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Observations of nearbed turbulence over mobile bedforms in combined, collinear wave-current flows

Observations of nearbed turbulence over mobile bedforms in combined, collinear wave-current flows
Observations of nearbed turbulence over mobile bedforms in combined, collinear wave-current flows

Collinear wave-current shear interactions are often assumed to be the same for currents following or opposing the direction of regular wave propagation; with momentum and mass exchanges restricted to the thin oscillating boundary layer (zero-flux condition) and enhanced but equal wave-averaged bed shear stresses. To examine these assumptions, a prototype-scale experiment investigated the nature of turbulent exchanges in flows with currents aligned to, and opposing, wave propagation over a mobile sandy bed. Estimated mean and maximum stresses from measurements above the bed exceeded predictions by models of bed shear stress subscribing to the assumptions above, suggesting the combined boundary layer is larger than predicted by theory. The core flow experiences upward turbulent fluxes in aligned flows, coupled with sediment entrainment by vortex shedding at flow reversal, whilst downward fluxes of eddies generated by the core flow, and strong adverse shear can enhance near-bed mass transport, in opposing currents. Current-aligned coherent structures contribute significantly to the stress and energy dissipation, and display characteristics of wall-attached eddies formed by the pairing of counter-rotating vortices. These preliminary findings suggest a notable difference in wave-following and wave-opposing wave-current interactions, and highlight the need to account for intermittent momentum-exchanges in predicting stress, boundary layer thickness and sediment transport.

Bedforms, Boundary layers, Coherent turbulence structures, Sediment dynamics, Wave-current interactions
2073-4441
1-26
Kassem, Hachem
658efa7a-a02c-4b29-9d07-5d57e95a4b51
Thompson, Charlotte
2a304aa6-761e-4d99-b227-cedb67129bfb
Amos, Carl
d0a18a13-bccd-4fdc-8901-aea595d4ed5c
Townend, Ian
f72e5186-cae8-41fd-8712-d5746f78328e
Todd, David
728129e8-4f41-4ca2-afa4-01a439ffdf52
Whitehouse, Richard J.S.
885c1b2d-ddcd-44e7-a700-a56683e2dde7
Chellew, Elizabeth
bb5faa6a-ca37-438c-8779-09d0efedd086
Kassem, Hachem
658efa7a-a02c-4b29-9d07-5d57e95a4b51
Thompson, Charlotte
2a304aa6-761e-4d99-b227-cedb67129bfb
Amos, Carl
d0a18a13-bccd-4fdc-8901-aea595d4ed5c
Townend, Ian
f72e5186-cae8-41fd-8712-d5746f78328e
Todd, David
728129e8-4f41-4ca2-afa4-01a439ffdf52
Whitehouse, Richard J.S.
885c1b2d-ddcd-44e7-a700-a56683e2dde7
Chellew, Elizabeth
bb5faa6a-ca37-438c-8779-09d0efedd086

Kassem, Hachem, Thompson, Charlotte, Amos, Carl, Townend, Ian, Todd, David, Whitehouse, Richard J.S. and Chellew, Elizabeth (2020) Observations of nearbed turbulence over mobile bedforms in combined, collinear wave-current flows. Water, 12 (12), 1-26, [3515]. (doi:10.3390/w12123515).

Record type: Article

Abstract

Collinear wave-current shear interactions are often assumed to be the same for currents following or opposing the direction of regular wave propagation; with momentum and mass exchanges restricted to the thin oscillating boundary layer (zero-flux condition) and enhanced but equal wave-averaged bed shear stresses. To examine these assumptions, a prototype-scale experiment investigated the nature of turbulent exchanges in flows with currents aligned to, and opposing, wave propagation over a mobile sandy bed. Estimated mean and maximum stresses from measurements above the bed exceeded predictions by models of bed shear stress subscribing to the assumptions above, suggesting the combined boundary layer is larger than predicted by theory. The core flow experiences upward turbulent fluxes in aligned flows, coupled with sediment entrainment by vortex shedding at flow reversal, whilst downward fluxes of eddies generated by the core flow, and strong adverse shear can enhance near-bed mass transport, in opposing currents. Current-aligned coherent structures contribute significantly to the stress and energy dissipation, and display characteristics of wall-attached eddies formed by the pairing of counter-rotating vortices. These preliminary findings suggest a notable difference in wave-following and wave-opposing wave-current interactions, and highlight the need to account for intermittent momentum-exchanges in predicting stress, boundary layer thickness and sediment transport.

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More information

Accepted/In Press date: 30 November 2020
e-pub ahead of print date: 14 December 2020
Published date: December 2020
Keywords: Bedforms, Boundary layers, Coherent turbulence structures, Sediment dynamics, Wave-current interactions

Identifiers

Local EPrints ID: 445380
URI: http://eprints.soton.ac.uk/id/eprint/445380
ISSN: 2073-4441
PURE UUID: 47866870-f964-4f93-83d1-58bab329a040
ORCID for Hachem Kassem: ORCID iD orcid.org/0000-0002-5936-6037
ORCID for Ian Townend: ORCID iD orcid.org/0000-0003-2101-3858

Catalogue record

Date deposited: 07 Dec 2020 17:30
Last modified: 26 Nov 2021 03:01

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Contributors

Author: Hachem Kassem ORCID iD
Author: Carl Amos
Author: Ian Townend ORCID iD
Author: David Todd
Author: Richard J.S. Whitehouse
Author: Elizabeth Chellew

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