A discontinuous Galerkin method for inviscid low Mach number flows

In this work we extend the high-order discontinuous Galerkin (DG) finite element method to inviscid low Mach number flows. The method here presented is designed to improve the accuracy and efficiency of the solution at low Mach numbers using both explicit and implicit schemes for the temporal discretization of the compressible Euler equations. The algorithm is based on a classical preconditioning technique that in general entails modifying both the instationary term of the governing equations and the dissipative term of the numerical flux function (full preconditioning approach). In the paper we show that full preconditioning is beneficial for explicit time integration while the implicit scheme turns out to be efficient and accurate using just the modified numerical flux function. Thus the implicit scheme could also be used for time accurate computations. The performance of the method is demonstrated by solving an inviscid flow past a NACA0012 airfoil at different low Mach numbers using various degrees of polynomial approximations. Computations with and without preconditioning are performed on different grid topologies to analyze the influence of the spatial discretization on the accuracy of the DG solutions at low Mach numbers.     

Supersonic beam spectroscopy of low J transitions of the ν3 band of SF6: Rabi oscillations and adiabatic rapid passage with a CW laser

The P(3) and P(4) manifolds of the ν₃ band of SF₆ have been observed in a supersonic beam with a bolometric detection. The influence of the laser beam divergence on the excitation efficiency has been studied. Rabi oscillations are observed when the wavefront is flat in the interaction region whereas only adiabatic rapid passage occurs when the molecules see a curved wavefront.     

Structure and optical activity of a crystalline modification of isotactic poly-(S)-4-methyl-1-hexene

The results of an x-ray and polarimetric study of a crystalline modification (form I) of isotactic poly-(S)-4-methyl-1-hexene are reported and discussed. The x-ray fiber spectra of this polymer are practically indistinguishable from those of isotactic poly-(R)-(S)-4-methyl-1-hexene. Although the crystal structure of the latter can be described on the basis of helices of different screw sense packed in a P4 space group, the crystal structure of poly-(S)-4-methyl-1-hexene is better described on the basis of a P1 space group. The conclusion of the x-ray investigation, that in the crystals of the optically active polymer an equal number of right-handed and left-handed helices must be present, is supported by the polarimetric measurements, which have shown that the polymer in the crystalline form I possesses a rather low rotatory power.     

Spontaneous photon emission from a non-relativistic free charged particle in collapse models: A case study

We study the photon emission rate of a non-relativistic charged particle interacting with an external classical noise through its position. Both the particle and the electromagnetic field are quantized. Under only the dipole approximation, the equations of motion can be solved exactly for a free particle, or a particle bounded by an harmonic potential. The physical quantity we will be interested in is the spectrum of the radiation emitted by the particle, due to the interaction with the noise. We will highlight several properties of the spectrum and clarify some issues appearing in the literature, regarding the exact mathematical formula of a spectrum for a free particle.     

CRYSTAL AND MOLECULAR STRUCTURE OF BIS(TRIPHENYLBENZYLPHOSPHONIUM) TETRABROMOCADMATE

Abstract

The molecular and crystal structure of bis(triphenylbenzylphosphonium)tetrabromocadmate has been determined by x-ray diffractometer data. Crystals are triclinic, space-group Pl with two formula units in a unit-cell of dimensions a = 12.506(6), b = 10.471(5), c = 18.396(13) Å, α = 93.07(4)°, β = 105.75(5)°, γ = 92.58(4)°. The structure was solved by direct and Fourier methods and refined by least-squares techniques to R = 0.061 for 3723 independent observed reflections. The structure consists of tetrabromocadmate (II) anions and triphenylbenzylphosphonium cations, both with a quasi-perfect tetrahedral symmetry around the cadmium and phosphorus atoms. The most significant average bond distances are: Cd-Br, 2.588(2) Å, P[sbnd]C (Phen), 1.794(5) Å and P[sbnd]CH₂, 1.806(6) Å. The P[sbnd]C (Phen) bonds are in slightly distorted staggered conformation (gauche-, gauche +, and trans) in respect of the C (Phen)-CH₂ bonds of the benzyl residues. The interatomic distances between the ions correspond to the usual Van der Waals distances.     

Island organization of TiO2 hierarchical nanostructures induced by surface wetting and drying

We report on the reorganization and bundling of titanium oxide nanostructured layers, induced by wetting with different solvents and subsequent drying. TiO(2) layers are deposited by pulsed laser deposition and are characterized by vertically oriented, columnar-like structures resulting from assembling of nanosized particles; capillary forces acting during evaporation induce bundling of these structures and lead to a micrometer-size patterning with statistically uniform islands separated by channels. The resulting surface is characterized by a hierarchical, multiscale morphology over the nanometer-micrometer length range. The structural features of the pattern, i.e., characteristic length, island size, and channel width, are shown to depend on properties of the liquid (i.e., surface tension) and thickness and density of the TiO(2) layers. The studied phenomenon permits the controlled production of multiscale hierarchically patterned surfaces of nanostructured TiO(2) with large porosity and large surface area, characterized by superhydrophilic wetting behavior without need for UV irradiation.     

Strength and microstructures of Benzoic Acid-Benzamide system

Pure end products in their binary eutectic of a benzoic acid-benzamide system, grownunidirectionally and randomly, exhibited marked anisotropy in their peculiar properties particularly if the growth is carried out in some periodic manner. Modulus of ruptures of eutectic composite and the constituent materials for both the growths have been determined. Directional-growth of the composite yields a bending strength quite larger than that can be obtained from an anomalous growth. Pure components also offer slighly greater strength in an ordered growth. Microphotographs of grown materials are related to the mode of crystallization of the materials.     

Mode of solidification and strength properties(II). Naphthalene-benzil binary system

The tensile and compressive strengths of directionally and randomly grown samples of naphthalene, benzil, and their eutectic in the faceted-faceted benzil-naphthalene system have been determined. The tensile strength of naphthalene and the eutectic increased significantly on directional solidification. Directional solidification substantially enhanced the compressive strength of the eutetic whes rea decrease in compressiv strength was observed in case of directionally grown naphthalene. In case of benzil, no appreciable change in strength was observed on directional solidification. A comparison between the tensile and compressive strength was made. The compressive strength was 2.5–3.8 times more than the tensile strength of naphthalene, grown randomly, benzil and their eutectic grown randomly and directionally.