Stable Regime Research Articles

Multi-agent Reinforcement Learning for the Control of Three-Dimensional Rayleigh–Bénard Convection

AbstractDeep reinforcement learning (DRL) has found application in numerous use-cases pertaining to flow control. Multi-agent RL (MARL), a variant of DRL, has shown to be more effective than single-agent RL in controlling flows exhibiting locality and translational invariance. We present, for the first time, an implementation of MARL-based control of three-dimensional Rayleigh–Bénard convection (RBC). Control is executed by modifying the temperature distribution along the bottom wall divided into multiple control segments, each of which acts as an independent agent. Two regimes of RBC are considered at Rayleigh numbers $$\textrm{Ra}=500$$ Ra = 500 and 750. Evaluation of the learned control policy reveals a reduction in convection intensity by $$23.5\%$$ 23.5 % and $$8.7\%$$ 8.7 % at $$\textrm{Ra}=500$$ Ra = 500 and 750, respectively. The MARL controller converts irregularly shaped convective patterns to regular straight rolls with lower convection that resemble flow in a relatively more stable regime. We draw comparisons with proportional control at both $$\textrm{Ra}$$ Ra and show that MARL is able to outperform the proportional controller. The learned control strategy is complex, featuring different non-linear segment-wise actuator delays and actuation magnitudes. We also perform successful evaluations on a larger domain than used for training, demonstrating that the invariant property of MARL allows direct transfer of the learnt policy.

Capturing the Variability of the Nocturnal Boundary Layer through Localized Perturbation Modeling

Abstract A single-column model is used to investigate regime transitions within the stable atmospheric boundary layer, focusing on the role of small-scale fluctuations in wind and temperature dynamics and of turbulence intermittency as triggers for these transitions. Previous studies revealed abrupt near-surface temperature inversion transitions within a limited wind speed range. However, representing these transitions in numerical weather prediction (NWP) and climate models is a known difficulty. To shed light on boundary layer processes that explain these abrupt transitions, the Ekman layer height and its correlation with regime shifts are analyzed. A sensitivity study is performed with several types of perturbations of the wind and temperature tendencies, as well as with the inclusion of intermittent turbulent mixing through a stochastic stability equation. The effect of small fluctuations of the dynamics on regime transitions is thereby quantified. The combined results for all tested perturbation types indicate that small-scale phenomena can drive persistent regime transitions from very to weakly stable regimes, but for the opposite direction, no evidence of persistent regime transitions was found. The inclusion of intermittency prevents the model from getting trapped in the very stable regime, thus preventing the so-called ”runaway cooling”, an issue for commonly used short-tail stability functions. The findings suggest that using stochastic parameterizations of boundary layer processes, either through stochastically perturbed tendencies or parameters, is an effective approach to represent sharp transitions in the boundary layer regimes and is, therefore, a promising avenue to improve the representation of stable boundary layers in NWP and climate models.

Decadal Morphological Evolution and Governance Measures of the South Branch, Changjiang Estuary

Estuaries and deltas hold significant socioeconomic importance and immense ecological value due to their dynamic geomorphic processes and unique geographical advantages. However, in recent decades, delta recession and the instability of river regimes have become global challenges, driven by intensive human interventions in upstream river basins and local regions. This study examines the South Branch of the Changjiang Estuary as a typical case to investigate its morphological evolution over the past decades and project future trends, offering suitable solutions to enhance the river regime stability. Analysis of bathymetric data reveals substantial channel–shoal adjustments in the South Branch from 1958 to 2016, characterized by significant erosion and deposition on a decadal scale. After 1997, reduced fluvial sediment supply has led to widespread erosion in the South Branch. Further disturbances at the Baimao Shoal and Biandan Shoal have exacerbated the instability of the river regime. Numerical predictions indicate continued erosion in the South Branch over the next 20 years, accompanied by further channel–shoal pattern adjustments. Hydrodynamic modeling of proposed measures demonstrates an improved flow ratio for the North Baimao Shoal Channel, contributing to enhanced channel–shoal system stability. These integrated governance measures have been incorporated into the latest renovation plan for the Changjiang Estuary. The findings provide valuable scientific guidance for the comprehensive management of the Changjiang Estuary and offer insights applicable to other large estuaries facing similar challenges.

Suppression and excitation by collisions of two-stream and bump-on-tail instabilities

Two-stream (TS) and bump-on-tail (BOT) electron distributions in plasma can lead to electrostatic instabilities and turbulence, and they have been extensively studied. Collisions usually mitigate these instabilities since they tend to hinder the motion of the participating electrons. Here, we numerically solve the full Vlasov–Poisson equations with Krook collisions to reconsider the evolution of the TS and BOT instabilities. It is found that even in the stable parameter regime predicted by linear theory, during the initial evolution (i.e., damping) stage, collisions can excite the TS instability. The reason is that during the evolution, efficient Krook collisions cause rapid thermalization of the TS electrons, leading to broadening of the initial velocity distributions of the two beams and appearance of regimes with unstable velocity gradients and trapped electrons. On the contrary, such a behavior does not occur for the BOT instability.

The human body and the body elements—Conditions for their use in genetics under the French bioethics law and beyond

French legislation has strictly regulated genetic analysis since the adoption of the bioethics laws in 1994. Following a historical perspective, we discuss how French laws have provided a stable legal regime for access to and use of the human body and its elements until the recent revision of the laws. We present the main bioethical principles attached to the human body and their jurisprudential treatment. We balance this approach of stability by showing the strong evolution of the provisions regulating genetics in French laws. Initially based on and centred on the protection of the individual, we present and analyse the new possibilities for using genetic analysis in the name of solidarity with family members and with society as a whole, for research perspectives and for public health prevention.

Actual problems of modeling thermal-hydraulic processes in fast neutron reactors

The results of studies on hydrodynamics and heat transfer processes in fast neutron reactors are presented. Data on turbulent momentum transfer in rod bundles are analyzed. It is shown that the intensification of turbulent momentum transfer in the rod bundle channels is due to large-scale turbulent momentum transfer (secondary currents). The intensification of interchannel turbulent exchange in close-packed lattices of rods is explained. A dependence is obtained for the dissimilarity coefficients of interchannel convective exchange forced by wire wrapping in bundles of rods. The methods and results of numerical modeling of thermal hydraulics using the Monte Carlo method, thermomechanical analysis of the temperature field in fuel rod assemblies in the lifetime process are presented. The results of modeling based on a water model of temperature fields and the structure of coolant movement in the primary circuit of the reactor in various regimes are presented. A stable temperature stratification of the coolant was revealed in the peripheral zone of the upper chamber of the reactor above the side screens. It is shown that the process of boiling liquid metals in fuel assemblies has a complex structure, characterized by stable and pulsating regimes and a heat transfer crisis. The agreement between the results of experimental and numerical modeling is shown. A cartogram of the flow regimes of a two-phase flow of liquid metals in fuel rod assemblies has been plotted. The influence of the surface roughness of fuel elements on the boiling process and heat transfer during boiling of liquid metals is analyzed. Long-term cooling of a fuel assembly with a “sodium cavity” above the reactor core in accident regimes with boiling of liquid metals is shown. The objectives of further research are formulated.

Particle motion and thermal fluctuations of charged AdS black holes surrounded by exotic fluid with modified Chaplygin equation of state

This study explores the intricate relationships between thermodynamic properties and particle dynamics of charged AdS black holes, emphasizing the impacts of charge Q and higher-order corrections, particularly the parameter β, which is a modified Chaplygin gas parameter. Our analysis of entropy reveals that smaller black holes exhibit decreasing entropy variations with increased β, while larger black holes display a complex behavior characterized by initial entropy increases followed by decreases as the horizon radius expands. The Helmholtz free energy analysis suggests that higher-order corrections significantly modify energy behavior, indicating potential phase transitions influenced by gravitational forces, with increasing β values counteracting instability linked to larger charges. Internal energy assessments show an evolving stability of black holes, transitioning from negative to positive values as entropy increases, while also highlighting the destabilizing effects of electric charge. The specific heat analysis illustrates a complex thermal stability regime, where a rise in specific heat with increasing Q correlates with greater stability. However, critical points of instability arise with higher B values, where B is the BH model parameter. Moreover, the trajectories of particles around non-rotating charged black holes reveal that lower parameters result in unbounded motion. In contrast, higher parameters tend to restrict particles closer to the event horizon. Collectively, these findings underscore the complex interplay among charge, higher-order corrections, and stability in black hole thermodynamics, shedding light on the fundamental mechanisms governing black hole behavior and paving the way for future explorations using advanced models and simulations.

Russian Business and the Authoritarian Welfare State

Authoritarian regimes widely involve non-state actors in social welfare to increase output legitimacy and regime stability. Despite the volume of corporate social investments, the role of private companies in authoritarian welfare is still poorly understood. This study analyzes the functioning of corporate social programs in the Russian authoritarian welfare state by focusing on corporate grant competitions and corporate volunteering as means of engaging both the local population and the companies’ employees in the regions. Drawing on stakeholder theory, it investigates how companies position themselves as welfare providers and how they legitimize their role with regard to key stakeholders, including state officials, employees, and the local population. The analysis is based on in-depth interviews with company representatives and key stakeholders in four Russian regions: Kemerovo, Khanty-Mansi Autonomous Region, Tiumen′, and Volgograd. In addition, corporate social reports from 2018 and information for beneficiaries were evaluated to identify program guidelines, motivations, and stakeholder assessments. The study argues that companies play an important role as welfare providers through the financial support of social organizations and employee initiatives at the regional and the local level. By providing social investments in geographically defined “territories of presence,” Russian companies pursue a double legitimation strategy, both demonstrating their social responsibility and loyalty toward the regime and strengthening authoritarian welfare provision through private social investments. With a detailed analysis of corporate social programs, this study contributes to our understanding of the specific conditions and mechanisms of social policy in authoritarian regimes and the place that companies occupy within it.

A Dynamic Model of Authoritarian Social Control

Abstract Authoritarian regimes often use targeted social control - unequal application of the law to limit expressive freedom and enforce social conformity. At the same time, their methods appear less draconian than in the past. In this model, an authority structures punishments and rewards to compel adherence to its preferred norm. The authority’s commitment is time-limited and depends on imperfectly informative signals of a citizen’ s behavior. Given two citizens with the same observed behavior, the authority imposes harsher punishments on the poorer and/or ex ante dissident individual. Lighter punishments are imposed on the wealthier citizen to prevent “overcompliance.” Wealth inequality increases over time. Some citizens become prosperous “lackeys” while others become destitute from confiscation. In stable regimes with high state capacity, the authority reduces punishments and/or increases rewards to allow citizens to accumulate wealth, leading to social conformity and balanced growth in the long run. In unstable regimes with low capacity, the citizenry splits into groups of wealthy lackeys and destitute proles.

Semi-constant source radius and variable stress drop of repeating earthquakes: A fluid-invasion hypothesis for earthquake generation

Summary Earthquakes with very similar waveforms have been recognized as repeating earthquakes, or repeaters, which are interpreted as repeated ruptures of an isolated, frictionally locked asperity patch that are caused by stress loading due to aseismic slip in the surrounding stable regime of the fault. Ideally, the repeated ruptures of the same asperity result in the same source dimension and stress drop. However, significant variations in the stress drop have been observed during fluid-injection experiments even for a single repeater sequence. This study focuses repeater sequences in three different tectonic regimes beneath the Japanese Islands and investigates variations in the source dimension and stress drop in the same repeater sequences. The obtained results show that the variation in the source radius is mostly within 20–30% in individual repeater sequences but the stress drop is largely variable by one order of magnitude or more. We propose a fluid-invasion hypothesis to explain our observations. When pore-fluid pressures gradually invade into an asperity, the edge of the asperity tends to behave as aseismic because of the reduction of the effective normal stress to values below the critical value of effective normal stress, which makes the source radius somewhat smaller. Simultaneously, the amount of the fluid invasion into the asperity fluctuates the shear strength of the asperity, with the stress drop being smaller for higher pore-fluid pressures. The reduced source radius and stress drop expected from the fluid-invasion hypothesis can explain positive correlations between the source radius and seismic moment. We propose a conceptual model for earthquake generation by involving the triggering of aseismic slip by the enhancement of pore-fluid pressures and subsequent rupture of asperity by the stress loading due to the aseismic slip, which can be potentially applied to the genesis of repeaters and non-repeaters.

ELM-free H-mode phase and decoupling of peeling–ballooning stability boundary in the MAST Upgrade tokamak

Abstract A linear magnetohydrodynamic (MHD) peeling–ballooning stability analysis of the edge-localized mode (ELM)-free phase of a MAST Upgrade (MAST-U) H-mode plasma is presented. In contrast to other similar discharges, #47018 is found to have a significantly higher and wider pedestal during its ELM-free H-mode phase that lasts for approximately 80 ms; this is made possible by the reduced core MHD mode activity on the q = 2 surface. During this period, there is sustained decoupling of peeling and ballooning branches of the stability boundary on J–α space, opening an access channel to the second stability regime with higher peaks in pedestal current density J N , ped and pressure gradient (α). Such decoupling of the stability boundary has not previously been observed in MAST-U H-modes, and if such a condition can be readily reproduced, it opens a wide range of opportunities for MAST-U to explore low-collisionality peeling-limited pedestal regimes as well as advanced scenarios such as quiescent H-modes that are relevant to future reactors such as STEP and ITER.

Propaganda and thought work in the Mao era: Absoluteness, sharp transitions, and political instability

AbstractThis paper elucidates the reasons behind Deng Xiaoping's initiation of political reform from a propaganda perspective by examining the features of Mao's propaganda and thought work and their implications for the Chinese Communist Party, analyzing cases such as the “Two‐Hundred Policy,” the “Anti‐Rightist Campaign,” propaganda campaigns related to the USSR, and the transformation of Lin Biao's image. It argues that Mao's propaganda and thought work, characterized by their absoluteness and sharp transitions, were detrimental to the stability of the communist regime. These characteristics led to widespread indifference, distrust, and even resistance toward the Chinese Communist Party among the Chinese populace since the late Mao era. China had entered a post‐trust era, marked by a growing tendency among the populace to preemptively view official messages as untrustworthy. To restore the party's credibility, Deng Xiaoping initiated reforms in propaganda and thought work to address these shortcomings.

Numerical investigation of multiphase flow through self-affine rough fractures

Multiphase flow through fractures has great significance in subsurface energy recovery and gas storage applications. Different fracture and flow properties affect flow through a fracture which is difficult to control in laboratory experiments. Here, we perform lattice Boltzmann simulations in an ensemble of synthetically generated fractures. Drainage simulations are performed at different capillary numbers, wettability, and viscosity ratios. We track the invading front and quantify breakthrough saturations and show that roughness and wettability have a strong effect on fluid invasion through a complex fracture. Invading a more viscous fluid results in more stable displacement regardless of the capillary number while at very low capillary numbers, fluid migration is dependent on the inherent structure of the fracture. We develop a fluid displacement phase diagram in a single rough fracture and compare our results from that in the literature. Finally, we extend the phase diagrams across multiple fractures and demonstrate the importance of natural fracture features of roughness and wettability in identifying stable versus unstable displacement regimes during multiphase flow through rough fractures. Our work presents an end-to-end numerical pathway for testing on experimental data and expanding numerical data sets for testing combinations of different physical phenomenon and make valuable predictions on fluid flow through rough fractures.

Observation of self-excited oscillations in DC glow discharge dusty plasma

We present a detailed experimental study of discharge current oscillations in a planar cathode plasma with poly dispersed alumina dust particles. Our experimental measurements suggest that surface density of dust particles play an important role together with cathode spots, in deciding self excited oscillating regime or stable dust cloud regime. Experiments conducted with dust particles density below the critical limit shows self excited oscillations. Operating above the critical limit, a dust cloud levitated in the sheath region was observed.

Seven Decades of Surface Temperature Changes in Central European Lakes: What Is Next?

Lakes are vital components of the hydrosphere, holding both environmental and economic significance. In recent times, they have undergone transformations in one of their key characteristics—water temperature. Assessing the scale and pace of these changes depends on the length and accuracy of the available data. This study focuses on the two lakes in Poland (Białe Augustowskie and Studzieniczne) with the longest continuous water temperature records, ranging from 1954 to 2023. The results reveal a relatively stable thermal regime until the late 1980s (with changes that were statistically insignificant) and a significant shift over the past three decades, during which the water temperature increased at a rate of 0.5 °C per decade. Importantly, simulations indicate further warming of the water by the end of the 21st century. Depending on the chosen climate change scenario, the warming of both lakes is expected to continue, with the Shared Socioeconomic Pathways (SSP585) scenario projecting a steady increase of 0.5 °C per decade. Given the fundamental importance of water temperature in determining factors such as water quality, these future changes present a significant challenge for water management authorities in terms of maintaining and managing these ecosystems.

Influence of water saliny changes on the operation of the RAS biological treatment system under low temperature conditions

In fish-breeding circulation systems the change-over of a biofilter from operation in fresh water conditions to sea water conditions (or vice versa) requires a smooth gradual transition. However, a one-time water change is feasible faster and technologically easier. This determined the relevance of the study. The goal was to determine the influence of one-time water salinity changes (sea to fresh and fresh to sea) on the ability of the activated sludge biocenosis of biofilters to maintain or restore its activity in a cold-water RAS. Six model biofilters were filled with sea water and six with fresh water. With a simultaneous decrease of water temperature from 21 to 9 °C, at the first stage a start-up period of biological treatment was carried out. At the second stage — in a stable operating regime, sea water was simultaneously changed to fresh water, and fresh water to sea water. Concentrations of ammonium, nitrites and nitrates were determined at the start of experiment, then on the next day after a bacterial culture and ammonia as a source of ammonium nitrogen adding, and then — 2 times a week. A steady decrease in the concentration of ammonium in water to 0.2 mg/l, nitrite — to 0.3 mg/l was considered as the indicator of the end of each stage. The duration of the start period of biological treatment in both fresh and sea water was approximately the same and amounted to 138 and 134 days, respectively. With the simultaneous change of sea water to fresh or fresh water to sea, the complete destruction of the biofilter activated sludge biocenosis didn’t happen, that indicated by the continuing increase of the nitrates concentration. However, the ability of the biofilter biocenosis to purify circulating water from ammonia nitrogen and nitrites naturally decreases with the following gradual performance restoration. The adaptation of biofilter biocenosis to work under conditions of rapid desalination was 1.5 times faster than the adaptation of biocenosis to rapid salination.

Spatiotemporal patterns in a 2D lattice of Hindmarsh–Rose neurons induced by high-amplitude pulses

We present numerical results for the effects of influence by high-amplitude periodic pulse series on a network of nonlocally coupled Hindmarsh–Rose neurons with 2D geometry of the topology. We consider the case when the pulse amplitude is larger than the amplitude of oscillations in the autonomous network for a wide range of pulse frequencies. An initial regime in the network is a spiral wave chimera. We show that the effects of external influence strongly depend on a balance between the pulse frequency and frequencies of the spectral peaks of the autonomous network. Except for the destructive role of the pulses, when they lead to loss of stability of the initial regime, we have also revealed a constructive role. We have found for the first time the emergence of a new type of multi-front spiral waves, when the wavefront represents a set of several close fronts, and the wave dynamics are significantly different from common spiral waves: neurons oscillate independently to the wave rotation, the rotation velocity is in many times less than for the common spiral wave, etc. We have also discovered several types of cluster spatiotemporal structures induced by the pulses.

Political stability in authoritarian regimes: the case of Central Asia

ABSTRACT Existing research indicates that political stability is a prerequisite for good governance and economic development in Central Asia. All five countries in the region are authoritarian regimes but it is unclear which factors impact on political stability. Using data for the period 2002–2022, this paper examines the variables which make a significant contribution to political stability and finds that the rule of law, personalist rule, a free media, freedom of assembly, political polarization and gross domestic product (GDP) growth, respectively have a positive effect on political stability while government accountability, exclusions by social group and civil society participation have a negative effect. Some of the findings are at odds with other studies on autocratic regimes, illustrating the importance of geo-political context.

Wrinkling of differentially growing bilayers with similar film and substrate moduli

Growth-induced surface wrinkling in constrained bilayers comprising a thin film attached to a thick substrate is a canonical model for understanding pattern formation in many biological systems. While the bilayer model has received much prior attention, the nonlinear behaviour for arrangements with similar film and substrate properties, or substrate growth that outpaces film growth, remains poorly understood. This paper therefore focuses on these cases in which the substrate’s elasticity dominates surface wrinkling. By combining analytical modelling and finite element simulations incorporating advanced path-following techniques, we characterise critical wrinkling states and explore the initial and advanced post-buckling behaviour for a wide range of film-to-substrate modulus and growth rate ratios. It is shown that the classical leading-order asymptotic expression for the critical strain is not of sufficient accuracy for film-to-substrate modulus ratios below 50, and higher order correction terms are presented. Leveraging a weakly nonlinear analytical model, we introduce a phase diagram categorising stable and unstable post-buckling regimes. Advanced path-following simulations are employed to unveil the evolution of wrinkling patterns, from initial sinusoidal instabilities to complex formations involving period doubling, quadrupling, and eventual surface creasing. These comprehensive phase diagrams, parameterised by film-to-substrate modulus and growth rate ratios, provide new insights into the rich dynamics of surface wrinkling. Finally, we demonstrate the existence of multi-stability in the advanced post-buckling regimes for bilayers experiencing substrate-dominated growth. This work contributes to the understanding of the mechanics underlying pattern formation in growing bilayers over the entire parameter regime, with potential implications for explaining biological morphogenesis and informing the development of novel diagnostic tools and artificial flexible electronic skins.

Influence of flame stability on iron oxide nanoparticle growth during FSP

Flame stability during flame spray pyrolysis (FSP) remains an active topic of investigation due to its impact on synthesised particle attributes and purity. The unique feature of the burner investigated here is the ability to control flame stability over newly defined stability maps. The novelty of the current work lies in understanding the influence of these broad stability modes on nanoparticle growth during FSP and on the attributes of collected products. Several distinct flame configurations are selected for iron oxide nanoparticle synthesis, ranging from stable to highly unstable flames. The flame stability regimes are characterised by OH∗ chemiluminescence and broadband flame luminescence imaging. Stability is correlated with the coefficient of variation of flame luminescence (CV) and flame height with mean OH∗ chemiluminescence. Planar Mie scattering is then used to identify the effect of flame luminescence intermittency on spray atomisation and evaporation quality. For particle analysis, in-situ thermophoretic sampling is performed from 30 to 200 mm above the burner exit plane and analysed via transmission electron microscopy (TEM). Further ex-situ analysis is also performed on the bulk-collected product via high-resolution TEM, X-ray powder diffraction (XRD), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). It is demonstrated that flames with higher instability (CVmin ≥ 0.35) maintain increased spray heights (>26 %) and reduced flame heights (>79 %) compared to stable flames with the same precursor volume flowrate. This reduces the high-temperature particle residence time for primary particle growth and impacts subsequent agglomeration. For example, the mean diameter of gyration and primary particle diameter are found to vary by 44 % and 29 % depending on the flame regime, respectively. Ex-situ analysis also demonstrates that the dominant iron oxide phase produced is maghemite regardless of the stability regime. However, higher concentrations of organic impurities including methyl, methylene and carboxylate functional groups are found via ATR-FTIR with increased flame instability (CV).