Home – Home – Jornals – Thermophysics and Aeromechanics 2006 number 4

The problem of image reconstruction is considered for the case when the recording system point-spread function and the recorded image are known with random errors. An efficient computational image reconstruction algorithm that uses a 2D Discrete Fourier Transform and a Fast Fourier Transform algorithm is designed. An algorithm for reliable estimation of the optimal regularization parameter is proposed to choose the regularization parameter determining the reconstruction problem solution accuracy. The reconstruction algorithm can be adapted to identify a point-spread function of dynamic image recording systems.

Algorithms for time-finite signal restoration are developed in view of feasibility of the pulse characteristic of processing devices. Functions with a lower level of side lobes than in the known functions for single-channel and two-channel restoration systems with using signal samples and time-finite signal derivatives are synthesized. Optimum functions are calculated and quasi-optimum time-finite synthesizing functions of the interpolation algorithm are proposed, which allow consideration of time constraints of initial signal realization and feasibility of the processing devices. It is found that the proposed algorithms will reduce the root-mean-square error of interpolation by 45

A method is proposed to construct nonlinear nonparametric pattern recognition algorithms for efficient solution of classification problems in the conditions of small samples. Their properties are analyzed using results of computational experiments.

A system of likelihood equations for optimal estimation of optical pulse parameters in view of shot noise is obtained. Algorithms for estimates of the amplitude and duration of a Gaussian pulse, and also its position on the time axis are found. The Cramer-Rao bounds for non-joint and joint estimates are determined.

The main results of comprehensive analysis of possible ways for solving the problem of forecasting the annual inflow of the River Ob at the site of the Novosibirsk Hydroelectric Station are considered. The research is based on ideas and methods of variant modeling. A set of probabilistic models is constructed for inflow forecasting with different accuracy and term (the period between the date of forecast and the beginning of the forecast period). Ways for future research are outlined.

Projective mapping of physical space on the spheroid domain, that is, a hyperbolic Hilbert space model, is considered. Straight lines, diameters, perpendiculars, distances, and angles on the Hilbert plane are determined. An image of visual space inside the spheroid is shown as well as its conformal mapping on the Poincare model. A formula for apparent size of the Moon elevated over the horizon is presented.

A computer system modeling the effectiveness function by results of independent tests based on the nonparametric kernel regression estimate method is developed.

The influence of light divergence and the shape of light scatters of an object with volume scattering on its brightness is analyzed.

Numerical formulation of the incident wave source conditions for a domain enclosed in a nonuniform envelope within the scope of the TF/SF technique (Total-Field/Scattering- Field technique) is proposed. This approach is compared with a traditional analytical approach to defining the incident wave source conditions. Benefits of the approach are shown.

Temperature dependences of the Hall coefficient and magnetoresistance in heteroepitaxial structures of p-Cd_xHg_x-1Te (MCT) of X_CdTe = 0.22

Results of numerical modeling a new type of acousto-optic (AO) filters employing multibeam expanders are discussed. A planar filter design in thin m) optical waveguides of chalcogenide glass on lithium niobate is proposed. Analysis of the devices is carried out in a spectral approach with application of numerical experiments by the finite-difference time domain (FDTD) method. It is shown that the proposed multi-reflector optical elements possess high dispersion properties such that the AO filter with a small size (below 0.1 cm) ensures a record thin linewidth (~1 nm) at a wavelength of 1.54 m, a tuning range greater than 40 nm, and a switching time of less than 2 s. In addition, the normalized linewidth per unit length of the device is 20 times better than the theoretical limit for available types of AO filters developed on the same substrate and by a common technology but without multibeam expanders.

An improved optoelectronic small linear displacement sensor based on laser beam diffraction on two phase diffraction gratings is proposed and investigated. The possibility of doubling the region of linearity while detecting signals in the first two diffraction orders is shown. A device for measuring surface irregularities is designed using this sensor. Experiments on measuring surface profiles of relief structures are carried out.

A stabilized fiber optic sensor for remote measuring the angle of object inclination from the vertical has been developed. The measurement principle is based on beam refraction by a transparent free hanging plane-parallel plate as the angular orientation of the sensitive element case changes. Output signal stabilization is assured by using two spatially separated measuring channels.

The basic milestones of the life journey of S.P. Korolev (12.01.1907- 14.01.1966), the Designer General of rocket and space systems, are presented. Information on various missiles of the first generation R-1,
R-2, R-5, R-7, and R-9, launch vehicles Sputnik, Luna, Molniya, Vostok, Soyuz, and N-1, and also spacecraft designed for various purposes, which were developed under Korolev

Results on a hyperboloid-flare model tested in a new hypersonic wind tunnel with adiabatic compression AT-303 based at ITAM SB RAS at

The drag of an axisymmetric body of revolution in a nominally gradientless incompressible flow under the action of eddy breakup devices (EBU) is studied experimentally. It is demonstrated that the use of EBUs in the boundary layer may reduce the net drag of the body of revolution approximately by 1.75 %, as compared with the corresponding value for the original configuration. An increase in the chord length of single-element devices and the height of their location in the boundary layer is found to gradually reduce the EBU effectiveness and, hence, to increase the net drag, as compared with the original configuration.

Laminar-turbulent transition in a boundary layer of low-aspect-ratio wing was investigated. Experiments clarifying the flow structure, its mean and oscillatory characteristics were carried out accompanied by linear stability analysis of the wind tunnel data on the laminar flow velocity profiles. Theoretical results obtained in a parallel flow approximation are in a good agreement with the experimental data on disturbances evolution at the initial stage of transition to turbulence.

The two-dimensional boundary-value problem of the unsteady flow of an incompressible viscous gas moving behind the piston in a

A mathematical model is proposed to describe laser ablation of metals in vacuum under the action of nanosecond laser pulses of moderate intensity taking into account the processes of cluster formation and decay in the vapor cloud. To describe the laser radiation absorption and metal heating, the thermal model based on the unsteady one-dimensional heat equation with a volume heat source is used, and the method of statistical modelling is employed for modelling vapor expansion and the processes of cluster formation. The efficiency of the proposed complex model is considered by the example of pulsed laser ablation of a niobium target.

This publication is a continuation of works devoted to problems that arise in heterogeneous technologies. The matter of parameters that define the transition from surface erosion to particle spraying onto the surface is addressed. In particular, we experimentally examine the influence of temperature of the supersonic heterogeneous flow (carrier-gas temperature and particle temperature) interacting with the streamlined obstacle on the occurrence probability of either surface damage or particle spraying.

Results of experimental studies on temperature distribution over the surface of a complex-shape heat exchanger like

The temperature mode of a wall was studied experimentally for sub- and supercritical pressures of water in tubes with turbulence stimulators; these data were compared with similar results obtained for a smooth tube. An increase in heat transfer inside a turbulizing tube was revealed for a single- and two-phase water flows. Correlation dependencies for determination of heat transfer coefficient in a single-phase flow inside a tube with turbulence stimulators were obtained.

A mathematical model is presented for unsteady conjugate convective-conductive heat transfer in a closed volume with local sources of heat release under the conditions of a convective-radiation heat exchange on one of external faces of the solution region. The thermogravitation convection regime was analysed for moderate Grashof numbers. The typical temperature and velocity fields were obtained, and the fields of desired quantities were compared for the planar and three-dimensional models in one of the typical sections of the solution region.

The density of aqueous solutions of lithium bromide was studied by the monochromatic gamma-ray attenuation method at the saturation line from the liquidus temperature of up to 250 

New generalized formulas for calculation of thermal conductivity of aqueous solutions of binary and multicomponent inorganic substances under high values of state parameters were derived. New values of thermal conductivity were calculated for aqueous solutions of salts within the ranges of temperatures of 293- 473 K, concentrations of 0- 25 mass % and pressures

The experimental setup is described and results are presented for measuring average boiling-up lag time for superheated n-hexane mixed with solid structures (activated coal, cellulose, silica gel) as function of temperature under atmospheric pressure. The

The results of computing a stationary arc with annular attachment on the butt end of a solid cylindrical cathode are presented. The influence of the discharge external parameters on the characteristics of arc plasma and the anode thermal state are considered.

Carbon nanotubes are generally synthesized in a fine powder with a very high specific volume and a low apparent density, which render their manipulation extremely difficult and hazardous. Here we demonstrate a simple and reliable method for the direct synthesis of multi-walled carbon nanotubes (MWCNTs) supporting each other in a controlled macroscopic shape and size by applying an external constraint on the reactor during the synthesis. The MWCNTs are highly entangled allowing a natural compacting of the macroscopic pieces without fine formation, making subsequent handling easier. The present synthesis method addresses many critical problems encountered with the use of carbon nanotubes as a macroscopic building-block materials.

By choosing appropriate liquid oxidizing treatment, the carbonaceous surface of an activated carbon (AC) can be functionalised with carboxylic groups in a broad range of content. These carboxylic groups are necessary functions, in order to disperse, perfectly, palladium cations through exchange reactions: H⁺/Pd²⁺ or H⁺/Pd(OH)⁺. Reduction in liquid phase of a 4 mass %Pd/AC whose carbonaceous surface was previously oxidized by NaOCl leads to palladium particles with a particle size of 1.5 nm which are much smaller than Pd particles obtained from a 4 mass % Pd/AC whose carbon surface was not oxidized. The well dispersed catalyst was tested in hydrogenation of cinnamaldehyde and exhibits quite a high hydrogenation efficiency as compared to activities reported in the literature.

Carbon materials with different pore texture have been prepared by evaporative drying and pyrolysis of resorcinol-formaldehyde aqueous gels. These carbon xerogels have been characterised and used as copper catalyst supports in the Selective Catalytic Reduction of NO by NH₃ at low temperature. Without metal, some activity is observed due to reaction between NO and NH₃, but the NO conversion is strongly enhanced after copper introduction. High deNO_x efficiency was obtained on these Cu/carbon xerogels at very low temperature and some pore texture effect was observed. However, special cares have to be taken to avoid carbon consumption and preserve its surface by keeping reaction temperature below 215^oC.

The modeling of a low temperature nitrogen adsorption in graphite mesopores with rectangular sections having the aspect ratio of the side lengths more than 7:1 has been executed by using the grand canonical Monte Carlo (GCMC) method. The adsorption branches of obtained GCMC isotherms were processed with the purpose of calculation of the formal pore size distributions (PSDs) by the nonlocal density functional theory (NLDFT) and Derjaguin-Broekhoff-de Boer (D-BdB) method. It is shown that NLDFT underestimates the sizes of the model rectangular mesopores though gives good estimations of specific surface area and volume of the pores. Also, it is shown that D-BdB method very strongly underestimates specific surface area and overestimates total volume of pores. PSDs obtained from D-BdB method have a very wide distributive interval of apparent sizes and brightly expressed bimodal forms. Such completely wrong information about the size of the investigated model pores is a consequence of the macroscopic assumption of the D-BdB theory about the constancy of the capillary condensate density in pores at various pressures.

Porous carbon materials with tailored pore texture can be prepared by evaporative drying and pyrolysis of resorcinol-formaldehyde aqueous gels. The pore size is easily controlled by the pH of the precursor solution. These materials, called "carbon xerogels", can be used as catalyst supports, like activated carbons. Since the texture of these materials is adjustable, it is possible to avoid diffusional limitations that are often encountered when a microporous activated carbon is used as support by choosing a support with the appropriate pore size. The meso- or macropore size of carbon xerogels can be enlarged so as to simplify the reactant diffusion, which leads to better catalytic performances.

Oxidations of alcohols into carbonyl functionalities are key reactions in the synthesis of fine chemicals. The need to substitute stoichiometric oxidations (using mineral oxidants) with "green" oxidations using benign oxidizing agents like molecular oxygen or hydrogen peroxide has been often emphasized. Supported noble metal catalysts, e.g. Pd/C or Pt/C are known to catalyse the oxidation of alcohols in the presence of molecular oxygen. It was previously reported that benzyl alcohol (Bzol) can be very selectively oxidized to benzaldehyde (Bzal) in dioxane, whereas the use of a 50/50 mixture of dioxane and alkaline aqueous solution yielded benzoic acid (Bzoic) by further oxidation of Bzal. In this work, we examined the effect of the activation treatment of the synthetic carbon and of the method of deposition of the metal on the characteristics of the supported metallic catalyst and its activity in this reaction. The highest dispersion and best catalytic performances were obtained by impregnation with hexachloroplatinic acid of a CO₂ activated carbon.

A systematic investigation of carbon nanotubes (CNT) and nanofibre (CNF) growth in terms of structures of carbon nanomaterials and growth rates by chemical vapour deposition is summarized. Different catalysts such as unsupported Fe₃O₄ and supported Ni, Fe and NiFe catalysts and different carbon sources such as CO, CO/H₂, CH₄/H₂, C₂H₆/H₂ and C₂H₄/CO/H₂ have been applied. This short review has been addressed to identify the principles for controlled synthesis of carbon nanotube or nanofibre with well-defined structures. The production of relatively high yields of defined structures is obtained. The Ni catalyst produces almost exclusively fishbone CNFs, while the supported Fe catalyst produces multiwall CNTs (MWCNTs). The NiFe catalyst can give a structure intermediate of the two. Unsupported Fe₃O₄ results in the platelet structure. The diameter of the produced structures holds information on the growth mechanism. The diameters of the MWNTs are always smaller than the growth catalyst particles, while the fishbone CNFs always have larger diameter than the growth catalyst. The structure and growth rate reflect the different reactivity of the metals and the dependence on surface orientation of the catalyst particles. Higher temperature and lower partial pressure yield smaller diameter MWNTs with fewer walls and a larger inner hollow core. The space velocity and thereby the H₂ partial pressure in the reactor has been identified as the most important parameter for scale-up of the reactor.

Carbon-based catalysts in powder, briquette and monolith forms have been prepared. Powder and briquette samples have been obtained from low-rank coal whereas monolith samples were prepared by coating cordierite monoliths with a blend of two polymers, namely, furan resin and polyethylene glycol. Vanadium was impregnated in these carbon-based catalysts by equilibrium adsorption using the ashes of a petroleum coke (PCA) as precursor and they were tested in the SCR of NO at low temperature. PCA contains around 23%V, 3 % Fe and 3 % Ni among other transition metals and this use is an undeniable good way for its revalorisation. As-prepared catalysts have demonstrated a considerable efficiency which was enhanced by pre-oxidizing treatments and an increase of vanadium loading. The presence of oxygen surface functionalities is of key importance to achieve an adequate distribution and fixation of the active phase while an excess in vanadium loading can even cause pore blockage decreasing the total efficiency. External diffusion and pressure drop coefficients were calculated for the three kinds of catalysts. As expected, powder catalyst shows the highest pressure drop while briquette and monolith samples show lower pressure drop values with no sign of diffusion limitations. For this reason, these structurated catalysts are considered to be more suitable for industrial applications.

The carbon films grown on Pt(III) surface by the high temperature catalytic decomposition of methane and ethylene were investigated with aid of X-ray photoelectron spectroscopy (XPS). It was observed that photoelectron spectra of carbon films depend on the hydrocarbon used as a reagent. It was shown that in case of methane used as the reagent the flat graphite films are deposited on the Pt(III) surface while ethylene or ethylene/methane mixture stimulates the carbon films distortion resulting in curved fullerites/ like structures. The formation of curved carbon films occurs through the formation of flat graphenes as the intermediate structures. It was determined that this topographic transition is reversible and has dynamic character depending on the crystal temperature and ethylene presence in the gas phase. The role of the diffusion-segregation phenomenon of carbon atoms in the formation of the curved films was established.

Dispersed palladium catalysts supported on carbon were prepared using different strong oxidants for preliminary treatment of the support. The surface properties of carbon and its ability to give place to highly dispersed Pd were discussed. Oxidative pre-treatment was shown to promote the dispersion significantly increasing the catalytic activity.

We have supported platinum on a commercial activated carbon with different loadings. The as-prepared catalyst has been used for the dehydrogenation of decalin in a batch-type reactor under boiling and refluxing conditions. The impact of some variables such as temperature, % Pt loading and decalin/catalyst ratio was studied using experimental design tools. The experiments carried out enable us to find the decalin to catalyst ratio which led to the best catalytic performance in our system.

Influence of the process parameters on porous structure of monolithic and spherical resorcinol-formaldehyde (RF) carbon aerogels prepared by supercritical acetone and carbon dioxide drying of alcogels, followed by carbonization is presented. The prepared organic RF aerogels were carbonized at 600 ^oC to form carbon aerogels and their porous structure was characterized by nitrogen adsorption. It was stated that synthesis parameters of RF organic aerogels (catalyst type, gel curing time, R/C ratio, resorcinol concentration in water) and drying method (low- or high-temperature supercritical drying) strongly influence textural parameters of the final carbon aerogels. Application of KOH in RF aerogel preparation allows obtaining carbon aerogels of the most advantageous parameters irrespective of the used drying method. Elongation of curing time of RF gels results in an increase in specific surface area and total pore volume of the carbon aerogels obtained both in the course of low- and high-temperature drying. Low-temperature supercritical drying of RF aerogels gives carbon materials of more advantageous texture parameters in comparison to carbon aerogels obtained in high-temperature supercritical drying.