Photogrammetry for Free Surface Flow Velocity Measurement: From Laboratory to Field Measurements

Hang Trieu,Mikael Sjödahl,Henrik Lycksam,J Gunnar I Hellström,Patrik Andreasson,Per Bergström

doi:10.3390/w13121675

Hang Trieu, Mikael Sjödahl + Show 4 more

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https://doi.org/10.3390/w13121675

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Journal: Water	Publication Date: Jun 17, 2021
Citations: 5	License type: CC BY 4.0

Affiliation: Luleå University of Technology, Vattenfall (Sweden)

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This study describes a multi-camera photogrammetric approach to measure the 3D velocity of free surface flow. The properties of the camera system and particle tracking velocimetry (PTV) algorithm were first investigated in a measurement of a laboratory open channel flow to prepare for field measurements. The in situ camera calibration methods corresponding to the two measurement situations were applied to mitigate the instability of the camera mechanism and camera geometry. There are two photogrammetry-based PTV algorithms presented in this study regarding different types of surface particles employed on the water flow. While the first algorithm uses the particle tracking method applied for individual particles, the second algorithm is based on correlation-based particle clustering tracking applied for clusters of small size particles. In the laboratory, reference data are provided by particle image velocimetry (PIV) and laser Doppler velocimetry (LDV). The differences in velocities measured by photogrammetry and PIV, photogrammetry and LDV are 0.1% and 3.6%, respectively. At a natural river, the change of discharges between two measurement times is found to be 15%, and the corresponding value reported regarding mass flow through a nearby hydropower plant is 20%. The outcomes reveal that the method can provide a reliable estimation of 3D surface velocity with sufficient accuracy.

Highlights

In order to perform the measurements at natural flow, it is important to study the properties of the measurement system regarding camera flow, it is important to study the properties of the measurement system regarding camera setup, camera calibration, and particle tracking velocimetry (PTV) algorithm in a controlled environment
Even if further research is required to study the accuracy of the photogrammetrybased PTV system comprehensively, the investigation in this study demonstrates good potential applicability of using multiple cameras in free surface flow velocity estimation
In the field of view of 0.8 m × 0.8 m, the error related to the camera system is about 0.03%, while the random error is estimated to be 7.7 mm/s which corresponds to 8.7% on the average velocities

Summary

Introduction

The ability to measure flows in the vicinity of hydropower plants has undeniably wide applications. In cases of plant changes, optimization of water utilization, or changing operating conditions, flow-related parameters often need to be taken into account. Among those parameters, flow velocity distribution over a large area undoubtedly contributes to the understanding of natural phenomena and leads to a better basis for decision making. The measurement task usually requires extensive investigative work due to complicated geometry, large volumes, transient effects. The use of the image analysis technique makes it possible to capture the surface-water velocity distribution over a large area outdoors without interference of normal plant operation

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