Yttria-stabilized-zirconia hollow spheres prepared by atmospheric plasma spray

Yttria-stabilized-zirconia (YSZ) hollow spheres are widely utilized for their novel physical and chemical properties. However, developing a simple and low-cost method for preparing such hollow spheres still remains a great challenge. In this paper, an atmospheric plasma spray (APS) method is introduced, and the formation mechanism of hollow 7YSZ (ZrO₂-7wt%Y₂O₃) spheres is presented. The hollow sphere morphology was observed by scanning electron microscopy (SEM) when agglomerated and sintered 7YSZ powders were used. Additionally, additive composition changes, phase transformations, and the thermal behavior of 7YSZ powders were analyzed by energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD), thermogravimetric analysis (TG) and differential scanning calorimeter analysis (DSC). Furthermore, the phase transformations of agglomerated and sintered 7YSZ powders, 7YSZ hollow spheres that annealed at various temperatures for different times are analyzed.     

Generation of impulsive pressure wave by spark discharge in water and its propagation

An experimental investigation was performed to find a new method for diagnosing three-dimensional flows in which obstacle bodies or cavities are included by means of a pressure wave. In rectangular closed tanks 200 × 337 × 250 mm (acrylic resin) and 300 × 450 × 400 mm (brass) filled with water, an impulsive pressure wave was generated by an instantaneous small spark discharge. The pressure waveforms were measured at a point on the wall with a high-frequency pressure transducer, and the data were recorded with an A/D converter. The measured wave fluctuations differed depending on wall conditions of the tank and on whether there was a submerged body in the water. The size of the submerged body also affected the pressure fluctuation. When acrylic resin and brass were used as wall materials, both the phase and amplitude of the reflecting wave differed. When a stainless steel sphere of diameters of 50.8, 30.2, or 19.1 mm was submerged in the tank, two kinds of pressure waves were observed, one passing through the sphere and the other diffracted around it. These results suggest the possibility of identifying bodies of simple shape by interpreting the precisely measured output pressure wave signals.     

Time dependence of creep recovery in cross-linked polymers

Summary Transient creep recovery tests in simple tension were performed in a series of cross-linked high polymers in pure and plasticized states. A loading cycle was adopted according to which each specimen was loaded in creep for a time period of 16 followed by recovery for the same time period. This loading cycle was repeated for various steps of temperature from ambient temperature up to a temperature corresponding to the rubbery region of each polymer. It was shown that, while the 15 measurements of deformation after loading or unloading were approximately equal, recovery individual curves were always lagging in time the corresponding creep curves. Composite curves of creep or recovery deformation in semi-log time scale reduced to a temperature of 30 °C were plotted by applying the method of reduced variables. The creepdeformation composite curve was always advancing in time relatively to the respective recovery composite curve. This phenomenon was an accumulation of the partial difference of the corresponding individual curves.Time-shift factor versus temperature curves for creep and recovery were plotted and compared.It was established that the remaining deformation after 16 minutes of a 16 creep recovery normalized to the initial deformation at the beginning of recovery, if plotted versus temperature, presented the same shape of curves for all types of materials tested. The maxima of these curves were diminishing with addition of plasticizer according to a straight line.     

Experimental evaluation of side-loads in LE-7A prototype engine nozzle

During development tests of the LE-7A prototype engine, severe side-loads were observed. The side-load peaks appeared only in certain limited conditions during start-up and shut-down transients. To investigate phenomena causing those severe side-loads observed in the LE-7A prototype engine nozzle, series of cold-flow tests and hot-firing tests as well as CFD analyses were conducted. As a result of the hot-firing tests, two different phenomena were found to cause severe side-loads in the LE-7A prototype engine nozzle. One was a restricted shock separation (RSS) flow structure and the other was a phenomenon termed “separation jump,” the rapid movement of the separation location in the vicinity of the step. A step was installed in the LE-7A prototype to supply film-cooling gas. Hot-firing test results showed that RSS can occur for a limited mixture ratio. Detailed flow structure of RSS on the nozzle surface was revealed by the cold-flow tests. Measured pressures and visualized images of cold-flow tests clarified the mechanism causing the separation jump. The key phenomenon ruling the separation jump was found to be the base flow behind the step. Based on the results of the present study, the latest LE-7A engine nozzle design has been changed to eliminate the severe side-load.

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Effect of Porous Coatings on Stability of Hypersonic Boundary Layers

The influence of ultrasoundabsorbing coatings on stability of hypersonic boundary layers is considered. Two types of coatings were used in experiments: feltmetal with a random porous microstructure and a sheet perforated by blind cylindrical microchannels. The experiments were performed in a wind tunnel at a Mach number M = 5.95 on sharp cones with a 7° apex halfangle. Evolution of natural disturbances and artificially induced wave packets in the boundary layer was studied with the help of hotwire anemometry. Spatial characteristics of artificial disturbances were obtained. It is demonstrated that such coatings exert a stabilizing effect on secondmode disturbances.     

Using image analysis to measure two-dimensional soil deformation

A system for measuring two-dimensional deformation in a soil bin using video image analysis was developed and evaluated. Soil was placed in the bin [1.22 (wide)×9.1 m long] in 50 mm deep layers of nominally uniform bulk density (1.35 Mg m⁻³) to a total depth of 0.76 m. White polypropylene markers [6.4 (dia.)×25.4 mm] were placed in the soil such that vertical rectangular cross-sectional grids were exposed by removing selected bin modules. Video image analysis was used to determine vertical and horizontal coordinates of each marker which, in turn, were compared to measurements using a sonic digitizer which were accurate to ±0.5 mm.     

Ignition and combustion drive by powerful laser radiation acting on a water surface

The ignition and combustion effects in water vapor generated by the action of the powerful CO₂-laser radiation on a water surface are investigated experimentally. The regimes corresponding to these phenomena are discussed and substantiated theoretically Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 172–180, January–February, 1997. The investigations described were made possible by financial support from the International Science Foundation (grants N7B000 and N7B300).     

Suppression of Low-frequency Variations in Vortex Shedding by a Splitter Plate Behind a Bluff Body

Measurements of instantaneous pressure fluctuations on a trapezoidal cross-section cylinder indicate that the low-frequency variations embedded in the vortex-shedding process can be successfully suppressed by insertion of a splitter plate whose length is twice the maximum width of the trapezoidal cylinder. The experiments were performed at Reynolds numbers in a range of 5 × 10³ to 4·5 × 10⁴. Spanwise correlation of the pressure fluctuations measured on the cylinder further indicates that the suppression of low-frequency variations improves the degree of two-dimensionality of vortex shedding. These findings are attributed to the presence of the splitter plate having an effect on stabilizing the vortex formation length which is comparable to the length of the splitter plate, thus eliminating the low-frequency variations embedded in the base pressure.     

纯钛与Ti6Al7Nb合金氮离子注入层在小牛血清溶液中的扭动微动磨损试验研究

文中主要研究了纯钛、Ti6A17Nb基体以及经过氮离子注入后的改性层的扭动微动磨损行为.氮离子注入采用高能离子注入机及增强沉积系统,加速电压为50 kV,注入剂量分别为1×l017、5×1017和9×1017cm-2.采用球/平面接触模式,对纯钛和Ti6Al7Nb合金/(Zr2O球(直径为25.2 mm),接触副在小牛血清介质条件下进行了纯钛和Ti6Al7Nb合金及其改性层的了扭动微动磨损试验.借助X衍射仪(XRD)、拉曼光谱(Raman)、三维形貌仪(3D)和扫描电镜(SEM)分析了两种材料及其改性层的化学成分、微动磨损磨痕形貌和微观组织结构,探讨了纯钛和Ti6Al7Nb合金基材和改性层的扭动微动运行行为和损伤机制.研究结果表明:用氮离子注入方法处理钛及其合金,生成的氮化物层能够改善材料表面的综合性能,提高其扭动微动磨损的耐磨性.纯钛和Ti6Al7Nb基体的磨损现象为氧化和剥落,其离子注入层的磨损现象为犁沟.     

Investigation of temperature statistic characteristics in turbulent water flow with unsteady heat transfer

The results of an experimental study of a temperature field and its statistical characteristics in turbulent water flow upon a sudden change of heat generation in the channel wall are reported. Measurements were performed in 5 mm × 40 mm, 10 mm × 40 mm, and 20 mm × 40 mm channels in the regions of thermal stabilization and stabilized heat transfer at Reynolds numbers of (0.8–6.8) × 10⁴. The measurement results are generalized using a dimensionless time scale. The results of the calculation of heat transfer coefficients at unsteady heat transfer are presented.     

TURBULENT ELLIPTIC WAKES

This paper describes results of an experimental study on turbulent wake of an elliptic disk set normal to the main flow, whose major diameter is 2·0 or 3·0 minor diameters, Reynolds number being 2·0×10⁴on the basis of the minor diameter D. Two periodic components of velocity fluctuations were found in the wake. One is centred around the minor plane, being due to the alternate shedding of rolled-up, hairpin-like vortices. The other is centred around the major plane, which is likely to be due to a meandering motion of the wake. The axis switching, which is a cross-over of half-widths in the major and minor planes plotted against the streamwise distance, occurred at approximately 4·0 D downstream of the disk. The mechanism of the axis switching is different from that in elliptic jets, and it is proposed that it is due to a difference in the growth rate of the fundamental Fourier modes in the minor and major planes. The structure of the wake is studied by flow visualization and a survey of the time-mean velocity, turbulence intensities and Reynolds shear stresses. Wavelet analysis of the velocity fluctuations disclosed a low-frequency unsteadiness in the wake. This unsteadiness has different representative frequencies in the major and minor planes, being approximately one-fifth of the frequency of the corresponding periodic component in both planes.     

Bubble interaction in low-viscosity liquids

An experimental study investigated how freely rising ellipsoidal bubbles approach each other, make contact and coalesce or breakup. Pulsed planar swarms of 10–20 bubbles with Eötvös numbers from 6.0 to 27.5 were released simultaneously in aqueous solutions of 0–48 wt% sugar with Morton numbers from 3.2 × 10⁻¹¹ to 3.7 × 10⁻⁶. Bubble interaction was recorded by a video camera following the rising bubbles. Essentially, all coalescence and breakup events occurred after, not during, wake-induced collisions by a complex process related to the bubble vortex shedding cycle. This same process was also found in multi-bubble clusters and may account for excess turbulent kinetic energy generation in bubbly flow.     

Vortex in a Supersonic Flow and its Influence on Blunt Body Flow and Heat Transfer

The dissipation of a tip vortex generated by a rectangular wing and the stagnation pressure and temperature distributions within the vortex are experimentally investigated. The interaction of the vortex with the bow shock ahead of a sphere and the heat flux distribution over the spherical surface are studied. The experiments were carried out at the Mach number M=3 and the unit Reynolds numbers Re₁=1.1·10⁷ and 3.7·10⁷ 1/m.     

Effect of mass transfer on flow across a staggered tube bundle

Over a range of Reynolds numbers from 0·6 × 10⁵ to 1·75 × 10⁵ tests were made on a seven rows deep tube bank. These tests were made using a specially instrumented porous cylinder which could be located in any position within the bank. Mass transfer through the porous surface simulating the condensation process in a surface condenser, was applied, and its effect on local parameters investigated. The distribution of static pressure and skin friction was determined around tubes in different rows in the bank. From these measurements, the pressure drag and friction drag were estimated. The total pressure drop across the bank was also measured.

Results showed that, for typical steam condenser loadings, the contribution of the pressure drag to the total drag does not change appreciably with suction. However, the skin friction contribution does change considerably with suction.     

DDT in methane-air mixtures

Experimental results from a study on the critical condition for deflagration-to-detonation transition (DDT) in methane-air mixtures are presented. Experiments were carried out at 293 K and 1 atm using methane-air mixtures with methane concentrations ranging from 5.5 to 17% vol. The tests were performed in detonation tubes with inner-diameters of 174 mm and 520 mm. Detonation cell widths l\lambda were determined in the tests for a range of methane concentrations. The results of DDT tests indicate that for a tube cross-sectional area blockage ratio (BR) of 0.3 the critical condition for DDT can be characterized by the d/l = 1d/\lambda = 1 criterion. However, for a BR = 0.6 the critical value d/ld/\lambda was significantly higher. The data also show that the critical condition for DDT can be described by L/l = 7L/\lambda = 7, where L is the characteristic length-scale of the channel volume between orifice plates. This length-scale is defined by a grouping of the orifice plate dimensions (inner and outer-diameter) and plate spacing.     

Ultrasonic investigation of interpenetrating networks of poly(methyl methacrylate) and polyurethane

Interpenetrating networks are the most recent development in polymeric blend materials. Due to the crosslinking of both the continuous and dispersed phases, a high degree of molecular mixing is achieved in these materials. Notwith-standing that poly(methyl methacrylate)-polyurethane (PMMA-PUR) interpene-trating and semi-interpenetrating networks have been extensively investigated by Meyer et al., ultrasonic relaxation technique has been applied here for the first time. These materials were found to be highly ultrasound absorbing.It is observed that ultrasonic absorption has a peak at a particular composition of PMMA-PUR interpenetrating network. The absorption coefficient increases with frequencyf. The absorption is of relaxational nature and is not due to the scattering of ultrasonic waves by the domains of the dispersed phase. At every composition of the interpenetrating network, the/f ² vs.f curve indicates the presence of a relaxation frequency below 2 MHz and that the absorption increases with the temperature at some compositions which indicates the presence of thermal relaxation. An attempt is made to relate the absorption with the relaxation of pendent groups of polyurethane in the continuous phase.     

On a class of non-linear elastic materials

This article deals with a family of non-linear hyperelastic materials depending on a parameter varying from 0 to 1; is a masonry-like material and is linear elastic. Some properties of the function delivering the Cauchy stress corresponding to the infinitesimal strain E, are proved; in particular, it is shown that is strongly monotone for >0 and monotone for =0. Moreover, denoting by [u(·;), E(·;), T(·;)] the solution to the equilibrium problem for solids made of a material the convergence of [u(·;), E(·;), T(·;)] for going to 0 and 1, is investigated.     

Influence of the axle load, tyre size and configuration on the compaction of a freshly tilled clayey soil

Enhancement of the potential root growth volume is the main objective of farmers when they establish a conventional tillage system. Therefore, the main function of primary tillage is to increase soil’s structural macroporosity. In spite of this, during secondary tillage operations on these freshly tilled soils, the traffic on seedbeds causes significant increases in soil compaction. The aim of this paper was to quantify soil compaction induced by tractor traffic on a recently tilled non consolidated soil, to match ballast and tyre size on the tractors used during secondary tillage. The work was performed in the South of the Rolling Pampa region, Argentina. Secondary tillage traffic was simulated by one pass of a conventional 2WD tractor, using four configurations of bias-ply rear tyres: 18.4×34, 23.1×30, 18.4×38 and 18.4×38 duals, two ballast conditions were used in each configuration. Soil bulk density and cone index in a 0 to 600 mm profile were measured before and after traffic. Topsoil compaction increased as did ground pressure. Subsoil compaction increased as total axle load increased and was independent from ground pressure. At heavy conditions, topsoil levels always showed higher cone index values. From 150 to 450 mm depth, the same tendency was found, but with smaller increases in the cone index parameter, 22 to 48%, averaging 35%. Finally, at the deepest layer considered, 600 mm, differential increases due to the axle load are great enough as to be considered similar to those found in the upper horizon, 36 to 64%, averaging 55%. On the other hand, bulk density tended to be less responsive than cone index to the traffic treatments. Topsoil compaction can be reduced by matching conventional bias-ply tyres with an optimized axle weight.     

Combined effect of longitudinal riblets and LEBU-devices on turbulent friction on a plate

The possibility of reducing turbulent friction with the help of large-eddy-breakup devices (LEBUs) and riblets is studied experimentally. The tests were conducted in a low-turbulence wind tunnel on a flat plate for 2·10⁶ Re 7·10⁶. The local friction coefficient was measured using internal strain-gauge balances, and the total drag was estimated by the momentum-transfer method. It is shown that a combination of LEBUs and riblets makes it possible to reduce the total turbulent friction drag of a flat plate 1800 mm long by 16%. The effects of the length of a ribbed surface on the efficiency of friction reduction and of LEBUs and riblets on the structure of a turbulent boundary layer are analyzed.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, pp. 39–46, May–June, 1995.     

Feasability study of wall shear stress imaging using microstructured surfaces with flexible micropillars

A new optical sensor technique based on a sensor film with arrays of hair-like flexible micropillars on the surface is presented to measure the temporal and spatial wall shear stress field in boundary layer flows. The sensor principle uses the pillar tip deflection in the viscous sublayer as a direct measure of the wall shear stress. The pillar images are recorded simultaneously as a grid of small bright spots by high-speed imaging of the illuminated sensor film. Two different ways of illumination were tested, one of which uses the fact that the transparent pillars act as optical microfibres, which guide the light to the pillar tips. The other method uses pillar tips which were reflective coated. The tip displacement field of the pillars is measured by image processing with subpixel accuracy. With a typical displacement resolution on the order of 0.2 m, the minimum resolvable wall friction value is _w20 mPa. With smaller pillar structures than those used in this study, one can expect even smaller resolution limits.