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Chung Y.C.,Nanya Institute of Technology | Ooi J.Y.,University of Edinburgh
Granular Matter | Year: 2011

Over the past 30 years, the Discrete Element Method (DEM) has rapidly gained popularity as a tool for modelling the behaviour of granular assemblies and is being used extensively in both scientific and industrial applications. However, it is far from clear from reviewing the literature whether the large number of DEM codes have been verified and checked against fundamental benchmark problems. DEM simulates the dynamics of each particle in an assembly by calculating the acceleration resulting from all the contact forces and body forces. It is clearly necessary that such a model be validated or verified by comparing with experimental results, analytical solutions or other numerical results (e.g. Finite Element Analysis (FEA) results) at particle impact level. There appears to be no standard benchmark tests against which DEM codes can be verified. It is thus essential and useful to establish a set of standard benchmark tests to confirm that these DEM codes are modelling the particle dynamics as intended. This paper proposes a set of benchmark tests to verify DEM codes at particle impact level for spherical particles. The analytical solutions derived from elasticity theory for elastic normal collision of two spheres or a sphere with a rigid plane are first reviewed. These analytical solutions apply only to the elastic regime for normal impact. Secondly, the analytical solutions of frictional oblique impact between two spheres or a sphere with a rigid plane are scrutinized and derived. These analytical solutions originate from the dynamics principles and should be satisfied for any DEM contact force model with prescribed friction and restitution coefficients. A set of eight benchmark tests are designed and performed using commercial DEM codes. Test 1 and Test 2 consider the elastic normal impact of two spheres or a sphere with a rigid plane, whereas the other tests (Test 3-Test 8) investigate the energy dissipation due to the collision. These benchmark tests also involve different types of material. The DEM results were compared with the analytical solutions, experimental or FEA results found in the literature. All benchmark tests showed good to excellent match, providing a quantitative verification for the codes used in this study. These benchmark tests not only verify DEM codes but also enhance the understanding of fundamental impact phenomena for modelling a large number of particles. © 2011 Springer-Verlag. Source


Lin J.-R.,Nanya Institute of Technology
Tribology International | Year: 2010

Based on the magneto-hydrodynamic (MHD) thin-film lubrication theory, steady load capacity and dynamic characteristics of wide tapered-land slider bearings lubricated with an electrically conducting fluid by the application of an external magnetic field are presented. Compared with the non-conducting lubricant (NLC) tapered-land bearing by Lin et al. (2006), the MHD bearing provides higher values of load capacity and dynamic coefficients. Compared with the MHD inclined-plane bearing by Lin et al. (2009), the MHD tapered-land bearing how an increase in load capacity and dynamic coefficients as well as a reduction in the steady friction parameter. © 2010 Elsevier Ltd. Source


Shiue Y.-C.,National Central University | Chiu C.-M.,National Sun Yat - sen University | Chang C.-C.,Nanya Institute of Technology
Computers in Human Behavior | Year: 2010

The motivation to share members' knowledge is critical to an online community's survival and success. Previous research has established that knowledge sharing intentions are based on group cohesion. Several studies also suggested that social loafing behavior will seriously corrode group cohesion. Therefore, social loafing is a key obstacle to fostering online community development. Although substantial studies have been performed on the critical factors that affect social loafing in the learning group, those on online communities are still lacking. By integrating two perspectives, social capital and perceived risk, a richer understanding of social loafing behavior can be gained. In the research model, social ties and perceived risk have been driven by anonymity, offline activities, knowledge quality, and media richness. Social ties and perceived risk are hypothesized to affect social loafing in the online community, which, in turn, is hypothesized as negatively affecting group cohesion. Data collected from 323 online users in online communities provide support for the proposed model. The study shows that social loafing is a significant negative predictor of the users' group cohesion. The study also shows that social ties and perceived risk are important components of social loafing. Anonymity, offline activities, knowledge quality, and media richness all have strong effects on social ties and perceived risk in the online community. Implications for theory and practice and suggestions for future research are discussed. © 2009 Elsevier Ltd. All rights reserved. Source


Based upon the ferrohydrodynamic flow model, a lubrication equation in cylindrical coordinates for ferrofluid squeeze films including the effects of convective inertia forces and the presence of transverse magnetic fields has been derived for engineering application. As an application, the problem of parallel circular disks is illustrated. It is found that the ferrofluid circular squeeze film considering fluid inertia effects provides a higher load capacity and a longer elapsed time as compared to the non-inertia non-ferrofluid case. These improved characteristics are further emphasized for larger values of the density parameter, the Langevin parameter and the volume concentration of particles. © 2011 Elsevier Ltd. All rights reserved. Source


Yang C.-B.,Nanya Institute of Technology
Journal of Computational and Theoretical Nanoscience | Year: 2011

Currently, line segment fabrication using near field photolithographic microscopes can only generate nano-scale line segments of equal depth; fabrication of 3D shapes is impossible. The innovative line segment fabrication model of near field photolithography is a method used to analyze the exposure energy density of innovative line segment fabrication of near field photolithography by controlling exposure energy density through manipulation of the near field distance between the optical fiber probe and the photoresist, performing fabrication of profiles in the process. This study proposes that the difference between the target value of the profile where fabrication takes place and the simulation value is used as the target function. The Levenberg-Marquardt method and a reasonable convergence criterion are used to determine a suitable optical fiber movement speed of Vz. This value is then substituted into the innovative line segment fabrication model of near field photolithography where simulation is performed. Simulation results fall in the area of profile errors; therefore, the model proposed in this paper affirms the ability to perform near field line segment 3D shape fabrication. The innovative line segment fabrication model of near field photolithography proposed here is not suitable for cases where the distance between the photoresist and the optical fiber probe exceeds 35.6 nm. Otherwise, the fabrication would lose its near field photolithographic properties, and its base would become emerged. Copyright © 2011 American Scientific Publishers. Source

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