俯仰振荡翼型推阻力转变滞后机制数值研究

扑翼产生的反卡门涡街被认为是一种推力型尾迹,但已有研究指出,随着斯特劳哈尔数(St)增大,低雷诺数下俯仰振荡翼型的净推力产生明显滞后于反卡门涡街的出现.为探究该现象背后的物理机制,对NACA0012翼型在雷诺数1000条件下作简谐俯仰运动的流场进行了数值模拟.采用翼型表面积分方法和基于有限控制体的气动力估计方法分别研究...     

Aerodynamic performance of owl-like airfoil undergoing bio-inspired flapping kinematics

Natural flyers have extraordinary flight skills and their prominent aerodynamic performance has attracted a lot of attention. However, the aerodynamic mechanism of birds' flapping wing kinematics still lacks in-depth understanding. In this paper, the aerodynamic performance of owl-like airfoil undergoing bio-inspired flapping kinematics extracted from a free-flying owl wing has been numerically investigated. The overset mesh technique is used to deal with the large range movements of flapping airfoils. The bio-inspired kinematics consist of plunging and pitching movement. A pure sinusoidal motion and a defined motion composed of plunging of sinusoidal motion and pitching of the bio-inspired kinematics are selected for comparison. The other two NACA airfoils are also selected to figure out the advantages of the owl-like airfoil. It is found that the cambered owl-like airfoil can enhance lift during the downstroke. The bio-inspired kinematics have an obvious advantage in lift generation with a presence of higher peak lift and positive lift over a wider proportion of the flapping cycle. Meanwhile, the bio-inspired motion is more economical for a lower power consumption compared with the sinusoidal motion. The sinusoidal flapping motion is better for thrust generation for a higher peak thrust value in both upstroke and downstroke, while the bio-inspired kinematics mainly generate thrust during the downstroke but produce more drag during the upstroke. The defined motion has similar lift performance with the bio-inspired kinematics, while it consumes more energy and generates less thrust. The unsteady flow field around airfoils is also analyzed to explain the corresponding phenomenon. The research in this paper is helpful to understand the flight mechanism of birds and to design a micro air vehicle with higher performance.     

Effect of flexibility on unsteady aerodynamics forces of a purely plunging airfoil

Introducing flexibility into the design of a vertically flapping wing is an effective way to enhance its aerodynamic performance. As less previous studies on the aerodynamics of vertically flapping flexible wings focused on the lift generated in a wide range of angle of attack·a 2D numerical simulation of a purely plunging flexible airfoil is employed using a loose fluid–structure interaction method. The aerodynamics of a fully flexible airfoil are firstly studied with the flexibility and angle of attack. To verify whether an airfoil could get aerodynamic benefit from the change in structure, partially flexible airfoil with rigid leading edge and flexible trailing edge were further considered. Results show that flexibility could always reduce airfoil drag while lift and lift efficiency both peak at moderate flexibility. When freestream velocity is constant, lift is maximized at a high angle of attack about 40° while this optimal angle of attack reduces to 15° in drag-balanced status. The airfoil drag reduction, lift augmentation as well as efficiency enhancement mainly attribute to the passive pitching other than the camber deformation. Partially deformed airfoil with the longest length of moderate flexible trailing edge can achieve the highest lift. This study may provide some guidance in the wing design of Micro Air Vehicle (MAV).     

介质阻挡放电等离子体对翼型流动分离控制的实验研究

在低速开口风洞中进行了等离子体激励器对NACA0015翼型流动分离控制的实验研究。采用PIV技术,对翼型绕流流场进行了测量,显示了施加等离子体激励后流场的变化。通过五分量天平对升力和阻力的测量,研究了激励电压和激励频率对翼型流动分离控制的规律。研究表明,低风速下在翼型前缘施加等离子体激励,能够有效地控制翼型流动分离,在来流为20m／s时,最大升力系数增加11％,失速迎角增加6°;在给定的流动状态下,激励电压和激励频率存在一个阈值,不同迎角下该阈值不同,迎角越大,分离越严重,对激励强度的要求也越高。     

扑动翼型的低雷诺数气动特性分析

通过求解引入拟压缩项的不可压Navier-Stokes方程,数值模拟了绕扑动翼型的低雷诺数非定常流动。针对厚度在4%-12%之间的NACA对称翼型,分析了翼型厚度等参数对扑动翼型气动特性的影响。在低雷诺数条件下,对于纯俯仰运动,随着翼型厚度的减小,平均阻力系数也变小。而对于纯沉浮运动,发现翼型厚度对气动特性的影响和俯仰运动有很大的差别,平均阻力系数随着翼型厚度的减小而变大。通过对沉浮运动一个周期流线图的分析,认为这是翼型前缘涡的影响造成的。由于前缘涡的影响,翼型厚度增加,平均压差阻力系数变小,甚至会出现负值。雷诺数的影响研究表明,随着雷诺数的增加,扑动翼型的阻力系数减小的趋势越缓慢。     

Numerical Investigation of the Effects of Wind Tunnel Model Vibrations on the Measured Aerodynamic Properties of Airfoils and Wings

This paper presents a numerical prediction of the unsteady flow field around oscillating airfoils at high angles of attack by solving unsteady Reynolds-averaged Navier-Stokes equations with SST turbulence model in order to simulate the effects of wind tunnel model vibrations on the aerodynamic properties of airfoils,especially high-aspect-ratio wings in a wind tunnel.The effects of the phase lagging between different modes of oscillations,i.e.,the airfoil plunging oscillation mode,the pitching oscillation mode,and the forward-backward oscillation mode,are also studied.It is shown that the vibrations (oscillations) of airfoils can cause the unsteady shedding of large-size separated vortex to precede the stationary stall incidence,hence lead to a stall onset at some earlier (lower) incidence than that in the steady sense.The different phase lagging has different effect on the flow field.When the pitching oscillation mode has small phase lagging behind the plunging oscillation mode,the effect of vibrations is large.Besides,if the amplitude of the oscillations is large enough,and the different modes of vibrations match or combine appropriately,the unsteady stall may occur 2° earlier in angle of attack than the case where airfoils keep stationary.     

微型扑翼飞行器风洞试验初步研究

 为了对微型扑翼飞行器空气动力学基本特性进行定量研究，利用西北工业大学微型飞行器专用风洞对微型扑翼飞行器机翼进行初步风洞吹风试验。试验中进行了扑动频率、风速、迎角、机翼平面形状、翼型弯度对机翼气动特性影响的研究。通过试验得出了微型扑翼飞行器升力、推力产生的基本规律，为微型扑翼飞行器总体设计和气动设计提供了参考。     

Experiments on unsteady vortex flowfield of typical rotor airfoils under dynamic stall conditions

A new experiment for airfoil dynamic stall is conducted by employing the advanced particle image velocimetry(PIV) technology in an open-return wind tunnel. The aim of this experimental 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(Ao A) 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 convection velocity of LEV is estimated in this measurement, and the results demonstrate that the influence 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.     

Impact of reduced frequency on the time lag in pressure distribution over a supercritical airfoil in a pitch-pause-return motion

Effects of reduced frequency, stop angle, and pause duration have been studied on a thin supercritical airfoil undergoing a pitch-pause-return motion, which is one of the classic maneuvers introduced by the AIAA Fluid Dynamics Technical Committee. Experiments were conducted in a low-speed wind tunnel at both a constant mean angle of attack and an oscillation amplitude with a reduced frequency ranging from 0.01 to 0.12. The desired stop angles of the airfoil were set to occur during the upstroke motion. The unsteady pressure distribution on the airfoil was measured for below, near, and beyond static stall conditions. Results showed that the reduced frequency and stop angle were the dominant contributors to the time lag in the flowfield. For stop angles in both below- and post-stall regions, the time for the flowfield to reach its steady state conditions, known as the time lag, decreased as the reduced frequency was increased. However, in the static-stall region and for a certain value of reduced frequency, a resonance phenomenon was observed, and a minimum time lag was achieved. The pressure distribution in this condition was shown to be highly influenced by this phenomenon.     

Lift performance enhancement for flapping airfoils by considering surging motion

The flapping motion has a great impact on the aerodynamic performance of flapping wings. In this paper, a surging motion is added to an airfoil performing pitching-plunging combined motion to figure out how it influences the lift performance and flow pattern of flapping airfoils. Firstly, the numerical methods are validated by a NACA0012 airfoil pitching case and a NACA0012 airfoil plunging case. Then, the E377m airfoil which has typical geometric characteristics of the bird-like airfoil is selected as the calculation model to study how phase differences φ₁ between surging motion and plunging motion affect the aerodynamic performance of flapping airfoils. The results show that the airfoil with surging motion has comprehensively better lift performance and thrust performance than the airfoil without surging motion when 15°< φ₁ < 90°. It is demonstrated that surging motion has a powerful ability to improve the aerodynamic performance of flapping airfoil by adjusting φ₁. Finally, to further explore how flapping airfoil improves lift performance by considering surging motion, the flapping motions of E377m airfoil with the highest lift coefficient and lift efficiency are obtained through trajectory optimization. The surging motion is removed in the highest lift case and highest lift efficiency case respectively, and the mechanism that surging motion adjusts the aerodynamic force is analyzed in detail by comparing the vortex structure and kinematic parameters. The results of this paper help reveal the aerodynamic mechanism of bird flight and guide the design of Flapping wing Micro Air Vehicles (FMAV).     

微型扑翼飞行器推力特性试验

通过进行微型扑翼飞行器低速风洞试验,研究了扑动翼展弦比、刚度和弯度对其推力特性的影响.制作了展弦比为2,4.和7以及带弯度与不带弯度的矩形扑动翼,并对三种不同刚度大小的矩形扑动翼进行了结构变形分析;试验风速变化范围从4 m/s到10 m/s,扑动频率从4 Hz到8 Hz.风洞试验结果表明适当的增大扑动翼展弦比或减小扑动翼刚度有助于提高扑翼飞行器的推进效率;而扑动翼翼型弯度的增加不利于推力特性的提高.      

低雷诺数翼型局部振动非定常气动特性

针对低雷诺数翼型特殊的气动特性,采用基于动网格的非定常数值模拟方法,研究翼型表面不同弦向位置的局部蒙皮以不同频率及振幅振动时对低雷诺数翼型气动特性及流场结构的影响,揭示蒙皮振动增升减阻的机理。研究表明,在低雷诺数条件下局部蒙皮振动可有效提高翼型气动特性,与刚性翼型相比蒙皮局部振动可使翼型升力系数提高,阻力系数降低,升阻比提高。振动位置对翼型气动特性及流场结构有显著的影响,振动表面位于翼型前缘附近或位于层流分离泡中心时可有效控制翼型层流分离,从而提高翼型气动特性。振动频率对翼型表面层流分离及转捩位置均有显著的影响,随着振动频率增加,翼型气动特性出现最优值。与刚性翼型相比,表面振动使翼型转捩位置略向上游移动,摩擦阻力增加,但振动使等效翼型相对厚度减小,压差阻力明显减小。在小幅振动范围内,随着振幅增加,流场非定常特性更加显著,翼型升阻比增加。     

高升力下二维构型阻力准确测量研究

翼型风洞试验阻力测量常使用尾迹流场测量积分求取阻力的方法,但各积分公式均建立在一定的假设基础上,有一定适用范围。在多段翼型流场N-S方程数值模拟和风洞试验的基础上,研究高升力情况下低速风洞阻力精确测量技术。通过N-S方程数值模拟求解多段翼型绕流场,分析尾迹流场的特点和常规风洞试验阻力计算公式推导时所作假设,提出新的更为准确的型阻计算公式;利用多段翼型绕流的数值模拟结果,积分表面压力和摩擦力求得翼型的气动特性,并利用计算得到的尾迹流场信息按照常规和新提出的风洞试验型阻计算公式计算阻力,将三者进行比较,检验提出的新型阻计算公式的准确性;通过风洞试验检验数值模拟得到的流场特点和新型阻计算公式。研究表明：在高升力条件下,传统型阻计算公式有很大的局限性,必须进行改进;提出的考虑尾迹区流动特点的新型阻计算公式能够得到更准确的阻力值。     

微型扑翼飞行器扑翼/尾翼气动干扰的数值研究

以二维刚性约束条件下的微型扑翼飞行器模型为研究对象,在动网格技术基础上,应用非定常数值分析手段对比分析了单翼/纵列翼布局的气动性能,深入研究了纵列翼缩减频率、扑翼—尾翼无量纲水平间距、来流攻角对其气动性能的影响.结果表明:①纵列翼尾翼对扑翼产生正效应干扰,相对于单翼布局,扑翼—尾翼无量纲水平间距为0.5倍翼型弦长时的纵列翼布局的推力系数和推进效率分别增加28.7%和5.7%;②缩减频率是影响推力的关键参数,随着缩减频率的增加,脱落涡的强度增加,推力系数增大.对于单翼、纵列翼两种布局模式,当缩减频率在1.0附近时推进效率达到最优;③对于纵列翼布局,在扑翼—尾翼无量纲水平间距为1.1倍翼型弦长时推进效率达到峰值;④在0°~20°来流攻角变化范围内,随着来流攻角的增加,升力系数增加,推力系数减小,当来流攻角大于9°时,两种布局的推力均为负值.      

About classical problems of airfoil drag

The drag of airfoils has long been determined from measurements in the wake behind the airfoil. The well-known formula of Squire and Young can be used to transfer the measured data to infinity downstream of the airfoil, where an exact relation to the drag exists. The Squire–Young formula is reviewed and an uncertainty in its derivation is discussed. A new formula of the same type is then derived, which also contains an uncertainty. This uncertainty can be reduced by experimental and numerical investigations of the wake displacement. Conversely, the displacement and the momentum of the wake must be investigated to reduce the uncertainty of the Squire–Young formula. In the second part of the paper, the determination of the drag directly by integration of the skin friction is investigated. Such a calculation is not difficult if the boundary-layer characteristics are computed numerically. An additional pressure drag must be regarded in this case. A new formula for the pressure drag is derived which shows that the uncertainty of the Squire–Young type formulas is contained in the evaluation of the pressure drag. This is another reason for further research on wake displacement.     

总距突增时共轴式直升机瞬态操纵响应分析

从共轴双旋翼直升机的工程实际出发,建立了共轴式直升机上下旋翼非定常气动特性的计算模型.引入Leishman-Beddoes指数函数的半经验公式,建立了二维翼型非定常气动模型;分别从固定尾迹和自由尾迹,引入干扰因子到动态入流三种方法出发,建立了反映共轴双旋翼直升机上下旋翼气动干扰的诱导速度模型;从跷跷板式旋翼的挥舞动力学方程出发,利用4阶Runge-Kutta算法求解桨叶刚性挥舞角的数值解.通过计算分析,得到了悬停和前飞状态,总距突增时上下旋翼升力的动态响应特性,以及总距突增时上下旋翼桨叶铰链力矩的响应特性.      

模拟厚度翼型非定常运动的涡方法

本文研究二维厚度翼型的大攻角非定常运动。用沿翼型表面分布的源和面涡来模拟翼型的厚度效应和升力效应,应用离散涡方法模拟前缘分离涡层和尾涡层。计算了大攻角翼型作俯仰运动的情形,与实验结果符合较好。     

翼型近尾迹流动的PIV研究—动力学机制

利用在线式互相关PIV(ParticleImageVelocimetry)系统,在低速风洞中对NACA0012翼型在雷诺数2.39×105,0°和4°攻角下的近尾迹流动进行了详细测量。实验结果表明,翼型近尾迹存在有序的涡街结构,涡街在尾缘处形成后,在向下游的迁移中,会经历一个发展壮大、失稳破碎的演化过程,流动从有序走向无序。翼型的近尾迹是一种以旋涡的运动学特性和动力学机制为主导的流动现象。本文着重探讨了翼型尾缘处的涡街形成机理,尾迹内的流动机制,以及近尾迹的流动稳定性。      

风力机翼型动态测压试验技术研究

结合国家高技术研究发展计划课题＂风力机先进翼型族的设计与试验研究＂,针对动态试验设备研制、数据采集和处理方法,在西北工业大学1.6m×3.0m低速翼型风洞（NF-3风洞）开展了风力机翼型动态测压试验技术的研究。采用S809风力机翼型模型,在雷诺数0.75×106和1.4×106、迎角-2°～＋18°条件下,通过改变模型3个平均迎角、3个振荡频率和2个振幅角等状态,进行了动态测压试验,并与静态测压及国外试验结果进行了对比。结果表明：NF-3风洞研制的试验设备,采用的数据采集和处理方法能够应用于风力机翼型的动态测压试验,并可推广应用于其他的翼型动态测压试验研究。     

二维弹性扑翼沉浮运动流动特性的数值研究

采用非定常势流板块法和Euler-Bernoul1i梁振动微分方程,对后缘带弹性薄板的翼型在作沉浮运动时翼型绕流和薄板弹性振动之间的流-固耦合过程进行数值模拟,并计算流动特性、非定常气动载荷,尤其是沉浮运动的推力效应.用有限差分法求解薄板的弹性变形运动,气动弹性分析采用弱耦合迭代方式.通过对翼型非定常气动力(升力和推力)、推进效率、尾迹脱落涡计算结果的分析发现,通过调整弹性薄板的刚度,可以使得翼型产生最佳的非定常推力.