Function Of Transport Distance Research Articles

Velocity of ultrasound diffusion in heterogeneous media and its applications

This paper investigates the velocity of ultrasonic diffusion in heterogeneous media under the conditions where the diffuse wave approximation is valid (λ≤a). The diffusion velocity is defined as the moving speed of the peak of the energy evolution curve. The peak arrival time as a function of transport distance is calculated for infinite spaces and bounded domains. The results show that the peak arrival time is independent of the domain’s geometry, i.e. the size, shape, and boundary conditions. This is confirmed by comparing the arrival times in an infinite space and a bounded domain with a geometric feature – a surface-breaking crack of varying depth. Therefore, the velocity of ultrasonic diffusion is an intrinsic property of a medium, and the formula for the infinite three-dimensional space is sufficient to calculate the arrival time–transport distance relationship, given the diffuse properties of the medium. These findings eliminate the needs for the finite element numerical simulations in the applications to determine geometric parameters such as the crack depth using diffuse ultrasound. The diffusion velocity is a function of transport distance in general, while its far-field asymptotic value is constant regardless of the dimensionality and geometry of the domain.

Investigation on the transport efficiency of fast electrons with double-layer Kα fluorescence measurement

Ultraintense laser driven fast electrons play an increasingly important role in many applications. To predict and optimize the fast electron transport efficiency, we introduce a one-dimensional analytical model including resistive effects to estimate the transport efficiency as a function of transport distance and a key parameter named the penetration path. Based on the model, the transport efficiency of fast electrons with the same penetration coefficient can be calculated for different characteristic parameters including the target thickness and laser intensity. A double-layer Kα fluorescence measurement of fast electron transport efficiency is proposed to eliminate the influence of in-target electrons refluxing from the relative Kα photon yield of the rear and front sides of the target. By fixing the transport distance, we have experimentally measured the penetration path and the efficiency of planar Al2O3 targets, in good accordance with Monte Carlo simulations. The results show that the beam energy can be reduced to 25% in a penetration path of tens of microns. This measurement method provides a feasible route to characterize and compare the fast electron transport in various targets and laser conditions, making it possible to modulate and optimize the transport efficiency in actual research studies, which is of great significance in fast ignition, X-ray emission, positron–electron pair production, and many other applications.

2500-year paleotempestological record of intense storms for the northern Gulf of Mexico, United States

The northern Gulf of Mexico has been devastated by recent intense storms. Camille (1969) and Katrina (2005) are two notable hurricanes that made landfall in nearly the same location in Mississippi. Fully understanding the risks and processes associated with hurricane impacts are impeded by a short and fragmented instrumental record, however. Paleotempestology has the potential to employ modern analogues from intense storms in this region to extend the hurricane record beyond pre-observational time. Existing empirically-based models can back-calculate surge heights over coastal systems as a function of transport distance, particle settling velocity, and gravitational acceleration. We collected sediment cores in a pond (3) and adjacent beach (1) in Hancock County, Mississippi. Grain-size, loss-on-ignition, and microfossil analyses were conducted on cores in the context of a Bayesian statistical age model using 137Cs and 14C dating. Using Hurricane Camille to calibrate the archive, similar coarse-grained deposits were identified, and inverse sediment transport models calculated paleosurge intensities similar in magnitude to Camille over the 2500-yr record. Our multi-millennial annual average landfall probability (0.48%) closely matches previously published studies from the Gulf of Mexico, indicating that intense hurricanes have not varied over these timescales. Over centennial timescales, active intervals occurred between 900 to 600 and 2200 to 1900yr BP, with relative quiescence between 1900 to 900yr BP. Comparisons with other published sites support the notion that southerly shifts in the Loop Current may be responsible for the decline in activity around 600yr BP.

Diffusion–reaction studies in low permeability shale using X-ray radiography with cesium

An X-ray radiography method for the determination of diffusion coefficients in rock has been modified to provide an estimation of diffusion–reaction parameters for the sorbing tracer, cesium (Cs+). Cesium tracer diffusion and sorption on cation exchange sites can be monitored in intact rock samples using data extracted from radiographs to plot time-series relative-concentration profiles of Cs+ as a function of transport distance. Cesium was found to be a very good tracer for these experiments because of the sensitivity of the X-ray attenuation measurements to Cs+ concentration.Reactive-transport modeling coupled with parameter estimation software was used to match experimental data and estimate pore diffusion coefficients for Cs+ (Dp-Cs=7.6×10−11m2/s), single-site selectivity coefficients for Cs+ exchange (logKCs+/Na+=1.5) and cation exchange capacity (CEC=8.4meq/100g) for drill core samples of Queenston Formation shale, from the Michigan Basin in Ontario, Canada.

Transport of Escherichia coli strains isolated from natural spring water

We present a new methodology to scale up bacteria transport experiments carried out in the laboratory to practical field situations. The key component of the methodology is to characterize bacteria transport not by a constant sticking efficiency, but by a range of sticking efficiency values determined from laboratory column experiments. In this study, initially, we harvested six Escherichia coli strains from springs in Kampala, the capital of Uganda, and then we carried out a number of experiments with 1.5m high columns of quartz sand with various sampling ports in order to determine the fraction of bacteria as a function of sticking efficiency. Furthermore, we developed a simple mathematical formulation, based on the steady-state analytical solution for the transport of mass in the subsurface, to arrive at bacteria concentrations as a function of transport distance. The results of the quartz sand column experiments indicated that the fractional bacteria mass and sticking efficiency of most of the strains we harvested could be adequately described by a power law. When applying the power distributions to the field situation in Kampala, we found that the transport distance required to reduce bacteria concentrations with five log units ranged from 1.5 to 23m, and this was up to three times more than when using a constant sticking efficiency. The methodology we describe is simple, can be carried out in a spreadsheet, and in addition to parameters describing transport, like pore water flow velocity and dispersion, only two constants are required, which define the relation between sticking efficiency and percentage of bacteria mass.

Transport of Escherichia coli in saturated porous media: Dual mode deposition and intra-population heterogeneity

Because of heterogeneity among members of a bacteria population, deposition rates of bacteria may decrease upon the distance bacteria are transported in an aquifer. Such deposition rate decreases may result in retained bacteria concentrations, which decrease hyper-exponentially as a function of transport distance, and may therefore significantly affect the transport of colloids in aquifers. We investigated the occurrence of hyper-exponential deposition of Escherichia coli, an important indicator for fecal contamination, and the causes for such behavior. In a series of column experiments with glass beads of various sizes, we found that attachment of E. coli decreased hyper-exponentially, or, on logarithmic scale in a bimodal way, as a function of the transported distance from the column inlet. From data fitting of the retained bacteria concentration profiles, the sticking efficiency of 40% of the E. coli population was high ( α=1), while the sticking efficiency of 60% was low ( α=0.01). From the E. coli total population, an E. coli subpopulation consisting of slow attachers could be isolated by means of column passage. In subsequent column experiments this subpopulation attached less than the E. coli total population, consisting of both slow and fast attachers. We concluded that the main driver for the observed dual mode deposition was heterogeneity among members of the bacteria population. Intra-population may result in some microbes traveling surprisingly high distances in the subsurface. Extending the colloid filtration theory with intra-population variability may provide a valuable framework for assessing the transport of bacteria in aquifers.

Parent material controlled subsoil weathering in a chronosequence, the Allier terraces, Limagne Rift Valley, France

The chronosequence in the Allier terraces is subject to erratic changes in parent material composition with time. Approximately 65% of the total variance in the basaltic elements content in the Allier terrace sands can be explained by the combined effect of parent material and time related neoformation of clay in the subsoil. It is shown that parent material composition, as a function of transport distance, strongly influences weathering rate in the Allier terrace chronosequence. In order to evaluate parent material controlled weathering effects a process model simulation was made based on an exponential decrease of MgO and neoformation of clay. Long term simulations with this model suggest that parent material controlled subsoil weathering is significant for sediments with easy weatherable fragments. After prolonged weathering the role of parent material decreases and becomes untraceable in strongly weathered materials.

Transport of Magnetite and Ilmenite by Glaciers in the Adirondack Mountains of New York

Till collected from surface exposures within the valley of the Hudson River south of the Sanford Hill magnetite-ilmenite ore deposit in the Adirondack Mountains of New York is composed primarily of plagio-clase, pyroxene, garnet, magnetite, and ilmenite. The concentrations of magnetite and ilmenite in bulk till both decrease exponentially with increasing distance south of the ore bodies because of dilution by the entrainment of plagioclase and garnet + pyroxene from the bedrock and from older till deposits in the valley. Evidence for comminution of magnetite and ilmenite is provided by decreasing abundances of these minerals in the coarse fractions (1000-125 μm) and corresponding increases in the fine fractions (<125 μm). The apparent rate of comminution of ilmenite as a function of transport distance is significantly greater than that of magnetite, which causes ilmenite to be concentrated in the fine fraction of till compared to magnetite. The weight ratios of magnetite and ilmenite in the fine to coarse fractions of basal till increase exponentially with distance from the source and may be useful for estimating transport distances.

Facies sequences in large-volume debris- and turbidity-flow deposits from the Pyrenees (Cretaceous; France, Spain)

Facies types and depositional models for thick gravity-flow deposits are inferred from Cretaceous Pyrenean examples occurring in different sedimentary settings: basin-plain during relative low sea level stand; slope-apron or deep valleys during relative sea level rise; and slope during relative sea level rise. The depositional units are interpreted as debrites, debrite-turbidite couplets, and megaturbidites. They are a function of transport distance and hydraulic jump by liquefaction and phase separation in large subaqueous flows.

Mechanics of particle selection by tentaculate deposit-feeders

A variety of experiments with glass and plastic beads covering a wide range of particle sizes and weights are used to develop a two-step, stochastic model of particle selection by deposit-feeders. These experiments include measurements of selectivity as a function of transport distance along a terebellid (polychaete) tentacle, of the degree to which a food source can be depleted of the preferred particle type by a terebellid, of selectivity by spionids (polychaetes) having lost their tentacles, of selectivity by an ampharetid (polychaete) as a function of available particle types, and of “selectivity” by adhesive-coated microscope slides. A large component of mechanical selection is seen to be achieved at the point of particle pick-up. The first step of the heuristic model is particle contact by the tentacle. By appealing to the Delesse principle and to the results with adhesive-coated slides, the probability of particle contact is shown to be a function of particle size. On a per-particle basis, larger particles have a higher probability of being contacted by a tentacle than do smaller ones. The second step is the retention of the particle given that it has been contacted. It is suggested that the conditional (i.e., given contact) probability of such retention is a function of the adhesive strength of the mucus found on the tentacle and that it will depend on both particle weight per unit surface area and particle surface characteristics. In general, the conditional probability of retention will be a decreasing function of submerged particle weight per unit surface area. An important deduction from this model is that size-selection in tentaculate deposit-feeders need have no morphological correlates; the lower the adhesive strength of the mucus, the greater will be the animal's selectivity for smaller, lighter particles.