Environmental Wind Tunnels

Automotive technology & trade

Positron moderation and timing

The development of more intense and better resolved slow positron (e+) beams is one of the major goals in e+ research. One way to achieve this is by improving the efficiency of moderation techniques. Such improvements would allow the application of this unique probe to many areas of research which currently find the technology prohibitively complex and expensive. It may also open the way to more ambitious research projects requiring high intensity beams, for example the formation of anti-hydrogen. A technique employed m e+ physics which is often important m order to improve the quality of the detected signal is that of single particle timing. This technique relies on the fact that generally e+ beams are of low intensity and by resolving interactions in time it becomes possible to reduce spurious (random) background counts. The development of a new method of timing has been carried out which utilises the secondary e+s emitted on β+ impact with the moderator. The success of this method lies m the fact that the same component of the β+ energy spectrum which contributes most to the emitted slow e+ yield (le the those with low energy) is also most efficient at secondary e+ emission. Positron tagging efficiencies of greater than 20% were observed with a signal to background ratio much higher than that obtained with other β tagging techniques. There was also no loss of beam intensity using this technique, unlike that of timing at a remoderator. An investigation was performed on the e+ moderating properties of the rare gas solids (RGS). Neon has recently provided the highest currently quoted moderation efficiencies. The work presented in this thesis showed that the other RGS (Ar, Kr and Xe) could achieve comparable efficiencies and provide greater than a ten fold improvement on the commonly used metal foil or mesh type moderators. Importantly these RGS moderators are easily fabricated, rugged and may be replaced without loss of vacuum. The first observation of electric field assisted e+ extraction was made during the course of this study. This was achieved by surface charging of the RGS film by the trapping of electrons on overlayered oxygen molecules. Enhancement in the moderation efficiency of a factor of three was observed and was attributed to the electric field, of strength approximately 6kV/mm, across the film due to the trapped surface charge. This effect is not only of importance in the development of more efficient slow e+ moderators but may prove to be an interesting new field of research in its own right

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

Contact electrification in aerosolized monodispersed silica microspheres quantified using laser based velocimetry

The contact electrification of aerosolized micro particles has been studied using a novel technique involving laser velocimetry. This has allowed the simultaneous determination of size and electrical charge of individual silica microspheres (in the range 1 – 8 µm). Interestingly the particles interacting with the injector tube have been seen to become electrified with a relatively narrow range of surface charge concentration of around Q/4πr² ~ −100 e⁻/µm² (~ −0.02mC/m²) for all particle sizes. Several combinations of aerosol particle and injector tube composition were also investigated, some of which led to positive particle electrification and all of which resulted in similar values of measured surface charge concentration. The electrification was not seen to be strongly affected by gas composition and is in reasonable agreement with the expected maximum surface charge observed in previous experiments (< 0.1mC/m²). Possible explanations for this effect are discussed, including the possibility of field emission at the contact site. In the future this technique is intended also to be applied to particle-particle induced contact electrification and its material dependence

Mars og Marsaktiviteter - nye projekter under opsejling

Mars Simulerings Laboratoriet p&aring;&nbsp;Aarhus Universitet har bidraget&nbsp;med vigtig viden i forbindelse med&nbsp;Mars-missionerne, og der er ogs&aring;&nbsp;udsigt til, at igangv&aelig;rende projekter&nbsp;kommer i betragtning til NASAs og&nbsp;ESAs Mars-ekspeditioner i 2007 og&nbsp;2011

Ejected Particles after Impact Splash on Mars: Aggregates and Aerodynamics

Our earlier laboratory measurements showed that low-velocity sand impacts release fine <5 {\mu}m dust from a Martian simulant soil. This dust will become airborne in the Martian atmosphere. Here, we extend this study by measuring aerodynamic properties of ejecta and characterizing deviations from the behavior of spherical, monolithic grains. We observe the settling of particles emitted as part of an impact splash. The sizes (20 to 280 {\mu}m) and sedimentation velocities (0.1 to 0.8 ms^{-1} ) of the particles are deduced from high-speed videos while the particles sediment under low ambient pressure of about 1 mbar. The particles regularly settle slower than expected, down to a factor of about 0.3. Using optical microscopy, the shape of the captured particles is characterized by simple axis ratios (longest/smallest), which show that the vast majority of particles are irregular but typically not too elongated, with axis ratios below 2 on average. Electron microscopy further reveals that the particles are typically porous aggregates, which is the most likely reason for the reduction of the sedimentation velocity. Due to the reduced bulk density, aggregates up to 10 {\mu}m in diameter should regularly be a part of the dust in the Martian atmosphere.Comment: 8 pages, 7 figure

Влияние семантики локализованности на текстовую внешнетемпоральную транспозицию

Языковая временная семантика в лингвистических исследованиях последних лет рассматривается как широкая сфера языковых/речевых отношений различных категорий (грамматических, функционально- семантических, текстовых), т.е. как область пересечения, иногда концентрации аспектуального, собственно темпорального, таксисного и другого аналогичного содержания, где центральное место принадлежит глагольной единице, потенциальные функциональные возможности которой и определяют указанные грамматические отношения

Aeolian Remobilisation of Volcanic Ash: Outcomes of a Workshop in the Argentinian Patagonia

During explosive volcanic eruptions, large quantities of tephra can be dispersed and deposited over wide areas. Following deposition, subsequent aeolian remobilisation of ash can potentially exacerbate primary impacts on timescales of months to millennia. Recent ash remobilisation events (e.g., following eruptions of Cordón Caulle 2011; Chile, and Eyjafjallajökull 2010, Iceland) have highlighted this to be a recurring phenomenon with consequences for human health, economic sectors, and critical infrastructure. Consequently, scientists from observatories and Volcanic Ash Advisory Centers (VAACs), as well as researchers from fields including volcanology, aeolian processes and soil sciences, convened at the San Carlos de Bariloche headquarters of the Argentinian National Institute of Agricultural Technology to discuss the ?state of the art? for field studies of remobilised deposits as well as monitoring, modeling and understanding ash remobilisation. In this article, we identify practices for field characterisation of deposits and active processes, including mapping, particle characterisation and sediment traps. Furthermore, since forecast models currently rely on poorly-constrained dust emission schemes, we call for laboratory and field measurements to better parameterise the flux of volcanic ash as a function of friction velocity. While source area location and extent are currently the primary inputs for dispersion models, once emission schemes become more sophisticated and better constrained, other parameters will also become important (e.g., source material volume and properties, effective precipitation, type and distribution of vegetation cover, friction velocity). Thus, aeolian ash remobilisation hazard and associated impact assessment require systematic monitoring, including the development of a regularly-updated spatial database of resuspension source areas.Fil: Jarvis, Paul A.. Universidad de Ginebra. Facultad de Ciencias. Sección de Ciencias de la Tierra; SuizaFil: Bonadonna, Costanza. Universidad de Ginebra. Facultad de Ciencias. Sección de Ciencias de la Tierra; SuizaFil: Dominguez, Lucia. Universidad de Ginebra. Facultad de Ciencias. Sección de Ciencias de la Tierra; SuizaFil: Forte, Pablo Brian. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Frischknecht, Corine. Universidad de Ginebra. Facultad de Ciencias. Sección de Ciencias de la Tierra; SuizaFil: Bran, Donaldo. Instituto Nacional de Tecnología Agropecuaria; ArgentinaFil: Aguilar, Rigoberto. No especifíca;Fil: Beckett, Frances. No especifíca;Fil: Elissondo, Manuela. Secretaría de Industria y Minería. Servicio Geológico Minero Argentino; ArgentinaFil: Gillies, John. Desert Research Institute; Estados UnidosFil: Kueppers, Ulrich. Ludwig Maximilians Universitat; AlemaniaFil: Merrison, Jonathan. University Aarhus. Institut for Fysik Og Astronomi; DinamarcaFil: Varley, Nick. Universidad de Colima; MéxicoFil: Wallace, Kristi L.. United States Geological Survey; Estados Unido