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Direct numerical simulations of ripples in an oscillatory flow

Published online by Cambridge University Press:  28 January 2019

Marco Mazzuoli Open the ORCID record for Marco Mazzuoli [Opens in a new window] ,
Aman G. Kidanemariam Open the ORCID record for Aman G. Kidanemariam [Opens in a new window]  and
Markus Uhlmann Open the ORCID record for Markus Uhlmann [Opens in a new window]
Marco Mazzuoli*
Affiliation:
Department of Civil, Chemical and Environmental Engineering, University of Genoa, Via Montallegro 1, 16145 Genova, Italy
Aman G. Kidanemariam
Affiliation:
Institute for Hydromechanics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
Markus Uhlmann
Affiliation:
Institute for Hydromechanics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
*
†Email address for correspondence: marco.mazzuoli@unige.it
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Abstract

Sea ripples are small-scale bedforms which originate from the interaction of an oscillatory flow with an erodible sand bed. The phenomenon of sea ripple formation is investigated by means of direct numerical simulation in which the sediment bed is represented by a large number of fully resolved spherical grains (i.e. the flow around each individual particle is accounted for). Two sets of parameter values (differing in the amplitude and frequency of fluid oscillations, among other quantities) are adopted which are motivated by laboratory experiments on the formation of laminar rolling-grain ripples. The knowledge of the origin of ripples is presently enriched by insights and by providing fluid- and sediment-related quantities that are difficult to obtain in the laboratory (e.g. particle forces, statistics of particle motion, bed shear stress). In particular, detailed analysis of flow and sediment bed evolution has confirmed that ripple wavelength is determined by the action of steady recirculating cells which tend to accumulate sediment grains into ripple crests. The ripple amplitude is observed to grow exponentially, consistent with established linear stability analysis theories. Particles at the bed surface exhibit two kinds of motion depending on their position with respect to the recirculating cells: particles at ripple crests are significantly faster and show larger excursions than those lying in ripple troughs. In analogy with the segregation phenomenon of polydisperse sediments, the non-uniform distribution of the velocity field promotes the formation of ripples. The wider the gap between the excursion of fast and slow particles, the larger the resulting growth rate of the ripples. Finally, it is revealed that, in the absence of turbulence, the sediment flow rate is driven by both the bed shear stress and the wave-induced pressure gradient, the dominance of each depending on the phase of the oscillation period. In phases of maximum bed shear stress, the sediment flow rate correlates more with the Shields number while the pressure gradient tends to drive sediment bed motion during phases of minimum bed shear stress.

JFM classification

Multiphase and Particle-laden Flows: Particle/fluid flow Geophysical and Geological Flows: Sediment transport
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 863 , 25 March 2019 , pp. 572 - 600
Copyright
© 2019 Cambridge University Press 

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Footnotes

‡

Present address: Federal Waterways Engineering and Research Institute (BAW), 76152 Karlsruhe, Germany

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Mazzuoli et al. supplementary movie 1

Top view of the bed: particles are coloured according to their distance form the wall (increasing blue to red). Small panels indicate the time development of the particle flow rate (left panel) and of the free-stream velocity (right panel). PART 1.

Download Mazzuoli et al. supplementary movie 1(Video)
Video 9.5 MB

Mazzuoli et al. supplementary movie 2

Top view of the bed: particles are coloured according to their distance form the wall (increasing blue to red). Small panels indicate the time development of the particle flow rate (left panel) and of the free-stream velocity (right panel). PART 2.

Download Mazzuoli et al. supplementary movie 2(Video)
Video 9.4 MB

Mazzuoli et al. supplementary movie 3

Top view of the bed: particles are coloured according to their distance form the wall (increasing blue to red). Small panels indicate the time development of the particle flow rate (left panel) and of the free-stream velocity (right panel). PART 3.

Download Mazzuoli et al. supplementary movie 3(Video)
Video 10.1 MB

Mazzuoli et al. supplementary movie 4

Top view of the bed: particles are coloured according to their distance form the wall (increasing blue to red). Small panels indicate the time development of the particle flow rate (left panel) and of the free-stream velocity (right panel). PART 4.

Download Mazzuoli et al. supplementary movie 4(Video)
Video 9.8 MB

Mazzuoli et al. supplementary movie 5

Top view of the bed: particles are coloured according to their distance form the wall (increasing blue to red). Small panels indicate the time development of the particle flow rate (left panel) and of the free-stream velocity (right panel). PART 5.

Download Mazzuoli et al. supplementary movie 5(Video)
Video 9.3 MB

Mazzuoli et al. supplementary movie 6

Top view of the bed: particles are coloured according to their distance form the wall (increasing blue to red). Small panels indicate the time development of the particle flow rate (left panel) and of the free-stream velocity (right panel). PART 6.

Download Mazzuoli et al. supplementary movie 6(Video)
Video 9.2 MB

Mazzuoli et al. supplementary movie 7

Top view of the bed: particles are coloured according to their distance form the wall (increasing blue to red). Small panels indicate the time development of the particle flow rate (left panel) and of the free-stream velocity (right panel). PART 7.

Download Mazzuoli et al. supplementary movie 7(Video)
Video 9.4 MB

Mazzuoli et al. supplementary movie 8

Top view of the bed: particles are coloured according to their distance form the wall (increasing blue to red). Small panels indicate the time development of the particle flow rate (left panel) and of the free-stream velocity (right panel). PART 8.

Download Mazzuoli et al. supplementary movie 8(Video)
Video 8 MB