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

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 analysed 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 half-parabolic 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 behaviours. Furthermore, we observe that strongly forced long waves are more unstable at large italic Re <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:mrow> <mml:mrow> <mml:mtext mathvariant="italic" class="MJX-tex-mathit">Re</mml:mtext> </mml:mrow> </mml:mrow> </mml:math> ${\textit{Re}}$ than the inviscid value they latch on to as italic Re right arrow normal infinity <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:mrow> <mml:mtext mathvariant="italic" class="MJX-tex-mathit">Re</mml:mtext> </mml:mrow> <mml:mo stretchy="false">→</mml:mo> <mml:mi mathvariant="normal">∞</mml:mi> </mml:math> $\textit{Re} \to \infty$ . Growth-rate maps highlight smooth transitions between long-wave and rippling modes and reveal a 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.