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Poitiers, France

Charrier D.S.H.,CNRS Pprime Institute
Applied Physics Letters | Year: 2012

A low cost and light electromagnetic thruster, consisting in a disc rigidly attached with a coaxial coil, shows steady recoil by losing its linear momentum. The signal applied in the device is a square electric potential. A continuous thrust is observed on the center-of-mass in one single direction under electromagnetic excitation for various voltages and nominal high frequencies. At 1 kHz with 20 V amplitude, the recoil force reaches 4 μN (micronewton). The recoil is numerically quantified with induced electromotive and Lorentz forces. The presented device directly converts electric energy into kinetic energy. © 2012 American Institute of Physics. Source

Pizzagalli L.,CNRS Pprime Institute
Acta Materialia | Year: 2014

Large-scale first-principles calculations were performed to determine the stability and mobility properties of screw dislocations in common silicon carbide polytypes (4H, 2H and 3C). There is a profound lack of knowledge regarding these dislocations, although experimental observations show that they govern the plastic behavior of SiC at low temperature. Numerical simulations reported in this paper indicate that these dislocations are characterized by a shuffle core, the associated Peierls stress of which ranges from 8.9 to 9.6 GPa depending on the polytype. The only other stable dislocation core exhibits a reconstruction along the dislocation line, with a greater stability, but is also found to be sessile. Polytypism has a weak influence on these results, especially regarding dislocation core energies and Peierls stress. However, a qualitative difference is predicted between the cubic and the hexagonal systems regarding slip planes, with a possible dislocation displacement along a prismatic plane on average, which would result from a zigzag motion of the screw dislocations at the atomic scale. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

Rabier J.,CNRS Pprime Institute
Philosophical Magazine | Year: 2013

Different core structures have been evidenced in silicon which control, respectively, low and high temperature deformation. Indeed perfect shuffle dislocations have been found to build deformation microstructures in the temperature range and stress where silicon is usually brittle. In contrast, the plastic regime is controlled by dissociated glide set dislocations. This paper aims at relating the plastic properties of silicon to the core structure of dislocations and in particular to the existence and the properties of these two core structures. © 2013 Copyright Taylor and Francis Group, LLC. Source

Marx D.,CNRS Pprime Institute
Journal of Sound and Vibration | Year: 2012

Acoustic liners are used to reduce sound emission by turbofan engines. Under grazing flow they may sustain hydrodynamic instabilities and these are studied using a stability analysis, based on a simplified model: the liner is a mass-spring-damper system, the base channel flow is piecewise linear, and the inviscid, incompressible Rayleigh equation is used. The model is an extension to the channel case of a boundary layer model by Rienstra and Darau. The piecewise linear profile introduces a finite boundary layer thickness which ensures well-posedness, allowing an initial value problem to be conducted to investigate absolute stability. For typical values in aeronautics the flow above the liner is unstable. Absolute instability is obtained for somewhat extreme values of the mean flow (tiny boundary layer thickness), and under realistic conditions the flow is convectively unstable. The effect of finite channel height is investigated in both cases. In particular, for large boundary layer thicknesses associated with convective instability the channel height has little effect on the unstable mode. Favorable outcomes and failures of the model are shown by comparison to a published experimental work. © 2012 Elsevier Ltd. All rights reserved. Source

Ben-Abdallah P.,University Paris - Sud | Biehs S.-A.,University Paris - Sud | Biehs S.-A.,Carl von Ossietzky University | Joulain K.,CNRS Pprime Institute
Physical Review Letters | Year: 2011

In this Letter, an N-body theory for the radiative heat exchange in thermally nonequilibrated discrete systems of finite size objects is presented. We report strong exaltation effects of heat flux which can be explained only by taking into account the presence of many-body interactions. Our theory extends the standard Polder and van Hove stochastic formalism used to evaluate heat exchanges between two objects isolated from their environment to a collection of objects in mutual interaction. It gives a natural theoretical framework to investigate the photon heat transport properties of complex systems at the mesoscopic scale. © 2011 American Physical Society. Source

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