Ultrasound Measuring of Porosity in Porous Materials

This chapter provides a temporal method for measuring the porosity and the tortuosity of air-saturated porous materials using experimental reflected waves. The direct problem of reflection and transmission of acoustic waves by a slab of porous material is studied. The equivalent fluid model has considered in which the acoustic wave propagates only in the pore-space. Since the acoustic damping in air-saturated porous materials is important, only the reflected waves by the first interface are taken into account, and the multiple reflections are neglected. The study of the sensitivity analysis shows that porosity is much more sensitive than tortuosity to reflection, especially when the incident angle is less than its critical value, at which the reflection coefficient vanishes. The inverse problem is solved using experimental data at a different incidence angle in reflection. Some advantages and perspectives of this method are discussed

Inverse estimation of the permeability of porous materials using experimental data via reflected waves at low frequencies

International audienceAn acoustic reflectivity method is proposed for measuring the permeability or flow resistivity of air-saturated porous materials. In this method, a simplified expression of the reflection coefficient is derived in the Darcy's regime (low frequency range), which does not depend on frequency and porosity. Numerical simulations show that the reflection coefficient of a porous material can be approximated by its simplified expression obtained from its Taylor development to the first order. This approximation is good especially for resistive materials (of low permeability) and for the lower frequencies. The permeability is reconstructed by solving the inverse problem using waves reflected by plastic foam samples, at different frequency bandwidths in the Darcy regime. The proposed method has the advantage of being simple compared to the conventional methods that use experimental reflected data, and is complementary to the transmissivity method which is more adapted to low resistive materials (high permeability)

Nonlinear Acoustics in Higher-Order Approximation: Comment

Some useful expressions for the second- and third-order equations for harmonic generation of infinite plane acoustic waves in a nonlinear non-viscous fluid are corrected. The concern addressed in the present comment is to point out some typographical errors in the first-order velocity and pressure expressions intervening in the calculation of the secondorder nonlinear equations, as well as a miscalculation of the axial component of the third-order Lighthill tensor term and the resulting third-order velocity and pressure equations presented in that paper

High-order Bessel non-vortex beam of fractional type

Based on the vector Maxwell’s equations and Lorenz’ gauge condition, full vector-wave derivations for the electric and magnetic fields components of a high-order Bessel non-vortex beam of fractional type α (HOBNVB-Fα) are presented. The field corresponds to the most generalized case of quasi-standing waves that reduce to perfect (i.e. equi-amplitude) standing waves or progressive waves with appropriate choice of the quasi-standing wave coefficient Υ. Particular emphasis is given on the polarization states of the vector potentials used to derive the field’s components and the transition from the progressive to perfect standing wave behavior. The results are of particular importance in the study of the optical/electromagnetic wave scattering, radiation force and torque in dual-beam optical laser-wave tweezers operating with this fractional type of non-diffracting non-vortex beams

Adjustable vector Airy light-sheet single optical tweezers: negative radiation forces on a subwavelength spheroid and spin torque reversal

Generalized solutions of vector Airy light-sheets, adjustable per their derivative order m, are introduced stemming from the Lorenz gauge condition and Maxwell’s equations using the angular spectrum decomposition method. The Cartesian components of the incident radiated electric, magnetic and time-averaged Poynting vector fields in free space (excluding evanescent waves) are determined and computed with particular emphasis on the derivative order of the Airy light-sheet and the polarization on the magnetic vector potential forming the beam. Negative transverse time-averaged Poynting vector components can arise, while the longitudinal counterparts are always positive. Moreover, the analysis is extended to compute the optical radiation force and spin torque vector components on a lossless dielectric prolate subwavelength spheroid in the framework of the electric dipole approximation. The results show that negative forces and spin torques sign reversal arise depending on the derivative order of the beam, the polarization of the magnetic vector potential, and the orientation of the subwavelength prolate spheroid in space. The spin torque sign reversal suggests that counter-clockwise or clockwise rotations around the center of mass of the subwavelength spheroid can occur. The results find useful applications in single Airy light-sheet tweezers, particle manipulation, handling, and rotation applications to name a few examples