Scalar Profile Research Articles

On AdS3/ICFT2 with a dynamical scalar field located on the brane

We exploit the holographic duality to study the system of a one-dimensional interface contacting two semi-infinite two-dimensional CFTs. Central to our investigation is the introduction of a dynamical scalar field located on the bulk interface brane which breaks the scaling symmetry of the dual interface field theory, along with its consequential backreaction on the system. We define an interface entropy from holographic entanglement entropy, to construct a g-function. At zero temperature we construct several illustrative examples and consistently observe that the g-theorem is always satisfied. These examples also reveal distinct features of the interface entropy that are intricately linked to the scalar potential profiles. At finite temperature we find that the dynamical scalar field enables the bulk theory to have new configurations which would be infeasible solely with a tension term on the interface brane.

Scalar mixing and entrainment in an axisymmetric jet subjected to external turbulence

The present study aims to understand the process of turbulent entrainment into a jet, as affected by background turbulence, using scalar statistics. Planar laser-induced fluorescence was employed to capture the orthogonal cross sections of the jet at a fixed downstream station with varying background turbulence intensities and length scales. The conditional scalar profiles revealed that the thickness of the scalar turbulent/turbulent interface is greater than that of the traditional turbulent/non-turbulent interface, and the interfacial thickness is an increasing function of the background turbulence intensity. Although nibbling remains the primary entrainment mechanism in the far field, increased occurrence of concentration “holes” within the interfacial layer in the presence of ambient turbulence suggests a more significant role of large-scale engulfment in the turbulent/turbulent entrainment process (although still below 1% of the total mass flux). Enhanced contribution of the area of detached jet patches (i.e., “islands”) to that of the main jet is hypothesized to be evidence of intense detrainment events in the background turbulence. This can potentially contribute to a reduced net entrainment into the jet, which manifests as less negative values of scalar skewness within the jet core.

Thermal energy and electro-osmotic for biomimetic artificial olfactory cilia in tri-hybrid nanofluids: entropy-defying approaches

Biomimetic artificial olfactory cilia have demonstrated potential in identifying specific volatile organic compounds linked to various diseases, including certain cancers, metabolic disorders, and respiratory conditions. These sensors may facilitate non-invasive disease diagnosis and monitoring. Cilia Motility is the coordinated movement of cilia, which are hair-like projections present on the surface of particular cells in different species. Cilia serve an important part in several biological functions, including motility, fluid movement, and sensory reception. Cilia motility is a complicated process that requires the coordinated interaction of structural components and molecular pathways. Cilia are made up of a highly structured structure known as the axoneme, which is made up of microtubules grouped in a unique pattern. The axoneme is made up of nine outer doublet microtubules and a core pair of singlet microtubules. This arrangement offers structural support and serves as a scaffold for the proteins involved in ciliary movement. Our latest endeavors investigate these Multiphysics phenomena in ciliary beating flows that are inspired by biology, utilizing copper, gold, and titania nanoparticles. We examine their functions in biological systems such as peristaltic transport computationally. Our models give precise two- and three-dimensional velocity, temperature, and concentration solutions by integrating transverse magnetohydrodynamics with laser heating. Furthermore, at the channel wall expressions, the skin friction coefficient, Sherwood number, Nusselt number and optimization of entropy generation are acquired and analyzed. Important properties of the velocity and scalar profiles are revealed by a thorough analysis of dimensionless parameters. The simplified examination provides more insight into the trapping patterns that result from the complex interaction between nanofluid rheology and optics. These findings greatly contribute to our knowledge and improvement of nanofluidic transport technologies in a variety of fields supporting industry, sustainability, and medicine. Our combined computational and experimental methodology clarifies the complex dynamics in these systems and provides design guidance for the engineering of improved fluidic devices that make use of multifunctional nanomaterial interfaces and peristaltic motion.

Tidal Love numbers from EFT of black hole perturbations with timelike scalar profile

We study static tidal Love numbers (TLNs) of a static and spherically symmetric black hole for odd-parity metric perturbations.We describe black hole perturbations using the effective field theory (EFT), formulated on an arbitrary background with a timelike scalar profile in the context of scalar-tensor theories.In particular, we obtain a static solution for the generalized Regge-Wheeler equation order by order in a modified-gravity parameter and extract the TLNs uniquely by analytic continuation of the multipole index ℓ to non-integer values.For a stealth Schwarzschild black hole, the TLNs are vanishing as in the case of Schwarzschild solution in general relativity.We also study the case of Hayward black hole as an example of non-stealth background, where we find that the TLNs are non-zero (or there is a logarithmic running).This result suggests that our EFT allows for non-vanishing TLNs and can in principle leave a detectable imprint on gravitational waves from inspiralling binary systems, which opens a new window for testing gravity in the strong-field regime.

Exploring a novel feature of ellis spacetime: Insights into scalar field dynamics

We have studied neutral and charged massive particles dynamics in Ellis spacetime in the presence of the external scalar field. Focusing on the circular motion of massive particles, the impact of an external scalar field on the Innermost Stable Circular Orbit (ISCO) position is analyzed, revealing a non-linear relationship with the scalar field parameter. Perturbation techniques are employed to investigate oscillatory motion near stable orbits in the Ellis spacetime, yielding analytical expressions for radial and angular oscillations. The throat of the wormhole has been constrained by comparing theoretical and observational results for fundamental frequencies of particles from quasars. Finally, scalar and gravitational perturbations in the Ellis spacetime have been studied. It is shown that the equation for the scalar profile function is fully independent from the tensor functions and the solution can be represented in terms of the confluent Heun function. However, it has been shown that equations for the tensor profile functions strongly depend on the scalar profile functions in the Ellis spacetime and they are reduced to the Regge–Wheeler–Zerilli equation. Finally, numerical solutions to the Regge–Wheeler–Zerilli equation for the radial functions in the Ellis have been presented.

Generic predictions for primordial perturbations and their implications

We introduce a novel framework for studying small-scale primordial perturbations and their cosmological implications. The framework uses a deep reinforcement learning to generate scalar power spectrum profiles that are consistent with current observational constraints. The framework is shown to predict the abundance of primordial black holes and the production of secondary induced gravitational waves. We demonstrate that the set up under consideration is capable of generating predictions that are beyond the traditional model-based approaches.

Nonpremixed and Premixed FPV Modeling of a Turbulent CH4-H2 Bluff-Body Flame

ABSTRACT In this study, modeling of nonpremixed CH4/H2 bluff-body (HM1) flame was performed using nonpremixed and premixed-based flamelet progress variable (FPV) models. The models were developed in an OpenFOAM flow solver, and the performance of the different variants of the FPV model was evaluated in the prediction of flame structure. The premixed FPV tables were constructed by employing two different interpolation techniques, i.e. linear and cubic spline, to retrieve the reactive scalars outside the flammability range. A generalized progress variable equation was introduced in the flow solver, which can be used for both tabulation approaches. Inside the flammable range, the profiles of the reactive scalars obtained from premixed FPV were found to be similar to those of the nonpremixed FPV. However, there exists disparity in the profiles outside the flammable range, particularly in the fuel-rich side between the nonpremixed and premixed approaches. A modified k-ε model was found to be suitable for modeling the turbulence compared to standard k-ε and k-ω shear stress transport models. The predictions of mean reactive scalars by the premixed FPV model, when combined with the linear interpolation, agree very well with experimental data and with the nonpremixed FPV predictions. However, deviations were observed in the predictions of the mean reactive scalar while using the cubic spline method in premixed FPV. All three approaches exhibit almost similar variations in the predictions of minor species. The results obtained from the present approach depict that the developed solver can be extended for modeling turbulent-chemistry interactions arising in partially premixed turbulent flames.

Exploring self-gravitating cylindrical structures in modified gravity: Insights from scalar-vector-tensor theory

We investigate static cylindrical solutions within an extended theory of modified gravity. By incorporating various coupling functions through a straightforward boost symmetry approach, we establish the equations of motion in a self-consistent manner and subsequently determine the linear scalar field profile. Utilizing analytical methods, we solve the system of equations for the metric functions and the U(1) gauge field, revealing their dependence on Bessel's functions. To comprehend gravito-objects exhibiting cylindrical symmetry, we develop a perturbative framework aimed at identifying all nontrivial solutions for the scalar profiles. Introducing first-order truncated perturbation equations for the gauge field, synchronized with metric gauges and electromagnetic field considerations, we demonstrate their integrability and obtain solutions through quadrature. Our findings suggest the feasibility of obtaining self-gravitating cylindrical structures within the scalar-vector-tensor theory. These cylindrical structures could provide insights into the behavior of gravitational and gauge fields in modified gravity, potentially offering new perspectives on astrophysical phenomena such as cosmic strings and cylindrical gravitational waves.

Detecting dark domain walls through their impact on particle trajectories in tailored ultrahigh vacuum environments

Light scalar fields, with double well potentials and direct matter couplings, undergo density driven phase transitions, leading to the formation of domain walls. Such theories could explain dark energy or dark matter or source the nanohertz gravitational-wave background. We describe an experiment that could be used to detect such domain walls in a laboratory environment, solving for the scalar field profile and showing how the domain wall affects the motion of a test particle. We find that, in currently unconstrained regions of parameter space, the domain walls leave detectable signatures. Published by the American Physical Society 2024

Endowing black holes with beyond-Horndeski primary hair: An exact solution framework for scalarizing in every dimension

This work outlines a straightforward mechanism for endorsing primary hair into Schwarzschild black holes, resulting in a unique modification within the framework of a special scalar-tensor theory, the so-called beyond-Horndeski gravity. The derived solutions are exact, showcase primary hair with a regular scalar field profile everywhere, and continuously connect with the vacuum geometry. Initially devised to introduce primary hair in spherically symmetric solutions within general relativity in any dimension, our investigation explores the conditions under which spherically symmetric black holes in alternative gravitational theories become amenable to the endowment of primary hair through a similar pattern. As a preliminary exploration, we embark on the process of endowing primary hair to the Reissner-Nordström black hole. Subsequently, we extend our analysis to encompass spherically symmetric solutions within Lovelock and cubic quasitopological gravity theories. Published by the American Physical Society 2024

A generalized mixture fraction formulation for nonpremixed ammonia–hydrogen flames

For nonpremixed combustion, the mixture fraction variables are widely used as a conserved scalar to map out the flame structure in a unified manner. For ammonia–hydrogen flames, however, a proper definition of the mixture fraction variable is difficult due to the presence of fuel-bound nitrogen in ammonia. This work proposes a revised mathematical definition of the mixture fraction as a modified form of the Bilger’s formula in order to describe the characteristics of nonpremixed ammonia–hydrogen blend flames at general fuel mixture ratios. The newly defined mixture fraction properly accounts for differential diffusion effects in the presence of hydrogen, and thus serves as a convenient invariant coordinate for the flame manifolds. Counterflow nonpremixed flames with different ammonia–hydrogen ratios were computed and the validity of the new formulation is demonstrated. The analysis of the flame structure revealed the presence of double reaction zones, one dominated by the hydrogen oxidation and the other by ammonia oxidation. The role of differential diffusion is crucial in determining the existence of double reactive layers and its impact is evaluated by varying the strain rate. Detailed reaction analysis revealed that the hydrogen flame is related to HO2 reactions, while the ammonia flame to N, NH2 and NH3 reaction groups. As the strain rate is increased, OH reactions are responsible for shifting dominant heat release rate from the hydrogen flame to the ammonia flames. The scalar dissipation rate profile shows two distinct peaks, suggesting that a different functional description is needed in the flamelet modeling framework.

Direct numerical simulation of a zero-pressure-gradient turbulent boundary layer with passive scalars up to Prandtl number Pr = 6

The objective of the present study is to provide a numerical database of thermal boundary layers and to contribute to the understanding of the dynamics of passive scalars at different Prandtl numbers. In this regard, a direct numerical simulation (DNS) of an incompressible zero-pressure-gradient turbulent boundary layer is performed with the Reynolds number based on momentum thickness ${\textit {Re}}_{\theta }$ ranging up to $1080$ . Four passive scalars, characterized by the Prandtl numbers ${\textit {Pr}} = 1,2,4,6$ are simulated using the pseudo-spectral code SIMSON (Chevalier et al., SIMSON : a pseudo-spectral solver for incompressible boundary layer flows. Tech. Rep. TRITA-MEK 2007:07. KTH Mechanics, Stockholm, Sweden, 2007). To the best of our knowledge, the present DNS provides the thermal boundary layer with the highest Prandtl number available in the literature. It corresponds to that of water at ${\sim }24\,^{\circ }{\rm C}$ , when the fluid temperature is considered as a passive scalar. Turbulence statistics for the flow and thermal fields are computed and compared with available numerical simulations at similar Reynolds numbers. The mean flow and scalar profiles, root-mean-squared velocity and scalar fluctuations, turbulent heat flux, turbulent Prandtl number and higher-order statistics agree well with the numerical data reported in the literature. Furthermore, the pre-multiplied two-dimensional spectra of the velocity and of the passive scalars are computed, providing a quantitative description of the energy distribution at the different length scales for various wall-normal locations. The energy distribution of the heat-flux fields at the wall is concentrated on longer temporal structures with increasing Prandtl number. This is due to the thinner thermal boundary layer as thermal diffusivity decreases and, thereby, the longer temporal structures exhibit a different footprint at the wall.

Starting inflation from inhomogeneous initial conditions with momentum

We investigate the circumstances under which cosmic inflation can arise from very inhomogeneous initial conditions using numerical relativity simulations. Previous studies have not considered cases with non-zero momentum density due to technical challenges with solving the coupled Einstein constraint equations. Here we address these, introducing and comparing several different ways of constructing cosmological initial conditions with inhomogeneous scalar field and time derivative profiles. We evolve such initial conditions with large inhomogeneities in both single- and two-field inflationary models. We study cases where the initial gradient and kinetic energy are much larger than the inflationary energy scale, and black holes can form, as well as cases where the initial scalar potential energy is comparable, as in scenarios where inflation occurs at nearly Planckian densities, finding large-field inflation to be generally robust. We consider examples of initial conditions where a large scalar field velocity towards non-inflationary values would prevent inflation from occurring in the homogeneous case, finding that the addition of large gradients in the scalar field can actually dilute this effect, with the increased expansion and non-vanishing restoring force leading to inflation.

An explicit representation for mean profiles and fluxes in forced passive scalar convection

We derive explicit formulae for the mean profiles of passive scalars (either temperature or concentration of a diffusing substance), and their respective wall fluxes (either heat or mass fluxes), in forced turbulent convection, as a function of the Reynolds and Prandtl numbers. Direct numerical simulation data for turbulent flow within a smooth straight pipe of circular cross-section, at friction Reynolds number ${{Re}}_{\tau }=1140$ , in the range of Prandtl numbers from ${{Pr}}=0.00625$ to ${{Pr}}=16$ , are used to infer the proper analytical form of the eddy diffusivity. This is leveraged to derive accurate predictive formulae for the mean passive scalar profiles, and for the corresponding logarithmic offset function. Asymptotic scaling laws result for the thickness of the conductive (diffusive) layer, and for the Nusselt number, which significantly extend the predictive envelope of classical formulae.

Quasinormal modes from EFT of black hole perturbations with timelike scalar profile

The Effective Field Theory (EFT) of perturbations on an arbitrary background geometry with a timelike scalar profile was recently constructed in the context of scalar-tensor theories. In this paper, we use this EFT to study quasinormal frequencies of odd-parity perturbations on a static and spherically symmetric black hole background. Keeping a set of operators that can accommodate shift-symmetric quadratic higher-order scalar-tensor theories, we demonstrate the computation for two examples of hairy black holes, of which one is the stealth Schwarzschild solution and the other is the Hayward metric accompanied by a non-trivial scalar field. We emphasize that this is the first phenomenological application of the EFT, opening a new possibility to test general relativity and modified gravity theories in the strong gravity regime.

Quasinormal modes and grey-body factors of regular black holes with a scalar hair from the Effective Field Theory

The Effective Field Theory (EFT) of perturbations on an arbitrary background geometry with a timelike scalar profile has been recently constructed in the context of scalar-tensor theories. Unlike General Relativity, the regular Hayward metric is realized as an exact background metric in the Effective Field Theory with timelike scalar profile without resorting to special matter field, such as nonlinear electrodynamics. The fundamental quasinormal mode for axial graviational perturbations of this black hole has been considered recently with the help of various methods. Here we make a further step in this direction and find that, unlike the fundamental mode, a few first overtones deviate from their Schwarzschild limit at a much higher rate. This outburst of overtones occurs because the overtones are extremely sensitive to the least change of the near- horizon geometry. The analytical formula for quasinormal modes is obtained in the eikonal regime. In addition, we calculated grey-body factors and showed that the regular Hayward black hole with a scalar hair has a smaller grey-body factor than the Schwarzschild one. Integration of the wave-like equation in the time-domain shows that the power-law tails, following the ring-down phase, are indistinguishable from the Schwarzschild ones at late times.

Prandtl number effects on passive scalars in turbulent pipe flow

We study the statistics of passive scalars (either temperature or concentration of a diffusing substance) at friction Reynolds number ${Re}_{\tau }=1140$ , for turbulent flow within a smooth straight pipe of circular cross-section, in the range of Prandtl numbers from ${Pr}=0.00625$ , to ${Pr}=16$ , using direct numerical simulations (DNS) of the Navier–Stokes equations. Whereas the organization of passive scalars is similar to the axial velocity field at ${Pr} = O(1)$ , similarity is impaired at low Prandtl number, at which the buffer-layer dynamics is filtered out, and at high Prandtl number, at which the passive scalar fluctuations become confined to the near-wall layer. The mean scalar profiles at ${Pr} \gtrsim 0.0125$ are found to exhibit logarithmic overlap layers, and universal parabolic distributions in the core part of the flow. Near-universality of the eddy diffusivity is exploited to derive accurate predictive formulas for the mean scalar profiles, and for the corresponding logarithmic offset function. Asymptotic scaling formulas are derived for the thickness of the conductive (diffusive) layer, for the peak scalar variance, and its production rate. The DNS data are leveraged to synthesize a modified form of the classical predictive formula of Kader & Yaglom (Intl J. Heat Mass Transfer, vol. 15, 1972, pp. 2329–2351), which is capable of accounting accurately for the dependence on both Reynolds and Prandtl numbers, for ${Pr} \gtrsim 0.25$ .

Approximately stealth black hole in higher-order scalar-tensor theories

We investigate a generic quadratic higher-order scalar-tensor theory with a scordatura term, which is expected to provide a consistent perturbative description of stealth solutions with a timelike scalar field profile. In the DHOST subclass, exactly stealth solutions are known to yield perturbations infinitely strongly coupled and thus cannot be trusted. Beyond DHOST theories with the scordatura term, such as in ghost condensation and U-DHOST, we show that stealth configurations cannot be realized as exact solutions but those theories instead admit approximately stealth solutions where the deviation from the exactly stealth configuration is controlled by the mass scale M of derivative expansion. The approximately stealth solution is time-dependent, which can be interpreted as the black hole mass growth due to the accretion of the scalar field. From observed astrophysical black holes, we put an upper bound on M as ĉ D1 1/2 M ≲ 2 × 1011 GeV, where ĉ D1 is a dimensionless parameter of order unity that characterizes the scordatura term. As far as M is sufficiently below the upper bound, the accretion is slow and the approximately stealth solutions can be considered as stealth at astrophysical scales for all practical purposes while perturbations are weakly coupled all the way up to the cutoff M and the apparent ghost is as heavy as or heavier than M.

The gravitational afterglow of boson stars

In this work we study the long-lived post-merger gravitational wave signature of a boson-star binary coalescence. We use full numerical relativity to simulate the post-merger and track the gravitational afterglow over an extended period of time. We implement recent innovations for the binary initial data, which significantly reduce spurious initial excitations of the scalar field profiles, as well as a measure for the angular momentum that allows us to track the total momentum of the spatial volume, including the curvature contribution. Crucially, we find the afterglow to last much longer than the spin-down timescale. This prolonged gravitational wave afterglow provides a characteristic signal that may distinguish it from other astrophysical sources.

Predicting radial profiles for jets with arbitrary buoyancy

The present study investigates the profiles of statistically axisymmetric turbulent jets with arbitrary buoyancy. Analytical expressions for the shape of the radial velocity, Reynolds stress and radial scalar flux profiles are derived from the governing equations by assuming self-similar Gaussian mean velocity and scalar profiles. Previously these have only been derived for the special cases of pure jets and plumes, whereas the present study generalises them to arbitrary buoyancies. These are then used to derive analytical expressions for the turbulent Schmidt/Prandtl numbers, which, along with the mean profiles, are shown to give predictions in agreement with existing literature.