Numerical Study On Turbulent Drag Reduction In Grooves In Pipeline By Large Eddy Simulation

As an important means of transportation,the resistance reduction of surface in gas pipeline is a valued issue in relevant fields.The turbulent flow drag reduction technique of riblet structure is a new type of drag reduction technique.In this paper,Large Eddy Simulation(LES)method is adopted to carry out numerical calculation of gas pipeline with rib structure,to capture the details of flow field and analyze the calculation results,so as to explore the effect of drag reduction and its mechanism.The drag reduction effect of the traditional triangular rib was investigated for the large size pipeline,which is approximately flat.In this study,the air was made as material.By comparing the calculated results of the smooth wall with the theoretical results and previous DNS research results,the drag reduction rate was calculated and compared with the experimental drag reduction rate.The mechanism of drag reduction on riblet is put forward by Analyzing time-average statistics and instantaneous flow field.The unique groove structure of the riblet limits the outward and spanwise development of the fluid in the groove,so that the low-speed steady flow with low energy is maintained in the groove.While the highly disturbed turbulent flow is pushed away from the wall,which increases the thickness of the boundary layer,avoids the direct friction between the high-speed fluid and the solid wall,reduces the velocity gradient near the wall,and reduces the frictional resistance.To improve the effect of drag reduction,the traditional rib model was modified.The traditional rib was arranged along the flow direction,and the rectangular surface of 1mm thickness was blocked along the flow direction toform a new rib structure.By comparing the calculation results before and after the modification,the drag reduction effect is increased by 35.8% after the modification,and the difference in the flow field before and after the modification is compared and analyzed.After modification,the skin friction drag reduction was twice as much as before modification,but the increase of pressure drag weakens the effect of skin friction drag reduction,making the total drag reduction rate 1.35 times as much as before modification.Timeaverage statistics are basically the same in the two riblet cases,with only slight differences.The transverse discontinuous rib formes vortices above and behind the blocking area,which reduces the shear stress near the blocking surface of the rib wall,changes the pressure distribution state on the wall,and makes the pressure distribution more uneven.After adding discontinuities,the distribution of low-velocity bands near the wall is more obvious.For gas pipelines with small diameter,circular pipes are directly used for calculation.In this paper,the traditional triangular rib is applied to the small diameter pipeline to transport natural gas,and the drag reduction effect of-14% is obtained.It reduces the velocity and wall friction in a wide range near the wall of the pipe.Drag reduction mechanism of circular pipe in traditional riblet agree with that of riblet on flat.Riblet limits the low-speed fluid in the groove,pushes the high-speed and high-disturbance turbulence away from the wall,and increases the thickness of boundary layer.The sliding friction between high velocity fluid and low speed fluid replaces the direct friction between high velocity fluid and wall and reduces the wastage of fluid flowing through the wall.