Gad A. Pinhasi

Extinction of pool flames by means of a DC electric field

Abstract The application of an electric field to a combustion system can produce large and potentially useful effects, such as reducing carbon formation, affecting flame velocity, extending flammability limits, increasing flame luminosity, and stabilizing and extinguishing flame. The present study is concerned primarily with the corona discharge interaction with pool fires. The fuel surface served as the blunt electrode and several specially designed sharp probes have been examined as the high-voltage electrode. The most effective sharp electrode appeared to be a simple thin wire parallel to the liquid surface situated above it at a distance of several millimeters. The flame was repelled from the probe, thus creating a possible pool flame extinction device. Similar results were achieved with a mechanical blower that reproduced the velocity profile of the electric wind. The gas composition in different locations was examined for both the corona and blower cases. No significant difference was found, and it was concluded that ion pumping has no influence on the extinction performance. It is suggested that extinction by corona discharge is caused solely by the aerodynamic action of the electric wind with its remarkably flat, sharp velocity profile. Fire extinctions under hot and aggressive environments are possible applications of the present device.

Propagation analysis of ultrashort pulses in resonant dielectric media

The space-frequency theory of the propagation of an ultrawideband radiation in dielectric media is presented. Characterization of the material via its susceptibility leads to a transfer function, which describes the response of the medium in the frequency domain. This description enables the consideration of broadband signals, taking into account inhomogeneous absorptive and dispersive effects of the medium. Analytical expressions are derived when a pulse-modulated signal is propagating in a general dielectric material. Conditions for apparent “superluminal” and pulse compression effects are identified. The theory is applied for a special case of transmission inside a resonant medium, revealing analytical approximations for the parameters of a Gaussian propagating pulse in terms of initial pulse width, carrier frequency, and medium parameters. Constraints of the derived analytical expressions are discussed, pointing out conditions of approximation validity. We demonstrate the approach by studying the propagation of ultrawideband signals, while transmitted in the vicinity of the 60 GHz absorption peak of the atmospheric medium at millimeter wavelengths.

A Numerical Model Based on Variational Principle for Airfoil and Wing Aerodynamics

Over the last few years, finite element algorithms for solution of the Euler flow equations have gained increased popularity. The objective of the current research is to develop a new method to solve the Euler flow equations numerically using a variational technique for airfoil and wing aerodynamics. A new formulation of Eularian variational principle satisfying the Kutta condition is suggested, and a numerical implementation is presented. The proposed method can obtain improved solution, especially for complicated geometries. A computer code named FLUIDEX was developed to analyze barotropic fluid dynamics. The solution of the flow problem is obtained by using numerical algorithm to find the extremum value of an “Action” i.e. by a variational principle. Predictions of the FLUIDEX numerical model were analyzed for particular cases of potential flow (compressible and incompressible). The results were successfully compared against exact analytical solutions for potential flow test problems. The proposed method obtains fast and stable solutions without the need to integrate the equations in time and space, and thus enables a considerable reduction of the time and cost of the solution. I. INTRODUCTION It is well known that a presentation of a physical problem in terms of a variational principle can lead to a better understanding of the problem. Moreover, a variational principle combined with a numerical technique can lead to improved solution, especially for complicated geometries. Early attempts have been made to formulate Eulerian fluid dynamics in terms of a variational principle (Herivel, 1955; Serrin, 1959; Lin, 1963). However, the variational principles developed by the above authors are very cumbersome containing quite a few “Lagrange multipliers” and “potentials”. The range of the total number of independent functions in the above formulations ranges from eleven to seven which exceeds by many the four functions appearing in the Eulerian and continuity equations of a baratropic flow. Therefore they did not have any practical use or applications. A compact four function variational principle was obtained by Seliger and Whitham (1968) and thus a new way of simulating fluid dynamics was made possible. In previous works we developed a new method to solve the Euler flow equations numerically using Seliger and Whitham Variational technique (Yahalom and Pinhasi 2002; Yahalom 2003). A numerical implementation of their formulation of the Eularian Variational principle was suggested, and results for analysis of flows around various geometries including wing profiles were presented.

Atmospheric and Fog Effects on Ultra-Wide Band Radar Operating at Extremely High Frequencies

The wide band at extremely high frequencies (EHF) above 30 GHz is applicable for high resolution directive radars, resolving the lack of free frequency bands within the lower part of the electromagnetic spectrum. Utilization of ultra-wideband signals in this EHF band is of interest, since it covers a relatively large spectrum, which is free of users, resulting in better resolution in both the longitudinal and transverse dimensions. Noting that frequencies in the millimeter band are subjected to high atmospheric attenuation and dispersion effects, a study of the degradation in the accuracy and resolution is presented. The fact that solid-state millimeter and sub-millimeter radiation sources are producing low power, the method of continuous-wave wideband frequency modulation becomes the natural technique for remote sensing and detection. Millimeter wave radars are used as complementary sensors for the detection of small radar cross-section objects under bad weather conditions, when small objects cannot be seen by optical cameras and infrared detectors. Theoretical analysis for the propagation of a wide “chirped” Frequency-Modulated Continuous-Wave (FMCW) radar signal in a dielectric medium is presented. It is shown that the frequency-dependent (complex) refractivity of the atmospheric medium causes distortions in the phase of the reflected signal, introducing noticeable errors in the longitudinal distance estimations, and at some frequencies may also degrade the resolution.

A Combined Variational and Multi -Grid Approach for Fluid Dynamics Simulation

A new approach for solving Partial Differential Equations problems related to fluid dynamics problems is being proposed. The method merges two existing methods: Using the variational techniques of finding the Action Minimum and using the Multi -Grid techniques of solving th e equation on a coarse grid, interpolating it to finer grids, performing reduced computer work on finest grid, receiving the same accuracy. The proposed method simplifies the Multi -level adaptive techniques (MLAT) ideas. The experiments has shown that solv ing the variation problem on a grid with n2 is much less expensive (more than factor 4) than that on a grid with (2n) 2 grid -points. The method solves a problem on a coarse grid, interpolates it to the finer grid and uses the interpolated values as initial values – better guess values for the problem on the finer grid. The finer grid has changed its role in the second step, to a coarse grid. This process improves the solution duration. The interpolation computer -work maybe performed efficiently once and ther efore may be neglected compared to the time invested in the variation solution itself. In this work we improve on our previous results 17 and also investigate further aspects of our method including the effects of introducing a better initial configuration (“guess”) 7 , using the relaxation technique and the effect of the required accuracy on computation time. We also compare the resources needed for interior and boundary points by our algorithm.

Ultra wideband wireless satellite communications in the 94 GHz band

The growing demand for broadband wireless communication links and the lack of wide frequency bands within the conventional spectrum causes us to seek bandwidth in the higher microwave and millimetre-wave spectrum at Extremely High Frequencies (EHF) above 30GHz. One of the principal challenges in realizing modern wireless communication links in the EHF band is phenomenon occurring during electromagnetic wave propagation through the atmosphere. Space-frequency theory of the propagation of an ultra-wide band radiation in a inhomogeneous dielectric media is presented. Characterization of the atmospheric medium is via its refractivity leading to a transfer function, which describes the changing response of the medium in the frequency domain. This description enables the consideration of broadband signals taking into account inhomogeneous absorptive and dispersive effects of the medium. We demonstrate the approach by studying propagation of ultra-wide band signals, while transmitted in the vicinity of the 94GHz window of the atmospheric medium at millimetre wavelengths.

Modeling of Boiling Liquid Expanding Vapor Explosion (BLEVE): Plane, Cylindrical and Spherical 1D Model

A 1D plane, cylindrical and spherical numerical model was developed for estimating the thermodynamic and the dynamic state of the boiling liquid during a boiling liquid expanding vapor explosion (BLEVE) event. The model predicts, simultaneously, the flow properties of the expanding two-phase flashing mixture and its surrounding air. The possible presence of a shock wave formed by the fluid expansion through the air is accounted for in the model. Model predictions of the shock wave strengths, in terms of TNT equivalence for the various coordinate systems, were compared against those obtained by simple energy models. As expected, the simple energy models over predicts the shock wave strength. However, the simple model which accounts for the expansion irreversibility, produces results which are closer to current model predictions. For the 1D plane case the model simulates a BLEVE scenario in a tunnel, whereas for the spherical case the more realistic BLEVE scenario in free space is being studied.Copyright

Backward Wave Excitation and Generation of Oscillations in Free-Electron Lasers in the Absence of Feedback—Beyond the High Gain Approximation

Quantum and free-electron lasers (FELs) are based on distributed interactions between electromagnetic radiation and gain media. In an amplifier configuration, a forward wave is amplified while propagating in a polarized medium. Formulating a coupled mode theory for excitation of both forward and backward waves, we identify conditions, leading to efficient excitation of backward wave without any mechanism of feedback or resonator assembly. The excitations of incident and reflected waves are described by a set of coupled differential equations expressed in the frequency domain. The induced polarization is given in terms of an electronic susceptibility tensor. In quantum lasers the interaction is described by two first-order differential equations. In FELs, the excitation of the forward and backward modes is described by two coupled third-order differential equations. In our previous investigation analytical and numerical solutions of reflectance and transmittance for both quantum lasers and high-gain FELs were presented. In this work we extend the study to a general FEL without restriction of the high-gain approximation. It is found that when the solutions become infinite, the device operates as an oscillator, producing radiation at the output with no Held at its input, entirely without any localized or distributed feedback.

Millimeter Wave High Resolution Radar Accuracy in Fog Conditions—Theory and Experimental Verification

Attenuation and group delay effects on millimeter wave (MMW) propagation in clouds and fog are studied theoretically and verified experimentally using high resolution radar in an indoor space filled with artificial fog. In the theoretical analysis, the frequency-dependent attenuation and group delay were derived via the permittivity of the medium. The results are applied to modify the millimeter-wave propagation model (MPM) and employed to study the effect of fog and cloud on the accuracy of the Frequency-Modulated Continuous-Wave (FMCW) radar operating in millimeter wavelengths. Artificial fog was generated in the experimental study to demonstrate ultra-low visibility in a confined space. The resulted attenuation and group delay were measured using FMCW radar operating at 320–330 GHz. It was found that apart from the attenuation, the incremental group delay caused by the fog also played a role in the accuracy of the radar. The results were compared to the analytical model. It was shown that although the artificial fog has slight different characteristics compare to the natural fog and clouds, in particle composition, size, and density, the model predictions were good, pointing out that the dispersive effects should be considered in the design of remote sensing radars operating in millimeter and sub-millimeter wavelengths.

Boiling Liquid Expanding Vapor Explosion: Simulation and Risk Analysis

A model was developed for estimating the thermodynamic and the dynamic state of the boiling liquid during a boiling liquid expanding vapor explosion (BLEVE) event. The model predicts, simultaneously, the bubble growth processes in the liquid at the superheat-limit state, the front velocity of the expanding two-phase mixture, and the shock wave pressure formed by the fluid expansion through the air. The model predictions of the shock wave strengths, in terms of TNT equivalence, were compared against those obtained by simple energy models. The study reveals what are the important mechanisms that dominate two-phase blowdown and BLEVE accidents. The research presents an overview of the mechanism, the causes, the consequences, and the preventive strategies associated with BLEVEs. They are therefore important computational tools for environmental safety assessments.