Singular Fundamental Solutions Research Articles

Analysis of three-dimensional cracks horizontally placed in transversely isotropic FGMs under interior compressive stresses

This paper develops the dual boundary element method (DBEM) using a new fundamental singular solution. This new solution is the elastic fields of transversely isotropic multilayered materials of infinite extent under point concentrated loads. The discretization technique is utilized to approximate arbitrary variations of functionally graded materials (FGMs) with transverse isotropy in a graded direction. The proposed DBEM is used to analyze the three-dimensional crack problems in FGM halfspaces. The penny-shaped and elliptical cracks are horizontally placed on the interface between the FGM layer and homogeneous halfspace and are subjected to uniform compressive stresses on crack surfaces. The stress intensity factors are provided to understand the fracture behaviors of transversely isotropic FGMs. An influential factor is defined to describe the influence of the anisotropy of the FGMs on stress intensity factors. Results show that the anisotropy and heterogeneity of materials exert an obvious influence on the fracture properties of crack problems.

Size-dependent steady creeping microfluid flow based on the boundary element method

In order to investigate size-dependent creeping plane microfluidic flow, a boundary element method is implemented that involves the calculation of interior quantities and multi-domain problems. The governing equations are formulated using the skew-symmetric character of the couple-stress tensor and its energy conjugate mean-curvature tensor, as established in consistent couple stress theory. This theoretical formulation includes one characteristic material length scale parameter l that represents the size-dependency of the problem. Here, we present the boundary integral representation and numerical implementation for two-dimensional size-dependent steady state creeping incompressible flow, in which velocities, angular velocities, force- and couple-tractions are the primary variables. Beyond the previously known singular fundamental solution kernels for point force and point couple in the velocity and angular velocity integral equations, here we present the integral equations for calculating internal mean curvatures and couple-stresses, along with their corresponding kernels. The formulation is then applied to solve some computational problems both to investigate the size effects on the response resulting from the theory, and to validate the strength of the numerical method. For this purpose, we study the important effect of different boundary conditions on the flow pattern and consider a multi-fluid domain Couette flow problem.

Boundary mesh free method with distributed sources for Kirchhoff plate bending problems

A new method for static analysis of Kirchhoff plate under lateral uniform load is presented in this paper. This method called Boundary Distributed Source (BDS), in which Method of Fundamental solution is used with distributed sources instead of point sources. New kernels are computed analytically by integrating the original singular fundamental solution and its derivatives over a circular disc, except diagonal values of Kirchhoff equivalent shear force and its derivative (i.e., when the source point coincides with the field point) are evaluated indirectly by reference solution of rigid body motion or its equilibrium around problem domain boundary. Numerical examples of thin plates with different shapes and boundary conditions are tested to investigate the correctness and effectiveness of the BDS method.

Efficient simulation of inclusions and reinforcement bars with the isogeometric Boundary Element method

The paper is concerned with the development of efficient and accurate solution procedures for the isogeometric boundary element method (BEM) when applied to problems that contain inclusions that have elastic properties different to the computed domain. This topic has been addressed in previous papers but the approach presented here is a considerable improvement in terms of efficiency and accuracy.One innovation is that initial stresses instead of body forces are used, resulting in discretised equations that can be written using matrix–vector multiplications. This allowed a one step solution to be implemented for elastic inclusions. In addition, a novel approach is used for the computation of strains, that avoids the use of highly singular fundamental solutions. Finally, a new type of inclusion is presented that can be used to model reinforcement bars or rock bolts and where analytical integration can be used. Test examples, where results are compared with Finite Element simulations, show that the proposed approach is sound.

The stability study of numerical solution of Fredholm integral equations of the first kind with emphasis on its application in boundary elements method

In this paper stability of numerical solution of Fredholm integral equation of the first kind is studied for radial basis kernels which possess positive Fourier transform. As a result, the equivalence relation between strong and weak forms of partial differential equations (PDEs) is proved for some special radial test functions. Also the stability of boundary elements method (BEM) is proved analytically for Laplace and Helmholtz equations by obtaining Fourier transform of singular fundamental solutions applied in BEM. Analytical result presented in this paper is an extension of stability idea of radial basis functions (RBFs) used to interpolate scattered data described by Wendland in [51]. Similar to the interpolation, it is proved here mathematically that integral operators which have radial kernels with positive Fourier transform are strictly positive definite. Thanks to the stability idea presented in [51], a positive lower bound for eigenvalues of these integral operators is found here, explicitly.

Leading-order Stokes flows near a corner

Singular solutions of the Stokes equations play important roles in a variety of fluid dynamics problems. They allow the calculation of exact flows, are the basis of the boundary integral methods used in numerical computations, and can be exploited to derive asymptotic flows in a wide range of physical problems. The most fundamental singular solution is the flow's Green function due to a point force, termed the Stokeslet. Its expression is classical both in free space and near a flat surface. Motivated by problems in biological physics occurring near corners, we derive in this paper the asymptotic behaviour for the Stokeslet both near and far from a corner geometry by using complex analysis on a known double integral solution for corner flows. We investigate all possible orientations of the point force relative to the corner and all corner geometries from acute to obtuse. The case of salient corners is also addressed for point forces aligned with both walls. We use experiments on beads sedimenting in corn syrup to qualitatively test the applicability of our results. The final results and scaling laws will allow to address the role of hydrodynamic interactions in problems from colloidal science to microfluidics and biological physics.

Boundary mesh free method with distributed sources for 2D elasticity problems

This paper presents development Boundary Distributed Source method for 2D elasticity problems, in which sources are located at domain boundary, i.e., without fictitious boundary. Singularities of the kernel are totally removed by replacing the concentrated point sources with area-distributed sources–circular disc centered at the source point in this study–providing new kernels resulting from analytical integration of the original singular fundamental solution and its derivative on the area of the source disc. All the coefficients for Dirichlet and Neumann boundary conditions in the systems of equations can be determined analytically except diagonal coefficients for Neumann boundary conditions; they are calculated indirectly by the consideration of two reference solutions of rigid body motion in the two directions.

A generalized traction integral equation for Stokes flow, with applications to near-wall particle mobility and viscous erosion

A double-layer integral equation for the surface tractions on a body moving in a viscous fluid is derived which allows for the incorporation of a background flow and/or the presence of a plane wall. The Lorentz reciprocal theorem is used to link the surface tractions on the body to integrals involving the background velocity and stress fields on an imaginary bounding sphere (or hemisphere for wall-bounded flows). The derivation requires the velocity and stress fields associated with numerous fundamental singularity solutions which we provide for free-space and wall-bounded domains. Two sample applications of the method are discussed: we study the tractions on an ellipsoid moving near a plane wall, which provides a more detailed understanding of the well-studied glancing and reversing trajectories in the context of particle sedimentation, and the erosion of bodies by a viscous flow, in which the surface is ablated at a rate proportional to the local viscous shear stress. Simulations and analytical estimates suggest that a spherical body in a uniform flow first reduces nearly but not exactly to the drag minimizing profile and then vanishes in finite time. The shape dynamics of an eroding body in a shear flow and near a wall are also investigated. Stagnation points on the body surface lead generically to the formation of cusps, whose number depends on the flow configuration and/or the presence of nearby boundaries.

Yue’s solution of classical elasticity in n-layered solids: Part 1, mathematical formulation

This paper presents the exact and complete fundamental singular solutions for the boundary value problem of a n-layered elastic solid of either transverse isotropy or isotropy subject to body force vector at the interior of the solid. The layer number n is an arbitrary nonnegative integer. The mathematical theory of linear elasticity is one of the most classical field theories in mechanics and physics. It was developed and established by many well-known scientists and mathematicians over 200 years from 1638 to 1838. For more than 150 years from 1838 to present, one of the remaining key tasks in classical elasticity has been the mathematical derivation and formulation of exact solutions for various boundary value problems of interesting in science and engineering. However, exact solutions and/or fundamental singular solutions in closed form are still very limited in literature. The boundary-value problems of classical elasticity in n-layered and graded solids are also one of the classical problems challenging many researchers. Since 1984, the author has analytically and rigorously examined the solutions of such classical problems using the classical mathematical tools such as Fourier integral transforms. In particular, he has derived the exact and complete fundamental singular solutions for elasticity of either isotropic or transversely isotropic layered solids subject to concentrated loadings. The solutions in n-layered or graded solids can be calculated with any controlled accuracy in association with classical numerical integration techniques. Findings of this solution formulation are further used in the companion paper for mathematical verification of the solutions and further applications for exact and complete solutions of other problems in elasticity, elastodynamics, poroelasticty and thermoelasticity. The mathematical formulations and solutions have been named by other researchers as Yue’s approach, Yue’s treatment, Yue’s method and Yue’s solution.

Singular boundary method for water wave problems

This article presents the singular boundary method (SBM) for water wave-structure interaction analysis. The SBM is a novel recent boundary meshless collocation method, which uses the singular fundamental solutions of governing equation as basis function in the interpolation formulation and introduces the concept of source intensity factors to avoid the singularities of the fundamental solutions at origin. It is a strong-form meshless boundary collocation technique with the merits being easy-to-implement and integration-free. In this study, two types of water wave-structure interactions, water wave interaction of multiple-cylinder-array structure and semicircular harbor, are considered. Numerical results show the efficiency and accuracy of the present SBM scheme in comparison with the analytical solutions, the method of fundamental solutions and the null field boundary integral element method. Then the near trapped mode phenomenon of ten-cylinder structure and the focusing effect of the semicircular harbor are revisited by using the present SBM.

Burton–Miller-type singular boundary method for acoustic radiation and scattering

This paper proposes the singular boundary method (SBM) in conjunction with Burton and Miller׳s formulation for acoustic radiation and scattering. The SBM is a strong-form collocation boundary discretization technique using the singular fundamental solutions, which is mathematically simple, easy-to-program, meshless and introduces the concept of source intensity factors (SIFs) to eliminate the singularities of the fundamental solutions. Therefore, it avoids singular numerical integrals in the boundary element method (BEM) and circumvents the troublesome placement of the fictitious boundary in the method of fundamental solutions (MFS). In the present method, we derive the SIFs of exterior Helmholtz equation by means of the SIFs of exterior Laplace equation owing to the same order of singularities between the Laplace and Helmholtz fundamental solutions. In conjunction with the Burton–Miller formulation, the SBM enhances the quality of the solution, particularly in the vicinity of the corresponding interior eigenfrequencies. Numerical illustrations demonstrate efficiency and accuracy of the present scheme on some benchmark examples under 2D and 3D unbounded domains in comparison with the analytical solutions, the boundary element solutions and Dirichlet-to-Neumann finite element solutions.

Singular boundary method for modified Helmholtz equations

This study makes the first attempt to apply a recent strong-form boundary collocation method using the singular fundamental solutions, namely the singular boundary method (SBM), to 2D and 3D modified Helmholtz equations. By the desingularization of subtracting and adding-back technique, the corresponding nonsingular SBM formulations are derived based on null-field integral equations and an inverse interpolation technique. Numerical demonstrations show the feasibility and accuracy of the present SBM in some benchmark problems.

Effect of anisotropy in ground movements caused by tunnelling

This paper presents closed-form analytical solutions for estimating far-field ground deformations caused by shallow tunnelling in a linear elastic soil mass with cross-anisotropic stiffness properties. The solutions describe two-dimensional ground deformations for uniform convergence (uε) and ovalisation (uδ) modes of a circular tunnel cavity, based on the complex formulation of planar elasticity and superposition of fundamental singularity solutions. The analyses are used to interpret measurements of ground deformations caused by open-face shield construction of a Jubilee Line Extension (JLE) tunnel in London Clay at a well-instrumented site in St James's Park. Anisotropic stiffness parameters are estimated from hollow-cylinder tests on intact block samples of London Clay (from the Heathrow Airport Terminal 5 project), and the selection of the two input parameters is based on a least-squares optimisation using measurements of ground deformations. The results show consistent agreement with the measured distributions of surface and subsurface, vertical and horizontal displacement components, and anisotropic stiffness properties appear to have little effect on the pattern of ground movements. The results provide an interesting counterpoint to prior studies using finite-element analyses that have reported difficulties in predicting the distribution of ground movements for the instrumented section of the JLE tunnel.

Dynamic response to fluid extraction from a poroelastic half‐space

SUMMARYIn this study, the dynamic response of a poroelastic half‐space to a point fluid sink is investigated using Biot's dynamic theory of poroelasticity. Based on Biot's theory, the governing field equations are re‐formulated in frequency domain with solid displacement and pore pressure. In a cylindrical coordinate system, a method of displacement potentials for axisymmetric displacement field is proposed to decouple the Biot's field equations to three scalar Helmholtz equations, and then the general solution to axisymmetric problems are obtained. The full‐space fundamental singular solution for a point sink is also derived using potential methods. The mirror‐image method is finally applied to construct the fundamental solution for a point sink buried in a poroelastic half‐space. Furthermore, a numerical study is conducted for a rock, that is, Berea sandstone, as a representative example. Copyright © 2013 John Wiley & Sons, Ltd.

Dynamic Green’s functions for a poroelastic half-space

The dynamic responses of a poroelastic half-space to an internal point load and fluid source are investigated in the frequency domain in this paper. By virtue of a method of displacement potentials, the 3D general solutions of homogeneous wave equations and fundamental singular solutions of inhomogeneous wave equations are derived, respectively, in the frequency domain. The mirror-image technique is then applied to construct the dynamic Green’s functions for a poroelastic half-space. Explicit analytical solutions for displacement fields and pore pressure are obtained in terms of semi-infinite Hankel-type integrals with respect to the horizontal wavenumber. In two limiting cases, the solutions presented in this study are shown to reduce to known counterparts of elastodynamics and those of Lamb’s problem, thus ensuring the validity of our result.

A non-iterative sampling approach using noise subspace projection for EIT

This study concerns the problem of the reconstruction of inclusions embedded in a conductive medium in the context of electrical impedance tomography (EIT), which is investigated within the framework of a non-iterative sampling approach. This type of identification strategy relies on the construction of a special indicator function that takes, roughly speaking, small values outside the inclusion and large values inside. Such a function is constructed in this paper from the projection of a fundamental singular solution onto the space spanned by the singular vectors associated with some of the smallest singular values of the data-to-measurement operator. The behavior of the novel indicator function is analyzed. For a subsequent implementation in a discrete setting, the quality of classical finite-dimensional approximations of the measurement operator is discussed. The robustness of this approach is also analyzed when only noisy spectral information is available. Finally, this identification method is implemented numerically and experimentally, and its efficiency is discussed on a set of, partly experimental, examples.

Investigation on near-boundary solutions by singular boundary method

Abstract This study is to solve the dramatic drop of numerical solution accuracy of the singular boundary method (SBM) in near-boundary regions, also known as the boundary layer effect in the literature of boundary element method (BEM), where we encounter ‘nearly singular’ interpolation functions when the field point is close to the boundary source points. It is noted that the SBM uses the singular fundamental solution as its interpolation basis function. Different from singularity at origin, the fundamental solution interpolation at near-boundary regions remains finite. However, instead of being a flat function, the interpolation function develops a sharp peak as the field point approaches the boundary, namely, nearly singular behaviors. Consequently, the evaluation of the physical quantities at this point is much less accurate than at central region points. To remove or damp out the rapid variations of this nearly singular interpolation, this paper introduces a nonlinear transformation, based on the sinh function, to remedy this boundary layer effect. Our numerical experiments verify that the proposed approach can improve the SBM near-boundary solution accuracy by several orders of magnitude in terms of relative errors. The SBM solution appears accurate at a point as close to the boundary as 1.0E–10 scale.

The solitary wave solution of the two-dimensional regularized long-wave equation in fluids and plasmas

This paper investigates the solitary wave solutions of the two-dimensional regularized long-wave equation which is arising in the investigation of the Rossby waves in rotating flows and the drift waves in plasmas. The main idea behind the numerical solution is to use a combination of boundary knot method and the analog equation method. The boundary knot method is a meshless boundary-type radial basis function collocation technique. In contrast with the method of fundamental solution, the boundary knot method uses the non-singular general solution instead of the singular fundamental solution to obtain the homogeneous solution. Similar to method of fundamental solution, the radial basis function is employed to approximate the particular solution via the dual reciprocity principle. In the current paper, we applied the idea of analog equation method. According to the analog equation method, the nonlinear governing operator is replaced by an equivalent nonhomogeneous linear one with known fundamental solution and under the same boundary conditions. Furthermore, in order to show the efficiency and accuracy of the proposed method, the present work is compared with finite difference scheme. The new method is analyzed for the local truncation error and the conservation properties. The results of several numerical experiments are given for both the single and double-soliton waves.

A new RBF-Trefftz meshless method for partial differential equations

Based on the radial basis functions (RBF) and T-Trefftz solution, this paper presents a new meshless method for numerically solving various partial differential equation systems. First, the analog equation method (AEM) is used to convert the original patial differential equation to an equivalent Poisson's equation. Then, the radial basis functions (RBF) are employed to approxiamate the inhomogeneous term, while the homogeneous solution is obtained by linear combination of a set of T-Trefftz solutions. The present scheme, named RBF-Trefftz has the advantage over the fundamental solution (MFS) method due to the use of nonsingular T-Trefftz solution rather than singular fundamental solutions, so it does not require the artificial boundary. The application and efficiency of the proposed method are validated through several examples which include different type of differential equations, such as Laplace equation, Hellmholtz equation, convectin-diffusion equation and time-dependent equation.

A method of fundamental solutions without fictitious boundary

This paper proposes a novel meshless boundary method called the singular boundary method (SBM). This method is mathematically simple, easy-to-program, and truly meshless. Like the method of fundamental solutions (MFS), the SBM employs the singular fundamental solution of the governing equation of interest as the interpolation basis function. However, unlike the MFS, the source and collocation points of the SBM coincide on the physical boundary without the requirement of introducing fictitious boundary. In order to avoid the singularity at the origin, this method proposes an inverse interpolation technique to evaluate the singular diagonal elements of the MFS coefficient matrix. The SBM is successfully tested on a benchmark problems, which shows that the method has a rapid convergence rate and is numerically stable.