57 research outputs found
Analytical model for flux saturation in sediment transport
The transport of sediment by a fluid along the surface is responsible for
dune formation, dust entrainment and for a rich diversity of patterns on the
bottom of oceans, rivers, and planetary surfaces. Most previous models of
sediment transport have focused on the equilibrium (or saturated) particle
flux. However, the morphodynamics of sediment landscapes emerging due to
surface transport of sediment is controlled by situations out-of-equilibrium.
In particular, it is controlled by the saturation length characterizing the
distance it takes for the particle flux to reach a new equilibrium after a
change in flow conditions. The saturation of mass density of particles
entrained into transport and the relaxation of particle and fluid velocities
constitute the main relevant relaxation mechanisms leading to saturation of the
sediment flux. Here we present a theoretical model for sediment transport
which, for the first time, accounts for both these relaxation mechanisms and
for the different types of sediment entrainment prevailing under different
environmental conditions. Our analytical treatment allows us to derive a closed
expression for the saturation length of sediment flux, which is general and can
thus be applied under different physical conditions
The physics of wind-blown sand and dust
The transport of sand and dust by wind is a potent erosional force, creates
sand dunes and ripples, and loads the atmosphere with suspended dust aerosols.
This article presents an extensive review of the physics of wind-blown sand and
dust on Earth and Mars. Specifically, we review the physics of aeolian
saltation, the formation and development of sand dunes and ripples, the physics
of dust aerosol emission, the weather phenomena that trigger dust storms, and
the lifting of dust by dust devils and other small-scale vortices. We also
discuss the physics of wind-blown sand and dune formation on Venus and Titan.Comment: 72 journal pagers, 49 figure
Transverse instability of dunes
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.Comment: 4 pages, 3 figures; To appear in Physical Review Letter
Numerical modeling of the wind flow over a transverse dune
Transverse dunes, which form under unidirectional winds and have fixed
profile in the direction perpendicular to the wind, occur on all celestial
objects of our solar system where dunes have been detected. Here we perform a
numerical study of the average turbulent wind flow over a transverse dune by
means of computational fluid dynamics simulations. We find that the length of
the zone of recirculating flow at the dune lee --- the {\em{separation bubble}}
--- displays a surprisingly strong dependence on the wind shear velocity,
: it is nearly independent of for shear velocities within
the range between ms and $0.8\,$ms but increases linearly with
for larger shear velocities. Our calculations show that transport in
the direction opposite to dune migration within the separation bubble can be
sustained if is larger than approximately ms, whereas a
larger value of $u_{\ast}$ (about $0.49\,$ms) is required to initiate this
reverse transport.Comment: 11 pages, 8 figure
Particle-based simulation of powder application in additive manufacturing
Abstract The development of reliable strategies to optimize part production in additive manufacturing technologies hinges, to a large extent, on the quantitative understanding of the mechanical behavior of the powder particles during the application process. Since it is difficult to acquire this understanding based on experiments alone, a particle-based numerical tool for the simulation of powder application is required. In the present work, we develop such a numerical tool and apply it to investigate the characteristics of the powder layer deposited onto the part using a roller as the coating system. In our simulations, the complex geometric shapes of the powder particles are taken explicitly into account. Our results show that increasing the coating speed leads to an increase in the surface roughness of the powder bed, which is known to affect part quality. We also find that, surprisingly, powders with broader size distributions may lead to larger values of surface roughness as the smallest particles are most prone to form large agglomerates thus increasing the packing's porosity. Moreover, we find that the load on the part may vary over an order of magnitude during the coating process owing to the strong inhomogeneity of inter-particle forces in the granular packing. Our numerical tool can be used to assist -and partially replace -experimental investigations of the flowability and packing behavior of different powder systems as a function of material and process parameters
Saltation transport on Mars
We present the first calculation of saltation transport and dune formation on
Mars and compare it to real dunes. We find that the rate at which grains are
entrained into saltation on Mars is one order of magnitude higher than on
Earth. With this fundamental novel ingredient, we reproduce the size and
different shapes of Mars dunes, and give an estimate for the wind velocity on
Mars.Comment: 4 pages, 3 figure
DEM simulation of the powder application in powder bed fusion
The packing behavior of powders is significantly influenced by various types
of inter-particle attractive forces, including adhesion and non-bonded van der
Waals forces [1, 2, 3, 4, 5, 6]. Alongside particle size and shape
distributions, the inter-particle interactions, in particular frictional and
adhesive forces, play a crucial role in determining the flow behavior and
consequently the packing density of the powder layer. The impact of various
types of attractive forces on the packing density of powders with different
materials and particle size distributions remains largely unexplored and
requires further investigation. Accurately comprehending these effects through
experiments while considering specific particle size distributions and material
properties poses significant challenges. To address these challenges, we employ
Discrete Element Method (DEM) simulations to characterize the packing behavior
of fine powders. We can demonstrate quantitative agreement with experimental
results by incorporating the appropriate particle size distribution and using
an adequate model of attractive particle interactions. Furthermore, our
findings indicate that both adhesion, which is modeled using the
Johnson-Kendall-Roberts (JKR) model [7], and van der Waals interactions are
crucial factors that must be taken into account in DEM simulations.Comment: 22 pages, 14 figure
Dune formation on the present Mars
We apply a model for sand dunes to calculate formation of dunes on Mars under
the present Martian atmospheric conditions. We find that different dune shapes
as those imaged by Mars Global Surveyor could have been formed by the action of
sand-moving winds occuring on today's Mars. Our calculations show, however,
that Martian dunes could be only formed due to the higher efficiency of Martian
winds in carrying grains into saltation. The model equations are solved to
study saltation transport under different atmospheric conditions valid for
Mars. We obtain an estimate for the wind speed and migration velocity of
barchan dunes at different places on Mars. From comparison with the shape of
bimodal sand dunes, we find an estimate for the timescale of the changes in
Martian wind regimes.Comment: 16 pages, 12 figure
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