Accurate predictions of Shock Waves and Boundary Layer Interaction(SWBLI)and strong Shock Waves and Wake Vortices Interaction(SWWVI)in a highly-loaded turbine propose challenges to the currently widely used Reynolds-A...Accurate predictions of Shock Waves and Boundary Layer Interaction(SWBLI)and strong Shock Waves and Wake Vortices Interaction(SWWVI)in a highly-loaded turbine propose challenges to the currently widely used Reynolds-Averaged Navier-Stokes(RANS)model.In this work,the SWBLI and the SWWVI in a highly-loaded Nozzle Guide Vane(NGV)are studied using a hybrid RANS/LES strategy.The Turbulence Kinetic Energy(TKE)budget and the Proper Orthogonal Decomposition(POD)method are used to analyze flow mechanisms.Results show that this hybrid RANS/LES method can obtain detailed flow structures for flow mechanisms analysis.Strong shock waves induce boundary layer separation,while the presence of a separation bubble can in turn lead to a Mach reflection phenomenon.The shock waves cause trailing-edge vortices to break clearly,and the wakes,in turn,can change the shocks intensity and direction.Furthermore,the Entropy Generation Rate(EGR)is used to analyze the irreversible loss.It turns out that the SWWVI can reduce the flow field loss.There are several weak shock waves in the NGV flow field,which can increase the irreversible loss.This work offers flow mechanisms analysis and presents the EGR distribution in SWBLI and SWWVI areas in a transonic turbine blade.展开更多
Detailed experimental measurements were conducted to study the interactions between incoming wakes and endwall secondary flow in a high-lift Low-Pressure Turbine(LPT)cascade.All of the measurements were conducted in b...Detailed experimental measurements were conducted to study the interactions between incoming wakes and endwall secondary flow in a high-lift Low-Pressure Turbine(LPT)cascade.All of the measurements were conducted in both the presence and absence of incoming wakes,and numerical analysis was performed to elucidate the flow mechanism.With increasing Reynolds number,the influence of the incoming wakes on suppressing the secondary flow gradually increased owing to the greater influence of incoming wakes on reducing the negative incidence angle at higher Reynolds numbers,leading to a lower blade loading near the leading edge and suppression of the Pressure Side(PS)leg of the horseshoe vortex.However,the effect of unsteady wakes on suppressing the profile losses gradually became weaker owing to the reduced size of the Suction Side(SS)separation bubble and increased mixing loss in the free-flow region at high Reynolds numbers.Incoming wakes clearly improved the aerodynamic performance of the low-pressure turbine cascade at low Reynolds numbers of 25,000 and 50,000.In contrast,at the high Reynolds number of 100,000,the profile loss at the midspan and mass-averaged total losses downstream of the cascade were higher in the presence of wakes than in the absence of wakes,and the unsteady wakes exerted a negative influence on the aerodynamic performance of the LPT cascade.展开更多
A novel method of rotated window Radon transform is developed for identifying the linear texture in SAR image.It is applied to automatic detection of the ship wakes of SEASAT SAR image.The location and direction of th...A novel method of rotated window Radon transform is developed for identifying the linear texture in SAR image.It is applied to automatic detection of the ship wakes of SEASAT SAR image.The location and direction of the traveling ship can be quickly and accurately detectec,In some cases, the ship velocity can also be obtained.展开更多
Different turbulence closures were used to predict the flow interaction between the wakes created by compressor outlet guide vanes(OGVs) and a downstream annular pre-diffuser.Two statistical turbulence models were tes...Different turbulence closures were used to predict the flow interaction between the wakes created by compressor outlet guide vanes(OGVs) and a downstream annular pre-diffuser.Two statistical turbulence models were tested based on the classical Reynolds-averaged Navier-Stokes(RANS) approach.Both high-Re and low-Re(Launder-Sharma) versions of the k-ε model were applied to a selected test problem for OGV wake/diffuser flows.The test problem was specifically chosen because experimentally determined inlet conditions and both profile and performance data were available to validate predictions.A preliminary study was also reported of the more advanced large eddy simulation(LES) approach.The LES sub-grid-scale(SGS) model was the basic Smagorinsky eddy viscosity assumption,with a Van-Driest damping function for improved capture of near-wall viscous behaviour.Comparison between the two RANS models showed little difference in terms of velocity contours at OGV trailing edge and diffuser exit.In terms of overall diffuser performance(static pressure recovery and total pressure loss coefficients),the high-Re model was shown to agree well with experimental data.The preliminary LES study indicates the highly unsteady character of the OGV wake flow,but requires improved treatment of inlet conditions.展开更多
The study on the formation of vortex streets behind stationary circular cylinders is of substantial sense in engineering, since vortex streets play the leading role in introduction of vibration, generation of noise an...The study on the formation of vortex streets behind stationary circular cylinders is of substantial sense in engineering, since vortex streets play the leading role in introduction of vibration, generation of noise and wake instability, etc.. In the present paper, the Orr-Sommerfeld equation combined with measured velocity profiles is used to investigate the type of instability behind stationary cylinders, which determines the mechanism of vortex formation. The numerical calculations for Reynolds numbers of 56-140 000 imply that there is a range around the end of the dead water region in cylinder wakes where the instability belongs to the absolute type. Beyond the range, the flows show to be of the convective type. On the other hand, the flows tend to be of the convective instability when Reynolds number is below the subcritical value of forming vortex streets. All of the correspondent Strouhal numbers agree with experimental data very well. The formation of vortex streets behind stationary cylinders is proposed to be caused by an absolute instability in the near wake. There is always an absolute instability region for the Reynolds numbers from 56-140,000. Further, the experimental manipulation of vortex streets according to the stability analysis mentioned above is proved to be very effective.展开更多
In this paper,studies on offshore wind farm wakes observed by spaceborne synthetic aperture radar(SAR)are reviewed mainly based on our previous research.Particularly,we focus on investigating wind wakes and tidal curr...In this paper,studies on offshore wind farm wakes observed by spaceborne synthetic aperture radar(SAR)are reviewed mainly based on our previous research.Particularly,we focus on investigating wind wakes and tidal current wakes observed by spaceborne SAR of Terra SAR-X,Gaofen-3 and Radarsat-2 in high spatial resolution,in two offshores wind farms,i.e.,the Alpha Ventus in the North Sea and the one near Donghai bridge in the East China Sea.Representing examples of wind wakes and tidal current wakes observed by SAR in the two farms are presented and compared.A preliminary statistical analysis on morphology of wind feature downstream Alpha Ventus is presented as well.Besides these studies on wind wakes generated by a single offshore wind farm,we show an example of wakes downstream multiple wind farms in the North Sea to demonstrate"cluster"effect of multiple offshore wind farms on sea wind.展开更多
We consider the classification of wake structures produced by self-propelled fish-like swimmers based on local measurements of flow variables.This problem is inspired by the extraordinary capability of animal swimmers...We consider the classification of wake structures produced by self-propelled fish-like swimmers based on local measurements of flow variables.This problem is inspired by the extraordinary capability of animal swimmers in perceiving their hydrodynamic environments under dark condition.We train different neural networks to classify wake structures by using the streamwise velocity component,the crosswise velocity component,the vorticity and the combination of three flow variables,respectively.It is found that the neural networks trained using the two velocity components perform well in identifying the wake types,whereas the neural network trained using the vorticity suffers from a high rate of misclassification.When the neural network is trained using the combination of all three flow variables,a remarkably high accuracy in wake classification can be achieved.The results of this study can be helpful to the design of flow sensory systems in robotic underwater vehicles.展开更多
In a large wind farm,the wakes of upstream and downstream wind turbines can interfere with each other,affecting the overall power output of the wind farm.To further improve the numerical accuracy of the turbine wake d...In a large wind farm,the wakes of upstream and downstream wind turbines can interfere with each other,affecting the overall power output of the wind farm.To further improve the numerical accuracy of the turbine wake dynamics under atmosphere turbulence,this work proposes some improvements to the actuator line-large-eddy simulation(AL-LES)method.Based on the dynamic k-equation large-eddy simulation(LES),this method uses a precursor method to generate atmospheric inflow turbulence,models the tower and nacelle wakes,and improves the body force projection method based on an anisotropic Gaussian distribution function.For these three improvements,three wind tunnel experiments are used to validate the numerical accuracy of this method.The results show that the numerical results calculated in the far-wake region can reflect the characteristics of typical onshore and offshore wind conditions compared with the experimental results.After modeling the tower and nacelle wakes,the wake velocity distribution is consistent with the experimental result.The radial migration velocity of the tip vortex calculated by the improved blade body force distribution model is 0.32 m/s,which is about 6%different from the experimental value and improves the prediction accuracy of the tip vortex radial movement.The method proposed in this paper is very helpful for wind turbine wake dynamic analysis and wind farm power prediction.展开更多
In order to clarify the migration mechanism and wake behavior of a single bubble rising near a vertical wall,three-dimensional direct numerical simulations are implemented based on the open-source soft-ware Basilisk a...In order to clarify the migration mechanism and wake behavior of a single bubble rising near a vertical wall,three-dimensional direct numerical simulations are implemented based on the open-source soft-ware Basilisk and various types of migration paths like linear,zigzag and spiral are investigated.The volume of fluid(VOF)method is used to capture the bubble interface at a small scale,while the gas-liquid interface and high-velocity-gradient regions in the flow field are encrypted with the adaptive mesh refinement technology.The results show that the vertical wall has an obstructive effect on the diffusion of the vortex boundary layer on the surface of the bubble migrating in a straight line,and the resulting reaction force tends to push the bubbles away from the wall surface.For the zigzag or spiral movement of a bubble in the x-y plane,the perpendicular wall is an unstable factor,but on the contrary,the motion in the z-y plane is stabilized.展开更多
We measured flow structures with stereoscopic particle image velocimetry(stereo-PIV) in the turbulent wakes of three parachute canopies, which had the same surface area, but different geometries. The tested parachute ...We measured flow structures with stereoscopic particle image velocimetry(stereo-PIV) in the turbulent wakes of three parachute canopies, which had the same surface area, but different geometries. The tested parachute canopies included ribbon canopy, 8-branches canopy, and cross canopy. The obtained results showed that the geometry of the parachute canopies had significant influences on the flow structures in the wakes of these three canopies. In addition, the variation of Reynolds number did not lead to a dramatic change in the distributions of velocity, vorticity,Reynolds stress, and turbulent kinetic energy.展开更多
The study on ship wakes of synthetic aperture radar(SAR)images holds great importance in detecting ship targets in the ocean.In this study,we focus on the issues of low quantity and insufficient diversity in ship wake...The study on ship wakes of synthetic aperture radar(SAR)images holds great importance in detecting ship targets in the ocean.In this study,we focus on the issues of low quantity and insufficient diversity in ship wakes of SAR images,and propose a method of data augmentation of ship wakes in SAR images based on the improved cycle-consistent generative adversarial network(CycleGAN).The improvement measures mainly include two aspects:First,to enhance the quality of the generated images and guarantee a stable training process of the model,the least-squares loss is employed as the adversarial loss function;Second,the decoder of the generator is augmented with the convolutional block attention module(CBAM)to address the issue of missing details in the generated ship wakes of SAR images at the microscopic level.The experiment findings indicate that the improved CycleGAN model generates clearer ship wakes of SAR images,and outperforms the traditional CycleGAN models in both subjective and objective aspects.展开更多
This paper proposes a passive control method to reduce peak values of slipstream and turbulent kinetic energy in a high-speed train wake by attaching vortex generators(VGs)onto the upper surface of the tail car.The im...This paper proposes a passive control method to reduce peak values of slipstream and turbulent kinetic energy in a high-speed train wake by attaching vortex generators(VGs)onto the upper surface of the tail car.The impact of the VGs is assessed through the improved delayed detached eddy simulations(IDDES)after validating predictions against previous experimental measurements and other numerical predictions for the base case.The simulations indicate that strategically installed VGs can reduce the average slipstream velocity(U slipstream)and the upper limit of slipstream velocity(U_(slipstream,max))by~17%and~15%,respectively,as well as moving the peaks downstream by approximately train height,thus reducing the danger posed by slipstream to waiting passengers and trackside workers.Analysis shows that the wake turbulent kinetic energy diminishes as the vortex generators decelerate the downwash flow and reduce shear production in the wake.It is also found that the presence of VGs significantly impacts the flow on the upper surface near the tail by modifying the unsteady trailing longitudinal vortices through the formation of additional counter-rotating longitudinal vortices from the VGs.These latter vortices prevent the merging of vortical airflow around the trailing nose tip,which is otherwise induced by the longitudinal vortex of the train.They also reduce vortex intensity through cross-annihilation and cross diffusion as the wake advects downstream,limiting outwards advection through interaction with the image pair,and contributing to a decrease in the peak slipstream value.The method proposed offers a simple approach to wake control leading to significant slipstream benefits.展开更多
Vortical structures and wakes of bluff bodies in homogenous and stratified environment are common and important in ocean engineering.Based on the Boussinesq approximation,a thermocline model is proposed to deal with t...Vortical structures and wakes of bluff bodies in homogenous and stratified environment are common and important in ocean engineering.Based on the Boussinesq approximation,a thermocline model is proposed to deal with the variable density stratified fluid,and implemented in the commercial software Simcenter STAR-CCM+framework.The improved delayed detached eddy simulation(IDDES)modeling method is adopted to resolve the coherent vortical structures and turbulent wakes precisely and efficiently.Four conditions consisting of one homogenous and three stratified fluid cases with different density gradient past a sphere at Reynolds number 3700 are investigated.Results show that density stratification has a great impact on the vortical structures,the vertical motion is suppressed and internal waves will be induced and propagated,which is very different with that of homogenous situation.With the stratification strength increases,the vortical structures are gradually flattened,the asymmetry and anisotropy between vertical and horizontal motions are enhanced.展开更多
Turbulent flow in a 3-D blade passage of a Francis hydro turbine was simulated with the Large Eddy Simulation （LES） to investigate the spatial and temporal distributions of the turbulence when strongly distorted wak...Turbulent flow in a 3-D blade passage of a Francis hydro turbine was simulated with the Large Eddy Simulation （LES） to investigate the spatial and temporal distributions of the turbulence when strongly distorted wakes in the inflow sweep over the passage, In a suitable consideration of the energy exchanging mechanism between the large and small scales in the complicated passage with a strong 3-D curvature, one-coefficient dynamic Sub-Grid-Scale （SGS） stress model was used in this article. The simulations show that the strong wakes in the inflow lead to a flow separation at the leading zone of the passage, and to form a primary vortex in the span-wise direction. The primary span-wise vortex evolves and splits into smaller vortex pairs due to the constraint of no-slip wall condition, which triggers losing stability of the flow in the passage. The computed pressures on the pressure and suction sides agree with the measured data for a working test turbine model.展开更多
The wake of a submarine is a crucial element that greatly affects its stealth and hydrodynamic performance.To investigate the propagation and evolution characteristics of submarine wakes in a stratified fluid,particul...The wake of a submarine is a crucial element that greatly affects its stealth and hydrodynamic performance.To investigate the propagation and evolution characteristics of submarine wakes in a stratified fluid,particularly under free surface conditions,a numerical method to simulate the linearly stratified environment was developed.A thermocline model based on Boussinesq assumption was introduced to match the continuously stratified fluid.The volume of fluid(VOF)method was utilized to capture free surface waves,while a fully structured grid and the SST k-ωturbulence model were combined to solve the complex flow of submarines.Grid independence study in homogenous flow near the free surface was initially conducted.Furthermore,the hydrodynamic performance,wake evolution characteristics,free surface signatures as well as decay of velocity under different stratification levels were analyzed.Results demonstrated that the numerical method employed was efficient in simulating stratified flows.Moreover,it showed that density stratification had a significant impact on the hydrodynamics and wake characteristics of a submarine,especially under strong density stratification circumstances.展开更多
Highly efficient turbine exhaust diffuser cannot be designed without taking into account the unsteady interactions with the last rotating row of the turbine. Former investigations described in the literature show a ve...Highly efficient turbine exhaust diffuser cannot be designed without taking into account the unsteady interactions with the last rotating row of the turbine. Former investigations described in the literature show a very high potential compared to that of other parts of turbomachines for improving the diffuser. A scale model of a typical gas turbine exhaust diffuser is investigated experimentally. To investigate the influence of rotating wakes, measurements without a spoke wheel as well as measurements with a variable-speed rotating cylindrical spoke wheel with 2 mm- or 10 mm-spokes simulating turbine rotor wakes were made. Miniaturized 3-hole pneumatic probes as well as a 2D-Laser-Doppler-Velocimeter （LDV） were used to investigate velocity profiles. 122 static pressure tapings were used to measure several axial and circumferential static pressure distributions. Without a spoke-wheel the annular diffuser separates at the shroud for all swirl configurations. For the measurements with the 2 mm spoke wheel, the separating diffuser was unstable while keeping the test rig operating parameters constant. For a non-rotating 10 mm spoke wheel and at rotational speeds less than 1,000 rpm, the annular diffuser separated at the shroud. Increasing the rotational speed of the 10mm spoke wheel, flow did not separate at the shroud and much higher pressure recovery than without spoke wheel has achieved.展开更多
The molecular wake-assisted interaction between two collinear femotosecond laser pulses is investigated in air,which leads to the generation of a controllable 1.8 mJ super-continuum pulse with an elongated self-guided...The molecular wake-assisted interaction between two collinear femotosecond laser pulses is investigated in air,which leads to the generation of a controllable 1.8 mJ super-continuum pulse with an elongated self-guided channel due to the cross-phase modulation of the impulsively aligned diatomic molecules in air.For two parallel launched femtosecond laser pulses with a certain spatial separation,controllable attraction and repulsion of the pulses are observed due to the counter-balance among molecular wakes,Kerr and plasma effects,where the molecular wakes show a longer interaction distance than the others to control the propagation of the intense ultrashort laser pulses.展开更多
Wash waves produced by ships disintegrate river banks and coastal lines. This phenomenon of bank erosion is mainly due to the height of the waves. Various factors govern the formation of these waves and their amplitud...Wash waves produced by ships disintegrate river banks and coastal lines. This phenomenon of bank erosion is mainly due to the height of the waves. Various factors govern the formation of these waves and their amplitudes: the geometry of the water channel, the shape and the speed of the boat, etc.. These factors play an important role on the wave generation, in addition on the resistance of the ship and so on its fuel consumption. Whether to study the impact of wash waves on the ship's environment or its resistance, the analysis of the generated wake is essential. Hence a fine characterization of the wave field is necessary. This study proposes a comparison of wakes generated by two generic ships based on a Wigley hull with block coefficients 0.67 and 0.89 respectively representative of maritime and fluvial ships. The wakes generated in deep water and confined water configurations have been measured for different Froude numbers by a non-intrusive optical stereo-correlation method, giving access to a detailed and complete definition of the generated wave fields. The resistance of the ship hulls has been measured in deep and confined water configurations with a hydrodynamic balance. The results permit one to study the influence of both hull and water channel geometries on the ship wake, on the amplitude of the far-field generated waves and on the near-field hydrodynamic response. Moreover, resistance curves are obtained for both configurations and highlight the effect of both hull and water channel geometries on the resistance coefficient of the ship. A comparison of the resistance curves with or without the ship trim is conducted and shows the influence of the trim on the resistance coefficient in the different ship speed regimes.展开更多
The design of turbine exhaust diffusers is still commonly based upon conservative design charts which do not consider the influence of the components installed upstream such as the last turbine stage.Based on investig...The design of turbine exhaust diffusers is still commonly based upon conservative design charts which do not consider the influence of the components installed upstream such as the last turbine stage.Based on investigations with different spoke wheels which approximate the blade wakes of the last turbine stage,the influence of a rotating bladed disc on the flow in a exhaust diffuser is presented.The pressure recovery for an annual diffuser with 15° half cone angle and the 5 mm spoke wheel used in the model test is very high and nearly independent on the rotational speed and the corresponding Strouhal number.For a 20° annual diffuser,the pressure recovery increases with Strouhal number展开更多
The experiments for the sound scattering of the ship wakes are introduced in this article. The wakes are detected in both horizon and vertical in lake trials and its traces are shown in display model A or B, Its si...The experiments for the sound scattering of the ship wakes are introduced in this article. The wakes are detected in both horizon and vertical in lake trials and its traces are shown in display model A or B, Its size and sound scattering strength are measured also. The scattering signals are analysed with the instananeous frequency sequence statistics methods展开更多
基金supported by the National Natural Science Foundation of China(No.51876098).
文摘Accurate predictions of Shock Waves and Boundary Layer Interaction(SWBLI)and strong Shock Waves and Wake Vortices Interaction(SWWVI)in a highly-loaded turbine propose challenges to the currently widely used Reynolds-Averaged Navier-Stokes(RANS)model.In this work,the SWBLI and the SWWVI in a highly-loaded Nozzle Guide Vane(NGV)are studied using a hybrid RANS/LES strategy.The Turbulence Kinetic Energy(TKE)budget and the Proper Orthogonal Decomposition(POD)method are used to analyze flow mechanisms.Results show that this hybrid RANS/LES method can obtain detailed flow structures for flow mechanisms analysis.Strong shock waves induce boundary layer separation,while the presence of a separation bubble can in turn lead to a Mach reflection phenomenon.The shock waves cause trailing-edge vortices to break clearly,and the wakes,in turn,can change the shocks intensity and direction.Furthermore,the Entropy Generation Rate(EGR)is used to analyze the irreversible loss.It turns out that the SWWVI can reduce the flow field loss.There are several weak shock waves in the NGV flow field,which can increase the irreversible loss.This work offers flow mechanisms analysis and presents the EGR distribution in SWBLI and SWWVI areas in a transonic turbine blade.
基金provided by the National Natural Science Foundation of China(Nos.51876202 and 51836008)。
文摘Detailed experimental measurements were conducted to study the interactions between incoming wakes and endwall secondary flow in a high-lift Low-Pressure Turbine(LPT)cascade.All of the measurements were conducted in both the presence and absence of incoming wakes,and numerical analysis was performed to elucidate the flow mechanism.With increasing Reynolds number,the influence of the incoming wakes on suppressing the secondary flow gradually increased owing to the greater influence of incoming wakes on reducing the negative incidence angle at higher Reynolds numbers,leading to a lower blade loading near the leading edge and suppression of the Pressure Side(PS)leg of the horseshoe vortex.However,the effect of unsteady wakes on suppressing the profile losses gradually became weaker owing to the reduced size of the Suction Side(SS)separation bubble and increased mixing loss in the free-flow region at high Reynolds numbers.Incoming wakes clearly improved the aerodynamic performance of the low-pressure turbine cascade at low Reynolds numbers of 25,000 and 50,000.In contrast,at the high Reynolds number of 100,000,the profile loss at the midspan and mass-averaged total losses downstream of the cascade were higher in the presence of wakes than in the absence of wakes,and the unsteady wakes exerted a negative influence on the aerodynamic performance of the LPT cascade.
基金Supported by the National Natural Science Foundation of China(No.49831060,No.69771007),and National Defense Foundation
文摘A novel method of rotated window Radon transform is developed for identifying the linear texture in SAR image.It is applied to automatic detection of the ship wakes of SEASAT SAR image.The location and direction of the traveling ship can be quickly and accurately detectec,In some cases, the ship velocity can also be obtained.
文摘Different turbulence closures were used to predict the flow interaction between the wakes created by compressor outlet guide vanes(OGVs) and a downstream annular pre-diffuser.Two statistical turbulence models were tested based on the classical Reynolds-averaged Navier-Stokes(RANS) approach.Both high-Re and low-Re(Launder-Sharma) versions of the k-ε model were applied to a selected test problem for OGV wake/diffuser flows.The test problem was specifically chosen because experimentally determined inlet conditions and both profile and performance data were available to validate predictions.A preliminary study was also reported of the more advanced large eddy simulation(LES) approach.The LES sub-grid-scale(SGS) model was the basic Smagorinsky eddy viscosity assumption,with a Van-Driest damping function for improved capture of near-wall viscous behaviour.Comparison between the two RANS models showed little difference in terms of velocity contours at OGV trailing edge and diffuser exit.In terms of overall diffuser performance(static pressure recovery and total pressure loss coefficients),the high-Re model was shown to agree well with experimental data.The preliminary LES study indicates the highly unsteady character of the OGV wake flow,but requires improved treatment of inlet conditions.
文摘The study on the formation of vortex streets behind stationary circular cylinders is of substantial sense in engineering, since vortex streets play the leading role in introduction of vibration, generation of noise and wake instability, etc.. In the present paper, the Orr-Sommerfeld equation combined with measured velocity profiles is used to investigate the type of instability behind stationary cylinders, which determines the mechanism of vortex formation. The numerical calculations for Reynolds numbers of 56-140 000 imply that there is a range around the end of the dead water region in cylinder wakes where the instability belongs to the absolute type. Beyond the range, the flows show to be of the convective type. On the other hand, the flows tend to be of the convective instability when Reynolds number is below the subcritical value of forming vortex streets. All of the correspondent Strouhal numbers agree with experimental data very well. The formation of vortex streets behind stationary cylinders is proposed to be caused by an absolute instability in the near wake. There is always an absolute instability region for the Reynolds numbers from 56-140,000. Further, the experimental manipulation of vortex streets according to the stability analysis mentioned above is proved to be very effective.
基金Natural Science Foundation of Hainan Province(No.420RC675)National Natural Science Foundation of China(No.41471309)。
文摘In this paper,studies on offshore wind farm wakes observed by spaceborne synthetic aperture radar(SAR)are reviewed mainly based on our previous research.Particularly,we focus on investigating wind wakes and tidal current wakes observed by spaceborne SAR of Terra SAR-X,Gaofen-3 and Radarsat-2 in high spatial resolution,in two offshores wind farms,i.e.,the Alpha Ventus in the North Sea and the one near Donghai bridge in the East China Sea.Representing examples of wind wakes and tidal current wakes observed by SAR in the two farms are presented and compared.A preliminary statistical analysis on morphology of wind feature downstream Alpha Ventus is presented as well.Besides these studies on wind wakes generated by a single offshore wind farm,we show an example of wakes downstream multiple wind farms in the North Sea to demonstrate"cluster"effect of multiple offshore wind farms on sea wind.
基金the National Natural Science Foundation of China(Grants 11772338 and 11372331)Chinese Academy of Sciences(Grants XDB22040104 and XDA22040203).
文摘We consider the classification of wake structures produced by self-propelled fish-like swimmers based on local measurements of flow variables.This problem is inspired by the extraordinary capability of animal swimmers in perceiving their hydrodynamic environments under dark condition.We train different neural networks to classify wake structures by using the streamwise velocity component,the crosswise velocity component,the vorticity and the combination of three flow variables,respectively.It is found that the neural networks trained using the two velocity components perform well in identifying the wake types,whereas the neural network trained using the vorticity suffers from a high rate of misclassification.When the neural network is trained using the combination of all three flow variables,a remarkably high accuracy in wake classification can be achieved.The results of this study can be helpful to the design of flow sensory systems in robotic underwater vehicles.
基金Project supported by the National Key Research and Development Program of China(Nos.2019YFE0192600,2017YFE0132000,and 2019YFB1503700)the National Natural Science Foundation of China(Nos.51761135012 and 11872248)。
文摘In a large wind farm,the wakes of upstream and downstream wind turbines can interfere with each other,affecting the overall power output of the wind farm.To further improve the numerical accuracy of the turbine wake dynamics under atmosphere turbulence,this work proposes some improvements to the actuator line-large-eddy simulation(AL-LES)method.Based on the dynamic k-equation large-eddy simulation(LES),this method uses a precursor method to generate atmospheric inflow turbulence,models the tower and nacelle wakes,and improves the body force projection method based on an anisotropic Gaussian distribution function.For these three improvements,three wind tunnel experiments are used to validate the numerical accuracy of this method.The results show that the numerical results calculated in the far-wake region can reflect the characteristics of typical onshore and offshore wind conditions compared with the experimental results.After modeling the tower and nacelle wakes,the wake velocity distribution is consistent with the experimental result.The radial migration velocity of the tip vortex calculated by the improved blade body force distribution model is 0.32 m/s,which is about 6%different from the experimental value and improves the prediction accuracy of the tip vortex radial movement.The method proposed in this paper is very helpful for wind turbine wake dynamic analysis and wind farm power prediction.
基金supported by the National Natural Science Foundation of China(Grant No.51906262)the Hunan Provincial Natural Science Foundation of China(Grant No.2020JJ5735).
文摘In order to clarify the migration mechanism and wake behavior of a single bubble rising near a vertical wall,three-dimensional direct numerical simulations are implemented based on the open-source soft-ware Basilisk and various types of migration paths like linear,zigzag and spiral are investigated.The volume of fluid(VOF)method is used to capture the bubble interface at a small scale,while the gas-liquid interface and high-velocity-gradient regions in the flow field are encrypted with the adaptive mesh refinement technology.The results show that the vertical wall has an obstructive effect on the diffusion of the vortex boundary layer on the surface of the bubble migrating in a straight line,and the resulting reaction force tends to push the bubbles away from the wall surface.For the zigzag or spiral movement of a bubble in the x-y plane,the perpendicular wall is an unstable factor,but on the contrary,the motion in the z-y plane is stabilized.
基金supported by the Science and Technology Commission of Shanghai Municipality(Grant 15ZR1442700)the Fundamental Research Funds for the Central Universities
文摘We measured flow structures with stereoscopic particle image velocimetry(stereo-PIV) in the turbulent wakes of three parachute canopies, which had the same surface area, but different geometries. The tested parachute canopies included ribbon canopy, 8-branches canopy, and cross canopy. The obtained results showed that the geometry of the parachute canopies had significant influences on the flow structures in the wakes of these three canopies. In addition, the variation of Reynolds number did not lead to a dramatic change in the distributions of velocity, vorticity,Reynolds stress, and turbulent kinetic energy.
基金Industry-University-Research Cooperation Fund Project of the Eighth Research Institute of China Aerospace Science and Technology Corporation(No.USCAST2021-5)Aeronautical Science Foundation of China(No.20220001057001)。
文摘The study on ship wakes of synthetic aperture radar(SAR)images holds great importance in detecting ship targets in the ocean.In this study,we focus on the issues of low quantity and insufficient diversity in ship wakes of SAR images,and propose a method of data augmentation of ship wakes in SAR images based on the improved cycle-consistent generative adversarial network(CycleGAN).The improvement measures mainly include two aspects:First,to enhance the quality of the generated images and guarantee a stable training process of the model,the least-squares loss is employed as the adversarial loss function;Second,the decoder of the generator is augmented with the convolutional block attention module(CBAM)to address the issue of missing details in the generated ship wakes of SAR images at the microscopic level.The experiment findings indicate that the improved CycleGAN model generates clearer ship wakes of SAR images,and outperforms the traditional CycleGAN models in both subjective and objective aspects.
基金Project(52372370)supported by the National Natural Science Foundation of ChinaProject(2023ZZTS0379)supported by the Graduate Student Independent Innovation Project of Central South University,ChinaProject(202206370058)supported by the China Scholarship Council。
文摘This paper proposes a passive control method to reduce peak values of slipstream and turbulent kinetic energy in a high-speed train wake by attaching vortex generators(VGs)onto the upper surface of the tail car.The impact of the VGs is assessed through the improved delayed detached eddy simulations(IDDES)after validating predictions against previous experimental measurements and other numerical predictions for the base case.The simulations indicate that strategically installed VGs can reduce the average slipstream velocity(U slipstream)and the upper limit of slipstream velocity(U_(slipstream,max))by~17%and~15%,respectively,as well as moving the peaks downstream by approximately train height,thus reducing the danger posed by slipstream to waiting passengers and trackside workers.Analysis shows that the wake turbulent kinetic energy diminishes as the vortex generators decelerate the downwash flow and reduce shear production in the wake.It is also found that the presence of VGs significantly impacts the flow on the upper surface near the tail by modifying the unsteady trailing longitudinal vortices through the formation of additional counter-rotating longitudinal vortices from the VGs.These latter vortices prevent the merging of vortical airflow around the trailing nose tip,which is otherwise induced by the longitudinal vortex of the train.They also reduce vortex intensity through cross-annihilation and cross diffusion as the wake advects downstream,limiting outwards advection through interaction with the image pair,and contributing to a decrease in the peak slipstream value.The method proposed offers a simple approach to wake control leading to significant slipstream benefits.
基金the National Key Research and Development Program of China(Grant Nos.2019YFB1704200,2019YFC0312400)the National Natural Science Foundation of China(Grant Nos.52001210,51879159)the Oceanic Interdisciplinary Program of Shanghai Jiao Tong University(Grant No.SL2020PT104).
文摘Vortical structures and wakes of bluff bodies in homogenous and stratified environment are common and important in ocean engineering.Based on the Boussinesq approximation,a thermocline model is proposed to deal with the variable density stratified fluid,and implemented in the commercial software Simcenter STAR-CCM+framework.The improved delayed detached eddy simulation(IDDES)modeling method is adopted to resolve the coherent vortical structures and turbulent wakes precisely and efficiently.Four conditions consisting of one homogenous and three stratified fluid cases with different density gradient past a sphere at Reynolds number 3700 are investigated.Results show that density stratification has a great impact on the vortical structures,the vertical motion is suppressed and internal waves will be induced and propagated,which is very different with that of homogenous situation.With the stratification strength increases,the vortical structures are gradually flattened,the asymmetry and anisotropy between vertical and horizontal motions are enhanced.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 50579025 and 90210005 ).
文摘Turbulent flow in a 3-D blade passage of a Francis hydro turbine was simulated with the Large Eddy Simulation （LES） to investigate the spatial and temporal distributions of the turbulence when strongly distorted wakes in the inflow sweep over the passage, In a suitable consideration of the energy exchanging mechanism between the large and small scales in the complicated passage with a strong 3-D curvature, one-coefficient dynamic Sub-Grid-Scale （SGS） stress model was used in this article. The simulations show that the strong wakes in the inflow lead to a flow separation at the leading zone of the passage, and to form a primary vortex in the span-wise direction. The primary span-wise vortex evolves and splits into smaller vortex pairs due to the constraint of no-slip wall condition, which triggers losing stability of the flow in the passage. The computed pressures on the pressure and suction sides agree with the measured data for a working test turbine model.
基金supported by the National Natural Science Foundation of China(Grant Nos.52001210,52131102).
文摘The wake of a submarine is a crucial element that greatly affects its stealth and hydrodynamic performance.To investigate the propagation and evolution characteristics of submarine wakes in a stratified fluid,particularly under free surface conditions,a numerical method to simulate the linearly stratified environment was developed.A thermocline model based on Boussinesq assumption was introduced to match the continuously stratified fluid.The volume of fluid(VOF)method was utilized to capture free surface waves,while a fully structured grid and the SST k-ωturbulence model were combined to solve the complex flow of submarines.Grid independence study in homogenous flow near the free surface was initially conducted.Furthermore,the hydrodynamic performance,wake evolution characteristics,free surface signatures as well as decay of velocity under different stratification levels were analyzed.Results demonstrated that the numerical method employed was efficient in simulating stratified flows.Moreover,it showed that density stratification had a significant impact on the hydrodynamics and wake characteristics of a submarine,especially under strong density stratification circumstances.
基金German Ministry of Economics and Technology under the AG Turbo's COOREFF-T program
文摘Highly efficient turbine exhaust diffuser cannot be designed without taking into account the unsteady interactions with the last rotating row of the turbine. Former investigations described in the literature show a very high potential compared to that of other parts of turbomachines for improving the diffuser. A scale model of a typical gas turbine exhaust diffuser is investigated experimentally. To investigate the influence of rotating wakes, measurements without a spoke wheel as well as measurements with a variable-speed rotating cylindrical spoke wheel with 2 mm- or 10 mm-spokes simulating turbine rotor wakes were made. Miniaturized 3-hole pneumatic probes as well as a 2D-Laser-Doppler-Velocimeter （LDV） were used to investigate velocity profiles. 122 static pressure tapings were used to measure several axial and circumferential static pressure distributions. Without a spoke-wheel the annular diffuser separates at the shroud for all swirl configurations. For the measurements with the 2 mm spoke wheel, the separating diffuser was unstable while keeping the test rig operating parameters constant. For a non-rotating 10 mm spoke wheel and at rotational speeds less than 1,000 rpm, the annular diffuser separated at the shroud. Increasing the rotational speed of the 10mm spoke wheel, flow did not separate at the shroud and much higher pressure recovery than without spoke wheel has achieved.
基金partly funded by the National Natural Science Foundation of China(Grant Nos.10525416,10804032,and 10990101)the National Basic Research Program of China(Grant No.2006CB806005)+1 种基金Projectsfrom Shanghai Science and Technology Commission(Grant Nos.08ZR1407100 and 09QA1402000)Shanghai Educational Development Foundation(Grant No.2008CG29)
文摘The molecular wake-assisted interaction between two collinear femotosecond laser pulses is investigated in air,which leads to the generation of a controllable 1.8 mJ super-continuum pulse with an elongated self-guided channel due to the cross-phase modulation of the impulsively aligned diatomic molecules in air.For two parallel launched femtosecond laser pulses with a certain spatial separation,controllable attraction and repulsion of the pulses are observed due to the counter-balance among molecular wakes,Kerr and plasma effects,where the molecular wakes show a longer interaction distance than the others to control the propagation of the intense ultrashort laser pulses.
文摘Wash waves produced by ships disintegrate river banks and coastal lines. This phenomenon of bank erosion is mainly due to the height of the waves. Various factors govern the formation of these waves and their amplitudes: the geometry of the water channel, the shape and the speed of the boat, etc.. These factors play an important role on the wave generation, in addition on the resistance of the ship and so on its fuel consumption. Whether to study the impact of wash waves on the ship's environment or its resistance, the analysis of the generated wake is essential. Hence a fine characterization of the wave field is necessary. This study proposes a comparison of wakes generated by two generic ships based on a Wigley hull with block coefficients 0.67 and 0.89 respectively representative of maritime and fluvial ships. The wakes generated in deep water and confined water configurations have been measured for different Froude numbers by a non-intrusive optical stereo-correlation method, giving access to a detailed and complete definition of the generated wave fields. The resistance of the ship hulls has been measured in deep and confined water configurations with a hydrodynamic balance. The results permit one to study the influence of both hull and water channel geometries on the ship wake, on the amplitude of the far-field generated waves and on the near-field hydrodynamic response. Moreover, resistance curves are obtained for both configurations and highlight the effect of both hull and water channel geometries on the resistance coefficient of the ship. A comparison of the resistance curves with or without the ship trim is conducted and shows the influence of the trim on the resistance coefficient in the different ship speed regimes.
基金supported by the Bundesmin-isterium für Wirtschaft und Technologie (BMWi) as per resolution of the German Federal Parliament under grant number 0327717B
文摘The design of turbine exhaust diffusers is still commonly based upon conservative design charts which do not consider the influence of the components installed upstream such as the last turbine stage.Based on investigations with different spoke wheels which approximate the blade wakes of the last turbine stage,the influence of a rotating bladed disc on the flow in a exhaust diffuser is presented.The pressure recovery for an annual diffuser with 15° half cone angle and the 5 mm spoke wheel used in the model test is very high and nearly independent on the rotational speed and the corresponding Strouhal number.For a 20° annual diffuser,the pressure recovery increases with Strouhal number
文摘The experiments for the sound scattering of the ship wakes are introduced in this article. The wakes are detected in both horizon and vertical in lake trials and its traces are shown in display model A or B, Its size and sound scattering strength are measured also. The scattering signals are analysed with the instananeous frequency sequence statistics methods
