Le flux d'énergie en mécanique: Application aux turbo-machines

Summary The concept of flux of energy, first introduced in electrodynamics byJ. H. Poynting, can be extended to the domain of fluid-mechanics. It conduces to recognize in the flux of energy for a continuous portion of matter two parts: the first one, which can be called ‘transported’, corresponds to the energy localized in the matter and carried with it as a consequence of its displacement; the other results from the work developed by the stresses and can be called ‘transmitted’. The expression of this flux of energy in the general case of variable motion of a compressible fluid introduces the concept of enthalpy. Its application to an extension ofHelmholtz's theorem on the conservation of vorticity in a perfect fluid, according which bound vortexes have a definite relative motion across the fluid, contributes to explain the mechanism by which, even in the case of grids with a large pitch: chord ratio (Kaplan turbines, screw propellers), the energy of all parts of fluid is finally transmitted to the moving blades.      

Diode-pumped Yb:Sr(3)Y(BO(3))(3) femtosecond laser

We have developed a diode-pumped Yb(3+)Sr(3)Y(BO(3))(3) (Yb:BOYS) laser generating 69-fs pulses, at a central wavelength of 1062 nm. This laser is mode locked by use of a semiconductor saturable-absorber mirror and emits 80 mW of average power at 113 MHz. This is, to our knowledge, the first mode-locked Yb:BOYS laser and the shortest duration obtained from an ytterbium laser with a crystalline host. The central wavelength can be tuned from 1051 to 1070 nm, for sub-100-fs pulses. We have also achieved an average power as high as 300 mW with pulse duration of 86 fs at 1068 nm.     

Determination of the porosities of monolithic columns by inverse size-exclusion chromatography

Retention data of polystyrene samples of narrow molecular size distribution and known average molecular mass were measured on several monolithic columns (Chromolith Performance, Merck) and one conventional packed column (Luna C18, Phenomenex) by size-exclusion chromatography. These data were used to determine the external, the internal, and the total porosities of these columns. These data provided also information on the pore-size distribution of the adsorbent medium. The external and the total porosities of these columns are much higher than those of conventional packed columns. The results illustrate the profound changes brought by monolithic columns to the balance of the hydrodynamic and the mass transfer kinetic properties of chromatographic columns. Classical methods of comparison between column performance must be re-evaluated.     

Experimental validation of an affinity energy distribution calculated with the expectation maximization method

The difference between the average energies of the high-energy modes of the adsorption energy distributions of (S)-alprenolol and (R)-alprenolol on a chiral stationary phase calculated by the expectation maximization method agree well with the difference between the adsorption energies of these two compounds measured by isothermal titration calorimetry.     

Investigation of the optimum optical design for pulsed-laser crossed-beam thermal lens spectrometry in infinite and finite samples

Optimization of the optical design for pulsed-laser crossed-beam thermal lens (PLCBTL) spectrometry has been investigated. Experiments have been carried out with large samples as well as for very small samples in a microchannel and using different lens combinations to focus the probe and excitation beams. The results have been interpreted in terms of the influence of the excitation beam size as well as the degree of mode-mismatching of the excitation and probe beams on the optimum sample position and on the amplitude and decay of the photothermal signal. A semi-empirical formula that describes the influence of the sample position with respect to the probe beam waist has been established. We have shown that the optimum signal is inversely proportional to the waist of the excitation beam and is independent of the sample size as long as the size of the excitation beam is smaller than the microchannel. Time-resolved experiments have also shown that when the excitation beam is smaller than the sample, the signal decay depends not only on the size of the excitation beam but also on the mode-mismatching factor. Otherwise, the temporal characteristics are closely related to the size of the microchannel.     

Alkylated isocoumarins from Pituranthos scoparius

Two isocoumarins have been isolated from the ethyl acetate extract of the root of Pituranthos scoparius: 3-n-propyl-5-methoxy-6-hydroxy-isocoumarin and 3-n-propyl-5,7-dimethoxy-6-hydroxy-isocoumarin. Their structures were assigned by spectral analysis.     

Use of gel-casting to prepare HPLC monolithic silica columns with uniform mesopores and tunable macrochannels

Monolithic silica columns with defined mesopore and a tunable macropore structure were fabricated by the gel-casting of ordered mesoporous silica precursors.     

A Combinatorial Character Formula for Some Highest Weight Modules

We give a combinatorial formula for the weight multiplicities of some infinite-dimensional highest weight (n)-modules. Our proof, which does not rely on Kazhdan–Lusztig combinatorics, uses a reduction to finite characteristics. The character formula for the corresponding modular representations, which has been computed in a 1997 preprint by the authors, is based on a dual pair which has no obvious counterpart in characteristic zero.     

High-repetition-rate 300-ps pulsed ultraviolet source with a passively Q-switched microchip laser and a multipass amplifier

We demonstrate a new kind of picosecond laser source in the UV at a high repetition rate of -45 kHz , using only passive, compact, and simple elements. This system is based on a microchip laser and a very efficient multipass amplifier, both pumped with recently developed high-brightness laser diodes. The system has been optimized to deliver, at a high repetition rate, subnanosecond pulses at the wavelength 355 nm with an energy per pulse of close to 1 muJ (38-mW average power). This source is to our knowledge the first totally passive 300-ps UV laser source at this high repetition rate.