Zhang Zhen, Zhao Lijun, Liu Boyuan, Jiang Tiansheng, Cheng Ze
College of Computer and Information Engineering, Hohai University, Nanjing 211100, China.
Micromachines (Basel). 2022 Jul 23;13(8):1167. doi: 10.3390/mi13081167.
Particle image velocimetry (PIV) is a quantitative flow visualization technique, which greatly improves the ability to characterize various complex flows in laboratory and field environments. However, the deployment of reference objects or ground control points (GCPs) for velocity calibration is still a challenge for in situ free-surface velocity measurements. By combining space-time image velocimetry (STIV) with direct sensor orientation (DSO) photogrammetry, a laser distance meter (LDM)-supported photogrammetric device is designed, to realize the GCPs-free surface velocity measurement under an oblique shooting angle. The velocity calibration with DSO is based on the collinear equation, while the lens distortion, oblique shooting angle, water level variation, and water surface slope are introduced to build an imaging measurement model with explicit physical meaning for parameters. To accurately obtain the in situ position and orientations of the camera utilizing the LDM and its embedded tilt sensor, the camera's intrinsic parameters and relative position within the LDM are previously calibrated with a planar chessboard. A flume experiment is designed to evaluate the uncertainty of optical flow estimation and velocity calibration. Results show that the proposed DSO-STIV has good transferability and operability for in situ measurements. It is superior to propeller current meters and surface velocity radars in characterizing shallow free-surface flows; this is attributed to its non-intrusive, whole-field, and high-resolution features. In addition, the combined uncertainty of free-surface velocity measurement is analyzed, which provides an alternative solution for error assessment when comparing measurement failures.
粒子图像测速技术（PIV）是一种定量流动可视化技术，它极大地提高了在实验室和野外环境中表征各种复杂流动的能力。然而，在原位自由表面速度测量中，部署用于速度校准的参考物体或地面控制点（GCP）仍然是一个挑战。通过将时空图像测速技术（STIV）与直接传感器定向（DSO）摄影测量相结合，设计了一种由激光测距仪（LDM）支持的摄影测量装置，以实现斜射角度下无GCP的表面速度测量。基于共线方程进行DSO速度校准，同时引入镜头畸变、斜射角度、水位变化和水面坡度，建立一个对参数具有明确物理意义的成像测量模型。为了利用LDM及其嵌入式倾斜传感器准确获取相机的原位位置和方向，事先使用平面棋盘对相机的内参和在LDM中的相对位置进行校准。设计了一个水槽实验来评估光流估计和速度校准的不确定性。结果表明，所提出的DSO-STIV在原位测量中具有良好的可转移性和可操作性。在表征浅自由表面流方面，它优于螺旋桨式流速仪和表面速度雷达；这归因于其非侵入性、全场和高分辨率的特点。此外，分析了自由表面速度测量的合成不确定性，为比较测量失败时的误差评估提供了一种替代解决方案。
