An experimental study of boundary-layer transition induced vibrations on a hydrofoil

This paper aims at characterizing experimentally laminar to turbulent transition induced vibrations. Here, the transition is known to be triggered by a Laminar Separation Bubble that results from a laminar separation of the boundary-layer flow on a hydrofoil. In this study we consider two NACA66312 (Mod) laminar hydrofoils at low angles of incidence (mostly 2° and 4°) and Reynolds numbers ranging from Re=450 000 to 1 200 000, in order to get transitional regimes. The first hydrofoil, made of steel (E=2.1×1011 Pa), is referred to as the rigid hydrofoil, although it is seen to vibrate under the action of the LSB. To better understand the possible interaction between the flow and the foil vibrations, vibration measurements are repeated using a flexible hydrofoil (E=3×109 Pa) of same geometry (under zero loading) and in close configurations. The experiments are carried out at the French Naval Academy Research Institute (IRENav, France). Wall pressure and flow velocity measurements enable a characterization of the laminar separation bubble and the identification of a vortex shedding at a given frequency. It is hence shown that the boundary-layer transition induces important foil vibrations, whose characteristics in terms of frequency and amplitude depend on the vortex shedding frequency, and can be coupled with natural frequencies of the hydrofoils

Wave Finite Element based Strategies for Computing the Acoustic Radiation of Stiffened or Non-Stiffened Rectangular Plates subject to Arbitrary Boundary Conditions

20 pagesInternational audienceThe wave finite element method (WFE) is investigated for the computation of the acoustic radiation of stiffened or non-stiffened rectangular plates under arbitrary boundary conditions. The method aims at computing the forced response of periodic waveguides (e.g. rectangular plates that are homogeneous or that contain a periodic distribution of stiffeners) using numerical wave modes. A WFE-based strategy is proposed which uses the method of elementary radiators for expressing the radiation efficiencies of stiffened or non-stiffened baffled rectangular plates immersed in a light acoustic fluid. In addition, a model reduction strategy consisting in using reduced wave bases for computing these radiation efficiencies with small CPU times is proposed. Numerical experiments highlight the relevance of the strategies

Nonlinear disturbance evolution in a two-dimensional boundary layer along an elastic plate and induced radiated sound

The interaction between a boundary-layer flow and an elastic plate is addressed by direct numerical simulation, taking into account the full coupling between the fluid flow and the flexible wall. The convectively unstable flow state is harmonically forced and two-dimensional nonlinearly saturated wavelike disturbances are computed along archetype-plates with respect to stiffness and natural frequencies. In the aim of determining the low-Mach-number radiated sound for the system, the simulation data are used to solve the Lighthill’s equation in terms of a Green’s function in the wavenumber-frequency space. Different degrees of fluid-structure coupling are implemented in the radiated sound model and the resulting acoustic pressure levels are compared. The sound radiation levels are shown to be increased in the presence of flexible walls with however significant differences in the radiated pressure levels for different coupling assumptionsThe authors gratefully acknowledge Thales Underwater Systems and DCNS for their financial support to this work

A 2D wave finite element-based superelement formulation for acoustic analysis of cavities of arbitrary shapes

International audienceA substructuring technique is proposed which enables fast computation of the acoustic response of arbitrary-shaped 2D cavities subject to different kinds of excitations. It combines rectangular superelements which are modeled by means of the wave finite element (WFE) method, and arbitrary-shaped superelements modeled using component mode synthesis (CMS). Within the WFE framework, the so-called receptance matrices of rectangular superelements — which link the pressure vectors to the acoustic force vectors over the boundaries — can be derived in an efficient way in terms of wave modes, without the need of explicitly condensing the internal degrees of freedom of the systems. A model reduction strategy is proposed which aims at expressing the receptance matrices with a few wave modes only. The proposed strategy involves enclosing each rectangular superelement in a finite element (FE) layer with a small width. In this way, smoothed pressure fields are likely to occur over the WFE superelements, hence enabling these superelements to be described with a few wave modes only. By considering those WFE-based rectangular superelements surrounded by FE layers, this yields the so-called hybrid WFE/FE superelements whose dynamic stiffness matrices can be computed in a very fast way. Modeling a whole arbitrary-shaped acoustic cavity follows from conventional assembly procedure between hybrid WFE/FE superelements, CMS superelements and other FE components. Numerical experiments are carried out to highlight the relevance of the proposed substructuring technique

An experimental study of boundary-layer transition induced vibrations on a hydrofoil

International audienceThis paper aims at characterizing experimentally laminar to turbulent transition induced vibrations. Here, the transition is known to be triggered by a Laminar Separation Bubble that results from a laminar separation of the boundary-layer flow on a hydrofoil. In this study we consider two NACA66312 (Mod) laminar hydrofoils at low angles of incidence (mostly 2° and 4°) and Reynolds numbers ranging from Re=450 000 to 1 200 000, in order to get transitional regimes. The first hydrofoil, made of steel (E=2.1×1011 Pa), is referred to as the rigid hydrofoil, although it is seen to vibrate under the action of the LSB. To better understand the possible interaction between the flow and the foil vibrations, vibration measurements are repeated using a flexible hydrofoil (E=3×109 Pa) of same geometry (under zero loading) and in close configurations. The experiments are carried out at the French Naval Academy Research Institute (IRENav, France). Wall pressure and flow velocity measurements enable a characterization of the laminar separation bubble and the identification of a vortex shedding at a given frequency. It is hence shown that the boundary-layer transition induces important foil vibrations, whose characteristics in terms of frequency and amplitude depend on the vortex shedding frequency, and can be coupled with natural frequencies of the hydrofoils

Nonlinear disturbance evolution in a boundary layer along an elastic plate and induced radiated sound

The interaction between a boundary-layer flow and an elastic plate is addressed by direct numerical simulation, taking into account the full coupling between the fluid flow and the flexible wall. The convectively unstable flow state is harmonically forced and two-dimensional nonlinearly saturated wavelike disturbances are computed along archetype-plates with respect to stiffness and natural frequencies. In the aim of determining the low-Mach-number radiated sound for the system, the simulation data are used to solve the Lighthill’s equation in terms of a Green’s function in the wavenumber-frequency space.Different degrees of fluid-structure coupling are implemented in the radiated sound model and the resulting acoustic pressure levels are compared. The sound radiation levels are shown to be increased in the presence of flexible walls with however significant differences in the radiated pressure levels for different coupling assumptionsInternational audienceThe interaction between a boundary-layer flow and an elastic plate is addressed by direct numerical simulation, taking into account the full coupling between the fluid flow and the flexible wall. The convectively unstable flow state is harmonically forced and two-dimensional nonlinearly saturated wavelike disturbances are computed along archetype-plates with respect to stiffness and natural frequencies. In the aim of determining the low-Mach-number radiated sound for the system, the simulation data are used to solve the Lighthill’s equation in terms of a Green’s function in the wavenumber-frequency space.Different degrees of fluid-structure coupling are implemented in the radiated sound model and the resulting acoustic pressure levels are compared. The sound radiation levels are shown to be increased in the presence of flexible walls with however significant differences in the radiated pressure levels for different coupling assumption

Etude du bruit propre hydrodynamique généré par une couche limite instable sur une structure déformable

Ce travail vise à étudier l'interaction entre un écoulement de couche limite instable et une structure déformable dans le but d'évaluer le bruit hydrodynamique qui peut en résulter. Des simulations numériques directes de l'évolution de la couche limite totale sur une plaque mince élastique encastrée sont effectuées, la dynamique du couplage fort entre le déplacement de la paroi et la pression dans l'écoulement étant notamment prise en compte par une méthode de pas fractionnaire. Ces simulations, complétées par des analyses spectrales en espace et en temps, mettent en évidence des phénomènes de résonances et de masse ajoutée au niveau de la paroi et montrent que les vibrations propres de la paroi peuvent générer des oscillations supplémentaires de pression le long de la paroi à relativement hautes fréquences et petites échelles spatiales, sources potentielles de bruit hydrodynamique. Ce travail est effectué dans le cadre d'une bourse de thèse Cifre avec Thales Underwater Systems

Extracorporeal Membrane Oxygenation for Severe Acute Respiratory Distress Syndrome associated with COVID-19: An Emulated Target Trial Analysis.

RATIONALE: Whether COVID patients may benefit from extracorporeal membrane oxygenation (ECMO) compared with conventional invasive mechanical ventilation (IMV) remains unknown. OBJECTIVES: To estimate the effect of ECMO on 90-Day mortality vs IMV only Methods: Among 4,244 critically ill adult patients with COVID-19 included in a multicenter cohort study, we emulated a target trial comparing the treatment strategies of initiating ECMO vs. no ECMO within 7 days of IMV in patients with severe acute respiratory distress syndrome (PaO2/FiO2 <80 or PaCO2 ≥60 mmHg). We controlled for confounding using a multivariable Cox model based on predefined variables. MAIN RESULTS: 1,235 patients met the full eligibility criteria for the emulated trial, among whom 164 patients initiated ECMO. The ECMO strategy had a higher survival probability at Day-7 from the onset of eligibility criteria (87% vs 83%, risk difference: 4%, 95% CI 0;9%) which decreased during follow-up (survival at Day-90: 63% vs 65%, risk difference: -2%, 95% CI -10;5%). However, ECMO was associated with higher survival when performed in high-volume ECMO centers or in regions where a specific ECMO network organization was set up to handle high demand, and when initiated within the first 4 days of MV and in profoundly hypoxemic patients. CONCLUSIONS: In an emulated trial based on a nationwide COVID-19 cohort, we found differential survival over time of an ECMO compared with a no-ECMO strategy. However, ECMO was consistently associated with better outcomes when performed in high-volume centers and in regions with ECMO capacities specifically organized to handle high demand. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Étude de l'interaction entre un écoulement de couche limite instable et une structure déformable. Application à la prédiction du bruit propre hydrodynamique d'une antenne Sonar.

This work, supported by Thales Underwater Systems through a Cifre thesis grant, and by DCNS, aims at improving the prediction of the hydrodynamic self-noise of a sonar antenna, due to the pressure fluctuations in the boundary-layer flow that develops along the dome. Noise estimations are generally based on semi-empirical models, that take only partially into account the dome flexibility. The present study readdresses the issue of the hydrodynamic noise, in the simplified archetype case of an unstable boundary layer flow along an elastic flat plate. The first part of the study consists in the direct numerical simulation of a two-dimensional boundary layer flow, characterized by a highly supercritical Reynolds number, over an elastic clamped plate. The incompressible Navier-Stokes equations are solved by means of a time-dependent mapping, associated to a mixed finite differences – Chebyshev collocation spatial discretization. A fractional step method enables a full coupling between the plate model and the fluid system. A velocity forcing is introduced in the latter, at unstable frequencies and high amplitudes, in order to give rise to convective and non-linearly saturated instabilities, which interact with the plate motions. The latter vibrates around an initial bent state resulting from the coupling with the non perturbed flow. Plates of various materials and lengths are tested, to characterize the vibrations, in terms of levels and spatial structures, as well as their retro-action on the flow instabilities, depending on the values of the plate natural frequencies. For the cases considered in the present investigation, we observe that the wall pressure spectrum possesses additional modal components, with very low wavenumbers and relatively high frequencies, which may give rise to increased radiation. A post-treatment is implemented to estimate the noise generated by the boundary layer velocity fluctuations. The radiated pressure in the uniform flow domain is evaluated from the simulation data in the framework of the Lighthill's analogy. The calculations are performed in the spectral domain, using an appropriate Green function whose expression takes into account the presence of the elastic plate. In this approach, the plate vibrations induce at some frequencies, including the plate natural frequencies, higher radiated pressure levels than in the rigid case, by favouring larger spatial structures. Additional models, derived from the same formulation, but taking into account the boundary layer compressibility in the computation of the vibratory contributions, highlight a clear increase in the radiated pressure levels in a wide frequency range around the plate natural frequencies, when the latter are distinct from the forcing frequencies, as well as the benefit from considering a full coupling in comparison to classical approaches based on a weak coupling assumption. Finally, the fluid-structure system is investigated experimentally using a hydrodynamic tunnel setup, in order to measure the vibrations of an elastic clamped plate triggered by a transitional or turbulent boundary layer flow, as well as the radiated noise in the cavity beneath the plate, which is equipped with a hydrophone. The measurements, acquired using laser vibrometry and laser Doppler velocimetry, are detailed and analysed.Ce travail, réalisé dans le cadre d'une convention de thèse Cifre avec Thales Underwater Systems, et cofinancé par DCNS, vise à améliorer la prédiction du bruit propre hydrodynamique d'une antenne sonar, dû aux fluctuations de pression dans la couche limite qui se développe le long du dôme. Les estimations de bruit reposent en général sur des modèles semi-empiriques qui ne tiennent que partiellement compte de la flexibilité du dôme. La présente étude reconsidère le problème du bruit hydrodynamique dans le cas-type simplifié d'une couche limite instable le long d'une plaque plane élastique. La première partie du travail est consacrée à la simulation numérique directe d'un écoulement de couche limite bidimensionnel, caractérisé par un nombre de Reynolds hautement supercritique, le long d'une paroi élastique encastrée. La résolution des équations de Navier-Stokes incompressibles repose sur un changement de variables évolutif au cours du temps, associé à une discrétisation spatiale mixte différences finies – collocation Chebyshev. Une méthode de pas fractionnaire permet d'assurer un couplage fort entre le modèle de paroi élastique et le système fluide. Un forçage en vitesse est injecté dans ce dernier, à des fréquences instables et des amplitudes suffisamment élevées, afin de donner lieu à des instabilités convectives et saturées non linéairement, qui interagissent avec les mouvements de la paroi. Cette dernière vibre autour d'un état déformé initial résultant du couplage avec l'écoulement non perturbé. Des parois de longueurs et de matériaux variés ont été testées en vue de caractériser les vibrations, en termes de niveaux et de structures spatiales, ainsi que leur rétroaction sur les instabilités de l'écoulement, en fonction des valeurs des fréquences propres de la paroi. Dans les divers cas étudiés, on observe que le spectre de pression pariétale est enrichi par des composantes modales, à très bas nombres d'ondes et relativement hautes fréquences, qui peuvent donner lieu à un rayonnement plus important. Un post-traitement est mis en oeuvre afin d'estimer le bruit généré par les fluctuations de vitesse dans la couche limite. La pression rayonnée dans l'écoulement uniforme est évaluée à partir des données acquises au cours des simulations, dans le cadre de l'analogie de Lighthill. Le calcul, effectué dans le domaine spectral, repose sur l'utilisation d'une fonction de Green dont l'expression tient compte de la présence de la paroi souple. Dans cette approche, les vibrations de la paroi induisent à certaines fréquences, en particulier aux fréquences propres de la paroi, des niveaux de pression rayonnée supérieurs à ceux du cas rigide, en favorisant la présence de structures spatiales plus larges. D'autres modèles, élaborés à partir de la même formulation, mais tenant compte de la compressibilité de la couche limite dans le calcul des contributions vibratoires, mettent en évidence une nette augmentation des niveaux de pression rayonnée sur paroi souple dans une large gamme de fréquences, de part et d'autres des fréquences propres, lorsque ces dernières sont distinctes des fréquences de forçage, ainsi que l'apport de la prise en compte d'un couplage fort par rapport aux approches classiques basées sur une hypothèse de couplage faible. Enfin, une étude expérimentale en tunnel hydrodynamique est mise en place en vue de mesurer les vibrations d'une plaque élastique, encastrée dans un support rigide, et soumise à un écoulement transitionnel ou turbulent, ainsi que le bruit rayonné dans la cavité sous-jacente à la plaque, instrumentée avec un hydrophone. Les résultats de la campagne de mesures, qui repose sur les techniques de vibrométrie laser et de vélocimétrie laser Doppler, sont détaillés et analysés

Acoustic radiation of 2D nearly periodic metamaterial plates via finite element procedures and model reduction strategies

International audienceThe acoustic radiation of 2D nearly periodic plates interacting with light fluids is addressed. Metamaterial plates made up of resonant multi-layered cells/substructures with varying (random) geometrical properties, which are described via distorted finite element meshes, are dealt with. The proposed approach involves the Craig-Bampton method and an interpolation scheme for modeling the substructures and approximating the related reduced mass and stiffness matrices. In this paper, this approach is applied to compute the radiation efficiencies of such metamaterial plates with the method of elementary radiators. The relevance of the proposed approach is highlighted through numerical experiments. Especially the structural and acoustic responses of a nearly periodic plate are assessed and compared to the purely periodic case