Nowadays,several stern devices are attracting a great deal of attention.The control surface is an effective apparatus for improving the hydrodynamic performance of planing hulls and is considered an important element ...Nowadays,several stern devices are attracting a great deal of attention.The control surface is an effective apparatus for improving the hydrodynamic performance of planing hulls and is considered an important element in the design of planing hulls.Control surfaces produce forces and a pitching moment due to the pressure distribution that they cause,which can be used to change the running state of high-speed marine boats.This work elaborates a new study to evaluate the hydrodynamic performance of a planing boat with a trim tab and an interceptor,and optimizes them by using an optimization algorithm.The trim tab and the interceptor have been used to optimize the running trim and motion control of semi-planing and planing boats at various speeds and sea conditions for many years.In this paper,the usage of trim tab is mathematically verified and experimental equations are utilized to optimize the performance of a planing boat at a specificd trim angle by using an optimization algorithm.The genetic algorithm(GA)is one of the most useful optimizing methods and is used in this study.The planing boat equations were programmed according to Savitsky’s equations and then analyzed in the framework of the GA-based optimization for performance improvement of theplaning hull.The optimal design of trim tab and interceptor for planing boat can be considered a multiobjective problem.The input data of GA include different parameters,such as speed,longitudinal center of gravity,and deadrise angle.We can extract the best range of forecasting the planing boat longitudinal center of gravity,the angle of the trim,and the least drag force at the best trim angle of the boat.展开更多
A hybrid isothermal model for the homogeneous-heterogeneous reactions in ferrohydrodynamic boundary layer flow is established.The characteristics of Newtonian heating and magnetic dipole in a ferrofluid due to a stret...A hybrid isothermal model for the homogeneous-heterogeneous reactions in ferrohydrodynamic boundary layer flow is established.The characteristics of Newtonian heating and magnetic dipole in a ferrofluid due to a stretchable surface is analyzed for three chemical species.It is presumed that the isothermal cubic autocatalator kinetic gives the homogeneous reaction and the first order kinetics gives the heterogeneous(surface)reaction.The analysis is carried out for equal diffusion coefficients of all autocatalyst and reactions.Heat flux is examined by incorporating Fourier’s law of heat conduction.Characteristics of materialized parameters on the magneto-thermomechanical coupling in the flow of a chemically reactive species are investigated.Further,the heat transfer rate and friction drag are depicted for the ferrohydrodynamic chemically reactive species.It is evident that the Schmidt number has increasing behavior on the rate of heat transfer in the boundary layer.Comparison with available results for specific cases is found an excellent agreement.展开更多
In the present study, the subcritical flow past a generic side mirror on a base plane is investigated at the Reynolds number of 5.2 × 10~5 using delayed detached eddy simulation(DDES) turbulence model. Asides fro...In the present study, the subcritical flow past a generic side mirror on a base plane is investigated at the Reynolds number of 5.2 × 10~5 using delayed detached eddy simulation(DDES) turbulence model. Asides from the capability of capturing main features of the large recirculation vortex in the wake of the side mirror and the front horseshoe vortex, the accuracy of DDES estimation of recirculation length is significantly increased by over20%, compared to the detached eddy simulation(DES) estimation using the same grid. And DDES prediction of pressure coefficient at the trailing edge of the mirror is in good agreement with the experiments, which is more accurate than both DES and large eddy simulation(LES) results. The results verify the capacity of DDES turbulence model to solve the turbulent flow around the side mirror. This is a key foundation for possible future study of full simulation of external flow field of vehicle.展开更多
As a simplified model of artificial reefs,a series of plate models punched with square or circular openings are designed to investigate the effects of openings on the hydrodynamic characteristics of artificial reefs.T...As a simplified model of artificial reefs,a series of plate models punched with square or circular openings are designed to investigate the effects of openings on the hydrodynamic characteristics of artificial reefs.The models are grouped by various opening numbers and opening-area ratios.They are physically tested in a water flume or used in the numerical simulation to obtain the drag force in the uniform flow with different speeds.The simulation results are found in good agreement with the experimental measurements.By the non-dimensional analysis,the drag coefficient specified to each model is achieved and the effects of openings are examined.It is found that the key factor affecting the drag coefficient is the open-area ratio.Generally,the drag coefficient is a linear function of the open area ratio with a minus slope.The empirical formulae for the square and circular openings respectively are deduced by means of the multiple regression analysis based on the measured and numerical data.They will be good references for the design of new artificial reefs.As a result of numerical simulation,the vorticity contours and pressure distribution are also presented in this work to better understand the hydrodynamic characteristics of different models.展开更多
Most fluid flows in nature and engineering applications are in the state of turbulence.Turbulent motions usually exhibit a wide range of spatial and temporal scales,such as the flow of natural gas and oil in pipelines...Most fluid flows in nature and engineering applications are in the state of turbulence.Turbulent motions usually exhibit a wide range of spatial and temporal scales,such as the flow of natural gas and oil in pipelines,the wakes of cars and submarines,the boundary layer of an aircraft,the current in the ocean surface,the atmospheric boundary layer,the interstellar gas clouds(gaseous stars),and the Earth’s wake in the solar wind.Turbulence can greatly improve the heat and mass transfer efficiency of macroscopic flow.For example,chemical engineers use turbulence to mix up and homogenize fluid components and to increase chemical reaction rates in liquids or gases.However,turbulence can also lead to increases in drag,aerodynamic heat,and hydrodynamic and aerodynamic noise.For instance,the aerodynamic loading of high-speed aircraft can be significantly increased due to turbulence.展开更多
It is a very difficult task to develop a method of reducing turbulent boundary layer drag.However,in recent years,plasma flow control technology has demonstrated huge potential in friction drag reduction.To further in...It is a very difficult task to develop a method of reducing turbulent boundary layer drag.However,in recent years,plasma flow control technology has demonstrated huge potential in friction drag reduction.To further investigate this issue,a smooth plate model was designed as a testing object arranged with a bidirectional dielectric-barrier-discharge(DBD)plasma actuator.In addition,measurement of skin friction drag was achieved by applying hot wire anemometry to obtain the velocity distribution of the turbulent boundary layer.A method of quantifying the friction drag effect was adopted based on the Spalding formula fitted with the experiment data.When plasma actuation was conducted,a velocity defect occurred at the two measuring positions,compared with the no plasma control condition;this means that the DBD plasma actuation could reduce the drag successfully in the downstream of the actuator.Moreover,drag reduction caused by backward actuation was slightly more efficient than that caused by forward actuation.With an increasing distance from plasma actuation,the drag-reduction effect could become weaker.Experimental results also show that the improvement of drag-reduction efficiency using a DBD plasma actuator can achieve about 8.78%in the local region of the experimental flat model.展开更多
Reduction of drag torque in disengaged wet clutches is essential for transmission research because it is one of the potentials of e ciency improvement. Aeration of oil film between two closely rotating plates promotes...Reduction of drag torque in disengaged wet clutches is essential for transmission research because it is one of the potentials of e ciency improvement. Aeration of oil film between two closely rotating plates promotes the decrease of drag torque at high speed region. The e ects of surface tension and static contact angles during aeration are nonnegligible showed by test results. The traditional lubrication model does not adequately predict the experimental results with di erent surface tension and contact angles during aeration. Hence, in this present paper, contact angles between Aluminum and Teflon materials were firstly measured, and the drag torques under two di erent contact angles were examined experimentally. An improved lubrication model of drag torque based on Navier–Stokes equations at the gas-liquid interface was built. The lubrication boundary condition was modified to introduce the e ects of surface tension and contact angle. The model shows that the e ects at the beginning of aeration of oil film are significant. These e ects almost occur at stationary plate due to low Reynolds number and Weber number. The model shows that an increase in the surface tension promotes aeration, but does not a ect the peak drag torque. Increasing contact angle also promotes the aeration, and accelerates the decrease of drag torque. The larger contact angle is, the smaller the peak drag torque will be. A computational fluid dynamics(CFD) model based on volume of fluid(VOF) method was presented to validate the interface shape when aeration occurs. The model prediction has a good agreement with experimental observations for Aluminum plates and Teflon plates. The modified lubrication model of drag torque gives a convenient description of the e ects of surface tension and contact angel, and lays down a frame to understand the beginning of aeration.展开更多
To study the drag reduction of a jet surface,an experiment is conducted with a gyrorotor model.The aim of this study is to analyze the influence of the jet aperture,the jet angle and the rotational velocity coupled wi...To study the drag reduction of a jet surface,an experiment is conducted with a gyrorotor model.The aim of this study is to analyze the influence of the jet aperture,the jet angle and the rotational velocity coupled with the jet velocity on the friction torque acting on a jet surface model by using a jet surface drag reduction testing experimental platform.The drag reduction characteristics of the jet surface are also studied.The results show that the maximum drag reduction rate is attained when the jet diameter is 5 mm and the jet velocity is 1.2 m/s.The influence of the jet velocity on the drag reduction rate decreases as the diameter decreases.However,the maximum drag reduction rate can be attained when the angle of the jet is 30° and the velocity of the jet is 1.0 m/s.When the jet diameter is small,the influence of the jet velocity on the drag reduction rate is small,whereas the maximum drag reduction rate is attained when the jet angle is 30° and the jet velocity is 1.0 m/s.When the jet angle is 15°,the drag reduction rate fluctuation is distinct.The maximum drag reduction rate is attained when the rotational velocity is 2 100 r/min and the jet velocity is 1.0 m/s.The results show that the best scheme is A2B2D3 A maximum drag reduction rate of 8.57% can be attained.The effective control of the jet effect on the separation of the boundary layer demonstrates the drag reduction principle of a bionic jet surface.展开更多
The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery(ADV)measurements.The flow characteristics such as the energetics and momentum tr...The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery(ADV)measurements.The flow characteristics such as the energetics and momentum transfer derived from convcntional spectral and quadrant analyses are considered as the flow encounters a finite vegetation patch.Consistent with numerous canopy flow experiments,a shear layer and coherent vortex structures near the canopy top emerge caused by Kelvin-Helmholtz instabilities after the flow equilibrates with the vegetated layer.These in stabilities are commonly attributed to velocity differences between non-vegetated and vegetated canopy layers in agreement with numerous experiments and simulations conducted on dense rigid canopies.The power-spectral density function for vertical velocity turbulent fluctuations at different downstream positions starting from the edge of the vegetation layer are also computed.For a preset water depth,the dominant dimensionless frequency is found to be surprisingly invariant around 0.027 despite large differences in vegetation densities.The ejection and sweep events significantly contribute to the Reynolds stresses near the top of the vegetation.The momentum flux carried by ejections is larger than its counterpart carried by the sweeps above the canopy top.However,the momentum flux carried by sweeps is larger below the top of the canopy.展开更多
As the Reynolds number increases,the skin friction has been identified as the dominant drag in many practical applications.In the present paper,the effects of the Reynolds number on the mean skin friction decompositio...As the Reynolds number increases,the skin friction has been identified as the dominant drag in many practical applications.In the present paper,the effects of the Reynolds number on the mean skin friction decomposition in turbulent channel flows up to Reτ=5 200 are investigated based on two different methods,i.e.,the Fukagata-Iwamoto-Kasagi(FIK)identity(FUKAGATA,K.,IWAMOTO,K.,and KASAGI,N.Contribution of Reynolds stress distribution to the skin friction in wall-bounded flows.Physics of Fluids,14(11),L73–L76(2002))and the Renard-Deck(RD)identity(DECK,S.,RENARD,N.,LARAUFIE,R.,and WEISS,P.′E.Large-scale contribution to mean wall shear stress in high-Reynolds-number flat-plate boundary layers up to Reθ=13 650.Journal of Fluid Mechanics,743,202–248(2014)).The direct numerical simulation(DNS)data provided by Lee and Moser(LEE,M.and MOSER,R.D.Direct numerical simulation of turbulent channel flow up to Reτ≈5 200.Journal of Fluid Mechanics,774,395–415(2015))are used.For these two skin friction decomposition methods,their decomposed constituents are discussed and compared for different Reynolds numbers.The integrands of the decomposed constituents are locally analyzed across the boundary layer to assess the actions associated with the inhomogeneity and multi-scale nature of turbulent motion.The scaling of the decomposed constituents and their integrands are presented.In addition,the boundary layer is divided into three sub-regions to evaluate the contributive proportion of each sub-region with an increase in the Reynolds number.展开更多
Along with the survey on experimental investigations drawing attention to the drag and heat reduction mechanism, the authors simultaneously focus on the recent advances of numerical simulations on the schemes applied ...Along with the survey on experimental investigations drawing attention to the drag and heat reduction mechanism, the authors simultaneously focus on the recent advances of numerical simulations on the schemes applied to supersonic/hypersonic vehicles. The CFD study has evolved as an irreplaceable method in scheme evaluation and aircraft optimization. Similar to our previous experimental survey, the advances in drag and heat reduction schemes are reviewed by similar kinds of mechanism in this article, namely the forward-facing cavity, the opposing jet, the aerospike, the energy deposition and their combinational configurations. This review article puts an emphatic eye on the flow conditions, numerical methods, novel schemes and analytical conclusions given in the simulations. Further, the multi-objective design optimization concept has also been illustrated due to the observable advantages of using CFD over experimental method, especially those performances conducted in drag reduction and thermal protection practice, and this would possess reference value in the design of aircraft system.展开更多
Flow through submerged rigid vegetation has been studied both analytically and experimentally.The Reynolds stress,present in the governing equation,has been modeled using one of the turbulent stress equations,adopted ...Flow through submerged rigid vegetation has been studied both analytically and experimentally.The Reynolds stress,present in the governing equation,has been modeled using one of the turbulent stress equations,adopted in numerous cases.The adva ntage of this turbulent stress model is to replace the mixing length non linear term of the stress with a linear relatio n between stress and the velocity gradient.The velocity field and shear stress are obtained by solving the governing force balance equation numerically.A correlation,validated with the experimental results,has been developed for the relevant non-dimensional numbers.展开更多
In this paper,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with the direct numerical simulation(DNS)methods for different Reynolds...In this paper,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with the direct numerical simulation(DNS)methods for different Reynolds numbers.A formulation is derived to express the relation between the drag and the Reynolds shear stress.With the application of optimal electromagnetic force,the in-depth relations among characteristic structures in the flow field,mean Reynolds shear stress,and the effect of drag reduction for different Reynolds numbers are discussed.The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers.The regular quasi-streamwise vortex structures,which appear in the flow field,have the same period with that of the electromagnetic force.These structures suppress the random velocity fluctuations,which leads to the absolute value of mean Reynolds shear stress decreasing and the distribution of that moving away from the wall.Moreover,the wave number of optimal electromagnetic force increases,and the scale of the regular quasi-streamwise vortex structures decreases as the Reynolds number increases.Therefore,the rate of drag reduction decreases with the increase in the Reynolds number since the scale of the regular quasi-streamwise vortex structures decreases.展开更多
Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify t...Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.展开更多
A flow control technique by local vibration is proposed to improve the aerodynamic performance of a typical airfoil NACA 0012.Both wind-tunnel experiments and a large eddy simulation(LES)are carried out to study the e...A flow control technique by local vibration is proposed to improve the aerodynamic performance of a typical airfoil NACA 0012.Both wind-tunnel experiments and a large eddy simulation(LES)are carried out to study the effects of local vibration on drag reduction over a wide range of angles of attack.The application parameters of local vibration on the upper surface of the airfoil are first evaluated by numerical simulations.The mounted position is chosen at 0.065-0.09 of chord length from the leading edge.The influence of oscillation frequency is investigated both by numerical simulations and experiments.The optimal frequencies are near the dominant frequencies of shear layer vortices and wake vortices.The patterns of shear vortices caused by local vibration are also studied to determine the drag reduction mechanism of this flow control method.The results indicate that local vibration can improve the aerodynamic performance of the airfoil.In particular,it can reduce the drag by changing the vortex generation patterns.展开更多
We measured the intrinsic electrophoretic drag coefficient of a single charged particle by optically trapping the particle and applying an AC electric field,and found it to be markedly different from that of the Stoke...We measured the intrinsic electrophoretic drag coefficient of a single charged particle by optically trapping the particle and applying an AC electric field,and found it to be markedly different from that of the Stokes drag.The drag coefficient,along with the measured electrical force,yield a mobility-zeta potential relation that agrees with the literature.By using the measured mobility as input,numerical calculations based on the Poisson-Nernst-Planck equations,coupled to the Navier-Stokes equation,reveal an intriguing microscopic electroosmotic flow near the particle surface,with a well-defined transition between an inner flow field and an outer flow field in the vicinity of electric double layer’s outer boundary.This distinctive interface delineates the surface that gives the correct drag coefficient and the effective electric charge.The consistency between experiments and theoretical predictions provides new insights into the classic electrophoresis problem,and can shed light on new applications of electrophoresis to investigate biological nanoparticles.展开更多
文摘Nowadays,several stern devices are attracting a great deal of attention.The control surface is an effective apparatus for improving the hydrodynamic performance of planing hulls and is considered an important element in the design of planing hulls.Control surfaces produce forces and a pitching moment due to the pressure distribution that they cause,which can be used to change the running state of high-speed marine boats.This work elaborates a new study to evaluate the hydrodynamic performance of a planing boat with a trim tab and an interceptor,and optimizes them by using an optimization algorithm.The trim tab and the interceptor have been used to optimize the running trim and motion control of semi-planing and planing boats at various speeds and sea conditions for many years.In this paper,the usage of trim tab is mathematically verified and experimental equations are utilized to optimize the performance of a planing boat at a specificd trim angle by using an optimization algorithm.The genetic algorithm(GA)is one of the most useful optimizing methods and is used in this study.The planing boat equations were programmed according to Savitsky’s equations and then analyzed in the framework of the GA-based optimization for performance improvement of theplaning hull.The optimal design of trim tab and interceptor for planing boat can be considered a multiobjective problem.The input data of GA include different parameters,such as speed,longitudinal center of gravity,and deadrise angle.We can extract the best range of forecasting the planing boat longitudinal center of gravity,the angle of the trim,and the least drag force at the best trim angle of the boat.
基金Supported by the Higher Education Commission(HEC)under Grant No.6170/Fedral/NRPU/R&D/HEC/2016.
文摘A hybrid isothermal model for the homogeneous-heterogeneous reactions in ferrohydrodynamic boundary layer flow is established.The characteristics of Newtonian heating and magnetic dipole in a ferrofluid due to a stretchable surface is analyzed for three chemical species.It is presumed that the isothermal cubic autocatalator kinetic gives the homogeneous reaction and the first order kinetics gives the heterogeneous(surface)reaction.The analysis is carried out for equal diffusion coefficients of all autocatalyst and reactions.Heat flux is examined by incorporating Fourier’s law of heat conduction.Characteristics of materialized parameters on the magneto-thermomechanical coupling in the flow of a chemically reactive species are investigated.Further,the heat transfer rate and friction drag are depicted for the ferrohydrodynamic chemically reactive species.It is evident that the Schmidt number has increasing behavior on the rate of heat transfer in the boundary layer.Comparison with available results for specific cases is found an excellent agreement.
基金the National Key Research and Development Plan of China(No.2016YFB0101601-7)the Science Foundation of Chinese Academy of Sciences(No.51175214)the Special Planning Project of Jilin Province(No.SXGJSF2017-2-1-5).
文摘In the present study, the subcritical flow past a generic side mirror on a base plane is investigated at the Reynolds number of 5.2 × 10~5 using delayed detached eddy simulation(DDES) turbulence model. Asides from the capability of capturing main features of the large recirculation vortex in the wake of the side mirror and the front horseshoe vortex, the accuracy of DDES estimation of recirculation length is significantly increased by over20%, compared to the detached eddy simulation(DES) estimation using the same grid. And DDES prediction of pressure coefficient at the trailing edge of the mirror is in good agreement with the experiments, which is more accurate than both DES and large eddy simulation(LES) results. The results verify the capacity of DDES turbulence model to solve the turbulent flow around the side mirror. This is a key foundation for possible future study of full simulation of external flow field of vehicle.
基金Shandong Provincial Oceanic and Fishery Department(Ecological Simulation Test of the Offshore Area in Shandong Peninsula)the Primary Research and Development Program of Shandong Province(Nos.2016CYJS04A01 and 2017CXGC0107).
文摘As a simplified model of artificial reefs,a series of plate models punched with square or circular openings are designed to investigate the effects of openings on the hydrodynamic characteristics of artificial reefs.The models are grouped by various opening numbers and opening-area ratios.They are physically tested in a water flume or used in the numerical simulation to obtain the drag force in the uniform flow with different speeds.The simulation results are found in good agreement with the experimental measurements.By the non-dimensional analysis,the drag coefficient specified to each model is achieved and the effects of openings are examined.It is found that the key factor affecting the drag coefficient is the open-area ratio.Generally,the drag coefficient is a linear function of the open area ratio with a minus slope.The empirical formulae for the square and circular openings respectively are deduced by means of the multiple regression analysis based on the measured and numerical data.They will be good references for the design of new artificial reefs.As a result of numerical simulation,the vorticity contours and pressure distribution are also presented in this work to better understand the hydrodynamic characteristics of different models.
基金National Natural Science Foundation of China(No.91752000).
文摘Most fluid flows in nature and engineering applications are in the state of turbulence.Turbulent motions usually exhibit a wide range of spatial and temporal scales,such as the flow of natural gas and oil in pipelines,the wakes of cars and submarines,the boundary layer of an aircraft,the current in the ocean surface,the atmospheric boundary layer,the interstellar gas clouds(gaseous stars),and the Earth’s wake in the solar wind.Turbulence can greatly improve the heat and mass transfer efficiency of macroscopic flow.For example,chemical engineers use turbulence to mix up and homogenize fluid components and to increase chemical reaction rates in liquids or gases.However,turbulence can also lead to increases in drag,aerodynamic heat,and hydrodynamic and aerodynamic noise.For instance,the aerodynamic loading of high-speed aircraft can be significantly increased due to turbulence.
文摘It is a very difficult task to develop a method of reducing turbulent boundary layer drag.However,in recent years,plasma flow control technology has demonstrated huge potential in friction drag reduction.To further investigate this issue,a smooth plate model was designed as a testing object arranged with a bidirectional dielectric-barrier-discharge(DBD)plasma actuator.In addition,measurement of skin friction drag was achieved by applying hot wire anemometry to obtain the velocity distribution of the turbulent boundary layer.A method of quantifying the friction drag effect was adopted based on the Spalding formula fitted with the experiment data.When plasma actuation was conducted,a velocity defect occurred at the two measuring positions,compared with the no plasma control condition;this means that the DBD plasma actuation could reduce the drag successfully in the downstream of the actuator.Moreover,drag reduction caused by backward actuation was slightly more efficient than that caused by forward actuation.With an increasing distance from plasma actuation,the drag-reduction effect could become weaker.Experimental results also show that the improvement of drag-reduction efficiency using a DBD plasma actuator can achieve about 8.78%in the local region of the experimental flat model.
基金National Natural Science Foundation of China(Grant No.51305032).
文摘Reduction of drag torque in disengaged wet clutches is essential for transmission research because it is one of the potentials of e ciency improvement. Aeration of oil film between two closely rotating plates promotes the decrease of drag torque at high speed region. The e ects of surface tension and static contact angles during aeration are nonnegligible showed by test results. The traditional lubrication model does not adequately predict the experimental results with di erent surface tension and contact angles during aeration. Hence, in this present paper, contact angles between Aluminum and Teflon materials were firstly measured, and the drag torques under two di erent contact angles were examined experimentally. An improved lubrication model of drag torque based on Navier–Stokes equations at the gas-liquid interface was built. The lubrication boundary condition was modified to introduce the e ects of surface tension and contact angle. The model shows that the e ects at the beginning of aeration of oil film are significant. These e ects almost occur at stationary plate due to low Reynolds number and Weber number. The model shows that an increase in the surface tension promotes aeration, but does not a ect the peak drag torque. Increasing contact angle also promotes the aeration, and accelerates the decrease of drag torque. The larger contact angle is, the smaller the peak drag torque will be. A computational fluid dynamics(CFD) model based on volume of fluid(VOF) method was presented to validate the interface shape when aeration occurs. The model prediction has a good agreement with experimental observations for Aluminum plates and Teflon plates. The modified lubrication model of drag torque gives a convenient description of the e ects of surface tension and contact angel, and lays down a frame to understand the beginning of aeration.
基金the Zhejiang Provincial Natural Science Foundation of China (Grant Nos.LY19E050003,LQ15E050005)the National Natural Science Foundation of China (Grant No.51779226).
文摘To study the drag reduction of a jet surface,an experiment is conducted with a gyrorotor model.The aim of this study is to analyze the influence of the jet aperture,the jet angle and the rotational velocity coupled with the jet velocity on the friction torque acting on a jet surface model by using a jet surface drag reduction testing experimental platform.The drag reduction characteristics of the jet surface are also studied.The results show that the maximum drag reduction rate is attained when the jet diameter is 5 mm and the jet velocity is 1.2 m/s.The influence of the jet velocity on the drag reduction rate decreases as the diameter decreases.However,the maximum drag reduction rate can be attained when the angle of the jet is 30° and the velocity of the jet is 1.0 m/s.When the jet diameter is small,the influence of the jet velocity on the drag reduction rate is small,whereas the maximum drag reduction rate is attained when the jet angle is 30° and the jet velocity is 1.0 m/s.When the jet angle is 15°,the drag reduction rate fluctuation is distinct.The maximum drag reduction rate is attained when the rotational velocity is 2 100 r/min and the jet velocity is 1.0 m/s.The results show that the best scheme is A2B2D3 A maximum drag reduction rate of 8.57% can be attained.The effective control of the jet effect on the separation of the boundary layer demonstrates the drag reduction principle of a bionic jet surface.
基金the National Natural Science Foundation of China(Grant Nos.51439007,11672213,11872285 and 51809286).
文摘The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery(ADV)measurements.The flow characteristics such as the energetics and momentum transfer derived from convcntional spectral and quadrant analyses are considered as the flow encounters a finite vegetation patch.Consistent with numerous canopy flow experiments,a shear layer and coherent vortex structures near the canopy top emerge caused by Kelvin-Helmholtz instabilities after the flow equilibrates with the vegetated layer.These in stabilities are commonly attributed to velocity differences between non-vegetated and vegetated canopy layers in agreement with numerous experiments and simulations conducted on dense rigid canopies.The power-spectral density function for vertical velocity turbulent fluctuations at different downstream positions starting from the edge of the vegetation layer are also computed.For a preset water depth,the dominant dimensionless frequency is found to be surprisingly invariant around 0.027 despite large differences in vegetation densities.The ejection and sweep events significantly contribute to the Reynolds stresses near the top of the vegetation.The momentum flux carried by ejections is larger than its counterpart carried by the sweeps above the canopy top.However,the momentum flux carried by sweeps is larger below the top of the canopy.
基金the National Basic Research Program of China(973 Program)(No.2014CB744802)the National Natural Science Foundation of China(No.11772194).
文摘As the Reynolds number increases,the skin friction has been identified as the dominant drag in many practical applications.In the present paper,the effects of the Reynolds number on the mean skin friction decomposition in turbulent channel flows up to Reτ=5 200 are investigated based on two different methods,i.e.,the Fukagata-Iwamoto-Kasagi(FIK)identity(FUKAGATA,K.,IWAMOTO,K.,and KASAGI,N.Contribution of Reynolds stress distribution to the skin friction in wall-bounded flows.Physics of Fluids,14(11),L73–L76(2002))and the Renard-Deck(RD)identity(DECK,S.,RENARD,N.,LARAUFIE,R.,and WEISS,P.′E.Large-scale contribution to mean wall shear stress in high-Reynolds-number flat-plate boundary layers up to Reθ=13 650.Journal of Fluid Mechanics,743,202–248(2014)).The direct numerical simulation(DNS)data provided by Lee and Moser(LEE,M.and MOSER,R.D.Direct numerical simulation of turbulent channel flow up to Reτ≈5 200.Journal of Fluid Mechanics,774,395–415(2015))are used.For these two skin friction decomposition methods,their decomposed constituents are discussed and compared for different Reynolds numbers.The integrands of the decomposed constituents are locally analyzed across the boundary layer to assess the actions associated with the inhomogeneity and multi-scale nature of turbulent motion.The scaling of the decomposed constituents and their integrands are presented.In addition,the boundary layer is divided into three sub-regions to evaluate the contributive proportion of each sub-region with an increase in the Reynolds number.
基金the National Natural Science Foundation of China(Nos.11502291&11802340).
文摘Along with the survey on experimental investigations drawing attention to the drag and heat reduction mechanism, the authors simultaneously focus on the recent advances of numerical simulations on the schemes applied to supersonic/hypersonic vehicles. The CFD study has evolved as an irreplaceable method in scheme evaluation and aircraft optimization. Similar to our previous experimental survey, the advances in drag and heat reduction schemes are reviewed by similar kinds of mechanism in this article, namely the forward-facing cavity, the opposing jet, the aerospike, the energy deposition and their combinational configurations. This review article puts an emphatic eye on the flow conditions, numerical methods, novel schemes and analytical conclusions given in the simulations. Further, the multi-objective design optimization concept has also been illustrated due to the observable advantages of using CFD over experimental method, especially those performances conducted in drag reduction and thermal protection practice, and this would possess reference value in the design of aircraft system.
文摘Flow through submerged rigid vegetation has been studied both analytically and experimentally.The Reynolds stress,present in the governing equation,has been modeled using one of the turbulent stress equations,adopted in numerous cases.The adva ntage of this turbulent stress model is to replace the mixing length non linear term of the stress with a linear relatio n between stress and the velocity gradient.The velocity field and shear stress are obtained by solving the governing force balance equation numerically.A correlation,validated with the experimental results,has been developed for the relevant non-dimensional numbers.
基金the National Natural Science Foundation of China(No.11672135)the Foundation for the Author of National Excellent Doctoral Dissertation of China(No.201461).
文摘In this paper,the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with the direct numerical simulation(DNS)methods for different Reynolds numbers.A formulation is derived to express the relation between the drag and the Reynolds shear stress.With the application of optimal electromagnetic force,the in-depth relations among characteristic structures in the flow field,mean Reynolds shear stress,and the effect of drag reduction for different Reynolds numbers are discussed.The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers.The regular quasi-streamwise vortex structures,which appear in the flow field,have the same period with that of the electromagnetic force.These structures suppress the random velocity fluctuations,which leads to the absolute value of mean Reynolds shear stress decreasing and the distribution of that moving away from the wall.Moreover,the wave number of optimal electromagnetic force increases,and the scale of the regular quasi-streamwise vortex structures decreases as the Reynolds number increases.Therefore,the rate of drag reduction decreases with the increase in the Reynolds number since the scale of the regular quasi-streamwise vortex structures decreases.
基金the National Basic Research Program of China (973 program)(Grants 2014CB744802 and 2014CB744804)the National Natural Science Foundation of China (Grants 11772194 and 91441205).
文摘Direct numerical simulations (DNS) of turbulent flow over a drag-reducing and a drag-increasing riblet configuration are performed. Three-dimensional two-point statistics are presented for the first time to quantify the interaction of the riblet surfaces with the coherent, energy-bearing eddy structures in the near-wall region. Results provide statistical evidence that the averaged organization of the streamwise vortices in the drag-reducing case is lifted above the riblet tip, while in the drag-increasing case the streamwise vortices are embedded further into the riblet cove. In the spanwise direction, the cores of the streamwise vortices over the riblet surfaces are shown to be closer to each other than those for flow over the smooth wall, and wider riblet spacing leads to more reduction on their spanwise distances. In the cases with riblets the streamwise vortices have longer streamwise lengths, but their inclination angles do not change much.
基金the National Natural Science Foundation of China(No.11532011)the Fundamental Research Funds for the Central Universities(No.2017FZA4031).
文摘A flow control technique by local vibration is proposed to improve the aerodynamic performance of a typical airfoil NACA 0012.Both wind-tunnel experiments and a large eddy simulation(LES)are carried out to study the effects of local vibration on drag reduction over a wide range of angles of attack.The application parameters of local vibration on the upper surface of the airfoil are first evaluated by numerical simulations.The mounted position is chosen at 0.065-0.09 of chord length from the leading edge.The influence of oscillation frequency is investigated both by numerical simulations and experiments.The optimal frequencies are near the dominant frequencies of shear layer vortices and wake vortices.The patterns of shear vortices caused by local vibration are also studied to determine the drag reduction mechanism of this flow control method.The results indicate that local vibration can improve the aerodynamic performance of the airfoil.In particular,it can reduce the drag by changing the vortex generation patterns.
文摘We measured the intrinsic electrophoretic drag coefficient of a single charged particle by optically trapping the particle and applying an AC electric field,and found it to be markedly different from that of the Stokes drag.The drag coefficient,along with the measured electrical force,yield a mobility-zeta potential relation that agrees with the literature.By using the measured mobility as input,numerical calculations based on the Poisson-Nernst-Planck equations,coupled to the Navier-Stokes equation,reveal an intriguing microscopic electroosmotic flow near the particle surface,with a well-defined transition between an inner flow field and an outer flow field in the vicinity of electric double layer’s outer boundary.This distinctive interface delineates the surface that gives the correct drag coefficient and the effective electric charge.The consistency between experiments and theoretical predictions provides new insights into the classic electrophoresis problem,and can shed light on new applications of electrophoresis to investigate biological nanoparticles.