Sharp Gradients Research Articles

Effect of Permittivity of Plasma Medium on the Particle Properties and Electric Field in a Magnetized Plasma Sheath

The plasma sheath, a thin layer having sharp gradients, is necessary to be formed at any material wall in exposure with plasma for its stability. Characteristics of magnetized plasma sheath formed in front of a material wall for different permittivity of plasma medium has been studies using the kinetic trajectory simulation (KTS) model. The electron density, ion density, electric field and potential decreases on moving away from the sheath entrance but total charge density increases. As plasma is composed of charged particles, any variation in the permittivity of the medium has significant effects on the sheath properties. It is found that on increasing the permittivity of the medium, the electric potential strength and hence electric field decreases because of increased shielding ability of the plasma. The results are in qualitative agreement with earlier studies based on the fluid approach but the kinetic approach is more accurate quantitatively providing a better perception of plasma-wall transition phenomena.

Understanding developing turbulence by a study of the nonlinear energy transfer in the Navier–Stokes equation

In the present work, we investigate a numerical one-dimensional solver to the Navier–Stokes equation that retains all terms, including both pressure and dissipation. Solutions to simple examples that illustrate the actions of the nonlinear term are presented and discussed. The calculations take the full 4D flow as its starting point and continuously projects the forces acting on the fluid at a fixed Eulerian point in a stationary coordinate system onto the direction of the instantaneous velocity. Pressure is included through modeling. Adhering to the requirement that time must in general be considered an independent variable, the time development of the time records and power spectra of the velocity fluctuations are studied. It is found that the actions of the nonlinear term in the Navier–Stokes equation manifests itself by generating sharp pulses in the time traces, where the sharpness is bounded by the finite viscosity. In the spectral domain, the sharp gradients in the pulses generate energy contributions at high frequencies that yields a −2 slope across the inertial range. The −2 (or −6/3) slope is explained through a simple example and the classically expected −5/3 slope in the inertial range can be recovered from the pressure fluctuations from the full flow field that can be considered a noise contribution at the point considered. We also observe that the spectrum can in principle keep spreading to higher frequencies or wavenumbers without upper bound, as the viscosity is approaching the zero limit.

Hydrologic Control on Arsenic Cycling at the Groundwater-Surface Water Interface of a Tidal Channel.

Historical industrial activities have resulted in soil contamination at sites globally. Many of these sites are located along coastlines, making them vulnerable to hydrologic and biogeochemical alterations due to climate change and sea-level rise. However, the impact of hydrologic dynamics on contaminant mobility in tidal environments has not been well studied. Here, we collected data from pressure transducers in wells, multi-level redox sensors, and porewater samplers at an As-contaminated site adjacent to a freshwater tidal channel. Results indicate that sharp redox gradients exist and that redox conditions vary on tidal to seasonal timescales due to sub-daily water level fluctuations in the channel and seasonal groundwater-surface water interactions. The As and Fe2+ concentrations decreased during seasonal periods of net discharge to the channel. The seasonal changes were greater than tidal variations in both Eh and As concentrations, indicating that impacts of the seasonal mechanism are stronger than those of sub-daily water table fluctuations. A conceptual model describing tidal and seasonal hydro-biogeochemical coupling is presented. These findings have broad implications for understanding the impacts of sea-level rise on the mobility of natural and anthropogenic coastal solutes.

Kinetic modulation of bacterial hydrolases by microbial community structure in coastal waters.

In this study, we hypothesized that shifts in the kinetic parameters of extracellular hydrolytic enzymes may occur as a consequence of seasonal environmental disturbances and would reflect the level of adaptation of the bacterial community to the organic matter of the ecosystem. We measured the activities of enzymes that play a key role in the bacterial growth (leucine aminopeptidase, β- and α-glucosidases) in surface coastal waters of the Eastern Cantabrian Sea and determined their kinetic parameters by computing kinetic models of distinct complexity. Our results revealed the existence of two clearly distinct enzymatic systems operating at different substrate concentrations: a high-affinity system prevailing at low substrate concentrations and a low-affinity system characteristic of high substrate concentrations. These findings could be the result of distinct functional bacterial assemblages growing concurrently under sharp gradients of high-molecular-weight compounds. We constructed an ecological network based on contemporaneous and time-delayed correlations to explore the associations between the kinetic parameters and the environmental variables. The analysis revealed that the recurring phytoplankton blooms registered throughout the seasonal cycle trigger the wax and wane of those members of the bacterial community able to synthesize and secrete specific enzymes.

Potential benefits of using radioactive ion beams for range margin reduction in carbon ion therapy

Sharp dose gradients and high biological effectiveness make ions such as 12C an ideal tool to treat deep-seated tumors, however, at the same time, sensitive to errors in the range prediction. Tumor safety margins mitigate these uncertainties, but during the irradiation they lead to unavoidable damage to the surrounding healthy tissue. To fully exploit the Bragg peak benefits, a large effort is put into establishing precise range verification methods. Despite positron emission tomography being widely in use for this purpose in 12C therapy, the low count rates, biological washout, and broad activity distribution still limit its precision. Instead, radioactive beams used directly for treatment would yield an improved signal and a closer match with the dose fall-off, potentially enabling precise in vivo beam range monitoring. We have performed a treatment planning study to estimate the possible impact of the reduced range uncertainties, enabled by radioactive 11C ions treatments, on sparing critical organs in tumor proximity. Compared to 12C treatments, (i) annihilation maps for 11C ions can reflect sub- millimeter shifts in dose distributions in the patient, (ii) outcomes of treatment planning with 11C significantly improve and (iii) less severe toxicities for serial and parallel critical organs can be expected.

Impact of the paleo-river valleys on the chemical parameter distributions in the East Siberian Sea

The East Siberian Sea (ESS) is the largest shelf sea of the Arctic Ocean. The paleo-river valleys of Indigirka (InV) and Kolyma (KV) Rivers are the main features of the ESS bottom topography. Using the time series of geostrophic velocities derived from satellite altimetry and ship-borne data we demonstrate the influence of InV and KV on the water circulation and chemical parameter distributions in the ESS. The KV affects the alongshore flow jet in the ESS by deflecting its inshore (southward) and jet flow accelerating. The water upwelling channeled by KV provides the nearshore area of the southern ESS with an increased concentration of nutrients and high seawater pCO2. In the InV area, the upwelling leads to stronger halocline/pycnocline and more sharp vertical gradients of the nutrients and pCO2 in the 20–30 m layer. The upwelling and water mixing in the paleovalley zones should be considered as one of the main factors leading to the high concentration of nutrients and pCO2 in the surface waters of the ESS.

Географическое и сезонное распределение поля приливной объемной силы в Охотском море в контексте динамики внутренних волн

Estimates of the barotropic tidal body force for diurnal and semidiurnal tides are obtained for the Sea of Okhotsk for the summer and winter periods. It is shown that in the study area, the tidal body force for diurnal tides is significantly greater than for semidiurnal ones. The maximum values of this quantity can reach about 2-8 m2s-2, and these values are typical for areas with a sharp bathymetric gradient. A comparison of the tidal body force for the two seasons showed noticeable differences. The features of the transformation of a barotropic tidal wave propagating in the zone of large values of the tidal body force for the K1, O1, M2 tidal constituents are demonstrated. Numerical simulations indicate that baroclinic tidal waves are effectively generated in this area, and intense short-period internal waves are likely to occur.

Quartz-hosted inclusions and embayments reveal storage, fluxing, and ascent of the Mesa Falls Tuff, Yellowstone

Quartz-hosted glasses from the Mesa Falls Tuff provide a geochemical window into the pre-eruptive magmatic system from one of Yellowstone's largest-volume caldera-forming eruptions. H2O and CO2 concentrations, along with major and trace elements, were measured in both fully enclosed glass inclusions and partially enclosed embayments in the same quartz crystals. Major elements are largely consistent between the inclusions and embayments, except for K2O and Na2O. Of note, K2O is enriched by ∼1 wt.% in embayment interiors relative to inclusions. Most trace elements are also enriched in the embayment interiors compared to inclusions from the same crystals. Fractionation trends of trace elements are consistent with ∼30-60% crystallization. Quartz-hosted glass inclusions preserve 3.1±0.9 wt.% H2O and 493±227 ppm CO2 whereas embayment interiors have 0.9±0.1 wt.% H2O and 399±229 ppm CO2. The CO2 is roughly similar, but the distinct ∼2 wt.% discrepancy between inclusion and embayment interior H2O contents may have been produced by CO2 fluxing sourced from underplated Yellowstone basalts. H2O gradients within embayments are flat in their interiors and modified by sharp positive gradients near embayment exteriors which were produced by post-eruptive rehydration. CO2 gradients occur as gently sloping concentration gradients that extend inward 150 to 250 μm from the embayment exterior. Finite-difference 1D and 2D diffusion modeling indicates the distribution of H2O and CO2 in embayments was produced by slow, fluid-saturated decompression that preceded rapid ascent during the caldera-forming eruption.

On the role of wind and tides in shaping the Gironde River plume (Bay of Biscay)

We study the Gironde River plume in the Bay of Biscay (North-East Atlantic) using a high resolution modelling approach of the estuary-mouth-shelf continuum. The Gironde River plume flows over a gently sloping shelf where semi-diurnal tides reach an amplitude of 1.3–1.5 m on average; the Gironde River discharge undergoes a significant seasonal variability with floods (>1000 m3/s) in winter and spring. From two years of simulation (2011–12), we document the behavior of the plume under varying forcing conditions. We then discuss the dynamical mechanisms at play, from the estimate of some mechanical energy budget terms and using sensitivity runs: run with no tides, run with filtered atmospheric forcing (no variability at time scales smaller than 30 days) and run with no river runoff. We first show that the plume's properties are determined by the processes occurring in a transition area at the estuary outlet between the Royan and Cordouan sections. The outflow passes through a deep and narrow channel both because of bathymetric constraints and tides. Indeed, tides generate a residual circulation made of anticyclonic and cyclonic vortices with diameters of 5–12 km. Tides also induce intense mixing that strongly modifies the stratification between the two sections. The mean conditions lead to classify the plume as a narrow outflow which is a favorable condition for a bulge to be formed according to the literature. We indeed observe the recurrent formation of a bulge with sharp salinity gradients and a narrow coastal current. The bulge develops in moderate to high discharge conditions, during the spring to neap tides transition and when winds are weak. When the intensity of tides and wind increases, the bulge breaks down and the plume spreads offshore under upwelling wind conditions or along the coast (both in northward and southward directions) under strong eastward wind conditions. Tides are the main driver at the estuary outlet (with a bottom friction term larger by two orders of magnitude than over the adjacent shelf) but winds and atmospheric pressure are the main forcing over the inner shelf. These results evidence the need to model accurately the whole estuary-shelf continuum when studying the fate of riverine waters over the shelf.

2D nonlocal elasticity: Investigation of stress and strain fields in complex shape regions

AbstractThe paper presents the features of solutions to nonlocal elasticity problems in domains with stepped transitions and elliptical notches. A comparative analysis of the solutions obtained in the classical and nonlocal elasticity theories was carried out, as well as an analysis of the influence of the main parameters of the model on the final results. The presented solutions for the region with a step transition are in qualitative agreement with the experimental data and have similar portraits of deformations and resultant stresses along the loading axis. In regions with elliptical notches, similar effects are observed. In addition to qualitative agreement with experiments, solutions in the nonlocal formulation have lower stress values in the concentrators and exhibit edge effects, which are characterized by sharp stress and strain gradients at the domain boundaries. The solutions were carried out numerically by the finite element method.

Generating high-resolution climatological precipitation data using SinGAN

ABSTRACT High-resolution (HR) climate data are indispensable for studying regional climate trends, disaster prediction, and urban development planning in the face of climate change. However, state-of-the-art long-term global climate simulations do not provide appropriate HR climate data. Deep learning models are often used to obtain high-resolution climate data. However, due to the fact that these models require sufficient low-resolution (LR) and HR data pairs for the training process, they cannot be applied to scenario with inadequate training data. In this paper, we explore the applicability of a single image generative adversarial network (SinGAN) in generating HR climate data. SinGAN relies on single LR input data to obtain the corresponding HR data. To improve the performance for extreme-value regions, we propose a SinGAN combined with the weighted patchGAN discriminator (WSinGAN). The proposed WSinGAN outperforms comparable models in generating HR precipitation data, and its results are close to real HR data with sharp gradients and more refined small-scale features. We also test the scalability of the pre-trained WSinGAN for unseen samples and show that although only a single LR sample is used to train WSinGAN, it can still produce reliable HR data for unseen data.

Instabilities in three-dimensional boundary-layer flows with a highly non-ideal fluid

The present work investigates the linear instability of three-dimensional boundary layers in thermodynamically non-ideal regimes. As a representative fluid, we consider carbon dioxide at supercritical pressure (80 bar). The flow set-up is matched to the redesigned DLR (German Aerospace Center) experiment on cross-flow instability, with identical pressure-coefficient distribution (accelerating the flow), sweep angle and Reynolds number, at a low Mach number. The flow temperature relative to the Widom line – also known as the pseudocritical line – thus characterises the non-ideality of the flow. We consider supercritical (gas-like), subcritical (liquid-like) and transcritical (pseudoboiling) regimes, where the flow temperature remains above, below or crosses the Widom line. The stability analyses of the parabolised Navier–Stokes baseflows indicate that wall heating destabilises the flow in the supercritical regime while wall cooling stabilises both effects similar to the ideal-fluid situation but being stronger. On the contrary, wall heating/cooling exhibits reversed effects in the subcritical regime, like for an ideal liquid. In the transcritical regime, with its sharp gradients of the thermodynamic and transport properties, wall heating stabilises the flow. Most substantially, however, wall cooling provokes a changeover of the leading instability mechanism: the accelerated streamwise flow attains inflectional wall-normal profiles, and the invoked inviscid Tollmien–Schlichting instability prevails with growth rates up to one order of magnitude larger than those of the cross-flow mode. We establish a two-fold mathematical relation from the momentum equation that explains the consequence of non-ideality and wall heating/cooling. The streamwise perturbation patterns of the flows in their linear instability regime are shown by mimicking wave trains emanating from virtual point-disturbance sources. From the viewpoint of keeping laminar flows, the transcritical thermodynamic state with a cooling wall must be avoided.

Historical and future shifts of a sharp zonal aridity gradient: A case study of the Hu Line in China

Ongoing climate change considerably affects terrestrial aridity from multiple perspectives. As a typical sharp zonal aridity gradient, China’s Hu Line effectively divides the country into the arid northwest and the humid southeast and has exerted profound impacts on human settlement, agricultural and socio-economic development. Previous studies of regions experiencing sharp zonal aridity gradients (e.g., the 100th meridian in the USA and the Sahel belt in Northern Africa) reported significant and widespread aridity changes in the context of anthropogenic climate change, yet how changes in aridity may geographically shift the Hu Line remains unknown. Here we assess the historical and future shifts of the Hu Line from agroecological, atmospheric and hydrologic perspectives. Our results show that the effective arid-humid divide from the agroecological perspective has moved northwest with a mean shift of 0.48° (over 1982–2016), whereas from the atmospheric and hydrologic perspectives its position remains effectively unchanged (very small northwest movements) over the past 35 years. For future periods (under a high emission scenario, SSP585), the effective arid-humid divides from all three perspectives move northwest towards the end of the 21st century, suggesting an overall decrease of agroecological, atmospheric and hydrologic aridity in the region. In addition, we find that vegetation response to elevated atmospheric CO2 concentration plays a key role in determining the predicted shifts of the Hu Line, which alleviates aridity increases under warming and effectively drives the arid-humid divides toward more arid northwest. Our finding of the northwest movement of the Hu Line under future climate change, in particular due to rising CO2, suggests a future potential for expansion of China’s agricultural and socioeconomy northwest and also sheds light on studying other regions experiencing sharp zonal aridity gradients.

Xylem hydraulics strongly influence the niche differentiation of tree species along the slope of a river valley in a water-limited area.

Xylem hydraulic characteristics govern plant water transport, affecting both drought resistance and photosynthetic gas exchange. Therefore, they play critical roles in determining the adaptation of different species to environments with various water regimes. Here, we tested the hypothesis that variation in xylem traits associated with a trade-off between hydraulic efficiency and safety against drought-induced embolism contributes to niche differentiation of tree species along a sharp water availability gradient on the slope of a unique river valley located in a semi-humid area. We found that tree species showed clear niche differentiation with decreasing water availability from the bottom towards the top of the valley. Tree species occupying different positions, in terms of vertical distribution distance from the bottom of the valley, showed a strong trade-off between xylem water transport efficiency and safety, as evidenced by variations in xylem structural traits at both the tissue and pit levels. This optimized their xylem hydraulics in their respective water regimes. Thus, the trade-off between hydraulic efficiency and safety contributes to clear niche differentiation and, thereby, to the coexistence of tree species in the valley with heterogeneous water availability.

A discussion on the kinetic behavior of Ziegler–Natta ethylene polymerization at early moments of the reaction via modeling

AbstractThe gas‐phase ethylene polymerizations with SiO2‐supported Ziegler–Natta (ZN) catalyst at early moments of reaction is modeled. The experimental data used in this work show that at initial stages of the polymerization there is a sharp reduction in the reaction rate and a sharp rise in molecular weight. In the modeling, multi‐active sites assumption and diffusion limitations inside the particle are studied. Energy balance to calculate the temperature at early moment is applied. Kinetic model including initiation, propagation, chain transfers and active site deactivation steps are used to predict the reaction rate and molecular weights. Polymeric flow model (PFM) is applied for single particle model (SPM). The results show two distinct regions. First region with sharp gradient for reaction rate and molecular weights and then, shortly there is a smooth region in which the changes in rate and molecular weights are slow. To fit this sharp gradient followed by a nearly steady state behavior two types of active sites are necessary. A group of highly active sites which deactivated soon and the active sites with lower activity and relatively long‐lasting.

The Interconnected Relationship between Auxin Concentration Gradient Changes in Chinese Fir Radial Stems and Dynamic Cambial Activity

Auxin has been shown to exhibit a striking concentration gradient distribution in radial sections of angiosperm and gymnosperm species, in which peak auxin levels are concentrated in dividing cambial cells, while the absolute auxin concentration sharply declines toward developing secondary phloem and xylem regions. The coincidence of auxin concentration gradient across shoot tissues and xylem cell developmental gradient has prompted that auxin could act as “a plant morphogen” to provide a positional signal for cambial cell development. However, the specific location of vascular cambium and the lack of mutants altering auxin distribution in shoots of woody species made further verification experiments difficult to explore. To address this issue, different concentrations of exogenous IAA were applied to decapitated Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) trunks in this study to induce the change in the auxin concentration gradient in radial stems, and its effects on cambial activities were examined on the physiological, cellular and molecular levels. Our findings manifested that exogenous IAA treatments resulted in vast changes in endogenous hormone concentrations (including IAA, ZR, GA3 and ABA), cambial cell developmental behaviors and transcriptional activities of genes related to polar auxin transport (PAT), auxin signaling, the biosynthesis and signal transduction of other plant hormones and the genetic control of cambial activity. Based on above findings, we postulated a model of auxin concentration gradient involved in the control of cambial activity and secondary growth in tree trunks. In this model, the contrasting expression of AUX1/LAX and PIN family carriers in distinct Chinese fir wood-forming tissues dynamically modulates PAT into the cambial zone adjacent to the secondary phloem side and secondary xylem tissues, resulting in a sharp and wide auxin spatial gradient distribution across shoots in different stages of secondary growth, respectively. This change in auxin concentration gradient distribution in radial sections in turn acts on cambial developmental behaviors by modulating the expression of auxin signaling genes and key transcription factors and the production of other plant hormones in distinct woody tissues. Findings in this study provide important insights for understanding the biological significance of auxin concentration gradient existing in the radial stems of woody species.

Anisotropic response in mechanical behavior of additively manufactured Al–Mn-Sc alloys by in-situ EBSD tensile tests

The anisotropic of microstructure for additively manufactured Al alloys is attributed to sharp temperature gradient during laser melting, resulting in anisotropy of mechanical properties. The correlation of grain orientation, size and aspect ratio on mechanical properties were investigated using in-situ EBSD tensile method on selective laser melting Al–Mn-Sc alloys in longitudinal section and cross section. The lattice rotation behavior and the effects of grain orientation, grain size and aspect ratio on mechanical anisotropy were explored. The predominant <001> orientation of the columnar crystals in the longitudinal section and the <102 > and <101> orientation that dominates the equiaxed crystals in the cross-section had a certain influence on the anisotropy of mechanical properties. The grains with different shapes and sizes exhibited different rotation behavior, and lattice rotation of equiaxed crystals with smaller grain sizes was more complex. By introducing anisotropy coefficients (Ka), the anisotropy sensitivity to grain orientations was revealed. Based on the anisotropy coefficient, grain size and aspect ratio, the mechanical anisotropy in both directions was accurately predicted.

Formation of core-shell austenite as promoted by alloying solutes in hot-rolled medium Mn steel

Recently, there has been a new trend to design chemically heterogeneous austenite by adjusting the various thermal treatment processes in medium Mn steel. In this work, we found that the single or synergistic addition of ferrite stabilizers (e.g., Al) accelerated the heterogeneous distribution of Mn and the formation of a core-shell austenitic structure compared with the addition of an austenite stabilizer (e.g., Cu) under the conventional quenched-annealing process. In addition, the Mn-enriched austenite shell played a significant role in suppressing the martensitic transformation of the Mn-depleted austenite core upon cooling. The sharp Mn gradient distribution corresponded to the heterogeneous kinetics of the deformation-induced martensitic transformation, resulting in the gradual transformation-induced plasticity (TRIP) effect.

Measuring Estuarine Total Exchange Flow From Discrete Observations

The exchange between estuaries and the coastal ocean is a key dynamical driver impacting nutrient and phytoplankton concentrations and regulating estuarine residence time, hypoxia, and acidification. Estuarine exchange flows can be particularly challenging to monitor because many systems have strong vertical and lateral velocity shear and sharp gradients in water properties that vary over space and time, requiring high-resolution measurements in order to accurately constrain the flux. The total exchange flow (TEF) method provides detailed information about the salinity structure of the exchange, but requires observations (or model resolution) that resolve the time and spatial co-variability of salinity and currents. The goal of this analysis is to provide recommendations for measuring TEF with the most efficient spatial sampling resolution. Results from three realistic hydrodynamic models were investigated. These model domains included three estuary types: a bay (San Diego Bay), a salt-wedge (Columbia River), and a fjord (Salish Sea). Model fields were sampled using three different mooring strategies, varying the number of mooring locations (lateral resolution) and sample depths (vertical resolution) with each method. The exchange volume transport was more sensitive than salinity to the sampling resolution. Most (>90%) of the exchange flow magnitude was captured by three to four moorings evenly distributed across the estuarine channel with a minimum threshold of 1-5 sample depths, which varied depending on the vertical stratification. These results can improve our ability to observe and monitor the exchange and transport of water masses efficiently with limited resources.

Flapping and powering characteristics of a flexible piezoelectric nanogenerator at Reynolds number range simulating ocean current

For effective ocean energy harvesting, it is necessary to understand the coupled motion of the piezoelectric nanogenerator (PENG) and ocean currents. Herein, we experimentally investigate power performance of the PENG in the perspective of the fluid–structure interaction considering ocean conditions with the Reynolds number (Re) values ranging from 1 to 141,489. A piezoelectric polyvinylidene fluoride micromesh was constructed via electrohydrodynamic (EHD) jet printing technique to produce the β-phase dominantly that is desirable for powering performance. Water channel was set to generate water flow to vibrate the flexible PENG. By plotting the Re values as a function of nondimensional bending rigidity (KB) and the structure-to-fluid mass ratio (M*), we could find neutral curves dividing the stable and flapping regimes. Analyzing the flow velocities between the vortex and surroundings via a particle image velocimetry, the larger displacement of the PENG in the chaotic flapping regime than that in the flapping regime was attributed to the sharp pressure gradient. By correlating M*, Re, KB, and the PENG performance, we conclude that there is critical KB that generate chaotic flapping motion for effective powering. We believe this study contributes to the establishment of a design methodology for the flexible PENG harvesting of ocean currents.