http://hdl.handle.net/10397/211 Search the Channel search http://repository.lib.polyu.edu.hk/jspui/simple-search http://hdl.handle.net/10397/5391 Title: Dynamic response of a quantum wire structure<br/><br/>Authors: Yu, Yabin; Au Yeung, T. C.; Shangguan, W. Z.<br/><br/>Abstract: We present an investigation of the dynamical response for a quantum wire structure with reservoirs. The capacitance, admittance, and the distribution of internal potential and charge density are calculated. Our numerical calculation for internal potential and charge density shows that the induced charge density is mainly distributed in transition regions between the reservoirs and the wire, and that once any quantum channel opens, the potential drop is very sharp and occurs in the transition regions. Small Friedel oscillations in the charge density as well as charge peaks are observed. We show in our model that in the reservoirs the characteristic potentials tend to unity or zero. The results of capacitance and emittance show the resonant peaks due to the opening of an additional channel, and the oscillations are related to the longitudinal states of the quantum wire. For capacitance, a steplike behavior appears as the number of open channels increase, but for emittance such steplike structure is not observed. Furthermore, we found that the emittance curves may lie either below or above capacitance, so the charge transmission may give positive or negative contributions to the emittance.<br/><br/>Description: DOI: 10.1103/PhysRevB.66.235315 http://hdl.handle.net/10397/5389 Title: Dynamic response of a double barrier system: The effect of contacts<br/><br/>Authors: Au Yeung, T. C.; Yu, Yabin; Shangguan, W. Z.; Chow, Wan-Ki<br/><br/>Abstract: We study the dynamical response of a double-barrier conductor with two contacts to investigate the contact effect on ac conduction in the system. We have presented the calculation of various physical quantities such as the distributions of internal potential and charge density, capacitance and low-frequency ac conductance. We show that the characteristic potentials would tend to unity (zero) in the reservoirs. When the system is far away from resonance, the charge distribution exists only around the barrier regions as a response to the applied voltage, and hence the contacts almost have no effect on the results. In the case of small transmission probability, we find a considerable amount of charge distribution surrounding the double-barrier conductor. As for the resonant case or near the resonance, our results show that the charge distribution displays large fluctuations outside the conductor, but almost no charge distribution within the conductor. In this case, the effect of contacts on the charge and potential distributions is considerable. Moreover, we find that qualitatively the presence of contacts does not change the main features of the emittance without contacts. But the contact effect on the capacitance is significant when the chemical potential is very close to resonant energy: there is a sharp capacitance peak at resonance that does not exist in the case without contacts.<br/><br/>Description: DOI: 10.1103/PhysRevB.68.075316 http://hdl.handle.net/10397/5365 Title: Constructal design for pedestrian movement in living spaces: evacuation configurations<br/><br/>Authors: Lui, Che-heng Gigi; Fong, Nai-kong; Lorente, Sylvie Jeanne; Bejan, Adrian; Chow, Wan-Ki<br/><br/>Abstract: Here we show that the configuration of an inhabited area controls the time required by all the pedestrians to vacate the space. From the minimization of the global evacuation time emerges the optimal configuration of the area. This is a fundamental principle for designing living spaces with efficient evacuation quality, and it is demonstrated here with several simple building blocks that can be used as components of more complex living structures: single walkway, corner, and T-shaped walkway. We show analytically and numerically that the ratio of the widths of the stem and branches of the T-shaped walkway has an optimal value that facilitates the evacuation of all the inhabitants. This result is fundamental, and is the crowd-dynamics equivalent of the Hess-Murray rule for the ratio of diameters in bifurcated ducts with fluid flow.<br/><br/>Description: DOI: 10.1063/1.3689771 http://hdl.handle.net/10397/5278 Title: Numerical study of three-dimensional flows around two identical square cylinders in staggered arrangements<br/><br/>Authors: Niu, Jianlei; Zhu, Zuojin<br/><br/>Abstract: With the aim to understand air pollutant dispersion among high-density, high-rise buildings, this paper presents the numerical results of three-dimensional flows around two identical square cylinders (TISCs) in staggered arrangements at a Reynolds number of 250 and zero incident angle when the angle between the incoming velocity vector and the line connecting the centers of the cylinders is 45°. The dependence of the drag and lift, their root mean square values, and the Strouhal number on the horizontal spacing have been evaluated. It was found that the correlation coefficients between the drag and lift of the upstream square cylinder (SC-I) were close to unity, with those for the downstream square cylinder (SC-II) exhibiting a wavy form along the spanwise direction. The wavelength and the magnitude are closely related to the cylinder spacing. Secondary vortex coherent structures as well as the primary vortex structures were reported and discussed.<br/><br/>Description: DOI: 10.1063/1.2194077