# Streak generation in viscosity-stratified wall-bounded flows

## TL;DR

This paper investigates how viscosity stratification affects streak generation in wall-bounded flows, revealing a new mechanism and explaining flow behavior differences in Poiseuille and Couette flows through theoretical and numerical analysis.

## Contribution

It introduces a novel viscosity-stratified lift-up mechanism and analyzes its interaction with the classical lift-up effect in wall-bounded flows.

## Key findings

- Viscosity stratification significantly alters the classical lift-up mechanism.
- A new pathway for streak generation is identified.
- Flow trends in Poiseuille and Couette flows are explained by the interaction of mechanisms.

## Abstract

Streamwise elongated flow structures, or streaks, dominate wall-bounded flows. We show that the renowned lift-up mechanism, that generates streaks, is significantly altered by viscosity stratification. We additionally identify a novel pathway for streak generation: the viscosity-stratified lift-up mechanism. The competition and cooperation of the streaks generated by these mechanisms provides an explanation for trends in viscosity-stratified Poiseuille flows, and remarkably, the opposing trends in Couette flows as well. Our theoretical observations are substantiated by numerical experiments.