Boundary Integral Equation Method Research Articles

Generalized Boundary Integral Equation Method for Boundary Value Problems of Two-D Isotropic Lattice Laplacian

Generalized Boundary Integral Equation Method for Boundary Value Problems of Two-D Isotropic Lattice Laplacian

A boundary point interpolation method for acoustic problems with particular emphasis on the calculation of Cauchy principal value integrals

A boundary point interpolation method (BPIM) is presented in this paper for numerical solving acoustic problems. The BPIM is a boundary-type meshless method that combines the point interpolation technique for constructing approximate functions with boundary integral equations for reformulating boundary value problems. Since effective numerical integration techniques are crucial in the BPIM and boundary integral equation methods for calculating regular and singular integrals, two Clenshaw-Curtis integration techniques (CCITs) for regular and weakly singular integrals are derived in a unified manner with the same integration points on quadratic integration cells. In addition, a unilateral CCIT, a power series expansion technique, and an improved CCIT are proposed to effectively calculate strongly singular integrals or Cauchy principal value integrals in the BPIM. These integration techniques provide direct and efficient formulas for calculating boundary integrals on quadratic integration cells, and can be directly applied to the calculation of regular and singular integrals in the boundary element method and other meshless boundary integral equation methods. Explicit expressions of integration weights in all CCITs are derived, and the values of integration points and weights are listed. Numerical results are finally provided to validate the effectiveness of the present meshless method and integration techniques.

Potential method in the coupled theory of thermoelastic triple-porosity nanomaterials

In this paper, the coupled linear theory of thermoelasticity for nanomaterials with triple porosity is considered in which the combination of Darcy’s law and the volume fraction concept for three levels of pores (macro-, meso- and micropores) is provided. The 3D basic boundary value problems (BVPs) of steady vibrations of this theory are formulated and these BVPs are investigated using the potential method (boundary integral equation method) and the theory of singular integral equations. Namely, the formula of integral representation of regular vectors is obtained. The surface (single-layer and double-layer) and volume potentials are introduced and their basic properties are given. Some useful singular integral operators are defined for which Noether’s theorems are valid. The symbolic determinants and indexes of these operators are calculated. The BVPs of steady vibrations are reduced to the equivalent singular integral equations. Finally, with the help of the potential method, we prove the existence theorems for classical solutions of the aforementioned BVPs.

Dynamics of multi-mode nonlinear internal waves in fluids of great depth

This paper is mainly concerned with the dynamics of strongly nonlinear internal waves in two-dimensional fluids of great depth. A fully nonlinear model for a three-layer fluid of great depth containing topography, a Miyata–Choi–Camassa (MCC)-type system, is developed based on the generalization of the Ablowitz–Fokas–Musslimani global-relation formulation for free-surface water waves. Mode-1 internal solitary waves are numerically found in the MCC-type equation using the Petviashvili iteration technique and in the full Euler equations based on the boundary integral equation method. The comparison between the results validates the broad applicability of the MCC-type system. Mode-2 internal solitary waves are found in the MCC-type equations and are shown to be a type of gap solitary wave. Using the multi-mode internal solitary-wave solutions in the MCC-type equations, we apply the conformal mapping technique to calculate streamlines and particle trajectories. For unsteady simulations, we propose a pseudo-spectral algorithm to handle the time-coupled equations and investigate the generation mechanism of multi-mode internal solitary waves due to the resonance effect of local protrusion on the rigid wall, as well as their collisions when the wall is flat. For flows past protrusion on the rigid wall, the flow speed can be divided into subcritical, transcritical, and supercritical. The shedding of multi-mode internal solitary waves can occur only in the transcritical region of flow speed. We implement the carving of bifurcation curves with inflection to achieve the regional delimitation and, based on this, the interpretation of the generation mechanism from a physical point of view.

Fracture Analysis of Planar Cracks in 3D Thermal Piezoelectric Semiconductors

The study of piezoelectric semiconductor (PSC) materials and devices is experiencing rapid growth, giving rise to a burgeoning research domain known as piezotronics. Understanding the fracture behavior of PSC materials is essential for designing robust devices and ensuring their long-term functionality. A planar crack of arbitrary shape in the isotropic plane of a three-dimensional (3D) transversely isotropic thermal PSC body subjected to combined thermal-electrical-mechanical loadings is studied via the extended displacement discontinuity (EDD) boundary integral equation method. Under the framework of linearized basic equations and one-way thermal coupling, the hypersingular boundary integral equations with the volume integrals containing the temperature and carrier concentration are obtained and expressed by the EDDs which include displacement, electric potential, carrier concentration and temperature discontinuities across the crack surfaces and are the basic variables in analyzing crack problems. Based on the finite part of hypersingular integrals, the EDDs near the crack border are proved to have the classical behavior of O(r). Furthermore, the asymptotic solutions of extended stresses including the stress, electric displacement, current density and heat flux at the crack front are presented and display classical singular behavior of O(1/r). The extended stress intensity factors are defined and given in terms of EDDs. A 3D numerical model of a penny-shaped crack in the thermal PSC cylinder is established and used to verify the presented formulae.

A Boundary Integral Equation Method for the Complete Electrode Model in Electrical Impedance Tomography with Tests on Experimental Data

A Boundary Integral Equation Method for the Complete Electrode Model in Electrical Impedance Tomography with Tests on Experimental Data

Efficient convergent boundary integral methods for slender bodies

The interaction of fibers in a viscous (Stokes) fluid plays a crucial role in industrial and biological processes, such as sedimentation, rheology, transport, cell division, and locomotion. Numerical simulations generally rely on slender body theory (SBT), an asymptotic, nonconvergent approximation whose error blows up as fibers approach each other. Yet convergent boundary integral equation (BIE) methods which completely resolve the fiber surface have so far been impractical due to the prohibitive cost of layer-potential quadratures in such high aspect-ratio 3D geometries. We present a high-order Nyström quadrature scheme with aspect-ratio independent cost, making such BIEs practical. It combines centerline panels (each with a small number of poloidal Fourier modes), toroidal Green's functions, generalized Chebyshev quadratures, HPC parallel implementation, and FMM acceleration. We also present new BIE formulations for slender bodies that lead to well conditioned linear systems upon discretization. We test Laplace and Stokes Dirichlet problems, and Stokes mobility problems, for slender rigid closed fibers with (possibly varying) circular cross-section, at separations down to 1/20 of the slender radius, reporting convergence typically to at least 10 digits. We use this to quantify the breakdown of numerical SBT for close-to-touching rigid fibers. We also apply the methods to time-step the sedimentation of 512 loops with up to 1.65 million unknowns at around 7 digits of accuracy.

Bragg Resonance of Surface Gravity Waves by Surface-Piercing Vertical Multi-Plate Breakwaters

Abstract The interaction between surface gravity waves and surface-piercing vertical multi-plate breakwaters in finite water depth is investigated under the assumption of potential flow theory. The time-domain desingularized boundary integral equation method (DBIEM) is adopted to solve the boundary value problem. A decomposition method based on the superposition principle is used to obtain reflected waves. The effect of the vertical plates as a floating breakwater by making full use of Bragg resonance is analyzed. Initially, the waveforms calculated without breakwaters are compared with second-order Stokes regular wave to obtain the appropriate desingularization distance. Then, the interaction of surface gravity waves with the breakwaters with the single plate, double, and triple plates are analyzed. The transmission and reflection coefficients of the breakwater with the single plate are verified with the analytical solutions of previous literature. The transmission and reflection coefficients of the breakwaters with double and triple plates are verified with experimental results. Finally, the transmission and reflection coefficients of surface-piercing vertical multi-plate breakwaters with different plate numbers and drafts are also calculated. The occurring condition of Bragg resonance and its values affected by the plate number and the draft are analyzed. The present results are reference values for the optimal design of vertical plates as a floating breakwater for offshore and coastal engineering applications by fully using Bragg resonance.

Nachal'no-kraevye zadachi dlya odnorodnykh parabolicheskikh sistem v poluogranichennoy ploskoy oblasti i uslovie dopolnitel'nosti

We consider initial–boundary value problems for homogeneous parabolic systemswith coefficients satisfying the double Dini condition with zero initial conditions in a semiboundedplane domain with nonsmooth lateral boundary. The method of boundary integralequations is used to prove a theorem on the unique classical solvability of such problems in thespace of functions that are continuous together with their first spatial derivative in the closureof the domain. An integral representation of the obtained solutions is given. It is shown thatthe condition for the solvability of the posed problems considered in the paper is equivalent tothe well-known complementarity condition.

A highly accurate indirect boundary integral equation solution for three dimensional elastic scattering problem

In this paper, we propose a numerical formula for three dimensional elastic wave scattering problem. Different from the classical boundary integral equation method, the three dimensional elastic scattering problem is transformed into a coupled boundary value problem including a scalar Helmholtz equation and a vector Helmholtz equation by the decomposition of the displacement. The coupled boundary value problem can be solved by the boundary integral equations based on the Helmholtz equation. A denseness result is given to support this algorithm. A comparative study with the classical method of fundamental solutions getting from the boundary integral equation method is given to check the accuracy with different types of boundary conditions. From the numerical results, the present method can give more accurate results than the classical method of fundamental solutions method.

An Adaptive Fast-Multipole-Accelerated Hybrid Boundary Integral Equation Method for Accurate Diffusion Curves

In theory, diffusion curves promise complex color gradations for infinite-resolution vector graphics. In practice, existing realizations suffer from poor scaling, discretization artifacts, or insufficient support for rich boundary conditions. Previous applications of the boundary element method to diffusion curves have relied on polygonal approximations, which either forfeit the high-order smoothness of Bézier curves, or, when the polygonal approximation is extremely detailed, result in large and costly systems of equations that must be solved. In this paper, we utilize the boundary integral equation method to accurately and efficiently solve the underlying partial differential equation. Given a desired resolution and viewport, we then interpolate this solution and use the boundary element method to render it. We couple this hybrid approach with the fast multipole method on a non-uniform quadtree for efficient computation. Furthermore, we introduce an adaptive strategy to enable truly scalable infinite-resolution diffusion curves.

Robust fast direct integral equation solver for three-dimensional doubly periodic scattering problems with a large number of layers

Frequency-domain wave scattering problems that arise in acoustics and electromagnetism can be often described by the Helmholtz equation. This work presents a boundary integral equation method for the Helmholtz equation in 3-D multilayered media with many doubly periodic smooth layers. Compared with conventional quasi-periodic Green's function method, the new method is robust at all scattering parameters. A periodizing scheme is used to decompose the solution into near- and far-field contributions. The near-field contribution uses the free-space Green's function in an integral equation on the interface in the unit cell and its immediate eight neighbors; the far-field contribution uses proxy point sources that enclose the unit cell. A specialized high-order quadrature is developed to discretize the underlying surface integral operators to keep the number of unknowns per layer small. We achieve overall linear computational complexity in the number of layers by reducing the linear system into block tridiagonal form and then solving the system directly via block LU decomposition. The new solver is capable of handling a 100-interface structure with 961.3k unknowns to 10−5 accuracy in less than 2 hours on a desktop workstation.

An efficient method for transcranial ultrasound focus correction based on the coupling of boundary integrals and finite elements

Transcranial focused ultrasound is a novel technique for the noninvasive treatment of brain diseases. The success of the treatment greatly depends on achieving precise and efficient intraoperative focus. However, compensating for aberrated ultrasound waves caused by the skull through numerical simulation-based phase corrections is a challenging task due to the significant computational burden involved in solving the acoustic wave equation. In this article, we propose a promising strategy using the coupling of the boundary integral equation method (BIEM) and the finite element method (FEM) to overcome the above limitation. Specifically, we adopt the BIEM to obtain the Robin-to-Dirichlet maps on the boundaries of the skull and then couple the maps to the FEM matrices via a dual interpolation technique, resulting in a computational domain including only the skull. Three simulation experiments were conducted to evaluate the effectiveness of the proposed method, including a convergence test and two skull-induced aberration corrections in 2D and 3D ultrasound. The results show that the method’s convergence is guaranteed as the element size decreases, leading to a decrease in pressure error. The computation times for simulating a 500 kHz ultrasound field on a regular desktop computer were found to be 0.47 ± 0.01 s in the 2D case and 43.72 ± 1.49 s in the 3D case, provided that lower–upper decomposition (approximately 13 s in 2D and 2.5 h in 3D) was implemented in advance. We also demonstrated that more accurate transcranial focusing can be achieved by phase correction compared to the noncorrected results (with errors of 1.02 mm vs. 6.45 mm in 2D and 0.28 mm vs. 3.07 mm in 3D). The proposed strategy is valuable for enabling online ultrasound simulations during treatment, facilitating real-time adjustments and interventions.

Spectral boundary integral equation method for simulation of 2D and 3D slip ruptures at bi‐material interfaces

AbstractThis paper presents a 3D spectral numerical scheme that can accurately model planarcracks of any shape residing on an interfacial plane between two elastic solids. The cracks propagate dynamically under the action of interfacial loads and interfacial constitutive laws. The scheme is a spectral form of the boundary integral equations (BIE) that relate the displacement discontinuity fields along the interfacial plane to the stress fields at the plane. In the BIE, the displacement discontinuities are expressed as a spatio‐temporal convolution of the traction components at the interface. This is in contrast with most previous studies which use a spatio‐temporal convolution of the displacement discontinuities or of the displacements at the interface. Due to the continuity of tractions across the interface, the present formulation is simpler, resulting in convolution kernels that can be written in closed‐form. Previous studies have relied on numerically obtained convolution kernels. The accuracy of the proposed scheme is validated with the known analytical solution of the 3D Lamb problem. Furthermore, new algorithms are developed for coupling the BIE with a slip‐weakening friction law, both for homogeneous materials as well as for bi‐material interfaces. This results in a widely applicable formulation for studying spontaneous rupture propagation. The algorithms are validated by comparing rupture simulation exercises to benchmark problems from Southern California Earthquake Center (SCEC). Finally, the methodology is used to simulate 3D frictional rupture propagation along a bi‐material interface.

Are the direct and indirect BEM/BIEMs equivalent?

For a long time, direct and indirect BEM/BIEMs were always unified and seemed equivalent in real implementation for ensuring the solution. Regarding the solution space and the numerical aspects in linear algebra, it is interesting to find that they are not fully equivalent. To demonstrate this finding, antiplane shear problems containing the elliptical rigid inclusion and hole as well as rigid-line inclusion and line crack were given to demonstrate the non-equivalence between the direct and indirect BEM/BIEMs. It is interesting to find that nonunique boundary densities may yield the unique field solution in both analytical derivation and numerical implementation. The mechanism was analytically studied and numerically performed by using the degenerate kernel and the singular value decomposition technique, respectively. Besides, the Fredholm alternative theorem was employed to examine the solvability condition in both the indirect boundary integral equation method and indirect boundary element method. For continuous system and discrete system, we can explain why the boundary density does not match the exact solution but the field solution is always acceptable when setting ξ0 = 0 in the beginning of the derivation of BIE. Two approaches, direct and indirect BEM/BIEMs, show the non-equivalence of solution in case of degenerate boundary and degenerate scale.

Synthetic eddy generation and modelling of turbine operation in a turbulent tidal flow

In the operation of hydrokinetic turbines in tidal sites, the effect of onset flow turbulence on device performance and reliability deserves particular attention. Computational Fluid Dynamics (CFD) models capable to describe this complex phenomenology are necessary to predict harmful operating conditions and to implement design strategies to mitigate the impacts. Similarly, the description of the interaction between turbine wakes and eddies in the onset turbulent flow is important in the study of tidal arrays.
 An original CFD methodology to simulate the operation of a tidal turbine in an arbitrary turbulent flow is presented. The methodology falls within the class of Synthetic Methods (SM). A randomly fluctuating velocity field is generated by a volume force distribution acting as forcing term in the Navier-Stokes equations. With respect to existing volume-force SM, an original definition of the forcing terms, and a control strategy to enforce a prescribed turbulence metrics are proposed. Volume force terms are imposed in a thin layer in the upstream region of the computational domain by a sinusoidal distribution with random variation of both intensity and phase. A Proportional-Integral-Derivative (PID) control is applied to minimize the difference between the intensity of the generated flow turbulence and the prescribed conditions.
 In [1] the model was applied to describe a turbulent stream in an unbounded flow by Detached Eddy Simulation (DES). The capability to generate eddies that evolve into a homogeneous, isotropic turbulent flow was demonstrated. Here, the model is applied to analyze the interaction between the generated onset turbulent flow and a tidal turbine. A computationally efficient approach is used here in which the turbine is described by a volume force method, in a similar fashion as turbulence generation is modelled. Specifically, a hybrid viscous/inviscid methodology is applied in which the DES solver is strongly coupled with a Boundary Integral Equation Method (BIEM) solver. At each time step, the blade load distribution is calculated by a time-accurate BIEM solution and recasts as volume force terms that are plugged into the DES solution. Examples of BIEM validation studies are given in [2, 3, 4] and in the results of a recent blind-test benchmark to be presented at the EWTEC 2023 Conference [5].
 The numerical application described here deals with a 3-bladed horizontal-axis tidal turbine in a 16% intensity, isotropic turbulent onset flow. In the full paper, the overall methodology is described, and numerical results are presented and discussed. Particular attention is given to characterize the generated turbulent stream in terms of key metric quantities like turbulence intensity, Power Spectral Density (PSD), time and spatial scales, isotropy, Probability Density Function (PDF). The results are analyzed to discuss the capability of the methodology to render a physically-consistent description of the phenomenology governing turbine operation in a real tidal flow.
 
 Gregori, M. et al., Mar. Sci. Eng. 2022. (https://doi.org/10.3390/jmse10101332)
 Sarichloo, Z. et al., Marine Energy Journal, 2022. (https://doi.org/10.36688/imej.5.77-90)
 Dubbioso, G.A. et al., Proceedings of EWTEC 2019.
 Salvatore, F. et al., Mar. Sci. Eng. 2018, 6, 53. (https://doi.org/10.3390/jmse6020053)
 Willden, R. et al., Submitted to EWTEC 2023.

A new meshless local integral equation method

This paper proposes a novel meshless local integral equation (LIE) method for numerical solutions of two and three-dimensional Poisson equations. The proposed method can be regarded as a new variant of the meshless local Petrov-Galerkin (MLPG) method which already has six variants, and so the proposed method can be called MLPG7. A variant of MLPG called local boundary integral equation (LBIE) method, uses a localized fundamental solution of Laplace equation as test function. This test function vanishes on local boundaries and therefore, the derived local equations do not involve the gradient of field variables. The motivation for new formulation is using an average of LBIE equation over radius of local sub-domains instead of a single sub-domain. A new kernel is introduced which is a new modification of fundamental solutions of Laplace equation. It is proved that using the new kernel as test function leads to the same local equations as averaging. Infact, the new formulation is a modification of the LBIE method for which the boundary integral is replaced with a domain integral. It is proved that new local weak formulation is equivalent with strong form. The convergence of the method for the case of regularly spaced nodal points is proved. For trial approximation, this paper uses Gaussian radial basis functions (Gaussian RBF), however, derivative free property of the method allows the use of any non-differentiable basis functions. Stability and convergence of the method are numerically studied by considering both 2D and 3D problems with regular and irregular nodal points. The method is compared with LBIE, finite differences (FD) and RBF-FD methods and the numerical results reveal the significance of the proposed method.

Dynamic nucleation as a cascade-up of earthquakes depending on rupture propagation velocity

Earthquake dynamic rupture requires a nucleation process to provide sufficient energy to overcome the fracture energy. Large earthquakes may occur via a cascading rupture process, which includes many triggering processes that cascade from small to large sections of the fault system. During such a process, the nucleation of a large section of the fault plane may occur dynamically via the propagating rupture from a small section of the fault plane. A quasi-static view of seismic nucleation has been widely discussed in earthquake seismology; however, the dynamic nucleation process remains poorly known. Here, we investigate one aspect of the dynamic nucleation process by focusing on the rupture propagation velocity during the nucleation process. We simplify this process as self-similar crack propagation at a constant rupture velocity in a finite nucleation zone within a target region that possesses a uniform fracture energy. We numerically solve this elastodynamic problem in two dimensions for both the anti-plane and in-plane cases using the Boundary Integral Equation method. As the rupture velocity increases, the critical ratio of the fracture energy step to continue the rupture increases and the critical size of the dynamic nucleation zone decreases. The rapid increase in the ratio of the fracture energy step toward infinity could explain why earthquakes never propagate at slow rupture velocities. However, the effect on the size of the nucleation zone is rather limited, with the size of the dynamic nucleation zone decreasing to ~ 70% of the static nucleation zone size. However, such a small difference would result in a significant overall difference if such a dynamic nucleation process repeatedly occurred in the cascading rupture process of a large earthquake, which would be a difficult situation for earthquake early warning.Graphical

APPLICATION OF SPECTRAL METHODS OF BOUNDARY INTEGRAL EQUATIONS FOR MODELING OF NANOOPTICAL DEVICES

Efficiency of modeling of optical nanostructures depends not only on the accuracy of the description of the new physical processes which appear in the new configurations of resonantly scattering and resonantly absorbing structures, but also on the selection of appropriate algorithms for solving the corresponding mathematical problem and numerical parameters of modeling depending on parameters of elements. This is why solving complicated problems of modeling complex resonantly scattering and resonantly absorbing electrodynamic nanostructures involves deep learning of all groups of new unknown effects. In this work a semi-analytical algorithm is developed based on parametrized by conformal mapping techniques spectral method of boundary integral equations with analytical regularization based on singularity subtraction enhanced by Fast Fourier transform, that contrary to the classical schemes, which are based on finite difference and finite element methods, allows to take into account the complex-valued functional dependence of dielectric permittivity of plasmonic materials on the wavelength (even when its value is tabulated) and to solve the problems with static and dynamic singularities in integral equations. Due to sensibility of plasmon resonances to changes in external medium such nanostructures are used in modern medicine, pharmacy and also as chemical and biological sensors. In this work main efforts are directed on generation of algorithm for investigation of dielectric nanostructures with static singularities.

Anomalous absorption in TM polarization light by echelle gratings.

Echelle grating plays an essential role in spectral analysis due to its broadband blaze, high dispersion, and high resolution; however, its absorption properties have received no attention. In this paper, we investigate the effect of parameters including incident wavelength, incident angle, and blaze angle on the absorption properties of the Al echelle grating. Based on calculations by the boundary integral equation method, we observe that the Al grating with a large blaze angle has an absorption enhancement effect on TM-polarized waves, and its intensity increases as the incident angle approaches the pseudo-Brewster angle (maximum absorption over 87%). In particular, this absorption enhancement effect is present in the wideband and somewhat generalizable to other metallic materials. In addition, the potential physical mechanisms underlying the absorption enhancement of the echelle grating are analyzed in detail through the electric field distribution. The resonance between the grating anomaly and the pseudo-Brewster effect results in the appearance of surface plasmon polariton and strong absorption. These findings will bring new understanding to the study of echelle gratings in case of high energy loss when the light incidents with a high angle for high resolution and will also show potential applications in electromagnetic stealth, photothermal conversion, and photodetection.