Browsing by Subject "Problem solving"

Now showing 1 - 20 of 120

Open Access
Accuracy and efficiency considerations in the solution of extremely large electromagnetics problems
(IEEE, 2011) Gürel, Levent; Ergül, Özgür
This study considers fast and accurate solutions of extremely large electromagnetics problems. Surface formulations of large-scale objects lead to dense matrix equations involving millions of unknowns. Thanks to recent developments in parallel algorithms and high-performance computers, these problems can easily be solved with unprecedented levels of accuracy and detail. For example, using a parallel implementation of the multilevel fast multipole algorithm (MLFMA), we are able to solve electromagnetics problems discretized with hundreds of millions of unknowns. Unfortunately, as the problem size grows, it becomes difficult to assess the accuracy and efficiency of the solutions, especially when comparing different implementations. This paper presents our efforts to solve extremely large electromagnetics problems with an emphasis on accuracy and efficiency. We present a list of benchmark problems, which can be used to compare different implementations for large-scale problems. © 2011 IEEE.
Open Access
Adaptive decomposition and remapping algorithms for object-space-parallel direct volume rendering of unstructured grids
(Academic Press, 2007-01) Aykanat, Cevdet; Cambazoglu, B. B.; Findik, F.; Kurc, T.
Object space (OS) parallelization of an efficient direct volume rendering algorithm for unstructured grids on distributed-memory architectures is investigated. The adaptive OS decomposition problem is modeled as a graph partitioning (GP) problem using an efficient and highly accurate estimation scheme for view-dependent node and edge weighting. In the proposed model, minimizing the cutsize corresponds to minimizing the parallelization overhead due to the data communication and redundant computation/storage while maintaining the GP balance constraint corresponds to maintaining the computational load balance in parallel rendering. A GP-based, view-independent cell clustering scheme is introduced to induce more tractable view-dependent computational graphs for successive visualizations. As another contribution, a graph-theoretical remapping model is proposed as a solution to the general remapping problem and is used in minimization of the cell-data migration overhead. The remapping tool RM-MeTiS is developed by modifying the GP tool MeTiS and is used in partitioning the remapping graphs. Experiments are conducted using benchmark datasets on a 28-node PC cluster to evaluate the performance of the proposed models. © 2006 Elsevier Inc. All rights reserved.
Open Access
Algorithms for layout of disconnected graphs
(Elsevier, 2000) Doğrusöz, Uğur
We present efficient algorithms for the layout of disconnected objects in a graph (isolated nodes and components) for a specified aspect ratio. These linear and near-linear algorithms are based on alternate-bisection and two-dimensional packing methodologies. In addition, the parameters that affect the performance of these algorithms as well as the circumstances under which the two methodologies perform well are analyzed.
Open Access
Analysis of an arbitrary profile reflector antenna system having resistive type surface–E-polarization case
(IEEE, 2006-06) Oǧuzer, T.; Altıntaş, Ayhan; Nosich, A. I.
The regularized solution is performed for arbitrary shape conic section profile geometry. In this case the reflector surface is taken as made up of resistive type material. The problem is formulated depending on the circular In periodicity and then Fourier series coefficients of the surface current density are obtained. The resultant matrix equation is in the regularized form. Then the various numerical results are obtained for different eccentricity factor of the conic section and the resisitvity of the reflector surface. © 2006 IEEE.
Open Access
An analysis of heuristics in a dynamic job shop with weighted tardiness objectives
(Taylor & Francis, 1999) Kutanoglu, E.; Sabuncuoğlu, İ.
Meeting due dates as a reflection of customer satisfaction is one of the scheduling criteria that is frequently encountered in today's manufacturing environments. The natural quantification of this qualitative goal involves tardiness related measures. In this study, we consider the dynamic job shop scheduling problem with the weighted tardiness criterion. After we present a comprehensive literature survey on the topic, we measure the long-run performances of more than 20 single-pass dispatching rules under various experimental conditions. In this study, we pay special attention to recently proposed dispatching heuristics such as CEXSPT, CR+ SPT, S/RPT+ SPT, and Bottleneck Dynamics (BD). We also investigate the effects of six resource pricing schemes proposed recently for BD. Moreover, we extend the earlier versions of inserted idleness and identify the conditions in which these techniques can be applied without incurring too much computational cost. Future research directions are also outlined in light of the computational results. © 1999 Taylor & Francis Ltd.
Open Access
Analysis of the 2D nonconcentric large reflector antenna-in-radome system: H-polarization case
(IEEE, 2004) Oğuzer, Taner
2D nonconcentric reflector antenna-in-radome system is analyzed for H-polarization case. Rigorous formulation is performed depending on the MoR, Green's function and CSP methods. Larger geometries are solved in reasonable cpu times by using the FFT based algorithm in the computation of the Green's function.
Open Access
An animation system for fracturing of rigid objects
(Springer, Berlin, Heidelberg, 2005) Küçükyilmaz, Ayşe; Özgüç, Bülent
This paper describes a system for the animation of fracturing brittle objects. The system combines rigid body simulation methods with a constraint-based model to animate fracturing of arbitrary polyhedral shaped objects under impact. The objects are represented as sets of masses, where pairs of adjacent masses are connected via a distance-preserving linear constraint. Lagrange multipliers are used to compute the forces exerted by those constraints, where these forces determine how and where the object will break. However, a problem with existing systems is that the initial body models exhibit well-defined uniformity, which makes the generated animations unrealistic. This work introduces a method for generating more realistic cracks without any performance loss. This method is easy to implement and applicable on different models. © Springer-Verlag Berlin Heidelberg 2005.
Open Access
Approaches for inequity-averse sorting
(Elsevier, 2016) Karsu, Ö.
In this paper we consider multi-criteria sorting problems where the decision maker (DM) has equity concerns. In such problems each alternative represents an allocation of an outcome (e.g. income, service level, health outputs) over multiple indistinguishable entities. We propose three sorting algorithms that are different from the ones in the current literature in the sense that they apply to cases where the DM's preference relation satisfies anonymity and convexity properties. The first two algorithms are based on additive utility function assumption and the third one is based on the symmetric Choquet integral concept. We illustrate their use by sorting countries into groups based on their income distributions using real-life data. To the best of our knowledge our work is the first attempt to solve sorting problems in a symmetric setting.
Open Access
An approximation algorithm for computing the visibility region of a point on a terrain and visibility testing
(IEEE, 2014-01) Alipour, S.; Ghodsi, M.; Güdükbay, Uğur; Golkari, M.
Given a terrain and a query point p on or above it, we want to count the number of triangles of terrain that are visible from p. We present an approximation algorithm to solve this problem. We implement the algorithm and then we run it on the real data sets. The experimental results show that our approximation solution is very close to the real solution and compare to the other similar works, the running time of our algorithm is better than their algorithm. The analysis of time complexity of algorithm is also presented. Also, we consider visibility testing problem, where the goal is to test whether p and a given triangle of train are visible or not. We propose an algorithm for this problem and show that the average running time of this algorithm will be the same as running time of the case where we want to test the visibility between two query point p and q.
Open Access
Automatic multimedia cross-modal correlation discovery
(ACM, 2004-08) Pan, J.-Y.; Yang, H.-J.; Faloutsos, C.; Duygulu, Pınar
Given an image (or video clip, or audio song), how do we automatically assign keywords to it? The general problem is to find correlations across the media in a collection of multimedia objects like video clips, with colors, and/or motion, and/or audio, and/or text scripts. We propose a novel, graph-based approach, "MMG", to discover such cross-modal correlations. Our "MMG" method requires no tuning, no clustering, no user-determined constants; it can be applied to any multi-media collection, as long as we have a similarity function for each medium; and it scales linearly with the database size. We report auto-captioning experiments on the "standard" Corel image database of 680 MB, where it outperforms domain specific, fine-tuned methods by up to 10 percentage points in captioning accuracy (50% relative improvement).
Open Access
Batch scheduling to minimize the weighted number of tardy jobs
(Pergamon Press, 2007) Erel, E.; Ghosh, J. B.
In this paper, we address a single-machine scheduling problem with due dates and batch setup times to minimize the weighted number of tardy jobs. We give a pseudo-polynomial dynamic program and a fully-polynomial approximation scheme for the case where the due dates are uniform within a family.
Open Access
A beam search-based algorithm and evaluation of scheduling approaches for fexible manufacturing systems
(Taylor & Francis, 1998) Sabuncuoglu İ.; Karabuk, S.
This paper presents a new algorithm for the flexible manufacturing system (FMS) scheduling problem. The proposed algorithm is a heuristic based on filtered beam search. It considers finite buffer capacity, routing and sequence flexibilities and generates machine and automated guided vehicle (AGV) schedules for a given scheduling period. A new deadlock resolution mechanism is also developed as an integral part of the proposed algorithm. The performance of the algorithm is compared with several machine and AGV dispatching rules using mean flow time, mean tardiness and makespan criteria. It is also used to examine the effects of scheduling factors (i.e., machine and AGV load levels, routing and sequence flexibilities, etc.) on the system performance. The results indicate thai the proposed scheduling algorithm yields considerable improvements in system performance over dispatching rules under a wide variety of experimental conditions. © 1998 "IIE".
Open Access
Binary integer formulation for mixed-model assembly line balancing problem
(Pergamon Press, 1998-04-01) Gökçen, H.; Erel, E.
The assembly line balancing problem has been a focus of interest to the academicians of production/operations management for the last 40 years. Although there are numerous studies published on the various aspects of the problem, the number of studies on mixed-model assembly lines are relatively small. In this paper, a binary integer programming model for the mixed-model assembly line balancing problem is developed and some computational properties of the model are given.
Open Access
Capture resilient ElGamal signature protocols
(Springer, 2006-11) Acan, Hüseyin; Kaya, Kamer; Selçuk, Ali Aydın
One of the fundamental problems of public key cryptography is protecting the private key. Private keys are too long to be remembered by the user, and storing them in the device which performs the private key operation is insecure as long as the device is subject to capture. In this paper, we propose server-assisted protocols for the ElGamal signature scheme which make the system capture resilient in the sense that the security of the system is not compromised even if the signature device is captured. The protocols also have a key disabling feature which allows a user to disable the device's private key in case both the device and the password of the user are compromised simultaneously. © Springer-Verlag Berlin Heidelberg 2006.
Open Access
Chat mining for gender prediction
(Springer, 2006-10) Küçükyılmaz, Tayfun; Cambazoğlu, B. Barla; Aykanat, Cevdet; Can, Fazlı
The aim of this paper is to investigate the feasibility of predicting the gender of a text document's author using linguistic evidence. For this purpose, term- and style-based classification techniques are evaluated over a large collection of chat messages. Prediction accuracies up to 84.2% are achieved, illustrating the applicability of these techniques to gender prediction. Moreover, the reverse problem is exploited, and the effect of gender on the writing style is discussed. © Springer-Verlag Berlin Heidelberg 2006.
Open Access
Coaching engineering freshmen at Bilkent University
(Middle East Technical University, Faculty of Engineering, 2005) Özdemir, B. U.; Verhoeven, J. M.; Çelik, B. K.
In September 2003, the Faculty of Engineering at Bilkent University created an office to help first year students adapt to university life and start their academic studies effectively. For each of the three Departments of the Faculty, an 'academic student coordinator' offers personal coaching and consulting services for each individual student; the coordinator follows the progress of the students and contacts and advises them if necessary. The office offers an Engineering Orientation course that covers time management, study skills, problem solving and critical thinking. Besides this, the office offers tutorial sessions for physics courses, and organizes social activities.
Open Access
Çok kullanıcılı çok antenli sistemlerde iş birlikli iletim
(IEEE, 2008-04) Yazarel, Y. K.; Aktaş, Defne
Bu çalışmada işbirlikli, çok kullanıcılı, ve çok antenli bir haberleşme sisteminde telsiz erişim terminallerinin en iyi veri iletimi tekniğine ortaklaşa karar vermeleri problemini inceliyoruz. Burada pek çok çalışmadan farklı olarak kullanıcıların bireysel başarım hedefleri ve anten başına iletim gücü kısıtlamaları olduğu durumu ele alıyoruz. Önceki bir çalışmamızda bu kısıtlamalar altında en iyi sonucu bulan döngülü bir algoritma sunmuştuk. Ancak bu algoritma merkezi bir yapıda olduğu için tam anlamıyla dağıtılmış şekilde gerçeklenememektedir. Bununla birlikte basit yaklaşıklıklar kullanarak en iyiye yakın bir başarım sağlayan ve kısıtlı ve yerel veri iletimi ile gerçeklenebilecek etkin bir algoritma öneriyoruz.
Open Access
Combined-field solution of composite geometries involving open and closed conducting surfaces
(IEEE, 2005-04) Ergül, Özgür; Gürel, Levent
Combined-field integral equation (CFIE) is modified and generalized to formulate the electromagnetic problems of composite geometries involving both open and closed conducting surfaces. These problems are customarily formulated with the electric-field integral equation (EFIE) due to the presence of the open surfaces. With the new definition and application of the CFIE, iterative solutions of these problems are now achieved with significantly improved efficiency compared to the EFIE solution, without sacrificing the accuracy. © 2005 ACES.
Open Access
Comparison of surface-modeling techniques
(IEEE, 1997-07) Sertel, Kubilay; Gürel, Levent
Solution techniques based on surface integral equations are widely used in computational electromagnetics. The accurate surface models increase the accuracy solutions by using exact and flat-triangulation models for a sphere. For a required solution accuracy, the problem size is significantly reduced by using geometry models for the scatterers. The dependence of the accuracy of the solution on the geometry modeling is investigated.
Open Access
Componentwise bounds for nearly completely decomposable Markov chains using stochastic comparison and reordering
(Elsevier, 2005) Pekergin, N.; Dayar T.; Alparslan, D. N.
This paper presents an improved version of a componentwise bounding algorithm for the state probability vector of nearly completely decomposable Markov chains, and on an application it provides the first numerical results with the type of algorithm discussed. The given two-level algorithm uses aggregation and stochastic comparison with the strong stochastic (st) order. In order to improve accuracy, it employs reordering of states and a better componentwise probability bounding algorithm given st upper- and lower-bounding probability vectors. Results in sparse storage show that there are cases in which the given algorithm proves to be useful. © 2004 Elsevier B.V. All rights reserved.