A year of genomic surveillance reveals how the SARS-CoV-2 pandemic unfolded in Africa.

The progression of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic in Africa has so far been heterogeneous, and the full impact is not yet well understood. In this study, we describe the genomic epidemiology using a dataset of 8746 genomes from 33 African countries and two overseas territories. We show that the epidemics in most countries were initiated by importations predominantly from Europe, which diminished after the early introduction of international travel restrictions. As the pandemic progressed, ongoing transmission in many countries and increasing mobility led to the emergence and spread within the continent of many variants of concern and interest, such as B.1.351, B.1.525, A.23.1, and C.1.1. Although distorted by low sampling numbers and blind spots, the findings highlight that Africa must not be left behind in the global pandemic response, otherwise it could become a source for new variants

The near field in the mixing of a three-dimensional inclined pollutant jet with a crossflow

International audienceAn experimental investigation was carried out to study the structure of the flow field resulting from the interaction of an inclined pollutant jet with a crossflow. The study of the flow field was conducted in a wind tunnel test by means of particle image velocimetry (PIV) system. As the jet was discharged with a variable angle, the resulting flow as been found to be quite complex owing to its three-dimensional nature and the interaction between several flow regions. Results showed the dependence of the emerging jet flow structure on its ratio velocity and the Reynolds number. Extensive wind tunnel experimental results are presented; they concern the Kelvin-Helmholtz vortical structures and the effect of the velocity ratio $v_0/u_\infty$. on the interacting zone. A three-dimensional numerical model with a second-order turbulent model (RSM) and a nonuniform grid system is used to examine the behavior of the emerging jet. The comparison of the numerical and experimental results gives satisfactory agreement

Twin inclined jets in crossflow: experimental investigation of different flow regimes and jet elevations

International audienceThe present experimental study is dedicated to unfolding the mixing process generated by double-inclined, variably elevated jets in crossflow. Twin tandem jets in crossflow are very common in the industry and are closely dependent on several parameters. Detailing the induced interactions in such a model would ultimately enhance our understanding and help optimize related applications. The jets handled in the present work are placed three diameters apart, arranged in line with the oncoming crossflow and sent at variable levels (h = 0, 1, 2, 5 cm) from the ground of a wind tunnel and are discharged from elliptic discharging cross sections. Elliptic jets are of particular interest in applications like industrial and boat chimneys, and are practically easily obtained by razing circular cylinders at the desired inclination and height. Particle image velocimetry and coupled charge device CCD camera were used. The dynamic and turbulent behavior of the resulting flowfield was characterized in terms of streamlines, velocity components and vortical structures. The obtained data helped highlight the impact of jets' elevation over their mixing mechanism among the surrounding free stream: establishment of the resulting flowfield dynamics and settlement of the induced vortical structures. The mixing process evolution was also considered under different flow regimes. For the matter an injection ratio (defined as jets to the mainstream's velocity ratio) raging between 0.67, 1 and 1.29 was considered. These cases correspond to jets dynamically dominated by, equivalent to or more dominant than the mainstream. The impact of jets' elevation combined to that of the injection ratio affects both the developed vortical structures and the established dynamic fields, which in turn highly affects the induced mixing process

Dispersion of a bent chimney fume around a variably oriented building

International audienceIn addition to experimental investigations, a computational study was conducted in order to analyse the structure of the flow issued from a bent chimney around a parallelepiped obstacle. The particle image velocimetry (PIV) technique was applied to generate an experimental database suitable for validation of the authors' numerical simulations. The numerical model is based on the resolution of the Navier-Stokes governing equations by means of the finite volume method associated with the second-order model. This gives satisfying results in the exit region and in the trailing zone of the jet. Comparing the obtained numerical results with the experimentally tracked data confirmed the authors' opinion. The comparison concerned both the mean and the fluctuating dynamic flow features. Once validated, their model allowed the evaluation of the effect of the obstacle attack angle (alpha = 0 degrees, alpha = 15 degrees, alpha = 30 degrees, alpha = 45 degrees, and alpha = 75 degrees) on the dynamic and mass flow features

Dispersion of a bent chimney fume around a variably oriented building

International audienceIn addition to experimental investigations, a computational study was conducted in order to analyse the structure of the flow issued from a bent chimney around a parallelepiped obstacle. The particle image velocimetry (PIV) technique was applied to generate an experimental database suitable for validation of the authors' numerical simulations. The numerical model is based on the resolution of the Navier-Stokes governing equations by means of the finite volume method associated with the second-order model. This gives satisfying results in the exit region and in the trailing zone of the jet. Comparing the obtained numerical results with the experimentally tracked data confirmed the authors' opinion. The comparison concerned both the mean and the fluctuating dynamic flow features. Once validated, their model allowed the evaluation of the effect of the obstacle attack angle (alpha = 0 degrees, alpha = 15 degrees, alpha = 30 degrees, alpha = 45 degrees, and alpha = 75 degrees) on the dynamic and mass flow features

Experimental and numerical analysis of the jet dispersion from a bent chimney around an obstacle

International audienceAn experimental study and a numerical modelling analysis were carried out simultaneously to study the flow field structure issuing from a chimney around an obstacle. The main purpose of this study is to evaluate the impact of the jet emitted from a chimney (bent or straight) on the dynamics and the turbulent features of the surrounding flow. The consideration of these features is particularly pertinent to the understanding of mixing between the interacting flows which may be very important in controlling pollutant dispersion in the atmosphere. The experimental data are depicted by means of a PIV technique; whereas the numerical three-dimensional model is simulated through the resolution of the different governing Navier-Stokes equations. The volume finite method, together with the second order turbulent closure model (RSM), was adopted. Variations in obstacle form (cylindrical or parallelepiped) and chimney configuration (bent or straight) were tested and features studied were: the global jet plume, the windward and leeward jet spread; the size, location and magnitude of the reverse flow region; the penetration and the deflection of the jet trajectory around the obstacle. All these considerations allowed us to characterize well the impact of the injection of the jet emitted from the chimney within the crossflow, and its spreading around the obstacle and within the whole domain. Such characterization is very important with regard to pollutant dispersion and consequently to the environmental impact. Indeed, the different species contained within the emitted fumes are mainly directed by the velocity components and their mixing and progression within the domain and around the obstacle are closely related

Dynamic and mass transfer characteristics of the flow issued from a bent chimney around buildings

International audienceThe present work consists in an experimental investigation of the flow issuing from a bent chimney over a downstream obstacle. Our purpose is to explore the resulting flow field and its different characterizing features. These features were captured by means of the Particle Image Velocimetry technique. A numerical simulation of the problem has also been carried out and validated after comparison of the corresponding results to the experimental data. A good level of agreement was achieved between the experiments and the calculations. Then, we tried to upgrade our model by adopting large (real) scale dimensions. Our purpose consisted mainly in the observation and evaluation of the behavior of the incoming flow in presence of a double tandem obstacle. In a second step, we proposed to increase the number of the placed obstacles to four. The results given by the three-dimensional model are likely to highlight the dynamic features of the established field as well as the resulting mass transfer. Finally, we tried to evaluate the effect of further parameters on the characterizing features of the resulting flow filed such as the velocity ratio, the obstacles' gap, the arrangement of the obstacles and the obstacles' geometry

Temperature impact on the turbulence generated by the interaction of twin inline inclined jets in crossflow

International audienceConsideration is given to the interaction of twin tandem jets with an oncoming uniform crossflow. A variable temperature is assumed for the emitted jets while the crossflow is maintained constant, equivalent to the ambient temperature. Both jet nozzles are elliptic, as initially inclined with an angle of 60A degrees, placed three diameters apart in line with the crossflow and discharge a nonreactive fume. The handled configuration is numerically simulated in the present work, by means of the finite volume method together with a non uniform grid system. The model is first validated with reference to available experimental data, in the simple isothermal case of air jets in air crossflow. It is then upgraded by considering a nonreactive fume discharged at a variable temperature. The upgraded model turbulence is described by means of the Reynolds Stress Model second order turbulent closure model. The present work is to our knowledge pioneering in the introduction of this particular model is such a configuration and its introduction proved to be highly valuable since is described satisfyingly the turbulent behavior of the resulting flowfield. This behavior is, precisely, specified in terms of shear stress components whose evolutions, explored along the different directions of the domain, showed a more pronounced vertical mixing, and gave rise to more significant vortices in most characterizing zones: near the injection plane as well as within the discharging nozzles