Volume 116, Issue 4, October 2004
Index of content:
- NONLINEAR ACOUSTICS 
Adhesion and nonlinear scattering by rough surfaces in contact: Beyond the phenomenology of the Preisach–Mayergoyz framework116(2004); http://dx.doi.org/10.1121/1.1785616View Description Hide Description
Phenomenological models reproducing the elasticity and acoustic properties of geomaterials and materials with damage have been successfully developed. These models yield macroscopic stress–strain constitutive equations featuring hysteresis with end-point memory, and predict the efficient generation of higher harmonics accompanying the propagation of monochromatic waves. The assumption common to these models is that the material’s microstructure is characterized by nonlinear compliant components of an unspecified nature which can exist in two states: “open” or “closed.” The density of the compliant units is defined on a mathematical continuum (the Preisach–Mayergoyz space) whose elements identify the dynamic behavior of the components. In this work, adhesion is shown to introduce hysteresis with end-point memory in the macroscopic behavior of an interface between two rough surfaces in contact, and, upon scattering, to generate higher harmonics bearing a striking similarity to those observed in wave propagation phenomena in media with distributed damage and in geomaterials. It appears, therefore, that two rough surfaces interacting via adhesion forces offer a meaningful example of macroscopic interface or bond with dynamics resembling that of the fictitious elements of the Preisach–Mayergoyz space, and acoustic nonlinear properties similar to those of rocks and damaged materials.
116(2004); http://dx.doi.org/10.1121/1.1785614View Description Hide Description
In the simulation of fluid dynamics, one can either treat the fluid as a continuum or as discrete particles. Although popular for acoustics, the continuum model is limited to small Knudsen numbers (the ratio of mean free path to a length scale). Particle methods are necessary for, but not limited to, problems with Knudsen numbers greater than 0.1, which can occur in shockwaves, microdevices, high frequency sound or rarefied gases. Some well known particle methods include Monte Carlo,cellular automata, discrete velocity, lattice Boltzmann, and molecular dynamics. The direct simulation Monte Carlo (DSMC) method describes gas flows through direct physical modeling of particle motions and collisions. DSMC can model problems for the entire range of Knudsen numbers. In particular, DSMC is capable of simulating nonlinear acoustics, as well as the details of viscous dissipation, dispersion, nonequilibrium effects, and other physical properties. A DSMC method has been implemented for one-dimensional nonlinear acoustics problems on parallel computers using object-oriented and the message passing interface (MPI). DSMC results will be shown and compared with continuum theory and continuum simulations.
Microparticle concentration in short path length ultrasonic resonators: Roles of radiation pressure and acoustic streaming116(2004); http://dx.doi.org/10.1121/1.1785831View Description Hide Description
Acoustic streaming in ultrasonic (1.4–3.0 MHz) circular and rectangular resonators of path length approximately one-half or one quarter wavelength (λ) has been characterized by particle imagevelocimetry(PIV) using fluorescent 1 μm diam latex markers. Particles of all diameters examined (1, 24, 80 μm) moved into pressure node planes within 4 s of initiation of sonication. The larger particles then moved within that plane to one or more preferred positions. 1 μm particles in a λ/2 cylindrical resonator with a single nodal concentration region for larger particles were convected by Rayleigh-type streaming from the center of the node plane to its edge. In contrast, particles concentrated at many loci in two planes of a second cylindrical and a rectangular chamber. Small scale wall-associated Rayleigh-type vortices occurred in a λ/4 chamber. More unexpectedly, wall-independent bulk suspensionvortices, with circulation planes parallel to the transducer radiating surface, were recorded in both resonators. Tracer particles experienced radial forces that drove them towards or away from the center of the vortices to be concentrated at its center or entrained in a vortex perimeter ring. These different outcomes are discussed in terms of lateral radiation force distribution in the node planes.
Acoustic concentration of particles in piezoelectric tubes: Theoretical modeling of the effect of cavity shape and symmetry breaking116(2004); http://dx.doi.org/10.1121/1.1785613View Description Hide Description
A new class of simple, highly efficient, cylindrical acoustic concentration devices has been developed based upon cylindrical (or near cylindrical) geometries [Kaduchak et al., Rev. Sci. Instrum. 73, 1332–1336 (2002)] for aerosol concentration applications. The concentrators are constructed from single PZT tubes driven at or near the breathing mode resonance. Acoustic concentration of aerosols is performed within the tube cavity. It has been found that slight modifications to the cylindrical cavity geometry can significantly increase the collection efficiency and assist in precise particle positioning. This paper analyzes the theoretical framework for the acoustic concentration of particles in these devices for various geometrical perturbations. The cavity geometries studied are (1) hollow cylindrical piezoelectric tube, (2) hollow piezoelectric tube with an inner concentric solid cylinder insert, (3) a hollow piezoelectric tube with a concentric elliptic insert which breaks the circular-cylindrical symmetry, and (4) a hollow elliptic cylindrical piezoelectric tube. It is shown that breaking the circular symmetry within the cavity localizes the particles in small spatial regions within the cavity. This localization of particles may be very useful in applications requiring aerosol collection or particle stream positioning.