Numerical and experimental studies on the hydrodynamic damping of a zero-thrust propeller

Fluid added mass and damping are significant parameters when predicting the dynamic response of a submerged structure. The hydrodynamic damping of underwater rotating machinery is numerically and experimentally investigated by a zero-thrust propeller in this paper. The lifting surface method(LSM) combined with forced vibration was introduced as the numerical method to compute the corresponding unsteady thrust, while the experimental method of measuring added damping was accomplished by a propeller undergoing rotation combined heave motion. Results of the theoretical method are in good agreement with the experimental results before cavitation occurs, as cavitation is regarded to weaken the unsteady response of the propeller partly. The calculation results also show that both the frequency ratio(vibration frequency divided by rotation frequency) and the blade angle have a significant influence on the hydrodynamic damping. Therefore, the effect of blade angle on hydrodynamic damping should be considered during the design phase.     

On the use of bend–twist coupling in full-scale composite marine propellers for improving hydrodynamic performance

Marine propellers are designed to work for a particular operating condition. However, a propeller often requires to operate at different off-design conditions, when its hydrodynamic efficiency drops. In this paper, a comprehensive numerical study is presented on the use of bend–twist coupling of composite propeller blades for improving their hydrodynamic efficiency at off-design conditions. The analysis is carried out on a full-scale propeller of diameter 4.2 m, considering the complete viscous turbulent flow, as the loading and deformation of model propellers that have been typically studied in literature for this purpose cannot be extrapolated to a full-scale prototype propeller. The open water performance is estimated using the finite volume method employing the pressure based RANS equation for the steady, incompressible, turbulent flow. The deformation analysis is done using the finite element method based on the first order shear deformation theory for composite laminates. The fluid–structure interaction is incorporated in an iterative manner. The effect of laminate configurations on the maximum twist achieved in the blade is studied for four different composite materials. The numerical study reveals that, within the limits of material safety, the twist generated in the deformed propeller using commonly used composite materials is inadequate to create any noticeable change in the hydrodynamic efficiency. When the material failure is ignored, however, it is possible to generate sufficient deformation and twist that can cause appreciable improvement in the hydrodynamic performance.     

船用复合材料螺旋桨研究进展

复合材料具有比强度高,阻尼性能好及可调整纤维铺层以控制结构变形等优点.复合材料应用于螺旋桨将改善螺旋桨的推进性能和振动特性.通过对国内外复合材料螺旋桨研究成果的回顾、总结和归纳,得出了传统的算法已不满足复合材料螺旋桨的设计和预报要求,复合材料螺旋桨的设计和预报算法需考虑桨叶变形引起的空间流场变化的结论.分析了可借助纤维增强材料所具有的弯扭耦合特性,调整桨叶纤维材料铺层和桨叶结构形式来提高螺旋桨推进效率的规律性.总结了复合材料螺旋桨研究中的关键技术和复合材料螺旋桨设计流程,并指出了复合材料螺旋桨未来研究的趋势.      

Analysis of unstable vortical structure in a propeller wake affected by a simulated hull wake

A two-frame PIV (particle image velocimetry) technique was used to investigate the flow characteristics of a complicated propeller wake influenced by a hull wake. As the propeller is significantly affected by the hull wake of a marine vessel, measurements of the propeller wake under the hull wake are certainly needed for more reliable validation of numerical predictions. Velocity field measurements were conducted in a cavitation tunnel with a simulated hull wake. Generally, the hull wake generated by the hull of a marine ship may cause different loading distributions on the propeller blade in both the upper and the lower propeller planes. The unstable propeller wake caused by the ship’s hull was interpreted in terms of turbulent kinetic energy (T _KE) to obtain useful information for flow modeling. The unstable or unsteady phenomenon in the upper propeller wake was identified by using the proper orthogonal decomposition (POD) method to characterize the coherent flow structure with turbulent kinetic energy. Strong unsteadiness appeared in the second and higher modes, largely affecting the downstream flow characteristics. The first eigenmode can be used to appropriately identify the tip vortex positions even in the unstable downstream region, which are helpful for establishing reliable wake modeling.     

Numerical investigation of cavitating flows for marine propulsors using an unstructured mesh technique

In the present study, the cavitating flows around marine propulsors have been numerically investigated by using a multi-phase RANS flow solver based on pseudo-compressibility and a homogeneous mixture model using unstructured meshes. To handle the relative motion between the rotating rotor and the stator, an overset mesh technique was adopted. The mass transfer rate between the liquid and vapor phases was determined by Merkle’s cavitation model based on the difference between the local and vapor pressure. The calculations were made for the P4381 marine propeller with different cavitation numbers at several advancing ratios. It was shown that the vapor structure, such as cavitation size and shape, was well captured at cavitating flow conditions. It was observed that the cavitation breakdown behavior was also well captured by the present method. Good agreement was obtained between the present results and the experiment for the integrated blade loadings, such as thrust and torque. The calculations were also made for a water-jet pump configuration at several flow conditions, and the cavitation breakdown behaviors for total headrise, power and thrust were validated by comparing the results with the experiment. The blade area covered by the cavitation and the shape of tip leakage cavitation were also compared with the experiment. Reasonable agreement between the predicted results and the experiment was obtained.     

Oblique entry of a slender body in an incompressible fluid

The plane problem of oblique penetration of a slender semiinfinite body in an ideal, weightless, and incompressible fluid is examined. Detailed numerical computations are performed for a wedge with rectilinear sides. The formulas obtained are applicable also for the calculation of the hydrodynamic reactions during emergence of a body from a fluid or during transverse motion of a half-blunt body with a low relative velocity. Moreover, the results of the present paper can be used to evaluate the hydrodynamic forces acting on underwater wings or propeller blades during intersection with a free surface.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 16–24, September–October, 1977.     

Hydroelastic Effects of the Camber Ratio on A Ducted Marine Propeller in A Wake Flow

The effects of the camber ratio on the hydrodynamic and structural behaviors of a NACA-based ducted marine propeller in the wake flow behind an underwater axisymmetric body are numerically studied by computational fluid dynamics methods, in particular, the finite element method. The results are presented in terms of the efficiency, deflection, pressure coefficient, and natural frequencies. It is shown that the wake flow strongly affects the performance of the selected propulsion system. It is shown that the distributions of the camber ratio over the blades of the propeller nonlinearly changes its resistance against cavitation occurrence and deflection, and also changes its hydrodynamic performance and vibrational behavior.     

Flow and migration of nanoparticle in a single channel

A numerical simulation based on a combined Euler and Lagrange method is investigated in this work to simulate the flow and migration of nanoparticles in a single channel. The motion of discrete nanoparticles is determined by the Lagrangian trajectory method based on the Newton’s second law that includes the influence of the body force, various hydrodynamic forces, the Brownian motion and the thermophoresis force. The coupling of discrete particles with continuous flow is realized through the modification of the source term of the continuous equation. The results reveal the two-phase flow nature of nanoparticle suspensions and their implications to the convective heat transfer of nanofluids.     

Non-linear hydrodynamics of thin laminae undergoing large harmonic oscillations in a viscous fluid

Smoothed Particle Hydrodynamics is implemented to study the motion of a thin rigid lamina undergoing large harmonic oscillations in a viscous fluid. Particularly, the flow physics in the proximity of the lamina is resolved and contours of non-dimensional velocity, vorticity and pressure are presented for selected oscillation regimes. The computation of the hydrodynamic load due to the fluid–structure interaction is carried out using Fourier decomposition to express the total fluid force in terms of a non-dimensional complex-valued hydrodynamic function, whose real and imaginary parts identify added mass and damping coefficients, respectively. For small oscillations, the hydrodynamic force reflects the harmonic nature of the displacement, whereas multiple harmonics are observed as both the amplitude and frequency of oscillation increase. We propose a novel formulation of hydrodynamic function that incorporates added mass and damping coefficients for a thin rigid lamina spanning large amplitudes in viscous fluids in a broad range of the oscillation frequencies. Results of the simulations are validated against numerical and experimental works available in the literature in addition to theoretical predictions for the limit case of zero-amplitude oscillations.     

Hydrodynamic interaction between two vertical cylinders in water waves

I.IntroductionWiththeconstructionoflargeoffshorestructures-wavediffractionandradiationproblemscausedbyseveralbodiesbecomeincreasinglyimportant.LargeoffShoreplatforms,wave-powerextractiondevices,Iargestoragefacilitiesandoffshorefloatingairportsl']havebeenp…     

Time-dependent hydroelastic analysis of cavitating propulsors

A 3-D potential-based boundary element method (BEM) is coupled with a 3-D finite element method (FEM) for the time-dependent hydroelastic analysis of cavitating propulsors. The BEM is applied to evaluate the moving cavity boundaries and fluctuating pressures, as well as the added mass and hydrodynamic damping matrices. The FEM is applied to analyze the dynamic blade deformations and stresses due to pressure fluctuations and centrifugal forces. The added mass and hydrodynamic damping matrices are superimposed onto the structural mass and damping matrices, respectively, to account for the effect of fluid–structure interaction. The problem is solved in the time-domain using an implicit time integration scheme. An overview of the formulation for both the BEM and FEM is presented, as well as the BEM/FEM coupling algorithm. Numerical and experiment validation studies are shown. The effects of fluid–structure interaction on the propeller performance are discussed.     

Investigation of a fluidized bed in a vortex chamber

Results on the tangential velocity in a fluidized bed measured by a laser Doppler velocimeter and by a mechanical propeller are presented. The reasons for the difference in the bed rotation velocities obtained by these measurement techniques are analyzed on the basis of numerical calculations of the multiphase flow dynamics.     

Utilization of bend–twist coupling for performance enhancement of composite marine propellers

Self-twisting composite marine propellers, when subject to hydrodynamic loading, will not only automatically bend but also twist due to passive bend–twist (BT) coupling characteristics of anisotropic composites. To exploit the BT coupling effects of self-twisting propellers, a two-level (material and geometry) design methodology is proposed, formulated, and implemented. The material design is formulated as a constrained, discrete, binary optimization problem, which is tackled using an enhanced genetic algorithm equipped with numerical and analytical tools as function evaluators. The geometry design is formulated as an inverse problem to determine the unloaded geometry, which is solved using an over-relaxed, nonlinear, iterative procedure. A sample design is provided to illustrate the design methodology, and the predicted performance is compared to that of a rigid propeller. The results show that the self-twisting propeller produced the same performance as the rigid propeller at the design flow condition, and it produced better performance than the rigid propeller at off-design flow conditions, including behind a spatially varying wake.     

Microstructural transition in an ordered set of magnetic spheres immersed in a carrier liquid

This work explores the physics of an ordered set of interacting spheres immersed in a carrier liquid. We present numerical simulations that compute the translational and rotational motion of N interacting spheres based on classical principles of Stokesian dynamics. The spheres are assumed to be made of a magnetizable material, subjected to magnetic and hydrodynamic long range interactions. We explore structure transition using a Lagragian approach of a continuum volume of fluid containing micrometric magnetic particles. We present local maps of particle volume fraction within the calculation Lattice. In this condition, considering the presence and absence of an applied magnetic field, instantaneous snapshots of the local microstructure are taken. Thus, different possibilities of long range interactions are considered. We also complement these results with meaningful statistics of time series obtained through our simulations, such as the correlation time of velocity fluctuations and their self-correlation functions. The data analyzed in the present work sustain the fact that initially ordered neutrally buoyant suspensions have an anisotropic memory-like behavior in the direction of an applied field. It is also observed that particles tend to form small isotropic clusters in the absence of an external field. However, hydrodynamic interactions tend to disperse the particulate phase, avoiding the formation of clusters. This finding suggests that hydrodynamic interactions may play a relevant role on the magnetization dynamics of ferrofluids.     

Inelastic impact dynamics of ships with one-sided barriers. Part I: analytical and numerical investigations

This two-part paper deals with impact interaction of ships with one-sided ice barrier during roll dynamics. The first part presents analytical and numerical studies for the case of inelastic impact. An analytical model of a ship roll motion interacting with ice is developed based on Zhuravlev and Ivanov non-smooth coordinate transformations. These transformations have the advantage of converting the vibro-impact oscillator into an oscillator without barriers such that the corresponding equation of motion does not contain any impact term. Such approaches, however, account for the energy loss at impact times in different ways. The present work, in particular, demonstrates that the impact dynamics may have qualitatively different response characteristics to different dissipation models. The difference between localized and distributed equivalent damping approaches is discussed. Extensive numerical simulations are carried out for all initial conditions covered by the ship grazing orbit for different values of excitation amplitude and frequency of external wave roll moment. The basins of attraction of safe operation are obtained and reveal the coexistence of different response regimes such as non-impact periodic oscillations, modulation impact motion, period added impact oscillations, chaotic impact motion and rotational motion. The second part will consider experimental validations of predicted results.     

Hydrodynamic Interaction Between a Slightly Distorted Sphere and a Fixed Sphere

The planar motion of a slightly distorted sphere around a fixed sphere in an unbounded fluid is investigated by a perturbation approach. An approximate velocity potential is derived in terms of sets of singularities by using the successive potential method. In a relative coordinate system moving with the uniform stream, the kinetic energy of the fluid is expressed as a function of 15 added masses. Approximate analytical solutions of added masses in series form are obtained and applied to determine the trajectories of the slightly distorted sphere around a fixed sphere. The hydrodynamic interaction between two bodies is computed based on the dynamical equations of motion. It is found that the presence of a sphere generates an effect on the planar motion of the slightly distorted sphere and the initial configuration of the slightly distorted sphere has a decisive influence on the development of its subsequent rotational motion. Received 24 August 2000 and accepted 8 February 2001     

黏弹性阻尼作用下轴向运动Timoshenko梁振动特性的研究

黏弹性阻尼一直是轴向运动系统的研究热点之一.以往研究轴向运动系统大都没有考虑黏弹性阻尼的影响.但在工程实际中, 存在黏弹性阻尼的轴向运动体系更为普遍.本文研究了黏弹性阻尼作用下轴向运动Timoshenko梁的振动特性.首先, 采用广义Hamilton原理给出了轴向运动黏弹性Timoshenko梁的动力学方程组和相应的简支边界条件.其次, 应用直接多尺度法得到了轴速和相关参数的对应关系, 给出了前两阶固有频率和衰减系数在黏弹性作用下的近似解析解.最后, 采用微分求积法分析了在有无黏弹性作用下前两阶固有频率和衰减系数随轴速的变化; 给出了前两阶固有频率和衰减系数在黏弹性作用下的近似数值解, 验证了近似解析解的有效性.结果表明: 随着轴速的增大, 梁的固有频率逐渐减小.梁的固有频率和衰减系数随着黏弹性系数的增大而逐渐减小, 其中衰减系数与黏弹性系数成正比关系, 黏弹性系数对第一阶衰减系数和固有频率的影响很小, 对第二阶衰减系数和固有频率的影响较大.      

Fluid-structure-acoustic coupling for a flat plate

The present work deals with the aeroacoustic sound radiated by a forward–backward facing step in combination with a flexible wall behind the step. A numerical flow computation with coupled aeroacoustic and vibroacoustic simulation was carried out. The structural deformations of the oscillating plate like structure in the wake of the forward–backward facing step were considered to be small and therefore not affecting the flow field. The presented approach enables a separate consideration for the aeroacoustic as well as the structural borne noise. The influence of the interactions of the acoustic medium with the flexible structure on the vibroacoustic sound radiation is investigated. One-sided and two-sided coupling approaches for the vibroacoustic analysis are introduced. The two-sided vibroacoustic computation allows for considering the damping influence of the ambient fluid on the flexible plate vibration and therefore on the sound radiation. Additional to the simulations, aeroacoustic measurements in an acoustic wind tunnel were performed for validation purposes.     

混合驱动水下滑翔机水动力参数辨识

与传统水下滑翔机相比,混合驱动水下滑翔机可以分别利用头部的浮力驱动单元和机身尾部的螺旋桨推进单元进行驱动从而实现不同形式的运动,具备低功耗、长航程、良好机动性等特点,具有广泛的应用前景. 准确的动力学模型以及精确的水动力参数是实现混合驱动水下滑翔机控制系统设计以及精确导航的基础. 在混合驱动水下滑翔机动力学模型已知的前提下获得准确的水动力参数是本文主要研究的问题. 本文以天津大学研制的混合驱动水下滑翔机“海燕” 作为研究对象,提出一种在有限航行参数条件下,基于大数据统计分析的计算流体力学(computational fluid dynamics, CFD) 和参数辨识相结合来获取混合驱动水下滑翔机的水动力参数的方法. 即首先建立滑翔机的动力学模型,推导出稳态数据与所求水动力参数的关系;然后采用CFD 的方法得到其升力系数,根据大量稳态纯滑翔实验数据,结合大数据统计分析,辨识出其剩余水动力参数;最后,根据混合驱动模式下的实验数据辨识出与螺旋桨相关的参数,从而得到其整套的水动力参数. 该方法不仅结合了CFD 方法具有获取复杂外形结构航行水动力的特点,而且可以有效利用大量现场实验数据,因而能够更加准确地辨识其实际运动. 通过运动仿真与试验对比,验证了该辨识方法的正确性和有效性,对滑翔机的研究具有指导意义.