Finite-Gap Solutions of the Vortex Filament Equation: Isoperiodic Deformations

We study the topology of quasiperiodic solutions of the vortex filament equation in a neighborhood of multiply covered circles. We construct these solutions by means of a sequence of isoperiodic deformations, at each step of which a real double point is “unpinched” to produce a new pair of branch points and therefore a solution of higher genus. We prove that every step in this process corresponds to a cabling operation on the previous curve, and we provide a labelling scheme that matches the deformation data with the knot type of the resulting filament.     

Vortex Filament Method

In this paper, the convergence of the vortex filament methodfor three-dimensional incompressible and inviscid fluid flowis proved. Properties of consistency and stability are analysed.The foundation for studying these properties is based on thecubature formulae with lines as well as on the specific useof the vector measure that transports the vorticity by the flow,preserving the filament structure of the solution of the problem.In this way, the method takes into account the stretching termimplicitly.     

Collisions of Vortex Filament Pairs

We consider the problem of collisions of vortex filaments for a model introduced by Klein et al. (J Fluid Mech 288:201–248, 1995) and Zakharov (Sov Phys Usp 31(7):672–674, 1988, Lect. Notes Phys 536:369–385, 1999) to describe the interaction of almost parallel vortex filaments in three-dimensional fluids. Since the results of Crow (AIAA J 8:2172–2179, 1970) examples of collisions are searched as perturbations of antiparallel translating pairs of filaments, with initial perturbations related to the unstable mode of the linearized problem; most results are numerical calculations. In this article, we first consider a related model for the evolution of pairs of filaments, and we display another type of initial perturbation leading to collision in finite time. Moreover, we give numerical evidence that it also leads to collision through the initial model. We finally study the self-similar solutions of the model.     

The Equation of Motion of a Vortex Layer of Small Thickness

The equation governing the evolution of a vortex layer whose thickness is small compared to its radius of curvature and in which the vorticity is everywhere the same is obtained, viscous and compressibility effects being neglected. The method of matched asymptotic expansions is applied and results in an extension, which is unexpectedly simple, of Birkhoff's integro-differential equation for vortex sheets. The equation is applied to long waves on a straight vortex layer of uniform thickness.     

Desingularization of a Steady Vortex Pair in the Lake Equation

Potential Analysis - We construct a family of steady solutions of the lake model perturbed by some small Coriolis force, that converge to a singular vortex pair. The desingularized solutions are...     

The Liouville Equation as a Hamiltonian System

Mathematical Notes - Smooth dynamical systems on closed manifolds with invariant measure are considered. The evolution of the density of a nonstationary invariant measure is described by the...     

与多值随机微分方程相关马氏半群的无穷小生成元（英文）

当多值随机微分方程的扩散及漂移系数满足利普希兹连续性条件时,我们考虑其解的无穷小生成元问题.为了找出该无穷小生成元的核,我们研究了对应的多值椭圆方程及其粘性解.     

Vortex breakdown as a fundamental element of vortex dynamics

Summary Keller et al. (1985) have shown that, in analogy to the theory of gravity currents, there exist two types of force-free transitions (i.e. vortex breakdown phenomena) between axisymmetric vortex-flow states in tubes.This paper presents an extension of our earlier analysis, which is based on a variational principle and does not account for viscous effects and shear layer instabilities, to axisymmetric vortex flows in tubes with varying cross-sectional area.A broad investigation into the properties of steady axisymmetric vortex flows is given, in so far as they can be represented by perfect-fluid theory and simple extensions of it. It is argued that the basic physics of such flows, including vortex breakdown phenomena, can be explained with the help of the relatively simple theoretical concepts proposed. Numerical results are presented which show complete flow fields of loss-free transition in diffusers for various types of vortex flows. LDA-measurements and flow visualization experiments proove the existence of two types of transitions, as proposed by the analysis, and show the significance of critical cross-sections in the flow.

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Zusammenfassung Keller et al. (1985) haben gezeigt, daß in Analogie zur Theorie der Schwereströmungen zwei Arten von impulserhaltenden Übergängen (i.e. Vortex-Breakdown-Phänornene) zwischen axisymmetrischen Wirbelströmungszuständen existieren.Diese frühere Theorie, die auf einem einfachen Variationsprinzip basiert und Reibungseffekte sowie Scherschichtsinstabilitäten nicht berücksichtigt, wird in dieser Arbeit auf axisymmetrische Wirbelströmungen in Rohren mit veränderlicher Querschnittsfläche erweitert. Die Eigenschaften axisymmetrischer Wirbelströmungen werden in ihrer vollen Breite untersucht, soweit sie der Theorie der idealen Strömungen und ihren einfachsten Verallgemeinerungen zugänglich sind. Wie es scheint, kann mit hinreichender Rechtfertigung festgestellt werden, daß die grundlegende Physik solcher Strömungen (einschließlich der Vortex-Breakdown-Phänome) mit den relativ einfachen theoretischen Konzepten erklärt werden kann, die in dieser Arbeit diskutiert werden. Es werden numerische Ergebnisse gezeigt, welche die vollständigen Strömungsfelder verlustfreier Übergänge in Diffusoren für verschiedenartige Wirbelströmungen beinhalten. LDA-Messungen und Strömungsbeobachtungen beweisen die Existenz der von der Theorie vorausgesagten übergangsarten und zeigen die zentrale Bedeutung der kritischen Strömungsquerschnitte.

     

On a Pseudorandom Property of Binary Sequences

C. Mauduit and A. Sárközy proposed the use of well-distribution measure and correlation measure as measures of pseudorandomness of finite binary sequences. In this paper we will introduce and study a further measure of pseudorandomness: the symmetry measure. First we will give upper and lower bounds for the symmetry measure. We will also show that there exists a sequence for which each of the well-distribution, correlation and symmetry measures are small. Finally we will compare these measures of pseudorandomness.     

Vortex Motion Law of an Evolutionary Ginzburg-Landau Equation in 2 Dimensions

We study the asymptotic behavior of solutions to an evolutionary Ginzburg-Landau equation. We also study the dynamical law of Ginzburg-Landau vortices of this equation under the Neuman boundary conditions. Away from the vortices, we use some measure theoretic arguments used by F.H.Lin in [1] to show the strong convergence of solutions. This is a continuation of our earlier work [2].     

Convergence of the Point Vortex Methods for Euler Equation on Half Plane

1.IntroductionThevortexmethodsareefficientnumericalmethodofsimulatingincompressibleflowathighReynold'snumber-TheconvergenceofthevortexmethodsfortheinitialvalueproblemsofEulerequationwasfirstobtainedbyHald[41,thentheresultswereimprovedbyseveralauthors[1'2'3'5].Butinfact,manypracticalproblemsareconsideredinaboundeddomainoranexteriordomain,andthenumericalboundaryconditionhasanimportanteffectonnumericalresult.Theparticletrajectoriesofexactsolutionwillnotgooutfromthedomain,butitisnotthecaseinprat…     

The Scattering of Sound by a Vortex Sheet

The effect of the radiation from an acoustic point source onthe vortex sheet separating two fluids in relative motion isinvestigated. A supersonic speed of flow is allowed for andsources which are either harmonically varying or pulsed in timeare included. It is discovered that waves due to Helmholtz instabilityare always present and dominate other disturbances whateverthe Mach number M. This contrasts strongly with excitation bya line source where instability disappears when M > 22. There is a bow wave moving ahead of other disturbances and,if M > 2, a second bow wave appears, behind the first bowwave but in front of other waves. If M > 2, there is a neutralstability wave which is singular at its front. In addition,if M > 22, further waves are generated which display thesame kind of singularity.     

The Motion of a Vortex Pair in a Stratified Atmosphere

The movement of a horizontal vortex pair through an inhomogeneous fluid is considered. The problem is formulated first for the case when the ambient fluid is uniform, the fluid moving with the vortex pair has a different density, and the motion is supposed laminar and inviscid. An approximate solution is obtained, which predicts that the distance between the vortices stays constant and the vortices accelerate at a constant rate. This solution is then applied to motion in a stratified atmosphere and it is found that the vortices oscillate vertically with a frequency and amplitude depending on the initial conditions and the stratification. Finally, approximate equations are constructed to describe the effects of turbulent entrainment into the fluid moving with the vortex pair, and an estimate of the damping is obtained.     

Optimal Kinematics of a Looped Filament

New kinematics of supercoiling of closed filaments as solutions of the elastic energy minimization are proposed. The analysis is based on the thin rod approximation of the linear elastic theory, under conservation of the self-linking number. The elastic energy is evaluated by means of bending contribution and torsional influence. Time evolution functions are described by means of piecewise polynomial transformations based on cubic spline functions. In contrast with traditional interpolation, the parameters, which define the cubic splines representing the evolution functions, are considered as the unknowns in a nonlinear optimization problem. We show how the coiling process is associated with conversion of mean twist energy into bending energy through the passage by an inflexional configuration in relation to geometric characteristics of the filament evolution. These results provide new insights on the folding mechanism and associated energy contents and may find useful applications in folding of macromolecules and DNA packing in cell biology.     

一类差分方程收敛性的证明

运用特征值以及级数展开的方法证明了一类差分方程的收敛性，在章的最后给出了一个一般的定理。     

一类Riccati方程的推广

把 Riccati方程 y′=Py2 + Qy+ R推广成 Riccati型方程 :f′( y) dydx=Pf 2 ( y) + Qf ( y) + R.并给出其可积的条件及其对应的通积分 .