Hypergraph-partitioning-based remapping models for image-space-parallel direct volume rendering of unstructured grids

In this work, image-space-parallel direct volume rendering (DVR) of unstructured grids is investigated for distributed-memory architectures. A hypergraph-partitioning-based model is proposed for the adaptive screen partitioning problem in this context. The proposed model aims to balance the rendering loads of processors while trying to minimize the amount of data replication. In the parallel DVR framework we adopted, each data primitive is statically owned by its home processor, which is responsible from replicating its primitives on other processors. Two appropriate remapping models are proposed by enhancing the above model for use within this framework. These two remapping models aim to minimize the total volume of communication in data replication while balancing the rendering loads of processors. Based on the proposed models, a parallel DVR algorithm is developed. The experiments conducted on a PC cluster show that the proposed remapping models achieve better speedup values compared to the remapping models previously suggested for image-space-parallel DVR. © 2007 IEEE

Multimodel Operability Framework for Design of Modular and Intensified Energy Systems

In this dissertation, a novel operability framework is introduced for the process design of modular and intensified energy systems that are challenged by complexity and highly constrained environments. Previously developed process operability approaches are reviewed and further developed in terms of theory, application, and software infrastructure. An optimization-based multilayer operability framework is introduced for process design of nonlinear energy systems. In the first layer of this framework, a mixed-integer linear programming (MILP)-based iterative algorithm considers the minimization of footprint and achievement of process intensification targets. Then, in the second layer, an operability analysis is performed to incorporate key features of optimality and feasibility accounting for the system achievability and flexibility. The outcome of this framework consists of a set of modular designs, considering both the aspects of size and process operability. For this study and throughout this dissertation, the nonlinear system is represented by multiple linearized models, which results in lower computational expense and more efficient quantification of operability regions. A systematic techno-economic analysis framework is also proposed for costing intensified modular systems. Conventional costing techniques are extended to allow estimation of capital and operating costs of modular units. Economy of learning concepts are included to consider the effect of experience curves on purchase costs. Profitability measures are scaled with respect to production of a chemical of interest for comparison with plants of traditional scale. Scenarios in which the modular technology presents break-even or further reduction in cost when compared to the traditional process are identified as a result. A framework for the development of process operability algorithms is provided as a software infrastructure outcome. Generated codes from the developed approaches are included in an open-source platform that will give researchers from academia and industry access to the algorithms. This platform has the purpose of dissemination and future improvement of process operability algorithms and methods. To show versatility and efficacy of the developed approaches, a variety of applications are considered as follows: a membrane reactor for direct methane aromatization conversion to hydrogen and benzene (DMA-MR), the classical shower problem in process operability, a power plant cycling application for power generation with penetration of renewable energy sources, and a newly developed modular hydrogen unit. Applications to DMA-MR subsystems demonstrate employment of the multilayer framework to find a region with modular design candidates, which are then ranked according to an operability index. The most operable design is determined and contrasted with the optimal design with respect to process intensification in terms of footprint minimization, showing that optimality at fixed nominal operations does not necessarily ensure the best system operability. For the modular hydrogen unit application, the developed process operability framework provides guidelines for obtaining modular designs that are highly integrated and flexible with respect to disturbances in inlet natural gas composition. The modular hydrogen unit is also used for demonstration of the proposed techno-economic analysis framework. A comparison with a benchmark conventional steam methane reforming plant shows that the modular hydrogen unit can benefit from the economy of learning. An assembled modular steam methane reforming plant is used to map the decrease in natural gas price that must be needed for the plant to break even when compared to traditional technologies. Scenarios in which the natural gas price is low allow break-even cost for both individual hydrogen units and the assembled modular plant. The economy of learning must produce a reduction of 40% or less in capital cost when the natural gas price is under 0.02 US$/Sm3. This result suggests that the synthesized modular hydrogen process has potential to be economically feasible under these conditions. The developed tools can be used to accelerate the deployment and manufacturing of standardized modular energy systems

Multigrid methods in convex optimization with application to structural design

This dissertation has investigated the use of multigrid methods in certain classes of optimization problems, with emphasis on structural, namely topology optimization. We have investigated the solution bound constrained optimization problems arising in discretization by the finite element method, such as elliptic variational inequalities. For these problems we have proposed a "direct" multi grid approach which is a generalization of existing multigrid methods for variational inequalities. We have proposed a nonlinear first order method as a smoother that reduces memory requirements and improves the efficiency of the resulting algorithm compared to the second order method (Newton's methods), as documented on several numerical examples. The project further investigates the use of multigrid techniques in topology optimization. Topology optimization is a very practical and efficient tool for the design of lightweight structures and has many applications, among others in automotive and aircraft industry. The project studies the employment of multigrid methods in the solution of very large linear systems with sparse symmetric positive definite matrices arising in interior point methods where, traditionally, direct techniques are used. The proposed multigrid approach proves to be more efficient than that with the direct solvers. In particular, it exhibits linear dependency of the computational effort on the problem size

Integration of optimal cleaning scheduling and control of heat exchanger networks undergoing fouling: Model and formulation

The performance and operability of heat exchanger networks (HENs) is strongly affected by fouling, which involves the deposition of unwanted material, which reduces the heat-transfer rate and increases the pressure drop, the operational costs, and the environmental impact of the process. Periodical cleaning and control of the flow rate distribution in the HEN are used to mitigate the effects of fouling and restore the performance of the units. The optimal cleaning scheduling has been formulated as a mixed-integer linear programming (MILP) or mixed-integer nonlinear programming (MINLP) problem and is solved using various approaches. The optimal control has been formulated as a nonlinear programming (NLP) problem and is used to define the flow rate distribution of the network. Both problems share the same objective: minimization of the total cost of the operation. In principle, the simultaneous solution of the optimal control problem and the optimal cleaning scheduling problem should provide greater savings than the independent or sequential solution of the two problems, since the interactions of the two mitigation alternatives are considered. However, these two problems have been typically considered separately, because of modeling and solution challenges. Also, it is not quite clear what additional benefit a simultaneous solution may bring. The challenges for solving the integrated problem are the large scale of the associated optimization problem and the different time scales involved in each operational layer. Here, a general and efficient formulation is proposed, using a continuous time discretization scheme for the integrated problem of scheduling and control of HENs subject to fouling. A dynamic model of the heat exchangers is proposed that is sufficiently detailed to represent the physics of interest with novel modifications to address simultaneously their control and scheduling in a network. The problem is formulated as a MINLP and solved using deterministic optimization algorithms. The flexibility of the model and variations of the formulation are demonstrated with two small case studies. The formulation complexity versus scale and advantages are analyzed. The results show that considering the two problems simultaneously has a very strong synergistic effect, with over a 20% decrease in operational cost achieved, in comparison to using either fouling mitigation alternative individually

Semiannual final report, 1 October 1991 - 31 March 1992

A summary of research conducted at the Institute for Computer Applications in Science and Engineering in applied mathematics, numerical analysis, and computer science during the period 1 Oct. 1991 through 31 Mar. 1992 is presented

Power market models for the clean energy transition: State of the art and future research needs

As power systems around the world are rapidly evolving to achieve decarbonization objectives, it is crucial that power system planners and operators use appropriate models and tools to analyze and address the associated challenges. This paper provides a detailed overview of the properties of power market models in the context of the clean energy transition. We review common power market model methodologies, their readiness for low- and zero‑carbon grids, and new power market trends. Based on the review, we suggest model improvements and new designs to increase modeling capabilities for future grids. The paper highlights key modeling concepts related to power system flexibility, with a particular focus on hydropower and energy storage, as well as the representation of grid services, price formation, temporal structure, and the importance of uncertainty. We find that a changing resource mix, market restructuring, and growing price uncertainty require more precise modeling techniques to adequately capture the new technology constraints and the dynamics of future power markets. In particular, models must adequately represent resource opportunity costs, multi-horizon flexibility, and energy storage capabilities across the full range of grid services. Moreover, at the system level, it is increasingly important to consider sub-hourly time resolution, enhanced uncertainty representation, and introduce co-optimization for dual market clearing of energy and grid services. Likewise, models should capture interdependencies between multiple energy carriers and demand sectors.publishedVersio

Green Forestry? Case Studies of Sustainable Forestry and Forest Certification

Abstract This dissertation explored sustainable forest management from multiple perspectives: a literature-based investigation to define management practices that sustain ecological, economic, and social forest resources over time; a field-based research project to identify management practice differences between Forest Stewardship Council (FSC) certified, Sustainable Forestry Initiative (SFI) certified, and uncertified properties in Maine; and a field-based research project to identify stand structural differences between FSC certified and uncertified properties in Vermont. Based on an extensive literature review, we developed an iterative decision-making framework of goal-setting/implementation/ monitoring/review that could assist forest owners in choosing management practices to sustain ecological, economic, and/or social capital over multiple time frames. Our unique contribution is the identification of six concrete management concepts at the implementation phase: (1) BMPs/RIL, (2) biodiversity conservation, (3) community forestry, (4) forest protection, (5) sustained forest product yield, and (6) triad forestry. Forest owners can implement practices under one or more of these concepts to achieve their sustainability goals. We illustrate a hypothetical application of our framework with a case study of an FSC certified managed natural forest in the lowland tropical region of Costa Rica. In the white pine forests of south-central Maine, we compared three FSC, SFI, and uncertified private properties against local scale Montreal criteria using triangulation of evidence from management documents, staff interviews, and field inspections. Certified properties were associated with improved internal management systems and improved practices for biodiversity conservation. However, our data suggest that certification does not necessarily involve fulfillment of all Montreal criteria, such as adherence to sustained timber yield, consideration of multiple social issues, or ecological monitoring at multiple temporal and spatial scales. In northern hardwood stands in central Vermont, we compared three FSC certified and three uncertified that were analogous in terms harvesting date, silvicultural treatment type, forest type, and general location. The uncertified sites were randomly selected to remove bias. We conducted stand structural analysis of both live trees and standing and downed coarse woody debris, and also developed 10-year growth projections using FVS/NE-TWIGS. Our data suggest that FSC certified stands had similar timber economic value, similar live tree structure, and similar tree carbon storage, but significantly greater residual coarse woody debris than comparable uncertified harvested stands