A laboratory study of the wave-induced mean flow and set-down in unidirectional surface gravity wavepackets on finite water depth

Ross Calvert, Colin Whittaker, Alison Raby, Paul H. Taylor, Alistair Borthwick, Ton Van Den Bremer

Research output: Contribution to journalArticlepeer-review

Abstract

The net movement of Lagrangian particles under water waves comprises a Stokes drift in the direction of wave propagation and an Eulerian return flow in the opposing direction. Accurate prediction of the Eulerian return flow in the ocean is of importance in modelling the transport of plastic pollution, oil, wreckage, and sediment. Herein, we derive a multiple-scales solution for the Eulerian mean flow under wavepackets that is valid for all water depths, both relative to the length of the wave and the length of the wavepacket. To validate this solution, we carry out Particle Tracking Velocimetry experiments in a long flume to extract the mean motion from Lagrangian seeding particles under wavepackets, finding good agreement. The extraction technique is able to deal with small background motion and sub-harmonic error waves associated with wave generation by the paddle, the latter being relatively large in finite-depth flume experiments. In finite depth, the return flow is forced by both the divergence of the Stokes transport on the wavepacket scale and the formation of a non-negligible mean set-down underneath the packet, which acts like a bounding streamtube in the form of a convergent-divergent duct. The magnitude of the horizontal return flow is thus enhanced, with particular relevance to transport in the finite-depth coastal environment.
Original languageEnglish
Article number114801
JournalPhysical Review Fluids
Volume4
Issue number11
DOIs
Publication statusPublished - 22 Nov 2019

Keywords

  • Wave-induced transport
  • Stokes drift
  • Lagrangian displacement
  • wavepackets
  • set-down
  • FINITE DEPTH

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