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.      

伯努利定理适用条件分析

本文分析了以理想流体的运动微分方程推导伯努利定理时所利用的限制条件,对有势力作用下不可压缩流动的流场中应用伯努利定理的矢径微分量、速度、涡量之间的关系进行了深入讨论.分析结果表明当流动为无旋流动或螺旋流时,伯努利常数在流场各点取相同值;伯努利常数在流涡面上各点取值相等,沿流线或涡线积分均为在流涡面上积分的特殊情况.     

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.     

考虑气核生长溃灭效应的螺旋桨梢涡空化初生数值模拟研究

梢涡空化是螺旋桨最早发生的空化类型,其一旦发生会显著增强舰船辐射噪声水平.因此,螺旋桨梢涡空化初生的预报是军舰临界航速确定的关键,长期以来受到船舶领域诸多专家学者的重点关注.微观气核受涡心低压作用而发生暴发式生长是梢涡空化初生的重要机制,而传统欧拉框架下的宏观空化模型用经验参数模化了微观气核的影响而无法对该过程准确模拟,影响对螺旋桨空化初生的准确预报.为了弥补传统模拟方法的不足,本研究发展并使用一种基于气泡动力学并考虑水相可压缩效应的欧拉-拉格朗日空化初生数值预报方法对螺旋桨梢涡空化初生进行了数值模拟研究.与实验结果对比表明,该模型能够准确地预报螺旋桨梢涡空化初生.此外,本研究不仅从微观气核角度探究不同来流气核尺寸对空化初生的影响,还进一步研究梢涡流动特性对气核演变的影响机制,初步探究初生空化在螺旋桨梢涡流场中的发声机理.在空化初生光学判断准则下尺寸越大的气核越容易被梢涡捕获而暴发式生长.气核在梢涡卷吸作用下逐渐靠近涡心低压区.在涡心低压区的持续作用下气核开始暴发式生长,并在半径达到最大后迅速收缩溃灭,产生强烈的正声压脉冲信号.     

基于速度梯度张量特征值的陷窝内旋涡分析

作为一种新型的涡流发生器,陷窝具有流动阻力小、综合传热性能高的特点,是现代高性能涡轮叶片内部冷却新技术. 旋涡的定量分析是陷窝强化传热优化设计的重要依据. 针对在不同陷窝模型下的旋涡结构、分离方式和背景压力变化引起的旋涡强度无法定量分析的问题,本文提出采用涡核速度和 涡核速度梯度张量特征值来定量分析旋涡的方法. 通过采用涡核处局部坐标系表示的速度矢量和速度梯度张量,得到了涡核的轴 向速度、径向速度、旋转角速度、轴向加速度和径向加速度,并在此基础上简化出了用最大轴向速度、最大轴向加速度和最大旋 转角速度综合表示的旋涡强度的定量分析方法. 用该方法分析了不同深宽比陷窝诱导的旋涡结构,随着深宽比的增大,最大轴向 速度、最大轴向加速度和最大旋转角速度均呈现明显的增大趋势,旋涡强度增大. 研究表明此方法具有数据处理简单、通用性强、 不受分离方式限制、不受背景压力影响的特点,且提取到的数据具有明确的物理意义,适用于各类旋涡定量分析.      

旋翼翼型动态失速流场特性PIV试验研究及L-B模型修正

为测量翼型动态失速的非定常涡流场特性,采用3D-PIV 技术,对典型直升机旋翼翼型SC1095 的动态失速流场特性进行测量,发现涡在不同位置处的输运速度不同:位于翼型表面的涡的无量纲速度为0.39,位于尾迹区的涡的无量纲速度为0.55. 利用前缘涡输运速度变化这一特征,改进了经典的翼型动态失速利什曼-贝多斯（Leishman-Beddoes,L-B）模型,将该模型中固定的涡时间常数修正为可以随涡位置变化的时变函数,修正后的模型计算得到翼型法向力峰值相对原L-B 模型提升5%,力矩系数负峰值相对原L-B 模型提升13%,与试验值相比更加吻合,表明修正后的翼型动态失速模型更好地体现了翼型前缘涡的物理特征.      

基于速度梯度张量特征值的陷窝内旋涡分析

作为一种新型的涡流发生器,陷窝具有流动阻力小、综合传热性能高的特点,是现代高性能涡轮叶片内部冷却新技术.旋涡的定量分析是陷窝强化传热优化设计的重要依据.针对在不同陷窝模型下的旋涡结构、分离方式和背景压力变化引起的旋涡强度无法定量分析的问题,本文提出采用涡核速度和涡核速度梯度张量特征值来定量分析旋涡的方法.通过采用涡核处局部坐标系表示的速度矢量和速度梯度张量,得到了涡核的轴向速度、径向速度、旋转角速度、轴向加速度和径向加速度,并在此基础上简化出了用最大轴向速度、最大轴向加速度和最大旋转角速度综合表示的旋涡强度的定量分析方法.用该方法分析了不同深宽比陷窝诱导的旋涡结构,随着深宽比的增大,最大轴向速度、最大轴向加速度和最大旋转角速度均呈现明显的增大趋势,旋涡强度增大.研究表明此方法具有数据处理简单、通用性强、不受分离方式限制、不受背景压力影响的特点,且提取到的数据具有明确的物理意义,适用于各类旋涡定量分析.     

气固两相混合层二维涡配对的数值研究

采用双向耦合模型中的速度耦合模型,数值模拟了气固两相混合层中涡的配对、合并过程,文中采用拟谱方法对流场进行了直接数值模拟,用Lagrange模型跟踪固粒,结果发现,在双向耦合过程中Stokes数仍然是重要的参数,但不是唯一影响流场的参数,流场的发展还与固粒的体积浓度、固粒的相对密度以及固粒大小等因素有关。固粒抑制流场的发展,阻碍涡的配对、合并,加快了涡量的扩散;小St数的固粒仍然跟随流体运动,大St数的固粒趋向于低涡量区的趋势减弱。     

分离涡方法对梢涡中脉动量的数值模拟

分别用RANS-SA方法和DES方法对NACA0012翼端梢涡进行模拟计算,分析了梢涡区域网格局部加密对梢涡计算结果的影响,并与实验结果进行了对比.相比于RANS-SA方法,DES方法在梢涡流场计算中具有更好的适用性,能够得到更准确的流动信息和更精细的涡结构;另外,网格局部加密对脉动量的计算影响很大.通过对脉动量的分析发现,在近尾缘处,几股涡的融合产生了比较强烈的脉动,随着梢涡的逐渐稳定,脉动量也逐渐减小;现有的实验结果显示在偏下游处会产生梢涡的振荡现象,使统计脉动量增大,而本文计算中未发现该现象.     

全附体潜艇粘性流场的RANS模拟及场量和涡量的校验分析

采用相同拓扑结构和相近网格质量的4套网格和5种湍流模型,对全附体Suboff潜艇粘性流场进行RANS模拟,分析了网格密度、节点空间分布规律和湍流模型对计算精度的影响,详细校验了其力积分量、速度场量和涡量特征。结果表明:网格密度最大的G4网格(140万)计算精度最高,总阻力较实验值误差为0.723%,其采用SST湍流模型时最优。计算得到的压力系数和剪切应力系数分布均与实验值吻合很好;桨盘面速度等值线分布计算精度与文献相当,轴向相对速度0.9以上的计算半径稍大于实验值,其余半径与实验吻合较好;桨盘面上0.25倍半径处速度分量沿周向分布计算精度较文献高,轴向分量与实验值吻合较好,径向分量峰值稍小于实验值,但峰值所处周向位置与实验值一致。成功捕捉到了附体端面绕流诱导对旋涡、附体叶根截面下游处项链形涡对、尾翼端面尾缘上方附着涡蹄、附体马蹄涡系、尾翼截面通道流体挤压作用诱导涡以及桨盘面涡量汇集的潜艇涡量场特征,且围壳端面绕流诱导对旋涡沿流动方向持续稳定,不影响桨盘面涡量场,均与文献中由大涡模拟模拟得到的定性结论一致。研究表明,在网格密度较大、节点分布合理、网格质量较高、湍流模型选取适当和壁面函数使用有效的条件下,RANS模拟潜艇粘性流场的场量和涡量特征同样具有很高的计算精度,能够在工程应用中有力支撑新型艇型设计与性能分析。     

Volumetric three-component velocimetry measurements of the turbulent flow around a Rushton turbine

Volumetric three-component velocimetry measurements have been taken of the flow field near a Rushton turbine in a stirred tank reactor. This particular flow field is highly unsteady and three-dimensional, and is characterized by a strong radial jet, large tank-scale ring vortices, and small-scale blade tip vortices. The experimental technique uses a single camera head with three apertures to obtain approximately 15,000 three-dimensional vectors in a cubic volume. These velocity data offer the most comprehensive view to date of this flow field, especially since they are acquired at three Reynolds numbers (15,000, 107,000, and 137,000). Mean velocity fields and turbulent kinetic energy quantities are calculated. The volumetric nature of the data enables tip vortex identification, vortex trajectory analysis, and calculation of vortex strength. Three identification methods for the vortices are compared based on: the calculation of circumferential vorticity; the calculation of local pressure minima via an eigenvalue approach; and the calculation of swirling strength again via an eigenvalue approach. The use of two-dimensional data and three-dimensional data is compared for vortex identification; a ‘swirl strength’ criterion is less sensitive to completeness of the velocity gradient tensor and overall provides clearer identification of the tip vortices. The principal components of the strain rate tensor are also calculated for one Reynolds number case as these measures of stretching and compression have recently been associated with tip vortex characterization. Vortex trajectories and strength compare favorably with those in the literature. No clear dependence of trajectory on Reynolds number is deduced. The visualization of tip vortices up to 140° past blade passage in the highest Reynolds number case is notable and has not previously been shown.     

Topological visualisation of focal structures in free shear flows

This paper describes a method for identifying and visualising the three-dimensional geometry of focal (vortex) structures in complex flows. The method is based primarily on the classification of the local topology as it is identified from the values of the velocity gradient tensor invariants. The identification of the local topology is reference frame invariant. Therefore, focal (vortex) structures can be unambiguously identified in these flows. A novel flow visualisation method is introduced whereby focal structures are rendered using a solid model view of the local topology. This new approach is applied to the identification of focal structures in three-dimensional plane mixing layer and plane wake flows.     

Local vorticity computation approach in double distribution functions based lattice Boltzmann methods for flow and scalar transport

Computation of vorticity, or the skew-symmetric velocity gradient tensor, in conjunction with the strain rate tensor, plays an important role in the flow classification, in vortical structure identification and in the modeling of various complex fluids and flows. For the simulation of flows accompanied by the advection-diffusion transport of a scalar field (e.g., temperature), double distribution functions (DDF) based lattice Boltzmann (LB) methods, involving a pair of LB schemes are commonly used. We present a new local vorticity computation approach by introducing an intensional anisotropy of the scalar flux in the third order, off-diagonal moment equilibria of the LB scheme for the scalar field, and then combining the second order non-equilibrium components of both the LB methods. As such, any pair of lattice sets in the DDF formulation that can independently support the third order off-diagonal moments would enable local determination of the complete flow kinematics, with the LB methods for the fluid motion and the transport of the passive scalar respectively providing the necessary moment relationships to determine the symmetric and skew-symmetric components of the velocity gradient tensor. Since the resulting formulation is completely local and do not rely on any finite difference approximations for velocity derivatives, it is by design naturally suitable for parallel computation. As an illustration of our approach, we formulate a DDF-LB scheme for local vorticity computation using a pair of multiple relaxation times (MRT) based collision approaches on two-dimensional, nine velocity (D2Q9) lattices, where the necessary moment relationships to determine the velocity gradient tensor and the vorticity are established via a Chapman-Enskog analysis. Simulations of various benchmark flows demonstrate good accuracy of the predicted vorticity fields using our approach against available solutions, including numerical results, with a second order convergence. Furthermore, extensions of our formulation for a variety of collision models, including those based on cascaded and non-cascaded central moments, to enable local vorticity computation are presented.     

Flow Features in a Fully Developed Ribbed Duct Flow as a Result of MILES

The present contribution describes the topology associated with the turbulent flow in a square duct partially blocked by a rib of square section mounted on a single wall. The flow is simulated by means of a MILES method and the resulting velocity fields are analysed using the concepts of stream surface, vortex core detection, wall streamline and bifurcation line. Instantaneous and time averaged coherent structures are extracted applying the second scalar invariant of the velocity gradient tensor (so-called $Q$ criterion), respectively, to the instantaneous and time averaged velocity fields. This postprocessing reveals significant 3D effects induced by the geometry, namely the influence of the side walls, which is clearly identified. The combination of the different visualisation techniques offers a complement to the standard representation based on Eulerian statistics and contributes to a deeper understanding of this complex flow.     

Aerodynamic characteristics of flapping motion in hover

The aim of the present work is to understand the aerodynamic phenomena and the vortex topology of an unsteady flapping motion by means of numerical and experimental methods. Instead of the use of real insect/bird wing geometries and kinematics which are highly complex and difficult to imitate by an exact modeling, a simplified model is used in order to understand the unsteady aerodynamics and vortex formation mechanisms during the different phases of the flapping motion. The flow is assumed to be laminar with a Reynolds number of 1,000. Direct numerical simulations, laser sheet visualizations and particle image velocimetry (PIV) measurements are performed for the phenomenological analysis of the flow. The vortex dynamics and their identification are put in evidence with PIV measurements by considering velocity magnitude, streamlines, second invariant of velocity gradient (Q-criteria), vorticity contours and Eurlerian accelerations.     

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.     

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.     

Measurement of mean rotation and strain-rate tensors by using stereoscopic PIV

A technique is described for measuring the mean velocity gradient (rate-of-displacement) tensor by using a conventional stereoscopic particle image velocimetry (SPIV) system. Planar measurement of the mean vorticity vector, rate-of-rotation and rate-of-strain tensors and the production of turbulent kinetic energy can be accomplished. Parameters of the Q criterion and negative λ₂ techniques used for vortex identification can be evaluated in the mean flow field. Experimental data obtained for a circular turbulent jet issuing normal to a crossflow in a low speed wind tunnel for a jet-to-crossflow velocity ratio of 3.3 are presented to show the applicability of the proposed technique. The results reveal the presence of a secondary counter-rotating vortex pair (SCVP) which is located within the jet core and has a sense of rotation opposite to that of the primary one (PCVP). Consistency of the measurements is verified by the agreement of data obtained in two perpendicular planes. Accuracy of the data is discussed and algebraic relations for some measurement uncertainties are presented.