3-D scanning-particle-image-velocimetry: Technique and application to a spherical cap wake flow

A 3-D time-resolving and whole-volume Digital-Particle-Image-Velocimetry (DPIV) technique based on the concept of a scanning light-sheet is presented and applied here to the 3-D transient wake phenomena in the spherical cap wake flow. The technique uses a scanning light-sheet for rapid sampling of the flow in depth and a two-camera recording system for stereoscopic 3-D DPIV. Application of a correlation technique in combination with a calibration yields, aside from the correct in-plane displacement, also the out of plane component and thus the total velocity vectors within the planes of the scanning light-sheet. With a high scanning rate in comparison to the characteristic time scales the method provides the 3-D velocity field in space and time. Through the use of conventional video-techniques the temporal evolution of the complete velocity and vorticity field can be obtained quantitatively from experiments. This is demonstrated for the 3-D starting flow around a spherical cap at Re=300. During the starting process, the flow in the wake evolves into a spherical vortex ring where the velocity distribution is very close to the theoretical solution of the Hill-type vortex. Later on, the Hill-type vortex ring deforms and the flow changes from a rotational symmetric stage to a planar symmetric flow with a double-threaded vortical structure which consists of two counter-rotating streamwise vortices similar to the ones observed in sphere wake flow.Presented at the EUROMECH Coll. 335, Image Techniques and Analysis in Fluid Dynamics, 5–7 June 1995, Roma, Italy. A version of this paper has been published in Proc. 7th Int. Symp. Flow Visualization (ed. J.P. Crowder), Begell House Inc., New York, 1995, pp. 715–720.     

Wave drag of an ellipsoid of revolution rapidly moving in a horizontal direction in a uniformly stratified fluid

The problem of the wave drag of ellipsoids moving in a uniformly stratified ideal fluid is considered by means of modeling the bodies by surface distributions of mass sources. Analytical results are obtained using the distributions known from the theory of a uniform fluid, which make it possible to describe the emission of internal waves by rapidly moving ellipsoids of revolution (spheroids) in the limit of large Froude numbers. An asymptotically simplified form of the dependence of the wave drag on the Froude number and the spheroid axis ratio is found. In the particular case of a sphere, the result confirmed earlier by Greenslade by making comparisons with a numerical calculation and experimental data is obtained.     

Stokes flow of a rotating sphere around the axis of a circular orifice or a circular disk

This paper presents an infinite series solution to the creeping flow equations for the axisymmetric motion of a sphere of arbitrary size rotating in a quiescent fluid around the axis of a circular orifice or a circular disk whose diameters are either larger or smaller than that of the sphere. Numerical tests of the convergence are passed and the comparison with the exact solution and other computational results shows an agreement to five significant figures for the torque coefficients in both cases. The torque coefficients are obtained for the sphere located up to a position tangent to the wall plane containing either the orifice or the disk. It is concluded that the torque coefficients of the sphere and the disk are monotonically increasing with the decrease of the distance from the disk or the orifice plane in both cases.     

The arbitrarily and the periodically laminated elastic hollow sphere: exact solutions and homogenization

Summary The aim of this paper is (1) to develop a rational method for the analysis of an arbitrarily laminated elastic, isotropic or transversely isotropic hollow sphere under internal and/or external pressure, (2) to solve the problem of a periodically layered sphere consisting of many equal groups of n different thin layers. The transfer matrix method is used, and exact closed-form solutions are worked out, supplemented by a numerical example. It turns out that by means of the proposed homogenization an originally (periodically) inhomogeneous isotropic sphere is replaced by a homogeneous anisotropic one belonging to the type of spherical symmetric anisotropy. Received 20 October 1997; accepted for publication 15 December 1997     

Simulation of Three-Dimensional Bluff-Body Flows Using the Vortex Particle and Boundary Element Methods

Recent contributions to the 3-D vortex method for bluff-body flows are presented. The numerical method--a vortex method combined with a boundary element method--is briefy reviewed. It is applied to direct numerical simulation (DNS) of the flow past a sphere (Re= 300, 500 and 1000). The on-going work to extend the method towards vortex-based large-eddy simulation (LES) for high Reynolds number flows is also presented. Preliminary results for the flow past a hemisphere are discussed.     

中空碳球合成方法及其电化学应用进展

中空碳球由于具有大的比表面积、良好的导电性以及高的化学稳定性等优异性能,现已被广泛地用于电化学储能、电化学催化和电化学传感领域。基于此,本文主要概述了中空碳球的各种合成方法及在电化学领域的研究进展,并对该领域面临的挑战及未来的发展方向进行了讨论。     

基于三维花状五氧化二铌及活性炭的锂离子混合电容器

锂离子混合电容器由于兼备锂离子电池和超级电容器的优势,即较高的能量密度和功率密度,而成为当前能量存储体系的研究热点。本工作合成了具有三维花状微纳结构的正交相五氧化二铌(T-Nb_2O_5),并将其与活性炭(AC)相匹配,设计出一种新型的T-Nb_2O_5/AC锂离子混合电容器。循环伏安和恒电流充放电的测试结果表明该锂离子混合电容器具有较好的电化学性能,如在碳酸酯类的有机电解液中,工作电压可达到3.0 V;在100 m A·g~(-1)的电流密度下,电容器的比能量和比功率密度可达到53.79 Wh·kg~(-1)和294 W·kg~(-1);在200 m A·g~(-1)的电流密度下,经过1000次充放电循环后,该电容器的比能量保持率为73%。由此可见,本工作开发的T-Nb_2O_5/AC锂离子混合电容器将在高功率的储能设备中有很好地应用前景。     

球形和花形ZnO纳米结构的可控制备及其光催化机理

报道了一种简单的制备球形和花形ZnO纳米结构的无模板水热法。在Zn(NO_3)_2·6H_2O与NH_3·H_2O的体系中,通过改变乙二醇和水的体积比,140℃水热反应2 h分别制备出球形和花形ZnO纳米结构。采用SEM、XRD、UV-Vis DRS和PL对其进行了表征和分析,结果表明球形ZnO纳米结构的直径为500 nm~1μm,它是由纳米颗粒自组装而成;花形ZnO纳米结构是由长度为300~700 nm,直径为100~300 nm的纳米棒组装而成。研究了乙二醇在球形和花形ZnO纳米结构形成过程中的作用并提出了可能的生长机理。室温下分别以球形和花形ZnO纳米结构为光催化剂,以偶氮染料甲基橙MO作为光催化研究对象,紫外光照2 h,对MO的降解率分别为83%和55%。以叔丁醇(t-Butanol)、乙二胺四乙酸二钠(EDTA-2Na)为自由基和空穴的捕获剂,推测其催化机理主要为空穴氧化和自由基协同氧化历程。     

(H3O)3Sb2Br9: the first member of the M3E2X9 structure family with oxonium cations

(H₃O)₃Sb₂Br₉ [trihydroxonium enneabromidodiantimonate(III)] is the first representative of the M₃E₂X₉ family (M = cation, E = Sb and Bi, and X = Br and I) with oxonium cations. The metastable compound was obtained in trace amounts from a solution of CsBr and SbBr₃ in concentrated aqueous HBr. Single crystals were isolated from the mother liquor and investigated by single‐crystal X‐ray diffraction at 100 K. (H₃O)₃Sb₂Br₉ crystallizes with the Tl₃Bi₂I₉ structure type, which is a distorted defect variant of cubic perovskite. The crystal structure comprises characteristic ²_∞[SbBr₃Br_3/2] double layers of corner‐sharing SbBr₆ octahedra with a [001] stacking direction. Due to the small size of the H₃O⁺ cation and O—H…Br hydrogen bonding, the octahedra are tilted.