This study investigates cavitating swirling flow in a diffuser,i.e.,a simplified model of a Francis turbine draft tube,using proper orthogonal decomposition(POD)and dynamic mode decomposition(DMD)applied to velocity a...This study investigates cavitating swirling flow in a diffuser,i.e.,a simplified model of a Francis turbine draft tube,using proper orthogonal decomposition(POD)and dynamic mode decomposition(DMD)applied to velocity and pressure field data.The interaction between vortex rope precession and cavitation surge under varying swirl and cavitation numbers is analyzed.The modal analysis results depicted the coherent structures correlated to the vortex rope precession near the diffuser inlet and the diffuser outlet,and cavitation surge in the diffuser.The POD analysis accurately revealed the flow features in the diffuser:The conical structure represents the flow diffusion with vortex rope precession and the reverse core indicates the backflow in the diffuser for the averaged flow,and the double helical structure near the diffuser inlet for the representative flow oscillation.The typical coherent structures obtained by the DMD for the cavitating swirling flow in the diffuser are the double helical structure concentrated near the diffuser inlet.The double helical structure also appears near the diffuser outlet where the breakdown of vortex rope occurs and the flow oscillation slows down.Once cavitation occurs,the mode induced by cavitation surge and its corresponding coherent structure may change according to the operating condition.The flow oscillation can be changed from the double helical mode to the axial oscillation caused by cavitation surge named breathing mode if cavitation surge becomes strong enough at a small cavitation number or large swirl number.展开更多
Draft tube vortex rope is considered a special cavitation flow phenomenon in tubular turbine units.Cavitation vortex rope is one of the most detrimental factors affecting the safety of hydraulic turbines.In this study...Draft tube vortex rope is considered a special cavitation flow phenomenon in tubular turbine units.Cavitation vortex rope is one of the most detrimental factors affecting the safety of hydraulic turbines.In this study,ANSYS CFX software was utilized to numerically simulate the internal cavitation flow of a hydraulic turbine draft tube.The evolution of the cavitation vortex core was characterized by vortex line distribution and vorticity transport equation.The shape and number of blades influenced the revolving direction and distribution characteristics of the vortex close to the runner cone,which formed a counterclockwise-clockwise-counterclockwise distribution pattern.Simultaneously,there were many secondary flows in the draft tube.Mutual cancellation and dissipation between the flows was one of the reasons for reduction in vorticity.When the cross-sectional shape of the draft tube was changed,the vorticity was distributed from the center of the vortex rope to all parts of the cross-sectional draft tube,with extreme values at the center and at the walls.The vortex stretching and dilatation terms played a major role in the change in vorticity,with the baroclinic torque having an effect at the center of the vortex rope,this study is helpful to understand the flow of water in the draft tube and guide the design and optimization of the draft tube in engineering application.展开更多
The vortex rope usually occurs in the draft tube of the Francis turbine operated under part-load conditions,to induce strong low-frequency pressure vibrations,and therefore,is very harmful to the safety of the hydropo...The vortex rope usually occurs in the draft tube of the Francis turbine operated under part-load conditions,to induce strong low-frequency pressure vibrations,and therefore,is very harmful to the safety of the hydropower unit.In the present work,three kinds of strategies are extensively investigated,i.e.,the installations of the ventilation and the fin,as well as the hybrid strategy of the air admission through a fin,so as to effectively suppress the vortex rope oscillation and the pressure vibration in the draft tube of a Francis turbine,whose specific speed is 125 m-kW.For the unsteady flow simulation,the Reynolds averaged Navier-Stokes(RANS)method is applied coupled with the k-ω SST turbulence model and a homogeneous cavitation model.The flow analysis confirms that the low-frequency pressure vibrations are originated from the periodical oscillation of the vortex rope,and the cavitation usually enhances the vortex rope oscillation in the draft tube.Under the part-load condition,the dominant component of the pressure vibration in the draft tube has a frequency,for example,f_(1),lower than the runner rotating frequency f_(n).It is shown that all three strategies can be adopted to alleviate the vortex rope oscillation and the pressure vibrations in the draft tube,but their suppression mechanisms are quite different.The ventilation of an adequate amount from the turbine runner cone can change the vortex rope geometry from the spiral type to the cylindrical type,suppress the vortex rope oscillation,and consequently create the homogeneous distributions of the pressure and the pressure gradient in the draft tube.On the other hand,a fin installed at the draft tube wall can induce a small extra rope,and the interaction between the main vortex rope and the extra rope changes the flow field and alleviates the pressure vibration in the draft tube.It should be noted that a fin is much more effective to suppress the pressure vibration in the draft tube under the cavitation condition than under the non-cavitation condition.A better effect of suppressing the vortex rope oscillation can be achieved by the air admission through a fin,which is studied numerically in this paper.The result indicates that the air admission can further improve the effect of a fin for suppressing the pressure vibration in the inlet cone of the draft tube.This improvement is due to the stronger interaction between the main vortex rope and the extra air rope.However,the air admission through a fin should be carefully treated because the strong interaction may induce a larger pressure vibration in the elbow of the draft tube.Finally,it is clear that any strategy for suppressing the pressure vibration hardly changes the dominant component frequency f_(1),which is in the range of 0.22 f_(n)-0.23 f_(n) due to the main vortex rope oscillation in this study.The current results may be used in various engineering applications,where the active control of the vortex oscillation and the pressure vibrations with or without the cavitation is necessary.展开更多
Pressure fluctuations induced by a vortex rope are the major causes of hydraulic turbine vibration in partial load operating conditions. Hence, an effective control strategy should be adopted to improve rotating chara...Pressure fluctuations induced by a vortex rope are the major causes of hydraulic turbine vibration in partial load operating conditions. Hence, an effective control strategy should be adopted to improve rotating characteristics of the vortex rope and reduce the corresponding pressure fluctuation. In this study, two new types of runner cones(i.e., abnormally shaped and long straight cones) were proposed to optimize the pressure distribution in the draft tube, and unsteady numerical simulations were performed to determine their mechanism of action. Numerical results were validated using flow observation and pressure fluctuation experiments. Detailed analyses were conducted to understand the effects of the helical vortex rope operating conditions. The results indicated that pressure fluctuations in the draft tube at partial load operation result primarily from low frequency fluctuations induced by the rotation of the helical vortex rope, whose amplitudes are related to the rotating radius of the helical vortex rope. Both runner cone types could effectively reduce the pressure-fluctuation amplitude. The long straight type could reduce the amplitude of low-frequency fluctuation induced by vortex rope to a maximum of 74.08% and the abnormalshape type to 38.31%. Thus, the effective optimization of the runner cone can potentially reduce pressure-fluctuation amplitudes.Our research findings were applied to a real hydraulic plant in China.展开更多
Three-stranded rope is widely used in fishing gear and mooring system. Results of numerical simulation are presented for flow around a three-stranded rope in uniform flow. The simulation was carried out to study the h...Three-stranded rope is widely used in fishing gear and mooring system. Results of numerical simulation are presented for flow around a three-stranded rope in uniform flow. The simulation was carried out to study the hydrodynamic characteristics of pressure and velocity fields of steady incompressible laminar and turbulent wakes behind a three-stranded rope. A three-cylinder configuration and single circular cylinder configuration are used to model the three-stranded rope in the two-dimensional simulation. The governing equations, Navier-Stokes equations, are solved by using two-dimensional finite volume method. The turbulence flow is simulated using Standard κ-ε model and Shear-Stress Transport κ-ω(SST) model. The drag of the three-cylinder model and single cylinder model is calculated for different Reynolds numbers by using control volume analysis method. The pressure coefficient is also calculated for the turbulent model and laminar model based on the control surface method. From the comparison of the drag coefficient and the pressure of the single cylinder and three-cylinder models, it is found that the drag coefficients of the three-cylinder model are generally 1.3–1.5 times those of the single circular cylinder for different Reynolds numbers. Comparing the numerical results with water tank test data, the results of the three-cylinder model are closer to the experiment results than the single cylinder model results.展开更多
基金Project supported by the National Natural Science Foundation of China(Grant No.52336001)supported by the Tsinghua National Laboratory for Information Science and Technology.
文摘This study investigates cavitating swirling flow in a diffuser,i.e.,a simplified model of a Francis turbine draft tube,using proper orthogonal decomposition(POD)and dynamic mode decomposition(DMD)applied to velocity and pressure field data.The interaction between vortex rope precession and cavitation surge under varying swirl and cavitation numbers is analyzed.The modal analysis results depicted the coherent structures correlated to the vortex rope precession near the diffuser inlet and the diffuser outlet,and cavitation surge in the diffuser.The POD analysis accurately revealed the flow features in the diffuser:The conical structure represents the flow diffusion with vortex rope precession and the reverse core indicates the backflow in the diffuser for the averaged flow,and the double helical structure near the diffuser inlet for the representative flow oscillation.The typical coherent structures obtained by the DMD for the cavitating swirling flow in the diffuser are the double helical structure concentrated near the diffuser inlet.The double helical structure also appears near the diffuser outlet where the breakdown of vortex rope occurs and the flow oscillation slows down.Once cavitation occurs,the mode induced by cavitation surge and its corresponding coherent structure may change according to the operating condition.The flow oscillation can be changed from the double helical mode to the axial oscillation caused by cavitation surge named breathing mode if cavitation surge becomes strong enough at a small cavitation number or large swirl number.
基金the National Natural Science Foundation,China(Grant No.52079118)Key Research and Development Plan of Sichuan Provincial Department of Science and Technology(Grant No.2023YFQ0021)+1 种基金Qinghai Province“Kunlun Talents High-end Innovation and Entrepreneurship Talent Program”Qinghai University of Science and Technology talent introduction of scientific research special grants,Central leading local(scientific and technological innovation base construction)project XZ202201YD0017CJiangsu South-North Water Diversion Science and Technology R&D Project(Grant No.JSNSBD202303).
文摘Draft tube vortex rope is considered a special cavitation flow phenomenon in tubular turbine units.Cavitation vortex rope is one of the most detrimental factors affecting the safety of hydraulic turbines.In this study,ANSYS CFX software was utilized to numerically simulate the internal cavitation flow of a hydraulic turbine draft tube.The evolution of the cavitation vortex core was characterized by vortex line distribution and vorticity transport equation.The shape and number of blades influenced the revolving direction and distribution characteristics of the vortex close to the runner cone,which formed a counterclockwise-clockwise-counterclockwise distribution pattern.Simultaneously,there were many secondary flows in the draft tube.Mutual cancellation and dissipation between the flows was one of the reasons for reduction in vorticity.When the cross-sectional shape of the draft tube was changed,the vorticity was distributed from the center of the vortex rope to all parts of the cross-sectional draft tube,with extreme values at the center and at the walls.The vortex stretching and dilatation terms played a major role in the change in vorticity,with the baroclinic torque having an effect at the center of the vortex rope,this study is helpful to understand the flow of water in the draft tube and guide the design and optimization of the draft tube in engineering application.
基金Projects supported by the National Natural Science Foundation of China(Grant Nos.91852103,51776102)the Beijing Natural Science Foundation(Grant No.3182014)+1 种基金This work was supported by the Institute for Guo Qiang,Tsinghua University(Grant No.2019GQG1019)the Tsinghua National Laboratory for Information Science and Technology.
文摘The vortex rope usually occurs in the draft tube of the Francis turbine operated under part-load conditions,to induce strong low-frequency pressure vibrations,and therefore,is very harmful to the safety of the hydropower unit.In the present work,three kinds of strategies are extensively investigated,i.e.,the installations of the ventilation and the fin,as well as the hybrid strategy of the air admission through a fin,so as to effectively suppress the vortex rope oscillation and the pressure vibration in the draft tube of a Francis turbine,whose specific speed is 125 m-kW.For the unsteady flow simulation,the Reynolds averaged Navier-Stokes(RANS)method is applied coupled with the k-ω SST turbulence model and a homogeneous cavitation model.The flow analysis confirms that the low-frequency pressure vibrations are originated from the periodical oscillation of the vortex rope,and the cavitation usually enhances the vortex rope oscillation in the draft tube.Under the part-load condition,the dominant component of the pressure vibration in the draft tube has a frequency,for example,f_(1),lower than the runner rotating frequency f_(n).It is shown that all three strategies can be adopted to alleviate the vortex rope oscillation and the pressure vibrations in the draft tube,but their suppression mechanisms are quite different.The ventilation of an adequate amount from the turbine runner cone can change the vortex rope geometry from the spiral type to the cylindrical type,suppress the vortex rope oscillation,and consequently create the homogeneous distributions of the pressure and the pressure gradient in the draft tube.On the other hand,a fin installed at the draft tube wall can induce a small extra rope,and the interaction between the main vortex rope and the extra rope changes the flow field and alleviates the pressure vibration in the draft tube.It should be noted that a fin is much more effective to suppress the pressure vibration in the draft tube under the cavitation condition than under the non-cavitation condition.A better effect of suppressing the vortex rope oscillation can be achieved by the air admission through a fin,which is studied numerically in this paper.The result indicates that the air admission can further improve the effect of a fin for suppressing the pressure vibration in the inlet cone of the draft tube.This improvement is due to the stronger interaction between the main vortex rope and the extra air rope.However,the air admission through a fin should be carefully treated because the strong interaction may induce a larger pressure vibration in the elbow of the draft tube.Finally,it is clear that any strategy for suppressing the pressure vibration hardly changes the dominant component frequency f_(1),which is in the range of 0.22 f_(n)-0.23 f_(n) due to the main vortex rope oscillation in this study.The current results may be used in various engineering applications,where the active control of the vortex oscillation and the pressure vibrations with or without the cavitation is necessary.
基金This work was supported by the National Natural Science Foundation of China(Grant No.51806044)the China Postdoctoral Science Foundation Funded Projection(Grant No.2018M630353)the Industrial Prospect and Key Core Technology of Jiangsu Province(Grant No.BE2019009-1)。
文摘Pressure fluctuations induced by a vortex rope are the major causes of hydraulic turbine vibration in partial load operating conditions. Hence, an effective control strategy should be adopted to improve rotating characteristics of the vortex rope and reduce the corresponding pressure fluctuation. In this study, two new types of runner cones(i.e., abnormally shaped and long straight cones) were proposed to optimize the pressure distribution in the draft tube, and unsteady numerical simulations were performed to determine their mechanism of action. Numerical results were validated using flow observation and pressure fluctuation experiments. Detailed analyses were conducted to understand the effects of the helical vortex rope operating conditions. The results indicated that pressure fluctuations in the draft tube at partial load operation result primarily from low frequency fluctuations induced by the rotation of the helical vortex rope, whose amplitudes are related to the rotating radius of the helical vortex rope. Both runner cone types could effectively reduce the pressure-fluctuation amplitude. The long straight type could reduce the amplitude of low-frequency fluctuation induced by vortex rope to a maximum of 74.08% and the abnormalshape type to 38.31%. Thus, the effective optimization of the runner cone can potentially reduce pressure-fluctuation amplitudes.Our research findings were applied to a real hydraulic plant in China.
基金supported by the National Natural Science Foundation of China (31072246, 30972256)Special Fund for Research on high efficient techniques for Antarctic Krill (20150256)+1 种基金Agro-Scientific Research in the Public Interest (201203018, 201303050-02)the Fundamental Research Funds for Chinese Academy of Fishery Sciences (CAFS) (2012A1301)
文摘Three-stranded rope is widely used in fishing gear and mooring system. Results of numerical simulation are presented for flow around a three-stranded rope in uniform flow. The simulation was carried out to study the hydrodynamic characteristics of pressure and velocity fields of steady incompressible laminar and turbulent wakes behind a three-stranded rope. A three-cylinder configuration and single circular cylinder configuration are used to model the three-stranded rope in the two-dimensional simulation. The governing equations, Navier-Stokes equations, are solved by using two-dimensional finite volume method. The turbulence flow is simulated using Standard κ-ε model and Shear-Stress Transport κ-ω(SST) model. The drag of the three-cylinder model and single cylinder model is calculated for different Reynolds numbers by using control volume analysis method. The pressure coefficient is also calculated for the turbulent model and laminar model based on the control surface method. From the comparison of the drag coefficient and the pressure of the single cylinder and three-cylinder models, it is found that the drag coefficients of the three-cylinder model are generally 1.3–1.5 times those of the single circular cylinder for different Reynolds numbers. Comparing the numerical results with water tank test data, the results of the three-cylinder model are closer to the experiment results than the single cylinder model results.
