Aeraulic transfer mechanisms through openings on enclosures in maintenance and dismantling sites : evaluation of aerosol aerodynamic behavior contribution to backflow phenomenon

The context of my thesis concerns the safety of ventilated enclosures implemented on maintenance and dismantling sites of nuclear sites. It aims to characterize the efficiency of dynamic containment of ventilated airlocks by studying the backflow of gaseous and particulate pollutants. Backflow phenomenon can occur through nominal or accidental openings on ventilated enclosures under the effect of an internal or external aeraulic disturbance. Our works are the continuation of S. Kaissoun1 works who studied the phenomenon of local backflow of a gaseous pollutant at a rectangular opening on a reduced-scale enclosure. As part of my thesis, we added an external envelope around the experimental enclosure in order to characterize the overall quantity of pollutants coming from the backflow. The main objectives of my thesis are as follow. On one hand, we aim to characterize the backflow phenomenon experimentally using laser visualization techniques (Schlieren, PIV) and to measure the local and global quantities of gaseous and 5 μm particulate pollutants emitted through an opening using dedicated tracing techniques. On the other hand, we aim to validate the capacity of hybrid CFD turbulence models (SST-DES) to qualitatively and quantitatively transcribe the unsteady backflow phenomena of gaseous and particulate pollutants. The experimental and numerical results obtained allow us to draw the following two conclusions. First, the aeraulic behavior of gas and 5 μm aerosol is similar near the opening for our aeraulic conditions. Secondly, the capacity of SST-DES hybrid model to reproduce the backflow phenomenon is qualitatively and quantitatively validated. Experimental and numerical studies of the backflow phenomenon were finally carried out on a full-scale airlock with flexible vinyl walls. Preliminary works show that quantitative results obtained experimentally matches those obtained from numerical simulations

3D measurements of inclined vortex rings interacting with a density stratification

Vortex rings are coherent vortical structures that dominate the dynamics of numerous flows as they are generated each time an impulsive jet occurs in a homogeneous fluid. They are also considered as elementary bricks of turbulence. Their faculty to propagate along their revolution axis by self-induction confers to such structures interesting transport properties, namely, transport of momentum, mass and heat. They are therefore often qualified as good candidates for mixing. From this perspective, the present study addresses the interaction of a vortex ring with a density stratification in order to get a better understanding of the subsequent mixing mechanisms. A new 3D time-resolved technique is used and gives a highlight at short timescale on the 3D vorticity reorganization and at larger timescale on the 3D patterns of internal gravity waves forced by the impacting/penetrating vortex. The influence of the Reynolds number of the vortex ring and its angle of attack relative to isopycnals will be detailed

Experimental investigation of mixing efficiency in particle‑laden Taylor–Couette flows

This paper reports on original experimental data of mixing in two-phase Taylor–Couette flows. Neutrally buoyant particles with increasing volume concentration enhance significantly mixing of a passive tracer injected within the gap between two concentric cylinders. Mixing efficiency is measured by planar laser-induced fluorescence coupled to particle image velocimetry to detect the Taylor vortices. To achieve reliable experimental data, index matching of both phases is used together with a second PLIF channel. From this second PLIF measurements, dynamic masks of the particle positions in the laser sheet are determined and used to calculate accurately the segregation index of the tracer concentration. Experimental techniques have been thoroughly validated through calibration and robustness tests. Three particle sizes were considered, in two different flow regimes to emphasize their specific roles on the mixing dynamics

Dynamics of an inclined Vortex Ring interacting with a density stratification

Vortex Rings are coherent vortical structures that dominate the dynamics of numerous flows as they are generated each time an impulsive jet occurs in a homogeneous fluid (for instance, plumes can be considered as Vortex Rings). Such structures have the faculty to self-propagate along their revolution axis, conferring them capacities of transport and mixing that could be exploited. Among applications, one can mention nuclear safety and the need to mix fluids of different density to prevent explosion hazard. The scope of the present study is to identify and evaluate the mixing mechanisms associated with a Vortex Ring interacting with a density stratification, in particular, the reorganization of the flow and the generation of internal waves. The influence of the Vortex Ring propagation speed and propagation angle relative to the density gradient on its dynamics and mixing power are studied thanks to 2D and 3D time-resolved TOMO-PIV

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Mécanismes de transfert aéraulique au travers d’ouvertures d’enceintes de confinement : évaluation de la contribution du comportement aérodynamique des aérosols au phénomène de rétrodiffusion

Le contexte de ma thèse concerne la sûreté des enceintes ventilées mises en oeuvre sur les chantiers de maintenance et de démantèlement de sites nucléaires. Il s’agit de caractériser l'efficacité du confinement dynamique des enceintes de type « sas » en étudiant le phénomène de rétrodiffusion d’un polluant gazeux ou particulaire. Ce phénomène de rétrodiffusion peut se produire à travers des ouvertures nominales ou accidentelles sur ces dispositifs de confinement suite à une perturbation aéraulique se produisant à l'intérieur ou à l’extérieur de l'enceinte. Nos travaux s’inscrivent dans la continuité de ceux de S. Kaissoun1 qui a étudié expérimentalement et numériquement le phénomène de la rétrodiffusion locale d’un polluant gazeux au niveau d’une ouverture rectangulaire d’une maquette expérimentale à échelle réduite. Dans le cadre de ma thèse, nous avons ajouté une enveloppe externe autour de la maquette expérimentale afin de caractériser la quantité globale des polluants rétrodiffusés. Les principaux objectifs de ma thèse sont les suivants. D’une part, on veut caractériser expérimentalement la rétrodiffusion à l’aide de techniques laser de visualisation (Schlieren, PIV) et mesurer les quantités locales et globales de polluants gazeux et particulaires pour un aérosol de diamètre aérodynamique 5 μm émis au travers d’une ouverture en utilisant les techniques de traçage dédiées. D’autre part, on veut valider la capacité des modèles hybrides CFD de turbulence (DES-SST) à retranscrire la rétrodiffusion des polluants gazeux et particulaires. Les résultats expérimentaux et numériques finalement obtenus permettent de dresser les deux conclusions suivantes. Les comportements aérauliques du gaz et de l’aérosol de 5 μm sont similaires à proximité de l’ouverture pour nos conditions aérauliques. La capacité du modèle hybride DES-SST à reproduire le phénomène de rétrodiffusion est validée qualitativement et quantitativement. Des études expérimentales et numériques du phénomène de rétrodiffusion ont finalement été réalisées sur un sas à échelle réelle présentant des parois souples en vinyle, à la fois pour un polluant gazeux et un polluant particulaire. Les résultats quantitatifs préliminaires obtenus expérimentalement correspondent à ceux issus des simulations numériques pour le sas à échelle réelleThe context of my thesis concerns the safety of ventilated enclosures implemented on maintenance and dismantling sites of nuclear sites. It aims to characterize the efficiency of dynamic containment of ventilated airlocks by studying the backflow of gaseous and particulate pollutants. Backflow phenomenon can occur through nominal or accidental openings on ventilated enclosures under the effect of an internal or external aeraulic disturbance. Our works are the continuation of S. Kaissoun1 works who studied the phenomenon of local backflow of a gaseous pollutant at a rectangular opening on a reduced-scale enclosure. As part of my thesis, we added an external envelope around the experimental enclosure in order to characterize the overall quantity of pollutants coming from the backflow. The main objectives of my thesis are as follow. On one hand, we aim to characterize the backflow phenomenon experimentally using laser visualization techniques (Schlieren, PIV) and to measure the local and global quantities of gaseous and 5 μm particulate pollutants emitted through an opening using dedicated tracing techniques. On the other hand, we aim to validate the capacity of hybrid CFD turbulence models (SST-DES) to qualitatively and quantitatively transcribe the unsteady backflow phenomena of gaseous and particulate pollutants. The experimental and numerical results obtained allow us to draw the following two conclusions. First, the aeraulic behavior of gas and 5 μm aerosol is similar near the opening for our aeraulic conditions. Secondly, the capacity of SST-DES hybrid model to reproduce the backflow phenomenon is qualitatively and quantitatively validated. Experimental and numerical studies of the backflow phenomenon were finally carried out on a full-scale airlock with flexible vinyl walls. Preliminary works show that quantitative results obtained experimentally matches those obtained from numerical simulations

Experimental and numerical studies of gaseous and particulate pollutants backflow through an opening from a depressurized enclosure: application to nuclear dismantling operations

International audienceDuring nuclear maintenance and dismantling operations, a dynamic confinement due to inflow passing through openings is applied in order to prevent the leakage of pollutants from depressurized enclosures. This ensures better safety for operating people. To guaranty an efficient dynamic confinement, three values for the velocity of the inward flow near the opening were required in the past, depending on the dangerousness of the radioactive pollutants. Nowadays the ISO standard (ISO, 2004) advises to study every situation independently from the other (Lafanechere, 2009).However, some aeraulic environments induced by activities inside the enclosure (such as countercurrent blowing works facing the opening) could generate local and temporary flow reversals near the opening which would then lead to leaks of pollutants outward the enclosure. Preliminary laser visualizations (Figure 1) have shown that the presence of an additional countercurrent turbulent jet flow in competition with the mean inward flow is among the main causes of backflow through the opening. The main purposes of this work are to detect and measure experimentally the backflow of gaseous and particulate pollutants to outside the depressurized enclosure induced by a countercurrent jet. It aims also at verifying the ability of CFD (Computational Fluids Dynamics) simulations to predict these flow inversions near the opening by using SST-DES turbulent model. Experiments were performed on an enclosure with small opening (0.03 × 0.1) 〖cm〗^2 on its frontal wall. Gas and particles tracing techniques highlight and contribute to quantify the backflow phenomenon near the opening. Experimental results show that the behavior of the gaseous and particulate pollutants (aerosol of aerodynamic diameter 5 microns) is similar in terms of backflow magnitude near the opening. CFD results (Figure 2) confirmed the reliability of the hybrid model SST-DES to quantify the backflow phenomenon near the opening. This model has been successfully compared to experimental results on gaseous and particulate leakage.We found that in addition to the ISO recommendations on the inflow velocity at the opening, a new aeraulic criterion based on the disturbance and the inflow velocities ratio at the opening should be taken into account to ensure an efficient confinement. The perspectives of this work are to perform experiments on a real depressurized airlock, while varying the disturbance type (countercurrent or parietal jet, inside or outside the enclosure, disturbance related to the motion of an external obstacle..). Simultaneously, we aim to test the ability of numerical simulations to predict the backflow phenomenon at this scale

Experimental and numerical studies of gaseous and particulate pollutants backflow through an opening from a depressurized enclosure: application to nuclear dismantling operations

International audienceDuring nuclear maintenance and dismantling operations, a dynamic confinement due to inflow passing through openings is applied in order to prevent the leakage of pollutants from depressurized enclosures. This ensures better safety for operating people. To guaranty an efficient dynamic confinement, three values for the velocity of the inward flow near the opening were required in the past, depending on the dangerousness of the radioactive pollutants. Nowadays the ISO standard (ISO, 2004) advises to study every situation independently from the other (Lafanechere, 2009).However, some aeraulic environments induced by activities inside the enclosure (such as countercurrent blowing works facing the opening) could generate local and temporary flow reversals near the opening which would then lead to leaks of pollutants outward the enclosure. Preliminary laser visualizations (Figure 1) have shown that the presence of an additional countercurrent turbulent jet flow in competition with the mean inward flow is among the main causes of backflow through the opening. The main purposes of this work are to detect and measure experimentally the backflow of gaseous and particulate pollutants to outside the depressurized enclosure induced by a countercurrent jet. It aims also at verifying the ability of CFD (Computational Fluids Dynamics) simulations to predict these flow inversions near the opening by using SST-DES turbulent model. Experiments were performed on an enclosure with small opening (0.03 × 0.1) 〖cm〗^2 on its frontal wall. Gas and particles tracing techniques highlight and contribute to quantify the backflow phenomenon near the opening. Experimental results show that the behavior of the gaseous and particulate pollutants (aerosol of aerodynamic diameter 5 microns) is similar in terms of backflow magnitude near the opening. CFD results (Figure 2) confirmed the reliability of the hybrid model SST-DES to quantify the backflow phenomenon near the opening. This model has been successfully compared to experimental results on gaseous and particulate leakage.We found that in addition to the ISO recommendations on the inflow velocity at the opening, a new aeraulic criterion based on the disturbance and the inflow velocities ratio at the opening should be taken into account to ensure an efficient confinement. The perspectives of this work are to perform experiments on a real depressurized airlock, while varying the disturbance type (countercurrent or parietal jet, inside or outside the enclosure, disturbance related to the motion of an external obstacle..). Simultaneously, we aim to test the ability of numerical simulations to predict the backflow phenomenon at this scale

Experimental study of backflow air leakage through an opening from a depressurized enclosure

International audienceIn the nuclear decommissioning and dismantling operations, a dynamic confinement is applied to all openings in order to prevent the transfer of pollutants outside depressurized enclosures and to insure the safety of workers. To guaranty an efficient dynamic confinement, ISO 16647 and ISO 17873 standards recommend to maintain a constant value for the inward flow velocity near the opening depending on the level of radioactive pollution hazard. The main purpose of this work is to identify the possible conditions under which flow inversions near the opening may lead to gaseous pollutant leakage and then failure of the dynamic confinement. We aim at quantifying the amount of this pollutant backflow. The leakage from an experimental ventilated enclosure with a small opening on its frontal wall has been investigated. Laser flow visualizations and Particle Image Velocimetry (PIV) measurements showed that the presence of an additional turbulent jet flow in competition with the inward confinement flow is among the main causes leading to the leakage through the opening. The gas tracing technique has provided experimental data to quantify the pollutant backflow and allowed to compare the different scenarios. We conclude that a new criterion based on local aeraulic conditions near the opening is relevant to guaranty an efficient confinement