基于1.5 μm多普勒激光雷达的飞机尾涡探测技术研究

为规避尾涡威胁,保障飞行安全,研究了飞机尾涡的激光探测技术.介绍了尾涡探测基本原理,给出了激光探测方式设计和探测系统参量选择.基于设计的尾涡激光探测方案,研究了飞机尾涡回波多普勒谱与机型参量、飞行参量以及环境参量间的关系,获得了尾涡的径向速度分布规律,建立了尾涡回波多普勒谱模型,选取最佳尾涡参量估计算法用于尾涡的全面表征|通过开展A340的尾涡探测外场实验验证了激光探测尾涡的可行性和尾涡参量估计算法的有效性.研究表明,尾涡回波多普勒谱值与径向速度的三次方成反比,与涡流环量的二次方成正比.     

基于1.5μm多普勒激光雷达的飞机尾涡探测技术研究

为规避尾涡威胁,保障飞行安全,研究了飞机尾涡的激光探测技术.介绍了尾涡探测基本原理,给出了激光探测方式设计和探测系统参量选择.基于设计的尾涡激光探测方案,研究了飞机尾涡回波多普勒谱与机型参量、飞行参量以及环境参量间的关系,获得了尾涡的径向速度分布规律,建立了尾涡回波多普勒谱模型,选取最佳尾涡参量估计算法用于尾涡的全面表...     

基于相干多普勒激光雷达的飞机尾涡观测

相干多普勒激光雷达(Coherent Doppler Lidar, CDL)凭借高精度、高时空分辨率等优势已成为晴空尾涡探测的主要手段。为了提高飞机尾涡观测效率并实现数据的快速处理和实时显示,基于2018年在四川开展的尾涡观测实验,提出了一种基于CDL频谱宽度和径向风速的尾涡快速识别方法,并基于此方法分析了典型机型的尾涡演化过程。结果表明,不同机型的尾涡演化趋势相似,均在距地面约50 m处产生并向下、向外扩散,尾涡环量逐渐减小,但由于近地面效应的影响,会伴有短时间环量增加现象。随着飞机重量和翼展的减小,尾涡的耗散时间缩短,左右涡初始涡核的间距减小,频谱宽度融合现象更明显,该现象消失所历经的时间增加。     

涡环发展及其与气泡相互作用的数值模拟

本文发展了三维离散涡模型用于模拟无粘不可压缩涡环的发展及其与气泡的相互作用,涡环被离散为平均分布在其中心线上的球形涡元,由Biot-Sarvart定律叠加每个涡元的诱导速度得到涡环的运动速度.涡环横截面上的涡量分布由二阶高斯分布来近似,而涡环随时间的演化则采用具有二阶精度的预测-校正算法.利用本文发展的三维离散涡模型,首先对圆涡环的发展进行了模拟,运动速度同理论解对比非常吻合,验证了模型的有效性.随后分别模拟了不同强度、不同核径比、不同长短轴比单个椭圆涡环的运动,成功再现了椭圆涡环的长短轴交替互换等现象,得到了不同工况下椭圆涡环随时间发展和演化的定性规律.最后本文将三维离散涡法与气泡运动方程相耦合,对涡环和气泡的相互作用进行了模拟,得到的气泡运动轨迹结果与实验结果非常一致,说明本文提出的三维涡模型可以推广到三维两相的情形.     

正弦高斯涡旋光束的远场传输特性

根据角谱法和稳相法,推导了正弦高斯涡旋光束TE波和TM波在远场传输和能流密度的解析表达式,研究了正弦高斯涡旋光束在远场中的相位奇点和能流密度分布.结果表明:正弦高斯涡旋光束的远场特性与高斯光束的束腰宽度、涡旋离轴量、坐标位置以及与正弦项相关的参量有关.在一定条件下,远场中会出现相位奇点和能流密度黑核;当控制参量改变时,相位奇点和黑核的位置会发生移动,但原点处不受影响.相位奇点和能流密度的对称性主要受涡旋离轴量影响,当涡旋离轴量为0时,相位奇点和能流密度分布关于原点对称;当涡旋离轴量改变时,相位奇点和能流密度分布呈现出非对称性.     

基于射流冲击横流原理产生涡旋及其测量

采用射流冲击横流的方法产生涡旋,模拟水岸和流水域的涡旋生成.通过CFD软件模拟真实的水流,研究射流和横流不同流速比以及射水口的位置对涡旋产生的影响,确定涡旋生成的最佳参量.实验中在水槽里产生稳定的涡旋,加入红色泡沫颗粒实现涡旋可视化,并采用双摄像头和3D-PIV技术对涡旋的三维流场特征和结构研究分析,计算得到涡旋的速度环量场、线速度场、涡量场以及黏度.     

基于自适应窗的合成孔径激光雷达联合时频成像方法

传统距离-多普勒算法中观测期间多普勒频率为常量的假设在合成孔径激光雷达系统中不再成立.根据合成孔径激光雷达回波信号模型,推导计算了振动对合成孔径激光雷达回波相位的影响.根据实际情况引入振动频率不超过500 Hz、振幅不超过0.2 mm的机载平台微振动参量,结果表明载机微振动对合成孔径激光雷达成像的方位分辨率影响很大,而对距离分辨率影响不大.采用时频分析成像方法,能够有效克服传统距离-多普勒算法对时变信号处理的局限性.分析比较Wigner-Ville分布、伪WWigner-Ville分布和平滑伪Wigner-Ville分布作为核函数的时频成像结果,均无法兼顾成像分辨率和成像效率.据此本文提出了一种基于自适应窗的联合时频成像方法,根据变异系数和变异阈值的比较自适应地调整子区域窗口大小,从而达到成像分辨率和算法效率的平衡优化.仿真结果表明,本算法对成像分辨率影响不大,成像时间减少近69.87%,适用于合成孔径激光雷达对成像高分辨率和成像效率的要求.     

基于自适应窗的合成孔径激光雷达联合时频成像方法

传统距离-多普勒算法中观测期间多普勒频率为常量的假设在合成孔径激光雷达系统中不再成立.根据合成孔径激光雷达回波信号模型,推导计算了振动对合成孔径激光雷达回波相位的影响.根据实际情况引入振动频率不超过500Hz、振幅不超过0.2mm的机载平台微振动参量,结果表明载机微振动对合成孔径激光雷达成像的方位分辨率影响很大,而对距离分辨率影响不大.采用时频分析成像方法,能够有效克服传统距离-多普勒算法对时变信号处理的局限性.分析比较Wigner-Ville分布、伪WWigner-Ville分布和平滑伪Wigner-Ville分布作为核函数的时频成像结果,均无法兼顾成像分辨率和成像效率.据此本文提出了一种基于自适应窗的联合时频成像方法,根据变异系数和变异阈值的比较自适应地调整子区域窗口大小,从而达到成像分辨率和算法效率的平衡优化.仿真结果表明,本算法对成像分辨率影响不大,成像时间减少近69.87%,适用于合成孔径激光雷达对成像高分辨率和成像效率的要求.     

环形与平面光学涡旋共线叠加的复合涡旋

将环形与平面涡旋核重合并共线叠加,在多个直径上得到圆对称分布的复合涡旋,模拟计算结果表明,其强度也呈圆对称分布.对具有任意参量的上述叠加方式得到的复合涡旋,提出了一种求解共线叠加复合涡旋位置的图解方法,利用该方法可以得到准确的涡旋位置分布特性,且确定的涡旋位置与数值模拟结果一致.     

米散射激光雷达重叠因子及全程回波信号标定技术研究

由于激光雷达光轴夹角的存在和光束质量分布的非均匀性, 导致理论回波信号与实际回波信号有较大差别, 需要对其进行校正. 基于数学推导和软件仿真, 给出了激光雷达重叠因子的数学表达式, 分析了光轴夹角导致的系统重叠因子在全程探测空间的变化情况; 分析了光束分布为高斯分布和均匀分布时的重叠因子变化情况; 对激光雷达的距离校正信号和Klett算法公式进行了重叠因子修正, 最后在对激光雷达系统参数标定的基础上, 在重叠区域利用修正的Klett公式对其所测得到的回波信号进行了修正, 在探测盲区利用斜率法修正距离校正信号, 进而得到了符合理论与实际情况的激光雷达在全程上校正的消光系数曲线. 关键词： 激光雷达 标定 重叠因子 消光系数     

基于涡流发生器的风洞试验段附面层控制数值模拟

沿试验段侧壁发展的附面层是影响飞行器半模型实验数据精准度的主要因素之一.利用数值模拟方法验证了涡流发生器减小附面层影响的可行性,重点分析了安装角度、结构尺寸、安装位置及个数等设计参数对附面层内速度分布的影响规律,对涡流发生器尾涡强度以及沿流向的发展规律进行了初步探讨.结果表明,涡流发生器产生的尾涡能够有效改善附面层内的速度分布,进而减小附面层厚度,降低附面层影响;涡流发生器的后缘应略高于当地附面层厚度,安装角度、位置、个数等参数必须合理设计以减小涡流发生器对试验段主气流的影响.基于计算结果初步设计了可用于2.4 m跨声速风洞半模试验段的涡流发生器,在亚声速范围内能够减小模型区侧壁附面层厚度66%左右,对核心流Mach数影响小于0.003,为涡流发生器的实际应用提供了依据.      

Wake structure of a deformable Joukowski airfoil

We examine the vortical wake structure shed from a deformable Joukowski airfoil in an unbounded volume of inviscid and incompressible fluid. The deformable airfoil is considered to model a flapping fish. The vortex shedding is accounted for using an unsteady point vortex model commonly referred to as the Brown-Michael model. The airfoil’s deformations and rotations are prescribed in terms of a Jacobi elliptic function which exhibits, depending on a dimensionless parameter m, a range of periodic behaviors from sinusoidal to a more impulsive type flapping. Depending on the parameter m and the Strouhal number, one can identify five distinct wake structures, ranging from arrays of isolated point vortices to vortex dipoles and tripoles shed into the wake with every half-cycle of the airfoil flapping motion. We describe these regimes in the context of other published works which categorize wake topologies, and speculate on the importance of these wake structures in terms of periodic swimming and transient maneuvers of fish.     

Nature of the source of vortex sound flowing around a cylindrical profile

This paper is devoted to refining the nature of a vortex sound source and validly estimating the parameters of the region of source origination in a wake behind a cylindrical profile depending on the incoming flow velocity and profile diameter. Based on experimental measurements of the rms values of pressure pulsations on the surface and in the wake behind the profile and hydrodynamic laws for 2D fluid flows, the position of the origin of the vortex street in the wake and the size of the region where the street is still irregular are estimated. In this region, the street dimensions and pressure pulsation amplitudes change with distance from the profile. It is found that the maximum of the pressure pulsation in the wake approaches the profile surface in the range of Reynolds numbers (4.7 × 10³?1.5 × 10⁴); the amplitude of pressure pulsations on the profile and vortex sound intensity also increase. Based on the relationship between the source’s position and size and the width of the vortex street, as well as taking into account the decay of vortex circulation in the street with increasing distance to the profile, it is shown that the distance from the source to the surface of the profile should not exceed two gages. It is shown that an obstacle in the wake in the region of its irregularity causes a decrease in pressure pulsations on the profile and attenuation of emitted sound. Sound emission ceases completely when the obstacle comes in direct contact with the region of origination of the vortex street. Theoretical estimates satisfactorily agree with the measurement results.     

Riemannian geometry of irrotational vortex acoustics

We consider acoustic propagation in an irrotational vortex, using the technical machinery of differential geometry to investigate the "acoustic geometry" that is probed by the sound waves. The acoustic space-time curvature of a constant circulation hydrodynamical vortex leads to deflection of phonons at appreciable distances from the vortex core. The scattering angle for phonon rays is shown to be quadratic in the small quantity Gamma/2pi(cb), where Gamma is the vortex circulation, c the speed of sound, and b the impact parameter.     

Drogue detection for vision-based autonomous aerial refueling via low rank and sparse decomposition with multiple features

The technology of vision-based probe-and-drogue autonomous aerial refueling is an amazing task in modern aviation for both manned and unmanned aircraft. A key issue is to determine the relative orientation and position of the drogue and the probe accurately for relative navigation system during the approach phase, which requires locating the drogue precisely. Drogue detection is a challenging task due to disorderly motion of drogue caused by both the tanker wake vortex and atmospheric turbulence. In this paper, the problem of drogue detection is considered as a problem of moving object detection. A drogue detection algorithm based on low rank and sparse decomposition with local multiple features is proposed. The global and local information of drogue is introduced into the detection model in a unified way. The experimental results on real autonomous aerial refueling videos show that the proposed drogue detection algorithm is effective.     

收缩螺旋涡线诱导速度的快速收敛法

给出了一种计算收缩涡线(桨尖涡和内部涡线)诱导速度所有组元的快速收敛方法,该方法是由Rand和Chiu的方法(计算螺旋涡线诱导速度快速收敛法)发展而来的,它以寻求上下界截断积分解析表达式为基础。其优点是能计算尾迹中任意一点的诱导速度,并且考虑了尾迹收缩。该方法不仅避免了离散涡线引起的截断误差。而且减小了沿有限方位角所产生的误差。     

Vortex sink under conditions of a thermal choking with regard to the real properties of gases

Changes in the formulation of the problem for a vortex source and a vortex sink upon taking into account the change in the heat capacity and the adiabatic exponent for a diatomic gas (for the example of air) in response to an increase in the temperature from 300 K to a few thousands of Kelvin are discussed. A thermal choking is studied for a vortex sink, and critical values of the energy parameter are calculated. It is shown that the minimal radius of the vortex sink decreases upon a heat release. Similarity parameters including the dimensionless circulation (or mass flow), the energy parameter, and the position and thickness of the heat-release region are varied. Errors of the gas model that assumes constant heat capacities and a constant adiabatic exponent are estimated.     

Cylinder wake in lattice gas hydrodynamics

We measure, using a lattice gas algorithm, the time development of a wake behind a cylinder at Reynolds numbers equal to 30 and 60. We measure also drag at these values and determine Re90 to be the Reynolds number where vortex shedding starts. Agreement with experimental results is good. We also measure vortex shedding frequency at Re = 108 from transverse velocity and at Re = 180 from lift on the cylinder. The corresponding Strouhal numbers are obtained and discussed.     

Phase averaged velocity field in the near wake of a square cylinder obtained by a PIV method

Phase averaged velocity fields in the near wake region behind a square cylinder have been successfully obtained using randomly sampled PIV data sets. The Reynolds number based on the flow velocity and the model height was 3,900. To identify the phase information, we examined the magnitude of circulation and the center of peak vorticity. The center of vorticity was estimated from lowpass filtered vorticity contours adopting a sub-pixel searching algorithm. Due to the sinusoidal nature of circulation which is closely related to the instantaneous vorticity, the location of peak vorticity fits well with a sine curve of the circulation magnitude. Conditionally averaged velocity fields represent the Karman Vortex shedding phenomenon quite successfully within ±5° phase uncertainty. The oscillating nature of the separated shear layers and the separation bubble at the upper and lower surfaces are clearly observed. With the hot-wire measurement of Strouhal frequency, we found that the convection velocity changes its magnitude very rapidly from 25 to 75 percent of the free stream velocity along the streamwise direction when the flow passes the recirculation region.