow that guarantees stability. If this threshold is crossed--either due to nonmodal growth, exponential growth, or a bypass transition scenario--our analysis predicts a loss of stability that may lead to transition to turbulence. We consider three approximated models for nonlinearity: unstructured, structured with non-repeated blocks, and structured with repeated blocks. We show that the imposed threshold obtained by these three methods complies with a hierarchical relationship, where the unstructured case is the most conservative, imposing the lowest bound on disturbance magnitude. We apply this approach to three canonical and well-studied base flows: Couette, plane Poiseuille, and Blasius. For these three base flows, we compare our results with experiments, direct numerical simulation results, nonmodal nonlinear stability results, and linear stability theory (LST). In the limit of infinitesimally small perturbation magnitude, our stability criterion for the unstructured case recovers the results of LST. For finite perturbations, the structured cases that account for nonlinear interactions provided stability thresholds that are consistent with experimental observations and simulation results of transition at both subcritical and post-critical Reynolds numbers for the considered base flows in our study. In particular, we utilize our stability criterion to demonstrate that Couette flow can become unstable and transition can be triggered at different Reynolds numbers, which is consistent with past experimental observations.