| The incipient and transport of sediment particles is one of the essential issue of bed load transport theory and river dynamics.The bed load formulas have been studied deeply by researchers from home and abroad and their achievements have been widely used in the relative numerical simulation study of scour pit,bridge pier local scour and river bed revolution.As one of the key point of the study of sediments transport,the forces acted on particles have been studied and discussed by researchers.In water flow,sediment particles are forced by submerged gravity,the interaction force between particles,as well as the lift and drag forces exerted by water flow.The forces exerted on particles are affected by various factors such as the state of flow field,particle size and gradation and relative positions between particles as well.Based on previous studies,in this paper the theory derivation method is adopted to study the bed load transport formula;based on the numerical simulation method,the software FLUENT is adopted to simulated the three dimensional flow around sediment particles with different positions on bed surface,the forces coefficients as well as flow field are analyzed.The main research contents and conclusions are as fllows:(1)The theoretical framework of bed load sediment transport rate formula came up by H.A.Einstein has been widely recognized,but still has many defects in its details.Many researchers have carried out many corrected and improved works.But the randomness characteristic of the position on bed surface has not been considered until now.Based on previous study,the relative exposure degree is adopted to describe the relative position of sediment particles on bed surface,based on rolling model,the random distribution characteristics of both bed surface sediment and water flow are considered at the same time in this paper,then a joint threshold probability of sediment is carried out,from which a new bed load transport formula is also derived.Einstein's theory of bed load transport rate is corrected and improvement as well.The revised formula is in good arrangement with measured data.(2)The LES and RANS turbulence model are used to simulate the three dimensional flow pass a single sediment particle on the smooth flume and the simulation results are compared.It shows that the LES model can reflect the pulsating characteristics of forces acted on particle more clearly and a more realistic flow field distribution can be obtained,which is better than the RANS model in all aspects.(3)The physical models of sediment particle both completely exposed and completely hided in the rough bed are constructed.The LES model is used to simulate the three-dimensional flow pass the research particles under different inlet velocity and the variation of CL and CD as well as the flow field are analyzed.Results show that there are significant differences existed in the value of CL and CD of particles and their changing rules at different relative positions with flow velocity.Compared with relevant experimental and simulation results,the results of this study exist differences,but the changing trend can consistent with other research results.When water flows through the rough surface,a low-speed zone is formed along the flow direction.The thickness of low-speed zone increases gradually along the flow direction and develops to the end of the flume.With the increase of flow velocity,the number of low-speed belt increases but the single width decreases.In addition,the existence of surface particles could reduce the regularity of the shape and spatial distribution of the low-speed zone. |
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