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

Abstract

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 areflenhanced while the small-scale vortices are suppressed. This observation refleals 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.