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Hans J. Herrmann - One of the best experts on this subject based on the ideXlab platform.
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2. Programa de Pós-Graduação em Engenharia Química,
2013Co-Authors: Eric J R Parteli, Jose S Andrade, Hans J. HerrmannAbstract:The simplest type of dune is the transverse one, which propagates with invariant profile orthogonally to a fixed wind direction. Here we show numerically and with a linear stability analysis that transverse dunes are unstable with respect to along-axis perturbations in their profile and decay on the bedrock into Barchan dunes. Any forcing modulation amplifies exponentially with growth rate determined by the dune turnover time. We estimate the distance covered by a transverse dune before fully decaying into Barchans and identify the patterns produced by different types of perturbation. PACS numbers: 45.70.-n, 45.70.Qj, 05.65.+b, 45.70.Mg Wind directionality and sand availability are the main factors dictating dune morphology. Bimodal and multidirectional wind systems form longitudinal and star dunes, respectively[1]. Underunimodalwinds, twotypes of dune may occur, depending on the amount of sand: crescent-shaped Barchans, evolving on the bedrock, and transverse dunes, which appear when the ground is covered with sand [1–3]. Studies of dune genesis have focuse
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A continuous model for sand dunes: Review, new developments and application to Barchan dunes and Barchan dune fields
Earth Surface Processes and Landforms, 2010Co-Authors: Orencio Duran, Eric J R Parteli, Hans J. HerrmannAbstract:Basically, sand dunes are patterns resulting from the coupling of hydrodynamic and sediment transport. Once grains move, they modify the surface topography which in turns modifies the flow. This important feedback mechanism lies at the core of continuous dune modelling. Here we present an updated review of such a model for aeolian dunes, including important modifications to improve its predicting power. For instance, we add a more realistic wind model and provide a self-consistent set of parameters independently validated. As an example, we are able to simulate realistic Barchan dunes, which are the basic solution of the model in the condition of unidirectional flow and scarce sediments. From the simulation, we extract new relations describing the morphology and dynamics of Barchans that compare very well with existing field data. Next, we revisit the problem of the stability of Barchan dunes and argue that they are intrinsically unstable bed-forms. Finally, we perform more complex simulations: first, a Barchan dune under variable wind strength and, second, Barchan dune fields under different boundary conditions. The latter has important implications for the problem of the genesis of Barchan dunes. Copyright © 2010 John Wiley & Sons, Ltd.
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a model of Barchan dunes including lateral shear stress
European Physical Journal E, 2005Co-Authors: Veit Schwammle, Hans J. HerrmannAbstract:Barchan dunes are found where sand availability is low and wind direction quite constant. The two dimensional shear stress of the wind field and the sand movement by saltation and avalanches over a Barchan dune are simulated. The model with one dimensional shear stress is extended including surface diffusion and lateral shear stress. The resulting final shape is compared to the results of the model with a one dimensional shear stress and confirmed by comparison to measurements. We found agreement and improvements with respect to the model with one dimensional shear stress. Additionally, a characteristic edge at the center of the windward side is discovered which is also observed for big Barchans. Diffusion effects reduce this effect for small dunes.
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wind velocity and sand transport on a Barchan dune
Geomorphology, 2003Co-Authors: G Sauermann, Jose S Andrade, Luis Parente Maia, U M S Costa, Ascânio D Araujo, Hans J. HerrmannAbstract:We present measurements of wind velocity and sand flux performed on the windward side of a large Barchan dune in Jericoacoara, northeastern Brazil. From the measured profile, we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model. We find that the results from this approach agree well with our field data. Moreover, using the calculated shear velocity, we predict the sand flux according to well-known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions. Based on the field data and theoretical predicted results, we indicate the principal differences between saturated and nonsaturated sand flux models. Finally, we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations.
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modelling a dune field
arXiv: Condensed Matter, 2002Co-Authors: A R Lima, Hans J. Herrmann, G Sauermann, Klaus KroyAbstract:We present a model to describe the collective motion of Barchan dunes in a field. Our model is able to reproduce the observation that a typical dune stays confined within a stripe. We also obtain some of the pattern structures which ressemble those observed from aerial photos which we do analyse and compare with the specific field of La\^ayounne.
Stephane Douady - One of the best experts on this subject based on the ideXlab platform.
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formation and stability of transverse and longitudinal sand dunes
Geology, 2010Co-Authors: E Reffet, Pascal Hersen, Courrech S Du Pont, Stephane DouadyAbstract:The shape of dunes depends on the history of wind regimes and sand availability. In deserts exposed to winds from two different directions but with comparable magnitude, dunes are found to be linear ridges, which are either perpendicular or parallel to the mean wind direction, depending on the angle between the two wind directions. These dunes, respectively observed for small and large angles between winds, are called transverse and longitudinal dunes. In both cases, their large width (hundreds of meters) and evolution time scale (years) strongly limit the investigation of their dynamics and thus our understanding of such structures. Here we show that, under water, similar structures can be obtained but at much smaller space and time scales. Performing controlled experiments together with numerical simulations, we highlight the physical mechanisms at play in the formation and long-term evolution of these structures. We show in particular that, while longitudinal dunes are stable and extend in time, transverse dunes are unstable. They evolve into wavy ridges and eventually break into Barchans if the sand supply is too low. This fundamental difference is understood through the study of single sand piles and bars exposed to two winds. In the case of a large angle between winds, a sand pile grows a finger pointing in the average wind direction and transforms into a longitudinal dune. Such an elongation does not occur for a small angle where a sand pile evolves into a Barchan. These results explain the morphological differences between straight and long longitudinal dunes and sinuous transverse dunes, while giving keys to infer the wind history or pattern state of development from the observation of dune shapes in the field.
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collision of Barchan dunes as a mechanism of size regulation
Geophysical Research Letters, 2005Co-Authors: Pascal Hersen, Stephane DouadyAbstract:[1] Barchans are propagating crescentic dunes that form in arid regions where strong, unidirectional winds blow across a firm soil lightly covered with sand. Recently, solitary Barchan dunes have been shown to be unstable towards sand flux variation either growing to form mega-dunes or shrinking and disappearing when perturbed from equilibrium. However, observations of large corridors of Barchan dunes in the field suggest that these bedforms are stable over long periods of time. Since dunes' migration rates are inversely proportional to their size, Barchans of different size must collide and these collisions may be crucial in maintaining the stability of dunes in nature. Here, we first explain the unstable behavior of solitary Barchans and then illustrate, using down-scaled physical experiments, the qualitative dynamics of binary collisions. A stability analysis, inspired by these experiments, suggests that collisions may indeed regulate the size of Barchans migrating in a corridor by redistributing sand from large dunes to smaller ones.
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relevant length scale of Barchan dunes
Physical Review Letters, 2002Co-Authors: Pascal Hersen, Stephane Douady, Bruno AndreottiAbstract:A new experiment can create small scale Barchan dunes under water: some sand is put on a tray moving periodically and asymmetrically in a water tank, and Barchans rapidly form. We measure basic morphological and dynamical properties of these dunes and compare them to field data. These favorable results demonstrate experimentally the relevance of the so-called ``saturation length'' for the control of the dunes physics.
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selection of dune shapes and velocities part 1 dynamics of sand wind and Barchans
European Physical Journal B, 2002Co-Authors: Bruno Andreotti, Philippe Claudin, Stephane DouadyAbstract:Almost fifty years of investigations of Barchan dunes morphology and dynamics is reviewed, with emphasis on the physical understanding of these objects. The characteristic quantities measured on the field (shape, size, velocity) and the physical problems they rise are presented. Then, we review the dynamical mechanisms explaining the formation and the propagation of dunes. In particular a complete and original approach of the sand transport over a flat sand bed is proposed and discussed. We conclude on open problems by outlining future research directions.
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selection of dune shapes and velocities part 1 dynamics of sand wind and Barchans
arXiv: Condensed Matter, 2002Co-Authors: Bruno Andreotti, Philippe Claudin, Stephane DouadyAbstract:Almost fifty years of investigations of Barchan dunes morphology and dynamics is reviewed, with emphasis on the physical understanding of these objects. The characteristics measured on the field (shape, size, velocity) and the physical problems they rise are presented. Then, we review the dynamical mechanisms explaining the formation and the propagation of dunes. In particular a complete and original approach of the sand transport over a flat sand bed is proposed and discussed. We conclude on open problems by outlining future research directions.
Pascal Hersen - One of the best experts on this subject based on the ideXlab platform.
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Spatial structuring and size selection as collective behaviours in an agent-based model for Barchan fields.
European Physical Journal B: Condensed Matter and Complex Systems, 2013Co-Authors: Mathieu Génois, Sylvain Courrech Du Pont, Pascal Hersen, Guillaume GrégoireAbstract:In order to test parameters of the peculiar dynamics occurring in Barchan fields, and compute statistical analysis over large numbers of dunes, we build and study an agent-based model, which includes the well-known physics of an isolated Barchan, and observations of interactions between dunes. We showed in a previous study that such a model, where Barchans interact through short-range sand recapture and collisions, reproduces the peculiar behaviours of real fields, namely its spatial structuring along the wind direction, and the size selection by the local density. In this paper we focus on the mechanisms that drives these features. In particular, we show that eolian remote sand transfer between dunes ensures that a dense field structures itself into a very heterogeneous pattern, which alternates dense and diluted stripes in the wind direction. In these very dense clusters of dunes, the accumulation of collisions leads to the local emergence of a new size for the dunes.
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formation and stability of transverse and longitudinal sand dunes
Geology, 2010Co-Authors: E Reffet, Pascal Hersen, Courrech S Du Pont, Stephane DouadyAbstract:The shape of dunes depends on the history of wind regimes and sand availability. In deserts exposed to winds from two different directions but with comparable magnitude, dunes are found to be linear ridges, which are either perpendicular or parallel to the mean wind direction, depending on the angle between the two wind directions. These dunes, respectively observed for small and large angles between winds, are called transverse and longitudinal dunes. In both cases, their large width (hundreds of meters) and evolution time scale (years) strongly limit the investigation of their dynamics and thus our understanding of such structures. Here we show that, under water, similar structures can be obtained but at much smaller space and time scales. Performing controlled experiments together with numerical simulations, we highlight the physical mechanisms at play in the formation and long-term evolution of these structures. We show in particular that, while longitudinal dunes are stable and extend in time, transverse dunes are unstable. They evolve into wavy ridges and eventually break into Barchans if the sand supply is too low. This fundamental difference is understood through the study of single sand piles and bars exposed to two winds. In the case of a large angle between winds, a sand pile grows a finger pointing in the average wind direction and transforms into a longitudinal dune. Such an elongation does not occur for a small angle where a sand pile evolves into a Barchan. These results explain the morphological differences between straight and long longitudinal dunes and sinuous transverse dunes, while giving keys to infer the wind history or pattern state of development from the observation of dune shapes in the field.
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collision of Barchan dunes as a mechanism of size regulation
Geophysical Research Letters, 2005Co-Authors: Pascal Hersen, Stephane DouadyAbstract:[1] Barchans are propagating crescentic dunes that form in arid regions where strong, unidirectional winds blow across a firm soil lightly covered with sand. Recently, solitary Barchan dunes have been shown to be unstable towards sand flux variation either growing to form mega-dunes or shrinking and disappearing when perturbed from equilibrium. However, observations of large corridors of Barchan dunes in the field suggest that these bedforms are stable over long periods of time. Since dunes' migration rates are inversely proportional to their size, Barchans of different size must collide and these collisions may be crucial in maintaining the stability of dunes in nature. Here, we first explain the unstable behavior of solitary Barchans and then illustrate, using down-scaled physical experiments, the qualitative dynamics of binary collisions. A stability analysis, inspired by these experiments, suggests that collisions may indeed regulate the size of Barchans migrating in a corridor by redistributing sand from large dunes to smaller ones.
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relevant length scale of Barchan dunes
Physical Review Letters, 2002Co-Authors: Pascal Hersen, Stephane Douady, Bruno AndreottiAbstract:A new experiment can create small scale Barchan dunes under water: some sand is put on a tray moving periodically and asymmetrically in a water tank, and Barchans rapidly form. We measure basic morphological and dynamical properties of these dunes and compare them to field data. These favorable results demonstrate experimentally the relevance of the so-called ``saturation length'' for the control of the dunes physics.
Dong Guangrong - One of the best experts on this subject based on the ideXlab platform.
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study on formation mechanism of feather shaped sand ridge in kumtag desert
Journal of Desert Research, 2007Co-Authors: Dong GuangrongAbstract:Kumtag Desert,like a fan,covers on the inclined plain in front of Altun Mountain.There distributes many kinds of complex sand dunes,among which feather-shaped sand ridge is the unique sand form in China.The literatures on the formation and development mechanism of feather-shaped sand ridge are scarce in recent ten years.The authors has conducted field observation,aerial photograph interpretation and simulation experiment in wind tunnel,it was found out that the forming factors of feather-shaped sand ridge are as follows: ① extensive inclined plain,② non-abundant sand materials on surface,③ winds in two similar directions as dynamic condition,④ tongue-liked sand dune is the sub-form of Barchan sand ridge.Both tongue-liked sand dune and Barchan sand ridge combine the feather-shaped sand ridge.The developing process of feather-shaped sand ridge can be summarized as the follow stages: Barchan sand dune→Barchan sand ridge→tongue-liked sand dune→feather-shaped sand ridge.
Gian Gabriele Ori - One of the best experts on this subject based on the ideXlab platform.
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dunes on planet tatooine observation of Barchan migration at the star wars film set in tunisia
Geomorphology, 2013Co-Authors: Ralph D Lorenz, Nabil Gasmi, J Radebaugh, Jason W Barnes, Gian Gabriele OriAbstract:Abstract Sand dune migration is documented with a readily-available tool (Google Earth) near Chott El Gharsa, just north-west of Tozeur, Tunisia. As fiducial markers we employ a set of buildings used to portray the fictional city Mos Espa. This set of ~ 20 buildings over roughly a hectare was constructed in 1997 for the movie Star Wars Episode 1 — The Phantom Menace. The site now lies between the arms of a large “pudgy” Barchan dune, which has been documented via satellite imaging in 2002, 2004, 2008 and 2009 to have moved from ~ 140 m away to only ~ 10 m away. Visits by the authors to the site in 2009 and 2011 confirm the Barchan to be in a threatening position: a smaller set nearby was substantially damaged by being overrun by dunes circa 2004. The migration rate of ~ 15 m/yr decreases over the observation period, possibly as a result of increased local rainfall, and is consistent with Barchan migration rates observed at other locations worldwide. The migration rate of this and two other Barchans suggests sand transport of ~ 50 m3/m/yr, somewhat higher than would be suggested by traditional wind rose calculations: we explore possible reasons for this discrepancy. Because of the link to popular science fiction, the site may be of pedagogical interest in teaching remote sensing and geomorphic change. We also note that nearby playa surfaces and agricultural areas have a time-variable appearance. The site's popularity as a destination for Star Wars enthusiasts results in many photographs being posted on the internet, providing a rich set of in-situ imagery for continued monitoring in the absence of dedicated field visits.
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Ripple migration and dune activity on Mars: Evidence for dynamic wind processes
Geophysical Research Letters, 2010Co-Authors: Simone Silvestro, Lori K. Fenton, D. A. Vaz, Nathan T. Bridges, Gian Gabriele OriAbstract:[1] In this report we show evidence of widespread ripple migration over the stoss side of dark Barchan dunes in Nili Patera on Mars. The measured average migration of ∼1.7 meters in less than 4 terrestrial months clearly indicates that active sand saltation is occurring in the study area. In addition, we document widespread changes in the dune base-ground surface contact and in the slip face structures, showing that not only the ripples, but the whole dunes are actually migrating in the present-day atmospheric setting. These results provide unequivocal evidence of recent aeolian activity and suggest that other dunes and ripples on Mars may also be active.