Incompressible flows with combustion simulated by a preconditioning method using multigrid acceleration and MUSCL reconstruction

A numerical algorithm for the steady state solution of three‐dimensional incompressible flows is presented. A preconditioned time marching scheme is applied to the conservative form of the governing equations. The preconditioning matrix multiplies the time derivatives of the system and circumvents the eigenvalue‐caused stiffness at low speed. The formulation is suitable for constant density flows and for flows where the density depends on non‐passive scalars, such as in low‐speed combustion applications. The k–ε model accounts for turbulent transport effects. A cell‐centred finite volume formulation with a Runge–Kutta time stepping scheme for the primitive variables is used. Second‐order spatial accuracy is achieved by developing for the preconditioned system an approximate Riemann solver with MUSCL reconstruction. A multi‐grid technique coupled with local time stepping and implicit residual smoothing is used to accelerate the convergence to the steady state solution. The convergence behaviour and the validation of the predicted solutions are examined for laminar and turbulent constant density flows and for a turbulent non‐premixed flame simulated by a presumed probability density function (PDF) model. Copyright © 2001 John Wiley & Sons, Ltd.     

Evaluation of a PbTe detector for infrared imaging purposes

Summary An infra-red detector produced by the Elettronica SPA Company for the European Space Agency to be used in the Earth surface mapping from aircraft has been tested in our laboratory. We present the characteristics of the detector working in the spectral range (1÷2.5) μm. The acquisition system and the electronics are discussed and a detailed study of all the noise sources is presented; the measured NEP is 3.2·10⁻¹⁴W Hz^−1/2 at a temperature of 108 K. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Jets physics in comet P/Halley

Summary CCD images of comet P/Halley taken from Catania Observatory in April 1986 with IHW filters are analysed. On scale distances up to and larger than 200·10³ km these images show the presence of jets in the Cont 6850, H₂O⁺ and C₂ filters. The jet maximum intensity is at <1000 km from the nucleus for dust, at (20÷30)·10³km for H₂O⁺ and at (60÷90)·10³km for C₂, implying 2·10⁴s or 10⁵s formation time for the last two molecules. Comparison of various images shows that H₂O⁺ and C₂ jets are in very different directions, the first one coinciding with the main dust jet, the second one appearing as a continuation in space of a very short dust jet. It is speculated that dust grains have very different sizes and chemical composition from the moment they leave the cometary nucleus, and that grains in the small dust jet undergo fragmentation and chemical reactions to produce C₂ jets.     

Raman investigation of femtosecond laser-induced graphitic columns in single-crystal diamond

We report on the fabrication of graphitic columns induced in single-crystal diamond plates using 100 fs laser pulses at 800 nm wavelength. Different values of laser fluence (0.6–1.2 J/cm²) and graphitization speed (1–100 μm/s) were used for the laser treatment. A Raman investigation was performed aimed at evaluating the structural properties of the fabricated columns, showing that a lower laser fluence and a proper choice of graphitization speed may improve the degree of graphite crystallinity, and suppress the residual diamond content.     

Magnetohydrodynamic instabilities at Comet P/Halley: Giotto observations

Summary We examine the plasma parameters observed by the ion mass spectrometer of the Giotto JPA experiment, downstream the bow shock and up to the closest approach of comet P/Halley. From the analysis of the observations we have identified two regions where the ΔV between the proton and water group ion bulk velocities is first parallel and later perpendicular, respectively, to the magnetic-field direction. In the parallel region, a strong MHD turbulence is observed that we suppose to be generated by a firehose instability mechanism driven by the velocity difference. At about 5·10⁵ km from closest approach, the center of mass switches from solar-wind protons to the cometary ions, while the velocity difference becomes perpendicular to the magnetic field, causing the quenching of the isstability and the disappearing of the plasma fluctuations. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Experimental study for the feasibility of a crystalline undulator

We present an idea for creation of a crystalline undulator and report its first realization. One face of a silicon crystal was given periodic microscratches (grooves) by means of a diamond blade. The x-ray tests of the crystal deformation due to a given periodic pattern of surface scratches have shown that a sinusoidal-like shape is observed on both the scratched surface and the opposite (unscratched) face of the crystal; that is, a periodic sinusoidal-like deformation goes through the bulk of the crystal. This opens up the possibility for experiments with high-energy particles channeled in a crystalline undulator, a novel compact source of radiation.     

Temperature polarization effects in thermo-osmosis

Water transport induced by heat flow across cellophane and cellulose acetate membranes has been extensively studied in recent years. In comparing the results obtained by the various authors, remarkable differences emerge among the apparent behaviour of otherwise similar membranes. Discrepancies are so large that in some cases the very existence of this transport phenomenon has been questioned.In this paper, an effect is discussed which appears significantly to affect heat-and-mass-transfer-coupled phenomena. A temperature polarization concept is used as an analogy to concentration polarization in reverse osmosis. This appears to account for most of the discrepancies in the published data. However, there remain differences in the experimental results which cannot be explained solely on the basis of “temperature polarization”. Thus, it appears necessary to perform further experimental investigations on thermo-osmosis through these membranes under well controlled thermal conditions. In this regard, a standard procedure is suggested which should allow determination of true thermo-osmotic transport coefficients, by accounting for thermal polarization effects.A physical interpretation of mass transfer across membranes induced by heat flow is also introduced. This will be further developed in subsequent papers.