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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.

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.

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.

Benard N.,CNRS Pprime Institute | Moreau E.,CNRS Pprime Institute
Applied Physics Letters | Year: 2012

The paper compares the influence of different waveforms as input for a dielectric barrier discharge plasma actuator investigated in context of plasma-assisted flow control. The electrical aspects, the plasma morphology, the body force production, and the two-component time-resolved electric wind produced over a single ac period of signal are investigated. Results shown that square waveform is optimal in terms of body force and mean electric wind production, but the velocity fluctuations are enhanced by using a sinusoidal waveform. The time-resolved measurements of the produced velocity demonstrate that the electromechanical conversion mechanism is quasi-linear in the vicinity of the discharge. © 2012 American Institute of Physics.

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.

Brunetiere N.,CNRS Pprime Institute | Tournerie B.,CNRS Pprime Institute
Tribology International | Year: 2012

This paper presents a numerical study of the behavior of a mechanical seal with textured surfaces. It is used to analyze the mechanisms underlying the enhancement of the hydrodynamic lift associated with surface texture in mechanical seals. The model solves the Reynolds equation coupled with a mass-conservative cavitation algorithm and takes into account asperity contact. It is shown that, unlike rough-textured surfaces, smooth-textured surfaces are unable to generate a load. The performance of two rough surfaces are compared with those of the same surfaces equipped with dimples. The effect of texture density and aspect ratio are studied as well. © 2012 Elsevier Ltd. All rights reserved.

Piriou B.,CIRAD - Agricultural Research for Development | Vaitilingom G.,CIRAD - Agricultural Research for Development | Veyssiere B.,CNRS Pprime Institute | Cuq B.,Montpellier SupAgro | Rouau X.,Montpellier SupAgro
Progress in Energy and Combustion Science | Year: 2013

The direct use of dry biomass dust as a fuel in reciprocating engines could be of great interest because of the large availability of plant matter and the versatility of Internal Combustion Engines (ICE). Coal dust was used in the past and mostly in slurries because of large production during industrial era in Europe but led to many problems caused by fuel handling and wear in ICE. In comparison, biomass has a CO2 neutral impact, and is almost ash and sulphur free. Biomass pulverization technologies are now mature and the raw material can be reduced to micronic size or even smaller. Among the various new and renewable fuels under research and development, solid raw biomass is certainly the most promising advanced biofuel. It requires no or little thermochemical or biological processing or upgrading and potentially does not generate waste, detrimental to the environment. After a general overview of the past attempts to run reciprocating engines with coal dust, this paper will assess the so far unconsidered use of dry biomass dust as a fuel in engines instead of abrasive, less volatile and more polluting coal dust. © 2012 Elsevier Ltd. All rights reserved.

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.

Babonneau D.,CNRS Pprime Institute
Journal of Applied Crystallography | Year: 2010

A software package for performing modelling and analysis of GISAXS (grazing-incidence small-angle X-ray scattering) data within the distorted-wave Born approximation has been developed using the IGOR Pro scripting language (http://www.wavemetrics.com). The tool suite uses a slab-model approach with the Abéls matrix method to calculate X-ray reflectivity curves, electric field intensity distributions and GISAXS intensities from supported or buried scatterers arranged in two or three dimensions in a stratified medium. Models are included to calculate the scattered intensity for monodisperse, polydisperse and interacting particles with various size distributions, form factors and structure factors. The source code for the entire package is freely available, allowing anyone to develop additional tools. © 2010 International Union of Crystallography Printed in Singapore - all rights reserved.

Benard N.,CNRS Pprime Institute | Moreau E.,CNRS Pprime Institute
Flow, Turbulence and Combustion | Year: 2011

Commercial and military aircrafts or miniature aerial vehicles can suffer from massive flow separation when high angles of attack are required. Single dielectric barrier discharge (DBD) actuators have demonstrated their capability of controlling such a separated flow at low external velocity. However, the processes resulting in the improvement of the flight performances remain unclear. In the present study, the reattachment process along the suction side of a NACA 0015 placed at an angle of attack of 16° is experimentally investigated for an external velocity of 20 m/s (Re = 260,000). A single DBD actuator is mounted at the leading edge of the model. The velocity fields above the suction side of the airfoil are measured by a high-speed acquisition system (3 kHz). The results indicate that the baseline flow presents shed vortices that form at the leading edge and linearly grow along the free shear layer axis. This vortex shedding is organized and exhibits a specific frequency of about 90 Hz. The continuous actuation produces a partial flow reattachment up to 70% of the chord length. Temporal cross-correlation function indicates the presence of a vortex shedding at the trailing edge of the controlled flow. Finally, the temporal analysis demonstrates that the reattachment process requires 50 ms to reach a stabilized attached flow. The time-resolved analysis of the reattachment suggests that the actuation by plasma discharge acts as a catalyser by reinforcing one of the coherent flow structures already existing in the natural flow. © 2011 Springer Science+Business Media B.V.

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