The role of Lagrangian drift in the geometry, kinematics and dynamics of surface waves

• Published in: Journal of fluid mechanics Vol. 954
• Main Authors: Pizzo, Nick, Lenain, Luc, Rømcke, Olav, Ellingsen, Simen Å., Smeltzer, Benjamin K.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 10.01.2023
• Subjects: Deep water; Drift; Dynamics; Finite amplitude waves; Flow stability; Fluid flow; Free surfaces; Geometry; Gravity waves; JFM Rapids; Kinematics; Labeling; Laboratories; Laboratory experimentation; Ocean models; Ocean waves; Phase velocity; Progressive waves; Qualitative analysis; Remote observing; Remote sensing; Surface geometry; Surface gravity waves; Surface waves; Theories; Velocity; Vorticity; Water waves; Wave dispersion; ocean processes; air/sea interactions; surface gravity waves
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/jfm.2022.1036

The role of the Lagrangian mean flow, or drift, in modulating the geometry, kinematics and dynamics of rotational and irrotational deep-water surface gravity waves is examined. A general theory for permanent progressive waves on an arbitrary vertically sheared steady Lagrangian mean flow is derived in the Lagrangian reference frame and mapped to the Eulerian frame. A Lagrangian viewpoint offers tremendous flexibility due to the particle labelling freedom and allows us to reveal how key physical wave behaviour arises from a kinematic constraint on the vorticity of the fluid, inter alia the nonlinear correction to the phase speed of irrotational finite amplitude waves, the free surface geometry and velocity in the Eulerian frame, and the connection between the Lagrangian drift and the Benjamin–Feir instability. To complement and illustrate our theory, a small laboratory experiment demonstrates how a specially tailored sheared mean flow can almost completely attenuate the Benjamin–Feir instability, in qualitative agreement with the theory. The application of these results to problems in remote sensing and ocean wave modelling is discussed. We provide an answer to a long-standing question: remote sensing techniques based on observing current-induced shifts in the wave dispersion will measure the Lagrangian, not the Eulerian, mean current.