151
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Wang P, Zheng X. Saltation transport rate in unsteady wind variations. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2014; 37:40. [PMID: 24853633 DOI: 10.1140/epje/i2014-14040-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/23/2013] [Revised: 04/23/2014] [Accepted: 04/25/2014] [Indexed: 06/03/2023]
Abstract
Wind flow in the atmospheric boundary layer is usually turbulent. The gusty wind significantly influences the saltation transport which is treated as equilibrium saltation. This study performs one-dimension numerical simulations of unsteady sand saltation to discuss the effects of parameters of periodical wind variations on saltation response and sand transport rate prediction. The results show that unsteady transport rates are larger than steady rates of equivalent mean wind velocity. The ratio of unsteady/steady transport rates increases with the increase of amplitude and frequency. For the average wind velocities much larger than the threshold value, the errors of transport rates predicted by unsteady and steady model are about 10%, while for a wind velocity slightly larger than saltation threshold, the errors will be more than 200%. The sand transport rates are not zero even though the average wind velocity equals (is even smaller than) the threshold value, whereas Q must be zero in the steady model. Finally, an unsteady transport rate prediction formula is proposed which takes mean velocity, fluctuating intensity and period as independent variables.
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Affiliation(s)
- Ping Wang
- Key Laboratory of Mechanics on Disaster and Environment in Western China, Lanzhou University, 730000, Gansu, China,
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152
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Pähtz T, Parteli EJR, Kok JF, Herrmann HJ. Analytical model for flux saturation in sediment transport. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 89:052213. [PMID: 25353793 DOI: 10.1103/physreve.89.052213] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/27/2014] [Indexed: 06/04/2023]
Abstract
The transport of sediment by a fluid along the surface is responsible for dune formation, dust entrainment, and 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 thus can be applied under different physical conditions.
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Affiliation(s)
- Thomas Pähtz
- Ocean College, Zhejiang University, 310058 Hangzhou, China and State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, 310012 Hangzhou, China
| | - Eric J R Parteli
- Institute for Multiscale Simulation, Universität Erlangen-Nürnberg, Nägelsbachstraße 49b, 91052 Erlangen, Germany
| | - Jasper F Kok
- Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California 90095, USA
| | - Hans J Herrmann
- Departamento de Física, Universidade Federal do Ceará, 60451-970 Fortaleza, Ceará, Brazil and Computational Physics, IfB, ETH Zürich, Schafmattstraße 6, 8093 Zürich, Switzerland
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153
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Pähtz T, Kok JF, Parteli EJR, Herrmann HJ. Flux saturation length of sediment transport. PHYSICAL REVIEW LETTERS 2013; 111:218002. [PMID: 24313529 DOI: 10.1103/physrevlett.111.218002] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/14/2012] [Indexed: 06/02/2023]
Abstract
Sediment transport along the surface drives geophysical phenomena as diverse as wind erosion and dune formation. The main length scale controlling the dynamics of sediment erosion and deposition is the saturation length Ls, which characterizes the flux response to a change in transport conditions. Here we derive, for the first time, an expression predicting Ls as a function of the average sediment velocity under different physical environments. Our expression accounts for both the characteristics of sediment entrainment and the saturation of particle and fluid velocities, and has only two physical parameters which can be estimated directly from independent experiments. We show that our expression is consistent with measurements of Ls in both aeolian and subaqueous transport regimes over at least 5 orders of magnitude in the ratio of fluid and particle density, including on Mars.
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Affiliation(s)
- Thomas Pähtz
- Department of Ocean Science and Engineering, Zhejiang University, 310058 Hangzhou, China and State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, 310012 Hangzhou, China
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154
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Abstract
Coastal dunes, in particular foredunes, support a resilient ecosystem and reduce coastal vulnerability to storms. In contrast to dry desert dunes, coastal dunes arise from interactions between biological and physical processes. Ecologists have traditionally addressed coastal ecosystems by assuming that they adapt to preexisting dune topography, whereas geomorphologists have studied the properties of foredunes primarily in connection to physical, not biological, factors. Here, we study foredune development using an ecomorphodynamic model that resolves the coevolution of topography and vegetation in response to both physical and ecological factors. We find that foredune growth is eventually limited by a negative feedback between wind flow and topography. As a consequence, steady-state foredunes are scale invariant, which allows us to derive scaling relations for maximum foredune height and formation time. These relations suggest that plant zonation (in particular for strand "dune-building" species) is the primary factor controlling the maximum size of foredunes and therefore the amount of sand stored in a coastal dune system. We also find that aeolian sand supply to the dunes determines the timescale of foredune formation. These results offer a potential explanation for the empirical relation between beach type and foredune size, in which large (small) foredunes are found on dissipative (reflective) beaches. Higher waves associated with dissipative beaches increase the disturbance of strand species, which shifts foredune formation landward and thus leads to larger foredunes. In this scenario, plants play a much more active role in modifying their habitat and altering coastal vulnerability than previously thought.
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155
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Araújo AD, Parteli EJR, Pöschel T, Andrade JS, Herrmann HJ. Numerical modeling of the wind flow over a transverse dune. Sci Rep 2013; 3:2858. [PMID: 24091456 PMCID: PMC3790208 DOI: 10.1038/srep02858] [Citation(s) in RCA: 43] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2013] [Accepted: 07/30/2013] [Indexed: 11/18/2022] Open
Abstract
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 separation bubble — displays a surprisingly strong dependence on the wind shear velocity, u*: it is nearly independent of u* for shear velocities within the range between 0.2 m/s and 0.8 m/s but increases linearly with u* for larger shear velocities. Our calculations show that transport in the direction opposite to dune migration within the separation bubble can be sustained if u* is larger than approximately 0.39 m/s, whereas a larger value of u* (about 0.49 m/s) is required to initiate this reverse transport.
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Affiliation(s)
- Ascânio D Araújo
- Departamento de Física, Universidade Federal do Ceará, Campus do Pici, 60451-970 Fortaleza, Ceará, Brazil
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156
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Wang ZT, Zhang CL, Wang HT. Forces on a saltating grain in air. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2013; 36:112. [PMID: 24091939 DOI: 10.1140/epje/i2013-13112-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/25/2013] [Accepted: 09/06/2013] [Indexed: 06/02/2023]
Abstract
A wind tunnel experiment was performed to measure the trajectories of individual sand grains. Then, the acceleration given by the numerical differentiation was used to assess the relative importance of different external forces on a saltating sand grain in air. It is reconfirmed that the gravitational force and drag are the most important to grain motion. The lift also has certain influence. However, the present research does not support that the electrostatic force is significant.
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Affiliation(s)
- Zhen-Ting Wang
- State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, 100875, Beijing, P.R. China,
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157
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Gu Z, Wei W, Su J, Yu CW. The role of water content in triboelectric charging of wind-blown sand. Sci Rep 2013; 3:1337. [PMID: 23434920 PMCID: PMC3580324 DOI: 10.1038/srep01337] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2012] [Accepted: 02/07/2013] [Indexed: 11/09/2022] Open
Abstract
Triboelectric charging is common in desert sandstorms and dust devils on Earth; however, it remains poorly understood. Here we show a charging mechanism of sands with the adsorbed water on micro-porous surface in wind-blown sand based on the fact that water content is universal but usually a minor component in most particle systems. The triboelectric charging could be resulted due to the different mobility of H(+)/OH(-) between the contacting sands with a temperature difference. Computational fluid dynamics (CFD) and discrete element method (DEM) were used to demonstrate the dynamics of the sand charging. The numerically simulated charge-to-mass ratios of sands and electric field strength established in wind tunnel agreed well with the experimental data. The charging mechanism could provide an explanation for the charging process of all identical granular systems with water content, including Martian dust devils, wind-blown snow, even powder electrification in industrial processes.
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Affiliation(s)
- Zhaolin Gu
- Institute of Global Environmental Change, Xi'an Jiaotong University, Xi'an, Shanxi, 710049, P.R. China.
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158
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Carneiro MV, Araújo NAM, Pähtz T, Herrmann HJ. Midair collisions enhance saltation. PHYSICAL REVIEW LETTERS 2013; 111:058001. [PMID: 23952445 DOI: 10.1103/physrevlett.111.058001] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/19/2012] [Indexed: 06/02/2023]
Abstract
Here we address the old question in aeolian particle transport about the role of midair collisions. We find that, surprisingly, these collisions do enhance the overall flux substantially. The effect depends strongly on restitution coefficient and wind speed. We can explain this observation as a consequence of a soft bed of grains which floats above the ground and reflects the highest flying particles. We make the unexpected observation that the flux is maximized at an intermediate restitution coefficient of about 0.7, which is comparable to values experimentally measured for collisions between sand grains.
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Affiliation(s)
- M V Carneiro
- Institut für Baustoffe, ETH-Hönggerberg, Schafmattstrasse 6, 8093 Zürich, Switzerland
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159
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Ageing dynamics of ion bombardment induced self-organization processes. Sci Rep 2013; 3:1850. [PMID: 23685386 PMCID: PMC3657716 DOI: 10.1038/srep01850] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2012] [Accepted: 03/15/2013] [Indexed: 11/24/2022] Open
Abstract
Instabilities caused during the erosion of a surface by an ion beam can lead to the formation of self-organized patterns of nanostructures. Understanding the self-organization process requires not only the in-situ characterization of ensemble averaged properties but also probing the dynamics. This can be done with the use of coherent X-rays and analyzing the temporal correlations of the scattered intensity. Here, we show that the dynamics of a semiconductor surface nanopatterned by normal incidence ion beam sputtering are age-dependent and slow down with sputtering time. This work provides a novel insight into the erosion dynamics and opens new perspectives for the understanding of self-organization mechanisms.
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160
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Berger KJ, Anand A, Metzger PT, Hrenya CM. Role of collisions in erosion of regolith during a lunar landing. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2013; 87:022205. [PMID: 23496503 DOI: 10.1103/physreve.87.022205] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/11/2012] [Indexed: 06/01/2023]
Abstract
The supersonic gas plume of a landing rocket entrains lunar regolith, which is the layer of loose solids covering the lunar surface. This ejection is problematic due to scouring and dust impregnation of surrounding hardware, reduction in visibility for the crew, and spoofing of the landing sensors. To date, model predictions of erosion and ejection dynamics have been based largely on single-trajectory models in which the role of interparticle collisions is ignored. In the present work, the parameters affecting the erosion rate of monodisperse solids are investigated using the discrete element method (DEM). The drag and lift forces exerted by the rocket exhaust are incorporated via one-way coupling. The results demonstrate that interparticle collisions are frequent in the region immediately above the regolith surface; as many as 20% of particles are engaged in a collision at a given time. These collisions play an important role both in the erosion dynamics and in the final trajectories of particles. In addition, a direct assessment of the influence of collisions on the erosion rate is accomplished via a comparison between a "collisionless" DEM model and the original DEM model. This comparison shows that the erosion dynamics change drastically when collisions are considered and that the erosion rate is dependent on the collision parameters (coefficient of restitution and coefficient of friction). Physical explanations for these trends are provided.
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Affiliation(s)
- Kyle J Berger
- Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80303, USA
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