On the application of the asymptotic method of global instability in aeroelasticity problems

The asymptotic method of global instability developed by A.G. Kulikovskii is an effective tool for determining the eigenfrequencies and stability boundary of one-dimensional or multidimensional systems of sufficiently large finite length. The effectiveness of the method was demonstrated on a number of one-dimensional problems; and since the mid-2000s, this method has been used in aeroelasticity problems, which are not strictly one-dimensional: such is only the elastic part of the problem, while the gas flow occupies an unbounded domain. In the present study, the eigenfrequencies and stability boundaries predicted by the method of global instability are compared with the results of direct calculation of the spectra of the corresponding problems. The size of systems is determined starting from which the method makes a quantitatively correct prediction for the stability boundary.     

Problem of determining the speed of sound and the memory of an anisotropic medium

Theoretical and Mathematical Physics - We consider the inverse problems of simultaneously determining two unknowns: the wave propagation velocity and the memory of a layered medium. To find them,...     

On the effectiveness of projection methods for convex feasibility problems with linear inequality constraints

The effectiveness of projection methods for solving systems of linear inequalities is investigated. It is shown that they often have a computational advantage over alternatives that have been proposed for solving the same problem and that this makes them successful in many real-world applications. This is supported by experimental evidence provided in this paper on problems of various sizes (up to tens of thousands of unknowns satisfying up to hundreds of thousands of constraints) and by a discussion of the demonstrated efficacy of projection methods in numerous scientific publications and commercial patents (dealing with problems that can have over a billion unknowns and a similar number of constraints).     

Problems and methods of averaged optimization

Typical extremal problems containing the mean values of unknowns or of some functions of unknowns are considered. Relationships between these problems and cyclic modes of dynamical systems are revealed, and optimality conditions for such modes are found.     

Calculating the vertex unknowns of nine point scheme on quadrilateral meshes for diffusion equation

In the construction of nine point scheme,both vertex unknowns and cell-centered unknowns are introduced,and the vertex unknowns are usually eliminated by using the interpolation of neighboring cell-centered unknowns,which often leads to lose accuracy.Instead of using interpolation,here we propose a different method of calculating the vertex unknowns of nine point scheme,which are solved independently on a new generated mesh.This new mesh is a Vorono¨i mesh based on the vertexes of primary mesh and some additional points on the interface.The advantage of this method is that it is particularly suitable for solving diffusion problems with discontinuous coeffcients on highly distorted meshes,and it leads to a symmetric positive definite matrix.We prove that the method has first-order convergence on distorted meshes.Numerical experiments show that the method obtains nearly second-order accuracy on distorted meshes.     

Technique for the numerical analysis of the riblet effect on temporal stability of plane flows

Problems related to the temporal stability of laminar viscous incompressible flows in plane channels with ribbed walls are formulated, justified, and numerically solved. A new method is proposed whereby the systems of ordinary differential and algebraic equations obtained after a spatial approximation are transformed into systems of ordinary differential equations with a halved number of unknowns. New algorithms that effectively calculate stability characteristics, such as the critical Reynolds numbers, the maximum amplification of the disturbance kinetic energy density, and optimal disturbances are described and substantiated. The results of numerical experiments with riblets similar in shape to those used in practice are presented and discussed.     

一类多维非线性反应扩散方程有限差分格式的稳定性问题(英文)

本文研究了一类多维线性反应扩散方程差分格式的稳定性.利用量未知元方法,建立了具有增量未知元的有限差分格式;然后利用非线性Galerkin方法,得到该差分格式的稳定性条件.通过对该格式的稳定性分析,说明和经典的差分格式的稳定性相比较,带有增量未知元的有限差分格式的稳定性得到了提高.     

Free vibrations and stability of hydraulic cylinder fixed elastically on both ends

The boundary value problem of the stability and free vibration of a hydraulic cylinder has been formulated and solved in this paper. The considered hydraulic cylinder has been elastically fixed at both ends and loaded by Euler force. An elastical fixation has been modelled by rotational springs. The mentioned above hydraulic cylinder consists of a piston rod and cylinder replaced by rods. There are conditions of continuity between the rods. The boundary value problem has been formulated on the basis of minimum potential energy (static problem) and on the basis of Hamilton's principle (free vibration problem). In this paper example results of numerical calculations of the stability and free vibration have been presented. Experimental research has been performed in order to verify the correctness of the assumed mathematical model. Professional measuring apparatus and special stand for research into the slender systems have been used in experiment. Natural frequencies have been measured in dependence on the values of an external load. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Unique discrete harmonic continuation and data completion problems using the fading regularization method

A data completion method is proposed for solving Cauchy problems for which the number of data is less than the number of unknowns. This method is presented on the Cauchy problem for the Laplace equation in 2D situations. The idea is to search the solution in a space of discrete harmonic functions for which the existence and the uniqueness of a discrete harmonic continuation are guaranteed. Many numerical simulations using the finite element method highlight the efficiency, accuracy, and stability of this method even when data are noisy.     

Incremental unknowns for solving partial differential equations

Summary Incremental unknowns have been proposed in [T] as a method to approximate fractal attractors by using finite difference approximations of evolution equations. In the case of linear elliptic problems, the utilization of incremental unknown methods provides a new way for solving such problems using several levels of discretization; the method is similar but different from the classical multigrid method.In this article we describe the application of incremental unknowns for solving Laplace equations in dimensions one and two. We provide theoretical results concerning two-level approximations and we report on numerical tests done with multi-level approximations.     

Derivation of the model of elastic curved rods from three-dimensional micropolar elasticity

In this paper we derive a model of curved elastic rods from the threedimensional linearized micropolar elasticity. Derivation is based on the asymptotic expansion method with respect to the thickness of the rod. The method is used without any a priori assumption on the scaling of the unknowns. The leading term, displacement and microrotation, is identified as the unique solution of a certain one-dimensional problem. Appropriate convergence results are proved.      

The random product homotopy and deficient polynomial systems

Summary Most systems ofn polynomial equations inn unknowns arising in applications aredeficient, in the sense that they have fewer solutions than that predicted by the total degree of the system. We introduce the random product homotopy, an efficient homotopy continuation method for numerically determining all isolated solutions of deficient systems. In many cases, the amount of computation required to find all solutions can be made roughly proportional to the number of solutions.     

On a stability criterion for creep

Stability in the presence of creep has been studied by different authors; some of these investigations were made as much as ten years ago.

The majority of the published papers are concerned with problems of the stability of longitudinally compressed rods, since this represents the simplest formulation of the problem by which many of the particular characteristics of stability can be elucidated.

It has to be noted that there exists a series of principally different formulations of the problems of stability in the presence of creep. In this paper, consideration will be restricted to stability of rectilinear forms of equilibrium.     

提高反应—扩散方程有限差分格式的稳定性问题

This paper deals with the special nonlinear reaction-diffusion equation.The finite difference scheme with incremental unknowns approximating to the differential equation (2.1) is set up by means of introducing incremental unknowns methods.Through the stability analyzing for the scheme,it was shown that the stability conditions of the finite difference schemes with the incremental unknowns are greatly improved when compared with the stability conditions of the corresponding classic difference scheme.     

A Shooting Algorithm for Optimal Control Problems with Singular Arcs

In this article, we propose a shooting algorithm for a class of optimal control problems for which all control variables appear linearly. The shooting system has, in the general case, more equations than unknowns and the Gauss–Newton method is used to compute a zero of the shooting function. This shooting algorithm is locally quadratically convergent, if the derivative of the shooting function is one-to-one at the solution. The main result of this paper is to show that the latter holds whenever a sufficient condition for weak optimality is satisfied. We note that this condition is very close to a second order necessary condition. For the case when the shooting system can be reduced to one having the same number of unknowns and equations (square system), we prove that the mentioned sufficient condition guarantees the stability of the optimal solution under small perturbations and the invertibility of the Jacobian matrix of the shooting function associated with the perturbed problem. We present numerical tests that validate our method.     

Study of the effect of test piece geometry on the propagation velocity and logarithmic decrement of longitudinal vibrations in fluoroplastics

The effect of the geometry of teflon test pieces on the speed of sound and the logarithmic decrement of longitudinal vibrations has been investigated. Conditions for the determination of the speed of sound in an infinite medium and in a rod have been obtained. A method is given for determining the dynamic moduli of longitudinal elasticity, the shear modulus, and the dynamic Poisson's ratio on test pieces in the form of rods and prisms, starting with a length of 30 mm.Mekhanika Polimerov, Vol. 2, No. 4, pp. 557–564, 1966     

A unified analysis of the local discontinuous Galerkin method for a class of nonlinear problems

In this paper we analyze the main features of the local discontinuous Galerkin method applied to nonlinear boundary value problems in the plane. We consider a class of nonlinear elliptic problems arising in heat conduction and fluid mechanics. The approach, which has been originally applied to several linear boundary value problems, is based on the introduction of additional unknowns given by the flux and the gradient of the temperature (velocity) for diffusion problems (fluid mechanics), and considers convex and nonconvex bounded domains with polygonal boundaries. Our present analysis unifies and simplifies the derivation of the results given in previous works. Several numerical examples are presented, which validate our theoretical results.     

An iterative approach for cone complementarity problems for nonsmooth dynamics

Aiming at a fast and robust simulation of large multibody systems with contacts and friction, this work presents a novel method for solving large cone complementarity problems by means of a fixed-point iteration. The method is an extension of the Gauss-Seidel and Gauss-Jacobi method with overrelaxation for symmetric convex linear complementarity problems. The method is proved to be convergent under fairly standard assumptions and is shown by our tests to scale well up to 500,000 contact points and more than two millions of unknowns.     

A stable approach to Newton's method for general mathematical programming problems inR n

The usual approach to Newton's method for mathematical programming problems with equality constraints leads to the solution of linear systems ofn +m equations inn +m unknowns, wheren is the dimension of the space andm is the number of constraints. Moreover, these linear systems are never positive definite. It is our feeling that this approach is somewhat artificial, since in the unconstrained case the linear systems are very often positive definite. With this in mind, we present an alternate Newton-like approach for the constrained problem in which all the linear systems are of order less than or equal ton. Furthermore, when the Hessian of the Lagrangian at the solution is positive definite (a situation frequently occurring), all our systems will be positive definite. Hence, in all cases, our Newton-like method offers greater numerical stability. We demonstrate that the convergence properties of this Newton-like method are superior to those of the standard approach to Newton's method. The operation count for the new method using Gaussian elimination is of the same order as the operation count for the standard method. However, if the Hessian of the Lagrangian at the solution is positive definite and we use Cholesky decomposition, then the order of the operation count for the new method is half that for the standard approach to Newton's method. This theory is generalized to problems with both equality and inequality constraints.     

Approximate factorizations with modified S/P consistently ordered M-factors

Preconditioned iterative methods are widely used to solve linear systems such as those arising from the finite element formulation of boundary value problems and approximate factorizations are widely used as preconditioners. The ordering of the unknowns is therefore an important issue because it has a strong influence on the convergence behaviour of the iteration method while it is also a decisive aspect for their parallel implementation. Consistent orderings are attractive for parallel implementations and it has been shown that some subclasses of these orderings also enhance the convergence behaviour of the associated iteration methods. This has in particular been shown for the so-called S/P consistent orderings. A wider definition of this class of orderings has recently been proposed and we investigate here how approximate factorizations should be implemented when using such more general orderings (still called S/P consistent) in order to keep their expected high convergence properties. A simple practical conclusion is suggested, supported by both theoretical and numerical arguments.