A method for dynamic local adaptation of graded Cartesian trees for the numerical solution of fluid dynamics problems is presented. Local wavelet analysis of a gas-dynamic field based on nonuniform ...B-splines is applied independently to each cell of the computational grid and makes it possible to identify nonsmooth or significantly nonlinear sections of the solution (or, vice versa, sufficiently smooth and linear ones) and modify the grid to calculate the next time step so that nonuniformly scaled flow features had adequate grid resolution. In combination with other computational fluid dynamics methods, such as the free boundary method, the presented technique allows one to effectively solve nonstationary problems involving flow around moving bodies. The operation of the proposed version of wavelet adaptation is demonstrated using a number of such problems.
Computer simulation of liquid fuel jet injection into heated atmosphere of combustion chamber, mixture formation, ignition and combustion need adequate modeling of evaporation, which is extremely ...important for the curved surfaces in the presence of strong heat and mass diffusion fluxes. Combustion of most widely spread hydrocarbon fuels takes place in a gas-phase regime. Thus, evaporation of fuel from the surface of droplets turns to be one of the limiting factors of the process as well. The problems of fuel droplets atomization, evaporation being the key factors for heterogeneous reacting mixtures, the non-equilibrium effects in droplets atomization and phase transitions will be taken into account in describing thermal and mechanical interaction of droplets with streaming flows. In the present paper processes of non-equilibrium evaporation of small droplets will be discussed. As it was shown before, accounting for non-equilibrium effects in evaporation for many types of widely used liquids is crucial for droplet diameters less than 100
μm, while the surface tension effects essentially manifest only for droplets below 0.1
μm. Investigating the behavior of individual droplets in a heated air flow allowed to distinguish two scenarios for droplet heating and evaporation. Small droplets undergo successively heating, then cooling due to heat losses for evaporation, and then rapid heating till the end of their lifetime. Larger droplets could directly be heated up to a critical temperature and then evaporate rapidly. Droplet atomization interferes the heating, evaporation and combustion scenario. The scenario of fuel spray injection and self-ignition in a heated air inside combustion chamber has three characteristic stages. At first stage of jet injection droplets evaporate very rapidly thus cooling the gas at injection point, the liquid jet is very short and changes for a vapor jet. At second stage liquid jet is becoming longer, because evaporation rate decreases due to decrease of temperature. But combustion of fuel vapor begins which brings to increase of heat flux to droplets and accelerates evaporation. The length of the liquid jet decreases again and remains constant slightly oscillating.
► Mathematical model and simulator for sprays evaporation and ignition in combustion chamber. ► Non-equilibrium effects increase droplet lifetime as compared with equilibrium models. ► Scenario of fuel spray ignition in a heated combustion chamber has three characteristic stages. ► At first stage of jet injection droplets evaporate very rapidly thus cooling the gas at injection point. ► At second stage liquid jet becomes longer, evaporation rate decreasing in cool gas. ► Finally jet shortens as combustion of fuel vapor increases heat flux and accelerates evaporation.
The paper gives coverage of recent advances in mathematical modeling of long term orbital debris evolution within the frames of continua approach. Under the approach the evolution equations contain a ...number of source terms responsible for the variations of quantities of different fractions of orbital debris population due to fragmentations and collisions. Mechanisms of hypervelocity collisions of debris fragments with pressurized vessels are investigated. The spacecraft shield honeycomb concept is suggested based on principles of impact energy conversion and redistribution and consumption by destroyable structures. The paper is devoted to the 100th anniversary of the founder of space debris research in Moscow State University Prof. G.A. Tyulin.
•A new concept for spacecraft shielding from space debris is suggested.•Effects of energy dissipation in collisions with fluid filled vessels are studied.•Using gas-filled bumpers for impact energy redistribution is studied.
The structural characteristics of loose zinc deposits obtained in pulse-potential modes are calculated using a phenomenological model. Increasing the duty cycle leads to intensified anodic ...dissolution during pauses and obtaining of denser deposits, due to the formation of dendrites with fewer tips, yet, a larger diameter as compared to the deposits obtained in the potentiostatic mode. The linear dependence of the diameter of the tips of dendrites forming a loose zinc deposit on the duty cycle is found. There is a critical time corresponding to the achievement of zero deposit growth rate when the metal deposited during the pulse will completely dissolve during the pause.
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•Key factors affecting the activity of phenoxy-imine titanium catalysts in ethylene polymerization were revealed theoretically.•Quantum chemistry calculations of a large series of ...phenoxy-imine ligands, octahedral titanium dichloride complexes, active cationic species, and their π‐complexes with ethylene were performed on DFT/BP86/SVP level.•It was found that the electronic structure of the active cation, determined by the energy gap between frontier orbitals (HOMO and LUMO), directly correlates with experimental values of the catalytic activity.
The mechanism of ethylene polymerization on phenoxy-imine (FI) titanium catalysts was studied theoretically to identify the major factors affecting the catalytic activity. Geometry optimizations of FI ligands, octahedral titanium dichloride complexes, active cationic species, and their π‐complexes with ethylene as well as calculations of the energy profile of chain propagation were performed at the BP86-D3 level. We found that the calculated energy gaps between frontier orbitals (HOMO and LUMO) in the active cations of the catalysts correlate with the experimental activity values. High activities of FI catalysts with α‐Cumyl groups were attributed to smaller HOMO-LUMO gaps due to hyperconjugation between π-systems of α‐Cumyl and (N‐aryl)salicylaldimine moieties in the active cations. The correlation provides a qualitative estimate of the catalytic activity for further design of new FI titanium complexes.
Severe course of COVID-19 is largely determined by hyperactivation of the immune system, or cytokine storm, in which immune cells (lymphocytes, monocytes,
etc
.) play a major role. Using low-voltage ...scanning electron microscopy, we studied the morphology of lymphocytes and monocytes during cytokine storm. Monocytes and lymphocytes were isolated by fluorescence sorting from the blood of healthy volunteers (
n
=6) and patients with COVID-19 (
n
=5) during cytokine storm (IL-6>23 ng/ml, smear positive for SARS-CoV-2). For each patient, 11-32 individual cells were analyzed at magnification of 18-32,000 times. Measurements showed that monocyte size was increased during cytokine storm (
p
=0.0001).
Paper presents the results of numerical studies of hybrid rocket engines operating cycle including unsteady-state transition stage. A mathematical model is developed accounting for the peculiarities ...of diffusion combustion of fuel in the flow of oxidant, which is composed of oxygen-nitrogen mixture. Three dimensional unsteady-state simulations of chemically reacting gas mixture above thermochemically destructing surface are performed. The results show that the diffusion combustion brings to strongly non-uniform fuel mass regression rate in the flow direction. Diffusive deceleration of chemical reaction brings to the decrease of fuel regression rate in the longitudinal direction.
The paper presents the results of developing of physical and mathematical model making it possible to take into account the effect of droplets non-uniformity in space and size distribution on ...ignition conditions for fuel sprays. The influence of condensed phase volume fraction on ignition and combustion of sprays was studied, physical and mathematical models for multi-phase flows, mixture formation and combustion of liquid fuels based on solving Navier–Stokes equations for gas phase accounting for thermal and mechanical interaction with poly-dispersed droplets array. The problems of particulate phase dynamics are regarded accounting for the interaction with gas phase atomization, evaporation and combustion.
It was shown that depending on droplet size distribution and aerosol cloud density different flow scenarios were possible.
Several ignition zones could be formed behind incident shock wave depending on mixture properties and initiation parameters. The possibility of numerical simulation permitting variation of definite parameters only made it possible to explain this fact.
► Droplets non-uniformity in space and size effect on fuel sprays ignition was simulated numerically. ► Several ignition zones could be formed behind incident shock wave depending on spray properties. ► Increase of cloud density and decrease of droplet size create more favorable ignition conditions. ► Shock wave attenuation in combustible spray is studied.
Differential RCS of RFID tag NIKITIN, P. V; RAO, K. V. S; MARTINEZ, R. D
Electronics letters,
04/2007, Letnik:
43, Številka:
8
Journal Article
Recenzirano
The differential radar cross-section (RCS) of an RFID tag is an important parameter which determines the power of the modulated backscattered tag signal. The vector differential RCS of an RFID tag as ...seen by the reader is analysed and, for the first time, compared with experimental results in UHF band.
The branching ratios and differential distributions for the four-leptonic decays
B
−
→
μ
+
μ
−
v
¯
e
e
−
,
B
−
→
e
+
e
−
v
¯
μ
μ
−
, and
B
−
→
μ
+
v
¯
μ
μ
−
μ
−
are calculated within the Standard ...Model. The branching ratios for the rare decays
B
d,s
→
e
+
e
−
μ
+
μ
−
and
B
d,s
→
μ
+
μ
−
μ
+
μ
−
are estimated. Methods for testing the lepton universality in rare multileptonic decays of charged and neutral
B
mesons are proposed.
