Rozprawy doktorskie na temat „Leader discharge”

The research work presented here is concerned with numerical simulations of two different electrical phenomena: Long gap electrical discharges under switching impulses and the lightning attachment process associated with positive upward leaders. The development of positive upward leaders and the progression of discharges in long gaps are attributable to two intertwined physical phenomena, namely, the leader channel and the streamer zone. The physical description and the proposed calculations of the above-mentioned phenomena are based on experimental tests conducted in long spark gaps. The methodology presented here proposes a new geometrical approximation for the representation of the streamer and the calculation of the accumulated electrical charge. Furthermore, two different approaches to representing the leader channel are applied and compared. Statistical delays before the inception of the first corona, and random distributions to represent the tortuous nature of the path taken by the leader channel were included based on the behavior observed in experimental tests, with the intention of ensuring the discharge behaved in a realistic manner. A reasonable agreement was found between the physical model and the experimental test results. A model is proposed to simulate the negative discharges produced by switching impulses using the methodology developed to simulate positive leader discharges and the physics underlying the negative leader phenomena. The validation of the method demonstrated that phenomena such as the pilot leader and negative leader currents are successfully represented. In addition, based on previous work conducted on the physics of lightning and the lightning attachment process, a new methodology is developed and tested. In this new approach, the background electric field and the ionized region, considered in conjunction with the advance of the leader segment, are computed using a novel method. The proposed methodology was employed to test two engineering methods that are accepted in international standards, the mesh method and the electro-geometrical method. The results demonstrated that the engineering approximations are consistent with the physical approach. In addition to the electrical phenomena mentioned above, one should remember that, to simplify the calculation, there are certain real effects arising from the lightning attachment process that have not been considered. In fact, when a structure is subjected to a strong electric field, it is possible to generate multiple upward leaders from that structure. This effect has not been taken into account in the numerical models available previously, and therefore the process of generating multiple upward leaders incepted over a structure is incorporated here. The results have shown that a slight advantage from the background electric field is enough for one upward connecting leader to take over, thereby forcing the others to abort the attachment process.

Electrical discharges with a variety of different forms (streamers, glow corona, leaders, etc.) broadly exist in nature and in industrial applications. Under certain conditions, one electrical discharge can be transformed into another form. This thesis is aimed to develop and use numerical simulation models in order to provide a better physical understanding of two of such transitions, namely the glow-to-streamer and the streamer-to-leader transitions in air. In the first part, the thesis includes the two-dimensional simulation of the glow-to-streamer transition under a fast changing background electric field. The simulation is performed with a fluid model taking into account electrons. An efficient semi-Lagrangian algorithm is proposed to solve the convection-dominated continuity equations present in the model. The condition required for the glow-to-streamer transition is evaluated and discussed. In order to enable such simulations for configurations with large interelectrode gaps and long simulation times, an efficient simplified model for glow corona discharges and their transition into streamers is also proposed. The second part of the thesis is dedicated to investigate the dynamics of the streamer-to-leader transition in long air gaps at atmospheric pressure. The transition is studied with a one-dimensional thermo-hydrodynamic model and a detailed kinetic scheme for N2/O2/H2O mixtures. In order to evaluate the effect of humidity, the kinetic scheme includes the most important reactions with the H2O molecule and its derivatives. The analysis includes the simulation of the corresponding streamer bursts, dark periods and aborted leaders that may occur prior to the inception of a stable leader. The comparison between the proposed model and the widely-used model of Gallimberti is also presented.
Elektriska urladdningar av olika former (streamers (från engelska), glöd-korona, ledare, etc.) förekommer i stor utsträckning i naturen och i industriella applikationer. Under vissa förhållanden kan en elektrisk urladdning omvandlas till en annan form av elektrisk urladdning. Denna avhandling syftar till att utveckla och använda numeriska simuleringsmodeller för att ge en bättre fysikalisk förståelse av två sådana övergångar, nämligen glöd-till-streamer- och streamer-till-ledar-övergångar, i luft. I den första delen, avhandlas en tvådimensionell simulering av glöd-till-streamer-övergången med ett hastigt föränderligt elektriskt fält i bakgrunden. Simuleringen utförs med en flödesmodell som tar hänsyn till elektronerna. En effektiv semi-Lagrangesk algoritm föreslås för att lösa de konvektionsdominerade kontinuitetsekvationerna i modellen. Vidare utvärderas och diskuteras förutsättningarna för glöd-till-streamer-övergången. För att möjliggöra sådana simuleringar i konfigurationer med stora elektrodavstånd och långa simuleringstider, föreslås också en effektiv och förenklad modell för glöd-korona-urladdningar samt deras övergång till streamers. Den andra delen av avhandlingen är tillägnad att undersöka dynamiken i streamer-till-ledar-övergångar över långa avstånd i luft, under atmosfäriskt tryck. Övergången studeras med en endimensionell termohydrodynamisk modell och en detaljerad kinetisk modell för blandningar av N2/O2/H2O. För att utvärdera effekten av luftfuktighet, innefattar den kinetiska modellen de viktigaste reaktionerna med H2O-molekylen och dess derivat. Analysen innefattar simuleringen av motsvarande streamer-kedjor, mörka perioder och avbrutna ledare som kan förekomma före starten av en stabil ledare. En jämförelse mellan den föreslagna modellen och den allmänt använda modellen av Gallimberti presenteras också.

QC 20170418

This dissertation deals with research on the numerical modelling of electrical discharges in laboratory long air gaps excited with positive switching impulses. It begins with the preliminary work of several scientists during the last decades, making a detailed analysis of different approaches for modelling all the stages in a full discharge. The relations between these models are identified as well as the effect on the outcome when modifying some important input parameters. The general concept describing the discharge phenomenon usually includes three main elements: the streamer inception, the streamer-to-leader transition and the stable leader propagation. These elements are present in many of the analysed models and the main differences between them are the assumptions and simplifications made by each author at a specific point in their methodologies. The models are usually simplified by assigning experimentally determined values to physical constants pertinent to different stages of the full discharge. These constants are the potential gradient in the leader-corona region to sustain the leader propagation, the charge per unit length along the leader channel which depends on the atmospheric conditions and the voltage impulse wave shape; and the leader propagation velocity, which is closely related to the discharge current. The dissertation includes the results of laboratory work related the study of leaders in long gap discharges, electrical parameters and optical records. By reconstructing the three-dimensional leader propagation for the rod-to-plane configuration, it was possible to study the random tortuous path followed by the leader as it propagates. One important element included in the discharge modelling is the representation of the leader-corona region in front of the leader tip as it propagates towards the grounded electrode. For the calculation of the net charge available in the leader-corona region, two new methodologies were pro-posed based on the electrostatic potential distribution obtained from a finite element method solver. This allowed the inclusion of more elements representing different parts of the discharge in the simulation domain. In the final part, all the analysed elements and the new proposed ones were included in a new methodology for the modelling of electrical dis-charges in long air laboratory gaps. The results obtained from this methodology were compared to experimental data. A good agreement was found between the simulation results and the experimental data.

Cette thèse est consacrée à l’étude de la décharge positive dans les grands intervalles d’air. L’objectif initial est de se doter d’un modèle capable de déterminer la tension U50 ainsi que le facteur d’intervalle k, deux facteurs importants entrant dans le dimensionnement des ouvrages hautes tensions, dans des géométries complexes. Pour ce faire un bilan des modèles de la décharge disponibles dans la littérature a été fait. Notre choix s’est porté sur le modèle de type circuit développé par le groupe du professeur Beroual et appliqué aussi bien à la décharge positive qu’à la décharge négative. Ce modèle est basé sur un schéma électrique équivalent, ses paramètres variant avec le temps en fonction des caractéristiques du canal et de la géométrie de la décharge. La propagation du leader est basée sur un critère lié au calcul du champ à sa tête et où le caractère aléatoire du trajet de la décharge est pris en compte. Comme la plupart des modèles présents dans la littérature, ce modèle ne s’intéresse qu’à l’intervalle de type pointe-plan. Une partie de ce travail a consisté à étendre le domaine d’applicabilité de ce modèle aux intervalles de géométrie complexes. Une étude détaillée de la géométrie pointe-pointe nous a permis de valider notre modèle dans sa capacité à simuler un exemple d’intervalle complexe. De plus, des essais expérimentaux nous ont permis de comparer la valeur de la U50 déterminée à l’aide de notre modèle et celle issue des expériences pour cette géométrie pointe-pointe. Les résultats obtenus sont en bon accord avec une marge d’erreur inférieure à 5%. Au vu du bon accord résultant de la confrontation modèle - expérience de laboratoire et de la grande similarité, grande étincelle - décharge atmosphérique, un modèle de foudre positif a été élaboré. Ce modèle nous a permis d’utiliser pour la première fois le modèle circuit pour modéliser le processus d’attachement de la foudre. Les résultats obtenues permettent de valider la faisabilité de trouver, aux méthodes standards de protection contre la foudre, une alternative numérique basé sur une simulation de la foudre. Une estimation du champ rayonné par le canal du leader a été effectuée. Ce qui ouvre une voie à l’étude de l’interaction de ces champs rayonnés avec les systèmes d’ingénierie
This thesis is devoted to the study of the positive discharge in large air gaps. The initial goal is to develop a model able to determine the voltage U50 and the k-factor, two important factors involved in the design of high voltage structures in complex geometries. To do this, a review of the discharge models available in the literature was done. We chose the circuit model developed by Professor Beroual’s group and applied to the positive discharge and also to the negative discharge. This model is based on an equivalent circuit diagram, his parameters varying with time according to the leader channel characteristics and the geometry of the discharge. The spread of the leader is based on a criterion related to the calculation of the field at its head and where the randomness of the discharge path is taken into account. As most of models found in the literature, this model applies only to the point-plane type interval. Part of this work was to extend the domain of applicability of this model to complex geometries. A detailed study of the rod-rod gap allowed us to validate our model in its ability to simulate an example of complex geometry. Moreover, experimental tests allowed us to compare the value of the U50 voltage determined using our model and the one from experiments for to this rod-rod geometry. The results are in good agreement with a margin of error of less than 5%. Given the good agreement resulting from the confrontation model - laboratory experience and the great similarity, large spark - atmospheric discharge, a positive lightning model was developed. This model allowed us to use for the first time the circuit model to model the lightning attachment process. The results obtained allow to validate the feasibility to find, to the standard methods of protection against lightning, digital alternative based on a simulation of lightning. An estimate of the field radiated by the leader channel was performed. This opens a way for the study of the interaction of these fields radiated with engineering systems

Thesis: M.B.A., Massachusetts Institute of Technology, Sloan School of Management, in conjunction with the Leaders for Global Operations Program at MIT, 2018.
Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, in conjunction with the Leaders for Global Operations Program at MIT, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (page 67).
In the last few years, US hospitals have faced severe challenges with bed capacity management that leads to capacity congestion. Delivering patients to the right bed at the right time is very important to patient care quality. However, the current process employs a self-reporting system to receive bed availability from each unit. This method does not provide consistent estimates nor does it provide a standardized, proactive bed capacity management perspective. In addition, the Department of Medicine (DOM) has a very complex patient population, both clinically and non-clinically. Various team structure and uneven distributed bed resources introduce additional challenge on patient discharges. The project aims to develop a predictive analytics tool that consistently and reliably identifies potential patient discharges in the next 24 hours. The prediction tool allows hospitals to incorporate a more proactive bed capacity management process. Every day, a ranked list with each patient's likelihood to be discharged will be the output. This list guides a more focused conversation within the care team to make patient discharge decisions. In addition, the prediction tool provides a comprehensive summary of barriers to discharge. In this work, we extended the model developed by Zanger [9] for predicting surgical patients' discharges to medicine inpatients' discharge prediction. By partitioning the training and validation set by the date on 12/31/2017, the current performance for the full model on January 2018 medicine inpatients has a prediction power of - 0.74 (Area Under Curve of a Receiver Operating Characteristic curve - AUC ROC there onwards). We further evaluated the model performance for specific patient populations. With patients' Length-Of-Stay (LOS) up to 3 days, the model's performance in terms of AUC ROC can reach ~ 0.8; 0.78 for model with patients' LOS up to 5 days, 0.77 for model with patients' LOS up to 7 days, and 0.72 for model with patients' performance up to 12 days. In addition, the model can capture 57.8% discharges in the next 48 hours, and 33.1% discharges in the next 24 hours.
by Ling Qi.
M.B.A.
S.M.

Thesis: M.B.A., Massachusetts Institute of Technology, Sloan School of Management, in conjunction with the Leaders for Global Operations Program at MIT, 2018.
Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, in conjunction with the Leaders for Global Operations Program at MIT, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 121-124).
In the last few years, MGH has grappled with severe bed capacity management problems. As a result, delays occur in delivering the patient to the right bed at the right time, hindering patient care. One of the root causes for those delays is the mismatch between the timing of admissions and discharges. Particularly, while bed managers know about most admissions well in advance, there is a prevalent lack of central transparency regarding which patients might be ready to leave the hospital and what are the barriers that may delay their discharge. This project aims to improve MGHs bed management processes by introducing a predictive model (based on neural network) that identifies, in real time, surgical inpatients discharges that will occur in the next 24 hours. As part of this research, we present a new modeling methodology, formalizing concepts of 'Milestones to Post-Operative Recovery' and 'Barriers to Discharge', which systematically track patients progress towards discharge. For every admitted surgical patient, our solution outputs a score that is correlated with the likelihood for discharge within 24 hours, and derives a list of barriers to discharge ranked by their significance. In addition, the solution predicts with high accuracy (R-Square 0.86) the total number of daily surgical inpatient discharges, a key piece of information for bed managers. Given training population of 15,553 surgical inpatients admitted to MGH between May 2016 and August 2017, and test population (out-of-sample) of 1,151 surgical inpatients hospitalized during September 2017, the model achieved remarkable performance with ROC of 0.857. During non-holiday weekdays, among the top 10 ranked surgical inpatients identified by the algorithm to have the highest probability of being discharged, 90% were discharged within 24 hours and 97% were discharged within 48 hours, capturing 23% of the hospital's daily surgical discharges. Among the top 30 patients ranked by the algorithm, 69% were discharged within 24 hours and 89% were discharged within 48 hours, capturing 53% of the hospital's daily surgical discharges. The model was implemented as a web-based tool and is currently being piloted at MGH. Preliminary results show potential to promote proactive discharge processes to eliminate unnecessary delays. The implemented solution is using standard EMR data streams, and can be generalized across hospitals.
by Jonathan Zanger.
M.B.A.
S.M.

This research is divided into two main parts. Chapters 1 and 2 address the effect of thermal addition on the breakdown of Boxelder leaves (Acer negundo) in insitu and artificial stream studies. In both studies the rates for the decline in penetrance values decreased significantly in the thermally influenced treatments. Reduction in mass-loss rate coefficients was significant in the in-situ study, but not in the artificial streams. This result was due to differences in flow and macroinvertebrate colonization. Chapters 3 and 4 address the development of a methodology for testing 24-48 hr old juvenile Asiatic clams (Corbicula fluminea) to sediments in 10-day bioassays. These organisms had acceptable survivorship in the controls (>70%), excellent recovery of all exposed organisms (97.5%) and were sensitive to copper-spiked sediments. Comparitive sensitivity of this age class demonstrated them to be equally or more sensitive than other commonly used sediment test organisms including Chironomus tentans, Daphnia magna and Hyalella azteca for survival and growth endpoints in 10-day exposures. Survival and growth effects for Corbicula juveniles were seen at 25 ppb copper in the sediment.
Master of Science

Lead-acid battery serves as the primary power source for vehicles. The total lifetime is determined by the parameters identified in the production, but also handling the operation and maintenance. This work deals with the effect of additives in the electrolyte added before and after the formation of cells lead-acid battery. The work deals with the influence of these additives during cyclic mode.

The project considers realization system, which allows us to test six different operation modes on lifetime of accumulators. The system will be connected to computer, that will perform measurement of current and voltage and switch betwen charge and discharge modes. The measured values will be recorded in certain time intervals to computer memory. After some time these measured data will be analised and the most optimal operational mode will be set.

The project considers realization system, which will allow us to test six different operation modes on lifetime accumulators. The system will link to computer, which will execute measurement current and tension and switch betwencharging and discharging modes. The measured values will record in someone time intervals in to computer memory. After someone time will analise these measured valuer and will provide optional operation mode.

Air transport plays a vital role in global economic growth and long-distance commutation. The aviation industry is found to double its fleet size every fifteen years. According to Air Transport Action Group (ATAG), 45 million aircraft took off worldwide in 2019, which translates to 1.5 lakh per day. Similar numbers are reported for other years under normal circumstances. Therefore, aviation appears to be an indispensable part of modern human civilization. Lightning is known to be one of the serious environmental threats to aircraft. Past incidents show that lightning strikes can lead to structural damage, operational interruption, and loss of lives. Field data suggest that, on average, an aircraft can get struck by lightning once or twice a year. Further, according to NOAA, The lightning strikes typically cost approximately two billion dollars to airline operators annually. Therefore, lightning protective measures are considered a crucial aspect of aircraft design. Design of suitable lightning protective measures involves Zoning of aircraft’s outer surface. Aircraft Zoning intends to differentiate lightning attachment points, channel slipping regions, and regions that carry just the stroke current. The first step in Zoning is to identify the initial attachment points. For the same, different methods like laboratory experiments, similarity principle, Rolling Sphere Method (RSM), and field-based approach are suggested in the standard, Aerospace Recommended Practice (ARP)-5414. Several aircraft accidents attributed to lightning strikes during the mid-20th century encouraged engineers to investigate the phenomena more closely. Therefore, several in-flight measurement campaigns were carried out in the late 80's, where the aircraft were flown inside the thunderstorm with the intention of getting struck by lightning. Field observation from these campaigns suggests two modes of lightning attachment, Aircraft-initiated and aircraft-intercepted. In the former one, under the influence of a thundercloud or descending lightning leader, the aircraft initiates bipolar leaders that lead to a strike. These leaders are deemed to propagate hundreds of meters to complete the lightning strike. In aircraft-intercepted strikes, the aircraft intercepts a descending lightning leader and hence gets struck. The methods suggested in the standard for identifying initial attachment points on aircraft are simple and have limitations. − The laboratory experiments on scaled aircraft models or isolated aircraft parts cannot portray all the aspects of discharges leading to the attachment. Therefore, the laboratory results cannot be directly extended to actual aircraft. − The similarity principle suggested in the standard is qualitative and can’t be extended to aircraft of any size and shape. − The field-based approach is not properly described in the standard, and hence, lacks clarity. − The 25 m Rolling Sphere Method (RSM) is routinely employed to determine the initial attachment points. Being a striking-distance-based approach, RSM only depicts the last stage of aircraft- intercepted attachment and, thus, doesn’t consider aircraft-initiated leaders. However, it is reported that 90% of the lightning strikes to aircraft are attributed to aircraft-initiated mode, which involves significant connecting leader activities. Therefore, precise assessment of initial attachment points requires considering the aircraft-initiated leader discharges. From the above discussion, it is evident that modeling bipolar leader discharges from aircraft is imperative in the context of lightning protection design. In literature, it is difficult to find a model for bipolar leader discharges from aircraft. However, works on either negative or positive leader discharge from energized electrodes in laboratory gaps and their extension to grounded objects are well-regarded in the literature. Knowledge from these works is found to be helpful to the present work. In spite of being responsible for most attachments, a model for bipolar leader discharges from aircraft is hard to find in the literature. Therefore, this work aims to develop a model for the inception and propagation of bipolar leaders from aircraft. Electrical discharges being field-driven phenomena, field computation is essential. Identifying the problem in hand as an open-geometry problem, a boundary-based method, Surface Charge Simulation Method (SCSM), is chosen for field computation. SCSM provides the global field distribution around the aircraft. Aircraft extremities are the most probable regions that can initiate discharges and, therefore, requires capturing the field around them in detail. The same is achieved by employing sub-modeling at the extremities. Sub-model charges are calculated using Charge Simulation Method (CSM), while the boundary condition on the sub-model is extracted from the global field solution. Modeling aircraft-initiated leader discharges involve modeling positive and negative leader discharges. Several models for positive and negative leader discharges in laboratory gaps are available in the literature. The latest model available in the literature for a positive leader discharge was developed by Becerra and Cooray. To reduce the computational burden, a simplified version of the model, which is also suggested by them, is considered in the present work. For negative leader discharge, a simplified physical model proposed by Z.Guo et al. is considered. Using the constructed model, the mechanism involved in the inception and propagation of the aircraft-initiated leader discharges is investigated and quantified. In contrast with discharges from energized electrodes or objects on the grounds, a few salient aspects of bipolar leader discharge from aircraft are pointed out. It is shown that aircraft potential changes with the development of connecting leaders, which modifies the field around it. As a consequence of the same, unipolar stable leader discharge from an aircraft is not viable. Therefore, the aircraft-initiated positive and negative leader discharges in a mutually supporting form are essential for the stable propagation of connecting leaders. The minimum ambient fields required for the stable propagation of bipolar leaders from a medium (DC-10) and a small aircraft (SDM) are determined. The values are well within the fields measured during different measurement campaigns. Subsequently, the dependency of this threshold field on permissible pitch and roll angle, aircraft flying altitude, and humidity are quantified. The aircraft-intercepted lightning strikes are also accounted for in this work. It is shown that, being electrically floating, the magnitude of aircraft potential increases, keeping the polarity the same as the descending leader tip potential. However, it is not the case for objects on the ground (i.e., buildings, towers, etc.). Therefore, the striking-distance-based approach, routinely employed for designing lightning protection for grounded objects, cannot be directly extended to aircraft. This also indicates a possible limitation of RSM while applied on aircraft. Further, The critical stroke current below which aircraft-intercepted mode of attachment is most probable is determined for two aircraft models, DC-10 and SDM. Unlike structures on the ground, the weight and volume of the lightning protection for aircraft should be constrained. Therefore, to provide adequate protection, it is absolutely essential to quantify the probability of lightning strikes to aircraft. Thus, based on the above model for lightning attachment, this work develops a methodology for estimating the rate of lightning strikes to aircraft. This method takes aircraft dimensions and spatial densities of lightning flashes and thunderstorms along its route as input. The proposed method is employed to estimate the average annual number of strikes to aircraft worldwide. Subsequently, the dependency of the strike rate on aircraft size and flying altitude are investigated. The entire exercise is carried out for medium-sized (DC-10) and small (SDM) aircraft. The estimated strike rates are well within the range of reported field data. Further, the estimated variation of strike rates with altitudes (below 3 km) correlates well with the data published by Boeing. The small deviations observed in the estimated strike rates are attributed to the assumption of cloud heights and takeoff/landing trajectory. Therefore, given exact data on thunderstorms, lightning flashes, and the operational behavior of an aircraft, the methodology can reliably estimate the rate of lightning strikes to aircraft. In summary, this work has developed a model for the inception and propagation of bipolar leaders from aircraft and also correctly picturizes the direct streamer mode of bridging involved in aircraft-intercepted attachment. The role of the air density (hence, altitude) is incorporated in the model, along with selected humidity values. The proposed model provides a discharge-physics-based method of identifying initial attachment points on aircraft. Therefore, the limitations of the methods suggested in the standard are overcome.The work has also developed a methodology for estimating the strike rate as a function of altitude, aircraft size, thunderstorms, and lightning flash density. The estimated strike rates correlate well with the reported data from field observation.