A symmetry-preserving Cartesian grid method for computing a viscous flow past a circular cylinder

This article deals with a numerical method for solving the unsteady, incompressible Navier–Stokes equations in domains with arbitrarily-shaped boundaries, where the boundary is represented using the Cartesian grid approach. We introduce a novel cut-cell discretization which preserves the spectral properties of convection and diffusion. Here, convection is discretized by a skew-symmetric operator and diffusion is approximated by a symmetric, positive-definite coefficient matrix. Such a symmetry-preserving discretization conserves the kinetic energy (if the dissipation is turned off) and is stable on any grid. The method is successfully tested for an incompressible, unsteady flow around a circular cylinder at $\mathit{Re}=100$. To cite this article: R. Verstappen, M. Dröge, C. R. Mecanique 333 (2005).     

In-source decay of hyperbranched polyesteramides in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Hyperbranched polyesteramides (DA2), prepared from hexahydrophthalic anhydride (D) and diisopropanolamine (A) have been characterized, by use of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), field desorption (FD)-MS, and electrospray ionization (ESI)-MS. MALDI of polyesteramides produces protonated molecules. The spectra show a complex chemical composition distribution and end-group distribution which are mainly composed of two series of homologous oligomers DnA(n)+1 - mH2O and DnA(n) - mH2O, where m = 1-2. Signals from protonated molecules DnAn+1 and DnAn are almost absent in the MALDI spectrum, whereas these ions are responsible for the base peak of DnA(n)+1 - mH2O and DnA(n) - mH2O (m = 1-2) clusters in the ESI spectrum. The absence of -OH end-groups signals in the MALDI spectrum is due to a metastable decay of protonated DnA(n)+1 and DnAn ions in the ion source of the MALDI mass spectrometer prior to ion extraction. In-source decay results in the formation of protonated lower DnA(n)+1 - mH2O and DnA(n) - mH2O oligomers and their corresponding neutrals, leading to wrong conclusions concerning the relative end-group distribution as a function of the degree of polymerization and the chemical composition.     

A new perspective on nonrelativistic gravity

The geometric reformulation of Newton??s gravity is known as Newton-Cartan theory. We compare the traditional derivation of this theory with a new, algebraic derivation, based on the gauging of a centrally extended Galilean symmetry algebra. In this comparison the role of the central charge gauge field will be explained. In particular, we show that the scalar potential following from this procedure coincides with the one given by the theory of Cartan. Our procedure can be generalized to describe other nonrelativistic limits of gravity involving gravitating strings.     

Vibronic coupling in the ground and excited states of oligoacene cations

The vibrational coupling in the ground and excited states of positively charged naphthalene, anthracene, tetracene, and pentacene molecules is studied on the basis of a joint experimental and theoretical study of ionization spectra using high-resolution gas-phase photoelectron spectroscopy and first-principles correlated quantum-mechanical calculations. Our theoretical and experimental results reveal that, while the main contribution to relaxation energy in the ground state of oligoacene systems comes from high-energy vibrations, the excited-state relaxation energies show a significant redistribution toward lower-frequency vibrations. A direct correlation is found between the nature of the vibronic interaction and the pattern of the electronic state structure.     

Adaptive spatial resolution: application to surface plasmon waveguide modes

In order to simulate surface plasmon waveguide structures we have utilized and improved the adaptive spatial resolution technique and combined it with PML boundary conditions. Using this new technique we have developed a novel concept for an integrated surface plasmon biosensor.     

Addition of carbon to anatase TiO2 by n-hexane treatment—surface or bulk doping?

Anatase TiO₂ can be sensitized to visible light by adding carbon as a dopant. Towards this end, TiO₂ photoelectrodes were subjected to a thermal treatment in a hexane-rich environment. By comparing the optical and photoelectrochemical characteristics of both thin film and nanocrystalline nanoporous photoelectrodes, carbon is found to be located mainly at the surface of the TiO₂. The amount of carbon that diffuses into the bulk of the material is too small to significantly enhance the visible light response and only a small shift of the absorption edge towards higher wavelengths is observed. The presence of carbon in TiO₂ shifts the anatase-to-rutile transformation temperature beyond 800 °C, and X-ray diffraction shows that spray deposition of TiO₂ under a CO₂ atmosphere results in a higher bulk carbon concentration than a post-deposition thermal treatment in a hexane-rich environment.     

N,N-acetals as N-acyliminium ion precursors: synthesis and absolute stereochemistry of epiquinamide

A stereoselective synthesis of (+)-epiquinamide is presented in combination with determination of the absolute configuration of the natural product. Key steps in the sequence involved chemoenzymatic formation of an enantiomerically pure cyanohydrin, reductive cyclization to the corresponding cyclic N,N-acetal, and subsequent conversion into a suitable N-acyliminium ion precursor to enable construction of the second ring.