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Search Results: 1 - 10 of 144195 matches for " B Hallen "
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“Ethnophilosophy” Redefined?
B Hallen
Thought and Practice: A Journal of the Philosophical Association of Kenya , 2010,
Abstract: The meaning of the term “ethnophilosophy” has evolved in both a significant and controversial variety of ways since it was first introduced by Paulin Hountondji in 1970. It was first challenged by the Kenyan philosopher, H. Odera Oruka, as based upon Hountondji’s unfair appreciation of Africa’s indigenous cultural heritage. Barry Hallen and J. Olubi Sodipo, using a form of analytic philosophy as foundational, thereafter argued that Yoruba ordinary language discourse also served to undermine Hountondji’s critique. The later work of the Ghanaian philosopher, Kwame Gyekye, and the Kenyan D. A. Masolo have further legitimized the epistemological status of elements of African culture that once would have been labeled as of no genuine philosophical significance because they were ‘ethnophilosophical’ in character. The end result of this debate seems to be that both the form and content of philosophy in culture generally must be relativized. The most significant consequence of this would be that African and non-Western philosophy generally would finally be culturally liberated from the oppressive influence, indeed dominance, of what has conventionally come to be known as ‘mainstream’ (Western) philosophy.
Degradación de la tenacidad al impacto durante el envejecimiento acelerado de soldadura en acero microaleado
Vargas-Arista, B.,Hallen, J. M.,Albiter, A.,ángeles-Chávez, C.
Revista de Metalurgia , 2008,
Abstract: The effect of accelerated aging on the toughness and fracture of the longitudinal weld metal of an API5L-X52 linepipe steel was evaluated by Charpy V-notch impact test, fracture analysis and transmission electron microscopy. Aging was performed at 250 °C for 100 to 1000 h. The impact results indicated a significant reduction in the fracture energy and impact toughness as a function of aging time, which were achieved by the scanning electron microscope fractographs that showed a decrease in the vol fraction of microvoids by Charpy ductile failure with the aging time, which favored the brittle fracture by transgranular cleavage. The minimum vol fraction of microvoids was reached at 500 h due to the peak-aged. The microstructural analysis indicated the precipitation of transgranular iron nanocarbides in the aged specimens, which was related to the deterioration of toughness and change in the ductile to brittle behavior. Se evaluó, mediante el ensayo de impacto Charpy, fractografía y microscopia electrónica de transmisión el efecto del envejecimiento acelerado sobre la tenacidad y fractura de la soldadura en tubería de acero API5L-X52. El envejecimiento se realizó a 250 °C por 1.000 h, con control cada 100 h. Los resultados de impacto indicaron una disminución en la energía de fractura y tenacidad al impacto en función del tiempo del envejecimiento, los cuales se evidenciaron mediante fractografía, por la reducción en la fracción volumétrica de microhuecos por fractura dúctil con el tiempo, favoreciendo la fractura frágil por clivaje transgranular. Sin embargo, a 500 h, se observó la fracción volumétrica mínima debido al pico del envejecimiento. El análisis microestructural evidenció la precipitación de nanocarburos de hierro transgranulares en las muestras envejecidas, la cual se relaciono con la pérdida de tenacidad y cambio en el comportamiento dúctil a frágil, confirmado por fractografía.
Anastral spindle assembly and γ-tubulin in Drosophila oocytes
Sharyn A Endow, Mark A Hallen
BMC Cell Biology , 2011, DOI: 10.1186/1471-2121-12-1
Abstract: We show, for the first time, using a bright GFP fusion protein and live imaging, that the Drosophila maternally-expressed γTub37C is present at low levels in oocyte meiosis I spindles. Despite this, we find that formation of bipolar meiosis I spindles does not require functional γTub37C, extending previous findings by others. Fluorescence photobleaching assays show rapid recovery of γTub37C in the meiosis I spindle, similar to the cytoplasm, indicating weak binding by γTub37C to spindles, and fits of a new, potentially more accurate model for fluorescence recovery yield kinetic parameters consistent with transient, diffusional binding.The FRAP results, together with its mutant effects late in meiosis I, indicate that γTub37C may perform a role subsequent to metaphase I, rather than nucleating microtubules for meiosis I spindle formation. Weak binding to the meiosis I spindle could stabilize pre-existing microtubules or position γ-tubulin for function during meiosis II spindle assembly, which follows rapidly upon oocyte activation and completion of the meiosis I division.Anastral spindles assemble without centrosomes by a pathway that is still not fully understood. In particular, the mechanism by which microtubule nucleation occurs has not been well defined. Although chromatin has been shown to play an essential role [1], the involvement of the microtubule nucleator, γ-tubulin, is still an open question. γ-Tubulin localizes to centrosomes and other microtubule organizing centers in mitosis and is essential for nucleating microtubules in organisms as diverse as yeast, Drosophila, Xenopus, humans, and higher plants [2-5]. γ-Tubulin is also found on spindle microtubules, where it has been proposed to nucleate microtubules for spindle maintenance by functioning in a chromatin-mediated nucleation pathway that augments the dominant pathway of nucleation by centrosomes [6,7].γ-Tubulin is present in cells as a large ring complex, γTuRC, comprising 12-13 γ-tubulin molecules a
CSI-aided MAC with Multiuser Diversity for Cognitive Radio Networks
Yuan Lu,Alexandra Duel-Hallen
Computer Science , 2011,
Abstract: Cognitive Radio (CR) aims to increase the spectrum utilization by allowing secondary users (SU) to access unused licensed spectrum bands. To maximize the throughput given limited sensing capability, SUs need to strike a balance between sensing the channels that are not heavily used by primary users (PU) and avoiding collisions with other SUs. To randomize sensing decisions without resorting to multiuser sensing policies, it is proposed to exploit the spatially-variant fading channel conditions on different links by adapting the reward to the channel state information (CSI). Moreover, the proposed channel-adaptive policy favors links with high achievable transmission rate and thus further improves the network throughput.
Channel-Adaptive Sensing Strategy for Cognitive Radio Ad Hoc Networks
Yuan Lu,Alexandra Duel-Hallen
Computer Science , 2012,
Abstract: In Cognitive Radio (CR) ad hoc networks, secondary users (SU) attempt to utilize valuable spectral resources without causing significant interference to licensed primary users (PU). While there is a large body of research on spectrum opportunity detection, exploitation, and adaptive transmission in CR, most existing approaches focus only on avoiding PU activity when making sensing decisions. Since the myopic sensing strategy results in congestion and poor throughput, several collision-avoidance sensing approaches were investigated in the literature. However, they provide limited improvement. A channel-aware myopic sensing strategy that adapts the reward to the fading channel state information (CSI) of the SU link is proposed. This CSI varies over the CR spectrum and from one SU pair to another due to multipath and shadow fading, thus randomizing sensing decisions and increasing the network throughput. The proposed joint CSI adaptation at the medium access control (MAC) and physical layers provides large throughput gain over randomized sensing strategies and/or conventional adaptive transmission methods. The performance of the proposed CSI-aided sensing strategy is validated for practical network scenarios and demonstrated to be robust to CSI mismatch, sensing errors, and spatial channel correlation.
Adaptation to the Primary User CSI in Cognitive Radio Sensing and Access
Yuan Lu,Alexandra Duel-Hallen
Computer Science , 2013,
Abstract: In Cognitive Radio (CR) networks, multiple secondary network users (SUs) attempt to communicate over wide potential spectrum without causing significant interference to the Primary Users (PUs). A spectrum sensing algorithm is a critical component of any sensing strategy. Performance of conventional spectrum detection methods is severely limited when the average SNR of the fading channel between the PU transmitter and the SU sensor is low. Cooperative sensing and advanced detection techniques only partially remedy this problem. A key limitation of conventional approaches is that the sensing threshold is determined from the miss detection rate averaged over the fading distribution. In this paper, the threshold is adapted to the instantaneous PU-to-SU Channel State Information (CSI) under the prescribed collision probability constraint, and a novel sensing strategy design is proposed for overlay CR network where the instantaneous false alarm probability is incorporated into the belief update and the reward computation. It is demonstrated that the proposed sensing approach improves SU confidence, randomizes sensing decisions, and significantly improves SU network throughput while satisfying the collision probability constraint to the PUs in the low average PU-to-SU SNR region. Moreover, the proposed adaptive sensing strategy is robust to mismatched and correlated fading CSI and improves significantly on conventional cooperative sensing techniques. Finally, joint adaptation to PU channel gain and SU link CSI is explored to further improve CR throughput and reduce SU collisions.
Altered Nucleotide-Microtubule Coupling and Increased Mechanical Output by a Kinesin Mutant
Hong-Lei Liu, Mark A. Hallen, Sharyn A. Endow
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0047148
Abstract: Kinesin motors hydrolyze ATP to produce force and do work in the cell – how the motors do this is not fully understood, but is thought to depend on the coupling of ATP hydrolysis to microtubule binding by the motor. Transmittal of conformational changes from the microtubule- to the nucleotide-binding site has been proposed to involve the central β-sheet, which could undergo large structural changes important for force production. We show here that mutation of an invariant residue in loop L7 of the central β-sheet of the Drosophila kinesin-14 Ncd motor alters both nucleotide and microtubule binding, although the mutated residue is not present in either site. Mutants show weak-ADP/tight-microtubule binding, instead of tight-ADP/weak-microtubule binding like wild type – they hydrolyze ATP faster than wild type, move faster in motility assays, and assemble long spindles with greatly elongated poles, which are also produced by simulations of assembly with tighter microtubule binding and faster sliding. The mutated residue acts like a mechanochemical coupling element – it transmits changes between the microtubule-binding and active sites, and can switch the state of the motor, increasing mechanical output by the motor. One possibility, based on our findings, is that movements by the residue and the loop that contains it could bend or distort the central β-sheet, mediating free energy changes that lead to force production.
Comportamiento Mecánico de Grietas No Coplanares en Tubos Aplicando el Método de los Elementos Finitos/[title] [title language=en]Mechanical Behavior of Non-Coplanar Cracks in Pipes Applying the Finite Elements Method
Morales,A.; González,J. L.; Hallen,J. M.;
Información tecnológica , 2004, DOI: 10.4067/S0718-07642004000600005
Abstract: planar-type cracks formed at different depths within tube walls, which are generally caused by hydrogen-induced cracking were analyzed. modeling was carried out on the interaction of pressurized cracks contained within the tube wall with internal pressure prior to their coalescence, varying the radius of the defects. the finite elements method was applied under non-linear conditions of the material following the isotropic hardening law and considering the properties of the material (api 5l-x52). the results suggested the evolution of stress fields and deformations in the crack tips area as a function of the pressure of the defect. defects of less than 38.1 mm resist a pressure of 70 to 124 mpa, and those larger than 63.5 mm severely affect the mechanical integrity. the function of the critical pressure supported by symmetrical cracks is of the potential type, and this pressure produces the interaction which plastifies the region among the cracks, prior to stepping
Comportamiento Mecánico de Grietas No Coplanares en Tubos Aplicando el Método de los Elementos Finitos/[title] [title language=en]Mechanical Behavior of Non-Coplanar Cracks in Pipes Applying the Finite Elements Method
A. Morales,J. L. González,J. M. Hallen
Información Tecnológica , 2004,
Abstract: Se analizan grietas del tipo planar formadas a diferentes profundidades del espesor del tubo y que generalmente son causadas por el agrietamiento inducido por hidrógeno. Se modeló la interacción de grietas presurizadas contenidas en la pared del tubo con presión interna previas a su coalescencia, variando el radio de los defectos. Se aplicó el método de los elementos finitos en condiciones no lineales del material bajo la ley de endurecimiento isotrópico y considerando las propiedades del material (API 5L-X52). Los resultados indican la evolución de los campos de esfuerzos y deformaciones en el área de puntas de grieta en función de la presión del defecto. Defectos menores a 38.1 mm resisten una presión de 70 hasta 124 MPa, y mayores de 63.5 mm afectan severamente la integridad mecánica. La función de la presión critica que soportan grietas simétricas es de tipo potencial y esta presión produce la interacción que plastifica la región entre las grietas, previamente al escalonamiento Planar-type cracks formed at different depths within tube walls, which are generally caused by hydrogen-induced cracking were analyzed. Modeling was carried out on the interaction of pressurized cracks contained within the tube wall with internal pressure prior to their coalescence, varying the radius of the defects. The finite elements method was applied under non-linear conditions of the material following the isotropic hardening law and considering the properties of the material (API 5L-X52). The results suggested the evolution of stress fields and deformations in the crack tips area as a function of the pressure of the defect. Defects of less than 38.1 mm resist a pressure of 70 to 124 MPa, and those larger than 63.5 mm severely affect the mechanical integrity. The function of the critical pressure supported by symmetrical cracks is of the potential type, and this pressure produces the interaction which plastifies the region among the cracks, prior to stepping
Enabling Adaptive Rate and Relay Selection for 802.11 Mobile Ad Hoc Networks
Neil Mehta,Alexandra Duel-Hallen,Wenye Wang
Computer Science , 2011,
Abstract: Mobile ad hoc networks (MANETs) are self-configuring wireless networks that lack permanent infrastructure and are formed among mobile nodes on demand. Rapid node mobility results in dramatic channel variation, or fading, that degrades MANET performance. Employing channel state information (CSI) at the transmitter can improve the throughput of routing and medium access control (MAC) protocols for mobile ad hoc networks. Several routing algorithms in the literature explicitly incorporate the fading signal strength into the routing metric, thus selecting the routes with strong channel conditions. While these studies show that adaptation to the time-variant channel gain is beneficial in MANETs, they do not address the effect of the outdated fading CSI at the transmitter. For realistic mobile node speeds, the channel gain is rapidly varying, and becomes quickly outdated due the feedback delay. We analyze the link throughput of joint rate adaptation and adaptive relay selection in the presence of imperfect CSI. Moreover, for an 802.11 network that employs geographic opportunistic routing with adaptive rate and relay selection, we propose a novel method to reduce the effect of the feedback delay at the MAC layer in the presence of Rayleigh fading. This method exploits channel reciprocity and fading prediction and does not require significant modification to the existing 802.11 frame structure. Extensive network simulations demonstrate that the proposed approach significantly improves the throughput, delay, and packet delivery ratio for high mobile velocities relative to previously proposed approaches that employ outdated CSI at the transmitter.
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