Matched Asymptotic Expansions Research Articles

Unsteady fluid flow in a slightly curved annular pipe: The impact of the annulus on the flow physics

The motivation of the present study is threefold. Mainly, the etiological explanation of the Womersley number based on physical reasoning. Next, the extension of a previous work [Messaris, Hadjinicolaou, and Karahalios, “Unsteady fluid flow in a slightly curved pipe: A comparative study of a matched asymptotic expansions solution with a single analytical solution,” Phys. Fluids 28, 081901 (2016)] to the annular pipe flow. Finally, the discussion of the effect of the additional stresses generated by a catheter in an artery and exerted on the arterial wall during an in vivo catheterization. As it is known, the square of the Womersley number may be interpreted as an oscillatory Reynolds number which equals to the ratio of the inertial to the viscous forces. The adoption of a modified Womersley number in terms of the annular gap width seems therefore more appropriate to the description of the annular flow than an ordinary Womersley number defined in terms of the pipe radius. On this ground, the non-dimensional equations of motion are approximately solved by two analytical methods: a matched asymptotic expansions method and a single. In the first method, which is valid for very large values of the Womersley number, the flow region consists of the main core and the two boundary layers formed at the inner and outer boundaries. In the second, the fluid is considered as one region and the Womersley number can vary from finite values, such that they fit to the blood flow in the aorta and the main arteries, to infinity. The single solution predicts increasing circumferential and decreasing axial stresses with increasing catheter radius at a prescribed value of the Womersley parameter in agreement with analogous results from other theoretical and numerical solutions. It also predicts the formation of pinches on the secondary flow streamlines and a third boundary layer, additional to those formed at the boundary walls. Finally, we show that the insertion of a catheter in an artery may trigger possible disastrous side effects. It may cause unexpected damage to a predisposed but still dormant location of the arterial wall due to high additional radial pressure that induces an excessive distension of the artery.

On radiating solitary waves in bi-layers with delamination and coupled Ostrovsky equations.

We study the scattering of a long longitudinal radiating bulk strain solitary wave in the delaminated area of a two-layered elastic structure with soft ("imperfect") bonding between the layers within the scope of the coupled Boussinesq equations. The direct numerical modelling of this and similar problems is challenging and has natural limitations. We develop a semi-analytical approach, based on the use of several matched asymptotic multiple-scale expansions and averaging with respect to the fast space variable, leading to the coupled Ostrovsky equations in bonded regions and uncoupled Korteweg-de Vries equations in the delaminated region. We show that the semi-analytical approach agrees well with direct numerical simulations and use it to study the nonlinear dynamics and scattering of the radiating solitary wave in a wide range of bi-layers with delamination. The results indicate that radiating solitary waves could help us to control the integrity of layered structures with imperfect interfaces.

Scattering of bulk strain solitary waves in bi-layers with delamination

We study the scattering of longitudinal bulk strain solitary waves in delaminated bi-layers with different types of bonding. The direct numerical modelling of these problems is challenging and has natural limitations. We develop a semi-analytical approach, based on the use of several matched asymptotic multiple-scale expansions and the Integrability Theory of the Korteweg - de Vries equation by the Inverse Scattering Transform. We show that the semi-analytical approach agrees well with the direct numerical simulations and use it to study the scattering of different types of longitudinal bulk strain solitary waves in a wide range of bi-layers with delamination. In particular, we model the dynamics of a long longitudinal strain solitary wave in a symmetric perfectly bonded bi-layer with delamination. The numerical modelling confirms that delamination causes fission of an incident solitary wave and, thus, can be used to detect the defect. We then extend our approaches to the modelling of the waves in bi-layers with soft (“imperfect”) bonding, described by a system of coupled Boussinesq equations and supporting radiating solitary waves. The results may help us to control the integrity of layered structures.

On Saint Venant - Kirchhoff imperfect interfaces

Using matched asymptotic expansions with fractional exponents, we obtain original transmission conditions describing the limit behavior for soft, hard and rigid thin interphases obeying the Saint Venant-Kirchhoff material model. The novel transmission conditions, generalizing the classical linear imperfect interface model, are discussed and compared with existing models proposed in the literature for thin films undergoing finite strain. As an example of implementation of the proposed interface laws, the uniaxial tension and compression responses of butt joints with soft and hard interphases are given in closed form.

Homogenization of ultrathin metallo-dielectric structures leading to transmission conditions at an equivalent interface

We present a method of homogenization of thin metallo-dielectric structures as used in the design of artificial surfaces, or metasurfaces. The approach is based on a so-called matched asymptotic expansion technique, leading to parameters being characteristic of an equivalent interface associated to jump conditions. It is applied to an array of metal strips on top of a metal-backed dielectric slab, with the strips having a small but possibly finite thickness. Solving the equivalent problem provides explicit expressions (1) of the reflection coefficient for a wave at oblique incidence and (2) of the dispersion relation of the surface waves. The results are shown to be in agreement with results coming from the transmission line theory in the limit of vanishing thickness of the metallization and for normal incidence of the wave. The influence of the finite thickness of the metallization is exemplified and validated by comparison with full wave simulations.

Multilayer Asymptotic Solution for Wetting Fronts in Porous Media with Exponential Moisture Diffusivity

We study the asymptotic behavior of sharp front solutions arising from the nonlinear diffusion equation , where the diffusivity is an exponential function . This problem arises, for example, in the study of unsaturated flow in porous media where θ represents the liquid saturation. For physical parameters corresponding to actual porous media, the diffusivity at the residual saturation is so that the diffusion problem is nearly degenerate. Such problems are characterized by wetting fronts that sharply delineate regions of saturated and unsaturated flow, and that propagate with a well‐defined speed. Using matched asymptotic expansions in the limit of large β, we derive an analytical description of the solution that is uniformly valid throughout the wetting front. This is in contrast with most other related analyses that instead truncate the solution at some specific wetting front location, which is then calculated as part of the solution, and beyond that location, the solution is undefined. Our asymptotic analysis demonstrates that the solution has a four‐layer structure, and by matching through the adjacent layers, we obtain an estimate of the wetting front location in terms of the material parameters describing the porous medium. Using numerical simulations of the original nonlinear diffusion equation, we demonstrate that the first few terms in our series solution provide approximations of physical quantities such as wetting front location and speed of propagation that are more accurate (over a wide range of admissible β values) than other asymptotic approximations reported in the literature.

A benchmark for the surface Cahn–Hilliard equation

We study the surface Cahn–Hilliard equation, an often used model for spinodal decomposition and coarsening effects on surfaces. As the width of the diffuse interface shrinks to zero one expects a convergence towards a surface Hele-Shaw model. While results from formal matched asymptotic expansions confirm this conjecture, a numerical reproduction of this asymptotic behavior has not been provided, yet. It is the purpose of this contribution, to fill this gap and present a rotationally symmetric example on the standard sphere, where analytic expressions for the surface Hele-Shaw model allow a comparison with numerical results for the surface Cahn–Hilliard equation.

Boundary-value problems for the Schröinger equation with rapidly oscillating and delta-liked potentials

This paper deals with boundary-value problems on the closed interval [a, b] for the Schrodinger equation with potential of the form q(x, μ−1x) + e−1Q(e−1x), where q(x, ζ) is a 1-periodic (in ζ) function, Q(ξ) is a compactly supported function, 0 ∈ (a, b), and μ, e are small positive parameters. The solutions of these boundary-value problemsup to O(e +μ) are constructed by combining the homogenization method and the method of matching asymptotic expansions.

New asymptotic analysis method for phase field models in moving boundary problem with surface tension

In this paper, we give an asymptotic analysis of the phase field Allen-Cahn and Cahn-Hilliard models of free surfaces with surface tension. Unlike the traditional approach that approximates the solution by the so-called matched asymptotic expansion involving outer expansion, inner expansion and matching, our new approach utilizes a uniform double asymptotic expansion to expand the whole phase field function directly. Although the main result is not new, we would like to emphasize that we derive the result under a uniform double asymptotic expansion. Thus, in this paper the detailed structure of the phase field functions in the equilibrium state is obtained, and the consistency of the phase field models with the corresponding sharp interface models is discussed, including the free surface Allen-Cahn model, Cahn-Hilliard model, and the Allen-Cahn model with volume constraint. The explicit asymptotic expansion of the phase field function reveals rich details of its structures. Moreover, it nicely explains some unusual phenomena we observed in numerical experiments. The theory introduced in this paper can be applied to guide the future modeling and simulation of other moving boundary problems by phase field models.

Controlled Topological Transitions in Thin-Film Phase Separation

In this paper the evolution of a binary mixture in a thin-film geometry with a wall at the top and bottom is considered. By bringing the mixture into its miscibility gap so that no spinodal decomposition occurs in the bulk, a slight energetic bias of the walls toward each one of the constituents ensures the nucleation of thin boundary layers that grow until the constituents have moved into one of the two layers. These layers are separated by an interfacial region where the composition changes rapidly. Conditions that ensure the separation into two layers with a thin interfacial region are investigated based on a phase-field model. Using matched asymptotic expansions a corresponding sharp-interface problem for the location of the interface is established. It is then argued that this newly created two-layer system is not at its energetic minimum but destabilizes into a controlled self-replicating pattern of trapezoidal vertical stripes by minimizing the interfacial energy between the phases while conserving their area. A quantitative analysis of this mechanism is carried out via a thin-film model for the free interfaces, which is derived asymptotically from the sharp-interface model.

An introduction to the mechanics of the lasso.

Trick roping evolved from humble origins as a cattle-catching tool into a sport that delights audiences all over the world with its complex patterns or 'tricks'. Its fundamental tool is the lasso, formed by passing one end of a rope through a small loop (the honda) at the other end. Here, we study the mechanics of the simplest rope trick, the Flat Loop, in which the rope is driven by the steady circular motion of the roper's hand in a horizontal plane. We first consider the case of a fixed (non-sliding) honda. Noting that the rope's shape is steady in the reference frame rotating with the hand, we analyse a string model in which line tension is balanced by the centrifugal force and the rope's weight. We use numerical continuation to classify the steadily rotating solutions in a bifurcation diagram and analyse their stability. In addition to Flat Loops, we find planar 'coat-hanger' solutions, and whirling modes in which the loop collapses onto itself. Next, we treat the more general case of a honda that can slide due to a finite coefficient of friction of the rope on itself. Using matched asymptotic expansions, we resolve the shape of the rope in the boundary layer near the honda where the rope's bending stiffness cannot be neglected. We use this solution to derive a macroscopic criterion for the sliding of the honda in terms of the microscopic Coulomb static friction criterion. Our predictions agree well with rapid-camera observations of a professional trick roper and with laboratory experiments using a 'robo-cowboy'.

The effect of slip distribution on flow past a circular cylinder

A slip boundary has been shown to have a significant impact on flow past bluff bodies. In this work and using a circular cylinder as a model system, the effects of various slip configurations on the passing flow are investigated. A theoretical analysis using matched-asymptotic expansion is first performed in the small-Reynolds number regime following Stokes and Oseen. A slip boundary condition is shown to lead to only higher-order effects (~1/ln(Re)) on the resulting drag coefficient. For higher Reynolds numbers (100–500), the effects of five types of symmetric slip boundary conditions, namely, no slip, fore-side slip, aft-side slip, flank slip, and all slip on the flow field and pertinent parameters are investigated with numerical simulations. Detailed results on the flow structure and force distribution are presented. Flank slip is found to have the best effect for drag reduction with comparable coverage of slip area. For asymmetric slip distributions, torque and lift are found to generally occur.

A new finite elements method for transient thermal analysis of thin layers

This work introduces a new finite element method for the modeling of the transient thermal behavior of mediums containing thin layers such as adhesive joints without any required mesh refinement in their vicinity. The methodology is based on the recent MAX-FEM model introduced in a previous authors' work. This model consists in correcting the classical Finite Element Method (FEM) by correction matrices taking into account the presence of thin layers without any mesh refinement. The proposed correction is issued from an analytical approach based on Matching Asymptotic Expansions (MAE) and by pretending the principle of the numerical eXtended Finite Elements Method (X-FEM). To describe the thermal behavior of 1D-adhesively bonded structures, the developed hybrid model has been implemented under Matlab code to perform transient explicit analyses. The efficiency of the model is then systematically tested in terms of time computation. All the derived results are compared to FEM, and some proposed applications deal with harmful conditions, material properties or specific boundary layers such as coating systems.

Asymptotic analysis of small defects near a singular point in antiplane elasticity, with an application to the nucleation of a crack at a notch

We use matching asymptotic expansions to treat the antiplane elastic problem associated with a small defect located at the tip of a notch. In a first part, we develop the asymptotic method for any type of defect and present the sequential procedure which allows us to calculate the different terms of the inner and outer expansions at any order. This requires in particular separating in each term its singular part from its regular part. In a second part, the asymptotic method is applied to the case of a crack of variable length located at the tip of a given notch. We show that the first two nontrivial terms of the expansion of the energy release rate are sufficient to well approximate the dependence of the energy release rate on the crack length in the range of values of the length which are sufficient to treat the problem of nucleation. This problem is considered in the last part where we compare the nucleation and the propagation of a crack predicted by two different models: the classical Griffith law and the Francfort–Marigo law based on an energy minimization principle. Several numerical results illustrate the interest of the method.

The history force on a small particle in a linearly stratified fluid

AbstractThe hydrodynamic force experienced by a small spherical particle undergoing an arbitrary time-dependent motion in a weakly density-stratified fluid is investigated theoretically. The study is carried out under the Oberbeck–Boussinesq approximation and in the limit of small Reynolds and small Péclet numbers. The force acting on the particle is obtained by using matched-asymptotic expansions. In this approach, the small parameter is given by$a/\ell $, where$a$is the particle radius and$\ell $is the stratification length, as defined by Ardekani & Stocker (Phys. Rev. Lett., vol. 105, 2010, article 084502), which depends on the Brunt–Väisälä frequency, on the fluid kinematic viscosity and on the thermal or the concentration diffusivity (depending on the case considered). The matching procedure used here, which is based on series expansions of generalized functions, slightly differs from that generally used in similar problems. In addition to the classical Stokes drag, it is found that the particle experiences a memory force given by two convolution products, one of which involves, as usual, the particle acceleration and the other one, the particle velocity. Owing to the stratification, the transient behaviour of this memory force, in response to an abrupt motion, consists of an initial fast decrease followed by a damped oscillation with an angular frequency corresponding to the Brunt–Väisälä frequency. The perturbation force eventually tends to a constant which provides us with correction terms that should be added to the Stokes drag to accurately predict the settling time of a particle in a diffusive stratified fluid.

Expansions at small Reynolds numbers for the locomotion of a spherical squirmer

The locomotion of a spherical squirmer — a model organism that achieves self-propulsion via steady tangential movement of its surface — is quantified at small Reynolds number R. Matched asymptotic expansions are employed to calculate the swimming velocity of the squirmer through O(R2). Approximations to the velocity and vorticity fields around the squirmer that are uniformly valid to O(R) are also constructed.

Influence of thin Slip Bands on Grain Boundary Stress Fields and Microcrack Initiation: Analytical and Numerical Approaches

Strain localization is often observed in single and poly-crystals, for instance forming clear bands or persistent slip bands respectively during post-irradiation tensile loading or cyclic loading. This concerns FCC, BCC or HCP crystallographic structures. At the intersection of the so-called slip bands (SBs) and grain boundaries (GBs), stress concentration areas emerge contributing to intergranular microcrack initiation. Indeed, GB stress fields show singularities due to the SB plastic shearing of the surrounding elastic matrix. Since Stroh's work [[?]], GB normal stress fields have been computed using pile-up theory assumptions. However, as these approaches assume that slip occurs along only one single atomic plane, they overestimate GB normal stress fields because SBs generally display finite thickness, from a few ten nm to a few μm. Crystalline finite elements (FE) computations are carried out using the Cast3M software. Meshed microstructure includes an elastoplastic SB within a main elastic grain which which is embedded at free surface of an elastic matrix. Results show in fact that using the pile-up formalism leads to large GB stress field overestimations. Therefore, an analytical formula describing the normal GB stress fields is proposed in the current paper, extend- ing the pile-up solution in the neighbourhood of the SB corner. The relationships between SB length and thickness and stress singularities are established in the light of the theory of matching asymptotic expansions. A double fracture criterion allows the prediction of intergranular microcrack initiation based on the computed stress singularities. Another analytical model based on the solution of the thermoelastic problem allow to propose a more general formula of the grain boundary stress fields accounting for the unloading effects. Using only one stress field computed by FE, the prefactors are computed and both analytical formulae are shown to be validated for all the range of grain size, SB thickness and remote applied stress.

Prediction of grain boundary stress fields and microcrack initiation induced by slip band impingement

Slip localization is widely observed in metallic polycrystals undergoing cyclic deformation or post-irradiation tensile deformation, whatever their crystallographic structure. Hence, strong strain localization occurs in thin slip bands (SBs) inducing by the way local stress concentrations at their intersections with grain boundaries (GBs). Many GB stress field formulae based on the dislocation pile-up theory have been proposed since the pionnering work of Stroh and others. These allow the use of the Griffith criterion for prediction GB fracture initiation. However, recent observations show that assuming that slip is localized on a single atomic plane leads to unrealistic results. In fact, a large number of slip planes are plastically activated and then finite slip band thickness should be accounted for. Numerous crystalline finite element (FE) computations have been carried out using considering a slip bands with low critical resolved shear stress embedded in an elastic matrix. The computed GB normal and shear stress fields: are considerable lower than the pile-up ones and exhibit strong dependency on the slip band thickness close to the SB corner but are in fair agreement with the solution predicted by the pile-up theory far away Since the pile-up theory leads to the overestimation of the local GB stress fields, the main goal of the current paper is to perform analytical model of GB stress components based upon FE calculations. The effect of various parameters can be understood in the framework of matching asymptotic expansions which is usually applied to cracks with V notches of finite thickness. Finally, the predicted remote stresses to GB fracture in pre-irradiated austenitic stainless steels subjected to tensile loading in various environment are compared to experimental data and the pile-up based predictions.

Dispersive shock wave interactions and asymptotics

Dispersive shock waves (DSWs) are physically important phenomena that occur in systems dominated by weak dispersion and weak nonlinearity. The Korteweg-de Vries (KdV) equation is the universal model for systems with weak dispersion and weak, quadratic nonlinearity. Here we show that the long-time-asymptotic solution of the KdV equation for general, steplike data is a single-phase DSW; this DSW is the "largest" possible DSW based on the boundary data. We find this asymptotic solution using the inverse scattering transform and matched-asymptotic expansions. So while multistep data evolve to have multiphase dynamics at intermediate times, these interacting DSWs eventually merge to form a single-phase DSW at large time.

A Two-Scale Computational Model of pH-Sensitive Expansive Porous Media.

We propose a new two-scale model to compute the swelling pressure in colloidal systems with microstructure sensitive to pH changes from an outer bulk fluid in thermodynamic equilibrium with the electrolyte solution in the nanopores. The model is based on establishing the microscopic pore scale governing equations for a biphasic porous medium composed of surface charged macromolecules saturated by the aqueous electrolyte solution containing four monovalent ions [Formula: see text]. Ion exchange reactions occur at the surface of the particles leading to a pH-dependent surface charge density, giving rise to a nonlinear Neumann condition for the Poisson-Boltzmann problem for the electric double layer potential. The homogenization procedure, based on formal matched asymptotic expansions, is applied to up-scale the pore-scale model to the macroscale. Modified forms of Terzaghi's effective stress principle and mass balance of the solid phase, including a disjoining stress tensor and electrochemical compressibility, are rigorously derived from the upscaling procedure. New constitutive laws are constructed for these quantities incorporating the pH-dependency. The two-scale model is discretized by the finite element method and applied to numerically simulate a free swelling experiment induced by chemical stimulation of the external bulk solution.