a review of particle damping modeling and testing

Abstract This survey provides an overview of the different approaches seen in the literature concerning particle damping. The emphasis is on particle dampers used on beams vibrating at frequencies between 10 Hz and 1 kHz. Design examples, analytical formulations, numerical models, and experimental setups for such dampers are gathered. Modeling approaches are presented both for particle interaction and for systems equipped with particle dampers. The consequences of the nonlinear behavior of particle dampers are brought to attention. As such, the apparent contradictions of the conclusions and approaches presented in the literature are highlighted. A list of particle simulation software and their use in the literature is provided. Most importantly, a suggested approach to create a sound numerical simulation of a particle damper and the accompanying experimental tests is given. It consists of setting up a discrete element method simulation, calibrating it with literature data and a representative damper experiment, and testing it outside of the range of operation used for the tuning

Elastic wave propagation in confined granular systems

We present numerical simulations of acoustic wave propagation in confined granular systems consisting of particles interacting with the three-dimensional Hertz-Mindlin force law. The response to a short mechanical excitation on one side of the system is found to be a propagating coherent wavefront followed by random oscillations made of multiply scattered waves. We find that the coherent wavefront is insensitive to details of the packing: force chains do not play an important role in determining this wavefront. The coherent wave propagates linearly in time, and its amplitude and width depend as a power law on distance, while its velocity is roughly compatible with the predictions of macroscopic elasticity. As there is at present no theory for the broadening and decay of the coherent wave, we numerically and analytically study pulse-propagation in a one-dimensional chain of identical elastic balls. The results for the broadening and decay exponents of this system differ significantly from those of the random packings. In all our simulations, the speed of the coherent wavefront scales with pressure as $p^{1/6}$; we compare this result with experimental data on various granular systems where deviations from the $p^{1/6}$ behavior are seen. We briefly discuss the eigenmodes of the system and effects of damping are investigated as well.Comment: 20 pages, 12 figures; changes throughout text, especially Section V.

Competition of mixing and segregation in rotating cylinders

Using discrete element methods, we study numerically the dynamics of the size segregation process of binary particle mixtures in three-dimensional rotating drums, operated in the continuous flow regime. Particle rotations are included and we focus on different volume filling fractions of the drum to study the interplay between the competing phenomena of mixing and segregation. It is found that segregation is best for a more than half-filled drum due to the non-zero width of the fluidized layer. For different particle size ratios, it is found that radial segregation occurs for any arbitrary small particle size difference and the final amount of segregation shows a linear dependence on the size ratio of the two particle species. To quantify the interplay between segregation and mixing, we investigate the dynamics of the center of mass positions for each particle component. Starting with initially separated particle groups we find that no mixing of the component is necessary in order to obtain a radially segregated core.Comment: 9 pages, 12 figures (EPIC/EEPIC & EPS, macros included), submitted to Physics of Fluid

A strategy to determine DEM parameters for spherical and non-spherical particles

In Discrete element method (DEM) simulations the choice of appropriate contact parameters is significant to obtain reasonable results. Particularly, for the determination of DEM parameters for non-spherical particles a general straightforward procedure is not available. Therefore, in a first step of the investigation here, methods to obtain the friction and restitution coefficients experimentally for single particles [Polyoxymethylene (POM) spheres and quartz gravel] will be introduced. In the following, these predetermined DEM coefficients are used as initial values for the adjustment of bulk simulations to respective experiments. In the DEM simulations, the quartz gravel particles are represented by non-spherical particles approximated by clustered spheres. The best fit approximation of the non-spherical particles is performed automatically by a genetic algorithm. In order to optimize the sliding and rolling friction coefficients for DEM simulations, the static and dynamic angle of repose are determined from granular piles obtained by slump tests and rotating drum experiments, respectively. Additionally, a vibrating plate is used to obtain the dynamic bed height which is mainly influenced by the coefficient of restitution. The adjustment of the results of the bulk simulations to the experiments is conducted automatically by an optimization tool based on a genetic algorithm. The obtained contact parameters are later used to perform batch-screening DEM simulations and lead to accurate results. This underlines the applicability of the in parts automated strategy to obtain DEM parameters for particulate processes like screening.DFG, SPP 1679, Dynamische Simulation vernetzter Feststoffprozess

ENERGY DISSIPATION IN A SAND DAMPER UNDER CYCLIC LOADING

Various seismic and wind engineering designs and retrofit strategies have been in development to meet structures\u27 proper and safe operation during earthquake and wind excitation. One such method is the addition of fluid and particle dampers, such as sand dampers, in an eort to reduce excessive and dangerous displacements of structures. The present study implements the discrete element method (DEM) to assess the performance of a pressurized sand damper (PSD) and characterize the dissipated energy under cyclic loading. The idea of a PSD is to exploit the increase in shearing resistance of sand under external pressure and the associated ability to dissipate energy through interparticle contact sliding. The dissipated energy in the pressurized sand during cyclic motion results in a reduction of excessive displacement. The advantage of using the DEM is that applying a simple linear contact model for the entire contacts assembly and also utilizing the advantage of irregular-shaped particles to mimic the behavior of actual sand grains. The series of DEM simulations reported herein examine the effects of multiple factors on the magnitude of dissipated energy. These factors include stroke amplitude, grain size distribution, the magnitude of pressure imposed on the sand, and different configurations of the PSD. The results reveal that the main energy dissipation mechanism is generated through interparticle frictional sliding in the sand. Additionally, the magnitude of cumulative dissipated energy increases with the pressure level applied to the sand damper, as well as with the stroke amplitude of the loading. Moreover, operating the piston with multiple spheres leads to a significant increase in the magnitude of dissipated energy. However, the soil exhibits similar behavior to the case of one sphere where a strain hardening behavior was noticed. A noticeable increase in the piston capacity was observed when the sphere size was increased by 10%, and the rest of the response patterns remained unchanged. According to the results, by increasing the sphere friction, the piston capacity remains almost the same. It is also worth mentioning that when a wider range of particle sizes was employed, the capacity of the maximum force considerably increased. A significant increase in the piston capacity was clearly noticed when a boxed-shaped piston configuration was utilized at the origin of the pressurized sand damper instead of a single sphere. The results of the conducted simulations were quantitatively compared with experimental data obtained from physical modeling of a similar pressurized sand damper which revealed a fairly good agreement. This confirms the ability of the proposed framework to satisfactorily analyze complex geotechnical problems involving soil interaction and large deformations. The proposed sand damper model is shown to be a promising device that mitigates vibrations in structural systems subject to seismic and wind loading

Particle Enrichment in Longitudinal Standing Bulk Acoustic Wave Microfluidics

Separation, isolation, and enrichment of targeted nano- and microparticles are critical to a variety of biomedical applications from clinical research (development of therapeutics and diagnostics) to fundamental investigations that require concentration of specific cells from culture, separation of target species from heterogenous mixtures, or controlled perturbation of cells and microorganisms to determine their response to stimuli. Numerous techniques are available for bench-scale and medical settings; however, these traditional approaches are often labor intensive, time-consuming, costly, and/or require modification of the target. Efficiency and specificity are also lacking. Recently, techniques that exploit the similar scales of microfluidic technologies and the intrinsic properties of cells have allowed for increased automation, reduced reagent waste, and decreased cost, as well as improved performance. So-called lab-on-a-chip (LOC) approaches enable rapid fabrication and optimization of small-scale, low-volume microchannels capable of high performance enrichment and separation owing to precise control of the forces driving the manipulation. Depending on the physics underlying a particular method, devices are classified as optical, hydrodynamic, dielectrophoretic, magnetic, or acoustic. Acoustics, and specifically ultrasound, permits noncontact cell separation and retention, which reduces the potential for undesirable surface interactions and physical stress on sensitive biological samples. Typically, separation is achieved by pinning a standing wave perpendicular (conventional lateral acoustophoresis) or parallel (longitudinal acoustic trapping) to the direction of flow. In this thesis, we report a novel longitudinal standing bulk acoustic wave (LSBAW) microfluidic channel that incorporates pairs of pillar arrays oriented perpendicular to the inflow direction. The pillar arrays act as ‘pseudo walls’ that locally amplify the pressure in the enrichment zone, which can be tuned to overcome the drag force for particles of size greater than a critical diameter. Thus, these particles are preferentially retained within the nodes of the local pressure field. In our study, model predictions were used to guide experimental trapping of particles in microchannels with two pillar configurations. We created six different microfluidic channels with varying inlet/outlet geometries, widths, and pillar shapes. Model results showed pressure field amplification caused by the ‘pseudo walls’ bounding the enrichment zone of each design. We also demonstrated trapping of polystyrene beads (5 μm and 20 μm) and 10 μm fluorescent hollow glass spheres during actuation at various predicted half-wave resonances of these devices. Certain channel architectures achieved acoustic field amplification suitable for particle trapping at flow rates up to ~20 μL/min. In addition, the simulated pressure fields (eigenmodes) were consistent with experimentally observed mode shapes, which validated our modeling approach. Computational and experimental results suggest that LSBAW pillar geometries and flow parameters can be tuned to achieve enhanced enrichment of targeted particles in a predefined region