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Sand Grains (sand + grain)
Selected AbstractsPositron Imaging Studies of Rotating DrumsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 1 2005David J. Parker Abstract The potential of the radioisotope tracer technique of positron emission tomography (PET) and the related techniques of positron emission projection imaging (PEPI) and positron emission particle tracking (PEPT) is illustrated with reference to laboratory scale studies of particulate motion in rotating drums, operating either in batch or continuous flow modes. Sand grains, glass beads and TiO2 granules down to 0.5mm diameter were labelled. Using PEPT the transition between rolling and slumping modes has been identified and the velocity profile within the active layer has been determined for a range of drum diameters. PEPI has been used to measure and explain residence time distributions, while all three techniques have been used to study segregation based on particle size, both radially and axially within the drum. Data on particle motion within a novel baffled drum is also presented. Le potentiel de la technique de traçage par radio-isotopes en tomographie par émission de positrons (PET) et les techniques associées de l'imagerie par projection des émissions de positrons (PEPI) ou le traçage des particules par émission de positrons (PEPT), est illustré en référence à des études à l'échelle de laboratoire du déplacement de particules dans des tambours rotatifs, fonctionnant soit en mode d'écoulement discontinu ou continu. Des grains de sable, des billes de verre et des granules de TiO2 aussi petits que 0,5 mm de diamètre ont été marqués. À l'aide de la technique PEPT, la transition entre les modes roulant et glissant a été identifiée et le profil de vitesse à l'intérieur de la couche active a été déterminé pour une gamme de diamètres de tambours. La technique PEPI a été utilisée pour mesurer et expliquer les distributions de temps de séjour, tandis que les trois techniques ont servi pour l'analyse de la ségrégation d'après la taille des particules, à la fois radialement et axialement dans le tambour. Des données sur le déplacement des particules dans un nouveau tambour à chicanes sont également présentées. [source] Expressing sand supply limitation using a modified Owen saltation equation ,EARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2008Dale A. Gillette Abstract An analysis of saltation data led us to modify the theory of P. R. Owen using a soil-related parameter ,A' that gave us the possibility of expressing limitation of sand grains of saltation-size in the underlying soil. The value of ,A' was set equal to the ratio of the horizontal flux of saltating particles to Owen's function of wind, times air density divided by gravitational acceleration. Values of A can be used to: (1) characterize the efficiency of the wind to move sand by saltation for different soil textures and aggregations; and (2) to make practical predictions of sand movement based on the condition of the surface soil. Values for A in a range from 1 to 10 are usually associated with supply-unlimited saltation and are usually associated with loose, sandy-textured soils. Values for A in a range from 0·25 to 1 are associated with finer soils that contain more silt and clay. The range of A values between 0 and 0·25 usually reflects finer textured soils that are packed, aggregated, or crusted. A decrease of A to a smaller value is a sign of supply limitation and usually to the soil changing from a looser state to a more aggregated state or more depleted state. Likewise, an increase of A usually corresponds to soil changing from an aggregated state to a looser state. Copyright © 2008 John Wiley & Sons, Ltd. [source] Crust strength: a wind tunnel study of the effect of impact by saltating particles on cohesive soil surfacesEARTH SURFACE PROCESSES AND LANDFORMS, Issue 7 2001M. A. Rice Abstract A wind tunnel study examined the effect of distributions of saltating particles on sediment surfaces which were characterized by distributions of their tensile strength. The sediments consisted of varying proportions of large sand-sized particles with a fine particle cement. The energies of the impacting particles and the surface strengths were compared with the mass of material lost from the surface. It is important to consider distributions of parameters rather than mean values only, since abrasion and erosion may occur from surfaces not predicted from average strength and saltation velocities. At the impact velocities used in this study (mean velocity 4·4 m s,1, with standard deviation of 0·51), surfaces containing less than 12 per cent fine material were easily eroded, but insignificant erosion occurred when the fine particle content exceeded 60 per cent. Small amounts of cementing material were easily ruptured, allowing the large sand grains to be moved (largely in creep) by the bombarding particles. A significant amount of energy was lost to the bed. As the percentage of fine material increased, the surface became more difficult to break up and less energy was lost to the bed. The probability that erosion will occur for known energy distributions of impacting particles and surface strength can be calculated and the mass loss increases exponentially with a decrease in the percentage of fine cementing particles. Copyright © 2001 John Wiley & Sons, Ltd. [source] Life in Darwin's dust: intercontinental transport and survival of microbes in the nineteenth centuryENVIRONMENTAL MICROBIOLOGY, Issue 12 2007Anna A. Gorbushina Summary Charles Darwin, like others before him, collected aeolian dust over the Atlantic Ocean and sent it to Christian Gottfried Ehrenberg in Berlin. Ehrenberg's collection is now housed in the Museum of Natural History and contains specimens that were gathered at the onset of the Industrial Revolution. Geochemical analyses of this resource indicated that dust collected over the Atlantic in 1838 originated from the Western Sahara, while molecular-microbiological methods demonstrated the presence of many viable microbes. Older samples sent to Ehrenberg from Barbados almost two centuries ago also contained numbers of cultivable bacteria and fungi. Many diverse ascomycetes, and eubacteria were found. Scanning electron microscopy and cultivation suggested that Bacillus megaterium, a common soil bacterium, was attached to historic sand grains, and it was inoculated onto dry sand along with a non-spore-forming control, the Gram-negative soil bacterium Rhizobium sp. NGR234. On sand B. megaterium quickly developed spores, which survived for extended periods and even though the numbers of NGR234 steadily declined, they were still considerable after months of incubation. Thus, microbes that adhere to Saharan dust can live for centuries and easily survive transport across the Atlantic. [source] Stratigraphic landscape analysis: Charting the Holocene movements of the Nile at Karnak through ancient Egyptian timeGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 3 2008J. M. Bunbury Geological analysis of 5,10-m-long sediment cores in the context of the anthropologically derived materials within them has allowed us to identify ancient landscape features in the Theban area around Luxor, Egypt. From these observations we propose a sequence of island formation and northwestward movement of the Nile from the Middle Kingdom onward in the area of the temple complexes of Karnak. The geoarchaeological techniques used appear to document the Holocene lateral migration and vertical aggradation of the Nile. Our method can be used to test postulated movements and is applicable to sites in river or coastal plains where sediments were being deposited during the occupation of the site. The sediments were sieved to retrieve sherds and numerous other small items (2 mm and larger), which included worked stone fragments, rootlet concretions (rhizocretions), desert polished sand grains, and occasionally beads. The small stone fragments can be correlated with buildings and sherds of known age within the site, while the rhizocretions and desert sand grains indicate environmental conditions prevailing at the time of deposition. © 2008 Wiley Periodicals, Inc. [source] Quantitative detection of fluid distribution using time-lapse seismicGEOPHYSICAL PROSPECTING, Issue 2 2007Futoshi Tsuneyama ABSTRACT Although previous seismic monitoring studies have revealed several relationships between seismic responses and changes in reservoir rock properties, the quantitative evaluation of time-lapse seismic data remains a challenge. In most cases of time-lapse seismic analysis, fluid and/or pressure changes are detected qualitatively by changes in amplitude strength, traveltime and/or Poisson's ratio. We present the steps for time-lapse seismic analysis, considering the pressure effect and the saturation scale of fluids. We then demonstrate a deterministic workflow for computing the fluid saturation in a reservoir in order to evaluate time-lapse seismic data. In this approach, we derive the physical properties of the water-saturated sandstone reservoir, based on the following inputs: VP, VS, , and the shale volume from seismic analysis, the average properties of sand grains, and formation-water properties. Next, by comparing the in-situ fluid-saturated properties with the 100% formation-water-saturated reservoir properties, we determine the bulk modulus and density of the in-situ fluid. Solving three simultaneous equations (relating the saturations of water, oil and gas in terms of the bulk modulus, density and the total saturation), we compute the saturation of each fluid. We use a real time-lapse seismic data set from an oilfield in the North Sea for a case study. [source] Solute transport in sand and chalk: a probabilistic approachHYDROLOGICAL PROCESSES, Issue 5 2006E. Carlier Abstract A probabilistic approach is used to simulate particle tracking for two types of porous medium. The first is sand grains with a single intergranular porosity. Particle tracking is carried out by advection and dispersion. The second is chalk granulates with intergranular and matrix porosities. Sorption can occur with advection and dispersion during particle tracking. Particle tracking is modelled as the sum of elementary steps with independent random variables in the sand medium. An exponential distribution is obtained for each elementary step and shows that the whole process is Markovian. A Gamma distribution or probability density function is then deduced. The relationships between dispersivity and the elementary step are given using the central limit theorem. Particle tracking in the chalky medium is a non-Markovian process. The probability density function depends on a power of the distance. Experimental simulations by dye tracer tests on a column have been performed for different distances and discharges. The probabilistic approach computations are in good agreement with the experimental data. The probabilistic computation seems an interesting and complementary approach to simulate transfer phenomena in porous media with respect to the traditional numerical methods. Copyright © 2006 John Wiley & Sons, Ltd. [source] Fine structure and 18S rDNA phylogeny of a marine araphid pennate diatom Plagiostriata goreensis gen. et sp. nov. (Bacillariophyta)PHYCOLOGICAL RESEARCH, Issue 1 2009Shinya Sato SUMMARY A marine araphid pennate diatom Plagiostriata goreensis is described from the sand grains of Goree Island, Dakar, Republic of Senegal, based on observations of fine structure of its frustule. The most striking feature of the species is its striation, which is angled at approximately 60° across the robust sternum. The other defining features of the species are its one highly reduced rimoportula and apical pores located at both ends of the valve margin. In the 18S rDNA phylogeny, the species appears as a member of a ,small-celled clade' of araphid pennate diatoms that consist of Nanofrustulum, Opephora and Staurosira. The results of the phylogenetic analyses suggest that the distinct characters of the diatom; namely, oblique striae and apical pores, may have been acquired independently. However, it remains unclear whether the rimoportula of P. goreensis is a reduced state or P. goreensis acquired its morphologically curious rimoportula independently after the loss of an ancient rimoportula at the root of the small-celled clade. [source] | |||||||||||||