Adiabatic Index Research Articles

Isotropic compact stars admitting Heintzmann solution in Rastall gravity

This paper is devoted to observe the physical attributes of static spherically symmetric isotropic compact stellar candidates in the context of Rastall theory of gravity. In order to inspect the structural composition of compact objects, the Heintzmann ansatz is taken into account. The unknown parameters associated with Heintzmann ansatz are evaluated through matching conditions with derived values of radii and masses of some specific star models. The consistency of the adopted ansatz is analyzed via graphical interpretation of matter variables, equation of state parameter, mass components, energy constraints, causality condition, stellar equation and adiabatic index for several choices of Rastall parameter. It is observed that the stars under consideration manifest stable structures corresponding to Heintzmann ansatz in this framework. Moreover, it is shown that for vanishing Rastall coupling parameter, the standard outcomes of general relativity can be retrieved.

Heat supply to and hydrogen desorption from magnesium hydride in a thermally insulated container with hot gas flow

We experimentally studied hydrogen desorption from MgH2 by supplying heat via a hot gas flow. Porous sheets of MgH2 held in a sponge-like carbon nanotube (CNT) matrix were developed and placed in a coaxial double-tube reactor. Ar gas was heated using a cylindrical heater and then flowed alongside the MgH2-CNT sheets, thereby increasing the temperature of the sheets and enabling the desorption of hydrogen into the gas flow. The total energy efficiency was approximately 6.2% when 64% of hydrogen in MgH2 was desorbed. A numerical simulation was conducted for the heat transfer and hydrogen desorption, and the obtained results were consistent with the experimental results. According to the simulation, the low energy efficiency was attributed to the small heat capacity ratio of MgH2 to the reactor (0.082) and considerable radiative heat loss (53%). The simulation was used to predict energy efficiency improvements, and the efficiency was considered to increase to 12% upon increasing the heat capacity ratio from 0.082 to 1.4, and further to 21% upon doubling the Ar flow rate, which enhanced the convective heat transfer from the heater to MgH2.

Impact of [formula omitted] theory on the geometry of charged stellar objects

The main objective of this article is to study the viable charged compact stellar structures in f(R,φ,χ) theory, where R, φ and χ denote the Ricci scalar, scalar field and kinetic term, respectively. For this purpose, a static spherical metric with anisotropic matter configuration is considered to analyze the compact stellar structures. We consider a particular model of this modified theory to formulate the explicit expressions of energy density and pressure components that govern the relationship between matter and geometry. The unknown constants in the Finch–Skea solutions are determined by the smooth matching of interior and exterior spacetimes to analyze the configuration of the proposed stellar structures. Physical parameters such as matter variables, energy bounds and equation of state parameters are analyzed to examine the viability of the considered stellar objects. Further, we use sound speed and adiabatic index methods to examine the stability of the compact objects. The rigorous analysis and satisfaction of necessary conditions lead to the conclusion that the stellar objects studied in this framework are viable and stable in the presence of charge.

Numerical study on heat transfer characteristics of regenerator based on a simplified geometry

The heat transfer characteristics of regenerator in regenerative coolers such as pulse tube and Stirling coolers is important. Various methodologies of characterizing the Nusselt number between the regenerator matrix and working fluid have been established and applied. However, most of the existing formulas are based on the study of steady flow or oscillating flow with low pressure ratios at above ambient temperature. In this paper, a tube model with finite wall thickness was constructed to numerically simulate simplified #400 SS mesh and packed HoCu2 sphere, with the aim of investigating the Nusselt number in regenerators under various flow conditions, including different Valensi numbers and temperature ranges. The results provide more evidence to support that within the operating conditions, including Valensi numbers and pressure ratios typically used in coolers, the Nusselt number of oscillating flow and steady flow are similar. Additionally, a heat transfer deterioration effect is observed below 40 K, which is related to the heat capacity ratio between the solid matrix and working fluid. Based on these findings, the study emphasizes the importance of further investigating the heat transfer characteristics of oscillating flow at low temperatures.

A comparative study of embedding class one, conformally flat, vanishing complexity factor and conformally symmetric solutions and their impacts on compact star structure

For the first time, we have presented in detail the comparative studies of embedding class one (CO), conformally flat (CF), vanishing complexity factor (COM), and conformally symmetric (CS) solutions, which are the easiest way of exploring new solutions of the field equations. All these solutions simplify the two metric potentials problem to one metric potential through specific bridge equations. To compare on the same footing, we have assumed the same grr metric function and solved the gtt metric functions via the bridge equations. We then compare all the physical quantities like density, pressure, anisotropy, adiabatic index, etc. To check which type of matter is appropriate for these solutions, we have plotted the equation of states (EoSs) and found the best-fitted functions. We have found that CO solution obeys quadratic EoS, the COM solution best fits with linear EoS, CF solution contains normal and exotic matters, while the CS solution follows a cubic polynomial, and also includes normal, stiff, and exotic matters. In the end, we have plotted the M−R curves and fitted them with observed masses of a few neutron stars to predict their radii.

Morphology and Mach Number Dependence of Subsonic Bondi–Hoyle Accretion

Abstract We carry out three-dimensional computations of the accretion rate onto an object (of size R sink and mass m) as it moves through a uniform medium at a subsonic speed v ∞. The object is treated as a fully absorbing boundary (e.g., a black hole). In contrast to early conjectures, we show that for an accretor with R sink ≪ R A = 2 Gm / v ∞ 2 in a gaseous medium with adiabatic index γ = 5/3, the accretion rate is independent of Mach number and is determined only by m and the gas entropy. Our numerical simulations are conducted using two different numerical schemes via the Athena++ and Arepo hydrodynamics solvers, which reach nearly identical steady-state solutions. We find that pressure gradients generated by the isentropic compression of the flow near the accretor are sufficient to suspend much of the surrounding gas in a near-hydrostatic equilibrium, just as predicted from the spherical Bondi–Hoyle calculation. Indeed, the accretion rates for steady flow match the Bondi–Hoyle rate, and are indicative of isentropic flow for subsonic motion where no shocks occur. We also find that the accretion drag may be predicted using the Safronov number, Θ = R A /R sink, and is much less than the dynamical friction for sufficiently small accretors (R sink ≪ R A ).

Capacity configuration and economic analysis of integrated wind–solar–thermal–storage generation system based on concentrated solar power plant

As the proportion of wind and photovoltaic power plants characterized by intermittency and volatility in the electric power system is increasing continuously, it restricts the development of renewable energy at a higher proportion and a larger scale. CSP has the dual functions of peak-regulating power supply and energy storage, which can provide important support for grid access and power regulation for renewable energy power. In this study, the capacity configuration and economy of integrated wind–solar–thermal–storage power generation system were analyzed by the net profit economic model based on the adaptive weight particle swarm algorithm. A case study was conducted on a 450 MW system in Xinjiang, China. The effects of heat storage capacity, capacity ratio of wind power and photovoltaic to molten salt parabolic trough power generation on the economy of the integrated power generation system were obtained under the condition that the system output should meet the power load. The results show that when the capacity ratio is fixed and the heat storage capacity increases, the net profit of the system increases first and then decreases. When the heat storage capacity is fixed, the net profit of the system increases with the increase of the capacity ratio, but when the capacity ratio exceeds 5:1, the upward trend is gradually slowed down. When there are different heat storage capacities and capacity ratios, the lowest peak-shaving electricity prices corresponding to the net profit break-even point of the integrated system are obtained.

Discussion of singularity-free embedding stellar structures in f(R) gravity utilizing scalar potential

In this paper, our purpose is to investigate the anisotropic stellar structure caused by [Formula: see text] modified gravity, where [Formula: see text] represents the Ricci scalar and [Formula: see text] represents the scalar potential. This study emphasizes the impact of static spherically symmetric stellar structures using anisotropic distribution. Furthermore, an anisotropic matter source is employed to explore the stability of star formations. To get the value of unknown parameters of the compact structures, we compare the Schwarzschild metric to external geometry. Moreover, we examine the physical attributes of compact objects by presuming a viable [Formula: see text] gravity model. We analyze the graphical behavior of density and pressure, the Tolman–Oppenheimer–Volkov equation, energy conditions, mass function, surface redshift, and adiabatic index. It is recognized that all the obtained results deliver emphatic evidence for the stability of our considered realistic stars.

Spherically symmetric wormhole solutions admitting Karmarkar condition

This paper investigated the viable traversable wormhole solutions through Karmarkar condition in the context of f(G,T) theory. A static spherical spacetime with anisotropic matter configuration is used to study the wormhole geometry. Karmarkar condition is used to develop a viable shape function for a static wormhole structure. A wormhole geometry is constructed using the resulting shape function that satisfies all the required conditions and connects the asymptotically flat regions of the spacetime. To assess the viability of traversable wormhole geometries, the energy conditions are analyzed by various models of this theory. Further, their stable state is investigated through sound speed and adiabatic index. This investigation demonstrates the presence of viable traversable wormhole solutions in the modified theory.

Safe Operating Conditions of isoperiodic Homogeneous Semi-Batch Reaction Based on Parameter Uncertainties Analysis

Proper operating conditions have a significant impact on the process state of semi-batch reaction. However, various error and bias lead to the uncertainties of parameter, which affect process safety of the semi-batch reaction. In this work, the effect of dimensionless parameters dimensionless adiabatic temperature rise (), Westerterp number (Wt), , dimensionless activation energy (γ), volumetric heat capacity ratio (RH) and relative volume increase (ε) on the temperature of the isoperiodic homogeneous semi-batch reaction are investigated by local sensitivity analysis. The sensitivity of the dimensionless maximum temperature to global parameters is analyzed using the scatter plot method based on Monte-Carlo sampling. Parameter sensitivity results are further quantitatively calculated by the eFAST method. The results indicate that there is a strong linear relationship between the maximum reaction temperature and △τad,0, which exhibits the highest sensitivity index compared to the other parameters. The safety boundary diagram and adiabatic temperature diagram are constructed by considering parameter uncertainty and the effect of parameter uncertainty on the critical criteria is analyzed.

Неоднозначность решений для ударно-волновых структур, образующихся в высокоскоростных потоках газа с малым показателем адиабаты

We considered branched shock-wave structures (triple configurations of shock waves) which forms in supersonic flows of perfect gas, mainly at high flow Mach numbers and low values of the adiabatic index. Ambiguity of the solution for triple configurations of steady shock waves formed at Mach reflection, including for configurations with a negative slope angle of the reflected shock, was studied analytically and numerically. The conditions for the coexistence of triple configurations which correspond to Mach reflection with other shock-wave structures are derived and graphically demonstrated.

Charged gravastars with conformal motion in the Finslerian space-time

In this article, we investigate the charged gravastar under conformal motion with the background of Finsler geometry. Mazur and Mottola pioneered the concept of the gravastar (gravitational vacuum star) for the first time. This vacuum object consists of three distinct regions, that is, (i) interior de Sitter region, (ii) thin shell consisting of ultrarelativistic stiff, and (iii) exterior vacuum Schwarzschild region. The nature of these regions can be analyzed by considering different equations of state parameters. We have studied various physical features of the gravastar, such as length, energy, entropy, stability, and the adiabatic index, both graphically and analytically within the Finslerian framework. Also, we have obtained the exact and non-singular solution for the gravastar model.

Characterizing and visualizing the direct injection of hydrogen into high-pressure argon and nitrogen environments

Using argon as the working fluid in an internal combustion engine holds the potential of substantially enhancing thermal efficiency because of its high specific heat ratio. Burning hydrogen, not with air but with pure oxygen, in a closed-loop argon power cycle would lead to an emission-free exhaust composition, effectively containing only water and argon, which can be separated by condensation. One of the current challenges is the high-pressure direct injection of both fuel and oxidizer, which directly controls the combustion process. To support the development of such an injection strategy, high-pressure injections of hydrogen into pressurized argon and nitrogen are investigated using high-speed Schlieren and pressure transducers in a non-heated constant-volume chamber at varying conditions. In this work it is shown that hydrogen mass flow increases linearly with injection pressure and that it is unaffected by chamber pressure as expected based on choked flow theory, while jet penetration and cone angle are determined by the pressure ratio between the fuel line and the ambient. A correlation is presented between jet penetration and pressure ratio for argon or nitrogen environments at room temperature.

Numerical investigation in effects of refrigerants’ thermodynamic properties on performance of supersonic ejector

ABSTRACT Using zeotropic mixtures with unique temperature glide characteristics as the working medium in heat-driven ejector refrigeration system will potentially reduce the irreversible losses in heat exchange process. However, the intrinsic link between the pure/mixture refrigerants’ thermophysical properties and the entrainment performance of supersonic ejector has not yet been elucidated. In this study, a novel ejector CFD model is established by coupling the real-gas thermodynamic equation and species transport equation, then the relationships between thermophysical properties of pure/mixture refrigerants and ejector’s dimensionless operational parameters were thoroughly investigated. The results indicated that the ejector expansion ratio rises as primary flow inlet temperature or refrigerant’s normal boiling point increases, while the dimensionless parameters of primary nozzle are relatively insensitive to the variation of primary flow inlet temperature and refrigerant’s boiling point, resulted in changes of supersonic jet expansion behaviour. Moreover, the specific volume and adiabatic index change characteristics resulted in minor fluctuations of primary nozzle’s dimensionless parameters. When the bubble point temperature is fixed at 30℃ in the condenser, the saturated vapor/liquid phase pressure glide characteristics of R600a/R1234ze mixture may lead to an increasement of ejector’s compression ratio.

Breaking the limitation of thermodynamic cycle efficiency of the plasma synthetic jet actuator: Noble gases

The low thermodynamic cycle efficiency of the plasma synthetic jet actuator (PSJA) limits its application in flow control of supersonic aircraft. To improve the thermodynamic cycle efficiency of the PSJA, the energy conversion process of the PSJA under single discharge has been investigated by thermodynamic theory analysis and numerical simulation. The relations between thermodynamic cycle efficiency, specific heat ratio of gas medium, non-dimensional energy deposition and heating efficiency are established by theoretical deduction. Employing the three-dimensional nonconstant electrode center energy deposition model, energy conversion processes in the PSJA with six different gas mediums have been investigated. The simulation results show that the thermodynamic cycle efficiency of the PSJA with noble gas medium can exceed 3.2 %, which is 1.92 times of the PSJA with air medium due to their low volume specific heat. The maximum jet x-velocity of helium can reach 493.11 m/s, which is 3.3 times higher than that of air. Consequently, noble gases prove to be effective in improving the thermodynamic cycle efficiency of the PSJA and enhancing the strength of the synthetic jet. The advancement in the performance of the PSJA contributes to their enhanced capabilities in the field of active flow control.

Charged anisotropic fluid sphere in [formula omitted] gravity satisfying Vaidya - Tikekar metric

In this paper, we study the properties of electrically charged anisotropic and spherically symmetric compact stars in f(Q) gravity theory. The field equations have been solved using the MIT-Bag Equation of State (EoS) along with the Vaidya–Tikekar potential. However, the specific form of charge E2(r)=q0L2r2(1+Lr2)2 has been used to study the charged compact star model. The interior charge anisotropic solution has been matched with the charged Schwarzchild-(Anti)-de-Sitter metric. In particular, this study has been conducted on different compact stars viz, EXO 1785-248, PSR J1614-2230, PSR J0740+620 and SMC X-1. The mass–radius ratio of these compact stars has been predicted, and the physical viability of the model has been checked through the behaviour of causality condition, matter variables, energy constraints, modified TOV equation, and adiabatic index of the star. In conclusion, we observe a well-behaved model of a compact star in f(Q) gravity with charged matter distribution.

Exploring Challenging Properties of Liquid Metallic Systems through Machine Learning: Liquid La and Li4Pb Systems.

In this machine learning (ML) study, we delved into the unique properties of liquid lanthanum and the Li4Pb alloy, revealing some unexpected features and also firmly establishing some of the debated characteristics. Leveraging interatomic potentials derived from ab initio calculations, our investigation achieved a level of precision comparable to first-principles methods while at the same time entering the hydrodynamic regime. We compared the structure factors and pair distribution functions to experimental data and unearthed distinctive collective excitations with intriguing features. Liquid lanthanum unveiled two transverse collective excitation branches, each closely tied to specific peaks in the velocity autocorrelation function spectrum. Furthermore, the analysis of the generalized specific heat ratio in the hydrodynamic regime investigated with the ML molecular dynamics simulations uncovered a peculiar behavior, impossible to discern with only ab initio simulations. Liquid Li4Pb, on the other hand, challenged existing claims by showcasing a rich array of branches in its longitudinal dispersion relation, including a high-frequency LiLi mode with a nonhydrodynamic optical character that maintains a finite value as q → 0. Additionally, we conducted an in-depth analysis of various transport coefficients, expanding our understanding of these liquid metallic systems. In summary, our ML approach yielded precise results, offering new and captivating insights into the structural and dynamic aspects of these materials.

A comprehensive study of massive compact star admitting conformal motion under bardeen geometry

This article primarily investigates the existence of the charged compact star under the conformal motion treatment within the context of f(Q) gravity. We have developed two models by implementing the power-law and linear form of conformal factor, enabling an in-depth comparison in our study. We have selected the MIT Bag model equation of state to describe the connection between pressure and energy density and matched the interior spherically symmetric space-time with the Bardeen space-time. In addition, the present research examines various physically valid characteristics of realistic stars, such as PSR J1614−2230, PSR J1903+327, Vela X-1, Cen X-3, and SMC X-1. We compare two constructed models by attributing the behavior of density, pressure, equilibrium conditions, and the adiabatic index. We have additionally included a brief analysis of the scenario involving Reissner-Nordstrom spacetime as an external geometry for the matching condition. In contrast to the Reissner-Nordstrom instance, the Bardeen model with the extra term in the asymptotic representations yields a more intriguing and viable result. The current analysis reveals that the resulting compact star solutions are physically acceptable and authentic when considering the presence of charge with conformal motion in f(Q) gravity.

Impact of energy-momentum squared gravity on the geometry of stellar objects

This paper investigates the impact of energy–momentum squared gravity on the geometry of anisotropic compact stellar objects. In this perspective, we use the Tolman IV solution to examine the configuration of static spherically symmetric structures. The values of unknown constants are found by the first fundamental form of Darmois junction conditions. We consider four different models of this theory to analyze the behavior of physical quantities such as effective fluid parameters, anisotropy, energy constraints and equation of state parameters in the interior of considered compact stars. Sound speed and adiabatic index methods are used to determine the stability of proposed compact stars. It is found that the proposed compact stars are viable and stable in energy–momentum squared gravity.

Nano-imprint lithography of broad-band and wide-angle antireflective structures for high-power lasers.

We demonstrate efficient anti reflection coatings based on adiabatic index matching obtained via nano-imprint lithography. They exhibit high total transmission, achromaticity (99.5% < T < 99.8% from 390 to 900 nm and 99% < T < 99.5% from 800 to 1600 nm) and wide angular acceptance (T > 99% up to 50 degrees). Our devices show high laser-induced damage thresholds in the sub-picosecond (>5 J/cm2 at 1030 nm, 500 fs), nanosecond (>150 J/cm2 at 1064 nm, 12 ns and >100 J/cm2 at 532 nm, 12 ns) regimes, and low absorption in the CW regime (<1.3 ppm at 1080 nm), close to those of the fused silica substrate.