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| 1 | 第三代涡识别方法及其应用综述显示文摘根据Liu等的划分,涡识别方法的发展可以分为三代。第一代方法基于涡量,并将Cauchy-Stokes分解得到的速度梯度张量反对称部分(涡量张量)视为旋转。然而众多研究人员已发现涡量与涡的关联性并不是很高。为了克服第一代方法的缺陷,以Q、l2、D和lci等方法为代表的第二代涡识别方法应运而生。但是第二代方法在实际使用中依赖于与具体算例相关的阈值并且会被剪切影响。为了解决阈值问题,2016年发展起来的Ω方法将涡定为涡量大于变形的联通区域并以相对值表示。而2017年和2018年发展起来的Liutex向量则提供了一个系统化数学化的流体局部刚体转动定义,包括局部旋转轴和转动强度,Ω方法,Liutex向量及一系列有关方法被定义为第三代涡识别方法。该文回顾了三代涡识别方法的发展历程,系统讨论了如何精确地从流体运动中分解得到刚性旋转部分,重点介绍由美国德州大学阿灵顿分校刘超群教授及其团队主导的第三代涡识别方法所取得的突破及其应用。主要突破包括:①Ω涡识别方法克服了传统涡识别方法需要针对具体流动调整阈值的问题,并能同时捕捉到强涡和弱涡;②基于Liutex的思想,能够严格获得流体运动的局部刚性旋转部分,包括标量、矢量和张量形式;③第三代方法能够合理地回答涡的六大要素问题,即涡的绝对强度、相对强度、当地旋转轴、涡核中心、涡核大小和涡边界,而前两代方法则无法处理。该文详细介绍了第三代方法的六个具体应用,其结果表明:①在涡识别方面,第三代方法大大优于前两代方法;②第三代方法不仅是涡识别和显示的后处理方法,而且是精确表示涡的物理量;③第三代方法能够定量化地表示涡的特性(六大要素)。Liutex作为一个新生的物理量,有方向有大小。Liutex的方向就是当地涡线和涡强度(此处区别于第一代涡量线和涡量强度的定义)等值面法线的方向,其大小准确代表当地旋转角速度(涡的绝对强度),且在当地非零Liutex方向上仅有瞬时的拉压形变,流体旋转均发生在与Liutex方向垂直的平面内。在等值面缩小变为一条线的地方,它就是涡核中心。它唯一代表涡核,唯一代表湍流的涡结构,与阈值完全无关。更重要的是它准确代表一个力,代表湍流产生的原动力,直接影响湍流生成。Liutex和其他第三代涡识别方法的发现代表湍流研究的一个崭新时代的来临,它开启了湍流研究的新纪元。 | 王义乾 桂南 | 2019 | 水动力学研究与进展(A辑)2019，34，4： | 36 |
| 2 | Liutex-涡定义和第三代涡识别方法显示文摘本文回顾了涡定义和涡识别方法的发展历史,着重介绍了作者UTA(University of Texas at Arlington)团队及其合作者在涡科学和湍流研究的一些最新学术创新成果。UTA团队发现了可以定量描述流体刚性转动部分的物理量——Liutex向量,其主要思想是把流体刚性转动从流体运动中提取出来,进而用Liutex来定义和识别涡结构,并已在广泛应用中证明了其作为涡识别方法的优越性。基于Liutex向量可以进一步研究涡量分解、速度梯度张量分解、流体运动分解、湍流结构、湍流生成机理以及旋涡的科学识别,为流体运动学的发展开辟了广阔的研究空间。区别于第一代涡识别方法和第二代涡识别方法,Liutex是一个向量,其方向代表当地转轴,大小代表当地流体刚性旋转角速度的二倍。本文详细介绍了基于Liutex向量的第三代涡的定义和识别方法,包括Liutex等值面、Liutex-Omega等值面、Liutex向量线、Liutex涡核线、以及最新发现的中低雷诺数湍流边界层中的Liutex-5/3幂次相似律,其发现很大程度上扩大了传统湍流能谱幂次律的适用范围,对建立湍流模型具有重要意义。 | 刘超群 | 2020 | 空气动力学学报2020，38，3： | 31 |
| 3 | A review of transient flow structure and unsteady mechanism of cavitating flow显示文摘The flow structure and the unsteady mechanism of the unsteady cavitating flow are reviewed in this paper. The flow patterns and structures in different cavitation regime, for the attached cavitation and the vortical cavitation, are shown with both the visualization and the quantitative information. The attached cavitating flow around the Clark-Y hydrofoil and the vortical cavitating flow around the Tulin hydrofoil are considered. In particular, the phenomena such as the large-scale vortex structure and the re-entrant flow associated with the cloud cavitation, and the cavitating vortex street's forming and crumbling are described. The evolution of the cavitation structure in the transient sheet/cloud cavity forming, along with the cavity collapse induced by the re-entrant flow and the shock wave propagation are discussed. The perspective future research of higher fidelity simulations, and the accurate identifications of the cavitating vortex structure is commented. | Biao Huang Si-cong Qiu Xiang-bin Li Qin Wu Guo-yu Wang | 2019 | Journal of Hydrodynamics2019，31，3： | 24 |
| 4 | 第二代涡识别方法在混流泵内部流场中的适用性分析显示文摘流体机械内部涡的产生会严重影响流体机械的内流场及外性能,速度、压力等传统方式并不能全面揭示流场真实的流动情况,而涡识别方法可以表征混流泵内部涡结构,识别涡的大小、位置和演变规律。采用大涡模拟对混流泵内部流场进行高精度数值模拟,由此获得不同涡识别方法(涡量、螺旋度方法、λ2准则和Q准则)在混流泵内流场中识别出的涡结构。结果表明:与涡量准则法相比,采用第二代涡识别方法更能获得详细的混流泵内部的涡结构。其中,Q准则能剔除绝大部分的剪切层影响,较其他涡识别方法更好地捕捉混流泵内部的涡结构,显示的涡大小更加精确。通过Q准则的识别,提取了混流泵叶轮流道内规则的通道涡,此类型涡结构均匀分布在各个流道,叶片叶顶泄漏涡结构清晰,摆脱了壁面剪切层的干扰;导叶叶片靠近轮毂处同样获取了规则的通道涡,通道涡均匀的分布在各个流道内,叶片尾缘脱落涡摆脱了壁面剪切层的干扰,可以看到清晰的涡结构。Q准则在第二代涡识别方法中脱颖而出,更适用于混流泵内流场的涡识别,在涡结构以及涡大小的显示方面都具有优势,可用于旋转机械内流场的分析。 | 赵斌娟 谢昀彤 廖文言 韩璐遥 付燕霞 黄忠富 | 2020 | 机械工程学报2020，56，14： | 23 |
| 5 | Liutex (vortex) core definition and automatic identification for turbulence vortex structures显示文摘As a milestone research in vortex identification(VI),the physical quantity of Liutex,including its forms of scalar,vector and tensor,was systematically explored and rigorously obtained as the third-generation(3G)of the vortex definition and identification methods distinguished from the first generation(1G)by vorticity and the second generation(2G)by the vortex identification(VI)criteria solely dependent on the velocity gradient tensor eigenvalues.Based on these findings,the vortex-core lines were abstracted from the well-defined Liutex,and for the first time,were automatically generated and massively visualized using computer.The distinctive characteristics of these vortex cores with the intriguing threshold-independency make them be the uniquely appropriate entity to represent and to depict the vortex structures in turbulence.The letter made use of the DNS data for the natural transition in a zero-pressure gradient flat-plate(Type-A turbulent boundary layer(TBL))and the fully-developed turbulence in a square annular duct(Type-B TBL)to demonstrate the vortex structure represented by the vortex-core lines.The 3G VI approach based on the vortex-core lines is capable of profoundly uncovering the vortex natures.Moreover,the capability of automatically identifying the vortex cores and massively visualizing the large number of vortex-core behaviors in a transient way will enable the fluid-mechanics and other related-science communities to step into a new era to explore the intrinsic natures of the centennial puzzle of turbulence and other vortex-related phenomena in future. | Hongyi Xu Xiao-shu Cai Chaoqun Liu | 2019 | Journal of Hydrodynamics2019，31，5： | 21 |
| 6 | A Liutex based definition and identification of vortex core center lines显示文摘Six core issues for vortex definition and identification concern with (1) the absolute strength,(2) the relative strength,(3) the rotational axis,(4) the vortex core center,(5) the vortex core size, and (6) the vortex boundary (Liu C. 2019). However, most of the currently popular vortex identification methods, including the Q criterion, the criterion and the Acj criterion etc., are Eulerian local region-type vortex identification criteria and can only approximately identify the vortex boundary by somewhat arbitrary threshold. On the other hand, the existing Eulerian local line-type methods, which seek to extract line-type features such as vortex core line, are not entirely satisfactory since most of these methods are based on vorticity or pressure minimum that will fail in many cases. The key issue is the lack of a reasonable mathematical definition for vortex core center. To address this issue, a Liutex (previously named Rortex) based definition of vortex core center is proposed in this paper. The vortex core center, also called vortex rotation axis line here, is defined as a line where the Liutex magnitude gradient vector is aligned with the Liutex vector, which mathematically implies that the cross product of the Liutex magnitude gradient vector and the Liutex vector on the line is equal to zero. Based on this definition, a novel three-step method for extracting vortex rotation axis lines is presented. Two test cases, namely the Burgers vortex and hairpin vortices, are examined to justify the proposed method. The results demonstrate that the proposed method can successfully identify vortex rotation axis lines without any user-specified threshold, so that the proposed method is very straightforward, robust and efficient. | Yi-sheng Gao Jian-ming Liu Yi-fei Yu Chaoqun Liu | 2019 | Journal of Hydrodynamics2019，31，3： | 19 |
| 7 | Liutex theoretical system and six core elements of vortex identification显示文摘The third-generation vortex identification method of Liutex(previously called Rortex)was introduced by the team led by Prof.Chaoqun Liu from University of Texas at Arlington to mathematically extract the rigid rotation part from the fluid motion,and thus to define and visualize vortices.Unlike the vorticity-based first generation and the scalar-valued second generation,Q,λ2,Δandλci methods for example,the Liutex vector provides a unique,mathematical and systematic way to define vortices and visualize vortical structures from multiple perspectives without ambiguity.In this article,we summarize the recent developments of the Liutex framework and discuss the Liutex theoretical system including its existence,uniqueness,stability,Galilean invariance,locality and globality,decomposition in tensor and vector forms,Liutex similarity in turbulence,and multiple Liutex-based vortex visualization methods including Liutex lines,Liutex magnitude iso-surfaces,Liutex-Ωmethod,and Liutex core line method,etc..Thereafter,the six core elements of vortex identification,including(1)absolute strength,(2)relative strength,(3)local rotational axis,(4)vortex rotation axes,(5)vortex core size,(6)vortex boundary,are used as touchstones against which the Liutex vortex identification system is examined.It is demonstrated with illustrative examples that the Liutex system is able to give complete and precise information of all six core elements in contrast to the failure and inaccuracy of the first and second-generation methods.The important concept that vorticity cannot represent vortex and the superiority of the Liutex system over previous methods are reiterated and stated in appropriate places throughout the paper.Finally,the article concludes with future perspectives,especially the application of the Liutex system in studying turbulence mechanisms encouraged by the discovery of Liutex similarity law.As a newly defined physical quantity,Liutex may open a door for quantified vortex and turbulence research including Liutex(vortex)dynamics and lead the community out of the shadow of turbulence research which traditionally relies on observations,graphics,assumptions,hypotheses,and other qualitative analyses.An optimistic projection is that the Liutex system could be critical to investigation of the vortex dynamics in applications from hydrodynamics,aerodynamics,oceanography,meteorology,etc.and to research of the generation,sustenance,modelling and controlling of turbulence. | Yi-qian Wang Yi-sheng Gao Hongyi Xu Xiang-rui Dong Jian-ming Liu Wen-qian Xu Meng-long Chen Chaoqun Liu | 2020 | Journal of Hydrodynamics2020，32，2： | 17 |
| 8 | Vortex identification methods in marine hydrodynamics显示文摘In this paper,several commonly used vortex identification methods for marine hydrodynamics are revisited.In order to extract and analyse the vortical structures in marine hydrodynamics,the Q,λ2-criterion and modified normalized Liutex/RortexΩR method are utilized for vortex identification for propeller open water test,ship drag test,ship propeller-rudder interaction,VIV of a marine riser and VIM of a Spar platform.The limitation of Q andλ2-criterion is discussed.The Liutex/RortexΩR method is promising for convenient and accurate vortex identification and visualization.However,care should be taken when choosing the small parameter b0 forΩR.We proposed recommended values of b0 for marine hydrodynamic problems. | Wei-wen Zhao Jian-hua Wang De-cheng Wan | 2020 | Journal of Hydrodynamics2020，32，2： | 14 |
| 9 | Cavitation vortex dynamics of unsteady sheet/cloud cavitating flows with shock wave using different vortex identification methods显示文摘Cavitation is a complex multiphase flow phenomenon with an abrupt transient phase change between the liquid and the vapor, including multiscale vortical motions. The transient cavitation dynamics is closely associated with the evolution of the cavitation vortex structures. The present paper investigates the cavitation vortex dynamics using different vortex identification methods, including the vorticity method, the Q criterion method, the Omega method (Ω), the method and the Rortex method. The Q criterion is an eigenvalue-based criterion, and in the Ω method, the parameter is normalized, is independent of the threshold value and in most conditions Ω= 0.52 . The Rortex method is based on an eigenvector-based criterion. Numerical simulations are conducted using the implemented compressible cavitation solver in the open source software OpenFOAM for the sheet/cloud cavitating flows around a NACA66 (mod) hydrofoil fixed at a = 6°,= 1.25 and Re = 7.96 × 10^5 . The flow is characterized by the alternate interactions of the re-entrant flow and the collapse induced shock wave. Results include the vapor structures and the vortex dynamics in the unsteady sheet/cloud cavitating flows, with emphasis on the vortex structures in thecavitation region, the cavity interface, the cavity closure, the cavity wakes, and the foil wakes with the shedding cavity. The comparisons of the various methods, including that the vorticity method, the Q criterion method, the Ω method, the λ2 method and the Rortex method, show the performances of different methods in identifying the cavitation vortex structures. Generally, during the attached cavity growth stage, the Q criteria can well predict the vortex structures in the cavitation region and at the foil trailing edge in the pure liquid region, while with the Ω method and the Rortex method, the vortex structures outside the attached cavity and on the foil pressure side can also be predicted. The λ2 method can well predict the vortex structures in the cavity closure region. During the re-entrant jet development stage, the vortex structures in the re-entrant jet region is weak. During the cavity cloud shedding stage, the vortex dynamics at the foil leading edge covered by newly grown cavity sheet is different from that during the attached cavity sheet growth stage. During the shock wave formation and propagation stage, strong vortex structures with both the size and the strength are observed owing to the cavity cloud shedding and collapse behavior. The influence of the small parameter ε in the Ω method on the cavitation vortex identification is discussed. | Chang-chang Wang Ying Liu Jie Chen Fu-yi Zhang Biao Huang Guo-yu Wang | 2019 | Journal of Hydrodynamics2019，31，3： | 12 |
| 10 | LES method of the tip clearance vortex cavitation in a propelling pump with special emphasis on the cavitation-vortex interaction显示文摘The turbulent cavitating flow around the propelling pump tip clearance is numerically simulated using the large eddy simulation(LES)method coupled with the Zwart-Gerber-Belamri(ZGB)cavitation model to investigate the cavitation-vortex interaction mechanism.The calculated cavitation structures around the blades are in a remarkable agreement with the experimental results.The distributions of the tip clearance vortex under two cavitation conditions are obtained and compared.The results show that the cavitation development enhances the vorticity generation and the flow unsteadiness around the tip clearance of a propelling pump.Vortices are basically located around the cavitation areas,particularly in the tip clearance region,during the cavitation.The relative vorticity transport equation is applied for the cavitating turbulent flows and it is further indicated that the vortex stretching term makes the main contribution in the vortex production,and the baroclinic torque and dilation terms are important source terms for the vorticity generation in the cavitating flow.In addition,the viscous diffusion term and the Coriolis force term are significant in modifying the vorticity field inside the blade tip clearance. | Cheng-zao Han Shun Xu Huai-yu Cheng Bin Ji Zhi-yuan Zhang | 2020 | Journal of Hydrodynamics2020，32，6： | 9 |
| 11 | Liutex-based vortex dynamics:A preliminary study显示文摘Vortex dynamics,with the possibility of efficient flow control,is explored in this study based on the new introduced vortex definition and identification system of Liutex.With the six core elements of vortex identification,including(1)absolute strength,(2)relative strength,(3)local rotational axis,(4)global rotational axis,(5)vortex core size and(6)vortex boundary,provided by the Liutex system,it is possible to numerically devise strategies,primarily by introducing additional source terms in Navier-Stokes equations,which we call Liutex force field model here,to control the vortex regions.Two methodologies of centripetal force model and counter-rotation force model are preliminarily investigated in a cavitating flow around two-dimensional Clark-Y hydrofoil.It is found that Liutex based models are capable of illustrating the vortex dynamics and possibly strengthening or weakening the vortices. | Hai-dong Yu Yi-qian Wang | 2020 | Journal of Hydrodynamics2020，32，6： | 9 |
| 12 | The applicability of vortex identification methods for complex vortex structures in axial turbine rotor passages显示文摘The complex vortex structures in the flow around turbine rotor passages, with weak or strong, large or small vortices, interacting with each other, often generate most of aerodynamic loss in turbomachines. Therefore, it is important to identify the vortex structures accurately for the flow field analysis and the aerodynamic performance optimization for turbomachines. In this paper, by using 4 vortex identification methods (the Q criterion, the Q method, the Liutex method and the Q -Liutex method), the vortices are identified in turbine rotor passages. In terms of the threshold selection, the results show that the D method and the Q -Liutex method are more robust, by which strong and weak vortices can be visualized simultaneously over a wide range of thresholds. As for the display consistency of the vortex identification methods and the streamlines, it is shown that the Liutex method gives results coinciding best with the streamlines in identifying strong vortices, while the Q -Liutex method gives results the most consistent with the streamlines in identifying weak vortices. As to the relationship among the loss, the vortices and the shear, except for the Q criterion, the other three methods can distinguish the vortical regions from the high shear regions. And the flow losses in turbine rotor passages are often related to high shear zones, while there is a small loss within the core of the vortex. In order to obtain the variation of vortices in the turbine rotor passages at different working points, the Liutex method is applied in 2 cases of a turbine with different angles of attack. The identification results show that the strengths of the tip leakage vortex and the upper passage vortex are weaker and the distance between them is closer at a negative angle of attack. This indicates that the Liutex method is an effective method, and can be used to analyze the vortex structures and their evolution in turbine rotor passages. | Yu-fan Wang Wei-hao Zhang Xia Cao Hong-kai Yang | 2019 | Journal of Hydrodynamics2019，31，4： | 8 |
| 13 | Liutex similarity in turbulent boundary layer显示文摘Kolmogorov's 1941 theory(K41)of similarity hypotheses and the-5/3 law for energy spectrum are considered as the most important theoretical achievement in turbulence research and the success of the modem turbulence theory.The assumptions of sufficient high Reynolds number and isotropy of turbulence that K41 based upon,however,cannot generally be met in practice,and thus discrepancy is often observed between the f law and direct numerical simulation(DNS)results of boundary layers in wall bounded turbulence,especially for moderate to low Reynolds number flows.Liutex vector is a recently defined new physical quantity which is extracted from turbulent flow to represent the rigid rotation part of fluid motion.Actually,Liutex is free from viscous dissipation and thus independent of Reynolds number,relaxing the very high Reynold number assumption of K41.Liutex similarity has been solidly demonstrated by DNS for a moderate Reynolds number turbulent boundary layer(Reθ≈1000),both the frequency and wavenumber spectrum of Liutex accurately matches the-5/3 law,which is obviously much better than the turbulence energy spectrum,while vorticity and other popular vortex identification methods,Q criterion for example,do not possess such a distinguished feature due to stretching and shearing contamination. | Wen-qian Xu Yi-qian Wang Yi-sheng Gao Jian-ming Liu Hua-shu Dou Chaoqun Liu | 2019 | Journal of Hydrodynamics2019，31，6： | 8 |
| 14 | 流体的旋涡特征提取方法综述显示文摘近年来,流体可视化已成为计算机图形学领域的一个研究热点,其最重要的目的之一是旋涡特征的提取与可视化。由于目前仍未有一个通用的定义描述旋涡,导致文献对旋涡是否存在的判断依据各不相同。为了对流体的旋涡特征提取方法进行较为系统的综述,首先对旋涡提取研究方向的相关概念进行解释,回顾流体旋涡特征提取方法的发展情况再进行总结,将常用的旋涡提取方法分为基于点、线、几何和基于机器学习的方法。对于新近提出的参考系不变性,将旋涡提取方法分为伽利略不变性、旋转不变性和拉格朗日不变性。为了比较不同方法的优势和缺陷,在综述每一类方法时分别给出若干经典方法,为研究者提供了一个清晰的研究思路。最后总结每类方法存在的难点和问题,并指出今后的研究重点。 | 邵绪强 刘艺林 杨艳 林丽娜 | 2020 | 图学学报2020，41，5： | 7 |
| 15 | A general alternate loading technique and its applications in the inverse designs of centrifugal and mixed-flow pump impellers显示文摘For the inverse designs of centrifugal and mixed-flow pump impellers,clarifying the generation process of secondary flows and putting forward corresponding suppression measures is an important approach to improve the impeller performance.In this paper,to provide a better qualitative insight into the generation mechanism of secondary flows in the impeller,a simple kinematic equation is derived based on the ideal assumptions,which indicates that the potential rothalpy gradient(PRG)is the most important dynamic source that actively induces secondary vortical flows.Induced by the natural adverse PRG on the S1 and S2 stream surfaces,two typical secondary flows,H-S and P-S secondary flows,are clearly presented.To specially suppress these typical secondary flows,a general alternate loading technique(GALT)is proposed,aiming to adjust the real blade loadingδp to control the PRG features.At the blade fore part,theδp on the hub streamline should be slowly increased to avoid breakneck growth of the potential rothalpy to reduce adverse streamwise PRG on the S2 streamsurface.At the blade middle part,theδp should be moderately decreased to reduce adverse streamwise PRG on the S1 streamsurface.At the blade aft part,the difference in theδp between the shroud and hub streamlines should be decreased faster to control the exit uniformity.By applying the GALT to the impeller designs of three typical pump types in hydraulic engineering,the organizational effect of the PRG on fundamental flow structures is proven.The GALT can effectively control the PRG distributions and suppress the secondary flows,thereby widening the pump’s high-efficiency zone,improving flow uniformity and suppressing pressure fluctuations.Compared with the current Z-G method and the ALT,the GALT can meet the requirements of'de-experience'better,thereby enabling the designers to obtain good products explicitly and quickly. | WANG ChaoYue WANG FuJun AN DongSen YAO ZhiFeng XIAO RuoFu LU Li HE ChengLian ZOU ZhiChao | 2021 | Science China(Technological Sciences)2021，64，4： | 7 |
| 16 | New ideas on governing equations of fluid dynamics显示文摘Classical fluid kinematics or Cauchy-Stokes decomposition mistreated vorticity as fluid rotation and mixed flow stretching with shearing.Classical fluid dynamics or Navier-Stokes(N-S)equations are based on the classical kinematics and treated vorticity as null in contribution of forces and mixed the stretching force with the shearing force,which is not consistent with the Galilean invariancy.N-S equations also neglect the flow rotation.It is believed that N-S equations may work for incompressible and laminar flow but are not satisfied for turbulent flow and compressible flow especially for high-speed flow.Based on Liutex,new fluid kinematics has been established by Liu in 2021,which gives a Liutex-based principal coordinate system and a new principal decomposition in that system,which has been transferred back to the original Cartesian coordinate system.The principal decomposition of velocity gradient tensor has four parts which are called rotation,stretching,anti-symmetric shearing and symmetric shearing.Four forces are derived according to the four parts of the velocity gradient tensor.According to the new fluid kinematics,it is reported in this letter that based on the principal decomposition,a new relation between the velocity gradient tensor and stress-rate tensor has been established to form a new fluid dynamics equation to govern fluid flow.The new governing equation may be applicable to both laminar flow and turbulent flow,and both incompressible flow and compressible flow including high-speed flow for reasonable results with reasonable grids.Further numerical experiment is needed to verify. | Chaoqun Liu | 2021 | Journal of Hydrodynamics2021，33，4： | 7 |
| 17 | Liutex-based vortex control with implications for cavitation suppression显示文摘Viewed as sinews and muscles of fluid motion,coherent vortical structures with their interactions are key to understanding the flow dynamics.Based upon this observation,we explore the possibility of efficient flow control by directly manipulating vortices numerically inside the flow field based on the vortex definition and identification system of Liutex.The objective is twofold:(1)to study the vortex dynamics,for example,by observing the response of the flow to strengthening or weakening of certain vortices,and(2)to obtain efficient vortex-based control strategies which might lead us to practical applications.In the present numerical study,the manipulating of vortices is achieved by introducing additional source(force)terms to the Navier-Stokes equations,which hereafter will be collectively called Liutex force field model.Methodologies including controlling the rotation strength and centripetal force of particular vortices are detailed in a flow past a cylinder with different control purposes at Reynolds number of 200.Further examples are provided with a cavitating flow around two-dimensional Clark-Y hydrofoil,with particular interests on cavitation suppression.It is illustrated particular vortex with cavitation encircled could be effectively suppressed. | Yi-qian Wang Hai-dong Yu Wei-wen Zhao De-cheng Wan | 2021 | Journal of Hydrodynamics2021，33，1： | 7 |
| 18 | Prediction of the precessing vortex core in the Francis-99 draft tube under off-design conditions by using Liutex/Rortex method显示文摘The turbulent flow in the draft tube of a Francis turbine is very complicated while working under off-design conditions. Although the off-design conditions were widely studied, the vortex core line in the draft tube of a Francis turbine with splitter blades is not well understood, especially the vortex rope property. This letter presents a prediction of the behavior of the vortex rope in the draft tube of the Francis-99 turbine obtained by the computational fluid dynamics (CFD), where the Liutex/Rortex method, as the most recent vortex definition, is applied to analyze the periodical precession of the vortex rope in the draft tube cone. The advantage of this Liutex/Rortex method is shown by its enhanced ability to represent the vortex rope structurewith the vortex-core lines. Furthermore, since it seems to be very hard to define a sharp boundary surface for the whole vortex structure, it is advantageousfocusing only on the vortex core line,by which different vortex structures can be clearly differentiated. The evolution of the vortex core and the process of the vortex breakdown in the draft tube are revealed, which might help to comprehend the development of the turbulent flow in the draft tube. | Cong Trieu Tran Xin-ping Long Bin Ji Chaoqun Liu | 2020 | Journal of Hydrodynamics2020，32，3： | 6 |
| 19 | Vortical structures and wakes of a sphere in homogeneous and density stratified fluid显示文摘Vortical structures and wakes of bluff bodies in homogenous and stratified environment are common and important in ocean engineering.Based on the Boussinesq approximation,a thermocline model is proposed to deal with the variable density stratified fluid,and implemented in the commercial software Simcenter STAR-CCM+framework.The improved delayed detached eddy simulation(IDDES)modeling method is adopted to resolve the coherent vortical structures and turbulent wakes precisely and efficiently.Four conditions consisting of one homogenous and three stratified fluid cases with different density gradient past a sphere at Reynolds number 3700 are investigated.Results show that density stratification has a great impact on the vortical structures,the vertical motion is suppressed and internal waves will be induced and propagated,which is very different with that of homogenous situation.With the stratification strength increases,the vortical structures are gradually flattened,the asymmetry and anisotropy between vertical and horizontal motions are enhanced. | Liu-shuai Cao Feng-lai Huang Cheng Liu De-cheng Wan | 2021 | Journal of Hydrodynamics2021，33，2： | 6 |
| 20 | Correlation analysis between underwater noise and Liutex for DTMB4119 propeller显示文摘Propeller is an important equipment of ocean structures.It has characteristics of complex geometry and periodic vortex shed,which means high research value.This paper takes DTMB419 propeller as the research object.A detailed study is conducted on the wake vortices in non-cavitation state and cavitation state.Three vortex identification methods are used to compare the vortices capture effects from downstream sections and three-dimensional perspectives.The results show that,the third-generation vortex identification technologyΩ_(R)shows obvious advantages.At the same time,FW-H equation is used to predict propeller noise,and the relationship between nonlinear sound source distribution and vortices distribution is analyzed.It is found that the distribution of Lighthill tensor is consistent withΩ_(R),which proves the advantages and potential of third-generation vortex identification technology from acoustic perspective. | Lian-jie Yu Jian-wei Wu De-cheng Wan | 2022 | Journal of Hydrodynamics2022，34，4： | 5 |
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