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Topographic perturbation of turbulent boundary layers by low-angle, early-stage aeolian dunes

Topographic perturbation of turbulent boundary layers by low-angle, early-stage aeolian dunes
Topographic perturbation of turbulent boundary layers by low-angle, early-stage aeolian dunes

Decimeter-scale early-stage aeolian bedforms represent topographic features that differ notably from their mature dune counterparts, with nascent forms exhibiting more gently sloping lee sides and a reverse asymmetry in their flow-parallel bed profile compared to mature dunes. Flow associated with the development of these “protodunes,” wherein the crest gradually shifts downstream towards its mature state, was investigated by studying the perturbation of the turbulent boundary layer over a succession of representative bedforms. Rigid, three-dimensional models were studied in a refractive-index-matched experimental flume that enabled near-surface quantification of mean velocities and Reynolds stresses using particle-image velocimetry in wall-normal and wall-parallel measurement planes. Data indicate strong, topographically induced flow perturbations over the protodunes, to a similar relative degree to that found over mature dunes, despite their low-angled slopes. The shape of the crest is found to be an important factor in the development of flow perturbations, and only in the case with the flattest crest was maximal speed-up of flow, and reduction in turbulent stresses, found to occur upstream of the crest. Investigation of the log-linearity of the boundary layer profile over the stoss sides showed that, although the profile is strongly perturbed, a log-linear region exists, but is shifted vertically. A streamwise trend in friction velocity is thus present, showing a behavior similar to the trends in mean velocity. Analysis of the growth of the internal boundary layer on the dune stoss sides, beginning at the toe region, reveals a similar development for all dune shapes, despite clear differences in mean velocity and turbulent stress perturbations in their toe regions. The data presented herein provide the first documentation of flow over morphologies broadly characteristic of subtle, low-angle, aeolian protodunes, and indicate key areas where further study is required to yield a more complete quantitative understanding of flow–form–transport couplings that govern their morphodynamics.

boundary lay­ers, early­-stage aeolian dunes, particle image velocimetry, protodunes, turbulence, early-stage aeolian dunes, boundary layers
0197-9337
1439
Bristow, N.R.
a03ed508-7716-4b03-a681-c0ad4591474a
Best, James L.
06bd4666-518e-469f-b2ce-76c92ea3d5ef
Wiggs, Giles F.S.
0b574ec8-fcd5-43b8-8b0b-0c84a01499d4
Nield, Joanna
173be2c5-b953-481a-abc4-c095e5e4b790
Baddock, Matthew C.
55f062fc-50a1-4c2d-83ff-a8cc92562346
Delorme, Pauline
d7e1a2d1-82e0-4c82-ae92-75d8ada3e51e
Christensen, K.T.
17ea786f-2dc0-4317-aa30-cd638c082011
Bristow, N.R.
a03ed508-7716-4b03-a681-c0ad4591474a
Best, James L.
06bd4666-518e-469f-b2ce-76c92ea3d5ef
Wiggs, Giles F.S.
0b574ec8-fcd5-43b8-8b0b-0c84a01499d4
Nield, Joanna
173be2c5-b953-481a-abc4-c095e5e4b790
Baddock, Matthew C.
55f062fc-50a1-4c2d-83ff-a8cc92562346
Delorme, Pauline
d7e1a2d1-82e0-4c82-ae92-75d8ada3e51e
Christensen, K.T.
17ea786f-2dc0-4317-aa30-cd638c082011

Bristow, N.R., Best, James L., Wiggs, Giles F.S., Nield, Joanna, Baddock, Matthew C., Delorme, Pauline and Christensen, K.T. (2022) Topographic perturbation of turbulent boundary layers by low-angle, early-stage aeolian dunes. Earth Surface Processes and Landforms, 47 (6), 1439. (doi:10.1002/esp.5326).

Record type: Article

Abstract

Decimeter-scale early-stage aeolian bedforms represent topographic features that differ notably from their mature dune counterparts, with nascent forms exhibiting more gently sloping lee sides and a reverse asymmetry in their flow-parallel bed profile compared to mature dunes. Flow associated with the development of these “protodunes,” wherein the crest gradually shifts downstream towards its mature state, was investigated by studying the perturbation of the turbulent boundary layer over a succession of representative bedforms. Rigid, three-dimensional models were studied in a refractive-index-matched experimental flume that enabled near-surface quantification of mean velocities and Reynolds stresses using particle-image velocimetry in wall-normal and wall-parallel measurement planes. Data indicate strong, topographically induced flow perturbations over the protodunes, to a similar relative degree to that found over mature dunes, despite their low-angled slopes. The shape of the crest is found to be an important factor in the development of flow perturbations, and only in the case with the flattest crest was maximal speed-up of flow, and reduction in turbulent stresses, found to occur upstream of the crest. Investigation of the log-linearity of the boundary layer profile over the stoss sides showed that, although the profile is strongly perturbed, a log-linear region exists, but is shifted vertically. A streamwise trend in friction velocity is thus present, showing a behavior similar to the trends in mean velocity. Analysis of the growth of the internal boundary layer on the dune stoss sides, beginning at the toe region, reveals a similar development for all dune shapes, despite clear differences in mean velocity and turbulent stress perturbations in their toe regions. The data presented herein provide the first documentation of flow over morphologies broadly characteristic of subtle, low-angle, aeolian protodunes, and indicate key areas where further study is required to yield a more complete quantitative understanding of flow–form–transport couplings that govern their morphodynamics.

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Accepted/In Press date: 11 January 2022
e-pub ahead of print date: 15 February 2022
Published date: 14 May 2022
Keywords: boundary lay­ers, early­-stage aeolian dunes, particle image velocimetry, protodunes, turbulence, early-stage aeolian dunes, boundary layers

Identifiers

Local EPrints ID: 454475
URI: http://eprints.soton.ac.uk/id/eprint/454475
ISSN: 0197-9337
PURE UUID: 2c045826-8008-4ae1-9f7b-7ae2ce55ed4c
ORCID for Joanna Nield: ORCID iD orcid.org/0000-0002-2657-0525

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Date deposited: 10 Feb 2022 17:40
Last modified: 11 Jan 2023 05:01

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Contributors

Author: N.R. Bristow
Author: James L. Best
Author: Giles F.S. Wiggs
Author: Joanna Nield ORCID iD
Author: Matthew C. Baddock
Author: Pauline Delorme
Author: K.T. Christensen

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