Abstract: Abstract We model shallow-water waves using a one-dimensional Korteweg–de Vries equation with the wave generation parameterized by random wave amplitudes for a predefined sea state. These wave amplitudes define the high-dimensional stochastic input vector for which we estimate the short-term wave crest exceedance probability at a reference point. For this high-dimensional and complex problem, most reliability methods fail, while Monte Carlo methods become impractical due to the slow convergence rate. Therefore, first within offshore applications, we employ the dimensionality reduction method called Active-Subspace Analysis. This method identifies a low-dimensional subspace of the input space that is most significant to the input–output variability. We exploit this to efficiently train a Gaussian process (i.e., a kriging model) that models the maximum 10-min crest elevation at the reference point, and to thereby efficiently estimate the short-term wave crest exceedance probability function. The active low-dimensional subspace for the Korteweg–de Vries model also exposes the expected incident wave groups associated with extreme waves and loads. Our results show the advantages and the effectiveness of the active-subspace analysis against the Monte Carlo implementation for offshore applications. PubDate: 2021-01-13

Abstract: Abstract In the present study, the effects of different types of basic temperature and concentration gradients on a layer of reactive fluid under variable gravity field are analyzed using linear and non-linear analysis. Energy method is applied to obtain the non-linear energy threshold below which the solution is globally stable. It is found that the linear and non-linear analysis are not in agreement for the considered models of temperature and concentration gradients. The obtained results of non-linear analysis for different values of reaction terms and variable gravity coefficients in each given model of temperature and concentration gradients are compared with the linear instability results. The Chebyshev pseudospectral method is used to obtain the numerical and graphical results of subsequent analysis. PubDate: 2020-10-29

Abstract: Abstract In this paper, we investigate the limits of Riemann solutions to the Euler equations of compressible fluid flow with a source term as the adiabatic exponent tends to one. The source term can represent friction or gravity or both in Engineering. For instance, a concrete physical model is a model of gas dynamics in a gravitational field with entropy assumed to be a constant. The body force source term is presented if there is some external force acting on the fluid. The force assumed here is the gravity. Different from the homogeneous equations, the Riemann solutions of the inhomogeneous system are non self-similar. We rigorously proved that, as the adiabatic exponent tends to one, any two-shock Riemann solution tends to a delta shock solution of the pressureless Euler system with a Coulomb-like friction term, and the intermediate density between the two shocks tends to a weighted \(\delta \) -mesaure which forms the delta shock; while any two-rarefaction-wave Riemann solution tends to a two-contact-discontinuity solution of the pressureless Euler system with a Coulomb-like friction term, whose intermediate state between the two contact discontinuities is a vacuum state. Moreover, we also give some numerical simulations to confirm the theoretical analysis. PubDate: 2020-10-22

Abstract: Abstract It is a well-known fact that the onset of Rayleigh–Bénard convection occurs via a long-wavelength instability when the horizontal boundaries are thermally insulated. The aim of this paper is to quantify the exact dimensions of a cylinder of rectangular cross-section wherein stable three-dimensional Rayleigh–Bénard convection sets in via a long-wavelength instability from the motionless state at the same value of the critical Rayleigh number as the corresponding horizontally unbounded problem when the bounding horizontal walls have infinite thermal conductance. Hence, we consider three-dimensional Rayleigh–Bénard convection in a cell of infinite extent in the x-direction, confined between two vertical walls located at \(y= \pm H\) and horizontal boundaries located at \(z=0\) and \(z=d\) . Our analysis predicts the existence of the sought stable state for experimental velocity boundary conditions at the vertical walls provided the aspect ratio \(\delta = H/d\) takes a certain value. In the limit \(H \rightarrow \infty \) , we retrieve the stability characteristics of the horizontally unbounded problem. As expected, the analysis predicts two counter-rotating rolls aligned along the y-direction of period \(2 \pi /\delta \) equal to the period of the roll in the y-direction of the corresponding unbounded problem. A long-scale asymptotic analysis leads to the derivation of an evolution partial differential equation (PDE) that is fourth order in space and contains a single bifurcation parameter. The PDE, valid for a specific value of \(\delta \) , is analyzed analytically and numerically as function of the bifurcation parameter and for a variety of velocity boundary conditions at the vertical walls to seek the stable steady-state solutions. The same analysis is also extended to the case of convection in a fluid-saturated porous medium. PubDate: 2020-10-04

Abstract: Abstract The purpose of this article is to construct a firm mathematical foundation for the boundary value problem associated with a generalized Emden equation that embraces Thomas–Fermi-like theories. Boundary value problems for the relativistic and non-relativistic Thomas–Fermi equations are included as special cases. Questions of existence and uniqueness of solutions to these boundary value problems form a fundamental and important area of investigation regarding whether these mathematical models for physical phenomena are actually well-posed. However, these questions have remained open for the generalized Emden problem and the relativistic Thomas–Fermi problem. Herein we advance current understanding of existence and uniqueness of solutions by proving that these boundary value problems each admit a unique solution. Our methods involve an analysis of the problems through arguments that apply differential inequalities and fixed-point theory. The new results guarantee the existence of a unique solution, ensuring the generalized Emden equation that embraces Thomas–Fermi theory sits on a firm mathematical foundation. PubDate: 2020-09-24

Abstract: Abstract In the present work, the band-pass filter characteristics of a coaxial waveguide with an impedance coated groove on the inner wall and perfectly conducting outer wall is analyzed rigorously through the Wiener–Hopf technique. By using the direct Fourier transform, the related boundary value problem is reduced to the Wiener–Hopf equation whose solution contains infinitely many constants satisfying an infinite system of linear algebraic equations. These equations are solved numerically and the field terms, which depends on the solution obtained numerically, are derived explicitly. The problem is also analyzed by applying a mode-matching technique and the results are compared numerically. Besides, some computational results illustrating the effects of parameters such as depth of the groove, surface impedances and radii of the walls are also presented. PubDate: 2020-09-15

Abstract: Abstract Investigated in this paper is the coupled fluid–body motion of a thin solid body undergoing a skimming impact on a shallow-water layer. The underbody shape (the region that makes contact with the liquid layer) is described by a smooth polynomic curve for which the magnitude of underbody thickness is represented by the scale parameter C. The body undergoes an oblique impact (where the horizontal speed of the body is much greater than its vertical speed) onto a liquid layer with the underbody’s trailing edge making the initial contact. This downstream contact point of the wetted region is modelled as fixed (relative to the body) throughout the skimming motion with the liquid layer assumed to detach smoothly from this sharp trailing edge. There are two geometrical scenarios of interest: the concave case ( \(C<0\) producing a hooked underbody) and the convex case ( \(C > 0\) producing a rounded underbody). As C is varied the rebound dynamics of the motion are predicted. Analyses of small-time water entry and of water exit are presented and are shown to be broadly in agreement with the computational results of the shallow-water model. Reduced analysis and physical insights are also presented in each case alongside numerical investigations and comparisons as C is varied, indicating qualitative analytical/numerical agreement. Increased body thickness substantially changes the interaction structure and accentuates inertial forces in the fluid flow. PubDate: 2020-09-08

Abstract: Abstract Because of geometric or material discontinuities, stress singularities can occur around the vertex of a V-notch. The singular order is an important parameter for characterizing the degree of the stress singularity. The present paper focuses on the calculation of the singular order for the anti-plane propagating V-notch in a composite material structure. Starting from the governing equation of elastodynamics and the displacement asymptotic expansion, an ordinary differential eigen equation with respect to the singular order is proposed. The interpolating matrix method is then employed to solve the established eigen equation to conduct the singular orders. The effects of the material principal axis direction, shear modulus, and propagation velocity and acceleration on the singular orders of the V-notch are respectively investigated, and some conclusions are drawn. The singular orders of the V-notches decrease with an increase in the material principal axis direction angle, except for the crack, whose singular orders do not change with the principal axis direction. The singular orders increase with an increase in the shear modulus \(G_{13} \) , while they decrease with an increase in the shear modulus \(G_{23} \) . The singular orders increase as the magnitude of the propagation velocity increases, while they decrease as the direction of the propagation velocity increases. The singular orders increase with an increase in the value of the propagation acceleration, while they decrease with an increase in the direction of the propagation acceleration. The singular orders become larger when the ratio of \(G_{13}^{(2)} /G_{13}^{(1)} \) increases, while they become smaller when the ratio of \(G_{23}^{(2)} /G_{23}^{(1)} \) increases, and they increase with an increase in the mass density ratio for the bi-material V-notch. PubDate: 2020-09-03

Abstract: Abstract A ferrofluid saturated porous layer convection problem is studied in the variable gravitational field for a local thermal nonequilibrium (LTNE) model. Internal heating and variation (increasing or decreasing) in gravity with distance through the layer affected the stability of the convective system. The Darcy model is employed for the momentum equation and the LTNE model for the energy equation. The boundaries are considered to be rigid-isothermal and paramagnetic. For the linear stability analysis of the three-dimensional problem, the normal mode has been applied and the eigenvalue problem is solved numerically using Chebyshev pseudospectral method, while weakly nonlinear analysis is carried out with a truncated Fourier series. The effect of different dimensionless parameters on the Rayleigh number has also been studied. We found that the system becomes unstable on the increasing value of nonlinearity index of magnetization ( \(M_3\) ), porosity-modified conductivity ratio ( \(\beta \) ), and internal heat parameter ( \(\xi \) ). It is observed that the system is stabilized by increasing the value of the Langevin parameter ( \(\alpha _\mathrm{{L}}\) ), variable gravity coefficient ( \(\delta \) ), and effective heat transfer parameter ( \(H_1\) ). Runge–Kutta–Gill method has been used for solving the finite-amplitude equations to study the transient behavior of the Nusselt number. Streamlines and isotherms patterns are determined for the steady case and are presented graphically. PubDate: 2020-09-01

Abstract: Abstract A third-order highly non-linear ODE that arises in applications of non-Newtonian boundary-layer fluid flow, governed by a power-law Ostwald–de Waele rheology, is considered. The model appears in many disciplines related to applied and engineering mathematics, in addition to engineering and industrial applications. The aim is to use a new set of variables, defined via the first and second-order derivatives of the dependent variable, to transform the problem to a bounded domain, where we study properties of solutions, discuss existence and uniqueness of solutions, and investigate some physical parameter values and limitations leading to non-existence of solutions. PubDate: 2020-08-13

Abstract: Abstract For a pulsating free surface source in a three-dimensional finite depth fluid domain, the Green function of the source presented by John [Communs. Pure Appl. Math. 3:45–101, 1950] is superposed as the Rankine source potential, an image source potential and a wave integral in the infinite domain \((0, \infty )\) . When the source point together with a field point is on the free surface, John’s integral and its gradient are not convergent since the integration \(\int ^\infty _\kappa \) of the corresponding integrands does not tend to zero in a uniform manner as \(\kappa \) tends to \(\infty \) . Thus evaluation of the Green function is not based on direct integration of the wave integral but is obtained by approximation expansions in earlier investigations. In the present study, five images of the source with respect to the free surface mirror and the water bed mirror in relation to the image method are employed to reformulate the wave integral. Therefore the free surface Green function of the source is decomposed into the Rankine potential, the five image source potentials and a new wave integral, of which the integrand is approximated by a smooth and rapidly decaying function. The gradient of the Green function is further formulated so that the same integration stability with the wave integral is demonstrated. The significance of the present research is that the improved wave integration of the Green function and its gradient becomes convergent. Therefore evaluation of the Green function is obtained through the integration of the integrand in a straightforward manner. The application of the scheme to a floating body or a submerged body motion in regular waves shows that the approximation is sufficiently accurate to compute linear wave loads in practice. PubDate: 2020-08-12

Abstract: Abstract An effective-medium theory for the electrical conductivity of Ohmic dispersions taking explicit account of particle shape and spatial distribution independently is available from the work of Ponte Castañeda and Willis [J Mech Phys Solids 43:1919–1951, 1996]. When both shape and distribution take particular “ellipsoidal” forms, the theory provides analytically explicit estimates. The purpose of the present work is to evaluate the predictive capabilities of these estimates when dispersions exhibit dissimilar particle shape and distribution. To this end, comparisons are made with numerical bounds for coated ellipsoid assemblages computed via the finite element method. It is found that estimates and bounds exhibit good agreement for the entire range of volume fractions, aspect ratios, and conductivity contrasts considered, including those limiting values corresponding to an isotropic distribution of circular cracks. The fact that the explicit estimates lie systematically within the numerical bounds hints at their possible realizability beyond the class of isotropic dispersions. PubDate: 2020-08-01

Abstract: Abstract A recent asymptotic model for the operation of a vanadium redox flow battery (VRFB) is extended to include the dissociation of sulphuric acid—a bulk chemical reaction that occurs in the battery’s porous flow-through electrodes, but which is often omitted from VRFB models. Using asymptotic methods and time-dependent two-dimensional numerical simulations, we show that the charge–discharge curve for the model with the dissociation reaction is almost identical to that for the model without, even though the concentrations of the ionic species in the recirculating tanks, although not the state of charge, are considerably different in the two models. The ability of the asymptotic model to extract both the qualitative and quantitative behaviour of the considerably more time-consuming numerical simulations correctly indicates that it should be possible to add further physical phenomena to the model without incurring significant computational expense. PubDate: 2020-08-01

Abstract: Abstract As an extension of the discrete Sommerfeld problems on lattices, the scattering of a time-harmonic wave is considered on an infinite square lattice when there exists a pair of semi-infinite cracks or rigid constraints. Due to the presence of stagger, also called offset, in the alignment of the defect edges the asymmetry in the problem leads to a matrix Wiener–Hopf kernel that cannot be reduced to scalar Wiener–Hopf in any known way. In the corresponding continuum model the same problem is a well-known formidable one which possesses certain special structure with exponentially growing elements on the diagonal of kernel. From this viewpoint the present paper tackles a discrete analog of the same by reformulating the Wiener–Hopf problem and reducing it to a finite set of linear algebraic equations; the coefficients of which can be found by an application of the scalar Wiener–Hopf factorization. The considered discrete paradigm involving lattice waves is relevant for modern applications of mechanics and physics at small length scales. PubDate: 2020-07-31

Abstract: Abstract The algebraic and semi-algebraic formulations of a new multiscale elliptic solver based on a non-overlapping domain decomposition method are presented. By algebraic we mean that all information needed to implement the proposed procedure can be extracted from the underlying fine grid finite volume linear system. In addition to the entries of this linear system, the proposed semi-algebraic procedure will also use the coefficients of the elliptic equation at subdomain boundaries. Initially we construct multiscale basis functions (or local solutions) subject to Robin boundary conditions. Although the implementation of Robin conditions in the formulation of the multiscale method uses the coefficients of the elliptic equation of interest, we modify it such that only entries of the finite volume linear system appear in the calculation of local solutions. A linear combination of local solutions gives a (discontinuous at subdomain boundaries) solution that is refined in a smoothing step. To this end we propose an algebraic scheme to remove discontinuities that needs a staggered (or dual) coarse grid. By iterating with the defect correction scheme, we construct an effective two-stage preconditioner that combines both local solutions and the smoothing step; the final result is obtained in a small number of iterations. Our focus in this work is the presentation of the algebraic and semi-algebraic formulations of the multiscale elliptic solver, which are carefully explained, and in verifying their accuracy and efficiency in the solution of challenging problems. We consider two- and three-dimensional problems with coefficients exhibiting high-contrast, anisotropic, and channelized structures that are solved to reveal the good properties of the proposed schemes. PubDate: 2020-07-31

Abstract: Abstract Hyperbolic towers are towers in the shape of a single-sheet hyperboloid, and they are interesting in architecture. In this paper, we deal with the infinitesimal bending of a curve on a hyperboloid of one sheet; that is, we study the flexibility of the net-like structures used to make a hyperbolic tower. Visualization of infinitesimal bending has been carried out using Mathematica, and some examples are presented and discussed. PubDate: 2020-07-31

Abstract: Abstract We find a general method to obtain the radially symmetric solutions of Dirichlet problem for Pennes bioheat equation in the exterior domain of a circle through the computation of Green’s function of a naturally related operator. We apply this technique to solve a problem in radio-frequency ablation. PubDate: 2020-07-30

Abstract: Abstract The radiative–conductive transfer equation in the \(S_N\) approximation for spherical geometry is solved using a modified decomposition method. The focus of this work is to show how to distribute the source terms in the recursive equation system in order to guarantee arithmetic stability and thus numerical convergence of the obtained solution, guided by a condition number criterion. Some examples are compared with results from literature and parameter combinations are analyzed, for which the condition number analysis indicates convergence of the solutions obtained by the recursive scheme. PubDate: 2020-07-30

Abstract: Abstract In geo-engineering, mechanical problems of surcharge loads acting on ground surface and shallow tunnel excavation are often encountered. When the complex variable method is applied, such problems turn to non-zero resultant issues and generally result in infinite displacement singularity in geomaterial at infinity, which violates the fact that displacement components at infinity should be zero in field observations. The displacement singularity at infinity caused by non-zero resultants corresponds non-zero coefficients of logarithmic items in complex potentials. To eliminate the singularity at infinity, the symmetrical method is adopted so that the far-field displacement obtained from the modified displacement solution conducted by the complex variable method would be logically identical to the reality. Two fundamental cases of single surcharge load acting on ground surface and single tunnel excavation are presented to verify the modified displacement solution, and good agreements are observed. The main advantage that the proposed solution eliminates displacement singularity at infinity and makes far-field displacement logically accord with field observation is presented by comparing to the original solution. The drawback that the proposed solution is dependent on modified depth is discussed as well. PubDate: 2020-06-06

Abstract: Abstract In the hot end of a 3-D printer, polymer feedstock flows through a heated cylinder in order to become pliable. This setup determines a natural upper limit to the speed at which the polymer may be extruded. The case of polymers which undergo the crystalline-melt transition is considered; the resulting mathematical model is a Stefan-like moving boundary-value problem for the polymer temperature. Using the heat balance integral method provides an analytical approximation for the temperature. Several different conditions which use this temperature to establish the maximum velocity are considered; using a pointwise polymer exit temperature in the hot end matches well with experimental data. PubDate: 2020-04-13