Study On Drag-reducing Mechanism Of Surfactant Drag-reducing Fluid By Experiments And Dns

The presence of small amounts of certain additives (such as polymers orsurfactants) in water can result in considerable drag reduction in turbulentflow which is called as additive drag-reducing fluid. The technique ofadditive drag reduction is widely applied in pipe flow transportation acrossthe long distance, which is of great energy value. Compared with the polymer,surfactant solution has the strong self-repairing ability after the mechanicaldegradation for the special molecular structure so that it is suitable forapplication in closed circulatory system. Nowadays, drag reducingmechanism of surfactant solution is still not very clear in the limit ofmeasurement facilities and the complexity of research in turbulence andnon-Newtonian fluids.This paper aims at the study of drag-reducing mechanism in surfactantsolution by the test of rheological characteristics, the measurement ofvelocity vectors in the channel flow and the direct numerical simulation of the drag-reducing channel flow. The applicability of the model in simulationof drag reduction is evaluated. The surfactant used as the additives in theexperiments is etyltrimethyl ammonium chloride, CTAC, which is one kindof cationic surfactants.AR-G2 rheometer is used in the rheological experiment forCTAC/NaSal solution. The apparent viscosity decreases when thetemperature raises and the concentration of the solution becomes small. Theshear-thinning and shear-thickening phenomenon both appear in the profilesof apparent viscosity of CTAC surfactant solution. Giesekus viscoelasticmodel is easy to simulate the shear-thinning phenomenon except theshear-thickening phenomenon. Double parallel Giesekus viscoelastic modelwith two relaxation factors is introduced and established because it isprobable to fit the two parts of the curve accompanying shear-thinning.Particle image velocimetry is used in the measurement of velocityvectors in x-y plane in the channel with CTAC surfactant solution. The newpattern of turbulence transportation appears in the experiments that Reynoldstransportation is damped in wall-normal direction, the stripes of velocityfluctuations are developed in streamwise direction with small vibration inwall-normal direction, and consequently zero Reynolds shear stress appears.Direct numerical simulation is carried out on the drag-reducing channel flow based on Giesekus viscoelastic model. Compared with the experiments,the simulation exhibits the similar statistic characteristics of the turbulentflow. Especially after the multiresolution analysis by wavelets transform, thedistribution of spacial scales and fluctuations under one scale is uniform forboth Newtonian and non-Newtonian flow.Moreover, the results in numerical simulation show that the elastic stressin streamwise direction displays the effect of "retarded viscosity" on thedelay between the stress and deformation under the high shear rate whereasno effect of "retarded viscosity" happens in wall-normal direction whichcauses to prevent the transportation and diffusion of the turbulence. Thischaracteristic coincides with the experimental results.Quadrant analysis of the data in experiment and simulation shows thatthe uniform distribution of streamwise and wall-normal velocity fluctuationsappears in viscoelasitc flow and CTAC drag-reducing flow compared withNewtonian flow as that the inclined angle of the major axis of ellipsedecreases. Compared with the experimental results, evident distribution offluctuations in simulation appears in quadrant III which indicates thetendency of bulk turbulence transportation leaving the wall surface. However,the distribution of fluctuations of CTAC solution in quadrantⅣindicatesthat the stripes of low speed will move toward the wall which result in the phenomenon of zero Reynolds shear stress in the experiment.The banlance analysis of mean kinetic energy and turbulent kineticenergy indicates that the mean kinetic energy consists of the works byReynolds shear stress, viscous shear stress and elastic shear stress. And thework by Reynolds shear stress is dissipated by the viscosity and elasticity. Asa result, the mean kinetic energy is divided into two parts, viscous item andelastic item, whose distribution is controlled by viscosity ratio,β.Direct numerical simulation based on double parallel Giesekusviscoelastic model is carried out with tow Wessenberg numbers, 10 and 100.The problem in numerical calculation of Giesekus model when Weτ=100 isresolved by the double parallel modal without the appearance ofunreasonable extremum and floating point overflow. Laminar pattern andhigh drag reduction, 86% appear in the simulation of the double parallelmodal with Weτ=10, 100. More simulations is carried out based on doubleparallel modal with Weτ=10, 40 andβ= 0.8, 0.6 individually. The results showthat the coherent structures are enhanced and drag reduction is improvedcompared with the single branch of the model.