Browsing by Subject "Model"

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Open Access
Assessing creativity in design education: Analysis of creativity factors in the first-year design studio
(Elsevier, 2012-05) Demirkan, H.; Afacan, Yasemin
The aim of this study is to explore creativity in design education and identify the creativity assessment indicators in the first-year design studio. A measurement tool of 41 items that consists of the artifact creativity, design elements and assembly of design elements were utilised for the assessment of 210 artifacts. Results oldie exploratory and confirmatory factor analysis indicated three main design creativity factors. The primary factor consists of the novelty and affective characteristics of artifact that are associated with its shape. The second factor has the elaboration characteristics that are integrated with its geometric and figure-ground relations and harmony of design elements. The third factor consists of rhythm, repetition, unity, order and number of design elements. (c) 2011 Elsevier Ltd. All rights reserved.
Open Access
Capital dependent population growth induces cycles
(Elsevier, 2011-09) Yuksel, M. K.
Cobb–Douglas type production functions and time-delay are not sufficient for the economy to behave cyclic. However, capital dependent population dynamics can enforce Hopf bifurcation. 2011 Elsevier Ltd. All rights reserved.
Open Access
Charged bosons in a quasi-one-dimensional system
(American Physical Society, 2000) Tanatar, Bilal; Davoudi, B.; Kohandel, M.
The ground-state properties of a system of charged bosons in a quasi-one-dimensional model with a neutralizing background are investigated within the hypernetted-chain approximation. Strong correlation effects drive the system from a homogeneous fluid phase toward a more ordered structure akin to Wigner crystallization in higher-dimensional charged quantum systems. The ordered phase of charged bosons is signaled by the development of a peak in the static structure factor, which is analyzed as a function of the density and the lateral width of the one-dimensional structure. We also calculate the pair-distribution function, the ground-state energy, and the local-field correction, and compare our results with other theoretical approaches.
Open Access
Comments on effective forecasting and judgmental adjustments: an empirical evaluation and strategies for improvement in supply-chain planning
(Elsevier, 2009) Önkal, D.
Demand forecasting is a crucial aspect of the planning process in supply-chain companies. The most common approach to forecasting demand in these companies involves the use of a computerized forecasting system to produce initial forecasts and the subsequent judgmental adjustment of these forecasts by the company's demand planners, ostensibly to take into account exceptional circumstances expected over the planning horizon. Making these adjustments can involve considerable management effort and time, but do they improve accuracy, and are some types of adjustment more effective than others? To investigate this, we collected data on more than 60,000 forecasts and outcomes from four supply-chain companies. In three of the companies, on average, judgmental adjustments increased accuracy. However, a detailed analysis revealed that, while the relatively larger adjustments tended to lead to greater average improvements in accuracy, the smaller adjustments often damaged accuracy. In addition, positive adjustments, which involved adjusting the forecast upwards, were much less likely to improve accuracy than negative adjustments. They were also made in the wrong direction more frequently, suggesting a general bias towards optimism. Models were then developed to eradicate such biases. Based on both this statistical analysis and organisational observation, the paper goes on to analyse strategies designed to enhance the effectiveness of judgmental adjustments directly.
Open Access
Dating the sea of Marmara sediments by a uniform mixing model
(Elsevier, 1996) Gökmen, A.; Yıldız, M.; Erten, H. N.; Salihoğlu, İ.
The sedimentation rates and ‘t”Pb fluxes on sediment surfaces were measured in the north, northwestern and southwestern parts of the Sea of Marmara. Each core had varying thickness of constant 2’0Pb activity regions followed by a decreasing part with sediment depth. The sedimentation rates of the samples collected from the Bosporus and the Dardanelles could not be analysed due to the homogenization of activity in the strong currents of these straits. A untform mixing model is proposed for the simultaneous analysis of sedimentation rates, “‘Pb fluxes and mixing depths from the experimental data. In this model, the parameters were obtained by minimizing the multi-dimensional parameter space using a grid search algorithm. The 2’0Pbfluxes were found to be about 0.048 Bqcm-’ year-t for all sampling sites. The mass sedimentation rates were 0.19 and 0.073gcm-2 year-’ at the shelves of the Bosporus and the Dardanelles and 0.055 and 0.064 g cm-’ year -’ in the northwestern and middle northern basins, respectively, of the Sea of Marmara.
Open Access
Detecting squeezed phonons through an indirect radiative transition
(A I P Publishing LLC, 1997-04-27) MÜstecaplioglu, Ö. E.; Shumovsky, A. S.
A model of the effect of the longitudinal optical phonon number distribution on the Rabi oscillations of the photons involved in the associated indirect transition in a semiconductor is presented. It is shown that a faster cavity photon revival rate is obtained given an initial squeezed thermal phonon state. This is the result of pairwise correlations of phonons due to squeezing and also holds true when phonons are in squeezed vacuum or in squeezed number states; It may be possible to use this effect to detect squeezing in phonons. (C) 1997 American Institute of Physics.
Open Access
The effectiveness of foreign exchange interventions under a floating exchange rate regime for the Turkish economy: a post-crisis period analysis
(Routledge, 2006) Akinci, Ö.; Çulha, O. Y.; Özlale, Ü.; Şahinbeyoǧlu, G.
The reported study has two purposes: first, it attempts to improve the literature on foreign exchange interventions of the central banks for the emerging market economies, an area not previously studied in detail. The Turkish economy in the post-crisis period constitutes a good example in this context. Second, it proposes a new methodology, a time-varying parameter model, to analyse the effectiveness of the foreign exchange interventions. When the results from such an exercise are compared with those obtained from an event-study analysis, it is found that purchase-based interventions seem to be successful, especially after stabilization of the financial markets. In that sense, an asymmetry is detected regarding the effectiveness of interventions. Concerning the relationship between interest rates and exchange rates, it is found that the uncovered interest rate parity condition operates in an unconventional way, supporting the views put forward by recent emerging markets literature. © 2006 Taylor & Francis.
Open Access
Effects of screening on the two-dimensional electron transport properties in modulation doped heterostructures
(Elsevier, 1998-06) Sen, O.; Besikci, C.; Tanatar, Bilal
The effects of screening on the polar optical phonon scattering rates and on the transport properties of the two-dimensional electron gas in AlGaAs/GaAs modulation doped heterostructures have been investigated through Monte Carlo simulations incorporating the three valleys of the conduction band, size quantization in the Gamma valley and the lowest three subbands in the quantum-well. At typical sheet densities observed in modulation doped field-effect transistors, screening considerably affects the electron transport properties under moderately large fields and at low temperatures, by lowering the intrasubband polar-optical phonon scattering rates especially in the first subband. The results show that screening, which is usually ignored in device Monte Carlo simulations, should be included in the simulation in order to be able to predict the device performance correctly. (C) 1998 Elsevier Science Ltd. All rights reserved.
Open Access
Electronic excited states of the CP29 antenna complex of green plants: a model based on exciton calculations
(Springer / Kluwer Academic Publishers, 2000) İşerı, E. İ.; Albayrak, D.; Gülen, D.
We have suggested a model for the electronic excited states of the minor plant antenna, CP29, by incorporating a considerable part of the current information offered by structure determination, site-directed mutagenesis, and spectroscopy in the modeling. We have assumed that the electronic excited states of the complex have been decided by the chlorophyll-chlorophyll (Chl) and Chl-protein interactions and have modeled the Coulombic interaction between a pair of Chls in the point-dipole approximation and the Chl-protein interactions are treated as empirical fit parameters. We have suggested the Qy dipole moment orientations and the site energies for all the chlorophylls in the complex through a simultaneous simulation of the absorption and linear dichroism spectra. The assignments proposed have been discussed to yield a satisfactory reproduction of all prominent features of the absorption, linear and circular dichroism spectra as well as the key spectral and temporal characteristics of the energy transfer processes among the chlorophylls. The orientations and the spectral assignments obtained by relatively simple exciton calculations have been necessary to provide a good point of departure for more detailed treatments of structure-function relationship in CP29. Moreover, it has been discussed that the CP29 model suggested can guide the studies for a better understanding of the structure-function relationship in the major plant antenna, LHCII.
Open Access
Experimental and model based investigation of period doubling phenomenon in human steady state visual evoked potential responses
(2018-07) Tuncel, Yiğit
Objective. Previous human Steady State Visual Evoked Potential (SSVEP) experiments have yielded different results regarding the range of stimulus frequencies in which Period Doubling (PD) behavior is observed. There also is lacking information about the consistency and repeatability of the occurences of subharmonic oscillations. The neural mechanism of such oscillations have also not been explored. To elaborate these rather unknown aspects of the PD behavior in SSVEP responses, an experimental and model based approach has been taken. Approach. The experimental side of the study aims at obtaining experimental and statistical data regarding the frequency range of PD generation and also investigates other characteristics of PD. In two sets of experiments, seven subjects were presented a sinusoidal flickering light stimulus with frequencies varying from 15 Hz to 42 Hz. To observe the short term repeatability in PD generation, another set of 5 successive experiments performed on five subjects with 10-minute breaks in between. To obtain the SSVEP responses, filtering, signal averaging and Power Spectral Density (PSD) estimation were applied to the recorded EEG. From the PSD estimates, Subharmonic Occurrence Rates (SORs) were calculated for each experiment and were used along with ANOVA for interpreting the outcomes of the short term repeatability experiments. The model based side of the study aims at explaining the observed phenomena in mathematical terms. For this purpose, Robinson’s Corticothalamic Model was implemented in both C and Simulink. The experimental procedure was reanimated on the model and the subharmonic generation in the model depending on different values for parameters was observed. The feedback loop that is responsible for the generation of subharmonic components was identified in the model, and this loop was isolated from the rest of the model and further analyzed with a describing function approach. Main Results. The experimental results showed that although fundamental (excitation frequency) and second harmonic components appear in almost all SSVEP spectra, there is considerable inter-subject and intra-subject variability regarding PD occurrence. PD occurs for all stimulus frequencies from 15 Hz to 42 Hz when all subjects are considered together. Furthermore, the statistical analyses of short term repeatability experiments suggest that in the short term, PD generation is consistent when all frequencies are considered together but for a single frequency significant short term differences occur. There also is considerable variation in the ratio of subharmonic amplitude to fundamental amplitude across different frequencies for a given subject. The modelling results showed that the subharmonic oscillations in the model are of resonance nature and that they can be obtained virtually in any frequency interval depending on the values of the parameters in the system. The intra-thalamic feedback loop in the model is identified to be the potential source of subharmonic oscillations in the system output. When isolated from the rest of the model and examined by itself, it has been found that this feedback loop can show a resonance phenomenon at the subharmonic frequency. By deriving a set of equations containing the necessary conditions for this resonance phenomenon, a semi-analytical method by which one can find the existence of these oscillations has been developed. Significance. From the experimental studies, important results and statistical data are obtained regarding PD generation. Our results indicate that modelling studies should attempt to generate PD for a broader range of stimulus frequencies by adjusting the parameter values. It is argued that SSVEP based BCI applications would likely benefit from the utilization of subharmonics in classification. Our modelling study is the first to investigate the source of subharmonic oscillations on a mathematical brain model. An experimental verification of the potential origin of such oscillations, which was identified to be the intra-thalamic loop, would be an important work. The proposed semi-analytical method could potentially be used to speed up a future parameter sweep study. We observed that in the current model alpha oscillation and subharmonic oscillations are in some way interrelated and they can not be generated together for any stimulation frequency. This is referred to as alpha entrainment, and is visible only for some stimulation frequencies in experimental results. Thus, we claim that the model is insufficient in explaining the PD phenomenon in SSVEP responses.
Open Access
Formation of quantum structures on a single nanotube by modulating hydrogen adsorption
(American Physical Society, 2003) Gülseren, O.; Yildirim, T.; Çıracı, Salim
Using first-principles density functional calculations we showed that quantum structures can be generated on a single carbon nanotube by modulating the adsorption of hydrogen atoms. The band gap of the hydrogen-free zone of the tube widens in the adjacent hydrogen covered zone. The sudden variation of the band gap leads to band offsets at the conduction- and valence-band edges. At the end, the band gap of the whole system is modulated along the axis of the tube, which generates quantum wells or quantum dots. Specific electronic states are confined in these quantum wells. The type and radius of the nanotube and the extent and sequence of hydrogen-free and hydrogen-covered zones can provide several options to design a desired optoelectronic nanodevice.
Open Access
Grace periods in sovereign debt
(Wiley-Blackwell Publishing Ltd., 1999) Bac, M.
This paper presents a theoretical analysis of grace periods in the context of an overhang of external debt creating a tax on domestic investment. The grace period arises as a Nash equilibrium strategy of the creditor in a dynamic, noncooperative game. Its length is shown to depend on the planning horizon of the parties, the discount factor and the growth prospect of the debtor country.
Open Access
Hydrogenation of naphthalene and methylnaphthalene: modeling and spectroscopy
(Elsevier BV, 2002) Sayan, Ş.; Paul, J.
In situ infrared spectra of 1-methylnaphthalene (1-MeNapht)hydrogenation, over sulfided NiMo/Al2O3-TiO2 catalysts, were compared with theoretically derived properties of methylnaphthalene and its bicyclic products: MeDilin, MeTetralin, MeOctalin and MeDecalin, and with conversion data from literature. Comparisons were also made between the un-substituted and methyl-substituted two-rings, and between the 1- and 5-methyl isomers of 1,4-dihydronaphthalene (dilin) and 1,2,3,4-tetrahydronaphthalene (tetralin). IR spectra of MeNapht adsorption, on the sulfided catalyst, were matched with data for adsorption on the catalyst without sulfidation and the empty support. Surface bound MeNapht is observed below 250°C on all catalysts. MeNapht adsorption suppresses OH groups nondiscriminatory on the empty support and the metal loaded catalyst. We relate the results to previous data on the interaction between the supported metal sulfides and titanium modified aluminas. Calculated total energies, and experimentally derived heats of formation, pointed at decahydronaphthalene (decalin) as the dominant product of naphthalene hydrogenation, with tetralin as an abundant intermediate, and dilin and 1,2,3,4,5,6,7,8-octahydronaphthalene (octalin) as short lived transient stages. The spectroscopic modeling showed that the orbital fingerprints of the five bicyclic compounds were not distinctly different, nor more than marginally modified by methyl substitution or isomerization. The only significant difference came at the highest occupied orbital, where a high naphthalene density of states (DoS) overlapped with the valence bands of metal or metal sulfide catalysts. The vibrational bands for naphthalene, dilin, tetralin and octalin were well separated. Octalin and decalin, alone, have similar vibrational spectra. Upheaval of ring degeneracy for methyl-substituted two-ring structures broadened all infrared bands in a characteristic way.
Open Access
The influence of early efficacy beliefs on teams' reactions to failing to reach performance goals
(Blackwell Publishing Ltd, 2011) Porter, C. O. L. H.; Gogus, I.; Yu, R. C. F.
Although a considerable amount of theoretical and empirical attention has been devoted to understanding individuals' responses to goal–performance discrepancies (GPDs), little attention has been devoted to examining how teams respond to GPDs. The present research sought to examine how teams responded to negative GPDs. We predicted that failing to reach higher goals would be perceived as less negative than failing to reach lower goals, and we examined the moderating influence of setting higher versus lower goals on how teams responded to performance that fell short of those goals. We also examined the role that efficacy beliefs that were formed early in those teams played in further explaining these effects. Results from 94 teams who all failed to reach self-set goals revealed that teams that failed to reach higher goals downwardly revised their goals less than teams that failed to reach lower goals. Early efficacy beliefs further explained these effects. High efficacy beliefs lessened the negative effects of failing to reach lower goals on subsequent goals. High efficacy beliefs also lessened the negative effects of failing to reach higher goals while low efficacy beliefs strengthened the negative effects of failing to reach higher goals. The implications of these findings for theory, research, and practice are discussed.
Open Access
An interdisciplinary heuristic evaluation method for universal building design
(Elsevier, 2009) Afacan, Yasemin; Erbug, C.
This study highlights how heuristic evaluation as a usability evaluation method can feed into Current building design practice to conform to universal design principles. It provides a definition of universal usability that is applicable to an architectural design context. It takes the seven universal design principles as a set of heuristics and applies an iterative sequence of heuristic evaluation in a shopping mall, aiming to achieve a cost-effective evaluation process. The evaluation was composed of three consecutive sessions. First, five evaluators from different professions were interviewed regarding the construction drawings in terms of universal design principles. Then, each evaluator was asked to perform the predefined task scenarios. In subsequent interviews, the evaluators were asked to reanalyze the construction drawings. The results showed that heuristic evaluation could successfully integrate universal usability into Current building design practice in two ways: (i) it promoted an iterative evaluation process combined with multi-sessions rather than relying on one evaluator and on one evaluation session to find the maximum number of usability problems, and (ii) it highlighted the necessity of an interdisciplinary ad hoc committee regarding the heuristic abilities of each profession. A multi-session and interdisciplinary heuristic evaluation method can save both the project budget and the required time, while ensuring a reduced error rate for the universal usage of the built environments. (C) 2008 Elsevier Ltd. All rights reserved.
Open Access
On the mechanisms of InGaN electron cooler in InGaN/GaN light-emitting diodes
(Optical Society of America, 2014) Zhang, Z. H.; W. L.; Tan, S. T.; Ju, Z.; Ji, Y.; Kyaw, Z.; Zhang, X.; Hasanov, N.; Zhu, B.; Lu, S.; Zhang, Y.; Sun, X. W.; Demir, Hilmi Volkan
Electron overflow limits the quantum efficiency of InGaN/GaN light-emitting diodes. InGaN electron cooler (EC) can be inserted before growing InGaN/GaN multiple quantum wells (MQWs) to reduce electron overflow. However, detailed mechanisms of how the InGaN EC contributes to the efficiency improvement have remained unclear so far. In this work, we theoretically propose and experimentally demonstrate an electron mean-free-path model, which reveals the InGaN EC reduces the electron mean free path in MQWs, increases the electron capture rate and also reduces the valence band barrier heights of the MQWs, in turn promoting the hole transport into MQWs. (C) 2014 Optical Society of America
Open Access
Orbital magnetization of single and double quantum dots in a tight-binding model
(American Physical Society, 2003) Aldea, A.; Moldoveanu, V.; Niţǎ, M.; Manolescu, A.; Gudmundsson, V.; Tanatar, Bilal
We calculate the orbital magnetization of single and double quantum dots coupled both by Coulomb interaction and by electron tunneling. The electronic states of the quantum dots are calculated in a tight-binding model, and the magnetization is discussed in relation to the energy spectrum and to the edge and bulk states. We identify effects of chirality of the electronic orbits and of the anticrossing of the energy levels when the magnetic field is varied. We also consider the effects of detuning the energy spectra of the quantum dots by an external gate potential. We compare our results with the recent experiments of Oosterkamp et al. [Phys, Rev. Lett. 80, 4951 (1998)].
Open Access
Profile-encoding reconstruction for multiple-acquisition balanced steady-state free precession imaging
(John Wiley and Sons Inc., 2017) Ilicak, Efe; Senel, Lutfi Kerem; Biyik, Erdem; Çukur, Tolga
Purpose: The scan-efficiency in multiple-acquisition balanced steady-state free precession imaging can be maintained by accelerating and reconstructing each phase-cycled acquisition individually, but this strategy ignores correlated structural information among acquisitions. Here, an improved acceleration framework is proposed that jointly processes undersampled data across N phase cycles. Methods: Phase-cycled imaging is cast as a profile-encoding problem, modeling each image as an artifact-free image multiplied with a distinct balanced steady-state free precession profile. A profile-encoding reconstruction (PE-SSFP) is employed to recover missing data by enforcing joint sparsity and total-variation penalties across phase cycles. PE-SSFP is compared with individual compressed-sensing and parallel-imaging (ESPIRiT) reconstructions. Results: In the brain and the knee, PE-SSFP yields improved image quality compared to individual compressed-sensing and other tested methods particularly for higher N values. On average, PE-SSFP improves peak SNR by 3.8 ± 3.0 dB (mean ± s.e. across N = 2–8) and structural similarity by 1.4 ± 1.2% over individual compressed-sensing, and peak SNR by 5.6 ± 0.7 dB and structural similarity by 7.1 ± 0.5% over ESPIRiT. Conclusion: PE-SSFP attains improved image quality and preservation of high-spatial-frequency information at high acceleration factors, compared to conventional reconstructions. PE-SSFP is a promising technique for scan-efficient balanced steady-state free precession imaging with improved reliability against field inhomogeneity. Magn Reson Med 78:1316–1329, 2017.
Open Access
Resistive Switching based electro-optical modulation
(Wiley, 2014-09-08) Battal, E.; Ozcan, A.; Okyay, Ali Kemal
Resistive switching enables optical modulation via atomic scale modifications that induce change in the refractive index of active device materials. The formation of filaments and migration of atoms around these filaments between high resistance and low resistance states results in the modulation of the free carrier concentration and, hence, the optical constants of the material.
Open Access
Smart markers for watershed-based cell segmentation
(Public Library of Science, 2012-11-12) Koyuncu, C. F.; Arslan, S.; Durmaz, I.; Cetin Atalay, R.; Gunduz Demir, C.
Automated cell imaging systems facilitate fast and reliable analysis of biological events at the cellular level. In these systems, the first step is usually cell segmentation that greatly affects the success of the subsequent system steps. On the other hand, similar to other image segmentation problems, cell segmentation is an ill-posed problem that typically necessitates the use of domain-specific knowledge to obtain successful segmentations even by human subjects. The approaches that can incorporate this knowledge into their segmentation algorithms have potential to greatly improve segmentation results. In this work, we propose a new approach for the effective segmentation of live cells from phase contrast microscopy. This approach introduces a new set of "smart markers'' for a marker-controlled watershed algorithm, for which the identification of its markers is critical. The proposed approach relies on using domain-specific knowledge, in the form of visual characteristics of the cells, to define the markers. We evaluate our approach on a total of 1,954 cells. The experimental results demonstrate that this approach, which uses the proposed definition of smart markers, is quite effective in identifying better markers compared to its counterparts. This will, in turn, be effective in improving the segmentation performance of a marker-controlled watershed algorithm.