Asymptotic ideals (ideals in the ring of Colombeau generalized constants with continuous parametrization)

We study the asymptotics at zero of continuous functions on (0, 1] by means of their asymptotic ideals, i.e., ideals in the ring of continuous functions on (0, 1] satisfying a polynomial growth condition at 0 modulo rapidly decreasing functions at 0. As our main result, we characterize maximal and prime ideals in terms of maximal and prime filters

Annular interdigital transducer focuses piezoelectric surface waves to a single point

We propose and demonstrate experimentally the concept of the annular interdigital transducer that focuses acoustic waves on the surface of a piezoelectric material to a single, diffraction-limited, spot. The shape of the transducing fingers follows the wave surface. Experiments conducted on lithium niobate substrates evidence that the generated surface waves converge to the center of the transducer, producing a spot that shows a large concentration of acoustic energy. This concept is of practical significance to design new intense microacoustic sources, for instance for enhanced acouto-optical interactions

Modulation of the extraordinary optical transmission by surface acoustic waves

International audienceThe numerical study of periodically nanostructured metallic films exhibiting extraordinary optical transmission (EOT) deposited onto the top of a piezoelectric material is reported. Surface acoustic waves are generated in the piezoelectric substrate and their influence in the transmission spectrum of the EOT structure is studied. It is shown that low frequency acoustic waves can significantly tune the resonance frequency of the EOT structure

Investigations about the modelling of acoustic properties of periodic porous materials with the shift cell approach

The main advantage of designing sound packages with periodic arrangements is that they can provide a combination of absorption effects, resonance effects and wave interferences effects. This offers different applications in transportation (aeronautics, space, automotive, railway), energy and civil engineering sectors, where both weight and space, as well as vibroacoustic quality of performance and comfort, still remain as critical issues. The application of shift cell technique is presented and discussed for periodic porous media described with equivalent fluid models: it consists in a reformulation of classical Floquet-Bloch (F-B) conditions, whose major advantage stands in allowing the introduction of any frequency dependence of porous material behavior, through the resolution a quadratic eigenvalue problem, providing an efficient way to compute the dispersion curves of a porous material modelled as an equivalent fluid. The central part of this work shows the results, in terms of absorption coefficient and transmission loss curves, obtained through a numerical test campaign involving different melamine and polyurethane foams. The 48 test cases involve a cubic unit cell of porous material with a cylindrical inclusion. Furthermore, some absorption coefficient and transmission loss comparisons are shown, between a homogeneous unit cell and a unit cell with a perfectly rigid inclusion; the comparisons are carried out at fixed dimensions, then at fixed mass and then at fixed performance in the periodicity peak range. The results clearly point out the advantage of designing foam layer with periodic inclusion patterns in order to improve the performances in a specific range of frequencies, allowing a save both in terms of thickness and, most of all, mass, respect to a classical homogeneous foam layer

Design guidelines for the acoustic performance improvement of a periodic porous material

In this paper, some guidelines are provided in order to predict at which frequency the 1st performance peak (related to periodicity effects: half of the wavelength = periodicity dimension) appears, together with its amplitude, as functions of the unit cell dimensions. Conversely, also the link between the unit cell dimensions and the 1st performance peak amplitude as functions of the design frequency is shown. Furthermore, some additional guidelines are provided in order to predict at which frequency the 1st performance peak appears, together with its amplitude, as functions of the foam airflow resistivity

Heat fluxes over weak SST heterogeneity

The spatial variability of turbulence and surface heat flux are examined for the case of small air-surface temperature difference and modest sea-surface temperature variability. As a result of nonlinearities in the bulk formula, the heterogeneity is predicted to shift the area-averaged heat flux toward more significant upward values compared to that computed from the usual bulk formula and the area-averaged air-surface temperature difference. This prediction is supported by a case study analysis of aircraft data collected over heterogeneity of the sea-surface temperature. Traditional approaches for computing the surface flux are found to be unreliable over surface heterogeneity where the scales of the transport vary horizontally. A method based on a multiresolution wavelet transform reveals the spatial variability of the flux for different scales. This information together with several additional scale-dependent indices are used to select the range of scales included in the flux computation. This approach provides more reliable estimates of the spatial variation of the flux, although uncertainties remain. Required improvements in observation strategies are discussed for quantitative evaluation of the bulk formula in conditions of surface heterogeneity

Modified ultrafast thermometer UFT-M and temperature measurements during Physics of Stratocumulus Top (POST)

A modified UFT-M version of the ultrafast airborne thermometer UFT, aimed at in-cloud temperature measurements, was designed for the Physics of Stratocumulus Top (POST) field campaign. Improvements in its construction resulted in the sensor's increased reliability, which provided valuable measurements in 15 of the 17 flights. Oversampling the data allowed for the effective correction of the artefacts resulting from the interference with electromagnetic transmissions from on-board avionic systems and the thermal noise resulting from the sensor construction. The UFT-M records, when averaged to the 1.4 and 55 m resolutions, compared to the similar records of a thermometer in a Rosemount housing, indicate that the housing distorts even low-resolution airborne temperature measurements. Data collected with the UFT-M during the course of POST characterise the thermal structure of stratocumulus and capping inversion with the maximum resolution of ~1 cm. In this paper, examples of UFT-M records are presented and discussed

The Fermion Monte Carlo revisited

In this work we present a detailed study of the Fermion Monte Carlo algorithm (FMC), a recently proposed stochastic method for calculating fermionic ground-state energies [M.H. Kalos and F. Pederiva, Phys. Rev. Lett. vol. 85, 3547 (2000)]. A proof that the FMC method is an exact method is given. In this work the stability of the method is related to the difference between the lowest (bosonic-type) eigenvalue of the FMC diffusion operator and the exact fermi energy. It is shown that within a FMC framework the lowest eigenvalue of the new diffusion operator is no longer the bosonic ground-state eigenvalue as in standard exact Diffusion Monte Carlo (DMC) schemes but a modified value which is strictly greater. Accordingly, FMC can be viewed as an exact DMC method built from a correlated diffusion process having a reduced Bose-Fermi gap. As a consequence, the FMC method is more stable than any transient method (or nodal release-type approaches). We illustrate the various ideas presented in this work with calculations performed on a very simple model having only nine states but a full sign problem. Already for this toy model it is clearly seen that FMC calculations are inherently uncontrolled.Comment: 49 pages with 4 postscript figure

Computation of wave dispersion characteristics in periodic porous materials modeled as equivalent fluids

This paper starts with the presentation of the shift cell technique, which allows the description of the propagation of all existing waves starting from the unit cell through a quadratic eigenvalue problem. Its major advantage is that it allows the implementation of any frequency dependence and damping in the problem: this is a fundamental advantage when computing the dispersion curves of a porous material modeled as an equivalent fluid. The second part of this work concerns the investigation of the link between the dispersion curves and the acoustic properties of the material. Deriving the equivalent acoustic properties of the unit cell from its dispersion characteristics, indeed, could be a very efficient approach for designing the sound packages with a simple a preliminary eigenvalue analysis. Proceedings of ISMA 2018 - International Conference on Noise and Vibration Engineering and USD 2018 - International Conference on Uncertainty in Structural Dynamics. All rights reserved

Entrainment rates and microphysics in POST stratocumulus

The article of record as published may be located at http://dx.doi.org/10.1002/jgrd.50878An aircraft field study (POST; Physics of Stratocumulus Top) was conducted off the central California coast in July and August 2008 to deal with the known difficulty of measuring entrainment rates in the radiatively important stratocumulus (Sc) prevalent in that area. The Center for Interdisciplinary Remotely-Piloted Aircraft Studies Twin Otter research aircraft flew 15 quasi-Lagrangian flights in unbroken Sc and carried a full complement of probes including three high-data-rate probes: ultrafast temperature probe, particulate volume monitor probe, and gust probe. The probes’ colocation near the nose of the Twin Otter permitted estimation of entrainment fluxes and rates with an in-cloud resolution of 1m. Results include the following: Application of the conditional sampling variation of classical mixed layer theory for calculating the entrainment rate into cloud top for POST flights is shown to be inadequate for most of the Sc. Estimated rates resemble previous results after theory is modified to take into account both entrainment and evaporation at cloud top given the strong wind shear and mixing at cloud top. Entrainment rates show a tendency to decrease for large shear values, and the largest rates are for the smallest temperature jumps across the inversion. Measurements indirectly suggest that entrained parcels are primarily cooled by infrared flux divergence rather than cooling from droplet evaporation, while detrainment at cloud top causes droplet evaporation and cooling in the entrainment interface layer above cloud top.NSF supported H. Gerber, G. Frick, and S. Malinowski (ATM-0735121, AGS-1020445), D. Khelif (ATM-0734323), and S. Krueger (ATM-0735118). The Office of Naval Research and the Naval Postgraduate School supported in part the deployment of the Twin Otter aircraft