# On the generation of free-surface waves by instabilities in quadratic shear flows

## TL;DR

This study explores how linear instabilities in quadratic shear flows generate free-surface waves, revealing the effects of flow curvature, viscosity, and forcing on stability and identifying five distinct instability mechanisms.

## Contribution

It provides a comprehensive analysis of free-surface wave generation in quadratic shear flows, including new stability boundaries and the identification of five instability types.

## Key findings

- Convex curvature suppresses long-wave instabilities.
- Concave curvature suppresses short-wave instabilities.
- Strong forcing increases long-wave instability at high Reynolds numbers.

## Abstract

This paper investigates the generation of free-surface waves in a liquid layer driven by linear instabilities in Couette-Poiseuille (quadratic) shear flows. The base velocity profiles are characterized by a curvature parameter, and two-dimensional viscous and inviscid perturbations are analyzed across a wide parameter space of curvature, wavenumber, and Reynolds number, for fixed Froude and Bond numbers. In the inviscid limit, analytical solutions of the Rayleigh equation reveal that velocity profiles ranging from Nusselt to linear flows remain stable against the rippling instability, with long-wave growth occurring only under strong interfacial forcing, whereas weaker forcing produces well-defined stability boundaries. For the viscous problem, Orr-Sommerfeld computations and asymptotic analyses reveal that a slight convex curvature of the shear flow suppresses long-wave instabilities, while a slight concave curvature suppresses short-wave instabilities, so even small deviations from a linear profile produce qualitatively different behaviors. Furthermore, we observe that strongly forced long waves are more unstable at large $Re$ than the inviscid value they latch on to as $Re \to \infty$. Growth-rate maps highlight smooth transitions between long-wave and rippling modes and reveal an additional shear instability near the linear profile at high Reynolds numbers. Based on energy transfers and eigenfunction structures, five distinct instability types are identified: shear, rippling, long-wave interfacial, short-wave interfacial, and a composite mode that combines features of shear, rippling and long-wave interfacial instabilities at large Reynolds numbers.