A decoupled method to identify affecting mechanism of crosswind on performance of a natural draft dry cooling tower

The natural draft dry cooling tower (NDDCT) has been increasingly used for cooling in power generation in arid area. As crosswind affects the performance of a NDDCT in a complicated way, and the basic affecting mechanism is unclear, attempts have been made to improve the performance of a NDDCT based on limited experiences. This paper introduces a decoupled method to study the complicated crosswind effects on the inlet and outlet of a NDDCT separately by computational fluid dynamics (CFD) modeling and hot state experiments. Accordingly, the basic affecting mechanism of crosswind on the NDDCT performance is identified. Crosswind changes the inlet flow field of a NDDCT and induces mainstream vortices inside the tower, so as to degrade the ventilation. Besides, low crosswind deflects the upward plume at the outlet to further degrade the ventilation, while high crosswind induces the low pressure area at the outlet to reduce the ventilation degradation.     

Wind loads on flat roofs of low-rise buildings with rounded leading edge

Using wind tunnel experiments, wind loads on the flat roofs of low-rise buildings with rounded leading edges were investigated under the effects of separation bubbles and conical vortices. First, pressure distributions on the flat roofs were presented. Second, total uplift forces and overturning moments were calculated by the introduction of area-averaged pressures. Third, eigenvalues and eigenvectors of fluctuating pressure were obtained by proper orthogonal decomposition. Finally, wind pressure characteristics at the rounded edges were examined. The results indicate that after the use of a rounded leading edge, suctions induced by separation bubble and conical vortices increase near the chamfer, decrease beneath the vortices, and stay invariant far from the leading edge. The area involved and the energy of pressure fluctuation decrease, with the largest decrease occurring on a flat roof with a larger fillet radius under conical vortices. In addition, the maximum decreases in peak total uplift forces and overturning moments reach 11% and 62%, respectively, and appear at skewed flows. The maximum suction and low-frequency spectral peak of pressure fluctuations at the rounded leading edge may exceed those at the neighboring region. As the fillet radius increases, the maximum suction and low-frequency pressure fluctuation energy at the chamfer decrease.     

Direct observation of the interaction between vortices and dislocations in superconducting crystals by a cryo-Lorentz EM (interaction between vortices and dislocations)

Quantized magnetic flux lines (vortices) in a Nb foil were directly observed in different magnetic fields up to 200 G by a cryo-Lorentz electron microscope. The interaction of vortices with dislocations in the specimen was examined and clarified; edge-on dislocations weakly pin individual vortices at magnetic fields below 100 G. In higher magnetic fields the formation of a regular hexagonal vortex lattice started preferentially at in-plane dislocations. At 200 G the Abrikosov vortex lattice was formed with small domains, whose centre included the dislocations, showing their important role on the formation of the vortex lattice. For a NbTi foil no clear image of vortices could be seen, because the surface was rough due to the formation of fine grains and precipitates.     

涡旋光束和光学涡旋

就涡旋光束和光学涡旋的基本特征和原理进行了概述,对其产生、传播及应用进行了介绍.对涡旋光束和光学涡旋的研究动态进行了叙述,并对其未来的研究和应用前景进行了展望.     

Wind tunnel study on the influence of different parapets on the roof pressure distribution of low-rise buildings

High suction loads appear on roofs of low-height buildings. The use of parapets with appropriate height at the roof edges alleviates these loads. The performance of six parapet configurations to decrease the suction loads induced on roofs by oblique winds has been studied in a low speed wind tunnel. The studied parapet configurations include vertical wall parapets, either solid or porous, and cantilevered parapets formed by a small horizontal roof close to the building roof. Low-height parapets with a medium porosity and cantilevered parapets are more efficient than solid parapets to reduce the wind suctions generated on the roofs by conical vortices.     

Recombinant Landscapes in the American City

How are urban actors, such as landscape designers, community groups, developers and local politicians, actively restructuring their environments to meet the challenges of the American city in the new global context? Grahame Shane outlines the approaches to landscape that have been emerging since the mid-20th century and are set to recombine urban assemblages whether they are located in historic city centres, postindustrial waterfronts or suburban sprawl. Copyright © 2007 John Wiley & Sons, Ltd.     

Visualization of internal energy flows in optical fields carrying a pair of fractional vortices

The internal energy flows resulting from the azimuthal component of optical currents in beams carrying a pair of fractional vortices during propagation have been experimentally detected in a knife edge test. Two optical vortices of fractional charge in a beam pair up by means of a connecting dark intensity line between intensity zeros of the vortices. Independent of the polarity of the fractional vortices, this pairing up process occurs. This is in contrast to the isolated intensity null points of vortices of integer charges and thus calls for a study on the internal energy flows in such beams.     

激光中的涡旋与孤子

本文概述了在有源非线性光学共振腔中形成的图样类型及特点,这些特征可用激光流体动力学描述;介绍了在这一领域内的最新发展和应用前景。     

不同翼片扰流特性的PIV对比实验

利用粒子成像测速（PIV）技术对圆管内置梯形翼片后方流场进行了测量,分析了翼片迎流（UFW）和顺流（DFW）两种放置方式对流场的扰动特性。结果表明,迎流翼片形成的涡沿周向延展范围较大,持续性好,涡偶内侧为向壁流;顺流翼片形成的涡沿径向延展范围较大,在较短距离内扰动较为明显,涡偶内侧为背壁流。两种流动结构都能有效提高壁面附近的速度分量,促进主流和壁面附近流体的质量交换。随着Reynolds数增大,纵向涡的稳定性减弱,在Re=3000时,翼片的扰流效果均较好。     

A parametric, experimental analysis of conical vortices on curved roofs of low-rise buildings

Different methods to reduce the high suction caused by conical vortices have been reported in the literature: vertical parapets, either solid or porous, placed at the roof edges being the most analysed configuration. Another method for alleviating the high suction peaks due to conical vortices is the use of some non-standard parapet configuration like cantilever parapets. In this paper the influence of roof curvature on the conical vortex pattern appearing on a curved roof (Fig. 1) when subject to oblique winds is experimentally analysed by testing the mean pressure distribution on the curved roofs of low-rise building models in a wind tunnel. Also, the efficiency of cantilever parapets to reduce mean suction loads on curved roofs is experimentally checked. Very high suction loads have been measured on curved roofs, the magnitude of these high suction loads being significantly decreased when cantilever parapets are used. Thus, the suitability of these parapets to reduce wind pressure loads on curved roofs is demonstrated.