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In this paper an extension of the theoretical model of Molin (J. Fluid Mech., vol. 430, 2001, pp. 27–50) is proposed, where the assumptions of infinite depth and infinite horizontal extent of the support are released. The fluid domain is decomposed into two subdomains: the moonpool (or the gap) and a lower subdomain bounded by the seafloor and by an outer cylinder where the linearized velocity potential is assumed to be nil. Eigenfunction expansions are used to describe the velocity potential in both subdomains. Garrett’s method is then applied to match the velocity potentials at the common boundary and an eigenvalue problem is formulated and solved, yielding the natural frequencies and associated modal shapes of the free surface. Applications are made, first in the case of a circular moonpool, then in the rectangular gap and moonpool cases. Based on so-called single-mode approximations, simple formulas are proposed that give the resonant frequencies.
The Herschel Dwarf Galaxy Survey investigates the interplay of star formation activity and the the metal-poor gas and dust of local universe dwarf galaxies using FIR and submillimetre imaging spectroscopic and photometric observations in the 50 to 550 μm window of the Herschel Space Observatory. The dust spectral-energy distributions are well constrained with the new Herschel and MIR Spitzer data. A submillimetre excess is often found in low metallicity galaxies, which, if tracing very cold dust, would highlight large dust masses not easily reconciled in some cases, given the low metallicities and expected gas-to-dust mass ratios. The galaxies are also mapped in the FIR fine-structure lines (63 and 145 μm OI, 158 μm CII, 122 and 205 μm NII, 88 μm OIII) probing the low density ionised gas, the HII regions and photodissociation regions. While still early in the mission we can already see, along with earlier studies, that line ratios in the metal-poor ISM differ remarkably from those in the metal-rich starburst environments. In dwarf galaxies, L[CII]/L(CO) (≥104) is at least an order of magnitude greater than in the most metal-rich starburst galaxies. The 88 μm [OIII] line usually dominates the FIR line emission over galaxy-wide scales, not the 158 μm [CII] line which is the dominant FIR cooling line in metal-rich galaxies. All of the FIR lines together can contribute 1% to 2% of the LTIR. The Herschel Dwarf Galaxy survey will provide statistical information on the nature of the dust and gas in low metallicity galaxies and place constraints on chemical evolution models of galaxies.
We present preliminary results of a 4-month campaign carried out in the framework of the Mons project, where time-resolved Hα observations are used to study the wind and circumstellar properties of a number of OB stars.
We present the results from the spectroscopic follow-up of WR140 (WC7 + O4-5) during its last periastron passage in january 2009. This object is known as the archetype of colliding wind binaries and has a relatively large period (≃8 years) and eccentricity (≃0.89). We provide updated values for the orbital parameters, new estimates for the WR and O star masses and new constraints on the mass-loss rates.
Results from experiments on wave interaction with a rigid vertical plate are reported. The 5m long plate is set against the wall of a 30m wide basin, at 100m from the wavemaker. This set-up is equivalent to a 10m plate in the middle of a 60m wide basin. Regular waves are produced, with wavelengths of 1.6m, 1.8m and 2m, and steepnesses H/L (H being the double amplitude and L being the wavelength) ranging from 2% to 5%. Free-surface elevations along the plate are measured with a row of 20 gauges. The focus is on the time evolution of the free-surface profile along the plate. At all steepnesses, strong deviations from the predictions of linear theory gradually take place as the reflected wave field develops in the basin. This phenomenon is attributed to third-order interactions between the incoming and reflected wave systems, on the weather side of the plate. The measured profiles along the plate are compared with the predictions of two numerical models: an approximate model based on the tertiary interaction theory of Longuet-Higgins & Phillips (J. Fluid Mech., vol. 12, 1962, p. 333) for plane waves, which provides a steady-state solution, and a fully nonlinear numerical wavetank based on extended Boussinesq equations. In most of the experimental tests, despite the large distance from the wavemaker to the plate and the small amplitude of the incident wave, no steady state is attained by the end of the exploitable part of the records.
Fatigue design of structures for high temperature service
resorts to modelling based on elasto-viscoplasticity constitutive
laws. The new model describes complex anisothermal cyclic
loadings, accounting for viscosity on a large range of strain.
The new constitutive law that has been developed for nodular
cast iron, is intended for the design of automotive parts for
high temperature service.
The snow surface roughness at centimetre and millimetre scales is an important parameter related to wind transport, snowdrifts, snowfall, snowmelt and snow grain size. Knowledge of the snow surface roughness is also of high interest for analyzing the signal from radar sensors such as SAR, altimeters and scatterometers. Unfortunately, this parameter has seldom been measured over snow surfaces. The techniques used to measure the roughness of other surfaces, such as agricultural or sand soils, are difficult to implement in polar regions because of the harsh climatic conditions. In this paper we develop a device based on a laser profiler coupled with a GPS receiver on board a snowmobile. This instrumentation was tested successfully in midre Lovénbreen, Svalbard, in April 2006. It allowed us to generate profiles of 3 km sections of the snow-covered glacier surface. Because of the motion of the snowmobile, the roughness signal is mixed with the snowmobile signal. We use a distance/frequency analysis (the empirical mode decomposition) to filter the signal. This method allows us to recover the snow surface structures of wavelengths between 4 and 50 cm with amplitudes of >1 mm. Finally, the roughness parameters of snow surfaces are retrieved. The snow surface roughness is found to be dependent on the scales of the observations. The retrieved RMS of the height distribution is found to vary between 0.5 and 9.2 mm, and the correlation length is found to be between 0.6 and 46 cm. This range of measurements is particularly well adapted to the analysis of GHz radar response on snow surfaces.
The effect of spatial variations in ice thickness, accumulation rate and lateral flow divergence on radar-detected isochrone geometry in ice sheets is computed using an analytical method, under assumptions of a steady-state ice-sheet geometry, a steady-state accumulation pattern and a horizontally uniform velocity shape function. By using a new coordinate transform, we show that the slope of the isochrones (with a normalized vertical coordinate) depends on three terms: a principal term which determines the sign of the slope, and two scale factors which can modify only the amplitude of the slope. The principal term depends only on a local characteristic time (ice thickness divided by accumulation rate minus melting rate) between the initial and final positions of the ice particle. For plug flow, only the initial and final values have an influence. Further applications are a demonstration of how the vertical velocity profile can be deduced from sharp changes in isochrone slopes induced by abrupt steps in bedrock or mass balance along the ice flow. We also demonstrate ways the new coordinate system may be used to test the accuracy of numerical flow models.
Results from experiments on wave interaction with a rigid plate are reported. The plate is projected from one of the sidewalls of the basin. The sidewall acts as a plane of symmetry, thereby doubling the widths of the plate and of the basin. The tests are carried out in regular waves of varying periods and steepnesses. At wavelengths comparable with the width of the plate, strong run-ups are observed at the plate–wall intersection, increasing with the wave steepness. These run-ups take many wave cycles to develop, with no steady state being reached in some cases. It is advocated that these phenomena result from third-order interactions between the incident and reflected wave fields, over a wide area on the weather side of the plate. A theoretical model is proposed, based on tertiary wave interaction. A parabolic equation is derived that describes the transformation of the incoming waves through their interaction with the reflected wave field. A steady-state solution is obtained through iterations. Results from the theoretical model are compared with the experimental data, with good agreement.
Deconvolution is a necessary tool for the exploitation of adaptive optics
corrected images, because the correction is partial. The Maximum A
Posteriori (MAP) framework is used to derive a deconvolution method
combines the data with our knowledge of the noise statistics as well as our
prior information about the object and the variability of the Point Spread
Function. The deconvolution of experimental and scientific data illustrates the capabilities
of this method.
Circumstellar disks from early accretion stages to debris disks motivate an
active theoretical and observational research. Their physics is quite complex and implies the
study of various domains such as the physics of grains, radiative transfert at various
wavelengths, collisional and radiative destructive processes, dynamics, long term evolution...
Such disks provide valuable information on the conditions of exoplanet formation, but also
on the interaction between disks and already formed planets;
they may be in some cases the indicators of the presence of planets
or, on contrary, they may prevent from direct planet detections depending on the observational technique.
We review the current most critical issues relative to disk studies, and discuss the need
and interest for various observational techniques. We conclude on the importance for high
dynamic observations and typical specifications.
Highly performing adaptive optical systems are required for next
generation giant telescopes as well as next generation
instrumentation for 10 m-class telescopes. Different types of AO
systems are currently under study, including Multi-Conjugate AO
(MCAO), high dynamic range AO, and low-order AO for distributed
partial correction AO. These systems require a large variety of
deformable mirrors with very challenging parameters. The
development of new technologies based on
micro-opto-electro-mechanical systems (MOEMS) is promising for
future deformable mirrors. We are currently developing a MDM based
on an array of electrostatic actuators with attachments to a
continuous mirror on top. The most challenging building block for
this device is the high optical quality mirror. Our first results
show a very efficient planarization of the surface with our
design. The integration of this mirror surface on top of an
actuator array is under investigation.
Hot massive stars constitute an important class of objects among
stellar populations. Diffraction limited high dynamic imaging
(HDI) with 8-10 m class telescopes of fine structures which often
characterize their extended nebulosities can trace back the
history of their mass-loss, therefore the evolution of the central
star across the HRD. Since these stars are among the most luminous
stars and display unique emission spectral features in galaxies at
high redshift, they can serve as natural candles to estimate
cosmological distances through the Wind-momentum Luminosity
Relation (WLR, Kudritzki et al. 2000). In this
paper I develop the context of massive stars HDI and derive some
general instrumental specifications for coronagraphic adaptive
Improved knowledge of geometrical boundary conditions, such as bedrock geometry and surface topography, can contribute significantly to glaciological studies including ice-sheet-flow modelling. Precise thickness and altimetric data allow an estimation of ice-flow direction, the balance velocity and the basal shear stress. These parameters are calculated along a 1160 km profile in East Antarctica using a relationship between shear stress, basal temperature, the Glen flow exponent and a parameter related to strain rate. Strong variations of the flow-law parameters and basal conditions are found to play a major role in the ice-flow pattern. Sliding, anisotropy and longitudinal stress strongly perturb the validity of the law, but their signature can be identified.
A study has been carried out on the acid soluble fraction (hydrosysis at pH 1.5, 3N HCl, 20 % KCl) of organic matter (OM) in the highly calcareous lacustrine sediments of the Jura region, France. Amino acids (AA's) were quantified and determined ninhydrin photometry and HPLC respectively. There was a high degree of interference by calcium in both procedures. The mechanism of interference is explained and methodology adopted to overcome it, by removal of calcium, are dealt with. The results of both methods are compared.