Shape and Dimensions of Ripples

Niels Gjøl Jacobsen, Jørgen Fredsøe (Supervisor)

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In this work, the flow and bed load transport over ripple profiles under the influence of oscillatory will be investigated. The investigation is made through a parametric study, where the bed shape and the ripple steepness are varied. For the sediment transport, the Shields parameter relative to the critical one is varied in the range 1.31-3.45. A detailed flow description in terms of phase resolved quantities and period averaged quantities are given. The former are phase lag between the free stream flow and the separation and a description of the lee side vortices behind vortex ripples. The latter are the recirculation zone and its penetration into the main flow. This will be compared to the period averaged bed shear stress. An analysis of the bed load transport, both as a transport capacity
in the phase resolved space and an analysis of possible stable ripples is conducted. This leads to some stable ripples and the general conclusion is that sharp crested ripples is thought unlikely to become stable under oscillatory flow due to the large increase in bed shear stresses because of the rapid contraction of the flow around the crest. Further, a dynamically moving ripple crest is investigated. The key focus is to investigate how a perturbation on top of the ripple influences the flow. It will be seen that both the shape and the velocity of the crest are affecting the period averaged flow globally.
Original languageEnglish
PublisherDTU Mechanical Engineering
Number of pages84
Publication statusPublished - 2007

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