Lluís M. García-Raffi

Broadband quasi perfect absorption using chirped multi-layer porous materials

This work theoretically analyzes the sound absorption properties of a chirped multi-layer porous material including transmission, in particular showing the broadband unidirectional absorption properties of the system. Using the combination of the impedance matching condition and the balance between the leakage and the intrinsic losses, the system is designed to have broadband unidirectional and quasi perfect absorption. The transfer and scattering matrix formalism, together with numerical simulations based on the finite element method are used to demonstrate the results showing excellent agreement between them. The proposed system allows to construct broadband sound absorbers with improved absorption in the low frequency regime using less amount of material than the complete bulk porous layer.

Nonlinear propagation and control of acoustic waves in phononic superlattices

Abstract The propagation of intense acoustic waves in a one-dimensional phononic crystal is studied. The medium consists in a structured fluid, formed by a periodic array of fluid layers with alternating linear acoustic properties and quadratic nonlinearity coefficient. The spacing between layers is of the order of the wavelength, therefore Bragg effects such as band gaps appear. We show that the interplay between strong dispersion and nonlinearity leads to new scenarios of wave propagation. The classical waveform distortion process typical of intense acoustic waves in homogeneous media can be strongly altered when nonlinearly generated harmonics lie inside or close to band gaps. This allows the possibility of engineer a medium in order to get a particular waveform. Examples of this include the design of media with effective (e.g., cubic) nonlinearities, or extremely linear media (where distortion can be canceled). The presented ideas open a way towards the control of acoustic wave propagation in nonlinear regime.

Strongly focused vortex beams by using flat Fresnel-spiral lenses

We report geometrically-optimal diffraction gratings for sharp vortex beam focusing using Fresnel-spiral curves. The lenses are built based on the Fresnel-spiral, a spiral curve that combine the focusing properties of Fresnel zone plates and the phase dislocations produced by spiral gratings. On the one hand, the constructive and destructive interferences between open and opaque zones in the grating, in analogy to the Fresnel zone plate, allow sharp beam focusing. On the other hand, the spiral shape of the grating retains the helicity, rotating the phase of the diffracted waves and creating a phase dislocation along the axis. This allows the generation of geometrically optimal focused vortex beams, enhancing the field intensity at the focus up to 170 times. In particular, this system offers a tunable topological charge of the vortex beam by using different arms in the Fresnel-spiral diffraction grating, being the topological charge equal to the number of arms. Two different Fresnel-spiral diffraction gratings with topological charge of 1 and 5 are experimentally tested showing excellent agreement with theory and simulations. These diffraction gratings will allow the design of effective wave-matter interaction systems, with direct applications in industry and biomedical engineering.We report geometrically-optimal diffraction gratings for sharp vortex beam focusing using Fresnel-spiral curves. The lenses are built based on the Fresnel-spiral, a spiral curve that combine the focusing properties of Fresnel zone plates and the phase dislocations produced by spiral gratings. On the one hand, the constructive and destructive interferences between open and opaque zones in the grating, in analogy to the Fresnel zone plate, allow sharp beam focusing. On the other hand, the spiral shape of the grating retains the helicity, rotating the phase of the diffracted waves and creating a phase dislocation along the axis. This allows the generation of geometrically optimal focused vortex beams, enhancing the field intensity at the focus up to 170 times. In particular, this system offers a tunable topological charge of the vortex beam by using different arms in the Fresnel-spiral diffraction grating, being the topological charge equal to the number of arms. Two different Fresnel-spiral diffraction grat...

Design and Implementation of a Tool for Analysing Student Products When They Solve Fermi Problems

In this chapter, we present a tool for analysing the work of secondary-level students from two different schools when they solve a type of Fermi problem. The tool is based on the characterisation of the concepts, procedures and language used to construct the models. Our results show that the proposed tool is useful to describe the models and to distinguish different aspects between the models produced by students without any previous modelling experience and those obtained by students who were already acquainted with working on modelling activities.

Modelización de sistemas elásticos continuos mediante el Método de Elementos Finitos

Propagation of mechanical waves in unidimensional systems is a fundamental part of physics, necessary for learning subjects such as acoustics and vibrations. The vibration of transverse waves in strings is the easiest case of elastic system. Usually, this is the first continuous elastic system in which students apply fundamental mathematical concepts as vibration mode, equation of motion and boundary condition. In this work the use of simulation methods is proposed to reinforce the understanding of vibratory and acoustic simple phenomena. This will be applied to the case of a string, a beam and a membrane of finite length with different physical characteristics and boundary conditions. La propagación de ondas mecánicas en sistemas unidimensionales es una parte fundamental de la f́ısica, necesaria para el aprendizaje de asignaturas como acústica y vibraciones. La vibración de ondas transversales en cuerdas es el caso más sencillo de sistema elástico. Habitualmente, este es el primer sistema elástico continuo en el cual los alumnos aplican conceptos matemáticos fundamentales como modo de vibración, ecuación de movimiento y condición de contorno. En este trabajo se propone el uso de los métodos de simulación para reforzar la comprensión de fenómenos simples en acústica y vibraciones. Lo aplicaremos a los casos de cuerda vibrante, barras y membranas de longitud finita con diferentes caracteŕısticas f́ısicas y condiciones de contorno.

Analysis of the reflection properties of sonic crystal devices.

The phenomenon of band‐gaps typical of wave propagation in periodic materials has been a subject of extensive investigations. In acoustics, the interest is mainly related with the ability of such structures, the so‐called sonic crystals, of blocking the propagation of sound at certain frequencies, acting as filters. Recently, there is an increasing interest in the use of sonic crystals to control the beam propagation, and a number of interesting features as focusing and collimation have been predicted and observed. At the root of these phenomena is the angle‐dependent dispersion introduced by the crystal. In propagating regimes, for frequencies outside the gap, the particular form of the dispersion relations in the wavenumber (reciprocal) space allows us to determine the spatial characteristics of the transmitted field. In this work, we merge the two ideas above, and investigate the spatial characteristics of the reflected acoustic field when its frequency belongs to a band‐gap. In a band‐gap, the propaga...

Sonic crystal barriers with absorbent and resonant scatterers.

The ability of periodic distributions of absorbent and resonant scatterers embedded in air for attenuating a wide range of frequencies is studied. The scatterers consist of a rigid core with a resonant cavity, covered by a layer of absorbent material. A preliminary numerical analysis shows the resulting combination of the three main physical phenomena appearing in such a periodic system: scattering, absorptions, and resonance. The scattering reproduces the so‐called band gaps, ranges of frequencies related with the periodicity of the crystal. In addition, resonant scatterers have been designed to present resonances in the range of frequencies below the band gap of the crystal, such that there appear attenuation peaks for the low frequencies. Finally, the resonant scatterers are covered by a layer of absorbent material, a fibrous woolen felt that produces an increase of the whole attenuation spectra without removing the multiple scattering and the resonance phenomena.