TRPIV Experimental Study On Drag-reduction Mechanism In Turbulent Boundary Layer With Polymer Additives

The phenomenon of drag reduction in turbulence with polymer additives has received considerable attention, since it was discovered over half a century ago. However, owing to the complexity of the physical system composed by polymers and turbulence, a thorough understanding of the hydrodynamic interactions between them has not been clearly revealed yet. The mechanism of drag reduction in polymer solution is still at a stage of exploration and expected to be further researched. At present, experimental studies on drag reduction in the turbulent boundary layer are lacking for the comprehensive analysis of coherent structures, especially of the multi-scale coherent structures. As the important turbulent structures, the coherent structures have significant impact on the maintenance, evolution and development of the turbulence. In this paper, the turbulent flow field with polymer additives was investigated, combined with the analysis for coherent structures. The dynamical influence of polymers on the wall-bounded turbulence was discussed in order to provide more insight into the cause of the decline in friction drag after adding polymers.Experiments were performed using time-resolved particle image velocimetry(TRPIV) to investigate the mechanism of drag reduction by polymers from the viewpoint of the multi-scale coherent structures manipulation in turbulent boundary layer. The fully developed near-wall turbulent flow fields with and without polymer additives at the same Reynolds number were measured by TRPIV system from the side and top views. The two-dimensional instantaneous time-series of velocity vectors in turbulent boundary layer were recorded and processed.Compared with the water case, the log-law profiles with the same slope in the drag reducing flow are displaced upward, which is associated with the increase in the thickness of the buffer layer. This effect reflects the character of drag reduction near the wall. Though the maximum value of streamwise velocity fluctuation is not affected by the viscoelasticity of polymers, the peaks of the wall-normal velocity fluctuation, Reynolds shear stress, and the spanwise vorticity greatly diminish with polymer additives. In addition, the peaks of the turbulent statistics distribute broader in the wall-normal direction and move farther away from the wall for the solution. Due to the effect of turbulent flow, the long-chain polymers stretch along the streamwise. For the reaction against the turbulence, they sharply weaken the energy of coherent motions in the wall-normal and spanwise direction, and hinder the transport of turbulent kinetic energy from the streamwise component to the other two components. Moreover, the stretched polymers store the kinetic energy which would originally been dissipated by small-scale eddy because of the molecular viscosity. This results in a change of the turbulent energy cascade at small-scales. As the solution concentration increases, the impact of polymer additives on the turbulent statistics is becoming evident at the same Reynolds numbers. And this changing process slows down at high concentration.The correlation functions of fluctuating velocities demonstrate that accompanied by the increasing concentration, the coherent structures in the polyacrylamide(PAM) solution stretch along the streamwise direction, but compress along the wall-normal direction. The conditional sampling and linear stochastic estimation(LSE) were employed to extract the hairpin vortex and vortex packet. The result shows that polymers suppress the energy transfer to hairpin vortices from the ensemble average movement, giving rise to the decline in intensity of vortices. For this reason, the generation of new hairpin vortices is inhibited, and the number of individual hairpin vortices in the vortex packet also decreases. Since the wall-normal motion of fluid is impeded, the angles of inclination of the hairpin vortices and packets, relative to the wall, are all smaller than those for the water case. With the weakened vortices’ strength, the induction to the adjacent fluids is attenuated in polymer solution, such as the reduction in the intensity of ejection under the hairpin head. The suppression of burst events makes the low-speed streaks more stable, leading to the decrease in the possibility for the generation of new coherent structures owing to the streak breakup. This implies that the polymer additives retard the self-sustaining mechanism of turbulent boundary layer. It is through the coherent structures in the boundary layer that polymers control turbulent flow to lead to drag reduction effect.The multi-scale analysis of wall turbulence with and without PAM has been carried out by the new mu-level method, based on locally averaged velocity structure function, to identify the coherent structures’ bursting and to extract the spatial topologies of turbulent characteristic quantity. Although the polymer solution does not much affect the topological shape, the fluctuating velocity and velocity derivatives of coherent structures drastically decreases during the burst events in the solution. By virtue of the capability against deformations, viscoelastic polymers damp the deformation degree of surrounding fluid, and cause the fluid flow to be uniform in the wall-normal direction. It indicates that the polymer additives make the turbulent motion less chaotic near the wall. Furthermore, the scales of flow field were separated by using proper orthogonal decomposition(POD) according to their contributions of turbulent kinetic energy. The three-segment feature of POD spectra manifests the different dominant mechanisms of large- and small-scale coherent structures. The distribution of energy contained in large-scale structure agrees with the power-law, while that of small-scale structure’ energy conforms to exponent law. The energy of the transitional region between them also follows a power-law distribution, but with the different coefficient from the large-scale. The change of coefficient in distribution after the addition of PAM denotes the influence of polymers on the transport of turbulent energy. In the large-scale segment, the energy further concentrates to large-scale structure, and the decay of small-scale’s energy slows down due to the existence of polymer additives. The different effects of polymers for the large- and small-scale confirm that the mechanisms of drag reduction in polymer solution at large- and small-scale coherent structures are different.