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A simulation based on the cosserat continuum model of the vortex structure in granular materials

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Acta Mechanica Solida Sinica 2015年第2期28卷 182-186页

作者： Cun Yu Xihua Chu Yuanjie Xu Department of Engineering Mechanics Wuhan University

Displacement fluctuation is the difference between the real displacement and the affine displacement in deforming granular materials. The discrete element method （DEM） is widely used along with experimental approaches to investigate whether the displacement fluctuation represents the vortex structure. Current research suggests that the vortex structure is caused by the cooperative motion of particle groups on meso-scales, which results in strain localization in granular materials. In this brief article, we investigate the vortex structure using the finite element method （FEM） based on the Cosserat cor[tinuum model. The numerical example focuses on the relationship between the vortex structure and the shear bands under two conditions：（a） uniform granular materials; （b） granular materials with inclusions. When compared with distributions of the effective strain and the vortex structure, we find that the vortex structure coexists with the strain localization and originates from the stiffness cooperation of different locations in granular materials at the macro level.

关键词： granular materials vortex structure strain localization Cosserat continuum model

Stereoscopic Particle Image Velocimetry Investigation of Three-Dimensional Characteristics of vortex structure in a Turbulent Channel Flow

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Chinese Physics Letters 2005年第3期22卷 624-627页

作者： Feng-ChenLI YasuoKAWAGUCHI TakehikoSEGAWA KoichiHISHIDA TurbomachineryGroup EnergyTechnologyResearchInstituteNationalInstituteofAdvancedIndustrialScienceandTechnology(AIST)Tsukuba305-8564Japan DepartmentofSystemDesignEngineering KeioUniversityYokohama223-8522Japan

We investigate the three-dimensional characteristics of turbulence structures in a turbulent channel flow by employing stereoscopic particle image velocimetry (SPIV). The SPIV measurement was performed for a water flow at the Reynolds number Re = 1.1×104 in both the streamwise–spanwise (x–z) and spanwise–wall-normal (z–y) planes, respectively. A three-dimensional image of the vortex tubes or the hairpin vortex leg(s) inclined toward the wall was obtained. A series of wall-normal vortex cores were found to align with the low-speed streaks with opposite vorticity signals at both sides of the streaks in the x-z plane, which were cross-section slices of the inclined hairpin vortex leg(s).

关键词： Stereoscopic Particle Image Velocimetry Investigation hree-Dimensional Characteristics vortex structure

vortex structures of dynamic pure yaw test using DDES approach and vortex identification method

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Journal of Hydrodynamics 2022年第2期34卷 226-233页

作者： Wei-wen Zhao Qing-jie Meng De-cheng Wan Yi-qian Wang Computational Marine Hydrodynamics Lab(CMHL) School of Naval ArchitectureOcean and Civil EngineeringShanghai Jiao Tong UniversityShanghai200240China Wuhan Second Ship Design and Research Institute Wuhan430205China School of Mathematical Science Soochow UniversitySuzhou215006China

Considered as the building blocks,vortex structures with variety of sizes and intensity are widely recognized in the viscous flow field around ship.In this paper,the computational fluid dynamics(CFD)solver,naoe-FOAM-SJTU,coupled with delayed detached-eddy simulation(DDES)is adopted to analyze the vortex structures around the benchmark model Yupeng Ship in dynamic pure yaw tests,which are captured by third generation of vortex identification method.The good agreement of the predicted force/moment by DDES method with the experimental data indicates that the present numerical schemes are reliable and robust.Three vortex identification methods,Q-criteria,Ω_(R) and Liutex,are used to capture the vortex structures around the hull.The large separated flow is able to be investigated by these three methods,in which more vortex structures are captured byΩ_(R) approach and Liutex method with scalar,vector and tensor form seems to be more suitable for analyzing the flow mechanism around the hull in dynamic pure yaw test.In general,each vortex structure corresponds to a dominant positive/negative axial Liutex and a bound vortex pair.The streamlines are spiral in the large separated flow,indicating that the flow in corresponding region is rotational.But the rotation of the flow is not directly related to the intensity of Liutex.

关键词： Dynamic pure yaw test delayed detached-eddy simulation(DDES) vortex identification method vortex structure flow mechanism

Analysis of the structure of Different Tibetan Plateau vortex Types

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Journal of Meteorological Research 2017年第3期31卷 514-529页

作者： Xinyuan FENG Changhai LIU Guangzhou FAN Jie ZHANG College of Atmospheric Sciences Chengdu University of Information Technology/Plateau Atmosphere and Environment Key Laboratory of Sichuan Province National Center for Atmospheric Research Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters Nanjing University of Information Science &Technology

Knowledge of the structure of the Tibetan Plateau vortex （TPV） is of considerable importance for understanding the generation and development mechanisms of this mesoscale system. However, our understanding of vortex struc- tures and our ability to classify them on a physical basis is limited due to insufficient observations. The high- resolution new-generation NCEP-CFSR （Climate Forecast System Reanalysis） dataset is used in the present paper to investigate the general structural features of various types of mature TPV through classification and composite struc- ture analysis. Results indicate that the dynamic and thermodynamic structures show regional and seasonal depend- ency, as well as being influenced by attributes of translation, associated precipitation, and the South Asian high （SAH）. The common precipitating TPV （type I）, frequently occurring in the west-east-oriented zonal region between 33° and 36°N, is a notably low-level baroclinic and asymmetric system. It resides within a large-scale confluent zone and preferentially travels eastward, potentially moving out of the plateau. The heavy rain vortex （type II） corresponds to a deep vortex circulation occurring in midsummer. The low-level baroclinic sub-category （type IIa） is associated with a low-level jet and mainly originates in the area 32°-35°N, 86°-94°E, preferentially moving east of 90°E and even away from the plateau; meanwhile, the nearly upright sub-category （type IIb）, which has a cold center at low levels and a warm center at mid-upper levels, is a quasi-stationary and quasi-symmetric system favorably occurring west of 92°E. A western-pattern SAH exists in the upper troposphere for these two sub-categories. The springtime dry vortex in the western plateau （type III） is warm and shallow （approximately 100 hPa deep）, and zonal circulation dominates the large-scale environmental flows in the middle and upper troposphere. The precipitating vortex in the southern plateau occurring during July-August （type IV） is not affected by northerly flow at l

关键词： Tibetan Plateau vortex vortex structure composite analysis physical classification Climate ForecastSystem Reanalysis

A letter for objective Liutex

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Journal of Hydrodynamics 2022年第5期34卷 965-969页

作者： Yifei Yu Yi-qian Wang Chaoqun Liu Department of Mathematics University of Texas at ArlingtonArlingtonTexas76019USA School of Mathematical Science Soochow UniversitySuzhou215006China

vortex structures,detected by the existing popular vortex identification methods,of the same case are generally different if the results are measured by different observers,who are moving with different coordinate systems.These coordinate systems can be inertial or non-inertial.Galilean invariance can solve the moving observer problems in different inertial coordinate systems.A variable is invariant under Galilean transformation is called Galilean invariant.Galilean invariant vortex identification methods can provide invariant vortex structures based on different inertial observers.However,for some situations,the observers are non-inertial,e.g.,the observer is sitting on an aircraft which is accelerating or rotating.In such a situation,it requires to use an objective method.Objectivity represents a property that one variable is not observer-dependent(the observer’s motion can be either inertial or non-inertial).A strategy based on a zero-vorticity(measured in inertial frame)reference point is proposed in this letter to find the objective vortex structure.Liutex is chosen as the vortex indicator and two numerical examples are used to test the proposed strategy.The results show that the strategy is effective.

关键词： Objective Liutex vortex structure Galilean invariance

Investigations on vortex structures for undulating fin propulsion using phase-locked digital particle image velocimetry

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Journal of Hydrodynamics 2021年第3期33卷 572-582页

作者： Ya-qiang Bai Jun Zhang Shu-cheng Zhai Guo-ping Zhang China Ship Scientific Research Center Wuxi214082China National Key Laboratory on Ship Vibration and Noise Wuxi214082China

The Gymnarchus niloticus fish can swim in surging and heaving directions only with a long undulating ribbon fin while keeping its body along almost straight line.These features substantially inspire the design of underwater vessels with high maneuverability and station keeping performance,which is characterized by peculiar vortex structures induced by undulating fin propulsion.To reveal the propulsion mechanism under the evolution of these complex vortex structures,the variation of velocity field with the undulating fin’s wave phase on cross section and mid-sagittal plane at wave amplitude of 85°is investigated by phase-locked digital particle image velocimetry(DPIV).Through experimental flow field images,two typical vortex structures are clearly identified,i.e.,streamwise vortex and crescent vortex,which is further explained by supplemental numerical simulations using large eddy simulation.vortex characteristic and its evolution on cross sections and mid-sagittal planes is investigated,and its relationship with thrust,heave force is also analyzed.It is found that the two kinds of vortexes induce the main hydrodynamic forces in two directions synchronously,which brings the undulating fin propulsion an extra-ordinal maneuverability.The research will be useful for understanding the potential mechanism of this novel propulsion and is of great application prospect in designing more maneuverable underwater vehicles.

关键词： Undulating fin propulsion vortex structure phase-locked digital particle image velocimetry(DPIV) hydrodynamic mechanism

LES investigation of cavitating flows around a sphere with special emphasis on the cavitation–vortex interactions

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Acta Mechanica Sinica 2020年第6期36卷 1238-1257,I0002页

作者： Yun Long Xinping Long Bin Ji State Key Lab of Water Resources and Hydropower Engineering Science School of Power and Mechanical EngineeringWuhan UniversityWuhan430072China Key Lab of Jet Theory and New Technology of Hubei Province Wuhan UniversityWuhan430072China

Large eddy simulation(LES)was coupled with a homogeneous cavitation model to study turbulent cavitating flows around a sphere.The simulations are in good agreement with available experimental data and the simulated accuracy has been evaluated using the LES verification and validation method.Various cavitation numbers are simulated to study important flow characteristics in the sphere wake,e.***.periodic cavity growth/contraction,interactions between the cloud and sheet cavitations and the vortex structure evolution.The spectral characteristics of the wake for typical cloud cavitation conditions were classified as the periodic cavitation mode,high Strouhal number mode and low Strouhal number mode.Main frequency distributions in the wake were analyzed and different dominant flow structures were identified for each of the three modes.Further,the cavitation and vortex relationship was also studied,which is an important issue associated with complex cavitating sphere wakes.Three types of cavitating vortex structures alternate,which indicates that three different cavity shedding regimes may exist in the wake.Analysis of vorticity transport equation shows a significant vorticity increase at the cavitation closure region and in the vortex cavitation region.This study provides a physical perspective to further understand the flow mechanisms in cavitating sphere wakes.

关键词： Cavitation Cavitating flow Sphere Large eddy simulation(LES) vortex structure

Particle image velocimetry measurements of vortex structures in stilling basin of multi-horizontal submerged jets

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Journal of Hydrodynamics 2013年第4期25卷 556-563页

作者：陈剑刚张建民许唯临栗帅何小泷 State Key Laboratory of Hydraulics and Mountain River Engineering Sichuan University Institute of Mountain Hazards and Environment Chinese Academy of Sciences

Measurements of turbulent flow fields in a stilling basin of multi-horizontal submerged jets were made with the single- camera Particle Image Velocimetry （PIV）. The particle images were captured, processed, and subsequently used to characterize the flow in terms of the 2-D velocity and vorticity distributions. This study shows that the maximum close-to-bed velocity in the stilling basin is approximately reduced by 60%, comparing to the jet velocity at the outlet of orifices. The jet velocity is distributed evenly at the latter half of the stilling basin and the time-averaged velocity of the cross section is reduced by 77%-85%, comparing to the jet velocity at the outlet of orifices. These results show that the vortices with horizontal axes are continuously repeated during the form-merge-split-disappear process. The vertical vortices are continuously formed and disappeared, they appear randomly near the slab and intermittently reach the slab of the stilling basin. The range of these vortices is small. Vortices with horizontal axes and ver- tical vortices do not coincide in space and the vortices with horizontal axes only affect the position of the tail of the vertical vortices attached to the slab of the stilling basin.

关键词： multi-horizontal submerged jets Particle Image Velocimetry （PIV） vortex structure stilling basin hydraulic jump

STUDY OF vortex CHARACTERISTICS OF THE FLOW AROUND A HORIZONTAL CIRCULAR CYLINDER AT VARIOUS GAP-RATIOS IN THE CROSS-FLOW

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Journal of Hydrodynamics 2006年第3期18卷 334-340页

作者： QI E-rong LI Guo-ya LI Wei WU Jian ZHANG Xin State Key Laboratory of Water Resources and Hydropower Engineering Science Wuhan University Wuhan430072 China China Three Gorges Project Corporation Yichang 443002 China Central South Building Design Institute Wuhan 430071 China

In the gap-ratio range of 0.0≤G≤7.0, a particle image velocimetry PIV is applied to conduct a systematic experimental research of the flow around a horizontal circular cylinder in the cross-flow of shallow water. The velocity distribution of transient flow field at various gap-ratios is obtained. Based on these data, the phenomena and rules of the vortex and its course of generation, development and evolvement at various gap-ratios are analyzed, and it is found that there are similar unshedding vortex structures at G = 0.0 and G = 7.0, and others are structures of shedding vortex. The figures of typical vortex movements are given. Based on this, the differences between the transient flow field and the time-averaged flow field and the characteristics of the vortex structures are analyzed. In addition when the Strouhal number keeps constant （about 0.2） concerning vortex shedding have been discussed. The findings of this paper are of guiding significance for engineering issues with similar flowing features.

关键词： PIV flow around a circular cylinder vortex structure Strouhal number