Hole Boundary Research Articles

Quantification of low-cycle fatigue damage accumulation in stress concentration area by local strain evolution

The novel non-destructive method for quantitative description of low-cycle fatigue damage accumulation is proposed and implemented. The key point, that defines scientific novelty and powerfulness of this approach, consists of involving various deformation parameters referred to critical point belonging to the hole boundary as current damage indicator. Required strain values follow from distributions of all three displacement components along the hole edge measured by reflection hologram interferometry. The derived data at different stage of low-cycle fatigue lead to normalized dependencies of various strain parameters from number of loading cycle, which are a source of damage accumulation functions.

Stress analysis of a circular hole in an orthotropic plate with finite width using complex potential method

AbstractOpen holes are essential design features in technical structures. However, stress concentrations arise and for a safe and lightweight optimal design, precise stress analysis means are crucial. Scope of this paper is the stress field determination of an open hole in an orthotropic rectangular plate of finite width using complex potential method. To derive results of high precision, the stress boundary conditions at the straight edges as well as along the hole boundary need to be fulfilled. In doing so, the stress field of the corresponding infinite‐dimensions problem is supplemented with auxiliary functions, which are based on a novel periodic arrangement technique and enable to take into account the stress‐free straight edges of the finite‐domain problem.

Dynamic stress responses of defects in periodic heterogeneous media under SH waves

Due to the complexity of heterogeneous medium models, the corresponding research methods still have great limitations, and hence, there is a dearth of research findings. To study the problem of dynamic stress concentration in heterogeneous media, the scattering of SH waves by circular holes in a full-space periodic heterogeneous continuum medium is investigated based on the propagation characteristics of elastic waves in complex heterogeneous media. The wave equation of SH waves propagating in heterogeneous media with the elastic shear modulus and density varying in the form of trigonometric functions is solved by using the complex function method, the wave function expansion method, and the auxiliary function method. On this basis, the wave field and stress field are derived, and the expression of the dynamic stress concentration factor (DSCF) on the boundary of a circular hole is given. The effects of the reference wavenumber and the variation period of the heterogeneous medium on the distribution of the DSCF on the boundary of a circular hole are analyzed through numerical examples. The results show that when the circular hole defect is within one heterogeneous period of the medium, shortening of the heterogeneous period of the medium reduces the peak of the DSCF due to high-frequency waves on the boundary of the circular hole.

Infrared computer vision in non-destructive imaging: Sharp delineation of subsurface defect boundaries in enhanced truncated correlation photothermal coherence tomography images using K-means clustering

This work investigates the non-destructive detection of defects in thermal images of industrial materials based on segmentation of images generated using enhanced truncated-correlation photothermal coherence tomography (eTC-PCT). eTC-PCT is an active infrared thermography modality, which is being applied to the field of non-destructive testing (NDT) and in biomedical & dental thermophotonic imaging. In this report, we combine eTC-PCT with a computer vision algorithm to sharply delineate holes and manufacturing defects (cracks) inside industrial materials. To this end, the eTC-PCT reconstructed image is processed through three consecutive algorithm stages: A threshold selection filter is followed by filtered image segmentation using the K-means algorithm (clustering method) and the outcome is applied to the delineation of (otherwise blurred) discontinuity boundaries by means of the Canny edge detection algorithm. The role of each method is described and it is demonstrated that the combination of these three algorithms is optimal for achieving significant delineation enhancement (sharpness) of blind hole and crack boundaries in industrial materials.

Ultimate fate of apparent horizons during a binary black hole merger. II. The vanishing of apparent horizons

In this second part of a two-part paper, we discuss numerical simulations of a head-on merger of two non-spinning black holes. We resolve the fate of the original two apparent horizons by showing that after intersecting, their world tubes "turn around" and continue backwards in time. Using the method presented in the first paper to locate these surfaces, we resolve several such world tubes evolving and connecting through various bifurcations and annihilations. This also draws a consistent picture of the full merger in terms of apparent horizons, or more generally, marginally outer trapped surfaces (MOTSs). The MOTS stability operator provides a natural mechanism to identify MOTSs which should be thought of as black hole boundaries. These are the two initial ones and the final remnant. All other MOTSs lie in the interior and are neither stable nor inner trapped.

What Happens to Apparent Horizons in a Binary Black Hole Merger?

We resolve the fate of the two original apparent horizons during the head-on merger of two nonspinning black holes. We show that, following the appearance of the outer common horizon and subsequent interpenetration of the original horizons, they continue to exist for a finite period of time before they are individually annihilated by unstable marginally outer trapped surfaces (MOTSs). The inner common horizon vanishes in a similar, though independent, way. This completes the understanding of the analog of the event horizon's "pair of pants" diagram for the apparent horizon. Our result is facilitated by a new method for locating MOTSs based on a generalized shooting method. We also discuss the role played by the MOTS stability operator in discerning which among a multitude of MOTSs should be considered as black hole boundaries.

Generalized holographic cosmology: low-redshift observational constraint

Four-dimensional cosmological models are studied on a boundary of a five-dimensional Anti-de Sitter (AdS5) black hole with AdS Reissner-Nordström and scalar charged Reissner-Nordström black hole solutions, where we call the former a “Hairless” black hole and the latter a “Hairy” black hole. To obtain the Friedmann-Robertson-Walker (FRW) spacetime metric on the boundary of the AdS5 black hole, we employ Eddington-Finkelstein (EF) coordinates to the bulk geometry. We then derive modified Friedmann equations on a boundary of the AdS5 black hole via AdS/CFT correspondence and discuss its cosmological implications. The late-time acceleration of the universe is investigated in our models. The contributions coming from the bulk side is treated as dark energy source, and we perform MCMC analyses using observational data. Compared to the ΛCDM model, our models contain additional free parameters; therefore, to make a fair comparison, we use the Akaike information criterion (AIC) and the Bayesian information criterion (BIC) to analyze our results. Our numerical analyses show that our models can explain the observational data as reliable as the ΛCDM model does for the current data.

Analytical stress solution for rock mass containing two holes based on an improved Schwarz alternating method

In this study, the accuracy and generality of stress calculation for a plate containing double holes via Schwarz alternating method was investigated. The original Schwarz alternating method with inverse mapping function shows a good performance in terms of the stress solution for a plate containing two arched holes. However, for a plate containing two holes with corners, evitable error may be produced during the calculation because of the unsatisfying accuracy of inverse mapping function. This error is particularly obvious in the stress calculation for a plate containing two triangular holes. To address the problem, an improved Schwarz alternating method without the consideration of inverse mapping function was proposed in this study. The coordinate transformation of points around hole boundaries is realized by an optimization method instead of the inverse mapping function, which leads to little error during the calculation and thus improves the stress solution accuracy especially for plates containing holes with corners. With the employment of the improved method, the influences of connecting angle and lateral pressure coefficient on stress solutions for plates containing double U-shaped holes were discussed. In addition, DEM simulations were further conducted to study the influence of stress characteristics on the failure patterns of rock mass. The result shows that the hoop stress linearly relates to lateral pressure coefficient in the elastic plate and fractures always initiate around hole boundaries where high stress concentrates.

Oscillations in the extreme mass-ratio inspiral gravitational wave phase correction as a probe of a reflective boundary of the central black hole

We discuss the energy loss due to gravitational radiation of binaries composed of exotic objects whose horizon boundary conditions are replaced with reflective ones. Our focus is on the extreme mass-ratio inspirals, in which the central heavier black hole is replaced with an exotic compact object. We show, in this case, a modulation of the energy loss rate depending on the evolving orbital frequency occurs and leads to two different types of modifications to the gravitational wave phase evolution; the oscillating part directly corresponding to the modulation in the energy flux, and the non-oscillating part coming from the quadratic order in the modulation. This modification can be sufficiently large to detect with future space-borne detectors.

Solar wind rotation rate and shear at coronal hole boundaries

Context.In situ measurements by several spacecraft have revealed that the solar wind is frequently perturbed by transient structures that have been interpreted as magnetic folds, jets, waves, and flux ropes that propagate rapidly away from the Sun over a large range of heliocentric distances. Parker Solar Probe (PSP), in particular, has detected very frequent rotations of the magnetic field vector at small heliocentric radial distances, accompanied by surprisingly large solar wind rotation rates. The physical origin of such magnetic field bends and switchbacks, the conditions for their survival across the interplanetary space, and their relation to solar wind rotation are yet to be clearly understood.Aims.We aim to characterise the global properties of the solar wind flows crossed by PSP, to relate those flows to the rotational state of the low solar corona, and to identify regions of the solar surface and corona that are likely to be sources of switchbacks and bends.Methods.We traced measured solar wind flows from the spacecraft position down to the surface of the Sun to identify their potential source regions, and used a global magneto-hydrodynamic model of the corona and solar wind to analyse the dynamical properties of those regions. We identify regions of the solar corona for which solar wind speed and rotational shear are important and long-lived that can be favourable to the development of magnetic deflections and to their propagation across extended heights in the solar wind.Results.We show that coronal rotation is highly structured, and that enhanced flow shear and magnetic field gradients develop near the boundaries between coronal holes and streamers, and around and above pseudo-streamers, even when such boundaries are aligned with the direction of solar rotation. The exact properties and amplitudes of the shears are a combined effect of the forces exerted by the rotation of the corona and of its magnetic topology. A large fraction of the switchbacks identified by PSP map back to these regions, both in terms of instantaneous magnetic field connectivity and of the trajectories of wind streams that reach the spacecraft.Conclusions.We conclude that these regions of strong shears are likely to leave an imprint on the solar wind over large distances and to increase the transverse speed variability in the slow solar wind. The simulations and connectivity analysis suggest they could be a source of the switchbacks and spikes observed by PSP.

A novel auxiliary hole localization algorithm based on multidimensional scaling for wireless sensor networks in complex terrain with holes

As the most critical part of wireless sensor networks, nodes localization plays a decisive role in determining the validity and value of the data collected by the sensors. A novel auxiliary hole localization algorithm based on multidimensional scaling has been proposed in the present study to overcome the poor nodes localization for wireless sensor networks in complex terrain with holes. In the algorithm research, the types of the holes in wireless sensor networks are originally classified and defined, and the localization strategy for the hole boundary nodes is designed. On this basis, an improved distance optimization method is employed to derive the formula of the distance between nodes blocked by holes. Finally, on the strength of the improved distance value, the nodes localization task is completed through the multidimensional scaling-MAP algorithm. As shown by the simulation results, the proposed algorithm is suitable for locating the nodes of wireless sensor networks in complex terrain with different types of holes. In addition, under the condition of each type of hole localization, the localization effect is better than the same type of extended Kalman filter multidimensional scaling algorithm.

Plasma properties, switchback patches, and low α-particle abundance in slow Alfvénic coronal hole wind at 0.13 au

ABSTRACT The Parker Solar Probe (PSP) mission presents a unique opportunity to study the near-Sun solar wind closer than any previous spacecraft. During its fourth and fifth solar encounters, PSP had the same orbital trajectory, meaning that solar wind was measured at the same latitudes and radial distances. We identify two streams measured at the same heliocentric distance (∼0.13 au) and latitude (∼–$3{_{.}^{\circ}}5$) across these encounters to reduce spatial evolution effects. By comparing the plasma of each stream, we confirm that they are not dominated by variable transient events, despite PSP’s proximity to the heliospheric current sheet. Both streams are consistent with a previous slow Alfvénic solar wind study once radial effects are considered, and appear to originate at the Southern polar coronal hole boundary. We also show that the switchback properties are not distinctly different between these two streams. Low α-particle abundance (∼0.6 per cent) is observed in the encounter 5 stream, suggesting that some physical mechanism must act on coronal hole boundary wind to cause α-particle depletion. Possible explanations for our observations are discussed, but it remains unclear whether the depletion occurs during the release or the acceleration of the wind. Using a flux tube argument, we note that an α-particle abundance of ∼0.6 per cent in this low-velocity wind could correspond to an abundance of ∼0.9 per cent at 1 au. Finally, as the two streams roughly correspond to the spatial extent of a switchback patch, we suggest that patches are distinct features of coronal hole wind.

Uniform convergence and asymptotics for problems in domains finely perforated along a prescribed manifold in the case of the homogenized Dirichlet condition

A boundary value problem for a second-order elliptic equation with variable coefficients is considered in a multidimensional domain which is perforated by small holes along a prescribed manifold. Minimal natural conditions are imposed on the holes. In particular, all of these are assumed to be of approximately the same size and have a prescribed minimal distance to neighbouring holes, which is also a small parameter. The shape of the holes and their distribution along the manifold are arbitrary. The holes are divided between two sets in an arbitrary way. The Dirichlet condition is imposed on the boundaries of holes in the first set and a nonlinear Robin boundary condition is imposed on the boundaries of holes in the second. The sizes and distribution of holes with the Dirichlet condition satisfy a simple and easily verifiable condition which ensures that these holes disappear after homogenization and a Dirichlet condition on the manifold in question arises instead. We prove that the solution of the perturbed problem converges to the solution of the homogenized one in the -norm uniformly with respect to the right-hand side of the equation, and an estimate for the rate of convergence that is sharp in order is deduced. The full asymptotic solution of the perturbed problem is also constructed in the case when the holes form a periodic set arranged along a prescribed hyperplane. Bibliography: 32 titles.

Multi-channel coronal hole detection with convolutional neural networks

Context.A precise detection of the coronal hole boundary is of primary interest for a better understanding of the physics of coronal holes, their role in the solar cycle evolution, and space weather forecasting.Aims.We develop a reliable, fully automatic method for the detection of coronal holes that provides consistent full-disk segmentation maps over the full solar cycle and can perform in real-time.Methods.We use a convolutional neural network to identify the boundaries of coronal holes from the seven extreme ultraviolet (EUV) channels of the Atmospheric Imaging Assembly (AIA) and from the line-of-sight magnetograms provided by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). For our primary model (Coronal Hole RecOgnition Neural Network Over multi-Spectral-data; CHRONNOS) we use a progressively growing network approach that allows for efficient training, provides detailed segmentation maps, and takes into account relations across the full solar disk.Results.We provide a thorough evaluation for performance, reliability, and consistency by comparing the model results to an independent manually curated test set. Our model shows good agreement to the manual labels with an intersection-over-union (IoU) of 0.63. From the total of 261 coronal holes with an area > 1.5 × 1010km2identified during the time-period from November 2010 to December 2016, 98.1% were correctly detected by our model. The evaluation over almost the full solar cycle no. 24 shows that our model provides reliable coronal hole detections independent of the level of solar activity. From a direct comparison over short timescales of days to weeks, we find that our model exceeds human performance in terms of consistency and reliability. In addition, we train our model to identify coronal holes from each channel separately and show that the neural network provides the best performance with the combined channel information, but that coronal hole segmentation maps can also be obtained from line-of-sight magnetograms alone.Conclusions.The proposed neural network provides a reliable data set for the study of solar-cycle dependencies and coronal-hole parameters. Given the fast and robust coronal hole segmentation, the algorithm is also highly suitable for real-time space weather applications.

Latitude Variations in Primary and Secondary Polar Crown Polarity Inversion Lines and Polar Coronal Hole Boundaries over Five Solar Cycles

Latitude Variations in Primary and Secondary Polar Crown Polarity Inversion Lines and Polar Coronal Hole Boundaries over Five Solar Cycles

Composition of Coronal Hole Boundary Layers at Low Heliographic Latitudes

AbstractIn some polar coronal holes, previous studies have identified a coronal hole boundary layer (CHBL), which is different in composition from the coronal hole core region. However, compositionally distinct CHBL structure at low heliographic latitudes remains underexplored. Using solar wind composition measurements from the Advanced Composition Explorer (ACE), we study two particular low‐latitude coronal holes of 2003: a large polar hole extension with outward magnetic polarity and a smaller equatorial hole with inward magnetic polarity. For both of these coronal holes, we examine the distribution of two composition parameters, the O7+ to O6+ charge state density ratio (n(O7+)/n(O6+)) and the Fe to O abundance ratio (n(Fe)/n(O)), and find that each hole has an identifiable CHBL based on an anticorrelation of solar wind speed and n(O7+)/n(O6+). We also examine the FIP (first ionization potential) effect in the CHBL using n(Fe)/n(O), and find that there is no significant change from the CHBL to the core region for one of the coronal holes we study, while the other coronal hole has a noticeable decrease in n(Fe)/n(O) values in its core. Finally, we discuss our results in the context of previous work studying CHBL structure.

First law of entanglement entropy in AdS black hole backgrounds

The first law for entanglement entropy in CFT in an odd-dimensional asymptotically AdS black hole is studied by using the AdS/CFT duality. The entropy of CFT considered here is due to the entanglement between two subsystems separated by the horizon of the AdS black hole, which itself is realized as the conformal boundary of a black droplet in even-dimensional global AdS bulk spacetime. In $(2+1)$-dimensional CFT, the first law is shown to be always satisfied by analyzing a class of metric perturbations of the exact solution of a $4$-dimensional black droplet. In $(4+1)$-dimensions, the first law for CFT is shown to hold under the Neumann boundary condition at a certain bulk hypersurface anchored to the conformal boundary of the boundary AdS black hole. From the boundary view point, this Neumann condition yields there being no energy flux across the boundary of the boundary AdS black hole. Furthermore, the asymptotic geometry of a $6$-dimensional small AdS black droplet is constructed as the gravity dual of our $(4+1)$-dimensional CFT, which exhibits a negative energy near the spatial infinity, as expected from vacuum polarization.

Radio evidence for a shock wave reflected by a coronal hole

We report the first unambiguous observational evidence in the radio range of the reflection of a coronal shock wave at the boundary of a coronal hole. The event occurred above an active region located at the northwest limb of the Sun and was characterized by an eruptive prominence and an extreme-ultraviolet wave steepening into a shock. The EUV observations acquired by the Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory and the Extreme Ultraviolet Imager instrument on board the Solar TErrestrial RElations Observatory were used to track the development of the EUV front in the inner corona. Metric type II radio emission, a distinguishing feature of shock waves propagating in the inner corona, was simultaneously recorded by ground-based radio spectrometers. The radio dynamic spectra displayed an unusual reversal of the type II emission lanes, together with type III-like herringbone emission, indicating shock-accelerated electron beams. Combined analysis of imaging data from the two space-based EUV instruments and the Nançay Radioheliograph evidences that the reverse-drifting type II emission was produced at the intersection of the shock front, reflected at a coronal hole boundary, with an intervening low-Alfvén-speed region characterized by an open field configuration. We also provide an outstanding data-driven reconstruction of the spatiotemporal evolution in the inner corona of the shock-accelerated electron beams produced by the reflected shock.

Analytical solutions and boundary element analysis for holes and cracks in anisotropic viscoelastic solids via time-stepping method

In this study the time-stepping method, which can convert the problem of anisotropic viscoelasticity into two sets of elastic-like system, is employed for both analytical solutions and boundary element method (BEM) to solve the two-dimensional problems of holes and cracks in anisotropic viscoelastic solids. Based upon this method and the available solutions for the corresponding elastic problems, some analytical solutions for the two-dimensional problems of holes and cracks in anisotropic viscoelastic solids are presented, such as the one with an elliptical hole, a straight crack, two collinear cracks, or collinear periodic cracks under uniform load at infinity. We also present the fundamental solution for the problems with an elliptical hole, and use this solution to construct a special boundary element method (SBEM). Further extension of SBEM is made to develop the boundary-based finite element method (BFEM) to deal with the problems with multiple holes and cracks. The advantage of SBEM and BFEM is that no meshes are required on the boundaries of holes and cracks. To show the accuracy and efficiency of the proposed method, several representative examples are implemented and compared with those obtained by the finite element method and the elastic-viscoelastic correspondence principle. The results show that the solutions obtained by different methods are in well agreement with each other, and the ones obtained by the time-stepping method are around 3 to 36 times faster than the others depending on the problems and methods.

Trapped region in Kerr–Vaidya space–time

We review the basic definitions and properties of trapped surfaces and discuss them in the context of Kerr-Vaidya line-element. Our study shows that the apparent horizon does not exist in general for axisymmetric space-times. The reason being the surface at which the null tangent vectors are geodesic and the surface at which the expansion of such vectors vanishes do not coincide. Calculation of an approximate apparent horizon for space-times that ensure its existence seems to be the only way to get away with this problem. The approximate apparent horizon, however, turned out to be non-unique. The choice of the shear free null geodesics, at least in the leading order, seem to remove this non-uniqueness. We also propose a new definition of the black hole boundary.