In hydraulic engineering,free-surface aeration is a natural phenomenon occurring in smooth channel flows.In self-aerated flows,a key aspect that has not yet been well understood is the formation mechanism of free-surf...In hydraulic engineering,free-surface aeration is a natural phenomenon occurring in smooth channel flows.In self-aerated flows,a key aspect that has not yet been well understood is the formation mechanism of free-surface air entrainment.In this research,the process of free-surface entrapped deformation is analyzed theoretically and the critical radius of curvature for air entrainment is obtained,affected by flow mean velocity and depth.When the severity of local free-surface deformation exceeds the critical condition,the entrapped free surface encounters closure in the unstable deformation movement process,resulting in air entrainment.This inference agrees well with observed experimental results that are obtained from the processes of surface entrapped deformation and air entrainment captured by a high-speed camera-based data acquisition system.This agreement indicates that self-aeration occurs in low-velocity open-channel flows.It is also confirmed that free-surface turbulent deformation provides a mechanism for air entrainment.展开更多
The study of flow diversions in open channels plays an important practical role in the design and management of open-channel networks for irrigation or drainage. To accurately predict the mean flow and turbulence char...The study of flow diversions in open channels plays an important practical role in the design and management of open-channel networks for irrigation or drainage. To accurately predict the mean flow and turbulence characteristics of open-channel dividing flows, a hybrid LES-RANS model, which combines the large eddy simulation （LES） model with the Reynolds-averaged Navier-Stokes （RANS） model, is proposed in the present study. The unsteady RANS model was used to simulate the upstream and downstream regions of a main channel, as well as the downstream region of a branch channel. The LES model was used to simulate the channel diversion region, where turbulent flow characteristics are complicated. Isotropic velocity fluctuations were added at the inflow interface of the LES region to trigger the generation of resolved turbulence. A method based on the virtual body force is proposed to impose Reynolds-averaged velocity fields near the outlet of the LES region in order to take downstream flow effects computed by the RANS model into account and dissipate the excessive turbulent fluctuations. This hybrid approach saves computational effort and makes it easier to properly specify inlet and outlet boundary conditions. Comparison between computational results and experimental data indicates that this relatively new modeling approach can accurately predict open-channel T-diversion flows.展开更多
A part of mean kinetic energy in a main-channel is used for production of a large-scale horizontal circulation in the side cavity. However, the details of the mechanism such as energy transport are poorly understood. ...A part of mean kinetic energy in a main-channel is used for production of a large-scale horizontal circulation in the side cavity. However, the details of the mechanism such as energy transport are poorly understood. Therefore, we conducted PIV mea- surements in a laboratory flume and compared space distributions of mean velocity components and Reynolds stress by varying a cavity geometry. In particular, a practical calculation method of Reynolds stress was also developed and its accuracy was examined by comparison with the measured data. Furthermore, contributions of components in an energy transport equation were revealed quantitatively.展开更多
This paper aims to numerically explore the characteristics of unsteady cavitating flow around a NACA0015 hydrofoil,with a focus on vorticity attributes.The simulation utilizes a homogeneous mixture model coupled with ...This paper aims to numerically explore the characteristics of unsteady cavitating flow around a NACA0015 hydrofoil,with a focus on vorticity attributes.The simulation utilizes a homogeneous mixture model coupled with a filter-based density correction turbulence model and a modified Zwart cavitation model.The study investigates the dynamic cavitation features of the thermal fluid around the hydrofoil at various incoming flow velocities.It systematically elucidates the evolution of cavitation and vortex dynamics corresponding to each velocity condition.The results indicate that with increasing incoming flow velocity,distinct cavitation processes take place in the flow field.展开更多
Large size titanium alloy parts are widely used in aerospace.However,they are difficult to manufacture using mechanical cutting technology because of severe tool wear.Electrochemical jet machining is a promising techn...Large size titanium alloy parts are widely used in aerospace.However,they are difficult to manufacture using mechanical cutting technology because of severe tool wear.Electrochemical jet machining is a promising technology to achieve high efficiency,because it has high machining flexibility and no machining tool wear.However,reports on the macro electrochemical jet machining of large size titanium alloy parts are very scarce,because it is difficult to achieve effective constraint of the flow field in macro electrochemical jet machining.In addition,titanium alloy is very sensitive to fluctuation of the flow field,and a turbulent flow field would lead to serious stray corrosion.This paper reports a series of investigations of the electrochemical jet machining of titanium alloy parts.Based on the flow analysis and experiments,the machining flow field was effectively constrained.TB6 titanium alloy part with a perimeter of one meter was machined.The machined surface was smooth with no obvious machining defects.The machining process was particularly stable with no obvious spark discharge.The research provides a reference for the application of electrochemical jet machining technology to achieve large allowance material removal in the machining of large titanium alloy parts.展开更多
Incipient sediment motion plays a key role in scouring and bed load transport. However, the incipient sediment motion in the vegetated open-channel flows has yet to be fully understood. This study aims to quantify the...Incipient sediment motion plays a key role in scouring and bed load transport. However, the incipient sediment motion in the vegetated open-channel flows has yet to be fully understood. This study aims to quantify the critical conditions of the sediment particle movement in the presence of emergent and submerged vegetation. A new formula of the critical flow velocity was proposed to predict the incipient sediment motion based on the force balance equation of a sediment particle and the assumption that the velocity distribution in the bed roughness boundary layer fits the logarithmic law. Analysis of the derived formula revealed that the critical flow velocity for incipient sediment motion decreases with the increase in vegetation density. The proposed formula agrees well with the experimental data in the literature, thereby implying that the critical flow velocity can effectively quantify the incipient sediment motion in the vegetated open channel flows.展开更多
The modern definition of the wave concept,which is based on the functional connection between the parameters of the spatial structure of an instantaneous flow pattern and the characteristics of the temporal variabilit...The modern definition of the wave concept,which is based on the functional connection between the parameters of the spatial structure of an instantaneous flow pattern and the characteristics of the temporal variability at a given point,is discussed.The dispersion relation for 2D plane periodic perturbations on the surface of viscous stratified fluid is selected as the characteristic function defining the wave motion.Using the theory of singular perturbations,a method for calculating complete solutions to the dispersion relations of periodic flows,including regular wave and singular ligament solutions is presented.Properties of the complete exact solution of the dispersion relation containing regular and singular functions are compared with asymptotic solutions.In limiting cases,obtained dispersion relations are matched with well⁃known expressions for waves in homogeneous viscous and ideal liquids.展开更多
The formation,evolution,and dynamics of flow structures in wall-bounded turbulence have long been central themes in fluid-mechanics research.Over the past three decades,Soliton-like Coherent Structures(SCSs)have emerg...The formation,evolution,and dynamics of flow structures in wall-bounded turbulence have long been central themes in fluid-mechanics research.Over the past three decades,Soliton-like Coherent Structures(SCSs)have emerged as a ubiquitous and unifying feature across a wide range of shear flows,including K-type,O-type,N-type,and bypass transitional boundary layers,as well as fully developed turbulent boundary layers,mixing layers,and pipe flows.This paper presents a systematic review of the fundamental properties of SCSs and highlights their fundamental role in multiple transition scenarios.The analysis further explores the connection between SCSs and low-speed streaks,offering insight into their coupled dynamics.The phenomenon of turbulent bursting is also examined within the context of SCS dynamics.Together,these studies underscore the potential of SCSs to serve as a coherent dynamical framework for understanding turbulence generation mechanisms in wall-bounded flows.Finally,the review extends to the manifestation of SCSs in other canonical flows,including mixing layers,stratified shear flows,and jets,confirming their universality and significance in fluid dynamics.These findings not only advance our understanding of turbulence generation but also offer a promising theoretical foundation for future research in transitional and turbulent flows.展开更多
This study investigates the air–water interaction dynamics in jet streams,with particular emphasis on the transition from the cavity to the far-field regions.A dual-tip conductivity phase-detection probe was employed...This study investigates the air–water interaction dynamics in jet streams,with particular emphasis on the transition from the cavity to the far-field regions.A dual-tip conductivity phase-detection probe was employed to analyze four distinct downstream water levels.Based on the development of the cross-sectional mean air concentration,the jet flow was divided into four distinct regions:the jet length region,impact region,splash region,and far-field region.The results demonstrate varying trends in the evolution of the mean air concentration and maximum bubble frequency.Downstream water levels exerted a significant influence on these parameters in the splash and far-field regions,whereas minimal variation was observed in the impact region.Additionally,notable differences were identified in the probability density function of water droplets between the cavity and downstream regions.Furthermore,downstream water depth was found to have a negligible effect on the proportion of small-sized droplets in both the impact and splash regions.展开更多
The variable salinity in stored reservoirs connected by a long channel attracts the attention of scientists worldwide,having applications in environmental and geophysical engineering.This study explores the impact of ...The variable salinity in stored reservoirs connected by a long channel attracts the attention of scientists worldwide,having applications in environmental and geophysical engineering.This study explores the impact of Navier slip conditions on exchange flows within a long channel connecting two large reservoirs of differing salinity.These horizontal density gradients drive the flow.We modify the recent one-dimensional theory,developed to avoid runaway stratification,to account for the presence of uniform slip walls.By adjusting the parameters of the horizontal density gradient based on the slip factor,we resolve analytically various flow regimes ranging from high diffusion to transitional high advection.These regimes are governed by physical parameters like channel aspect ratio,slip factor,Schmidt number,and gravitational Reynolds number.Our solutions align perfectly with ones in the no-slip limit.More importantly,under the conditions of no net flow across the channel and high Schmidt number(where stratification is concentrated near the channel’s mid-layer),we derive a closed-form solution for the slip parameter,aspect ratio,and gravitational Reynolds number that describes the interface’s behavior as a sharp interface separating two distinct zones.This interface,arising from hydrostatic wall gradients,ultimately detaches the low-and high-density regimes throughout the channel when the gravitational Reynolds number is inversely proportional to the aspect ratio for a fixed slip parameter.This phenomenon,observed previously in 2D numerical simulations with no-slip walls in the literature,is thus confirmed by our theoretical results.Our findings further demonstrate that wall slip leads to distinct and diverse flow regimes.展开更多
The increasing performance demands of modern aero engines necessitate the integrated design of compressor transition ducts with upstream components to reduce the axial length of the engine.However,this design approach...The increasing performance demands of modern aero engines necessitate the integrated design of compressor transition ducts with upstream components to reduce the axial length of the engine.However,this design approach narrows the spacing between the stator and the strut,making traditional research on transition ducts only with struts unsuitable.The numerical results and experimental oil flow visualization results were utilized to reconstruct the three-dimensional flow structures in the stator passages under various operating conditions.Additionally,numerical methods were employed to analyze the mechanisms of the strut's effect on the upstream stator in an aggressive transition duct.The results show that the strut potential field increases the load on the upstream stator,leading to severe blade surface separation and corner separation/stall,and redistributes the inflow angle of the upstream stators circumferentially,resulting in significant differences in the flow structures within the stator passages on both sides.The separation flows within the stator passages mainly manifest in five types:pressure surface separation vortex,suction surface concentrated shedding vortex,suction surface separation vortex,suction surface-corner stall separation vortex,and suction surface separation vortex pair.Under different operating conditions,the separation flows within the stator passages are always composed of a part of these five types or a transitional state between two of them.展开更多
Buoyancy-driven flows are prevalent in a wide range of geophysical and astrophysical systems. In this review, we focus on threepivotal effects that significantly influence the dynamics and transport properties of buoy...Buoyancy-driven flows are prevalent in a wide range of geophysical and astrophysical systems. In this review, we focus on threepivotal effects that significantly influence the dynamics and transport properties of buoyancy-driven flows and may have impli-cations for natural systems. These effects pertain to the role of boundary conditions, the impact of rotation, and the effect offinite size. Boundary conditions represent how the fluid flow interacts with different kinds of surfaces. Rotation, as the Earth’srotation in geophysical systems or the whirling of astrophysical systems, introduces Coriolis and centrifugal forces, leading tothe profound vortical structure and distinct transport property. Finite size, representing geometrical constraints, influences thebehavior of buoyancy-driven flows across varying geometrical settings. This review aims to provide a holistic understanding ofthe intricate interplay of these factors, offering insights into the complex natural phenomena from the perspectives of the threeeffects.展开更多
Bridge pier failures from granular flow impacts are common.Installing defense piles upstream is an effective mitigation strategy,yet their protective mechanisms and standardized design guidelines are unclear.This stud...Bridge pier failures from granular flow impacts are common.Installing defense piles upstream is an effective mitigation strategy,yet their protective mechanisms and standardized design guidelines are unclear.This study employed 3D discrete element method to analyze the influence of defense pile size and placement on its performance across 219 scenarios,providing a detailed examination of their protective mechanisms.Results show that optimizing these factors can reduce the maximum impact force on bridge piers by up to 94%.In terms of size,a critical height threshold is identified,beyond which increasing pile height does not enhance protection.This threshold depends on the movement height of granular particles at the slope base.Protection effectiveness varies with pile size:when H≤0.05 h(H is the height of defense piles,h is the height of bridge),protection marginally improves with increasing height and diameter;for 0.05 h<H<0.15 h,protection strongly correlates with both parameters;for H≥0.15 h,diameter becomes the dominant factor.In terms of placement,an optimal longitudinal distance exists between the defense pile and the bridge pier.The larger the diameter,the greater the optimal longitudinal distance.However,the transverse distance is inversely related to protection effectiveness.Mechanistic analysis shows that defense piles are more effective at redirecting particles to prevent direct collisions with the pier(contributing 100%impact energy reduction before the non-dimensional travel time t*=7.01 and 63%–100%afterward)than at reducing particle velocity.This study provides insights into the protective mechanisms of defense piles and informs strategies for optimizing bridge pier protection in granular flow-prone regions.展开更多
The impacts of natural boulders carried by debris flows pose serious risks to the safety and reliability of structures and buildings.Natural boulders can be highly random and unpredictable.Consequently,boulder control...The impacts of natural boulders carried by debris flows pose serious risks to the safety and reliability of structures and buildings.Natural boulders can be highly random and unpredictable.Consequently,boulder control during debris flows is crucial but difficult.Herein,an eco-friendly control system featuring anchoring natural boulders(NBs)with(negative Poisson's ratio)NPR anchor cables is proposed to form an NB-NPR baffle.A series of flume experiments are conducted to verify the effect of NB-NPR baffles on controlling debris flow impact.The deployment of NB-NPR baffles substantially influences the kinematic behavior of a debris flow,primarily in the form of changes in the depositional properties and impact intensities.The results show that the NB-NPR baffle matrix successfully controls boulder mobility and exhibits positive feedback on solid particle deposition.The NB-NPR baffle group exhibits a reduction in peak impact force ranging from 29%to 79%compared to that of the control group in the basic experiment.The NPR anchor cables play a significant role in the NB-NPR baffle by demonstrating particular characteristics,including consistent resistance,large deformation,and substantial energy absorption.The NB-NPR baffle innovatively utilizes the natural boulders in a debris flow gully by converting destructive boulders into constructive boulders.Overall,this research serves as a basis for future field experiments and applications.展开更多
This study examines the causal relationship between financial technology startup venture capital(VC)financing and its deals with domestic credit provided by the banking sector and equity market movement.Despite the ri...This study examines the causal relationship between financial technology startup venture capital(VC)financing and its deals with domestic credit provided by the banking sector and equity market movement.Despite the rise of alternative finance,such as fintech venture capital(it is the fund that venture capital firms put into young,promising fintech companies so that they can help them expand and scale quickly),which is yet underexplored,borrowers still heavily rely on banks and the stock market for financing.We use panel data from 57 countries from 2010 to 2020 and an advanced econometric method called the cross-sectional autoregressive distributed lag model(CS-ARDL)to determine how the size and number of fintech equity funds dealt with by venture capital firms,banking sector credit,and stock market returns are interrelated at the global level and across regional,income,and economic levels.Our results reveal a cointegrating relationship between fintech venture capital funding and deals with bank loans and equity market returns.However,this relationship varies across the regions studied and between developed and developing economies.Our findings provide crucial guidelines for policymakers to create policies that support balanced financial development by highlighting the global interaction of equity market movements,banking credit,and fintech venture capital investment and lay the groundwork for internationally aligned policies to guarantee the optimal distribution of financial capital and improve economic stability and adaptability by illustrating how these links differ across geographical locations and economic conditions.展开更多
International capital flows play a crucial role in the process of globalization,presenting both opportunities and challenges to the financial stability of emerging economies.This article sorts out the positive effects...International capital flows play a crucial role in the process of globalization,presenting both opportunities and challenges to the financial stability of emerging economies.This article sorts out the positive effects and potential risks of international capital flows on the financial stability of emerging economies.By combining case studies in recent years,it analyzes the complex relationship between cross-border capital flows and financial stability,and proposes policy paths for emerging economies to cope with the shock of capital flows,providing references for enhancing financial resilience and achieving sustainable development.展开更多
Ecosystem service flows(ESFs)can reveal the interrelationships and impacts between natural systems and human activities.We can improve the stability and sustainability of ecosystems,more effectively utilize natural re...Ecosystem service flows(ESFs)can reveal the interrelationships and impacts between natural systems and human activities.We can improve the stability and sustainability of ecosystems,more effectively utilize natural resources,protect the environment,and enhance the harmonious coexistence of humans and nature by comprehending ESFs.However,few studies have examined ESFs across scales and evaluated their sustainability;most have concentrated on regional scales.In order to quantify and analyze ESFs within the Jing River Basin(JRB)and between the JRB and the adjacent and distant regions from a water-food-energy perspective,this paper employs a meta-coupling framework.Additionally,it evaluates the sustainability of these flows using a techno-ecological synergy framework.The results show that the ESFs within the JRB was significant in 2020.Water production services were concentrated in the southern part of the JRB,while the distribution of food supply and carbon supply services was relatively even.Huan County emerged as the largest exporting county,providing 1.46×10^(8)kg of food to other counties and exporting 2.97×10^(6)kg of energy.The ESFs in the JRB primarily moved towards the neighboring and distant systems.Water production services flowed into the Guanzhong Plain Urban Agglomeration(GPUA),amounting to 5.8×10^(6)kg.Carbon supply services flowed out at 2.4×10^(5)kg,and food exports were the highest,reaching 5.0×10^(7)kg.The ecosystem service flows from the JRB to both the neighboring and distant systems enhanced food security and ecological resilience.The basin itself demonstrated good sustainability in food supply services,with an index value reaching 48.19.In crossscale calculations of food production sustainability with the adjacent GPUA,the index value increased from 48.19 to 52.99,indicating a significant improvement.These findings demonstrate that applying the meta-coupling framework provides an effective approach to quantify ESFs and assess their sustainability across scales.展开更多
To address the early separation problem in the Menter Shear-Stress Transport(SST)turbulence model,a correction for the Turbulent Kinetic Energy(TKE)production term,P_(k),is introduced to account for the effect of the ...To address the early separation problem in the Menter Shear-Stress Transport(SST)turbulence model,a correction for the Turbulent Kinetic Energy(TKE)production term,P_(k),is introduced to account for the effect of the Adverse Pressure Gradient(APG).The correction is determined based on the distribution of Pkin the APG region before separation.When the friction coefficient C_(f) is decomposed,its direct dependence on Pkis clearly observed.However,with the introduction of Bradshaw’s assumption,Pkin the SST turbulence model is over-suppressed,resulting in a lower inner peak or no significant inner peak distribution at all.To address this problem,this paper proposes a Gaussian function,HGauss,which corrects the numerical values of P_(k) involved in the calculation of the Menter SST model by focusing on the inner peak region of P_(k).The modified SST model is then applied to four cases with APGs.The modification leads to an increase in the wall friction coefficient C_(f)in the APG region and causes a downstream shift in the separation location,improving the model’s consistency with high-accuracy data and experimental results.It is demonstrated that this correction can improve the early separation problem in the Menter SST turbulence model.展开更多
Efficient and accurate simulation of unsteady flow presents a significant challenge that needs to be overcome in computational fluid dynamics.Temporal discretization method plays a crucial role in the simulation of un...Efficient and accurate simulation of unsteady flow presents a significant challenge that needs to be overcome in computational fluid dynamics.Temporal discretization method plays a crucial role in the simulation of unsteady flows.To enhance computational efficiency,we propose the Implicit-Explicit Two-Step Runge-Kutta(IMEX-TSRK)time-stepping discretization methods for unsteady flows,and develop a novel adaptive algorithm that correctly partitions spatial regions to apply implicit or explicit methods.The novel adaptive IMEX-TSRK schemes effectively handle the numerical stiffness of the small grid size and improve computational efficiency.Compared to implicit and explicit Runge-Kutta(RK)schemes,the IMEX-TSRK methods achieve the same order of accuracy with fewer first derivative calculations.Numerical case tests demonstrate that the IMEX-TSRK methods maintain numerical stability while enhancing computational efficiency.Specifically,in high Reynolds number flows,the computational efficiency of the IMEX-TSRK methods surpasses that of explicit RK schemes by more than one order of magnitude,and that of implicit RK schemes several times over.展开更多
基金supported by the National Natural Science Foundation of China (Grant No. 51379138)the National Basic Research Program of China (“973” Project) (Grant No. 2013CB035905)
文摘In hydraulic engineering,free-surface aeration is a natural phenomenon occurring in smooth channel flows.In self-aerated flows,a key aspect that has not yet been well understood is the formation mechanism of free-surface air entrainment.In this research,the process of free-surface entrapped deformation is analyzed theoretically and the critical radius of curvature for air entrainment is obtained,affected by flow mean velocity and depth.When the severity of local free-surface deformation exceeds the critical condition,the entrapped free surface encounters closure in the unstable deformation movement process,resulting in air entrainment.This inference agrees well with observed experimental results that are obtained from the processes of surface entrapped deformation and air entrainment captured by a high-speed camera-based data acquisition system.This agreement indicates that self-aeration occurs in low-velocity open-channel flows.It is also confirmed that free-surface turbulent deformation provides a mechanism for air entrainment.
文摘The study of flow diversions in open channels plays an important practical role in the design and management of open-channel networks for irrigation or drainage. To accurately predict the mean flow and turbulence characteristics of open-channel dividing flows, a hybrid LES-RANS model, which combines the large eddy simulation （LES） model with the Reynolds-averaged Navier-Stokes （RANS） model, is proposed in the present study. The unsteady RANS model was used to simulate the upstream and downstream regions of a main channel, as well as the downstream region of a branch channel. The LES model was used to simulate the channel diversion region, where turbulent flow characteristics are complicated. Isotropic velocity fluctuations were added at the inflow interface of the LES region to trigger the generation of resolved turbulence. A method based on the virtual body force is proposed to impose Reynolds-averaged velocity fields near the outlet of the LES region in order to take downstream flow effects computed by the RANS model into account and dissipate the excessive turbulent fluctuations. This hybrid approach saves computational effort and makes it easier to properly specify inlet and outlet boundary conditions. Comparison between computational results and experimental data indicates that this relatively new modeling approach can accurately predict open-channel T-diversion flows.
文摘A part of mean kinetic energy in a main-channel is used for production of a large-scale horizontal circulation in the side cavity. However, the details of the mechanism such as energy transport are poorly understood. Therefore, we conducted PIV mea- surements in a laboratory flume and compared space distributions of mean velocity components and Reynolds stress by varying a cavity geometry. In particular, a practical calculation method of Reynolds stress was also developed and its accuracy was examined by comparison with the measured data. Furthermore, contributions of components in an energy transport equation were revealed quantitatively.
文摘This paper aims to numerically explore the characteristics of unsteady cavitating flow around a NACA0015 hydrofoil,with a focus on vorticity attributes.The simulation utilizes a homogeneous mixture model coupled with a filter-based density correction turbulence model and a modified Zwart cavitation model.The study investigates the dynamic cavitation features of the thermal fluid around the hydrofoil at various incoming flow velocities.It systematically elucidates the evolution of cavitation and vortex dynamics corresponding to each velocity condition.The results indicate that with increasing incoming flow velocity,distinct cavitation processes take place in the flow field.
基金the National Natural Science Foundation of China(No.52205468)China Postdoctoral Science Foundation(No.2022M710061 and No.2023T160277)Natural Science Foundation of Jiangsu Province(No.BK20210755)。
文摘Large size titanium alloy parts are widely used in aerospace.However,they are difficult to manufacture using mechanical cutting technology because of severe tool wear.Electrochemical jet machining is a promising technology to achieve high efficiency,because it has high machining flexibility and no machining tool wear.However,reports on the macro electrochemical jet machining of large size titanium alloy parts are very scarce,because it is difficult to achieve effective constraint of the flow field in macro electrochemical jet machining.In addition,titanium alloy is very sensitive to fluctuation of the flow field,and a turbulent flow field would lead to serious stray corrosion.This paper reports a series of investigations of the electrochemical jet machining of titanium alloy parts.Based on the flow analysis and experiments,the machining flow field was effectively constrained.TB6 titanium alloy part with a perimeter of one meter was machined.The machined surface was smooth with no obvious machining defects.The machining process was particularly stable with no obvious spark discharge.The research provides a reference for the application of electrochemical jet machining technology to achieve large allowance material removal in the machining of large titanium alloy parts.
基金supported by the National Natural Science Foundation of China(Grant Nos.52020105006,11872285).
文摘Incipient sediment motion plays a key role in scouring and bed load transport. However, the incipient sediment motion in the vegetated open-channel flows has yet to be fully understood. This study aims to quantify the critical conditions of the sediment particle movement in the presence of emergent and submerged vegetation. A new formula of the critical flow velocity was proposed to predict the incipient sediment motion based on the force balance equation of a sediment particle and the assumption that the velocity distribution in the bed roughness boundary layer fits the logarithmic law. Analysis of the derived formula revealed that the critical flow velocity for incipient sediment motion decreases with the increase in vegetation density. The proposed formula agrees well with the experimental data in the literature, thereby implying that the critical flow velocity can effectively quantify the incipient sediment motion in the vegetated open channel flows.
基金Sponsored by Ministry of Science and Higher Education within the Framework of Russian State Assignment(Grant No.124012500442⁃3).
文摘The modern definition of the wave concept,which is based on the functional connection between the parameters of the spatial structure of an instantaneous flow pattern and the characteristics of the temporal variability at a given point,is discussed.The dispersion relation for 2D plane periodic perturbations on the surface of viscous stratified fluid is selected as the characteristic function defining the wave motion.Using the theory of singular perturbations,a method for calculating complete solutions to the dispersion relations of periodic flows,including regular wave and singular ligament solutions is presented.Properties of the complete exact solution of the dispersion relation containing regular and singular functions are compared with asymptotic solutions.In limiting cases,obtained dispersion relations are matched with well⁃known expressions for waves in homogeneous viscous and ideal liquids.
基金supported by the National Key Project(GJXM92579).
文摘The formation,evolution,and dynamics of flow structures in wall-bounded turbulence have long been central themes in fluid-mechanics research.Over the past three decades,Soliton-like Coherent Structures(SCSs)have emerged as a ubiquitous and unifying feature across a wide range of shear flows,including K-type,O-type,N-type,and bypass transitional boundary layers,as well as fully developed turbulent boundary layers,mixing layers,and pipe flows.This paper presents a systematic review of the fundamental properties of SCSs and highlights their fundamental role in multiple transition scenarios.The analysis further explores the connection between SCSs and low-speed streaks,offering insight into their coupled dynamics.The phenomenon of turbulent bursting is also examined within the context of SCS dynamics.Together,these studies underscore the potential of SCSs to serve as a coherent dynamical framework for understanding turbulence generation mechanisms in wall-bounded flows.Finally,the review extends to the manifestation of SCSs in other canonical flows,including mixing layers,stratified shear flows,and jets,confirming their universality and significance in fluid dynamics.These findings not only advance our understanding of turbulence generation but also offer a promising theoretical foundation for future research in transitional and turbulent flows.
基金supported by the National Natural Science Foundation of China(52479068)Open Fund Research from the State Key Laboratory of Hydraulics and Mountain River Engineering(SKHL2323).
文摘This study investigates the air–water interaction dynamics in jet streams,with particular emphasis on the transition from the cavity to the far-field regions.A dual-tip conductivity phase-detection probe was employed to analyze four distinct downstream water levels.Based on the development of the cross-sectional mean air concentration,the jet flow was divided into four distinct regions:the jet length region,impact region,splash region,and far-field region.The results demonstrate varying trends in the evolution of the mean air concentration and maximum bubble frequency.Downstream water levels exerted a significant influence on these parameters in the splash and far-field regions,whereas minimal variation was observed in the impact region.Additionally,notable differences were identified in the probability density function of water droplets between the cavity and downstream regions.Furthermore,downstream water depth was found to have a negligible effect on the proportion of small-sized droplets in both the impact and splash regions.
文摘The variable salinity in stored reservoirs connected by a long channel attracts the attention of scientists worldwide,having applications in environmental and geophysical engineering.This study explores the impact of Navier slip conditions on exchange flows within a long channel connecting two large reservoirs of differing salinity.These horizontal density gradients drive the flow.We modify the recent one-dimensional theory,developed to avoid runaway stratification,to account for the presence of uniform slip walls.By adjusting the parameters of the horizontal density gradient based on the slip factor,we resolve analytically various flow regimes ranging from high diffusion to transitional high advection.These regimes are governed by physical parameters like channel aspect ratio,slip factor,Schmidt number,and gravitational Reynolds number.Our solutions align perfectly with ones in the no-slip limit.More importantly,under the conditions of no net flow across the channel and high Schmidt number(where stratification is concentrated near the channel’s mid-layer),we derive a closed-form solution for the slip parameter,aspect ratio,and gravitational Reynolds number that describes the interface’s behavior as a sharp interface separating two distinct zones.This interface,arising from hydrostatic wall gradients,ultimately detaches the low-and high-density regimes throughout the channel when the gravitational Reynolds number is inversely proportional to the aspect ratio for a fixed slip parameter.This phenomenon,observed previously in 2D numerical simulations with no-slip walls in the literature,is thus confirmed by our theoretical results.Our findings further demonstrate that wall slip leads to distinct and diverse flow regimes.
基金supported by the National Natural Science Foundation of China(No.52276025)the Science Center for Gas Turbine Project of China(Nos.P2022-A-Ⅱ-001-001,P2022-A-Ⅱ-002-001 and P2022-B-Ⅱ-002-001)。
文摘The increasing performance demands of modern aero engines necessitate the integrated design of compressor transition ducts with upstream components to reduce the axial length of the engine.However,this design approach narrows the spacing between the stator and the strut,making traditional research on transition ducts only with struts unsuitable.The numerical results and experimental oil flow visualization results were utilized to reconstruct the three-dimensional flow structures in the stator passages under various operating conditions.Additionally,numerical methods were employed to analyze the mechanisms of the strut's effect on the upstream stator in an aggressive transition duct.The results show that the strut potential field increases the load on the upstream stator,leading to severe blade surface separation and corner separation/stall,and redistributes the inflow angle of the upstream stators circumferentially,resulting in significant differences in the flow structures within the stator passages on both sides.The separation flows within the stator passages mainly manifest in five types:pressure surface separation vortex,suction surface concentrated shedding vortex,suction surface separation vortex,suction surface-corner stall separation vortex,and suction surface separation vortex pair.Under different operating conditions,the separation flows within the stator passages are always composed of a part of these five types or a transitional state between two of them.
基金supponted by the National Natural Science Foundation of China (Grant Nos 12202173,12072144,12232010,12372219,and 12302282).
文摘Buoyancy-driven flows are prevalent in a wide range of geophysical and astrophysical systems. In this review, we focus on threepivotal effects that significantly influence the dynamics and transport properties of buoyancy-driven flows and may have impli-cations for natural systems. These effects pertain to the role of boundary conditions, the impact of rotation, and the effect offinite size. Boundary conditions represent how the fluid flow interacts with different kinds of surfaces. Rotation, as the Earth’srotation in geophysical systems or the whirling of astrophysical systems, introduces Coriolis and centrifugal forces, leading tothe profound vortical structure and distinct transport property. Finite size, representing geometrical constraints, influences thebehavior of buoyancy-driven flows across varying geometrical settings. This review aims to provide a holistic understanding ofthe intricate interplay of these factors, offering insights into the complex natural phenomena from the perspectives of the threeeffects.
基金supported by the National Natural Science Foundation of China(Grant numbers 41977233)。
文摘Bridge pier failures from granular flow impacts are common.Installing defense piles upstream is an effective mitigation strategy,yet their protective mechanisms and standardized design guidelines are unclear.This study employed 3D discrete element method to analyze the influence of defense pile size and placement on its performance across 219 scenarios,providing a detailed examination of their protective mechanisms.Results show that optimizing these factors can reduce the maximum impact force on bridge piers by up to 94%.In terms of size,a critical height threshold is identified,beyond which increasing pile height does not enhance protection.This threshold depends on the movement height of granular particles at the slope base.Protection effectiveness varies with pile size:when H≤0.05 h(H is the height of defense piles,h is the height of bridge),protection marginally improves with increasing height and diameter;for 0.05 h<H<0.15 h,protection strongly correlates with both parameters;for H≥0.15 h,diameter becomes the dominant factor.In terms of placement,an optimal longitudinal distance exists between the defense pile and the bridge pier.The larger the diameter,the greater the optimal longitudinal distance.However,the transverse distance is inversely related to protection effectiveness.Mechanistic analysis shows that defense piles are more effective at redirecting particles to prevent direct collisions with the pier(contributing 100%impact energy reduction before the non-dimensional travel time t*=7.01 and 63%–100%afterward)than at reducing particle velocity.This study provides insights into the protective mechanisms of defense piles and informs strategies for optimizing bridge pier protection in granular flow-prone regions.
基金financial support from the National Natural Science Foundation of China(Grant No.41941018).
文摘The impacts of natural boulders carried by debris flows pose serious risks to the safety and reliability of structures and buildings.Natural boulders can be highly random and unpredictable.Consequently,boulder control during debris flows is crucial but difficult.Herein,an eco-friendly control system featuring anchoring natural boulders(NBs)with(negative Poisson's ratio)NPR anchor cables is proposed to form an NB-NPR baffle.A series of flume experiments are conducted to verify the effect of NB-NPR baffles on controlling debris flow impact.The deployment of NB-NPR baffles substantially influences the kinematic behavior of a debris flow,primarily in the form of changes in the depositional properties and impact intensities.The results show that the NB-NPR baffle matrix successfully controls boulder mobility and exhibits positive feedback on solid particle deposition.The NB-NPR baffle group exhibits a reduction in peak impact force ranging from 29%to 79%compared to that of the control group in the basic experiment.The NPR anchor cables play a significant role in the NB-NPR baffle by demonstrating particular characteristics,including consistent resistance,large deformation,and substantial energy absorption.The NB-NPR baffle innovatively utilizes the natural boulders in a debris flow gully by converting destructive boulders into constructive boulders.Overall,this research serves as a basis for future field experiments and applications.
基金financially supported by Doctoral Fellowship of the University Grants Commission,Bangladesh.
文摘This study examines the causal relationship between financial technology startup venture capital(VC)financing and its deals with domestic credit provided by the banking sector and equity market movement.Despite the rise of alternative finance,such as fintech venture capital(it is the fund that venture capital firms put into young,promising fintech companies so that they can help them expand and scale quickly),which is yet underexplored,borrowers still heavily rely on banks and the stock market for financing.We use panel data from 57 countries from 2010 to 2020 and an advanced econometric method called the cross-sectional autoregressive distributed lag model(CS-ARDL)to determine how the size and number of fintech equity funds dealt with by venture capital firms,banking sector credit,and stock market returns are interrelated at the global level and across regional,income,and economic levels.Our results reveal a cointegrating relationship between fintech venture capital funding and deals with bank loans and equity market returns.However,this relationship varies across the regions studied and between developed and developing economies.Our findings provide crucial guidelines for policymakers to create policies that support balanced financial development by highlighting the global interaction of equity market movements,banking credit,and fintech venture capital investment and lay the groundwork for internationally aligned policies to guarantee the optimal distribution of financial capital and improve economic stability and adaptability by illustrating how these links differ across geographical locations and economic conditions.
文摘International capital flows play a crucial role in the process of globalization,presenting both opportunities and challenges to the financial stability of emerging economies.This article sorts out the positive effects and potential risks of international capital flows on the financial stability of emerging economies.By combining case studies in recent years,it analyzes the complex relationship between cross-border capital flows and financial stability,and proposes policy paths for emerging economies to cope with the shock of capital flows,providing references for enhancing financial resilience and achieving sustainable development.
基金supported by the National Natural Science Foundation of China[Grant NO.42361040]。
文摘Ecosystem service flows(ESFs)can reveal the interrelationships and impacts between natural systems and human activities.We can improve the stability and sustainability of ecosystems,more effectively utilize natural resources,protect the environment,and enhance the harmonious coexistence of humans and nature by comprehending ESFs.However,few studies have examined ESFs across scales and evaluated their sustainability;most have concentrated on regional scales.In order to quantify and analyze ESFs within the Jing River Basin(JRB)and between the JRB and the adjacent and distant regions from a water-food-energy perspective,this paper employs a meta-coupling framework.Additionally,it evaluates the sustainability of these flows using a techno-ecological synergy framework.The results show that the ESFs within the JRB was significant in 2020.Water production services were concentrated in the southern part of the JRB,while the distribution of food supply and carbon supply services was relatively even.Huan County emerged as the largest exporting county,providing 1.46×10^(8)kg of food to other counties and exporting 2.97×10^(6)kg of energy.The ESFs in the JRB primarily moved towards the neighboring and distant systems.Water production services flowed into the Guanzhong Plain Urban Agglomeration(GPUA),amounting to 5.8×10^(6)kg.Carbon supply services flowed out at 2.4×10^(5)kg,and food exports were the highest,reaching 5.0×10^(7)kg.The ecosystem service flows from the JRB to both the neighboring and distant systems enhanced food security and ecological resilience.The basin itself demonstrated good sustainability in food supply services,with an index value reaching 48.19.In crossscale calculations of food production sustainability with the adjacent GPUA,the index value increased from 48.19 to 52.99,indicating a significant improvement.These findings demonstrate that applying the meta-coupling framework provides an effective approach to quantify ESFs and assess their sustainability across scales.
基金supported by the National Natural Science Foundation of China(No.92252201)。
文摘To address the early separation problem in the Menter Shear-Stress Transport(SST)turbulence model,a correction for the Turbulent Kinetic Energy(TKE)production term,P_(k),is introduced to account for the effect of the Adverse Pressure Gradient(APG).The correction is determined based on the distribution of Pkin the APG region before separation.When the friction coefficient C_(f) is decomposed,its direct dependence on Pkis clearly observed.However,with the introduction of Bradshaw’s assumption,Pkin the SST turbulence model is over-suppressed,resulting in a lower inner peak or no significant inner peak distribution at all.To address this problem,this paper proposes a Gaussian function,HGauss,which corrects the numerical values of P_(k) involved in the calculation of the Menter SST model by focusing on the inner peak region of P_(k).The modified SST model is then applied to four cases with APGs.The modification leads to an increase in the wall friction coefficient C_(f)in the APG region and causes a downstream shift in the separation location,improving the model’s consistency with high-accuracy data and experimental results.It is demonstrated that this correction can improve the early separation problem in the Menter SST turbulence model.
基金supported by the National Natural Science Foundation of China(No.92252201)the Fundamental Research Funds for the Central Universitiesthe Academic Excellence Foundation of Beihang University(BUAA)for PhD Students。
文摘Efficient and accurate simulation of unsteady flow presents a significant challenge that needs to be overcome in computational fluid dynamics.Temporal discretization method plays a crucial role in the simulation of unsteady flows.To enhance computational efficiency,we propose the Implicit-Explicit Two-Step Runge-Kutta(IMEX-TSRK)time-stepping discretization methods for unsteady flows,and develop a novel adaptive algorithm that correctly partitions spatial regions to apply implicit or explicit methods.The novel adaptive IMEX-TSRK schemes effectively handle the numerical stiffness of the small grid size and improve computational efficiency.Compared to implicit and explicit Runge-Kutta(RK)schemes,the IMEX-TSRK methods achieve the same order of accuracy with fewer first derivative calculations.Numerical case tests demonstrate that the IMEX-TSRK methods maintain numerical stability while enhancing computational efficiency.Specifically,in high Reynolds number flows,the computational efficiency of the IMEX-TSRK methods surpasses that of explicit RK schemes by more than one order of magnitude,and that of implicit RK schemes several times over.
