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 rotatio...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.展开更多
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 identific...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.展开更多
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 con...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.展开更多
The motion of fluid consists of different scales of coherent vortical structures.These vortical structures determine the characteristics of fluid motion and are key to understand fluid dynamics.In this paper,we study ...The motion of fluid consists of different scales of coherent vortical structures.These vortical structures determine the characteristics of fluid motion and are key to understand fluid dynamics.In this paper,we study the fine control method of vortical structures based on the Liutex force field model.This is achieved by constructing a source term using Liutex and directly add it to the Navier-Stokes equations.To investigate the influence of the constructed Liutex force model on vortical structures,a flow past a cylinder at Reynolds number of 100 is numerically studied with different source term magnitude and region.The drag and lift forces on the cylinder,as well as the flow field near and behind the cylinder are compared and analyzed.Results show that Liutex force model can effectively strengthen or weaken the vortical structures based on different purpose.展开更多
In this study,the new method of the vortex core line based on Liutex definition,also known as Liutex core line,is applied to support the hypothesis that the vortex ring is not a part of theΛ-vortex and the formation ...In this study,the new method of the vortex core line based on Liutex definition,also known as Liutex core line,is applied to support the hypothesis that the vortex ring is not a part of theΛ-vortex and the formation of the ring-like vortex is formed separately from theΛ-vortex.The proper orthogonal decomposition(POD)is also applied to analyze the Kelvin-VHelmholtz(K-H)instability happening in hairpin ring areas of the flow transition on the flat plate to understand the mechanism of the ring-like vortex formation.The new vortex identification method named modified Liutex-Omega method is efficiently used to visualize and observe the shapes of vortex structures in 3-D.The streamwise vortex structure characteristics can be found in POD mode one as the mean flow.The other POD modes are in stremwise and spanwise structures and have the fluctuation motions,which are induced by K-H instability.Moreover,the result shows that POD modes are in pairs and share the same characteristics such as amplitudes,mode shapes,and time evolutions.The vortex core and POD results confirm that theΛ-vortex is not self-deformed to a hairpin vortex,but the hairpin vortex is formed by the K-H instability during the development of Lambda vortex to hairpin vortex in the boundary layer flow transition.展开更多
For complex aerodynamic and hydrodynamic problems,the analysis of vortex is very important.The Liutex method is an eigenvalue-based method which is local,accurate,and unique,which can give an accurate definition of vo...For complex aerodynamic and hydrodynamic problems,the analysis of vortex is very important.The Liutex method is an eigenvalue-based method which is local,accurate,and unique,which can give an accurate definition of vortex,so the control of vortex can be implemented and effectively guaranteed.Based on Liutex method,two methodologies of centripetal force model and counter-rotation force model were proposed to illustrate the vortex dynamics and possibly strengthen or weaken the vortices.In this paper,the Liutex-based centripetal force model is applied by adding a source term to the Navier-Stokes equations.In order to investigate the influence of the constructed Liutex force model on the 3-dimensional flow around a slow-fat ship,the calm-water drag calculation result of JBC ship is regarded as the initial flow field,and the new resistance and wake performances of the ship are obtained after applying the centripetal force model to the flow field with different strengths.Several views of the comparisons of the new steady flow fields are shown,and the parametric study results indicate that the Liutex-based centripetal force model can effectively change the resistance and wake performances of the JBC ship,which provides a new idea and theoretical basis for the comprehensive hydrodynamic performance optimization of the ship hull.展开更多
Recently,the Liutex-based force field models are proposed and applied to some two-dimensional flows to explore the possibility of direct vortex control strategies.In this paper,we applied the Liutex-based centripetal ...Recently,the Liutex-based force field models are proposed and applied to some two-dimensional flows to explore the possibility of direct vortex control strategies.In this paper,we applied the Liutex-based centripetal force model to three-dimensional flow around a circular cylinder at Re=3900.The turbulent flows around the three-dimensional cylinder are modeled by the delayed detached-eddy simulations based on the two-equation k-ω shear stress transport(SST)model.The cylinder flow without adding the force field model is simulated in the first place.The statistical flow quantities are compared with experimental data to validate the accuracy of the current numerical models.Then the force field model is applied to the momentum equation in a specific control region near the cylinder to study the change of flow field and hydrodynamic performance.The effect of different control region locations with the same force field strength is discussed to find out the most influenced locations on the drag and lift.Based on the screened control region,the influence source term strength is further studied.Variations on the global statistical flow quantities,local instantaneous three-dimensional flow structures are analyzed and discussed.It is concluded that the Liutex-based force field model can effectively change the hydrodynamic behavior of flow past a cylinder and may provide a new direction for performance improvement and optimization.展开更多
The vortex core detection method based on the Liutex vector is utilized to investigate the alternation of vortical structures on a boundary layer transition subjected to spanwise-wall oscillation.Compared with iso-sur...The vortex core detection method based on the Liutex vector is utilized to investigate the alternation of vortical structures on a boundary layer transition subjected to spanwise-wall oscillation.Compared with iso-surface based methods,the Liutex core line method is shown to be precise,free of threshold and capable to capture both strong and weak vortices simultaneously.Tollmien-Schlichting(T-S)waves in the linear growth region,A-and hairpin vortices in the transition region and twisted vortices in the turbulent region are all well captured by Liutex core lines.The cyclic wall movement accelerates the transition process while reducing the turbulent drag by 21.8%with selected parameters.For the wall oscillation case,the development from T・S wave to Avortex is advanced about one T・S wave length in the stream wise direction.In the transition region,the A-vortex and legs of hairpin vortex are shortened in the wall oscillation case,and the symmetry of the vortical structures is lost in the late transition region since the introduction of asymmetry disturbances by the cyclic wall movement.Extrusions of weak vortices at the edge of boundary layer are found in the turbulent section which is often omitted by iso-surface based vortex identification method.Thus,it is demonstrated that for the transitional boundary layer the Liutex core line method provides a systematic and thresholdvortex definition,which could serve as a powerful tool to understand and guide flow control.展开更多
Riblets are a series of small protrusions formed along the flow direction,which have been extensively studied as a passive turbulent drag reduction technique.Experiments and numerical simulations have shown that well-...Riblets are a series of small protrusions formed along the flow direction,which have been extensively studied as a passive turbulent drag reduction technique.Experiments and numerical simulations have shown that well-designed riblets can significantly reduce drag in turbulent flows,making them highly promising and valuable for various applications.In this study,we focus on a scalloped riblet,which is designed by smoothly connecting two third-order polynomials,and thus the sharpness of the tip and the curvature of the valley can be well defined.We conduct direct numerical simulations of turbulent channel with smooth plate,scalloped riblet-mounted and triangular riblet-mounted walls.Width in wall units of W^(+)=20 and height-width ratio ofγ=0.5 are selected for both riblet cases.Compared with the smooth plate case,the scalloped riblet case achieves an 8.68%drag reduction,while the triangular riblet case achieves a 4.79%drag reduction.The obtained drag reduction rate of the triangular riblet is consistent with previous experiments and simulations,and the results indicate that the scalloped riblet is more effective in reducing drag and deserves further investigation.We compare turbulent statistics of the scalloped riblet case with those of the triangular riblet case.The mean velocity profiles of riblets are similar,but both the Reynolds shear stress and second-order statistics of velocity fluctuations and Liutex are significantly reduced in the scalloped riblets controlled turbulent channel,indicating that the scalloped riblet can more effectively suppress the spanwise and wall-normal turbulent intensity near the wall.We also compare the pre-multiplied spectra of streamwise velocity and streamwise Liutex component for the three cases to investigate the energy distribution and characteristics of Liutex distribution.The Liutex vortex identification method is also utilized to analyze the instantaneous flow field,which provides insights into the flow field and could be beneficial for the further optimization of riblet.展开更多
The dynamic mechanism of the vortex generation and evolution process in a fully developed turbulent boundary layer with Reθ=97-194 is experimentally investigated.In this study,a moving single-frame and long-exposure(...The dynamic mechanism of the vortex generation and evolution process in a fully developed turbulent boundary layer with Reθ=97-194 is experimentally investigated.In this study,a moving single-frame and long-exposure(MSFLE)imaging method and a moving particle image velocimetry/particle tracing velocimetry(M-PIV/PTV)are designed and implemented for measuring the temporal and spatial evolution of vortex cores in both qualitative and quantitative ways,respectively.On the other hand,the Liutex vector,which is a new mathematical definition and identification of the vortex core proposed by Liu’s group,is first applied in the experiment for the structural visualization and quantitative analysis of the local fluid rotation.The results show that an intuitional process of vortex evolution can be clearly observed by tracking the vortex using MSFLE and verify that the roll-up of the shear layer induced by shear instability is the origin of vortex formation in turbulence.Furthermore,a quantitative investigation in terms of the critical vortex core boundary(size)and its accurate rotation strength is carried out based on the Liutex vector field analysis by M-PIV/PTV.According to statistics of the relation between vortex core size and the rotation strength during the whole process,the physical mechanism of vortex generation and evolution in a turbulent boundary layer of low Reynolds number can be summarized as a four-dominant-state course consisting of the“synchronous linear segment(SL)-absolute enhancement segment(AE)-absolute diffusion segment(AD)-skewing dissipation segment(SD)”.展开更多
基金This work was mainly supported by the Department of Mathematics of University of Texas at Arlington where the corresponding author,Dr.Chaoqun Liu,is the full-time professor。
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.
基金Project supported by the National Natural Science Foundation of China(Grant No.11702159).
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.
基金supported by the National Nature Science Foundation of China(Grant Nos.11702159,51879159 and 51909160).
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.
基金supported by the National Natural Science Foundation of China(Grant Nos.51909160,51879159)the National Key Research and Development Program of China(Grant Nos.2019YFB1704200,2019YFC0312400).
文摘The motion of fluid consists of different scales of coherent vortical structures.These vortical structures determine the characteristics of fluid motion and are key to understand fluid dynamics.In this paper,we study the fine control method of vortical structures based on the Liutex force field model.This is achieved by constructing a source term using Liutex and directly add it to the Navier-Stokes equations.To investigate the influence of the constructed Liutex force model on vortical structures,a flow past a cylinder at Reynolds number of 100 is numerically studied with different source term magnitude and region.The drag and lift forces on the cylinder,as well as the flow field near and behind the cylinder are compared and analyzed.Results show that Liutex force model can effectively strengthen or weaken the vortical structures based on different purpose.
基金The authors thank the Department of Mathematics of University of Texas at Arlington and Royal Thai Government for the financial support.
文摘In this study,the new method of the vortex core line based on Liutex definition,also known as Liutex core line,is applied to support the hypothesis that the vortex ring is not a part of theΛ-vortex and the formation of the ring-like vortex is formed separately from theΛ-vortex.The proper orthogonal decomposition(POD)is also applied to analyze the Kelvin-VHelmholtz(K-H)instability happening in hairpin ring areas of the flow transition on the flat plate to understand the mechanism of the ring-like vortex formation.The new vortex identification method named modified Liutex-Omega method is efficiently used to visualize and observe the shapes of vortex structures in 3-D.The streamwise vortex structure characteristics can be found in POD mode one as the mean flow.The other POD modes are in stremwise and spanwise structures and have the fluctuation motions,which are induced by K-H instability.Moreover,the result shows that POD modes are in pairs and share the same characteristics such as amplitudes,mode shapes,and time evolutions.The vortex core and POD results confirm that theΛ-vortex is not self-deformed to a hairpin vortex,but the hairpin vortex is formed by the K-H instability during the development of Lambda vortex to hairpin vortex in the boundary layer flow transition.
基金Projects supported by the National Key Research and Development Program of China(Grant Nos.2019YFB1704200,2019YFC0312400)the National Natural Science Foundation of China(Grant Nos.51879159,51909160).
文摘For complex aerodynamic and hydrodynamic problems,the analysis of vortex is very important.The Liutex method is an eigenvalue-based method which is local,accurate,and unique,which can give an accurate definition of vortex,so the control of vortex can be implemented and effectively guaranteed.Based on Liutex method,two methodologies of centripetal force model and counter-rotation force model were proposed to illustrate the vortex dynamics and possibly strengthen or weaken the vortices.In this paper,the Liutex-based centripetal force model is applied by adding a source term to the Navier-Stokes equations.In order to investigate the influence of the constructed Liutex force model on the 3-dimensional flow around a slow-fat ship,the calm-water drag calculation result of JBC ship is regarded as the initial flow field,and the new resistance and wake performances of the ship are obtained after applying the centripetal force model to the flow field with different strengths.Several views of the comparisons of the new steady flow fields are shown,and the parametric study results indicate that the Liutex-based centripetal force model can effectively change the resistance and wake performances of the JBC ship,which provides a new idea and theoretical basis for the comprehensive hydrodynamic performance optimization of the ship hull.
基金Projects supported by the National Natural Science Foundation of China(Grant Nos.51909160,51879159)the National Key Research and Development Program of China(Grant Nos.2019YFB1704200,2019YFC0312400).
文摘Recently,the Liutex-based force field models are proposed and applied to some two-dimensional flows to explore the possibility of direct vortex control strategies.In this paper,we applied the Liutex-based centripetal force model to three-dimensional flow around a circular cylinder at Re=3900.The turbulent flows around the three-dimensional cylinder are modeled by the delayed detached-eddy simulations based on the two-equation k-ω shear stress transport(SST)model.The cylinder flow without adding the force field model is simulated in the first place.The statistical flow quantities are compared with experimental data to validate the accuracy of the current numerical models.Then the force field model is applied to the momentum equation in a specific control region near the cylinder to study the change of flow field and hydrodynamic performance.The effect of different control region locations with the same force field strength is discussed to find out the most influenced locations on the drag and lift.Based on the screened control region,the influence source term strength is further studied.Variations on the global statistical flow quantities,local instantaneous three-dimensional flow structures are analyzed and discussed.It is concluded that the Liutex-based force field model can effectively change the hydrodynamic behavior of flow past a cylinder and may provide a new direction for performance improvement and optimization.
基金Project supported by the National Natural Science Foundation of China(Grant No.11702159)The work was supported by the European Commission and Ministry of Industry and Information Technology(MIIT)of China through the Research and Innovation action DRAGY(Grant No.690623).This investigation is accomplished by using code DNSUTA developed by Dr.Chaoqun Liu at the University of Texas at Arlington.We also thank Prof.Hongyi Xu from Fudan University for providing the executable files to automatically detect Liutex core lines.Helpful discussions with Prof.Lian-di Zhou are highly appreciated by the authors.
文摘The vortex core detection method based on the Liutex vector is utilized to investigate the alternation of vortical structures on a boundary layer transition subjected to spanwise-wall oscillation.Compared with iso-surface based methods,the Liutex core line method is shown to be precise,free of threshold and capable to capture both strong and weak vortices simultaneously.Tollmien-Schlichting(T-S)waves in the linear growth region,A-and hairpin vortices in the transition region and twisted vortices in the turbulent region are all well captured by Liutex core lines.The cyclic wall movement accelerates the transition process while reducing the turbulent drag by 21.8%with selected parameters.For the wall oscillation case,the development from T・S wave to Avortex is advanced about one T・S wave length in the stream wise direction.In the transition region,the A-vortex and legs of hairpin vortex are shortened in the wall oscillation case,and the symmetry of the vortical structures is lost in the late transition region since the introduction of asymmetry disturbances by the cyclic wall movement.Extrusions of weak vortices at the edge of boundary layer are found in the turbulent section which is often omitted by iso-surface based vortex identification method.Thus,it is demonstrated that for the transitional boundary layer the Liutex core line method provides a systematic and thresholdvortex definition,which could serve as a powerful tool to understand and guide flow control.
基金supported by the Jiangsu Shuangchuang Project(Grant No.JSSCTD202209)the National Science Foundation of the Jiangsu Higher Education Institutions of China(Grant No.22KJB130011).
文摘Riblets are a series of small protrusions formed along the flow direction,which have been extensively studied as a passive turbulent drag reduction technique.Experiments and numerical simulations have shown that well-designed riblets can significantly reduce drag in turbulent flows,making them highly promising and valuable for various applications.In this study,we focus on a scalloped riblet,which is designed by smoothly connecting two third-order polynomials,and thus the sharpness of the tip and the curvature of the valley can be well defined.We conduct direct numerical simulations of turbulent channel with smooth plate,scalloped riblet-mounted and triangular riblet-mounted walls.Width in wall units of W^(+)=20 and height-width ratio ofγ=0.5 are selected for both riblet cases.Compared with the smooth plate case,the scalloped riblet case achieves an 8.68%drag reduction,while the triangular riblet case achieves a 4.79%drag reduction.The obtained drag reduction rate of the triangular riblet is consistent with previous experiments and simulations,and the results indicate that the scalloped riblet is more effective in reducing drag and deserves further investigation.We compare turbulent statistics of the scalloped riblet case with those of the triangular riblet case.The mean velocity profiles of riblets are similar,but both the Reynolds shear stress and second-order statistics of velocity fluctuations and Liutex are significantly reduced in the scalloped riblets controlled turbulent channel,indicating that the scalloped riblet can more effectively suppress the spanwise and wall-normal turbulent intensity near the wall.We also compare the pre-multiplied spectra of streamwise velocity and streamwise Liutex component for the three cases to investigate the energy distribution and characteristics of Liutex distribution.The Liutex vortex identification method is also utilized to analyze the instantaneous flow field,which provides insights into the flow field and could be beneficial for the further optimization of riblet.
基金supported by the National Natural Science Foundation of China(Grants Nos.51906154,51576130)the National Science and Technology Major Project(Grant No.2017-V-0016-0069).
文摘The dynamic mechanism of the vortex generation and evolution process in a fully developed turbulent boundary layer with Reθ=97-194 is experimentally investigated.In this study,a moving single-frame and long-exposure(MSFLE)imaging method and a moving particle image velocimetry/particle tracing velocimetry(M-PIV/PTV)are designed and implemented for measuring the temporal and spatial evolution of vortex cores in both qualitative and quantitative ways,respectively.On the other hand,the Liutex vector,which is a new mathematical definition and identification of the vortex core proposed by Liu’s group,is first applied in the experiment for the structural visualization and quantitative analysis of the local fluid rotation.The results show that an intuitional process of vortex evolution can be clearly observed by tracking the vortex using MSFLE and verify that the roll-up of the shear layer induced by shear instability is the origin of vortex formation in turbulence.Furthermore,a quantitative investigation in terms of the critical vortex core boundary(size)and its accurate rotation strength is carried out based on the Liutex vector field analysis by M-PIV/PTV.According to statistics of the relation between vortex core size and the rotation strength during the whole process,the physical mechanism of vortex generation and evolution in a turbulent boundary layer of low Reynolds number can be summarized as a four-dominant-state course consisting of the“synchronous linear segment(SL)-absolute enhancement segment(AE)-absolute diffusion segment(AD)-skewing dissipation segment(SD)”.
