Laboratory studies of aeolian sediment transport processes on planetary surfaces

International audienceWe 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 1–4m 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 linearlywith 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 reachmuch higher than Bagnold's focal point is virtually independent of Shields parameter—at least for lowor 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 0.25–0.40mmindicate that ripple spacing depends on friction velocity in a similar way as particle jump length. The observations are thus in agreementwith 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 onMars still lacks important elements. Based upon image analysis and numerical predictions, aeolian ripples have been thought to consist of relatively large grains (diameter N 0.6mm) and that saltation occurs at high wind speeds (N26 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 b20 m/s, with even turbulent gusts not exceeding 25 m/s. Electrification is seen as a dominant factor in the transport dynamics of dust onMars, 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

MWCNTs of different physicochemical properties cause similar inflammatory responses, but differences in transcriptional and histological markers of fibrosis in mouse lungs

Multi-walled carbon nanotubes (MWCNTs) are extensively produced and used in composite materials and electronic applications, thus increasing risk of worker and consumer exposure. MWCNTs are an inhomogeneous group of nanomaterials that come in various lengths, shapes and with different metal contaminations, which makes hazard evaluation difficult. However, several studies suggest that length plays an important role in the toxicity induced by MWCNTs. How the length influences toxicity at the molecular level is yet to be characterized. Female C57BL/6 mice were exposed by single intratracheal instillation to 18, 54 or 162 µg/mouse of a short MWCNT (NRCWE-026, 847±102 nm in length) or long MWCNT (NM-401, 4048±366 nm in length). The two MWCNTs were extensively characterized. Lung tissues were harvested 24 h, 3 d and 28 d after exposure. We employed DNA microarrays, bronchoalveolar lavage fluid analysis, comet assay and dichlorodihydrofluorescein assay in order to profile the pulmonary responses. Bioinformatics tools were then applied to compare and contrast the expression profiles and to build a length dependent property-response matrix for gene-by-gene comparison. The toxicogenomic analysis of the global mRNA changes after exposure to the short, entangled NRCWE-026 or the longer, stiffer NM-401 showed high degree of similarities. The toxicity of both MWCNTs was driven by strong inflammatory and acute phase responses, which peaked at day 3 and was observed both in bronchoalveolar lavage cell influx and in gene expression profiles. The inflammatory response was sustained at post-exposure day 28. Also, at the sub-chronic level, we identified a sub-set of 14 fibrosis related genes that were uniquely differentially regulated after exposure to NM-401. Acellular ROS production occurred almost exclusively with NRCWE-026, however the longer NM-401 induced in vivo DNA strand breaks and differential regulation of genes involved in free radical scavenging more readily than NRCWE-026. Our results indicate that the global mRNA response after exposure to MWCNTs is length independent at the acute time points, but that fibrosis may be length dependent sub-chronic end point.JRC.H.6-Digital Earth and Reference Dat

The vertical variation of particle speed and flux density in aeolian saltation: measurement and modeling

Particle dynamics in aeolian saltation has been studied in a boundary layer wind tunnel above beds composed of quartz grains having diameters of either 254 µm or 320 µm. The cross section of the tunnel is 600 mm × 900 mm and its thick boundary layer allows precise estimation of the fluid friction speed. Saltation is modelled using a numerical saltation model and predicted grain speeds agree fairly well with experimental results obtained from laser-Doppler anemometry. At 80 mm height the ratio between air speed and grain speed is about 1.1 and from there it increases towards the bed so that at 5 mm it is about 2.0. All grain speed profiles converge towards a common value of about 1 m/s at 2-3 mm height. Flux density profiles, measured with a laser-Doppler appear to be similar to most other density profiles measured with vertical array compartment traps, i.e. two exponential segments will fit data between heights from a few mm to 100-200 mm. The experimental flux density profiles are found to agree well with model predictions. Generally validation rates are low from 30-50 % except at the highest level of 80 mm where they approach 80 %. When flux density profiles based on the validated data are used to estimate the total mass transport rate results are in fair agreement with measured transport rates except for conditions near threshold where as much as 50 % difference is observed

Saltation threshold for pyroclasts at various bedslopes: Wind tunnel measurements

AbstractPyroclastic density currents represent one of the most destructive hazards associated with explosive volcanism. This destructive nature does not only urge the need for but also prevents the obtainment of in situ measurements of their physical characteristics. The resulting deposits offer, however, evidence of the physics of their sedimentation phase. Deposits of dilute pyroclastic density currents frequently exhibit repeated cycles of deposition and erosion, yielding insights into the turbulent shearing along the ground. The utilization of such field observations can be greatly enhanced by the calibration of physical properties of such flows under well-constrained laboratory conditions. Here, wind tunnel measurements were performed using pyroclastic particles. The saltation threshold and surface roughness length were calculated for wind above a pyroclastic bed. The results serve as an aid in linking field observations to quantitative values of turbulent shear at the base of a flow. Scoria and pumice particles were investigated as a function of grain size (1 ϕ fractions between 0.125 and 4mm), as well as the influence of bedslope (−20° to +25° in 10° steps). The results point to the dominant control of density, grain size and, contrary to previous assumptions, differ moderately from results obtained for round beads. Properly utilized, the dataset enables the establishment of a link between the grain size of natural deposits and the shearing extant during their emplacement. Depending on the type of sedimentary structure observed in the field, the saltation threshold can be used as a minimal or a maximal shearing limit during emplacement of dilute pyroclastic density current deposits. Stoss-aggrading laminations likely involve the saltation threshold as an upper limit, whereas for truncation events it must have been overcome. The effect of particle concentration within the flow, a critical parameter for pyroclastic density currents and the extent of validity of the data, are discussed

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Nanomaterial grouping: Existing approaches and future recommendations

The physico-chemical properties of manufactured nanomaterials (NMs) can be fine-tuned to obtain different functionalities addressing the needs of specific industrial applications. The physico-chemical properties of NMs also drive their biological interactions. Accordingly, each NM requires an adequate physico-chemical characterization and potentially an extensive and time-consuming (eco)toxicological assessment, depending on regulatory requirements. Grouping and read-across approaches, which have already been established for chemicals in general, are based on similarity between substances and can be used to fill data gaps without performing additional testing. Available data on “source” chemicals are thus used to predict the fate, toxicokinetics and/or (eco)toxicity of structurally similar “target” chemical(s). For NMs similar approaches are only beginning to emerge and several challenges remain, including the identification of the most relevant physico-chemical properties for supporting the claim of similarity. In general, NMs require additional parameters for a proper physico-chemical description. Furthermore, some parameters change during a NM's life cycle, suggesting that also the toxicological profile may change. This paper compares existing concepts for NM grouping, considering their underlying basic principles and criteria as well as their applicability for regulatory and other purposes. Perspectives and recommendations based on experiences obtained during the EU Horizon 2020 project NanoReg2 are presented. These include, for instance, the importance of harmonized data storage systems, the application of harmonized scoring systems for comparing biological responses, and the use of high-throughput and other screening approaches. We also include references to other ongoing EU projects addressing some of these challenges.JRC.F.2-Consumer Products Safet