Página 22 dos resultados de 2122 itens digitais encontrados em 0.027 segundos

‣ Aquatic gravity currents through emergent vegetation

Tanino, Yukie, 1980-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 155 leaves; 5179212 bytes; 5218566 bytes; application/pdf; application/pdf
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Differential heating and cooling can generate density-driven, lateral exchange flows in aquatic systems. Despite the ubiquity of wetlands and other types of aquatic canopies, few studies have examined the hydrodynamic effects of aquatic vegetation on these currents. This study investigates the dynamics of lock-exchange flows, a particular class of density currents, propagating through rigid emergent vegetation. First, previous mathematical formulation is extended to develop theoretical models of vegetated lock-exchange flows. The regime in which stem drag is inversely proportional to velocity is considered as a special case. Lock-exchange flows were generated in a laboratory flume with rigid cylindrical dowels as model vegetation. Experimental observations were consistent with the theory. Under high stem drag or low stem Reynolds number conditions, the interface deviated from the well-documented block profile associated with unobstructed lock-exchange flows and approached a linear profile. Criteria are developed to categorize all flow conditions as inertial or non-inertial and the interface profile as linear, transitional, or non-linear, respectively, based on (a) the evolution of the velocity of the leading edge of the undercurrent and (b) the interface shape. Finally...

‣ Versioning of Web service interfaces

Agarwal, Anamika, 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 91 leaves; 4532469 bytes; 4542677 bytes; application/pdf; application/pdf
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This thesis investigates the problem of "design for change" in the context of Web Service based information systems. It describes the current status of architecting Web Services, an implementation of the Service Oriented Architecture. It also discusses the availability and support for higher level specifications, such as security, that leverage the baseline standards like SOAP (Simple Object Access Protocol), WSDL (Web Service Description Language), and UDDI (Universal Discovery, Description and Integration). This thesis examines versioning as an additional specification. It discusses the Web Service enabled programming environment by addressing the various causes of change. Two levels of versioning are identified to deal with this problem--Schema level and Interface level versioning. For Schema level versioning, the extensibility models and their versioning support are discussed. Other techniques of using distinct namespaces and custom versioning tags are presented by means of samples. The second level of versioning, the Interface Level Versioning, discusses implementation of versioning logic using the current standards of WSDL 1.1. In addition, the ongoing recommendations and efforts in this field are stated.; by Anamika Agarwal.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ Modeling lane-changing behavior in presence of exclusive lanes

Choudhury, Charisma Farheen, 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 122 p.; 5489452 bytes; 5504638 bytes; application/pdf; application/pdf
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Driving behavior is significantly affected by the presence of exclusive lanes. Particularly, unlimited access to exclusive lanes result significant amount of special type of lane-changing actions. The objective of this thesis is to develop an improved lane-changing model that has a generalized structure and is flexible enough to capture the lane-changing behavior in all situations including the presence of unlimited access exclusive lanes. A new lane-changing model with explicit choice of target lane is proposed in this regard. The target lane is the lane the driver perceives as the best to be in taking a wide range of factors and goals into account. The direction of the immediate lane change is based on the choice of this target lane rather than myopic evaluation of adjacent lanes. A lane change occurs in the direction implied by the chosen target lane depending upon gap availability. The parameters of the model are jointly estimated with detailed vehicle trajectory data and calibrated for a situation with unlimited access High Occupancy Vehicle (HOV) lane. Estimation results show that the target lane choice is affected by lane-specific attributes, such as average speed and density, variables that relate to the path plan and the interactions of the vehicle with other vehicles surrounding it. The model is validated and compared with an existing lane-changing model using a microscopic traffic simulator in an HOV lane situation. The results indicate that the proposed model is significantly better than the previous model.; by Charisma Farheen Choudhury.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ Software development of the Automated Law Review Submission System; Software development of the ALRSS

Gottlieb, David Bendix, 1982-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 80 p.; 3263781 bytes; 3263585 bytes; application/pdf; application/pdf
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Organizing article data and information as well as streamlining communications are the most important goals of any submission system. The submission process used by law professors and law review journals severely fails to meet those goals. The Automated Law Review Submission System (ALRSS) has been developed in order to meet those goals. Many potential ALRSS users were interviewed and a rigorous set of requirements was developed. ALRSS successfully provides an easily accessible, central document and information repository, and also would nearly eliminate the use of phone, fax, mail or personal email. Furthermore, the system enables editors of law reviews l;o control the workflow of the review process. With further development, ALRSS will compete commercially with ExpressOTM in the law article community and strive to operate as effectively as MonsterTrakTM and LSDASTM do in their respective communities. The long term goal of ALRSS is to function as a general submission system that can be customized for a specific submission processes.; by David Bendix Gottlieb.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2004.; Includes bibliographical references (p. 79-80).

‣ Design considerations for retractable-roof stadia

Frazer, Andrew H., 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 58 leaves; 3623011 bytes; 3628429 bytes; application/pdf; application/pdf
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As existing open-air or fully enclosed stadia are reaching their life expectancies, cities are choosing to replace them with structures with moving roofs. This kind of facility provides protection from weather for spectators, a natural grass playing surface for players, and new sources of revenue for owners. The first retractable-roof stadium in North America, the Rogers Centre, has hosted numerous successful events but cost the city of Toronto over CA$500 million. Today, there are five retractable-roof stadia in use in America. Each has very different structural features designed to accommodate the conditions under which they are placed, and their individual costs reflect the sophistication of these features. These stadia also share some noticeable characteristics, particularly in their retractable mechanisms. There are currently five more projects for retractable-roof stadia in planning or construction in this country. These new structures will utilize design features from their predecessors, but the prices of these new projects are growing disproportionately to their technology and costing as much as $800 million. Beyond its multipurpose capabilities, a retractable-roof stadium carries an image of technological sophistication and distinction to it home city and thereby sells itself. This thesis attempts to identify the important design features of existing retractable-roof stadia that are or will likely be repeated in new ones. Recommendations for the future of this building type will be made in terms of possible improvements to design and of potential future markets for construction. A financial study will also be made to observe the funding processes for these structures and the escalating trend in total cost.; by Andrew H. Frazer.; Thesis (M. Eng.)--Massachusetts Institute of Technology...

‣ A solution to the braced excavation collapse in Singapore

Artola, Javier, 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 86 leaves; 5385436 bytes; 5395189 bytes; application/pdf; application/pdf
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At about 3:30pm on April 20, 2004, a 30m deep excavation adjacent to Nicoll Highway in Singapore collapsed, resulting in four casualties and a delay of part of a US$4.14 billion subway project. This thesis examines the flaws in the original design of the bracing system, which have been cited as causes of the failure. The Author then proposes a revised design for the braced excavation system. The Plaxis finite element program was used to simulate the excavation process and compute forces on the major structural elements in the original design. Some pertinent background information on this program is provided throughout the thesis in order to better understand the significance of certain errors in the input data of the original model that ultimately led to the incorrect assumptions and calculations of the original design. A new model using this same program was regenerated with a corrected set of input assumptions, thereby leading to reasonable estimates of structural forces. These results were then used to propose a revised design of the excavation support system and compare this design to the original used in the excavation project. There are several lessons that could be learned from this structural failure, one being the need to acknowledge the limitations built in advanced analysis software systems...

‣ Managing large-scale systems with automated, centralized applications : using the Automated Submission System for law reviews

Crumley Alvarez, Patricia A. (Patricia Alexandra), 1981-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 113 leaves; 3263452 bytes; 3263258 bytes; application/pdf; application/pdf
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Through the power of the internet, documents can be delivered almost instantaneously. Communications processes can happen faster. Existing systems may be centralized (or made sharable) and automated applications can be created. The concept of automated applications can be extended to many fields of endeavor, so that an Automated Submission System for Law Reviews is an attractive project. A distinctive feature of Law Reviews is that students are reviewing submissions from professors, other professionals, or even from other students. The product of this research is the compilation of system requirements, which dictated the data and business model. By contacting law students actively participating in Law Review Boards, as well as professors, the data model was enhanced. It was extremely important to assure that the system was useful enough so that it generated workflow benefits, to entice Law Reviews to adopt the system. The Automated Submission System for Law Reviews would facilitate the submission process and communication between Authors and Reviewers. The Automated Submission System for Law Review Articles will prove to be extremely convenient and attractive for both, Law Reviews and Authors, since it is a centralized system that provides information for all parties involved. Efficiency and usability are the key; the system provides these two essential elements.; by Patricia A. Crumley Alvarez.; Thesis (M. Eng.)--Massachusetts Institute of Technology...

‣ A client-server software application for statistical analysis of fMRI data

Choudhary, Vijay Singh, 1979-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 66 leaves; 2781086 bytes; 2780893 bytes; application/pdf; application/pdf
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Statistical analysis methods used for interrogating functional magnetic resonance imaging (fMRI) data are complex and continually evolving. There exist a scarcity of educational material for fMRI. Thus, an instructional based software application was developed for teaching the fundamentals of statistical analysis in fMRI. For wider accessibility, the application was designed with a client/server architecture. The Java client has a layered design for flexibility and a nice Graphical User Interface (GUI) for user interaction. The application client can be deployed to multiple platforms in heterogeneous and distributed network. The future possibility of adding real-time data processing capabilities in the server led us to choose CGI/Perl/C as server side technologies. The client and server communicates via a simple protocol through the Apache Web Server. The application provides students with opportunities for hands-on exploration of the key concepts using phantom data as well as sample human fMRI data. The simulation allows students to control relevant parameters and observe intermediate results for each step in the analysis stream (spatial smoothing, motion correction, statistical model parameter selection etc.). Eventually this software tool and the accompanying tutorial will be disseminated to researchers across the globe via Biomedical Informatics Research Network (BIRN) portal.; by Vijay Singh Choudhary.; Thesis (S.M.)--Massachusetts Institute of Technology...

‣ Acoustical wave propagation in buried water filled pipes

Kondis, Antonios, 1980-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 151 p.; 6032755 bytes; 6053197 bytes; application/pdf; application/pdf
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This thesis presents a comprehensive way of dealing with the problem of acoustical wave propagation in cylindrically layered media with a specific application in water-filled underground pipes. The problem is studied in two stages: First the pipe is considered to be very stiff in relation to the contained fluid and then the stiffness of the pipe and the soil are taken into account. In both cases the solution process can take into account signals of any form, generated in any point inside the pipe. The simplified method provides the basic understanding on wave propagation and noise generation in the pipe in relation to pipe radius and frequency of excitation. Following the simplified analysis, the beam forming method is discussed and applied in order to reduce the noise in the pipe. Moving on to the complete analysis of the pipe, the stiffness matrix method is used to take into account the properties of the system. The solution time is proven to be much higher in this case, but the results vary from the simplified case in many real value problems. The results of the two methods are compared in more detail and then a decision making process for the choice of method is developed. This decision process is based on the frequency of the excitation...

‣ The dynamics of Internet publishing on the computer book industry

Allen, Robbie (Robbie C.)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 64 p.; 3612710 bytes; 3614619 bytes; application/pdf; application/pdf
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The Internet has been a disruptive force for many industries, but perhaps none more so than in the publishing business. While many segments of the publishing industry have made attempts to use the Internet to augment or replace existing revenue sources, none has done as little with the Internet as book publishers. This thesis will examine the computer book niche of the publishing business, review various Internet publishing models that have been employed to date, and outline opportunities and challenges that computer book publishers should consider to stay viable in the Internet age. An analysis of various Internet publishing business models will be covered along with a discussion of how Internet publishing can facilitate better methods and processes for developing content.; by Robbie Allen.; Thesis (S.M.)--Massachusetts Institute of Technology, System Design and Management Program; and, (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2006.; Includes bibliographical references (p. 61-64).

‣ Dependable system architecture for businesses : analysis of an enterprise resource planning system

Kittusamy, Kannakumar
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 128 p.; 6240684 bytes; 6247201 bytes; application/pdf; application/pdf
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This thesis defines the concept of Dependable System Architecture (DSA), evaluation criteria for DSAs and applies those criteria to evaluate SAP, a leading ERP system. The evaluation criteria employ system frameworks, concepts, tools, components and methodologies. During the process of evaluation, this thesis touches upon many different architectural evaluations such as technical, operational, implementation and service and support. Finally, this thesis concludes whether SAP provides Dependable System Architecture for businesses.; by Kannakumar Kittusamy.; Thesis (S.M.)--Massachusetts Institute of Technology, System Design and Management Program; and, (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2006.; Page 128 blank.; Includes bibliographical references (p. 125-127).

‣ The effect of anthropogenic development on sediment loading to bays on St. John, U.S. Virgin Islands

McCreery, Helen F
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 83 p.
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In order to assess the impact of anthropogenic development on sediment delivery rates to bays on St. John, U.S.V.I., I developed a sediment loading prediction model. Based on the modified universal soil loss equation, this model predicts sediment loading using watershed parameters, including the soil type, topography, and land use of the watershed, and also storm based parameters, including the total storm runoff and peak discharge. Sediment loads I predicted on a storm-by-storm basis, and annual sediment loads were calculated by summing the loads from each storm in a year. This was performed from historical rainfall data for three years: 1990, which had an approximately average total rainfall; 1986, which had the highest total rainfall in the period for which I had rainfall data; and 1989, which was the year of Hurricane Hugo, and therefore had the largest single storm. I calculated sediment loads for multiple bays, with varying levels of development. The sediment loads to the developed bays were as much as or even more than an order of magnitude greater than sediment loads to undeveloped bays. Additionally, I calculated the sediment loads to each bay under entirely undeveloped conditions, comparing these loads to the loads under current development conditions. Again...

‣ Appropriate technology water treatment processes for MaeLa Temporary Shelter, Thailand

Vater, Katherine Ann
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 74 p.
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This thesis recommends the use of horizontal-flow roughing filters to treat spring water of variable annual quality in MaeLa Temporary Shelter, Thailand. The public drinking water system for 45,000 refugees is overseen by Aide Medicale Intemacionale, with which this project was conducted. Half the drinking water for the camp is provided by thirteen springs. The volume and turbidity of these springs varies annually, correlating with the rainy and dry seasons. Treating the varying turbidity and volume at these sources so that the water can be effectively disinfected is the treatment goal. Available materials and operation and maintenance capabilities are also design parameters. Horizontal-flow roughing filtration was determined to fit these parameters and a design with two equivalent filters operating in parallel is recommended. One important feature of the filters is baffles that dictate the flow path of water through the filter. A second feature is an outflow at the top of the filter that will maintain a constant water volume in the filter. The feasibility of the design is based on flow tests and turbidity measurements taken on site as well as weekly flow rates and turbidities for 2007 provided by AMI. The requirements for mechanical regeneration of the filter are also determined.; by Katherine Ann Vater.; Thesis (M. Eng.)--Massachusetts Institute of Technology...

‣ The role of topography in the emergence of African savannas

Kim, Yeonjoo, 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 96 leaves
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The coexistence of trees and grasses in savannas are not well understood even though savannas occupy a wide area of West Africa. In this study, a hypothesis is proposed to investigate the question of how trees and grasses coexist in a region. The hypothesis suggests that the variation in elevation leads to the variation in soil moisture, which in turn can explain the coexistence of trees and grasses in savannas. To test this hypothesis, experimental simulations are performed using biospheric model, IBIS, and distributed hydrologic model, SHE. We, first, estimate the amount of rainfall required for trees and grasses under a certain atmospheric condition. Here, the variation of rainfall is prescribed to force a similar variation of soil moisture. A 30% decrease in rainfall is sufficient to simulate grasses at 9°N. A 100% increase in rainfall is sufficient to simulate trees at II°N. However, even with a five fold increase in rainfall, the model fails to simulate trees at 13°N. To study the influences of topography explicitly, a distributed hydrologic modeling is performed using SHE. The results suggest that the variation of the depth to water table induced by the varying elevation is highly correlated with the variation of soil moisture. Consequently...

‣ Airline revenue management based on dynamic programming incorporating passenger sell-up behavior

Tam, Chiu Fai Wilson
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 147 p.
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Low-fare carriers with simplified and unrestricted fare structures have rapidly grown and captured an important share of demand in the markets they enter, forcing legacy carriers to inevitably simplify their fare structures to avoid distraction of their competitiveness. Consequently, traditional Revenue Management (RM) systems, which assume independent demand of fare classes, have become less effective for legacy carriers in dealing with passengers who tend to purchase the lowest fare available in the absence of distinctions among fare products. This thesis studies two RM optimization algorithms based on dynamic programming, Lautenbacher DP (DPL) and Gallego-Van Ryzin DP (DP-GVR), that aim to control fare class closure using maximum expected revenue. The underlying principle of both DP methods considers the actual arrival pattern of passengers as a Markov decision process. DPL assumes independence of fare classes as do traditional RM methods, and determines which classes should be open for a given time frame. DP-GVR considers the fact that passengers may sell-up or buy down between fare classes, and determines which fare class should be the lowest class open for a given time frame. The goal of this thesis is to evaluate the effectiveness of DPL and DP-GVR when they account for sell-up...

‣ RM methods for multiple fare structure environments; Revenue Management methods for multiple fare structure environments

Kayser, Matthew R. (Matthew Russell)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 115 p.
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The rapid growth of Low Cost Carriers (LCC) and their simplified fare structures has created "semi-restricted" fare structures where lower classes are undifferentiated except for price, while higher fare classes are still differentiated by various advance purchase and booking restrictions. The problem this causes is two-fold: first, traditional revenue management systems, which operate based on the assumption of demand independence, will see demand "spiral down" into the lowest booking classes as passengers will buy the lowest available fare in the absence of fare restrictions. Second, airlines must maximize network revenues across two different fare structures, a more-restricted structure used on markets without an LCC presence, and the semirestricted structure for markets where LCC competition exists. This thesis describes methods of dealing with these two problems: Hybrid Forecasting (HF), which forecasts "product-oriented" demand using traditional forecasting methods while simultaneously forecasting "price-oriented" demand for those passengers who will buy the lowest available fare, and Fare Adjustment (FA), which is used at the booking limit optimizer level to account for the sell-up potential of passengers (probability a passenger will book in a higher class if his original booking class is denied). Fare Adjustment allows the airline to deal with multiple fare structures separately. The goal of this thesis is to provide a comprehensive summary of results when an airline uses HF and FA simultaneously in two different multiple fare structure...

‣ Decision algorithms for Unmanned Underwater Vehicles during offensive operations; Decision algorithms for UUVs during offensive operations

Smith, Tyler B. (Tyler Bradford)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 117 p.; 15141342 bytes; 15140838 bytes; application/pdf; application/pdf
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The field of research involving autonomous vehicles has expanded greatly over the past decade. This thesis addresses the case of a system of Unmanned Underwater Vehicles (UUVs) operating in littoral areas in an offensive capacity. A series of complementary algorithms were designed to collect information about an enemy vessell, and subsequently use this information to both select and move to a prefered intercept location that maximizes the opportunity to both re-acquire and destroy an enemy vessel. Additionally, within the context of a specifically designed simulation, key parameter changes were analyzed to determine their effectiveness to improve the system's performance as measured by four measures of effectiveness. A methodology was also designed to optimize the location of the engaging UUVs to maximize their effectiveness, and capitalize on the enemy movement within the operational area. Results are presented for both original locations and optimized locations, and initial findings provide insight into the effectiveness of the designed algorithms and statistical inference of these key parameter changes.; by Tyler B. Smith.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2006.; Includes bibliographical references (p. 115-117).

‣ Estimating evapotranspiration from the Amazon Basin using the atmospheric water balance; Estimating ET from the Amazon Basin using the atmospheric water balance

Karam, Hanan Nadim
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 109 p.; 27195954 bytes; 27195517 bytes; application/pdf; application/pdf
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The spatio-temporal patterns of evapotranspiration (ET) in the Amazon basin are still poorly understood. Field studies in the Amazonian forest have shown that at some sites, deep roots allow trees to sustain elevated transpiration during several months of minimal rainfall, whereas at others, trees experience evident dry season water limitation. However, the few sites investigated are inadequate to characterize the conditions of transpiration throughout the basin. As a result of this uncertainty in modeling trees' access to deep soil moisture, land surface models cannot provide reliable estimates of transpiration in the region. From a basin-averaged perspective, it remains uncertain whether transpiration is water-limited, peaking during the basin's wet season, or energy-limited, peaking during the dry season when clearer skies allow for higher surface radiation. In this work, we investigate an approach to deriving a spatially-averaged ET estimate for the Amazon basin, which avoids modeling the forest's terrestrial hydrology. ET is computed as a residual of the atmospheric water balance, using basin-averaged convergence of atmospheric water vapor flux [C], precipitation [P], and tendency of total atmospheric water vapor [dw/dt] as inputs.; (cont.) As our resulting estimate of ET is only as good as the input estimates of the other hydrologic components...

‣ A .net based resource sharing framework

Lin, Xiaohan
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 124 p.
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This thesis presents an Internet resource sharing architecture. It allows users to access and utilize unused computer resources, such as CPU cycles and storage, without an expert's knowledge. It achieves this by providing a number of abstract services that hide some of the complexity inherent in distributed computing. In recent years, Grid Computing has been proposed as a solution for Internet resource sharing. However, Grid Computing as presently implemented does not address the need of the large majority of the users. In this thesis, we propose a different approach to achieve Internet resource sharing called the Realm. The Realm Framework offers a lightweight layer on top of the Microsoft .Net Framework so that the programs that can be migrated to .Net Framework can also utilize the shared resources through the Realm Framework. By leveraging the Microsoft .Net Framework, the Realm Framework avoids tedious re-working in this fast-paced world of technology by sitting on the top of the full-featured, coherent and up-to-date development platform. The Realm Framework applies current technologies such as Web Services, the Common Language Runtime (CLR) and popular encryption algorithms.; (cont.) In this thesis a versatile runtime system and a set of extension interfaces in C# programming language is developed. The modularized software package offers a layered programming model for distributed-application developers with different levels of proficiency. Two utilities that are helpful for maintaining a distributed system are also developed...

‣ Multi-sensor large scale land surface data assimilation using ensemble approaches

Zhou, Yuhua, Ph. D. Massachusetts Institute of Technology
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 234 p.
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One of the ensemble Kalman filter's (EnKF) attractive features in land surface applications is its ability to provide distributional information. The EnKF relies on normality approximations that improve its efficiency but can also compromise the accuracy of its distributional estimates. The effects of these approximations are evaluated by comparing the conditional marginal distributions and moments estimated by the EnKF to those obtained from an SIR particle filter, which gives exact solutions for large ensemble sizes. The results show that overall the EnKF appears to provide a good approximation for nonlinear, non-normal land surface problems. A difficulty in land data assimilation problems results from the high dimensionality of states created by spatial discretization over large computational grids. The high dimensionality can be reduced by exploiting the fact that soil moisture field may have significant spatial correlation structure especially after extensive rainfall while it may have local structure determined by soil and vegetation variability after prolonged drydown. This is confirmed by SVD of the replicate matrix produced in an ensemble forecasting experiment. Local EnKF's are suitable for problems during dry periods but give less accurate results after rainfall.; (cont.) The most promising option is to develop a generalized method that reflects structural changes in the ensemble. A highly efficient ensemble multiscale filter (EnMSF) is then proposed to solve large scale nonlinear estimation problems with arbitrary uncertainties. At each prediction step realizations of the state variables are propagated. At update times...