The effects of forced boundary conditions on flow within a cubic cavity using digital particle image thermometry and velocimetry (DPITV)

Buoyancy-driven convection within a cavity, whose sidewalls are heated and cooled, is a problem of great interest, because it has applications in heat transfer and mixing. Most studies to date have studied one of two cases: the steady-state case or the development of the transient flow as it approaches steady state. Our main concern was to study the response of the cavity to time-varying thermal boundary conditions. We therefore decided to observe the flow phenomena within a convection cavity under sinusoidal thermal forcing of the sidewalls. To map the flow properly, it is necessary to have simultaneous kinematic and thermal information. Therefore, the digital particle image thermometry and velocimetry (DPITV) is used to acquire data. Implementing this technique requires seeding the flow with encapsulated liquid crystal particles and illuminating a cross section of the flow with a sheet of white light. Extraction of the thermal and kinematic content is in two parts. For the first, the liquid crystals will reflect different colors of the visible spectrum, depending on the temperatures to which they are subjected. Therefore, calibrating their color reflection with temperature allows for the extraction of the thermal content. For the second part, the kinematic information is obtained through the use of a digital cross-correlation particle image velocimetry technique. With the use of DPITV, the flow within a convection cavity is mapped and studied under steady forcing and sinusoidally forced boundary conditions at the Brunt-Väisälä frequency. For the sinusoidally forced case, three cases are studied. In the first, the heating between the two walls is in phase. In the second, the heating between the two walls is 180° out of phase. In the third, the heating between the two walls is 90° out of phase. For steady forcing, the thermal plots show that the flow develops a linearly stratified profile within the center of the cell. At the sidewalls, however, owing to forcing, hot/cold thermal boundary layers develop at the left/right walls. These hot/cold thermal boundary layers then turn around the upper-left/lower-right corners and develop into intrusion layers that extend across the top and bottom walls. The vorticity and streamlines show that the bulk of the fluid motion is concentrated around the walls, whereas the fluid within the center of the cell remains stationary. For the sinusoidally forced cases, the thermal plots show the existence of many thermal “islands,” or pockets of fluid where the temperature is different with respect to its surroundings. The vorticity plots show that the center of the cell is mostly devoid of vorticity and that the vorticity is mainly confined to the sidewalls, with some vorticity at the top and bottom walls. For the 0° forcing, the streamlines show the development of two counterrotating rollers. For the 180° forcing, the streamlines show the development of only one roller. Finally, for the 90° forcing, the streamlines show the development of both a two-roller and a one-roller system, depending on the position within the forcing cycle.     

Mobility and aggregation of free clusters soft landed on amorphous and crystalline carbon substrates

Neutral antimony clusters produced by a gas aggregation source have been deposited at room temperature on thin films of amorphous carbon and cleavage surfaces (0001) of graphite. Antimony islands generated from different mean size distributions of preformed clusters Sb_n(n = 4, 90, 150, 250, 600, 2000) have been investigated by transmission electron microscopy. Only compact islands have been observed on amorphous carbon, whereas an evolution from compact to dendritic shapes occurs on graphite substrate as the mean size of the deposited clusters increases. For clusters containing more than 150 atoms the dendritic islands exhibit a fractal character whose dimensional analysis yields a fractal dimension of 1.63 ± 0.07. The different models for island growth are discussed in the light of these results.     

Structures and Mutagenic Properties of Products Obtained by C-Nitrosation of Opipramol

Reaction of the tricyclic psychotropic drug opipramol ( 1 ) with an excess of HNO₂ affords a product mixture mutagenic for Salmonella typhimurium. The main product is a tetracyclic furoxan 2 (yield ca. 80%), resulting from nitrosation at C(10) and C(11) of 1 . Compound 2 is not mutagenic. The essential mutagen is a nitroarene 3 formed via contraction of the central ring of 1 , and nitrosation at C(2). Its yield is extremely low (<0.1%). Nitroarenes have previously not been encountered as mutagenic products of the interaction of drugs with nitrite.     

A combined laser-induced breakdown and Raman spectroscopy Echelle system for elemental and molecular microanalysis

Raman and laser-induced breakdown spectroscopy is integrated into a single system for molecular and elemental microanalyses. Both analyses are performed on the same ~ 0.002 mm² sample spot allowing the assessment of sample heterogeneity on a micrometric scale through mapping and scanning. The core of the spectrometer system is a novel high resolution dual arm Echelle spectrograph utilized for both techniques. In contrast to scanning Raman spectroscopy systems, the Echelle–Raman spectrograph provides a high resolution spectrum in a broad spectral range of 200–6000 cm^? 1 without moving the dispersive element. The system displays comparable or better sensitivity and spectral resolution in comparison to a state-of-the-art scanning Raman microscope and allows short analysis times for both Raman and laser induced breakdown spectroscopy. The laser-induced breakdown spectroscopy performance of the system is characterized by ppm detection limits, high spectral resolving power (15,000), and broad spectral range (290–945 nm). The capability of the system is demonstrated with the mapping of heterogeneous mineral samples and layer by layer analysis of pigments revealing the advantages of combining the techniques in a single unified set-up.     

Generation of continuously tunable IR radiation by means of difference-frequency mixing of nitrogen-laser and dye-laser radiation in LiIO3

The paper describes the generation of difference frequencies from nitrogen-laser and dye-laser radiation in LiIO₃ which was accomplished for the first time. IR radiation in the range from 0.7 to 2.2 μm is generated, the radiant power being in the watt range. Comparison measurements in LiIO₃, ADP and KDP indicate the benefits of LiIO₃. An analysis of the influence of the bandwidth on the conversion efficiency shows that below 2 μm an efficient wideband transformation is also possible. Experimentally, it was possible to realize IR radiation with bandwidths of 90 Å. Possibilities of improving the efficiency of transformation are discussed.     

Coherent vortices in a turbulent and rotating fluid

The emergence of coherent, intense and quasi-steady vortices in a rotating fluid under the action of turbulence is a phenomenon which is not yet well accounted for. A first deterministic approach by Maxworthy, Hopfinger and Redekopp (1985) related it to the occurence of mixed density fluid intrusions in a linearly stratified fluid. In the first part we present visualizations of the vortex genesis verifying qualitatively Maxworthy et al.'s ideas. In the second part, a linear instability model is proposed, very similar to the thermal convection instability in a rotating fluid. For marginal stability conditions, the model shows the occurrence of a regular spatial distribution of steady vortices. This new approach does not contradict the visualizations presented in the first part.     

Preparation of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles for superthermites by the detonation of an explosive nanocomposite material

This article reports on the preparation of chromium(III) oxide nanoparticles by detonation. For this purpose, a high explosive—hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)—has been solidified from a solution infiltrated into the macro- and mesoporosity of Cr₂O₃ powder obtained by the combustion of ammonium dichromate. The resulting Cr₂O₃/RDX nanocomposite material was embedded in a cylindrical charge of pure explosive and detonated in order to fragment the metallic oxide into nanoparticles. The resulting soot contains Cr₂O₃ nanoparticles, nanodiamonds, amorphous carbon species and inorganic particles resulting from the erosion by the blast of the detonation tank wall. The purification process consists in (i) removing the carbonaceous species by an oxidative treatment at 500 °C and (ii) dissolving the mineral particles by a chemical treatment with hydrofluoric acid. Contrary to what could be expected, the Cr₂O₃ particles formed during the detonation are twice larger than those of initial Cr₂O₃. The detonation causes the fragmentation of the porous oxide and the melting of resulting particles. Nanometric droplets of molten Cr₂O₃ are ejected and quenched by the water in which the charge is fired. Despite their larger size, the Cr₂O₃ nanoparticles prepared by detonation were found to be less aggregated than those of the initial oxide used as precursor. Finally, the Cr₂O₃ synthesized by detonation was used to prepare a superthermite with aluminium nanoparticles. This material possesses a lower sensitivity and a more regular combustion compared to the one made of initial Cr₂O₃.