Previous studies showed that 4 D-Var technique used for data assimilation could be modified for weather control. This study demonstrates the ability of 4 D-Var to influence the future path of a tropical cyclone by cal...Previous studies showed that 4 D-Var technique used for data assimilation could be modified for weather control. This study demonstrates the ability of 4 D-Var to influence the future path of a tropical cyclone by calculating perturbations in WRF simulation. Given the background error covariance matrix, the initial field is improved by the vortex dynamic initialization technique. Our results show that 4 D-Var can be applied to control the trajectory of simulated tropical cyclones by producing "optimal" perturbations. In the numerical simulation experiment of Typhoon Mitag in 2019, after this kind of weather control similar to data assimilation, the tropical cyclone moved obviously,and the damaging wind over the coastline weakened. The prediction results after the initial field modified by 4 D-Var have a great change, and the position of the tropical cyclone moved about 0.5° southeastward after assimilation,which misses the southeast coast of China. Moreover, the damaging wind is also weakened. Since the 4 D-Var is premised on the assumption that the model is perfect and does not consider the model error, then the research plan to consider model error and introduce new methods is discussed in the paper.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
The transition process within a Laminar Separation Bubble(LSB)that formed on a compressor blade surface was investigated using Large Eddy Simulations(LESs)at a Reynolds number of 1.5×10;and incidence angles of 0&...The transition process within a Laminar Separation Bubble(LSB)that formed on a compressor blade surface was investigated using Large Eddy Simulations(LESs)at a Reynolds number of 1.5×10;and incidence angles of 0°,+3°,and+5°.The vortex dynamics in the separated shear layers were compared at various incidence angles and its effects on the loss generation were clarified through entropy analysis.Results showed that transition onset,which was accurately identified by the Linear Stability Theory(LST),was significantly promoted at the increased incidence angle.As such,the development of LSB was suppressed and the relative role of viscous instability played in the transition process was weakened.At the incidence angle of 0°,two-dimensional spanwise vortices detached from the blade surface and roiled up periodically,which were further stretched and eventually evolved into large-scale hairpin vortices.As time passed,the fully developed hairpin vortices broke down into small-scale eddies.Meanwhile,the flow near the wall reversely ejected into the outer separated shear layers and a sweeping process happened subsequently,forcing the separated shear layers to reattach and accelerating the generation of turbulent fluctuations.By comparison,the strength of vortex rolling-up was weakened at higher incidence angles,and the vortex pairing and breakdown of large-scale vortices were less pronounced.Therefore,the level of turbulent fluctuations that generated in the separated shear layers was reduced.Detailed entropy analysis showed that the turbulent dissipation effect related to the Reynolds shear stresses determined the largest amount of positive entropy generation,which declined to a lower level as the incidence angle increased from 0°to+5°.Correspondingly,the profile loss was reduced by 50.4%.展开更多
High quality Nb films were successfully prepared on both flexible polyimide(PI)and rigid Al2O3substrates and their transport properties were systematically studied at various applied currents,external magnetic fields,...High quality Nb films were successfully prepared on both flexible polyimide(PI)and rigid Al2O3substrates and their transport properties were systematically studied at various applied currents,external magnetic fields,and sample orientations.It is found that a curved Nb/PI film exhibits quite different superconducting transition and vortex dynamics compared to the flat Nb/Al2O3film.For the curved Nb/PI film,smooth superconducting transitions were obtained at low currents,while unexpected cascade structures were revealed in theρ(T)curves at high currents.We attribute this phenomenon to the gradient distribution of vortex density together with a variation of superconductivity along the curved film.In addition,reentrant superconductivity was induced in the curved Nb/PI thin film by properly choosing the measurement conditions.We attribute this effect to the vortex pinning from both in-plane vortices and out-of-plane vortices.This work reveals the complex transport properties of curved superconducting thin films,providing important insights for further theoretical investigations and practical developments of flexible superconductors.展开更多
The flow around a square cylinder with a synthetic jet positioned at the rear surface is numerically investigated with the unsteady Reynolds-averaged Navier-Stokes(URANS)method.Instead of the typical sinusoidal wave,a...The flow around a square cylinder with a synthetic jet positioned at the rear surface is numerically investigated with the unsteady Reynolds-averaged Navier-Stokes(URANS)method.Instead of the typical sinusoidal wave,a bi-frequency signal is adopted to generate the synthetic jet.The bi-frequency signal consists of a basic sinusoidal wave and a high-frequency wave.Cases with various amplitudes of the high-frequency component are simulated.It is found that synthetic jets actuated by bi-frequency signals can realize better drag reduction with lower energy consumption when appropriate parameter sets are applied.A new quantity,i.e.,the actuation efficiency Ae,is used to evaluate the controlling efficiency.The actuation efficiency Ae reaches its maximum of 0.2668 when the amplitude of the superposed high-frequency signal is 7.5%of the basic signal.The vortex structures and frequency characteristics are subsequently analyzed to investigate the mechanism of the optimization of the bi-frequency signal.When the synthetic jet is actuated by a single-frequency signal with a characteristic velocity of 0.112 m/s,the wake is asymmetrical.The alternative deflection of vortex pairs and the peak at half of the excitation frequency in the power spectral density(PSD)function are detected.In the bi-frequency cases with the same characteristic velocity,the wake gradually turns to be symmetrical with the increase in the amplitude of the high-frequency component.Meanwhile,the deflection of the vortex pairs and the peak at half of the excitation frequency gradually disappear as well.展开更多
Superconducting wire-networks are paradigms to study Cooper pairing issues,vortex dynamics and arrangements.Recently,emergent low-dimensional crystalline superconductors were reported in the minimal-disorder limit,pro...Superconducting wire-networks are paradigms to study Cooper pairing issues,vortex dynamics and arrangements.Recently,emergent low-dimensional crystalline superconductors were reported in the minimal-disorder limit,providing novel platforms to reveal vortices-related physics.Study on superconducting loops with high-crystallinity is thus currently demanded.Here,we report fabrication and transport measurement of finite square-network based on two-dimensional crystalline superconductor Mo_(2)C.We observe oscillations in the resistance as a function of the magnetic flux through the loops.Resistance dips at both matching field and fractional fillings are revealed.Temperature and current evolutions are carried out in magnetoresistance to study vortex dynamics.The amplitude of oscillation is enhanced due to the interaction between thermally activated vortices and the currents induced in the loops.The driving current reduces the effective activation energy for vortex,giving rise to stronger vortex interaction.Moreover,by the thermally activated vortex creep model,we derive the effective potential barrier for vortex dissipation,which shows well-defined correspondence with structures in magnetoresistance.Our work shows that low-dimensional crystalline superconducting network based on Mo_(2)C possesses pronounced potential in studying the modulation of vortex arrangements and dynamics,paving the way for further investigations on crystalline superconducting network with various configurations.展开更多
The flow around the slat cove of a two-dimensional 30P30N multi-element airfoil is investigated with time-resolved particle image velocimetry(TR-PIV)at low Reynolds number(Rec=2.41×10^(4)and 4.61×10^(4)).The...The flow around the slat cove of a two-dimensional 30P30N multi-element airfoil is investigated with time-resolved particle image velocimetry(TR-PIV)at low Reynolds number(Rec=2.41×10^(4)and 4.61×10^(4)).The effects of angle of attack(α=8°,12°,and 16°)on the mean flow characteristics and vortex dynamics are discussed.The size of the recirculation within the slat cove and the intensity of the shed vortices originating from the slat cusp shear layer are found to generally decrease as the angle of attack increases.The joint time-frequency analyses show that disturbances of different frequencies exist in the slat cusp shear layer and they trigger the different vortex shedding patterns of the slat cusp shear layer.The self-sustained oscillation within the slat cove,normally observed at high Reynolds number(Re_(c)~10^(6)),is proved to be responsible for the disturbances of different frequencies and the related vortex dynamics in the current study.展开更多
Vortex beams with fractional topological charge(FTC) have many special characteristics and novel applications.However, one of the obstacles for their application is the difficulty of precisely determining the FTC of f...Vortex beams with fractional topological charge(FTC) have many special characteristics and novel applications.However, one of the obstacles for their application is the difficulty of precisely determining the FTC of fractional vortex beams. We find that when a vortex beam with an FTC illuminates a dynamic angular double slit(ADS), the far-field interference patterns that include the information of the FTC of the beam at the angular bisector direction of the ADS vary periodically. Based on this property, a simple dynamic ADS device and data fitting method can be used to precisely measure the FTC of a vortex light beam with an error of less than 5%.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 individual influence of pitching and plunging motions on flow structures is studied experimentally by changing the phase lag between the geometrical angle of attack and the plunging angle of attack.Five phase lags...The individual influence of pitching and plunging motions on flow structures is studied experimentally by changing the phase lag between the geometrical angle of attack and the plunging angle of attack.Five phase lags are chosen as the experimental parameters,while the Strouhal number,the reduced frequency and the Reynolds number are fixed.During the motion of the airfoil,the leading edge vortex,the reattached vortex and the secondary vortex are observed in the flow field.The leading edge vortex is found to be the main flow structure through the proper orthogonal decomposition.The increase of phase lag results in the increase of the leading edge velocity,which strongly influences the leading edge shear layer and the leading edge vortex.The plunging motion contributes to the development of the leading edge shear layer,while the pitching motion is the key reason for instability of the leading edge shear layer.It is also found that a certain increase of phase lag,around 34.15°in this research,can increase the airfoil lift.展开更多
Hemodynamics plays a crucial role in the growth of an abdominal aortic aneurysm(AAA)and its possible rupture.Due to the serious consequences that arise from the aneurysm rupture,the ability to predict its evolution an...Hemodynamics plays a crucial role in the growth of an abdominal aortic aneurysm(AAA)and its possible rupture.Due to the serious consequences that arise from the aneurysm rupture,the ability to predict its evolution and the need for surgery are of primary importance in the medical field.Furthermore,the presence of intraluminal thrombus(ILT)strongly affects the evolution of the pathology.In this study,we analyzed the influence of hemodynamics on the growth and possible rupture of AAAs.Numerical investigations of pulsatile non-Newtonian blood flow were performed in six patient-specific AAAs reconstructed from diagnostic images,having different sizes and shapes,and with or without ILT.Wall shear stress and vorticity distribution in the bulge and their evolution during the cardiac cycle were analyzed.The results indicate that blood flow dynamics acts synergistically with atherosclerotic degeneration in the development of the disease.The high surface complexity and tortuosity of the aneurysms significantly affect the blood motion,and the presence of inflection in the aneurysm centerline has a noticeable effect on the vortex dynamics.Links between regions of slow recirculating flows,low values of time-averaged wall shear stress,high values of oscillatory shear index,and zones of ILT deposition were found.In the absence of ILT,possible thrombus accumulation areas and consequent aneurysm growth were identified.The findings of this study highlight the importance of hemodynamics in assessing the vulnerability of the aortic wall and underline the crucial role of patient-specific investigations in predicting the rupture of individual aneurysms.展开更多
We propose criteria of tracking vortex surfaces in complex flows based on the vortex-surface field (VSF). The criteria characterize the accuracy and Lagrangian tracking performance of the numerical VSF solution, and d...We propose criteria of tracking vortex surfaces in complex flows based on the vortex-surface field (VSF). The criteria characterize the accuracy and Lagrangian tracking performance of the numerical VSF solution, and determine the time period when the vortex surface tracking is satisfactory. Moreover, we develop a turbulent-like flow combining large-scale coherent structures in the Taylor-Green flow and small-scale turbulent structures in homogeneous isotropic turbulence (HIT). From tracking of vortex surfaces during the effective tracking period, we find that the imposed HIT disturbance significantly wrinkles vortex surfaces. Subsequently, the wrinkled vortex tube with large vorticity magnitude tends to be further twisted, contributing to energy cascade, while the wrinkling is mitigated in the region with small vorticity magnitude.展开更多
In this paper,we introduce a three-dimensional numerical method for computing the wake behind a flat plate advancing perpendicular to the flow.Our numerical method is inspired by the panel method of J.Katz and A.Plotk...In this paper,we introduce a three-dimensional numerical method for computing the wake behind a flat plate advancing perpendicular to the flow.Our numerical method is inspired by the panel method of J.Katz and A.Plotkin[J.Katz and A.Plotkin,Low-speed Aerodynamics,2001]and the 2D vortex blob method of Krasny[R.Krasny,Lectures in Appl.Math.,28(1991),pp.385–402].The accuracy of the method will be demonstrated by comparing the 3D computation at the center section of a very high aspect ratio plate with the corresponding two-dimensional computation.Furthermore,we compare the numerical results obtained by our 3D numerical method with the corresponding experimental results obtained recently by Ringuette[M.J.Ringuette,Ph.D.Thesis,2004]in the towing tank.Our numerical results are shown to be in excellent agreement with the experimental results up to the so-called formation time.展开更多
A better understanding of the mixing behavior of excited turbulent mixing layers is critical to a number of aerospace applications.Previous studies of excited turbulent mixing layers focused on single frequency excita...A better understanding of the mixing behavior of excited turbulent mixing layers is critical to a number of aerospace applications.Previous studies of excited turbulent mixing layers focused on single frequency excitation or the excitation with fundamental and its second harmonic frequency.There is a lack of detailed studies on applying low and higher frequency excitation.In this study,we have performed large-eddy simulations of periodically excited turbulent mixing layers.The excitation consists of a fundamental frequency and its third harmonic.We have used phase-averaging to identify the vortex structure and strength in the mixing layer,and we have studied the vortex dynamics.Two different vortex paring mechanisms are observed depending on the phase shift between the two excitation frequencies.The influence of these two mechanisms on the mixing of a passive scalar is also studied.It is found that exciting the mixing layer with these low and high frequencies has initially an adverse influence on the mixing process;however,it improves the mixing further downstream of the splitter plate with the excitation using a phase shift ofΔφ=πshowing the best mixing performance.The present works shed lights on the fundamental vortex dynamics,and has great potential for aeronautical,automotive and combustion engineering applications.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
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
Based on the boundary vorticity-flux theory,topology optimization of the caudal fin of the three-dimensional self-propelled swimming fish is investigated by combining unsteady computational fluid dynamics with moving ...Based on the boundary vorticity-flux theory,topology optimization of the caudal fin of the three-dimensional self-propelled swimming fish is investigated by combining unsteady computational fluid dynamics with moving boundary and topology optimization algorithms in this study.The objective functional of topology optimization is the function of swimming efficiency,swimming speed and motion direction control.The optimal caudal fin,whose topology is different from that of the natural fish caudal fin,make the 3D bionic fish achieve higher swimming efficiency,faster swimming speed and better maneuverability.The boundary vorticity-flux on the body surface of the 3D fish before and after optimization reveals the mechanism of high performance swimming of the topology optimization bionic fish.The comparative analysis between the swimming performance of the 3D topology optimization bionic fish and the 3D lunate tail bionic fish is also carried out,and the wake structures of two types of bionic fish show the physical nature that the swimming performance of the 3D topology optimization bionic fish is significantly better than the 3D lunate tail bionic fish.展开更多
基金National Natural Science Foundation of China(41405062, 41775017)。
文摘Previous studies showed that 4 D-Var technique used for data assimilation could be modified for weather control. This study demonstrates the ability of 4 D-Var to influence the future path of a tropical cyclone by calculating perturbations in WRF simulation. Given the background error covariance matrix, the initial field is improved by the vortex dynamic initialization technique. Our results show that 4 D-Var can be applied to control the trajectory of simulated tropical cyclones by producing "optimal" perturbations. In the numerical simulation experiment of Typhoon Mitag in 2019, after this kind of weather control similar to data assimilation, the tropical cyclone moved obviously,and the damaging wind over the coastline weakened. The prediction results after the initial field modified by 4 D-Var have a great change, and the position of the tropical cyclone moved about 0.5° southeastward after assimilation,which misses the southeast coast of China. Moreover, the damaging wind is also weakened. Since the 4 D-Var is premised on the assumption that the model is perfect and does not consider the model error, then the research plan to consider model error and introduce new methods is discussed in the paper.
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co-supported by the National Natural Science Foundation of China(No.51836008)the National Science and Technology Major Project of China(No.2017-II-0010-0024)。
文摘The transition process within a Laminar Separation Bubble(LSB)that formed on a compressor blade surface was investigated using Large Eddy Simulations(LESs)at a Reynolds number of 1.5×10;and incidence angles of 0°,+3°,and+5°.The vortex dynamics in the separated shear layers were compared at various incidence angles and its effects on the loss generation were clarified through entropy analysis.Results showed that transition onset,which was accurately identified by the Linear Stability Theory(LST),was significantly promoted at the increased incidence angle.As such,the development of LSB was suppressed and the relative role of viscous instability played in the transition process was weakened.At the incidence angle of 0°,two-dimensional spanwise vortices detached from the blade surface and roiled up periodically,which were further stretched and eventually evolved into large-scale hairpin vortices.As time passed,the fully developed hairpin vortices broke down into small-scale eddies.Meanwhile,the flow near the wall reversely ejected into the outer separated shear layers and a sweeping process happened subsequently,forcing the separated shear layers to reattach and accelerating the generation of turbulent fluctuations.By comparison,the strength of vortex rolling-up was weakened at higher incidence angles,and the vortex pairing and breakdown of large-scale vortices were less pronounced.Therefore,the level of turbulent fluctuations that generated in the separated shear layers was reduced.Detailed entropy analysis showed that the turbulent dissipation effect related to the Reynolds shear stresses determined the largest amount of positive entropy generation,which declined to a lower level as the incidence angle increased from 0°to+5°.Correspondingly,the profile loss was reduced by 50.4%.
基金Project supported by the National Key Basic Research Program of China(Grant Nos.2021YFA0718700,2018YFB0704102,2017YFA0303003,2017YFA0302902,2016YFA0300301,and 2021YFA0718802)the National Natural Science Foundation of China(Grant Nos.11927808,11834016,118115301,119611410,11961141008,61727805+5 种基金11961141002)the Key Research Program of Frontier Sciences,Chinese Academy of Sciences(CAS)(Grant Nos.QYZDB-SSW-SLH008 and QYZDY-SSW-SLH001)CAS Interdisciplinary Innovation Team,the Strategic Priority Research Program(B)of CAS(Grant Nos.XDB25000000and XDB33000000)the Beijing Natural Science Foundation(Grant No.Z190008)the Key-Area Research and Development Program of Guangdong Province,China(Grant No.2020B0101340002)the support from the China Postdoctoral Science Foundation(Grant No.2022M711497)。
文摘High quality Nb films were successfully prepared on both flexible polyimide(PI)and rigid Al2O3substrates and their transport properties were systematically studied at various applied currents,external magnetic fields,and sample orientations.It is found that a curved Nb/PI film exhibits quite different superconducting transition and vortex dynamics compared to the flat Nb/Al2O3film.For the curved Nb/PI film,smooth superconducting transitions were obtained at low currents,while unexpected cascade structures were revealed in theρ(T)curves at high currents.We attribute this phenomenon to the gradient distribution of vortex density together with a variation of superconductivity along the curved film.In addition,reentrant superconductivity was induced in the curved Nb/PI thin film by properly choosing the measurement conditions.We attribute this effect to the vortex pinning from both in-plane vortices and out-of-plane vortices.This work reveals the complex transport properties of curved superconducting thin films,providing important insights for further theoretical investigations and practical developments of flexible superconductors.
基金supported by the National Natural Science Foundation of China(No.11721202)。
文摘The flow around a square cylinder with a synthetic jet positioned at the rear surface is numerically investigated with the unsteady Reynolds-averaged Navier-Stokes(URANS)method.Instead of the typical sinusoidal wave,a bi-frequency signal is adopted to generate the synthetic jet.The bi-frequency signal consists of a basic sinusoidal wave and a high-frequency wave.Cases with various amplitudes of the high-frequency component are simulated.It is found that synthetic jets actuated by bi-frequency signals can realize better drag reduction with lower energy consumption when appropriate parameter sets are applied.A new quantity,i.e.,the actuation efficiency Ae,is used to evaluate the controlling efficiency.The actuation efficiency Ae reaches its maximum of 0.2668 when the amplitude of the superposed high-frequency signal is 7.5%of the basic signal.The vortex structures and frequency characteristics are subsequently analyzed to investigate the mechanism of the optimization of the bi-frequency signal.When the synthetic jet is actuated by a single-frequency signal with a characteristic velocity of 0.112 m/s,the wake is asymmetrical.The alternative deflection of vortex pairs and the peak at half of the excitation frequency in the power spectral density(PSD)function are detected.In the bi-frequency cases with the same characteristic velocity,the wake gradually turns to be symmetrical with the increase in the amplitude of the high-frequency component.Meanwhile,the deflection of the vortex pairs and the peak at half of the excitation frequency gradually disappear as well.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11974026,11774005,and 51802314)the National Key Research and Development Program of China(Grant No.2017YFA0303304)+1 种基金Science Foundation of Jihua Laboratory(Grant No.2021B0301030003-03)the Strategic Priority Research Program of Chinese Academy of Sciences(Grant No.XDB30000000)。
文摘Superconducting wire-networks are paradigms to study Cooper pairing issues,vortex dynamics and arrangements.Recently,emergent low-dimensional crystalline superconductors were reported in the minimal-disorder limit,providing novel platforms to reveal vortices-related physics.Study on superconducting loops with high-crystallinity is thus currently demanded.Here,we report fabrication and transport measurement of finite square-network based on two-dimensional crystalline superconductor Mo_(2)C.We observe oscillations in the resistance as a function of the magnetic flux through the loops.Resistance dips at both matching field and fractional fillings are revealed.Temperature and current evolutions are carried out in magnetoresistance to study vortex dynamics.The amplitude of oscillation is enhanced due to the interaction between thermally activated vortices and the currents induced in the loops.The driving current reduces the effective activation energy for vortex,giving rise to stronger vortex interaction.Moreover,by the thermally activated vortex creep model,we derive the effective potential barrier for vortex dissipation,which shows well-defined correspondence with structures in magnetoresistance.Our work shows that low-dimensional crystalline superconducting network based on Mo_(2)C possesses pronounced potential in studying the modulation of vortex arrangements and dynamics,paving the way for further investigations on crystalline superconducting network with various configurations.
基金supported by the National Natural Science Foundation of China(Grant Nos.12102024 and 11721202)the China Postdoctoral Science Foundation(Grant Nos.2021M700010 and 2022T150036)。
文摘The flow around the slat cove of a two-dimensional 30P30N multi-element airfoil is investigated with time-resolved particle image velocimetry(TR-PIV)at low Reynolds number(Rec=2.41×10^(4)and 4.61×10^(4)).The effects of angle of attack(α=8°,12°,and 16°)on the mean flow characteristics and vortex dynamics are discussed.The size of the recirculation within the slat cove and the intensity of the shed vortices originating from the slat cusp shear layer are found to generally decrease as the angle of attack increases.The joint time-frequency analyses show that disturbances of different frequencies exist in the slat cusp shear layer and they trigger the different vortex shedding patterns of the slat cusp shear layer.The self-sustained oscillation within the slat cove,normally observed at high Reynolds number(Re_(c)~10^(6)),is proved to be responsible for the disturbances of different frequencies and the related vortex dynamics in the current study.
基金Fundamental Research Funds for the Central UniversitiesNational Natural Science Foundation of China(NSFC)(11374008,11374238,11374239,11534008)
文摘Vortex beams with fractional topological charge(FTC) have many special characteristics and novel applications.However, one of the obstacles for their application is the difficulty of precisely determining the FTC of fractional vortex beams. We find that when a vortex beam with an FTC illuminates a dynamic angular double slit(ADS), the far-field interference patterns that include the information of the FTC of the beam at the angular bisector direction of the ADS vary periodically. Based on this property, a simple dynamic ADS device and data fitting method can be used to precisely measure the FTC of a vortex light beam with an error of less than 5%.
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(Nos.GZ 1280,11722215 and 11721202)。
文摘The individual influence of pitching and plunging motions on flow structures is studied experimentally by changing the phase lag between the geometrical angle of attack and the plunging angle of attack.Five phase lags are chosen as the experimental parameters,while the Strouhal number,the reduced frequency and the Reynolds number are fixed.During the motion of the airfoil,the leading edge vortex,the reattached vortex and the secondary vortex are observed in the flow field.The leading edge vortex is found to be the main flow structure through the proper orthogonal decomposition.The increase of phase lag results in the increase of the leading edge velocity,which strongly influences the leading edge shear layer and the leading edge vortex.The plunging motion contributes to the development of the leading edge shear layer,while the pitching motion is the key reason for instability of the leading edge shear layer.It is also found that a certain increase of phase lag,around 34.15°in this research,can increase the airfoil lift.
文摘Hemodynamics plays a crucial role in the growth of an abdominal aortic aneurysm(AAA)and its possible rupture.Due to the serious consequences that arise from the aneurysm rupture,the ability to predict its evolution and the need for surgery are of primary importance in the medical field.Furthermore,the presence of intraluminal thrombus(ILT)strongly affects the evolution of the pathology.In this study,we analyzed the influence of hemodynamics on the growth and possible rupture of AAAs.Numerical investigations of pulsatile non-Newtonian blood flow were performed in six patient-specific AAAs reconstructed from diagnostic images,having different sizes and shapes,and with or without ILT.Wall shear stress and vorticity distribution in the bulge and their evolution during the cardiac cycle were analyzed.The results indicate that blood flow dynamics acts synergistically with atherosclerotic degeneration in the development of the disease.The high surface complexity and tortuosity of the aneurysms significantly affect the blood motion,and the presence of inflection in the aneurysm centerline has a noticeable effect on the vortex dynamics.Links between regions of slow recirculating flows,low values of time-averaged wall shear stress,high values of oscillatory shear index,and zones of ILT deposition were found.In the absence of ILT,possible thrombus accumulation areas and consequent aneurysm growth were identified.The findings of this study highlight the importance of hemodynamics in assessing the vulnerability of the aortic wall and underline the crucial role of patient-specific investigations in predicting the rupture of individual aneurysms.
基金National Numerical Wind Tunnel Project(Grant No.NNW2021ZT6-B36)National Natural Science Foundation of China(Grant Nos.11925201,91541204,and 91841302),and the Xplore Prize.
文摘We propose criteria of tracking vortex surfaces in complex flows based on the vortex-surface field (VSF). The criteria characterize the accuracy and Lagrangian tracking performance of the numerical VSF solution, and determine the time period when the vortex surface tracking is satisfactory. Moreover, we develop a turbulent-like flow combining large-scale coherent structures in the Taylor-Green flow and small-scale turbulent structures in homogeneous isotropic turbulence (HIT). From tracking of vortex surfaces during the effective tracking period, we find that the imposed HIT disturbance significantly wrinkles vortex surfaces. Subsequently, the wrinkled vortex tube with large vorticity magnitude tends to be further twisted, contributing to energy cascade, while the wrinkling is mitigated in the region with small vorticity magnitude.
基金supported by NSF ITR Grant No.ACI-0204932NSF FRG Grant No.DMS-0353838.
文摘In this paper,we introduce a three-dimensional numerical method for computing the wake behind a flat plate advancing perpendicular to the flow.Our numerical method is inspired by the panel method of J.Katz and A.Plotkin[J.Katz and A.Plotkin,Low-speed Aerodynamics,2001]and the 2D vortex blob method of Krasny[R.Krasny,Lectures in Appl.Math.,28(1991),pp.385–402].The accuracy of the method will be demonstrated by comparing the 3D computation at the center section of a very high aspect ratio plate with the corresponding two-dimensional computation.Furthermore,we compare the numerical results obtained by our 3D numerical method with the corresponding experimental results obtained recently by Ringuette[M.J.Ringuette,Ph.D.Thesis,2004]in the towing tank.Our numerical results are shown to be in excellent agreement with the experimental results up to the so-called formation time.
基金This work was supported by the National Natural Science Foundation of China(Grant Nos.12102029,11902019 and 11721202)the Postdoctoral Science Foundation Grant of China(Grant No.2021M690301).
基金the financial support provided by the Deakin University,Australiathe University of Canterbury,New Zealand (No. 452DISDZ)
文摘A better understanding of the mixing behavior of excited turbulent mixing layers is critical to a number of aerospace applications.Previous studies of excited turbulent mixing layers focused on single frequency excitation or the excitation with fundamental and its second harmonic frequency.There is a lack of detailed studies on applying low and higher frequency excitation.In this study,we have performed large-eddy simulations of periodically excited turbulent mixing layers.The excitation consists of a fundamental frequency and its third harmonic.We have used phase-averaging to identify the vortex structure and strength in the mixing layer,and we have studied the vortex dynamics.Two different vortex paring mechanisms are observed depending on the phase shift between the two excitation frequencies.The influence of these two mechanisms on the mixing of a passive scalar is also studied.It is found that exciting the mixing layer with these low and high frequencies has initially an adverse influence on the mixing process;however,it improves the mixing further downstream of the splitter plate with the excitation using a phase shift ofΔφ=πshowing the best mixing performance.The present works shed lights on the fundamental vortex dynamics,and has great potential for aeronautical,automotive and combustion engineering applications.
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This work was supported by the National Natural Science Foundation of China(NSFC)(Grant No.91941301)the Key Research and Development Project of Sichuan Province(Grant No.2019ZYZF0002)。
基金support of National Natural Science Foundation of China(Grant No.11302071)and National Postdoctoral Foundation of China(Grant No.2013M541597).Our deep appreciation goes to Professor Wei SHYY of Hong Kong University of Science and Technology,for detailed discussion and kindly help.
文摘Based on the boundary vorticity-flux theory,topology optimization of the caudal fin of the three-dimensional self-propelled swimming fish is investigated by combining unsteady computational fluid dynamics with moving boundary and topology optimization algorithms in this study.The objective functional of topology optimization is the function of swimming efficiency,swimming speed and motion direction control.The optimal caudal fin,whose topology is different from that of the natural fish caudal fin,make the 3D bionic fish achieve higher swimming efficiency,faster swimming speed and better maneuverability.The boundary vorticity-flux on the body surface of the 3D fish before and after optimization reveals the mechanism of high performance swimming of the topology optimization bionic fish.The comparative analysis between the swimming performance of the 3D topology optimization bionic fish and the 3D lunate tail bionic fish is also carried out,and the wake structures of two types of bionic fish show the physical nature that the swimming performance of the 3D topology optimization bionic fish is significantly better than the 3D lunate tail bionic fish.