Recently, various studies of micro air vehicle （MAV） and unmanned air vehicle （UAV） have been reported from wide range points of view. The aim of this study is to research the aerodynamic improvement of delta wing...Recently, various studies of micro air vehicle （MAV） and unmanned air vehicle （UAV） have been reported from wide range points of view. The aim of this study is to research the aerodynamic improvement of delta wing in low Reynold＇s number region to develop an applicative these air vehicle. As an attractive tool in delta wing, leading edge flap （LEF） is employed to directly modify the strength and structure of vortices originating from the separation point along the leading edge. Various configurations of LEF such as drooping apex flap and upward deflected flap are used in combination to enhance the aerodynamic characteristics in the delta wing. The fluid force measurement by six component toad ceil and particle image velocimetry （PIV） analysis are performed as the experimental method. The relations between the aerodynamic superiority and the vortex behavior around the models are demonstrated.展开更多
A new experiment for airfoil dynamic stall is conducted by employing the advanced par- ticle image velocimetry （PIV） technology in an open-return wind tunnel. The aim of this experimen- tal investigation is to demon...A new experiment for airfoil dynamic stall is conducted by employing the advanced par- ticle image velocimetry （PIV） technology in an open-return wind tunnel. The aim of this experimen- tal investigation is to demonstrate the influences of different motion parameters on the convection velocity, position and strength of leading edge vortex （LEV） of airfoil under different dynamic stall conditions. Two different typical rotor airfoils, OA209 and SC1095, are measured at different free stream velocities, oscillation frequencies, and angles of attack. It is demonstrated by the measured data that the airfoil with larger leading edge radius could notably decrease the strength of LEV. The angle of attack （AoA） of airfoil can obviously influence the dynamic stall characteristics of airfoil, and the LEV would be effectively inhibited by decreasing the mean pitch angle. In addition, the con- vection velocity of LEV is estimated in this measurement, and the results demonstrate that the influ- ence of airfoil shape on convection velocity of LEV is limited, but the convection velocity of LEV would be increased by enlarging the oscillation frequency. Meanwhile, the convection velocity of LEV is a time variant value, and this value would increase as the LEV convects to the trailing edge of airfoil.展开更多
The leading edge vortex(LEV)is one of the most important lift augmentation mechanisms in flapping wing aerodynamics.We propose a methodology that aims to provide a quantitative description of the LEV.The first step of...The leading edge vortex(LEV)is one of the most important lift augmentation mechanisms in flapping wing aerodynamics.We propose a methodology that aims to provide a quantitative description of the LEV.The first step of the method consists of the identification of the vortical structures surrounding the wing using the Q criterion.The impact of the employed threshold is shown to be minor,not influencing the observed trends.In the second step,we identify the core of the LEV using a thinning algorithm,discriminating the LEV using the orientation of the locally averaged vorticity vector.Finally,we compute relevant flow quantities along the LEV core,by averaging in planes perpendicular to the local vorticity at the LEV core points.We have applied this methodology to flow data corresponding to a pair of wings performing a flapping motion in forward flight at moderate Reynolds number.We have performed a geometrical characterization of the LEV and we have computed several flow quantities along the LEV core.For the particular configuration under study,we have shown that the LEV,during the first half of the downstroke,develops and grows,increasing its circulation smoothly.Approximately at middownstroke,the leading edge vortex starts splitting and its downstream part is advected towards the wake while keeping its circulation rather constant.Finally,we have briefly explored the link between the sectional lift on the wing and the local circulation obtained with the present methodology.展开更多
The individual influence of pitching and plunging motions on flow structures is studied experimentally by changing the phase lag between the geometrical angle of attack and the plunging angle of attack.Five phase lags...The individual influence of pitching and plunging motions on flow structures is studied experimentally by changing the phase lag between the geometrical angle of attack and the plunging angle of attack.Five phase lags are chosen as the experimental parameters,while the Strouhal number,the reduced frequency and the Reynolds number are fixed.During the motion of the airfoil,the leading edge vortex,the reattached vortex and the secondary vortex are observed in the flow field.The leading edge vortex is found to be the main flow structure through the proper orthogonal decomposition.The increase of phase lag results in the increase of the leading edge velocity,which strongly influences the leading edge shear layer and the leading edge vortex.The plunging motion contributes to the development of the leading edge shear layer,while the pitching motion is the key reason for instability of the leading edge shear layer.It is also found that a certain increase of phase lag,around 34.15°in this research,can increase the airfoil lift.展开更多
Morphology as well as kinematics is a critical determinant of performance in flapping flight.To understand the effects of the structural traits on aerodynamics of bioflyers,three rectangular wings with aspect ratios...Morphology as well as kinematics is a critical determinant of performance in flapping flight.To understand the effects of the structural traits on aerodynamics of bioflyers,three rectangular wings with aspect ratios（AR）of1,2,and 4 performing hovering-like sinusoidal kinematics at wingtip based Reynolds number of 5 300 are experimentally investigated.Flow structures on sectional cuts along the wing span are compared.Stronger K-H instability is found on the leading edge vortex of wings with higher aspect ratios.Vortex bursting only appears on the outer spanwise locations of high-aspect-ratio wings.The vortex bursting on high-aspect-ratio wings is perhaps one of the reasons why bio-flyers normally have low-aspect-ratio wings.Quantitative analysis exhibits larger dimensionless circulation of the leading edge vortex（LEV）over higher aspect ratio wings except when vortex bursting happens.The average dimensionless circulation of AR1 and AR2 along the span almost equals the dimensionless circulation at the 50%span.The flow structure and the circulation analysis show that the sinusoidal kinematics suppresses breakdown of the LEV compared with simplified flapping kinematics used in similar studies.The Reynolds number effect results on AR4 show that in the current Re range,the overall flow structure is not sensitive to Reynolds number.展开更多
This paper focuses on flow structures of the wing-wake interaction between the hind wing and the wake of the forewing in hovering flight of a dragonfly since there are arguments whether the wing-wake interaction is us...This paper focuses on flow structures of the wing-wake interaction between the hind wing and the wake of the forewing in hovering flight of a dragonfly since there are arguments whether the wing-wake interaction is useful or not.A mechanical flapping model with two tandem wings is used to study the interaction.In the device,two identical simplified model wings are mounted to the flapping model and they are both scaled up to keep the Reynolds number similar to those of dragonfly in hovering flight since our experiment is conducted in a water tank.The kinetic pattern of dragonfly(Aeschna juncea) is chosen because of its special interesting asymmetry.A multi-slice phase-locked stereo particle image velocimetry(PIV) system is used to record flow structures around the hind wing at the mid downstroke(t/T=0.25) and the mid upstroke(t/T=0.75).To make comparison of the flow field between with and without the influence of the wake,flow structures around a single flapping wing(hind wing without the existence of the forewing) at these two stroke phases are also recorded.A local vortex identification scheme called swirling strength is applied to determine the vortices around the wing and they are visualized with the iso-surface of swirling strength.This paper also presents contour lines of z at each spanwise position of the hind wing,the vortex core position of the leading edge vortex(LEV) of hind wing with respect to the upper surface of hind wing,the circulation of the hind wing LEV at each spanwise position and so on.Experimental results show that dimension and strength of the hind wing LEV are impaired at the mid stroke in comparison with the single wing LEV because of the downwash from the forewing.Our results also reveal that a wake vortex from the forewing traverses the upper surface of the hind wing at the mid downstroke and its distance to the upper surface is about 40% of the wing chord length.At the instant,the distance of the hind wing LEV to the upper surface is about 20% of the wing chord length.Thus,there must be a wing-wake interaction mechanism that makes the wake vortex become an additional LEV of the hind wing and it can partly compensate the hind wing for its lift loss caused by the downwash from the forewing.展开更多
Dynamic stall under large Reynolds numbers and large reduced frequencies has a significant effect on the performance of the wind turbine blades,helicopter rotors,etc.So the dynamic stall physics of the NACA0012 airfoi...Dynamic stall under large Reynolds numbers and large reduced frequencies has a significant effect on the performance of the wind turbine blades,helicopter rotors,etc.So the dynamic stall physics of the NACA0012 airfoil under a large Reynolds number of Re=1.5×10^(6) was studied using experimental and numerical methods.The reduced frequency range was k=0.035-0.1.The unsteady flow field in dynamic stall was studied in detail by using the transient pressure measurement and the numerical simulation based on the unsteady Reynolds-averaged Navier-Stokes(URANS)equation.And the time-frequency characteristics of the dynamic stall were studied using the wavelet analysis.The study showed that the aerodynamic performance during the dynamic stall was dominated by the shear layer vortex(SLV)and the leading edge vortex(LEV),and the phase difference between the SLV and the LEV was the key factor in the exist-ence of the bimodal characteristics of the aerodynamic force/moment.There was a significant linear correlation between the negative peak of the vortex-induced Cp and the Cn in the reduced frequency range studied in this paper.During the convection of the near-wall LEV to the trailing edge,the high-frequency features firstly decay,and the multi-scale structures of the LEV become more significant as the reduced frequency gradually increases.展开更多
文摘Recently, various studies of micro air vehicle （MAV） and unmanned air vehicle （UAV） have been reported from wide range points of view. The aim of this study is to research the aerodynamic improvement of delta wing in low Reynold＇s number region to develop an applicative these air vehicle. As an attractive tool in delta wing, leading edge flap （LEF） is employed to directly modify the strength and structure of vortices originating from the separation point along the leading edge. Various configurations of LEF such as drooping apex flap and upward deflected flap are used in combination to enhance the aerodynamic characteristics in the delta wing. The fluid force measurement by six component toad ceil and particle image velocimetry （PIV） analysis are performed as the experimental method. The relations between the aerodynamic superiority and the vortex behavior around the models are demonstrated.
基金supported by the National Natural Science Foundation of China(No.11272150)
文摘A new experiment for airfoil dynamic stall is conducted by employing the advanced par- ticle image velocimetry （PIV） technology in an open-return wind tunnel. The aim of this experimen- tal investigation is to demonstrate the influences of different motion parameters on the convection velocity, position and strength of leading edge vortex （LEV） of airfoil under different dynamic stall conditions. Two different typical rotor airfoils, OA209 and SC1095, are measured at different free stream velocities, oscillation frequencies, and angles of attack. It is demonstrated by the measured data that the airfoil with larger leading edge radius could notably decrease the strength of LEV. The angle of attack （AoA） of airfoil can obviously influence the dynamic stall characteristics of airfoil, and the LEV would be effectively inhibited by decreasing the mean pitch angle. In addition, the con- vection velocity of LEV is estimated in this measurement, and the results demonstrate that the influ- ence of airfoil shape on convection velocity of LEV is limited, but the convection velocity of LEV would be increased by enlarging the oscillation frequency. Meanwhile, the convection velocity of LEV is a time variant value, and this value would increase as the LEV convects to the trailing edge of airfoil.
基金supported by grant DPI2016-76151-C2-2-R(AEI/FEDER,UE)。
文摘The leading edge vortex(LEV)is one of the most important lift augmentation mechanisms in flapping wing aerodynamics.We propose a methodology that aims to provide a quantitative description of the LEV.The first step of the method consists of the identification of the vortical structures surrounding the wing using the Q criterion.The impact of the employed threshold is shown to be minor,not influencing the observed trends.In the second step,we identify the core of the LEV using a thinning algorithm,discriminating the LEV using the orientation of the locally averaged vorticity vector.Finally,we compute relevant flow quantities along the LEV core,by averaging in planes perpendicular to the local vorticity at the LEV core points.We have applied this methodology to flow data corresponding to a pair of wings performing a flapping motion in forward flight at moderate Reynolds number.We have performed a geometrical characterization of the LEV and we have computed several flow quantities along the LEV core.For the particular configuration under study,we have shown that the LEV,during the first half of the downstroke,develops and grows,increasing its circulation smoothly.Approximately at middownstroke,the leading edge vortex starts splitting and its downstream part is advected towards the wake while keeping its circulation rather constant.Finally,we have briefly explored the link between the sectional lift on the wing and the local circulation obtained with the present methodology.
基金supported by the National Natural Science Foundation of China(Nos.GZ 1280,11722215 and 11721202)。
文摘The individual influence of pitching and plunging motions on flow structures is studied experimentally by changing the phase lag between the geometrical angle of attack and the plunging angle of attack.Five phase lags are chosen as the experimental parameters,while the Strouhal number,the reduced frequency and the Reynolds number are fixed.During the motion of the airfoil,the leading edge vortex,the reattached vortex and the secondary vortex are observed in the flow field.The leading edge vortex is found to be the main flow structure through the proper orthogonal decomposition.The increase of phase lag results in the increase of the leading edge velocity,which strongly influences the leading edge shear layer and the leading edge vortex.The plunging motion contributes to the development of the leading edge shear layer,while the pitching motion is the key reason for instability of the leading edge shear layer.It is also found that a certain increase of phase lag,around 34.15°in this research,can increase the airfoil lift.
基金supported by the Innovation Technology Commission(ITC)of the Government of the Hong Kong Special Administrative Region(HKSAR)with Project(ITS/115/13FP)Hong Kong Ph.D.Fellowship Scheme from the Research Grants Council(RGC)
文摘Morphology as well as kinematics is a critical determinant of performance in flapping flight.To understand the effects of the structural traits on aerodynamics of bioflyers,three rectangular wings with aspect ratios（AR）of1,2,and 4 performing hovering-like sinusoidal kinematics at wingtip based Reynolds number of 5 300 are experimentally investigated.Flow structures on sectional cuts along the wing span are compared.Stronger K-H instability is found on the leading edge vortex of wings with higher aspect ratios.Vortex bursting only appears on the outer spanwise locations of high-aspect-ratio wings.The vortex bursting on high-aspect-ratio wings is perhaps one of the reasons why bio-flyers normally have low-aspect-ratio wings.Quantitative analysis exhibits larger dimensionless circulation of the leading edge vortex（LEV）over higher aspect ratio wings except when vortex bursting happens.The average dimensionless circulation of AR1 and AR2 along the span almost equals the dimensionless circulation at the 50%span.The flow structure and the circulation analysis show that the sinusoidal kinematics suppresses breakdown of the LEV compared with simplified flapping kinematics used in similar studies.The Reynolds number effect results on AR4 show that in the current Re range,the overall flow structure is not sensitive to Reynolds number.
基金supported by the National Natural Science Foundation of China (Grant Nos. 10772017,10472011)
文摘This paper focuses on flow structures of the wing-wake interaction between the hind wing and the wake of the forewing in hovering flight of a dragonfly since there are arguments whether the wing-wake interaction is useful or not.A mechanical flapping model with two tandem wings is used to study the interaction.In the device,two identical simplified model wings are mounted to the flapping model and they are both scaled up to keep the Reynolds number similar to those of dragonfly in hovering flight since our experiment is conducted in a water tank.The kinetic pattern of dragonfly(Aeschna juncea) is chosen because of its special interesting asymmetry.A multi-slice phase-locked stereo particle image velocimetry(PIV) system is used to record flow structures around the hind wing at the mid downstroke(t/T=0.25) and the mid upstroke(t/T=0.75).To make comparison of the flow field between with and without the influence of the wake,flow structures around a single flapping wing(hind wing without the existence of the forewing) at these two stroke phases are also recorded.A local vortex identification scheme called swirling strength is applied to determine the vortices around the wing and they are visualized with the iso-surface of swirling strength.This paper also presents contour lines of z at each spanwise position of the hind wing,the vortex core position of the leading edge vortex(LEV) of hind wing with respect to the upper surface of hind wing,the circulation of the hind wing LEV at each spanwise position and so on.Experimental results show that dimension and strength of the hind wing LEV are impaired at the mid stroke in comparison with the single wing LEV because of the downwash from the forewing.Our results also reveal that a wake vortex from the forewing traverses the upper surface of the hind wing at the mid downstroke and its distance to the upper surface is about 40% of the wing chord length.At the instant,the distance of the hind wing LEV to the upper surface is about 20% of the wing chord length.Thus,there must be a wing-wake interaction mechanism that makes the wake vortex become an additional LEV of the hind wing and it can partly compensate the hind wing for its lift loss caused by the downwash from the forewing.
基金the Key Laboratory of Flow Visualization and Measurement Techniques,AVIC Aerodynamics Research Institute(XFX20220502)the foundation of National Key Laboratory of Science and Technology on Aerodynamic Design and Research(No.61422010401).
文摘Dynamic stall under large Reynolds numbers and large reduced frequencies has a significant effect on the performance of the wind turbine blades,helicopter rotors,etc.So the dynamic stall physics of the NACA0012 airfoil under a large Reynolds number of Re=1.5×10^(6) was studied using experimental and numerical methods.The reduced frequency range was k=0.035-0.1.The unsteady flow field in dynamic stall was studied in detail by using the transient pressure measurement and the numerical simulation based on the unsteady Reynolds-averaged Navier-Stokes(URANS)equation.And the time-frequency characteristics of the dynamic stall were studied using the wavelet analysis.The study showed that the aerodynamic performance during the dynamic stall was dominated by the shear layer vortex(SLV)and the leading edge vortex(LEV),and the phase difference between the SLV and the LEV was the key factor in the exist-ence of the bimodal characteristics of the aerodynamic force/moment.There was a significant linear correlation between the negative peak of the vortex-induced Cp and the Cn in the reduced frequency range studied in this paper.During the convection of the near-wall LEV to the trailing edge,the high-frequency features firstly decay,and the multi-scale structures of the LEV become more significant as the reduced frequency gradually increases.
