Stiff Fluid Research Articles

Transition analysis of the non-OT G 2 stiff fluid spike solution

We use the technique developed in Moughal’s doctoral thesis to analyse the joint spike transition, revealing new groups of worldlines which undergo distinct transitions, and correcting misconceptions about spikes.

The Sharma–Mittal Model’s Implications on FRW Universe in Chern–Simons Gravity

The Sharma–Mittal holographic dark energy model is investigated in this paper using the Chern–Simons modified gravity theory. We investigate several cosmic parameters, including the deceleration, equation of state, square of sound speed, and energy density. According to the deceleration parameter, the universe is in an decelerating and expanding phase known as de Sitter expansion. The Sharma–Mittal HDE model supports a deceleration to acceleration transition that is compatible with the observational data. The EoS depicts the universe’s dominance era through a number of components, such as ω=0, 13, 1, which indicate that the universe is influenced by dust, radiation, and stiff fluid, while −1<ω<13, ω=−1, and ω<−1 are conditions for quintessence DE, ΛCDM, and Phantom era dominance. Our findings indicate that the universe is in an accelerated expansion phase, and this is similar to the observational data.

Moduli variations in two-phase fluid rocks considering squirt flow and patchy saturation

The complex pore structure and fluid distribution in underground rocks bring difficulties to rock physics modeling and reservoir interpretation. The existence of cracks and ellipsoidal pores will cause squirt flow during wave propagation, and the fluid may be present as patchy saturation. By considering the effects of both squirt flow and patchy saturation, a new rock physics model is proposed. The fluids in inclusions include a soft fluid such as gas and a stiff fluid such as water. The novelty is that an explicit expression of patch saturation with frequency is introduced. Thus, the proposed model is helpful to analyze the variations of moduli with frequency caused by squirt flow and patchy saturation simultaneously. By comparison, the proposed model matches well with existed boundary models and covers all possible behavior of the moduli. Then, variations in moduli with water saturation and frequency are analyzed in detail, as well as the impact of patchy saturation on the attenuation induced by the squirt flow. The analysis shows that a small amount of gas contained in inclusions can cause a large change in moduli, especially the bulk modulus. The effect of patchy saturation on the moduli increases with the increasing frequency and when the frequency is low, the effect only occurs at very high water saturation. Besides, at high water saturation, the squirt flow can cause a sharp change in bulk modulus. The proposed model is helpful for understanding wave propagation in partially saturated rock with cracks.

Stable bounded excursion gravastars with regular black holes

This paper explores the possible existence of stable regions of bounded excursion gravastars in the background of regular black holes (Bardeen and Bardeen-de Sitter black holes). For this purpose, we match internal de Sitter geometry with an external regular black hole through an intermediate shell with an equation of state. The matter surface located at the shell greatly affects the dynamical configuration of the shell. We consider stiff and dust fluids to discuss the outcomes of the developed structures. We then determine the critical values of physical parameters at which the shell neither collapses nor expands for both cases with different choices of external and internal geometries. It is found that there does not exist any possible region of stable bounded excursion gravastar for the interior flat and exterior regular geometries. However, the stable bounded excursion gravastar is obtained for the interior de Sitter and exterior regular black holes with a suitable choice of physical parameters for both stiff and dust shells. We conclude that the possibility of the existence of both bounded excursion gravastars as well as black holes cannot be excluded in the dynamical model.

Intravesical Mucoadhesive Hydrogel Induces Chemoresistant Bladder Cancer Ferroptosis through Delivering Iron Oxide Nanoparticles in a Three-Tier Strategy.

Bladder cancer (BCa) is the most costly solid tumor owing to its high recurrence. Relapsed cancer is known to acquire chemoresistant features after standard intravesical chemotherapy. This cancer state is vulnerable to ferroptosis, which occurs when lipid peroxides generated by iron metabolism accumulate to lethal levels. Increasing the labile iron pool (LIP) by iron oxide nanoparticles (IONPs) promises to inhibit chemoresistant BCa (CRBCa), but systemically administered IONPs do not sufficiently accumulate at the tumor site. Therefore, their efficacy is weakened. Here, we present a three-tier delivery strategy through a mucoadhesive hydrogel platform conveying hyaluronic acid-coated IONPs (IONP-HA). When instilled, the hydrogel platform first adhered to the interface of the tumor surface, sustainably releasing IONP-HA. Subsequently, the tumor stiffness and interstitial fluid pressure were reduced by photothermal therapy, promoting IONP-HA diffusion into the deep cancer tissue. As CRBCa expressed high levels of CD44, the last delivery tier was achieved through antibody-mediated endocytosis to increase the LIP, ultimately inducing ferroptosis. This three-tiered strategy delivered the IONPs stepwise from anatomical to cellular levels and increased the iron content by up to 50-fold from that of systematic administration, which presents a potential regimen for CRBCa.

Realistic anisotropic Karmarkar stars in Rastall gravitational framework

In this study, we discuss the anisotropic matter distribution of compact stars in the context of Rastall modified theory of gravity by applying Karmarkar condition of embedding technique. For this purpose, we consider the spherical symmetric geometry and solve the resulting Rastall field equations by taking different values of Rastall parameter, i.e., ξ=0.05,ξ=−0.05, and ξ=−0.15 into account. Since the Karmarkar condition links both metric potentials through a differential equation and allows one to pick one of the metric potential, so we pick a well-known interesting form of grr component of metric as eλ(r)=1+cr21+ar2n(1+br2)2. We elaborate the properties of compact stars Vela X-1, 4U 1820-30 and SAX J 1808.4-3658 for different positive values of n, i.e., 1.8≤n<7, where n≠2,4,6. It is interesting to mention here that results are not justified for n≥7. Also, we find no solution for even integral values of n, but for odd integers, we obtain physically admissible solutions representing the compact stars. For greater values of n, we obtain stiff fluid equations of state in which adiabatic index shows the increasing behavior and speed of sound approximates to speed of light. It is seen that the causality condition is violated for all values of n≥7 and the values of n between 0 and 1.8 generate complex values of transverse sound speed vt.

Homogeneous perfect fluid collapse in five-dimensional spherically symmetric spacetime

In this paper, the higher-dimensional collapse of homogeneous isotropic perfect fluid is studied by considering the geometry of five-dimensional spherically symmetric metric. Using equations of state for different fields like dust, radiation and stiff fluid with and without cosmological constant [Formula: see text], the gravitational collapse is studied. The results are compared with the usual four-dimensional study in [A. V. Astashenok, K. Mosani, S. D. Odintsov and G. C. Samanta, Int. J. Geom. Methods Mod. Phys. 16, 1950035 (2019)]. It is found that the collapse rate is faster in five-dimensional spacetime as compared to four-dimensional case supporting the cosmic censorship hypothesis.

Study of gravastars in Rastall gravity

Gravastars have been considered as a feasible alternative to black holes in the past couple of decades.Stable models of gravastar have been studied in many of the alternative gravity theories besides standard GeneralRelativity (GR). The Rastall theory of gravity is a popular alternative to GR, specially in the cosmological andastrophysical context. Here, we propose a stellar model under the Rastall gravity following Mazur-Mottola's [1,2]conjecture. The gravastar consists of three regions, viz., (I) Interior region, (II) Intermediate shell region,and (III) Exterior region. The pressure within the interior core region is assumed with a constant negativematter-energy density which provides a repulsive force over the entire thin shell region. The shell is assumed tobe made up of fluid of ultrarelativistic plasma which follows the Zel'dovich's conjecture of stiff fluid [3,4].It is also assumed that the pressure is proportional to the matter-energy density according to Zel'dovich's conjecture,which cancel the repulsive force exerted by the interior region. The exterior region is completely vacuum which isdescribed by the Schwarzschild-de Sitter solution. Under all these specifications we obtain a set of exact andsingularity-free solutions of the gravastar model presenting several physically valid features within the frameworkof Rastall gravity. The physical properties of the shell region namely, the energy density, proper length, totalenergy and entropy are explored. The stability of the gravastar model is investigated using the surface redshiftagainst the shell thickness and maximizing the entropy of the shell within the framework of Rastall gravity.

Performance‐based seismic retrofitting of frame structures using negative stiffness devices and fluid viscous dampers via optimization

AbstractIn this paper, an optimization approach for the seismic retrofitting of inelastic moment resisting frames (MRFs) equipped with nonlinear fluid viscous dampers (FVDs) and negative stiffness devices (NSDs) is presented. The goal of the optimization is to minimize a cost‐related objective function, which accounts for the cost of the supplemental FVDs and NSDs. The mechanical properties and the topological layout of both the FVDs and NSDs are considered as design variables and simultaneously optimized during the optimization process. A loss estimation analysis is utilized as a performance constraint, where the PEER framework is adopted. The structural response is evaluated using a nonlinear time history analysis (NTHA), which accounts for the nonlinear behavior of the MRF elements, FVDs, and NSDs. The optimization problem is formulated using differentiable functions only, making it suitable for an efficient gradient‐based optimization solution scheme. The gradients are efficiently derived using the adjoint sensitivity analysis approach. Two numerical examples show the robustness and efficiency of the presented methodology, where the optimized design relies on NSDs and FVDs in both examples.

Characterization of Lorentzian manifolds with a semi-symmetric linear connection

Our aim is to characterize the Lorentzian manifolds endowed with a type of semi-symmetric non-metric connection. It is proven that a Lorentzian manifold with a type of semi-symmetric non-metric connection is a GRW space-time. To minimize the gap between the RW space-time and the GRW space-time, we establish a bridge between them. It is shown that a weakly Ricci symmetric GRW space-time is a stiff matter fluid. We also classify the space-times by using the Petrov's classification. Finally, we construct an example of a 4-dimensional Lorentzian manifold admitting a semi-symmetric non-metric connection whose associated vector field is a unit timelike concircular vector field.

Spherical accretion: Bondi, Michel, and rotating black holes

ABSTRACT In this work, we revisit the steady-state, spherically symmetric gas accretion problem from the non-relativistic regime to the ultrarelativistic one. We first perform a detailed comparison between the Bondi and Michel models, and show how the mass accretion rate in the Michel solution approaches a constant value as the fluid temperature increases, whereas the corresponding Bondi value continually decreases, the difference between these two predicted values becoming arbitrarily large at ultrarelativistic temperatures. Additionally, we extend the Michel solution to the case of a fluid with an equation of state corresponding to a monoatomic, relativistic gas. Finally, using general relativistic hydrodynamic simulations, we study spherical accretion on to a rotating black hole, exploring the influence of the black hole spin on the mass accretion rate, the flow morphology and characteristics, and the sonic surface. The effect of the black hole spin becomes more significant as the gas temperature increases and as the adiabatic index γ stiffens. For an ideal gas in the ultrarelativistic limit (γ = 4/3), we find a reduction of 10 per cent in the mass accretion rate for a maximally rotating black hole compared to a non-rotating one, while this reduction is of up to 50 per cent for a stiff fluid (γ = 2).

LRS Bianchi I Model with Bulk Viscosity in $$\boldsymbol{f(R,T)}$$ Gravity

Locally-rotationally-symmetric Bianchi type-I viscous and non -viscous cosmological models are explored in general relativity (GR) and in f(R,T) gravity. Solutions are obtained by assuming that the expansion scalar is proportional to the shear scalar which yields a constant value for the deceleration parameter (q=2). Constraints are obtained by requiring the physical viability of the solutions. A comparison is made between the viscous and non-viscous models, and between the models in GR and in f(R,T) gravity. The metric potentials remain the same in GR and in f(R,T) gravity. Consequently, the geometrical behavior of the $f(R,T)$ gravity models remains the same as the models in GR. It is found that f(R,T) gravity or bulk viscosity does not affect the behavior of effective matter which acts as a stiff fluid in all models. The individual fluids have very rich behavior. In one of the viscous models, the matter either follows a semi-realistic EoS or exhibits a transition from stiff matter to phantom, depending on the values of the parameter. In another model, the matter describes radiation, dust, quintessence, phantom, and the cosmological constant for different values of the parameter. In general, f(R,T) gravity diminishes the effect of bulk viscosity.

Cylindrically symmetric unimodular f(R) black holes

In this paper, we examine massless scalar field by using unimodular [Formula: see text] theory. It is taken into account unimodular and cylindrically symmetric spacetime which provides convenience in researching black hole. The field equations in unimodular [Formula: see text] theory for given spacetime with massless scalar field and additional Bianchi identities are solved. Cylindrically symmetric anti-de Sitter (AdS)–Schwarzschild-like and AdS–Reissner–Nordström-like black hole spacetimes are achieved. Equations of motion are derived by using Hamiltonian. Orbits of massless test particles are depicted. Obtained line element asymptotically converges to dS/AdS spacetime. Weak and strong energy conditions of the massless scalar field are obtained with Raychaudhuri equations in unimodular [Formula: see text] theory. Also, stiff fluid interpretation of scalar field is reviewed.

Perfect fluid spacetimes and Yamabe solitons

This paper deals with the study of perfect fluid spacetimes. It is proven that a perfect fluid spacetime is Ricci recurrent if and only if the velocity vector field of perfect fluid spacetime is parallel and α = β. In addition, in a stiff matter perfect fluid Yang pure space with p + σ ≠ 0, the integral curves generated by the velocity vector field are geodesics. Moreover, it is shown that in a generalized Robertson–Walker perfect fluid spacetime, the Weyl tensor is divergence-free and the gradient of the potential function of the concircular vector field is pointwise collinear with the velocity vector field of perfect fluid spacetime. We also characterize the perfect fluid spacetimes whose Lorentzian metrics are Yamabe and gradient Yamabe solitons, respectively.

Well-Doublets: A First-Order Assessment of Geothermal SedHeat Systems

Renewable and sustainable energy sources can play an important role in meeting the world’s energy needs and also in addressing environmental challenges such as global warming and climate change. Geothermal well-doublet systems can produce both electrical and thermal energy through extracting heat from hot-water aquifers. In this paper, we examine some potential challenges associated with the operation of well-doublet systems, including heat conductivity, chemical, and mechanical issues. In these systems, geomechanics issues such as thermal short-circuiting and induced seismicity arise from temperature and pressure change impacts on the stress state in stiff rocks and fluid flow in fractured rock masses. Coupled chemical processes also can cause fluid channeling or formation and tubular goods plugging (scaling) with precipitates. Mechanical and chemical disequilibrium conditions lead to increased production uncertainties; hence risk, and therefore coupled geo-risk assessments and optimization analyses are needed for comparative commercialization evaluations among different sites. The challenges related to heat transfer processes are also examined. These studies can help better understand the issues that may arise during the operation of geothermal well-doublet systems and improve their effectiveness, subsequently reducing associated costs and risks.

Accretion onto a born-Infeld black hole

This study deals with the accretion of an isotropic fluid onto a Born-Infeld black hole. For this purpose, we have formulated the fluid velocity u(r), energy density ϱ(r) and rate of mass accretion for a singularity free black hole with the help of the barotropic equation of state and conservation laws. We have plotted the governing results to inspect the physical significance of the matter flow around the specific black hole. The spherically symmetric accretion has been analyzed for a stiff fluid, dust fluid, quintessence fluid and phantom fluid. The plotted critical quantities have been discussed by finding the critical radius, and all the plots have been shown in two regions. The fluid velocity and the energy density have positive and negative regions. It has been found that the black hole mass increases for the stiff fluid, dust fluid and quintessence fluid, but it decreases for the phantom fluid.

Generalized scalar field cosmologies: a global dynamical systems formulation

Local and global phase-space descriptions and averaging methods are used to find qualitative features of solutions for the FLRW and the Bianchi I metrics in the context of scalar field cosmologies with arbitrary potentials and arbitrary couplings to matter. The stability of the equilibrium points in a phase-space as well as the dynamics in the regime where the scalar field diverges are studied. Equilibrium points that represent some solutions of cosmological interest such as: several types of scaling solutions, a kinetic dominated solution representing a stiff fluid, a solution dominated by an effective energy density of geometric origin, a quintessence scalar field dominated solution, the vacuum de Sitter solution associated to the minimum of the potential, and a non-interacting matter dominated solution are obtained. All of which reveal a very rich cosmological phenomenology.

Isotropic Gravastar Model in Rastall Gravity

In the present paper, we have introduced a new model of gravastar with an isotropic matter distribution in Rastall gravity by the Mazur–Mottola (2004) mechanism. Mazur–Mottola approach is about the construction of gravastar which is predicted as an alternative to black hole. By following this convention, we define gravastar in the form of three phases. The first one is an interior phase which has negative density; the second part consists of thin shell comprising ultrarelativistic stiff fluid for which we have discussed the length, energy, and entropy. By the graphical analysis of entropy, we have shown that our proposed thin shell gravastar model is potentially stable. The third phase of gravastar is defined by the exterior Schwarzschild geometry. For the interior of gravastar, we have found the analytical solutions free from any singularity and the event horizon in the framework of Rastall gravity.

Relic gravitons from stiff curvature perturbations

The tensor modes reentering the Hubble radius when the plasma is dominated by a stiff fluid lead to a spectral energy density whose blue slope depends on the total post-inflationary sound speed. This result gets however corrected by a secondary (gauge-dependent) term coming from the curvature inhomogeneities that reenter all along the same stage of expansion. In comparison with the first-order result, the secondary contribution is shown to be always suppressed inside the sound horizon and its effect on the total spectral energy density of the relic gravitons is therefore negligible for all phenomenological purposes. It is also suggested that the effective anisotropic stress of the curvature inhomogeneities can be obtained from the functional derivative of the second-order action of curvature inhomogeneities with respect to the background metric.

Generalising certain cosmological solutions sourced by a stiff perfect fluid

In this article, we show that a number of well-known time-dependent, spherically symmetric solutions of the Einstein’s field equations sourced by a stiff, perfect fluid with a cosmological constant can be generalised into a solution with arbitrary metric functions. This metric can be applied to construct stiff, perfect fluid metrics with (or without) a cosmological constant. We also explore the possibility that this metric may allow us to generate a singularity-free, spherically symmetric cosmological model.