| While the country’s economy is growing rapidly,it is also facing a series of problems such as energy shortage and environmental pollution.To this end,the country has set the energy saving and emission reduction targets of "carbon peaking" and "carbon neutral",with the aim of saving energy and protecting the environment.Related research shows that the frictional resistance of aircraft fuselages,ship surfaces and the inner walls of long-distance pipelines can account for40%,60% and 80% of the total resistance respectively,and if a part of the frictional resistance can be reduced,a large amount of energy can be saved,so the development of turbulence drag reduction technology has high engineering value.Many of the current common drag reduction techniques are developed on the basis of bionics,for example,scholars have proposed the theory of micro-groove drag reduction by observing the epidermal structure of sharks,and the theory of additive drag reduction by studying the mucus attached to the epidermis of clown fish and catfish,etc.Previously,scholars have conducted more research on single drag reduction techniques and have rarely investigated how the combination of multiple drag reduction methods can improve the drag reduction effect.In this thesis,two types of drag reduction are combined,micro-groove and high-molecular polymer,and the effect of a single micro-groove on drag reduction and the improvement of the combined drag reduction on the single drag reduction are investigated.After reading and summarizing a large amount of literature,the micro-groove structure was determined to be a triangular micro-groove with h=s=0.2mm and the high-molecular polymer was chosen to be a polyacrylamide(PAM)solution with concentrations of 15mg/L,30mg/L and 60mg/L respectively.For the choice of mathematical models,the RNG k-ε turbulence model was used to describe the flow in the micro-groove line and the Cross model was used to describe the non-Newtonian fluid PAM solution with viscous properties.In this thesis,we first simulated the drag reduction of a single micro-groove(when the flow mass is water),studied the influence of Reynolds number on the drag reduction effect of the groove,and compared the velocity field,turbulent kinetic energy and wall stresses near the micro-groove surface and the smooth surface.The results show that:(1)In a certain Reynolds number range,the micro-groove designed in this thesis achieves a good resistance reduction effect,with a maximum resistance reduction rate of 10.23%.(2)The resistance reduction rate of micro-grooves increases and then decreases with the increase of Reynolds number,in accordance with the law described by the "Reynolds number effect".(3)Observation of the flow velocity distribution cloud shows that a strip of low flow velocity exists in all the valleys of the micro-grooves,increasing the thickness of the viscous bottom layer,which is equivalent to adding a buffer layer between the high velocity fluid and the wall,reducing the dissipation of energy;The analysis of the spreading velocity distribution shows that the shape of the micro-grooves restricts the development of the spreading flow,so that the spreading flow is confined within the respective micro-grooves and does not spread to other areas.(4)A region of high turbulent energy appears at the crest of the micro-groove,but the turbulent energy inside the trough is lower,and the average turbulent energy at the overall micro-groove is lower than at the smooth wall.(5)The wall shear stress on the sloping surface of the micro-groove gradually increases from the bottom of the groove to the peak of the groove over the smooth wall,but the average wall shear stress on the sloping surface as a whole is smaller than that of the smooth wall,which is the direct cause of resistance reduction.The relative wall shear stresses are lower for higher resistance reduction rates,indicating that the negative impact on resistance reduction at the peak of the trough is better coordinated with the positive impact on resistance reduction inside the trough at high resistance reduction rates.In order to investigate the effect of the combination of the two types of drag reduction on the drag reduction effect,the micro-groove pipeline model was kept unchanged,and the fluid water was replaced by high-molecular polymer PAM solution for simulation,and the simulation results were compared with water to analyse the flow characteristics.The results show that:(1)The combination of PAM solution and micro-grooves achieved better drag reduction than a single micro-groove,with a maximum drag reduction of 30.48%.(2)Within a certain range of incoming flow velocity,the joint drag reduction rate increases and then decreases with increasing incoming flow velocity,and the drag reduction rate increases with increasing concentration for the several PAM solutions studied.(3)The flow velocity is gradually increased from the wall to the centre of the pipeline,the higher the resistance reduction rate,the greater the flow velocity at the centre of the pipeline,the higher the concentration of PAM solution at the wall,the smaller the initial flow velocity.(4)The turbulence intensity near the wall of the PAM solution is always lower than that of water,while in the middle of the pipeline it is slightly higher than that of water,indicating that the area where the high-molecular polymer acts is mainly near the wall and dominates,making the overall turbulence intensity in the flow space lower than that of water.(5)Compared to water,the wall shear stresses generated by the PAM solution are significantly reduced,and the higher the concentration the lower the wall shear stresses.(6)The vortex variation near the wall of a solution with high resistance reduction is relatively smooth and the average vortex is small,making the turbulence less violent and the flow more stable. |
