The quickest transfer of a multidimensional dynamic object onto the surface of an ellipsoid

The time-optimal control of motion of a multidimensional dynamic object is investigated. Both open-loop and feedback controls are considered. It is assumed that the force vector is constrained by a non-degenerate ellipsoid. The terminal set is described by the surface of an ellipsoid that allows degeneracy, and the terminal velocity is not fixed. The initial position of the object can be either outside or inside the ellipsoid. Using the maximum principle, the necessary and sufficient conditions for the optimality of the control are established in the form of a system of polynomial equations, the orders of which depend on the dimension of the problem and the degree of degeneracy of the terminal ellipsoid. Iterative procedures and a technique for continuation with respect to the parameters of the approximating ellipsoid in the admissable region of change of the phase vector are proposed. Control problems for the limiting shapes of the terminal ellipsoid are investigated. The qualitative effect of the discontinuity of the functional and the control defined as functions of the phase vector is established.     

The time-optimal transfer of a perturbed dynamical object to a given position

The problem of transferring a dynamical object (a point mass) of arbitrary dimensional to a required position in a time-optimal manner by means of a force of limited modulus is solved. The velocity of the object at the final instant is not specified. It is assumed that an arbitrary known perturbation, with a magnitude strictly less than that of the control, acts on the controlled system. For clarity when analysing the optimal controlled motion, considerable attention is paid to the case of a steady perturbation. A constructive procedure for finding the optimal response time and the control is developed for arbitrary permissible values of the governing parameters. The Bellman function and the feedback control are constructed over the whole of the phase space. The structural properties of the solution are established and an asymptotic analysis is carried out by small-parameter methods. The extremal directions of the perturbation vector and the corresponding response time and optimal control are found. A modification of the time-optimal problem to the case of a non-stationary perturbation is presented and the basic properties of the optimal solution are investigated.     

A factorization method for elliptic problems in a circular domain

We present a method to factorize a second order elliptic boundary value problem in a circular domain, in a system of uncoupled first order initial value problems. We use a space invariant embedding technique along the radius of the circle, in a decreasing way. This technique is inspired in the temporal invariant embedding used by J.-L. Lions for the control of parabolic systems. The singularity at the origin for the initial value problems is studied. A formal calculation for more general star-shaped domains is presented. To cite this article: J. Henry et al., C. R. Acad. Sci. Paris, Ser. I 339 (2004).     

Torsion of a circular cone with static and dynamic loading

Integral transformation methods—the Mellin transform for statics and the Lebedev-Kontorovich transform for dynamics—are used to construct analytic solutions of the problem of the torsion of an elastic circular cone. Assuming that external forces are concentrated in the neighbourhood of the vertex of the cone, the asymptotic behaviour of the far field is investigated. It is shown that the leading term of the asymptotic expansion is governed by the magnitude of the moment of the external forces, so that the St Venant principle is satisfied in the cases under consideration.     

Electroosmotic flow and heat transfer in a heterogeneous circular microchannel

Asymmetries in boundary condition are inevitable in practice in microfluidic channels, despite being rarely addressed from theoretical perspectives. Here, by arriving at closed form analytical solutions, we bring out a unique coupling between asymmetries in surface charge and heat transfer in electroosmotically driven microchannel flows. For illustration, we assume that the channel is laterally composed of two parts, each having specified values of the zeta potential and the wall heat flux. Considering low zeta potentials, we obtain analytical solutions in terms of infinite series for the dimensionless forms of the electric potential, the velocity, and the temperature distributions. We demonstrate that, by carefully adjusting the governing parameters, a variety of flow patterns may be achieved, a property that is crucial in applications such as liquid-phase transportation and mixing. Moreover, we show that the average velocity is a linear function of both the zeta potential ratio and the coverage factor. We further show that the average Nusselt number increases when part of the channel having the larger heat flux enlarges and the zeta potential of the part having the smaller surface charge increases. Hence, the maximum heat transfer rates are achieved when the boundary conditions are symmetrical.     

Capital accumulation for production in a dynamic spatial monopoly

This paper characterises the dynamics of capacity accumulation in a spatial monopoly, contrasting the socially optimal behavior of a benevolent planner against that of a profit-seeking monopolist. In steady state, the monopolist always distorts its investment as compared with the social optimum, except for those situations where, under both monopoly and social planning, either the equilibrium is driven by the Ramsey golden rule or consumers’ reservation price is sufficiently high to induce the profit-seeking firm to serve all of them. Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 61, Optimal Control, 2008.     

Parameter-uniform numerical method for singularly perturbed convection-diffusion problem on a circular domain

A linear singularly perturbed elliptic problem, of convection-diffusion type, posed on a circular domain is examined. Regularity constraints are imposed on the data in the vicinity of the two characteristic points. The solution is decomposed into a regular and a singular component. A priori parameter-explicit pointwise bounds on the partial derivatives of these components are established. By transforming to polar co-ordinates, a monotone finite difference method is constructed on a piecewise-uniform layer-adapted mesh of Shishkin type. Numerical analysis is presented for this monotone numerical method. The numerical method is shown to be parameter-uniform. Numerical results are presented to illustrate the theoretical error bounds established.     

Unsteady heat transfer from a circular cylinder in a low Reynolds number flow

This paper examines the temperature distribution in a low Reynolds number flow past a circular cylinder following a sudden change in the temperature of the cylinder. Solutions describing the initial development of the temperature field and the final approach to the eventual steady state are obtained. Expressions for the Nusselt number for small and large values of time are determined.     

Stationary solutions of the non-linear Boltzmann equation in a bounded spatial domain

A contraction mapping (or, alternatively, an implicit function theory) argument is applied in combination with the Fredholm alternative to prove the existence of a unique stationary solution of the non-linear Boltzmann equation on a bounded spatial domain under a rather general reflection law at the piecewise C¹ boundary. The boundary data are to be small in a weighted L_∞-norm.     

Stability of the Thomson vortex polygon with evenly many vortices outside a circular domain

We consider the stability problem for the stationary rotation of a regular point vortex n-gon lying outside a circular domain. After the article of Havelock (1931), the complete solution of the problem remains unclear in the case 2 ≤ n ≤ 6. We obtain the exhaustive results for evenly many vortices n = 2, 4, 6.     

The circular dimension of a graph

A graph is a pair (V, I), V being the vertices and I being the relation of adjacency on V. Given a graph G, then a collection of functions {f_i}^m_n=1, each f_i mapping each vertex of V into anarc on a fixed circle, is said to define an m-arc intersection model for G if for all x,y ? V, xly ? (∨_i?m)(f_i(x)∩f_i(y)≠Ø). The circular dimension of a graph G is defined as the smallest integer m such that G has an m-arc intersection model. In this paper we establish that the maximum circular dimension of any complete partite graph having n vertices is the largest integer p such that 2^p+p?n+1.     

On steady irrotational flow of incompressible ideal fluid in a circular domain with free surface

We study irrotational steady flows of an incompressible ideal fluid in a two-dimensional circular domain, with free surface. Stationary solutions have been studied as bifurcation problems assuming the obstacle exactly circular, and in the presence of surface tension. In this paper, disregarding the surface tension, for non circular obstacles, we analize the set of solutions close to bifurcation points letting the Froud number as driving parameter.

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Sunto Si studiano moti irrotazionali stazionari di un fluido ideale incomprimibile in un dominio bidimensionale circolare con superficie libera. Di recente, le soluzioni stazionarie sono considerate come problemi di biforcazione assumendo che l’ostacolo sia un cerchio, supponendo che vi sia una tensione superficiale sulla superficie libera. In questo lavoro trascurando la tensione superficiale e considerando ostacoli non circolari, analizziamo l’insieme delle soluzioni vicino punti di biforcazione al variare del numero di Fround.

     

The circular chromatic number of a digraph

We introduce the circular chromatic number χ_c of a digraph and establish various basic results. They show that the coloring theory for digraphs is similar to the coloring theory for undirected graphs when independent sets of vertices are replaced by acyclic sets. Since the directed k‐cycle has circular chromatic number k/(k – 1), for k ≥ 2, values of χ_c between 1 and 2 are possible. We show that in fact, χ_c takes on all rational values greater than 1. Furthermore, there exist digraphs of arbitrarily large digirth and circular chromatic number. It is NP‐complete to decide if a given digraph has χ_c at most 2. © 2004 Wiley Periodicals, Inc. J Graph Theory 46: 227–240, 2004     

Skin friction and heat transfer on a circular cylinder moving in a fluid at rest

Summary The incompressible laminar stationary flow along a circular cylinder drawn through an orifice with constant velocity is studied. The surrounding fluid is at rest. (Manufacture of fibers.) For skin friction and heat transfer, a solution for high values of Reynolds number is given in the form of an asymptotic expansion. The special second-order effects, entrainment and curvature, are studied. It is shown that the entrainment effect becomes important for skin friction when the distance from the orifice is small, whereas the influence on heat transfer is small. The curvature effect on skin friction and heat transfer is given as a function of Reynolds number and distance from the orifice.

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Zusammenfassung Es wird die inkompressible, laminare und stationäre Strömung entlang eines mit konstanter Geschwindigkeit bewegten Kreiszylinders untersucht, der aus einer Öffnung hervortritt (Spinnprozeß). Das umgebende Fluid in großer Entfernung vom Zylinder befinde sich dabei in Ruhe. Für die Schubspannung und den Wärmeübergang bei großen Reynoldszahlen wird eine Lösung in Form einer asymptotischen Entwicklung angegeben. Insbesondere werden die Effekte 2. Ordnung, Verdrängung und Krümmung, untersucht. Es wird gezeigt, daß der Verdrängungseffekt für kleine Abstände von der Zylinderaustrittsöffnung für die Schubspannung von Bedeutung wird, wohingegen der Wärmeübergang dadurch nur wenig beeinflußt wird. Der Einfluß der Krümmung auf die Wandschubspannung und den Wärmeübergang wird als Funktion der Reynoldszahl und des Abstandes von der Zylinderaustrittsöffnung angegeben.