A note for analysis of thermo-mechanical contact problems

A discussion about the bifurcation and non-uniqueness of solutions in the analysis of thermo-mechanical contact problems with initial gap is given. Without loss of generality, a mechanical contact problem coupled with steady heat transfer is studied and an example of non-uniqueness of solutions caused by the thermo-mechanical mechanism is presented. The important work is that the non-uniqueness of solutions, which is different from that found in the analysis of the traditional frictional contact problems, is studied in detail. The possible oscillation and non-convergence problems in the iteraction process of the numerical computation are discussed, and an enhanced algorithm is put forward to overcome the difficulties. Project sypported by the National Natural Science Foundation of China (Nos. 50178016, 10225212 and 19872016), the National Key Basic Research Special Foundation (No. G1999032805) and the Foundation for University Key Teacher by the Ministry of Education.     

Broadwell Model and Conservative Supersonic Boundary

In this paper, we consider an initial-boundary value problem for the Broadwell model with a supersonic physical boundary. Here, we pose a conservative boundary condition which preserves the total number of particles. We show that the solution converges pointwise to a boundary layer exponentially, when the perturbations of the initial data to the equilibrium state are sufficiently small, by using Green’s function as established in Lan et al. (Netw Heterog Media 1:167–183, 2006), weighted energy estimates and by constructing a pair of anti-derivatives to convert the conservative boundary condition into a dissipative boundary condition with conservation laws together with an a priori chosen boundary layer.     

Wave structure induced by fluid-dynamic limits in the Broadwell model

Consider the fluid-dynamic limit problem for the Broadwell system of the kinetic theory of gases, for Maxwellian Riemann initial data. The formal limit is the Riemann problem for a pair of conservation laws and is invariant under dilations of coordinates. The approach of self-similar fluid-dynamic limits consists in replacing the mean free path in the Broadwell model so that the resulting problem preserves the invariance under dilations. The limiting procedure was justified in [ST]. Here, we study the structure of the emerging solutions. We show that they consist of two wave fans separated by a constant state. Each wave fan is associated with one of the characteristic fields and is either a rarefaction wave or a shock wave. The shocks satisfy the Lax shock conditions and have the internal structure of a Broadwell shock profile.     

Transition of unsteady flows of evaporation to steady state

We investigate the half-space problem of evaporation and condensation in the scope of discrete kinetic theory. Exact solutions are found to the boundary value problem and the initial boundary value problems of the flow in the half space for a discrete velocity model. The results are used to analyze the transition of the unsteady solutions towards steady states. To cite this article: A. d'Almeida, C. R. Mecanique 336 (2008).     

L2 Well-Posedness of Planar Div-Curl Systems

Criteria for the existence and uniqueness of solutions of div-curl boundary value problems on bounded planar regions with nice boundaries are developed. The boundary conditions to be treated include prescribed normal component of the field, tangential component of the field and disjoint combinations of these conditions. Under natural assumptions on the data, when either tangential or normal components are given on the whole boundary, weak (finite-energy) solutions exist if and only if a compatibility condition holds. If the region is simply connected this solution is unique. When the region is multiply connected, there is a finite-dimensional family of solutions. The dimension of the solution space is the Betti number of the region. The problem is well-posed with a unique solution when certain line integrals are further prescribed. L ² estimates of the solutions are given. If mixed tangential, and normal, components of the field are specified on different parts of the boundary, no compatibility condition is required for solvability. In general, though, there is considerable non-uniqueness of solutions. Well-posedness is recovered by specifying certain line integrals. L ² estimates of the solutions are given. The dimensionality of the solution space depends on the topology of the boundary data. These results depend on certain weighted orthogonal decompositions of L ² vector fields on the region which are related to classical Hodge-Weyl decomposition results.     

Wave equations and reaction-diffusion equations with several nonlinear source terms

The initial boundary value problem of wave equations and reaction-diffusion equations with several nonlinear source terms in a bounded domain is studied by potential well method.The invariance of some sets under the flow of these problems and the vac- uum isolation of solutions are obtained by introducing a family of potential wells.Then the threshold result of global existence and nonexistence of solutions are given.Finally, the problem with critical initial conditions are discussed.     

Stability of Navier-Stokes equation (I)

In this paper we give an example of non-uniqueness of local solution for some kinds of boundary value problem of Navier-Stokes equation.     

Flow of viscoelastic fluids through a porous channel—I

The flow of viscoelastic fluids through a porous channel with one impermeable wall is computed. The flow is characterized by a boundary value problem in which the order of the differential equation exceeds the number of boundary conditions. Three solutions are developed: (i) an exact numerical solution, (ii) a perturbation solution for small R, the cross-flow Reynold's number and (iii) an asymptotic solution for large R. The results from exact numerical integration reveal that the solutions for a non-Newtonian fluid are possible only up to a critical value of the viscoelastic fluid parameter, which decreases with an increase in R. It is further demonstrated that the perturbation solution gives acceptable results only if the viscoelastic fluid parameter is also small. Two more related problems are considered: fluid dynamics of a long porous slider, and injection of fluid through one side of a long vertical porous channel. For both the problems, exact numerical and other solutions are derived and appropriate conclusions drawn.     

A stability study of Navier-Stokes equations (III)

In this paper, the necessary conditions of the existence of C² solutions in some initial problems of Navier-Stokes equations are given, and examples of instability of initial value (at t=0) problems are also given. The initial value problem of Navier-Stokes equation is one of the most fundamental problem for this equation various authors studies this problem and contributed a number of results. J. Lerav, a French professor, proved the existence of Navier-Stokes equation under certain defined initial and boundary value conditions. In this paper, with certain rigorously defined key concepts, based upon the basic theory of J. Hadamard partial differential equations¹, gives a fundamental theory of instability of Navier-Stokes equations. Finally, many examples are given, proofs referring to Ref. [4].     

Stability of Navier-Stokes Equation(Ⅰ)

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Non-Uniqueness of Minimizers for Strictly Polyconvex Functionals

In this note we solve a problem posed by Ball (in Philos Trans R Soc Lond Ser A 306(1496):557–611, 1982) about the uniqueness of smooth equilibrium solutions to boundary value problems for strictly polyconvex functionals,

Loss of uniqueness of numerical solutions of the borehole problem modelled with enhanced media

It is well known that, initial boundary value problems involving constitutive equations modeling the degradation of the strength of materials are not well posed, which renders computations questionable. To overcome this issue it is necessary to enhance the models by incorporating some internal length. It has been shown that such an enhancement restores the objectivity of the computation as spurious mesh dependency is avoided. However, at least for simple problems (e.g. one dimensional ones), it has been proven that uniqueness of the underlying mathematical problem itself is not restored. Moreover numerical modeling of element tests yields several solutions. This paper demonstrates that several numerical solutions can be obtained also for less simple problems, namely the borehole problems. Even when a defect is introduced in the computed problems, different numerical solutions are found. Contrary to the one dimensional problem there is no proof that this loss of uniqueness comes from the underlying mathematical problem. It is our opinion that this is an inherent property of initial boundary value problems where, broadly speaking, strong degradation of the mechanical properties is modeled. In any case, it is necessary to be aware of this issue. For problems involving constitutive equation modeling strength degradation, it is important to try to find other solutions than the one obtained by using routinely a numerical code. The failure patterns of the different solutions found are however similar to experimental observations. This possible loss of uniqueness can then be seen as a counterpart of the difficulties encountered when attempting to reproduce experiments. This is crucial when dealing with geomaterials.     

Boundary integral equations of unique solutions in elasticity

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On the approximate solution of elliptic,self adjoint boundary value problems

Summary Integral representations for the solutions to linear elliptic self adjoint boundary value problems are derived in terms of two functions which are generalisations of the single and double layer potentials used in the theory of harmonic functions. The generalised potentials are constructed in terms of a fundamental solution which is an approximation to the exact kernel of the boundary value problem in question. The representations so obtained are shown to provide a basis from which strict approximations to the solutions of boundary value problems can be developed. In particular the structure of the integral equation representing the given boundary value problem is precisely determined.     

Harten solution for one-dimensional unsteady equation

In order to use the second-order 5-point difference scheme mentioned to compute the solution of one dimension unsteady equations of the direct reflection of the strong plane detonation wave meeting a solid wall barrier, in this paper, we technically construct the difference schemes of the boundary and sub-boundary of the problem, and deduce the auto-analogue analytic solutions of the initial value problem, and at the same time, we present a method for the singular property of the initial value problem, from which we can get a satisfactory computation result of this difficult problem. The difference scheme used in this paper to deal with the discontinuity problems of the shock wave are valuable and worth generalization.     

MARTEN SOLUTION FOR ONE-DIMENSIONAL UNSTEADY EQUATION

In order to use the second-order5-point difference scheme mentioned to computethe solution of one dimension unsteady equations of the direct reflection of the strongplane detonation wave meeting a solid wall barrier,in this paper,we technicallyconstruct the difference schemes of the boundary and sub-boundary of the problem,and deduce the auto-analogue analytic solutions of the initial value problem,and at thesame time,we present a method for the singular property of the initial value problem,from which we can get a satisfactory computation result of this difficult problem.The difference scheme used in this paper to deal with the discontinuity problemsof the shock wave are valuable and worth generalization.     

Nonstationary ideal incompressible fluid flows: Conditions of existence and uniqueness of solutions

The problem of formulating minimal conditions on input data that can guarantee the existence and uniqueness of solutions of the boundary value problems describing non-one-dimensional ideal incompressible fluid flow is considered using as an example the initial boundary value problem in a space-time cylinder constructed on a bounded flow domain with the nonpenetration condition on its boundary (which corresponds to fluid flow in a closed vessel). The existence problems are considered only for plane flows, and the uniqueness issues for three-dimensional flows as well. The required conditions are obtained in the form of conditions specifying that the vorticity belongs to definite functional Orlicz spaces. The results are compared with well-known results. Examples are given of admissible types of singularities for which the obtained results are valid, which is a physical interpretation of these results. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 4, pp. 130–145, July–August, 2008.     

粘弹性问题的插值型无单元伽辽金比例边界法

作为一种最近发展起来的半解析数值方法,插值型无单元伽辽金比例边界法不仅无需基本解,且在处理应力奇异性问题和无限域问题时十分有效．为了更有效地求解粘弹性问题,对插值型无单元伽辽金比例边界法应用于此类问题进行了研究,并发展了相应的算法. 通过时域分段展开,将时空耦合的初边值问题转化为一系列递推形式的边值问题,然后采用插值型无单元伽辽金比例边界法进行自适应计算．在径向保持解析特性的基础上,环向采用无单元伽辽金法离散可简化前处理和后处理工作量．此外,改进的插值型移动最小二乘法形函数具有插值性,有效地解决了本质边界条件不能直接施加的困难．最后给出了数值算例,并验证了所提方法的有效性和正确性.     

A Legendre wavelet spectral collocation technique resolving anomalies associated with velocity in some boundary layer flows of Walter-B liquid

A Legendre wavelet spectral collocation method is proposed here to solve three boundary layer flow problems of Walter-B fluid namely the stagnation point flow, Blasius flow and Sakiadis flow. In the proposed method, we first transform the boundary value problems into initial value problems using shooting method. We then split the semi infinite domain into subintervals and the governing initial value problems are transformed to system of algebraic equations in each subinterval. The solutions of these algebraic equations yield an approximate solution of the differential equation in each subinterval. The overshoot in the velocity profile associated with the stagnation point and Blasius flows and undershoot in the Sakiadis flow is controlled. Physically realistic solutions are presented for both weakly and strongly viscoelastic parameters. The residual error validates the correctness, convergence and accuracy of the obtained solutions.