Wave propagation in periodic beam networks – mechanism of local resonance

We study the wave propagation in infinite two-dimensional periodic beam networks using finite element simulations and experimental techniques. Although the characteristic pass and stop bands (i.e., bandgaps) of these systems have been heavily investigated in earlier research efforts, more careful scrutiny on the problem leads us to the conclusion that the bandgaps in the triangular beam lattice are because of the localized resonance, similar to the mechanism in acoustic metamaterials with artificial resonators made of multiple constituent materials. In this study, we show, for the first time, that flat phonon modes and locally resonant bandgaps can be generated in a system comprised only one material phase. In addition, we conduct a parametric study on the effects of network global topology, local geometry as well as defect density on the locally resonant bandgaps

Combustion at reduced gravitational conditions

The theoretical structures needed for the predictive analyses and interpretations for flame propagation and extinction for clouds of porous particulates are presented. Related combustion theories of significance to reduced gravitational studies of combustible media are presented. Nonadiabatic boundaries are required for both autoignition theory and for extinction theory. Processes that were considered include, pyrolysis and vaporization of particulates, heterogeneous and homogeneous chemical kinetics, molecular transport of heat and mass, radiative coupling of the medium to its environment, and radiative coupling among particles and volume elements of the combustible medium

Three-dimensional numerical study of flow characteristic and membrane fouling evolution in an enzymatic membrane reactor

In order to enhance the understanding of membrane fouling mechanism, the hydrodynamics of granular flow in a stirred enzymatic membrane reactor was numerically investigated in the present study. A three-dimensional Euler-Euler model, coupled with k-e mixture turbulence model and drag function for interphase momentum exchange, was applied to simulate the two-phase (fluid-solid) turbulent flow. Numerical simulations of single- or two-phase turbulent flow under various stirring speed were implemented. The numerical results coincide very well with some published experimental data. Results for the distributions of velocity, shear stress and turbulent kinetic energy were provided. Our results show that the increase of stirring speed could not only enlarge the circulation loops in the reactor, but it can also increase the shear stress on the membrane surface and accelerate the mixing process of granular materials. The time evolution of volumetric function of granular materials on the membrane surface has qualitatively explained the evolution of membrane fouling.Comment: 10 panges, 8 figure

Effects of geometric and material nonlinearities on tunable band gaps and low-frequency directionality of phononic crystals

We investigate the effects of geometric and material nonlinearities introduced by deformation on the linear dynamic response of two-dimensional phononic crystals. Our analysis not only shows that deformation can be effectively used to tune the band gaps and the directionality of the propagating waves, but also reveals how geometric and material nonlinearities contribute to the tunable response of phononic crystals. Our numerical study provides a better understanding of the tunable response of phononic crystals and opens avenues for the design of systems with optimized properties and enhanced tunability.Engineering and Applied Science

A Comparative Study on Spin-Orbit Torque Efficiencies from W/ferromagnetic and W/ferrimagnetic Heterostructures

It has been shown that W in its resistive form possesses the largest spin-Hall ratio among all heavy transition metals, which makes it a good candidate for generating efficient dampinglike spin-orbit torque (DL-SOT) acting upon adjacent ferromagnetic or ferrimagnetic (FM) layer. Here we provide a systematic study on the spin transport properties of W/FM magnetic heterostructures with the FM layer being ferromagnetic Co$_{20}$Fe$_{60}$B$_{20}$ or ferrimagnetic Co$_{63}$Tb$_{37}$ with perpendicular magnetic anisotropy. The DL-SOT efficiency $|\xi_{DL}|$, which is characterized by a current-induced hysteresis loop shift method, is found to be correlated to the microstructure of W buffer layer in both W/Co$_{20}$Fe$_{60}$B$_{20}$ and W/Co$_{63}$Tb$_{37}$ systems. Maximum values of $|\xi_{DL}|\approx 0.144$ and $|\xi_{DL}|\approx 0.116$ are achieved when the W layer is partially amorphous in the W/Co$_{20}$Fe$_{60}$B$_{20}$ and W/Co$_{63}$Tb$_{37}$ heterostructures, respectively. Our results suggest that the spin Hall effect from resistive phase of W can be utilized to effectively control both ferromagnetic and ferrimagnetic layers through a DL-SOT mechanism

Topological Phononic Crystals with One-Way Elastic Edge Waves

We report a new type of phononic crystals with topologically nontrivial band gaps for both longitudinal and transverse polarizations, resulting in protected one-way elastic edge waves. In our design, gyroscopic inertial effects are used to break the time-reversal symmetry and realize the phononic analogue of the electronic quantum (anomalous) Hall effect. We investigate the response of both hexagonal and square gyroscopic lattices and observe bulk Chern numbers of 1 and 2, indicating that these structures support single and multimode edge elastic waves immune to backscattering. These robust one-way phononic waveguides could potentially lead to the design of a novel class of surface wave devices that are widely used in electronics, telecommunication, and acoustic imaging.National Science Foundation (U.S.) (Grant CMMI-1120724)National Science Foundation (U.S.) (Grant CMMI-1149456)National Institutes of Health (U.S.) (Grant DMR-1420570)United States. Army Research Office (Grant W911NF-13-D-0001)National Science Foundation (U.S.) (Grant DMR-1419807

Nonlinear wave propagation in periodic multilayer of polymers

The ultimate goal of this research is to investigate the propagation of large-amplitude elastic waves in periodic multilayer structures. Sources of nonlinearity are associated with large-strain kinematics, material nonlinearity, and bifurcation paths. In this study, we use a numerical approach to investigate the propagation of nonlinear waves of finite deformation in polymeric structures of finite size. Insights on the dispersion properties of such systems, and their functional dependence on the strain levels, are obtained by postprocessing the time-history results obtained through time-domain simulations. In particular, we highlight the nonlinear effects of amplitude parameters on the bandgaps and wave directionality of the considered systems