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Jasper F Kok - One of the best experts on this subject based on the ideXlab platform.
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size independent susceptibility to transport in aeolian Saltation
Journal of Geophysical Research, 2019Co-Authors: Raleigh L Martin, Jasper F KokAbstract:Natural wind-eroded soils contain a mixture of particle sizes. However, models for aeolian Saltation are typically derived for sediment bed surfaces containing only a single particle size. To nonetheless treat natural mixed beds, models for Saltation and associated dust aerosol emission have typically simplified aeolian transport either as a series of non-interacting single particle size beds or as a bed containing only the median or mean particle size. Here, we test these common assumptions underpinning aeolian transport models using measurements of size-resolved Saltation fluxes at three natural field sites. We find that a wide range of sand size classes experience "equal susceptibility" to Saltation at a single common threshold wind shear stress, contrary to the "selective susceptibility" expected for treatment of a mixed bed as multiple single particle size beds. Our observation of equal susceptibility refutes the common simplification of Saltation as a series of non-interacting single particle sizes. Sand transport and dust emission models that use this incorrect assumption can be both simplified and improved by instead using a single particle size representative of the mixed bed.
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wind invariant Saltation heights imply linear scaling of aeolian Saltation flux with shear stress
Science Advances, 2017Co-Authors: Raleigh L Martin, Jasper F KokAbstract:Wind-driven sand transport generates atmospheric dust, forms dunes, and sculpts landscapes. However, it remains unclear how the flux of particles in aeolian Saltation-the wind-driven transport of sand in hopping trajectories-scales with wind speed, largely because models do not agree on how particle speeds and trajectories change with wind shear velocity. We present comprehensive measurements, from three new field sites and three published studies, showing that characteristic Saltation layer heights remain approximately constant with shear velocity, in agreement with recent wind tunnel studies. These results support the assumption of constant particle speeds in recent models predicting linear scaling of Saltation flux with shear stress. In contrast, our results refute widely used older models that assume that particle speed increases with shear velocity, thereby predicting nonlinear 3/2 stress-flux scaling. This conclusion is further supported by direct field measurements of Saltation flux versus shear stress. Our results thus argue for adoption of linear Saltation flux laws and constant Saltation trajectories for modeling Saltation-driven aeolian processes on Earth, Mars, and other planetary surfaces.
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aeolian Saltation on mars at low wind speeds
LPI, 2017Co-Authors: R Sullivan, Jasper F KokAbstract:Laboratory experiments indicate that the fluid threshold friction speed, u*tf, required to initiate fully developed aeolian Saltation is much higher on Mars than on Earth. A discrepancy exists between Mars climate models that do not predict winds this strong, and observations that sand-sized particles are indeed moving. This paper describes how wind friction speeds well below u*tf, but above the impact threshold, u*ti, required to sustain Saltation, can initiate sustained Saltation on Mars, but at relatively low flux. Numerical experiments indicate that a sand grain on Mars mobilized sporadically between u*ti and u*tf will develop, over fetch lengths longer than generally available within low-pressure wind tunnels, trajectories capable of splashing grains that propagate Saltation and collectively form a cluster of saltating grains that migrate downwind together. The passage of a Saltation cluster should leave behind a narrow zone of affected surface grains. The cumulative effect of many clusters represents a low-flux phenomenon that should produce slow changes to aeolian bedforms over periods in which winds remain close to u*ti and never or rarely reach u*tf. Field evidence from small impact ripples along rover traverses is consistent with effects of Saltation at these low friction speeds, without obvious evidence for events ≥ u*tf. The potential utility of this grain mobility process is that it can operate entirely at more common winds well below u*tf, and so help explain widespread sand movements observed on Mars wherever evidence might be mostly absent for u*tf being exceeded.
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field measurements demonstrate distinct initiation and cessation thresholds governing aeolian sediment transport flux
arXiv: Geophysics, 2016Co-Authors: Raleigh L Martin, Jasper F KokAbstract:Wind-blown sand and dust models depend sensitively on the threshold wind stress. However, laboratory and numerical experiments suggest the coexistence of distinct "fluid" and "impact" thresholds for the initiation and cessation of aeolian Saltation, respectively. Because aeolian transport models typically use only the fluid threshold, existence of a separate lower impact threshold complicates the prediction of wind-driven transport. Here, we derive the first field-based estimates of distinct fluid and impact thresholds from high-frequency Saltation measurements at three field sites. Our measurements show that, when Saltation is mostly inactive, its instantaneous occurrence is governed primarily by wind exceedance of the fluid threshold. As Saltation activity increases, so too does the relative importance of the impact threshold, until it dominates under near-continuous transport conditions. Although both thresholds are thus important for high-frequency Saltation prediction, we find that the time-averaged Saltation flux is primarily governed by impact threshold.
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wind invariant Saltation heights imply linear scaling of aeolian Saltation flux with shear stress
2016Co-Authors: Raleigh L Martin, Jasper F KokAbstract:Wind-driven sand transport generates atmospheric dust, forms dunes, and sculpts landscapes. However, it remains unclear how the sand flux scales with wind speed, largely because models do not agree on how particle speed changes with wind shear velocity. Here, we present comprehensive measurements from three new field sites and three published studies, showing that characteristic Saltation layer heights, and thus particle speeds, remain approximately constant with shear velocity. This result implies a linear dependence of Saltation flux on wind shear stress, which contrasts with the nonlinear 3/2 scaling used in most aeolian process predictions. We confirm the linear flux law with direct measurements of the stress-flux relationship occurring at each site. Models for dust generation, dune migration, and other processes driven by wind-blown sand on Earth, Mars, and several other planetary surfaces should be modified to account for linear stress-flux scaling.
Hans J. Herrmann - One of the best experts on this subject based on the ideXlab platform.
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bursts in discontinuous aeolian Saltation
Scientific Reports, 2015Co-Authors: M V Carneiro, K R Rasmussen, Hans J. HerrmannAbstract:Close to the onset of Aeolian particle transport through Saltation we find in wind tunnel experiments a regime of discontinuous flux characterized by bursts of activity. Scaling laws are observed in the time delay between each burst and in the measurements of the wind fluctuations at the fluid threshold Shields number θc. The time delay between each burst decreases on average with the increase of the Shields number until sand flux becomes continuous. A numerical model for Saltation including the wind-entrainment from the turbulent fluctuations can reproduce these observations and gives insight about their origin. We present here also for the first time measurements showing that with feeding it becomes possible to sustain discontinuous flux even below the fluid threshold.
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bursts in discontinuous aeolian Saltation
arXiv: Atmospheric and Oceanic Physics, 2014Co-Authors: M V Carneiro, K R Rasmussen, Hans J. HerrmannAbstract:Close to the onset of Aeolian particle transport through Saltation we find in wind tunnel experiments a regime of discontinuous flux characterized by bursts of activity. Scaling laws are observed in the time delay between each burst and in the measurements of the wind fluctuations at the fluid threshold Shields number $\theta_c$. The time delay between each burst decreases on average with the increase of the Shields number until sand flux becomes continuous. A numerical model for Saltation including the wind-entrainment from the turbulent fluctuations can reproduce these observations and gives insight about their origin. We present here also for the first time measurements showing that with feeding it becomes possible to sustain discontinuous flux even below the fluid threshold.
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Modeling spatial nonequilibrium Saltation under turbulent water flow at low Shields stress
2013Co-Authors: Thomas Pähtz, Hans J. HerrmannAbstract:In this study, we extend the mechanistic equilibrium Saltation model of Ashida and Michiue (1972) for nearly horizontal sand beds to the case of spatial nonequilibrium sand transport, which allows us to calculate sand transport rates over varying mildly-sloped topographies. Our model incorporates a generalized description of the reduction of the Shields stress within the Saltation layer due to momentum transfer from the fluid to the grains. We show that the stronger the reduction of the Shields stress the weaker is the influence of the Saltation step length on the spatial transition from a nonequilibrium to an equilibrium state. We validate our model with different experimental data sets and show that it can reasonably explain the recently measured dependency of the minimal length of barchan dunes on the shear velocity (Franklin and Charru, 2011)
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midair collisions enhance Saltation
Physical Review Letters, 2013Co-Authors: M V Carneiro, Thomas Pähtz, N A M Araujo, Hans J. HerrmannAbstract: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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simulation of aeolian Saltation
POWDERS AND GRAINS 2013: Proceedings of the 7th International Conference on Micromechanics of Granular Media, 2013Co-Authors: M V Carneiro, Hans J. HerrmannAbstract:We reveal that the transition in the saturated flux for aeolian Saltation is generically discontinuous by explicitly simulating particle motion in turbulent flow in two dimensions. The discontinuity is followed by a coexistence interval with two metastable solutions. We found also that the mere presence of mid-air collisions, surprisingly, enhances the saturated flux, which has a peak for an intermediate restitution coefficient of 0.65.
Alexandre Valance - One of the best experts on this subject based on the ideXlab platform.
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The threshold for continuing Saltation on Earth and other solar system bodies
Journal of Geophysical Research : Solid Earth, 2017Co-Authors: Diego Berzi, Alexandre Valance, Jim T. JenkinsAbstract:We predict the threshold for continuing Saltation of spheres in a turbulent fluid that explicitly accounts for the influence of fluid drag, lubrication forces, bed roughness, and interparticle cohesion. This reduces the need for the fitting parameters employed in existing formulations. The theory is based on a highly idealized model of steady Saltation as a collection of particles that follow the same average, periodic trajectory—a succession of identical jumps, collisions with the bed, and rebounds from it. The Saltation threshold is first derived in the limit of large particle inertia and, then, extended to infer results when the viscous forces of the fluid and interparticle cohesion are not negligible. The theory is successfully compared with existing discrete element simulations of spheres interacting with a Reynolds-averaged turbulent fluid and with wind tunnel experiments in a range of particle-to-fluid density ratios and particle Reynolds numbers for Saltation in terrestrial and extraterrestrial conditions.
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laboratory studies of aeolian sediment transport processes on planetary surfaces
Geomorphology, 2015Co-Authors: K R Rasmussen, Alexandre Valance, J P MerrisonAbstract:Abstract We review selected experimental Saltation studies performed in laboratory wind tunnels and collision experiments performed in (splash-) laboratory facilities that allow detailed observations between impinging particles on a stationary bed. We also discuss progress in understanding aeolian transport in nonterrestrial environments. Saltation studies in terrestrial wind tunnels can be divided into two groups. The first group comprises studies using a short test bed, typically 1–4 m long, and focuses on the transitional behavior near the upwind roughness discontinuity where Saltation starts. The other group focuses on studies using long test beds — typically 6 m or more — where the saturated Saltation takes place under equilibrium conditions between wind flow and the underlying rough bed. Splash studies using upscaled model experiments allow collision simulations with large spherical particles to be recorded with a high speed video camera. The findings indicate that the number of ejected particles per impact scales linearly with the impact velocity of the saltating particles. Studies of saturated Saltation in several facilities using predominantly Particle Tracking Velocimetry or Laser Doppler Velocimetry indicate that the velocity of the (few) particles having high trajectories increases with increasing friction velocity. However, the speed of the majority of particles that do not reach much higher than Bagnold's focal point is virtually independent of Shields parameter — at least for low or intermediate u ⁎ -values. In this case mass flux depends on friction velocity squared and not cubed as originally suggested by Bagnold. Over short beds particle velocity shows stronger dependence on friction velocity and profiles of particle velocity deviate from those obtained over long beds. Measurements using horizontally segmented traps give average Saltation jump-lengths near 60–70 mm and appear to be only weakly dependent on friction velocity, which is in agreement with some, but not all, older or recent wind tunnel observations. Similarly some measurements performed with uniform sand samples having grain diameters of the order of 0.25–0.40 mm indicate that ripple spacing depends on friction velocity in a similar way as particle jump length. The observations are thus in agreement with a recent ripple model that link the typical jump length to ripple spacing. A possible explanation for contradictory observations in some experiments may be that long observation sequences are required in order to assure that equilibrium exists between ripple geometry and wind flow. Quantitative understanding of Saltation characteristics on Mars still lacks important elements. Based upon image analysis and numerical predictions, aeolian ripples have been thought to consist of relatively large grains (diameter > 0.6 mm) and that Saltation occurs at high wind speeds (> 26 m/s) involving trajectories that are significantly longer than those on Earth (by a factor of 10–100). However, this is not supported by recent observations from the surface of Mars, which shows that active ripples in their geometry and composition have characteristics compatible with those of terrestrial ripples (Sullivan et al., 2008). Also the highest average wind speeds on Mars have been measured to be Electrification is seen as a dominant factor in the transport dynamics of dust on Mars, affecting the structure, adhesive properties and detachment/entrainment mechanisms specifically through the formation of aggregates (Merrison et al., 2012). Conversely for terrestrial conditions electric fields typically observed are not intense enough to significantly affect sand transport rates while little is known in the case of extra-terrestrial environments.
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laboratory studies of aeolian sediment transport processes on planetary surfaces
Geomorphology, 2015Co-Authors: K R Rasmussen, Alexandre Valance, J P MerrisonAbstract:Abstract We review selected experimental Saltation studies performed in laboratory wind tunnels and collision experiments performed in (splash-) laboratory facilities that allow detailed observations between impinging particles on a stationary bed. We also discuss progress in understanding aeolian transport in nonterrestrial environments. Saltation studies in terrestrial wind tunnels can be divided into two groups. The first group comprises studies using a short test bed, typically 1–4 m long, and focuses on the transitional behavior near the upwind roughness discontinuity where Saltation starts. The other group focuses on studies using long test beds — typically 6 m or more — where the saturated Saltation takes place under equilibrium conditions between wind flow and the underlying rough bed. Splash studies using upscaled model experiments allow collision simulations with large spherical particles to be recorded with a high speed video camera. The findings indicate that the number of ejected particles per impact scales linearly with the impact velocity of the saltating particles. Studies of saturated Saltation in several facilities using predominantly Particle Tracking Velocimetry or Laser Doppler Velocimetry indicate that the velocity of the (few) particles having high trajectories increases with increasing friction velocity. However, the speed of the majority of particles that do not reach much higher than Bagnold's focal point is virtually independent of Shields parameter — at least for low or intermediate u ⁎ -values. In this case mass flux depends on friction velocity squared and not cubed as originally suggested by Bagnold. Over short beds particle velocity shows stronger dependence on friction velocity and profiles of particle velocity deviate from those obtained over long beds. Measurements using horizontally segmented traps give average Saltation jump-lengths near 60–70 mm and appear to be only weakly dependent on friction velocity, which is in agreement with some, but not all, older or recent wind tunnel observations. Similarly some measurements performed with uniform sand samples having grain diameters of the order of 0.25–0.40 mm indicate that ripple spacing depends on friction velocity in a similar way as particle jump length. The observations are thus in agreement with a recent ripple model that link the typical jump length to ripple spacing. A possible explanation for contradictory observations in some experiments may be that long observation sequences are required in order to assure that equilibrium exists between ripple geometry and wind flow. Quantitative understanding of Saltation characteristics on Mars still lacks important elements. Based upon image analysis and numerical predictions, aeolian ripples have been thought to consist of relatively large grains (diameter > 0.6 mm) and that Saltation occurs at high wind speeds (> 26 m/s) involving trajectories that are significantly longer than those on Earth (by a factor of 10–100). However, this is not supported by recent observations from the surface of Mars, which shows that active ripples in their geometry and composition have characteristics compatible with those of terrestrial ripples (Sullivan et al., 2008). Also the highest average wind speeds on Mars have been measured to be Electrification is seen as a dominant factor in the transport dynamics of dust on Mars, affecting the structure, adhesive properties and detachment/entrainment mechanisms specifically through the formation of aggregates (Merrison et al., 2012). Conversely for terrestrial conditions electric fields typically observed are not intense enough to significantly affect sand transport rates while little is known in the case of extra-terrestrial environments.
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aeolian sand transport length and height distributions of Saltation trajectories
Aeolian Research, 2014Co-Authors: Alexandre Valance, Pascal Dupont, Ould El A MoctarAbstract:Abstract We report wind-tunnel measurements on aeolian sand transport aiming at characterizing the distribution of the length and height of trajectories of the saltating particles. We employ a simple horizontal sand trap device to assess the distribution of Saltation length while the distribution of Saltation height is inferred from the measurements of the particle lift-off velocity by means of particle velocimetry tracking techniques. Our measurements reveal that the Saltation length and height present a continuum distribution which decreases monotonously and exhibits a long tail that can be well described by a lognormal law. Interestingly, these distributions are found almost invariant with the flow strength. As a consequence, the mean Saltation length ( l ¯ ) and height ( h ¯ ) are independent of the flow strength confirming previous indirect measurements. The influence of the flow strength is only seen through the tail of the Saltation length distribution: the higher the Shields number, the flatter the distribution tail. Finally, experiments carried out with sand of different sizes show that the mean Saltation length and height are not related to the sand grain size through a simple manner but depend instead linearly with the height z f of the Bagnold focus point: l ‾ ≈ 6 z f and h ‾ ≈ 0.6 z f . This last result emphasizes that the focus height is an important characteristic length scale of the Saltation transport.
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Particle velocity distribution in Saltation transport.
Physical Review E, 2012Co-Authors: T. D. Ho, Pascal Dupont, A. Ould El Moctar, Alexandre ValanceAbstract:We report on wind-tunnel measurements of particle velocity distribution in aeolian transport. By performing extended statistics, we show that for Saltation occurring over an erodible bed the vertical lift-off velocity distributions deviate significantly from a Gaussian law and exhibit a long tail accurately described by a lognormal law. In contrast, Saltation over a rigid bed produces Gaussian velocity distributions. These results strongly suggest that the deviation from Gaussian distributions is a consequence of the splash process which is exclusively present in Saltation transport over an erodible bed. We further suggest that the non-Gaussian statistics is intimately related to the statistical properties of a single splash event which produces ejection of particles with lift-off velocities distributed according to a lognormal law. This lognormal behavior can be simply inferred from the propagation process of the impact energy through the granular bed which can be viewed as the analog of a fragmentation process. These findings emphasize the crucial role of the splash process in Saltation transport.
J P Merrison - One of the best experts on this subject based on the ideXlab platform.
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laboratory studies of aeolian sediment transport processes on planetary surfaces
Geomorphology, 2015Co-Authors: K R Rasmussen, Alexandre Valance, J P MerrisonAbstract:Abstract We review selected experimental Saltation studies performed in laboratory wind tunnels and collision experiments performed in (splash-) laboratory facilities that allow detailed observations between impinging particles on a stationary bed. We also discuss progress in understanding aeolian transport in nonterrestrial environments. Saltation studies in terrestrial wind tunnels can be divided into two groups. The first group comprises studies using a short test bed, typically 1–4 m long, and focuses on the transitional behavior near the upwind roughness discontinuity where Saltation starts. The other group focuses on studies using long test beds — typically 6 m or more — where the saturated Saltation takes place under equilibrium conditions between wind flow and the underlying rough bed. Splash studies using upscaled model experiments allow collision simulations with large spherical particles to be recorded with a high speed video camera. The findings indicate that the number of ejected particles per impact scales linearly with the impact velocity of the saltating particles. Studies of saturated Saltation in several facilities using predominantly Particle Tracking Velocimetry or Laser Doppler Velocimetry indicate that the velocity of the (few) particles having high trajectories increases with increasing friction velocity. However, the speed of the majority of particles that do not reach much higher than Bagnold's focal point is virtually independent of Shields parameter — at least for low or intermediate u ⁎ -values. In this case mass flux depends on friction velocity squared and not cubed as originally suggested by Bagnold. Over short beds particle velocity shows stronger dependence on friction velocity and profiles of particle velocity deviate from those obtained over long beds. Measurements using horizontally segmented traps give average Saltation jump-lengths near 60–70 mm and appear to be only weakly dependent on friction velocity, which is in agreement with some, but not all, older or recent wind tunnel observations. Similarly some measurements performed with uniform sand samples having grain diameters of the order of 0.25–0.40 mm indicate that ripple spacing depends on friction velocity in a similar way as particle jump length. The observations are thus in agreement with a recent ripple model that link the typical jump length to ripple spacing. A possible explanation for contradictory observations in some experiments may be that long observation sequences are required in order to assure that equilibrium exists between ripple geometry and wind flow. Quantitative understanding of Saltation characteristics on Mars still lacks important elements. Based upon image analysis and numerical predictions, aeolian ripples have been thought to consist of relatively large grains (diameter > 0.6 mm) and that Saltation occurs at high wind speeds (> 26 m/s) involving trajectories that are significantly longer than those on Earth (by a factor of 10–100). However, this is not supported by recent observations from the surface of Mars, which shows that active ripples in their geometry and composition have characteristics compatible with those of terrestrial ripples (Sullivan et al., 2008). Also the highest average wind speeds on Mars have been measured to be Electrification is seen as a dominant factor in the transport dynamics of dust on Mars, affecting the structure, adhesive properties and detachment/entrainment mechanisms specifically through the formation of aggregates (Merrison et al., 2012). Conversely for terrestrial conditions electric fields typically observed are not intense enough to significantly affect sand transport rates while little is known in the case of extra-terrestrial environments.
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laboratory studies of aeolian sediment transport processes on planetary surfaces
Geomorphology, 2015Co-Authors: K R Rasmussen, Alexandre Valance, J P MerrisonAbstract:Abstract We review selected experimental Saltation studies performed in laboratory wind tunnels and collision experiments performed in (splash-) laboratory facilities that allow detailed observations between impinging particles on a stationary bed. We also discuss progress in understanding aeolian transport in nonterrestrial environments. Saltation studies in terrestrial wind tunnels can be divided into two groups. The first group comprises studies using a short test bed, typically 1–4 m long, and focuses on the transitional behavior near the upwind roughness discontinuity where Saltation starts. The other group focuses on studies using long test beds — typically 6 m or more — where the saturated Saltation takes place under equilibrium conditions between wind flow and the underlying rough bed. Splash studies using upscaled model experiments allow collision simulations with large spherical particles to be recorded with a high speed video camera. The findings indicate that the number of ejected particles per impact scales linearly with the impact velocity of the saltating particles. Studies of saturated Saltation in several facilities using predominantly Particle Tracking Velocimetry or Laser Doppler Velocimetry indicate that the velocity of the (few) particles having high trajectories increases with increasing friction velocity. However, the speed of the majority of particles that do not reach much higher than Bagnold's focal point is virtually independent of Shields parameter — at least for low or intermediate u ⁎ -values. In this case mass flux depends on friction velocity squared and not cubed as originally suggested by Bagnold. Over short beds particle velocity shows stronger dependence on friction velocity and profiles of particle velocity deviate from those obtained over long beds. Measurements using horizontally segmented traps give average Saltation jump-lengths near 60–70 mm and appear to be only weakly dependent on friction velocity, which is in agreement with some, but not all, older or recent wind tunnel observations. Similarly some measurements performed with uniform sand samples having grain diameters of the order of 0.25–0.40 mm indicate that ripple spacing depends on friction velocity in a similar way as particle jump length. The observations are thus in agreement with a recent ripple model that link the typical jump length to ripple spacing. A possible explanation for contradictory observations in some experiments may be that long observation sequences are required in order to assure that equilibrium exists between ripple geometry and wind flow. Quantitative understanding of Saltation characteristics on Mars still lacks important elements. Based upon image analysis and numerical predictions, aeolian ripples have been thought to consist of relatively large grains (diameter > 0.6 mm) and that Saltation occurs at high wind speeds (> 26 m/s) involving trajectories that are significantly longer than those on Earth (by a factor of 10–100). However, this is not supported by recent observations from the surface of Mars, which shows that active ripples in their geometry and composition have characteristics compatible with those of terrestrial ripples (Sullivan et al., 2008). Also the highest average wind speeds on Mars have been measured to be Electrification is seen as a dominant factor in the transport dynamics of dust on Mars, affecting the structure, adhesive properties and detachment/entrainment mechanisms specifically through the formation of aggregates (Merrison et al., 2012). Conversely for terrestrial conditions electric fields typically observed are not intense enough to significantly affect sand transport rates while little is known in the case of extra-terrestrial environments.
K R Rasmussen - One of the best experts on this subject based on the ideXlab platform.
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laboratory studies of aeolian sediment transport processes on planetary surfaces
Geomorphology, 2015Co-Authors: K R Rasmussen, Alexandre Valance, J P MerrisonAbstract:Abstract We review selected experimental Saltation studies performed in laboratory wind tunnels and collision experiments performed in (splash-) laboratory facilities that allow detailed observations between impinging particles on a stationary bed. We also discuss progress in understanding aeolian transport in nonterrestrial environments. Saltation studies in terrestrial wind tunnels can be divided into two groups. The first group comprises studies using a short test bed, typically 1–4 m long, and focuses on the transitional behavior near the upwind roughness discontinuity where Saltation starts. The other group focuses on studies using long test beds — typically 6 m or more — where the saturated Saltation takes place under equilibrium conditions between wind flow and the underlying rough bed. Splash studies using upscaled model experiments allow collision simulations with large spherical particles to be recorded with a high speed video camera. The findings indicate that the number of ejected particles per impact scales linearly with the impact velocity of the saltating particles. Studies of saturated Saltation in several facilities using predominantly Particle Tracking Velocimetry or Laser Doppler Velocimetry indicate that the velocity of the (few) particles having high trajectories increases with increasing friction velocity. However, the speed of the majority of particles that do not reach much higher than Bagnold's focal point is virtually independent of Shields parameter — at least for low or intermediate u ⁎ -values. In this case mass flux depends on friction velocity squared and not cubed as originally suggested by Bagnold. Over short beds particle velocity shows stronger dependence on friction velocity and profiles of particle velocity deviate from those obtained over long beds. Measurements using horizontally segmented traps give average Saltation jump-lengths near 60–70 mm and appear to be only weakly dependent on friction velocity, which is in agreement with some, but not all, older or recent wind tunnel observations. Similarly some measurements performed with uniform sand samples having grain diameters of the order of 0.25–0.40 mm indicate that ripple spacing depends on friction velocity in a similar way as particle jump length. The observations are thus in agreement with a recent ripple model that link the typical jump length to ripple spacing. A possible explanation for contradictory observations in some experiments may be that long observation sequences are required in order to assure that equilibrium exists between ripple geometry and wind flow. Quantitative understanding of Saltation characteristics on Mars still lacks important elements. Based upon image analysis and numerical predictions, aeolian ripples have been thought to consist of relatively large grains (diameter > 0.6 mm) and that Saltation occurs at high wind speeds (> 26 m/s) involving trajectories that are significantly longer than those on Earth (by a factor of 10–100). However, this is not supported by recent observations from the surface of Mars, which shows that active ripples in their geometry and composition have characteristics compatible with those of terrestrial ripples (Sullivan et al., 2008). Also the highest average wind speeds on Mars have been measured to be Electrification is seen as a dominant factor in the transport dynamics of dust on Mars, affecting the structure, adhesive properties and detachment/entrainment mechanisms specifically through the formation of aggregates (Merrison et al., 2012). Conversely for terrestrial conditions electric fields typically observed are not intense enough to significantly affect sand transport rates while little is known in the case of extra-terrestrial environments.
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laboratory studies of aeolian sediment transport processes on planetary surfaces
Geomorphology, 2015Co-Authors: K R Rasmussen, Alexandre Valance, J P MerrisonAbstract:Abstract We review selected experimental Saltation studies performed in laboratory wind tunnels and collision experiments performed in (splash-) laboratory facilities that allow detailed observations between impinging particles on a stationary bed. We also discuss progress in understanding aeolian transport in nonterrestrial environments. Saltation studies in terrestrial wind tunnels can be divided into two groups. The first group comprises studies using a short test bed, typically 1–4 m long, and focuses on the transitional behavior near the upwind roughness discontinuity where Saltation starts. The other group focuses on studies using long test beds — typically 6 m or more — where the saturated Saltation takes place under equilibrium conditions between wind flow and the underlying rough bed. Splash studies using upscaled model experiments allow collision simulations with large spherical particles to be recorded with a high speed video camera. The findings indicate that the number of ejected particles per impact scales linearly with the impact velocity of the saltating particles. Studies of saturated Saltation in several facilities using predominantly Particle Tracking Velocimetry or Laser Doppler Velocimetry indicate that the velocity of the (few) particles having high trajectories increases with increasing friction velocity. However, the speed of the majority of particles that do not reach much higher than Bagnold's focal point is virtually independent of Shields parameter — at least for low or intermediate u ⁎ -values. In this case mass flux depends on friction velocity squared and not cubed as originally suggested by Bagnold. Over short beds particle velocity shows stronger dependence on friction velocity and profiles of particle velocity deviate from those obtained over long beds. Measurements using horizontally segmented traps give average Saltation jump-lengths near 60–70 mm and appear to be only weakly dependent on friction velocity, which is in agreement with some, but not all, older or recent wind tunnel observations. Similarly some measurements performed with uniform sand samples having grain diameters of the order of 0.25–0.40 mm indicate that ripple spacing depends on friction velocity in a similar way as particle jump length. The observations are thus in agreement with a recent ripple model that link the typical jump length to ripple spacing. A possible explanation for contradictory observations in some experiments may be that long observation sequences are required in order to assure that equilibrium exists between ripple geometry and wind flow. Quantitative understanding of Saltation characteristics on Mars still lacks important elements. Based upon image analysis and numerical predictions, aeolian ripples have been thought to consist of relatively large grains (diameter > 0.6 mm) and that Saltation occurs at high wind speeds (> 26 m/s) involving trajectories that are significantly longer than those on Earth (by a factor of 10–100). However, this is not supported by recent observations from the surface of Mars, which shows that active ripples in their geometry and composition have characteristics compatible with those of terrestrial ripples (Sullivan et al., 2008). Also the highest average wind speeds on Mars have been measured to be Electrification is seen as a dominant factor in the transport dynamics of dust on Mars, affecting the structure, adhesive properties and detachment/entrainment mechanisms specifically through the formation of aggregates (Merrison et al., 2012). Conversely for terrestrial conditions electric fields typically observed are not intense enough to significantly affect sand transport rates while little is known in the case of extra-terrestrial environments.
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bursts in discontinuous aeolian Saltation
Scientific Reports, 2015Co-Authors: M V Carneiro, K R Rasmussen, Hans J. HerrmannAbstract:Close to the onset of Aeolian particle transport through Saltation we find in wind tunnel experiments a regime of discontinuous flux characterized by bursts of activity. Scaling laws are observed in the time delay between each burst and in the measurements of the wind fluctuations at the fluid threshold Shields number θc. The time delay between each burst decreases on average with the increase of the Shields number until sand flux becomes continuous. A numerical model for Saltation including the wind-entrainment from the turbulent fluctuations can reproduce these observations and gives insight about their origin. We present here also for the first time measurements showing that with feeding it becomes possible to sustain discontinuous flux even below the fluid threshold.
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bursts in discontinuous aeolian Saltation
arXiv: Atmospheric and Oceanic Physics, 2014Co-Authors: M V Carneiro, K R Rasmussen, Hans J. HerrmannAbstract:Close to the onset of Aeolian particle transport through Saltation we find in wind tunnel experiments a regime of discontinuous flux characterized by bursts of activity. Scaling laws are observed in the time delay between each burst and in the measurements of the wind fluctuations at the fluid threshold Shields number $\theta_c$. The time delay between each burst decreases on average with the increase of the Shields number until sand flux becomes continuous. A numerical model for Saltation including the wind-entrainment from the turbulent fluctuations can reproduce these observations and gives insight about their origin. We present here also for the first time measurements showing that with feeding it becomes possible to sustain discontinuous flux even below the fluid threshold.
