Experimental Study On Coherent Structures And The Impact On The Bed Load Movement

Turbulence is filled with different types and scales of coherent structure,which control the generation?transmission and dissipation of turbulent kinetic energy.In nature,rivers and oceans are important forms of turbulence.Turbulence flow can carry sediment transport along the flow direction at the bottom of the riverbed to form bedload movement.Bed load movement is closely related to the coherence structure of turbulent flow.Therefore,it is important to study the interaction between turbulent coherent structures and sediment particles movement,which is of great significance to fluid mechanics and sediment mechanics.The turbulence coherent structure is a random and complex structure and the different coherent structures do not exist independently,but interact with each other.At present,there is still some controversy about this problem,which needs further study.Additional formula of bedload transport rate can be fitted with the initial parameters of fluid intensity and sediment particles;however,there is little research on the characteristics of sediment particle movement.The purpose of this paper is to deepen the understanding of turbulent coherent structures and to analyze the relationship between coherent structure and sediment particle movement.The main results can summarize as follows:(1)The two-dimensional PIV system was employed to obtain the study the flow field and study the influence of the probe on the front.The relation between the influence range and the diameter of the model is established.The correlation between the instrument design and the flow Study to provide reference.(2)It is found that the development of streaky structure of turbulence from the inner region to the outer region is a continuous process;and the characteristic scales?PDF and percentage area of high-and low-speed streaky structure are analyzed along the y direction in both open channel flow and closed channel flow;In addition,from the formation,development and distribution of streaky structures,it can demonstrate that the coherent structure models such as attached vortex,streamwise vortex and hairpin vortex are reasonable.(3)Under the influence of turbulent coherent structure,the sparse sediment parti-cles moving along the flow direction can form a strip-like structure.The streak width is symmetric with respect to the midline of the bed,attaining its maximum value at the center and decreasing gradually toward the sidewall.The model of streamwise vortex can explain the streaky structure of sediment particles,that is,the reverse rotation of nearby streamwise vortices forms the streaky structure.The accumulative distribution curve of the sparse sediment motion trajectory is smooth,which shows that the thin sediment is better to follow the turbulent flow,which has little effect on the turbulence structure.(4)The existence of the turbulent coherent structure leads to the fact that the bed load motion is not an average process.After comparing the three-dimensional reconstruction coordinates of bed load with the sediment movement rate,the region of high sediment movement rate forms the groove structure,and the region of low sediment movement rate forms the ridge structure.Due to bed morphology of the groove and ridge structure,the original turbulent coherent structure is changed.Moreover,a fixed sediment position results in the formation of one kind vortex structure similar to the secondary flow.