Subcritical State Research Articles

Noise and spike-time-dependent plasticity drive self-organized criticality in spiking neural network: Toward neuromorphic computing

Self-organized criticality (SoC) may optimize information transmission, encoding, and storage in the brain. Therefore, the underlying mechanism of the SoC provides significant insight for large-scale neuromorphic computing. We hypothesized that noise and stochastic spiking plays an essential role in SoC development in spiking neural networks (SNNs). We demonstrated that under appropriate noise levels and spike-time-dependent plasticity (STDP) parameters, an SNN evolves a SoC-like state characterized by a power-law distribution of neuronal avalanche size in a self-organized manner. Consistent with the physiological findings, the development of SNN was characterized by a transition from a subcritical state to a supercritical state and then to a critical state. Excitatory STDP with an asymmetric time window dominated the early phase of development; however, it destabilized the network and transitioned to the supercritical state. Synchronized bursts in the supercritical state enable inhibitory STDP with a symmetric time window, induce the development of inhibitory synapses, and stabilize the network toward the critical state. This sequence of transitions was observed when the appropriate noise level and STDP parameters were set to the initial conditions. Our results suggest that noise or stochastic spiking plays an essential role in SoC development and self-optimizes SNN for computation. Such neural mechanisms of noise harnessing would offer insight into the development of energy-efficient neuromorphic computing.

Investigation of the free overfall for discharge measurement in stone bed triangular channels

This study utilizes the freefall as a simple and useful tool for measuring end depth discharge EDD in triangular channels having stone beds. For this task, forty-five triangular channel models are constructed and examined all having 360 cm length and end drop fall of 20 cm. In these channel models, three channel side slopes z = 0.7, 1.0 and 1.3, five bed slopes St = 0.00, 0.0033, 0.005, 0.010 and 0.020 and three bed stone diameters ds = 5 mm, 10 mm and 15 mm are tested. The laboratory results demonstrated straight line relationships for the variation of end depth ye with critical depth yc and the variation of end depth with uniform normal depth yn for different values of z, St and ds tested in this study, while, the relation between ye/yc and bed slope St showed a quadratic expression. Moreover, the relationship for the dimensionless discharge with both channel bed slope St and the ratio of end water depth ye to bed stone diameter ds(ye/ds) has been presented by a simple empirical power equation with high determination coefficient which can be applied for subcritical and supercritical states of flow for the range of upstream Froude number Frn between 0.14 and 1.27.

X-Ray Polarimetry of the Accreting Pulsar 1A 0535+262 in the Supercritical State with PolarLight

The X-ray pulsar 1A 0535+262 exhibited a giant outburst in 2020, offering us a unique opportunity for X-ray polarimetry of an accreting pulsar in the supercritical state. Measurement with PolarLight yielded a nondetection in 3–8 keV; the 99% upper limit of the polarization fraction (PF) is found to be 0.34 averaged over spin phases, or 0.51 based on the rotating vector model. No useful constraint can be placed with phase-resolved polarimetry. These upper limits are lower than a previous theoretical prediction of 0.6–0.8, but consistent with those found in other accreting pulsars, like Her X-1, Cen X-3, 4U 1626−67, and GRO J1008−57, which were in the subcritical state, or at least not confidently in the supercritical state, during the polarization measurements. Our results suggest that the relatively low PF seen in accreting pulsars cannot be attributed to the source not being in the supercritical state, but could be a general feature.

A scoping review for building a criticality-based conceptual framework of altered states of consciousness.

The healthy conscious brain is thought to operate near a critical state, reflecting optimal information processing and high susceptibility to external stimuli. Conversely, deviations from the critical state are hypothesized to give rise to altered states of consciousness (ASC). Measures of criticality could therefore be an effective way of establishing the conscious state of an individual. Furthermore, characterizing the direction of a deviation from criticality may enable the development of treatment strategies for pathological ASC. The aim of this scoping review is to assess the current evidence supporting the criticality hypothesis, and the use of criticality as a conceptual framework for ASC. Using the PRISMA guidelines, Web of Science and PubMed were searched from inception to February 7th 2022 to find articles relating to measures of criticality across ASC. N = 427 independent papers were initially found on the subject. N = 378 were excluded because they were either: not related to criticality; not related to consciousness; not presenting results from a primary study; presenting model data. N = 49 independent papers were included in the present research, separated in 7 sub-categories of ASC: disorders of consciousness (DOC) (n = 5); sleep (n = 13); anesthesia (n = 18); epilepsy (n = 12); psychedelics and shamanic state of consciousness (n = 4); delirium (n = 1); meditative state (n = 2). Each category included articles suggesting a deviation of the critical state. While most studies were only able to identify a deviation from criticality without being certain of its direction, the preliminary consensus arising from the literature is that non-rapid eye movement (NREM) sleep reflects a subcritical state, epileptic seizures reflect a supercritical state, and psychedelics are closer to the critical state than normal consciousness. This scoping review suggests that, though the literature is limited and methodologically inhomogeneous, ASC are characterized by a deviation from criticality, though its direction is not clearly reported in a majority of studies. Criticality could become, with more extensive research, an effective and objective way to characterize ASC, and help identify therapeutic avenues to improve criticality in pathological brain states. Furthermore, we suggest how anesthesia and psychedelics could potentially be used as neuromodulation techniques to restore criticality in DOC.

A unified discharge capacity formula of clogged grate inlets

ABSTRACT To investigate the influence of different clogging degrees of a street inlet on its discharge capacity, an experimental campaign with the clogging degree of inlet grate and connection tube varying from 0.0 to 0.75 were conducted in a double-layer flume platform with a horizontal bottom. A total of 608 experimental runs were conducted in a subcritical state, with the inflow discharge and surface water depth varying from 24 L/s to 45 L/s and from 0.022 m to 0.38 m, respectively. To facilitate the application of the unified formula under different clogging scenarios, a parameter termed the clogging decay coefficient was introduced. The relationship between clogging decay coefficient and clogging degree was proposed respectively for the clogging of grate and connection tube. The proposed unified formula has several advantages, including a simple structure, a better accuracy, and without the necessity to select different discharge coefficients.

The sub-critical illusion: synthetic Zeeman effect observations from galactic zoom-in simulations

ABSTRACT Mass-to-flux ratios measured via the Zeeman effect suggest the existence of a transition from a magnetically sub-critical state in H i clouds to a supercritical state in molecular clouds. However, due to projection, chemical, and excitation effects, Zeeman measurements are subject to a number of biases, and may not reflect the true relations between gravitational and magnetic energies. In this paper, we carry out simulations of the formation of magnetized molecular clouds, zooming in from an entire galaxy to sub-pc scales, which we post-process to produce synthetic H i and OH Zeeman measurements. The mass-to-flux ratios we recover from the simulated observations show a transition in magnetic criticality that closely matches observations, but we find that the gravitational-magnetic energy ratios on corresponding scales are mostly supercritical, even in the H i regime. We conclude that H i clouds in the process of assembling to form molecular clouds are already supercritical even before H2 forms, and that the apparent transition from sub- to supercriticality between H i and H2 is primarily an illusion created by chemical and excitation biases affecting the Zeeman measurements.

A spurious-count effect observed in Rossi-α and Feynman-α analysis using fission counter for Kyoto University Reactor

ABSTRACT For a subcritical state of Kyoto University Reactor (KUR), Rossi-α and Feynman-α analyses were carried out using a water-surrounded fission counter assigned as reactor startup channel, where the Rossi-α analysis had a long time-interval range from 0.5 µs to 2 ms and the Feynman-α analysis had a wide gate-width range from 1 µs to 5 ms. The result in a shorter time range than several microseconds shows clearly a rise in a positive correlation amplitude resulting from spurious count, which is a well-known characteristic of fission counter. A time constant of the generating process of the spurious count and a probability of occurrence of a spurious count subsequently to a neutron count can be determined from these analyses. The time constant is around 1 µs, and the occurrence probability is around 0.1%, which is one digit smaller than that evaluated from a previous Feynman-α analysis using a graphite-surrounded fission counter. The considerable difference between the present and the previous occurrence probability indicates that the probability must strongly depend on surroundings of fission counter.

Identification of implicit dynamics of supercritical CO2 invasion in sub-regions of bench micromodels

The ambiguous dynamics of invading carbon dioxide in subcritical and supercritical states, as well as the response of pore-scale resident fluids, play a key role in understanding CO2 capture and storage (CCS) and the corresponding phase equilibrium mechanisms. This paper reveals the implicit dynamics of invading supercritical carbon dioxide (sCO2) in deionized water (DIW)-saturated micromodels using a variant of Lattice-Boltzmann Color Fluid model and descriptive experimental data. The breakthrough time is evaluated by characterizing the displacement velocity, the capillary to pressure-difference ratio, and the transient saturation at a series of micromodels with scaling pore-throats. The recorded sub-regimes are remarkably categorized as oscillatory while the interfacial velocity of sCO2/DIW is jumping into oscillatory magnitudes. Hence, the transient saturation would be significantly accelerated with decreasing pore-throats, demonstrating increased invasion efficiency. Accordingly, a renovated model is established to account for the transient dynamics of invading sCO2 towards efficient geological sequestration.

Experimental study on coal seam permeability enhancement and CO2 permeability caused by supercritical CO2

Permeability is one of the most important parameters for characterizing fluid flow and production from reservoirs. In this paper, experimental studies on the percolation, permeability, and adsorption of supercritical CO2 in coal seams were carried out, taking into account the effects of injection pressure and temperature, comparing the changes in longitudinal wave velocity of specimens before and after the tests, and analyzing the permeability effect of supercritical CO2 on raw coal specimens. The test results showed that when the volume stress was 36 MPa, the permeability of supercritical CO2 in coal increased by 93%, on average, compared with that of CO2. The modified D-R model was used to fit the adsorption data, and it was found that the excess adsorption capacity of supercritical CO2 by coal decreased with increased pressure, with a maximum value of approximately 8 MPa. When the temperature increased by 10°C, the adsorption capacity decreased by 8.3%, on average. In the subcritical CO2 state, the trend of excess CO2 adsorption in coal was consistent with that of absolute adsorption, which was 16% higher than that of excess adsorption, on average. After the action of supercritical CO2, the propagation velocity of the longitudinal wave in the sample decreased significantly, indicating that supercritical CO2 can effectively promote the development of pores and fractures in the coal sample, with an obvious anti-reflection effect on the coal seam and the best permeability enhancement effect at 35°C.

Measurement of delayed neutrons in a thermal nuclear reactor by means of a long run pile noise experiment in sub-critical state

The transfer function of a zero-power thermal reactor was successfully measured thanks to neutron noise techniques from 1 mHz to 160 Hz. During a month-long experimental campaign, the fluctuations of the neutron population in critical and subcritical configurations of the core were acquired using excore fission chambers and analysed through Cross-Power Spectral Density (CPSD) methodology. Firstly, the reactor’s kinetic parameters, i.e. prompt decay constant, effective delayed neutron fraction and generation time, were obtained at critical state. It required calibrating the reactor’s power, which was done by metal foil activation and measurement of the 235U fission rate. Secondly, these parameters were used to estimate the groups’ abundances of delayed neutrons from the CPSD measured in a sub-critical state. Fitting data with a point kinetic model was done with Bayesian inference - CONRAD and Stan programs were used. A very good agreement was found between experimental abundances and the ones computed with TRIPOLI-4 Monte-Carlo transport code and JEFF3.1.1 nuclear data library. Uncertainties on prior abundances between 6 % to 101 %, held mainly by nuclear data, were lowered down to 4 % to 54 %, depending on the delayed group.

Feynman-α Analysis Using BGO Gamma-Ray Detector in a University Training and Research Reactor

Feynman-α analyses for a critical state and several subcritical states of the UTR-KINKI reactor have been carried out using two Bi14Ge3O12 (BGO) gamma-ray detectors free from radioactivation of the scintillator. As a statistical index of the analyses, the covariance-to-mean ratio of gamma counts between these detectors instead of the variance-to-mean ratio of each of the detectors is employed to get rid of a large negative correlation originating from the counting loss of a signal processing circuit. In the gate width dependence of the covariance-to-mean ratio measured at each reactor state, not only a familiar neutron-correlation component but also another small positive correlation between prompt gammas can clearly be observed. The prompt-neutron decay constant α determined considering the positive gamma correlation agrees very well with that obtained from a conventional Feynman-α analysis based on neutron detection. Neglecting the gamma correlation term, the decay constant is much overestimated with an increase in subcriticality, and the maximum overestimation reaches about 24% at a shutdown state with a subcriticality of 1.49%Δk/k.

Steady state bifurcation of a population model with chemotaxis

In this paper, we are concerned with the pattern dynamics of a population model with chemotaxis. The existence of the solution and the global stability of the coexistence equilibrium are performed. In the sequel, we find that chemotaxis can induce the steady state bifurcation of the spatiotemporal model, and there are multiple thresholds of the steady state bifurcation with different assumptions. We then give the amplitude equation to determine the direction of the steady state bifurcation via the multiple time scales. It is found that the population model admits the supercritical or subcritical steady state bifurcation. Numerical results check the validity of the theoretical analysis.

On the nonlinear post-critical responses of VAT sandwich beams with variable stiffness composite skins under axial compression

In this article, the nonlinear post-critical problem of Variable Angle Tow (VAT) sandwich beams with variable stiffness composite skins under axial compression is studied for the first time. The present sandwich beam model is constructed from a modified version of extended high-order sandwich panel theory (EHSAPT), in which the first-order shear deformation theory with von Kármán’s strain–displacement relation is utilized for the skin layer, whereas the high-order theory with linear strain–displacement is applied to characterize the deformation of the core layer. Based on the principal of virtual work, a system of differential equations with variable coefficients is firstly formulated for the postbuckling problem of VAT sandwich beams with variable stiffness composite skins under plane-strain state. Meanwhile, a more efficient Rayleigh–Ritz approach combined with variational formulas is employed to derive the nonlinear algebraic equations in weak forms, and thereafter a sophisticated path-tracing technique is applied to determine the postbuckling equilibrium path. Both super-critical and sub-critical postbuckling states of VAT sandwich beams can be readily captured under the present analytical model framework. The accuracy and effectiveness of the developed Rayleigh–Ritz procedure are validated by comparing against the results available in literature. Effects of variation in skin stiffness on both the critical instability and post-critical behaviours of VAT sandwich beams are discussed in numerical examples. It is found that the change in skin stiffness plays a determinative role in the formation of nonperiodic wrinkling patterns of VAT sandwich beams. In addition, the change in skin stiffness may cause global and local instability loads to be close to each other, and accordingly, the possibility of encountering interactive buckling is increased. The results presented herein may be beneficial for the design of VAT sandwich structures under compression.

Influence of Surface Processes on Strain Localization and Seismic Activity in the Longmen Shan Fold‐and‐Thrust Belt: Insights From Discrete‐Element Modeling

AbstractWe investigated interactions between structural deformation and surface processes in the Longmen Shan fold‐and‐thrust belt and the adjacent western Sichuan foreland basin (WSFB) in the eastern margin of the Tibetan Plateau. The discrete‐element modeling (DEM) method was used to study the influences of the various mechanical properties of the detachments and syn‐tectonic erosion/deposition on the structural evolution of the Longmen Shan and WSFB. DEM simulations demonstrated two stages of surface processes during the Late Cenozoic profoundly influenced thrusting sequences and strain localization in the hinterland and foreland portions of the Longmen Shan. Models indicate that the fold‐and‐thrust belt lacking surface processes propagate in a forward‐breaking manner, whereas those with surface processes develop following an out‐of‐sequence thrusting pattern. We infer that large‐scale erosion propagation from the Sichuan Basin westward to the Tibetan Plateau since Late Cenozoic caused the Longmen Shan hinterland to reach a subcritical wedge state. Tectonic activity retreats to the edge of Plateau, enhancing the rapid uplift of the Longmen Shan and inhibiting the propagation of substantial shortening deformation to the foreland basin. The foreland thrust belt slides stably along the shallow detachment, causing the initiation and growth of the Longquan fault in the leading front. These results explain why both the Longmen Shan hinterland and the western Sichuan foreland thrust belts are currently in a state of simultaneous seismic activity. Our findings offer important implications regarding the seismic potentials of other fold‐and‐thrust belts that interact with dynamic surface processes.

Discrete scale invariance of the quasi-bound states at atomic vacancies in a topological material

Recently, log-periodic quantum oscillations have been detected in the topological materials zirconium pentatelluride (ZrTe5) and hafnium pentatelluride (HfTe5), displaying an intriguing discrete scale invariance (DSI) characteristic. In condensed materials, the DSI is considered to be related to the quasi-bound states formed by massless Dirac fermions with strong Coulomb attraction, offering a feasible platform to study the long-pursued atomic-collapse phenomenon. Here, we demonstrate that a variety of atomic vacancies in the topological material HfTe5 can host the geometric quasi-bound states with a DSI feature, resembling an artificial supercritical atom collapse. The density of states of these quasi-bound states is enhanced, and the quasi-bound states are spatially distributed in the "orbitals" surrounding the vacancy sites, which are detected and visualized by low-temperature scanning tunneling microscope/spectroscopy. By applying the perpendicular magnetic fields, the quasi-bound states at lower energies become wider and eventually invisible; meanwhile, the energies of quasi-bound states move gradually toward the Fermi energy (EF). These features are consistent with the theoretical prediction of a magnetic field-induced transition from supercritical to subcritical states. The direct observation of geometric quasi-bound states sheds light on the deep understanding of the DSI in quantum materials.

Pseudo-optimal discharge pressure analysis of transcritical CO2 electric vehicle heat pumps due to temperature glide

• A pseudo-optimal discharge pressure for electric vehicle heat pumps was proposed. • The principle of pseudo-optimal discharge pressure was explained. • Pseudo-optimal discharge pressure exists in subcritical and transcritical states. • The effects of conditions on pseudo-optimal discharge pressures were revealed. Performance improvement of electric vehicle heat pumps can enhance thermal comfort and reduce the power consumption of electric vehicles, hence stimulating their development. In this paper, the cyclic characteristics of an electric vehicle heat pump were studied under different operating modes and working conditions. Different from the optimal discharge pressure of conventional CO 2 systems which was determined by the variation gradient characteristics of the transcritical isotherm, the discharge pressure of electric vehicle heat pumps at the peak COP value was defined as a pseudo-optimal discharge pressure and was determined by the obvious temperature glide at the outlet of the gas cooler. This pseudo-optimal discharge pressure existed at both transcritical and subcritical states. The difference in the pseudo-optimal discharge pressure between the subcritical or transcritical states was first evaluated. Then an in-depth analysis of the influence of key parameters on the pseudo-optimal discharge pressure was studied to develop an accurate prediction method since the traditional prediction methods were not applicable. The results showed that pseudo-optimal discharge pressure increased with the ambient temperature, inlet air temperature, and supply air temperature. This study provides valuable information for the comprehensive performance optimization of an electric vehicle thermal management system.

Preliminary measurements of transient boundary heat transfer process under supercritical pressures using pixelated phase-shifting interferometry

The boundary heat transfer dynamics of supercritical carbon dioxide (CO2) is vital to its application in solar heater, extraction, refrigeration, heat pumps, power generations, nuclear systems and many other occasions. In this study, an experiment system using pixelated-array masked phase-shifting interferometer has been established for the visualization of temperature of supercritical CO2. The laser diagnostic process has been operated with a carefully designed visualization chamber to give clear transient results of the density and temperature fields for the first time. Those field information has been used to calculate the heat transfer characteristic parameters of the supercritical CO2 fluid in different boundary conditions. The current quantitative visualization analysis by the utilization of pixelated methods is shown capable of giving transient analysis of detailed field evolutions. The results show that (1) small temperature differences are generated in supercritical boundary due to the typical large thermal capacity; (2) supercritical convection boundary is with much quicker thermal quenching than subcritical states, while the subcritical states boundary parameter show localized characteristics while supercritical cases show fluctuation phenomena; (3) the boundary fluctuations heat transfer field exist for supercritical states and the fluctuations affect the overall heat transfer behaviour and the field evolution in supercritical states.

Neuronal avalanche dynamics and functional connectivity elucidate information propagation in vitro.

Cascading activity is commonly observed in complex dynamical systems, including networks of biological neurons, and how these cascades spread through the system is reliant on how the elements of the system are connected and organized. In this work, we studied networks of neurons as they matured over 50 days in vitro and evaluated both their dynamics and their functional connectivity structures by observing their electrophysiological activity using microelectrode array recordings. Correlations were obtained between features of their activity propagation and functional connectivity characteristics to elucidate the interplay between dynamics and structure. The results indicate that in vitro networks maintain a slightly subcritical state by striking a balance between integration and segregation. Our work demonstrates the complementarity of these two approaches—functional connectivity and avalanche dynamics—in studying information propagation in neurons in vitro, which can in turn inform the design and optimization of engineered computational substrates.

Theoretical and Numerical Modeling of Multicomponent Transcritical Diffuse Interfaces Under LRE Conditions

In this study, a theoretical and numerical framework for simulating transcritical flows under a variety of conditions of interest for aerospace propulsion applications is presented. A real-fluid multicomponent and multiphase thermodynamic model, based on a cubic equation of state (EoS) and vapor–liquid equilibrium (VLE) assumptions, is presented to describe transcritical mixtures properties. The versatility of this thermodynamic model is reported since it can represent at the same time the supercritical states as well as subcritical stable two-phase states at equilibrium, via a homogeneous mixture approach. The effect this model has on the evaluation of the thermophysical variables will be emphasized. From the Computational Fluid Dynamics (CFD) point of view, the well-known numerical challenges that arise with the coupling between real-fluid thermodynamics and governing equations under transcritical conditions, are addressed by comparing a fully conservative (FC) to a quasi-conservative (QC) numerical schemes, in the context of the advection problem of a transcritical contact discontinuity.

Acoustic emission behavior of rock materials containing two preexisting flaws and an opening subjected to uniaxial compression: insights into self-similarity, chaotic, and fractal features

To investigate the acoustic emission (AE) self-similarity, chaotic, and fractal features and precursory anomalies of the surrounding rock of underground caverns, uniaxial compression tests were performed on prepared granite specimens containing two preexisting flaws and an opening. Combined with AE observations, and the variations in the self–similarity coefficient, the entropy value, and correlation dimension were analyzed. The results show that the minimum value of the self–similarity coefficient probably indicates the macroscopic fracture of the sample, and the critical recognition mode before specimen failure can be expressed as a “declining-minimum value-rising” type of the self–similarity coefficient. Entropy values ranging from 0.8 to 1.2 can be used as the critical interval to evaluate the mechanical state of the specimen. When the entropy is greater than 0.8, the possibility of sample failure increases, and an entropy larger than 1.2 implies that the sample is at risk of failure at any time. The failure of samples is a process of increasing order and decreasing correlation dimension. The samples have the same identification pattern in the critical fracture state, namely, the correlation dimension “fluctuating-continuing decrease (or drop to the minimum and then rise slightly)”. The continuous decline stage of the correlation dimension can be regarded as the precursor information of sample failure, and the decline to the lowest value indicates that the specimen is in a subcritical or critical failure state. The change in flaw angle affects the curve shape of the three characteristic parameters but does not change their overall evolution pattern.