A two-dimensional（2-D） incompressible plane jet is investigated using the lattice Boltzmann method（LBM） for low Reynolds numbers of 42 and 65 based on the jet-exit-width and the maximum jet-exit-velocity. The resu...A two-dimensional（2-D） incompressible plane jet is investigated using the lattice Boltzmann method（LBM） for low Reynolds numbers of 42 and 65 based on the jet-exit-width and the maximum jet-exit-velocity. The results show that the mean centerline velocity decays as x-1/3 and the jet spreads as x2/3 in the self-similar region, which are consistent with the theoretical predictions and the experimental data. The time histories and PSD analyses of the instantaneous centerline velocities indicate the periodic behavior and the interaction between periodic components of velocities should not be neglected in the far field region, although it is invisible in the near field region.展开更多
Micro air vehicles （MAV＇s） have the potential to revolutionize our sensing and information gathering capabilities in environmental monitoring and homeland security areas. Due to the MAV＇s＇ small size, flight regi...Micro air vehicles （MAV＇s） have the potential to revolutionize our sensing and information gathering capabilities in environmental monitoring and homeland security areas. Due to the MAV＇s＇ small size, flight regime, and modes of operation, significant scientific advancement will be needed to create this revolutionary capability. Aerodynamics, structural dynamics, and flight dynamics of natural flyers intersects with some of the richest problems in MAV＇s, inclu- ding massively unsteady three-dimensional separation, transition in boundary layers and shear layers, vortical flows and bluff body flows, unsteady flight environment, aeroelasticity, and nonlinear and adaptive control are just a few examples. A challenge is that the scaling of both fluid dynamics and structural dynamics between smaller natural flyer and practical flying hardware/lab experiment （larger dimension） is fundamentally difficult. In this paper, we offer an overview of the challenges and issues, along with sample results illustrating some of the efforts made from a computational modeling angle.展开更多
The aerodynamic force and flow structure of NACA 0012 airfoil performing an unsteady motion at low Reynolds number (Re = 100) are calculated by solving Navier-Stokes equations. The motion consists of three parts: the ...The aerodynamic force and flow structure of NACA 0012 airfoil performing an unsteady motion at low Reynolds number (Re = 100) are calculated by solving Navier-Stokes equations. The motion consists of three parts: the first translation, rotation and the second translation in the direction opposite to the first. The rotation and the second translation in this motion are expected to represent the rotation and translation of the wing-section of a hovering insect. The flow structure is used in combination with the theory of vorticity dynamics to explain the generation of unsteady aerodynamic force in the motion. During the rotation, due to the creation of strong vortices in short time, large aerodynamic force is produced and the force is almost normal to the airfoil chord. During the second translation, large lift coefficient can be maintained for certain time period and (C) over bar (L), the lift coefficient averaged over four chord lengths of travel, is larger than 2 (the corresponding steady-state lift coefficient is only 0.9). The large lift coefficient is due to two effects. The first is the delayed shedding of the stall vortex. The second is that the vortices created during the airfoil rotation and in the near wake left by previous translation form a short 'vortex street' in front of the airfoil and the 'vortex street' induces a 'wind'; against this 'wind' the airfoil translates, increasing its relative speed. The above results provide insights to the understanding of the mechanism of high-lift generation by a hovering insect.展开更多
Spanwise flexibility is a key factor influencing propulsion performance of pectoral foils. Performances of bionic fish with oscillating pectoral foils can be enhanced by properly selecting the spanwise flexibility. Th...Spanwise flexibility is a key factor influencing propulsion performance of pectoral foils. Performances of bionic fish with oscillating pectoral foils can be enhanced by properly selecting the spanwise flexibility. The influence law of spanwise flexibility on thrust generation and propulsion efficiency of a rectangular hydro-foil is discussed. Series foils constructed by the two-component silicon rubber are developed. NACA0015 shape of chordwise cross-section is employed. The foils are strengthened by fin rays of different rigidity to realize variant spanwise rigidity and almost the same chordwise flexibility. Experiments on a towing platform developed are carried out at low Reynolds numbers of 10 000, 15 000, and 20 000 and Strouhal numbers from 0.1 to 1. The following experimental results are achieved: (1) The average forward thrust increases with the St number increased; (2) Certain degree of spanwise flexibility is beneficial to the forward thrust generation, but the thrust gap is not large for the fins of different spanwise rigidity; (3) The fin of the maximal spanwise flexibility owns the highest propulsion efficiency; (4) Effect of the Reynolds number on the propulsion efficiency is significant. The experimental results can be utilized as a reference in deciding the spanwise flexibility of bionic pectoral fins in designing of robotic fish prototype propelled by flapping-wing.展开更多
A series of three-dimensional numerical simulations is carried out to investigate the effect of inclined angle on flow behavior behind two side-by-side inclined cylinders at low Reynolds number Re=100 and small spacin...A series of three-dimensional numerical simulations is carried out to investigate the effect of inclined angle on flow behavior behind two side-by-side inclined cylinders at low Reynolds number Re=100 and small spacing ratio T/D=1.5 (T is the center-to-center distance between two side-by-side cylinders, D is the diameter of cylinder). The instantaneous and time-averaged flow fields, force coefficients and Strouhal numbers are analyzed. Special attention is focused on the axial flow characteristics with variation of the inclined angle. The results show that the inclined angle has a significant effect on the gap flow behaviors behind two inclined cylinders. The vortex shedding behind two cylinders is suppressed with the increase of the inclined angle as well as the flip-flop gap flow. Moreover, the mean drag coefficient, root-mean-square lift coefficient and Strouhal numbers decrease monotonously with the increase of the inclined angle, which follows the independent principle at small inclined angles.展开更多
On the basis of the studies on the high unsteady aerodynamic mechanisms of the fruit fly hovering the aerodynamic advantages and disadvantages of the fruit fly flapping motion were analyzed. A new bionic flapping moti...On the basis of the studies on the high unsteady aerodynamic mechanisms of the fruit fly hovering the aerodynamic advantages and disadvantages of the fruit fly flapping motion were analyzed. A new bionic flapping motion was proposed to weaken the disadvantages and maintain the advantages, it may be used in the designing and manufacturing of the micro air vehicles （MAV＇s）. The translation of the new bionic flapping motion is the same as that of fruit fly flapping motion. However, the rotation of the new bionic flapping motion is different. It is not a pitching-up rotation as the fruit fly flapping motion, but a pitching-down rota- tion at the beginning and the end of a stroke. The numerical method of 3rd-order Roe scheme developed by Rogers was used to study these questions. The correctness of the numerical method and the computational program was justified by comparing the present CFD results of the fruit fly flapping motion in three modes, i.e., the advanced mode, the symmetrical mode and the delayed mode, with Dickinson＇s experimental results. They agreed with each other very well. Subsequently, the aerodynamic characteristics of the new bionic flapping motion in three modes were also numerically simulated, and were compared with those of the fruit fly flap- ping. The conclusions could be drawn that the high unsteady lift mechanism of the fruit fly hovering is also effectively utilized by this new bionic flapping. Compared with the fruit fly flapping, the unsteady drag of the new flapping decreases very much and the ratio of lift to drag increases greatly. And the great discrepancies among the mean lifts of three flapping modes of the fruit fly hovering are effectively smoothed in the new flapping. On the other hand, this new bionic flapping motion should be realized more easily. Finally, it must be pointed out that the above conclusions were just drawn for the hovering flapping motion. And the aerodynamic characteristics of the new bionic flapping motion in forward flight are going to be studied in the next step.展开更多
High-speed rotor rotation under the low-density condition creates a special low-Reynolds compressible flow around the rotor blade airfoil where the compressibility effect on the laminar separated shear layer occurs. H...High-speed rotor rotation under the low-density condition creates a special low-Reynolds compressible flow around the rotor blade airfoil where the compressibility effect on the laminar separated shear layer occurs. However, the compressibility effect and shock wave generation associated with the increase in the Mach number (M) and the trend change due to their interference have not been clarified. The purpose is to clear the compressibility effect and its impact of shock wave generation on the flow field and aerodynamics. Therefore, we perform a two-dimensional unsteady calculation by Computational fluid dynamics (CFD) analysis using the CLF5605 airfoil used in the Mars helicopter Ingenuity, which succeeded in its first flight on Mars. The calculation conditions are set to the Reynolds number (Re) at 75% rotor span in hovering (Re = 15,400), and the Mach number was varied from incompressible (M = 0.2) to transonic (M = 1.2). The compressible fluid dynamics solver FaSTAR developed by the Japan aerospace exploration agency (JAXA) is used, and calculations are performed under multiple conditions in which the Mach number and angle of attack (α) are swept. The results show that a flow field is similar to that in the Earth’s atmosphere above M = 1.0, such as bow shock at the leading edge, whereas multiple λ-type shock waves are observed over the separated shear layer above α = 3° at M = 0.80. However, no significant difference is found in the C<sub>p</sub> distribution around the airfoil between M = 0.6 and M = 0.8. From the results, it is found that multiple λ-type shock waves have no significant effect on the airfoil surface pressure distribution, the separated shear layer effect is dominant in the surface pressure change and aerodynamic characteristics.展开更多
This study experimentally investigates aerodynamic characteristics and flow fields of a smooth owl-like airfoil without serrations and velvet structures. This biologically inspired airfoil design is intended to serve ...This study experimentally investigates aerodynamic characteristics and flow fields of a smooth owl-like airfoil without serrations and velvet structures. This biologically inspired airfoil design is intended to serve as the main-wing for low-Reynolds-number aircrafts such as micro air vehicles. Reynolds number dependency on aerodynamics is also evaluated at low Reynolds numbers. The results of the study show that the owl-like airfoil has high lift performance with a nonlinear lift increase due to the presence of a separation bubble on the suction side. A distinctive flow feature of the owl airfoil is a separation bubble on the pressure side at low angles of attack. The separation bubble switches location from the pressure side to the suction side as the angle of attack increases and is continuously present on the surface within a wide range of angles of attack. The Reynolds number dependency on the lift curves is insignificant, although differences in the drag curves are especially pronounced at high angles of attack. Eventually, we obtain the geometric feature of the owl-like airfoil to increase aerodynamic performance at low Reynolds numbers.展开更多
Oscillation phenomena in far field region of plane jets are studied by lattice Boltzmann method over a range of Reynolds numbers （Re） from 16 to 65. Numerical results show that the instantaneous centerline velocitie...Oscillation phenomena in far field region of plane jets are studied by lattice Boltzmann method over a range of Reynolds numbers （Re） from 16 to 65. Numerical results show that the instantaneous centerline velocities show periodic oscillation behavior in far field region when Re〉38. In contrast, the periodic behavior is invisible in corresponding flow field when Re≤38. For the cases of Re≤38, the exchange of momentum due to straining mo- tion gradually dominates the downstream flow filed, which qualitatively suggests the possibility of iet instability.展开更多
A dual-jet consisting of a wall jet and an offset jet has been numerically simulated using lattice Boltzmann method to examine the effects of jet spacing between two jet centerlines,defined as s.The Reynolds number ba...A dual-jet consisting of a wall jet and an offset jet has been numerically simulated using lattice Boltzmann method to examine the effects of jet spacing between two jet centerlines,defined as s.The Reynolds number based on jet-exit-width dis set to be Re = 56 and the jet spacing is set to be less than or equal 10 times the jet-exitwidth.Computational results reveal that the flow field displays periodic vortex shedding when the jet spacing is in the range of 9≤s/d ≤ 10,while it remains steady with two counter-rotating vortices in the converging region when s/d ≤ 8.When s/d = 9,the power spectral analyses indicate that the vortex shedding phenomenon has specific frequency.The significant oscillation stresses induced by the periodic components of velocities are found to mainly exist in the inner shear layer regions,implying stronger momentum transfer occuring in these regions.展开更多
To achieve high-performance compressor cascades at low Reynolds number(Re),it is important to organize the boundary layer transition and separation processes efficiently and reasonably.In this study,the airfoil is foc...To achieve high-performance compressor cascades at low Reynolds number(Re),it is important to organize the boundary layer transition and separation processes efficiently and reasonably.In this study,the airfoil is focused on at a 5%blade height at the root of the orthogonal blade in the downflow passage of the high-load booster stage.The bionics modeling design is carried out for the leading edge of the original blade cascade;the response characteristics of laminar transition and separation to blades with different leading edge shapes at low Reynolds numbers are studied by using large eddy simulations combined with Omega vortex identification.The findings of this study demonstrate that bionic leading edge modeling can significantly improve the aerodynamic performance of blades at low Reynolds numbers.The blades effectively suppress the formation of separation bubbles at low Reynolds numbers and weaken or even eliminate large-scale flow separation at the trailing edge.In addition,the blades can weaken the vortex intensity on the blade surface,reduce the areas of high-velocity fluctuations,and minimize aerodynamic losses caused by turbulence dissipation.These results should serve as a valuable reference for the aerodynamic design and flow control of the high-load booster stage blade at low Re.展开更多
The onset of instability with respect to the spatio-temporally growing disturbance in a viscosity-stratified two-layer liquid film flow is analyzed. The known results obtained from the temporal theory of instability s...The onset of instability with respect to the spatio-temporally growing disturbance in a viscosity-stratified two-layer liquid film flow is analyzed. The known results obtained from the temporal theory of instability show that the flow is unstable in the limit of zero Reynolds numbers. The present theory predicts the neutral stability in the same limit. The discrepancy is explained. Based on the mechanical energy equation, a new mechanism of instability is found. The new mechanism is associated with the convective nature of the disturbance that is not Galilei invariant.展开更多
基金Supported by the National Nature Science Foundation of China(10472046)the Scientific Innova-tion Research of College Graduate in Jiangsu Province(CX08B-035Z)the Innovation and Excellence Foundation of Doctoral Dissertation of Nanjing University of Aeronautics and Astronautics(BCXJ08-01)~~
文摘A two-dimensional（2-D） incompressible plane jet is investigated using the lattice Boltzmann method（LBM） for low Reynolds numbers of 42 and 65 based on the jet-exit-width and the maximum jet-exit-velocity. The results show that the mean centerline velocity decays as x-1/3 and the jet spreads as x2/3 in the self-similar region, which are consistent with the theoretical predictions and the experimental data. The time histories and PSD analyses of the instantaneous centerline velocities indicate the periodic behavior and the interaction between periodic components of velocities should not be neglected in the far field region, although it is invisible in the near field region.
基金a Multidisciplinary University Research Initiative (MURI) project sponsored by AFOSR
文摘Micro air vehicles （MAV＇s） have the potential to revolutionize our sensing and information gathering capabilities in environmental monitoring and homeland security areas. Due to the MAV＇s＇ small size, flight regime, and modes of operation, significant scientific advancement will be needed to create this revolutionary capability. Aerodynamics, structural dynamics, and flight dynamics of natural flyers intersects with some of the richest problems in MAV＇s, inclu- ding massively unsteady three-dimensional separation, transition in boundary layers and shear layers, vortical flows and bluff body flows, unsteady flight environment, aeroelasticity, and nonlinear and adaptive control are just a few examples. A challenge is that the scaling of both fluid dynamics and structural dynamics between smaller natural flyer and practical flying hardware/lab experiment （larger dimension） is fundamentally difficult. In this paper, we offer an overview of the challenges and issues, along with sample results illustrating some of the efforts made from a computational modeling angle.
基金The project supported by the National Natural Science Foundation of China (19725210)
文摘The aerodynamic force and flow structure of NACA 0012 airfoil performing an unsteady motion at low Reynolds number (Re = 100) are calculated by solving Navier-Stokes equations. The motion consists of three parts: the first translation, rotation and the second translation in the direction opposite to the first. The rotation and the second translation in this motion are expected to represent the rotation and translation of the wing-section of a hovering insect. The flow structure is used in combination with the theory of vorticity dynamics to explain the generation of unsteady aerodynamic force in the motion. During the rotation, due to the creation of strong vortices in short time, large aerodynamic force is produced and the force is almost normal to the airfoil chord. During the second translation, large lift coefficient can be maintained for certain time period and (C) over bar (L), the lift coefficient averaged over four chord lengths of travel, is larger than 2 (the corresponding steady-state lift coefficient is only 0.9). The large lift coefficient is due to two effects. The first is the delayed shedding of the stall vortex. The second is that the vortices created during the airfoil rotation and in the near wake left by previous translation form a short 'vortex street' in front of the airfoil and the 'vortex street' induces a 'wind'; against this 'wind' the airfoil translates, increasing its relative speed. The above results provide insights to the understanding of the mechanism of high-lift generation by a hovering insect.
基金supported by National Hi-tech Research and Development Program of China(863 Program, Grant No. 2006AA04Z252)National Natural Science Foundation of China(Grant No. 51005006)+1 种基金Research Fund for the Doctoral Program of Higher Education of China(Grand No. 20101102110022)Innovation Foundation of Beihang University for PhD Graduates, China
文摘Spanwise flexibility is a key factor influencing propulsion performance of pectoral foils. Performances of bionic fish with oscillating pectoral foils can be enhanced by properly selecting the spanwise flexibility. The influence law of spanwise flexibility on thrust generation and propulsion efficiency of a rectangular hydro-foil is discussed. Series foils constructed by the two-component silicon rubber are developed. NACA0015 shape of chordwise cross-section is employed. The foils are strengthened by fin rays of different rigidity to realize variant spanwise rigidity and almost the same chordwise flexibility. Experiments on a towing platform developed are carried out at low Reynolds numbers of 10 000, 15 000, and 20 000 and Strouhal numbers from 0.1 to 1. The following experimental results are achieved: (1) The average forward thrust increases with the St number increased; (2) Certain degree of spanwise flexibility is beneficial to the forward thrust generation, but the thrust gap is not large for the fins of different spanwise rigidity; (3) The fin of the maximal spanwise flexibility owns the highest propulsion efficiency; (4) Effect of the Reynolds number on the propulsion efficiency is significant. The experimental results can be utilized as a reference in deciding the spanwise flexibility of bionic pectoral fins in designing of robotic fish prototype propelled by flapping-wing.
基金financially supported by Joint Key Funds of Zhejiang Provincial Natural Science Foundation of China and Powerchina Huadong Engineering Corporation Limited(Grant No.LHZ19E090004)the National Key R&D Program of China(Grant No.2018YFD0900901)
文摘A series of three-dimensional numerical simulations is carried out to investigate the effect of inclined angle on flow behavior behind two side-by-side inclined cylinders at low Reynolds number Re=100 and small spacing ratio T/D=1.5 (T is the center-to-center distance between two side-by-side cylinders, D is the diameter of cylinder). The instantaneous and time-averaged flow fields, force coefficients and Strouhal numbers are analyzed. Special attention is focused on the axial flow characteristics with variation of the inclined angle. The results show that the inclined angle has a significant effect on the gap flow behaviors behind two inclined cylinders. The vortex shedding behind two cylinders is suppressed with the increase of the inclined angle as well as the flip-flop gap flow. Moreover, the mean drag coefficient, root-mean-square lift coefficient and Strouhal numbers decrease monotonously with the increase of the inclined angle, which follows the independent principle at small inclined angles.
基金The project supported by the National Natural Science Foundation of China10232010The project supported by the National Natural Science Foundation of China10032060The project supported by the National Natural Science Foundation of China90605005
文摘On the basis of the studies on the high unsteady aerodynamic mechanisms of the fruit fly hovering the aerodynamic advantages and disadvantages of the fruit fly flapping motion were analyzed. A new bionic flapping motion was proposed to weaken the disadvantages and maintain the advantages, it may be used in the designing and manufacturing of the micro air vehicles （MAV＇s）. The translation of the new bionic flapping motion is the same as that of fruit fly flapping motion. However, the rotation of the new bionic flapping motion is different. It is not a pitching-up rotation as the fruit fly flapping motion, but a pitching-down rota- tion at the beginning and the end of a stroke. The numerical method of 3rd-order Roe scheme developed by Rogers was used to study these questions. The correctness of the numerical method and the computational program was justified by comparing the present CFD results of the fruit fly flapping motion in three modes, i.e., the advanced mode, the symmetrical mode and the delayed mode, with Dickinson＇s experimental results. They agreed with each other very well. Subsequently, the aerodynamic characteristics of the new bionic flapping motion in three modes were also numerically simulated, and were compared with those of the fruit fly flap- ping. The conclusions could be drawn that the high unsteady lift mechanism of the fruit fly hovering is also effectively utilized by this new bionic flapping. Compared with the fruit fly flapping, the unsteady drag of the new flapping decreases very much and the ratio of lift to drag increases greatly. And the great discrepancies among the mean lifts of three flapping modes of the fruit fly hovering are effectively smoothed in the new flapping. On the other hand, this new bionic flapping motion should be realized more easily. Finally, it must be pointed out that the above conclusions were just drawn for the hovering flapping motion. And the aerodynamic characteristics of the new bionic flapping motion in forward flight are going to be studied in the next step.
文摘High-speed rotor rotation under the low-density condition creates a special low-Reynolds compressible flow around the rotor blade airfoil where the compressibility effect on the laminar separated shear layer occurs. However, the compressibility effect and shock wave generation associated with the increase in the Mach number (M) and the trend change due to their interference have not been clarified. The purpose is to clear the compressibility effect and its impact of shock wave generation on the flow field and aerodynamics. Therefore, we perform a two-dimensional unsteady calculation by Computational fluid dynamics (CFD) analysis using the CLF5605 airfoil used in the Mars helicopter Ingenuity, which succeeded in its first flight on Mars. The calculation conditions are set to the Reynolds number (Re) at 75% rotor span in hovering (Re = 15,400), and the Mach number was varied from incompressible (M = 0.2) to transonic (M = 1.2). The compressible fluid dynamics solver FaSTAR developed by the Japan aerospace exploration agency (JAXA) is used, and calculations are performed under multiple conditions in which the Mach number and angle of attack (α) are swept. The results show that a flow field is similar to that in the Earth’s atmosphere above M = 1.0, such as bow shock at the leading edge, whereas multiple λ-type shock waves are observed over the separated shear layer above α = 3° at M = 0.80. However, no significant difference is found in the C<sub>p</sub> distribution around the airfoil between M = 0.6 and M = 0.8. From the results, it is found that multiple λ-type shock waves have no significant effect on the airfoil surface pressure distribution, the separated shear layer effect is dominant in the surface pressure change and aerodynamic characteristics.
文摘This study experimentally investigates aerodynamic characteristics and flow fields of a smooth owl-like airfoil without serrations and velvet structures. This biologically inspired airfoil design is intended to serve as the main-wing for low-Reynolds-number aircrafts such as micro air vehicles. Reynolds number dependency on aerodynamics is also evaluated at low Reynolds numbers. The results of the study show that the owl-like airfoil has high lift performance with a nonlinear lift increase due to the presence of a separation bubble on the suction side. A distinctive flow feature of the owl airfoil is a separation bubble on the pressure side at low angles of attack. The separation bubble switches location from the pressure side to the suction side as the angle of attack increases and is continuously present on the surface within a wide range of angles of attack. The Reynolds number dependency on the lift curves is insignificant, although differences in the drag curves are especially pronounced at high angles of attack. Eventually, we obtain the geometric feature of the owl-like airfoil to increase aerodynamic performance at low Reynolds numbers.
基金Supported by the National Natural Science Foundation of China(10472046)the Priority Academic Program Development of Jiangsu Higher Education Institutions+1 种基金the Postgraduate Research and Innovation Project of Jiangsu Province(CX08B-035Z)the PhD Thesis Innovation and Excellence Fund of Nanjing University of Aeronautics and Astronautics(BCXJ08-01)
文摘Oscillation phenomena in far field region of plane jets are studied by lattice Boltzmann method over a range of Reynolds numbers （Re） from 16 to 65. Numerical results show that the instantaneous centerline velocities show periodic oscillation behavior in far field region when Re〉38. In contrast, the periodic behavior is invisible in corresponding flow field when Re≤38. For the cases of Re≤38, the exchange of momentum due to straining mo- tion gradually dominates the downstream flow filed, which qualitatively suggests the possibility of iet instability.
基金supported by the National Natural Science Foundation of China(No.11402124)the Natural Science Foundation of Jiangsu Province(No.BK20140985)+1 种基金the Natural Science Foundation of Jiangsu Higher Education Institutions of China(No.14KJB130002)the Startup Foundation for Introducing Talent of NUIST(No.2013x031)
文摘A dual-jet consisting of a wall jet and an offset jet has been numerically simulated using lattice Boltzmann method to examine the effects of jet spacing between two jet centerlines,defined as s.The Reynolds number based on jet-exit-width dis set to be Re = 56 and the jet spacing is set to be less than or equal 10 times the jet-exitwidth.Computational results reveal that the flow field displays periodic vortex shedding when the jet spacing is in the range of 9≤s/d ≤ 10,while it remains steady with two counter-rotating vortices in the converging region when s/d ≤ 8.When s/d = 9,the power spectral analyses indicate that the vortex shedding phenomenon has specific frequency.The significant oscillation stresses induced by the periodic components of velocities are found to mainly exist in the inner shear layer regions,implying stronger momentum transfer occuring in these regions.
基金financially supported by the National Science and Technology Major Project(2019-Ⅱ-0004-0024)Youth Innovation Promotion Association CAS(No.2020148)。
文摘To achieve high-performance compressor cascades at low Reynolds number(Re),it is important to organize the boundary layer transition and separation processes efficiently and reasonably.In this study,the airfoil is focused on at a 5%blade height at the root of the orthogonal blade in the downflow passage of the high-load booster stage.The bionics modeling design is carried out for the leading edge of the original blade cascade;the response characteristics of laminar transition and separation to blades with different leading edge shapes at low Reynolds numbers are studied by using large eddy simulations combined with Omega vortex identification.The findings of this study demonstrate that bionic leading edge modeling can significantly improve the aerodynamic performance of blades at low Reynolds numbers.The blades effectively suppress the formation of separation bubbles at low Reynolds numbers and weaken or even eliminate large-scale flow separation at the trailing edge.In addition,the blades can weaken the vortex intensity on the blade surface,reduce the areas of high-velocity fluctuations,and minimize aerodynamic losses caused by turbulence dissipation.These results should serve as a valuable reference for the aerodynamic design and flow control of the high-load booster stage blade at low Re.
基金supported by the National Natural Science Foundation of China (Nos. 10702038 and 10772107)the National Science Foundation of USA (No. CTS-0138057)the Shanghai Leading Academic Discipline Project (No. Y0103)
文摘The onset of instability with respect to the spatio-temporally growing disturbance in a viscosity-stratified two-layer liquid film flow is analyzed. The known results obtained from the temporal theory of instability show that the flow is unstable in the limit of zero Reynolds numbers. The present theory predicts the neutral stability in the same limit. The discrepancy is explained. Based on the mechanical energy equation, a new mechanism of instability is found. The new mechanism is associated with the convective nature of the disturbance that is not Galilei invariant.
