scholarly journals Dipolar Excitation of a Perfectly Electrically Conducting Spheroid in a Lossless Medium at the Low-Frequency Regime

2018 ◽  
Vol 2018 ◽  
pp. 1-20 ◽  
Author(s):  
Panayiotis Vafeas
Keyword(s):  
Electric Fields ◽  
Vector Fields ◽  
Dimensional Space ◽  
Scattering Problem ◽  
Low Frequency ◽  
Main Idea ◽  
Wave Number ◽  
Low Frequencies ◽  
Static Approximation ◽  
Frequency Regime

The electromagnetic vector fields, which are scattered off a highly conductive spheroid that is embedded within an otherwise lossless medium, are investigated in this contribution. A time-harmonic magnetic dipolar source, located nearby and operating at low frequencies, serves as the excitation primary field, being arbitrarily orientated in the three-dimensional space. The main idea is to obtain an analytical solution of this scattering problem, using the appropriate system of spheroidal coordinates, such that a possibly fast numerical estimation of the scattered fields could be useful for real data inversion. To this end, incident and scattered as well as total fields are written in a rigorous low-frequency manner in terms of positive integral powers of the real-valued wave number of the exterior environment. Then, the Maxwell-type problem is converted to interconnected Laplace’s or Poisson’s equations, complemented by the perfectly conducting boundary conditions on the spheroidal object and the necessary radiation behavior at infinity. The static approximation and the three first dynamic contributors are sufficient for the present study, while terms of higher orders are neglected at the low-frequency regime. Henceforth, the 3D scattering boundary value problems are solved incrementally, whereas the determination of the unknown constant coefficients leads either to concrete expressions or to infinite linear algebraic systems, which can be readily solved by implementing standard cut-off techniques. The nonaxisymmetric scattered magnetic and electric fields follow and they are obtained in an analytical compact fashion via infinite series expansions in spheroidal eigenfunctions. In order to demonstrate the efficiency of our analytical approach, the results are degenerated so as to recover the spherical case, which validates this approach.

scholarly journals A seismic vertical vibrator driven by linear synchronous motors

Geophysics ◽  
2015 ◽  
Vol 80 (2) ◽  
pp. EN57-EN67 ◽  
Author(s):  
Rik Noorlandt ◽  
Guy Drijkoningen ◽  
Johan Dams ◽  
Rob Jenneskens
Keyword(s):  
Low Frequency ◽  
Field Tests ◽  
Seismic Source ◽  
Neutral Position ◽  
Low Frequencies ◽  
Synchronous Motors ◽  
Frequency Regime ◽  
Linear Synchronous Motors ◽  
Spring Mechanism ◽  
The Individual

A linear synchronous motor (LSM) is an electric motor that can produce large controllable forces and is therefore suitable as a driving engine for a seismic vibrator. This motor consists of two independent elements, a magnet track and a coil track, allowing practically unlimited motor displacements. This makes the LSM very suitable for expanding the source frequency band to the lower frequencies in which larger strokes are needed. In contrast to hydraulic engines, the LSM performs equally well over the whole frequency range, making possible a smaller amount of signal distortion, especially at the low frequencies. To find the feasibility of an LSM-driven vibrator, we successfully designed and built a multi-LSM prototype vibrator of some 1200 kg. We addressed the synchronization between the individual motor tracks and the different motors. To lower the energy consumption, a spring mechanism was implemented that delivered the force needed to lift the vibrator mass to its neutral position. The resonance belonging to this spring mechanism was successfully suppressed with the help of a position feedback control that also suppressed the temperature effects. The seismic data acquired in the field tests proved that the prototype LSM vibrator acted very well as a seismic source. It has no trouble generating pseudorandom sweeps, and even given its limited size, it generated signals within the low-frequency regime, down to 2 Hz, rather easily.

scholarly journals Electromagnetic Low-Frequency Dipolar Excitation of Two Metal Spheres in a Conductive Medium

2012 ◽  
Vol 2012 ◽  
pp. 1-37 ◽  
Author(s):  
Panayiotis Vafeas ◽  
Polycarpos K. Papadopoulos ◽  
Dominique Lesselier
Keyword(s):  
Magnetic Dipole ◽  
Dimensional Space ◽  
Three Dimensional ◽  
Low Frequency ◽  
Wave Number ◽  
Conductive Medium ◽  
Time Harmonic ◽  
Frequency Interaction ◽  
Minor Significance ◽  
Infinite Linear Systems

This work concerns the low-frequency interaction of a time-harmonic magnetic dipole, arbitrarily orientated in the three-dimensional space, with two perfectly conducting spheres embedded within a homogeneous conductive medium. In such physical applications, where two bodies are placed near one another, the 3D bispherical geometry fits perfectly. Considering two solid impenetrable (metallic) obstacles, excited by a magnetic dipole, the scattering boundary value problem is attacked via rigorous low-frequency expansions in terms of integral powers(ik)n, wheren≥0,kbeing the complex wave number of the exterior medium, for the incident, scattered, and total non-axisymmetric electric and magnetic fields. We deal with the static (n=0) and the dynamic (n=1,2,3) terms of the fields, while forn≥4the contribution has minor significance. The calculation of the exact solutions, satisfying Laplace’s and Poisson’s differential equations, leads to infinite linear systems, solved approximately within any order of accuracy through a cut-off procedure and via numerical implementation. Thus, we obtain the electromagnetic fields in an analytically compact fashion as infinite series expansions of bispherical eigenfunctions. A simulation is developed in order to investigate the effect of the radii ratio, the relative position of the spheres, and the position of the dipole on the real and imaginary parts of the calculated scattered magnetic field.

Choice of Electrode Material for Detecting Low Frequency Electric Field in Sea Water

2011 ◽  
Vol 239-242 ◽  
pp. 137-140 ◽  
Author(s):  
Yu Su Song ◽  
Kun Zhang ◽  
Pan Zuo
Keyword(s):  
Solid State ◽  
Electric Field ◽  
Electric Fields ◽  
Electrode Material ◽  
Sea Water ◽  
Low Frequency ◽  
Low Frequencies ◽  
Voltage Difference ◽  
Frequency Electric Field ◽  
Sensitive Electrode

Seven types of electrodes have been selected. DC resistances, AC impedances and voltage difference of the electrode pairs have been studied, according to the requirement of low and extra low frequencies electric fields detection in sea water. The feasibilities of these electrodes used for detecting these signals have been studied and explained on theories. The results show that the all-solid-state Ag/AgCl electrode is the most sensitive electrode with its very low DC resistances, AC impedances and the voltage difference controlled within 20μV, which makes it detecting low frequency electric field accurately.

scholarly journals Blazars in the LOFAR Two-Metre Sky Survey first data release

2019 ◽  
Vol 622 ◽  
pp. A14 ◽  
Author(s):  
S. Mooney ◽  
J. Quinn ◽  
J. R. Callingham ◽  
R. Morganti ◽  
K. Duncan ◽  
...  
Keyword(s):  
Spectral Index ◽  
Low Frequency ◽  
Value Added ◽  
Low Frequencies ◽  
Sample Distribution ◽  
Sky Survey ◽  
Data Release ◽  
Frequency Regime ◽  
Source Confusion ◽  
Bl Lacs

Historically, the blazar population has been poorly understood at low frequencies because survey sensitivity and angular resolution limitations have made it difficult to identify megahertz counterparts. We used the LOFAR Two-Metre Sky Survey (LoTSS) first data release value-added catalogue (LDR1) to study blazars in the low-frequency regime with unprecedented sensitivity and resolution. We identified radio counterparts to all 98 known sources from the Third Fermi-LAT Point Source Catalogue (3FGL) or Roma-BZCAT Multi-frequency Catalogue of Blazars (5th edition) that fall within the LDR1 footprint. Only the 3FGL unidentified γ-ray sources (UGS) could not be firmly associated with an LDR1 source; this was due to source confusion. We examined the redshift and radio luminosity distributions of our sample, finding flat-spectrum radio quasars (FSRQs) to be more distant and more luminous than BL Lacertae objects (BL Lacs) on average. Blazars are known to have flat spectra in the gigahertz regime but we found this to extend down to 144 MHz, where the radio spectral index, α, of our sample is −0.17 ± 0.14. For BL Lacs, α = −0.13 ± 0.16 and for FSRQs, α = −0.15 ± 0.17. We also investigated the radio-to-γ-ray connection for the 30 γ-ray-detected sources in our sample. We find Pearson’s correlation coefficient is 0.45 (p = 0.069). This tentative correlation and the flatness of the spectral index suggest that the beamed core emission contributes to the low-frequency flux density. We compare our sample distribution with that of the full LDR1 on colour-colour diagrams, and we use this information to identify possible radio counterparts to two of the four UGS within the LDR1 field. We will refine our results as LoTSS continues.

scholarly journals The Power of Low Frequencies: Faraday Tomography in the Sub-GHz Regime

Galaxies ◽  
2018 ◽  
Vol 6 (4) ◽  
pp. 112 ◽  
Author(s):  
Cameron Van Eck
Keyword(s):  
Magnetic Fields ◽  
Interstellar Medium ◽  
Frequency Dependence ◽  
Faraday Rotation ◽  
Low Frequency ◽  
Low Frequencies ◽  
Polarized Emission ◽  
Frequency Regime ◽  
Nearby Galaxies ◽  
Murchison Widefield Array

Faraday tomography, the study of the distribution of extended polarized emission by strength of Faraday rotation, is a powerful tool for studying magnetic fields in the interstellar medium of our Galaxy and nearby galaxies. The strong frequency dependence of Faraday rotation results in very different observational strengths and limitations for different frequency regimes. I discuss the role these effects take in Faraday tomography below 1 GHz, emphasizing the 100–200 MHz band observed by the Low Frequency Array and the Murchison Widefield Array. With that theoretical context, I review recent Faraday tomography results in this frequency regime, and discuss expectations for future observations.

Microparticle Trapping in Streaming Flows in Open Rectangular Chambers Undergoing Low Frequency Vertical Vibrations

2014 ◽  
Author(s):  
Prashant Agrawal ◽  
Prasanna S. Gandhi ◽  
Adrian Neild
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Mode Switching ◽  
Small Particles ◽  
Low Frequencies ◽  
Different Types ◽  
Frequency Regime ◽  
Vertical Vibrations ◽  
Rectangular Chamber ◽  
Streaming Flows

Microparticle collection in microfluidic systems via mechanical vibrations has been demonstrated in both low frequency systems (in the range of 100Hz) and in the high frequency regime (in the range of 1MHz). However, in most systems, collection of particles with lower inertia is hindered by second order time-averaged streaming flows. In our approach, we experimentally demonstrate collection of small particles (about 3 μm in diameter) by trapping them in streaming flows in a liquid filled open rectangular chamber undergoing vertical vibrations at low frequencies. The proposed method is then utilized to separate two different types of particles in distinct patterns through mode-switching.

AC conductivity of superionic thallium sulfide crystals exposed to γ-irradiation

2021 ◽  
Author(s):  
R. M. Sardarly ◽  
F. T. Salmanov ◽  
N. A. Aliyeva ◽  
R. N. Mehdiyeva ◽  
S. M. Gakhramanova
Keyword(s):  
Charge Transfer ◽  
Electric Fields ◽  
Low Frequency ◽  
Frequency Range ◽  
Γ Irradiation ◽  
Low Frequencies ◽  
Warburg Impedance ◽  
Transfer Processes ◽  
Alternating Electric Fields ◽  
Superionic Conduction

The frequency dependence of the thallium sulfide (TlS) crystal impedance is analyzed in wide frequency and temperature range for the hopping and superionic conduction mechanisms. It has been established that in weak alternating electric fields, there is a hopping mechanism of charge transfer. The use of impedance spectroscopy methods in TlS crystals, at temperatures of 300, 350 and 400 K in the frequency range of 2*106 Hz and subjected to [Formula: see text]-irradiation doses 0, 0.25 and 0.75 MGy charge transfer processes, has been investigated. Hodographs constructed from the data of experimental measurements of 400 K, in the low-frequency region ([Formula: see text] Hz) and regardless of the absorbed gamma quanta, indicating additional contributions to the conductivity, presumably corresponds to the fact that in the frequency range of the applied signal, carrier diffusion does not reach the diffuse layer. This type of hodographs at low frequencies is characteristic of the Warburg impedance.

Numerical analysis of wave–structure interaction of regular waves with surface-piercing inclined plates

2021 ◽  
Author(s):  
C. P. Cummins ◽  
G. T. Scarlett ◽  
C. Windt
Keyword(s):  
Excellent Agreement ◽  
Stokes Equations ◽  
Scattering Problem ◽  
Low Frequency ◽  
Wave Structure ◽  
Finite Depth ◽  
Linear Potential ◽  
Low Frequencies ◽  
Resonant Wave ◽  
Non Linear

AbstractThe Mocean wave energy converter consists of two sections, hinged at a central location, allowing the device to convert energy from the relative pitching motion of the sections. In a simplified form, the scattering problem for the device can be modelled as monochromatic waves incident upon a thin, inclined, surface-piercing plate of length $$L'$$ L ′ in a finite depth $$d'$$ d ′ of water. In this paper, the flow past such a plate is solved using a Boundary Element Method (BEM) and Computational Fluid Dynamics (CFD). While the BEM solution is based on linear potential flow theory, CFD directly solves the Navier–Stokes equations. Problems of this type are known to exhibit near-perfect reflection (indicated by a reflection coefficient $$|R|\approx 1$$ | R | ≈ 1 ) of waves at specific wavenumbers $$k'$$ k ′ . In this paper, we show that the resonant motion of the fluid induces large hydrodynamic forces on the plate. Furthermore, we argue that this low-frequency resonance resembles Helmholtz resonance, and that Mocean’s device being able to tune to these low frequencies does not act like an attenuator. For the case where the water is deep ($$d'>\lambda '/2$$ d ′ > λ ′ / 2 , where $$\lambda '=2\pi /k'$$ λ ′ = 2 π / k ′ ), we find excellent agreement between our simulations and previous semi-analytical studies on the value of the resonant wave periods in deep water. We also find excellent agreement between the excitation forces on the plate computed using the BEM model, analytical results, and CFD for large inclination angles ($$\alpha > 45^\circ $$ α > 45 ∘ ). For $$\alpha \le 15^\circ $$ α ≤ 15 ∘ , both methods show the same trend, but the CFD predicts a significantly smaller peak in the excitation force compared with BEM, which we attribute to non-linear effects such as the non-linear Froude–Krylov force

Impedance spectroscopy of (TlGaSe2)1−x(TlInSe2)x solid solutions in radio frequency range

2020 ◽  
Vol 34 (11) ◽  
pp. 2050113
Author(s):  
R. M. Sardarli ◽  
F. T. Salmanov ◽  
N. A. Alieva ◽  
R. M. Abbasli
Keyword(s):  
Charge Transfer ◽  
Impedance Spectroscopy ◽  
Electric Fields ◽  
Solid Solutions ◽  
Effective Medium Theory ◽  
Low Frequency ◽  
Localized States ◽  
Electrode Impedance ◽  
Frequency Range ◽  
Low Frequencies

The processes of charge transport on alternating current in [Formula: see text] solid solutions have been studied. It has been established that in weak alternating electric fields, there is a hopping mechanism of charge transfer over localized states in the vicinity of the Fermi level. A quantitative assessment of parameters was made in the framework of the effective medium theory and the Mott approximation. The use of impedance spectroscopy methods in [Formula: see text] solid solutions in the frequency range of 25–106 Hz, at temperatures of 180, 240 and 300 K charge transfer processes has been investigated. It was found that at 300 K in the low-frequency region, there are additional contributions to the conductivity, which, apparently, is associated with diffuse ion transfer near the boundary of the solid electrolyte and the electrode. Impedance locus curves, at low frequencies and at a temperature of 300 K, are characteristic of Warburg diffuse impedance.

scholarly journals Analysis of electromagnetic scattering from plasmonic inclusions beyond the quasi-static approximation and applications

2019 ◽  
Vol 53 (4) ◽  
pp. 1351-1371 ◽  
Author(s):  
Hongjie Li ◽  
Shanqiang Li ◽  
Hongyu Liu ◽  
Xianchao Wang
Keyword(s):  
Surface Plasmon ◽  
Electromagnetic Scattering ◽  
Scattering Problem ◽  
Spherical Geometry ◽  
Layer Potential ◽  
Static Approximation ◽  
Quasi Static Approximation ◽  
Plasmonic Structures ◽  
Time Harmonic ◽  
Frequency Regime

This paper is concerned with the analysis of time-harmonic electromagnetic scattering from plasmonic inclusions in the finite frequency regime beyond the quasi-static approximation. The electric permittivity and magnetic permeability in the inclusions are allowed to be negative-valued. Using layer potential techniques for the full Maxwell system, the scattering problem is reformulated into a system of integral equations. We derive the complete eigensystem of the involved matrix-valued integral operator within spherical geometry. As applications, we construct two types of plasmonic structures such that one can induce surface plasmon resonances within finite frequencies and the other one can produce invisibility cloaking. It is particularly noted that the cloaking effect is a newly found phenomenon and is of different nature from those existing ones for plasmonic structures in the literature. The surface plasmon resonance result may find applications in electromagnetic imaging.

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