A Ghost Fluid/Level Set Method for boiling flows and liquid evaporation: Application to the Leidenfrost effect.

The development of numerical methods for the direct numerical simulation of two-phase flows with phase change, in the framework of interface capturing or interface tracking methods, is the main topic of this study. We propose a novel numerical method, which allows dealing with both evaporation and boiling at the interface between a liquid and a gas. Indeed, in some specific situations involving very heterogeneous thermodynamic conditions at the interface, the distinction between boiling and evaporation is not always possible. For instance, it can occur for a Leidenfrost droplet; a water drop levitating above a hot plate whose temperature is much higher than the boiling temperature. In this case, boiling occurs in the film of saturated vapor which is entrapped between the bottom of the drop and the plate, whereas the top of the water droplet evaporates in contact of ambient air. The situation can also be ambiguous for a superheated droplet or at the contact line between a liquid and a hot wall whose temperature is higher than the saturation temperature of the liquid. In these situations, the interface temperature can locally reach the saturation temperature (boiling point), for instance near a contact line, and be cooler in other places. Thus, boiling and evaporation can occur simultaneously on different regions of the same liquid interface or occur successively at different times of the history of an evaporating droplet. Standard numerical methods are not able to perform computations in these transient regimes, therefore, we propose in this paper a novel numerical method to achieve this challenging task. Finally, we present several accuracy validations against theoretical solutions and experimental results to strengthen the relevance of this new method

Comparison between the FLUIDICS experiment and direct numerical simulations of fluid sloshing in spherical tanks under microgravity conditions

The fluids behaviour within a spherical tank under microgravity conditions is investigated through a comparison between the original data from the FLUIDICS experiment carried out in the ISS and Direct Numerical Simulations for two-phase flows. The study case consists in the rotation of a spherical tank around a fixed axis. The tank is filled with a liquid with physical properties similar to those of liquid propellants and gases used in the space industry. Two tanks with different filling ratios have been tested in space. Cameras and sensors allow extracting the fluids dynamics and the temporal evolution of the force and torque exerted by the fluids on the tank wall. Several manoeuvres corresponding to different angular velocities and angular accelerations are submitted on both tanks. The velocity profile is divided into four phases: from zero, the angular velocity around the vertical axis increases linearly until it reaches the required constant value for which the fluids stabilise in the second phase, then the angular velocity decreases until it recovers zero. Numerical simulations are computed with the home-made code DIVA which is based on the Level Set method coupled with the Ghost Fluid Method. The force in the radial direction gives the value of the centrifugal force during the constant angular velocity phase. The average centrifugal force is well predicted by the simulations, the comparison with the experimental data exhibits errors lower than 3% for the half-filled tank. Considering the vertical torque, the effect of the Euler acceleration is clearly visible through the important peaks of opposite sign observed during the acceleration and the deceleration phases. Moreover, the oscillations of the gas bubble during the second phase can be observed from the torque evolution. Their magnitude decreases throughout time until the steady state is reached. The measured and predicted temporal evolutions match together until the magnitude of the oscillations reaches the noise level of the data. The bubble oscillations are much more damped for the tank containing a larger amount of liquid (75%). The frequency of these oscillations are investigated applying the Fourier transform of the torque signals and by looking at the videos taken during the experiment. Similar oscillation frequencies are observed with the experimental setup and the numerical simulations, even for the manoeuvre with the lower Bond numbers. We verify that the oscillation frequency increases with the angular velocity. Finally, the comparison exhibits that the numerical simulations provide an accurate prediction of the fluids behaviours in microgravity conditions for this range of Bond numbers

EuReCa ONE—27 Nations, ONE Europe, ONE Registry A prospective one month analysis of out-of-hospital cardiac arrest outcomes in 27 countries in Europe

AbstractIntroductionThe aim of the EuReCa ONE study was to determine the incidence, process, and outcome for out of hospital cardiac arrest (OHCA) throughout Europe.MethodsThis was an international, prospective, multi-centre one-month study. Patients who suffered an OHCA during October 2014 who were attended and/or treated by an Emergency Medical Service (EMS) were eligible for inclusion in the study. Data were extracted from national, regional or local registries.ResultsData on 10,682 confirmed OHCAs from 248 regions in 27 countries, covering an estimated population of 174 million. In 7146 (66%) cases, CPR was started by a bystander or by the EMS. The incidence of CPR attempts ranged from 19.0 to 104.0 per 100,000 population per year. 1735 had ROSC on arrival at hospital (25.2%), Overall, 662/6414 (10.3%) in all cases with CPR attempted survived for at least 30 days or to hospital discharge.ConclusionThe results of EuReCa ONE highlight that OHCA is still a major public health problem accounting for a substantial number of deaths in Europe.EuReCa ONE very clearly demonstrates marked differences in the processes for data collection and reported outcomes following OHCA all over Europe. Using these data and analyses, different countries, regions, systems, and concepts can benchmark themselves and may learn from each other to further improve survival following one of our major health care events

Simulation numérique directe de gouttes et de groupes de gouttes qui s'évaporent dans un écoulement laminaire ou turbulent

The vaporisation of injected fuel in a combustion chamber is a crucial phenomenon in an aeronautical motor because it determines the vapour quantity which will be burned to bring energy to the motor. Still, this phenomenon is not well understood due to the difficulties to measure on experiments vaporisation rates of injected sprays. Moreover, hypothesis of theoretical models are not representatives of conditions encountered in combustion furnaces. The thesis takes place in an effort to analyse the evaporation of droplet groups in a turbulent flow by mean of Direct Numerical Simulation (DNS). Indeed, during droplet group evaporation, collective effects can modify single droplet rates of vaporisation and the group global rate of vaporisation. The DNS approach should allow to quantify precisely this effect and leads to an improvement of actual models of evaporation. Firstly, algorithms are developed and used in a 1D spherical configuration to describe the evaporation of a single static droplet without gravity. Secondly, the vaporisation of a droplet in a laminar flow has been studied. The analysis focused on heat exchanges between the droplet and the external environment as well as the exerted force on the droplet by the laminar flow. In this part, it has been highlighted that the evaporation induced a decrease in thermal exchanges and drag. In some cases of strong evaporation, the drag of the droplet has been observed to be negative. It means that the evaporation can cause a propulsion phenomenon of the droplet. A theoretical analysis allows to link this behaviour to an asymmetry of the vaporisation rate. Thirdly, a study of the turbulence influence on the evaporation of a droplet has been carried out. A generator of turbulent fluctuations has been implemented and parallel approaches have been introduced to reduced computational time. It allows to analyse thermal exchanges and drag behaviour of a droplet interacting with a turbulent flow. The analysis showed that these two variables increase with turbulence. Lastly, the evaporation of groups of droplets has been studied. For three different groups of droplets, trajectories of droplets have been analysed with heat exchanges when they are put in a laminar or a turbulent flow with or without phase change. In presence of evaporation, the analysis pointed out that trajectories were different from cases without evaporation and so that phase change modifies collective effects. Moreover, these collective effects have also been observed on thermal exchanges.L’évaporation du carburant injecté dans une chambre de combustion est un phénomène crucial dans un foyer aéronautique car elle détermine la quantité de vapeur qui sera ensuite brûlée pour fournir de l’énergie au moteur. Cependant, ce phénomène reste mal décrit du fait des difficultés de mesurer expérimentalement les taux d’évaporation des gouttes appartenant à un brouillard. D’autre part, les hypothèses des modèles théoriques ne sont toujours pas représentatives des conditions rencontrées dans les foyers de combustion.La thèse s’inscrit dans une démarche visant à étudier l’évaporation d’un groupe de gouttes dans un écoulement turbulent au moyen de la Simulation Numérique Directe (SND). En effet, lors de l’évaporation de groupes de gouttes, des effets collectifs peuvent influer sur le taux d’évaporation de chaque goutte ou sur le taux d’évaporation global du nuage de gouttes. L’approche SND permet de quantifier précisément ces effets afin d’améliorer les modèles actuels d’évaporation. Dans un premier temps, des algorithmes ont été développés et utilisés dans une configuration 1D sphérique pour décrire l’évaporation d’une goutte statique isolée et sans gravité.Puis dans un second temps, l’évaporation d’une goutte a été étudiée dans un écoulement laminaire. Une analyse des échanges de chaleur entre la goutte et le milieu extérieur ainsi que de la force de traînée exercée par l’écoulement laminaire sur la goutte a été effectuée. Dans cette partie, il a été mis en évidence que l’évaporation induit une diminution des échanges thermiques et de la traînée. Il a notamment été observé que dans certains cas de forte évaporation, la traînée de la goutte peut devenir négative. Cela implique que l’évaporation peut être à l’origine d’un phénomène de propulsion de la goutte. Une analyse théorique permet de lier se comportement à une asymétrie du débit d’évaporation.Dans un troisième temps, l’influence de la turbulence sur l’évaporation d’une goutte a été étudiée. Pour cela, un générateur de fluctuations turbulentes a été implémenté et des techniques de calculs parallèles ont été introduites pour réduire les temps de calcul. Cela a permis d’analyser les échanges thermiques et le comportement de la traînée d’une goutte en interaction avec un écoulement turbulent. Il a été montré que ces deux grandeurs ont tendance à être amplifiées par la turbulence.Enfin, dans un dernier temps, l’évaporation de groupes de gouttes a été étudiée. Pour trois groupes de gouttes différents, les déplacements des gouttes ont été analysés avec les échanges de chaleur lorsque ceux-ci sont placés dans un écoulement laminaire ou turbulent avec ou sans changement de phase. En présence d’évaporation, il a été mis en évidence que les déplacements sont différents des cas sans évaporation et donc que le changement de phase modifie les effets collectifs. De plus, ces effets de groupes ont aussi été observés sur les échanges thermiques

Direct numerical simulation of droplets and droplet groups vaporizing in a laminar or turbulent flow

L’évaporation du carburant injecté dans une chambre de combustion est un phénomènecrucial dans un foyer aéronautique car elle détermine la quantité de vapeur qui sera ensuite brûléepour fournir de l’énergie au moteur. Cependant, ce phénomène reste mal décrit du fait desdifficultés de mesurer expérimentalement les taux d’évaporation des gouttes appartenant à unbrouillard. D’autre part, les hypothèses des modèles théoriques ne sont toujours pas représentativesdes conditions rencontrées dans les foyers de combustion.La thèse s’inscrit dans une démarche visant à étudier l’évaporation d’un groupe de gouttesdans un écoulement turbulent au moyen de la Simulation Numérique Directe (SND). En effet, lorsde l’évaporation de groupes de gouttes, des effets collectifs peuvent influer sur le taux d’évaporationde chaque goutte ou sur le taux d’évaporation global du nuage de gouttes. L’approche SNDpermet de quantifier précisément ces effets afin d’améliorer les modèles actuels d’évaporation.Dans un premier temps, des algorithmes ont été développés et utilisés dans une configuration1D sphérique pour décrire l’évaporation d’une goutte statique isolée et sans gravité. Puisdans un second temps, l’évaporation d’une goutte a été étudiée dans un écoulement laminaire.Une analyse des échanges de chaleur entre la goutte et le milieu extérieur ainsi que de la force detraînée exercée par l’écoulement laminaire sur la goutte a été effectuée. Dans cette partie, il a étémis en évidence que l’évaporation induit une diminution des échanges thermiques et de la traînée.Il a notamment été observé que dans certains cas de forte évaporation, la traînée de la gouttepeut devenir négative. Cela implique que l’évaporation peut être à l’origine d’un phénomène depropulsion de la goutte. Une analyse théorique permet de lier ce comportement à une asymétriedu débit d’évaporation. Dans un troisième temps, l’influence de la turbulence sur l’évaporationd’une goutte a été étudiée. Pour cela, un générateur de fluctuations turbulentes a été implémentéet des techniques de calculs parallèles ont été introduites pour réduire le temps des calculs. Celaa permis d’analyser les échanges thermiques et le comportement de la traînée d’une goutte eninteraction avec un écoulement turbulent. Il a été montré que ces deux grandeurs ont tendanceà être amplifiées par la turbulence. Enfin, dans un dernier temps, l’évaporation de groupes degouttes a été étudiée. Pour trois groupes de gouttes différents, les déplacements des gouttes ontété analysés avec les échanges de chaleur lorsque ceux-ci sont placés dans un écoulement laminaireou turbulent avec ou sans changement de phase. En présence d’évaporation, il a été mis enévidence que les déplacements sont différents des cas sans évaporation et donc que le changementde phase modifie les effets collectifs. De plus, ces effets de groupes ont aussi été observés sur leséchanges thermiques.The vaporisation of injected fuel in a combustion chamber is a crucial phenomenon inan aeronautical motor because it determines the vapour quantity which will be burned to bringenergy to the motor. Still, this phenomenon is not well understood due to the difficulties tomeasure on experiments vaporisation rates of injected sprays. Moreover, hypothesis of theoriticalmodels are not representatives of conditions encountered in combustion furnaces.The thesis take place in an effort to analyse the evaporation of droplet groups in a turbulentflow by mean of Direct Numerical Simulation (DNS). Indeed, during droplet group evaporation,collective effects can modify single droplet rates of vaporisation and the group global rate ofvaporisation. The DNS approach should allows to quantify precisely this effect and leads to animprovement of actual models of evaporation.Firstly, algorithms are developped and used in a 1D spherical configuration to describe theevaporation of a single static droplet without gravity. Secondly, the vaporistion of a droplet in alaminar flow has been studied. The analysis focus on heat exchanges between the droplet and theexternal environment as well as the force exerced on the droplet by the laminar flow. In this part,it has been highlighted that the evaporation induced a decrease in thermal exchanges and drag.In some cases of strong evaporation, the drag of the droplet has been observed to be negative.It means that the evaporation can cause a propulsion phenomenon of the droplet. A theoriticalanalysis allows to link this behaviour to an asymetry of the vaporisation rate. Thirdly, a studyof the turbulence influence on the evaporation of a droplet has been carried out. A generator ofturbulent fluctuations has been implemented and parallel approaches have been introduced toreduced computational time. It allowes to analyse thermal exchanges and drag behaviour of adroplet interacting with a turbulent flow. The analysis showed that theses two variables increasewith turbulence. Lastly, the evaporation of groups of droplets has been studied. For three differentgroups of droplets, trajectories of droplets have been analysed with heat exchanges when they areput in a laminar or a turbulent flow with or without phase change. In presence of evaporation,the analysis pointed out that trajectories were different from cases whitout evaporation and sothat phase change modifies collective effects. Moreover, these collective effects have also beenobserved on thermal exchanges

Prévention du suicide et santé connectée

L’évaluation ponctuelle du risque suicidaire habituellement conduite aux urgences, après un geste suicidaire, ne rend pas compte de son évolution après la sortie des soins, alors même que le risque de récidive reste important plusieurs mois après. Dans ces conditions, les possibilités d’identification, et donc de prise en charge, des patients à risque suicidaire sont limitées. Le développement de la santé connectée (eHealth) donne désormais accès en temps réel à des informations sur l’état de santé d’un patient entre deux séjours en centre de soins. Cette extension de l’évaluation clinique à l’environnement du patient permet de développer des outils d’aide à la décision face à la gestion du risque suicidaire

Effect of general anaesthesia on functional outcome in patients with anterior circulation ischaemic stroke having endovascular thrombectomy versus standard care: a meta-analysis of individual patient data

Background: General anaesthesia (GA) during endovascular thrombectomy has been associated with worse patient outcomes in observational studies compared with patients treated without GA. We assessed functional outcome in ischaemic stroke patients with large vessel anterior circulation occlusion undergoing endovascular thrombectomy under GA, versus thrombectomy not under GA (with or without sedation) versus standard care (ie, no thrombectomy), stratified by the use of GA versus standard care. Methods: For this meta-analysis, patient-level data were pooled from all patients included in randomised trials in PuMed published between Jan 1, 2010, and May 31, 2017, that compared endovascular thrombectomy predominantly done with stent retrievers with standard care in anterior circulation ischaemic stroke patients (HERMES Collaboration). The primary outcome was functional outcome assessed by ordinal analysis of the modified Rankin scale (mRS) at 90 days in the GA and non-GA subgroups of patients treated with endovascular therapy versus those patients treated with standard care, adjusted for baseline prognostic variables. To account for between-trial variance we used mixed-effects modelling with a random effect for trials incorporated in all models. Bias was assessed using the Cochrane method. The meta-analysis was prospectively designed, but not registered. Findings: Seven trials were identified by our search; of 1764 patients included in these trials, 871 were allocated to endovascular thrombectomy and 893 were assigned standard care. After exclusion of 74 patients (72 did not undergo the procedure and two had missing data on anaesthetic strategy), 236 (30%) of 797 patients who had endovascular procedures were treated under GA. At baseline, patients receiving GA were younger and had a shorter delay between stroke onset and randomisation but they had similar pre-treatment clinical severity compared with patients who did not have GA. Endovascular thrombectomy improved functional outcome at 3 months both in patients who had GA (adjusted common odds ratio (cOR) 1·52, 95% CI 1·09–2·11, p=0·014) and in those who did not have GA (adjusted cOR 2·33, 95% CI 1·75–3·10, p<0·0001) versus standard care. However, outcomes were significantly better for patients who did not receive GA versus those who received GA (covariate-adjusted cOR 1·53, 95% CI 1·14–2·04, p=0·0044). The risk of bias and variability between studies was assessed to be low. Interpretation: Worse outcomes after endovascular thrombectomy were associated with GA, after adjustment for baseline prognostic variables. These data support avoidance of GA whenever possible. The procedure did, however, remain effective versus standard care in patients treated under GA, indicating that treatment should not be withheld in those who require anaesthesia for medical reasons