Unstable Regime Research Articles

De Sitter Versus Anti‐de Sitter in Horndeski‐Like Gravity

AbstractThe general solutions of Horndeski‐like gravity that can interpolate between the de Sitter and anti‐de Sitter regimes are presented. In particular, the first‐order formalism with two scalar fields is developed, and considering a black hole ansatz with flat slicing, three different cases, namely exponential, vacuum, and smooth superpotential solutions, with no Minkowski extrema are investigated. Furthermore, these solutions show that a Renormalization Group flow is established, and a turnaround in the warp factor is obtained, where the transition is bounded by the area low. The ideal regimes to trap gravity, which are constructed using the holographic function, which provides stable and unstable regimes to localize gravity are discussed. Finally, it is shown that no ghost appear and that the matter sector that violates the ‐theorem is physical.

Virtual Meter with Flow Pattern Recognition Using Deep Learning Neural Networks: Experiments and Analyses

Summary Operators often require real-time measurement of fluid flow rates in each well of their fields, which allows better control of production. However, petroleum is a complex multiphase mixture composed of water, gas, oil, and other sediments, which makes its flow challenging to measure and monitor. A critical issue is how the liquid component interacts with the gaseous phase, also known as the flow pattern. For example, sometimes liquids can accumulate in the lower part of the pipeline and block the flow completely, causing a gas pressure buildup that can lead to unstable flow regimes or even accidents (blowouts). On the other hand, some flow patterns can also facilitate sediment deposition, leading to obstructions and reduced production. Thus, this work aims to show that deep neural networks can act as a virtual flowmeter (VFM) using only a history of production, pressure, and temperature telemetry, accurately estimating the flow of all fluids in real time. In addition, these networks can also use the same input data to detect and recognize flow patterns that can harm the regular operation of the wells, allowing greater control without requiring additional costs or the installation of any new equipment. To demonstrate the feasibility of this approach and provide data to train the neural networks, a water-air loop was constructed to resemble an oil well. This setup featured inclined and vertical transparent pipes to generate and observe different flow patterns and sensors to record temperature, pressure, and volumetric flow rates. The results show that deep neural networks achieved up to 98% accuracy in flow pattern prediction and 1% mean absolute prediction error (MAPE) in flow rates, highlighting the capability of this technique to provide crucial insights into the behavior of multiphase flow in risers and pipelines.

A Theory for the Balance between Warm Rain and Ice Crystal Processes of Precipitation in Mixed-Phase Clouds

Abstract Mixed-phase clouds contain both supercooled cloud liquid and ice crystals. In principle, precipitation may be initiated either by the liquid phase or by the ice phase. Ice crystals may grow by vapor diffusion to become snow (“ice crystal process”), forming “cold” precipitation. Equally, cloud droplets, when large enough, coalesce to form “warm” precipitation by the “warm rain process.” Warm rain could be supercooled and freeze as “warm” graupel. In the present paper, a new simplified theoretical analysis is provided to examine the microphysical system consisting of three species of hydrometeor, namely, cloud liquid, “cold ice” (crystals, snow), and “warm rain” (frozen or supercooled). This is obtained by nondimensionalizing and simplifying the evolution equations for the mass of each species. Analytical formulas are given for equilibria. Feedback analysis shows that the sign of the feedback is linked to the abundance of precipitation, with a neutral surface in the 3D phase space. The system’s precipitation amount explodes while in the initial unstable regime, crossing the neutral surface and approaching the equilibrium point that is a stable attractor. Positive and negative feedbacks are elucidated. In a standard case, the cold ice mass is about 1000 times larger than the warm rain mass. To illustrate the physical behavior of the theory, sensitivity tests are performed with respect to environmental conditions (e.g., aerosol, updraft speed) and microphysical parameters (e.g., riming and sedimentation rates for cold ice). Cold ice prevails, especially in fast ascent, due to its low bulk density, favoring slow sedimentation and a wide cross-sectional area for riming. Significance Statement The theory elucidates how the ice phase can prevail in the precipitation from any mixed-phase clouds with supercooled cloud liquid and crystals. The ice phase radically suppresses cloud liquid by riming when active and “wins” the competition against coalescence. This prevalence of ice is shown to arise from the low bulk density of snow. The cloud is viewed as a system of negative and positive feedbacks that prevail in realms of stability and instability in a 3D phase space.

Водозаборное сооружение для целей орошения земель в горных условиях Северного Кавказа

Purpose: to develop a water intake structure for land reclamation purpose under the conditions of water withdrawal from mountainous natural water sources. Materials and methods. The paper is based on the theoretical analysis of the river hydrological and geological characteristics, which made it possible to calculate the minimum flow required for the stable operation of water intake structure and guaranteed water withdrawal during the growing seasons. The design parameters of the water intake structure were developed using the method of expert assessment with non-parametric statistics by comparing several dependent variables. Having determined the required qualities for a water intake structure, the design of a reclamation facility adapted for the conditions of the mountain and foothill zones of the North Caucasus was developed. Results. To achieve this goal, a static method for analyzing water intake structures was adapted, and statements with a factorial rating scale for expert assessment were developed. Kendall's concordance coefficient was 0.726, the data were consistent, the significance level P was less than 0.05, the differences between the factors were significant. The layer-by-layer latticed type of water intake structure has maximum efficiency. The design of a bottom-type water intake structure has been developed on the basis of the river hydrological regime and the method of water supply to the system. This type of structure demonstrates high efficiency in conditions of difficult terrain and unstable water source regimes during the growing seasons, ensuring stable water intake with minimal impact on the ecosystem. Conclusions. The proposed design of a water intake structure can be adapted to different types of water resources and their fluctuations, which makes it acceptable for mountainous areas with variable water levels in natural water sources. Minimal impact on the ecosystem is ensured by a high degree of water purification without significant changes in river channels.

Waterfowl and Shorebirds: Formation and evolution of “cold” wintering grounds in Inner Asia (Baikal Siberia)

Despite the widespread occurrence of “cold” wintering of waterfowl, the causes of their formation have been studied very poorly. Very harsh winter conditions are typical for Eastern Siberia, but wintering grounds with a high number of 55.0-60.0 thousand birds are also formed here. Such wintering grounds include the source and upper course of the Angara River (thermal refugium). The high death rate of nests in the summer due to the unstable mountain floodplain water regime of the rivers of the region is compensated by the repeated nesting of birds. As a result, their total breeding season is greatly increased. By the time of migration, the late broods do not have time to accumulate enough plastic substances (fat) necessary for migration to the southern wintering grounds. Favorable conditions at the beginning of winter in thermal refugiums contribute to long delays of birds in feeding and resting places. However, by the time they are able to continue migration, it becomes impossible. Snow falls in the adjacent territories, reservoirs are covered with ice and a constant negative air temperature is established. Consequently, the “cold” wintering of shorebirds and waterfowl is forced. The basis of their species composition is formed by the animal-eating diving duck species. In this regard, their protection and maintenance of numbers in extreme years are difficult and expensive tasks.

Recent near-shore ostracod fauna of the Caspian Sea

The work is based on the E.A.Gofman’s collection of recent ostracods. Here we present summarized data on our results of studying 45 near-shore sampling sites and previously published data, which were the first one to investigate the distribution of recent ostracods in the Caspian Sea and to discuss the influence of such factors as bottom sediments type, water temperature and salinity. Overall, distribution of the ostracods in the Caspian Sea shows a remarkable depth control. The near-shore ostracod assemblage of the Caspian Sea indicates shallow water conditions. Predominant species tolerates temperature and salinity seasonal changes. In northern area assemblage is dominated by Cyprideis torosa and contains abundant euryhaline species tolerant to reduced salinity ranges reflecting the river influence, significant temperature changes and unstable hydrological regime. In the Central and Southern Caspian Sea, the assemblage is mostly represented by the Tyrrhenocythere amnicola donetziensis and other stenohaline species. The analysis of the recent near-shore ostracods of the Caspian Sea allows to conclude that salinity, along with water depth, are the leading factors determining the composition of ostracod assemblages.

TRANSFORMATION OF THE AMUDARYA RIVER BASIN IN THE CONTEXT OF CLIMATE CHANGE

The article assesses the impact of climate change on the water resources of the Amudarya River basin. The Amudarya River is fed by thawed snow and glacial waters. Glaciers have a significant role in the water supply of agriculture, as well as hydropower. Due to the intensive melting of glaciers in the short term, the flow of some rivers will increase. However, in the long term, the runoff will decrease, and some glaciers may disappear altogether. An important task of this study was to take into account the work of researchers working on environmental problems in the Amudarya basin. Since the ecology of the transboundary Amudarya River is in the sphere of national interests of several countries: Tajikistan, Uzbekistan, Turkmenistan and Kazakhstan, certain conditions for cross-border cooperation are being formed. The main problem of cooperation is unstable river regimes, which require continuous planning and negotiations on the allocation of resources, and which are exacerbated by climate change. The main consequences of the influence of climate change on the formation of water resources and water use in the Amudarya River basin are highlighted.

Impact of Schwoebel Barriers on the Step-Flow Growth of a Multicomponent Crystal

The step-flow and spiral growth of a multicomponent crystal are considered from vapors, taking into account the different possible Schwoebel barriers for each component within the Burton-Cabrera-Frank model. Analytic expressions for the final growth rates of such a multicomponent crystal are determined while considering the kinetic properties of all the individual components and growth conditions. Possible instabilities inherent in the presence of several components are studied, and a stability criterion for the multicomponent case is proposed. It is shown that, in certain cases, nucleation of nanoislands of pure components behind the moving steps can initiate, significantly distorting the growth process. The criterion for the occurrence of such an unstable regime is found.

Effect of Adhesion Layer at the Pt/TiO2 Interface on the Degradation Pathways of TiO2-Protected III-V Water-Splitting Photocathodes

Photoelectrochemical (PEC) water splitting has been identified as a promising route towards the production of green hydrogen as a solar fuel to meet the global energy demands [1]. Highly efficient photoelectrodes made of III-V compounds are promising candidates to reach the goal of >15% solar-to-hydrogen efficiency [2]. However, the main bottle neck of these materials is their poor stability in direct contact with acidic electrolytes, which is needed to ensure efficient mass transport and an efficient hydrogen evolution reaction (HER). Atomic layer deposited (ALD) titanium dioxide (TiO2) is commonly utilized as a protective layer to shield the absorber from the electrolyte [3]. To reduce HER overpotentials, the TiO2 film needs to be modified with a catalyst layer. In a demonstration of a TiO2 protected III-V PEC device, Cheng et al. proposed that the detachment of the catalyst layer induced the degradation of the protective TiO2 film in a pH 0 electrolyte which eventually led to the corrosion of the photoabsorber [4].In this work, GaInP/GaAs/Ge photocathodes for water reduction were protected with ALD TiO2 layers deposited from tetrakis(dimethylamido)titanium (TDMAT) and water precursors with varying thicknesses. A 2 nm catalytic Pt layer was deposited via electron beam evaporation (EBE) to reduce the overpotential of the HER. The conformal coverage of the absorber with TiO2 was confirmed with atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). None of the TiO2 films were able to stabilize the illuminated III-V electrodes for 60 min at 0 V versus the counter electrode (CE) in a pH 0 electrolyte (1 M H2SO4). Characterization of the electrochemical potential at the electrode’s front side revealed an unstable regime for the TiO2 protection layer. ICP-MS of the electrolytes, SEM/EDX and electrochemical analyses revealed the detachment of the catalytic Pt layer that led to the dissolution of the TiO2 and the III-V degradation (Figure 1A). The adhesion of Pt on TiO2 was improved by depositing a 2 nm Ti layer via EBE at the Pt/TiO2 interface (Figure 1B). The adhesion layer acts as a wetting layer for Pt, reducing the nucleation energy barrier and increasing the number of nucleation sites compared to Pt/TiO2. The enhanced wetting is due to the formation of Ti−Pt chemical bonds [5] and the overpotential @ 10 mA/cm2 to perform HER was 60 mV, similar to that of a pristine EBE Pt layer. The Ti layer greatly improves the stability of TiO2/III-V photoelectrodes, confirming the results of Cheng et al. and illustrating the importance of a stable catalyst layer.Figure 1. A) SEM micrograph of degradation of Ge/GaAs/GaInP/TiO2/Pt photocathode after 60 min. of photocurrent measurements at 0 V vs. CE in 1 M H2SO4. B) Accelerated stability test in dark for 2 nm Pt/15 nm TiO2 on FTO with (red curve) and without (black curve) a 2 nm Ti adhesion layer in between the Pt and TiO2 layers.

Harmonizing sound and light: X-ray imaging unveils acoustic signatures of stochastic inter-regime instabilities during laser melting

Laser powder bed fusion (LPBF) is a metal additive manufacturing technique involving complex interplays between vapor, liquid, and solid phases. Despite LPBF’s advantageous capabilities compared to conventional manufacturing methods, the underlying physical phenomena can result in inter-regime instabilities followed by transitions between conduction and keyhole melting regimes — leading to defects. We investigate these issues through operando synchrotron X-ray imaging synchronized with acoustic emission recording, during the remelting processes of LPBF-produced thin walls, monitoring regime changes occurring under constant laser processing parameters. The collected data show an increment in acoustic signal amplitude when switching from conduction to keyhole regime, which we correlate to changes in laser absorptivity. Moreover, a full correlation between X-ray imaging and the acoustic signals permits the design of a simple filtering algorithm to predict the melting regimes. As a result, conduction, stable keyhole, and unstable keyhole regimes are identified with a time resolution of 100 µs, even under rapid transitions, providing a straightforward method to accurately detect undesired processing regimes without the use of artificial intelligence.

Scaling of surface roughness in film deposition with height-dependent step edge barriers.

We perform kinetic Monte Carlo simulations of film growth in simple cubic lattices with solid-on-solid conditions, Ehrlich-Schwoebel (ES) barriers at step edges, and a kinetic barrier related to the hidden off-plane diffusion at multilayer steps. Broad ranges of the diffusion-to-deposition ratio R, detachment probability per lateral neighbor, ε, and monolayer step crossing probability P=exp[-E_{ES}/(k_{B}T)] are studied. Without the ES barrier, four possible scaling regimes are shown as the coverage θ increases: nearly layer-by-layer growth with damped roughness oscillations; kinetic roughening in the Villain-Lai-Das Sarma (VLDS) universality class when the roughness is W∼1 (in lattice units); unstable roughening with mound nucleation and growth, where slopes of logW×logθ plots reach values larger than 0.5; and asymptotic statistical growth with W=θ^{1/2} solely due to the kinetic barrier at multilayer steps. If the ES barrier is present, the layer-by-layer growth crosses over directly to the unstable regime, with no transient VLDS scaling. However, in simulations up to θ=10^{4} (typical of films with a few micrometers), low temperatures (small R,ε, or P) may suppress the two or three initial regimes, while high temperatures and P∼1 produce smooth surfaces at all thicknesses. These crossovers help to explain proposals of nonuniversal exponents in previous works. We define a smooth film thickness θ_{c} where W=1 and show that VLDS scaling at that point indicates negligible ES barriers, while rapidly increasing roughness indicates a small ES barrier (E_{ES}∼k_{B}T). θ_{c} scales as ∼exp(const×P^{2/3}) if the other parameters are kept fixed, which represents a high sensitivity on the ES barrier. The analysis of recent experimental data in the light of our results distinguishes cases where E_{ES}/(k_{B}T) is negligible, ∼1, or ≪1.

Groundwater dynamics in intertidal zones of beach aquifers during salt-fingering flow

At aquifer-sea interfaces, submarine groundwater discharge (SGD) and related chemical loads are closely related to groundwater environment in the intertidal zone of coastal aquifers. Although unstable salinity distribution in the intertidal zone associated with salt-fingering flow has been known as a natural phenomenon for some coastal aquifers, a quantitative understanding of groundwater dynamics in the intertidal zone during salt-fingering flow is still lacking. This study investigated the movement features of salt fingers and the response laws of freshwater discharge zone (FDZ) in the intertidal zone, considering the fixed and seasonal inland conditions. The results revealed that during salt-fingering flow, the finger was repeatedly generated and detached from the intertidal zone. Horizontally, the finger moved toward the sea, and the finger speed first increased at the development stage, then remained unchanged at the separation stage, and finally decreased at the mergence stage. The movement speed of salt fingers was closely related to ambient freshwater flow in the FDZ. In the vertical direction, the finger first permeated downward with a decreasing speed, and then the upward discharge of fresh groundwater in the FDZ region with an increased flow velocity pushed the finger upwards with an increasing speed. In contrast to the results in a stable intertidal regime, the width of FDZ at the aquifer-sea interface fluctuated over time, and the variation trends possessed a well agreement with the flux of fresh groundwater discharge. This suggested that the width of FDZ at the aquifer-sea interface can be used as an indicator of fresh groundwater discharge, even in an unstable regime, to provide a basis for field monitoring. The intensity of salt-fingering flow varied over time under the condition of seasonal freshwater inflow. The mean duration of salt fingers decreased slightly with the increase of seasonal freshwater periods due to a decrease in the interface perturbation for the intertidal environment. These findings lay the foundation for gaining a better understanding of the potential implications of salt-fingering flow on SGD and related solute transports.

Modeling of heat transfer at local convection over a vertical throughflow in a two-layered air-heat generating porous system

Convective heat transfer in an air layer partially filled with a heat-generating granular porous medium is studied. There is a slow seepage of air through the layer in the vertical direction with a constant velocity. Equal temperatures are maintained at the outer solid permeable boundaries, and the heat source strength is constant within the porous sublayer and is proportional to the solid volume fraction. The permanent heat generation within the porous sublayer, combined with the vertical throughflow, causes a nonlinear thermal profile which is conducive for convection to occur. The Boussinesq approximation and Darcy's law are used to describe this convection. Numerical solution of the nonlinear convective problem is obtained on the basis of Newton's method. In the limiting case, the numerical data for the onset of convection are compared with the results of the earlier paper of the authors, where a linear stability theory and a method for constructing of the fundamental system of partial solution vectors have been applied, and with the data by other authors. The stationary regimes of local convection, which occurs in an “air – heat-generating porous medium-air” system over the basic vertical throughflow, and its effect on the heat transfer from the porous air sublayer with the growth of supercriticality are studied. It is shown that, depending on the velocity of the basic throughflow (the Peclet number), convection excitation can be both soft (due to supercritical pitchfork bifurcation) and hard (when the loss of stability of the basic throughflow is accompanied by subcritical pitchfork bifurcation that gives rise to an unstable secondary convective regime). This secondary regime is replaced by a stable tertiary convective regime with increasing supercriticality. It has been found that the total heat transfer rate for the upward basic throughflow exceeds that for the downward basic throughflow significantly, and that local convection at any direction of this throughflow increases the heat transfer rate in the system. An increase in the Nusselt number with the growth of supercriticality is recorded. However, a noticeable contribution of local convection to the total heat transfer is observed only when all values of the Pecklet number are negative and its positive values are lower than 2.

Self-adaptive early warning of undesirable gas-liquid flow pattern in offshore oil and gas pipeline-riser system

It is an important function of submarine pipeline flow assurance system to monitor undesirable gas-liquid flow patterns in pipeline in real time, and release early warning ahead of the formation of these flow patterns. In order to tackle the problem of existing method that requires a large training set, and A method of self-adaptive early warning of transition to unstable regime is developed based on transient experiment of gas-liquid two-phase flow performed on a 150-m-long laboratorial pipeline-riser system. The responses of different differential pressure signals were investigated first, so as to pick out the optimum ones used for early warning. Then, the long-short time memory (LSTM) neural network combined with empirical mode decomposition (EMD) was adopted to judge transient condition. In the next step, a series of EMD-LSTM models were trained in order to predict whether undesirable flow pattern would occur, and the warning would be released if the result was ‘yes’. Within the range of experiments, all transient cases were correctly judged. The principles of proposed method were then tested and verified by data from a longer laboratorial pipeline-riser system and an offshore oil field, indicating good adaptability of the method.

Nonequilibrium dynamics of the Jaynes-Cummings dimer.

We investigate the nonequilibrium dynamics of a Josephson-coupled Jaynes-Cummings dimer in the presence of Kerr nonlinearity, which can be realized in the cavity and circuit quantum electrodynamics systems. The semiclassical dynamics is analyzed systematically to chart out a variety of photonic Josephson oscillations and their regime of stability. Different types of transitions between the dynamical states lead to the self-trapping phenomenon, which results in photon population imbalance between the two cavities. We also study the dynamics quantum mechanically to identify characteristic features of different steady states and to explore fascinating quantum effects, such as spin dephasing, phase fluctuation, and revival phenomena of the photon field, as well as the entanglement of spin qubits. For a particular "self-trapped" state, the mutual information between the atomic qubits exhibits a direct correlation with the photon population imbalance, which is promising for generating photon mediated entanglement between two non interacting qubits in a controlled manner. Under a sudden quench from stable to unstable regime, the photon distribution exhibits phase space mixing with a rapid loss of coherence, resembling a thermal state. Finally, we discuss the relevance of the new results in experiments, which can have applications in quantum information processing and quantum technologies.

The late Quaternary palynological record of the northern Yangtze Delta: Implication for palaeoclimate change in East Asia

Marine Isotope Stage (MIS) 3 is the latest interstadial period with unstable climate regimes that provides an indispensable key to better predict future climate change. However, paleoclimate reconstruction in the Yangtze Delta covering MIS 3 remains elusive due to limited archives with reliable age. Here, we derived a 53,000–year–long paleoclimate record from the northern Yangtze Delta, using luminescence age–based palynological data of a coastal sediment core (YZ07). The results show that the climate was relatively warm and wet during ∼53.0–37.5 ka, followed by a slightly cool but wetter condition between ∼37.5 ka and ∼33.2 ka. Afterwards, a cool and dry climate prevailed from ∼33.2 ka to ∼30.7 ka. Three cold events were identified around 48.0 ka, 39.0 ka and 31.0 ka, corresponding to the timings of Heinrich Event (HE 5), HE 4 and HE 3, respectively. From ∼30.7 ka to ∼11.5 ka, broadleaved forests and wetland herbs obviously contracted in the Yangtze Delta region, indicating cold and dry climate conditions. From the late Pleistocene to Holocene, the pollen variations in this core site were linked to coastal geomorphology under climate–induced sea–level changes. Besides, our findings suggest that the percentages of emergent vegetation in this work were negatively correlated with El Niño–Southern Oscillation (ENSO)–linked flood events during the Holocene. Overall, from mid–MIS 3 to MIS 2, the climate exhibits a ‘warm–cool–cold’ pattern in the Yangtze Delta, broadly concordant with other climate proxies in East Asian monsoon domain and beyond on suborbital or millennial timescales. We argue that the northern hemisphere summer insolation may be responsible for the paleoclimate variability and paleovegetation successions in the Yangtze Delta. However, the variability of precipitation in MIS 3 might be governed by interhemispheric insolation forcing, which accounts for a wetter climate phase occurring during late MIS 3 rather than mid–MIS 3.

Jeans Gravitational Instability of a Rotating Collisionless Magnetized Plasma with Anisotropic Pressure

The problem of self-gravitational instability of an astrophysical rotating plasma in a strong magnetic field with an anisotropic pressure tensor is studied on the basis of the Chew–Goldberger–Low (CGL) quasi-hydrodynamic equations modified by generalized polytropic laws. Using the general form of a dispersion relation obtained by the normal-mode perturbation method, a discussion is provided of the propagation of small-amplitude perturbation waves in an infinite homogeneous plasma medium for transverse, longitudinal, and oblique directions with respect to the magnetic field vector. It is shown that different polytropic indices and anisotropic pressures not only change the classical Jeans instability condition but also cause the appearance of new unstable regions. Modified Jeans instability criteria are obtained for isotropic MHD equations and anisotropic CGL equations owing to the influence of the polytropic indices on gravitational and firehose instabilities for astrophysical plasma. It is shown that in the case of a longitudinal mode of perturbation wave propagation, the Jeans instability criterion does not depend on uniform rotation. In the case of the transverse propagation regime, the presence of rotation reduces the critical wave number and exerts a stabilizing effect on the growth rate of the unstable regime.

Emissive cathode immersed in a plasma: plasma–cathode interactions, operation and stability

Thermionic emission from a polycrystalline tungsten emissive cathode immersed in a magnetized plasma column is investigated experimentally and numerically. Electrical and optical measurements of the cathode temperature show a highly inhomogeneous cathode temperature profile due to plasma–cathode interactions. The spatially and temporally resolved cathode temperature profile provides an in-depth understanding of the thermionic electron current, in excellent agreement with experimental data. The plasma-cathode coupling leads to a sharp and heterogeneous rise in temperature along the cathode, which can eventually lead to unstable cathode operation, with divergent current growth. A detailed thermal modeling accurately reproduces the experimental measurements, and allows to quantify precisely the relative importance of heating and cooling mechanisms in the operation of the cathode immersed in the plasma. Numerical resolution of the resulting integro-differential equation highlights the essential role of heterogeneous ohmic heating and the importance of ion bombardment heating in the emergence of unstable regimes. Detailed thermal modelling enables operating regimes to be predicted in excellent agreement with experimental results.

Compressible Large Eddy Simulation of Thermoacoustic Instabilities in the PRECCINSTA Combustor Using Flamelet Generated Manifolds With Dynamic Thickened Flame Model

Abstract The fully compressible, density-based CFD-solver TRACE has been extended for simulations of turbulent reacting flows in aero engine gas turbine combustors. The flamelet generated manifolds combustion model is utilized to account for detailed chemical kinetics and combined with the dynamically thickened flame model to resolve the flame front on the large eddy simulation (LES) mesh. The chemistry tabulation is coupled with the LES solver by inversion of the transported energy equation using tabulated mixture averaged NASA polynomial coefficients. LES of the PRECCINSTA test case, a lean, partially premixed swirl combustor are performed and the two distinctive regimes are correctly predicted: a stable regime with a “quite” stable flame and an unstable regime with an oscillating flame driven by self-excited thermoacoustic instabilities. Statistics collected from the simulations, mean, and root-mean-square values are in good agreement with the experimental reference data for both operating conditions. The dominant frequency of the unstable flame deviates from the measurement by about 100 Hz and requires further investigation. The results demonstrate the general suitability of the simulation framework for reacting flow simulations in gas turbine combustion systems and the prediction of self-excited thermoacoustic oscillations.

Star-disk interactions in the strongly accreting T Tauri star S CrA N

Context. Classical T Tauri stars are thought to accrete material from their surrounding protoplanetary disks through funnel flows along their magnetic field lines. The classical T Tauri stars with high accretion rates (∼10−7 M⊙ yr−1) are ideal targets for testing this magnetospheric accretion scenario in a sustained regime. Aims. We constrained the accretion-ejection phenomena around the strongly accreting northern component of the S CrA young binary system (S CrA N) by deriving its magnetic field topology and its magnetospheric properties, and by detecting ejection signatures, if any. Methods. We led a two-week observing campaign on S CrA N with the ESPaDOnS optical spectropolarimeter at the Canada-France-Hawaii Telescope. We recorded 12 Stokes I and V spectra over 14 nights. We computed the corresponding least-squares deconvolution (LSD) profiles of the photospheric lines and performed Zeeman-Doppler imaging (ZDI). We analyzed the kinematics of noticeable emission lines, namely He I λ5876 and the first four lines of the Balmer series, which are known to trace the accretion process. Results. We found that S CrA N is a low-mass (0.8 M⊙) young (∼1 Myr) and fully convective object exhibiting strong and variable veiling (with a mean value of 7 ± 2), which suggests that the star is in a strong accretion regime. These findings could indicate a stellar evolutionary stage between Class I and Class II for S CrA N. We reconstructed an axisymmetric large-scale magnetic field (∼70% of the total energy) that is primarily located in the dipolar component, but has significant higher poloidal orders. From the narrow emission component radial velocity curve of He I λ5876, we derived a stellar rotation period of P* = 7.3 ± 0.2 days. We found a magnetic truncation radius of ∼2 R* which is significantly closer to the star than the corotation radius of ∼6 R*, suggesting that S CrA N is in an unstable accretion regime. That the truncation radius is quite smaller than the size of the Brγ line emitting region, as measured with the GRAVITY interferometer (∼8 R*), supports the presence of outflows, which is nicely corroborated by the line profiles presented in this work. Conclusions. The findings from spectropolarimetry are complementary to those provided by optical long-baseline interferometry, allowing us to construct a coherent view of the innermost regions of a young, strongly accreting star. The strong and complex magnetic field reconstructed for S CrA N is inconsistent with the observed magnetic signatures of the emission lines associated with the postshock region, however. We recommend a multitechnique synchronized campaign of several days to place more constrains on a system that varies on a timescale of about one day.