Stealth and equiluminous materials for scattering cancellation and wave diffusion

[EN] We report a procedure to design two-dimensional acoustic structures with prescribed scattering properties. The structures are designed from targeted properties in the reciprocal space so that their structure factors, i.e. their scattering patterns under the Born approximation, exactly follow the desired scattering properties for a set of wavelengths. The structures are made of a distribution of rigid circular cross-sectional cylinders embedded in air. We demonstrate the efficiency of the procedure by designing two-dimensional stealth acoustic materials with broadband back-scattering suppression independent of the angle of incidence and equiluminous acoustic materials exhibiting broadband scattering of equal intensity also independent of the angle of incidence. The scattering intensities are described in terms of both single and multiple scattering formalisms, showing excellent agreement with each other, thus validating the scattering properties of each material.This work has been funded by the project Conseil Regional des Pays de la Loire HYPERMETA under the program Etoiles Montantes of the Region Pays de la Loire, by the project Agence Nationale de la Recherche ANR-RGC METARoom [grant number (ANR-18-CE08-0021)] and by the project PID2020112759GB-I00 of the Ministerio de Ciencia e Innovacion.Kuznetsova, S.; Groby, JP.; García-Raffi, LM.; Romero-García, V. (2021). Stealth and equiluminous materials for scattering cancellation and wave diffusion. Waves in Random and Complex Media. https://doi.org/10.1080/17455030.2021.194863

Evidences of evanescent Bloch waves in Phononic Crystals

We show both experimentally and theoretically the evanescent behaviour of modes in the Band Gap (BG) of finite Phononic Crystal (PC). Based on experimental and numerical data we obtain the imaginary part of the wave vector in good agreement with the complex band structures obtained by the Extended Plane Wave Expansion (EPWE). The calculated and measured acoustic field of a localized mode out of the point defect inside the PC presents also evanescent behaviour. The correct understanding of evanescent modes is fundamental for designing narrow filters and wave guides based on Phononic Crystals with defects.Comment: 8 pages, 3 figure

URANS Analysis of a Launch Vehicle Aero-Acoustic Environment

[EN] Predicting and mitigating acoustic levels become critical because of the harsh acoustic environment during space vehicle lift-off. This paper aimed to study the aero-acoustic environment during a rocket lift-off. The sound propagation within a launch event was studied using dedicated computational fluid dynamics (CFD). The resolution of all the phenomena that occur is unfeasible. We discuss the turbulence simplification and propose a feasible simulation through an unsteady Reynolds-averaged Navier¿Stokes (URANS) model. The results were validated with experimental data showing a good correlation near the fairing surface and an improvable accuracy in the far field. To assess noise generation, the main shock waves were identified, and the evolution of the generated sound pressure was assessed. Moreover, vertical directivity was revealed by data analysis of the pressure field surrounding the fairing.This research was funded by the European Space Agency of Project REDLAUCH: Launch Sound Level Reduction under contract 4000126316/19/NL/LvH. The work was supported by the MICINN (grants: DIN2019-010877 and RTI2018-102256-B-100) and by the Barcelona Supercomputing Center under Project IM-2021-2-0017 Rocket launch aeroacoustics.Escartí-Guillem, MS.; García-Raffi, LM.; Hoyas, S. (2022). URANS Analysis of a Launch Vehicle Aero-Acoustic Environment. Applied Sciences. 12(7):1-9. https://doi.org/10.3390/app120733561912

Kaplan-Meier type survival curves for COVID-19: a health data based decision-making tool

Countries are recording health information on the global spread of COVID-19 using different methods, sometimes changing the rules after a few days. They are all publishing the number of new individuals infected, cured and dead, along with some supplementary data. These figures are often recorded in a non-uniform manner and do not match the standard definitions of these variables. However, in this paper we show that the Kaplan-Meier curves calculated with them could provide useful information about the dynamics of the disease in different countries. Our aim is to present a robust and simple model to show certain characteristics of the evolution of the dynamic process, showing that the differences of evolution among the countries is reflected in the corresponding Kaplan-Meier-type curves. We compare the curves obtained for the most affected countries so far, proposing possible interpretations of the properties that distinguish them.Comment: 12 pages, 6 figure

Rocket plume URANS simulation using OpenFOAM

[EN] A key aspect in the launch environment of a space vehicle is the generation and propagation of noise, due to its effect in the payload. Obtaining experimental measures is an extremely difficult, so CFD techniques appear as a useful tool to estimate both the spatial location of noise sources and their spectral power. In this paper, a numerical simulation has been carried out to simulate the supersonic impingement of the plume jet generated by the engine and the corresponding acoustic load generated in the launch platform of the VEGA rocket using a URANS model in OpenFOAM.This work was partially funded by MINECO / FEDER, under project TI2018-102256-B-I00.Escartí-Guillem, MS.; Hoyas, S.; García-Raffi, LM. (2019). Rocket plume URANS simulation using OpenFOAM. Results in Engineering. 4:1-3. https://doi.org/10.1016/j.rineng.2019.100056S13

Sharp acoustic vortex focusing by Fresnel-spiral zone plates

[EN] We report the optimal focusing of acoustic vortex beams by using flat lenses based on a Fresnelspiral diffraction grating. The flat lenses are designed by spiral-shaped Fresnel zone plates composed of one or several arms. The constructive and destructive interferences of the diffracted waves by the spiral grating result in sharp acoustic vortex beams, following the focal laws obtained in analogy with the Fresnel zone plate lenses. In addition, we show that the number of arms determines the topological charge of the vortex, allowing the precise manipulation of the acoustic wave field by flat lenses. The experimental results in the ultrasonic regime show excellent agreement with the theory and full-wave numerical simulations. A comparison with beam focusing by Archimedean spirals also showing vortex focusing is given. The results of this work may have potential applications for particle trapping, ultrasound therapy, imaging, or underwater acoustic transmitters.This work was supported by the Spanish Ministry of Economy and Innovation (MINECO) and European Union FEDER through Project Nos. FIS2015-65998-C2-1 and FIS2015-65998-C2-2. N.J. acknowledges financial support from Generalitat Valenciana through Grant No. APOSTD-2017-042.Jimenez, N.; Romero García, V.; García-Raffi, LM.; Camarena Femenia, F.; Staliunas, K. (2018). Sharp acoustic vortex focusing by Fresnel-spiral zone plates. Applied Physics Letters. 112(20):204101-1-204101-5. https://doi.org/10.1063/1.5029424S204101-1204101-511220J. Nye and M. Berry ,Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences(The Royal Society, 1974), Vol.336, pp. 165–190.Grier, D. G. (2003). A revolution in optical manipulation. Nature, 424(6950), 810-816. doi:10.1038/nature01935Volke-Sepúlveda, K., Santillán, A. 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