舵几何特征对桨-舵系统尾流场演化的影响

发生在桨和舵之间的干扰会影响螺旋桨尾流的演化,导致尾流场中的湍流在下游增强,恶化船舶的振动和噪声性能,深入分析舵几何参数对桨-舵系统尾流场演化的影响能够为推进器尾流场的调节和减振降噪提供新思路.因此,从弦长、剖面和梯形舵入手分析不同的舵几何参数对螺旋桨尾流场演化特性的影响,使用大漩涡模拟方法模拟流场中的湍流结构,对不同舵弦长、剖面下的螺旋桨尾涡结构演化进行了分析,在舵弦长、剖面影响螺旋桨尾流场演化的研究的基础上分析了梯形舵对螺旋桨尾涡结构的影响,进一步分析了梯形舵影响下的螺旋桨尾流场中湍动能的分布.结果表明舵的弦长和剖面均会影响螺旋桨尾流场的演化,这种影响表现为更大的弦长和更厚的剖面会促进螺旋桨梢涡在舵压力面上的偏移,更薄的舵剖面会带来更强烈的螺旋桨毂涡偏移;涡管轮廓和舵表面脉动压力的对比均表明梯形舵会促进螺旋桨尾流场沿逆舵梯度方向偏移,从而导致螺旋桨的尾涡结构在舵两侧及下游呈现不对称分布,桨-舵系统下游的湍流结构与螺旋桨尾涡-舵碰撞过程、螺旋桨尾涡-舵随边涡干扰过程、螺旋桨梢涡-螺旋桨毂涡干扰有关,偏移更大的螺旋桨尾涡结构会在尾流场中更早地引起湍动能增强.     

The wake flow downstream of a propeller-rudder system

We report wall-resolved, large-eddy simulations for the case of a propeller operating upstream of a hydrofoil, mimicking a rudder. Our primary objective is the identification of wake features that are unique to this coupled system, when compared to open-water cases, which are usually the focus of experiments and computations in the literature. We were able to achieve unprecedented levels of numerical resolution, which capture the dynamics of all energetic eddies in the flow by using a scalable, conservative, structured solver in cylindrical coordinates. The boundary conditions on the rotating propeller and hydrofoil were enforced via an immersed boundary formulation. The largest values of turbulent stresses in the wake of the hydrofoil are achieved outwards from the radial coordinate of the tip of the propeller blades. This is due to spanwise gradients across the hydrofoil (in the direction parallel to the span of the hydrofoil), producing a displacement of the pressure side legs of the tip vortices towards outer coordinates, where they experience shear with the wake of the hydrofoil. The evolution of turbulence is non-monotonic across the streamwise direction. This is a consequence of the growing shear resulting from the complex interactions involving the shear layers from the trailing edge, the tip vortices and the two branches of the hub vortex coming from the two sides of the hydrofoil. Such a shear is reinforced by the spanwise velocities developed by the two branches of the propeller wake across the hydrofoil. Compared to an isolated propeller, these phenomena enhance turbulence production. The present results highlight that a downstream hydrofoil, typical of surface ships, is able to significantly intensify the wake signature of a propeller.     

Propeller tip and hub vortex dynamics in the interaction with a rudder

In the present paper, the interaction mechanisms of the vortices shed by a single-screw propeller with a rudder installed in its wake are addressed; in particular, following the works by Felli et al. (Exp Fluids 6(1):1–11, 2006a, Exp Fluids 46(1):147–1641, 2009a, Proceedings of the 8th international symposium on particle image velocimetry: Piv09, Melbourne, 2009b), the attention is focused on the analysis of the evolution, instability, breakdown and recovering mechanisms of the propeller tip and hub vortices during the interaction with the rudder. To investigate these mechanisms in detail, a wide experimental activity consisting in time-resolved visualizations, velocity measurements by particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) along horizontal chordwise, vertical chordwise and transversal sections of the wake have been performed in the Cavitation Tunnel of the Italian Navy. Collected data allows to investigate the major flow features that distinguish the flow field around a rudder operating in the wake of a propeller, as, for example, the spiral breakdown of the vortex filaments, the rejoining mechanism of the tip vortices behind the rudder and the mechanisms governing the different spanwise misalignment of the vortex filaments in the pressure and suction sides of the appendage.     

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.     

Flow measurement around a model ship with propeller and rudder

For the design of hull forms with better resistance and propulsive performance, it is essential to understand flow characteristics, such as wave and wake development, around a ship. Experimental data detailing the local flow characteristics are invaluable for the validation of the physical and numerical modeling of computational fluid dynamics (CFD) codes, which are recently gaining attention as efficient tools for hull form evaluation. This paper describes velocity and wave profiles measured in the towing tank for the KRISO 138,000 m³ LNG carrier model with propeller and rudder. The effects of propeller and rudder on the wake and wave profiles in the stern region are clearly identified. The results contained in this paper can provide an opportunity to explore integrated flow phenomena around a model ship in the self-propelled condition, and can be added to the International Towing Tank Conference benchmark data for CFD validation as the previous KCS and KVLCC cases.     

Experimental analysis of the flow field around a propeller–rudder configuration

The problem of an isolated free-running propeller–rudder combination is tackled in the present paper. The activity concerns phase-averaged velocity measurements by LDV along two transversal planes of the wake, just in the front and behind the rudder. In addition, visualizations of the chordwise interaction between the tip vortex filaments and the rudder, performed using a time resolved CMOS camera, are presented. The major phenomena that affect the performance of a rudder in the race of a propeller, with special emphasis on unsteady-flow aspects, are highlighted in this paper.     

Experimental investigation of a blunt trailing edge flow field with application to sound generation

The unsteady lift generated by turbulence at the trailing edge of an airfoil is a source of radiated sound. The objective of the present research was to measure the velocity field in the near wake region of an asymmetric beveled trailing edge in order to determine the flow mechanisms responsible for the generation of trailing edge noise. Two component velocity measurements were acquired using particle image velocimetry. The chord Reynolds number was 1.9 × 10⁶. The data show velocity field realizations that were typical of a wake flow containing an asymmetric periodic vortex shedding. A phase average decomposition of the velocity field with respect to this shedding process was utilized to separate the large scale turbulent motions that occurred at the vortex shedding frequency (i.e., those responsible for the production of tonal noise) from the smaller scale turbulent motions, which were interpreted to be responsible for the production of broadband sound. The small scale turbulence was found to be dependent on the phase of the vortex shedding process implying a dependence of the broadband sound generated by the trailing edge on the phase of the vortex shedding process.     

PIV analysis of flow around a container ship model with a rotating propeller

The flow characteristics of the propeller wake behind a container ship model with a rotating propeller were investigated using a two-frame PIV (Particle Image Velocimetry) technique. Ensemble-averaged mean velocity fields were measured at four different blade phases and ensemble-averaged to investigate the flow structure in the near-wake region. The mean velocity fields in longitudinal planes show that a velocity deficit is formed in the regions near the blade tips and hub. As the flow develops in the downstream direction, the trailing vortices formed behind the propeller hub move upward slightly due to the presence of the hull wake and free surface. Interaction between the bilge vortices and the incoming flow around the hull causes the flow structure to be asymmetric. Contour plots of the vorticity give information on the radial distribution of the loading on the blades. The radial velocity profiles fluctuate to a greater extent under the heavy (J=0.59) and light loading (J=0.88) conditions than under the design loading condition (J=0.72). The turbulence intensity has large values around the tip and trailing vortices. As the wake develops in the downstream direction, the strength of the vorticity diminishes and the turbulence intensity increases due to turbulent diffusion and active mixing between the tip vortices and the adjacent wake flow.     

Measurements in the flow around a marine propeller at the stern of an axisymmetric body

An experimental study of the flow around and behind an axisymmetric body driven by a marine propeller is reported. Experiments were performed in a wind tunnel to document this complex, unsteady, three-dimensional, turbulent shear flow. Measurements were made in the boundary layer and wake of the bare body with a fixed dummy hub for the propeller, with the dummy hub rotating, and finally, with the propeller in operation. A five-hole yaw probe was employed for the mean-flow measurements, and two- and threesensor hotwires were used to obtained the mean and turbulent velocity fields. Part 1 of this two-part paper describes the experimental arrangement and circumferentially-averaged results which clarify certain overall aspects of the flow when it is viewed as a rotationally-symmetric flow. These are of special interest in marine hydrodynamics. In Part 2, the triple-sensor hotwire data are analyzed using phase-averaging techniques to reconstruct the instantaneous velocity and Reynolds-stress fields downstream of the propeller to show the evolution of the wakes of individual blades, blade-tip vortices, and the complex flow associated with vortices generated at hub-blade junctions.     

Three-dimensional evolution of vortices and early features of coherent structure in the turbulent wake behind a 2-D circular cylinder

3-D evolution of Kármán vortex filaments and vortex filaments in braid regions in the turbulent wake of a 2-D circular cylinder is investigated numerically based on inviscid vortex dynamics by analyzing the response of the initially 2-D spanwise vortex filaments to periodic spanwise disturbance of varying magnitude, wavelength and initial phase angles. Our results reveal a kind of 3-D vortex system in the wake which consists of large scale horseshoe-shaped vortices and small scale γ-shaped vortex filaments as well as vortex loops. The mechanism and the dynamic process about the generation of streamwise vortical structure and the 3-D coherent structure are reported. currently published in the Chinese Edition of Acta Mechanica Sinica, Vol.25, No.3, 1993 The project supported by National Natural Science Foundation of China and the National Basic Research Project “Nonlinear Science”     

涡识别技术及其在船用螺旋桨伴流场中的应用

涡识别是很重要的流体问题, 为了在船用螺旋桨伴流场中寻找一种合理的涡识别方法, 本文结合实践, 研究了六种涡识别技术理论, 其中使用Burgers涡流和Lamb-Oseen涡流作了必要的解释, 讨论了各种识别方法的优缺点. 局部低压标准比较直观, 但深究其黏性和非定常影响后, 明显不足; 迹线或流线显然不能满足伽利略不变性, 会使辨别变得混乱; 涡度大小需要规定其阈值, 具有一定不确定性, 且也会识别不是涡的涡片; 速度梯度张量的复特征值也会有识别不出的区域; 速度梯度张量的第二不变量标准和对称张量的第二特征值标准能更好地识别涡核, 这两种标准有时等效. 螺旋桨伴流场的数值模拟是在开源软件OpenFOAM平台上实现的, 湍流大涡模型由一种局部动态方程建模, 此模型优于动态Smagorinsky模型. 最终的结果显示: 对于船用螺旋桨伴流场中的涡, 采用速度梯度张量的第二不变量和对称张量的第二特征值的结果基本一致, 而最小压力标准、流线或迹线标准、涡度值标准和速度张量的复特征值标准都存在一定的缺陷, 不适用于船用螺旋桨伴流场中的涡识别.      

Wake visualization of a heaving and pitching foil in a soap film

Many fish depend primarily on their tail beat for propulsion. Such a tail is commonly modeled as a two-dimensional flapping foil. Here we demonstrate a novel experimental setup of such a foil that heaves and pitches in a soap film. The vortical flow field generated by the foil correlates with thickness variations in the soap film, which appear as interference fringes when the film is illuminated with a monochromatic light source (we used a high-frequency SOX lamp). These interference fringes are subsequently captured with high-speed video (500 Hz) and this allows us to study the unsteady vortical field of a flapping foil. The main advantage of our approach is that the flow fields are time and space resolved and can be obtained time-efficiently. The foil is driven by a flapping mechanism that is optimized for studying both fish swimming and insect flight inside and outside the behavioral envelope. The mechanism generates sinusoidal heave and pitch kinematics, pre-described by the non-dimensional heave amplitude (0–6), the pitch amplitude (0°–90°), the phase difference between pitch and heave (0°–360°), and the dimensionless wavelength of the foil (3–18). We obtained this wide range of wavelengths for a foil 4 mm long by minimizing the soap film speed (0.25 m s⁻¹) and maximizing the flapping frequency range (4–25 Hz). The Reynolds number of the foil is of order 1,000 throughout this range. The resulting setup enables an effective assessment of vortex wake topology as a function of flapping kinematics. The efficiency of the method is further improved by carefully eliminating background noise in the visualization (e.g., reflections of the mechanism). This is done by placing mirrors at an angle behind the translucent film such that the camera views the much more distant and out-of-focus reflections of the black laboratory wall. The resulting high-quality flow visualizations require minimal image processing for flow interpretation. Finally, we demonstrate the effectiveness of our setup by visualizing the vortex dynamics of the flapping foil as a function of pitch amplitude by assessing the symmetry of the vortical wake.     

螺旋桨梢涡不稳定性机理与演化模型研究

螺旋桨尾流场的涡流特性是一个基础但又十分复杂的流体力学问题, 它的复杂性源于其蕴含复杂的漩涡系统, 且该漩涡系统会在高速的剪切层流动中不断演化, 其流体动力学行为, 如由稳定态演变为不稳定态的机理以及复杂工况环境中的流动现象, 一直是流体力学领域的难点和备受关注的热点问题. 从工程应用的角度看, 桨后梢涡的演化特性与船舶结构物的宏观特性直接相关, 更好地理解多工况下螺旋桨尾流的动力学特性, 将有助于改善与振动、噪声以及结构问题等相关的推进器性能, 对综合性能优良的下一代螺旋桨的设计和优化有着重要的现实意义. 本文基于延迟分离涡模拟、大涡模拟和无湍流模型模拟方法以及粒子图像测速流场测试分别开展了螺旋桨尾流动力学特性的数值与试验研究, 对螺旋桨尾流不稳定性的触发机理进行了揭示. 基于均匀来流中螺旋桨梢涡的演化机理, 提出了螺旋桨梢涡演化模型. 该模型能够较为准确地模拟螺旋桨梢涡的演化过程, 预测螺旋桨梢涡融合的时间和位置, 对螺旋桨流噪声预报和控制以及性能优良的螺旋桨设计具有重要意义.      

某调距导管桨系泊工况水力性能和流激直发声预报

采用经验证的计算流体力学方法,对某艏辅推调距导管桨设计螺距和系泊工况螺距的水动力性能进行了有效预报,并对系泊工况装船桨流激噪声进行了分析。系泊工况下,由于导管桨的抽吸作用在导管外壁近壁面区域存在与导管内部流动方向相反的逆向流动,且导管桨尾流场速度梯度分布不均匀、流动紊乱,此时桨叶与导管的推力之比约为1.2∶1。系泊工况船+桨的瞬态流场脉动信息表明,导管桨各部件噪声源强度均表现出从1倍到4倍叶频依次下降的规律,最强幅值集中在桨叶导边和导管内壁;在远场声源级频谱曲线中轴向测点线谱较高峰值位置体现出导管桨进流流场的流动特性。对比分析该艏辅推整体和各部件宽带声源级指向性,可知旋转部件(桨叶、桨榖)对总噪声级的贡献较大,静止部件是径向测点噪声的主要贡献源。     

Uncovering large-scale coherent structures in natural and forced turbulent wakes by combining PIV,POD, and FTLE

Planar velocity data of the unsteady separated flow in the turbulent wake of a circular cylinder obtained by particle image velocimetry (PIV) are analyzed in order to visualize the large-scale coherent structures associated with alternating vortex shedding at a Reynolds number of 2,150. Two different cases are examined: unforced vortex shedding in the natural wake and vortex lock-on incited by forced perturbations superimposed in the inflow velocity. Proper orthogonal decomposition (POD) is employed to reconstruct the low-order wake dynamics from randomly sampled snapshots of the velocity field. The reconstructed flow is subsequently used to determine the evolution of the finite-time Lyapunov exponent (FTLE) fields which identify the Lagrangian coherent structures. The results demonstrate that the combination of methods employed offers a powerful visualization tool to uncover large-scale coherent structures and to exemplify vortex dynamics in natural and forced bluff-body wakes.     

单一水平轴风电机组尾迹的模拟方法与流动机理研究综述

为实现碳达峰、碳中和“3060”目标, 风能将在我国能源体系发挥重要作用. 风力机尾迹是影响风电性能和度电成本的关键因素, 需在风力机布置和控制设计中充分考虑. 本文首先介绍风力机尾迹的数值模拟方法, 包括解析模型、低阶模型、大涡模拟和来流湍流生成方法. 解析模型和低阶模型可快速计算风力机尾迹, 但依赖于模型参数, 且不能或不能准确预测尾迹湍流特性. 结合风力机参数化模型的大涡模拟可准确预测尾迹蜿蜒等湍流特征, 是流动机理研究的有力工具, 可为发展快速预测模型提供数据和理论支撑. 接着, 本文介绍了叶尖涡、中心涡和尾迹蜿蜒并讨论其产生机理. 对于湍流来流, 叶尖涡主要存在于近尾迹. 蜿蜒是远尾迹的主要特征, 影响下游风力机的来流特征. 尾迹蜿蜒的产生有两种机制: 来流大尺度涡和剪切层失稳. 数值和观测结果显示两种机制共同存在. 机舱和中心涡对尾迹蜿蜒有重要影响. 采用叶片和机舱的致动面模型可准确预测尾迹蜿蜒. 研究显示不同风力机尾迹间的湍流特征存在相似性, 为发展尾迹湍流的快速预测模型提供了理论依据. 当前研究多关注平坦地形上的风力机尾迹, 复杂地形和海洋环境下的大气湍流和风力机尾迹的机理复杂, 现有工程模型无法准确预测, 有待深入研究.      

Phase sampling in the analysis of a propeller wake

A phase sampling procedure is used for the analysis of the non-steady, periodic flow field in the near wake of a marine propeller. This method allows to obtain a true ensemble averaging of the experimental measurements. The average is made over a large number of repeated experiments each of which is taken during a complete revolution of the propeller. The measurements are carried out in a recirculating water tunnel with a two-channel laser Doppler velocimeter. The computer-aided evaluation of the experimental results visualizes the following characteristic features of the wake: (1) the vortex sheet developing from the trailing edge; (2) a sudden increase of the axial velocity in the core of the tip vortex; (3) a boundary layer effect near the shaft of the propeller. From the analysis of the direction of vortex rotation along the radial direction of the blade, it is possible to derive information on the working conditions of the propeller.     

POD analysis of a finite-length cylinder near wake

The near wake of a wall-mounted finite-length square cylinder with an aspect ratio of 7 is investigated based on the proper orthogonal decomposition (POD) of the PIV data measured in three spanwise planes, i.e., z/d = 6, 3.5 and 1.0, near the cylinder free end, mid-span and fixed end (wall), respectively. The Reynolds number based on free-stream velocity (U _∞) and cylinder width (d) is 9,300. A two-dimensional (2D) square cylinder wake is also measured and analyzed at the same Reynolds number for the purpose of comparison. The structures of various POD modes show marked differences between the two flows. While the coefficients, a ₁ and a ₂, of the POD modes 1 and 2 occur within an annular area centered at a ₁ = a ₂ = 0 in the 2D wake, their counterparts are scattered all over the entire circular plane at z/d = 1.0 and 3.5 of the finite-length cylinder wake. Flow at z/d = 6 is dominated by POD mode 1, which corresponds to symmetrical vortex shedding and accounts for 54.0 % of the total turbulent kinetic energy (TKE). On the other hand, the POD modes 1 and 2, corresponding to anti-symmetrical vortex shedding, are predominant, accounting for about 45.0 % of the total TKE, at z/d = 3.5 and 1. It has been found that the flow structure may be qualitatively and quantitatively characterized by the POD coefficients. For example, at z/d = 6, a larger a ₁ corresponds to a smaller length of flow reversal zone and a stronger downwash flow. At z/d = 3.5 and 1, two typical flow modes can be identified from a ₁ and a ₂. While large a ₁ and/or a ₂ correspond to anti-symmetrical vortex shedding, as in a 2D cylinder wake, small a ₁ and a ₂ lead to symmetrical vortex shedding. Any values between the large and small a ₁ and/or a ₂ correspond to the flow structure between these two typical flow modes. As such, the probability of occurrence of a flow structure may be determined from the distribution of the POD coefficients.     

Effects of Periodic Excitation on the Flow Around a D-Shaped Cylinder at Low Reynolds Numbers

The baseline and forced flow around a bluff body with semi-elliptical D-shape was investigated by solving the 2D Navier–Stokes equations at low Reynolds numbers. A D-shape rather than the canonic circular-cylinder was selected due to the fixed separation points in the latter, enabling to study a pure wake rather than boundary-layer control. The correlation between Strouhal and Reynolds numbers, the mean drag, the lift and drag oscillations vs. the Reynolds number and wake structure were investigated and compared to experimental and numerical data. Effects of open-loop forcing, resulting from the influence of zero-mass-flux actuators located at the fixed separation points, were studied at a Reynolds number of 150. Fluidic rather than body motion or volume forcing was selected due to applicability considerations. The motivation for the study was to quantify the changes in the flow field features, as captured by Proper Orthogonal Decomposition (POD) analysis, due to open-loop forcing, inside and outside the “lock-in” regime. This is done in order to evaluate the suitability of low-order-models based on POD modes of this changing flow field, for future feed-back flow control studies. The evolution of the natural and the excited vortices in the Kármán wake were also investigated. The formation and convection regions of the vortex evolution were documented. It was found that the forcing causes an earlier detachment of the vortices from the boundary-layers, but does not affect their circulation or convection speeds. The results of the POD analysis of the near-wake flow show that the influence of the bluff body shape (“D”-shaped versus circular cylinder) on the baseline POD wake modes is small. It was found that the eigenfunctions (mode-shapes) of the POD velocity modes are less sensitive to slot excitation than the vorticity modes. As a result of the open-loop excitation, two types of mode-shape-change were observed: a mode can be exchanged with a lower-energy mode or shifted to a low energy level. In the latter case, the most energetic mode becomes the “actuator” mode. The evolution of one-slot excitation on still fluid (“Synthetic jet”) was studied and compared to published data and to “actuator” modes with external flow present. Based on the current findings, it is hypothesized that the cross-flow velocity POD modes are suitable for feedback control of wake flow using periodic excitation, due to their low sensitivity to the excitation as compared to the streamwise velocity or vorticity modes.