Large Eddy Simulation Of Sediment Transport On Different River Bed Morphologies

Natural river beds are usually consisted of variaties of bedforms.In the mountainous streams,boulders and gravels with diameter scaling from centemeters to meters cover river beds.Big boulders of different arrange types could form several boulder bedforms,such as boulder arrays and step-pools.While in the alluvial rivers,bed materials are non-uniform sand with diameter ranging between centemeters and millimetres.The river beds are presented either in rough-flat shape or in three dimensional sediment dunes.The presence of bedforms could greatly influence mean flow fields,turubulent intensities and vortex structures near beds,thus affecting sediment transport process on these bedforms.It is found that sediment transport process on bedforms plays an important role in river stability,river biodiversity,shipping security and security of hydraulic structures.However several important issues of sediment transport process on river bedforms still remains unknown,such as the relationship between sediment transport and flow conditions,spacital variety of sediment transport on bedforms and the mechanics in forming three dimensional sediment dunes.In this paper,we conducted several large eddy simulations to study the issues aforementioned.We develope a large eddy simulation model to calculate bedload transport,suspended sediment transport and bed morphology change on bedforms.A well-development in-house LES code,Hydro3 D,is used to simulate turbulent flow over bedforms.Bedload transport model,suspended sediment transport model and bed morphology change model are embedded by the current author.We then apply this code on turbulent flows over boulder-array bedforms,rough-flat river beds and three dimensional sediment dune bedforms and further study sediment transport process on these bedforms.The results show that sediment transport process is affected both by spacial variables,such as wake-zone,boulder spacing and ratio of width to depth,and time variables,such as instantaneous velocities,turbulent events and vortex structures.Our results show that flow over boulder-array bedforms is strongly turbulent.Boulders cause flow decelaration around boulders,form wakes downstream and redistribute bed stresses.Two kinds of vortices with different frequence,hairpin vortices and gap vortices,are generated by the boulder edge and gaps between boulders and gravels respectively.The fluid exchange between porous bed and the main channel enhanced secondary flow in the spanwise direction.We further calculate bedload transport rate among boulder array beds.The results show great spacial variety near boulders.Based on near bed velocities,a wake-zone low bedload transport region and a boulder-side high bedload transport region are predicted.Based on bed shear stresses,a wake-zone low bedload transport region,a boulder-side high bedload transport region,a boulder-upstream high bedload transport region and a gap vortex high transport region are predicted.Based on near bed turbulent events,the bedload transport rate is stongly suppressed downstream of the boulder.In terms of flow and suspended sediment transport over flat rough beds,our results show that suspended sediment transport is mainly controlled by local roughness height,vertical velocities and elongated vortices.Ratios of width to depth also influence suspended sediment transport via affecting mean flow velocity,secondary flow and Reynolds shear stresses.We successfully simulated the forming process of three dimensional sediment dunes from flat beds.We found that minor sand waves are formed by three steps,local defects formed by high instantaneous velocity,local defects merged together within the region of low instantaneous velocity and formation of wake flow.After minro sand wave forms,these sand waves merge in the streamwise direction and extend in the spanwise direction.And three dimensional sediment dunes are results of this merging and extending processes.