Generalized Contour Dynamics: A Review

Contour dynamics is a computational technique to solve for the motion of vortices in incompressible inviscid flow. It is a Lagrangian technique in which the motion of contours is followed, and the velocity field moving the contours can be computed as integrals along the contours. Its best-known examples are in two dimensions, for which the vorticity between contours is taken to be constant and the vortices are vortex patches, and in axisymmetric flow for which the vorticity varies linearly with distance from the axis of symmetry. This review discusses generalizations that incorporate additional physics, in particular, buoyancy effects and magnetic fields, that take specific forms inside the vortices and preserve the contour dynamics structure. The extra physics can lead to time-dependent vortex sheets on the boundaries, whose evolution must be computed as part of the problem. The non-Boussinesq case, in which density differences can be important, leads to a coupled system for the evolution of both mean interfacial velocity and vortex sheet strength. Helical geometry is also discussed, in which two quantities are materially conserved and whose evolution governs the flow.     

Thin Films and Interfaces of Electroactive Organic Materials: The Surface Science Approach

 We highlight the importance of interfacial properties in determining the performance of devices based on electroactive organic materials. Investigations of the interfaces of benzene with Al(111) and In₂O₃ are presented as a model of interface properties for devices based on complex aromatic molecules. At both interfaces the binding is shown to be electrostatic, with the resulting interface dipole determining the band alignment. It is also argued that chemical modification of substrates can be used to tailor both electronic and structural properties.     

Surfactant-driven instability in two-fluid pipe and channel flows

Recent work on the stability of two-layer channel flow, with either smooth or corrugated walls, and core-annular flow is discussed with the emphasis on the potentially destabilising effect of an insoluble surfactant. Linearised normal mode analyses are conducted to identify unstable parametric conditions, and these are followed up by computer simulations which confirm the linear results and, furthermore, reveal a rich array of dynamical behaviour in the nonlinear regime. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Free-surface film flow over topography under electric fields

We study a gravity-driven flow of a liquid down an inclined uneven wall in the presence of a normal electric field. We derive an evolution equation for the free-surface, and analyse steady-state solutions for flows into a rectangular trench and over a rectangular mound. The results are corroborated by boundary-element calculations for Stokes flow. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Perfect elements in Dubreil-Jacotin regular semigroups

IfS is a strong Dubreil-Jacotin regular semigroup thenx∈S is said to beperfect ifx=x(ξ∶x)x where ζ is the bimaximum element ofS. It is shown that the setP(S) of perfect elements is an ideal ofS, and is also a strong Dubreil-Jacotin subsemigroup. It is then proved that every element ofS is perfect if and only ifS is naturally ordered. Finally, necessary and sufficient conditions forP(S) to be orthodox are determined.     

Analyse der Milch

Ohne Zusammenfassung     

Computing the nonabelian tensor squares of polycyclic groups

In this paper we develop a theory for computing the nonabelian tensor square and related computations for finitely presented groups and specialize it to polycyclic groups. This theory provides a framework for making nonabelian tensor square computations for polycyclic groups and is the basis of an algorithm for computing the nonabelian tensor square for any polycyclic group.     

Measurement of forward-backward asymmetry and Wilson coefficients in B --> K*l+l-

We report the first measurement of the forward-backward asymmetry and the ratios of Wilson coefficients and in A9/A7 and A10/A7 in B --> K*l+l-, where l represents an electron or a muon. We find evidence for the forward-backward asymmetry with a significance of 3.4sigma. The results are obtained from a data sample containing 386 x 10(6) BB pairs that were collected on the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider.     

Analysis of the electronic structure of Hf(Si0.5As0.5)As by X-ray photoelectron and photoemission spectroscopy

The ternary hafnium silicon arsenide, Hf(Si_xAs_1−x)As, has been synthesized with a phase width of 0.5?x?0.7. Single-crystal X-ray diffraction studies on Hf(Si_0.5As_0.5)As showed that it adopts the ZrSiS-type structure (Pearson symbol tP6, space group P4/nmm, Z=2, a=3.6410(5) Å, c=8.155(1) Å). Physical property measurements indicated that it is metallic and Pauli paramagnetic. The electronic structure of Hf(Si_0.5As_0.5)As was investigated by examining plate-shaped crystals with laboratory-based X-ray photoelectron spectroscopy (XPS) and synchrotron radiation photoemission spectroscopy (PES). The Si 2p and As 3d XPS binding energies were consistent with assignments of anionic Si¹⁻ and As^1-. However, the Hf charge could not be determined by analysis of the Hf 4f binding energy because of electron delocalization in the 5d band. To examine these charge assignments further, the valence band spectrum obtained by XPS and PES was interpreted with the aid of TB-LMTO band structure calculations. By collecting the PES spectra at different excitation energies to vary the photoionization cross-sections, the contributions from different elements to the valence band spectrum could be isolated. Fitting the XPS valence band spectrum to these elemental components resulted in charges that confirm that the formulation of the product is Hf²⁺[(Si_0.5As_0.5)As]²⁻.     

The effect of melt processing conditions on the hydrogen-bonded sheet orientation and polarization of nylon 11 films

It is known that melt-quenched, cold-drawn, and then annealed nylon 11 films possess a particular doubly oriented hydrogen-bonded sheet structure: the hydrogen-bonded sheets being in the plane of the film and the molecular chain direction being in the direction of draw. These films have been shown to be ferroelectric. In order to clarify the role of this special double orientation on polarization mechanisms and, in addition, provide insight into the reasons for this unique orientation, a systematic study was undertaken. Nylon 11 was melted at 210°C in a hot press for different melting times (ranging from 30 s to 20 min) prior to quenching into an ice-water bath. The resulting orientation of the hydrogen-bonded sheet structure in these films was examined using wide-angle X-ray diffraction and FTIR spectroscopy. For undrawn, melt-quenched films with short times in the melt, a degree of preferred orientation of the hydrogen-bonded sheets in the plane of the film was observed. As time in the melt increased, this preferred orientation in the plane decreased. This was also observed for films which were cold drawn before annealing. However, following cold drawing and several cycles of polarization using a maximum field of 150 MV/m at room temperature, the uniaxially drawn films with different times in the melt possessed the same remanent polarization and the same final orientation of the hydrogen-bonded sheets in the film thickness direction. © 1996 John Wiley & Sons, Inc.