# On the three-dimensional stability of Poiseuille flow in a finite-length duct

## TL;DR

This paper investigates the three-dimensional stability of Poiseuille flow in a finite-length duct, revealing boundary-layer structures in eigenmodes at high Reynolds numbers and questioning the linear model's validity under such conditions.

## Contribution

It introduces a SUPG-based scheme for eigenvalue problems and uncovers the boundary-layer structure of least-stable eigenmodes at high Reynolds numbers.

## Key findings

- Flows are asymptotically stable up to Re=2500.
- Eigenmodes develop boundary-layer structures at high Re.
- Linear models may be inadequate at high Re due to singular eigenmodes.

## Abstract

The stability of a three-dimensional, incompressible, viscous flow through a finite-length duct is studied. A divergence-free basis technique is used to formulate the weak form of the problem. A SUPG (streamingline upwind Petrov-Galerkin) based scheme for eigenvalue problems is proposed to stabilize the solution. With proper boundary condtions, the least-stable eigenmodes and decay rates are computed. It is again found that the flows are asymptotically stable for all $Re$ up to $2500$. It is discovered that the least-stable eigenmodes have a boundary-layer-structure at high $Re$, although the Poiseuille base flow does not exhibits such structure. At these Reynolds numbers, the eigenmodes are dominant in the vicinity of the duct wall and are convected downstream. The boundary-layer-structure brings singularity to the modes at high $Re$ with unbounded perturbation gradient. It is shown that due to the singular structure of the least-stable eigenmodes, the linear Navier-Stoker operator tends to have pseudospectrua and the nonlinear mechanism kicks in when the perturbation energy is still small at high $Re$. The decreasing stable region as $Re$ increases is a result of both the decreasing decay rate and the singular structure of the least-stable modes. The result demonstrated that at very high $Re$, linearization of Navier-Stokes equation for duct flow may not be a good model problem with physical disturbances.