1,720,972 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Numerical characterization of high-frequency flow fluctuations in intracranial aneurysms
Full text linkRupture of intracranial aneurysms (IAs), saccular outpouching of blood vessels, can lead to death or permanent disability in majority of the patients. Hemodynamic forces are thought to play a key role in the process, but, currently, can only be estimated through computational fluid dynamic (CFD) simulations of patient's cerebrovasculature. IA flows have conventionally been considered laminar and stable; however, recent studies highlighted the presence of high-frequency flow fluctuations consistent with clinical evidence — a discrepancy attributed to conventional use of low-order CFD. The goal of this doctoral thesis was to investigate the nature and characteristic of these flow fluctuations, and its prevalence in IAs.
Towards that goal, we performed the first mesh, time, and solver refinement study to investigate the isolated impact of each modelling choice (Chapter 2). With 320-fold increase in mesh size, 25-fold increase in time-steps, and considering both a low-order solver and a high-order solver, we found that solver choice had a relatively higher impact on hemodynamics compared to other modelling assumptions.
Finding of these flow fluctuations also meant rethinking the conventional assumption of Newtonian rheology for blood. As a first step, we performed a controlled numerical experiment on an idealized stenotic vessel for Re=500-1000, which suggested that shear-thinning non-Newtonian rheology caused a delay of 10% in the critical Reynolds number (Chapter 3). We later investigated the practical impact of these findings on IA hemodynamics (Chapter 4). These findings suggested that assumption of shear- thinning rheology had a negligible impact, particularly when compared to effects of low-order solvers.
Due to limitations of current hemodynamic metrics in quantifying flow fluctuations, we proposed a frequency-based (Chapter 5). Using this tool and aforementioned findings, we performed high-resolution simulations of 19 IAs to characterize and investigate the prevalence of flow fluctuations (Chapter 6). Through proper orthogonal decomposition, we showed that, depending on the patient, a wide range of spectra exists, some with high concentration of energy in higher modes. We also found that approximately half of IA aneurysms are ‘turbulent-like and 30% can have cycle-to-cycle fluctuations. Further investigation is needed to establish the clinical impact of these flow fluctuations in IA rupture.Ph.D
Numerical characterization of high-frequency flow fluctuations in intracranial aneurysms
No full textRupture of intracranial aneurysms (IAs), saccular outpouching of blood vessels, can lead to death or permanent disability in majority of the patients. Hemodynamic forces are thought to play a key role in the process, but, currently, can only be estimated through computational fluid dynamic (CFD) simulations of patient's cerebrovasculature. IA flows have conventionally been considered laminar and stable; however, recent studies highlighted the presence of high-frequency flow fluctuations consistent with clinical evidence — a discrepancy attributed to conventional use of low-order CFD. The goal of this doctoral thesis was to investigate the nature and characteristic of these flow fluctuations, and its prevalence in IAs.
Towards that goal, we performed the first mesh, time, and solver refinement study to investigate the isolated impact of each modelling choice (Chapter 2). With 320-fold increase in mesh size, 25-fold increase in time-steps, and considering both a low-order solver and a high-order solver, we found that solver choice had a relatively higher impact on hemodynamics compared to other modelling assumptions.
Finding of these flow fluctuations also meant rethinking the conventional assumption of Newtonian rheology for blood. As a first step, we performed a controlled numerical experiment on an idealized stenotic vessel for Re=500-1000, which suggested that shear-thinning non-Newtonian rheology caused a delay of 10% in the critical Reynolds number (Chapter 3). We later investigated the practical impact of these findings on IA hemodynamics (Chapter 4). These findings suggested that assumption of shear- thinning rheology had a negligible impact, particularly when compared to effects of low-order solvers.
Due to limitations of current hemodynamic metrics in quantifying flow fluctuations, we proposed a frequency-based (Chapter 5). Using this tool and aforementioned findings, we performed high-resolution simulations of 19 IAs to characterize and investigate the prevalence of flow fluctuations (Chapter 6). Through proper orthogonal decomposition, we showed that, depending on the patient, a wide range of spectra exists, some with high concentration of energy in higher modes. We also found that approximately half of IA aneurysms are ‘turbulent-like and 30% can have cycle-to-cycle fluctuations. Further investigation is needed to establish the clinical impact of these flow fluctuations in IA rupture.Ph.D
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Sources of Error in Image-based Computational Fluid Dynamics Modeling of Common Carotid Arteries
Full text linkMagnetic resonance imaging is often used as a source for reconstructing vascular anatomy for the purpose of computational fluid dynamics (CFD) analysis. We recently observed large discrepancies in such “image-based” CFD models of the normal common carotid artery (CCA) derived from contrast enhanced MR angiography (CEMRA). A novel quantitative comparison of velocity profile shape of N=20 cases revealed an average 25% overestimation of velocities by CFD, attributed to a corresponding underestimation of lumen area in the CEMRA-derived geometries. We hypothesized that this was due to blurring of edges in the images caused by dilution of contrast agent during the relatively long elliptic centric CEMRA acquisitions, and confirmed this with MRI simulations. CFD simulations incorporating realistic inlet velocity profiles and non-Newtonian rheology had a negligible effect on velocity profile skewing, suggesting a role for other sources of error or modeling assumptions.MAS
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Improving the performance and efficiency of wireless networks using rate adaptation
Full text linkRecent years have seen a staggering increase in the deployment and utilization of wireless networks. More and more devices are being equipped with Wireless LAN (WLAN) cards to take advantage of the omnipresence of WLAN networks. Therefore, it has become necessary that the protocols used by WLANs are efficient and provide good performance. Rate Adaptation protocols are an important mechanism employed by WLANs to improve network performance. This dissertation develops three complementary techniques, which use rate adaptation to optimize and improve performance by i) performing rate adaptation to optimize energy consumption, ii) developing a more accurate technique to predict the frame delivery ratio that is used by rate adaptation protocols, and iii) jointly optimizing rate adaptation with data retransmission to maximize throughput.
More specifically, in i), we use extensive measurements to develop a simple yet accurate energy consumption model for 802.11n wireless cards. We use the model to drive the design of an energy aware rate adaptation scheme. A major benefit of a model-based rate adaptation is that applying a model allows us to eliminate frequent probes required in many existing rate adaptation schemes.
In ii), we find that the accuracy of existing delivery ratio calculation techniques is still limited due to bursty errors inherent to the wireless channel. We develop a new method for computing packet delivery rate that captures the burstiness of errors. Furthermore, we propose a new data interleaving technique, which leverages our framework to reduce the burstiness of errors, thereby improving frame delivery ratio.
Finally, in iii), we address the susceptibility of wireless networks to transmission failures due to dynamic channel conditions and unpredictable interference. To efficiently recover from failures, we propose a retransmission scheme where the receiver combines information received from multiple failed transmissions associated with the same frame. The protocol has two distinguishing features. First, it simultaneously supports partial retransmission and combines bits with low confidence. Second, it jointly optimizes the data rate of the retransmission and the information to be retransmitted to maximize throughput.Electrical and Computer Engineerin
Sources of Error in Image-based Computational Fluid Dynamics Modeling of Common Carotid Arteries
No full textMagnetic resonance imaging is often used as a source for reconstructing vascular anatomy for the purpose of computational fluid dynamics (CFD) analysis. We recently observed large discrepancies in such “image-based” CFD models of the normal common carotid artery (CCA) derived from contrast enhanced MR angiography (CEMRA). A novel quantitative comparison of velocity profile shape of N=20 cases revealed an average 25% overestimation of velocities by CFD, attributed to a corresponding underestimation of lumen area in the CEMRA-derived geometries. We hypothesized that this was due to blurring of edges in the images caused by dilution of contrast agent during the relatively long elliptic centric CEMRA acquisitions, and confirmed this with MRI simulations. CFD simulations incorporating realistic inlet velocity profiles and non-Newtonian rheology had a negligible effect on velocity profile skewing, suggesting a role for other sources of error or modeling assumptions.MAS
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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