2,311 research outputs found
Observation of resonant interactions among surface gravity waves
We experimentally study resonant interactions of oblique surface gravity
waves in a large basin. Our results strongly extend previous experimental
results performed mainly for perpendicular or collinear wave trains. We
generate two oblique waves crossing at an acute angle, while we control their
frequency ratio, steepnesses and directions. These mother waves mutually
interact and give birth to a resonant wave whose properties (growth rate,
resonant response curve and phase locking) are fully characterized. All our
experimental results are found in good quantitative agreement with four-wave
interaction theory with no fitting parameter. Off-resonance experiments are
also reported and the relevant theoretical analysis is conducted and validated.Comment: 11 pages, 7 figure
Maneuvers automation for agricultural vehicle in headland
International audienceThis paper addresses the problem of path generation and motion control for the autonomous maneuvers of agricultural vehicle in headland. A reverse turn planner is firstly presented, based on primitives connected together to easily generate the reference motion. Next, the steering and speed control algorithms are considered. To perform accurate path following, the sliding conditions are taken into account with a kinematic model extended with sliding parameters. In addition, predictive actions are developed to anticipate for vehicle steering and speed variations. The capabilities of the proposed algorithms are finally investigated through full-scale experiments. Fish-tail maneuvers are autonomously performed with an experimental mobile robot, and promising results are reported during reverse turn maneuvers with a vehicle-trailer system
Assessing the benefits of approximately exact step sizes for Picard and Newton solver in simulating ice flow (FEniCS-full-Stokes v.1.3.2)
Solving the momentum balance is the computationally expensive part of simulating the evolution of ice sheets. The momentum balance is described by the nonlinear full-Stokes equations, which are solved iteratively. We use the Picard iteration and Newton's method combined with Armijo step sizes and approximately exact step sizes, respectively, to solve these equations. The Picard iteration uses either no step size control or the approximately exact step sizes. We compare the variants of Newton's method and the Picard iteration in benchmark experiments, called ISMIP-HOM experiments A, B, E1, and E2. The ISMIP-HOM experiments consist of a more realistic domain and are designed to test the quality of ice models. For an even more realistic test case, we simulate the experiments E1 and E2 with a time-dependent surface. We obtain that approximately exact step sizes greatly reduce the necessary number of iterations for the Picard iteration and Newton's method with nearly no increase in the computation time for each iteration.</p
Covalent vs. non-covalent redox functionalization of C-LiFePO4 based electrodes
During high rate utilization of porous Li battery, Li+ refuelling from the electrolyte limits the discharge kinetics of positive electrodes. In the case of thick electrodes a strategy to buffer the resulting sharp drop of Li+ concentration gradient would be to functionalize the electrode with anionic based redox molecules (RMR) that would be therefore able to relay intercalation process. The occurrence of these RMR in the electrode should not however, induce adverse effect on Li intercalation processes. In this respect, this work studies the effect of functionalizing LFPC based electrodes by either covalent or non-covalent chemistry, on Li intercalation kinetics. To do so, model molecules containing a nitro group were introduced at the surface of both carbon conducting additives and active material (C-LiFePO4). It is shown that presumably due to formation of sp(3) defects, covalent anchoring using diazonium chemistry inhibits the intercalation kinetics in C-FePO4. On the contrary, if molecules such as pyrene derivatives are immobilized by pi-staking interactions, Li intercalation is not impeded. Therefore non-covalent functionalization of pyrene based RMR appears as a promising route to relay Li intercalation reaction during high power demand. The framework for future development of this strategy is discussed. (C) 2013 Elsevier B.V. All rights reserved
Characterizing the fungal and bacterial microflora and concentrations in fitness centres
Fitness centres are special places where conditions for microbiological proliferation should be considered. Moisture due to human perspiration and water condensation as a result of human physical activities are prevalent in this type of buildings. Exposure to microbial contaminants is clinically associated with respiratory disorders and people who work out in polluted environments would be susceptible to contaminants. This work studied the indoor air contamination in three gymnasiums in Lisbon. The sampling was performed at two periods: at the opening (morning) and closing (night) of the three gymnasiums. The airborne bacterial and fungal populations were sampled by impaction directly onto Tryptic Soy Agar (for bacteria) and Malt Extract Agar (for fungi) plates, using a Merck MAS-100 air sampler. Higher bacterial concentrations were found at night as compared to the morning but the same behaviour was not found for fungal concentrations. Gram-negative catalase positive cocci were the dominant bacteria in indoor air samples of the studied gymnasiums. In this study, 21 genera/species of fungal colonies were identified. Chrysosporium sp., Chrysonilia sp., Neoscytalidium hialinum, Sepedonium sp. and Penicillium sp. were the most prevalent species identified in the morning, while Cladosporium sp., Penicillium sp., Chrysosporium sp., Acremonium sp. and Chrysonilia sp. were more prevalent at night. A well-designed sanitation and maintenance program for gymnasiums is needed to ensure healthier space for indoor physical activity
Electron glass effects in amorphous NbSi films
We report on non equilibrium field effect in insulating amorphous NbSi thin
films having different Nb contents and thicknesses. The hallmark of an electron
glass, namely the logarithmic growth of a memory dip in conductance versus gate
voltage curves, is observed in all the films after a cooling from room
temperature to 4.2 K. A very rich phenomenology is demonstrated. While the
memory dip width is found to strongly vary with the film parameters, as was
also observed in amorphous indium oxide films, screening lengths and
temperature dependence of the dynamics are closer to what is observed in
granular Al films. Our results demonstrate that the differentiation between
continuous and discontinuous systems is not relevant to understand the
discrepancies reported between various systems in the electron glass features.
We suggest instead that they are not of fundamental nature and stem from
differences in the protocols used and in the electrical inhomogeneity length
scales within each material.Comment: Submission SciPos
Momentum Distributions of Particles from Three--Body Halo Fragmentation: Final State Interactions
Momentum distributions of particles from nuclear break-up of fast three-body
halos are calculated consistently, and applied to Li. The same two-body
interactions between the three particles are used to calculate the ground state
structure and the final state of the reaction processes. We reproduce the
available momentum distributions from Li fragmentation, together with
the size and energy of Li, with a neutron-core relative state containing
a -state admixture of 20\%-30\%. The available fragmentation data strongly
suggest an -state in Li at about 50 keV, and indicate a -state
around 500 keV.Comment: 11 pages (RevTeX), 3 Postscript figures (uuencoded postscript file
attached at the end of the LaTeX file). To be published in Phys. Rev.
A confined–unconfined aquifer model for subglacial hydrology and its application to the Northeast Greenland Ice Stream
Subglacial hydrology plays an important role in ice sheet dynamics as it determines
the sliding velocity. It also drives freshwater into the ocean, leading to undercutting of calving fronts by plumes.
Modeling subglacial water has been a challenge for decades. Only
recently have new approaches been developed such as representing subglacial channels
and thin water sheets by separate layers of variable hydraulic conductivity.
We extend this concept by modeling a confined–unconfined
aquifer system (CUAS) in a single layer of an equivalent porous medium (EPM). The advantage of this formulation is
that it prevents unphysical values of pressure at reasonable computational
cost. We performed sensitivity tests to investigate the effect of
different model parameters. The strongest influence of model parameters was
detected in terms of governing the opening and closure of the system. Furthermore,
we applied the model to the Northeast Greenland Ice Stream, where
an efficient system independent of seasonal input was identified about
500 km downstream from the ice divide.
Using the effective pressure from the hydrology model, the Ice
Sheet System Model (ISSM) showed considerable improvements in
modeled velocities in the coastal region.</p
Experimental Method to Determine the Energy Envelope Performance of Buildings
In France, buildings represent 40% of the annual energy consumption. This sector represents an important stack to achieve the objective of reducing by 4 the greenhouse gas emissions by 2050. Knowledge of construction techniques and the use of equipments are the main keys to realize low energy buildings. To achieve this aim, we monitored 24 experimental buildings. In order to evaluate these experimental buildings we compare the monitored energy performance to the predicted energy performance and explain the differences between both performances. Therefore, we developed an in-situ method to determine the thermal envelope performance of buildings (Ubuilding). The buildings are monitored in order to know the followings inputs: Occupancy rate; Heat supply; Solar supply; Ventilation and airflow losses; Distributions losses.
The aim of this paper is to present the developed method and monitoring protocol. In order to validate the proposed experimental approach, we will present applications on different monitoring buildings in context of the project PREBAT (Research Program on Building's Evaluation)
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