Development And Application Of Large Scale Surface Velocimetry

Large-scale surface velocimetry provides a novel method for both two-dimensional(2D)and three-dimensional(3D)measurements,enabling interpretation of coherent structures on river surface,quantification of complex characteristics in weir flows and dam-break flows,and collection of velocity data for various engineering purposes.In recent years the new method has been drawing much attention in hydraulic research as a promising velocity measurement technique.The study presents progress of development and application of the large-scale surface velocimetry in four aspects,summarized as follows.1.Feature matching velocimetry(FMV)is introduced to enhance computational efficiency in 2D measurement.The influence of image processing and parameter settings is investigated and recommendations are given with respect to the choice of parameters.The precision of averaged and fluctuating velocities of FMV reaches ±2% and ±10%,respectively,while the computational speed is doubled compared to FMV and PIV/PTV.The validity of FMV is tested in weir flow and jet flow.2.An airborne image velocimetry(AIV)system is developed for field measurement.For cruise flying mode,an improved Sf M-PIV method is applied to long river reaches in both urban and rural areas,achieving high coverage of velocity field with an accuracy of ±10%.For hover flying mode,a rapid measuring technique based on feature matching is presented for measuring local surface velocity field in an urban river reach with enhanced computational efficiency.The measured data enable qualitative description of the turbulence characteristics including large-scale coherent structures on the river surface.3.A new indirect method is introduced to reconstruct complex three dimensional surface velocity field in steady flows.The new method maps the curved flow surface with two coplanar cameras and achieves measurement of 2D velocity field with one camera.The measured 2D velocity field is projected onto the 3D flow surface based on complex linear mapping relationship between the object space and image space.Application of this method to weir flow measurement yields satisfactory results.4.A large scale 3D surface particle tracking velocimetry(3DSPTV)is developed based on two cameras.Its application to weir flow reveals that the nappe of the weir flow keeps to a uniform shape under different conditions.The longitudinal velocity shows rapid increase followed by slow drop along the nappe,while the vertical velocity approximately fits free fall motion.Application of the 3DSPTV to earthen embankment overtopping flow yields 3D surface morphology and velocity field,based on which four typical stages of earthen embankment can be clearly recognized.