276 research outputs found
Experimental characterization and modelling of a cavitating centrifugal pump operating in fast start-up conditions
The start-up of rocket engine turbopumps is generally performed only in a few seconds. It implies that these pumps reach their nominal operating conditions after only a few rotations. During these first rotations of the blades, the flow evolution in the pump is governed by transient phenomena, based mainly on the flow rate and rotation speed evolution. These phenomena progressively become negligible when the steady behavior is reached. The pump transient behaviour induces significant pressure fluctuations which may result in partial flow vaporization, i.e. cavitation. An existing experimental test rig has been updated in the LML laboratory (Lille, France) for the start-ups of a centrifugal pump. The study focuses on cavitation induced during the pump start-up. Instantaneous measurement of torque, flow rate, inlet and outlet unsteady pressures, and pump rotation velocity enable to characterize the pump behaviour during rapid starting periods. Three different types of fast start-up behaviours have been identified and have been presented at ISROMAC 12 (Duplaa et al, 2008). According to the final operating point, the start-up is characterized either by a single drop of the delivery static pressure, by several low-frequency drops, or by a water hammer phenomenon that can be observed both a the inlet and outlet of the pump. A physical analysis to explain these three different types of transient flow behaviour has been recently proposed (Duplaa et al, 2010). In the present paper, a modelling of the fast start ups in cavitating conditions is proposed. It consists of a two steps adaptation of fast start-up model in non cavitating conditions proposed by Dazin et al (2007). For that, fast X-rays imaging has been performed in the impeller with the collaboration of the French Atomic Agency (CEA) in order to determinate the high frequency evolution of the volume fraction during fast the start-ups. Although the results of the modelling presented here are not definitive, they are very promising
Experimental and Numerical Investigation of Unforced unsteadiness in a Vaneless Radial Diffuser
The paper reports combined experimental and numerical investigations of unforced un- steadiness in a vaneless radial diffuser. Experimental data were obtained within the diffuser using stereoscopic time resolved Particle Image Velocimetry (PIV) recording three velocity components in a plane (2D/3C), coupled with unsteady pressure transducers. To characterize the inception and the evolution of the unsteady phenomena, spectral analyses of the pressure signals were carried out both in frequency and time-frequency domains and the PIV results were post processed by an original averaging method. Two partial flow rates were investigated in detail in this paper. A single unforced unsteadiness was identified for the lowest flow rate, whereas, two competitive intermittent modes were recognized for the higher mass flow. Numerical analyses were carried out on the same pump by the commercial code CFX. All the computations were performed using the unsteady transient model and the turbulence was modelled by the Scale-Adaptive Simulation (SAS) model. Numerical pressure signals were compared with the experimental data to verify the development of the same pressure fluctua- tions
Cavitation inception in fast startup
The start-up of rocket engine turbopumps is generally performed only in a few seconds. It implies that these pumps reach their nominal operating conditions after only a few rotations. During these first rotations of the blades, the flow evolution in the pump is governed by transient phenomena, based mainly on the flow rate and rotation speed evolution. These phenomena progressively become negligible when the steady behaviour is reached. The pump transient behaviour induces significant pressure fluctuations which may result in partial flow vaporization, i.e. cavitation. An existing experimental test rig has been updated in the LML laboratory (Lille, France) for the start-ups of a centrifugal pump. The study focuses on cavitation induced during the pump start-up. Instantaneous measurement of torque, mass flow rate, inlet and outlet unsteady pressures, and pump rotation velocity enable to characterize the pump behaviour during rapid starting periods
Crisis del feudalismo: economĂa rural y demografĂa en NormandĂa Oriental desde comienzos del siglo XIV a mediados del siglo XVI : ParĂs, 1976
Este libro naciĂł en realidad a partir del vĂ©rtigo experimentado frente a un gran vacĂo teĂłrico: el desconocimiento de la vida econĂłmica de las saciedades precapitalistas y tambiĂ©n de una certeza, de que si desconocemos "el modo de producciĂłn" de una sociedad no podemos realizar ningun análisis histĂłrico satisfactorio.Departamento de Histori
La radio PĂo XII vue Ă travers l'altĂ©ritĂ©, la thĂ©ologie de la libĂ©ration et l'espace public
La Radio PĂo XII de Siglo XX en Bolivie a Ă©tĂ© financĂ©e en bonne partie par les catholiques du QuĂ©bec pour combattre l'alcoolisme, l'analphabĂ©tisme et le communisme. Construite en 1959, elle s'inscrivait dans un anti-communisme primaire oĂą l'Occident chrĂ©tien affrontait l'Orient communiste et athĂ©e. Cette radio devait cependant connaĂ®tre un revirement radical Ă partir de 1961 sous la triple pression d'un durcissement des rĂ©gimes militaires qui allaient se succĂ©der en Bolivie et sur l'ensemble du continent; d'une vaste rĂ©forme de l'Église catholique sous l'impulsion de Vatican II dont l'option prĂ©fĂ©rentielle pour les pauvres devait ĂŞtre radicalisĂ©e et systĂ©matisĂ©e pour devenir en AmĂ©rique latine la ThĂ©ologie de la libĂ©ration; et enfin, d'un monde en pleine Ă©bullition oĂą de Prague Ă Los Angeles, en passant par La Havane et Paris, on entrait dans une pĂ©riode de turbulence sociale hors de l'ordinaire. Ces jeunes prĂŞtres -QuĂ©bĂ©cois, AmĂ©ricains, Espagnols -devaient ĂŞtre des acteurs importants de ce revirement qu'allait connaĂ®tre cette radio, dans la rĂ©gion la plus pauvre de Bolivie. Une rĂ©gion oĂą le syndicat des mineurs avait Ă©tĂ© un des principaux artisans de la rĂ©volution bolivienne de 1952 qui avait menĂ©e Ă la nationalisation des mines et Ă une vaste rĂ©forme agraire. La Radio PĂo XlI devenait dans ce contexte le lieu Ă la fois physique et symbolique oĂą s'est cristallisĂ©e cette rencontre entre ces prĂŞtres nouveau genre et la rĂ©sistance bolivienne. Une radio que les militaires ont fait sauter Ă cinq reprises parce qu'elle planifiait des grèves, organisait des rĂ©seaux radiophoniques de rĂ©sistance aux nombreux coups d'État, cachait des armes et faisait sortir des dissidents clandestinement du pays. Ces prĂŞtres oblats ont Ă©tĂ© arrĂŞtĂ©s, emprisonnĂ©s et battus parce qu'ils Ă©taient les protagonistes d'une nouvelle Ă©vangĂ©lisation dite libĂ©ratrice, dont la radio constituait le principal outil. L'histoire de la Radio PĂo XII et de son revirement inĂ©dit seront analysĂ©s Ă travers les concepts d'altĂ©ritĂ© (Todorov), de l'histoire politique de la chrĂ©tientĂ© (monde-plein de Gauchet), de la ThĂ©ologie de la libĂ©ration (GutiĂ©rrez et L. Boff) et de l'espace public (Habermas). Une radio qui Ă©voluait dans un microcosme qui concentrait alors les conflits et les contradictions qui traversaient l'ensemble du continent sud-amĂ©ricain (mĂ©socosme). ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : AltĂ©ritĂ©, ThĂ©ologie de la libĂ©ration, Espace public, Monde-plein, Sujets-acteurs
Transient behavior of a radial vaneless diffuser
The paper refers to the behavior of a radial flow pump vaneless diffuser during a starting period. Results obtained with a 1D numerical model are compared with some new experimental data which have been obtained using 2D/3C High repetition rate PIV within the diffuser coupled with unsteady pressure measurements. These tests have been performed on a test rig with a radial impeller matched with a vaneless diffuser. They have been made in air, on a test rig well adapted for studies on interactions between impeller and diffuser, as well as for the use of optical methods and especially Particle Image Velocimetry (PIV) as there is no volute downstream of the diffuser. The present study refers to new experiments combining pressure measurements and 2D/3C High Speed PIV at partial flow rates within a vaneless diffuser with a large outlet radius. Four BrĂĽel & Kjaer condenser microphones are used for the unsteady pressure measurements. They were flush mounted on the shroud side of the diffuser wall and on the suction pipe of the pump. The sampling frequency was 2048 Hz. For PIV measurements, the laser sheet was generated by a Darwin PIV ND:YLF Laser at three heights within the diffuser. PIV snapshots have been recorded by two identical CMOS cameras. A home made software has been used for the images treatment. The results consist in fields of 80 x 120 mm2 and 81 x 125 velocity vectors with a temporal resolution of 250 velocity maps per second. For each flow rate and each laser sheet height in the diffuser, two acquisitions of about 1500 velocity maps have been realised. The experimental data are compared with the ones provided by a 1D transient model of the flow within the diffuser
On the Scott-continuity of tagged signal processes
Process networks are frequently used to model signal processing and multimedia
applications. It is important to ensure that a process network has
a uniquely defined behavior in order to correctly model such deterministic
systems. Furthermore, a constructive procedure to determine this unique
behavior is necessary for its simulation and execution.
By the Kahn principle, the unique behavior of a process network is known
to be the least fixed point of the network functional when every process
computes a Scott-continuous function. The Kahn principle is used in a
recent timed extension of the least fixed point semantics of Kahn process
networks, using the tagged signal model. In this extension, processes compute
a function from input signals to output signals, where a signal is defined as a
partial function from a down set of tags to a set of values. However, it is often
tedious to formally prove that a tagged signal process is Scott-continuous.
This paper presents theorems on Scott-continuity that simplify such proofs.
Thus, a general theorem on the necessary and sufficient conditions for the
Scott-continuity of tagged signal processes is proven. Scott-continuity is then
proven for broad classes of processes, namely the classes of exactly causal
processes and of domain-warping processes, which include stateless processes,
delays as well as a subset of discrete-event processes
Shigella applies molecular mimicry to subvert vinculin and invade host cells
Shigella flexneri, the causative agent of bacillary dysentery, injects invasin proteins through a type III secretion apparatus upon contacting the host cell, which triggers pathogen internalization. The invasin IpaA is essential for S. flexneri pathogenesis and binds to the cytoskeletal protein vinculin to facilitate host cell entry. We report that IpaA harbors two vinculin-binding sites (VBSs) within its C-terminal domain that bind to and activate vinculin in a mutually exclusive fashion. Only the highest affinity C-terminal IpaA VBS is necessary for efficient entry and cell–cell spread of S. flexneri, whereas the lower affinity VBS appears to contribute to vinculin recruitment at entry foci of the pathogen. Finally, the crystal structures of vinculin in complex with the VBSs of IpaA reveal the mechanism by which IpaA subverts vinculin's functions, where S. flexneri utilizes a remarkable level of molecular mimicry of the talin–vinculin interaction to activate vinculin. Mimicry of vinculin's interactions may therefore be a general mechanism applied by pathogens to infect the host cell
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