International Journals
Fluid Mechanics
– Granular size segregation in under-water sand ripples.
Germain Rousseaux, Hervé Caps and José-Eduardo Wesfreid.
European Physical Journal E, Volume 13, Number 2, p. 213-219, January 2004. –Wavelength selection of rolling-grain ripples in laboratory.
Germain Rousseaux, Alexandre Stegner and José-Eduardo Wesfreid.
Physical Review E, Volume 69, Issue 3, p. 031307-1/6, March 2004.
– Dynamics of transient eddy above rolling-grain ripples.
Germain Rousseaux, Harunori Yoshikawa, Alexandre Stegner and José-Eduardo Wesfreid.
Physics of Fluids, Volume 16, Number 4, p. 1049-1058, April 2004.– Résolution de l’équation de Young-Laplace par une méthode géométrique utilisant la courbure.
Mathieu Gentes, Germain Rousseaux, Pierre Coullet et Pierre-Gilles de Gennes.
Comptes-Rendus Physique, Volume 6, Issue 9, p. 1027-1033, November 2005.– Physical distinction between rolling-grain ripples and vortex ripples : an experimental study.
Germain Rousseaux.
Physical Review E, Volume 74, Number 6, p. 066305-1/7, December 2006.– On the Lamb vector and the hydrodynamic charge.
Germain Rousseaux, Shahar Seifer, Victor Steinberg and Alexander Wiebel.
Experiments in Fluids, Volume 42, Issue 2, p. 291-299, February 2007.– Viscous fingering in packed chromatographic columns: linear stability analysis.
Germain Rousseaux, Michel Martin and Anne De Wit.
Journal of Chromatography A, Volume 1149, Issue 2, p. 254-273, May 2007.
– Observation of negative phase velocity waves in a water tank: A classical analogue to the Hawking effect ?
Germain Rousseaux, Christian Mathis, Philippe Maïssa, Thomas Philbin and Ulf Leonhardt.
New Journal of Physics, Volume 10, 053015, May 2008.
Best Of 2008 NJP & IOP Select : General and Fluid Mechanics & >250 downloadings IOP & Europhysics News & Pour La Science– Oscillation-induced sand ripples in a circular geometry.
(PRE Kaleidoscope Select)
Germain Rousseaux, Joachim Kruithof, Patrice Jenffer and José-Eduardo Wesfreid.
Physical Review E, Volume 78, Number 1, p. 016302-1/5, July 2008.– Wave-Current Interaction as a Spatial Dynamical System : Analogies with Rainbow and Black Hole Physics.
Jean-Charles Nardin, Germain Rousseaux and Pierre Coullet.
Physical Review Letters, Volume 102, Issue 12, p. 124504-1/4, March 2009.– Horizon effects with surface waves on moving water.
Germain Rousseaux, Christian Mathis, Philippe Maïssa, Pierre Coullet, Thomas Philbin and Ulf Leonhardt.
New Journal of Physics, Volume 12, 095018, September 2010.
Best Of 2010 NJP & Selected as a NJP Video Abstract.
CNRS Journal & Science et Vie & Physics– Pattern formation in bubbles emerging periodically from a liquid free surface.
Harunori Yoshikawa, Christian Mathis, Philippe Maïssa, Germain Rousseaux and Stephane Douady.
European Physical Journal E, Volume 33, p. 11-18, September 2010.– Experimental study of dispersion and miscible viscous fingering of initially circular samples in Hele-Shaw cells.
Renaud Maes, Germain Rousseaux, Benoit Scheid, Manoranjan Mishra, Pierre Colinet and Anne De Wit.
Physics of Fluids, Volume 22, Number 12, p. 123104-1/12, December 2010. – Experimental demonstration of the supersonic-subsonic bifurcation in the circular jump: a hydrodynamic white hole.
Gil Jannes, Romain Piquet, Philippe Maïssa, Christian Mathis and Germain Rousseaux.
Physical Review E, Volume 83, Issue 5, p. 056312-1/5, May 2011.
New Scientist.– Viscous fingering in packed chromatographic columns: non-linear dynamics.
Germain Rousseaux, Michel Martin and Anne De Wit.
Journal of Chromatography A, Volume 1218, Issue 46, p. 8353-8361, November 2011.– Application of two–color LIF thermometry to nucleate boiling.
Nadine Kosseifi, Harunori Yoshikawa, Pascal Biwole, Christian Mathis, Germain Rousseaux, Séverine Boyer and Thierry Coupez.
Journal of Material Science and Engineering, Volume 3, Number 5B, p. 281-290 May 2013.
– Spectral analysis of ship waves in deep water from accurate measurements of the free surface elevation by optical methods.
Guillaume Gomit, Germain Rousseaux, Ludovic Chatellier, Damien Calluaud and Laurent David.
Physics of Fluids, Volume 26, Issue 12, 122101-1/11, December 2014.
– Wave blocking and partial transmission in subcritical flows over an obstacle.
Léo-Paul Euvé, Florent Michel, Renaud Parentani and Germain Rousseaux.
Physical Review D, Volume 91, Issue 02, 024020-1/12, January 2015.
– A Theoretical and Numerical Determination of Optimal Ship Froms based on Michell’s Wave Resistance.
Julien Dambrine, Morgan Pierre and Germain Rousseaux.
ESAIM: Control, Optimisation and Calculus of Variations (ESAIM: COCV)
– A novel method to generate tidal-like bores in the laboratory.
Germain Rousseaux, Jean-Marc Mougenot, Ludovic Chatellier, Laurent David and Damien Calluaud.
European Journal of Mechanics/B, Volume 55, Part 1, p. 31- 38, January 2016.
– Wave blocking phenomenon of surface waves on a shear flow with constant vorticity.
Philippe Maïsa, Germain Rousseaux and Yury Stepanyants.
Physics of Fluids, Volume 28, Issue 032102, March 2016.
– Negative energy waves in shear flow with a linear profile.
Philippe Maïssa, Germain Rousseaux and Yury Stepanyants.
European Journal of Mechanics-B/Fluids, Volume 56, p. 192- 196, March-April 2016.
– Analogue Wormholes and Black Hole LASER Effect in Hydrodynamics.
Cédric Peloquin, Léo-Paul Euvé, Thomas Philbin and Germain Rousseaux.
Physical Review D, Volume 93 Issue 08, 084032-1/12 April 2016.
– Observation of noise correlated by the Hawking effect in a water.
Léo-Paul Euvé, Florent Michel, Renaud Parentani, Thomas Philbin and Germain Rousseaux.
Physical Review Letters, Volume 117, Issue 12, 121301, September 2016.
– Energy distribution in shallow water ship wakes from a spectral analysis of the wave field.
Clément Caplier, Germain Rousseaux, Damien Calluaud and Laurent David.
Physics of Fluids, Volume 28, Issue 10, p. 107104, October 2016.
– Scattering of long water waves in a canal with suddenly changed cross-section in the presence of a current.
Semyon Churilov, Andri Ermakov, Germain Rousseaux and Yury Stepanyants.
Physics Review Fluids, Volume 2, Issue 9, P.094805, Septembre 2017.
– Classical analoque of an interstellar travel through a hydrodynamic wormhole.
(PRD Kaleidoscope Select)
Léo-Paul Euvé, Germain Rousseaux.
Physical Review D, Volume 96 Issue 06, 064042-1/15 Septembre 2017.
– Flow fields and particle trajectories beneath a tidal bore: a numerical study.
Adrien berchet, Bruno Simon, Anthony Beaudouin , Pierre Lubin, Germain Rousseaux and Serge Huberson.
International Journal of Sediment Research, Volume 33, Issue 3, p. 351-370, Septembre 2018.
– 2D numerical contributions for the study of non-cohesive sediment transport beneath tidal bores.
Yoga Satria Putria, Anthony Beaudoin, Germain Rousseaux, Lionel Thomas and Serge Huberson.
Comptes Rendus Mécanique, Volume 347, Issue 2, p. 166-180, February 2019.
– Effects of finite water depth and lateral confinement on ships wakes and drag.
Clément Caplier, Germain Rousseaux, Damien Calluaud and Laurent David.
Journal of Hydrodynamics, online 06/06, p. 1-8, June 2019.
– Calibrating and measuring wakes and drag forces of inland vessels in confined water in a towing tank.
Clément Caplier, Germain Rousseaux, Damien Calluaud and Laurent David.
Ocean Engineering, Volume 186, p. 106134, August 2019.
– Scattering of co-current surface waves on an analogue black hole.
Léo-Paul Euvé, Scott Robertson, Nicolas James, Alessandro Fabbri and Germain Rousseaux.
Physical Review Letters, Volume 124, Issue 14, p. 141101, April 2020.
– Classical Hydrodynamics for Analogue Space-times : open channels and thin films.
Germain Rousseaux and Hamid Kellay.
Philosophical Transactions of the Royal Society A, Volume 378, Issue 2177, 20190233, July 2020.
– The dual nature of the dead-water phenomenology: Nansen versus Ekman wave-making drags.
Johan Fourdrinoy, Julien Dambrine, Madalina Petcu, Morgan Pierre and Germain Rousseaux.
Proceedings of the National Academy of Sciences, Volume 117, Issue 29, p. 16739-16742, July 2020.
– Correlations on weakly time-dependent transcritical white-hole flows.
Johan Fourdrinoy, Scott Robertson, Nicolas James, Alessandro Fabbri and Germain Rousseaux.
Physical Review D, Volume 105, Issue 08, 085022-1/14 April 2022.
Nonlinear Physics
– Amplitude Equations for Mechanical Analogues of Faraday and Non-linear Optical Rotations.
Germain Rousseaux, Pierre Coullet and Jean-Marc Gilli.
European Journal of Physics, Volume 26, Number 6, p. 1065-1078, November 2005.– Robert Hooke’s conical pendulum from the modern viewpoint of amplitude equations and its optical analogues.
Germain Rousseaux, Pierre Coullet and Jean-Marc Gilli.
Proceedings of the Royal Society of London A, Volume 462, Number 2066, p. 531-540, February 2006. – Chaos in Robert Hooke’s inverted cone.
Médéric Argentina, Pierre Coullet, Jean-Marc Gilli, Marc Monticelli and Germain Rousseaux.
Proceedings of the Royal Society of London A, Volume 463, Number 2081, p. 1259-1269, May 2007.– Bead, hoop and spring…: some theoretical remarks.
Germain Rousseaux.
European Journal of Physics, Volume 28, L7-L9, May 2007.
>250 downloadings IOP
– On the Bead, Hoop and Spring (BHS) dynamical system.
Germain Rousseaux.
Nonlinear Dynamics, Volume 56, Issue 3, p. 315-323, May 2009.– Robert Hooke’s three-body problem.
Médéric Argentina, Pierre Coullet, Jean-Marc Gilli, Marc Monticelli and Germain Rousseaux.
International Journal of Bifurcations and Chaos, Volume 19, No. 10, p. 1-10, October 2009.– On the critical equilibrium of the spiral spring pendulum
Pierre Coullet, Jean-Marc Gilli and Germain Rousseaux.
Proceedings of the Royal Society of London A, Volume 466, Number 2114, p. 407-421, February 2010.
Classical Electromagnetism
– Lorenz or Coulomb in Galilean Electromagnetism ?
Germain Rousseaux.
EuroPhysics Letters, Volume 71, Number 1, p. 15-20, July 2005.– On the electrodynamics of moving bodies at low velocities.
Marc de Montigny and Germain Rousseaux.
European Journal of Physics, Volume 27, Number 4, p. 755-768, July 2006.
>250 downloadings IOP– On some applications of Galilean electrodynamics of moving bodies.
Marc de Montigny and Germain Rousseaux.
American Journal of Physics, Volume 75, Issue 1, p. 984-992, November 2007.
– The Maxwell-Lodge effect : significance of electromagnetic potentials in the classical theory.
Germain Rousseaux, Richard Kofman and Olivier Minazzoli.
European Physical Journal D, Volume 42, Number 2, p. 249-256, September 2008.
Dedicated Web Site on the Maxwell-Lodge effect (in French)– On the electrodynamics of Minkowski at low velocities.
Germain Rousseaux.
EuroPhysics Letters, Volume 84, Number 2, 20002, 4 p., October 2008.– Implications of Galilean Electromagnetism in Numerical Modeling.
Francesca Rapetti and Germain Rousseaux.
Presented at CEM 2011 (Eigth International Conference on Computational Electromagnetics), 11-14 April 2011, Wroclaw, Poland.
Applied Computational Electromagnetics Society journal, Vo. 26, No. 9, pp. 784-791, September 2011.– On quasistatic models hidden in Maxwell’s equations.
Francesca Rapetti and Germain Rousseaux.
Applied Numerical Mathematics, Vol. 79, p. 92-106, May 2014.– Forty Years of Galilean Electromagnetism (1973-2013).
Germain Rousseaux.
European Physical Journal EPJ-Plus, Review Paper, Vol. 128, No 8, p. 1-14, August 2013.
Relativity
– Comment on « Momentum Transfer from Quantum Vacuum to Magnetoelectric Matter ».
Germain Rousseaux.
Physical Review Letters, Volume 100, Number 24, p. 248901, June 2008.
– Hawking radiation and the boomerang behavior of massive modes near a horizon.
Gil Jannes, Philippe Maïssa, Thomas Philbin and Germain Rousseaux.
Physical Review D, Volume 83, Issue 10, p. 104028-1/11, May 2011.
– Visco-elastic Cosmology for a Sparkling Universe ?
Germain Rousseaux and Stefan Mancas.
General Relativity and Gravitation, Volume 52, Issue 6, Article 55, May 2020.
International Proceedings
Fluid Mechanics
– Transient eddy above rolling-grain ripples.
Harunori Yoshikawa, Germain Rousseaux, Joachim Kruithof, Alexandre Stegner and José Eduardo Wesfreid.
Proceedings of the International Workshop « Marine Sandwave and River Dune Dynamics II », Enschede, The Netherlands, 1-2 April 2004.– Tapping wave energy through Longuet-Higgins microseism effect.
Bernard Molin, David Lajoie, Nicolas Jarry and Germain Rousseaux.
Proceedings of the 23rd International Workshop on Water Waves and Floating Bodies, Jeju, Korea, 13-16 April 2008.– Flow visualizations and PIV analysis above vortex ripples.
Germain Rousseaux.
Proceedings of the 13th International Symposium on Flow Visualization and 12th French Congress Fluvisu, Nice, France, 1-4 July 2008.– Horizon effects for surface waves in wave channels and circular jumps.
Gil Jannes, Romain Piquet, Jennifer Chaline, Philippe Maïssa, C. Mathis and Germain Rousseaux.
Spanish Relativity Meeting (ERE2010) : Gravity as a crossroad in Physics, Journal of Physics (Conference Series), Volume 314, 012031, 6th-10th September 2010, Granada, Spain.
– Spiral pattern formation in a simple two-phase flow system.
Harunori Yoshikawa, Christian Mathis, Philippe Maïssa and Germain Rousseaux.
Proceedings of the Conference Chaos, Complexity and Transport CCT’11, 23th-27th May 2011, Marseille France, p. 113-122 (World Scientific, 2012).
– Recent results on the problem of wave-current interaction including water depth, surface tension and vorticity effects.
P. Maïssa, G. Rousseaux and Y. Stepanyants.
Coastal Dynamics 2013 CD2013, 7th International Conference on Coastal Dynamics (Arcachon), p. 1137 – 1146, June 2013.– Flukeprint of cetaceans and the corresponding shear flow phenomena.
Germain Rousseaux, David Uminsky and Rachel Levy.
Proceedings of the International Symposium on Turbulence and Shear Flow Phenomena, TSFP8 (Poitiers), P11, August 2013.
– TR-PIV measurements in open channel flow for the analysis of undular tidal bores.
Laurent David, Ludovic Chatellier, Damien Calluaud, Young Jin Jeon, Germain Rousseaux and Lionel Thomas.
Proceedings of the 17th International Symposium on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2014.
– Cut-cell method: application to water waves generated by a submerged obstacle.
Julien Dambrine, Nicolas James and Germain Rousseaux.
6th European Conference on Computational Fluid Dynamics — Barcelona, Spain, 20-25 July 2014.
– An experimental study of the effects of finite water depth and lateral confinement on ships wake and drag.
Clément Caplier, Germain Rousseaux, Damien Calluaud and Laurent David.
E-proceedings of the 36th IAHR World Congress, 28 June – 3 July, 2015, The Hague, the Netherlands, 2015.
– Study of different type of tidal bore in the laboratory.
Laurent David, Ludovic Chatellier, Damien Calluaud, Germain Rousseaux and Lionel Thomas.
E-proceedings of the 36th IAHR World Congress, 28 June – 3 July, 2015, The Hague, the Netherlands, 2015.
– An experimental study of the effects of finite water depth and lateral confinement on ships wake and drag.
Clément Caplier, Germain Rousseaux, Damien Calluaud, Laurent David.
Congrès SHF « Hydrodynamics and simulation applied to inland waterways and port approaches », Meudon 18-19 November 2015.
– The effets of river counter-current on ship wakes : an experimental approach.
Clément Caplier, Germain Rousseaux, Damien Calluaud, Laurent David.
Congrès SHF « Hydrodynamics and simulation applied to inland waterways and port approaches », Meudon 18-19 November 2015.
– Optimisation of ship hulls considered as slender-bodies.
Julien Dambrine, Morgan Pierre, Germain Rousseaux.
Congrès SHF « Hydrodynamics and simulation applied to inland waterways and port approaches », Meudon 18-19 November 2015.
– The Battle of Actium and the Myth of the Ship-Holder : the effect of bathymetry.
Johan Fourdrinoy, Clément Caplier, Yann Devaux, Germain Rousseaux, Julien Dambrine, Morgan Pierre, Madalina Petcu, Areti Giani, Ierotheos Zacharias, Isabelle Jouteur and Paul Martin.
Mashcon V : 5th International Conference on Ship Manoeuvring in Shallow and Confined Water, Ostend, Belgium. 19 – 23 May 2019.
Electromagnetism
– Implications of Galilean Electromagnetism in Numerical Modeling.
Francesca Rapetti and Germain Rousseaux.
CEM 2011 (Eigth International Conference on Computational Electromagnetics), Wroclaw, Poland, 11-14 April 2011.
Relativity
– Hawking tunneling and boomerang behaviour of massive particles with E < m.
Gil Jannes, Thomas G. Philbin, Germain Rousseaux
Spanish Relativity Meeting (ERE2011), AIP Conference Series, 29th August-2nd September 2011, Madrid, Spain.
French Journals
Fluid Mechanics
– Dynamique de l’interface sable-fluide : des rides de plage aux dunes sous-marines.
Alexandre Stegner et Germain Rousseaux.
Bulletin de la Société Française de Physique, Numéro 129, p. 4-7, Mai 2001.– Dynamique d’une interface eau-granulaire.
Germain Rousseaux, Alexandre Stegner et José Eduardo Wesfreid.
Proceedings du 15ème Congrés Français de Mécanique à Nancy en Septembre 2001.– Une vision naïve du transport sédimentaire induit par un mascaret ondulant.
A. Berchet, A. Beaudoin, S. Huberson, G. Rousseaux, B. Simon et P. Lubin.
Proceedings du 21ème Congrés Français de Mécanique à Bordeaux en Août 2013.
Session d’Hydrodynamique Côtiére et Fluviale.
– Interaction houle-courant : Mesure de la Relation de dispersion.
Pierre-Jean Faltot, Romain Bellanger, Jean-Marc Mougenot et Germain Rousseaux.
Proceedings des 13èmes Journées Génie Côtier Génie Civil à Dunkerque en Juillet 2014.
Session d’Hydrodynamique Côtière, p. 79-92, Editions Paralia du C.F.L..
Editeurs : Daniel Levacher, Martin Sanchez, Arnaud Hequette et Yves Lalaut.
– Etude de mascaret en laboratoire.
Laurent David, Ludovic Chatellier, Damien Calluaud, Germain Rousseaux et Lionel Thomas.
Proceedings du 14ème Congrès Francophone de Techniques Laser à Marseille en Septembre 2014.
– Etude expérimental des effets de hauteur d’eau finie et de confinement latéral sur les sillages de navire en bassin des carènes.
Clément Caplier, Germain Rousseaux, Damien Calluaud et Laurent David.
Proceedings des 14èmes Journées de l’Hydrodynamique, 18-20 Novembre 2014, DGA, Val de Reuil.
– An experimental study of the effect of finite water depth and lateral confinement on ships wake and drag.
Clément Caplier, Germain Rousseaux, Damien Calluaud et Laurent David.
Congrès SHF « Hydrodynamics and simulation applied to inland waterways and port approaches », Meudon 18-19 November 2015.– The effets of river counter-current on ship wakes : an experimental approach.
Clément Caplier, Germain Rousseaux, Damien Calluaud, Laurent David.
Congrès SHF « Hydrodynamics and simulation applied to inland waterways and port
approaches », Meudon 18-19 November 2015.
– Optimisation of ship hulls considered as slender-bodies.
Julien Dambrine, Morgan Pierre, Germain Rousseaux.
Congrès SHF « Hydrodynamics and simulation applied to inland waterways and port
approaches », Meudon 18-19 November 2015.
– Génération non-linéaire d’harmoniques après une conversion linéaire en interaction houle-courant.
Léo-Paul Euve et Germain Rousseaux.
Proceeding d’hydrodynamique Côtière, P. 29-38, Editions Paralia du C.F.L..
Editeurs: Daniel Levacher, Martin Sanchez et Vincent Rey.
– Etude comparative de deux modes de génération de mascaret par vélocimétrie par imagerie de particules résolues en temps et stéréo-réfraction.
Laurent David, Guillaume Gomit, Alessia Simonini, Germain Rousseaux, Ludovic Chatellier, Damien Calluaud.
Congrès Francophone de Techniques Laser, CFTL 2016, Toulouse, 13 – 16 Septembre 2016.
– Estimation of sediment particule trajectories in tidal bores using numerical modelling.
Yoga Satria Putra, Anthony Beaudoin, Mélissa Ramos Ortega, Germain Rousseaux and Serge Huberson.
Proceedings du 23ème Congrès Français de Mécanique à Lille, 28 Août – 1 Septembre 2017.
– Générateur de houle à contre-courant via un déversoir oscillant.
Johan Fourdrinoy, Léo-Paul Euvé, Jean-Marc Mougenot et Germain Rousseaux.
Proceedings des XVèmes Journées Nationales Génie Côtier – Génie Civil à La Rochelle en Mai 2018.
Session d’Hydrodynamique Côtière, p. 57-66, Editions Paralia du C.F.L..
Fluid Mechanics and Electromagnetism– Les équations de Maxwell sont-elles incomplètes ?
Germain Rousseaux.
Annales de la Fondation Louis de Broglie, Volume 26, Numéro 4, p. 673-682, 2001.– On the physical meaning of the gauge conditions of Classical Electromagnetism :
the hydrodynamics analogue viewpoint.
Germain Rousseaux.
Annales de la Fondation Louis de Broglie, Volume 28, Numéro 2, p. 261-270, 2003.– A propos d’une analogie entre la mécanique des fluides et l’électromagnétisme.
Germain Rousseaux et Etienne Guyon.
Bulletin de l’Union des Physiciens Français, 841 (2), p. 107-136, Février 2002.– Remarques supplémentaires sur l’approximation des régimes quasi-stationnaires en électromagnétisme.
Germain Rousseaux et André Domps.
Bulletin de l’Union des Professeurs de Physique et de Chimie, Volume 98, 868 (2), p. 71-86, Novembre 2004.
This article is at the origin of the examination paper of the French Agrégation (high level competitive examination for the recruitment of teachers) for the year 2008 (correction) as discussed in the report of the board of examiners.
Electromagnetism
– Sur la théorie de Riemann-Lorenz de l’Electromagnétisme Classique.
Germain Rousseaux.
Bulletin de l’Union des Professeurs de Physique et de Chimie, Volume 98, 868 (2), p. 41-56, Novembre 2004.
– The gauge non-invariance of Classical Electromagnetism.
Germain Rousseaux.
Annales de la Fondation Louis de Broglie, Volume 30, Numéro 3, p. 387-397, 2005.– Sur Un Effet Physique Attribuable Uniquement Au Potentiel Vecteur en Electromagnétisme Classique.
Germain Rousseaux, Richard Kofman et Olivier Minazzoli.
Bulletin de l’Union des Professeurs de Physique de Spéciales, 20 p., Janvier 2006. – Sur l’électrodynamique des corps en mouvement selon Einstein.
Germain Rousseaux.
Bulletin de l’Union des Professeurs de Physique de Spéciales, 6 p., Avril 2006.– Sur le caractère galiléen du magnétisme.
Germain Rousseaux.
Bulletin de l’Union des Professeurs de Physique de Spéciales, 3 p., Novembre 2006.
germain.rousseaux (at) cnrs.fr