Second-order hydroelastic analysis of a floating plate in multidirectional irregular waves

Based on an analysis of second-order hydrodynamic forces induced by coupling of first-order wave potentials, second-order hydroelastic equations are established and solved in the frequency domain. The responses of a very large floating structure in multidirectional irregular waves are studied. The characteristics of the difference and sum frequency coordinates are discussed in detail; peaks can be found at the difference and sum frequencies close to the wet resonant frequencies of each mode. We present and analyze the maximum vertical displacement of different points as well as the time history of the vertical displacements of selected points. The differences of the combined (the summation of the linear and non-linear responses) and linear displacements of the selected points are calculated. Our results demonstrate that non-linear fluid forces influence the total responses of the referenced floating structure.     

An acoustic approach to determine tip vortex cavitation inception for an elliptical hydrofoil considering nuclei-seeding

Tip vortex cavitation is usually the first type of propeller cavitation to appear with intense noise. The precise prediction of tip vortex cavitation inception is of great importance. In this paper, via an acoustic approach an experimental investigation on tip vortex cavitation inception for an elliptical hydrofoil has been done in a cavitation mechanism tunnel. For non-nuclei-seeding conditions, the sound level “collapses” when the tip vortex cavitation approaches desinence and there exists an inverse N-shape curve between the sound pressure level and cavitation number, drastically different from the generally known S-shape curve. Three nuclei-seeding conditions are then investigated to study the nuclei effects on the tip vortex cavitation inception. We propose an acoustic criterion to determine tip vortex cavitation inception applicable to both non-nuclei-seeding and nuclei-seeding conditions. The results confirm that the nuclei content and distribution in water indeed play an important role in the cavitation inception process. Supplemental observations from the high-speed video camera validate the proposed acoustic method.     

Lagrangian renormalized approximation of turbulence

A review is made of a two-point closure approximation called Lagrangian renormalized approximation (LRA), which is derived by a simple truncation of systematic Lagrangian renormalized expansions and is free from any ad hoc adjusting parameters or quantities. Emphasis is given to three key ideas: (i) representative, (ii) mapping and (iii) renormalized expansion underlying the LRA. The nature of the method of the expansion is explained by simple models, and general properties of the expansion as well as the truncated approximation are discussed in a general framework. A brief survey of the consequences of the LRA applied to three- and two-dimensional turbulences and passive scalar fields convected by turbulence is also presented.     

Apparent and effective drag for circular cylinders oscillating transverse to a free stream

This communication brings forward the concept of an effective drag for circular cylinders undergoing controlled or self-excited oscillation transverse to a free stream. A relationship between the effective drag and the apparent drag customarily measured by force transducers, is derived on the basis of the average rate of energy dissipation in the fluid. The effective drag is employed to gain insight into the fluid dynamics of vortex-induced vibrations using available data from the published literature. It is shown that the effective drag varies almost continuously as a function of the true reduced velocity except for a sudden decrease near conditions of maximum amplitude. The variation displays similarities but also important differences among independent experiments, which highlight the importance of various influencing parameters whose effect on the free response is currently not well understood. Some evidence is found that self-excited oscillations occur at points in the frequency–amplitude space where the effective drag is comparatively low avoiding the region where energy transfer from the fluid to the structure is most efficient. A total energy balance is employed to reveal the effect of mechanical damping on the free response which indicates that the amplitude scales with the inverse of the square root of damping in conformity with experimental data. The dimensionless damping parameters proposed by Vandiver are examined in light of the present analysis, and alternative empirical formulas are suggested for organizing the free vibration response.     

Study on prediction of noise produced by stern wake-duct-propeller interaction

The mechanism and prediction methods of low-frequency discrete spectrmu noise of ducted propellers are studied. The acoustic pressure field of ducted propellers is divided into the propeller radiated field and the duct scattered field based on the linear acoustic theory. First of all, the flow field is analyzed with unsteady panel method to get the unsteady force, and then the pressure data are used as the sources for Ffowcs Williams-Hawings formulation to predict the propeller radiated field, meanwhile, the duct scattered field is obtained by Kirchoff integral equation. Through the analysis and calculation of a ducted propeller case, it is found that the duct scattered sound field is a small quantity compared with propeller radiated sound field under low-frequency and short duct condition; Also the predicted discrete spectrum noise level of ducted propellers is compared with that of full-scale propellers by experiment, verifying the predicted results are able to reflect the full-scale propellers＇ discrete spectrum noise level properly.     

湍流边界层激励下腔体内水动力自噪声预报与测量

循环水槽中试验测量了腔体内水动力自噪声,并与模态法建立的湍流脉动压力引起腔体自噪声预报进行比较验证。透声窗振动以简支边界为条件,腔体内部声波以刚性边界为条件模态展开,通过辐射边界条件建立模态耦合振动方程。在随机湍流脉动压力作用下,推导了模态振动方程在随机力激励下的自噪声功率谱响应。对循环水槽中5 m/s和8 m/s两种流速工况下的腔体水动力自噪声和湍流脉动压力进行了测试,结合测量的脉动压力预报方法可以计算腔体水动力自噪声量值,理论预报与试验测量结果大致吻合,趋势一致,为声呐罩材料选取及声学环境控制提供了一种分析方法。      

Power extraction efficiency improvement of a fully-activated flapping foil: With the help of an auxiliary rotating foil

The power extraction efficiency improvement of a fully-activated flapping foil with the help of an auxiliary rotating foil is numerically examined in this work. A NACA0015 airfoil is placed in a two-dimensional laminar flow and synchronously performs the imposed pitching and plunging motions. An auxiliary smaller foil, which rotates about its center, is arranged below the flapping foil. As a consequence, the vortex interaction between the flapping foil and the rotating foil occurs. At a Reynolds number of 1100 and the position of the pitching axis at one-third chord, the effects of the distance between the flapping foil and the auxiliary foil, the phase difference between the rotating motion and the flapping motion as well as the frequency of flapping motion on the power extraction performance are systematically investigated. It is found that compared to the single flapping foil, the efficiency of power extraction for the flapping foil with an auxiliary device can be improved. Based on the numerical analysis, it is indicated that the enhanced plunging contribution, which is caused by the increased lift force owing to the vortex interaction, directly helps the efficiency improvement.     

A circular cylinder undergoing large-amplitude transverse oscillations in a slow uniform cross flow

This study explores the vortex patterns formed by a circular cylinder undergoing lateral cylinder oscillations with large amplitudes and in the presence of a slow uniform cross flow. It is an extension of our previous study (Lam et al., 2010b) in which formation of the 2S, 2P and P+S vortex modes were discussed from the viewpoint of interaction of a uniform cross-flow with the vortex street patterns of a cylinder oscillating in an otherwise quiescent fluid at Keulegan–Carpenter numbers up to KC=8.9. The present paper reports three additional experimental sets in which the amplitudes of cylinder oscillations have even larger values, at A/D>2.5, and lie beyond the vortex mode map usually quoted from Williamson and Roshko (1988). It is found that the slow uniform cross-flow at λ/D≈3 and Reynolds number based on cross-flow velocity at 232 acts to convect the corresponding vortex patterns in the absence of cross-flow downstream across the line of cylinder oscillation. Vortex–vortex interaction and vortex–cylinder interaction are observed to affect the subsequent development of vortices. The P+S vortex mode is found to occur up to KC=16. At KC between 16 and 24, a new vortex mode is observed in which only one vortex pair can be convected downstream every cylinder oscillation cycle. Another new vortex mode with two vortex pairs and two stationary vortices are found at KC>24.     

船用金属材料声发射信号特性研究

利用声发射技术对6种船用金属材料拉伸试验过程中产生的声发射信号进行了详细研究。通过对材料从弹性、屈服、强化直至破坏过程中的声发射信号参数统计分析,总结了船用金属材料拉伸过程中声发射信号事件数量、幅度、频率随拉力变化的分布规律,为在船用金属材料与结构缺陷中进行声发射检测的设备参数的配置、传感器的选择和危险等级的划分提供了基础数据。     

浮筏的主动吸振控制实验研究

浮筏在低频减振方面存在较大不足,而这恰是主动减振的优势.将两者相结合,研究了浮筏的主动吸振控制问题.研制了一种主动吸振器,建立了以DSP F2812为核心的控制系统;提出了符号LMS算法,与广泛使用的滤波x-LMS等算法相比,该算法不需要精确的模型信息;在此基础上设计了自适应前馈主动吸振控制律,并在浮筏实验平台上进行主动吸振控制实验.实验结果表明,通过主动吸振控制,浮筏的低频减振能力得到了显著的改善.