Volume 25, Nº 2 (2016)

We investigate a vertically discharged shallow wall-bounded turbulent water jet both experimentally (LIF, TomoPIV) and numerically (LES). We identify the well-known meandering motion of the jet core generating large-scale planar vortices similar to the Karman vortex street. The modulation of the meandering amplitude is identified in experiments and simulations, which is attributed to the competition between sinusoidal and symmetric instability modes. TomoPIV data confirms that elongated streamwise vortices represent the smaller scale vortical structure of the jet in the near and far fields.

Results of measuring the thermal conductivity coefficient of silica dioxide Tarkosil T-50 nanopowder by a laser flash technique are reported. Experimental data on thermal conductivity of nanopowder in vacuum, in different gas media, and at different temperatures were obtained. Comparative analysis of thermal conductivity was performed for the T-50 nanopowder and materials with close density values. Based on the experimental data, we obtained a mathematical model to determine the thermal conductivity coefficient of T-50 nanopowder in different gas media.

Perforated steel plates are structural components widely employed in engineering. In several applications these panels are subjected to axial compressive load, being undesired the occurrence of buckling. The present work associates the computational modeling and the constructal design method to obtain geometries, which maximizes the mechanical behavior for these components. A numerical model was used to tackle with elastic and elasto-plastic buckling. Square and rectangular plates with centered elliptical cutouts were considered and several hole volume fractions and ratios between the ellipse axes (H₀/L₀) were taken into account. Stress limit improvements around 100% were achieved depending only on the cutout shape.

Densities, viscosities, and ultrasonic velocities of the binary mixtures of acetophenone with ethyl butyrate were measured over the entire mole fractions at 303.15, 313.15, and 323.15 K. From these experimental results, excess molar volume V^E, viscosity deviation Δη, deviations in isentropic compressibility Δκ_s, excess intermolecular free length ΔL_f, and excess Gibbs free energy ΔG*^E were calculated. The viscosity values were fit to the models of Krishnan–Laddha and McAllister. The thermophysical properties under study were fit to the Jouyban–Acree model. The excess values were correlated using Redlich–Kister polynomial equation to obtain their coefficients and standard deviations. It was found that in all cases, the data obtained fitted with the values correlated by the corresponding models very well. The results are interpreted in terms of molecular interactions occurring in the solution.

Silicon oxide nanowires were synthesized from monosilane–argon–hydrogen mixture by the gas-jet electron-beam plasma chemical deposition method with simultaneous oxygen injection into the vacuum chamber. The synthesis was performed on monocrystalline silicon substrates covered with micron and nanometer tin catalyst particles. The nanowires are formed the via vapor–liquid–solid mechanism in the “catalyst-on-bottom” mode, in which many nanowires grow from one catalyst particle. The process of synthesizing nanowires on a substrate with catalyst consists of three stages: heating to synthesis temperature, hydrogen plasma treatment, and nanowire growth. In the substrate region corresponding to the jet axis, different structures are formed depending on the catalyst particle size. For catalyst particles under 100 nm, there are formed structures of chaotically oriented and interlaced bundles of silica nanowires. For catalyst particles of 0.3–1 micron, there are formed oriented arrays of cylindrically shaped nanowire bundles (“microropes”). Cocoon-like structures are formed for catalyst particles of more than 1 micron.We propose a model of nanowire growth by this method, which is based on nonuniform heating of a catalyst particle by a directed plasma flow. It was found that for synthesis of oriented microrope arrays the initial tin film thickness should be less than 100 nm and the synthesis process should include a hydrogen plasma treatment stage.

The turbojet engine is used to propel the aircraft by accelerating the combustion gases in the opposite direction to motion. This is accomplished by greatly accelerating a small mass of fluid. In this work, an energy analysis is carried out under operating conditions of 13000-m altitude and 0.8 Mach number to determine the variation of specific thrust and the specific fuel consumption with different compressor pressure ratios and turbine inlet temperatures. The exergy analysis is then applied to show more realistic performance. Thereby, the specific thrust showed an increase by 1.7%, and the specific fuel consumption was decreased by the same ratio as a result of 10% drop in optimum pressure ratio.

Magnetohydrodynamics flow of a visco-elastic incompressible fluid (Walter’s B′ model) past an infinite porous plate in porous medium under the action of transverse uniform magnetic field in the presence of heat source and chemical reaction is investigated. The governing equations of the motion, energy and concentration are solved by a successive perturbation technique. The flow phenomenon is characterized by suction parameter, magnetic parameter, porosity parameter, Grashoff number, modified Grashoff number, Prandtl number, heat source parameter, chemical reaction parameter and Schmidt number. The expressions for skin friction coefficient, Nusselt number, and Sherwood number on the surface are also discussed.