Sound radiation characteristics of a box-type structure

The finite element and boundary element methods are employed in this study to investigate the sound radiation characteristics of a box-type structure. It has been shown [T.R. Lin, J. Pan, Vibration characteristics of a box-type structure, Journal of Vibration and Acoustics, Transactions of ASME 131 (2009) 031004-1–031004-9] that modes of natural vibration of a box-type structure can be classified into six groups according to the symmetry properties of the three panel pairs forming the box. In this paper, we demonstrate that such properties also reveal information about sound radiation effectiveness of each group of modes. The changes of radiation efficiencies and directivity patterns with the wavenumber ratio (the ratio between the acoustic and the plate bending wavenumbers) are examined for typical modes from each group. Similar characteristics of modal radiation efficiencies between a box structure and a corresponding simply supported panel are observed. The change of sound radiation patterns as a function of the wavenumber ratio is also illustrated. It is found that the sound radiation directivity of each box mode can be correlated to that of elementary sound sources (monopole, dipole, etc.) at frequencies well below the critical frequency of the plates of the box. The sound radiation pattern on the box surface also closely related to the vibration amplitude distribution of the box structure at frequencies above the critical frequency. In the medium frequency range, the radiated sound field is dominated by the edge vibration pattern of the box. The radiation efficiency of all box modes reaches a peak at frequencies above the critical frequency, and gradually approaches unity at higher frequencies

Existence of Bistable Waves in a Competitive Recursion System with Ricker Nonlinearity

Using an abstract scheme of monotone semiflows, the existence of bistable traveling wave solutions of a competitive recursion system with Ricker nonlinearity is established. The traveling wave solutions formulate the strong inter-specific actions between two competitive species

The Impact of Group Diversity on Performance and Knowledge Spillover -- An Experiment in a College Classroom

An important yet under-explored question in the teamwork literature concerns how group characteristics affect productivity. Within a given teamwork setting, it is not obvious how group member diversity affects the performance of the individual and the group. The group may gain from knowledge transfer and sharing while it may be crippled by communication and coordination problems that are prevalent in heterogeneous groups. In this study, we combine class performance data from an undergraduate management class with students%u2019 personal records to explore diversity and knowledge spillover effects. A major advantage of our dataset is the exogenous assignment of groups, which rules out the troublesome yet common self-selection issue in team literature. Our results indicate that male-dominant groups performed worse both in group work and in individually taken exams than female-dominant and equally-mixed gender groups after controlling for other group characteristics. Individual members from a group with more diversity in age and gender scored higher in exams. However, we did not find any significance of a group%u2019s racial composition over group and individual performances. Another novel aspect of this natural experiment is that each group chooses their own group contract form %u2013 members of %u201Cautonomous%u201D groups receive equal grade for their group work while those in "democratic" groups can adopt differentiated point allocation, thus, providing a proper mechanism to punish free riders. Our estimation results show a significant correlation between the choice of a democratic contract and the group and individual performance. To address the endogeneity problem in groups%u2019 contract choices, we use a maximum likelihood treatment effect model and found that the democratic group contract has a positive and significant effect on group performance.

UNDERSTANDING ELECTRICAL CONDUCTION IN LITHIUM ION BATTERIES THROUGH MULTI-SCALE MODELING

Silicon (Si) has been considered as a promising negative electrode material for lithium ion batteries (LIBs) because of its high theoretical capacity, low discharge voltage, and low cost. However, the utilization of Si electrode has been hampered by problems such as slow ionic transport, large stress/strain generation, and unstable solid electrolyte interphase (SEI). These problems severely influence the performance and cycle life of Si electrodes. In general, ionic conduction determines the rate performance of the electrode, while electron leakage through the SEI causes electrolyte decomposition and, thus, causes capacity loss. The goal of this thesis research is to design Si electrodes with high current efficiency and durability through a fundamental understanding of the ionic and electronic conduction in Si and its SEI. Multi-scale physical and chemical processes occur in the electrode during charging and discharging. This thesis, thus, focuses on multi-scale modeling, including developing new methods, to help understand these coupled physical and chemical processes. For example, we developed a new method based on ab initio molecular dynamics to study the effects of stress/strain on Li ion transport in amorphous lithiated Si electrodes. This method not only quantitatively shows the effect of stress on ionic transport in amorphous materials, but also uncovers the underlying atomistic mechanisms. However, the origin of ionic conduction in the inorganic components in SEI is different from that in the amorphous Si electrode. To tackle this problem, we developed a model by separating the problem into two scales: 1) atomistic scale: defect physics and transport in individual SEI components with consideration of the environment, e.g., LiF in equilibrium with Si electrode; 2) mesoscopic scale: defect distribution near the heterogeneous interface based on a space charge model. In addition, to help design better artificial SEI, we further demonstrated a theoretical design of multicomponent SEIs by utilizing the synergetic effect found in the natural SEI. We show that the electrical conduction can be optimized by varying the grain size and volume fraction of two phases in the artificial multicomponent SEI

On a State-sponsored Sport System in China

The gold medal success of China in recent Olympic Games can be traced to the advancement of the state-sponsored sport system (SSSS). While the program was developed initially through socialist ideals, it is more than a centralized government system to monopolize resources for glorified sport performance. Participation in competition is an inherent part of the human condition. Success in athletics is associated with national identity and has economic, social, and cultural implications. Because of this, it is essential that the SSSS adjust and improve to keep pace with other facets of China’s quickly changing national reform. In association with emerging economic reform, some sports now receive equal or more funds from private investments compared to government allocation. The state-sponsored sport system must continue to adapt to maintain the Chinese tradition of excellence in competition