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Assessment of coupled lagrangian Eulerian finite element simulations to model suction forces during hydrodynamic impacts
No full textInternational audienceDuring the emergency landing of an aircraft on water, the structure may experience critical forces and could eventually fails. The appropriate design of the structure should minimize the risk of occupant injuries. The recent progress in computation capabilities led to the increased use of numerical simulations in the certification process of aircraft. A specific challenge concerns the modelisation of suction forces that develop near the aircraft tail, where the first contact with water occurs. This phenomenon is due to the high horizontal velocity of the structure at impact and the longitudinal curvature of the fuselage. It can affect the overall aircraft kinematics during ditching. In this work, as an effort to improve aircraft ditching simulations and to assess the capabilities of numerical models to describe suction forces, the simple test case of the wedge water entry and subsequent exit is considered. Numerical simulations with the Eulerian formulation for the fluid and the Lagrangian formulation for the structure are used. The method used for the fluid-structure interaction is based on an immersed contact interface with penalty forces. The present work focuses on impact and suction forces modelling. Results show a satisfying capacity of the numerical approach to model negative hydrodynamic force (suction)
Actions of the hyperoctahedral group to compute minimal contractors
No full textInternational audienceThe hyperoctahedral group Bn is the group of symmetries of the hypercube [−1,1]n of Rn. For instance permutations, or symmetries along each of the n canonical planes of Rn all belong to Bn. Now, many sets of equations contain symmetries in Bn. This is the case of the addition constraint: x1+x2=x3 or the multiplication x1⋅x2=x3. In robotics, many specific geometrical constraints such as for instance constraints involving distances or angles used for localization also have these symmetries. This paper shows the fundamental role of the hyperoctahedral group for interval-based methods. These methods use operators, called contractors, which contract axis-aligned boxes, without removing any point of the solution set defined by a conjunction of constraints (typically equations, or inequalities). More precisely, the paper presents an algorithm which allows us to build minimal contractors associated to constraints with symmetries in Bn. As an application, we will consider the geometrical constraint associated to the angle between vectors. The corresponding contractor will then be used in a constraint propagation framework in order to localize a robot using several radars
Sequential sensor selection for the localization of acoustic sources by sparse Bayesian learning
No full textInternational audienceThis paper deals with the design of sensor arrays in the context involving the localization of a few acoustic sources. Sparse approximation is known to be effective to find the source locations, but it depends on different array characteristics, such as the number of sensors and the array geometry. The present paper tackles this array design problem under the form of a sequential sensor selection procedure. The proposed method alternates between two steps. One step involves a source localization estimator, given a current set of measurement points, to obtain the estimation variance. Then, the other step selects the new point where a future measurement will maximally decrease the variance from the previous step. As such, the procedure can be applied online. Both numerical and experimental studies are conducted in an indoor nearfield configuration. Results show that the proposed approach performs better than offline state-of-the-art methods, and the presented empirical study reveals a better robustness to the model mismatches originating from the room reflections
Femtosecond soft x-ray lasing in dense collisionaly-pumped plasma
No full textInternational audienceWe report the first experimental measurement of a strong pulse shortening of a seeded plasma-based soft x-ray laser at high electron densities. As a consequence of the gain gating caused by subsequent collisional ionization, the duration drops from 1.4 ps to an unprecedented value of 520 fs RMS when the electron density increases from 4 to 7.6×1019cm-3. These measurements, performed with an original single-shot method, are in good agreement with results from 3D Maxwell-Bloch simulations. © 2022 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DO
On the dynamics of a collapsing bubble in contact with a rigid wall
No full textInternational audienceThis work reveals that the dynamic response of a spherical cap bubble in contact with a rigid wall depends on the effective contact angle at the instant prior to collapse. This parameter allows us to discriminate between two regimes in which the mechanisms of interaction between the collapsing bubble and its surrounding medium differ significantly: when the contact angle is smaller than 90∘ , a classical jet directed towards the wall is observed, whereas if the initial contact angle is larger than 90∘ , an annular re-entrant jet parallel to the wall appears. We show that this change of behaviour can be explained using the impulse potential flow theory for small times, which shows the presence of a singularity on the initial acceleration of the contact line when the contact angle is larger than 90∘ . Direct numerical simulations show that although viscosity regularises the solution at t>0 , the solution remains singular at t=0 . In these circumstances, numerical and experimental results show that the collapse of flat bubbles can eventually lead to the formation of a vortex ring that unexpectedly induces long-range effects. The role of the bubble geometry at the instant of maximum expansion on the overall collapse process is shown to be well captured by the impulse potential flow theory, which can be generalised easily to other bubble shapes. These results may find direct application in the interpretation of geophysical flows as well as the control and design of biomedical, naval, manufacturing and sonochemistry applications
High-performance language virtual machines: an analysis and challenges
No full textA presentation presenting scientific challenges for modern high-performance language virtual machine
Analyse des effets de taille : Comparaison entre le modèle TLS (Thick Level Set) et le Critère Couplé
No full textInternational audienceThe prediction of the load of a structure with a defect remains to this day a great challenge. In this study, we propose to highlight the harmfulness of a sharp V-notch through a comparative study of the stress to failure obtained by the TLS model and by the coupled criterion. Indeed, the harmfulness of the notch is more important when its size is higher than the characteristic length of the material. Moreover,we showed that the prediction of the stress to failure by the coupled criterion is overestimated when the size of the notch becomes smaller than the characteristic length of the material.La prédiction de la charge critique d’une structure présentant un défaut reste à ce jour un grand challenge. Dans cette étude, nous proposons de mettre en évidence la nocivité d’une entaille aigüe en V à travers une étude comparative de la contrainte à la rupture obtenue par le modèle TLS et par le critère couplé. En effet, la nocivité de l’entaille est plus importante quand sa taille est supérieure à la longueur caractéristique du matériau. Nous avons montré que les prédictions de la contrainte à la rupture par le critère couplé sont surestimées quand la taille de l’entaille devient plus petite par rapport à la longueur caractéristique du matériau
Determination of the ion collection efficiency of the Razor Nano Chamber for ultra‐high dose‐rate electron beams
No full textInternational audienceBackground: Ultra-high dose-rate (UHDR) irradiations (>40 Gy/s) have recently garnered interest in radiotherapy (RT) as they can trigger the so-called “FLASH” effect, namely a higher tolerance of normal tissues in comparison with conventional dose rates when a sufficiently high dose is delivered to the tissue. To transfer this to clinical RT treatments, adapted methods and practical tools for online dosimetry need to be developed. Ionization chambers remain the gold standards in RT but the charge recombination effects may be very significant at such high dose rates, limiting the use of some of these dosimeters. The reduction of the sensitive volume size can be an interesting characteristic to reduce such effects.Purpose: In that context, we have investigated the charge collection behavior of the recent IBA Razor™ Nano Chamber (RNC) in UHDR pulses to evaluate its potential interest for FLASH RT.Methods: In order to quantify the RNC ion collection efficiency (ICE), simultaneous dose measurements were performed under UHDR electron beams with dose-rate-independent Gafchromic™ EBT3 films that were used as the dose reference. A dose-per-pulse range from 0.01 to 30 Gy was investigated, varying the source-to-surface distance, the pulse duration (1 and 3 μs investigated) and the LINAC gun grid tension as irradiation parameters. In addition, the RNC measurements were corrected from the inherent beam shot-to-shot variations using an independent current transformer. An empirical logistic model was used to fit the RNC collection efficiency measurements and the results were compared with the Advanced Markus plane parallel ion chamber.Results: The RNC ICE was found to decrease as the dose-per-pulse increases, starting from doses above 0.2 Gy/pulse and down to 40% of efficiency at 30 Gy/pulse. The RNC resulted in a higher ICE for a given dose-per-pulse in comparison with the Markus chamber, with a measured efficiency found higher than 85 and 55% for 1 and 10 Gy/pulse, respectively, whereas the Markus ICE was of 60 and 25% for the same doses. However, the RNC shows a higher sensitivity to the pulse duration than the Advanced Markus chamber, with a lower efficiency found at 1 μs than at 3 μs, suggesting that this chamber could be more sensitive to the dose rate within the pulse.Conclusions: The results confirmed that the small sensitive volume of the RNC ensures higher ICE compared with larger chambers. The RNC was thus found to be a promising online dosimetry tool for FLASH RT and we proposed an ion recombination model to correct its response up to extreme dose-per-pulses of 30 Gy
Circularly Polarized In-Band Full-Duplex Antenna Array for Ka-Band Inter-CubeSat Links
No full textInternational audienceThis paper presents an 8 x 8 patch array with in-band full-duplex capabilities for Ka-band inter-CubeSat links that can be fitted on the surface of a 1U CubeSat. In simulation, the array can achieve 50 dB of self-interference cancellation and a very low axial ratio (< 1 dB) from 25 to 27 GHz by exploiting an approach of multilevel sequential rotation. The array also exhibits a wide matching bandwidth from 24.5 to 27.5 GHz and can achieve a gain higher than 20 dBi
Gouvernance et autonomie stratégique par l'innovation : Une étude exploratoire
No full textInternational audienceThis paper explores the association between governance and the innovation-based strategic autonomy using unbalanced panel data of 33 countries observed over the period 2000-2015. The econometric results show that the “voice and accountability” dimension of governance is correlated positively with the innovation-based strategic autonomy. A negative correlation is highlighted for the “political stability and absence of violence/terrorism” dimension. The implications, limitations of the research, and avenues for future research are discussed.Cet article explore l'association entre la gouvernance et l'autonomie stratégique par l'innovation en utilisant un panel non cylindré de 33 pays observés sur la période 2000-2015. Les résultats économétriques montrent que la dimension « voix et responsabilité » de la gouvernance est corrélée positivement avec l'autonomie stratégique par l'innovation. Une corrélation négative est obtenue pour la dimension « stabilité politique et absence de violence/terrorisme ». Les implications des résultats obtenus, les limites de la recherche menée, et les perspectives sont évoquées