Curvature singularity and film-skating during drop impact

We study the influence of the surrounding gas in the dynamics of drop impact on a smooth surface. We use an axisymmetric 3D model for which both the gas and the liquid are incompressible; lubrication regime applies for the gas film dynamics and the liquid viscosity is neglected. In the absence of surface tension a finite time singularity whose properties are analysed is formed and the liquid touches the solid on a circle. When surface tension is taken into account, a thin jet emerges from the zone of impact, skating above a thin gas layer. The thickness of the air film underneath this jet is always smaller than the mean free path in the gas suggesting that the liquid film eventually wets the surface. We finally suggest an aerodynamical instability mechanism for the splash.Comment: 5 figure

On alpha stable distribution of wind driven water surface wave slope

We propose a new formulation of the probability distribution function of wind driven water surface slope with an $\alpha$-stable distribution probability. The mathematical formulation of the probability distribution function is given under an integral formulation. Application to represent the probability of time slope data from laboratory experiments is carried out with satisfactory results. We compare also the $\alpha$-stable model of the water surface slopes with the Gram-Charlier development and the non-Gaussian model of Liu et al\cite{Liu}. Discussions and conclusions are conducted on the basis of the data fit results and the model analysis comparison.Comment: final version of the manuscript: 25 page

Experiments on wind-perturbed rogue wave hydrodynamics using the Peregrine breather model

International audienceBeing considered as a prototype for description of oceanic rogue waves (RWs), the Peregrine breather solution of the nonlinear Schrodinger equation (NLS) has been recently observed and intensely investigated experimentally in particular within the context of water waves. Here, we report the experimental results showing the evolution of the Peregrine solution in the presence of wind forcing in the direction of wave propagation. The results show the persistence of the breather evolution dynamics even in the presence of strong wind and chaotic wave eld generated by it. Furthermore, we have shown that characteristic spectrum of the Peregrine breather persists even at the highest values of the generated wind velocities thus making it a viable characteristic for prediction of rogue waves

In-training assessment using direct observation of single-patient encounters: a literature review

We reviewed the literature on instruments for work-based assessment in single clinical encounters, such as the mini-clinical evaluation exercise (mini-CEX), and examined differences between these instruments in characteristics and feasibility, reliability, validity and educational effect. A PubMed search of the literature published before 8 January 2009 yielded 39 articles dealing with 18 different assessment instruments. One researcher extracted data on the characteristics of the instruments and two researchers extracted data on feasibility, reliability, validity and educational effect. Instruments are predominantly formative. Feasibility is generally deemed good and assessor training occurs sparsely but is considered crucial for successful implementation. Acceptable reliability can be achieved with 10 encounters. The validity of many instruments is not investigated, but the validity of the mini-CEX and the ‘clinical evaluation exercise’ is supported by strong and significant correlations with other valid assessment instruments. The evidence from the few studies on educational effects is not very convincing. The reports on clinical assessment instruments for single work-based encounters are generally positive, but supporting evidence is sparse. Feasibility of instruments seems to be good and reliability requires a minimum of 10 encounters, but no clear conclusions emerge on other aspects. Studies on assessor and learner training and studies examining effects beyond ‘happiness data’ are badly needed

A new approach to estimation of global air-sea gas transfer velocity fields using dual-frequency altimeter backscatter

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C11003, doi:10.1029/2006JC003819.A new approach to estimating air-sea gas transfer velocities based on normalized backscatter from the dual-frequency TOPEX and Jason-1 altimeters is described. The differential scattering of Ku-band (13.6 GHz) and C-band (5.3 GHz) microwave pulses is used to isolate the contribution of small-scale waves to mean square slope and gas transfer. Mean square slope is derived for the nominal wave number range 40–100 rad m−1 by differencing mean square slope estimates computed from the normalized backscatter in each band, using a simple geometric optics model. Model parameters for calculating the differenced mean square slope over this wave number range are optimized using in situ optical slope measurements. An empirical relation between gas transfer velocity and mean square slope, also based on field measurements, is then used to derive gas transfer velocities. Initial results demonstrate that the calculated transfer velocities exhibit magnitudes and a dynamic range which are generally consistent with existing field measurements. The new algorithm is used to construct monthly global maps of gas transfer velocity and to illustrate seasonal transfer velocity variations over a 1-year period. The measurement precision estimated from >106 duplicate observations of the sea surface by TOPEX and Jason-1 altimeters orbiting in tandem is better than 10%. The estimated overall uncertainty of the method is ±30%. The long-term global, area-weighted, Schmidt number corrected, mean gas transfer velocity is 13.7 ± 4.1 cm h−1. The new approach, based on surface roughness, represents a potential alternative to commonly used parameterizations based on wind speed.Financial support for this research from the National Aeronautics and Space Administration through Jet Propulsion Laboratory contract 961425 and the NOAA Global Carbon Cycle Program under grant NA16GP2918, Office of Global Programs is gratefully acknowledged

LES INTERACTIONS ENTRE LES TRANSFERTS RADIATIFS ET TURBULENTS DE CHALEUR DANS LES BASSES COUCHES DE L'ATMOSPHERE CLAIRE

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La physique de la turbulence. Exposé de synthèse

L'exposé introductif rappelle tout d'abord les caractéristiques générales de la turbulence, phénomène à prévoir, mais aussi outil à utiliser. Puis sont présentées les idées récentes sur la transition et sa prévision, la dynamique de la turbulence homogène à deux ou trois dimensions, les structures cohérentes et leur rôle, l'organisation des zones à forts gradients et les progrès récents dans la description des couches limites turbulentes. Enfin, quelques interactions entre turbulence dynamique et autres processus physiques dans des situations complexes sont évoquées

Étude sur le fil chaud et le film chaud dans l'eau

The research carried out on this subject has helped in the development of methods of measurïng mean water flow velocities and turbulent fluctuations by means of hot wire and film anemometers to within a similar degree of accuracy to that attainable in air. For this type of measurement, it is ncecssary to know the cooling relationship for the sensitive element, which is a relationship between dimensionless Nusselt, Reynolds, Prandtl and Grashof numbers. Various difIiculties arise in the use of this type of probe in water, which can only be suceessfully oversome by special manufacturing methods, appropriate geometrical design features and by designing for low heating coefficients. Fluid temperature variations are liable to cause considerable error. The experimental rig comprised a tubular water tunnel for calibration and check measurements in turbulent flow of know characteristics, pressure measuring instrumentation, a constant-temperature anemometer and various items of auxiliary equipment. Fairly satisfactory results were obtained with bare hot wire pick-ups with low regular drift thanks to a specially developed manufacturing method. Various types of industrially produced hot-film anemometer with quartz insulation were also studied and calibrated and drift correction methods suggested. The relationship between the cooling of a conical film, flow velocity and water temperature was investigated. In the range of values covered, the Nusselt number seemed to solely depend on the Péclet number, and approximately on velocity. The relationship appears to be of the form P = D + C Pén = D + C' Vn, with the exponent n working out at 0.265. Turbulence strength and spectrum measurements were carried out to check the method in fully developed flow discharging from the cylindrical water tunnel with Reynold numbers varying between 15 000 and 150 000. The results agreed well with those of previous measurements in dynamically similar air f1ows. Summing up it seems that, with careful use, a hot wire or hot film anemometer can produce comparably accurate measurement results in water as in air

The use of N-Sulfenylimines in the beta-lactam synthon method: Staudinger reaction, oxidation of the cycloadducts and ring opening of beta-lactams .

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