考虑挡板对空化脱落的抑制效应数值分析

激波和回射流是空化脱落的两种主要机制，在相同攻角下，空化数越大，回射流对空化脱落的效果就越明显。为了研究长度不同、布置方式不同的挡板对空化回射流和空化脱落的抑制效应，分别在平板和水翼的吸力面添加挡板，用来阻挡逆流而上的回射流，从而观察空化脱落的情况。数值模拟采用一种新修正的稳定化SST k-ω湍流模型，空化模型则选取了比较常用的Schnerr-Sauer空化模型。研究结果表明：在水翼吸力面布置恰当的挡板可以以较小的阻力代价换取巨额的升力提升，从而提高升阻比，提高水翼的效率，同时挡板的存在可以有效地减小片空泡的脱落和云空化的产生。     

海洋模式中垂直混合参数化方案介绍

介绍了海洋垂直混合过程参数化方案的发展,以及不同参数化方案在海洋模式中的应用情况。首先,介绍不同垂直混合参数化方案的物理问题、理论依据、数学表达和特征,并对不同参数化方案进行了比较。其次,针对中尺度涡、亚中尺度涡以及波浪、潮流混合参数化的最新研究进展进行了总结并对垂直混合参数化的未来发展提出了一些建议。     

带前缘结节叶片导管桨尾流不稳定性分析

导管桨的尾流不稳定性在其性能评价中非常重要,不但是其能否提供稳定推力的保证,而且也与螺旋桨的尾流噪声直接相关。为了改善导管桨的尾流,提高尾流稳定性,并优化导管桨的流场脉动,根据座头鲸鳍肢前缘结节的仿生原理,对导管桨叶片的导边进行改进,提出了两种仿生桨型,采用IDDES湍流模型对低进速系数下常规导管桨和仿生叶片导管桨进行数值模拟,探究叶片构型对导管桨性能和尾流不稳定性的影响。计算结果表明,前缘结节可以有效降低叶片受力波动的幅值和叶片所受合力的主频域峰值,具有较大结节的叶片对导管桨尾流有明显的优化作用,在尾流远场中扩大了流动稳定区,延后了尾流处涡破碎的发生,改善了能量谱密度的频域分布。进一步,大前缘结节叶片导管桨应用在低速工况下时,可以大量减少尾流泄涡区域的二次涡产生,这是由于前缘结节提升了相邻涡互感的强度,使得尾流更加稳定,而小结节叶片仿生桨型对导管桨尾流则无明显优化作用。研究方法和成果可为螺旋桨尤其是导管桨尾流不稳定性研究提供参考,不仅验证了前缘结节在导管桨叶片应用的合理性,而且揭示了其优化尾流稳定性的机理。     

波浪滑翔机椭圆形后缘水翼动力特性研究

水翼是波浪滑翔机的重要动力转换部件。本文针对水翼平面形状的优化问题进行研究,首先,建立了波浪滑翔机水翼水动力分析模型,分析了展弦比和翼型对单个平直水翼升力特性的影响,选定了最佳展弦比和翼型;在此基础上,提出了一种椭圆形后缘的水翼平面外形技术方案,建立了水翼椭圆形后缘平面外形的控制方程;利用CFD方法,针对单个水翼和阵列化水翼2种条件,分析对比了平直水翼和具有椭圆形后缘水翼的升力和阻力特性。研究结果表明:在低雷诺数条件下,较大的展弦比有利于提高水翼的升力系数;攻角较大时NACA0012翼型截面形状的水翼性能优于平板翼;对于相同面积的水翼,当攻角较小时椭圆尾缘水翼的升力系数和阻力与平直水翼相同,而攻角较大时椭圆尾缘水翼的升/阻力特性优于平直水翼。     

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%。     

压力脉动对浮泥悬扬影响的实验研究

采用水槽实验和现场实测相结合的方法,在一个时均流速为零的无流水槽试验中采用振动发生器所产生脉动水流,探究不同的压力脉动对于底床泥沙悬扬的影响,分析水体的垂向浊度分布及相应的脉动水压力等,探明压力脉动对浮泥悬扬的重要性,为揭示风生波浪造成的压力脉动与浮泥悬扬间的固有响应关系奠定基础。     

波浪与渤海近岸海冰流固耦合特性的数值模拟

在波浪作用下,近岸海冰断裂发生时的极限波高和波长可以为大尺度海冰模型中的参数化提供依据。利用流固耦合数值模拟技术,结合渤海冬季近岸海冰、波浪的相关参数,模拟了波浪与小尺度近岸固定海冰的相互作用过程,得出了波浪作用下海冰的最大主应力、最大主应变和自由端位移峰值随入射波浪要素的变化规律。进一步结合渤海海冰的强度,分析了海冰由于波浪作用而发生断裂的极限波高及极限波长。结果表明:海冰最大主应力的最大值、最大主应变的最大值和自由端位移峰值的绝对值随波高、波长的增加而增大。辽东湾某典型海域,当水深为5m,海冰长为2m,冰厚为0.3m以及入射波周期为3s时,海冰发生断裂的极限波高为0.63m;入射波高0.6m时,海冰断裂极限波长为15.35m。     

区域海－气耦合模式对海表粗糙度参数化方案的敏感性研究

本文回顾了近年来海表粗糙度参数化研究成果,借助COARE算法对四种粗糙度参数化方案下海浪对海气界面通量及大气底层运动的影响进行分析,并对高海况条件下,区域海－气耦合模式对海表粗糙度参数化方案的敏感性进行了初步的探讨。研究表明：海表粗糙度受海面波浪状态的影响,在其参数化方案中考虑波龄的作用后,在高风速、低波龄区,波浪作用使海气界面上的动量和热量通量显著增加;波浪通过参与海气界面上的动力和热力作用影响低层大气运动,低波龄条件下,波浪作用使大气运动减弱,高波龄条件下,波浪作用较小;区域海－气耦合模式对海表粗糙度参数化方案较敏感,不同的参数化方案对台风路径影响不大,对台风强度影响显著,从本文的个例来看,采用Smith92和Liu07方案的模拟结果更接近实测值。本文的研究结果为更合理的参数化海气界面的物理过程,提高耦合模式的模拟准确率提供了一定的参考。     

规则波作用下台阶地形上墩柱绕流三维数值研究

利用Flow-3D建立三维数值波浪水槽,模拟波浪在不对称台阶地形上的传播。系统研究规则波作用下墩柱周围水流的流动特性,分析墩柱周围的瞬时速度场、涡量场以及KC值变化,不同相位时墩柱前、后水平流速分布情况。结果表明:波浪在台阶地形传播的过程中,墩柱迎水面的涡动结构不够明显;高涡量呈对称状聚集在墩柱的背水面,并形成一对旋转方向相反的涡结构;周期对KC值的影响比波高的影响要明显;墩柱迎水面水平方向流速变化较大,侧面水平流速变化最为剧烈,背面由于受到墩柱的掩护作用水平方向流速变化不大,在墩柱的正面和侧面竖向环流明显。     

可控深水破碎波的实验室生成方法研究

实验室一般采用波浪聚焦方法生成深水破碎波,通过各组分波浪的波幅叠加生成一个波高显著增大的大波,使其波陡超过极限波陡发生破碎。利用该方法生成深水破碎波浪的破碎次数通常并不唯一,导致波浪破碎后的流场特征不明显;造波参数不易于选取导致研究工况的设置难度大,直接影响深水破碎精细化实验的效果和效率。本文采用聚焦波理论计算波面,并利用上跨零点法定义的波高和波长计算理论波陡,结合物理模型实验统计波浪沿程破碎次数与剧烈程度,研究以JONSWAP谱为造波输入谱型时,聚焦波幅、谱峰频率、频宽等造波输入参数对于波浪破碎情况的影响,从而建立深水波浪破碎次数与造波输入参数之间的近似定量关系,为实验造波参数的选取提供参考,提高实验效率。     

Deep ocean wave energy conversion using a cycloidal turbine

A lift based wave energy converter, namely, a cycloidal turbine, is investigated. This type of wave energy converter consists of a shaft with one or more hydrofoils attached eccentrically at a radius. The main shaft is aligned parallel to the wave crests and submerged at a fixed depth. In the two-dimensional limit, i.e. for large spans of the hydrofoil (or an array of these), the geometry of the converter is suitable for wave termination of straight crested Airy waves. Results from two-dimensional potential flow simulations, with thin hydrofoils modeled as either a point vortex or discrete vortex panel, are presented. The operation of the cycloidal turbine both as a wave generator as well as a wave-to-shaft energy converter interacting with a linear Airy wave is demonstrated. The impact on the performance of the converter for design parameters such as device size, submergence depth, and number of hydrofoils is shown. For optimal parameter choices, simulation results demonstrate inviscid energy conversion efficiencies of more than 99% of the incoming wave energy to shaft energy. This is achieved using feedback control to synchronize the rotational rate, blade pitch angle, and phase of the cycloidal wave energy converter to the incoming wave. While complete termination of the incoming wave is shown, the remainder of the energy is lost to harmonic waves traveling in the up-wave and down-wave directions.     

Isoparametric Boundary Element Method Applied to the Rectangular and Delta Hydrofoils near the Free Surface

In this paper, the hydrodynamic characteristics and flow field around rectangular and delta hydrofoils, moving with a constant speed beneath the free surface are numerically studied by means of isoparametric boundary element method (IBEM). The quantities (source and dipole strengths) and the geometry of the elements are represented by a linear distribution. Two types of three-dimensional hydrofoils (rectangular and delta) are selected with NACA4412 and symmetric Joukowski sections. Some numerical results of pressure distribution, lift, wave-making drag coefficients and velocity field around the hydrofoils are presented. Also, the wave pattern due to moving hydrofoil is predicted at different operational conditions. Comparisons are made between computational results obtained through this method and those from the experimental measurements and other numerical results which reveal good agreement.     

动波浪壁圆柱绕流三维数值模拟

利用计算流体力学软件Fluent开展了三维动波浪壁圆柱绕流的数值模拟,建立了三维运动波浪壁圆柱模型,通过C语言自编程序实现波浪壁面的运动控制,并保证壁面变形时网格的高质量。在来流速度u=0.125 m/s、雷诺数Re=12 500的情况下,开展了动波浪壁波动速度w=0、0.062 5、0.125、0.187 5 m/s四个工况的计算分析,并比较了不同波动速度对流场结构、升力、阻力特性的影响。结果表明:动波浪壁圆柱能有效抑制流动的分离,消除交替脱落的尾涡,从而消除周期振荡的升力;在消除卡门涡街的同时,圆柱后驻点处的涡量值随波动速度增加而增加,其原因在于波形移动加大了壁面流体的速度,从而减小了圆柱前后的压力差,减小了阻力;随着波动速度的增大,平均阻力系数呈明显下降趋势,当波动速度为来流速度的1.5倍时,平均阻力系数相对于光滑圆柱下降了53.76%。     

High-speed submerged and surface piercing cavitating hydrofoils, including tandem case

The iterative method that is originally developed before both for two- and three-dimensional single cavitating hydrofoils moving with a constant speed under free surface is applied to the case of high-speed (Froude number up to 6.5) and some figures are given. The method is also extended to include the surface piercing hydrofoils (vertical struts) and the case of tandem hydrofoils into the calculations. The iterative nonlinear method based on the Green's theorem allows separating the cavitating hydrofoil problem(s) and the free surface problem. These two (or three in the case of tandem hydrofoil) problems are solved separately, with the effects of one on the other being accounted for in an iterative manner. The cavitating hydrofoil surface(s) and the free surface are modeled with constant strength dipole and constant strength source panels. The source strengths on the free surface are expressed in terms of perturbation potential by applying the linearized free surface conditions. No radiation condition is enforced for downstream and transverse boundaries. The cavitation number is expressed in terms of Froude number and the submergence depth of the hydrofoil from the free surface. An algebraic grid on the free surface has been described to get a smooth transition between the panels along the direction of uniform inflow and to have a long distance in the downstream direction depending on the wave-length (or Froude number) while keeping the number of panels fixed. First, the method is validated in the case of surface piercing hydrofoil. Then, the effects of high Froude number and the submergence depth of the hydrofoil from free surface on the results are discussed and some figures are given for interested engineers and designers. The method is later applied to the case of tandem hydrofoils and the effects of one hydrofoil on the other are discussed.     

A potential based panel method for 2-D hydrofoils

A potential based panel method for the hydrodynamic analysis of 2-D hydrofoils moving beneath the free surface with constant speed without considering cavitation is described. By applying Green's theorem and the Green function method, an integral equation for the perturbation velocity potential is obtained under the potential flow theory. Dirichlet type boundary condition is used instead of Neumann type boundary condition. The 2-D hydrofoil is approximated by line panels which have constant source strength and constant doublet strength distributions. The free surface condition is linearized and the method of images is used for satisfying this free surface condition. All the terms in fundamental solution (Green function) of perturbation potential are integrated over a line panel. Pressure distribution, lift, residual drag and free surface deformations are calculated for NACA4412, symmetric Joukowski and van de Vooren profile types of hydrofoil. The results of this method show good agreement with both experimental and numerical methods in the literature for the NACA4412 and symmetric Joukowski profile types. The lift and residual drag values of the van de Vooren profile are also presented. The effect of free surface is examined by a parametric variation of Froude number and depth of submergence.     

A potential based panel method for 2-D hydrofoils

A potential based panel method for the hydrodynamic analysis of 2-D hydrofoils moving beneath the free surface with constant speed without considering cavitation is described. By applying Green's theorem and the Green function method, an integral equation for the perturbation velocity potential is obtained under the potential flow theory. Dirichlet type boundary condition is used instead of Neumann type boundary condition. The 2-D hydrofoil is approximated by line panels which have constant source strength and constant doublet strength distributions. The free surface condition is linearized and the method of images is used for satisfying this free surface condition. All the terms in fundamental solution (Green function) of perturbation potential are integrated over a line panel. Pressure distribution, lift, residual drag and free surface deformations are calculated for NACA4412, symmetric Joukowski and van de Vooren profile types of hydrofoil. The results of this method show good agreement with both experimental and numerical methods in the literature for the NACA4412 and symmetric Joukowski profile types. The lift and residual drag values of the van de Vooren profile are also presented. The effect of free surface is examined by a parametric variation of Froude number and depth of submergence.     

Asymptotic analysis of the interaction between linear long waves and a submerged floating breakwater of wavy surfaces

In this work, we carried out an asymptotic analysis, up to the second order in a regular expansion, of the interaction of linear long waves with an impermeable, fixed, submerged breakwater composed of wavy surfaces. Below the floating breakwater, there is also a step with a wavy surface. The undulating surfaces are described by sinusoidal profiles. The effects of three different geometric parameters — the amplitude of the wavy surfaces and the submerged length and width of the structure — on the reflection and transmission coefficients are analyzed. The hydrodynamic forces are also determined. The governing equations are expressed in dimensionless form. Using the domain perturbation method, the small wavy surfaces of the breakwater are linearized. The wavy surfaces of the breakwater generate larger values of the reflection coefficient than those obtained for breakwaters with flat surfaces, and the largest values of this coefficient are obtained when the length of the breakwater is of the same order of magnitude as the wavelength. The asymptotic solution is compared with the theoretical solutions that have been reported in the specialized literature and with a numerical solution. The present mathematical model can be used as a practical reference for the selection of the geometric configuration of a submerged floating breakwater under shallow flow conditions.     

涡流发生器对潮流能水轮机翼型水动力学特性影响的数值模拟

为了改善潮流能水轮机叶片表面流动分离问题,提高其升阻比,本文通过在潮流能水轮机叶片表面加装涡流发生器,来研究涡流发生器对潮流能水轮机水动力学性能的影响。本文以NACA4418翼型为研究对象,分别建立了含VGs和不含VGs的三维模型,利用CFD方法研究了VGs的高度、长度以及相邻一对VGs之间的间距等多个方面对该翼型性能的影响。结果表明:VGs可以有效地提高翼型的最大升力系数;相邻VGs间距的增加对流动分离的抑制有积极影响。此外,通过对尾迹区流线和旋涡的分析,进一步揭示了尾迹区的流场特征。     

The hydrofoil in surface proximity, theory and experiment

Results of an experimental study of the effect of surface proximity on hydrofoil lift are presented. The biplane image theory, a horseshoe vortex model and momentum theory are described in relation to the effect of surface proximity on hydrofoil lift and drag. The biplane image theory and the horseshoe vortex model are shown to predict the same effect on lift, and are seen to be in good agreement with the experimental data. The Payne momentum theory is seen to differ significantly from the measured results. The data indicate a significant reduction in lift at depths less than two chords with very little effect at greater depth.     

Structure and mechanics of nonpiscine control surfaces

Animals display a variety of control surfaces that can be used for propulsion and maneuvering devises. For nonpiscine vertebrates, these control surfaces are primarily evolutionary modifications of the paired appendages (i.e., legs). The diversity of control surfaces can be classified with regard to the forces used for stability and maneuverability. For animals, the pertinent forces are pressure drag, acceleration reaction, and lift. These forces can be generated actively by motion of the control surfaces or passively from flows produced by movements of the body or external flow fields. Drag-based control surfaces are associated with paddling and rowing movements, where the limbs are oriented either in the vertical parasagittal plane or horizontal plane, respectively. The paddle is unstreamlined and has a triangular design with a broad distal end, thereby affecting a large mass of water. Appendages, which are used to generate lift-based forces, are relatively stiff hydrofoils. To maximize lift, the hydrofoil should have a crescent wing-like design with high aspect ratio. This shape provides the hydrofoil with a high lift-to-drag ratio and high propulsive efficiency. The tail flukes of cetaceans are streamlined control surfaces with a wing-like design. The flukes of cetaceans function in the hydrodynamic generation of forces for thrust, stability, and maneuverability. The three-dimensional geometry of flukes is associated with the production of lift and drag. Previous studies of fluke geometry have been limited in the number of species examined and the resolution of measurements.