Current Flow Models Research Articles

Measuring Simplified Pore-Throat Angularity Using Automated Mathematical Morphology

Summary Fluid flow in sedimentary rocks is controlled mainly by the morphology of pore-connecting throats. Pore throats (PTs) typically exhibit diverse converging/diverging morphologies such as biconic, parabolic, or hyperbolic geometries. These different geometries are defined by variable opening angle, or angularity, between the throat walls from the narrowest point of the throat toward the pore body. Importantly, each of these geometries imposes different constraints on fluid flow. However, current pore-level flow models usually favor simple cylindrical or biconic throat morphologies, in part because of the difficulty to extract the throat angularity from pore-space imagery. An image-analysis technique called mathematical morphology has been used to characterize porosity in laterally continuous pore networks (e.g., in sandstones) from thin-section microphotographs. This method allows the extraction of petrophysical parameters such as pore and throat diameters through successive image alterations—namely, erosion/dilation cycles using an expanding structuring element (SE). This study proposes a novel application of this technique and quantifies PT angularity. Angularity can be measured from the throat toward the pore body so that the true geometry—biconic, parabolic, or hyperbolic—can be recognized. The technique is tested on simple geometries to demonstrate the correctness of the mathematic equations involved. Because all equations assume perfect, nonpixelated geometries while images are composed of square pixels, the accuracy of measurements depends strongly on image resolution. Pixelation causes significant fluctuations of ±2 to 10° around the correct angularity values that decrease in amplitude as image resolution increases. Finally, potential implications of this parameter on fluid-flow modeling are discussed.

Empirical selection between least-cost and current-flow designs for establishing wildlife corridors in Gabon.

Corridors are intended to increase species survival by abating landscape fragmentation resulting from the conversion of natural habitats into human-dominated matrices. Conservation scientists often rely on 1 type of corridor model, typically the least-cost model or current-flow model, to construct a linkage design, and their choice is not usually based on theory or empirical evidence. We developed a method to empirically confirm whether corridors produced by these 2 models are used by target species under current landscape conditions. We applied this method in the Gamba landscape between 2 national parks in southwestern Gabon. We collected signs of presence of African forest elephant (Loxodonta cyclotis), forest buffalo (Syncerus caffer nanus), and 2 apes, western lowland gorilla (Gorilla gorilla gorilla) and central chimpanzee (Pan troglodytes troglodytes), on transects. We used patch-occupancy models to identify least-cost and current-flow corridors for these 4 species. On average, 28.7% of current-flow corridors overlapped with least-cost corridors, confirming that the choice of corridor model can affect the location of the resulting linkage design. We validated these corridors by monitoring signs and examining camera detections on new transects within and outside modeled corridors. Current-flow corridors performed better than least-cost corridors for elephants, whereas the opposite was found for buffalo and apes. Locations of the highest priority corridors for the 3 taxa did not overlap, and only 18.3% of their combined surface was common among 2 species. We used centrality metrics to calculate the average contribution of corridor pixels to landscape connectivity and derived an index that can be used to prioritize corridors. As a result, we recommend protecting at least 17.4% of the land surface area around Gamba town to preserve the preferred travel routes of the target species.

Inherent physiological artifacts in EEG during tDCS

Online imaging and neuromodulation is invalid if stimulation distorts measurements beyond the point of accurate measurement. In theory, combining transcranial Direct Current Stimulation (tDCS) with electroencephalography (EEG) is compelling, as both use non-invasive electrodes and image-guided dose can be informed by the reciprocity principle. To distinguish real changes in EEG from stimulation artifacts, prior studies applied conventional signal processing techniques (e.g. high-pass filtering, ICA). Here, we address the assumptions underlying the suitability of these approaches. We distinguish physiological artifacts - defined as artifacts resulting from interactions between the stimulation induced voltage and the body and so inherent regardless of tDCS or EEG hardware performance – from methodology-related artifacts - arising from non-ideal experimental conditions or non-ideal stimulation and recording equipment performance. Critically, we identify inherent physiological artifacts which are present in all online EEG-tDCS: 1) cardiac distortion and 2) ocular motor distortion. In conjunction, non-inherent physiological artifacts which can be minimized in most experimental conditions include: 1) motion and 2) myogenic distortion. Artifact dynamics were analyzed for varying stimulation parameters (montage, polarity, current) and stimulation hardware. Together with concurrent physiological monitoring (ECG, respiration, ocular, EMG, head motion), and current flow modeling, each physiological artifact was explained by biological source-specific body impedance changes, leading to incremental changes in scalp DC voltage that are significantly larger than real neural signals. Because these artifacts modulate the DC voltage and scale with applied current, they are dose specific such that their contamination cannot be accounted for by conventional experimental controls (e.g. differing stimulation montage or current as a control). Moreover, because the EEG artifacts introduced by physiologic processes during tDCS are high dimensional (as indicated by Generalized Singular Value Decomposition- GSVD), non-stationary, and overlap highly with neurogenic frequencies, these artifacts cannot be easily removed with conventional signal processing techniques. Spatial filtering techniques (GSVD) suggest that the removal of physiological artifacts would significantly degrade signal integrity. Physiological artifacts, as defined here, would emerge only during tDCS, thus processing techniques typically applied to EEG in the absence of tDCS would not be suitable for artifact removal during tDCS. All concurrent EEG-tDCS must account for physiological artifacts that are a) present regardless of equipment used, and b) broadband and confound a broad range of experiments (e.g. oscillatory activity and event related potentials). Removal of these artifacts requires the recognition of their non-stationary, physiology-specific dynamics, and individualized nature. We present a broad taxonomy of artifacts (non/stimulation related), and suggest possible approaches and challenges to denoising online EEG-tDCS stimulation artifacts.

Physics of Transcranial Direct Current Stimulation Devices and Their History.

Transcranial direct current stimulation (tDCS) devices apply direct current through electrodes on the scalp with the intention to modulate brain function for experimental or clinical purposes. All tDCS devices include a current controlled stimulator, electrodes that include a disposable electrolyte, and headgear to position the electrodes on the scalp. Transcranial direct current stimulation dose can be defined by the size and position of electrodes and the duration and intensity of current applied across electrodes. Electrode design and preparation are important for reproducibility and tolerability. High-definition tDCS uses smaller electrodes that can be arranged in arrays to optimize brain current flow. When intended to be used at home, tDCS devices require specific device design considerations. Computational models of current flow have been validated and support optimization and hypothesis testing. Consensus on the safety and tolerability of tDCS is protocol specific, but medical-grade tDCS devices minimize risk.

A study of cascading failures in real and synthetic power grid topologies

AbstractUsing the direct current power flow model, we study cascading failures and their spatial and temporal properties in the U.S. Western Interconnection (USWI) power grid. We show that yield (the fraction of demand satisfied after the cascade) has a bimodal distribution typical of a first-order transition. The single line failure leads either to an insignificant power loss or to a cascade which causes a major blackout with yield less than 0.8. The former occurs with high probability if line tolerance α (the ratio of the maximal load a line can carry to its initial load) is greater than 2, while a major blackout occurs with high probability in a broad range of 1 < α < 2. We also show that major blackouts begin with a latent period (with duration proportional to α) during which few lines overload and yield remains high. The existence of the latent period suggests that intervention during early stages of a cascade can significantly reduce the risk of a major blackout. Finally, we introduce the preferential Degree And Distance Attachment model to generate random networks with similar degree, resistance, and flow distributions to the USWI. Moreover, we show that the Degree And Distance Attachment model behaves similarly to the USWI against failures.

Effect of the landscape matrix condition for prioritizing multispecies connectivity conservation in a highly biodiverse landscape of Central Mexico

Implementing and monitoring long-term conservation strategies demands identifying priorities for preserving landscape connectivity. In this manuscript, we present an approach to prioritize areas for preserving landscape connectivity by using the landscape matrix in central-western Mexico and the connectivity for habitat patches considering ensembles of different terrestrial organisms. We aggregated three multispecies connectivity scenarios into a composite corridor scenario. To evaluate which corridors were more important to multispecies connectivity, we used the composite corridor model based on two ways: (1) the contribution of habitat patches that the corridor connects to overall connectivity and (2) the corridor’s capability for facilitating movement across the network of patches. Habitat patches were classified according to their value for the conservation of multispecies connectivity by hybridizing circuit-based and spatial prioritization models for connectivity conservation. We developed current flow models for each species (n = 40) and combined them in four prioritization models corresponding to the three multispecies groups and an all-species group. We found that the corridors having the highest accumulated importance (CI ≥ 58) are located along the protected areas of Pico de Tancitaro and the Monarch Butterfly Biosphere Reserve (Reserva de la Biosfera de la Mariposa Monarca–RBMM, Spanish acronym), which have relatively similar spatial distributions corridors compared to areas with priority for conservation (relative rank test = 0.6). Within those areas, there are permeable sectors with high connectivity retention values that could optimize their ecological function as multispecies corridors. Our approach is applicable to different landscapes, and it allows for identifying priorities for connectivity conservation by integrating landscape elements outside habitat patches.

ROAST: An Open-Source, Fully-Automated, Realistic Volumetric-Approach-Based Simulator For TES.

Research in the area of transcranial electrical stimulation (TES) often relies on computational models of current flow in the brain. Models are built on magnetic resonance images (MRI) of the human head to capture detailed individual anatomy. To simulate current flow, MRIs have to be segmented, virtual electrodes have to be placed on the scalp, the volume is tessellated into a mesh, and the finite element model is solved numerically to estimate the current flow. Various software tools are available for each step, as well as processing pipelines that connect these tools for automated or semi-automated processing. The goal of the present tool - ROAST - is to provide an end-to-end pipeline that can automatically process individual heads with realistic volumetric anatomy leveraging open-source software (SPM8, iso2mesh and getDP) and custom scripts to improve segmentation and execute electrode placement. When we compare the results on a standard head with other major commercial software for finite element modeling (ScanIP, Abaqus), ROAST only leads to a small difference of 9% in the estimated electric field in the brain. We obtain a larger difference of 47% when comparing results with SimNIBS, an automated pipeline that is based on surface segmentation of the head. We release ROAST as an open-source, fully-automated pipeline at https://www.parralab.org/roast/.

Minimal Heating at the Skin Surface During Transcranial Direct Current Stimulation

To assess if transcranial direct current stimulation (tDCS) produces a temperature change at the skin surface, if any change is stimulation polarity (anode or cathode) specific, and the contribution of passive heating (joule heat) or blood flow on such change. Temperature differences (ΔTs) in an agar phantom study and an in vivo study (forearm stimulation) including 20 volunteers with both experimental measures and finite element method (FEM) multiphysics prediction (current flow and bioheat) models of skin comprising three tissue layers (epidermis, dermis, and subcutaneous layer with blood perfusion) or of the phantom for active stimulation and control cases were compared. Temperature was measured during pre, post, and stimulation phases for both phantom and subject's forearms using thermocouples. In the phantom, ΔT under both anode and cathode, compared to control, was not significantly different and less than 0.1°C. Stimulation of subjects resulted in a gradual increase in temperature under both anode and cathode electrodes, compared to control (at t = 20 min: ΔTanode = 0.9°C, ΔTcathode = 1.1°C, ΔTcontrol = 0.05°C). The FEM phantom model predicted comparable maximum ΔT of 0.27°C and 0.28°C (at t = 20 min) for the control and anode/cathode cases, respectively. The FEM skin model predicted a maximum ΔT at t = 20 min of 0.98°C for control and 1.36°C under anode/cathode electrodes. Taken together, our results indicate a moderate and nonhazardous increase in temperature at the skin surface during 2 mA tDCS that is independent of polarity, and results from stimulation induced blood flow rather than joule heat.

Electricity transmission formulations in multi-sector national planning models: An illustration using the KAPSARC energy model

The purpose of this study is to assess the policy-relevant effects of incorporating a more proper representation of electricity transmission in multi-sector national policy models. This goal is achieved by employing the KAPSARC Energy Model (KEM), which is the first publicly available large-scale energy policy model for Saudi Arabia. Past studies using KEM have examined industrial pricing policy, residential energy efficiency, the prospects of power generation technologies, and residential electricity pricing. These studies have shown that under certain fuel pricing scenarios, significant renewable energy capacity is deployed. With large-scale renewable technologies introduced in the power system, representing transmission more appropriately becomes important. By incorporating a direct current optimal flow model, our results show •the optimal investment in utility-scale photovoltaics are 30 percent lower, and the weighted average marginal costs of delivering electricity are 100 percent higher, compared to a model that has a transshipment formulation,•a version of KEM with a single node in each region for transmission and without transmission losses provides valuable insight while keeping the model size tractable,•the market-clearing price of natural gas in a deregulated environment modifies slightly, demonstrating that a transmission component predominantly affects the operations of the power system. Although the scarcity of natural gas increases slightly due to lower PV deployment, its greater use by power generators is minor.

Early Oligocene partial melting via biotite dehydration melting and prolonged low-pressure–low-temperature metamorphism of the upper High Himalaya Crystalline Sequence in the far east of Nepal

Abstract Early Oligocene partial melting and prolonged low-pressure–low-temperature (low-P/T) metamorphism were investigated in migmatites and orthogneisses from the upper High Himalaya Crystalline Sequence (HHCS) in the far east of Nepal. The migmatites were formed by biotite dehydration melting at c. 800°C from 33 to 25 Ma. Cordierite was only produced at shallow crustal levels at pressures <6 kbar. After Early Oligocene partial melting, the low-P/T metamorphism continued until 17 Ma during exhumation of the cordierite-bearing migmatites. Early Oligocene biotite dehydration melting in the upper HHCS occurred at different times and locations from the Early Miocene muscovite dehydration melting in the underlying HHCS and the metamorphic discontinuity was accompanied by thrusting of the High Himalayan Discontinuity at c. 27–19 Ma. Pervasive partial melting and prolonged low-P/T metamorphism in the upper HHCS is more compatible with a lateral southwards channel flow of the upper HHCS along the High Himalayan Discontinuity, whereas current channel flow models explaining the exhumation of the HHCS as driven only by the coupled activity of the Main Central Thrust and South Tibetan Detachment have faced difficulties in explaining the timing of the low-P/T metamorphism observed in the upper HHCS. Supplementary material: The representative cordierite compositions from the cordierite migmatites, the far east of Nepal are available at https://doi.org/10.6084/m9.figshare.c.4068815

Electrochemomechanics in Doped Garnet Lithium-Ion Conductors

Solid electrolytes based on Li7La3Zr2O12 (LLZO) garnet may enable lithium-metal electrodes. The negatively charged crystal sublattices form channels that facilitate cation transport and produce room-temperature ionic conductivity of 0.4 mS/cm [1]. LLZO’s shear modulus is large enough to suppress dendrite nucleation arising from morphological instability [2, 3]. LLZO has cation transference near unity, so diffusion does not limit the current density as it does for liquid electrolytes. Although the modulus and transference of LLZO are both high, a ‘critical current’ is still observed, above which Li dendrites form [4]. Experiments show that preconditioning to improve interfacial contact can raise critical currents significantly [4]. Close inspection by TEM reveals an extended space charge layer [5] at the boundaries, which was predicted theoretically more than 30 years ago [6]. A recent model by Braun et al. [7] shows the importance of Lorentz forces and stress in explaining how apparent screening lengths are stretched for LLZO [5]. The role of mechanical stress in determining the critical current remains unclear. The study of space charging in lithium-ion conductors requires additional consideration of deformation stress, which is beyond the capability of typical Poisson-Boltzmann theory. Moreover, a dynamic model that accounts for the resistive character of the material is needed to rationalize the response when both current and surface charge are significant. Recently, Newman’s concentrated-solution theory was generalized in a thermodynamic consistent way to account for the coupling of all these electrical, mechanical, and electrochemical processes [8]. In this generalized scheme, the momentum balance links mechanical stress, related to thermodynamic pressure, with the Lorentz force, which arises from space charge. Onsager–Stefan–Maxwell multicomponent-diffusion theory brings in irreversible thermodynamics to allow modelling of current flow. We will discuss how this generalized framework can be applied to the study of Li|LLZO|Li cells. This model illustrates cation-concentration, stress, and electric-potential profiles that arise in solid electrolytes as they respond to potential bias and/or Faradaic currents. That information provides clues about the mechanism of mechanical failure that underpins the observations of critical current. We will discuss the key results of a parametric study of important mechanical properties, such as bulk modulus, critical stress, and partial molar volume of the crystal lattice. In addition, LLZO under different doping conditions will be simulated to understand the migration and segregation of doping cations (such as Al3+ and Ga+), and show how the presence of dopants may impact critical currents. Reference: R. Murugan, et al. Angew. Chem 46, 7778 (2007).C. Monroe and J. Newman, J. Electrochem. Soc. 151, A880 (2004).C. Monroe and J. Newman, J. Electrochem. Soc. 152, A396 (2005).A. Shara, et al. J. Power Sources 302, 135 (2016).K. Yamamoto, et al. Angew. Chem 49, 4414 (2010).A. A. Kornyshev and M. A. Vorotyntsev, Electrochim. Acta, 26(3), 303 (1981).S. Braun, et al. J. Phys. Chem. C 119, 22281 (2015).P. Goyal and C. W. Monroe, J. Electrochem. Soc. 164, E3647 (2017).

The influence of grain physicochemistry and biomass on hydraulic conductivity in sand-filled treatment wetlands

The flow of effluent through treatment wetlands is influenced by the infrastructure set-up, the effluent character, the type of hydraulic flow, the mode of operation, the type of substrate, and the type and quantity of biomass. Current flow models have not been well validated, and/or do not accurately account for biomass clogging. In this study, treatment wetlands containing Dune or River sand with similar particle size distributions exhibited significant disparities in achievable flow rates. To gain insight into this phenomenon, further investigations were conducted to compare: (i) sand particle characteristics (size, elemental and mineral composition, grain morphology), (ii) the relationships between mineral composition and shape of the sand particles, (iii) the hydraulic conductivity of the different sand types before and after inducement of biomass growth, and (iv) the measured hydraulic conductivities with those predicted using the fractional packing Kozeny-Carman model.Using automated scanning electron microscopy (QEMSCAN™) it was determined that the shape of the quartz particles of the River sand (98% quartz) and calcite particles of the Dune sand (81% quartz, 18% calcite) were less round and more angular than the quartz particles of the Dune sand, and that the River sand particles were conglomerate in nature and/or fractured. The hydraulic conductivities of the Dune and River sands were significantly different (0.284 and 0.015 mm s−1, respectively), and the hydraulic conductivity of the Dune sand decreased by 51% due to biomass accumulation. The fractional packing model overestimated the measured values.

Decomposition of Settlement Residues Using Loss Compensated DC-OPF Model

This letter presents an approach to decompose the post-settlement residues (SR) into network loss and congestion related components in a loss-compensated (LC) direct current (DC) optimal power flow (OPF) model. The LC-DC-OPF model is presented based on quadratically constrained quadratic programming. The Karush-Kuhn-Tucker conditions of this LC-DC-OPF model are used to derive the decomposed components of SR. The numerical efficiency and accuracy of the model are demonstrated by simulations on five benchmark test systems.

Impact of German Energiewende on transmission lines in the central European region

This paper analyses the impacts of the growth of renewable energy production and German nuclear phase-out on the electricity transmission systems in Central Europe. The principal concern is the significant disparity between the growth of renewable production and the pace at which new transmission lines for the transport of electricity have been built, especially in Germany. This imbalance profoundly endangers the system stability and reliability in the whole region. The assessment of these impacts on the transmission grid is analysed by the direct current load flow model ELMOD. Two development scenarios for the year 2025 are evaluated by 3 representative weeks. The results illustrate the issue from three different perspectives. First, the distribution of loads in the grids is shown. Second, hourly patterns during particular weeks are analysed. Third, a geographical decomposition is made, and problematic regions are identified. The high solar or wind power generation decrease the periods of very low transmission load and increase the mid- and high load on the transmission lines. High solar feed-in has less detrimental impacts on the transmission grid than high wind feed-in. High wind feed-in burdens the transmission lines in the north-south direction in Germany and water-pump-storage areas in Austria.

Incomplete evidence that increasing current intensity of tDCS boosts outcomes

BackgroundTranscranial direct current stimulation (tDCS) is investigated to modulate neuronal function by applying a fixed low-intensity direct current to scalp. ObjectivesWe critically discuss evidence for a monotonic response in effect size with increasing current intensity, with a specific focus on a question if increasing applied current enhance the efficacy of tDCS. MethodsWe analyzed tDCS intensity does-response from different perspectives including biophysical modeling, animal modeling, human neurophysiology, neuroimaging and behavioral/clinical measures. Further, we discuss approaches to design dose-response trials. ResultsPhysical models predict electric field in the brain increases with applied tDCS intensity. Data from animal studies are lacking since a range of relevant low-intensities is rarely tested. Results from imaging studies are ambiguous while human neurophysiology, including using transcranial magnetic stimulation (TMS) as a probe, suggests a complex state-dependent non-monotonic dose response. The diffusivity of brain current flow produced by conventional tDCS montages complicates this analysis, with relatively few studies on focal High Definition (HD)-tDCS. In behavioral and clinical trials, only a limited range of intensities (1-2 mA), and typically just one intensity, are conventionally tested; moreover, outcomes are subject brain-state dependent. Measurements and models of current flow show that for the same applied current, substantial differences in brain current occur across individuals. Trials are thus subject to inter-individual differences that complicate consideration of population-level dose response. ConclusionThe presence or absence of simple dose response does not impact how efficacious a given tDCS dose is for a given indication. Understanding dose-response in human applications of tDCS is needed for protocol optimization including individualized dose to reduce outcome variability, which requires intelligent design of dose-response studies.

A study on the effect of high liquid viscosity on slug flow characteristics in upward vertical flow

Understanding two-phase flow behavior in upward vertical wells is critical for proper well design and operation. Current two-phase slug flow models and their closure relationships are based on experimental data with low viscosity liquids (μL < 20 mPa s). However, recent studies show that two-phase flow behavior of high viscosity liquids is significantly different than that of low viscosity liquids. The objective of this paper is to experimentally and theoretically investigate the effect of high oil viscosity on slug characteristics in upward gas-oil vertical pipe flow. Slug flow characteristics such as translational velocity, slug frequency, and slug length were experimentally measured using a 50.8 mm ID vertical pipe for six different high oil viscosities, namely 586, 401, 287, 213, 162, and 127 mPa s. The experimental results were used to comparatively analyze the physical behavior of flow parameters and evaluate existing mechanistic models and closure relationships, where significant discrepancies are found in prediction of slug frequency and average slug length.Based on a dimensional analysis approach, a new empirical slug frequency closure relationship is developed for high viscosity liquid upward two-phase vertical flow. The new closure relationship is a function of both Froude and viscosity dimensionless numbers. A validation study of the proposed model showed significant improvement in slug frequency prediction for high viscosity when compared with existing slug frequency correlations.

TDCS changes in motor excitability are specific to orientation of current flow

BackgroundMeasurements and models of current flow in the brain during transcranial Direct Current Stimulation (tDCS) indicate stimulation of regions in-between electrodes. Moreover, the folded cortex results in local fluctuations in current flow intensity and direction, and animal studies suggest current flow direction relative to cortical columns determines response to tDCS. MethodsHere we test this idea by using Transcranial Magnetic Stimulation Motor Evoked Potentials (TMS-MEP) to measure changes in corticospinal excitability following tDCS applied with electrodes aligned orthogonal (across) or parallel to M1 in the central sulcus. ResultsCurrent flow models predicted that the orthogonal electrode montage produces consistently oriented current across the hand region of M1 that flows along cortical columns, while the parallel electrode montage produces non-uniform current directions across the M1 cortical surface. We find that orthogonal, but not parallel, orientated tDCS modulates TMS-MEPs. We also show modulation is sensitive to the orientation of the TMS coil (PA or AP), which is thought to select different afferent pathways to M1. ConclusionsOur results are consistent with tDCS producing directionally specific neuromodulation in brain regions in-between electrodes, but shows nuanced changes in excitability that are presumably current direction relative to column and axon pathway specific. We suggest that the direction of current flow through cortical target regions should be considered for targeting and dose-control of tDCS.

Adequacy and security analysis of interdependent electric and gas networks

In this article, adequacy and security assessments on the coupled operations of the electric and gas networks are performed. Extreme operating conditions and fault of components are considered as events that can impact the interdependent systems. The electric and gas networks are represented by an event-based direct current power flow model and by a transient one-dimensional mass flow model, respectively. Furthermore, the automations and safety strategies enforced by transmission system operators are represented within an original modelling approach. A quantitative analysis is performed with reference to the simplified energy infrastructures of Great Britain. Results highlight the contingencies which can jeopardize security and identify the components that are prone to fail and induce large gas pressure instabilities and loss of supply, and the locations in the gas grid that are susceptible to pressure violation. Moreover, a simulated 30% increase of the peak gas demand in 2015 is a limit for safe operations of the gas network, but the coupled systems are robust enough to avoid the spread of a cascading failure across networks. These results allow preventing critical operating conditions induced by the interaction between networks and can guide safety-based decisions on system reinforcements and the development of mitigating actions.

What Effect Does tDCS Have on the Brain? Basic Physiology of tDCS

Transcranial direct current stimulation (tDCS) can effectively modulate a wide range of clinical and cognitive outcomes by modulating cortical excitability. Here, we summarize the main findings from both animal and human neurophysiology literature, which have revealed mechanistic evidence for the acute and neuroplastic after-effects of tDCS. Insights into the magnitude and geometric orientation of transcranially induced currents have been provided by the combination of computational modeling of current flow in animal slice preparations and intracranial recordings in humans. In addition to its synaptic effects, stimulation also induces after-effects on the glial and vascular systems, the latter also observed in humans by magnetic resonance imaging. Several studies have also observed non-linear or antagonistic effects of tDCS parameters, which warrants further systematic studies to explore and understand the basic mechanisms. tDCS is a valuable and promising technique across the neurophysiological, cognitive neuroscience, and clinical domains of research. Primary and secondary effects of tDCS still remain to be completely understood. An important challenge for the field is advancing tDCS protocols forward for optimal intervention and treatment strategies.

Multiscale modeling of localized resistive heating in nanocrystalline metals subjected to electropulsing

Many electrically assisted processes have been reported to induce changes in microstructure and metal plasticity. To understand the physics-based mechanisms behind these interesting phenomena, however, requires an understanding of the interaction between the electric current and heterogeneous microstructure. In this work, multiscale modeling of the electric current flow in a nanocrystalline material is reported. The cellular automata method was used to track the nanoscale grain boundaries in the matrix. Maxwell's electromagnetic equations were solved to obtain the electrical potential distribution at the macro scale. Kirchhoff's circuit equation was solved to obtain the electric current flow at the micro/nano scale. The electric current distribution at two representative locations was investigated. A significant electric current concentration was observed near the grain boundaries, particularly near the triple junctions. This higher localized electric current leads to localized resistive heating near the grain boundaries. The electric current distribution could be used to obtain critical information such as localized resistive heating rate and extra system free energy, which are critical for explaining many interesting phenomena, including microstructure evolution and plasticity enhancement in many electrically assisted processes.