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Satellite Image Compression and Denoising With Neural Networks
Get PDFEarth observation through satellite images is crucial
to help economic activities as well as to monitor the impact of human activities on ecosystems. Current satellite systems are subjected to strong computational complexity constraints. Thus, image compression is perfomed onboard with specifically tailored algorithms while image denoising is performed on the ground. In this letter, we intend to address satellite image compression and denoising with neural networks. The first proposed approach uses a single neural architecture for joint onboard compression and denoising. The second proposed approach sequentially uses a first neural architecture for onboard compression and a second one for on ground denoising. For both approaches, the onboard architectures are lightened as much as possible, following the procedure proposed in [1]. The two approaches are shown to outperform the current satellite imaging system and their respective pros and cons are discussed
Microkinetic Modeling of Acetylene Hydrogenation Under Periodic Reactor Operation
Get PDFDynamic reactor operations will gain interest with the increase of intermittent electricity from renewable sources. Periodic operation of catalytic reactors can also be used for kinetic studies with better parameter estimation than steady-state operation. This methodology is exemplified for the case of acetylene hydrogenation over Pd/α-Al2O3 using small amplitude flow oscillations. It allows the evaluation of several elementary kinetic parameters from one set of dynamic data for a reaction mechanism based on a single catalytic site.
The optimized transient model is able to reproduce the time-evolution of the reactant and product molar fractions at the outlet of a fixed bed operated with modulations of different amplitudes and frequencies. It further shows that the acetylene conversion is controlled by two steps, the hydrogen adsorption step and the hydrogenation of the surface vinyl intermediate into adsorbed ethylen
Spontaneous spinning of a dichloromethane drop on an aqueous surfactant solution
Get PDFWe report a series of experiments carried out with a dichloromethane drop deposited on the surface of an aqueous solution containing a surfactant, cetyltrimethylammonium bromide. After an induction stage during which the drop stays axisymmetric, oscillations occur along the contact line. These oscillations are succeeded by a spectacular spontaneous spinning of the drop. The latter quickly takes the form of a two-tip ‘rotor’ and the spinning rate stabilizes at a constant value, no longer varying despite the gradual changes of the drop shape and size. The drop eventually disappears due to the continual dissolution and evaporation of dichloromethane. Schlieren visualizations and particle image velocimetry are used to establish a consistent scenario capable of explaining the evolution of the system. The Marangoni effect induced by the dissolution of dichloromethane in the drop vicinity is shown to be responsible for the observed dynamics. Arguments borrowed from dynamical systems theory and from an existing low-order model allow us to explain qualitatively why the system selects the spinning configuration. The geometry of the immersed part of the drop is shown to play a crucial role in this selection process, as well as in the regulation of the spinning rate
An unsupervised approach for health index building and for similarity-based remaining useful life estimation
Get PDFPredictive maintenance techniques attempt to trigger a maintenance intervention at the right moment by estimating the life expectation. Predictive maintenance is increasingly implemented by automated approaches able to perform diagnostics and prognostics. The main part of recent research in these approaches is focused in machine
learning structures whose reasoning is implicit and cannot be easily explained. This poses a problem for their
implementation in highly constrained area such as aeronautics. To overcome this constraint, explicit reasoning
approaches such as the Similarity-Based Model (SBM) can be implemented. The SBM has been widely used for fault diagnostics and the remaining useful life (RUL) estimation, but the development of SBM includes tasks that often rely on high skilled experts. For instance, data reduction techniques required for SBM are often performed by experts judgment whose outcomes are not always consistent. The produced features from these techniques are used to build the Health Index that can be used to create the degradation trends that serve as a reference for the SBM. To overcome these difficulties, an automatic and unsupervised approach based on the Kernel Principal Component Analysis is proposed to enhance the Health Index creation. It preserves as much of the sensor information as possible improving the similarity-based RUL estimation. Additionally, when estimating the RUL of a system, the most similar degradation trends stored in the SBM library are used to compute individual RULs, the final RUL is obtained by a fusion rule technique that combines all these individual RULs into a consolidated value. For the fusion rule techniques, a self-adaptive method that does not rely on human expertize is proposed. This fusion rule can benefit of the accumulated knowledge over the SBM operation. This unsupervised approach to develop a SBM is validated with promising results against an equivalent and supervised algorithm that came out best in the 2008 prognostic health management challenge
The Spectral Difference Raviart–Thomas Method for Two and Three-Dimensional Elements and Its Connection with the Flux Reconstruction Formulation
Get PDFThe purpose of this work is to describe in detail the development of the spectral difference Raviart–Thomas (SDRT) formulation for two and three-dimensional tensor-product elements and simplexes. Through the process, the authors establish the equivalence between the SDRT method and the flux reconstruction (FR) approach under the assumption of the linearity of the flux and the mesh uniformity. Such a connection allows building a new family of FR schemes for two and three-dimensional simplexes and also to recover the well-known FR-SD method with tensor-product elements. In addition, a thorough analysis of the numerical dissipation and dispersion of both aforementioned schemes and the nodal discontinuous Galerkin FR (FR-DG) method with two and three-dimensional elements is proposed through the use of the combined-mode Fourier approach. SDRT is shown to possess an enhanced temporal linear stability in comparison to FR-DG. On the contrary, SDRT displays larger dissipation and dispersion errors with respect to FR-DG. Finally, the study is concluded with a set of numerical experiments, the linear advection-diffusion problem, the Isentropic Euler Vortex, and the Taylor-Green Vortex (TGV). The latter test case shows that SDRT schemes present a non-linear unstable behavior with simplex elements and certain polynomial degrees. For the sake of completeness, the matrix form of the SDRT method is developed and the computational performance of SDRT with respect to FR schemes is evaluated using GPU architectures
Nonlinear Effects of Electromagnetic Pulse on a Plasma-Based Electrically Small VHF Antenna
Get PDFDemonstration of the electromagnetic hardening properties of a plasma-based electrically small VHF antenna is presented in this paper. It is shown that when illuminated by an electromagnetic pulse, the maximum voltage collected by the antenna grows linearly with the maximum magnitude of the incident electric field, but beyond a certain threshold around 70 kV/m, the maximum voltage collected ceases to grow linearly and seems to saturate around a value of 200 V
Influence of hydrogen on the stress-relaxation properties of 17-4PH martensitic stainless steel manufactured by laser powder bed fusion
Get PDFTo evaluate the interactions between hydrogen and the mechanical behavior of additively manufactured (AM) 17-4PH martensitic stainless steel (MSS), stress-relaxation tests were performed for both non-charged and hydrogen-charged samples. Similar tests were performed for the conventional 17-4PH MSS counterpart (CM). The results clearly indicated significant differences in the relaxation properties between the CM and AM MSSs in the H900 state, which were attributed to the higher reversed austenite amount in the AM MSS than that in the CM MSS. In particular, the influence of hydrogen on the dislocation mobility in the austenite was clearly shown
Stability of the Low Reynolds Number Compressible Flow Past a NACA0012 Airfoil
Get PDFWith stratospheric flight ormartian exploration in perspective, the compressibility influence on thewake dynamics of
a NACA0012 profile is investigated. The unsteady flow past the airfoil at Re = 1000 is characterized using direct
numerical simulations for various angles of attack α ∈ [0°; 20°] and Mach numbers up to M∞ = 0.5. Steady flows
obtained using the selective frequency damping (SFD) technique are used as base states for a global linear stability
analysis. The influence of both the angle of attack and the Reynolds number on the most amplifiedmode characteristics
is first investigated in the incompressible regime. Then, the compressibility effects in the subcritical regime are
considered. A stabilizing or a destabilizing effect of compressibility is observed depending on the angle of attack and
theReynolds number. For α < 20°, compressibility has a destabilizing effect close to the critical threshold,which results
in anearlierHopfbifurcation,whereas increasing theMach number always results in a decrease of the growth rate of the
mode well above the critical threshold. Finally, the mode frequency decreases with the Mach number
Optimal growth over a time-evolving variable-density jet at Atwood number |At|=0.25
Get PDFSecondary instabilities growing over a time-evolving variable-density round jet subject to the primary Kelvin–Helmholtz (KH) instability at Atwood number |At|=0.25 are investigated with a non-modal linear stability analysis. Despite local modifications of the base flow vorticity induced by the baroclinic torque, these disturbances experience a short-term universal growth due to a combination of the Orr and lift-up mechanisms, whatever the azimuthal wavenumber ????. At Re=1000, the secondary energy growth stems from the development of elliptical and hyperbolic instabilities, with an E-type-to-H-type transition as ???? and Re increase, as in the homogeneous case (Nastro et al., J. Fluid Mech., vol. 900, 2020, A13). In the light jet at Re=1000, after the KH mode saturation, the high-???? H-type instability is replaced by a perturbation organised as counter-rotating streamwise vortices located in the base flow region of promoted strain rate. Increasing the Reynolds number up to Re=10000 yields larger energy growths and a strong anisotropy among energy and enstrophy components with a preferential increase of axial velocity and azimuthal vorticity. Both come from the linearised baroclinic source that drives the optimal response towards folded sheets of axial velocity that differ from those observed in the variable-density plane shear layers. When the perturbation is injected around the KH saturation time for Re=10000, the response to optimal perturbation takes the form of fast growing secondary KH instabilities whatever ????. We find these three-dimensional secondary KH instabilities to be good candidates for the transition to turbulence in variable-density jet flows
The electrochemical potential is a key parameter for cell adhesion and proliferation on carbon surface
Get PDFThe Nernst potential of the support/cell interface is suspected to play a key role in cell adhesion and proliferation. However, the studies that have addressed this topic have generally varied the electrochemical potential of the interface by comparing different materials or by varying the chemical composition of the surface coating. It is consequently hard to definitively separate the actual effect of the potential from possible side-effects due to differences in the surface composition or topography. Here, a 3-electrode set-up was used to apply different values of potential to identical carbon electrodes. Potentials were applied in the range −200 to 400 mV vs. silver pseudo-reference (SPR), i.e. 90 to 690 mV/SHE, to screen-printed carbon electrodes used to grow Vero or Raw 264.7 cell lines. Values up to 200 mV/SPR prohibited cell adhesion and even caused detachment of cells that were previously adhered. The value of 400 mV/DRP allowed cell adhesion and proliferation, leading to confluent and sometimes very compact mats. The zero charge potential, measured around 200 mV/DRP, showed that the drastic effect of the applied potential was probably due to the negative/positive switch of the surface charge