柔性仿金枪鱼月牙形尾鳍水动力分析

为研究鱼类高速游动机理,以金枪鱼月牙形尾鳍为对象,采用面元法计算分析了其非定常水动力性能。假设尾鳍在横移和摇摆的同时,以某一匀速向前运动,并假设其在弦向和展向以某一给定规律发生变形,以模仿柔性变形。探讨了前进速度、横移和摇摆的幅度、频率及其相位差对柔性尾鳍推进性能的影响,并与做相应运动的刚性尾鳍进行了对比。     

Review of experimental work in biomimetic foils

Significant progress has been made in understanding some of the basic mechanisms of force production and flow manipulation in oscillating foils for underwater use. Biomimetic observations, however, show that there is a lot more to be learned, since many of the functions and details of fish fins remain unexplored. This review focuses primarily on experimental studies on some of the, at least partially understood, mechanisms, which include 1) the formation of streets of vortices around and behind two- and three-dimensional propulsive oscillating foils; 2) the formation of vortical structures around and behind two- and three-dimensional foils used for maneuvering, hovering, or fast-starting; 3) the formation of leading-edge vortices in flapping foils, under steady flapping or transient conditions; 4) the interaction of foils with oncoming, externally generated vorticity; multiple foils, or foils operating near a body or wall.     

仿生胸鳍的三维尾涡结构与参数影响分析

为研究仿胸鳍推进的机理和流体动力特性及缩小机器鱼与生物原型之间的性能差距,利用浸入边界法数值模拟了做耦合旋转运动胸鳍的非定常绕流问题。详细探讨胸鳍非定常运动的三维尾涡结构演化和推进机理,并开展胸鳍推进性能与尾涡结构的参数影响分析。结果表明:迎流面在背、腹侧边缘及鳍梢部显著涡旋结构的作用下所出现的低压力区,加之鳍表面和上游来流之间好的垂直度共同造成了在动力划水阶段的高推力;在恢复划水阶段的高升力与背侧边缘涡强度的持续增加,以及因鳍表面倾斜而引起的水动力被分解到竖直方向的比重提升有关;胸鳍尾流场被一个三维双环涡结构所支配;当前的模拟为仿胸鳍推进建立了一个最优的斯特劳哈尔数St范围(在0.55附近),在此之后平均推力仍随St的增大而增加,而推进效率则表现出一个缓慢降低的趋势;当前后拍动与纵倾运动之间的相位差为90度时,胸鳍同时取得最佳的推力和效率。     

仿鱼尾潜器推进系统的水动力分析

以开发适用于小型潜器的仿生操纵与推进系统为研究背景 ,对金枪鱼的月牙形尾鳍进行水动力分析。文中将金枪鱼的尾鳍处理为在做横移和摇摆的耦合运动的同时 ,以某一匀速向前运动的月牙形刚性尾翼。计算中应用了双曲面元和压力库塔条件 ,利用面元法计算分析该三维尾翼的非定常水动力性能。探讨了前进速度、横荡和摇首的幅度、频率及其相位差对推进性能的影响     

典型参量对全被动式振荡水翼获能规律的影响

全被动式振荡水翼是振荡水翼式潮流能装置的主要型式之一。对全被动式振荡水翼进行了数值模拟研究,构建了二维数值模型,研究了雷诺数及升沉刚度对全被动式振荡水翼水动力性能的影响。通过流场结构及水动力性能分析研究了典型参量对全被动式振荡水翼获能性能的影响机理,确定了维持其良好水动力性能的参数范围。研究发现,水翼的水动力性能对雷诺数及升沉刚度的变化较为敏感。雷诺数增大,水翼所需升沉刚度随之增加,且水翼可以在更大的参数范围下获得较优的水动力性能。另外,水翼可以在没有升沉刚度的情况下实现周期性运动,其获能甚至优于一些有升沉刚度的情况。最优工况下,平均功率系数和能量转换效率分别为1.07和27.48%。     

潮流能发电装置支撑结构对水轮机水动力学性能影响研究

水平轴潮流能水轮机在工作过程中,由于支撑结构的存在,会使水轮机周围流场中的潮流流向、流速等参数发生不同程度的改变,进而影响水轮机的性能和发电装置的稳定性。为了研究支撑结构对水轮机水动力学性能的影响规律,以某100 k W单立柱座底式潮流能发电装置的支撑结构为研究对象,采用CFD方法,分别在正、反向来流时采用不同支撑结构的共六种工况下,对潮流能水轮机模型的获能和受力进行数值模拟。通过水槽模型试验,验证数值模拟的可靠性。研究结果表明:支撑结构对水轮机的水动力学性能的影响不容忽视,针对所研究的支撑结构,在正向来流时水轮机的获能系数降幅约30%,轴向力系数降幅约28%;反向来流时的降幅更大,分别约为63%和41%。     

Effect of caudal on hydrodynamic performance of flapping foil in fish-like swimming

In the present study the effect of caudal length on hydrodynamic performance of flapping foil is investigated. According to reality of swimming of fishes, the kinematics of their oscillation tail is involved with two rotational motions where one of them causes the tail to move in circular direction and the other leads the tail to pitch around its pitch axis. With this concept, a generalized kinematic model is considered. According to simulation of the motion trajectory of flapping foil, it is shown that the length of caudal may affect the hydrodynamic performance. It is shown that at lower and higher Strouhal numbers (St < 0.2 and St > 0.6) the hydrodynamic performance of flapping foil is optimum when the length of caudal is infinitive. It should be noted that at higher caudal length the variation of propulsive efficiency and produced thrust are stopped and these hydrodynamic parameters are kept at constant values. Additionally, it is demonstrated that there is the possibility of improving propulsive efficiency at moderate Strouhal numbers (0.2 < St < 0.6) by manipulation of caudal length. Furthermore, it is shown that in some cases the manipulation of caudal length may increase thrust coefficient as the propulsive efficiency is also increased.     

Hydrodynamic coefficients for a thick-walled bottomless cylindrical body floating in water of finite depth

The paper deals with the linearized hydrodynamic forces acting on a thick-walled, bottomless cylindrical body having vertical symmetry axis and oscillating in water of finite depth. For the solution of the radiation problem, the flow field around the structure is subdivided into ring-shaped fluid regions, in each of which an axisymmetric eigenfunction expansion for the velocity potential is made. By implementing Galerkin's method the various potential solutions are then matched and numerical results concerning the hydrodynamic coefficients for heave, surge and pitch motions, as well as the coupling terms between the last two modes are obtained.     

Numerical investigation of auto-pitch wing-in-ground effect oscillating foil propulsor

The propulsive characteristics of auto-pitch wing-in-ground effect oscillating foil propulsors (APWIGs) were numerically investigated through an unsteady Reynolds Averaged Navier-Stokes solver. The kinematics of such a biplane configuration is characterized by the prescribed heave motion and flow-induced pitch motion restrained by a torsional spring for each foil. Based on the validated numerical model, the comparison of propulsive performance between APWIGs and single auto-pitch oscillating foil, as well as dual-foil heave-only configuration, was conducted at different advance speeds. Results show that APWIGs is advantageous in both thrust production and efficiency enhancement over other two configurations due to the resulting wing-in-ground effect and substantial reduction of flow separation by the flow-regulated pitch motion. Furthermore, the effect of torsional spring stiffness on the propulsion of APWIGs was studied under different loaded conditions. It was found that both the maximum pitching angle and phase difference of pitch with heave are dramatically affected by the spring stiffness, which has major contribution to the hydrodynamic behaviours of the foils. Under a certain operating speed, an optimal torsional spring stiffness that produces the best propulsive performance can be found. With respect to the parametric space in the current study, the APWIGs can achieve a constant high efficiency over 70% by employing an appropriate spring stiffness.     

Numerical modeling of perforated plates in oscillating flow

Perforated plates, relevant for several marine applications, are experimentally and numerically investigated. The numerical investigations are performed using a presently developed Navier–Stokes solver. Several comparison and sensitivity studies are presented, in order to validate and verify the solver. Forced heave experiments are performed on two perforated plates with perforation ratios 19% and 28%. Amplitude-dependent added mass and damping coefficients are presented. Good agreement is obtained between the solver and the present experiments. Consistent with existing data, the results show that the hydrodynamic coefficients of perforated plates are highly amplitude dependent. The damping force is found to dominate over added mass force. The damping force dominance increases with increasing perforation ratio. It is highlighted that plate-end flow separation has an important effect on the damping coefficient. The developed numerical solver is two-dimensional, but is found to yield reasonable estimates of hydrodynamic force coefficients when compared with a previous three-dimensional experimental investigation. This could indicate that three-dimensional effects are not dominant for the hydrodynamic forces of perforated plates, and that a two-dimensional viscous flow solver could have relevance as a tool for estimating hydrodynamic forces on three-dimensional perforated structures.     

A numerical model for predicting the jump of lift force on an oscillating cylinder

The fluid force coefficients on a transversely oscillating cylinder are calculated by applying two-dimensional large eddy simulation method. Considering the “jump” phenomenon of the amplitude of lift coefficient is harmful to the security of the submarine slender structures, the characteristics of this “jump” are dissertated concretely. By comparing with experiment results, we establish a numerical model for predicting the jump of lift force on an oscillating cylinder, providing consultation for revising the hydrodynamic parameters and checking the fatigue life scale design of submarine slender cylindrical structures.     

Hydrodynamic characteristics of a lunate shape oscillating propulsor

Research was conducted to study the hydrodynamic efficiency of a foil with aft-swept wing tips. A potential flow based time domain panel method was formulated to predict the performance of a lunate and rectangular foil in large amplitude, unsteady motion. Skin drag was approximated and boundary layer growth and separation were also estimated. Hydrodynamic efficiency was evaluated in terms of propulsive efficiency and thrust coefficient of the foil. Results are presented for a lunate shaped planform and for a rectangular foil. Predictions show that the lunate shaped planform has a substantially higher propulsive efficiency (13% higher) than the rectangular foil under heavy load conditions when the feathering parameter is zero, throughout a range of reduced frequencies (0.2 to 1.8). Under a medium load condition, however, the rectangular foil gave a higher propulsive efficiency at reduced frequencies less than 0.5 and the same efficiency value at a reduced frequency of 1.8. For a practical range of reduced frequencies between 0.5 and 1.0, the lunate tail gave higher propulsive efficiency. The lunate planform gave a lower thrust coefficient at a heavy load and higher thrust at a medium load condition than the rectangular planform for all reduced frequencies.     

Propulsive performance of a tanker hull form in damaged conditions

Many disastrous oil spill accidents from damaged vessels become worse especially when the early treatment is not prompt enough. To properly handle this type of accidents and prevent further disasters, International Maritime Organization establishes and imposes various rules and regulations. Better understanding of the propulsive performance of damaged vessels is important for containing the oil spill while the vessels are being towed or self-propelled. In the present study, both experimental and computational methods were used to investigate the flow phenomena around the hull and the hydrodynamic performances of a VLCC in various damaged conditions. From the resistance and self-propulsion test results, it is found that higher power is required to propel the ship especially with the bow trim. Wake measurement data provide physical insight into the factors to be considered for the propeller operation in damaged conditions.     

Fine Structure of a Stratified Flow around a Fixed and Slow-Moving Wedge

The dynamics of the establishment and spatial structure of flows of a continuously stratified fluid around a fixed and slow-moving horizontal wedge are studied using direct numerical simulation based on the fundamental system of inhomogeneous fluid mechanics equations. Large-scale components (eddies, internal waves, and the wake) and fine-structure components are isolated in the flow patterns in near and away from the obstacle. The mechanism of formation of the propulsive force generating the self-motion of a free body at a neutral-buoyancy horizon is determined. The dependence of the flow parameters on the shape of the obstacle is shown. The transformation of the medium perturbation field at the beginning of the induced slow movement of the wedge at the neutral-buoyancy horizon is traced. The complex structures of fields of different physical quantities and their gradients are visualized. The intrinsic temporal and spatial scales of the flow components are identified.     

The motions of a ship in swallow water

A general formulation is given of the hydrodynamic forces on a ship, oscillating about a state of rest in 6df in response to excitation by a harmonic wave in shallow water. A method is described to obtain a numerical approximation of the velocity potential, describing the flow around the moving ship by means of a distribution of discrete three-dimensional sources.With this method it is possible to take the influence of a quay into account.Calculated values of wave excited forces, hydrodynamic coefficients and motions of a 200,000 tdw tanker in shallow water are presented and compared with experimental results.     

Direct numerical simulations of flow over two-dimensional and three-dimensional ripples and implication to sediment transport: Steady flow

A well resolved and highly accurate direct numerical simulation (DNS) solver has been developed to understand the implication of hydrodynamics to sediment transport. In the first part of the study we focus on steady flow over two-dimensional and three-dimensional ripples at two Reynolds numbers Re_τ = 180 and 400 (defined by channel half-height and wall-friction velocity) in a channel geometry. The DNS scheme is based on a fourth-order vertical velocity and second-order vertical vorticity formulation, which resolves the difficulties in pressure boundary condition encountered when solving the Navier–Stokes equations. The complex boundary introduced due to the ripples has been imposed in the Cartesian domain using an elegant immersed boundary method. Detailed hydrodynamic analysis has revealed turbulence statistics (in particular, the higher order) and henceforth, the flow structures are sensitive — whether the ripples are two-dimensional or three-dimensional. The importance of fluctuating component of the bottom stress in addition to its mean component; and its significance to sediment transport and ripple migration speed have been investigated.     

SPH方法的孤立波与部分淹没结构物相互作用数值计算研究

为研究孤立波作用下结构物周围流场特征,基于无网格SPH方法,建立孤立波与海洋结构物相互作用模型,对不同波幅孤立波作用下部分淹没矩形结构物周围波面、流速、涡量及结构受力特征进行计算分析,探索了相对波高对非淹没结构物周围流场的影响规律。结果表明:流场特征与相对波高密切相关,相对波高较小时,波面、流速、涡量及结构荷载均较为光滑,相对波高在0.2以上时,波峰爬升至结构物顶部并在越过结构物后与水槽内水体碰撞造成流场波动,波面、流速、涡量及结构荷载的波动幅度随着相对波高增大而增大,流场更加复杂,结构物水平和垂向负压也越大,且结构物周围涡分布逐渐向深度方向和下游方向发展。     

Propulsive performance of three naturally occurring oscillating propeller planforms

A numerical method, the quasi-vortex-lattice method (QVLM), was applied to predict the propulsive performance of three naturally occurring oscillating propellers. These were cetacean flukes for a fin whale (Balaenoptera physalus); white-sided dolphin (Lagenorhynchus acutus); and white whale (Delphinapterus leucas). The fin whale's flukes had the highest aspect ratio (6.1) and moderate sweep angle (31°); the white-sided dolphin's flukes had the highest sweep angle (47°) and lowest aspect ratio (2.7); and the white whale's flukes had moderate aspect ratio (3.3) and the lowest sweep angle (28°). In the numerical simulations, the planforms were assumed to be rigid both in chordwise and spanwise directions, and to be oscillating harmonically in an irrotational, incompressible fluid. Calculation and comparisons of propulsive efficiency and thrust coefficient vs advance ratio for each of the planforms were made in three cases: varied heave amplitude; different pitching axis positions; and varied angular amplitude of pitch.     

Numerical investigation on the hydrodynamic characteristics of a marine propeller operating in oblique inflow

The hydrodynamic characteristics of a marine propeller operating in oblique inflow are investigated by using CFD method. Two propellers with different geometries are selected as the study subjects. RANS simulation is carried out for the propellers working at a wide range of advance coefficients and incidence angles. The effects of axial inflow and lateral inflow are demonstrated with the hydrodynamic force on the propeller under different working conditions. Based on the obtained flow field details, the hydrodynamic mechanism of propeller operating in oblique inflow is analyzed further. The trailing vortex wake of propeller is highly affected by the lateral inflow, resulting in the deflected development path and the circumferentially non-uniform structure, as well as the enhanced axial velocity in slipstream. Different flow patterns are observed on the propeller blade with the variation of circumferential position. Combined with the computed hydrodynamic forces and pressure distribution on propeller, the mechanism resulting in the increase of propulsive loads and the generation of propeller side force is explored. Finally, a systematic analysis is carried out for the propulsive loads and propeller side force as a function of axial and lateral advance coefficients. The major terms that play a dominant role in the modeling of propulsive loads and propeller side force are determined through the sensitivity analysis. This study provides a deeper insight into the hydrodynamic characteristics of propeller operating in oblique inflow, which is useful to the investigation of propeller performance during ship maneuvers.     

Three-dimensional numerical simulation of solitary wave run-up using the IB method

Although the finite difference method is computationally efficient, it is acknowledged to be inferior when dealing with flow-over on structures with a complex geometry because of its rectilinear grid system. Therefore, we developed a numerical procedure that can cope with flow over structures with complex shapes while, at the same time, retaining the simplicity and efficiency of a rectilinear grid system. We used the immersed boundary method, which involves application of immersed boundary forces at solid boundaries rather than conventional boundary conditions, to investigate wave interactions with coastal structures in a three-dimensional numerical wave tank by solving the Navier–Stokes equations for two-phase flows. We simulated the run-up of a solitary wave around a circular island. Maximum run-up heights were computed around the island and compared with available laboratory measurements and previous numerical results. The three-dimensional features of the run-up process were analyzed in detail and compared with those of depth-integrated equations models.