Fluids-membrane interaction with a full Eulerian approach based on the level set method

A fully Eulerian approach to predict fluids-membrane behaviours is presented in this paper. Based on the numerical model proposed by Ii et al. (2012), we present a sharp methodology to account for the jump conditions due to hyperelastic membranes. The membrane is considered infinitely thin and is represented by the level set method. Its deformations are obtained from the transport of the components of the left Cauchy-Green tensor throughout time. Considering the linear or a hyperelastic material law, the surface stress tensor is computed and gives the force exerted by the membrane on the surrounding fluids. The membrane force is taken into account in the Navier-Stokes equations as jump conditions on the pressure and on the velocity derivatives by imposing suitable singular source terms in cells crossed by the interface. To prevent stability issues, an extension algorithm has been developed to remove the normal derivatives of the scalar fields specific to the membrane. In particular, a subcell resolution at the interface of the extrapolated variable is proposed for increasing the accuracy of the extension algorithm. These improvements are validated by comparing our numerical results with benchmarks from the literature. Moreover, a new benchmark is proposed for fluids with both different viscosities and different densities to target applications where a gas and a liquid phase are separated by a membrane

Genetic diversification of an invasive honey bee ectoparasite across sympatric and allopatric host populations

Invasive parasites are major threats to biodiversity. The honey bee ectoparasite, Varroa destructor, has shifted host and spread almost globally several decades ago. This pest is generally considered to be the main global threat to Western honey bees, Apis mellifera, although the damages it causes are not equivalent in all its new host’s populations. Due to the high virulence of this parasite and the viruses it vectors, beekeepers generally rely on acaricide treatments to keep their colonies alive. However, some populations of A. mellifera can survive without anthropogenic mite control, through the expression of diverse resistance and tolerance traits. Such surviving colonies are currently found throughout the globe, with the biggest populations being found in Sub- Saharan Africa and Latin America. Recently, genetic differences between mite populations infesting surviving and treated A. mellifera colonies in Europe were found, suggesting that adaptations of honey bees drive mite evolution. Yet, the prevalence of such co-evolutionary adaptations in other invasive populations of V. destructor remain unknown. Using the previous data from Europe and novel genetic data from V. destructor populations in South America and Africa, we here investigated whether mites display signs of adaptations to different host populations of diverse origins and undergoing differing management. Our results show that, contrary to the differences previously documented in Europe, mites infesting treated and untreated honey bee populations in Africa and South America are genetically similar. However, strong levels of genetic differentiation were found when comparing mites across continents, suggesting ongoing allopatric speciation despite a recent spread from genetically homogenous lineages. This study provides novel insights into the co-evolution of V. destructor and A. mellifera, and confirms that these species are ideal to investigate coevolution in newly established host- parasite systems.Agencia Nacional de Investigación e InnovaciónPrograma ECOS-Su

Direct numerical simulation of a bubble motion in a spherical tank under external forces and microgravity conditions

We present, in this paper, numerical simulations of bubble sloshing in a spherical tank, resulting from a tank rotation around a fixed axis in microgravity conditions. This configuration is of great interest in space applications where sloshing can have harmful effects on the stability of satellites. Depending on the dimensionless numbers characterising this phenomenon, our study is focused on the motion and the deformation of a bubble, initially at rest, which is set in motion when the manoeuvre is starting until it reaches a constant rotation speed around the axis. It is shown in this article that, during the first stage of the manoeuvre, the motion of the bubble is essentially driven by the inertial force that depends on the angular acceleration. Next, when the angular velocity is increasing, the centrifugal force being dominant, the trajectory of the bubble is pushed towards the direction between the centre of the tank and the axis of rotation. Finally, when the angular velocity becomes constant, the bubble, reaching a quasi-steady position, is deformed and pressed against the solid boundary of the tank. A quantified description of these phenomena is proposed through a parametric study varying the essential dimensionless numbers, i.e. the Bond number based on the angular velocity, and another Bond number based on the angular acceleration. As the temporal evolution of the forces acting on the satellite wall is of utmost importance for designing satellites and manoeuvres, we also present an analysis characterising the latter. We also detail the first comparisons between the numerical simulations and the Fluidics experiment performed in the International Space Station (ISS) in microgravity conditions. Thanks to these comparisons, we can validate the simulations in configurations of interest

Genetic diversification of an invasive honey bee ectoparasite across sympatric and allopatric host populations.

Invasive parasites are major threats to biodiversity. The honey bee ectoparasite, Varroa destructor, has shifted host and spread almost globally several decades ago. This pest is generally considered to be the main global threat to Western honey bees, Apis mellifera, although the damages it causes are not equivalent in all its new host's populations. Due to the high virulence of this parasite and the viruses it vectors, beekeepers generally rely on acaricide treatments to keep their colonies alive. However, some populations of A. mellifera can survive without anthropogenic mite control, through the expression of diverse resistance and tolerance traits. Such surviving colonies are currently found throughout the globe, with the biggest populations being found in Sub-Saharan Africa and Latin America. Recently, genetic differences between mite populations infesting surviving and treated A. mellifera colonies in Europe were found, suggesting that adaptations of honey bees drive mite evolution. Yet, the prevalence of such co-evolutionary adaptations in other invasive populations of V. destructor remain unknown. Using the previous data from Europe and novel genetic data from V. destructor populations in South America and Africa, we here investigated whether mites display signs of adaptations to different host populations of diverse origins and undergoing differing management. Our results show that, contrary to the differences previously documented in Europe, mites infesting treated and untreated honey bee populations in Africa and South America are genetically similar. However, strong levels of genetic differentiation were found when comparing mites across continents, suggesting ongoing allopatric speciation despite a recent spread from genetically homogenous lineages. This study provides novel insights into the co-evolution of V. destructor and A. mellifera, and confirms that these species are ideal to investigate coevolution in newly established host-parasite systems

Mélange à l'aval d'une confluence d'écoulements à surface libre

Les travaux présentés ici traitent de la rencontre de deux écoulements à surface libre au sein d'une confluence en se focalisant sur l'évolution du mélange de ces écoulements au sein de la branche aval, par une approche numérique. Il apparaît qu'en fonction des caractéristiques des deux écoulements et de la géométrie de la confluence, l'efficacité du mélange varie très fortement et que cette variation dépend de la présence ou non d'un écoulement secondaire intense dans la branche aval et notamment de la présence d'une ou plusieurs cellules de recirculation de grande taille. Si ces cellules ont lieu, la mise en contact des deux écoulements a principalement lieu par advection au sein de ces cellules secondaires en forme d'hélices et le mélange est très rapide. Dans le cas contraire, le mélange a lieu par diffusion turbulente à travers l'interface entre les deux écoulements en présence et le mélange est alors beaucoup moins efficace. Nous montrons ici qu'un angle de confluence élevé, qu'un rapport de quantité de mouvement élevé ou qu'un rapport d'aspect de l'écoulement élevé tendent à favoriser l'apparition d'une cellule hélicoïdale et donc à accélérer le mélange dans la branche aval

Diversity and Global Distribution of Viruses of the Western Honey Bee, Apis mellifera

Funding: This research was permitted and funded by the COLOSS network (https://coloss.org/).Peer reviewedPublisher PD

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

Simulation numérique du ballottement d'ergol et modélisation de l'interaction fluides-membrane dans un réservoir de satellite

Propellant sloshing in tanks is one of the most important disturbances of satellite stability in orbit. Considering low-inertial manoeuvres, there is no analytical model and experimental facilities require long time period of microgravity conditions. Thus, this PhD thesis aims to predict this phenomenon by numerical simulations. The study is based on the DIVA code which solves the Navier-Stokes equations for two-phase flows with the level-set method and the Ghost Fluid method. Two tank technologies are studied: simple tanks, which only contain the liquid propellant and the gas maintaining the pressure, and diaphragm tanks, for which a hyperelastic membrane separates both fluids. In the first case, a parametric study on the sloshing effects is done considering rotational manoeuvres and the different behaviours observed are described in relation to the study parameters. Thereafter, the data from the FLUIDICS experiment, sent to the International Space Station, are compared to the numerical results and exhibit good agreement. In the second case, a fluids-membrane interaction model inspired from works on the deformation of biological cells is developed. The membrane strains and stresses are computed in an Eulerian way, from which the force exerted by the membrane on the surrounding fluids is deduced and integrated in the two-phase flows solver. The numerical results are validated by comparison with benchmarks from the literature.Le ballottement dans les réservoirs d'ergols est une des perturbations les plus importantes de la stabilité d'un satellite en orbite. En considérant des manœuvres faiblement inertielles, il n'existe pas de modèle analytique et l'expérimentation nécessite de longues périodes de temps en micro-gravité. Nous nous proposons donc, dans cette thèse, de réaliser des simulations numériques de ce phénomène. L'étude est basée sur le solveur DIVA résolvant les équations de Navier-Stokes diphasiques avec les méthodes level-set et Ghost Fluid. Deux technologies de réservoirs sont à l'étude : les réservoirs classiques, ne contenant que l'ergol liquide et le gaz pressurisant, et les réservoirs à membrane, pour lesquels une membrane hyperélastique sépare les deux fluides. Dans le premier cas, une étude paramétrique complète sur les effets du ballottement lors d'une manœuvre de rotation est menée et les différents régimes d'écoulement obtenus sont décrits par rapport aux paramètres d'études. Par la suite, les données de l'expérience FLUIDICS, envoyée à bord de la Station Spatiale Internationale, sont comparées aux résultats numériques et montrent un très bon accord. Par rapport au second cas, un modèle d'interaction fluides-membrane est développé en s'inspirant de travaux sur la déformation de cellules biologiques. Les déformations et contraintes propres à la membrane sont suivies de façon Eulérienne, les efforts exercés par la membrane sur les fluides environnants en sont déduits et intégrés au solveur diphasique. Les résultats obtenus sont validés par comparaison à des cas-tests de la littérature

Numerical simulation of propellant sloshing and modelling of fluids-membrane interaction in satellite tanks

Le ballottement dans les réservoirs d'ergols est une des perturbations les plus importantes de la stabilité d'un satellite en orbite. En considérant des manœuvres faiblement inertielles, il n'existe pas de modèle analytique et l'expérimentation nécessite de longues périodes de temps en micro-gravité. Nous nous proposons donc, dans cette thèse, de réaliser des simulations numériques de ce phénomène. L'étude est basée sur le solveur DIVA résolvant les équations de Navier-Stokes diphasiques avec les méthodes level-set et Ghost Fluid. Deux technologies de réservoirs sont à l'étude : les réservoirs classiques, ne contenant que l'ergol liquide et le gaz pressurisant, et les réservoirs à membrane, pour lesquels une membrane hyperélastique sépare les deux fluides. Dans le premier cas, une étude paramétrique complète sur les effets du ballottement lors d'une manœuvre de rotation est menée et les différents régimes d'écoulement obtenus sont décrits par rapport aux paramètres d'études. Par la suite, les données de l'expérience FLUIDICS, envoyée à bord de la Station Spatiale Internationale, sont comparées aux résultats numériques et montrent un très bon accord. Par rapport au second cas, un modèle d'interaction fluides-membrane est développé en s'inspirant de travaux sur la déformation de cellules biologiques. Les déformations et contraintes propres à la membrane sont suivies de façon Eulérienne, les efforts exercés par la membrane sur les fluides environnants en sont déduits et intégrés au solveur diphasique. Les résultats obtenus sont validés par comparaison à des cas-tests de la littérature.Propellant sloshing in tanks is one of the most important disturbances of satellite stability in orbit. Considering low-inertial manoeuvres, there is no analytical model and experimental facilities require long time period of microgravity conditions. Thus, this PhD thesis aims to predict this phenomenon by numerical simulations. The study is based on the DIVA code which solves the Navier-Stokes equations for two-phase flows with the level-set method and the Ghost Fluid method. Two tank technologies are studied: simple tanks, which only contain the liquid propellant and the gas maintaining the pressure, and diaphragm tanks, for which a hyperelastic membrane separates both fluids. In the first case, a parametric study on the sloshing effects is done considering rotational manoeuvres and the different behaviours observed are described in relation to the study parameters. Thereafter, the data from the FLUIDICS experiment, sent to the International Space Station, are compared to the numerical results and exhibit good agreement. In the second case, a fluids-membrane interaction model inspired from works on the deformation of biological cells is developed. The membrane strains and stresses are computed in an Eulerian way, from which the force exerted by the membrane on the surrounding fluids is deduced and integrated in the two-phase flows solver. The numerical results are validated by comparison with benchmarks from the literature

Intra-Colonial Viral Infections in Western Honey Bees (Apis Mellifera).

RNA viruses play a significant role in the current high losses of pollinators. Although many studies have focused on the epidemiology of western honey bee (Apis mellifera) viruses at the colony level, the dynamics of virus infection within colonies remains poorly explored. In this study, the two main variants of the ubiquitous honey bee virus DWV as well as three major honey bee viruses (SBV, ABPV and BQCV) were analyzed from Varroa-destructor-parasitized pupae. More precisely, RT-qPCR was used to quantify and compare virus genome copies across honey bee pupae at the individual and subfamily levels (i.e., patrilines, sharing the same mother queen but with different drones as fathers). Additionally, virus genome copies were compared in cells parasitized by reproducing and non-reproducing mite foundresses to assess the role of this vector. Only DWV was detected in the samples, and the two variants of this virus significantly differed when comparing the sampling period, colonies and patrilines. Moreover, DWV-A and DWV-B exhibited different infection patterns, reflecting contrasting dynamics. Altogether, these results provide new insight into honey bee diseases and stress the need for more studies about the mechanisms of intra-colonial disease variation in social insects