Étude du rayonnement acoustique d'instabilités hydrodynamiques de jets double-flux par les équations de stabilité parabolisées (PSE)

Dans le but de réduire le bruit de jet, source principale de nuisance sonore au décollage d'un avion, une compréhension fine des mécanismes aéroacoustiques mis en jeu est nécessaire. Les structures cohérentes de grande échelle se développant dans la couche de mélange d'un jet semblent responsables d'une part importante du bruit observé en champ lointain, surtout dans les basses fréquences. Une approche permettant d'étudier ces structures turbulentes est fournie par la théorie de stabilité, notamment au moyen des équations de stabilité parabolisées (PSE). L'étude de ces ondes d'instabilité est alors complémentaire d'autres approches (LES ou expériences), puisqu'elle permet de mettre en évidence la nature et la dynamique de ces structures, également présentes dans les résultats de simulations ou de mesures. Au cours de ces travaux de thèse, nous nous sommes intéressés aux structures cohérentes se développant dans des jets à double flux étudiés au cours du projet européen CoJeN (Coaxial Jet Noise). En particulier, nous avons exploité une base de données issues de mesures de fluctuations de pression réalisées en champ proche et en champ lointain de ces jets. Nous avons alors pu comparer les résultats de notre modélisation PSE à ces mesures en périphérie immédiate du jet, confirmant ainsi la pertinence d’un tel modèle, même dans des configurations aussi complexes. De plus, le calcul du rayonnement acoustique en champ lointain engendré par les fluctuations de pression modélisées nous a permis de faire des comparaisons directes avec les niveaux et les directivités mesurés. Nous avons ainsi pu mettre en évidence quantitativement la contribution de ces structures turbulentes de grande échelle au bruit total rayonné par le jet. ABSTRACT : Increasingly stringent aircraft noise regulations require the development of innovative noise reduction strategies. Jet noise is a dominant acoustic component during take-off and a fine understanding of the underlying aeroacoustics mechanisms is then necessary. Large-scale coherent structures that develop in the mixing layer of jets appear to be the dominant acoustic source responsible for the lowfrequency far-field noise observed at low emission angles. A stability analysis based on the parabolized stability equations (PSE) is a suitable tool for studying these coherent structures, revealing the nature and the dynamics of the fluctuations obtained by simulations or experiments. The present work is focused on coherent structures developing in the two mixing layers of dualstream jets studied in the course of the European project CoJeN (Coaxial Jet Noise). In particular, pressure fluctuations measurements acquired in the near and far fields of two coaxial jets have been thoroughly analyzed. A direct comparison of these experimental results with linear PSE calculations has been performed in the vicinity of the jets, referred to as the linear-hydrodynamic region, confirming the relevance of the approach even in such complex industrial configurations. Furthermore, the acoustic projection to the far-field of the wavepackets issued by this model and calibrated in the near-field allows a direct comparison of the acoustic levels and directivity with far field sound measurements. A quantitative assessment of the contribution of the instability waves to the total jet noise measured has therefore been obtained

Measurement of acoustic velocity components in a turbulent flow using LDV and high-repetition rate PIV

International audienceThe present study provides theoretical details and experimental validation results to the approach proposed by Minotti et al. (Aerosp Sci Technol 12(5):398–407, 2008) for measuring amplitudes and phases of acoustic velocity components (AVC) that are waveform parameters of each component of velocity induced by an acoustic wave, in fully turbulent duct flows carrying multi-tone acoustic waves. Theoretical results support that the turbulence rejection method proposed, based on the estimation of cross power spectra between velocity measurements and a reference signal such as a wall pressure measurement, provides asymptotically efficient estimators with respect to the number of samples. Furthermore, it is shown that the estimator uncertainties can be simply estimated, accounting for the characteristics of the measured flow turbulence spectra. Two laser-based measurement campaigns were conducted in order to validate the acoustic velocity estimation approach and the uncertainty estimates derived. While in previous studies estimates were obtained using laser Doppler velocimetry (LDV), it is demonstrated that high-repetition rate particle image velocimetry (PIV) can also be successfully employed. The two measurement techniques provide very similar acoustic velocity amplitude and phase estimates for the cases investigated, that are of practical interest for acoustic liner studies. In a broader sense, this approach may be beneficial for non-intrusive sound emission studies in wind tunnel testings

3D density reconstruction of a screeching supersonic jet by synchronized multi-camera Background Oriented Schlieren

International audienceThis work describes the application of 3D Background Oriented Schlieren (3DBOS) to the experimental analysis of the mean and instantaneous 3D structures of supersonic under-expanded screeching jets. These jets feature various dynamical states as the jet nozzle pressure ratio (NPR) is varied. Hence, six flow conditions were considered using acoustic measurements and 3D density reconstructions. Relying on the latter and on Proper Orthogonal Decomposition (POD) to isolate the main flow features, clear visualizations of the instantaneous 3D state and the reconstructed average dynamics of modes A1 and C, that are respectively axisymmetric and helical, were obtained

La Intelligences multiples comme facteurs prédictifs du sentiment d’efficacité personnelle des élèves en instance de formation pour le métier d’enseignant du primaire

Résumé : La recherche a identifié 9 facteurs déterminants du sentiment d’efficacité personnelle chez les apprenants en instance formation pour le métier d’enseignant de l’école primaire. Il s’agit des intelligences multiples découvertes par Gardner. L’hypothèse de recherche est que l’élève-maître ayant les traits sur toutes les intelligences multiples va développer un sentiment d’efficacité personnelle élevé. En cherchant à vérifier cette hypothèse, notre objectif général est de vérifier si les intelligences multiples peuvent constituer des facteurs de présage du sentiment d’efficacité personnelle dans le processus de formation des élèves-maîtres. Partant de cet objectif général, nous avons assigné à la présente étude, les objectifs spécifiques suivants : Identifier parmi les IM, celles qui sont saturées au SEP chez les apprenants de l’école normale de Mbankânà de Kinshasa, élaborer un instrument ou un modèle de recherche pouvant déterminer le SEP à partir des intelligences multiples. Un modèle mathématique de Bayes a été conçu à cet effet, dans l’optique de prédire ce sentiment sur base des intelligences multiples. Ce modèle a été évalué performant sur le plan interne et externe. Le système éducatif congolais, peut désormais, l’utiliser dans l’optique de recruter et de faire le suivi des élèves candidats enseignants

A cross-sectional survey of supports for evidence-informed decision-making in healthcare organisations: a research protocol

Background: This protocol builds on the development of a) a framework that identified the various supports (i.e. positions, activities, interventions) that a healthcare organisation or health system can implement for evidence-informed decision-making (EIDM) and b) a qualitative study that showed the current mix of supports that some Canadian healthcare organisations have in place and the ones that are perceived to facilitate the use of research evidence in decision-making. Based on these findings, we developed a web survey to collect cross-sectional data about the specific supports that regional health authorities and hospitals in two Canadian provinces (Ontario and Quebec) have in place to facilitate EIDM. Methods/design This paper describes the methods for a cross-sectional web survey among 32 regional health authorities and 253 hospitals in the provinces of Quebec and Ontario (Canada) to collect data on the current mix of organisational supports that these organisations have in place to facilitate evidence-informed decision-making. The data will be obtained through a two-step survey design: a 10-min survey among CEOs to identify key units and individuals in regard to our objectives (step 1) and a 20-min survey among managers of the key units identified in step 1 to collect information about the activities performed by their unit regarding the acquisition, assessment, adaptation and/or dissemination of research evidence in decision-making (step 2). The study will target three types of informants: CEOs, library/documentation centre managers and all other key managers whose unit is involved in the acquisition, assessment, adaptation/packaging and/or dissemination of research evidence in decision-making. We developed an innovative data collection system to increase the likelihood that only the best-informed respondent available answers each survey question. The reporting of the results will be done using descriptive statistics of supports by organisation type and by province. Discussion This study will be the first to collect and report large-scale cross-sectional data on the current mix of supports health system organisations in the two most populous Canadian provinces have in place for evidence-informed decision-making. The study will also provide useful information to researchers on how to collect organisation-level data with reduced risk of self-reporting bias

Experimental study of turbulent-jet wave packets and their acoustic efficiency

This paper details the statistical and time-resolved analysis of the relationship between the near-field pressure fluctuations of unforced, subsonic free jets (0.4 ≤ M ≤ 0.6) and their far-field sound emissions. Near-field and far-field microphone measurements were taken on a conical array close to the jets and an azimuthal ring at 20∘ to the jet axis, respectively. Recent velocity and pressure measurements indicate the presence of linear wave packets in the near field by closely matching predictions from the linear homogenous parabolized stability equations, but the agreement breaks down both beyond the end of the potential core and when considering higher order statistical moments, such as the two-point coherence. Proper orthogonal decomposition (POD), interpreted in terms of inhomogeneous linear models using the resolvent framework allows us to understand these discrepancies. A new technique is developed for projecting time-domain pressure measurements onto a statistically obtained POD basis, yielding the time-resolved activity of each POD mode and its correlation with the far field. A single POD mode, interpreted as an optimal high-gain structure that arises due to turbulent forcing, captures the salient near-field–far-field correlation signature; further, the signatures of the next two modes, understood as suboptimally forced structures, suggest that these POD modes represent higher order, acoustically important near-field behavior. An existing Green's-function-based technique is used to make far-field predictions, and results are interpreted in terms of POD/resolvent modes, indicating the acoustic importance of this higher order behavior. The technique is extended to provide time-domain far-field predictions

Global stability and control of the confined turbulent flow past a thick flat plate

This article investigates the structural stability and sensitivity properties of the confined turbulent wake behind an elongated D-shaped cylinder of aspect-ratio 10 at Re = 32 000. The stability analysis is performed by linearising the incompressible Navier-Stokes equations around the numerically computed and the experimentally measured mean flows. We found that the vortex-shedding frequency is very well captured by the leading unstable global mode, especially when the additional turbulent diffusion is modelled in the stability equations by means of a frozen eddy-viscosity approach. The sensitivity maps derived from the computed and the measured mean flows are then compared, showing a good qualitative agreement. The careful inspection of their spatial structure highlights that the highest sensitivity is attained not only across the recirculation bubble but also at the body blunt-edge, where tiny pockets of maximum receptivity are found. The impact of the turbulent diffusion on the obtained results is investigated. Finally, we show how the knowledge of the unstable adjoint global mode of the linearised mean-flow dynamics can be exploited to design an active feedback control of the unsteady turbulent wake, which leads, under the adopted numerical framework, to completely suppress its low-frequency oscillation

Utilisation et gestion des pesticides en cultures maraîchères : cas de la zone de Nkolo dans la province du Kongo Central, République démocratique du Congo

Objectifs: L’objectif de cette étude est d’évaluer les connaissances et les pratiques des maraîchers de Nkolo et ses environs quant à l’utilisation des pesticides.Méthodologie et Résultats: Une enquête a été réalisée auprès des 46 maraîchers. Le questionnaire a porté sur les pesticides utilisés, la dose appliquée, la préparation des solutions des pesticides, la fréquence d’application, la protection des maraîchers pendant l’usage des pesticides, la durée avant récolte (DAR), la gestion d’emballages des pesticides et les caractéristiques socio-professionnelles des maraîchers. Les résultats ont montrés que le maraîchage est généralement pratiqué par les hommes (93% contre 7% de femmes), et concerne principalement la culture des légumes fruits (tomate, aubergine et poivron). Les pesticides sont systématiquement appliqués, notamment les fongicides et les insecticides, à titre préventif et fréquemment à intervalle de 7 jours (saison pluvieuse) et 14 jours (saison sèche). Un à neuf pesticides différents peuvent être utilisés par cycle de culture, et sont généralement mélangés et appliqués en une fois à l’aide d’un pulvérisateur. Le dosage des pesticides et la DAR varient largement suivant les maraîchers. La majorité de maraîchers n’a pas suivi une formation sur les pesticides (83%), ne se protège pas adéquatement pendant l’usage des pesticides (72%) et abandonne les emballages des pesticides au champ (46%).Conclusion et applications des résultats: Les maraîchers de Nkolo et ses environs n’ont généralement pas une bonne connaissance des pesticides et ne respectent pas les recommandations des fabricants. L’application systématique des pesticides sans tenue de protection appropriée les met en contact régulier avec les pesticides et les expose à l’intoxication aux pesticides. Le manque de maîtrise de dosage des pesticides peut accroître le risque de résistance des ravageurs et pathogènes aux pesticides. En vue de réduire le risque d’intoxication des maraîchers et consommateurs des légumes, et de la pollution de l’environnement, il est d’ores et déjà, urgent et important de promouvoir la formation des maraîchers de Nkolo pour une utilisation judicieuse des pesticides.Mots clés: Pesticides, cultures maraîchères, pratiques culturales, enquête, Nkolo (RD Congo)English Title: Pesticides use and management in the vegetable crops at Nkolo in Kongo Central Province in the western DR CongoEnglish AbstractObjectives: Pesticides are systematically applied in vegetable crops at Nkolo (in Kongo Central province in the western DR Congo) in order to provide crop protection and improve the yield. This study aimed at assessing the knowledges and practices of gardeners in relation to of pesticides use.Methodology and Results: Forty-six gardeners were interviewed. The questionnaire focused on the pesticides types, solution procedures, application rate and frequency, gardener protective equipment, and the pre-harvest intervals. Socio-professional traits of gardeners were also recorded. Majority of gardeners was male (93% vs 7% of women). Pesticides, especially fungicides and insecticides are systematically applied, based on preventive application and generally in 7 (dry season) and 14 (rain season) days of intervals. Depending on the gardener, 1 to 9 pesticides are used per crop cycle. All the pesticides are generally mixed and applied together on the crops using a manual sprayer. Application rate and pre-harvest intervals varied with gardeners. Most of them do not have been trained on pesticide use (83%), do not use protective equipment (72%) and abandon pesticide containers in the field (46%).Conclusion and application of results: Gardeners in Nkolo and surroundings have poor knowledges on pesticides use and do not use it properly. Systematically application of pesticides without proper protective equipment result in regular contact of gardener with pesticides and exposes them to pesticide poisoning. Furthermore, improper pesticides use should increase the risk of pest resistance in the long term use. In order to mitigate the risk of poisoning for gardeners and consumers, and environmental pollution in Nkolo and surroundings, there is an urgent need to promote proper use of pesticides.Keywords: Pesticides, vegetables crops, agricultural practices, survey, Nkolo (DR Congo