104 research outputs found
Acoustic response of a feeding system to high-frequency transverse acoustic field
International audienceThe acoustic coupling between the injection system and the acoustic fluctuations in liquid rocket engine combustion chambers is an important issue in the understanding of the thermo-acoustic instability phenomenon. This paper presents results of a large parametric investigation of a two-phase injection system acoustic response, to the excitation produced by a high-amplitude transverse acoustic field forced into a main resonant cavity. Two domes, one for the gas and one for the liquid, were expressly designed to feed three identical coaxial injectors. Characterization of domes internal mode shapes were performed by measuring pressure signals at different locations in the domes. Experimental mode shapes showed good agreement with those predicted by numerical simulations. Acoustic pressure amplitudes up to 17% of the the one induced in the main cavity can be found in both gas and liquid dome. The maximum acoustic response is observed in a configuration in which acoustic boundary conditions does not correspond to the maximum injection system solicitation conditions
Acoustic response of a feeding system to high-frequency transverse acoustic field
International audienceThe acoustic coupling between the injection system and the acoustic fluctuations in liquid rocket engine combustion chambers is an important issue in the understanding of the thermo-acoustic instability phenomenon. This paper presents results of a large parametric investigation of a two-phase injection system acoustic response, to the excitation produced by a high-amplitude transverse acoustic field forced into a main resonant cavity. Two domes, one for the gas and one for the liquid, were expressly designed to feed three identical coaxial injectors. Characterization of domes internal mode shapes were performed by measuring pressure signals at different locations in the domes. Experimental mode shapes showed good agreement with those predicted by numerical simulations. Acoustic pressure amplitudes up to 17% of the the one induced in the main cavity can be found in both gas and liquid dome. The maximum acoustic response is observed in a configuration in which acoustic boundary conditions does not correspond to the maximum injection system solicitation conditions
Logic Programming and Logarithmic Space
We present an algebraic view on logic programming, related to proof theory
and more specifically linear logic and geometry of interaction. Within this
construction, a characterization of logspace (deterministic and
non-deterministic) computation is given via a synctactic restriction, using an
encoding of words that derives from proof theory.
We show that the acceptance of a word by an observation (the counterpart of a
program in the encoding) can be decided within logarithmic space, by reducing
this problem to the acyclicity of a graph. We show moreover that observations
are as expressive as two-ways multi-heads finite automata, a kind of pointer
machines that is a standard model of logarithmic space computation
Response of coaxial air-assisted liquid jets in an acoustic field: atomization and droplets clustering
International audienceHigh-frequency combustion instabilities have been proven to be extremely harmful to liquid rocket engine operation , even leading to the destruction of the combustion chamber. The coupling between acoustic field and combustion heat release rate in the combustion chamber is considered as the driving phenomenon. Experiments have shown that intense acoustic field can deeply affect atomization process thereby causing a non-uniform heat release distribution which can couple with the resonant mode shapes of the combustion chamber and consequently trigger or sustain combustion instability. The effects of acoustic acting on atomization of coaxial air-assisted liquid jets have been investigated experimentally and results are presented in this paper. The experimental setup is composed of three coaxial injectors installed on the roof of a semi-open resonant cavity provided with 4 compression drivers. An acoustic field corresponding to the 2 nd transverse mode of the cavity is forced into that at a frequency of 1 kHz. Acoustic levels up to 174 dB are produced. High speed visualizations are performed in order to observe the response of the jet to the acoustic perturbations. In the case of low Weber numbers (We < 30) the jet can be considered as cylindrical and depending on the position of the injector with respect to the acoustic axis different responses can be observed. If the injector is placed in correspondence of the velocity antinode the jet is flattened into a liquid sheet perpendicular to the acoustic axis, if the injector is located in correspondence of an intensity antinode the jet is deviated toward the velocity antinode. Combined response can be observed at intermediate positions. For higher Weber numbers the jet is no more cylindrical and a spray is formed, characterized by with a certain spray angle. Such a spray is can still be affected by the acoustics but it is not always possible to get evidence of this from observation of raw images. To quantify these effects, image analyses have been carried-out to determine how spatial distributions of droplets are affected by acoustics. Results are presented for Weber numbers ranging from 30 to 1500, with and without acoustic. Clustering of droplets is shown as well as improvement of atomization process
Acoustic response of an injection system to high-frequency transverse acoustic fields
International audienceThe acoustic coupling between the injection system and the acoustic fluctuations in liquid rocket engine combustion chambers is an important issue in the understanding of the thermo-acoustic instability phenomenon. This paper presents the results of a wide-ranging parametric investigation of the acoustic response of a two-phase injection system submitted to a forced high-amplitude transverse acoustic field. Two domes, one for the gas and one for the liquid, were expressly designed to feed three identical coaxial injectors. The internal mode shapes of the domes were characterized by measuring pressure signals at different locations in the domes. Experimental mode shapes showed good agreement with those predicted by numerical simulations. Acoustic pressure amplitudes up to 23% of those induced in the main cavity can be found in both the gas and liquid domes. The response efficiency in a dome depends on the position of the injectors' exit in the acoustic field
Resource Control for Synchronous Cooperative Threads
We develop new methods to statically bound the resources needed for the
execution of systems of concurrent, interactive threads. Our study is concerned
with a \emph{synchronous} model of interaction based on cooperative threads
whose execution proceeds in synchronous rounds called instants. Our
contribution is a system of compositional static analyses to guarantee that
each instant terminates and to bound the size of the values computed by the
system as a function of the size of its parameters at the beginning of the
instant. Our method generalises an approach designed for first-order functional
languages that relies on a combination of standard termination techniques for
term rewriting systems and an analysis of the size of the computed values based
on the notion of quasi-interpretation. We show that these two methods can be
combined to obtain an explicit polynomial bound on the resources needed for the
execution of the system during an instant. As a second contribution, we
introduce a virtual machine and a related bytecode thus producing a precise
description of the resources needed for the execution of a system. In this
context, we present a suitable control flow analysis that allows to formulte
the static analyses for resource control at byte code level
Analysing the Complexity of Functional Programs: Higher-Order Meets First-Order
International audienceWe show how the complexity of higher-order functional programs can be analysed automatically by applying program transformations to a defunctionalized versions of them, and feeding the result to existing tools for the complexity analysis of first-order term rewrite systems. This is done while carefully analysing complexity preservation and reflection of the employed transformations such that the complexity of the obtained term rewrite system reflects on the complexity of the initial program. Further, we describe suitable strategies for the application of the studied transformations and provide ample experimental data for assessing the viability of our method
Trouble with Bleeding: Risk Factors for Acute Hepatitis C among HIV-Positive Gay Men from Germany—A Case-Control Study
OBJECTIVES: To identify risk factors for hepatitis C among HIV-positive men who have sex with men (MSM), focusing on potential sexual, nosocomial, and other non-sexual determinants. BACKGROUND: Outbreaks of hepatitis C virus (HCV) infections among HIV-positive MSM have been reported by clinicians in post-industrialized countries since 2000. The sexual acquisition of HCV by gay men who are HIV positive is not, however, fully understood. METHODS: Between 2006 and 2008, a case-control study was embedded into a behavioural survey of MSM in Germany. Cases were HIV-positive and acutely HCV-co-infected, with no history of injection drug use. HIV-positive MSM without known HCV infection, matched for age group, served as controls. The HCV-serostatus of controls was assessed by serological testing of dried blood specimens. Univariable and multivariable regression analyses were used to identify factors independently associated with HCV-co-infection. RESULTS: 34 cases and 67 controls were included. Sex-associated rectal bleeding, receptive fisting and snorting cocaine/amphetamines, combined with group sex, were independently associated with case status. Among cases, surgical interventions overlapped with sex-associated rectal bleeding. CONCLUSIONS: Sexual practices leading to rectal bleeding, and snorting drugs in settings of increased HCV-prevalence are risk factors for acute hepatitis C. We suggest that sharing snorting equipment as well as sharing sexual partners might be modes of sexual transmission. Condoms and gloves may not provide adequate protection if they are contaminated with blood. Public health interventions for HIV-positive gay men should address the role of blood in sexual risk behaviour. Further research is needed into the interplay of proctosurgery and sex-associated rectal bleeding
Deep Brain Stimulation of Nucleus Accumbens Region in Alcoholism Affects Reward Processing
The influence of bilateral deep brain stimulation (DBS) of the nucleus nucleus (NAcc) on the processing of reward in a gambling paradigm was investigated using H2[15O]-PET (positron emission tomography) in a 38-year-old man treated for severe alcohol addiction. Behavioral data analysis revealed a less risky, more careful choice behavior under active DBS compared to DBS switched off. PET showed win- and loss-related activations in the paracingulate cortex, temporal poles, precuneus and hippocampus under active DBS, brain areas that have been implicated in action monitoring and behavioral control. Except for the temporal pole these activations were not seen when DBS was deactivated. These findings suggest that DBS of the NAcc may act partially by improving behavioral control
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