A fluid flow model consisting of Bernoulli’s law in its normal form, the equation of state of air, and the cross-stream force balance between a downward pressure gradient and the upward centrifugal force on fluid par...A fluid flow model consisting of Bernoulli’s law in its normal form, the equation of state of air, and the cross-stream force balance between a downward pressure gradient and the upward centrifugal force on fluid particles moving along curved streamlines over the top circular wing surface involving three equations in three unknowns (pressure, density and velocity) are solved to show that both density and pressure decrease upward as the inverse square of the distance from the circle’s center, and the velocity is independent of that dis-tance. These derived characteristics are used to explain the lift force on the wing in what is believed to be a novel way.展开更多
We consider, compare, and contrast various aspects of aerodynamic and ballistic flight. We compare the energy efficiency of aerodynamic level flight at a given altitude versus that of ballistic flight beginning and en...We consider, compare, and contrast various aspects of aerodynamic and ballistic flight. We compare the energy efficiency of aerodynamic level flight at a given altitude versus that of ballistic flight beginning and ending at this same altitude. We show that for flights short compared to Earth’s radius, aerodynamic level flight with lift-to-drag ratio L/D > 2?is more energy-efficient than ballistic flight, neglecting air resistance or drag in the latter. Smaller L/D suffices if air resistance in ballistic flight is not neglected. For a single circumnavigation of Earth, we show that aerodynamic flight with L/D > 4π is more energy-efficient than minimum-altitude circular-orbit ballistic spaceflight. We introduce the concept of gravitational scale height, which may in an auxiliary way be helpful in understanding this result. For flights traversing N circumnavigations of Earth, if then even minimum-altitude circular-orbit ballistic spaceflight is much more energy-efficient than aerodynamic flight because even at minimum circular-orbit spaceflight altitude air resistance is very small. For higher-altitude spaceflight air resistance is even smaller and the energy-efficiency advantage of spaceflight over aerodynamic flight traversing the same distance is therefore even more pronounced. We distinguish between the energy efficiency of flight per se and the energy efficiency of the engine that powers flight. Next we consider the effects of air density on aerodynamic level flight and provide a simplified view of drag and lift. We estimate the low-density/high-altitude limits of aerodynamic level flight (and for comparison also of balloons) in Earth’s and Mars’ atmospheres. Employing Mars airplanes and underwater airplanes on Earth (and hypo-thetically also on Mars) as examples, we consider aerodynamic level flight in rarefied and dense aerodynamic media, respectively. We also briefly discuss hydrofoils. We appraise the optimum range of air densities for aerodynamic level flight. We then consider flights of hand-thrown projec-tiles that are unpowered except for the initial throw. We describe how aerodynamically efficient ones (i.e., with large L/D) such as Frisbees, Aerobies, and boomerangs not only can traverse record horizontal distances, but (along with discuses) also can—since lift exceeds weight at achievable throwing speeds—maintain altitude farther if thrown horizontally against the wind than with it. Then we compare the energy efficiency of surface transportation versus that of both aerodynamic and ballistic flight.展开更多
Insects usually fly by passively rotating wings,which has been applied to the design of flapping-wing Micro-Air Vehicles(MAVs)to reduce mechanical complexity.In this paper,a robotic passive rotating-wing model is desi...Insects usually fly by passively rotating wings,which has been applied to the design of flapping-wing Micro-Air Vehicles(MAVs)to reduce mechanical complexity.In this paper,a robotic passive rotating-wing model is designed to investigate wing kinematics and lift generation,which are measured by a high-speed camera and a force transducer,respectively.In addition,flow fields are measured using the Particle Image Velocimetry(PIV).Experimental results demonstrate that passive rotating motion has a coordinative relationship with actively stroking motion.As the stroke amplitude or frequency increases,the rotating amplitude is enlarged.To characterize the active stroking motion,a driving Reynolds number Redrivingis defined,which varies from 68 to 366 in this study.Moving the gravity center of the wing towards trailing ed ge induces the increase of additional torque M,which decreases the wing rotating amplitude and promotes the advance of wing rotation.We find that the timing of wing rotation is gradually delayed and the mean lift coefficient C^(-)_(L)monotonously decreases as Redrivingincreases.By increasing the additional torque M,C^(-)_(L)is slightly improved and approaches to the lift coefficient of a real fruit fly at driving Re approximately equal to 230.The instantaneous lifts combined with the vortical structures further demonstrate that the lift generation associated with wing rotation is mainly attributed to the growth of the LeadingEdge Vortex(LEV)and the passive wake capture mechanism.Passive wake capture is influenced by LEV,reversal stroke motion and wing additional torque together,which can only maintain the lift at a high level for a considerable period.The high-lift generation mechanisms of flapping and passive rotating flight could shed light on the simplified design of MAVs and the improvement of their aerodynamic performance.展开更多
针对一个并联式涡轮基组合循环(Turbine Based Combined Cycle,TBCC)发动机排气系统的气动方案,对其在整个飞行包线范围内典型工作点上的流场进行了数值模拟研究,获得了飞行包线范围内排气系统相应的推力系数、升力、俯仰力矩随飞行马...针对一个并联式涡轮基组合循环(Turbine Based Combined Cycle,TBCC)发动机排气系统的气动方案,对其在整个飞行包线范围内典型工作点上的流场进行了数值模拟研究,获得了飞行包线范围内排气系统相应的推力系数、升力、俯仰力矩随飞行马赫数的变化关系。计算结果显示,在整个飞行包线范围内,排气系统的轴向推力系数随着飞行马赫数先减小后增大,在跨声速飞行时降到最低Ma=0.9,涡喷不加力时为0.562,加力时0.662),在设计点附近达到最大;升力和俯仰力矩性能在亚声速及跨声速飞行时较差,在超声速飞行时随着飞行马赫数增加逐渐好转。表明排气系统在跨声速飞行范围内工作时应采取措施以改善其性能。展开更多
文摘A fluid flow model consisting of Bernoulli’s law in its normal form, the equation of state of air, and the cross-stream force balance between a downward pressure gradient and the upward centrifugal force on fluid particles moving along curved streamlines over the top circular wing surface involving three equations in three unknowns (pressure, density and velocity) are solved to show that both density and pressure decrease upward as the inverse square of the distance from the circle’s center, and the velocity is independent of that dis-tance. These derived characteristics are used to explain the lift force on the wing in what is believed to be a novel way.
文摘We consider, compare, and contrast various aspects of aerodynamic and ballistic flight. We compare the energy efficiency of aerodynamic level flight at a given altitude versus that of ballistic flight beginning and ending at this same altitude. We show that for flights short compared to Earth’s radius, aerodynamic level flight with lift-to-drag ratio L/D > 2?is more energy-efficient than ballistic flight, neglecting air resistance or drag in the latter. Smaller L/D suffices if air resistance in ballistic flight is not neglected. For a single circumnavigation of Earth, we show that aerodynamic flight with L/D > 4π is more energy-efficient than minimum-altitude circular-orbit ballistic spaceflight. We introduce the concept of gravitational scale height, which may in an auxiliary way be helpful in understanding this result. For flights traversing N circumnavigations of Earth, if then even minimum-altitude circular-orbit ballistic spaceflight is much more energy-efficient than aerodynamic flight because even at minimum circular-orbit spaceflight altitude air resistance is very small. For higher-altitude spaceflight air resistance is even smaller and the energy-efficiency advantage of spaceflight over aerodynamic flight traversing the same distance is therefore even more pronounced. We distinguish between the energy efficiency of flight per se and the energy efficiency of the engine that powers flight. Next we consider the effects of air density on aerodynamic level flight and provide a simplified view of drag and lift. We estimate the low-density/high-altitude limits of aerodynamic level flight (and for comparison also of balloons) in Earth’s and Mars’ atmospheres. Employing Mars airplanes and underwater airplanes on Earth (and hypo-thetically also on Mars) as examples, we consider aerodynamic level flight in rarefied and dense aerodynamic media, respectively. We also briefly discuss hydrofoils. We appraise the optimum range of air densities for aerodynamic level flight. We then consider flights of hand-thrown projec-tiles that are unpowered except for the initial throw. We describe how aerodynamically efficient ones (i.e., with large L/D) such as Frisbees, Aerobies, and boomerangs not only can traverse record horizontal distances, but (along with discuses) also can—since lift exceeds weight at achievable throwing speeds—maintain altitude farther if thrown horizontally against the wind than with it. Then we compare the energy efficiency of surface transportation versus that of both aerodynamic and ballistic flight.
基金supported by the National Nature Science Foundation of China(Nos.12102259,12202273)the China Postdoctoral Science Foundation(No.2018M642007)。
文摘Insects usually fly by passively rotating wings,which has been applied to the design of flapping-wing Micro-Air Vehicles(MAVs)to reduce mechanical complexity.In this paper,a robotic passive rotating-wing model is designed to investigate wing kinematics and lift generation,which are measured by a high-speed camera and a force transducer,respectively.In addition,flow fields are measured using the Particle Image Velocimetry(PIV).Experimental results demonstrate that passive rotating motion has a coordinative relationship with actively stroking motion.As the stroke amplitude or frequency increases,the rotating amplitude is enlarged.To characterize the active stroking motion,a driving Reynolds number Redrivingis defined,which varies from 68 to 366 in this study.Moving the gravity center of the wing towards trailing ed ge induces the increase of additional torque M,which decreases the wing rotating amplitude and promotes the advance of wing rotation.We find that the timing of wing rotation is gradually delayed and the mean lift coefficient C^(-)_(L)monotonously decreases as Redrivingincreases.By increasing the additional torque M,C^(-)_(L)is slightly improved and approaches to the lift coefficient of a real fruit fly at driving Re approximately equal to 230.The instantaneous lifts combined with the vortical structures further demonstrate that the lift generation associated with wing rotation is mainly attributed to the growth of the LeadingEdge Vortex(LEV)and the passive wake capture mechanism.Passive wake capture is influenced by LEV,reversal stroke motion and wing additional torque together,which can only maintain the lift at a high level for a considerable period.The high-lift generation mechanisms of flapping and passive rotating flight could shed light on the simplified design of MAVs and the improvement of their aerodynamic performance.
文摘针对一个并联式涡轮基组合循环(Turbine Based Combined Cycle,TBCC)发动机排气系统的气动方案,对其在整个飞行包线范围内典型工作点上的流场进行了数值模拟研究,获得了飞行包线范围内排气系统相应的推力系数、升力、俯仰力矩随飞行马赫数的变化关系。计算结果显示,在整个飞行包线范围内,排气系统的轴向推力系数随着飞行马赫数先减小后增大,在跨声速飞行时降到最低Ma=0.9,涡喷不加力时为0.562,加力时0.662),在设计点附近达到最大;升力和俯仰力矩性能在亚声速及跨声速飞行时较差,在超声速飞行时随着飞行马赫数增加逐渐好转。表明排气系统在跨声速飞行范围内工作时应采取措施以改善其性能。
