Cross-waves induced by the vertical oscillation of a fully immersed vertical plate

Capillary waves excited by the vertical oscillations of a thin elongated plate below an air-water interface are analyzed using time-resolved measurements of the surface topography. A parametric instability is observed above a well defined acceleration threshold, resulting in a so-called cross-wave, a staggered wave pattern localized near the wavemaker and oscillating at half the forcing frequency. This cross-wave, which is stationary along the wavemaker but propagative away from it, is described as the superposition of two almost anti-parallel propagating parametric waves making a small angle of the order of $20^\mathrm{o}$ with the wavemaker edge. This contrasts with the classical Faraday parametric waves, which are exactly stationnary because of the homogeneity of the forcing. Our observations suggest that the selection of the cross-wave angle results from a resonant mechanism between the two parametric waves and a characteristic length of the surface deformation above the wavemaker.Comment: to appear in Physics of Fluid

Recent developments in CID imaging

Readout of CID imaging arrays was first performed by injecting and detecting the signal charge from each sensing site in sequence. A new readout method, termed parallel injection, has been developed in which the functions of signal charge detection and injection have been separated. The level of signal charge at each sensing site is detected during a line scan, and during the line retrace interval, all charge in the selected line is injected. The parallel injection technique is well adapted to TV scan formats in that the signal is read out at high speed, line by line. A 244 line by 248 element TV compatible imager, employing this technique and including an on chip preamplifier, has been constructed and operation demonstrated

Parametrically excited helicopter ground resonance dynamics with high blade asymmetries

The present work is aimed at verifying the influence of high asymmetries in the variation of in-plane lead-lag stiffness of one blade on the ground resonance phenomenon in helicopters. The periodical equations of motions are analyzed by using Floquet's Theory (FM) and the boundaries of instabilities predicted. The stability chart obtained as a function of asymmetry parameters and rotor speed reveals a complex evolution of critical zones and the existence of bifurcation points at low rotor speed values. Additionally, it is known that when treated as parametric excitations; periodic terms may cause parametric resonances in dynamic systems, some of which can become unstable. Therefore, the helicopter is later considered as a parametrically excited system and the equations are treated analytically by applying the Method of Multiple Scales (MMS). A stability analysis is used to verify the existence of unstable parametric resonances with first and second-order sets of equations. The results are compared and validated with those obtained by Floquet's Theory. Moreover, an explanation is given for the presence of unstable motion at low rotor speeds due to parametric instabilities of the second order

Organisation d’un contrôle de la pollution radioactive du lait. Cas des radioisotopes de courtes périodes

Michon Georges. Organisation d’un contrôle de la pollution radio¬ active du lait, cas des radioisotopes de courtes périodes. In: Bulletin de l'Académie Vétérinaire de France tome 116 n°6, 1963. pp. 283-285

Eloge de Jean Leplâtre

Michon Georges. Éloge de Jean Leplâtre. In: Bulletin de l'Académie Vétérinaire de France tome 137 n°2, 1984. pp. 216-218

Organisation d’un contrôle de la pollution radioactive du lait « cas des isotopes de longue période »

Michon G. Organisation d’un contrôle de la pollution du lait. «Cas des isotopes de longues périodes». In: Bulletin de l'Académie Vétérinaire de France tome 116 n°10, 1963. pp. 463-465

Teneur en radium 226 des laits en France I. — Intérêt de l'étude

Michon Georges. Teneur en radium 226 des laits en France. I. Intérêt de l’étude. In: Bulletin de l'Académie Vétérinaire de France tome 118 n°4, 1965. pp. 141-143