Delay-based congestion control: Flow vs. BitTorrent swarm perspectives

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Design Oriented Aerodynamic Modelling of Wind Turbine Performance

The development of a wind turbine aerodynamics model using a Boundary Integral Equation model (BIEM) is presented. The methodology is valid to study inviscid unsteady flows around three dimensional bodies of arbitrary shape and arbitrarily moving with respect to the incoming flow. The extension of this methodology to study viscosity effects in turbine blade flow at high angle of attack is addressed and an approach to determine aerodynamic loads over a wide range of turbine operating conditions is proposed. Numerical applications considering a selected test cases from the NREL experimental dataset are presented. Finally, the application of the proposed turbine aerodynamics model into a multi-disciplinary study including aeroelasticity of pylon-turbine assembly and aeroacoustics modelling of induced noise is briefly described

Le contrôle de congestion dans les applications Pair-à-Pair : le cas de LEDBAT

In the last years, Internet delays are considerably growing, causing a performance deterioration of interactive applications. This phenomenon is getting worse with the increasing popularity of bandwidth-intensive applications, as video streaming, remote backup and P2P systems. The cause of these delays has been identified with the excess buffering inside the network, called “bufferbloat”. Research efforts in this direction head toward active queue management techniques and end-to-end congestion control. In this context, we investigated LEDBAT, a low-priority delay-based transport protocol introduced by BitTorrent. This protocol is designed to transfer large amount of data without affecting the delay experienced by other applications or users. First we analysed transport-level performance of LEDBAT using experimental measurement, simulation and analytical model. Specifically, we evaluated LEDBAT as is, comparing its performance to standard TCP or to other low priority protocols. We then identified a later-comer advantage and we proposed fLEDBAT, which re-introduces intra-protocol fairness maintaining the original LEDBAT objectives. Finally we studied the impact of the LEDBAT protocol on BitTorrent performance. Through simulations and real network experiments, we analysed how BitTorrent impacts on the buffer occupancy of the access node. BitTorrent performance was evaluated in terms of completion time, the main metric to assess the user quality of experience. Results showed that LEDBAT decreases the completion time with respect to standard TCP and significantly reduces the buffer occupancy, that translates in lower delays experienced by competing interactive applications.Durant ces dernières années, les délais de transmission sur Internet ont augmenté de manière considérable, causant une détérioration de performances des applications interactives. La cause de ces augmentations de délais est l’excès de mémoire tampon à l’intérieur du réseau, appelé "bufferbloat". Les efforts de recherche dans cette direction vont vers des techniques de gestion des files d’attente actives et des techniques de contrôle de congestion de bout-à-bout. Dans ce contexte, nous avons examiné LEDBAT, un protocole introduit par BitTorrent qui se base sur le délai au niveau transport, et conçu pour transférer grandes quantités de données sans affecter les délais expérimentés par d’autres applications ou utilisateurs. Nous avons analysé la performance de niveau de transport de LEDBAT avec de mesures expérimentales, de simulations et de modèles analytiques, en comparant ses performances au standard TCP ou à d’autre protocoles de failbe priorité. Nous avons ensuite identifié un problème d’iniquité, et nous avons proposé fLEDBAT, qui ré-introduit l’équité intra-protocole. Dans un deuxième temps, nous avons étudié l’impact du protocole LEDBAT sur la performance de BitTorrent. Par des simulations et des expérimentations sur réseaux réelles, nous avons analysé les effets de LEDBAT sur le remplissage des tampons des noeuds d’accès. Les performances de BitTorrent ont été évaluées en termes de temps d’exécution, qui reflète la qualité de l’expérience utilisateur. Dans les deux cas, les résultats ont montré que LEDBAT diminue le temps de traitement par rapport à TCP et réduit de manière significative l’utilisation de tampons, ce qui se traduit par une baisse des délais

Musculoskeletal disorders among robotic surgeons: A questionnaire analysis

Objective: Robotic surgical systems offer better workplace in order to relieve surgeons from prolonged physical efforts and improve their surgical outcomes. However, robotic surgery could produce musculoskeletal disorders due to the prolonged sitting position of the operator, the fixed position of the console viewer and the movements of the limbs. Until today, no one study has been reported concerning the association between robotics and musculoskeletal pain. The aim of this work was verify the prevalence of musculoskeletal disorders among Italian robotic surgeons. Material and methods: Between July 2011 and April 2012 a modified Standardized Nordic Questionnaire was delivered to thirty-nine Italian robotic centres. Twentytwo surgeons (56%) returned the questionnaires but only seventeen questionnaires (43.5%) were evaluable. Results: Seven surgeons (41.2%) reported musculoskeletal disorders, by since their first use of the robot which significantly persisted during the daily surgical activity (P < 0.001). Regarding the body parts affected, musculoskeletal disorders were mainly reported in the cervical spine (29.4%) and in the upper limbs (23.5%). Six surgeons (35.3%) defined the robotic console as less comfortable or neither comfortable/uncomfortable with a negative influence on their surgical procedures. Conclusions: In spite of some important limitations, our data showed musculoskeletal disorders due to posture discomfort with negative impact on daily surgical activity among robotic surgeons. These aspects could be due to the lack of ergonomic seat and to the fixed position of the console viewer which could have produced an inadequate spinal posture. The evaluation of these postural factors, in particular the development of an integrated and more ergonomic chair, could further improve the comfort feeling of the surgeon at the console and probably his surgical outcomes

Theoretical and Numerical Hydromechanics Analysis of Self-Pitching Propellers

The development of design-oriented modelling techniques to predict Self-Pitching Propellers (SPP) hydrodynamic and dynamic behaviour is here addressed. Two hydrodynamics approaches are described and validated against literature data. Both Controllable-Pitch Propellers (CPP) and SPP test cases are considered. In addition, a general theoretical Reduced Order Model (ROM) for blade hydrodynamics unsteady loads is proposed to investigate blade equilibrium stability. Numerical results are shown in terms of performance, spindle moment and pitch setting equilibrium conditions. The proposed Blade Element Momentum Theory (BEMT) model shows to be well suited to capture SPP performance, spindle moment and blade equilibrium condition once a devoted tailoring process of model parameters is performed

Cell Attention Networks

Since their introduction, graph attention networks achieved outstanding results in graph representation learning tasks. However, these networks consider only pairwise relationships among nodes and then they are not able to fully exploit higher-order interactions present in many real world data-sets. In this paper, we introduce Cell Attention Networks (CANs), a neural architecture operating on data defined over the vertices of a graph, representing the graph as the 1-skeleton of a cell complex introduced to capture higher order interactions. In particular, we exploit the lower and upper neighborhoods, as encoded in the cell complex, to design two independent masked self-attention mechanisms, thus generalizing the conventional graph attention strategy. The approach used in CANs is hierarchical and it incorporates the following steps: i) a lifting algorithm that learns {\it edge features} from {\it node features}; ii) a cell attention mechanism to find the optimal combination of edge features over both lower and upper neighbors; iii) a hierarchical {\it edge pooling} mechanism to extract a compact meaningful set of features. The experimental results show that CAN is a low complexity strategy that compares favorably with state of the art results on graph-based learning tasks.Comment: Preprint, under revie