Browsing by Author "Kohler, Werner E."
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- Acoustic propagation in nonuniform circular ducts carrying near sonic mean flowsKelly, Jeffrey J. (Virginia Tech, 1981-01-05)A linear model based on the wave-envelope technique is used to study the propagation of axisymmetric and spinning acoustic modes in hard-walled and lined nonuniform circular ducts carrying near sonic mean flows. This method is valid for large as well as small axial variations, as long as the mean flow does not separate. The wave-envelope technique is based on solving for the envelopes of the quasiparallel acoustic modes that exist in the duct instead o£ solving for the actual wave, thereby reducing the computational time and the round-off error encountered in purely numerical techniques. The influence of the throat Mach number, frequency, boundary-layer thickness and liner admittance on both upstream and downstream propagation of acoustic modes is considered. A numerical procedure, which is stable for cases of strong interaction, for analysis of nonlinear acoustic propagation through nearly sonic mean flows is also developed. This procedure is a combination of the Adams-PECE integration scheme and the singular value decomposition scheme. It does not develop the numerical instability associated with the Runge-Kutta and matrix inversion methods for nearly sonic duct flows. The numerical results show that an impedance condition can be satisfied at the duct exit and a corresponding solution obtained. The numerical results confirm that the nonlinearity intensifies the acoustic disturbance in the throat region, reduces the intensity of the fundamental frequency at the duct exit, and increases the reflections. This implies that the mode conversion properties of variable area ducts can reflect and focus the acoustic signal to the vicinity of the throat in high subsonic flows. Also the numerical results indicate that a shock develops if certain limits on the input parameters are exceeded.
- Adaptive out-of-step relaying with phasor measurementZaldivar, Virgilio Antonio Centeno (Virginia Tech, 1995)This work describes the development of an adaptive out-of-step relay, from the formulation of its concept to its field implementation and one year testing at the Florida- Georgia interface. This dissertation describes the theory of such a relay, its hardware configuration, the system as it was installed in the field, the major results and improvements obtained after the one year field test, and the adaptive features developed after the analysis of the collected data. Most of the adaptive concepts applied on this relay were used on field application for the first time and proved their value through the one year field test. Synchronized phasor measurements were used for the first time for relaying application, proving their ability to detect and analyze system disturbances through the measurement of angle differences between any two points in a system. It is shown that for a system that behaves primarily as a two-machine power system, the out-of-step relay could be enhanced and made more secure by applying the principle of equal area criterion. The main contribution of this dissertation is the use and application of old and new adaptive concepts as well as new technology to the solution of the out-of-step problem for a system that behaves like a two machine system. This work provides a solution for the basic stability problem with currently available technology and knowledge. In addition, the data collected during this research has been and will be of great help for those studying the power system stability problem and those developing new adaptive relaying techniques.
- Adaptive power control as a fade countermeasure on satellite linksSweeney, Dennis G. (Virginia Tech, 1993)Satellite systems in the 20/30 GHz band are very susceptible to outages due to rain-induced fades. In order to reduce the impact of these fades, it has been proposed that the power of the uplink station transmitter be adjusted during the fade to compensate. This dissertation will explore some of the issues involved in implementing this uplink power control (ULPC). Fade slope is examined as a parameter to predict signal strength during a fade. A fade slope model based on fade physics is presented, but it strongly suggests that fade slope is not an appropriate parameter for ULPC. Real time scaling of attenuation from the downlink to the uplink shows more promise for ULPC. Differences in drop size distributions during a rain storm will result is different scaling factors. If the downlink attenuation is limited to 6 dB at 20 GHz, real time scaling can be accomplished. A scaling type ULPC algorithm driven by downlink attenuation is tested on 66 hours of OLYMPUS 20/30 GHz fade data. A similar algorithm driven by uplink attenuation is tested and the performance of the two algorithms is presented and compared.
- Adaptive power system controlManansala, Edgardo Celestino (Virginia Polytechnic Institute and State University, 1989)This work presents a centralized control scheme applied to a power system. The scheme has adaptive characteristics which allow the controller to keep track of the changing power system operating point and to control nonlinear functions of state variables. Feedback to the controller is obtained from phasor measurements at chosen power system buses, generator field voltage measurements, and state estimators. Control effort is aimed at minimizing the oscillations and influencing the power system state trajectory through the control of linear and nonlinear functions of state variables during a power system disturbance. The main contributions of this dissertation are the simultaneous introduction and utilization of measurement based terms in the state and output equations in the derivation and implementation of the control law, the study of limits on controller performance as the state residual vector becomes very large, and the simulation of the performance of local state estimators to prove the need for faster phasor measurement systems. The test system is a hypothetical 39-Bus AC power system consisting of typical components which have been sufficiently modelled for the simulation of power system performance in a dynamic stability study.
- An altimeter waveform model for combined surface and volume scatteringNewkirk, Michael Hayes (Virginia Tech, 1994)Radar altimeters were originally designed to study ocean geodynamics, where the scattering processes are governed entirely by the surface features. These same altimeters have recorded data from over the polar ice sheets, where the scattering processes cannot in general be limited to surface properties. Radar pulse penetration, which gives rise to volume scattering, must also be accounted for in these return waveforms. This pulse penetration affects the altimeter range measurements as well as other information that is derived from the altimeter waveform data. To aid in the study of pulse penetration effects, a waveform model for combining surface and volume scattering effects in the estimation of the radar altimeter returns recorded over continental ice and snow is developed and discussed. The surface scattered waveform model is based on the well-known impulse response method which is capable of accounting for arbitrary altitude, beamwidth, pulsewidth and pointing angle. The newly formulated volume scattered waveform model is also an impulse response based method which differs from previous versions in that it can also be applied to a general altimeter configuration. The two models are time registered and then added together in an arbitrary ratio representing the relative contributions of surface and volume scattering to the overall return power waveform. The combined model can be used to study actual altimeter waveforms by varying the important parameters, including surface roughness and effective extinction coefficient. The capabilities and limitations of this new combined model are also discussed and guidelines for its use are detailed. The combined model is tested by comparing it to the Multimode Aircraft Radar Altimeter (MARA) data which were recorded over and around the Greenland ice sheet in September 1991. Evaluation of this averaged waveform data identified problems that were encountered with the MARA design. A number of techniques are developed in an effort to account for and correct these problems, but none of these attempts were completely successful. The 1991 MARA data are considered usable for waveform analysis, but with the understanding that some error may be present in the final results. The MARA data obtained from the Greenland ice sheet are analyzed for estimates of surface roughness, effective extinction coefficient and ratio of surface to volume scattering strengths. A simple optimization method is employed which achieves a least-squares fit of the combined model to the altimeter data. The result is an estimate of these parameters as a function of location on the ice shelf. To the author's knowledge, this is the first time both surface- and volume-related parameters have been estimated simultaneously from Ka-band radar data.
- Analysis and Applications of Microstructure and Holey Optical FibersKim, Jeong I. (Virginia Tech, 2003-09-10)Microstructure and photonic crystal fibers with periodic as well as random refractive-index distributions are investigated. Two cases corresponding to fibers with one-dimensional (1D) radial index distributions and two-dimensional (2D) transverse index distributions are considered. For 1D geometries with an arbitrary number of cladding layers, exact analytical solutions of guided modes are obtained using a matrix approach. In this part, for random index distributions, the average transmission properties are calculated and the influence of glass/air ratio on these properties is assessed. Important transmission properties of the fundamental mode, including normalized propagation constant, chromatic dispersion, field distributions, and effective area, are evaluated. For 2D geometries, the numerical techniques, FDTD (Finite-Difference Time-Domain) method and FDM (Finite Difference Method), are utilized. First, structures with periodic index distributions are examined. The investigation is then extended to microstructure optical fibers with random index distributions. Design of 2D microstructure fibers with random air-hole distributions is undertaken with the aim of achieving single-mode guiding property and small effective area. The former is a unique feature of the holey fiber with periodic air-hole arrangement and the latter is a suitable property for nonlinear fiber devices. Measurements of holey fibers with random air-hole distributions constitute an important experimental task of this research. Using a section of a holey fiber fabricated in the draw tower facility at Virginia Tech, measurements of transmission spectra and fiber attenuation are performed. Also, test results for far-field pattern measurements are presented. Another objective of this dissertation is to explore new applications for holey fibers with random or periodic hole distributions. In the course of measuring the holey fibers, it was noticed that robust temperature-insensitive pressure sensors can be made with these fibers. This offers an opportunity for new low-cost and reliable pressure fiber-optic sensors. Incorporating gratings into holey fibers in conjunction with the possibility of dynamic tuning offers desirable characteristics with potential applications in communications and sensing. Injecting gases or liquids in holey fibers with gratings changes their transmission characteristics. These changes may be exploited in designing tunable optical filters for communication applications or making gas/liquid sensor devices.
- Analysis of Highly Coupled Wideband Antenna Arrays Using Scattering Parameter Network ModelsTakamizawa, Koichiro (Virginia Tech, 2002-12-10)Wideband phased arrays require very tight element spacing to permit wide angle scanning of the main beam over the wide bandwidth. The consequence of tight spacing is very high mutual coupling among the elements in the array. Previous efforts by Virginia Tech Antenna Group has shown that the strong coupling can be utilized in arrays to obtain broadband frequency response while maintaining a small element spacing. However, mutual coupling between elements in a tightly coupled array can sometimes dramatically change the operating frequency, bandwidth, and radiation pattern from that of the single isolated element. Thus, there are some fundamental questions that remain regarding the effective operation of highly coupled arrays for beam forming, beam scanning, and aperture reconfiguration. Existing antenna pattern analysis techniques including the active element pattern method are inadequate for the application in highly coupled arrays. This dissertation focuses on the development of a new antenna array analysis technique. The presented method is based on the scattering parameter network descriptions of the array elements, associated feed network and the active element patterns. The developed model is general. It can be applied to an array of any size and configuration. The model can be utilized to determine directivity, gain and realized gain of arrays as well as their radiation efficiency and impedance mismatch. Using the network model, the relationship between radiation pattern characteristics and the input impedance characteristics of the array antennas becomes clear. Three types of source impedance matching conditions for array antennas are investigated using the model. A numerically simulated array of strip dipole array is used to investigate the effects of various impedance matching methods on the radiation pattern and impedance bandwidth. An application of network analysis is presented on an experimental investigation of $3\times 3$ Foursquare array test bed to further verify the concepts.
- Analysis of surface pressure and velocity fluctuations in the flow over surface-mounted prismsGe, Zhongfu (Virginia Tech, 2004-12-14)The full-scale value of the Reynolds number associated with wind loads on structures is of the order of 10^7. This is further complicated by the high levels of turbulence fluctuations associated with strong winds. On the other hand, numerical and wind tunnel simulations are usually carried out at smaller values of Re. Consequently, the validation of these simulations should only be based on physical phenomena derived with tools capable of their identification. In this work, two physical aspects related to extreme wind loads on low-rise structures are examined. The first includes the statistical properties and prediction of pressure peaks. The second involves the identification of linear and nonlinear relations between pressure peaks and associated velocity fluctuations. The first part of this thesis is concerned with the statistical properties of surface pressure time series and their variations under different incident flow conditions. Various statistical tools, including space-time correlation, conditional sampling, the probability plot and the probability plot correlation coefficient, are used to characterize pressure peaks measured on the top surface of a surface-mounted prism. The results show that the Gamma distribution provides generally the best statistical description for the pressure time series, and that the method of moments is sufficient for determining its parameters. Additionally, the shape parameter of the Gamma distribution can be directly related to the incident flow conditions. As for prediction of pressure peaks, the results show that the probability of non-exceedence can best be derived from the Gumbel distribution. Two approaches for peak prediction, based on analysis of the parent pressure time series and of observed peaks, are presented. The prediction based on the parent time series yields more conservative estimates of the probability of non-exceedence. The second part of this thesis is concerned with determining the linear and nonlinear relations between pressure peaks and the velocity field. Validated by analytical test signals, the wavelet-based analysis is proven to be effective and accurate in detecting intermittent linear and nonlinear relations between the pressure and velocity fluctuations. In particular, intermittent linear and nonlinear velocity pressure relations are observed over the nondimensional frequency range fH/U<0.32. These results provide the basis for flow parameters and characteristics required in the simulation of the wind loads on structures.
- Analysis, Design and Performance Evaluation of Optical Fiber Spectrum-Sliced WDM SystemsArya, Vivek (Virginia Tech, 1997-06-03)This dissertation investigates the design and performance issues of a recently demonstrated technique, termed as spectrum-slicing, for implementing wavelength-division-multiplexing (WDM) in optical fiber systems. Conventional WDM systems employ laser diodes operating at discrete wavelengths as carriers for the different data channels that are to be multiplexed. Spectrum-slicing provides an attractive low-cost alternative to the use of multiple coherent lasers for such WDM applications by utilizing spectral slices of a broadband noise source for the different data channels. The principal broadband noise source considered is the amplified spontaneous emission (ASE) noise from an optical amplifier. Each slice of the spectrum is actually a burst of noise that is modulated individually for a high capacity WDM system. The stochastic nature of the broadband source gives rise to excess intensity noise which results in a power penalty at the receiver. One way to minimize this penalty, as proposed and analyzed for the first time in this work, is to use an optical preamplifier receiver. It is shown that when an optical preamplifier receiver is used, there exists an optimum filter bandwidth which optimizes the detection sensitivity (minimizes the average number of photons/bit) for a given error probability. Moreover the evaluated detection sensitivity represents an order of magnitude ( > 10 dB) improvement over conventional PIN receiver-based detection techniques for such spectrum-sliced communication systems. The optimum is a consequence of signal energy fluctuations dominating at low values of the signal time bandwidth product (m), and the preamplifier ASE noise dominating at high values of m. Operation at the optimum bandwidth renders the channel error probability to be a strong function of the optical bandwidth, thus providing motivation for the use of forward error correction coding (FEC). System capacity (for BER = ) is shown to be 23 Gb/s without coding, and 75 Gb/s with a (255,239) Reed Solomon code. The effect of non-rectangular spectra on receiver sensitivity is investigated for both OOK and FSK transmission, assuming the system (de)multiplexer filters to be N'th order Butterworth bandpass. Although narrower filters are recommended for improving power budget, it is shown that system penalty due to filter shape may be kept < 1 dB by employing filters with N > 2. Moreover spectrum-sliced FSK systems using optical preamplifier receivers are shown, for the first time, to perform better in a peak optical power limited environment. Performance-optimized spectrum-sliced WDM systems have potential use in both local loop and long-distance fiber communication systems which require low-cost WDM equipment for high data rate applications.
- Analytical Framework for the Performance Analysis of Multiple Antenna SystemsBae, Kyung Kyoon (Virginia Tech, 2005-09-09)There has been great interest in antenna array processing (diversity, beamforming, null steering, and spatial multiplexing) to enhance the received signal quality and the capacity of wireless communications systems. However, in order to properly exploit the characteristics of different array processing techniques, understanding trade-offs among different techniques and parametric investigation, which offers an insight as to what parameters determine system performance under different situations is necessary. In this study, we present analytical framework which can facilitate the performance analysis of systems with antenna array. Five original contributions to the performance analysis of antenna array processing are presented in this study. First, we present theoretical outage probability of a system equipped with an array which suppresses a few dominant interering signals in TDMA cellular networks when the fading statistics of interfering signals are independent but non-identically distributed. Most of the related previous works assumed either independent and identically distributed fading statistics among cochannel interferences (CCI) or Rayleigh fading when CCI signals are subject to i.n.d. fading statistics. Secondly, the performance of multi-branch predetection equal gain combiner for different modulation techniques in equally correlated Nakagami-m fading is presented through analytical analysis. Specifically, the characteristic function (CHF) and the moment generating function (MGF) of EGC output with correlated inputs are derived and used to evaluate the average symbol error probability (ASEP) and the outage probability performance, respectively. Thirdly, we derived analytical expression which can be used to analyze the performance of different types of diversity techniques in equally correlated Nakagami-m or Rice fading channels. Fourthly, asymptotic analysis on different types of diversity combiners in generalized fading channels is presented in a unifying way. Finally, we investigate and present the impact of transmit diversity at handsets on the reverse link DS/CDMA systems in terms of capacity and coverage over generalized fading channels through analytical approaches. Then, we validate the analytical results with simulation results and investigate practical issues which are hard to capture through analytical analysis using system level simulator we developed. Although we have mainly focused on applying the analytical framework we have derived in this work to the performance analysis of physical layer algorithms such as spatial diversity and adaptive null steering, the framework can be extended to assist the analysis and design of wireless communication systems such as, to name a few, distributed multiple input multiple output (MIMO) system in cooperative wireless networks, multipath routing protocol analysis in wireless fading channels, and antenna selection problems in MIMO system.
- Applications in Remote Sensing Using the Method of Ordered Multiple InteractionsWestin, Benjamin Alexander (Virginia Tech, 2013-04-24)The Method of Ordered Multiple Interactions provides a numerical solution to the integral
equations describing surface scattering which is both computationally efficient and reliably
convergent. The method has been applied in a variety of ways to solving the electromagnetic
scattering from perfectly-conducting rough surfaces. A desire to more accurately predict
the scattering from natural terrain has led to the representation of the surface material as
penetrable instead of conductive.
For this purpose, the Method of Ordered Multiple Interactions is applied to numerically
solve the electromagnetic scattering from randomly-rough dielectric surfaces. A primary
consequence of the penetrable surface material is the introduction of a pair of coupled integral equations in place of the single integral equation used to solve the problem with a perfectly conducting surface. The method is tested and analyzed by developing independent scattering solutions for canonical cases in a transform domain and by comparing results with solutions from other techniques.
The dielectric implementation of the Method of Ordered Multiple Interactions is used to solve
the electromagnetic scattering from a class of randomly-rough dielectric surfaces. This allows
for the characterization of the effect of a number of transmitter and surface parameters in the
scattering problem, observing bistatically and also specifically in the backscatter direction.
MOMI is then applied as a method to examine subsurface penetration characteristics from
a similar family of rough surfaces. Characteristics of the environment parameters and the
scattered field itself are examined, and the numerical challenges associated with observing
beneath the surface are identified and addressed.
The Method of Ordered Multiple Interactions is then incorporated as a major component of
a larger solution which computes the total scattering when a dielectric object is buried just
beneath the rough surface. This hyrid approach uses MOMI and the Method of Moments to
iteratively account for multiple interactions between the target and the dielectric interface,
enabling the study of scattering from the combined environment of a rough surface and the
embedded object, as well as the individual scattering events which combine to form the
steady-state solution. - Centralized Control of Power System StabilizersSanchez Ayala, Gerardo (Virginia Tech, 2014-10-09)This study takes advantage of wide area measurements to propose a centralized nonlinear controller that acts on power system stabilizers, to cooperatively increase the damping of problematic small signal oscillations all over the system. The structure based on decision trees results in a simple, efficient, and dependable methodology that imposes much less computational burden than other nonlinear design approaches, making it a promising candidate for actual implementation by utilities and system operators. Details are given to utilize existing stabilizers while causing minimum changes to the equipment, and warranting improvement or at least no detriment of current system behavior. This enables power system stabilizers to overcome their inherent limitation to act only on the basis of local measurements to damp a single target frequency. This study demonstrates the implications of this new input on mathematical models, and the control functionality that is made available by its incorporation to conventional stabilizers. In preparation of the case of study, a heuristic dynamic reduction methodology is introduced that preserves a physical equivalent model, and that can be interpreted by any commercial software package. The steps of this method are general, versatile, and of easy adaptation to any particular power system model, with the aggregated value of producing a physical model as final result, that makes the approach appealing for industry. The accuracy of the resulting reduced network has been demonstrated with the model of the Central American System.
- Characterization and modeling of magnetic materials and structuresAl-Mazroo, Abdulhameed Yousef (Virginia Polytechnic Institute and State University, 1988)This dissertation presents methods for wideband characterization and modeling of magnetic materials and structures over a wide frequency range (dc to a few GHz). A method for modeling the thick film inductor structures at high frequencies is presented in this dissertation. The thick film inductor under test is printed and located in shunt connection at the end of a reference transmission line. Time Domain Reflectometry (TDR) technique is used to measure the response waveform from the inductor under test. The response from a short circuit at the location of the inductor is acquired as the reference waveform. The two acquired waveforms are then transformed into the frequency domain using the Fast Fourier Transform algorithm (FFT). The reflection coefficient is then computed as the ratio between the Fourier Transforms of the response and reference waveforms. From the information contained, the complex impedance of the structure under study can be calculated. This information is used for modeling that structure by fitting the data to the network model using the computer network analysis program. Experimental and simulated response waveforms are compared and brought to a close match by changing the model components values. A cavity-like sample holder filled with ferrite material ls proposed in this dissertation to measure the complex permeability of the magnetic material filling this cavity. The cavity walls are deposited on a coaxially shaped sample using thick film techniques. The reflection coefficient from the cavity under study is measured by adapting the cavity to the end of a transmission line. The full field analysis of this proposed configuration is used to determine a relationship between the complex permeability of the ferrite material and the measured reflection coefficient. The method of moments ls used to achieve this task. Computer simulation experiments are performed to test the sensitivity of the technique and to predict the performance over the desired frequency range. Actual experimentation as well as verifications of these measurements are conducted to verify the merit of the proposed technique.
- Characterization of tropospheric scintillations on Earth-space paths in the Ku and Ka frequency bands using the results from the Virginia Tech OLYMPUS experimentHaidara, Fatim M. (Virginia Tech, 1993-05-15)There has been increasing interest in the use of Very Small Aperture Terminals (VSATs) in satellite communication links operating in the Ku and Ka bands. Such systems offers the advantage of low signal margins and therefore, low cost. For the same reason, however, VSATs links can be strongly impaired by tropospheric scintillations. The VA Tech OLYMPUS propagation experiment which includes 12, 20, 30 GHz beacon receivers at an elevation angle of 14° provides us with valuable multi-frequency scintillation data. This dissertation focuses on the characterization of tropospheric scintillations on earth-space paths. Tropospheric scintillations are rapid fluctuations of signal caused by multiple scattering from the sma11 scale turbulent refractive index inhomogeneities in the troposphere. Analysis of the scintillation results from the VA Tech OLYMPUS experiment for both short term and long term studies is presented. The results include spectral characteristics, frequency scaling and statistics of the scintillation intensity, statistics of scintillation fading, diurnal and meteorological trends as well as a comparison with the current CCIR predictive model. Sma11 scale spatial diversity is a potential means to counteract tropospheric scintillation fading; it can be cost effective because of the low cost of VSATs. The VA Tech experiment includes 12, 20, 30 GHz beacon receivers and colocated 20 and 30 GHz diversity receivers that allows us to study this restoration technique. In this dissertation the effects of spatial diversity are studied through a cross-correlation analysis. Experimental results are compared to a theoretical model in which the atmosphere is characterized by an "anisotropic" Kolmogorov spectrum. A method to determine minimum vertical spacing between terminals to achieve good diversity performance is introduced.
- Characterization of Ultra Wideband Communication ChannelsMuqaibel, Ali Hussein (Virginia Tech, 2003-03-05)Ultra-wideband (UWB) communication has been the subject of extensive research in recent years due to its unique capabilities and potential applications, particularly in short-range multiple access wireless communications. However, many important aspects of UWB-based communication systems have not yet been thoroughly investigated. The propagation of UWB signals in indoor environments is the single most important issue with significant impacts on the future direction, scope, and generally the extent of the success of UWB technology. The objective of this dissertation is to obtain a more thorough and comprehensive understanding of the potentials of UWB technology by characterizing the UWB communication channels. Channel characterization refers to extracting the channel parameters from measured data. The extracted parameters are used to quantify the effect of the channel on communication UWB systems using this channel as signal transmission medium. Data are measured in different ways using a variety of time-domain and frequency-domain techniques. The experimental setups used in channel characterization effort also include pulse generators and antennas as integral parts of the channel, since the pulse shape and antenna characteristics have significant impact on channel parameters. At a fundamental level, the propagation of UWB signals, as any electromagnetic wave, is governed, among other things, by the properties of materials in the propagation medium. One of the objectives of this research is to examine propagation through walls made of typical building materials and thereby acquire ultra-wideband characterization of these materials. The loss and the dielectric constant of each material are measured over a frequency range of 1 to 15 GHz. Ten commonly used building materials are chosen for this investigation. These include, dry wall, wallboard, structure wood, glass sheet, bricks, concrete blocks, reinforced concrete (as pillar), cloth office partition, wooden door, and styrofoam slab. The work on ultra-wideband characterization of building materials resulted in an additional interesting contribution. A new formulation for evaluating the complex dielectric constant of low-loss materials, which involves solving real equations and thus requiring only one-dimensional root searching techniques, was found. The results derived from the exact complex equation and from the new formulation are in excellent agreement. Following the characterization of building materials, an indoor UWB measurement campaign is undertaken. Typical indoor scenarios, including line-of-sight (LOS), non-line-of-sight (NLOS), room-to-room, within-the-room, and hallways, are considered. Results for indoor propagation measurements are presented for local power delay profiles (local-PDP) and small-scale averaged power delay profiles (SSA-PDP). Site-specific trends and general observations are discussed. The results for pathloss exponent and time dispersion parameters are presented. The analyses results indicate the immunity of UWB signals to multipath fading. The results also clearly show that UWB signals, unlike narrowband signals, do not suffer from small scale fading, unless the receiver is too close to walls. Multipath components are further studies by employing a deconvolution technique. The application of deconvolution results in resolving multipath components with waveforms different from those of the sounding pulse. Resolving more components can improve the design of the rake receiver. The final part of this research elaborates on the nature of multiple access interference and illustrates the application of multi-user detection to improve the performance of impulse radio systems. Measured dispersion parameters and their effects on the multiple access parameters are discussed.
- A Class of Robust and Efficient Iterative Methods for Wave Scattering ProblemsAdams, Robert John (Virginia Tech, 1998-12-17)Significant effort has recently been directed towards the development of numerically efficient iterative techniques for the solution of boundary integral equation formulations of time harmonic scattering problems. The primary result of this effort has been the development of several advanced numerical techniques which enable the dense matrix-vector products associated with the iterative solution of boundary integral equations to be rapidly computed. However, an important aspect of this problem which has yet to be adequately addressed is the development of rapidly convergent iterative techniques to complement the relatively more mature numerical algorithms which expedite the matrix-vector product operation. To this end, a class of efficient iterative methods for boundary integral equation formulations of two-dimensional scattering problems is presented. This development is based on an attempt to approximately factor (i.e., renormalize) the boundary integral formulation of an arbitrary scattering problem into a product of one-way wave operators and a corresponding coupling operator which accounts for the interactions between oppositely propagating waves on the surface of the scatterer. The original boundary integral formulation of the scattering problem defines the coupling between individual equivalent sources on the surface of the scatterer. The renormalized version of this equation defines the coupling between the forward and backward propagating fields obtained by re-summing the individual equivalent sources present in the original boundary integral formulation of the scattering problem. An important feature of this class of rapidly convergent iterative techniques is that they are based on an attempt to incorporate the important physical aspects of the scattering problem into the iterative procedure. This leads to rapidly convergent iterative series for a number of two-dimensional scattering problems. The iterative series obtained using this renormalization procedure are much more rapidly convergent than the series obtained using Krylov subspace techniques. In fact, for several of the geometries considered the number of iterations required to achieve a specified residual error is independent of the size of the scatterer. This desirable property of the iterative methods presented here is not shared by other iterative schemes for wave scattering problems. Moreover, because the approach used to develop these iterative series depends only on the assumption that the total field can be approximately represented by a summation of independent and oppositely directed waves (and not on the presence of special geometries, etc.), the proposed iterative methods are very general and are thus applicable to a large number of complex scattering problems.
- A comparison and study of the Born and Rytov expansionsBruce, Matthew F. (Virginia Tech, 1993-08-19)Since the introduction of the Born and Rytov approximations for use in random wave propagation some forty years ago, a controversy has boiled over the regions of validity and relative merits of the methods. Although the methods fail for strong fluctuations and distant path lengths, these two perturbation methods are the only approaches available for weak fluctuations in a random in homogeneous media. The approximations have also been applied to the inverse problem for optical and acoustical tomography. The intent of this thesis is to investigate the work of previous authors and attempt to clarify the distinctions of each method. The conclusion will be reached that neither approximation is necessarily better than the other in general for all applications. A careful consideration of the problem following the points given should point towards the use of one approximation over the other.
- Computerized Ultrasonic Raytracing Model for C-scans of Solid Steel Bridge PinsParikh, Sanjiv D. (Virginia Tech, 1998-08-06)This report describes the results of computerized ultrasonic C-scanning of solid steel bridge pins using a raytrace model. The raytrace model was developed to facilitate interpretation of data obtained from an ultrasonic C-scanning system for the Virginia Transportation Research Council (VTRC). The report discusses the reasons behind the development of the raytrace model, as well as specifications of the model, the input conditions, and the data output and visualization. The model uses as input, various "boundary" conditions of the solid steel pin with reduced diameter pin ends, as well as size and location information of a flaw or a wear groove placed within the main pin body. The model considers sound beams to be composed of rays and calculates ray reflections/conversions. This is done until the ray returns to a receiver location or is lost due to exceeding the time-of-flight. Once the model has returned with the received ray data, it uses the receiver conditions provided (transducer used, size of scanning grid, grid resolution, etc.), and calculates a 2-Dimensional C-scan image for each particular depth/time selected. Using PV-Wave visualization software, it is possible to plot the values for each depth to view a color graph. This graphical plot can then be analyzed/compared with the field C-scans to determine the closest match of a flaw or a wear groove inside the bridge pin. This helps in deciding if the condition of the pin is acceptable.
- A continuum Approach to Power system simulationDonolo, Marcos A. (Virginia Tech, 2006-09-04)The behavior of large and tightly interconnected power systems resembles, in certain circumstances, the behavior of a continuously distributed system. This resemblance motivated the derivation of continuum models, which were used to explain and predict disturbance propagation, un-damped power oscillations, and the stability of power systems. In this dissertation, we propose a one-dimensional continuum representation suitable for meshed power systems. Previous continuous representations of meshed power systems used two-dimensional spatial domains. Thus our approach has the potential to provide better resolution for comparable computational burden. It is important to note that, the computational burden required to obtain solutions for PDEs involved in the continuum representation varies notably with the solver implementation. The contributions of this dissertation are: a) Reviewing a previous continuum model and providing a detailed derivation for the one-dimensional version of it. b) Providing and describing in detail a parameter distribution technique adequate for the continuum approach. c) Identifying and documenting limitations on the continuum model voltage calculation. e) Providing a procedure to simulate the behavior of meshed power systems using the one dimensional continuum model. And f) Identifying and applying a numerical PDE solver for the continuum approach.
- Control of Switched Reluctance Motors Considering Mutual InductanceBae, Han-Kyung (Virginia Tech, 2000-08-09)A novel torque control algorithm, which adopts a two-phase excitation, is proposed to improve the performance of the Switched Reluctance Motor (SRM) drive. By exciting two adjacent phases instead of single phase, the changing rate and the magnitude of the phase currents are much reduced. Therefore the existing problems caused by the single-phase excitation such as large torque ripple during commutation, increased audible noise and fatigue of the rotor shaft are mitigated. The electromagnetic torque is efficiently distributed to each phase by the proposed Torque Distribution Function (TDF) that also compensates the effects of mutual coupling. To describe the effects of mutual coupling between phases, a set of voltage and torque equations is newly derived for the two-phase excitation. Parameters of the SRM are obtained by Finite Element Analysis (FEA) and verified by measurements. It is shown that the mutual inductance of two adjacent phases partly contributes to generate the electromagnetic torque and introduces coupling between two adjacent phases in the current or flux linkage control loop, which has been neglected in the single-phase excitation. The dynamics of the current or flux linkage loop are coupled and nonlinear due to the mutual inductance between two adjacent phases and the time varying nature of inductance. Each phase current or flux linkage needs to be controlled precisely to achieve the required performance. A feedback linearizing current controller is proposed to linearize and decouple current control loop along with a gain scheduling scheme to maintain performance of the current control loop regardless of rotor position as well as a feedback linearizing flux linkage controller. Finally, to reduce current or flux linkage ripple, a unipolar switching strategy is proposed. The unipolar switching strategy effectively doubles the switching frequency without increasing the actual switching frequency of the switches. This contributes to the mitigation of current or flux linkage ripple and hence to the reduction of the torque ripple.