A method based on potential theory for calculating air cavity formation of an air cavity resistance reduction ship

This research is intended to provide academic reference and design guidance for further studies to determine the most effective means to reduce a ship’s resistance through an air-cavity. On the basis of potential theory and on the assumption of an ideal and irrotational fluid, this paper drives a method for calculating air cavity formation using slender ship theory then points out the parameters directly related to the formation of air cavities and their interrelationships. Simulations showed that the formation of an air cavity is affected by cavitation number, velocity, groove geometry and groove size. When the ship’s velocity and groove structure are given, the cavitation number must be within range to form a steady air cavity. The interface between air and water forms a wave shape and could be adjusted by an air injection system.     

A method based on potential theory for calculating air cavity formation of an air cavity resistance reduction ship

This research is intended to provide academic reference and design guidance for further studies to determine the most effective means to reduce a ship’s resistance through an air-cavity. On the basis of potential theory and on the assumption of an ideal and irrotational fluid, this paper drives a method for calculating air cavity formation using slender ship theory then points out the parameters directly related to the formation of air cavities and their interrelationships. Simulations showed that the formation of an air cavity is affected by cavitation number, velocity, groove geometry and groove size. When the ship’s velocity and groove structure are given, the cavitation number must be within range to form a steady air cavity. The interface between air and water forms a wave shape and could be adjusted by an air injection system.     

采用微气泡缩减高速船拖曳阻力(英文)

Ship hull form of the underwater area strongly influences the resistance of the ship.The major factor in ship resistance is skin friction resistance.Bulbous bows,polymer paint,water repellent paint(highly water-repellent wall),air injection,and specific roughness have been used by researchers as an attempt to obtain the resistance reduction and operation efficiency of ships.Micro-bubble injection is a promising technique for lowering frictional resistance.The injected air bubbles are supposed to somehow modify the energy inside the turbulent boundary layer and thereby lower the skin friction.The purpose of this study was to identify the effect of injected micro bubbles on a navy fast patrol boat(FPB) 57 m type model with the following main dimensions:L=2 450 mm,B=400 mm,and T=190 mm.The influence of the location of micro bubble injection and bubble velocity was also investigated.The ship model was pulled by an electric motor whose speed could be varied and adjusted.The ship model resistance was precisely measured by a load cell transducer.Comparison of ship resistance with and without micro-bubble injection was shown on a graph as a function of the drag coefficient and Froude number.It was shown that micro bubble injection behind the mid-ship is the best location to achieve the most effective drag reduction,and the drag reduction caused by the micro-bubbles can reach 6%-9%.     

Experimental Research on Flash Boiling Spray of Dimethyl Ether

The high-speed digital imaging technique is applied to observe the developing process of flash boiling spray of dimethyl ether at low ambient pressure, and the effects of nozzle opening pressure and nozzle hole diameter on the spray shape, spray tip penetration and spray angle during the injection are investigated. The experimental results show that the time when the vortex ring structure of flash boiling spray forms and its developing process are determined by the combined action of the bubble growth and breakup in the spray and the air drag on the leading end of spray; with the enhancement of nozzle opening pressure, the spray tip penetration increases and the spray angle decreases. The influence of nozzle hole diameter on the spray tip penetration is relatively complicated, the spray tip penetration is longer with a smaller nozzle hole diameter at the early stage of injection, while the situation is just opposite at the later stage of injection. This paper establishes that the variation of spray angle is consistent with that of nozzle hole diameter.     

温度、盐度和流体类型对圆柱周围湍流声学特性的影响研究（英文）

The effects of the temperature,salinity,and fluid type on the acoustic characteristics of turbulent flow around a circular cylinder were numerically investigated for the Reynolds numbers of 2.25 × 10~4,4.5 × 10~4,and 9.0 × 10~4.Various hybrid methodsReynolds-averaged Navier-Stokes(BANS) with the Ffowcs Williams and Hawkings(FWH) model,detached-eddy simulation(DES) with FWH,and large-eddy simulation with FWH-were used for the acoustic analyses,and their performances were evaluated by comparing the predicted results with the experimental data.The DES-FWH hybrid method was found to be suitable for the aero-and hydro-acoustic analysis.The hydro-acoustic measurements were performed in a silent circulation channel for the Reynolds number of 2.25 × 10~4.The results showed that the fluid temperature caused an increase in the overall sound pressure levels(OASPLs) and the maximum sound pressure levels(SPL T) for the air medium;however,it caused a decrease for the water medium.The salinity had smaller effects on the OASPL and SPLT compared to the temperature.Moreover,the main peak frequency increased with the air temperature but decreased with the water temperature,and it was nearly constant with the change in the salinity ratio.The SPLT and OASPL for the water medium were quite higher than those for the air medium.     

Interaction of two three-dimensional explosion bubbles

The interaction of two underwater explosion bubbles was mathematically analyzed in this paper. Based on the assumption of potential flow, high-order curved elements were used to discretize the boundary integral equation and solve it. Assuming that gas inside the bubble follows the isentropic rule, the Euler-Lagrange method was used to trace the evolution of the bubble, and when calculating the singular integral, the singularity of the double-layer singular integral was eliminated by reconstructing a principal-value integral of double-layer potential so that a more precise result could be obtained. Elastic mesh technique (EMT) was also used when tracing the evolution of the bubble interface, and numerical smoothing wasn't needed. A comparison of calculations using this three-dimensional model with results of the Reyleigh-Plesset bubble model shows that the three-dimensional model and calculation method in this paper is practical. This three-dimensional model was applied to simulate the interaction of two bubbles under the action of gravity, and the dynamic characteristics of two bubbles near the surface was also analyzed. Bubbles influenced by surface effects and gravity present severe non-linearity. This paper provides a reference for research into the dynamics of multi-bubbles.     

CFD simulation of propeller and rudder performance when using additional thrust fins

To analyse a possible way to improve the propulsion performance of ships,the unstructured grid and the Reynolds Average Navier-Stokes equations were used to calculate the performance of a propeller and rudder fitted with additional thrust fins in the viscous flow field.The computational fluid dynamics software FLUENT was used to simulate the thrust and torque coefficient as a function of the advance coefficient of propeller and the thrust efficiency of additional thrust fins. The pressure and velocity flow behind the propeller was calculated. The geometrical nodes of the propeller were constituted by FORTRAN program and the NUMBS method was used to create a configuration of the propeller,which was then used by GAMMBIT to generate the calculation model. The thrust efficiency of fins was calculated as a function of the number of additional fins and the attack angles. The results of the calculations agree fairly well with experimental data,which shows that the viscous flow solution we present is useful in simulating the performance of propellers and rudders with additional fins.     

硬帆辅助推进散货船的稳定性和动力性能综合分析（英文）

The wind-assisted propulsion system is becoming one of the most popular and efficient ways to reduce both fuel consumption and carbon dioxide emission from the ships.In this study,several analyses have been carried out on a model of bulk carrier fitted with five rigid sails with a 180° rotating mechanism for maximum usage of wind power and a telescopic reefing mechanism for folding it during berthing.The stability of the ship has been verified through the calculations of initial stability,static stability,and dynamic stability through the fulfillment of the weather criterion using MAXSURF software.The structural analysis of the sail was carried out in ANSYS static structural module.Several flow simulations were carried out in ANSYS fluent module to predict the thrusts produced by the sails at different apparent wind angles,which would in turn reduce the thrust required from the propeller.In this way,the brake horse powers required for different sail arrangements were analyzed to find out a guideline for this wind propulsion system to generate better powering performances.To consider drift and yaw effect on propulsion system,an MMG mathematical model-based simulation was carried out for different drift angles of motion of the ship considering hard sail-based wind loads.Through these analyses,it has been found out that the hard sail-based wind-assisted propulsion system in some cases have produced a reduction of more than 30% brake power in straight ahead motion and around 20% reduction in case of drifting ships compared to the model having no sails.     

Course-keeping control of underactuated hovercraft

The course-keeping control of underactuated hovercraft with two aft propellers was considered. The control of the heading error and cross-track error was accomplished by the yaw torque merely in this case. The hovercraft dynamic model is nonlinear and underactuated. At first the Controllability of course-keeping control for hovercraft was proved, then a course-keeping control law was derived that keeps hovercraft heading constant as well as minimizes the lateral movement of hovercraft. The proposed law guarantees heading error and sway error all converge to zero exponentially. Simulation tests were carried out to illustrate the effectiveness of the proposed control law. For further research, the disturbance influence would be considered in the dynamic equations.     

水下点火推进尾空泡振荡的研究

本文通过对水下航行体垂直发射出筒后尾空泡内点火推进过程的数值模拟研究,获得了其尾空泡演化的一般规律,并与相应的试验结果进行了对比;进而给出了燃气尾空泡周期性地膨胀、颈缩、阻滞、脱落的演化规律,以及与尾空泡内的压力振荡、喷管扩张段内激波前后移动的相关性。研究表明:水下航行体尾空泡内点火与空中点火、水中直接点火形成的燃气射流存在明显差别,水下点火推进同时会形成对水下航行体额外的振动激励;尾空泡收缩阶段,亚音速射流在尾空泡颈缩处膨胀加速,使尾空泡持续颈缩进而形成对燃气的阻滞作用,是形成尾空泡近似周期性形态演化、空泡压力剧烈振荡的主因。     

单排与双排气幕防波堤消波性能数值模拟

将气液两相流看成是变密度单流体,以连续方程、雷诺平均方程和磊叩模型为控制方程,采用VOF方法追踪两相流界面,通过UDF在连续方程中添加附加质量源项的方法,建立了气幕防波堤数学模型。通过数值计算得到的波浪透射系数同试验数据吻合较好。在此基础上讨论了不同原型入射波浪要素、不同气幕间距以及不同供气量等对单排与双排气幕防波堤消波性能的影响,进而通过分析得出相应的结论,为气幕防波堤的设计提供有意义的依据。     

基于气液两相流解析模型的水冲压发动机推进特性分析

给出了喷管中气液两相流动等温正压模型的解析解。基于该解析模型以及水冲压发动机内流道几何参数,迭代求解了发动机在一系列工况下的推进特性。研究表明,水冲压发动机的推力、效率、比冲随着气相质量流量以及航速的共同提高而提高,单单增加航速或单单增加气体质量流率,都会使得比冲和效率先增后减。单单增加气体质量流率,水流流量减小,推力增加,但是耗气量显著增加。单单增加航速,水的流量增加,推力将先增后减。     

航行体头型对气泡附着影响的数值研究

水下航行体穿越水中膨胀气泡时,气泡在一定条件下有可能附着在航行体上形成附着空泡,从而改变航行体水动力性能,为此开展了头型因素对气团附着空泡影响的数值研究。利用数值模拟方法计算了四种头型航行体穿越膨胀气团的过程,结果发现头型是影响气泡附着的重要因素:对于分离角较大的钝头型,气泡容易附着在航行体上形成附着空泡;对于分离角较小的细长头型,气泡受到扰动后就很快与航行体分离而不会附着。     

船舶柴油机喷油泵柱塞穴蚀机理的数值仿真研究

针对船舶柴油机喷油泵柱塞螺旋槽上方壁面的穴蚀现象,建立了喷油泵柱塞与套筒模型,利用FLUENT软件对喷油泵内流场进行数值仿真计算,重点研究局部低压区、湍流涡团以及气泡析出、成长和溃灭过程。研究结果表明,柱塞螺旋槽与套筒回油孔构成的重叠开口的大小和形状对燃油流场具有决定性影响,产生低压区和湍流,生成的气泡溃灭时对柱塞螺旋槽上方壁面和回油孔内壁产生机械冲击,造成穴蚀。     

水下发射航行体空泡、气泡和自由面相互影响的理论研究

基于势流理论、细长体理论和奇点分布法,对水下发射过程中的航行体空泡、自由面和筒口气团的相互影响开展了理论研究。文中用一个奇点模拟气团、用沿轴线分布的奇点模拟细长体形状的航行体和空泡,建立了自由面影响下的气团和空泡相互耦合的动力学模型,提出了非定常流场中带气体泄漏现象的含气空泡的压力估算方法。实验结果验证了文中提出的模型的合理性。     

水下航行体垂直发射筒口压力场 L/E耦合数值模拟

采用 Lagrange 结构网格和 Euler 流场网格（L/E）耦合的数值仿真方法,对潜射航行器出筒后筒口压力场进行三维数值仿真分析。计算模型考虑海水的横向来流和航行体出筒过程对筒口气泡脉动的影响。仿真计算获得了筒口压力场气泡脉动主要特征,试验与计算的2个主要压力波峰曲线形状基本一致。通过对比计算表明,考虑横向流作用可以有效减少筒口压力场计算偏差。本文的计算方法及结论可为发射筒口压力场分析提供有益指导。     

输入扰动对波动鳍推进性能影响的研究

输入扰动对波动鳍推进性能影响是一个新兴的研究领域。本文分析了高频率、小幅度和短波长的正弦波形叠加于波动鳍上时对波动鳍推进力的影响。建立运动学模型,利用四面体网格对计算区域进行划分,采用非耦合隐式求解器求解非定常不可压缩N-S方程和连续性方程。给出计算条件,并对算法给予验证。比较等波幅和变波幅两种正弦扰动波形下,波动鳍的无量纲阻力系数、阻力系数均值以及涡量场随周期(0.05 s、0.1 s、0.3 s、0.5 s、0.8 s)、幅度(0.0008 m、0.001 m、0.0015 m)和波长(0.002 m、0.008 m、0.012 m)的变化情况,从涡动力学角度对该影响进行解释。结果显示:输入扰动不仅影响了波动鳍前缘涡的传递,并对波动鳍周边及尾迹区域漩涡数量和强度产生了改变。除在特定条件外,输入扰动对波动鳍推进力产生负面影响,变波幅扰动的影响要大于等波幅情况。该研究可为波动鳍选择合适参数的输入扰动以提高推进力提供参考。     

人工气泡船水动力性能试验研究

文章通过试验途径研究了人工大气泡船的水动力性能,试验包括气泡船流量测试试验、气泡船通气对比试验、气泡船与其原型船静水及波浪中对比试验。结果表明,气泡船充气后阻力能大大下降;对于气泡船有一最经济的充气流量;气泡船在静水及波浪中的综合性能均优于常规船型,特别是阻力下降很多。根据试验结果还分析了气泡降阻的原理:具有一定压力的气泡腔,除了减小船底的摩擦阻力之外,还支撑了艇的一部分重量,减小了艇体的吃水,从而阻力也相应地减少。