• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 140
  • 66
  • 44
  • 44
  • 17
  • 12
  • 9
  • 5
  • 5
  • 5
  • 5
  • 5
  • 5
  • 4
  • 3
  • Tagged with
  • 406
  • 77
  • 42
  • 41
  • 41
  • 40
  • 37
  • 36
  • 36
  • 35
  • 34
  • 32
  • 29
  • 28
  • 27
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Direct Time Domain Modelling Of First Return Stroke Of Lightning

Dileepkumar, K P 07 1900 (has links)
Being one of the most spectacular events in nature, lightning is basically a transient high current electric discharge in the atmosphere, which extends up to kilometres. Cloud to ground discharge is the most hazardous one as far as ground based structures are considered. Among the different phases of a lightning flash, return stroke is considered to be the most energetic phase and is basically responsible for most of the damages. Hence, much emphasis has been given to return stroke modeling. A more realistic modeling of return stroke is very essential to accurately study the interaction of return stroke with the structures on ground. As return stroke is dominated by electromagnetic phenomenon, an electromagnetic model will be the most suitable one. It does not call for any assumption on the mode of wave propagation, as well as, electromagnetic coupling between the different channel portions. There are mainly two approaches adopted for electromagnetic models i.e. frequency domain and time domain approach. Time domain approach is more reliable as it can handle, in principle, the nonlinear processes in the lightning channel. It is also free of numerical frequency domain to time domain inversion problem, which are found to be quite severe. However, most of the previous works on time domain electromagnetic models suffered from following two serious limitations - (i) the initial charge on the channel, which forms the true excitation for the problem, is not considered and (ii) instead of the non-linearly rising conductivity of the channel, a constant resistance is employed. For a realistic simulation of the interaction between the channel and any intercepting system, a time domain model with the above two major aspects being fully represented is very essential. In an earlier work, all these aspects have been fully considered but a domain based numerical modelling was employed. Consequently, it was difficult to consider the down conductor and further the number of unknowns was considerably large. In view of this, the present work is taken up and its scope is defined as to develop a boundary based numerical time domain electromagnetic model in which the initial charge on the channel and the non-linearly evolving channel conductance are fully considered. For the electrical engineering applications, electromagnetic aspects of the lightning phenomena is more important than the other associated physical processes and hence, importance is given only to the electromagnetic aspects. In other words, the light emission, thunder, chemical reactions at the channel etc. are not considered. Also, for most of the electrical engineering applications, the critical portion of current would be the region up to and around the peak and hence, modeling for this regime will be given prime importance. Owing to the complexity of the problem, some simplifying assumptions would be very essential. The literature indicates that these assumptions do not affect the adequate representation of the phenomena. Lightning channel is considered to be vertically straight without any branches. Earth is considered to be perfectly conducting. Explicit reference to dynamically varying channel radius, temperature and the air density is not made. However, it is assumed that the arc equation employed to describe the temporal changes in conductivity would adequately take care of these parameters. Lightning channel is represented by a highly conducting small core, which is surrounded by a weakly conducting corona sheath. The initial charge on the channel is deduced by solving for electrostatic field, with leader portion set to possess an axial gradient of 60 V/cm and the streamer portion to 5 – 10 kV/cm. The radius of the corona sheath is set iteratively by enforcing a gradient of 24 kV/cm up to its radial boundary. As analytical solution for the problem is impractical, suitable numerical solution is sought. Since the spatial extension of this time marching problem is virtually unbound and that the significant conduction is rather solely confined to an extremely small cross section of the channel core, a boundary-based method is selected. Amongst the numerical methods, the present work employs the moment method for the solution of the fields associated with the return strokes. A numerical solution of the Electric Field Integral Equation (EFIE) for thin structures has been developed in the literature. The same approach has been employed in the present work, however, with suitable modifications to suit the lightning problem. The code was written in MATLAB and integrations involved in the EFIE were solved using MATLAB symbolic computation. Before introducing the channel dynamic conductance and the initial charge on the channel, the code developed is validated by comparing the results for a center fed dipole antenna with that given in the literature. Also, NEC (Numeric Electromagnetic Code) simulations for various cases of monopole and dipole antenna were performed. The results from the code developed are shown to have good matching with that obtained from NEC based time domain results. In an earlier work, the dynamic conductance of the return stroke channel core, which is a high current electric arc, was represented by a first order arc equation. The same approach is employed in the present work also. Similarly, the transition from streamer to leader was modeled by Braginskii’s spark law and the same has been considered in the present work. A value of 10-5 S/m was used for minimum value of streamer conductance. For numerical stability, upper (Gmax = 3 S/m) and lower bounds (Gmin = 0.0083 S/m) for the channel conductance are forced. Preliminary simulations were run with and without dynamic channel conductance. The initial charge distribution along the channel formed the excitation. Results clearly show that without the dynamically varying channel conductance, no streamer to leader transition and hence, no return stroke evolution can occur. In other words, the non-linearly evolving channel conductance is mainly responsible for the evolution of the return stroke. In order to consider the charge neutralization by the return stroke, the charge deposited by it is diffused into the corona sheath. A fixed value of the corona sheath conductance is employed and the diffusion process is modeled by an equation derived from the continuity equation. To study the effect of corona sheath, simulations were run with and without corona. From the simulation results it was observed that the corona sheath causes increase in peak value of the stroke current, as well as, time to front and a decrease in the velocity of propagation. For the validation of the model, the basic characteristics of the return stroke current like the current wave shape, temporal variation of stroke current at different heights, velocity of propagation and the vertical electric fields at various radial distances were compared with available field/experimental data. A good agreement was seen and based on this, it is concluded that the present work has successfully developed a boundary based time domain numerical model for the lightning return stroke. Natural lightning being a stochastic process, the values of the parameters associated with it would differ in every event. On other hand, any deterministic model like the one developed in the present work predicts a fixed pattern of the simulated quantities. Therefore, it was felt that some of the model parameters must be permitted to vary so that a range of results could be obtained rather than a single pattern of results. Incidentally, the model parameters like arc time constant, settling value of arc conductivity/gradient, bounds for channel conductivity, streamer gradient, radius of the core etc. are not precisely known for the natural lightning environment. Further, some of them are known to vary within an event. Considering these and that simplicity is very important in already complex model, the above-mentioned parameters are taken as tunable parameters (of course to be varied within the prescribed range) for deducing the return stroke currents with some desired characteristics. A study on the influence of these parameters is made and suggestions are provided. Simulations for the nominal range of stroke currents are made and results are presented. These simulations clearly show the role of cloud potential, which in turn dictates the length of final bridging streamer, on the return stroke currents. The spatio-temporal variation of the current, charge deposited by the return stroke and the channel conductivity are presented which, reveal the dynamic processes leading to the evolution of return stroke current. Subsequently, simulations for two cases of stroke to elevated strike object are attempted. The upward leader was modeled quite similar to the descending one. Many interesting findings are made. In summary, the present work has successfully developed a boundary-based time domain numerical electromagnetic model for the lightning return stroke, wherein, the initial charge deposited on the channel and the non-linearly rising channel conductance have been appropriately considered. Simulation using the model clearly depicts the dynamic evolution of the return stroke. The characteristics of the simulated return strokes are in good agreement with the field data. Some of the parameters of the model are suggested as tunable parameters, which permit simulation of strokes with different characteristics.

Connection between Severe Weather and Intense Lightning

Yandulska, Kateryna 17 February 2010 (has links)
This Thesis researches, explains and provides conclusions for the question of correlation between data and criteria used by Environment Canada (EC) and Lightning Studies Research Group (LSG) of University of Toronto. The necessity of such correlation arises from the question of common criteria between severe weather, as it shown in graphical data from EC, and intense lighting storms, recorded by LSG, despite deep differences in area, time scale and object of observation used by those two organizations. The objective of the Thesis is not only to compare those two, very different sets of data and criteria and find out the common ground between them, but also to provide in-depth explanation of criteria, used by Lighting Studies Research Group, along with revisiting and establishing some of them. Eight storm cases, taken from years 2005-2008 provide cases for practical research, which affects intermediate Greater Toronto Area.

Connection between Severe Weather and Intense Lightning

Yandulska, Kateryna 17 February 2010 (has links)
This Thesis researches, explains and provides conclusions for the question of correlation between data and criteria used by Environment Canada (EC) and Lightning Studies Research Group (LSG) of University of Toronto. The necessity of such correlation arises from the question of common criteria between severe weather, as it shown in graphical data from EC, and intense lighting storms, recorded by LSG, despite deep differences in area, time scale and object of observation used by those two organizations. The objective of the Thesis is not only to compare those two, very different sets of data and criteria and find out the common ground between them, but also to provide in-depth explanation of criteria, used by Lighting Studies Research Group, along with revisiting and establishing some of them. Eight storm cases, taken from years 2005-2008 provide cases for practical research, which affects intermediate Greater Toronto Area.

The Current Status of Lightning Safety Knowledge and the Effects of Lightning Education Modes on College Students

Phillips, Melissa Catherine Koeka 18 July 2011 (has links)
No description available.


Nagler, Michael N. January 1981 (has links)
The design of BOLTS (Broad Area Lightning Telescope Sensor) is presented. This sensor will provide full-time (day/night) coverage of the continental U.S. from a geosynchronous orbit. The average ground resolution will be 8 km and the system will be able to detect ≃ 10⁷ watt strokes during nighttime and ≃ 4 x 10⁷ watt strokes during daytime with a probability of detection of 0.9. We present the system's requirements and projected performance, together with the design rationale. Contrast enhancement is achieved using a narrow band interference filter deposited on a curved surface inside the F/2.5, 101.7 mm optical system. Deposition of the interference layers on the curved surface reduce the passband wandering caused by off-axis bundles. The focal plane constitutes an 800 x 800 element virtual phase CCD array with a multiple outputs option. The central 800 x 400 elements are used for imaging while the outer 2x (800 x 200) elements serve as buffer memory for one frame storage. An additional 2x (800 x 200) array serves for storing a second frame. Signal detection is achieved via a frame-to-frame subtraction algorithm that is hardware implemented immediately following the CCD arrays. An integration time of 5 msec is used, which stems from SNR optimization requirements and from the fact that lightning strokes occur randomly in time and space. The data obtained after frame-to-frame subtraction is subjected to a threshold test and the resulting positive events are digitized and stored in an on-board digital memory using 48 bits/event. Each record contains intensity information over a dynamic range of 4000, location information and time of occurrence information. A prototype instrument built to perform measurements from aboard a U-2 plane is described. The purpose of this instrument is to refine some of the lightning data used in defining the system's parameters. A short discussion about the changes required to expand the design to either a global coverage instrument or a high resolution, smaller field instrument is presented.

Fulgurite Classification, Petrology, and Implications for Planetary Processes

Block, Kristin January 2011 (has links)
A variety of fulgurites from diverse locations have been studied. Morphological features were measured and physical properties documented, and a classification scheme was developed. Three major types are introduced and described: Type I, Type II, and Type III, along with two minor types: Type IV and Melt Droplets. Fulgurites representative of each major taxonomic type were investigated using electron microprobe point analyses and x-ray mapping. A range of compositions were found, including nearly pure glass, detrital zircons with baddeleyite rims, Fe-metal with P-rich rims, and unusual Fe-Si metals. The fulgurite formation process is considered within the planetary context through a discussion of lightning detection and potential for formation on other terrestrial bodies. Finally, suggestions for future investigations are presented and discussed.

Lightning return stroke electromagnetics - time domain evaluation and application

McAfee, Carson William Ian January 2016 (has links)
A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering, 2016 / The work presented extends and contributes to the research of modelling lightning return stroke (RS) electromagnetic (EM) fields in the time domain. Although previous work in this area has focused on individual lightning electromagnetic pulse (LEMP) modelling techniques, there has not been an investigation into the strengths and weaknesses of different methods, as well as the implementation considerations of the models. This work critically compares three unique techniques (Finite Antenna, FDTD, and Single Cell FDTD) under the same ideal simulation parameters. The research presented will evaluate the EM fields in the range of 50m to 500m from the lightning channel. This range, often referred to as the near field distance, has a significant effect on lightning induced overvoltages on distribution lines, which are primarily created by the horizontal EM fields of the RS channel. These close distances have a significant effect on the model implementations, especially with the FDTD method. Each of these modelling methods is explained and tested through examples. The models are implemented in C++ and have been included in the Appendix to aid in future implementation. From the model simulations it is clear that the FDTD method is the most comprehensive model available. It allows for non-ideal ground planes, as well as complex simulation environments. However, FDTD has a number of numerical related errors that the Finite Antenna method does not suffer from. The Single Cell FDTD method is simple to implement and does not suffer from the same numerical errors as a full FDTD implementation, but is limited to simple simulation environments. This work contributes to the research field by comparing and evaluating three techniques and giving consideration to the implementation and the applicability to lightning EM simulations. / MT2017

An approximation to the Heidler Function with an analytical integral for engineering applications using lightning currents

Terespolsky, Brett Ryan January 2015 (has links)
A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering in the Lightning and EMC Research Group School of Electrical and Information Engineering September 2015 / The work presented contributes to research in lightning protection simulations and focuses on approximating the Heidler function with an analytical integral and hence a frequency domain representation. The integral of lightning current models is required in the analysis of lightning events including the induced effects and frequency analyses of lightning strikes. Previous work in this area has produced very specific forms of the Heidler function that are used to represent lightning current waveshapes. This work however focuses on a generic solution with parameters that can be modified to produce any lightning current waveshape that is required. In the research presented, such an approximation is obtained. This function has an analytical solution to the integral and hence can be completely represented in the frequency domain. This allows for a true representation of Maxwell’s equations for Electromagnetic (EM) fields and for an analytical frequency domain analysis. It has parameters that can be changed to obtain different waveshapes (10/350, 0.25/100, etc.). The characteristics of the approximation are compared with those of the Heidler function to ascertain whether or not the function is applicable for use with the lightning protection standard (IEC 62305-1). It is shown that the approximation does represent the same characteristics as those of the Heidler function and hence can be used in IEC 62305-1 standardised applications. This represents a valuable contribution to engineers working in the field of lightning protection, specifically simulation models. / MT2017

Present-day and future lightning, and its impact on tropospheric chemistry

Finney, Declan Luke January 2017 (has links)
Lightning represents a key interaction with climate through its production of nitrogen oxides (NOx) which lead to ozone production. These NOx emissions are generally calculated interactively in chemistry-climate models but there has been little development of the representation of the lightning processes since the 1990s. In most models the parametrisation of lightning is based upon simulated cloud-top height. The aims of the thesis are: to explore existing schemes, and develop a new process-based scheme, to parametrise lightning; to use a new process-based lightning scheme to give insights regarding the role of lightning NOx in tropospheric chemistry; and to use alternative lightning schemes to improve the understanding of the response of lightning to climate change, and the consequent impacts on tropospheric chemistry. First, a new lightning parametrisation is developed using reanalysis data and satellite lightning observations which is based on upward cloud ice flux. This parametrisation is more closely linked to thunderstorm charging theory. It greatly improves the simulated zonal distribution of lightning compared to the cloud-top height approach, which overestimates lightning in the tropics. The new lightning scheme is then implemented in a chemistry-climate model, the UK Chemistry and Aerosol model (UKCA). It is evaluated against ozone sonde measurements with broad global coverage and improves the simulation of the annual cycle of upper tropospheric ozone concentration, compared to ozone simulated with the cloud-top height approach. This improvement in simulated ozone is attributed to the change in ozone production associated with the improved zonal distribution of simulated lightning. Subsequently, data from a chemistry-climate model intercomparison project (ACCMIP) are used to study the state-of-the-art in lightning NOx parametrisation along with its response to climate change. It is found that the models using the cloud-top height approach produce a very similar response of lightning NOx to changes in global mean surface temperature of +0.44± 0.05 TgNK-1, for a baseline emission of 5 TgN yr-1. However, two models using two alternative lightning schemes produce a weaker and a negative response of lightning to climate change. Finally, simulations in a future climate scenario for year 2100 in the UKCA model were performed with the cloud-top height and the ice flux parametrisations. The lightning response to climate change when using the cloud-top height scheme is in good agreement with the positive response found in the multi-model results of the cloud-top height approach. However, the new ice flux approach suggests that lightning will decrease in future. These opposing responses introduce large uncertainty into the projections of tropospheric ozone and methane lifetime in the future scenario. An analysis of the radiative forcing from these two species also shows the large uncertainty in the individual methane and ozone radiative forcings in the future. Due to the opposite effect that lightning NOx has on methane (loss) and ozone (production) the net radiative forcing effect of lightning in present-day and future is found to be close to zero. However, there is a small positive feedback suggested by the results of the cloud-top height approach, whereas no feedback is evident with the ice flux approach. These results show there are large and crucial uncertainties introduced by lightning parametrisation choice, not only in terms of the actual lightning distribution but also atmospheric composition and radiative forcing. The new ice-based parametrisation developed here offers a good alternative to the widely-used approach and can be used in future to model lightning and develop the understanding of associated uncertainties.

The evolution of total lightning and radar reflectivity characteristics of two mesoscale convective systems over Houston, Texas

Hodapp, Charles Lee 15 May 2009 (has links)
Two mesoscale convective systems (MCSs) passed over the Houston Lightning Detection and Ranging (LDAR) network on 31 October 2005 and 21 April 2006. As the MCSs traverse the LDAR network, the systems slowly mature with a weakening convective line and a developing stratiform region and radar bright band. The intensification of stratiform region precipitation, including the bright band, is thought to play an important role in stratiform lightning structure, charge structure, and total lightning production of MCSs. The stratiform areas quadruple in size and the mean reflectivity values increase substantially by ~ 6 dB. As the stratiform region matures, VHF source density plots show a lightning pathway that slopes rearward and downward from the back of the convective line and into the stratiform region. At early times for both MCSs, the pathway extends horizontally rearward 40 to 50 km into the stratiform region at an altitude of 9 to 12 km. Near the end of the analysis time period, the pathway slopes rearward 40 km and downward through the transition zone before extending 40 to 50 km in the stratiform region at an altitude of 4 - 7 km. The sloping pathway likely results from charged ice particles advected from the convective line by storm relative front-to-rear flow while the level pathway extending further into the stratiform region is likely caused by both charge advection and local in-situ charging. As the stratiform region matures, the stratiform flash rates double and lightning heights decrease. The percentage of lightning flashes originating in the stratiform region increases significantly from 10 - 20% to 50 - 60%. Overall, the number of positive cloud-to-ground flashes in the stratiform region also increases. Between both MCSs, 60% of the positive CGs originated in the convective or transition regions. Both in-situ charging mechanisms created by the development of the mesoscale updraft and charge advection by the front-to-rear flow likely contribute to the increased electrification and lightning in the stratiform region.

Page generated in 0.1271 seconds