Decomposition Based Search - A theoretical and experimental evaluation

In this paper we present and evaluate a search strategy called Decomposition Based Search (DBS) which is based on two steps: subproblem generation and subproblem solution. The generation of subproblems is done through value ranking and domain splitting. Subdomains are explored so as to generate, according to the heuristic chosen, promising subproblems first. We show that two well known search strategies, Limited Discrepancy Search (LDS) and Iterative Broadening (IB), can be seen as special cases of DBS. First we present a tuning of DBS that visits the same search nodes as IB, but avoids restarts. Then we compare both theoretically and computationally DBS and LDS using the same heuristic. We prove that DBS has a higher probability of being successful than LDS on a comparable number of nodes, under realistic assumptions. Experiments on a constraint satisfaction problem and an optimization problem show that DBS is indeed very effective if compared to LDS.Comment: 16 pages, 8 figures. LIA Technical Report LIA00203, University of Bologna, 200

Role of the tip vortex in the force generation of low-aspect-ratio normal flat plates

We investigate experimentally the force generated by the unsteady vortex formation of low-aspect-ratio normal flat plates with one end free. The objective of this study is to determine the role of the free end, or tip, vortex. Understanding this simple case provides insight into flapping-wing propulsion, which involves the unsteady motion of low-aspect-ratio appendages. As a simple model of a propulsive half-stroke, we consider a rectangular normal flat plate undergoing a translating start-up motion in a towing tank. Digital particle image velocimetry is used to measure multiple perpendicular sections of the flow velocity and vorticity, in order to correlate vortex circulation with the measured plate force. The three-dimensional wake structure is captured using flow visualization. We show that the tip vortex produces a significant maximum in the plate force. Suppressing its formation results in a force minimum. Comparing plates of aspect ratio six and two, the flow is similar in terms of absolute distance from the tip, but evolves faster for aspect ratio two. The plate drag coefficient increases with decreasing aspect ratio

Coronal MHD transport theory and phenomenology

In the presence of a weakly inhomogeneous background, magnetohydrodynamic fluctuations are transported, reflected and at small scales, dissipated. In contrast to orderings appropriate to outer solar wind conditions, here we explore transport in a regime relevant for solar coronal heating and solar wind acceleration, in which effects of the order of the Alfvén speed are retained while disregarding the solar wind velocity. We consider the general properties of the transport equations as well as some solutions of interest

Conditions for sustainment of magnetohydrodynamic turbulence driven by Alfvén waves

In a number of space and astrophysical plasmas,turbulence is driven by the supply of wave energy. In the context of incompressible magnetohydrodynamics (MHD) there are basic physical reasons, associated with conservation of cross helicity, why this kind of driving may be ineffective in sustaining turbulence. Here an investigation is made into some basic requirements for sustaining steady turbulence and dissipation in the context of incompressible MHD in a weakly inhomogeneous open field line region, driven by the supply of unidirectionally propagating waves at a boundary. While such wave driving cannot alone sustain turbulence, the addition of reflection permits sustainment. Another sustainment issue is the action of the nonpropagating or quasi-two dimensional part of the spectrum; this is particularly important in setting up a steady cascade. Thus, details of the waveboundary conditions also affect the ease of sustaining a cascade. Supply of a broadband spectrum of waves can overcome the latter difficulty but not the former, that is, the need for reflections. Implications for coronal heating and other astrophysical applications, as well as simulations, are suggested

MHD turbulence and heating of the open field-line solar corona

This paper discusses the possibility that heating of the solar corona in open field-line regions emanating from coronal holes is due to a nonlinear cascade, driven by low-frequency or quasi-static magnetohydrodynamic fluctuations. Reflection from coronal inhomogeneities plays an important role in sustaining the cascade. Physical and observational constraints are discussed. Kinetic processes that convert cascaded energy into heat must occur in regions of turbulent small-scale reconnection, and may be similar in some respects to ion heating due to intense electron beams observed in the aurora

Anisotropy-based mechanism for zigzag striped patterns in magnetic thin films

In this work we studied a two dimensional ferromagnetic system using Monte Carlo simulations. Our model includes exchange and dipolar interactions, a cubic anisotropy term, and uniaxial out-of-plane and in-plane ones. According to the set of parameters chosen, the model including uniaxial out-of-plane anisotropy has a ground-state which consists of a canted state with stripes of opposite out-of-plane magnetization. When the cubic anisotropy is introduced zigzag patterns appear in the stripes at fields close to the remanence. An analysis of the anisotropy terms of the model shows that this configuration is related to specific values of the ratio between the cubic and the effective uniaxial anisotropy. The mechanism behind this effect is related to particular features of the anisotropy's energy landscape, since a global minima transition as a function of the applied field is required in the anisotropy terms. This new mechanism for zigzags formation could be present in monocrystal ferromagnetic thin films in a given range of thicknesses.Comment: 910 pages, 10 figure

A reduced magnetohydrodynamic model of coronal heating in open magnetic regions driven by reflected low-frequency waves

A reduced magnetohydrodynamic (RMHD) description is employed to examine a suggestion made by W. H. Matthaeus and colleagues in 1999 that coronal heating might be sustained by a cascade of low-frequency MHD turbulence. Here RMHD simulations show that the low-frequency cascade to high transverse wavenumbers can be driven by an externally maintained flux of low-frequency propagating Alfvén waves, in combination with reflection caused by an inhomogeneous background medium. The simulations show that the suggestions made previously on the basis of a phenomenology are indeed realizable. In addition, the effect is seen to sensitively depend on the presence of reflection, as the background turbulence level needed to maintain the cascade can be sustained only when reflection is imposed. The steady level of turbulence is insensitive to the initial seed turbulence level (provided it is nonzero). Consequences of this model for realistic models of coronal heating in open field-line regions are discussed