A Thermal Infrared Imaging Observation System For Shallow Flow Velocity

Flow velocity is an important physical quantity to characterize the hydraulic characteristics of water flow.The accurate measurement of shallow flow velocities is of great significance to understand and model the sediment transport and soil erosion.In this study,a thermal infrared image observation system based on thermal infrared imaging technique and computer vision recognition technology was established to measure shallow flow velocities.The established observation system consists of three subsystems:thermal tracer control subsystem,image acquisition and transmission subsystem,and image calculation subsystem.The thermal tracer control subsystem is mainly responsible for providing constant temperature hot water,which consists of electric heater,temperature sensor and hot water pump.The image acquisition and transmission subsystem is responsible for the acquisition and transmission of the thermal infrared images of thermal tracer migration,which consists of a FLIR ONE3.1.0 thermal infrared camera,4 thermal infrared targets,and a wireless router.The image calculation subsystem is mainly responsible for extracting and calculating high frame rate thermal infrared images,which consists of data storage unit and computing matching unit.By means of automatic control of thermal tracer,instantaneous image acquisition,image correction,noise removal and centroid determination,the change of shallow flow velocity was dynamically monitored.To test the precision and accuracy of the established thermal infrared imaging system,a series of experiments were conducted in the Simulated Runoff Hall of the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau.The experimental tank was a 4.6×0.196×0.1 m³ glass tank set at a gradient of 15°to the horizontal,with different flows(0.2～1.5 L·s^-1).At the same time,the flow velocity measured by thermal infrared imaging technology is used to judge the flow pattern,and the flow velocity measured by traditional tracer technology is corrected.The effect of roughness and sediment content on the flow velocity was further discussed.To simulate the undisturbed soil and measure the flow velocity.The thermal infrared imaging observation system provided a reliable way to measure the shallow flow velocity.In the future,images obtained by thermal infrared imaging observation system can be used to trace the development process of water flow in different tracer section at different times,they can also be used to measure the tracer migration velocity along the direction of water flow and the diffusion velocity perpendicular to the direction of water flow.This technique can be applied to the study of rainfall erosion and runoff scour,which is of great significance to the further study of soil erosion process and mechanism.The main conclusions of this paper are as follows:（1）Established thermal infrared imaging observation system,which has high precision and accuracy.The measurement spatial resolution of the system was up to 2mm and the observation time resolution was 1/9 s.The standard deviation of the system was 0.020 m/s and the observation accuracy reached to 98.33%.Compared with traditional tracer techniques（dye tracer and salt tracer methods）,the accuracy of the thermal infrared imaging observation system was higher.The relative errors of thermal infrared imaging observation system were within±10%.The relative errors of dye tracer method were more than 10%.The relative errors of salt tracer method of the 52%samples had relative errors within±10%.（2）The thermal infrared target was developed and the coordinate system was established.It was convenient for all images to calculate parameters in the same coordinate system.The correction coefficient was proposed.On the underlying surface of glass,the ratio of the centroid velocity measured by thermal infrared imaging technology to the velocity measured by dye tracer method and salt tracer method was corrected byαandβ,respectively.The mean value ofαandβwas 0.783and 1.042 respectively.This indicated that dye tracers tend to overestimate the shallow flow velocity.（3）The effected of different underlay surfaces and different sediment content on flow velocity were analyzed.The result showed that under different underlay surfaces,flow velocity increased with the increased of slope and discharge.The flow velocity fluctuates up and down with little change under different sediment content.The water flow velocity of glass underlying surface was greater than that of sandpaper underlying surface and greater than that of artificial grass underlying surface.Most currents are turbulent and rapid.（4）The erosion experiment of undisturbed soil was simulated.The result showed that the flow velocity increased first,then decreased and finally increased gradually with the increased of time.The correction coefficient of soil underlying surface（α=0.171）was smaller than that of glass underlying surface（α=0.783）.The study of sediment rate showed that with the change of time,sediment rate increased first,then decreased and finally increased again.The velocity decreased with the change of sediment rate.