Observations and modeling of lightning leaders

The development of atmospheric lightning is initiated by a ‘leader’ phase during which ionized channels appear in virgin air. The use of rapid cameras, the measure of fields and currents associated with the discharge allow one to compare the propagation of laboratory leaders with those of natural or artificially triggered lightning. The corresponding physical processes can be analyzed with the help of models developed for laboratory leaders provided that the non linear effects due to the intense current circulation leading to lightning leader thermalization are taken into account. A self-coherent simulation of triggered lightning leaders for both polarities is presented is this paper. Furthermore, these models make it possible to define the ‘stabilization field’ concept, equal to the minimum ambient field allowing the stable progress of a leader from a ground structure, expressed as a height and curvature function of this structure. This concept can be validated through triggered lightning tests. Finally, the stabilization field analysis is completed by a simplified analytical model based upon an electrostatic approach of propagation equilibrium. To cite this article: P. Lalande et al., C. R. Physique 3 (2002) 1375–1392.     

Fundamental processes in long air gap discharges

The development of atmospheric lightning is initiated and sustained by the formation in virgin air of ‘streamer corona’ and ‘leader’ discharges, very similar to those observed in laboratory long sparks. Therefore, the experimental and theoretical investigations of these laboratory discharges have become of large interest to improve the physical knowledge of the lightning process and to develop self-consistent models that could be applied to new protection concepts.In the present paper the fundamental processes of the subsequent phases of long air gap discharges are analyzed, from the first corona inception and development to the leader channel formation and propagation. For all these processes simulations models are discussed that have been essentially derived and simplified by the authors, in order to develop sequential time-dependent simulation of the laboratory breakdown, with both positive and negative voltages. The possibility of extending these models to the case of natural lightning is discussed in the companion paper, presented in this same volume. To cite this article: I. Gallimberti et al., C. R. Physique 3 (2002) 1335–1359.     

A lightning initiation mechanism: application to a thunderstorm electrification model

The use of numerical models has greatly increased our understanding of the electrical and microphysical process within electrified clouds. We use the University of Washington, 1.5-dimensional thunderstorm model to examine the effects of including a runaway electron based lightning initiation mechanism. We find that this mechanism can significantly alter the electrification history of modeled storms and produce vertical electric field profiles that are very similar to those of observed storms. To cite this article: R. Solomon et al., C. R. Physique 3 (2002) 1325–1333.     

The physical origin of the land–ocean contrast in lightning activity

New tests and older ideas are explored to understand the origin of the pronounced contrast in lightning between land and sea. The behavior of islands as miniature continents with variable area supports the traditional thermal hypothesis over the aerosol hypothesis for lightning control. The substantial land–ocean contrast in updraft strength is supported globally by TRMM (Tropical Rainfall Measuring Mission) radar comparisons of mixed phase radar reflectivity. The land–ocean updraft contrast is grossly inconsistent with the land–ocean contrast in CAPE (Convective Available Potential Energy), from the standpoint of parcel theory. This inconsistency is resolved by the scaling of buoyant parcel size with cloud base height, as suggested by earlier investigators. Strongly electrified continental convection is then favored by a larger surface Bowen ratio, and by larger, more strongly buoyant boundary layer parcels which more efficiently transform CAPE to kinetic energy of the updraft in the moist stage of conditional instability. To cite this article: E. Williams, S. Stanfill, C. R. Physique 3 (2002) 1277–1292.     

Some microphysical and electrical aspects of a Cloud Resolving Model: description and thunderstorm case study

An electrification scheme, consistent with the mixed-phase microphysical parameterization, has been developed for the French cloud resolving model MésoNH. There are four successive steps: (i) charge separation is assumed to result only from non-inductive processes; (ii) electrical charges carried by the different hydrometeor species are transported along the air flow and redistributed according to the microphysical processes; (iii) the electric field is deduced from the integration of a modified Poisson equation; (iv) a lightning parameterization simulates triggering, propagation and pseudo-fractal branching of the flashes and associated charge neutralization. Two numerical experiments are conducted firstly to evaluate the performances of the lightning scheme, secondly to test the simulated evolution of the electrical characteristics of a idealized supercellular storm. To cite this article: G. Molinié et al., C. R. Physique 3 (2002) 1305–1324.     

Modelling of long term kinetic evolution: a fruitful relationship between experiment and theoretical development

Recent developments in multi-scale modelling, based on atomic scale calculations, are leading to a growing conviction that modelling will soon be used to design material components for nuclear reactors. In this article we discuss this assumption on the basis of the relationship between experimental studies and theoretical calculations of the microstructural evolution of materials under irradiation. In the first part of the paper, the available numerical models for long term microstructural evolutions are briefly reviewed. The experimental methods are presented in a second part. In the third part, several examples of fruitful relationships between modelling and experiments are discussed. To cite this article: A. Barbu, C. R. Physique 9 (2008).     

Traumatic vocal fold avulsion injury in a newborn

A full-term newborn developed respiratory compromise in the immediate postparturition period requiring urgent intubation. Evaluation of post-extubation stridor later the same day revealed an avulsion injury extending from the left vocal fold into the lateral glottic musculature. Primary repair was accomplished with anatomic realignment of the torn vocal fold and muscle. Endotracheal intubation was utilized for stenting and the patient was extubated following 3 days of paralysis with sedation. Follow-up examination revealed a reparative granuloma, which was lasered. Eight-week follow-up examination revealed normal vocal fold architecture. At 18 months the patient continues to have a normal voice and normal laryngeal development.     

Respiratory function during speaking and singing in professional country singers

Respiratory function during speaking and singing was investigated in six male professional country singers. Function was studied using magnetometers to transduce anteroposterior diameter changes of the rib cage and abdomen while subjects performed various respiratory maneuvers, speaking activities, and singing activities. Results indicated that respiratory behavior during speaking was generally the same as that of other normal subjects. Respiratory behavior during singing resembled that of speaking. Discussion includes comparison of respiratory performance of present singers with untrained singers and classically trained singers. Implications are offered regarding how the results might be applied to the prevention of voice disorders by education and training of country singers.     

Comparison of direct and indirect calculations of laryngeal airway resistance in connected speech

Direct measures of subglottal pressure obtained through a tracheal puncture were used to calculate laryngeal airway resistance. Six subjects completed tasks including syllable trains and more natural speech samples produced at three loudness levels. Direct calculations of natural speech resistance values were compared with indirect estimates obtained during syllable train production. The degree of correspondence between direct and indirect calculations varied by subject. Overall, the smallest relative errors among calculations occurred for syllable trains, with higher relative errors for the monologue and sentence. For loudness conditions, the smallest and largest relative errors occurred for soft and loud productions, respectively. The clinical utility of indirect estimation is questioned and suggestions for improving its validity are provided.     

Vocalization and breathing during the first year of life

Vocalization and breathing were studied in 40 healthy infants, including five boys and five girls each at ages 5 weeks, 2.5 months, 6.5 months, and 12 months. Breathing was monitored through the use of a variable inductance plethysmograph that enabled estimates of the volume changes of the rib cage, abdomen, and lung, as well as estimates of selected temporal features of the breathing cycle. Four vocalization types were studied intensively. These included cries, whimpers, grunts, and syllable utterances. Breathing behavior was highly variable across the four vocalization types, demonstrating the degrees of freedom of performance available to the infant to accomplish the aeromechanical drive required. Such behavior was influenced by body length, body position, and age, but not by vocalization type and sex. The protocol established is a useful tool for observing the natural course of the emergence of vocalization and breathing during the first year of life.     

Time-reversed waves and super-resolution

Time-reversal mirrors (TRMs) refocus an incident wavefield to the position of the original source regardless of the complexity of the propagation medium. TRMs have now been implemented in a variety of physical scenarios from GHz microwaves to MHz ultrasonics and to hundreds of Hz in ocean acoustics. Common to this broad range of scales is a remarkable robustness exemplified by observations at all scales that the more complex the medium (random or chaotic), the sharper the focus. A TRM acts as an antenna that uses complex environments to appear wider than it is, resulting for a broadband pulse, in a refocusing quality that does not depend on the TRM aperture.Moreover, when the complex environment is located in the near field of the source, time-reversal focusing opens completely new approaches to super-resolution. We will show that, for a broadband source located inside a random metamaterial, a TRM located in the far field radiated a time-reversed wave that interacts with the random medium to regenerate not only the propagating but also the evanescent waves required to refocus below the diffraction limit. This focusing process is very different from that developed with superlenses made of negative index material only valid for narrowband signals. We will emphasize the role of the frequency diversity in time-reversal focusing. To cite this article: M. Fink et al., C. R. Physique 10 (2009).     

Discrete dislocation dynamics: an important recent break-through in the modelling of dislocation collective behaviour

Recent results obtained by 3D discrete Dislocation Dynamics (DD) simulations are reviewed. Firstly, in the case of fatigued AISI 316L stainless steel, it is shown how DD simulations can both explain the formation of persistent slip bands and give a criterion for crack initiation. The same study is performed in the case of precipitate hardened metals where the precipitate size plays a crucial role. Secondly, we show how molecular dynamics (MD) simulations can feed the DD simulations for two applications. The first concerns the modelling of BCC Fe for which the dislocation mobility is derived from MD simulations. The second considers the modelling of irradiated stainless steels (FCC), where MD is used to define the local rules of interactions between dislocations and Frank loops. To cite this article: M.C. Fivel, C. R. Physique 9 (2008).     

Equivalence of star products on a symplectic manifold; an introduction to Deligne''s ech cohomology classes

These notes grew out of the Quantisation Seminar 1997–1998 on Deligne's paper [P. Deligne, Déformations de l'algèbre des fonctions d'une variété symplectique: Comparison entre Fedosov et De Wilde, Lecomte, Selecta Math. (New Series) 1 (1995) 667–697] and the lecture of the first author in the Workshop on Quantisation and Momentum Maps at the University of Warwich in December 1997.We recall the definitions of the cohomology classes introduced by Deligne for equivalence classes of differential star products on a symplectic manfold and show the properties of the relations between these classes by elementary methods based on ech cohomology.     

Differences in voice quality between men and women: Use of the long-term average spectrum (LTAS)

The goal of this study was to determine if there are acoustical differences between male and female voices, and if there are, where exactly do these differences lie. Extended speech samples were used. The recorded readings of a text by 31 women and by 24 men were analyzed by means of the Long-term Spectrum (LTAS), extracting the amplitude values (in decibels) at intervals of 160 Hz over a range of 8 kHz. The results showed a significant difference between genders, as well as an interaction of gender and frequency level. The female voice showed greater levels of aspiration noise, located in the spectral regions corresponding to the third formant, which causes the female voice to have a more “breathy” quality than the male voice. The lower spectral tilt in the women's voices is another consequence of this presence of greater aspiration noise.     

The effects of dissolved oxygen upon amide proton relaxation and chemical shift in a perdeuterated protein

The effects of dissolved molecular oxygen upon amide proton ((1)H(N)) longitudinal and transverse relaxation rates and chemical shifts were studied for a small protein domain, the second type 2 module of fibronectin ((2)F2)-isotopically enriched to 99% (2)H, 98% (15)N. Longitudinal relaxation rate enhancements, R(O(2))((1)H(N)), of individual backbone (1)H(N) nuclei varied up to 14 fold between a degassed and oxygenated (1 bar) solution, indicating that the oxygen distribution within the protein is inhomogeneous. On average, smaller relaxation rate enhancements were observed for (1)H(N) nuclei associated with the core of the protein compared to (1)H(N) nuclei closer to the surface, suggesting restricted oxygen accessibility to some regions. In agreement with an O(2)-(1)H(N) hyperfine interaction in the extreme narrowing limit, the (1)H(N) transverse relaxation rates showed no significant change, up to an oxygen pressure of 9.5 bar (the maximum pressure used in this study). For most (1)H(N) resonances, small deltadelta(O(2))((1)H(N)) hyperfine chemical shifts could be detected between oxygen pressures of 1 bar and 9.5 bar.     

Nucleation problems in metallurgy of the solid state: recent developments and open questions

Nucleation processes play a key role in the microstructure evolution of metallic alloys during thermomechanical treatments. These processes can involve phase transformations (such as precipitation) and structural instabilities (such as recrystallisation). Although the word ‘nucleation’ is used in both cases, the situation is profoundly different for precipitation and for recrystallisation on which this article is focussed. In the case of precipitation, species are conserved and the underlying physics is stochastic fluctuations, allowing the apparition of critical germs of the new phase. In the case of recrystallisation, the underlying physical phenomenon is the progressive growth of subgrain structures leading to an unstable configuration, allowing a dislocation free grain to grow at the expense of a dislocated one. The two cases require different types of modelling which are presented in the article. To cite this article: Y. Bréchet, G. Martin, C. R. Physique 7 (2006).     

Polarized confocal theta microscopy

We propose a comprehensive treatment of theta microscopy based on dipole emission, which better describes fluorescence emission than the isotropic emission model, as fluorescence emission is often polarized. Formulas describing the point spread function for polarized confocal fluorescence theta microscopy are given. Examples are given and some advantages of polarized theta fluorescence microscopy are presented. To cite this article: O. Haeberlé et al., C. R. Physique 3 (2002) 1445–1450.     

Metamaterials and invisibility

Metamaterials have significantly extended the range of electromagnetic properties available to device designers. An interesting application of these new materials is to the problem of cloaking, where the goal is to render an object invisible to electromagnetic radiation within a certain frequency range. Here, I review the concepts behind recently-proposed invisibility cloaks, and the way in which metamaterials can allow these designs to be realized. To cite this article: B. Wood, C. R. Physique 10 (2009).