Point Dipole Source Research Articles

An efficient predicting approach for blade frequency line-spectrum noise of marine centrifugal pumps

The paper introduces an innovative and efficient calculation method for the spectral flow noise of the pump system based on the spatial distribution dipole, which predicts the blade frequency radiation noise at the pipe flange of the pump system under different operating conditions. First, the unsteady flow simulation of the centrifugal pump is calculated using computational fluid dynamics (CFD). Then, the pressure on each segmented blade surface is decomposed into lift and drag forces. After that, each segment is decomposed into several dipole point sources based on the blade noise theory. Finally, the simplified dipole point sources are considered sound sources, and the blade frequency line spectrum noise at the pipe flange of the pump system is predicted using acoustic-structure coupling. The results indicate that the simplified method provides high prediction accuracy while significantly improving the computational efficiency of blade frequency line spectrum noise calculation for centrifugal pumps.

Variational study of a model for near-perfect transmission through lossy media with acoustic sources

Complementary acoustic metamaterials have been proposed as a means of compensating for the high impedance mismatches of aberrating layers that disrupt the acoustic field and hence distort acoustic images. Recently, a complementary acoustic metamaterial featuring active components was shown in principle to compensate for both the impedance mismatch and energy attenuation of lossy materials, but a physical realization of the concept has not yet been implemented. Here, we present results from a one-dimensional acoustic model showing how a plane wave incident on a lossy material can be augmented by point monopole and dipole sources to allow for near-perfect transmission, thus rendering the lossy medium acoustically transparent. We present general expressions for source magnitudes that are dimensionless with respect to frequency, material thickness, and the background medium. We explore the sensitivity of the performance to variations in each of the model parameters, considering both theoretical and practical limitations to the proposed method. We show that these findings are consistent with three-dimensional finite element simulations.

A Combined Magnetoelectric Sensor Array and MRI-Based Human Head Model for Biomagnetic FEM Simulation and Sensor Crosstalk Analysis.

Magnetoelectric (ME) magnetic field sensors are novel sensing devices of great interest in the field of biomagnetic measurements. We investigate the influence of magnetic crosstalk and the linearity of the response of ME sensors in different array and excitation configurations. To achieve this aim, we introduce a combined multiscale 3D finite-element method (FEM) model consisting of an array of 15 ME sensors and an MRI-based human head model with three approximated compartments of biological tissues for skin, skull, and white matter. A linearized material model at the small-signal working point is assumed. We apply homogeneous magnetic fields and perform inhomogeneous magnetic field excitation for the ME sensors by placing an electric point dipole source inside the head. Our findings indicate significant magnetic crosstalk between adjacent sensors leading down to a 15.6% lower magnetic response at a close distance of 5 mm and an increasing sensor response with diminishing crosstalk effects at increasing distances up to 5 cm. The outermost sensors in the array exhibit significantly less crosstalk than the sensors located in the center of the array, and the vertically adjacent sensors exhibit a stronger crosstalk effect than the horizontally adjacent ones. Furthermore, we calculate the ratio between the electric and magnetic sensor responses as the sensitivity value and find near-constant sensitivities for each sensor, confirming a linear relationship despite magnetic crosstalk and the potential to simulate excitation sources and sensor responses independently.

Duct Effects on Acoustic Source Radiation

This paper presents a detailed numerical investigation of the effects of a semi-infinite hard-walled duct on the radiation due to point dipole sources rotating around the duct axis. In particular, this study is concerned with the effect of source position relative to the duct open end and its comparison with radiation in a free field. This study was motivated by the increasing tendency of ducted propellers and fans to be located close to the open end, relative to the acoustic wavelength. In this study, the conditions under which duct effects on source radiation may be neglected are established.

Surface plasmon enhanced THz spontaneous emission near metal-cored wires coated with graphene

This work deals with the theoretical study of the spontaneous emission of a point dipole source placed near a cylindrical structure composed of a central metallic core and a dielectric shell covered with graphene. We show that the spontaneous emission rate is dominated by coupling into surface plasmon (SP) modes on graphene, which present high values of the propagation constant along the cylinder symmetry axis. When the dielectric shell thickness is reduced, the plasmon dispersion is modified into a linear dependence and the modal SP propagation constant is further incremented, involving an improved field confinement. This effect is manifested as a redshift of the emission frequency spectrum for each individual cylindrical harmonic mode. We propose a simple model that provides us a link between the dispersion characteristics and maximum emission rate for each SP cylindrical mode.

A model for near-perfect transmission through lossy media with acoustic sources

Acoustic metamaterials exhibit effective material properties not found in naturally occurring media, and, as such, have received considerable attention for their potential applications in noise and vibration control, diagnostic imaging, and nonreciprocal transmission. Complementary acoustic metamaterials have been proposed as a means of compensating for the high impedance mismatches of aberrating layers that disrupt the acoustic field and hence distort acoustic images. More recently, a complementary acoustic metamaterial featuring active components was shown in principal to compensate for both the impedance mismatch and energy attenuation of lossy materials, but a physical realization of the concept has not yet been implemented. Here, we present results from a one-dimensional acoustic model showing how a plane wave incident on a lossy material can be augmented by point monopole and dipole sources to allow for near perfect transmission, thus rendering the lossy medium acoustically transparent. We present general expressions for source magnitudes that are dimensionless with respect to frequency, material thickness, and the background medium. We show that these results are consistent with three-dimensional finite element simulations, where the appropriate monopolar and dipolar forces are generated using finite dimensional velocity sources with real loudspeaker characteristics mounted in an acoustic waveguide.

Establishing the excitation field in tip-enhanced Raman spectroscopy to study nanostructures within two-dimensional systems

The optical field generated by a nanoplasmonic probe is revealed in tip-enhanced Raman spectroscopy (TERS) experiments. The TERS intensity profile of nano-objects smaller than the probe’s apex has a donut-like shape which resembles the magnitude of the field generated by a point-dipole source, being well described by the Dyadic Green’s function. Having prior knowledge on the excitation field generated by the TERS probe, we measured the width of shear solitons caused by lattice reconstruction in low-angle twisted bilayer graphene, a prominent platform for twistronics, and the extend of defect-induced light emission from graphene edges.

A better understanding of terahertz emission from semiconductor surfaces with a phased-array effect

We studied the phased-array effects in terahertz emission from semiconductor surfaces upon femtosecond laser illumination. A finite-difference time-domain simulation and experimental observation of the radiation patterns were utilized to examine the optical excitation at normal to the semiconductor surface and oblique angles of 30° and 45° with diameters of ∼10 and 275 μm. The results revealed that there exists a clear phased-array effect for the defocusing conditions. The larger diameter induced a pronounced directivity of the emission owing to the constructive interference of multiple point sources obeying the law of linear superposition, whereas the radiation patterns at smaller diameters were explained as the dipole point source. This finding, in addition to previous studies, will provide a better understanding and contribute to applications of terahertz emission spectroscopy in the field of semiconductor research and development.

Modelling of ducted noise sources in the proximity of acoustic liners

This paper focuses on modelling and predicting the acoustic field generated by ducted point sources in close proximity to an acoustic liner. Two analytical models are presented. The first model comprises a point monopole or dipole source in an infinite lined duct based on an existing Green’s function. The predictions are compared with classic solutions for a source over an infinite lined plane and with a high frequency asymptotic duct approximation. The second model extends the Green’s function to include a liner section of finite length connected to hard-wall extensions by using mode-matching techniques. The new model features the inclusion of point sources in the vicinity of an impedance discontinuity. The accuracy of this model is demonstrated by comparison with reference FE solutions. Both models indicate a significant impact of the source proximity to the liner surface in the source power output. These models offer insight on the source modification effects beyond the conventional approach in the design of liners solely based on acoustic absorption.

Phase response reconstruction for non‐minimum phase systems using frequency‐domain magnitude values

It is often more complicated to measure the phase response of a large system than the magnitude. In that case, one can attempt to use the Kramers-Kronig (KK) relations for magnitude and phase, which relates magnitude and phase analytically. The advantage is that then only the magnitude of the frequency response needs to be measured. We show that the KK relations for magnitude and phase may yield invalid results when the transfer function has zeros located in the right half of the complex s-plane, i.e. the system is non-minimum phase. In this paper we propose a method which enables to determine these zeros, by using specific excitation signals and measuring the resulting time responses of the system. The method is verified using blind tests among the authors. When the locations of the zeros in the right half of the complex s-plane are known, modified KK relations can be successfully applied to non-minimum phase systems. We demonstrate this by computing the phase response of the electric field, excited by a point dipole source inside a closed cavity with Perfect Electrically Conducting (PEC) walls. Also, the effects of simulated measurement noise are considered for this example.

Two-point magnetic field positioning algorithm based on rotating magnetic dipole

The detection and localization of magnetic targets based on magnetic field signals have become a research hotspot in recent years. In this study, a magnetic field model is established for the rotating propeller, which is regarded as a magnetic dipole. Moreover, a positioning algorithm of two-point shaft-rate magnetic field is proposed according to the rotating magnetic dipole. In addition, the positioning accuracy of the algorithm and the influence of some factors such as uneven magnetic distribution of magnetic dipole and measurement noise on the accuracy are analyzed. The simulation results indicate that the positioning accuracy increases with the increase in the circular trajectory radius of magnetic moment when there is an uneven magnetic distribution. The error caused by measurement noise increases with the increase in the distance between the measurement point and magnetic dipole source. Finally, the feasibility of the algorithm is verified through experiments.

Analytic Sinusoidal Steady-State Electromagnetic Field Expressions for the Ideal Veselago Lens

Analytic expressions for the sinusoidal steady-state electromagnetic field associated with an ideal Veselago lens illuminated by either a line-source (2D case) or an electric dipole point-source (3D case) are obtained based on power flow considerations and the imposition of appropriate boundary conditions. The fields are written in terms of the standard free-space Green’s function except that both incoming and outgoing waves are utilized. Expressions for the Poynting vector are also provided. Some components of the field suffer a complex-conjugate discontinuity across two power-transfer singularity planes , one within the lens and one through the external focus point, so named because all the power generated by the point-source transfers through a singular point on each plane. The internal power-transfer singular point and the illuminating point-source are equidistant from the first lens boundary, whereas the external singular point, i.e., the focus point, and the internal singular point are equidistant from the second lens boundary. These singularities represent different physical properties than the traditional source or sink singularities. The new interpretation based on the concise analytic field expressions clears up the physically objectionable interpretation of these singularities as representing new sources of energy: no new sources of energy exist in our representations.

3D tomographical imaging of localized surface plasmon resonances of cylindrical and dimeric spherical nanocrescents

A novel dimeric arrangement consisting of two spherical nanocrescents with tips pointing at each other in nanometer proximities is proposed. Hybridization of the plasmonic modes in close proximities leads to a 48% enhancement in Purcell factor inside the gap compared to the single spherical nanocrescent. A nanometer resolved tomographic cathodoluminescence (CL) spectroscopy combined with radiative local density of optical states (LDOS) calculation have been utilized to investigate bright surface plasmon modes of the nanocrescent in three dimensions. In this work, the retarded full Maxwell equations are solved using the boundary element method (BEM) with the electron beam and point dipole sources. Then, tomographic reconstructions are produced from a set of 2D CL maps in different orientations via the simultaneous iterative reconstruction technique (SIRT). The procedure of achieving CL tomograms and its connection to radiative LDOS are, first, validated for well-established cylindrical nanocrescent and then applied to the spherical nanocrescent dimer. The new proposed plasmonic dimer with promising radiative optical properties obtained in three dimensions shows enormous potential to play the main role in applications such as in vivo biosensing based on plasmonic-enhanced fluorescence and also surface-enhanced Raman spectroscopy (SERS).

Speed-up of scattered field formulation of FDTD method combining with time-domain EFIE by using spherical harmonic expansion of Green’s function

This paper proposes a scattered field formulation of the finite-difference time domain method (S-FDTD) combining with a time-domain electric field integral equation (TD-EFIE) based on surface equivalence theorem. The scattered field formulation enables us to reduce computation cost effectively for the case that scatterers are located at a far distance from field sources of electromagnetic wave since we need to calculate the scattered field component only on the region in the vicinity of the scatterers. Then it is required for the use of the scattered field formulation that the field source for the incident field component has to be expressed by any analytical solutions such as a point dipole source, plane wave, and so on. In this work, the scattered field formulation of the FDTD method is applied to cases that there are no analytical expressions of the field sources, in which the field source contains conductors such as antennas, combining with the TD-EFIE on a virtual surface which encloses the field sources. Then, it is known that calculation of the TD-EFIE itself is time consuming. This paper considers speed-up of the S-FDTD simulation based on the TD-EFIE using a spherical harmonic expansion of Green’s function of Helmholtz equation, additionally.

Inverse obstacle scattering for Maxwell’s equations in an unbounded structure

This paper is concerned with the electromagnetic scattering of a point source by a perfectly electrically conducting obstacle which is embedded in a two-layered lossy medium separated by an unbounded rough surface. Given a dipole point source, the direct problem is to determine the electromagnetic wave field for the given obstacle and unbounded rough surface; the inverse problem is to reconstruct simultaneously the obstacle and unbounded rough surface from the reflected and transmitted fields measured on a plane surface which is above and below the unknown objects, respectively. For the direct problem, its well-posedness is established and a new boundary integral equation is proposed. The analysis is based on the exponential decay of the dyadic Green function for Maxwell’s equations in a lossy medium. For the inverse problem, the global uniqueness is proved and a local stability is discussed. A crucial step in the proof of the stability is to obtain the existence and characterization of the domain derivative of the electric field with respect to the shape of the obstacle and unbounded rough surface.

Hydrogen motional disorder in crystalline iron group chloride dihydrates.

The principal components and the relative orientation of the 2H paramagnetic shift and quadrupolar coupling tensors have been measured for the MCl2·2D2O family of compounds, M = Mn, Fe, Co, Ni, and Cu, using the two-dimensional shifting-d echo nuclear magnetic resonance experiment in order to determine (1) the degree of unpaired electron delocalization and (2) the number and location of crystallographically distinct hydrogen sites around oxygen and their fractional occupancies. Expressions for the molecular susceptibility of 3d ion systems, where the spin-orbit coupling is a weak perturbation onto the crystal field, are derived using the generalized Van Vleck equation and used to predict molecular susceptibilities. These predicted molecular susceptibilities are combined with various point dipole source configurations modeling unpaired electron delocalization to predict 2H paramagnetic shift tensors at potential deuterium sites. The instantaneous deuterium quadrupolar coupling and shift tensors are then combined with parameterized motional models, developed for trigonally (M = Mn, Fe, Co, and Cu) and pyramidally (M = Ni) coordinated D2O ligands, to obtain the best fit of the experimental 2D spectra. Dipole sources placed onto metal nuclei with a small degree of delocalization onto the chlorine ligands yield good agreement with the experiment for M = Mn, Fe, Co, and Ni, while good agreement for CuCl2·2D2O is obtained with additional delocalization onto the oxygen. Our analysis of the salts with trigonally coordinated water ligands (M = Mn, Fe, Co, and Cu) confirms the presence of bisector flipping and the conclusions from neutron scattering measurements that hydrogen bonding to chlorine on two adjacent chains leads to the water molecule in the [M(D2O)2Cl4] cluster being nearly coplanar with O-M-Cl involving the shortest metal-chlorine bonds of the cluster. In the case of NiCl2·2D2O, the experimental parameters were found to be consistent with a motional model where the D2O ligands are pyramidally coordinated to the metal and undergo bisector flipping while the water ligand additionally hops between two orientations related by a 120° rotation about the Ni-O bond axis. The position of the three crystallographically distinct hydrogen sites in the unit cell was determined along with fractional occupancies. This restricted water ligand motion is likely due to van der Waals interactions and is concerted with the motion of neighboring ligands.

The sound field of a rotating dipole in a plug flow.

An analytical far field solution for a rotating point dipole source in a plug flow is derived. The shear layer of the jet is modelled as an infinitely thin cylindrical vortex sheet and the far field integral is calculated by the stationary phase method. Four numerical tests are performed to validate the derived solution as well as to assess the effects of sound refraction from the shear layer. First, the calculated results using the derived formulations are compared with the known solution for a rotating dipole in a uniform flow to validate the present model in this fundamental test case. After that, the effects of sound refraction for different rotating dipole sources in the plug flow are assessed. Then the refraction effects on different frequency components of the signal at the observer position, as well as the effects of the motion of the source and of the type of source are considered. Finally, the effect of different sound speeds and densities outside and inside the plug flow is investigated. The solution obtained may be of particular interest for propeller and rotor noise measurements in open jet anechoic wind tunnels.

TMz/TEz Vector Potentials Due to an Arbitrarily Oriented Dipole in 3‐D Space in the Presence of an Infinite Dielectric Cylinder

AbstractThis manuscript provides a new formulation for the classic electromagnetic scattering problem of a point dipole source in the presence of an infinite lossy dielectric cylinder embedded within a lossy space. The formulation is in terms of the TMz and TEz vector potential components (scalar potentials) Az and Fz for an arbitrary dipole orientation. A new Bessel function identity, apparently derived here for the first time, shows that the new solution is equivalent to the previous dyadic vector wave function solutions that solve for the electric and magnetic fields directly. Expressions for the scattered potential coefficients, for a dipole inside or outside the cylinder, can be reduced to a linear set of two equations that has a simple closed‐form solution. The new potential formulation should find application in the numerical solution of various problems in electromagnetics and geophysics involving tunnels or other structures that can be modeled as cylinders.

Controlling surface charge and spin density oscillations by Dirac plasmon interaction in thin topological insulators

We study the selective excitation at infrared and THz frequencies of optical and acoustic plasmonic modes supported by thin topological insulators. These modes are characterized by effective net charge or net spin density, respectively, and we study their excitation by combining many-body and electromagnetic calculations. We first show that non-locality can significantly modify the plasmonic response: it changes the energy of propagating plasmons up to tens of percent. We then discuss how, by changing the distance between a dipolar source and a semi-infinite 10 nm thin film, it is possible to control the excitation of acoustic and optical propagating plasmons, which can propagate over a distance of several plasmonic wavelengths. Furthermore, we consider 10 nm thin TI nanodisks and study the excitation of acoustic and optical localized plasmon modes by a point dipole source and plane wave illumination, respectively. The resonant plasmonic modes appear at frequencies that strongly depends on the size of the disk, and that can be potentially tuned by applying electrostatic gating to modify the Fermi Energy of the conductive 2-dimensional layer that forms at the interfaces of the TI. We observe a spectral shift from ~29 $\mu$m to ~34 $\mu$m by changing the Fermi Energy from 250meV to 350meV. Last, the electromagnetic energy of these plasmonics modes can be confined to very small regions, of effective volume ~(120 nm)^3 for the smaller disk considered, much less than the free-space wavelength cubed $\lambda$^3 ~(35000 nm)^3. The strong confinement is desirable for achieving very efficient coupling with nearby systems. Our detailed study thus shows that thin topological insulators are a promising system to control both the spin and charge oscillations associated with the plasmonic resonances, with possible applications to fast, compact and electrically-controlled spintronics devices.

Mitigating VHF Lightning Source Retrieval Errors

AbstractThe problem of inferring the location and time of occurrence of a very high frequency (VHF) lightning source emission from Lightning Mapping Array (LMA) network time-of-arrival (TOA) measurements is closely examined in order to clarify the cause of retrieval errors and to determine how best to mitigate these errors. With regard to this inverse problem, the previous literature lacks a comprehensive discussion of the associated forward problem. Hence, the forward problem is analyzed in this study to better clarify why retrieval errors increase with increasing source horizontal range and/or decreasing source altitude. Further insight is obtained by performing carefully designed Monte Carlo inversion simulations that provide specific retrieval error plots, which in turn lead to clear recommendations for mitigating retrieval errors. Based on all of the numerical results, the following strategies are recommended for mitigating retrieval errors (when possible, and without obstructing the line of sight): expand the horizontal extent of the LMA network, maximize the vertical sensor baseline by using mountainous terrain if available, and improve TOA measurement timing accuracy. Adding sensors to the network is relatively ineffective, unless of course the addition of sensors expands the horizontal extent and/or vertical baseline of the network. It is also shown how the standard retrieval method can be generalized by considering, in addition to the regular (unpolarized) point VHF source, the polarized transient very low frequency/low frequency (VLF/LF) electric point dipole source. Multiple observations (i.e., VHF arrival time and power, and VLF/LF arrival time and electric field amplitude) are simultaneously implemented into the new generalized mathematical framework, and the potential benefits are indicated.