Direct Numerical Simulation Of Secondary Flow And Turbulence Coherent Structures In Compound Open Channel

Compound channel is a common channel type in flood plain area.In compound channel,both secondary flow out of the main flow plane and turbulent flow in the boundary layer can be observed.Despite their weak intensity,they have significant influences on the streamwise velocity distribution,pollutant concentration distribution and river bed stability.There exists some relationship between the secondary flow and the turbulence coherent structure.However,at the moment,the major of the researches has focused on the secondary flow in compound open channel.That on the interactions between turbulence coherent structures and secondary flow is rare.Therefore,a combined study on secondary flow and turbulence coherent structures in compound open channel is valuable in understanding the interactions between secondary flow and coherent structures.In this paper,direct numerical simulation of secondary flow and turbulence coherent structures in compound open channel with a moderate Reynolds number was carried out.The research background and trend of secondary flow and turbulence coherent structures in compound open channel were reviewed.Analysis on the direct numerical simulation results on flat-plate turbulence,rectangular open channel turbulence and compound open channel turbulence was performed.The main contents and conclusions are as follows.i.Direct numerical simulation of flat-plate turbulence with Re=180 was carried out,and results showed good agreement with those in literature.ii.Direct numerical simulation of turbulent flow with Re=180 in a rectangular open channel was carried out.Three major vortex-pairs were found and the max velocity of the secondary flow is approximate 3% of that of main flow.iii.Direct numerical simulation of turbulence flow with Re=180 in a compound open channel was conducted.It was found that there was a major vortex-pair near the junction of flood plain and deep trough.The max velocity of the secondary flow is approximate 4.5% of that of main flow.Near the junction of flood plain and deep trough,the turbulent intensities and Reynolds stresses were high,due to the large amount of turbulence coherent structures in that region.iv.The flow characteristics and the secondary flow structures were analyzed by comparing the direct numerical simulation results of flat-plate turbulence,rectangular open channel turbulence and compound open channel turbulence.The interactions between secondary flow and turbulent coherent structures were discussed.