Research On Image Measurement Technology Of Dam Break Wave And Hydrodynamic Characteristics Of Wave Front

The dam break wave is one of the important research topics in hydrodynamics,and velocity and water level,as the flow characteristics of dam break wave,are the two key physical quantities used to describe the hydrodynamic characteristics of dam break wave.The dam break wave is a kind of unsteady flow occurring in an open channel,and its flow characteristics change with time and space,so it is not easy to measure the speed or water level.In this paper,the image measurement technology of dam break wave velocity and water level were studied,and the hydrodynamic characteristics of the wave front area under typical working conditions were studied through experiments.The research in this paper has certain guiding significance for the experimental measurement of dam break wave and provides data support for verifying the calculation of the hydrodynamics method.The main research contents and achievements of this paper are as follows:In the aspect of measurement technology,first of all,according to the change of image gray level caused by dam break wave contour and internal bubble movement,optical flow image velocity measurement technology was introduced.The pyramid simulation evolution idea was used to optimize and improve the traditional optical flow method based on global smoothing,and the simulation image was used to verify the improvement effect.The results show that the optimization algorithm has obvious improvement over the traditional Horn-Schunck（HS）optical flow method in velocity measurement accuracy,no matter the uniform particle image or the complex water flow image.Then,a new water level measurement technique based on edge detection was developed by taking advantage of the obvious water level line at the interface between water flow and air.The measurement accuracy was verified by numerical simulation,the measured value and the simulated value were fitted linearly,and the goodness of fit R² was greater than 0.999,indicating that this technology could accurately measure the water level distribution of the dam break wave.And a water level measurement software was developed based on MATLAB.In the experiment,the hydrodynamic characteristics of the wave front region under dry and wet conditions downstream were studied by the image measurement of velocity and water level.For the downstream dry river bed,the measured water level is in good agreement with the numerical simulation,and the overall error is less than 5%.The experimental results show that the Chanson theory can predict the wavefront water level well when the friction coefficient f is precisely set.When the upstream initial water level h₀ is constant,the water level and the slope of the wave front region decrease with the increase of time t.When t is the same,the water level in the wavefront region increases with the increase of h₀.In addition,the increase of the bed surface roughness will cause the further increase of the wave front water level,and the wave front velocity decreases with the increase of t due to the existence of friction.For the downstream wet riverbed,the wave front area presents a mushroom jump,accompanied by enrolling and aeration.Results show that with the increase of water level ratioσ,mushroom structure dimensions of the structure are reduced,at the same time,aeration degree also decreased accordingly.The maximum water level in the mushroom-shaped wavefront region increases first and then decreases with time because of the direct internal conversion of mechanical energy.The instability of the mushroom structure in the wavefront region is also found.The wave front region is divided into initial formation section,transition section and stable section,and the flow characteristics of each phase are different.