Synthetic analysis is conducted to the wind tunnel experiment results of zero lift drag coefficient and lift coefficient for large aspect ratio winged rigid body.By means of wind tunnel experiment data,the dynamics mo...Synthetic analysis is conducted to the wind tunnel experiment results of zero lift drag coefficient and lift coefficient for large aspect ratio winged rigid body.By means of wind tunnel experiment data,the dynamics model of the zero lift drag coefficient and lift coefficient for the large aspect ratio winged rigid body is amended.The research indicates that the change trends of zero lift drag coefficient and lift coefficient to Mach number are similar.The calculation result and wind tunnel experiment data all verify the validity of the amended dynamics model by which to estimate the zero lift drag coefficient and lift coefficient for the large aspect ratio winged rigid body,and thus providing some technical reference to aerodynamics character analysis of the same types of winged rigid body.展开更多
To analyze the influence of the chord length ratio and angle of attack on lift coefficients and explore the interaction mechanism between the two,we established a calculation model of the pressure distribution coeffic...To analyze the influence of the chord length ratio and angle of attack on lift coefficients and explore the interaction mechanism between the two,we established a calculation model of the pressure distribution coefficient on the airfoil surface and lift coefficient of a dual-wing sail on the basis of the vortex panel method.Computational fluid dynamics was used in auxiliary calculation and analysis.Results revealed a reciprocal interference between the front-wing and rear-wing sails.The total lift coefficient of the dual-sail increased with an increase in the front sail chord length.The lift coefficient of the rear sail decreased with an increase in the front sail chord length or angle of attack.The front sail wake affected the pressure distribution on the upper and lower surfaces of the rear sail leading edge.展开更多
Sail is the core part of autonomous sailboat and wing sail is a new type of sail. Wing sail generates not only propulsion but also lateral force and heeling moment. The latter two will affect the navigation status and...Sail is the core part of autonomous sailboat and wing sail is a new type of sail. Wing sail generates not only propulsion but also lateral force and heeling moment. The latter two will affect the navigation status and bring resistance. Double sail can effectively reduce the center of wind pressure and heeling moment. In order to study the effect of distance between two sails, airfoil and attack angle on the total lift coefficient of double sail propulsion system, pressure coefficient distribution and lift coefficient calculation model have been established based on vortex panel method. By using the basic finite solution, the fluid dynamic forces on the two-dimensional sails are computed.The results show that, the distance in the range of 0 to 1 time chord length, when using the same airfoil in the fore and aft sail, the total lift coefficient of the double sail increases with the increase of distance, finally reaches a stable value in the range of one to three times chord length. Lift coefficients of thicker airfoils are more sensitive to the change of distance. The thicker the airfoil, the longer distance is required of the total lift coefficient toward stable.When different airfoils are adopted in fore and aft sail, the total lift coefficient increases with the increase of the thickness of aft sail. The smaller the thickness difference is, the more sensitive to the distance change the lift coefficient is. The thinner the fore sail is, the lower the influence will be on the lift coefficient of aft sail.展开更多
Swept wing is widely used in civil aircraft,whose airfoil is chosen,designed and optimized to increase the cruise speed and decrease the drag coefficient.The parameters of swept wing,such as sweep angle and angle of a...Swept wing is widely used in civil aircraft,whose airfoil is chosen,designed and optimized to increase the cruise speed and decrease the drag coefficient.The parameters of swept wing,such as sweep angle and angle of attack,are determined according to the cruise lift coefficient requirement,and the drag coefficient is expected to be predicted accurately,which involves the instability characteristics and transition position of the flow.The pressure coefficient of the RAE2822 wing with given constant lift coefficient is obtained by solving the three-dimensional Navier-Stokes equation numerically,and then the mean flow is calculated by solving the boundary layer（BL） equation with spectral method.The cross-flow instability characteristic of boundary layer of swept wing in the windward and leeward is analyzed by linear stability theory（LST）,and the transition position is predicted by eNmethod.The drag coefficient is numerically predicted by introducing a laminar/turbulent indicator.A simple approach to calculate the lift coefficient of swept wing is proposed.It is found that there is a quantitative relationship between the angle of attack and sweep angle when the lift coefficient keeps constant;when the angle of attack is small,the flow on the leeward of the wing is stable.when the angle of attack is larger than 3°,the flow becomes unstable quickly;with the increase of sweep angle or angle of attack the disturbance on the windward becomes more unstable,leading to the moving forward of the transition position to the leading edge of the wing;the drag coefficient has two significant jumping growth due to the successive occurrence of transition in the windward and the leeward;the optimal range of sweep angle for civil aircraft is suggested.展开更多
In this paper, Lorentz forces are proved to be able to suppress separation in flows over hydrofoils. Furthermore, a differential equation of pressure distributions on the hydrofoil surface is derived, from which it is...In this paper, Lorentz forces are proved to be able to suppress separation in flows over hydrofoils. Furthermore, a differential equation of pressure distributions on the hydrofoil surface is derived, from which it is found that BVF （boundary vortex flux） σ is a suitable criterion for describing the lift coefficient variations during the electromagnetic control process. According to our numerical results, the periodic variations of lift for a hydrofoil at an attack angle of 17 ° are analyzed and its inherent mechanism is discussed in detail with the concept of BVE On the other hand, the effects of Lorentz force on the hydrofoil＇s lift are investigated both experimentally and numerically for different magnitudes and locations.展开更多
Cylindrical cross sections are critical components in offshore structures, including jacket platform legs, pipelines, mooring lines, and risers. Thesecylindrical structures are subjected to vortex-induced vibrations (...Cylindrical cross sections are critical components in offshore structures, including jacket platform legs, pipelines, mooring lines, and risers. Thesecylindrical structures are subjected to vortex-induced vibrations (VIV) due to strong ocean currents, where vortices generated during fluid flowresult in significant vibrations in crossflow and in-flow directions. Such vibrations can lead to severe damage to platforms, cables, and risersystems. Consequently, mitigating VIV caused by vortex-induced forces is important. This study investigates the hydrodynamic performance offive strake models relative to a bare cylinder at moderate Reynolds numbers. The models encompass one conventional continuous helical strake(HS) and four helical discrete strake (HDS) with varying segment spacing between the fins. The hydrodynamic performance, specifically liftand drag force coefficients, was computed using a Reynolds averaged Navier –Stokes-based CFD solver and validated with experimentalmeasurements. The conventional HS suppresses 95% of the lift force but increases the drag force by up to a maximum of 48% in measurements.The HDS suppress the lift force by 70%–88% and increase the drag force by 15%–30%, which is less than the increase observed with the HS.Flow visualization showed that HS and HDS cylinders mitigate vortex-induced forces by altering the vortex-shedding pattern along the length ofthe cylinder. The HDS achieves a reduction in drag compared with the conventional continuous HS. The segment spacing is found to significantlyimpact the reduction in vortex-induced forces.展开更多
The drag and lift forces acting on a rotational spherical particle in a logarithmic boundary flow are numerically studied. The effects of the drag velocity and rotational speed of the sphere on the drag force are exam...The drag and lift forces acting on a rotational spherical particle in a logarithmic boundary flow are numerically studied. The effects of the drag velocity and rotational speed of the sphere on the drag force are examined for the particle Reynolds number from 50 to 300 and for the dimensionless rotational angular speed of 0≤Ω≤1.0. The influence of dimensionless roughness height Z0 of the wall is also evaluated for z0 ≤ 10. The results show that the drag forces on a sphere both in a logarithmic flow and in a uniform unsheared flow increase with the increase of the drag velocity. For 50≤Rep≤300, -↑CD increases with decreased roughness height z0. The time-averaged drag coefficient is also significantly affected by rotational speed of the sphere and roughness height zo. The lift coefficient -↑CL increases with increased rotational speed and decreases with increased roughness height.展开更多
A study of the effects of passive vortex generators （VGs） on Aludra unmanned aerial vehicle （UAV） aerodynamic characteristics is presented. Both experimental and numerical works are carried out where an array of V...A study of the effects of passive vortex generators （VGs） on Aludra unmanned aerial vehicle （UAV） aerodynamic characteristics is presented. Both experimental and numerical works are carried out where an array of VGs is attached on Aludra UAV＇s wing. The flow measurements are made at various angles of attack by using 3-axis component balance system. In the numerical investigation, the Reynolds-averaged Navier-Stokes （RANS） code FLUENT 6.3TM is used in the simulations with fully structured mesh with Spalart-Allmaras （S-A） turbulence model and standard wall function. The comparison between the experimental and numerical results reveals a satisfactory agreement. The parametric study shows that higher maximum lift coefficient is achieved when the VGs are placed nearer to the separation point. In addition to this, shorter spanwise distance between the VGs also increases the maximum lift coefficient, rectangular and curve-edge VG performs better than triangular VG.展开更多
The lattice Boltzmann method (LBM) is employed to simulate the uniform flow past a circular cylinder. The performance of the two-dimensional LBM model on the prediction of force coefficients and vortex shedding freque...The lattice Boltzmann method (LBM) is employed to simulate the uniform flow past a circular cylinder. The performance of the two-dimensional LBM model on the prediction of force coefficients and vortex shedding frequency is investigated. The local grid refinement technique and second-order boundary condition for curved walls are applied in the calculations. It is found that the calculated vortex shedding frequency, drag coefficient and lift coefficient are consistent with experimental results at Reynolds nu...展开更多
The flow past a primary cylinder with one tandem control rod and one staggered control rod is simulated in this paper through solving the Navier-Stokes equations. Two examples are simulated to validate the model, and ...The flow past a primary cylinder with one tandem control rod and one staggered control rod is simulated in this paper through solving the Navier-Stokes equations. Two examples are simulated to validate the model, and the results matched well with those of previous researches. The Reynolds number based on the diameter of the primary cylinder is 500. The diameter ratio between the control rod and the primary cylinder （d/D） is 0.25. It was found that the effect of the combination of one upstream tandem control rod and one staggered control rod on the hydrodynamics of the primary cylinder is a linear superposition of the effect of a corresponding single control rod, and the effect of the upstream tandem control rod is dominant at larger spacing ratios such as G/D=2. For the combination of a downstream tandem control rod and a staggered control rod, the effect of the control rods is different from that of the corresponding single control rod in the region of 0.2〈G/D〈0.5 ＆ 30°〈a〈120° and 0.9〈G/D〈1.4 ＆ 30°〈a〈50°, where the additional effect is obvious. In this case, the effect of the downstream tandem control rod is dominant at small spacing ratios （such as G/D=0.1）. At moderate spacing ratios such as G/D=0.4, the effects of the tandem control rod and the staggered control rod are comparable in both cases.展开更多
Using ANSYS-CFX, a general purpose fluid dynamics program, the vortex-induced vibration(VIV) of a variable cross-section cylinder is simulated under uniform current with high Reynolds numbers. Large eddy simulation(LE...Using ANSYS-CFX, a general purpose fluid dynamics program, the vortex-induced vibration(VIV) of a variable cross-section cylinder is simulated under uniform current with high Reynolds numbers. Large eddy simulation(LES) is conducted for studying the fluid-structure interaction. The vortex shedding in the wake, the motion trajectories of a cylinder, the variation of drag and lift forces on the cylinder are analyzed. The results show that the vortices of variable cross-section cylinder are chaotic and are varying along the cylinder. In places where cross-sections are changing significantly, the vortices are more irregular. The motion trail of the cylinder is almost the same but irregular. The drag and lift coefficients of the cylinder are varying with the changes of diameters.展开更多
Large eddy simulation cooperated with the second order full extension ETG(Euler-Taylor-Galerkin) finite element method was applied to simulate the flow around two square cylinders arranged side by side at a spacing ra...Large eddy simulation cooperated with the second order full extension ETG(Euler-Taylor-Galerkin) finite element method was applied to simulate the flow around two square cylinders arranged side by side at a spacing ratio of (1.5.) The second order full extension ETG finite element method was developed by Wang and He. By means of Taylor expansion of terms containing time derivative, time derivative is replaced by space derivative. The function of it is equal to introducing an artificial viscosity term. The streamlines of the flow at different moments were obtained. The time history of drag coefficient, lift coefficient and the streamwise velocity on the symmetrical points were presented. Furthermore, the symmetrical problem of the frequency spectrum of flow around two square cylinders arranged side by side were studied by using the spectral analysis technology. The data obtained at the initial stage are excluded in order to avoid the influence of initial condition on the results. The power spectrums of drag coefficient, lift coefficient, the streamwise velocity on the symmetrical points were analyzed respectively. The results show that although the time domain process of dynamic parameters is non-symmetrical, the frequency domain process of them is symmetrical under the symmetrical boundary conditions.展开更多
A dual-time method is introduced to calculate the unsteady flow in a certain vibrating flat cascade. An implicit lower-upper symmetric-gauss-seidel scheme（LU-SGS） is applied for time stepping in pseudo time domains,...A dual-time method is introduced to calculate the unsteady flow in a certain vibrating flat cascade. An implicit lower-upper symmetric-gauss-seidel scheme（LU-SGS） is applied for time stepping in pseudo time domains, and the convection items are discretized with the spatial three-order weighted non-oscillatory and non-free-parameter dissipation difference （WNND） scheme. The turbulence model adopts q-co low-Reynolds-number model. The frequency specmuns of lift coefficients and the unsteady pressure-difference coefficients at different spanwise heights as well as the entropy contours at blade tips on different vibrating instants, are obtained. By the analysis of frequency specmuns of lift coefficients at three spanwise heights, it is considered that there exist obvious non-linear perturbations in the flow induced by the vibrating, and the perturbation frequencies are higher than the basic frequency. The entropy contours at blade tips at different times display an intensively unsteady attribute of the flow under large amplitudes.展开更多
As the transport sector is responsible for the consumption of a vast proportion of the oil produced,it is mandatory to research feasible solutions to tackle this issue.The appli-cation of aerodynamic attachments for p...As the transport sector is responsible for the consumption of a vast proportion of the oil produced,it is mandatory to research feasible solutions to tackle this issue.The appli-cation of aerodynamic attachments for passiveflow control and reducing resisting aerodynamic forces such as drag and lift is one of the most practicable ways to minimize vehicle energy con-sumption.Theflaps are one of the most innovative aerodynamic attachments that can enhance theflow motion in the boundary layer at the trailing edge of the wings.In the present paper,theflap is designed and modeled for controlling the airflow at the roof-end of a 2D Ahmed body model,inspired by the schematic of theflap at the trailing edge of the wing.As a result,theflap’s geometry and position from the roof-end of the car model are parameterized,which leads to having four design variables.The objective functions of the present study are the vehicle’s drag coefficient and lift coefficient.25 Design of Experiment(DOE)points are considered enabling the Box-Behnken method.Then,each DOE point is modeled in the computational domain,and theflow-field around the model is simulated using Ansys Fluent software.The results obtained for the DOE points are employed by different regressors,and the relation between design variables and objective functions is extracted using GMDH-ANN.The GMDH-ANN is then coupled with three types of optimization algorithms,among which the Genetic algorithm proves to have the most ideal coupling process for optimization.Finally,af-ter analyzing the variations in the geometry and position of the roofflap from the car roof-end,the roof-flap with specifications of L=0.1726 m,a=5.0875°,H=0.0188 m,and d=0.241 m can optimize the car drag and lift coefficients by 21.27% and 19.91%,respec-tively.The present research discusses the opportunities and challenges of optimal design roof-flap geometry and its influence on car aerodynamic performance.展开更多
We report the results of accurate prediction of lift(C L)and drag(C D)coefficients of two typical airfoil flows(NACA0012 and RAE2822)by a new algebraic turbulence model,in which the eddy viscosity is specified by a st...We report the results of accurate prediction of lift(C L)and drag(C D)coefficients of two typical airfoil flows(NACA0012 and RAE2822)by a new algebraic turbulence model,in which the eddy viscosity is specified by a stress length(SL)function predicted by structural ensemble dynamics(SED)theory.Unprecedented accuracy of the prediction of C D with error of a few counts(one count is 10−4)and of C L with error under 1%-2%are uniformly obtained for varying angles of attack(AoA),indicating an order of magnitude improvement of drag prediction accuracy compared to currently used models(typically around 20 to 30 counts).More interestingly,the SED-SL model is distinguished with fewer parameters of clear physical meaning,which quantify underlying turbulent boundary layer(TBL)with a universal multi-layer structure,and is thus promising to be more easily generalizable to complex TBL.The use of the new model for the calibration of flow condition in experiment and the extraction of flow physics from numerical simulation data of aeronautic flows are discussed.展开更多
In view of the fact that large scale vortices play the substantial role of momentum transport in turbulent flows, large eddy simulation （LES） is considered as a better simulation model. However, the sub-grid scale ...In view of the fact that large scale vortices play the substantial role of momentum transport in turbulent flows, large eddy simulation （LES） is considered as a better simulation model. However, the sub-grid scale （SGS） models reported so far have not ascertained under what flow conditions the LES can lapse into the direct nu-merical simulation. To overcome this discrepancy, this paper develops a swirling strength based the SGS model to properly model the turbulence intermittency, with the primary characteristics that when the local swirling strength is zero, the local sub-grid viscosity will be vanished. In this paper, the model is used to investigate the flow characteris-tics of zero-incident incompressible turbulent flows around a single square cylinder （SC） at a low Reynolds number range Re ∈ [103, 104]. The flow characteristics investigated include the Reynolds number dependence of lift and drag coefficients, the distributions of time-spanwise averaged variables such as the sub-grid viscosity and the logarithm of Kolmogorov micro-scale to the base of 10 at Re=2 500 and 104, the contours of spanwise and streamwise vorticity components at t = 170. It is revealed that the peak value of sub-grid viscosity ratio and its root mean square （RMS） values grow with the Reynolds number. The dissipation rate of turbulent kinetic energy is larger near the SC solid walls. The instantaneous factor of swirling strength intermittency （FSI） exhibits some laminated structure involved with vortex shedding.展开更多
This study investigates the aerodynamic characteristics of a low-Reynolds-number airfoil at high angles of attack(AoA)from 0°to 90°,focusing on their relevance for micro and unmanned aerial vehicle(MAV/UAV)a...This study investigates the aerodynamic characteristics of a low-Reynolds-number airfoil at high angles of attack(AoA)from 0°to 90°,focusing on their relevance for micro and unmanned aerial vehicle(MAV/UAV)applications.Simulations are conducted using the k-ωshear stress transport(SST)turbulence model using ANSYS Fluent software.Among the key findings is that the lift coefficient CL increases from 1.2981 at 0°AoA to a peak of 2.034 at 11°before decreasing to 1.51 at 90°,indicating initial lift improvement followed by a reduction due to potential flow separation or stall.The drag coefficient CD increases from 0.0222 at 0°AoA to a peak of 0.3572 at 12°,and then decreases to 0.0467 at 90°,indicating initially increasing turbulence and separation,followed by stabilization in the flow regime.The lift-to-drag ratio L/D reaches its maximum of 32.334 at 90°AoA,highlighting improved aerodynamic efficiency at higher AoAs despite increased drag.The skin friction coefficient Cf shows a maximum of 0.046918 at the leading edge at 30°AoA and 0.0394262 at the trailing edge at 90°,indicating critical points of frictional drag.Additionally,the turbulence viscosity ratio at the LE peaks at 0.5586 at 30°AoA and drops to 0.004 at 90°,while it increases at the trailing edge,reaching 0.0394262 at 90°,showing heightened turbulence effects at high AoAs.The present numerical study,however,determines the lift coefficient to be 2.00.This yields a maximum percentage variation of 11.5%compared with the value in the literature.These results provide a comprehensive overview of how high-AoA conditions impact aerodynamic performance,offering valuable insights for optimizing airfoil design and improving MAV/UAV efficiency.展开更多
The 2D incompressible Navier-Stokes equations arc solved based on the finite Flexible structure;Airfoil;Lock-in phenomenon;Lift coefficient;volume method and dynamic mesh technique is used to carry out partial fluid s...The 2D incompressible Navier-Stokes equations arc solved based on the finite Flexible structure;Airfoil;Lock-in phenomenon;Lift coefficient;volume method and dynamic mesh technique is used to carry out partial fluid structure interaction.The local flexible structure(hereinafter termed as flexible structure)vibrates in a single mode located on the upper surface of the airfoil.The Influence of vibration frequency and amplitude are examined and the corresponding fluid flow characteristics are investigated Computational fluid dynamics(CFD)which add complexity to the inherent problem in unsteady flow.The study is conducted for flow over NACA0012 airfoil at 600≤Re≤3000 at a low angle of attack.Vibration of flexible structure induces a secondary vortex which modifies the pressure distribution and lift performance of the airfoil.At some moderate vibration amplitude,frequency synchronization frequency of rigid airfoil.Evolution and shedding of vortices corresponding to the deformation of flexible structure depends on the Reynolds number.In the case of Re≤1000,the deformation of flexible structure is considered in-phase with the vortex shedding i.e.,increasing maximum lift is linked with the positive deformation of flexible structure.At Re=1500 a phase shift of about 1/π exists while they are out-of-phase at Re>1500.Moreover,the oscillation amplitude of lift coefficient increases with increasing vibration amplitude for Re£1500 while it decreases with increasing vibration amplitude for Re>1500.As a result of frequency lock-in,the average lift coefficient is increased with increasing vibration amplitude for all investigated Reynolds numbers(Re).The maximum increase in the average liftcoefficient is 19.72% within the range of investigated parameters.展开更多
To control secondary flows, streamwise fences were attached to end wall of a linear turbine rotor cascade. The cascade had 8 blades of 400 mm long and 175 mm chord. The blades deflected the flow by 120°. The fenc...To control secondary flows, streamwise fences were attached to end wall of a linear turbine rotor cascade. The cascade had 8 blades of 400 mm long and 175 mm chord. The blades deflected the flow by 120°. The fences were made out of 0.7 mm thick brass sheet and the heights of the fences were 14 mm, 18 mm respectively. The curvature of the fences was the same as that of the blade camber line. The fences were fixed normal to the end wall and at half pitch away from the blades. The experimental program consists of total pressure, static pressure measurements at the inlet and outlet of the cascade, by using five-hole probe. In addition, static pressure on the blade suction surface and pressure surface was also obtained. Fences are effective in preventing the movement of the pressure side leg of the horseshoe vortex. Consequently the accumulation of low energy fluid on the suction surface is minimised. End wall losses are reduced by the fences due to weakening of the end wall cross flow.展开更多
文摘Synthetic analysis is conducted to the wind tunnel experiment results of zero lift drag coefficient and lift coefficient for large aspect ratio winged rigid body.By means of wind tunnel experiment data,the dynamics model of the zero lift drag coefficient and lift coefficient for the large aspect ratio winged rigid body is amended.The research indicates that the change trends of zero lift drag coefficient and lift coefficient to Mach number are similar.The calculation result and wind tunnel experiment data all verify the validity of the amended dynamics model by which to estimate the zero lift drag coefficient and lift coefficient for the large aspect ratio winged rigid body,and thus providing some technical reference to aerodynamics character analysis of the same types of winged rigid body.
基金the Foundation of State Key Laboratory of Robotics(No.2020-Z14)the Jiang Xin-song Innovation Foundation(No.Y8F7010701)+1 种基金the National Natural Science Foundation of China(No.41906173)the China Postdoctoral Science Foundation(No.2019M662874)。
文摘To analyze the influence of the chord length ratio and angle of attack on lift coefficients and explore the interaction mechanism between the two,we established a calculation model of the pressure distribution coefficient on the airfoil surface and lift coefficient of a dual-wing sail on the basis of the vortex panel method.Computational fluid dynamics was used in auxiliary calculation and analysis.Results revealed a reciprocal interference between the front-wing and rear-wing sails.The total lift coefficient of the dual-sail increased with an increase in the front sail chord length.The lift coefficient of the rear sail decreased with an increase in the front sail chord length or angle of attack.The front sail wake affected the pressure distribution on the upper and lower surfaces of the rear sail leading edge.
基金financially supported by the JIANG Xinsong Innovation Fund(Grant No.Y8F7010701)
文摘Sail is the core part of autonomous sailboat and wing sail is a new type of sail. Wing sail generates not only propulsion but also lateral force and heeling moment. The latter two will affect the navigation status and bring resistance. Double sail can effectively reduce the center of wind pressure and heeling moment. In order to study the effect of distance between two sails, airfoil and attack angle on the total lift coefficient of double sail propulsion system, pressure coefficient distribution and lift coefficient calculation model have been established based on vortex panel method. By using the basic finite solution, the fluid dynamic forces on the two-dimensional sails are computed.The results show that, the distance in the range of 0 to 1 time chord length, when using the same airfoil in the fore and aft sail, the total lift coefficient of the double sail increases with the increase of distance, finally reaches a stable value in the range of one to three times chord length. Lift coefficients of thicker airfoils are more sensitive to the change of distance. The thicker the airfoil, the longer distance is required of the total lift coefficient toward stable.When different airfoils are adopted in fore and aft sail, the total lift coefficient increases with the increase of the thickness of aft sail. The smaller the thickness difference is, the more sensitive to the distance change the lift coefficient is. The thinner the fore sail is, the lower the influence will be on the lift coefficient of aft sail.
基金co-supported by the National Natural Science Foundation of China(Nos.11332007,11672351)the Tianjin Natural Science Foundation of China(No.15JCYBJC19500)+1 种基金the Hebei Natural Science Foundation of China(No.A2015105073)an open fund from the State Key Laboratory of Aerodynamics of China(No.SKLA201601)
文摘Swept wing is widely used in civil aircraft,whose airfoil is chosen,designed and optimized to increase the cruise speed and decrease the drag coefficient.The parameters of swept wing,such as sweep angle and angle of attack,are determined according to the cruise lift coefficient requirement,and the drag coefficient is expected to be predicted accurately,which involves the instability characteristics and transition position of the flow.The pressure coefficient of the RAE2822 wing with given constant lift coefficient is obtained by solving the three-dimensional Navier-Stokes equation numerically,and then the mean flow is calculated by solving the boundary layer（BL） equation with spectral method.The cross-flow instability characteristic of boundary layer of swept wing in the windward and leeward is analyzed by linear stability theory（LST）,and the transition position is predicted by eNmethod.The drag coefficient is numerically predicted by introducing a laminar/turbulent indicator.A simple approach to calculate the lift coefficient of swept wing is proposed.It is found that there is a quantitative relationship between the angle of attack and sweep angle when the lift coefficient keeps constant;when the angle of attack is small,the flow on the leeward of the wing is stable.when the angle of attack is larger than 3°,the flow becomes unstable quickly;with the increase of sweep angle or angle of attack the disturbance on the windward becomes more unstable,leading to the moving forward of the transition position to the leading edge of the wing;the drag coefficient has two significant jumping growth due to the successive occurrence of transition in the windward and the leeward;the optimal range of sweep angle for civil aircraft is suggested.
文摘In this paper, Lorentz forces are proved to be able to suppress separation in flows over hydrofoils. Furthermore, a differential equation of pressure distributions on the hydrofoil surface is derived, from which it is found that BVF （boundary vortex flux） σ is a suitable criterion for describing the lift coefficient variations during the electromagnetic control process. According to our numerical results, the periodic variations of lift for a hydrofoil at an attack angle of 17 ° are analyzed and its inherent mechanism is discussed in detail with the concept of BVE On the other hand, the effects of Lorentz force on the hydrofoil＇s lift are investigated both experimentally and numerically for different magnitudes and locations.
文摘Cylindrical cross sections are critical components in offshore structures, including jacket platform legs, pipelines, mooring lines, and risers. Thesecylindrical structures are subjected to vortex-induced vibrations (VIV) due to strong ocean currents, where vortices generated during fluid flowresult in significant vibrations in crossflow and in-flow directions. Such vibrations can lead to severe damage to platforms, cables, and risersystems. Consequently, mitigating VIV caused by vortex-induced forces is important. This study investigates the hydrodynamic performance offive strake models relative to a bare cylinder at moderate Reynolds numbers. The models encompass one conventional continuous helical strake(HS) and four helical discrete strake (HDS) with varying segment spacing between the fins. The hydrodynamic performance, specifically liftand drag force coefficients, was computed using a Reynolds averaged Navier –Stokes-based CFD solver and validated with experimentalmeasurements. The conventional HS suppresses 95% of the lift force but increases the drag force by up to a maximum of 48% in measurements.The HDS suppress the lift force by 70%–88% and increase the drag force by 15%–30%, which is less than the increase observed with the HS.Flow visualization showed that HS and HDS cylinders mitigate vortex-induced forces by altering the vortex-shedding pattern along the length ofthe cylinder. The HDS achieves a reduction in drag compared with the conventional continuous HS. The segment spacing is found to significantlyimpact the reduction in vortex-induced forces.
文摘The drag and lift forces acting on a rotational spherical particle in a logarithmic boundary flow are numerically studied. The effects of the drag velocity and rotational speed of the sphere on the drag force are examined for the particle Reynolds number from 50 to 300 and for the dimensionless rotational angular speed of 0≤Ω≤1.0. The influence of dimensionless roughness height Z0 of the wall is also evaluated for z0 ≤ 10. The results show that the drag forces on a sphere both in a logarithmic flow and in a uniform unsheared flow increase with the increase of the drag velocity. For 50≤Rep≤300, -↑CD increases with decreased roughness height z0. The time-averaged drag coefficient is also significantly affected by rotational speed of the sphere and roughness height zo. The lift coefficient -↑CL increases with increased rotational speed and decreases with increased roughness height.
文摘A study of the effects of passive vortex generators （VGs） on Aludra unmanned aerial vehicle （UAV） aerodynamic characteristics is presented. Both experimental and numerical works are carried out where an array of VGs is attached on Aludra UAV＇s wing. The flow measurements are made at various angles of attack by using 3-axis component balance system. In the numerical investigation, the Reynolds-averaged Navier-Stokes （RANS） code FLUENT 6.3TM is used in the simulations with fully structured mesh with Spalart-Allmaras （S-A） turbulence model and standard wall function. The comparison between the experimental and numerical results reveals a satisfactory agreement. The parametric study shows that higher maximum lift coefficient is achieved when the VGs are placed nearer to the separation point. In addition to this, shorter spanwise distance between the VGs also increases the maximum lift coefficient, rectangular and curve-edge VG performs better than triangular VG.
基金Supported by the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20060056036)
文摘The lattice Boltzmann method (LBM) is employed to simulate the uniform flow past a circular cylinder. The performance of the two-dimensional LBM model on the prediction of force coefficients and vortex shedding frequency is investigated. The local grid refinement technique and second-order boundary condition for curved walls are applied in the calculations. It is found that the calculated vortex shedding frequency, drag coefficient and lift coefficient are consistent with experimental results at Reynolds nu...
基金the support from the National Natural Science Foundation of China(Nos.11372188,and 51490674)the National Basic Research Program of China(973 Program)(No.2015CB251203)
文摘The flow past a primary cylinder with one tandem control rod and one staggered control rod is simulated in this paper through solving the Navier-Stokes equations. Two examples are simulated to validate the model, and the results matched well with those of previous researches. The Reynolds number based on the diameter of the primary cylinder is 500. The diameter ratio between the control rod and the primary cylinder （d/D） is 0.25. It was found that the effect of the combination of one upstream tandem control rod and one staggered control rod on the hydrodynamics of the primary cylinder is a linear superposition of the effect of a corresponding single control rod, and the effect of the upstream tandem control rod is dominant at larger spacing ratios such as G/D=2. For the combination of a downstream tandem control rod and a staggered control rod, the effect of the control rods is different from that of the corresponding single control rod in the region of 0.2〈G/D〈0.5 ＆ 30°〈a〈120° and 0.9〈G/D〈1.4 ＆ 30°〈a〈50°, where the additional effect is obvious. In this case, the effect of the downstream tandem control rod is dominant at small spacing ratios （such as G/D=0.1）. At moderate spacing ratios such as G/D=0.4, the effects of the tandem control rod and the staggered control rod are comparable in both cases.
基金supported by the National Natural Science Foundation of China (Nos. 51179179 and 51079136)
文摘Using ANSYS-CFX, a general purpose fluid dynamics program, the vortex-induced vibration(VIV) of a variable cross-section cylinder is simulated under uniform current with high Reynolds numbers. Large eddy simulation(LES) is conducted for studying the fluid-structure interaction. The vortex shedding in the wake, the motion trajectories of a cylinder, the variation of drag and lift forces on the cylinder are analyzed. The results show that the vortices of variable cross-section cylinder are chaotic and are varying along the cylinder. In places where cross-sections are changing significantly, the vortices are more irregular. The motion trail of the cylinder is almost the same but irregular. The drag and lift coefficients of the cylinder are varying with the changes of diameters.
文摘Large eddy simulation cooperated with the second order full extension ETG(Euler-Taylor-Galerkin) finite element method was applied to simulate the flow around two square cylinders arranged side by side at a spacing ratio of (1.5.) The second order full extension ETG finite element method was developed by Wang and He. By means of Taylor expansion of terms containing time derivative, time derivative is replaced by space derivative. The function of it is equal to introducing an artificial viscosity term. The streamlines of the flow at different moments were obtained. The time history of drag coefficient, lift coefficient and the streamwise velocity on the symmetrical points were presented. Furthermore, the symmetrical problem of the frequency spectrum of flow around two square cylinders arranged side by side were studied by using the spectral analysis technology. The data obtained at the initial stage are excluded in order to avoid the influence of initial condition on the results. The power spectrums of drag coefficient, lift coefficient, the streamwise velocity on the symmetrical points were analyzed respectively. The results show that although the time domain process of dynamic parameters is non-symmetrical, the frequency domain process of them is symmetrical under the symmetrical boundary conditions.
基金This Project is supported by National Natural Science Foundation of China (No.50776056)National Hi-tech Research and Development Program of China (863 Program,No.2006AA05Z250).
文摘A dual-time method is introduced to calculate the unsteady flow in a certain vibrating flat cascade. An implicit lower-upper symmetric-gauss-seidel scheme（LU-SGS） is applied for time stepping in pseudo time domains, and the convection items are discretized with the spatial three-order weighted non-oscillatory and non-free-parameter dissipation difference （WNND） scheme. The turbulence model adopts q-co low-Reynolds-number model. The frequency specmuns of lift coefficients and the unsteady pressure-difference coefficients at different spanwise heights as well as the entropy contours at blade tips on different vibrating instants, are obtained. By the analysis of frequency specmuns of lift coefficients at three spanwise heights, it is considered that there exist obvious non-linear perturbations in the flow induced by the vibrating, and the perturbation frequencies are higher than the basic frequency. The entropy contours at blade tips at different times display an intensively unsteady attribute of the flow under large amplitudes.
文摘As the transport sector is responsible for the consumption of a vast proportion of the oil produced,it is mandatory to research feasible solutions to tackle this issue.The appli-cation of aerodynamic attachments for passiveflow control and reducing resisting aerodynamic forces such as drag and lift is one of the most practicable ways to minimize vehicle energy con-sumption.Theflaps are one of the most innovative aerodynamic attachments that can enhance theflow motion in the boundary layer at the trailing edge of the wings.In the present paper,theflap is designed and modeled for controlling the airflow at the roof-end of a 2D Ahmed body model,inspired by the schematic of theflap at the trailing edge of the wing.As a result,theflap’s geometry and position from the roof-end of the car model are parameterized,which leads to having four design variables.The objective functions of the present study are the vehicle’s drag coefficient and lift coefficient.25 Design of Experiment(DOE)points are considered enabling the Box-Behnken method.Then,each DOE point is modeled in the computational domain,and theflow-field around the model is simulated using Ansys Fluent software.The results obtained for the DOE points are employed by different regressors,and the relation between design variables and objective functions is extracted using GMDH-ANN.The GMDH-ANN is then coupled with three types of optimization algorithms,among which the Genetic algorithm proves to have the most ideal coupling process for optimization.Finally,af-ter analyzing the variations in the geometry and position of the roofflap from the car roof-end,the roof-flap with specifications of L=0.1726 m,a=5.0875°,H=0.0188 m,and d=0.241 m can optimize the car drag and lift coefficients by 21.27% and 19.91%,respec-tively.The present research discusses the opportunities and challenges of optimal design roof-flap geometry and its influence on car aerodynamic performance.
文摘We report the results of accurate prediction of lift(C L)and drag(C D)coefficients of two typical airfoil flows(NACA0012 and RAE2822)by a new algebraic turbulence model,in which the eddy viscosity is specified by a stress length(SL)function predicted by structural ensemble dynamics(SED)theory.Unprecedented accuracy of the prediction of C D with error of a few counts(one count is 10−4)and of C L with error under 1%-2%are uniformly obtained for varying angles of attack(AoA),indicating an order of magnitude improvement of drag prediction accuracy compared to currently used models(typically around 20 to 30 counts).More interestingly,the SED-SL model is distinguished with fewer parameters of clear physical meaning,which quantify underlying turbulent boundary layer(TBL)with a universal multi-layer structure,and is thus promising to be more easily generalizable to complex TBL.The use of the new model for the calibration of flow condition in experiment and the extraction of flow physics from numerical simulation data of aeronautic flows are discussed.
基金Project supported by the National Natural Science Foundation of China(No.11372303)
文摘In view of the fact that large scale vortices play the substantial role of momentum transport in turbulent flows, large eddy simulation （LES） is considered as a better simulation model. However, the sub-grid scale （SGS） models reported so far have not ascertained under what flow conditions the LES can lapse into the direct nu-merical simulation. To overcome this discrepancy, this paper develops a swirling strength based the SGS model to properly model the turbulence intermittency, with the primary characteristics that when the local swirling strength is zero, the local sub-grid viscosity will be vanished. In this paper, the model is used to investigate the flow characteris-tics of zero-incident incompressible turbulent flows around a single square cylinder （SC） at a low Reynolds number range Re ∈ [103, 104]. The flow characteristics investigated include the Reynolds number dependence of lift and drag coefficients, the distributions of time-spanwise averaged variables such as the sub-grid viscosity and the logarithm of Kolmogorov micro-scale to the base of 10 at Re=2 500 and 104, the contours of spanwise and streamwise vorticity components at t = 170. It is revealed that the peak value of sub-grid viscosity ratio and its root mean square （RMS） values grow with the Reynolds number. The dissipation rate of turbulent kinetic energy is larger near the SC solid walls. The instantaneous factor of swirling strength intermittency （FSI） exhibits some laminated structure involved with vortex shedding.
文摘This study investigates the aerodynamic characteristics of a low-Reynolds-number airfoil at high angles of attack(AoA)from 0°to 90°,focusing on their relevance for micro and unmanned aerial vehicle(MAV/UAV)applications.Simulations are conducted using the k-ωshear stress transport(SST)turbulence model using ANSYS Fluent software.Among the key findings is that the lift coefficient CL increases from 1.2981 at 0°AoA to a peak of 2.034 at 11°before decreasing to 1.51 at 90°,indicating initial lift improvement followed by a reduction due to potential flow separation or stall.The drag coefficient CD increases from 0.0222 at 0°AoA to a peak of 0.3572 at 12°,and then decreases to 0.0467 at 90°,indicating initially increasing turbulence and separation,followed by stabilization in the flow regime.The lift-to-drag ratio L/D reaches its maximum of 32.334 at 90°AoA,highlighting improved aerodynamic efficiency at higher AoAs despite increased drag.The skin friction coefficient Cf shows a maximum of 0.046918 at the leading edge at 30°AoA and 0.0394262 at the trailing edge at 90°,indicating critical points of frictional drag.Additionally,the turbulence viscosity ratio at the LE peaks at 0.5586 at 30°AoA and drops to 0.004 at 90°,while it increases at the trailing edge,reaching 0.0394262 at 90°,showing heightened turbulence effects at high AoAs.The present numerical study,however,determines the lift coefficient to be 2.00.This yields a maximum percentage variation of 11.5%compared with the value in the literature.These results provide a comprehensive overview of how high-AoA conditions impact aerodynamic performance,offering valuable insights for optimizing airfoil design and improving MAV/UAV efficiency.
基金This work is supported by National Science Foundation of Zhejiang Province(LZ13E060001)National Natural Science Foundation of China(51210011)Zhejiang Provincial Public Projects(analysis and test)of Zhejiang Province(2015C37027).
文摘The 2D incompressible Navier-Stokes equations arc solved based on the finite Flexible structure;Airfoil;Lock-in phenomenon;Lift coefficient;volume method and dynamic mesh technique is used to carry out partial fluid structure interaction.The local flexible structure(hereinafter termed as flexible structure)vibrates in a single mode located on the upper surface of the airfoil.The Influence of vibration frequency and amplitude are examined and the corresponding fluid flow characteristics are investigated Computational fluid dynamics(CFD)which add complexity to the inherent problem in unsteady flow.The study is conducted for flow over NACA0012 airfoil at 600≤Re≤3000 at a low angle of attack.Vibration of flexible structure induces a secondary vortex which modifies the pressure distribution and lift performance of the airfoil.At some moderate vibration amplitude,frequency synchronization frequency of rigid airfoil.Evolution and shedding of vortices corresponding to the deformation of flexible structure depends on the Reynolds number.In the case of Re≤1000,the deformation of flexible structure is considered in-phase with the vortex shedding i.e.,increasing maximum lift is linked with the positive deformation of flexible structure.At Re=1500 a phase shift of about 1/π exists while they are out-of-phase at Re>1500.Moreover,the oscillation amplitude of lift coefficient increases with increasing vibration amplitude for Re£1500 while it decreases with increasing vibration amplitude for Re>1500.As a result of frequency lock-in,the average lift coefficient is increased with increasing vibration amplitude for all investigated Reynolds numbers(Re).The maximum increase in the average liftcoefficient is 19.72% within the range of investigated parameters.
文摘To control secondary flows, streamwise fences were attached to end wall of a linear turbine rotor cascade. The cascade had 8 blades of 400 mm long and 175 mm chord. The blades deflected the flow by 120°. The fences were made out of 0.7 mm thick brass sheet and the heights of the fences were 14 mm, 18 mm respectively. The curvature of the fences was the same as that of the blade camber line. The fences were fixed normal to the end wall and at half pitch away from the blades. The experimental program consists of total pressure, static pressure measurements at the inlet and outlet of the cascade, by using five-hole probe. In addition, static pressure on the blade suction surface and pressure surface was also obtained. Fences are effective in preventing the movement of the pressure side leg of the horseshoe vortex. Consequently the accumulation of low energy fluid on the suction surface is minimised. End wall losses are reduced by the fences due to weakening of the end wall cross flow.
