Tangential Pressure Research Articles

Relativistic modeling of stellar objects in a Schwarzschild’s coordinates with embedded class one spacetime

In this study, we investigate a new relativistic anisotropic Einstein field equations solution for compact stars under embedding class 1 conditions. In order to do this, we apply the Karmarkar condition and the embedding class one technique. By taking Buchdahl type metric potential g_{rr} into consideration, the precise analytical solution has been investigated. We have studied physical characteristics of various compact star using this analytical solution. Central singularities are absent from the solution. We have explored thermodynamic observables inside the stellar models, such as radial and tangential pressures, matter density, anisotropic factor, energy conditions, TOV, red-shift, and the speed of sound, etc., after establishing this space-time geometry for the stellar models. From the graphical representation of various physical characteristics, it is demonstrated that our model meets all the specification for ultra-high density compact bodies.

Pulsar glitch in a strangeon star model – III. The recovery

ABSTRACT Strangeon star model has passed various observational tests, such as the massive pulsars and the tidal deformability during binary mergers. Pulsar glitch, as a useful probe for studying the interior structure of pulsars, has also been studied in strangeon star model in our previous papers, including the recovery coefficient, the waiting time of glitches, and glitch activity. In this paper, the recovery process of a glitch is described in the strangeon star model, based on the starquake picture established before (in Paper I). After the starquake, the inner motion of the stellar matter would reduce the tangential pressure in the cracked places at the equatorial plane. The recovery (increase) of the tangential pressure would be achieved by a viscous flow towards the cracked places at equatorial plane, which leads to the exponential recovery of the spin frequency. A uniform viscous flow can reproduce the single exponential decay observed in some glitches, and the viscous time-scale τ and the depth h of the cracking place below the surface can be fitted by the recovery data. It is found that h increases with glitch size Δν/ν, which is expected in the glitch scenario of strangeon stars. The magnitude of the recovery predicted in this recovery model is also consistent with that derived from observations. The single exponential decay reproduced by a uniform viscous flow can be generalized to two or more exponentials by the multicomponent of viscous flows.

The Effect of f(R, T) Modified Gravity on the Mass and Radius of Pulsar HerX1

Millisecond pulsars are the perfect testable to examine potential matter-geometry coupling and its physical consequences in the context of the recent Neutron Star Interior Composition Explorer discoveries. We apply the field equations of modified gravity, f(R, T) = R + α T, to a spherically symmetric spacetime, where R is the Ricci scalar, α is a dimensional parameter, and T is the matter of the geometry. Five unknown functions are present in the output system of differential equations, which consists of three equations. To close the system, we make explicit assumptions about the anisotropy and the radial metric potential, g rr . We then solve the output differential equations and derive the explicit forms of the components of the energy-momentum tensor, i.e., density, radial, and tangential pressures. We look into the possibility that all of the physical parameters in the star can be reexpressed in terms of α and the compactness parameters, C = 2 GM Rc−2. We show that, for a given mass, the size permitted by Einstein’s general relativity is less due to the matter-geometry coupling in f(R, T). The validity of the hypothesis was validated by observations from an extra 21 pulsars. To achieve a surface density that is compatible with a neutron core at nuclear saturation density, the mass–radius curve enables masses up to 3.35M ⊙. We emphasize that although there is no assumption of an equation of state, the model fits well with a linear behavior. When comparing the surface densities of these 20 pulsars, we divided them into three groups. We show that these three groups are compatible with neutron cores.

Regularizing the JNW and JMN naked singularities

We extend the method of Simpson and Visser (SV) of regularizing a black hole spacetime, to cases where the initial metric represents a globally naked singularity. We choose two particular geometries, the Janis–Newman–Winicour (JNW) metric representing the solution of an Einstein-scalar field system, and the Joshi–Malafarina–Narayan (JMN) metric that represents the asymptotic equilibrium configuration of a collapsing star supported by tangential pressures as the starting configuration. We illustrate several novel features for the modified versions of the JNW and JMN spacetimes. In particular, we show that, depending on the values of the parameters involved the modified JNW metric may represents either a two way traversable wormhole or it may retain the original naked singularity. On the other hand, the SV modified JMN geometry is always a wormhole. Particle motion and observational aspects of these new geometries are investigated and are shown to posses interesting features. We also study the quasinormal modes of different branches of the regularized spacetime and explore their stability properties.

Anisotropic star by gravitational decoupling: A vanishing complexity approach

A new static spherically symmetric interior solution has been derived using Gravitational Decoupling via Complete Geometric Deformation, satisfying the vanishing complexity which explores the fundamental physical acceptability of the stellar model. In the current analysis, we present a systematic approach proposed by L. Herrera [Phys Rev D 97: 044010, 2018] and Contreras-Stuchlik [Eur Phys J C 82: 706, 2022] to derive a complexity-free solution. We plotted the graphical behavior of the energy density, radial pressure, tangential pressure, equation of state parameters, anisotropy, and stability analysis, respectively to check the physical viability of the complexity-free solution. In addition, we also discuss the energy exchange between the relativistic fluids which shows that the decoupling constant controls the direction of the energy exchange.

Experimental and numerical investigation of vortex flows and pressure fluctuations in a high-head pump-turbine

Vortex flows and pressure fluctuations, especially in the draft tube and vaneless space, have crucial impact on stable and safety operation for a high-head pump-turbine due to be fast adjusted and frequently switched between pump and turbine modes and often down to the deep part load. To more accurately investigate the vortex flows and pressure fluctuations in a high-head pump-turbine with splitter blades runner, a reliable approach by combined experiments with numerical simulations is presented in this paper. Laser Doppler Velocimetry measurements and high-speed camera observations inside the draft tube have been conducted at all the investigated points. Velocity profiles, vortex ropes visualization, the instantaneous tangential velocity and pressure fluctuations in both measured and simulated are presented to validate the numerical simulations and classify the vortex flow patterns. Meanwhile, the relationship between formation, development and breakdown of the vortex rope, the processional frequency and swirl intensity has been linked up and experimentally validated. Variation regularity of pressure fluctuations along the flow passage at different operating conditions has been quantitatively substantiated with measurements and predictions. Validation of predicted results versus experimental data shows that the presented numerical simulations can give very good results of vortex structures and the predicted fluctuation's frequencies. The pressure fluctuations amplitude of both the measured and the predicted in turbine mode agree well in the vortex rope-free zone, but has relatively larger deviations out of the vortex rope-free zone. It also demonstrated that the runner with splitter blades can restrain development of vortex flow and decrease the amplitude of pressure fluctuations for a high-head pump-turbine. The presented approach has been applied to successfully developed the high-head pump-turbines for several Pumped Storage Power Plants in China and demonstrated very useful in hydraulic optimizations.

Relativistic stellar modeling with perfect fluid core and anisotropic envelope fluid

We investigate the effect of density perturbations and local anisotropy on the stability of stellar matter structures in general relativity using the concept of cracking. Adopting a core-envelope model of a super-dense star, we examine the properties and stability conditions by introducing anisotropic pressure to the envelope region. Furthermore, we propose self-bound compact stars with an anisotropic envelope as a potential progenitor for starquakes. We show how the difference between sound propagation in radial and tangential directions would be used to identify potentially stable regions within a configuration. Due to an increase in the anisotropic parameter, strain energy accumulates in the envelope region and becomes a potential candidate for building-up quake like situation. This stress-energy stored in the envelope region that would be released during a starquake of a self-bound compact star is computed as a function of the magnitude of anisotropy at the core-envelope boundary. Numerical studies for spherically asymmetric compact stars indicate that the stress energy can be as high as $$10^{50}$$ erg if the tangential pressure is slightly more significant than the radial pressure. It is happened to be of the same order as the energy associated with giant $$\gamma $$ -ray bursts. Thus, the present study will be useful for the correlation studies between starquakes and GRBs.

Description of the interior of the neutron star in EXO 1785-248 by mean of the Karmarkar condition

Starting from the known condition of Karmarkar, which guarantees that a static and spherically symmetrical space-time is embedded in a manifold of dimension 5, and that it generates a differential equation between the metric coefficients [Formula: see text] and [Formula: see text], we solve Einstein’s equations for a fluid with anisotropic pressures. This allows us to represent the interior of the neutron star EXO 1785-248, with observational data around the pair of mass and radius [Formula: see text], [Formula: see text] and [Formula: see text], [Formula: see text]. It is shown that the density, radial and tangential pressure are monotonically decreasing functions, while the radial and tangential speeds of sound satisfy the causality conditions. The model presented depends on the compactness [Formula: see text] and two other parameters that characterize the internal behavior of the Hydrostatic variables, in particular the values of the central density [Formula: see text]. In particular for the observational values of mass and radius [Formula: see text], we have [Formula: see text] meanwhile that for [Formula: see text] we have [Formula: see text]. In a complementary manner it is shown that the model satisfies the causality condition and that according to the stability criteria of Harrison–Zeldovich–Novikov and of cracking the solution is stable.

New anisotropic star solutions in mimetic gravity

We extract new classes of anisotropic solutions in the framework of mimetic gravity, by applying the Tolman–Finch–Skea metric and a specific anisotropy not directly depending on it, and by matching smoothly the interior anisotropic solution to the Schwarzschild exterior one. Then, in order to provide a transparent picture we use the data from the 4U 1608-52 pulsar. We study the profile of the energy density, as well as the radial and tangential pressures, and we show that they are all positive and decrease toward the center of the star. Furthermore, we investigate the anisotropy parameter and the anisotropic force that are both increasing functions of the radius, which implies that the latter is repulsive. Additionally, by examining the radial and tangential equation-of-state parameters, we show that they are monotonically increasing, not corresponding to exotic matter. Concerning the metric potentials, we find that they have no singularity, either at the center of the star or at the boundary. Furthermore, we verify that all energy conditions are satisfied, we show that the radial and tangential sound speed squares are positive and sub-luminal, and we find that the surface redshift satisfies the theoretical requirement. Finally, in order to investigate the stability we apply the Tolman–Oppenheimer–Volkoff equation, we perform the adiabatic index analysis, and we examine the static case, showing that in all cases the star is stable.

Boundary-value-problem examination of the stability of a symmetrical rotor partially filled with a viscous incompressible fluid

Instabilities in a rotor system partially filled with a fluid can have an exponentially increasing amplitude, and this can cause catastrophic damage. Numerous theoretical models have been proposed, and numerous experiments have been conducted to investigate the mechanisms of this phenomenon. However, the explanation of the existence of the first unstable region induced by a viscous incompressible fluid is unclear, and only one solving method, a standard finite difference procedure, was proposed in 1991 for solving the instabilities in a system containing a symmetric rotor partially filled with a viscous incompressible fluid. To better understand the mechanisms of the instability induced by the viscous fluid, based on the linearized two-dimensional Navier–Stokes equations, this system's differential equations are transferred to solve the characteristic equations with boundary conditions. A Matlab boundary value problem (BVP) solver bvp5c proposed in 2008 is an efficient tool to solve this problem by uncoupling the boundary conditions with unknown initial guess. Applying this approach to a rotor system allows the instability regions to be obtained. In this study, first, the radial and tangential velocities and pressure fluctuations along the radial direction of a disk filled with fluid were examined. Then, parametric analysis of the effect of the Reynolds number R e c r, filling ratio H, damping ratio C, and mass ratio m on the system's stability was conducted. Using this calculation method allowed the first exploration of some new laws regarding the instabilities. These results will benefit the further understanding of the existence of the first unstable region of a rotor partially filled with a viscous incompressible fluid.

Influence of anisotropic matter on the Alcubierre metric and other related metrics: revisiting the problem of negative energy

Negative energy scenarios are the most widely studied for the warp metric. In fact, the prevailing view in the community so far has been that the warp metric necessarily has negative energies. In this work it is shown that the issue of negative energy densities associated with the Alcubierre warp metric with a general form function and similar metrics can be addressed when the whole non--vacuum Einstein equations of the system are examined. To this end, we have considered matter content in the form of anisotropic fluids. We have succeeded in writing the Einstein equations in such a way that some general constraints on the material content become evident. This means that, in rectangular coordinates, the energy density depends necessarily on the tangential pressures of the fluid. For matter such as dust or isotropic fluids we find that density and other related quantities become identically zero. This makes the negative energy problem spurious. It is also revealed that constructing Alcubierre-based metrics using cylindrical and spherical coordinates results in a system of equations that are amenable to more systematic analysis. The field equations constrain the dependence of the form function and how this impacts the matter content. In all cases we determine that energy density is not mandatory negative, despite the recurrent claims in the literature.This result prompts a reevaluation of the negative energy requirements and underscore the importance of cylindrical and spherical type-warps to demonstrate that negative energy density is not an intrinsic unavoidable feature of warp drives.

A degraded adhesion model for creep force calculation based on the CONTACT algorithm

Degraded adhesion is observed extensively during the braking or traction process. An accurate degraded adhesion law, on the other hand, is difficult to acquire since it is characterized by a complicated non-linear behaviour involving various surface phenomena. This work provides a new local degraded adhesion model, its implementation in a wheel-rail contact model, and its application in a multibody railway vehicle model. The present degraded adhesion model takes into account large sliding and adhesion recovery phenomena, which are strictly related to the power dissipated at the contact interface. Implementation of the current local degraded adhesion model enhances Kalker’s CONTACT algorithm. The application into multibody dynamics simulation demonstrates the high accuracy of computations. The model is validated by comparing it to experimental findings obtained from on-field tests. Simulated results regarding the tangential pressures, the slip velocity, and the dissipated power under degraded adhesion and partial adherence conditions are reported.

Double relativistic master polytrope for anisotropic matter

We present a detailed analysis of a general relativistic static spherical symmetric distribution in which both the radial and tangential pressures follow a master polytropic equation of state that generalizes the standard treatment and avoids the appearance of singularities in the system. In particular, we find the corresponding Lane-Emden equation and integrate it for a wide range of values of the parameters involved. We explore the parameter space with the aim to find the set of parameters leading to reasonable physical solutions. Also, we considered the effect of spherically symmetric perturbations of the matter variables in order to analyze the possible apparition of cracking within the compact distribution.

Black holes surrounded by Einstein clusters as models of dark matter fluid

We construct a novel class of spherically symmetric and asymptotically flat black holes and naked singularities surrounded by anisotropic dark matter fluid with the equation of state (EoS) of the form P_t=omega rho . We assume that dark matter is made of weakly interacting particles orbiting around the supermassive black hole in the galactic center and the dark matter halo is formed by means of Einstein clusters having only tangential pressure. In the large distance from the black hole we obtain the constant flat curve with the upper bound for the dark matter state parameter omega lesssim 10^{-7}. To this end, we also check the energy conditions of the dark matter fluid outside the black hole/naked singularity, the deflection of light by the galaxy, and the shadow images of the Sgr A^star black hole using the rotating and radiating particles. For the black hole case, we find that the effect of dark matter fluid on the shadow radius is small, in particular the angular radius of the black hole is shown to increase by the order 10^{-4}, upmu arcsec compared to the vacuum solution. For the naked singularity we obtain significantly smaller shadow radius compared to the black hole case. Finally, we study the stability of the S2 star orbit around Sgr A^star black hole under dark matter effects. It is argued that the motion of S2 star orbit is stable for values omega lesssim 10^{-7}, however further increase of omega leads to unstable orbits. Using the observational result for the shadow images of the Sgr A^star reported by the EHT along with the tightest constraint for omega found from the constant flat curve, we show that the black hole model is consistent with the data while the naked singularity in our model can be ruled out.

Ricci inverse anisotropic stellar structures

This paper offers novel quintessence compact relativistic spherically symmetrical anisotropic solutions under the recently developed Ricci inverse gravity (Amendola et al., 2020), by employing Krori and Barua gravitational potentials, A r 2 = ν ( r ) , & B r 2 + C = μ ( r ) (with A, B, and C being real constants). For this objective, a specific explicit equation of state, connecting energy density and radial pressure, i.e., p r = ω ρ , such that 0 < ω < 1 , has been utilized with an anisotripic fluid source. Ricci inverse field equations are used to find the exclusive expressions of the energy density, radial and tangential stresses, and the quintessence energy density, the critical physical attributes reflecting the exceptional conduct of extremely dense matter configuration. For the observatory source stars H e r X − 1 , S A X J 1808 . 4 − 3658 & 4 U 1820 − 30 , all the important physical quantities like energy densities, tangential and radial pressures, energy conditions, gradients, anisotropy, redshift and mass–radius functions, and stellar compactness have been worked out and analyzed graphically. It has been concluded that all of the stellar formations under consideration remain free from any undesirable central singularity and are stable.

Effect of the Position of Overflow Pipe with Mixed Spiral Structures on the Separation Performance of Hydrocyclones

Overflow pipes are important components of hydrocyclones. The overflow products can carry huge amounts of residual energy when being discharged. In order to take full advantage of the residual energy and enhance hydrocyclone separation performance, this research designed a novel hydrocyclone by adding static mixing units with spiral elements in the overflow pipe. This study performed numerical simulations to investigate the effects of the install position of the spiral structure on the separation performance and inner flow field of the hydrocyclone. It can be concluded that both tangential velocity and pressure are first enhanced and then reduced by the elevation of the spiral structure. When the spiral structure is installed 30 mm away from the overflow pipe bottom, because of the hindrance of spiral elements, the discharge of coarse particles with the overflow are fully decreased and the quality of overflow products are enhanced.

Three-layered super dense star with charged anisotropic fluid

We generate a three-layered super dense charged star with Van der Waals equation of state at the intermediate layer of stellar spheres. Our model comprises of three interior layers: the core, the intermediate, and the envelope described by linear, Van der Waals, and Chaplygin equations of state, respectively. The plots for the physical variables such as energy density, radial and tangential pressures, mass, anisotropy, and metric potentials are well behaved and agree with the astrophysical results. The stability and equilibrium factors are continuous and defined at some values of the parameters. We have also generated new stellar masses and radii which are in acceptable ranges. We have also regained previous masses and radii for the stars like PSRJ1903+0327, PSRJ1614-2230, SAXJ1808.4-3658, EXO1745-248, and EXO1785-248.

Traversable Finslerian wormholes supported by phantom energy

This paper supplements the article (Eur. Phys. J. C 76: 246, 2016) by extending some more new wormhole solutions in the background of Finslerian geometry. Here, we present six more solutions by considering i) a linear equation of state (EoS) pr = ωρ and three different shape functions, ii) a linear relationship between radial and tangential pressure as pt = npr with two different redshift functions and iii) a specific density profile with a linear equation of state pr = ωρ. It is found that all these wormholes are violating null energy condition (NEC) signifying that the throats are opened by exotic matters. Since the equation of state parameters, ω or ωr = pr/ρ for all the solutions are &lt;−1, the corresponding exotic matter is none other than the phantom energy. Further, the two solutions in Case ii) are not asymptotically flat since limr→∞[b(r)/r]↛0. To make it asymptotically flat we matched it with the Schwarzschild vacuum.

Pulsating flows of viscous fluid in flat channel for given harmonic fluctuation of flow rate

In research work, it is pointed out the issue of a pulsating flow of a viscous incompressible fluid in a flat channel for a given harmonic fluctuation of the fluid flow rate. The study of the generation of pulsating current is used in biological mechanics, in particular, in the use of microchip systems. In addition, to ensure a constant flow of liquid, pneumatic micropumps that periodically squeeze liquid from empty volumes are widely used. In such systems, the installation of pulsating flow is shown to be economically beneficial. The transfer function of the amplitude-phase rate response is determined; with the help of these functions, the ratio of the tangential shear pressure on the channel fence to the average acceleration over the channel section is determined. In Figure 1, it can be seen that the ratio of KH = 0 of in τHC/τ0KC is close to one and α02 is less than one. If α02 takes on values greater than unity, then the ratio of to KH = 0 to τHC/τ0KC is greater than unity, and it has been shown to increase with increasing frequency of dimensionless oscillation. It was shown that in an unsteady flow of liquid, even in cases where liquid acceleration is equal to zero, it was studied that the stress on the wall of the channel exceeds its quasi-stationary value. τHC/τ0KC with the increase of the ratio KH, the increase of the parameter is explained by the fact that the change of the tensile stress on the wall advances in phases concerning the average speed along the section. Calculations determine that the non-stationary shear pressure on the channel fence increases non-monotonically with the acceleration of the liquid particle at low oscillation frequencies. The shear pressure reaches its maximum value, then decreases with increasing dimensionless oscillation rate, and asymptotically approaches the values without accelerated flow.

Relativistic Krori‐Barua Compact Stars in f(R,T)$f(R,T)$ Gravity

AbstractThis work aims to investigate the behaviour of compact stars in the background of theory of gravity. For current work, we consider the Krori‐Barua metric potential i.e., and , where, and C are constants. We use matching conditions of spherically symmetric space‐time with Schwarzschild solution as an exterior geometry and examine the physical behaviour of stellar structure by assuming the exponential type gravity model. In the present analysis, we discuss the graphical behaviour of energy density, radial pressure, tangential pressure, equation of state parameters, anisotropy and stability analysis respectively. Furthermore, an equilibrium condition can be visualized through the modified Tolman‐Oppenheimer‐Volkov equation. Some extra features of compact stars i.e. mass‐radius function, compactness factor and surface redshift have also been investigated. Conclusively, all the results in current study validate the existence of compact stars under exponential gravity model.