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Mechanical properties of graphene nanoplatelets reinforced epikote 828 under dynamic compression
No full textInternational audienceIn this experimental investigation, the influence of graphene nanoplatelets (GNPs) on the dynamic behavior of polymeric material such as diglycidyl ether of bisphenol A (DGEBA) epoxy is investigated using the Split Hopkinson Pressure Bar (SHPB). Nanocomposite samples with a different weight percentage of GNPs i.e. 0, 1, 2, 5 wt% were fabricated and tested under dynamic compression to understand the influence of nanofillers on the mechanical performance of the epoxy. The results established that changing the mass fraction of GNPs greatly influences the mechanical behavior of the epoxy and confirmed that the optimal mass fraction of GNPs was 1 wt% because of the good dispersion and viscosity. While, a further increase in the percentage of GNPs resulted in degradation of the mechanical strength of the material because of the agglomeration of graphene sheets, porosity, and their poor interfacial bonding. Moreover, each mass fraction of nanocomposites was tested at three different impact pressures i.e. 1.5, 2, and 4 bar. The main objective is to quantify the effect of the strain rate on the mechanical behavior and on the resulting damage modes. This study further confirmed that the high percentage of GNPs increases the viscosity of the epoxy resulting in porosity and void in the structure which generates high-localised stresses into the matrix causing premature failure under dynamic loading
Backbone curves, Neimark-Sacker boundaries and appearance of quasi-periodicity in nonlinear oscillators: application to 1:2 internal resonance and frequency combs in MEMS
No full textInternational audienceQuasi-periodic solutions can arise in assemblies of nonlinear oscillators as a consequence of Neimark-Sacker bifurcations. In this work, the appearance of Neimark-Sacker bifurcations is investigated analytically and numerically in the specific case of a system of two coupled oscillators featuring a 1:2 internal resonance. More specifically, the locus of Neimark-Sacker points is analytically derived and its evolution with respect to the system parameters is highlighted. The backbone curves, solution of the conservative system, are first investigated, showing in particular the existence of two families of periodic orbits, denoted as parabolic modes. The behaviour of these modes, when the detuning between the eigenfrequencies of the system is varied, is underlined. The non-vanishing limit value, at the origin of one solution family, allows explaining the appearance of isolated solutions for the damped-forced system. The results are then applied to a Micro-Electro-Mechanical System-like shallow arch structure, to show how the analytical expression of the Neimark-Sacker boundary curve can be used for rapid prediction of the appearance of quasiperiodic regime, and thus frequency combs, in Micro-Electro-Mechanical System dynamics
IWV observations from a network of ground-based GNSS receivers during EUREC4A
No full textInternational audienceIWV data were retrieved from a network of nearly fifty Global Navigation Satellite System (GNSS) stations distributed over the Caribbean arc for the period 1 January-29 February 2020 encompassing the EUREC4A field campaign. Two of the stations had been installed at the Barbados Cloud Observatory (BCO) during fall 2019 in the framework of the project and are still running. All other stations are permanent stations operated routinely from various geodetic and geophysical organisations in the region. High spatial and temporal Integrated Water Vapour (IWV) observations will be used to investigate the atmospheric environment during the life cycle of convection and its feedback on the large-scale circulation and energy budget.</p><p>This paper describes the ground-based GNSS data processing details and assesses the quality of the GNSS IWV retrievals as well as the IWV estimates from radiosoundings, microwave radiometer measurements and ERA5 reanalysis.</p><p>The GNSS results from five different processing streams run by IGN and ENSTA-B/IPGP are first intercompared. Four of the streams were run operationally, among one was in near-real time, and one was run after the campaign in a reprocessing mode. The uncertainties associated with each of the data sets, including the zenith tropospheric delay to IWV conversion methods and auxiliary data, are quantified and discussed. The IWV estimates from the reprocessed data set are compared to the Vaisala RS41 radiosonde measurements operated from the BCO and to the measurements from the operational radiosonde station at Grantley Adams international airport (GAIA). A significant dry bias is found in the GAIA humidity observations with respect to the BCO sondes (-2.9 kg/m2) and the GNSS results (-1.2 kg/m2). A systematic bias between the BCO sondes and GNSS is also observed (1.7 kg/m2) where the Vaisala RS41 measurements are moister than the GNSS retrievals. The HATPRO IWV estimates agree with the BCO soundings after an instrumental update on 27 January, while they exhibit a dry bias compared to GNSS and BCO sondes before that date. ERA5 IWV estimates are overall close to the GAIA observations, probably due to the assimilation of these observations in the reanalysis. However, during several events where strong peaks in IWV occurred, ERA5 is shown to significantly underestimate the IWV peaks. Two successive peaks are observed on 22 January and 23/24 January which were associated with heavy rain and deep moist layers extending from the surface up to altitudes of 3.5 and 5 km, respectively. ERA5 significantly underestimates the moisture content in the upper part of these layers. The origins of the various moisture biases are currently being investigated.</p>
Opportunistic IP Birthmarking using Side Effects of Code Transformations on High-Level Synthesis
No full textInternational audienceThe increasing design and manufacturing costs are leading to globalize the semiconductor supply chain. However, a malicious attacker can resell a stolen Intellectual Property (IP) core, demanding methods to identify a relationship between a given IP and a potentially fraudulent copy. We propose a method to protect IP cores created with highlevel synthesis (HLS): our method inserts a discrete birthmark in the HLS-generated designs that uses only intrinsic characteristics of the final RTL. The core of our process leverages the side effects of HLS due to specific source-code manipulations, although the method is HLS-tool agnostic. We propose two independent validation metrics, showing that our solution introduces minimal resource and delay overheads (< 6% and < 2%, respectively) and the accuracy in detecting illegal copies is above 96%
Non overlapping Domain Decomposition Methods for Time Harmonic Wave Problems
No full textThe domain decomposition method (DDM) initially designed, with the celebrated paper of Schwarz in 1870 as a theoretical tool for partial differential equations (PDEs) has become, since the advent of the computer and parallel computing techniques, a major tool for the numerical solution of such PDEs, especially for large scale problems. Time harmonic wave problems offer a large spectrum of applications in various domains (acoustics, electromagnetics, geophysics, ...) and occupy a place of their own, that shines for instance through the existence of a natural (possibly small) length scale for the solutions: the wavelength. Numerical DDMs were first invented for elliptic type equations (e.g. the Laplace equation), and even though the governing equations of wave problems (e.g. the Helmholtz equation) look similar, standard approaches do not work in general
Model-Based Adaptive Filtering of Dielectric Elastomer Loudspeakers
No full textDielectric elastomers are soft actuators that can reach deformations by more than 500% when a high voltage is applied. They have been considered for use as loudspeakers because of their quick response. One of their limitations is an inhomogeneous frequency response, due to the modal behavior of the membrane. In this study, we set up a sensor-free adaptive filtering strategy that relies on a finite element model of the loudspeaker, to improve the frequency response
Optimal Ciliary Locomotion of Axisymmetric Microswimmers
No full text17 pages, 6 figuresInternational audienceMany biological microswimmers locomote by periodically beating the densely-packed cilia on their cell surface in a wave-like fashion. While the swimming mechanisms of ciliated microswimmers have been extensively studied both from the analytical and the numerical point of view, the optimization of the ciliary motion of microswimmers has received limited attention, especially for non-spherical shapes. In this paper, using an envelope model for the microswimmer, we numerically optimize the ciliary motion of a ciliate with an arbitrary axisymmetric shape. The forward solutions are found using a fast boundary integral method, and the efficiency sensitivities are derived using an adjoint-based method. Our results show that a prolate microswimmer with a 2:1 aspect ratio shares similar optimal ciliary motion as the spherical microswimmer, yet the swimming efficiency can increase two-fold. More interestingly, the optimal ciliary motion of a concave microswimmer can be qualitatively different from that of the spherical microswimmer, and adding a constraint to the ciliary length is found to improve, on average, the efficiency for such swimmers
Does it work outside this benchmark? Introducing the Rigid Depth Constructor tool, depth validation dataset construction in rigid scenes for the masses.
No full textWe present a protocol to construct your own depth validation dataset for navigation. This protocol, called RDC for Rigid Depth Constructor, aims at being more accessible and cheaper than already existing techniques, requiring only a camera and a Lidar sensor to get started. We also develop a test suite to get insightful information from the evaluated algorithm. Finally, we take the example of UAV videos, on which we test two depth algorithms that were initially tested on KITTI and show that the drone context is dramatically different from in-car videos. This shows that a single context benchmark should not be considered reliable, and when developing a depth estimation algorithm, one should benchmark it on a dataset that best fits one's particular needs, which often means creating a brand new one. Along with this paper we provide the tool with an open source implementation and plan to make it as user-friendly as possible, to make depth dataset creation possible even for small teams. Our key contributions are the following: We propose a complete, open-source and almost fully automatic software application for creating validation datasets with densely annotated depth, adaptable to a wide variety of image, video and range data. It includes selection tools to adapt the dataset to specific validation needs, and conversion tools to other dataset formats. Using this application, we propose two new real datasets, outdoor and indoor, readily usable in UAV navigation context. Finally as examples, we show an evaluation of two depth prediction algorithms, using a collection of comprehensive (e.g. distribution based) metrics