Influence of confinement on a two-dimensional wake

The spatio-temporal development of an incompressible two-dimensional viscous wake flow confined by two flat slipping plates is investigated by means of direct numerical simulation (DNS), using a spectral Chebyshev multi-domain method. The limit between unstable and stable configurations is determined with respect to several non-dimensional parameters: the confinement, the velocity ratio and two different Reynolds numbers, $100$ and $500$. The comparison of such limit curves with theoretical results obtained by Juniper (J. Fluid Mech., vol.565, 2006, pp.171-195) confirms the existence of a region at moderate confinement where the instability is maximal. Moreover, instabilities are also observed under sustained co-flow, in the form of a vacillating front. Using a direct computation of the two-dimensional base flow, we perform a local linear stability analysis for several velocity profiles prevailing at different spatial locations, so as to determine the local spatio-temporal nature of the flow: convectively unstable or absolutely unstable. Comparisons of the DNS and local stability analysis results are provided and discusse

Influence du confinement sur un sillage bi-dimensionnel

Le problème modèle étudié est un sillage bidimensionnel confiné par deux parois latérales. Les équations de Navier-Stokes incompressibles sont résolues par un code spectral multi-domaine. L’objectif est de déterminer la limite entre les configurations instables et stables en fonction des differents paramètres adimensionnels : le paramètre de confinement et le rapport de vitesses. Les écoulements les plus instables se produisent pour des confinements d’ordre un

Solid Organ Transplantation During COVID-19 Pandemic: An International Web-based Survey on Resources’ Allocation

Background. Solid organ transplants (SOTs) are life-saving interventions, recently challenged by coronavirus disease 2019 (COVID-19). SOTs require a multistep process, which can be affected by COVID-19 at several phases. Methods. SOT-specialists, COVID-19-specialists, and medical ethicists designed an international survey according to CHERRIES guidelines. Personal opinions about continuing SOTs, safe managing of donors and recipients, as well as equity of resources' allocation were investigated. The survey was sent by e-mail. Multiple approaches were used (corresponding authors from Scopus, websites of scientific societies, COVID-19 webinars). After the descriptive analysis, univariate and multivariate ordinal regression analysis was performed. Results. There were 1819 complete answers from 71 countries. The response rate was 49%. Data were stratified according to region, macrospecialty, and organ of interest. Answers were analyzed using univariate- multivariate ordinal regression analysis and thematic analysis. Overall, 20% of the responders thought SOTs should not stop (continue transplant without restriction); over 70% suggested SOTs should selectively stop, and almost 10% indicated they should completely stop. Furthermore, 82% agreed to shift resources from transplant to COVID-19 temporarily. Briefly, main reason for not stopping was that if the transplant will not proceed, the organ will be wasted. Focusing on SOT from living donors, 61% stated that activity should be restricted only to "urgent"cases. At the multivariate analysis, factors identified in favor of continuing transplant were Italy, ethicist, partially disagreeing on the equity question, a high number of COVID-19- related deaths on the day of the answer, a high IHDI country. Factors predicting to stop SOTs were Europe except-Italy, public university hospital, and strongly agreeing on the equity question. Conclusions. In conclusion, the majority of responders suggested that transplant activity should be continued through the implementation of isolation measures and the adoption of the COVID-19-free pathways. Differences between professional categories are less strong than supposed

A pandemic recap : lessons we have learned

On January 2020, the WHO Director General declared that the outbreak constitutes a Public Health Emergency of International Concern. The world has faced a worldwide spread crisis and is still dealing with it. The present paper represents a white paper concerning the tough lessons we have learned from the COVID-19 pandemic. Thus, an international and heterogenous multidisciplinary panel of very differentiated people would like to share global experiences and lessons with all interested and especially those responsible for future healthcare decision making. With the present paper, international and heterogenous multidisciplinary panel of very differentiated people would like to share global experiences and lessons with all interested and especially those responsible for future healthcare decision making.Non peer reviewe

The influence of shear layer thickness on the stability of confined two-dimensional wakes

The influence of confinement onto the inviscid and incompressible linear stability of the family of wakes introduced by Monkewitz [Phys. Fluids 31, 999 (1988)] is examined. The nondimensional parameters of the model, the velocity ratio Λ, defined as ratio of the velocity gap to the mean velocity, the profile shape parameter, which controls the shear layer thickness δw, and the confinement parameter, are varied and their effect onto temporal and spatiotemporal stability properties is considered. Particularly, the limit between absolute (A) and convective (C) instability is investigated as a function of the different parameters. For a given confinement, there exists an optimal value of the shear layer thickness for which the absolute instability is maximal. The absolute frequency and complex wavenumber of the mode at the C/A transition are discussed. Furthermore, the continuous profiles are approximated by means of piecewise broken-line profile, with similar spatiotemporal properties. As a typical application, a few nonparallel base flows, computed by direct numerical simulation at Re = 500, are analyzed on a weakly nonparallel basis by plotting the locus of the local velocity profile in the (δw,Λ) plane. The absolute frequency at the C/A transition point is seen to predict accurately the frequency prevailing in the nonlinear direct numerical simulations. These results further help interpreting the influence of confinement on the Strouhal number measured experimentally in the wake of confined cylinders

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International audienceVoix de Erri de Luca traduit de l'italien par Angela Biancofior

Influence of confinement on the spatio-temporal development of wakes

This thesis focuses on the stability of wake flows, especially on the influence of confinement on their stability properties. Several analyses are conducted to this end. At first, a local stability analysis under the WKBJ approach is carried out on a family of inviscid confined jet/wakes flows with or without surface tension at interfaces. Both temporal and spatio-temporal behaviours of such flows are examined and commented. We are particularly interested in the destabilizing effect due to the blockage noticed for wake flows at medium confinement. This aspect is also investigated by means of the counter propagating Rossby waves (CRW) perspective. Furthermore, the influence of confinement on non-parallel viscous wakes is examined by means of direct numerical simulations (DNS). It turns out that when prescribing the Reynolds number to Re = 100 the destabilizing influence at medium confinement is confirmed either with free slip of not slip conditions at the walls. Increasing Re up to 500, more subtle destabilizing phenomena are encountered, under the form of a vacillating front. By means of large-eddy simulations (LES), we further examine the effect of confinement on turbulent wake flows for Re = 5000. Examing several quantities as turbulent kinetic energy, dissipation rate, one-dimensional spectrum and third order velocity structure function, we conclude that the confinement favours the development of fully 3D turbulence.L objet de ce travail est l étude de la stabilité de sillages et, en particulier, de l influence du confinement sur leurs propriétés de stabilité. Différentes analyses sont effectuées pour parvenir à ce but. Une analyse locale, sous la condition WKBJ, est effectuée en premier lieu, en utilisant comme modèle une famille de sillages non visqueux et confinés en présence ou pas d une tension de surface aux interfaces. Le développement temporel ainsi que spatio-temporel de tels écoulements sont examinés et commentés. En particulier, on s intéresse à l effet déstabilisant d un confinement modéré sur le développement d un sillage. Cet aspect est aussi étudié en utilisant la perspective des ondes Rossby contra-propagatives (CRW). L influence du confinement sur des sillages non parallèles et visqueux est également examinée par des simulations numériques directes (DNS). En fixant le nombre de Reynolds à Re = 100, l effet déstabilisant pour un confinement modéré est confirmé avec aux parois des conditions de glissement ou bien d adhérence. En augmentant le nombre de Reynolds à Re = 500, des phénomènes plus complexes ont été observés, sous la forme d un front vacillant. En outre, avec des simulations des grandes échelles (LES) , on examine l effet du confinement sur des sillages turbulents à Re = 5000. L étude de plusieurs quantités, comme l énergie cinétique turbulente, son taux de dissipation, des spectres de vitesse et la fonction de structure de troisième ordre, a mis en évidence que le confinement favorise le développement de la turbulence 3D.NICE-BU Sciences (060882101) / SudocSudocFranceF