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: economía rural y demografía en Normandía Oriental desde comienzos del siglo XIV a mediados del siglo XVI : París, 1976

Este libro nació en realidad a partir del vértigo experimentado frente a un gran vacío teórico: el desconocimiento de la vida económica de las saciedades precapitalistas y también de una certeza, de que si desconocemos "el modo de producción" de una sociedad no podemos realizar ningun análisis histórico satisfactorio.Departamento de Histori

La radio Pío XII vue à travers l'altérité, la théologie de la libération et l'espace public

La Radio Pío XII de Siglo XX en Bolivie a été financée en bonne partie par les catholiques du Québec pour combattre l'alcoolisme, l'analphabétisme et le communisme. Construite en 1959, elle s'inscrivait dans un anti-communisme primaire où l'Occident chrétien affrontait l'Orient communiste et athée. Cette radio devait cependant connaître un revirement radical à partir de 1961 sous la triple pression d'un durcissement des régimes militaires qui allaient se succéder en Bolivie et sur l'ensemble du continent; d'une vaste réforme de l'Église catholique sous l'impulsion de Vatican II dont l'option préférentielle pour les pauvres devait être radicalisée et systématisée pour devenir en Amérique latine la Théologie de la libération; et enfin, d'un monde en pleine ébullition où de Prague à Los Angeles, en passant par La Havane et Paris, on entrait dans une période de turbulence sociale hors de l'ordinaire. Ces jeunes prêtres -Québécois, Américains, Espagnols -devaient être des acteurs importants de ce revirement qu'allait connaître cette radio, dans la région la plus pauvre de Bolivie. Une région où le syndicat des mineurs avait été un des principaux artisans de la révolution bolivienne de 1952 qui avait menée à la nationalisation des mines et à une vaste réforme agraire. La Radio Pío XlI devenait dans ce contexte le lieu à la fois physique et symbolique où s'est cristallisée cette rencontre entre ces prêtres nouveau genre et la résistance bolivienne. Une radio que les militaires ont fait sauter à cinq reprises parce qu'elle planifiait des grèves, organisait des réseaux radiophoniques de résistance aux nombreux coups d'État, cachait des armes et faisait sortir des dissidents clandestinement du pays. Ces prêtres oblats ont été arrêtés, emprisonnés et battus parce qu'ils étaient les protagonistes d'une nouvelle évangélisation dite libératrice, dont la radio constituait le principal outil. L'histoire de la Radio Pío XII et de son revirement inédit seront analysés à travers les concepts d'altérité (Todorov), de l'histoire politique de la chrétienté (monde-plein de Gauchet), de la Théologie de la libération (Gutiérrez et L. Boff) et de l'espace public (Habermas). Une radio qui évoluait dans un microcosme qui concentrait alors les conflits et les contradictions qui traversaient l'ensemble du continent sud-américain (mésocosme). ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : Altérité, Théologie de la libération, 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 Brüel & 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