Unstructured Adaptive Grid Flow Simulations of Inert and Reactive Gas Mixtures

Unstructured adaptive grid flow simulation is applied to the calculation of high-speed compressible flows of inert and reactive gas mixtures. In the present case, the flowfield is simulated using the 2-D Euler equations, which are discretized in a cell-centered finite volume procedure on unstructured triangular meshes. Interface fluxes are calculated by a Liou flux vector splitting scheme which has been adapted to an unstructured grid context by the authors. Physicochemical properties are functions of the local mixture composition, temperature, and pressure, which are computed using the CHEMKIN-II subroutines. Computational results are presented for the case of premixed hydrogen–air supersonic flow over a 2-D wedge. In such a configuration, combustion may be triggered behind the oblique shock wave and transition to an oblique detonation wave is eventually obtained. It is shown that the solution adaptive procedure implemented is able to correctly define the important wave fronts. A parametric analysis of the influence of the adaptation parameters on the computed solution is performed.     

Energy Dissipation in a Medium with Turbulent Viscosity and the Hill Vortex

Doklady Physics - A new approach to calculation of the dissipation of mechanical energy in a medium with turbulent viscosity based on determination of the velocity-shear modulus is proposed. As an...     

Quenching Properties of an Arc in a Double Flow with a Vortex

The effect of a vortex in a gas flow on an air-blast arc is investigated. The radial density of a vortex in the compressible flow is evaluated with a simple model. The experiments show that the width of a low pressure channel on the axis of the nozzle is comparable to the theoretical values. The measured electric field strength profile is strongly influenced by the presence of such a vortex. In addition, the thermal interrupting capability is drastically lowered by vortex superimposed on the axial gas flow.     

Simulation of Vortex Convection in a Compressible Viscous Flow with Dynamic Mesh Adaptation

In this work, vortex convection is simulated using a dynamic mesh adaptation procedure. In each adaptation period, the mesh is refined in the regions where the phenomena evolve and is coarsened in the regions where the phenomena deviate since the last adaptation. A simple indicator of mesh adaptation that accounts for the solution progression is defined. The generation of dynamic adaptive meshes is based on multilevel refinement/coarsening. The efficiency and accuracy of the present procedure are validated by simulating vortex convection in a uniform flow. Two unsteady compressible turbulent flows involving blade-vortex interactions are investigated to demonstrate further the applicability of the procedure. Computed results agree well with the published experimental data or numerical results.     

Excitation of Eigenwaves by a Vortex Electromagnetic Beam in a Semi-Infinite Cylinder Filled with a Gyrotropic Medium

Radiophysics and Quantum Electronics - We study the diffraction of a Laguerre–Gaussian electromagnetic vortex beam by the end of a semi-infinite cylinder filled with a gyrotropic medium. A...     

Interruption Capability of Gases and Gas Mixtures in a Puffer-Type Interrupter

SF6 gas has been widely used in arc interruption applications for the past 20 years. Reported here is a systematic, comprehensive effort to search for and evaluate gases and gas mixtures suitable as an arc interruption medium potentially superior to SF6. The search began with several hundred gases, narrowed down to about forty, and finally fifteen gases and gas mixtures were evaluated in a full size puffer-type interrupter under 60-Hz high-power conditions. The results showed SF6 stood out as having the best interruption ability with several mixtures having ~80 percent of SF6' s performance.     

Destruction of a Stationary Vortex in an Elastic Medium by Energy Deposition

Technical Physics - The dynamics of the structure of an infinite vortex tube in ideal gas under explosive external energy deposition near its axis is studied. With neglect of viscosity and heat...     

Spectroscopic Detection of Oxygen in Nitrogeneous Gas Mixtures

A pulsed high-frequency discharge was used to excite a non-LTE (local thermal equilibrium) plasma with a high specific power density sufficiently high to dissociate oxygen and gaseous oxides. If traces of hydrocarbons were present in the gas mixture, in the spectrum the Violet system of CN appeared with a long radiation decay time. The CN bands are excited preferentially by inelastic collisions with metastable nitrogen molecules (A³σ). These levels can be efficiently depopulated by competing impact with atomic oxygen. By measuring the radiation signal during the radiation decay a dynamic range of 6 was achieved for a variation of the oxygen content between 1 and 100 ppm. The measured radiation decay of the violet CN 0–0-bands at 388 nm was simulated by a simple model of rate equations valid in the time region where diffusion processes may be neglected.     

Silicon Carbide Nanoparticles Produced by CO2 Laser Pyrolysis of SiH4/C2H2 Gas Mixtures in a Flow Reactor

Pulsed CO₂-laser-induced decomposition of different mixtures of SiH₄ and C₂H₂ in a flow reactor has been employed to produce silicon carbide clusters and nanoparticles with varying content of carbon. The as-synthesized species were extracted from the reaction zone by a conical nozzle and expanded into the source chamber of a cluster beam apparatus where, after having traversed a differential chamber, they were analyzed with a time-of-flight mass spectrometer. Thin films of silicon carbide nanoclusters were produced by depositing the clusters at low energy on potassium bromide and sapphire windows mounted into the differential chamber. At the same time, Si and SiC nanoparticles were collected in a filter placed into the exhaust line of the flow reactor. Both beam and powder samples were characterized by FTIR spectroscopy. The close resemblance of the spectra suggests that the composition of the beam and powder particles obtained during the same run is nearly identical. XRD spectroscopy could only be employed for the investigation of the powders. It was found that CO₂ laser pyrolysis is ideally suited to produce silicon carbide nanoparticles with a high degree of crystallinity. Nanopowders produced from the pyrolysis of a stoichiometric (2:1) mixture of SiH₄/C₂H₂ were found to contain particles or domains of pure silicon. The characteristic silicon features in the FTIR and XRD spectra, however, disappeared when C₂H₂ was applied in excess.     

涡流二极管三维强旋湍流流动的数值模拟

为了揭示涡流二极管内三维强旋流动的特性与其阻抗性能的关系,进一步提高涡流二极管正反向流动阻抗比,本文采用Reynolds应力模型对涡流腔内非对称强旋转湍流流动进行了数值模拟,分析了腔内矢量场与标量场的分布特性,基于涡流腔内的涡量分析,得到了涡流腔内旋流的强制涡与自由涡结构及各自区域范围,并研究了涡流腔径宽比对整体性能的...     

基于Omega涡识别方法的离心泵内非定常流动数值评价

离心泵内部非定常流动与流场中生成的旋涡密切相关.本文采用Omega涡识别方法研究离心泵流场涡结构及非定常流动特性,阈值选择为0.51.分析得出LES湍流模型相对比其他两种湍流模型涡识别结果更为细致,对叶片流道中的小涡结构可以很好地捕捉.设计工况下,LES模型识别出的涡面积数值是DDES模型的1.15倍,是SSTk-ω模...     

Flow Mechanisms and Diffusion Combustion of Turbulent Jets

The problem of flow and combustion of turbulent jets of fuel gas in the external medium of an oxidant (air) is solved with regard to the existence of the actual boundary of the turbulent flow region of a jet. Based on the ideas of the friction force of the external flow acting on the boundary of a jet, the entrainment equation for the external medium is derived that closes the system of equations of motion of turbulent jets. The physical meaning of the dissipation rate of the turbulent energy of a jet is interpreted as the work of the friction force. To describe the combustion kinetics, the limit of instantaneous reactions corresponding to the diffusion combustion mode is used. Calculations of the effective reaction rates for reactants and the volumes occupied by them are based on the representation of a turbulent medium as an aggregation of independent turbulent particles—vortices—whose random contacts lead to the mixing and combustion of reacting substances [31]. The concomitant phenomena of flow and combustion are analyzed, including radiation effects. In particular, it is shown that the apparent increase in the combustion temperature with increasing Reynolds number is in fact attributed to the relative decrease of thermal radiation losses. Qualitative agreement is obtained between the results of the theoretical calculations of the length of a combustion torch and experimental data.     

Shear Flow of a Viscous Fluid over a Cavity with a Pulsating Gas Bubble

Doklady Physics - For the first time, the effect of pressure fluctuations on the averaged shear stress in a shear viscous flow over a two-dimensional rectangular microcavity containing a pulsating...     

The Multifocus Structure of Radiation upon Femtosecond Filamentation of an Optical Vortex in a Medium with an Anomalous Group Velocity Dispersion

Optics and Spectroscopy - We analyze numerically the femtosecond filamentation of a vortex beam with the topological charge m = 1 at wavelength λ0 = 1800 nm in fused silica under anomalous...     

Vortex solutions with a smooth core

A Yang-Mills field is used to smooth over the core of a vortex solution.     

Nonlinear Couette Flow in a Low Density Granular Gas

A model kinetic equation is solved exactly for a special stationary state describing nonlinear Couette flow in a low density system of inelastic spheres. The hydrodynamic fields, heat and momentum fluxes, and the phase space distribution function are determined explicitly. The results apply for conditions such that viscous heating dominates collisional cooling, including large gradients far from the reference homogeneous cooling state. Explicit expressions for the generalized transport coefficients (e.g., viscosity and thermal conductivity) are obtained as nonlinear functions of the coefficient of normal restitution and the shear rate. These exact results for the model kinetic equation are also shown to be good approximations to the corresponding state for the Boltzmann equation via comparison with direct Monte Carlo simulation for the latter.     

Initiation of Kerosene Combustion in a Supersonic Air Flow by a Bunch of Gas-Dynamic Pulses

Doklady Physics - In this paper, we propose a method to initiate combustion of kerosene in a supersonic air flow in part of a combustion chamber with a constant cross section and experimentally...