Test And Numerical Simulation Of Flow Characteristics Over A Backward Facing Step

Separation and reattachment of flow are common flow phenomena in nature,industrial production and people's daily life.Despite its structure is simple,backward facing step flow contains all the characteristics of separation and reattachment.Therefore,it is a typical representative for the study of this flow.In water conservancy projects such as sudden expansion of river course and sudden expansion and separation flow after falling sill in spillway tunnel,the separation and re-attachment of flow often accompany the occurrence of cavitation and erosion.Therefore,it is very important to study the flow behind the step to find and improve the location where cavitation is prone to occur in hydraulic engineering.Based on the relevant theoretical knowledge,the flow characteristics over a backward facing step are studied by means of experiment and numerical simulation.Particle image velocimetry(PIV)was used to measure the flow over a backward facing step at 40 Reynolds numbers between 500 and 80000.The Realizable k-?turbulence model was used to simulate the flow over a backward facing step at these Reynolds numbers.The two-dimensional and three-dimensional flow characteristics over a backward facing step were analyzed.The large eddy numerical simulation method was used to simulate the back-step flow under Re=3040.Three groups of different computing conditions were set up to validate the model from the aspects of mesh size,subgrid mode and pressure-velocity coupling algorithm,and the simulation results were analyzed.The main conclusions were as follows:(1)With the increase of Reynolds number,the length of the reflux zone X_r/h first increases and then decreases,and finally stabilizes near 6.6.The length of the secondary reflux zone decreases gradually with the increase of Reynolds number,and the size of the secondary reflux vortex at the step decreases gradually.(2)At low Reynolds number,the turbulent kinetic energy below the step height presents a parabolic distribution,and decreases gradually along the way,and the maximum velocity is found at the central section of the reflux zone.The turbulent kinetic energy above the step height presents a S-shaped distribution,and the velocity at the central section is low.At high Reynolds number,the turbulent kinetic energy increases along the way below the step height,and decreases along the way after leaving the backflow zone.The turbulent kinetic energy increases gradually over the step height,and the velocity is less affected by the whirlpool in the backflow zone behind the step.The position of maximum turbulent kinetic energy decreases gradually along the way.(3)Different mesh sizes have a greater impact on the results of large eddy numerical simulation.The finer the mesh,the more accurate the results,but the longer the time consumed.Different SGS modes have a greater impact on the results of large eddy numerical simulation.The results obtained by using Smagorinsky-Lilly dynamic model are obviously better than those obtained by using Smagorinsky-Lilly model and WALE model,but the time spent on simulation has also increased.Different pressure-velocity coupling algorithms have less influence on the results of large eddy numerical simulation,and the results obtained by PISO algorithm are better than those obtained by SIMPLE algorithm and SIMPLEC algorithm.(4)The start-up process of the flow behind the step c an be divided into four stages:a)the generation stage of the starting vortices,b)the separation stage of the starting vortices,c)the generation stage of other vortices,d)the breakup stage of the starting vortices.The vortexs in the flow field over a backward facing step also experienced four stages:generation,development,migration and breakup.After recirculation zone,the whole process of water jump phenomenon is similar to the change of the vortexs.