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Coherent structures in wave boundary layers. Part 1. Oscillatory motion

Published online by Cambridge University Press:  08 March 2010

STEFAN CARSTENSEN ,
B. MUTLU SUMER  and
JØRGEN FREDSØE
STEFAN CARSTENSEN
Affiliation:
Technical University of Denmark, DTU Mekanik, Section of Coastal, Maritime and Structural Engineering, Building 403, 2800 Kgs. Lyngby, Denmark
B. MUTLU SUMER*
Affiliation:
Technical University of Denmark, DTU Mekanik, Section of Coastal, Maritime and Structural Engineering, Building 403, 2800 Kgs. Lyngby, Denmark
JØRGEN FREDSØE
Affiliation:
Technical University of Denmark, DTU Mekanik, Section of Coastal, Maritime and Structural Engineering, Building 403, 2800 Kgs. Lyngby, Denmark
*
‡Email address for correspondence: bms@mek.dtu.dk
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Abstract

This work concerns oscillatory boundary layers over smooth beds. It comprises combined visual and quantitative techniques including bed shear stress measurements. The experiments were carried out in an oscillating water tunnel. The experiments reveal two significant coherent flow structures: (i) Vortex tubes, essentially two-dimensional vortices close to the bed extending across the width of the boundary-layer flow, caused by an inflectional-point shear layer instability. The imprint of these vortices in the bed shear stress is a series of small, insignificant kinks and dips. (ii) Turbulent spots, isolated arrowhead-shaped areas close to the bed in an otherwise laminar boundary layer where the flow ‘bursts’ with violent oscillations. The emergence of the turbulent spots marks the onset of turbulence. Turbulent spots cause single or multiple violent spikes in the bed shear stress signal, which has profound implications for sediment transport (in both the laboratory and the field). The experiments also show that similar coherent flow structures exist in the case of combined oscillatory flow and current.

JFM classification

Boundary Layers: Boundary layer stability Turbulent Flows: Turbulent transition
Type
Papers
Information
Journal of Fluid Mechanics , Volume 646 , 10 March 2010 , pp. 169 - 206
Copyright
Copyright © Cambridge University Press 2010

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Footnotes

†

Present address: DHI, Agern Alle 5, 2970 Hørsholm, Denmark.

References

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Carstensen et al. supplementary movie 1

Movie 1. (Fig. 5) Time evolution of vortex tubves (plan view). Test 5, Half Cycle 5. Re = 2.9×105. ωt = 147 to 185 degrees. The flow near the bed (where the vortex tubes are located) is from right to left, and therefore the system of vortex tubes are in constant motion from right to left thorughout the sequence.

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Carstensen et al. supplementary movie 2

Movie 2. (Fig. 9) Video illustrating the time development of a turbulent spot. Test 6, Half Cycle 4. Re = 4.5×105. ωt = 50 to 200 degrees. Free-stream flow from right to left.

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Carstensen et al. supplementary movie 3

Movie 3. (N/A) Turbulent Spot (Side view). Test 10. Re = 3.3×105. Free-stream direction from left to right.

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Carstensen et al. supplementary movie 4

Movie 4. (Fig. 12) Turbulent spots and vortex tubes occur concurrently in space(plan view). Test 7, Half Cycle 3. Re = 3.0×105. ωt = 54 to 209 degrees. Free-stream direction from left to right.

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