高聚物减阻溶液对壁湍流输运过程的影响

Influnce of polymer additives on the transport process in drag reducing turbulent flow

基于相同雷诺数下清水和高分子聚合物溶液壁湍流的高时间分辨率粒子图像测速技术(time-resolved particle image velocimetry,TRPIV)的对比实验,从高聚物溶液对湍流边界层动量能量输运影响的角度分析其减阻的机理.对比两者的雷诺应力发现高聚物的存在抑制了湍流输运过程.这一影响与高聚物对壁湍流中占主导地位的涡旋运动和低速条带等相干结构的作用密切相关.运用条件相位平均、相关函数和线性随机估计(linear stochastic estimation,LSE)等方法,分析提取了高聚物溶液流场中的发卡涡和发卡涡包等典型相干结构的空间拓扑形态.相比于清水,高聚物溶液中相干结构的流向尺度增大,涡旋运动的发展及低速流体喷射的强度受到削弱,表明了添加的高聚物阻碍了湍流原有的能量传递和自维持的机理.正是通过影响相干结构,高聚物抑制了湍流边界层中近壁区与外区之间的动量和能量输运,使得湍流的无序性降低,从而减小了湍流流动的阻力.

The spatial-temporal sequence of velocity fields in wall turbulence with and without polymer additives at the same Reynolds number are measured by time-resolved particle image velocimetry （TRPIV） from the side and top views. Based on this experimental database of a water channel, the mechanism of drag reduction by polymers is explored from the viewpoint of the influence of polymer solution on the transport of momentum and energy in a turbulent boundary layer. Comparison of Reynolds stress profiles confirms that due to the existence of polymer additives, the transport of turbulent momentum is significantly inhibited, as if caused by the decrease of Reynolds shear stress. Furthermore, it is noted that these changes are closely related to the effect of polymer additives on the classical coherent structures, such as vortices and low-speed streaks, which are the dominant structures in near-wall turbulence. The spatial topological mode of hairpin vortex extracted by conditional sampling method shows that the intensity of vortices and ejection event are greatly suppressed by the polymer solution. Not only does the decline of turbulent kinetic energy production indicate that the energy of hairpin vortices that comes from the ensemble average movement is attenuated in the solution, but all this implys that the polymer additives hinder the self-sustaining mechanism, the inherent character of wall turbulence. Then, the analysis of linear stochastic estimation （LSE） suggests that the development of hairpin vortices in the packet is impeded, which is mainly reflected in the reduction of the number of hairpin vortices and the suppression of uplift in the wall-normal direction. To investigate the change of low-speed streaks after the addition of polymers, the spanwise autocorrelation function of streamwise fluctuating velocities has been calculated. In the polymer solution the large-scale vortices are enhanced while the small-scale vortices are suppressed. This observation reveals that the polymers disrupt the energy transport from large to small scales. To summarize, it is through the action on coherent structures that the polymer additives can damp the transport of momentum and energy between the near-wall region and outer region of the boundary layer. In this way, the polymer solution makes turbulent flow less chaotic, leading to the reduction of friction drag.