Ripple formation over a sand bed submitted to a laminar shear flow

Valance, Alexandre; Langlois, Vincent

doi:10.1140/epjb/e2005-00050-6

Cited by 34 publications

(22 citation statements)

References 20 publications

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“…Gravity effects are mainly related to a strong decrease (as a power of 3 of B g ) in the growth rate. Valance and Langlois (2005) obtained similar results, but for laminar shear flows. They found that the length-scale of the bedforms is given mainly by the relaxation effects if particle Reynolds number is relatively large ( Bagnold, 1941) .…”

Section: Results From the Stability Analysissupporting

confidence: 59%

“…They are three: the fluid flow perturbation by the shape of the bed, the relaxation effects related to the grains and the gravity effects. The fluid flow perturbation is known to be the unstable mechanism (Jackson et al, 1975;Hunt et al, 1988 andWeng et al, 1991), and the relaxation and the gravity effects are the stable mechanisms (Valance and Langlois (2005) and Charru (2006) in the case of viscous flows).…”

Section: Introductionmentioning

confidence: 99%

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Initial instabilities of a granular bed sheared by a turbulent liquid flow: length-scale determination

Franklin¹

2010

J. Braz. Soc. Mech. Sci. & Eng.

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Section: Results From the Stability Analysissupporting

confidence: 59%

Section: Introductionmentioning

confidence: 99%

Initial instabilities of a granular bed sheared by a turbulent liquid flow: length-scale determination

Franklin¹

2010

J. Braz. Soc. Mech. Sci. & Eng.

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“…In an aeolian context, this length clearly originates in the particle inertia, and can be estimated from the momentum conservation equation of an accelerated particle, giving l sat = c l ͑ p /͒d, ͑28͒ measurements giving c l Ϸ 4. 37 However, the inertial balance that justifies the derivation of ͑28͒ is questionable for liquid flows, in particular for laminar flows that, close to a boundary, are necessarily viscous. 34,35 The relaxation equation ͑28͒ has also been invoked for liquid FIG.…”

Section: The Erosion-deposition Equation As a Relaxation Equationmentioning

confidence: 99%

Selection of the ripple length on a granular bed sheared by a liquid flow

2006

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The stability of a granular bed sheared by a liquid flow is investigated using an erosion-deposition model for the moving grains. Extending to arbitrary wave number a previous long wave analysis [F. Charru and E. J. Hinch, J. Fluid Mech. 550, 111 (2006)], this study shows that short waves are stabilized by a crest-erosion mechanism that is absent when the local particle flux is assumed to be in equilibrium with the local bed shear stress. The cutoff length associated with this mechanism scales on a deposition length of the particles. This characteristic length is similar, although of a different physical nature, to the inertial length involved in relaxation models of dune dynamics under the wind. Two stabilizing mechanisms cooperate for the most amplified wave number: the new crest-erosion mechanism and the gravity force parallel to the local slope of the inclined bed. The predictions of this model are compared to observations, showing better agreement than previous stability analyses, which strongly underpredict the observed lengths.

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“…These first perturbations grow exponentially with time until a homogeneous ripple field is formed [ Betat , ; Fourrière et al , ]. During this phase the incipient ripple wavelength remains constant and is linked to the roughness of the bed, scaling whether on the viscous length or on the grain size, depending on the shear velocity [ Valance and Langlois , ; Valance , ; Charru , ]. Many laboratory and field experiments allowed for the observation of the initial ripples development, revealing wavelengths of the order of a few centimeters (of the order O (10 2 ) d 50 to O (10 3 ) d 50 ) and development time scales of a few tens of seconds [ Yalin , ; Coleman and Melville , ; Baas , ; Betat , ; Langlois and Valance , ; Fourrière et al , ].…”

Section: Introductionmentioning

confidence: 99%

Numerical modeling of subaqueous sand dune morphodynamics

et al. 2016

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The morphodynamic evolution of subaqueous sand dunes is investigated, using a 2‐D Reynolds‐averaged Navier‐Stokes numerical model. A laboratory experiment where dunes are generated under stationary unidirectional flow conditions is used as a reference case. The model reproduces the evolution of the erodible bed until a state of equilibrium is reached. In particular, the simulation exhibits the different stages of the bed evolution, e.g., the incipient ripple generation, the nonlinear bed form growing phase, and the dune field equilibrium phase. The results show good agreement in terms of dune geometrical dimensions and time to equilibrium. After the emergence of the first ripple field, the bed growth is driven by cascading merging sequences between bed forms of different heights. A sequence extracted from the simulation shows how the downstream bed form is first eroded before merging with the upstream bed form. Superimposed bed forms emerge on the dune stoss sides during the simulation. An analysis of the results shows that they emerge downstream of a slight deflection on the dune profile. The deflection arises due to a modification of the sediment flux gradient consecutive to a reduction in the turbulence relaxation length while the upstream bed form height decreases. As they migrate, superimposed bed forms grow on the dune stoss side and eventually provoke the degeneration of the dune crest. Cascading merging sequences and superimposed bed forms dynamics both influence the dune field evolution and size and therefore play a fundamental role in the dune field self‐organization process.

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Ripple formation over a sand bed submitted to a laminar shear flow

Cited by 34 publications

References 20 publications

Initial instabilities of a granular bed sheared by a turbulent liquid flow: length-scale determination

Initial instabilities of a granular bed sheared by a turbulent liquid flow: length-scale determination

Selection of the ripple length on a granular bed sheared by a liquid flow

Numerical modeling of subaqueous sand dune morphodynamics

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