用于无人机导航的光流测速优化方法

针对在无人机视觉导航过程中光流信息易受前景运动物体与噪声影响造成测速精度下降的问题,基于Mean-Shift算法提出了一种光流测速优化方法。该方法使用Lucas-Kanade光流算法计算出稀疏光流后,采用改进的Mean-Shift算法对光流数据进行聚类处理,根据速度的一致性剔除光流数据中的干扰项。实验结果表明,该优化方法可以有效地减少前景运动物体和噪声对无人机光流测速的影响,提高无人机测速的精度。

基于模糊核均值聚类优化的光流测速方法

光流算法作为一种载体速度测量方法,易受光照变化、物体相对移动影响,最终导致速度信息获取不准确。为了提高载体运动速度解算精度,提出了一种基于模糊核均值聚类算法优化的金字塔Lucas-Kanade光流测速方法(FKCM-金字塔LK)。该方法首先通过金字塔LK光流算法得到稀疏光流,然后使用模糊核均值聚类算法对稀疏光流数据进行聚类处理,最终建立光流与实验平台运动参数之间的关系,并得到准确的速度信息。实验结果表明,提出的基于模糊核均值聚类算法优化的金字塔LK光流测速方法不仅可以有效地减少物体相对移动对光流测速的影响,而且与现有其他光流测速方法相比,速度测量精度得到显著提高。

基于互相关的光流测速法在湍流热对流中的应用

同时准确地测量大尺度流动结构和小尺度湍流统计特性对于理解湍流的能量传递规律有着重要意义。尽管粒子成像测速(PIV)技术得到了广泛的应用,但由于算法原理的要求以及商用相机分辨率的限制,PIV技术在同时获取大、小尺度流动特性方面并不实用。另一方面,近年来逐渐引起关注的光流测速法虽然可以实现单像素分辨率的大区域速度场测量,但其准确性通常受限于应用场景而难以保证。本文采用一种基于互相关的光流测速法来分析RayleighBénard对流的粒子图像,进而同时获得全局速度场和小尺度湍流特性。结果表明,这种方法不仅可以获得与PIV技术一致的全局场和流动强度,还能有效地解析出湍流耗散区的速度结构函数。进一步的,分别基于速度结构函数和定义直接计算而获得的能量耗散率不仅在数值大小上互相吻合,还与直接数值模拟的结果以及理论预测相一致。这些结果表明基于互相关的光流测速法在同时测量大尺度流动结构和小尺度统计特性上有很好的应用潜力。

基于改进容积卡尔曼滤波的惯性/光流组合自主测速方法

针对容积卡尔曼滤波算法在惯性/光流组合测速数据融合时出现由于各系统输出数据频率不一致导致融合精度有限的问题,提出了一种基于多速率残差校正的改进容积卡尔曼滤波算法。通过当前时刻误差估算组合导航系统残差,再使用估算后的残差对速度估计值进行补偿,最终实现惯性/光流组合系统速度测量值的数据融合。实验结果表明,通过提出的改进容积卡尔曼滤波对惯性/光流数据进行融合后,东向速度均方根误差为0.2964m/s,北向速度均方根误差为0.06m/s,与现有其他卡尔曼滤波算法相比,此方法可显著提高惯性/光流组合系统的速度测量精度。

MEASUREMENT AND VISUALIZATION OF FLOW FIELD IN AXIAL FLOW FAN

An experimental investigation associated with the basic fluid mechanics in an axial flow fan is described in this paper. The flow field in the tip region has been studied by laser Doppler anemometer (LDA) and flow visualization technique. Some experimental data and images are interpreted to understand the complex interactions between the annulus wall boundary layer and the leakage flow. It shows that the vortex inside the blade passage is produced by the separation of annulus wall boundary layer rather than the rolling up of leakage flow.

Turbulent Energy Budgets of a Ground Vortex Flow

Turbulent kinetic energy budgets are presented for a highly curved flow generated by the collision of plane wall turbulent jet with a low-velocity boundary layer. The different terms are obtained in the vertical plane of symmetry by quadratic interpolation of the LDV （Laser Doppler Velocimetry） measurements, for a wall jet-to-boundary layer velocity ratio of 2. The results, which have relevance to flows encountered in powered-lift aircraft operating in ground effect, quantify the structure of the complex ground vortex flow. The analysis of turbulent energy equation terms using the measured data revealed that production by normal and shear stresses are both very important to the turbulent structure of the impact zone of the ground vortex. This is an indication that the modeling of turbulence of a ground vortex requires a good representation of the production by normal stresses which is most important in the collision zone.

Large eddy simulation of a round jet into a counterflow

A round jet into a counterflow under different jet-to-current velocity ratios was investigated using large eddy simulation.The results agree well with experimental measurements from laser-Doppler anemometry and laser-induced fluorescence that include velocity and mean concentrations along the centerline and radial direction.Vortex rings appear in the region near the jet exit and large-scale vortex structures still occur near the stagnation point.The flow becomes more chaotic and three-dimensional with the presence of these structures.In particular,their presence near the stagnation point results in large velocity fluctuations that enhance the mixing process and dilution.These fluctuations are described by probability density functions that deviate from Gaussian distribution.The three-dimensional streamlines indicate that the jet not only oscillates in three directions but also rotates about the jet axis and around the vortex.The second and third moments of the velocity or scalar fluctuations identify that the mixing processes are greater in the region before the stagnation point.

UV soaking for enhancing the photocurrent and response speed of Cs_(2)AgBiBr_(6)-based all-inorganic perovskite photodetectors

The response speed of the reported Cs_(2)AgBiBr_(6)-based photodetectors exhibits a wide variation ranging from microseconds to nanoseconds,while the reason is still unclear.Apart from the conventional approaches such as reducing effective area,new regulating approaches for response speed improvement have rarely been reported.On the other hand,it is generally believed that ultraviolet(UV)light has negative impact on perovskite devices resulting in performance degradation.In this work,we demonstrated that the response speed of the photodetector with FTO/Cs_(2)AgBiBr_(6)/Au structure can be effectively regulated by utilizing UV light-soaking effect without reducing the device area.Particularly,the decay time is efficiently modulated from 30.1μs to 340 ns.In addition,the−3 dB bandwidth of the device is extended from 5 to 20 kHz.It is worth mentioning that the light current is remarkably boosted by 15 times instead of any attenuation.Furthermore,we prove the universality of UV soaking treatment on Cs_(2)AgBiBr_(6)-based photodetectors with other all-inorganic structures,i.e.,FTO/TiO_(2)/Cs_(2)AgBiBr_(6)/Au,FTO/Cs_(2)AgBiBr_(6)/TiO_(2)/Au and FTO/TiO_(2)/Cs_(2)AgBiBr_(6)/CuSCN/Au.Our results demonstrate a new method to improve the response speed and light current of Cs_(2)AgBiBr_(6)-based perovskite all-inorganic photodetectors.