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Die von den CIO- und IT-Leiter:innen-Runden vorgelegte IT-Strategie der bayerischen Hochschulen[1] definiert vor dem Hintergrund aktueller Rahmenbedingungen Handlungsempfehlungen für die fortlaufende kooperative und umfassende Erneuerung und Optimierung der IT. Sie unterstützt gleichzeitig die Digitalisierung der Hochschulen im Freistaat Bayern und ist Grundlage zur umfassenden und kontinuierlichen digitalen Transformation von Forschung und Lehre, Verwaltung, Gesellschaft und Wirtschaft im Freistaat Bayern. [1] Hochschulen bezeichnet hierbei alle „Hochschulen des Freistaates Bayern (staatliche Hochschulen) und die nichtstaatlichen Hochschulen“ nach Art. 1 BayHSchG.

Streamer and breakdown behavior of transformer oil under impulse voltage are investigated with experimental observations and simulation. The optical observation by using Intensified Charge-coupled Device (ICCD) fast snapshot technique indicates that streamer channel branching is of a stochastic nature. The 2-D axially symmetric modeling based on hydrodynamic drift-diffusion model is employed to depict the streamer dynamics of transformer oil. The experimental and simulation results reveal that bushy negative streamer requires higher excitation than filamentary positive streamer. Positive streamer travels faster than negative streamer from experimental estimation.

A two dimensional axis-symmetric fluid model is developed to present the streamer discharge process in transformer oil under positive and negative sub-microsecond impulse voltage, which is based on the hydrodynamic drift-diffusion approximation to simulate the dynamics of discharge inception and propagation. The streamers' spatial evolution with time are obtained, i.e., the distribution of electric field and space charge density under different conditions such as the amplitude of applied voltage and rise time. Simulation results verify the breakdown voltage has a distinct polar effect: negative streamer requires more than 2 times voltage threshold than that of positive one. It is revealed that the negative streamer, which usually has a bushy profile, is much more diffusive than the positive one. The magnitude and rise time of the applied voltage have evident influences on the shape and speed of the streamer in oil. The formation of positive or negative streamers are both characterized by a self-generated field enhancement at the head, and space charges enhance the front field of ionization region to assist the growing discharge channel to propagate longer. In terms of the generation of charged particles, it is considered that field-dependent molecular ionization predominates the charge generation mechanism of streamer discharge processes in transformer oil.

Analyses of unipolar charge transport phenomena in series, two-region composite dielectric systems are presented for planar, coaxial cylindrical, and concentric spherical electrode geometries. Space charge limited condition is applied at the charge emitting electrode for steady state and transient. Method of characteristics is used to convert the governing partial differential equations into a set of ordinary differential equations that allow analytical solutions as functions of time and space for volume and interfacial surface charge densities, charge trajectories, electric field intensity, and voltage drop across the electrodes. Solutions given in this paper can be used in design of high-voltage capacitors, pulsed-power transmission lines, and other apparatus. These solutions have also been used as a way of gaining confidence in the correctness and accuracy of our charge transport and prebreakdown models in two-phase insulation systems.

The Kerr electro-optic technique is used to investigate electric field and space charge effects using large Kerr constant propylene carbonate as the dielectric liquid with Kerr constant B≈1.22×10-12 m/V2. However, improved accuracy of this technique to measure space charge distributions remains an important goal. This paper describes inversion algorithm research on obtaining accurate space charge distributions from Kerr measurements. The original charge-coupled device (CCD) image of light intensity was converted into grayscale values for improved calculation. The grayscale CCD image was transformed into the frequency domain through a two-dimensional Fourier transformation. Butterworth filtering techniques were employed for edge enhancement algorithms. The most important parameter for calculation of space charge distributions are the integer fringe numbers n in the sequence of light and dark fringes measured by the CCD camera. Comparison of the improved algorithm and traditional methods for calculating electric field and space charge distributions between a pair of stainless steel parallel plate electrodes was investigated. The space charge dynamics were also calculated after processing the CCD images with some improvements. The results showed the electric field was essentially uniform at early times of pulsed high voltages. This uniformity indicated the absence of space charge during the early transient interval, while a bipolar charge injection was found in the central region between electrodes at later times. For longer times the electric field and space charge distributions slowly became uniform again. Grayscale processing and Butterworth filtering played an important role in the improvement of the algorithm for better image quality and edge enhancement.

Moisture equilibrium characteristics of oil-paper insulation systems are important for insulation condition evaluation of oil-filled transformers. This paper presents moisture equilibrium curves of mineral and vegetable oil-paper insulation systems obtained by both direct and indirect methods. A new mathematical model is proposed to determine moisture content in oil-paper insulation systems. Experiments on moisture equilibrium in oil-paper insulation systems are conducted. The results estimated through the mathematical model agree with experimental results. The moisture equilibrium curves between vegetable and mineral oil-paper insulation systems are presented and compared. The moisture content in the vegetable oil-impregnated paper is significantly smaller than that in the mineral oil-impregnated paper, when both types of insulating oils have the same moisture content.

Investigations during the last decade have shown that conductive nanoparticles can be dispersed in transformer oils to form nanofluids. Well-dispersed nanoparticles are capable of increasing the breakdown voltage of the oil under power frequency and lightning impulses. They also increase the inception voltages for partial discharge [1]. With increasing moisture content, reduction of the breakdown voltage of the nanofluid at power frequency is significantly smaller than that in the corresponding transformer oil [1]. The electrical and thermal properties of four types of nanofluid, prepared by dispersing Al2O3, Fe3O4, SiO2, and SiC nanoparticles in transformer oils, were described in [2]. It has also been reported that the thermal conductivity of such oil was enhanced by 8% when aluminum nitride nanoparticles were dispersed in it at a loading of 0.5% by weight, and its cooling capability was improved by about 20% [3]. An electrodynamic model has been developed describing streamer formation in transformer oil-based nanofluids, which presents generation, recombination, and transport equations for each charge carrier type [4]. Vegetable insulation oils are based on natural ester oils, which are environmentally friendly and fire resistant [4]���[9]. At the moment, little is known about the preparation of nanofluids using natural ester oils and their dielectric, breakdown, and aging properties. Surface modification of nanoparticles is a very effective procedure to avoid nanoparticle agglomeration in insulating nanofluids [10]���[14]. However, the surface modification procedures used for mineral oils cannot be applied to vegetable oils because of their very different molecular structures. We therefore investigated new approaches to the preparation of vegetable oil-based nanofluids. This paper presents some of the results of a study of the breakdown voltages and dielectric properties of a vegetable oil-based nanofluid. The nanofluid was prepared by dispersing Fe3O4 nanoparticles in a vegetable insulation oil obtained from a laboratory at Chongqing University. Oleic acid was used for surface modification of the nanoparticles.

In order to study the flashover mechanism of polluted insulators under AC voltage, a new arc propagation criterion which is based on an arc root voltage gradient is proposed. This criterion can explain the variation of the arc root voltage gradient in the arc propagation process. Based on this criterion, a new distributed parameter electrical circuit flashover model of polluted insulators is presented. The arc channel is considered as an equivalent distributed parameter circuit model instead of using the arc voltage-gradient equation. The parameters of the arc model are obtained from the electromagnetic field distribution of the arc and the gas discharge theories. The arc root is considered as parallel paths including the polluted layer. The variation of the voltage on the arc root is related to the capability of arc propagation. This model takes the microscopic mechanism of arc root ionization into consideration, which can improve the accuracy of the flashover model. The results obtained from the presented model are in good agreement with other mathematical and experimental results., National Basic Research Program of China (973 Program) (2009CB724507), National 111 Project of China (B08036)

An electrothermal hydrodynamic model is presented to evaluate effects of the applied lightning impulse voltage parameters such as polarity, magnitude, and rise time on the initiation and propagation of the streamers formed in an IEC defined needle-sphere electrode geometry filled with transformer oil. Instantaneous velocity, column diameter, head curvature, maximum electric field, and the volume charge density have been investigated as the main characteristics of the streamer. Modeling results indicate that greater applied voltage peak amplitudes form streamers with higher velocity, greater head curvatures, and thicker columns. The bushy negative streamers usually initiate at almost twice the applied voltage magnitude and propagate slower than filamentary positive streamers. Results also show that in transformer oil at the same impulse voltage peak amplitude, shorter rise times create thicker positive and negative streamers., ABB Corporate Research Center (Vasteras, Sweden)

Comprehensive modeling and analysis is presented for the charge generation, recombination, attachment, and transport of positive ions, negative ions, and electrons in transformer oil between a positive high voltage sharp needle electrode and a large spherical ground electrode. Case studies show that a key mechanism in streamer development is field ionization, which is the direct ionization of molecules due to action of the electric field. Due to the high mobility electrons, which are about 1 × 105 times more mobile than the positive ions, the newly generated electrons quickly exit the high field ionization zone towards the anode leading to the development of a net positive space charge peak which subsequently creates an electric field enhancement in the oil. The process is driven by the applied high voltage creating temporally dynamic space charge and electric field distributions that develop an ionizing wave that drives streamer development. The pre-breakdown modeling and analysis elucidates the development of different streamer modes in transformer oil. In particular, the analysis focuses on mechanisms driving filamentary fast mode streamers discussed in the literature. The results demonstrate that streamer modes arise in transformer oil due to the ionization of different families of hydrocarbon molecules (i.e., aromatic, naphthenic, and paraffinic) at increasing electric field levels (or applied voltages). Ionization of the low concentration aromatic molecules in transformer oil, that generally have lower ionization energies than naphthenic/paraffinic molecules, leads to the propagation of streamers with velocities on the order of 1 km/s. As the applied voltage is increased, the ionization of the main hydrocarbon molecules in transformer oil, high concentration naphthenic/paraffinic molecules, dominates producing high electric field levels and space charge at the streamer tip. This results in the propagation of a very fast streamer with velocities on the order of 10 km/s. Furthermore, these streamers have protrusion spacing in the approximate range of 20-100 μm in ~36 ns like those of electrohydrodynamic instability of charged jets that may be the origin of streamer branching. A preliminary model based on earlier electrohydrodynamic stability analysis is presented that predicts protrusion spacing in the approximate range of 7-30 μm with growth rate 2.5-5 μs.

Analytical solutions to the continuum spin and linear velocities as well as the two-dimensional volume flow rate are solved and presented for negative electrorheological Poiseuille flows induced by internal micro-particle Quincke electrorotation using continuum anti-symmetric/couple stress theories with finite spin viscosities and small spin velocities. Theoretical predictions on the volume flow rate and linear velocity profile obtained from the continuum finite spin viscosity small spin velocity theory presented herein agree very well with the experimental measurements in the parametric regime of low or small driving pressure gradients as reported in current literature.

This paper presents a numerical method, combined the finite element method in the spatial domain with the finite difference time domain, to calculate the transient impulse response of grounding systems considering the ionization phenomenon. In this numerical method, space-time variable soil resistivity is used to simulate the soil ionization phenomenon where soil resistivity is controlled according to its relationship with the local instantaneous value of the electric field and no a priori hypothesis on the geometrical shape of the ionized region around the electrodes is necessary. Based on the widely accepted principle of dimensional similarity, this paper makes simulated experimental investigations on the impulse-current dispersal regularity of grounding electrodes with various structures. The proposed numerical scheme is validated by comparing computed results with experimental results and simulation results in literature. Based on the measurement and simulation results, the impulse response regularity of grounding electrodes is discussed and the effect of ionization on human and installations safety is reported.

The path of the lightning discharge shows characteristics of branching and tortuosity, and the lightning shielding failure of the transmission line also has statistical features. In this paper, an improved leader progression model using a fractal approach is introduced, which considers both the deterministic and stochastic features of the downward lightning leader. Based on the proposed model, the statistical features of lightning shielding failure on transmission lines will be analyzed. First, the fractal dimensions of the downward lightning trajectory in simulation are calculated and the value of breakdown probability constant η will be determined based on the fractal dimensions of natural lightning. Second, using the proposed model the shielding failure probability contour around the transmission line space is also obtained and used to describe the shielding failure zone and then the contour curve is applied to analyzing the characteristics of transmission lines that suffer from direct lightning strokes. Moreover, a method to estimate the lightning shielding failure rate of transmission lines is put forward, and the influence of the line height and the shielding angle of ground wires on the shielding failure rate of transmission lines are also investigated.

Ferrofluid spin-up flow is studied within a sphere subjected to a uniform rotating magnetic field from two surrounding spherical coils carrying sinusoidally varying currents at right angles and 90° phase difference. Ultrasound velocimetry measurements in a full sphere of ferrofluid shows no measureable flow. There is significant bulk flow in a partially filled sphere (1–14 mm/s) of ferrofluid or a finite height cylinder of ferrofluid with no cover (1–4 mm/s) placed in the spherical coil apparatus. The flow is due to free surface effects and the non-uniform magnetic field associated with the shape demagnetizing effects. Flow is also observed in the fully filled ferrofluid sphere (1–20 mm/s) when the field is made non-uniform by adding a permanent magnet or a DC or AC excited small solenoidal coil. This confirms that a non-uniform magnetic field or a non-uniform distribution of magnetization due to a non-uniform magnetic field are causes of spin-up flow in ferrofluids with no free surface, while tangential magnetic surface stress contributes to flow in the presence of a free surface. Recent work has fitted velocity flow measurements of ferrofluid filled finite height cylinders with no free surface, subjected to uniform rotating magnetic fields, neglecting the container shape effects which cause non-uniform demagnetizing fields, and resulting in much larger non-physical effective values of spin viscosity η ′∼10 −8 −10 −12 N s than those obtained from theoretical spin diffusion analysis where η ′≤10 −18 N s. COMSOL Multiphysics finite element computer simulations of spherical geometry in a uniform rotating magnetic field using non-physically large experimental fit values of spin viscosity η ′∼10 −8 −10 −12 N s with a zero spin-velocity boundary condition at the outer wall predicts measureable flow, while simulations setting spin viscosity to zero ( η ′ = 0) results in negligible flow, in agreement with the ultrasound velocimetry measurements. COMSOL simulations also confirm that a non-uniform rotating magnetic field or a uniform rotating magnetic field with a non-uniform distribution of magnetization due to an external magnet or a current carrying coil can drive a measureable flow in an infinitely long ferrofluid cylinder with zero spin viscosity ( η ′ = 0).

This paper proposes a complete and effective smart over-voltage monitoring and identification system. In recent years, smart grids are of the greatest interest in power system research. One of the main features of smart grid is their self-healing, which can continuously carry out online self-evaluation, discover existing faults, and correct them immediately. The over-voltage smart monitoring-identification-suppression systems play a key role in the construction of self-healing grids. In this paper, eight kinds of common over-voltage are discussed and analyzed. The S-transform algorithm is used to extract features of over-voltage. Aiming at the main features of each kind of over-voltage, six different characteristic quantities are proposed. A well designed fuzzy expert system and a support vector machine are employed as the classifiers to build a two-step identification model. The accuracy of the identification system is verified by field records. Results show that this system is feasible and promising for real applications., National Basic Research Program of China (973 Program) (2009CB724504), National 111 Project of China (B08036)

The multi-needle electrospinning system is a convenient way to produce fibers with special structures such as core-shell morphologies at a high production rate. In this paper, a specially designed multi-needle electrospinning system is presented. The spinnerets were built-up with an equilateral hexagon array. Each set of 3 needles of the spinnerets were distributed as an equilateral triangle. A coaxial shield ring was used to create an approximate uniform electric field near the tips of the needles and to restrict the collection area. The simulation results also show that the outside needles can help to create a more uniform electric field near the inside tips of the needles and restrict the path of the inside jets, which works almost the same as the additional shield ring. Based on the simulation results, several multi-needle systems were tested. A 7 cm diameter shield ring was used in a 7 needle system, a 9 cm diameter shield ring was used in a 19 needle system and a 10.5 cm diameter shield ring was used in a 37 needle system. Polyethylene Oxide (PEO) aqueous solution was used as the test solution in experiments. The electrospinning results demonstrated that the use of multi-needle spinnerets is robust and that uniform nanofibers can be produced. The more needles used, the smaller the mean fiber diameter for larger mean electric field strengths. These distributions of needles show the scale up possibility of special structure electrospun nanofiber manufacturing.

2011 March 1, In the presence of alternating-sinusoidal or rotating magnetic fields, magnetic nanoparticles will act to realign their magnetic moment with the applied magnetic field. The realignment is characterized by the nanoparticle's time constant, τ. As the magnetic field frequency is increased, the nanoparticle's magnetic moment lags the applied magnetic field at a constant angle for a given frequency, Ω, in rad/s. Associated with this misalignment is a power dissipation that increases the bulk magnetic fluid's temperature which has been utilized as a method of magnetic nanoparticle hyperthermia, particularly suited for cancer in low-perfusion tissue (e.g., breast) where temperature increases of between 4 and 7 degree Centigrade above the ambient in vivo temperature cause tumor hyperthermia. This work examines the rise in the magnetic fluid's temperature in the MRI environment which is characterized by a large DC field, B0. Theoretical analysis and simulation is used to predict the effect of both alternating-sinusoidal and rotating magnetic fields transverse to B0. Results are presented for the expected temperature increase in small tumors (approximately 1 cm radius) over an appropriate range of magnetic fluid concentrations (0.002–0.01 solid volume fraction) and nanoparticle radii (1–10 nm). The results indicate that significant heating can take place, even in low-field MRI systems where magnetic fluid saturation is not significant, with careful selection of the rotating or sinusoidal field parameters (field frequency and amplitude). The work indicates that it may be feasible to combine low-field MRI with a magnetic hyperthermia system using superparamagnetic iron oxide nanoparticles., National Institutes of Health (U.S.)

This work examines feasibility, practical advantages, and disadvantages of a combined MRI/magnetic particle hyperthermia (MPH) system for cancerous tumor treatment in low perfusion tissue. Although combined MRI/hyperthermia systems have been proposed and constructed, the current proposal differs because the hyperthermia system would be specifically designed to interact with the magnetic nanoparticles injected at the tumor site. The proposal exploits the physical similarities between the magnetic nanoparticles currently employed for MPH and those used as superparamagnetic iron oxide (SPIO) contrast agents in MR imaging. The proposal involves the addition of a rotating magnetic field RF hyper- thermia source perpendicular to the MRI B0 field which operates in a similar manner to the MRI RF excitation fi eld,B1, but at significantly higher frequency and field strength such that the magnetic nanoparticles are forced to rotate in its presence. This rotation is the source of increases in temperature which are of therapeutic benefit in cancer therapy. For rotating magnetic fields with amplitudes much smaller than B0, the nanoparticles' suspension mag- netization rapidly saturates with increasing B0. Therefore, the proposal is best suited to low-field MRI systems when magnetic saturation is incomplete. In addition, careful design of the RF hyperthermia source is required to ensure no physical or RF interference with the B1 field used for MRI excitation. Notwithstanding these caveats, the authors have shown that localized steady-state temperature rises in small spherical tumors of up to 108 Ca re con- ceivable with careful selection of the nanoparticle radius and concentration, RF hyperther- mia field amplitude and frequency. 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part A 36A: 36-47, 2010.

Traditional methods of measuring magnetization in magnetic fluid samples, such as vibrating sample magnetometry (VSM), are typically limited to maximum field strengths of about 1 T. This work demonstrates the ability of MRI to measure the magnetization associated with two commercial MRI contrast agents at 3 T by comparing analytical solutions to experimental imaging results for the field pattern associated with agents in cylindrical vials. The results of the VSM and fitted MRI data match closely. The method represents an improvement over VSM measurements since results are attainable at imaging field strengths. The agents investigated are Feridex, a superparamagnetic iron oxide suspension used primarily for liver imaging, and Magnevist, a paramagnetic, gadolinium-based compound used for tumors, inflammation and vascular lesions. MR imaging of the agents took place in sealed cylindrical vials in the presence of a surrounding volume of deionized water where the effects of the contrast agents had a measurable effect on the water's magnetization in the vicinity of the compartment of contrast agent. A pair of phase images were used to reconstruct a B0 fieldmap. The resultant B0 maps in the water region, corrected for shimming and container edge effects, were used to predict the agent's magnetization at 3 T. The results were compared with the results from VSM measurements up to 1.2 T and close correlation was observed. The technique should be of interest to those seeking quantification of the magnetization associated with magnetic suspensions beyond the traditional scope of VSM. The magnetization needs to be sufficiently strong (Ms ≳ 50 Am2/kg Fe for Feridex and Xm ≳ 5 × 10−5 m3/kg Gd for Magnevist) for a measurable dipole field in the surrounding water. For this reason, the technique is mostly suitable for undiluted agents. Copyright © 2009 John Wiley & Sons, Ltd.

This paper is aimed at exploring the effect of cortical brain atrophy on the currents induced by transcranial magnetic stimulation (TMS). We compared the currents induced by various TMS conditions on several different MRI derived finite element head models of brain atrophy, incorporating both decreasing cortical volume and widened sulci. The current densities induced in the cortex were dependent upon the degree and type of cortical atrophy and were altered in magnitude, location, and orientation when compared to healthy head models. Predictive models of the degree of current density attenuation as a function of the scalp-to-cortex distance were analyzed, concluding that those which ignore the electromagnetic field-tissue interactions lead to inaccurate conclusions. Ultimately, the precise site and population of neural elements stimulated by TMS in an atrophic brain cannot be predicted based on healthy head models which ignore the effects of the altered cortex on the stimulating currents. Clinical applications of TMS should be carefully considered in light of these findings.

Objectives Interest in transcranial direct current stimulation (tDCS) in clinical practice has been growing, however, the knowledge about its efficacy and mechanisms of action remains limited. This paper presents a realistic magnetic resonance imaging (MRI)-derived finite element model of currents applied to the human brain during tDCS. Experimental design Current density distributions were analyzed in a healthy human head model with varied electrode montages. For each configuration, we calculated the cortical current density distributions. Analogous studies were completed for three pathological models of cortical infarcts. Principal observations The current density magnitude maxima injected in the cortex by 1 mA tDCS ranged from 0.77 to 2.00 mA/cm2. The pathological models revealed that cortical strokes, relative to the non-pathological solutions, can elevate current density maxima and alter their location. Conclusions These results may guide optimized tDCS for application in normal subjects and patients with focal brain lesions.

New flows and instabilities are demonstrated for magnetic fluids and by dual analogy to dielectric fluids. If a fluid drop is contained in a thin gap between two glass plates (Hele–Shaw cell) with a simultaneously applied in-plane rotating field and a DC axial field, smooth spirals or an abrupt transformation to many small droplets can occur. A preliminary minimum magnetization and surface energy analysis is presented to model the abrupt transformation in ferrofluids. An analysis of effective DC magnetoviscosity is also presented for planar Couette flow with an applied uniform DC field transverse to a duct axis with the effective magnetoviscosity and flow spin velocity calculated as a function of field strength. Related Couette viscometer measurements of ferrofluid viscosity show zero and negative magnetoviscosity values for rotating magnetic fields.

Existence of a previously unreported flow of small current to and from a metallic electrode surface during transient air pressure change is reported and described. Measurements of transient current, humidity, and temperature with transient air pressure change are presented. The flow of electrode current is shown to be not due to capacitance and input offset voltage changes, since the calculated value is on the order of 10 −16 A which is much less than the measured currents of order 10 −13 A. Measurements using air of different levels of humidity and with dry nitrogen confirm that zero moisture results in essentially zero current, and that the amplitude of the current increases with increasing humidity.

Distilled water, tap water, Sargasso Sea water and 0.9% NaCl solution contained in a Nalgene 30 ml polyallomer centrifuge tube are found to maintain distinct negative non-zero equilibrium charges as measured within a grounded patented Faraday cage. Here, the Faraday cage is composed of two identical in-line hollow, gold-plated Faraday cup electrodes which enclose the sample that moves between them during each measurement under computer control. Charge measurements using various electrometers were conducted to rule out the possibility of false instrument readings due to input offset voltage and other experimental effects. Charge, voltage and capacitance are presented as a function of the water dielectric temperature.

This paper explores how transcranial magnetic stimulation (TMS) induced currents in the brain are perturbed by electrical and anatomical changes following a stroke in its chronic stage. Multiple MRI derived finite element head models were constructed and evaluated to address the effects that strokes can have on the induced stimulating TMS currents by comparing stroke models of various sizes and geometries to a healthy head model under a number of stimulation conditions. The TMS induced currents were significantly altered for stimulation proximal to the lesion site in all of the models analyzed. The current density distributions were modified in magnitude, location, and orientation such that the population of neural elements that are stimulated will be correspondingly altered. The current perturbations were minimized for conditions tested where the coil was far removed from the lesion site, including models of stimulation contralateral to the lesioned hemisphere. The present limitations of TMS to the peri-lesional cortex are explored, ultimately concluding that conventional clinical standards for stimulation are unreliable and potentially dangerous predictors of the site and degree of stimulation when TMS is applied proximal to infarction site.

The depth of penetration of periodic-field magnetoquasistatic sensors depends on two factors: the skin depth into the material under test and the spatial wavelength of the imposed periodic magnetic field. In applications where high depth of penetration is desirable, the second factor may result in the need for sensors with impractically large dimensions. We describe a way to overcome this problem by generating a field whose effective spatial wavelength is on the order of the sensor length. The magnetic field is shaped by a distributed primary winding that consists of multiple winding segments, with the total current amplitude in each segment following a sinusoidal envelope function. The effective spatial wavelength of the imposed magnetic field may be changed dynamically by changing the excitation current pattern in the primary windings. From a modeling perspective, an advantage of this kind of magnetic field excitation is that the drive current distribution is known from the beginning, since the width of the individual windings is small compared to the wavelength and may be approximated as being infinitely narrow. This greatly simplifies numerical computation, since it makes it possible to apply fast discrete Fourier transform methods directly. We discuss first sensors with Cartesian geometry. We then discuss cylindrical geometry sensors whose models use fast Hankel transforms.

Major recent advances Magnetic fluid rheology and flow advances in the past year include: (1) generalization of the magnetization relaxation equation by Shliomis and Felderhof and generalization of the governing ferrohydrodynamic equations by Rosensweig and Felderhof; (2) advances in such biomedical applications as drug delivery, hyperthermia, and magnetic resonance imaging; (3) use of the antisymmetric part of the viscous stress tensor due to spin velocity to lower the effective magnetoviscosity to zero and negative values; (4) and ultrasound velocity profile measurements of spin-up flow showing counter-rotating surface and co-rotating volume flows in a uniform rotating magnetic field. Recent advances in magnetic fluid rheology and flows are reviewed including extensions of the governing magnetization relaxation and ferrohydrodynamic equations with a viscous stress tensor that has an antisymmetric part due to spin velocity; derivation of the magnetic susceptibility tensor in a ferrofluid with spin velocity and its relationship to magnetically controlled heating; magnetic force and torque analysis, measurements, resulting flow phenomena, with device and biomedical applications; effective magnetoviscosity analysis and measurements including zero and negative values, not just reduced viscosity; ultrasound velocity profile measurements of spin-up flow showing counter-rotating surface and co-rotating volume flows in a uniform rotating magnetic field; theory and optical measurements of ferrofluid meniscus shape for tangential and perpendicular magnetic fields; new theory and measurements of ferrohydrodynamic flows and instabilities and of thermodiffusion (Soret effect) phenomena.

We study the response of magnetic nanoparticle suspensions (ferrofluids) to uniform rotating magnetic fields generated by a two-pole three-phase magnetic induction motor stator winding. Measurements of the torque required to rotate a polycarbonate spindle submerged in ferrofluid subjected to co-rotating and counter-rotating fields yield experimental observations of negative magnetoviscosity in a cylindrical Couette geometry, conceptually similar to the observations of Bacri et al. (Phys. Rev. Lett. 75 (1995) 2128) in a Poiseuille flow under an oscillating magnetic field. Further measurements are presented for the torque required to restrain a spindle when it is (i) entirely filled with ferrofluid, (ii) entirely surrounded with ferrofluid, and (iii) both entirely filled and surrounded with ferrofluid. Some of the results for the spindle either entirely filled or entirely surrounded with ferrofluid are compared to theoretical expressions obtained from the ferrohydrodynamic equations using a rigorous regular perturbation expansion in the small parameter Ω τ , where Ω is the applied field frequency and τ is the effective magnetic relaxation time of the suspension.

An optical system using reflections of a narrow laser beam to measure the height and shape of a ferrofluid meniscus in response to uniform applied magnetic fields finds that meniscus height on a vertical flat wall decreases in horizontal applied field and increases in vertical applied field. An approximate energy minimization analysis predicts meniscus height in directional agreement with measurements. This study is a first step in calculating the tangential surface force acting in flows where magnetization magnitude and direction lag a changing magnetic field direction, and the meniscus shape is magnetically perturbed.

This review paper focuses on interdigital electrodes-a geometric structure encountered in a wide variety of sensor and transducer designs. Physical and chemical principles behind the operation of these devices vary so much across different fields of science and technology that the common features present in all devices are often overlooked. This paper attempts to bring under one umbrella capacitive, inductive, dielectric, piezoacoustic, chemical, biological, and microelectromechanical interdigital sensors and transducers. The paper also provides historical perspective, discusses fabrication techniques, modeling of sensor parameters, application examples, and directions of future research.

Measurements of magnetic-field-induced torque in applied uniform rotating magnetic fields are presented and compared to theoretical analyses for water- and oil-based ferrofluids. These experiments measure the viscous torque on the inner wall of a stationary hollow polycarbonate spindle that is completely filled with ferrofluid and attached to a viscometer functioning as a torque meter. The spindle remains stationary and is centered inside a three-phase AC 2-pole motor stator winding, creating uniform time-varying rotating magnetic fields. The viscous torque is measured as a function of magnetic field amplitude, frequency, and direction of rotation. These measurements demonstrate that ferrofluid flow and torque are present even in the absence of free surfaces and agree with a recently derived analysis of the torque during spin-up flow of ferrofluids.

Measurement of multilayer material properties with fringing electric field dielectrometry sensors requires the processing of terminal admittance data from multiple electrode pairs. The choice of the algorithmic approach for data analysis is critical because of a high sensitivity to small measurement uncertainties. While individual channel uncertainties are usually within a reasonable instrumentation range, measurement of properties of multiple material layers fails unless special techniques are used to prevent numerical instabilities. This paper demonstrates the mechanism of uncertainty in a successive stair-step property estimation in which the estimates of one layer's properties are used to estimate the subsequent layer's properties. Three-wavelength measurements of dielectric permittivity and conductivity illustrate the problem in two-layer experimental setups. Limitations of the stair-step approach and possible future improvements are discussed.

Interdigital dielectrometry is used for non-destructive evaluation of material properties. Spectroscopic analysis of frequency-dependent dielectric properties increases sensitivity and selectivity of measurement of many physical variables (e.g. moisture, porosity, density, and aging status) that are correlated with changes of dielectric properties. This paper presents a convenient algorithmic methodology of analysis and visualization of properties of frequencydispersive materials. Analytical expressions for the simple case of a Maxwell parallel-plate capacitor are developed and contrasted withparametric numerical models of interdigital sensor probes. The visualization of these parametric spaces helps develop intuition and understanding of dielectric measurement results in more complex cases. In particular, loss of sensitivity in different frequency regimes, interpretation of negative transcapacitance values, and byproducts of fast inversion procedures are discussed. Experimental data withfluid and solid dielectric samples is presented for illustration of algorithms, proof of principle, and a basic discussion of multiple wavelength measurement sensitivity. References to descriptions of applications of the presented techniques to practical problems are provided. r 2002 Elsevier Science B.V. All rights reserved.

Closed form analytical equations are obtained for ferrofluid flow, spin velocity and volumetric flow rate in alternating and rotating uniform magnetic fields including the effects of spin viscosity.

The Kerr electro-optic field mapping technique has been used to study space charge effects in HV pulsed propylene carbonate (C/sub 4/H/sub 6/O/sub 3/) using stainless steel parallel-plane, aluminum blade-plane, and aluminum point-plane electrodes. Measurements presented here were taken using a circular polariscope with aligned polarizers at room temperature (T /spl sim/ 20 to 27/spl deg/C). Average peak space charge free electric field strengths were E/sub 0/ /spl sim/ 52 kV/cm for the parallel-plane electrodes, E/sub 0/ /spl sim/ 210 kV/cm at the blade electrode tip and E/sub 0/ /spl sim/ 240 kV/cm at the point electrode tip. The measurement times ranged from 10 /spl mu/s to 1 ms. For the parallel-plane electrodes, the light intensity pictures were analyzed along a line between electrodes in the center of the electrode gap and for the blade-plane and point-plane electrodes the light intensity pictures were analyzed both from the tip of the blade and point, to the plane, and along the plane surface. From the light intensity distribution for the parallel-plane and blade-plane electrodes, the electric field and space charge density were calculated as a function of position and time. The calculated space charge distributions show both positive and negative charge injections from the electrodes as well as bulk charge dissociation and recombination. The light intensity was recorded on Polaroid film for the parallel-plane, blade-plane and point-plane electrode measurements and also with a CCD camera for the point-plane electrodes. A CCD camera was used for the point-plane electrodes to resolve more accurately the gray-scale light intensity distribution because of the short optical path length. All the HV Kerr electro-optic measurements showed significant space charge effects in propylene carbonate because the light intensity distributions differed significantly from the calculated Kerr effect patterns from numerical solutions to Laplace's equation under space-charge free conditions.

Previous models of plane-Poiseuille flow of ferrofluids in alternating and rotating magnetic fields are extended by including the effects of spin diffusion and planar Couette flow. Accurate modeling of this problem is required for the design of experiments to determine key parameters for previously reported anomalous forward and backward ferrofluid pumping behavior in alternating and rotating magnetic fields. The previously derived zero-spin-viscosity analysis predicted multi-valued and singular flow solutions with possible zero and negative effective magnetoviscosity. This analysis calculates analytical expressions for the translational and spin velocity profiles, vorticity profiles, volumetric flow rate, and the shear force on a moving duct surface, comparing the effects of boundary conditions of zero spin velocity and spin velocity equal to half the vorticity at the duct walls. The analysis shows that the single singularity in flow behavior corresponding to zero magnetoviscosity in the zero-spin-viscosity analysis expands to multiple possible flow singularities for the nonzero spin viscosity case. Simple representative shearing experiments are proposed to differentiate between the zero and nonzero spin viscosity solutions, to calculate the values of key viscous parameters, and to show how wall boundary conditions can be determined from shear stress measurements.

Recently we used the onion peeling method to reconstruct the axisymmetric electric field distribution of point/plane electrodes from Kerr electro-optic measurements. The method accurately reconstructed the electric field from numerically generated data. However in the presence of experimental noise the performance was less satisfactory. The measurements were especially noisy and unstable near the needle tip which is also the interesting region since most charge injection initiates here. We develop a new algorithm for Kerr electro-optic reconstruction of space charge in axisymmetric point/plane electrode geometries. The algorithm is built on the finite element method (FEM) for Poisson's equation and will be called finite element based Kerr electro-optic reconstruction (FEBKER) hereafter. FEBKER calculates the space charge density directly to avoid the numerical problems associated with taking the divergence of the electric field, uses single parameter light intensity measurements to enable transient analysis, which otherwise is difficult since multiple parameter intensity measurements are slow due to the rotation of polarizers, and is capable of reconstruction even when the number and/or position of measurements are limited by the electrodes and/or the experimental setup. The performance of the algorithm is tested on synthetic Kerr electro-optic data obtained for an axisymmetric point/plane electrode geometry in transformer oil with specified space charge density distributions. The impact of experimental error is analyzed by incorporating random error to the synthetic data. Regularization techniques that decrease the impact of experimental error are applied. In principle FEBKER is applicable to arbitrary three-dimensional geometries as well.

It is important to monitor the moisture content of transformer oil in a transformer. One parameter of particular interest is the moisture solubility of transformer oil. It has been reported that transformer oils under different conditions have different solubility. Measurements of solubility for four different types of conditioned oil are presented in this paper: fresh Shell Diala AX oil, lab-aged Shell Diala A oil, Texas Utility used transformer oil, and Ramapo Substation used transformer oil. To avoid the difficulty of achieving full saturation, this paper proposes an alternative method of measuring the moisture solubility in transformer oil using a relative humidity sensor. It utilizes the linearity between the relative humidity of the oil and the moisture content of the oil, to measure the solubility indirectly. The measured results of fresh oil, lab-aged oil, and the Texas Utility oil are very close, and only Ramapo oil shows different sensor response characteristics and solubility.

Magnetic field based micro/nanoelectromechanical systems (MEMS/NEMS) devices are proposed that use 10 nm diameter magnetic particles, with and without a carrier fluid, for a new class of nanoduct flows, nanomotors, nanogenerators, nanopumps, nanoactuators, and other similar nanoscale devices. A few examples of macroscopic ferrohydrodynamic instabilities that result in patterns, lines, and structures are shown that can be scaled down to sub-micron dimensions.

Kerr electro-optic fringe patterns have long been used to study space charge injection and transport phenomena in highly birefringent materials such as nitrobenzene, Most past experimental work has been limited to 1 or 2-dimensional geometries where the electric field magnitude and direction have been constant along the light path such as two concentric or parallel cylinders or parallel plate electrodes. For these geometries the extrema in the fringe patterns can be used directly to find the electric field magnitude and direction. In this work we extend the fringe based Kerr electro-optic measurement technique to a point/plane electrode geometry which often is used in HV research to create large electric fields for charge injection at known location and at reasonable voltages. We calculate theoretical Kerr electro-optic fringe patterns for this point/plane electrode geometry with and without space charge distributions, for which the electric field magnitude and direction vary along the light path. We particularly compare the calculated space charge free optical patterns for the point/plane electrodes to the optical patterns of the 2-dimensional analog blade/plane geometry. We underline the differences and study how these fringe patterns can be used to reconstruct the axisymmetric electric field components in practice.

In this paper we extend the continuum model for interdigital dielectrometry sensors and propose a new, direct technique for estimating material electrical properties from measurements. Interdigital sensors consist of alternating pairs of long, thin electrodes on a plane. An ideal model assumes that the periodic structure extends to infinity and the electrodes have no thickness. We extend this ideal analysis to account for the physical thickness of the electrodes. We also present the model in a matrix form which is amenable to linear algebraic analysis techniques. In particular, the 'inverse problem' of estimating material properties is formulated as a generalized Eigenvalue problem, which avoids the convergence problems of previous iterative algorithms.

The response of a three-wavelength interdigital dielectrometry sensor to the moisture diffusion process in oil-impregnated transformer pressboard has been simulated using an empirical relationship between the moisture concentration and the dielectric properties of the pressboard. A benchmark test of the moisture diffusion process has been developed with the purpose of comparing alternative parameter estimation algorithms used in /spl omega/-/spl kappa/ (frequency wavenumber) dielectrometry. The results of simulations highlight characteristic features of the multi-wavelength sensor response, such as the sensor response delay from the start of the transient moisture diffusion process as a function of sensor wavelength, the influence of moisture boundary conditions, and the relation between the signal magnitude and variations of dielectric properties. One of the parameter estimation algorithms, linear calibrated admittance-based estimation (LCABE), has been applied to both simulated and measured data. Adequate performance of the LCABE approach in the absence of strong discontinuities of dielectric properties in the electric field penetration region is demonstrated and contrasted to an electrically shielded region case, in which the signal response becomes nonlinear. The proposed approach offers significant potential for the measurement of diffusion processes in various dielectrics, especially for cases with highly irregular geometry or material structure. Measurement results from moisture diffusion process monitoring are included. Parameter estimation of measurement results with LCABE confirm its applicability to the monitoring of moisture dynamics in transformer pressboard.