EXPERIMENTAL ESTIMATION OF A SCALING EXPONENT FOR TIP VORTEX CAVITATION VIA ITS INCEPTION TEST IN FULL- ANDMODEL-SHIP

Tip vortex cavitation noise of marine propeller became primary concerns to reduce hazardous environmental impacts from commercial ship or to keep the underwater surveillance of naval ships. The investigations of the tip vortex and its induced noise are normally conducted through the model test in a water cavitation tunnel. However the Reynolds number of model-test is much smaller than that of the full-scale, which subsequently results in the difference of tip vortex cavitation inception. Hence, the scaling law between model-and full-scales needs to be identified prior to the prediction and assessment of propeller noise in full scale. From previous researches, it is generally known that the incipient caivtation number of tip vortex can be represented as a power of the Reynolds number. However, the power exponent for scaling, which is the main focus of this research, has not been clearly studied yet. This paper deals with the estimation of scaling exponent based on tip vortex cavitation inception test in both full-and model-scale ships. Acoustical measurements as well as several kind of signal processing technique for an inception criterion suggest the scaling exponent as 0.30. The scaling value proposed in this study shows slight difference to the one of most recent research. Besides, extrapolation of model-ship noise measurement using the proposed one predicts the full-scale noise measurement with an acceptable discrepancy.     

NMR Lineshape in Anisotropic Superconductors with Nonregular Vortex Lattice

The nuclear magnetic resonance line shapes within a primitive cell of the vortex lattice of the type II anisotropic superconductors in a case when a vortex is displaced on small distance a from a regular position in a primitive cell are constructed. The results of the numerical calculations show that displacement of the flux line lattice essentially changes the NMR lineshape. The derivative of the power of the absorption energy with respect to the magnetic field is calculated. It allows to obtain more detailed information about the real vortex lattice of a superconductor.     

Effect of near-critical swirl on the Burke-Schumann reaction sheet

The influence of swirl on the shape of the Burke-Schumann reaction sheet in a straight cylindrical pipe is investigated by asymptotic and numerical means. Attention is confined to swirl levels that are near the critical value at which vortex breakdown occurs. A high-Reynolds-number, laminar, isothermal, low-Mach-number reacting flow is considered. An asymptotic analysis is developed to study the nonlinear interaction between near-critical swirl and mixture fraction distribution within the flow. It is first shown that leading-order perturbation of the velocity field from the columnar state, generated by the interaction of near-critical swirl and low viscosity, can be described by a nonlinear reduced-order model. This flow perturbation is computed, and then employed to determine the correction to the classical Burke-Schumann solution. Under lean conditions of reaction the reaction sheet becomes shorter and more compact as swirl is increased. For rich conditions of reaction, increasing swirl first causes the reaction-sheet length to decrease, and then increase after vortex breakdown has appeared. Numerical simulations of the flow and reaction-zone shape are substantiated by, and supplement, the asymptotic results.     

充液管路系统中阀门流噪声的研究

用二方程模型封闭的雷诺平均N-S方程组,对水管路系统中三种常见阀门的三维分离流动进行数值模拟.模拟结果表明,随着蝶阀、闸阀和球阀开度的减小,流体在蝶阀背面、球阀阀门内外分别形成两个方向相反的漩涡,闸阀的漩涡出现在挡板与管道的壁角处,并且漩涡在阀门下游逐渐消失.同时实验表明,阀门下游的流噪声大于阀门上游的流噪声,涡声是阀门噪声的主要来源.     

某型船尾部异常噪声研究

试航中发现某型油船主机在450～600r/min时,在螺旋桨的正上方能明显听到异常噪声.通过定性分析排除了螺旋桨上有缠绕物、轴系的配合问题,定量分析排除了螺旋桨出现"谐鸣"的可能性,实船测试分析结果表明产生噪声的原因是由于螺旋桨出现了梢涡空泡现象.研究为船舶的同类现象提供参考.     

Instability of the flow in the vicinity of trailing edge of a class of thin aerofoils

A numerical study has been undertaken to investigate the notion of absolute/convective instability in laminar incompressible trailing edge flows past wedge-like shapes with curved boundaries of the form y=α(−x)^m. The effects of various trailing edge shapes m and relative thickness α on the flow separation and the development of instabilities in the vicinity of trailing edge are investigated. The nonlinear viscous-inviscid interaction equations, which have been derived by means of the asymptotic theory of flow separation, are solved first numerically to construct genuine mean velocity profiles representing the correct flow in the vicinity of the trailing edge. The absolute/convective nature of the asymptotically formed velocity profiles via a composite expansion is then ascertained by using a spatio-temporal analysis based on the Briggs-Bers pinching criterion. Although no absolute growth is encountered upstream of the trailing edge of the airfoil shapes considered, in particular the wake region behind the trailing edge of Joukowski type profiles is found to be persistently susceptible to absolute instability. It is found that separation is enhanced as the relative thickness of the airfoil gets bigger. This, in turn, is shown to lead to an additional enhancement of the absolute instability character by both increasing the absolute growth rate as well as the extent of the unstable region. Shedding frequency of the Karman vortex street is also determined behind the trailing edge shapes considered.     

涡流式竖井溢洪道研究述评

近几年国内许多单位开展了溢洪道内消能工的研究，内容包括孔板消能工、空腔消能工、阶梯消能工、同轴反向内消能工、单向涡旋内消能工等．对涡流式竖井消能工的研究也比较集中，其所具有的良好的水力特性和消能效果受到工程界的重视．传统的泄流方式在高水头、大流量、高流速运行条件下，会受到空化空蚀及流动失稳等问题的困扰，修建涡流式竖井泄洪洞是解决上述难点的有效方法之一．由于它的综合消能率在５０％以上，能有效地降低洞内流速，因而选用涡流式竖井溢洪道不失为将导流洞改建成泄洪洞或单独修建泄洪洞的较佳选择．     

旋流燃烧器的冷态试验研究

通过轴向可动叶轮旋流燃烧器改造方案的冷态模拟试验，给出了冷态空气动力场的测量结果．对测量结果进行了有益的讨论，得到的结论对旋流燃烧器的改造提供了基本依据     

平面旋涡(中心型奇点)水力特性的探讨

本文讨论了平面旋涡(中心型奇点)的水力特性.说明了这种旋涡的中心被封闭的流线所包围，中心点的流速为零，封闭流线为椭圆曲线；给出了旋涡内流速与压强，旋涡的环量、涡量以及角动量的表达式；建立了旋涡水力参数和主流水力参数的连接条件，包括两个运动连接条件和两个动力连接条件；将得到的理论结果应用于侧墙突扩或跌坎流，得到的旋涡中心压强表明，在一般明渠条件下，主流流速大于20m/s时旋涡区有可能出现空蚀；应用于水跃时发现，跃长与跃高之比和突扩流旋涡的长宽比基本相同，估计了水跃旋滚区的紊流运动粘滞系数；应用于交岔渠道的旋涡时，指出了旋涡宽度与渠宽之比必须小于0.160，给出了一个数例