Resurrecting Dead-water Phenomenon

We revisit experimental studies performed by Ekman on dead-water using modern techniques in order to present new insights on this peculiar phenomenon. We extend its description to more general situations such as a three-layer fluid or a linearly stratified fluid in presence of a pycnocline, showing the robustness of dead-water phenomenon. We observe large amplitude nonlinear internal waves which are coupled to the boat dynamics, and we emphasize that the modeling of the wave-induced drag requires more analysis, taking into account nonlinear effects

Dead Waters: Large amplitude interfacial waves generated by a boat in a stratified fluid

We present fluid dynamics videos of the motion of a boat on a two-layer or three-layer fluid. Under certain specific conditions, this setup generates large amplitude interfacial waves, while no surface waves are visible. The boat is slowed down leading to a peristaltic effect and sometimes even stopped: this is the so-called dead water phenomenon

Bacterial Cell Morphogenesis Does Not Require a Preexisting Template Structure

SummaryMorphogenesis, the development of shape or form in cells or organisms, is a fundamental but poorly understood process throughout biology. In the bacterial domain, cells have a wide range of characteristic shapes, including rods, cocci, and spirals. The cell wall, composed of a simple meshwork of long glycan strands crosslinked by short peptides (peptidoglycan, PG) and anionic cell wall polymers such as wall teichoic acids (WTAs), is the major determinant of cell shape. It has long been debated whether the formation of new wall material or the transmission of shape from parent to daughter cells requires existing wall material as a template [1–3]. However, rigorous testing of this hypothesis has been problematical because the cell wall is normally an essential structure. L-forms are wall-deficient variants of common bacteria that have been classically identified as antibiotic-resistant variants in association with a wide range of infectious diseases [4–6]. We recently determined the genetic basis for the L-form transition in the rod-shaped bacterium Bacillus subtilis and thus how to generate L-forms reliably and reproducibly [7, 8]. Using the new L-form system, we show here that we can delete essential genes for cell wall synthesis and propagate cells in the long-term absence of a cell wall template molecule. Following genetic restoration of cell wall synthesis, we show that the ability to generate a classical rod-shaped cell is restored, conclusively rejecting template-directed models, at least for the establishment of cell shape in B. subtilis

Observation of the shock wave propagation induced by a high-power laser irradiation into an epoxy material

The propagation of laser-induced shock waves in a transparent epoxy sample is investigated by optical shadowgraphy. The shock waves are generated by a focused laser (3 ns pulse duration—1.2 to 3.4TWcm−2) producing pressure from 44 to 98.9 GPa. It is observed that the shock wave and the release wave created by the shock reverberation at the rear face are both followed by a dark zone in the pictures. This corresponds to the creation of a tensile zone resulting from the crossing on the loading axis of the release waves coming from the edge of the impact area (2D effects). After the laser shock experiment, the residual stresses in the targets are identified and quantified through a photoelasticimetry analysis of the recovered samples. This work results in a new set of original data which can be directly used to validate numerical models implemented to reproduce the behaviour of epoxy under extreme strain rate loading. The residual stresses observed prove that the high-pressure shocks can modify the pure epoxy properties, which could have an influence on the use made of these materials

An unstructured conservative level-set algorithm coupled with dynamic mesh adaptation for the computation of liquid-gas flows

International audienceAccurate and efficient simulations of 3D liquid-gas flows are of first importance in many industrial applications, such as fuel injection in aeronautical combustion chambers. In this context, it is mandatory to handle complex geometries. The use of unstructured grids for two-phase flow modeling fulfills this requirement and paves the way to isotropic adaptive mesh refinement. This work presents a narrow-band conservative level-set algorithm implemented in the YALES2 incompressible flow solver, which is combined to dynamic mesh adaptation. This strategy enables resolving the small physical scales at the liquid-gas interface at a moderate cost. It is applied to predicting the outcome of a droplet collision with reflexive separation. In the accurate conservative level set framework, the interface is represented using a hyperbolic tangent profile, which is advected by the fluid, and then reshaped using a reinitialization equation. The classical signed-distance function is reconstructed at nodes in the narrow band around the interface using a geometric projection/marker method (GPMM), to calculate the smallest distance to the interface. The interface normal and curvature are computed using this signed-distance function. Within a mesh cell, the interface is approximated by a segment (2D) or one or several triangles (3D). The distance at the nodes is simply obtained by projection to the closest surface elements. If a node is connected to n elements containing interface fragments, it has a n-marker list (a marker contains the coordinates of the crossing points and the distance). To speed-up the algorithm, the markers stored at each node are sorted based on their distance. Markers are propagated from one band to another: each node compares its markers to its neighbors' and keeps the closest only. The GPMM approach for the reconstruction of the level-set signed-distance function used in conjunction with the reinitialization of Chiodi et al. (2017) leads to significant improvement in the interface quality and overall accuracy compared to the reinitialization of Desjardins et al. (2008) in the calculations performed on unstructured grids. Since the accuracy of the interface normal and curvature directly depends on the signed-distance function reconstruction, less spurious currents occur on the implicit surface. The improved level-set algorithm leads to accurate predictions of the outcome of a droplet collision with reflexive separation, and is validated against the experimental results of Ashgriz et al. (1990). Introduction Two-phase flows are ubiquitous in nature and in industrial systems. The understanding of the various phenomena occurring in liquid-gas flows is crucial for aeronautical combustors, in which a fuel is injected in liquid form, goes under an atomization process, evaporation, mixing with air and eventually combustion. Understanding the atomization process and the resulting droplet distribution is of first importance for aircraft engine performance and operability. The prediction of the atomization process is complex, due to many non-linear phenomena such as interface break-up, droplet convection, or droplet collision. Atomization also involves a wide range of time and space scales, which leads to important calculation costs. Thus, the use of dynamic mesh adaptation for unstructured meshes is particularly helpful for simulating industrial liquid-gas flow problems, as it allows implicit interface dynamics calculation in complex geometries at a reasonable cost [1]. To capture the interface, the conservative level set method is used, which accurately predicts the interface dynamics while conserving liquid mass [2]. This article presents a method to compute the signed-distance function on unstructured grids, and an implementation of the reinitialization of [3], adapted to unstructured meshes. Classic test cases are run to check the overall accuracy and robustness of the method, and a droplet collision case is simulated to validate the global algorithm with a front merging scenario against the experimental results of [4]

Cell growth of wall-free L-form bacteria is limited by oxidative damage

SummaryThe peptidoglycan (PG) cell wall is a defining feature of the bacterial lineage and an important target for antibiotics, such as β-lactams and glycopeptides. Nevertheless, many bacteria are capable of switching into a cell-wall-deficient state, called the “L-form” [1–3]. These variants have been classically identified as antibiotic-resistant forms in association with a wide range of infectious diseases [4]. L-forms become completely independent of the normally essential FtsZ cell division machinery [3, 5]. Instead, L-form proliferation is driven by a simple biophysical process based on an increased ratio of surface area to cell volume synthesis [6, 7]. We recently showed that only two genetic changes are needed for the L-form transition in Bacillus subtilis [7]. Class 1 mutations work to generate excess membrane synthesis [7]. Until now, the function of the class 2 mutations was unclear. We now show that these mutations work by counteracting an increase in the cellular levels of reactive oxygen species (ROS) originating from the electron transport pathway, which occurs in wall-deficient cells. Consistent with this, addition of a ROS scavenger or anaerobic culture conditions also worked to promote L-form growth without the class 2 mutations in both Gram-positive B. subtilis and Gram-negative Escherichia coli. Our results suggest that physiological compensation for the metabolic imbalance that occurs when cell wall synthesis is blocked is crucial for L-form proliferation in a wide range of bacteria and also provide new insights into the mode of action of antibiotics that target the bacterial cell wall

Estimation of convection speed in underexpanded jets from schlieren pictures

In this paper, the quality of the estimation of the convection velocity in jet shear layers using schlieren pictures is investigated. The aim is to discuss whether the convection velocity is likely to be biased if determined from schlieren images obtained at a high framerate, as in previous experiments using the phase shift method. For this, a numerical procedure is developed in order to generate schlieren-like images on the basis of simulation data, and applied to the results provided by the large-eddy simulation (LES) of an under-expanded round jet at an ideally expanded Mach number of 1.56. The results obtained from the schlieren pictures are compared with those obtained directly from the LES density fields. It is notably found that the location of the maximum of gray level fluctuations in the schlieren pictures corresponds well to that of the maximum of density fluctuations, and that the convection velocity estimated for low frequencies using schlieren pictures is underestimated for small separation distances between the two points used for the phase shift calculation

Atténuation des Défauts dans les Réseaux sur Puce avec une Approche de Brassage de Bits Basée sur des Régions

National audienceThe technological evolution based on the reduction of the transistor size leads to a greater sensitivity leading to faults. Moreover, in a system-on-chip, a faulty interconnect network can largely impact the operation of an application since it carries a large amount of data. Given these two elements, data protection techniques must be developed to mitigate fault impacts on the application. Some works propose to shuffle the bits inside a flit, transferring the fault impacts on the least significant bits. However, these approaches are applied at a fine-grained level, providing effective fault mitigation, but with significant hardware costs. To address this limitation, this work proposes a region-based bit-shuffling technique which sacrifices efficiency in order to reduce hardware costs.L'évolution technologique s'appuyant sur la réduction de la taille des transistors conduit à une plus grande sensibilité pouvant conduire à des fautes. De plus, au sein d'un système sur puce, un réseaux d'interconnexion fautif peut largement impacter le fonctionnement d'une application puisqu'il transporte une grande quantité de données. Compte tenu des ces deux éléments, des techniques de protection des données doivent être développées pour atténuer l'impact des fautes sur l'application. Des travaux proposent de mélanger les bits à l'intérieur d'un flit, transférant l'impact des fautes sur les bits les moins significatifs. Cependant, ces approches sont appliquées à un niveau de grain fin, fournissant une atténuation des défauts efficace, mais avec des coûts matériels importants. Pour remédier à cette limitation, ce travail propose une technique de brassage de bit par région, qui sacrifie l'efficacité afin de réduire les coûts matériels