Experimental investigation on the drag reduction performance by periodically slow-releasing polymer solution through porous media

In practical engineering applications of polymer drag reduction (DR), the environment is often complex and dynamic. Factors such as water depth and flow velocity could interfere with the constant polymer releasing state, resulting in the occurrence of fluctuation in the polymer releasing process. An experimental study was conducted to evaluate the polymer DR performance when periodically released into turbulent channel flow in square and triangular waveform patterns. The polymer diffusion and the velocity profile were obtained using particle image velocimetry and planar laser-induced fluorescence measurements to further analyze the DR mechanism. For the square wave pattern with abrupt changes within a period, the existing rising and declining phases in DR and near-wall polymer concentration curves as a function of time always resulted in poor tracking performance. Although increasing the slow-release period shifted the peak point towards a plateau stage and elevated the peak DR, the gradual decline in trough DR and the extended duration of the decline still weakened the average DR efficiency over one period. In other words, the DR effects became increasingly weaker compared to those achieved by continuously releasing methods due to the extended period of ineffectiveness. Increasing the slow-release rate and the duty cycle could improve the average DR efficiency, but this was still inferior to the continuously releasing method because of the aforementioned poor tracking performance. Conversely, the DR and near-wall polymer concentration curves exhibited significant tracking performance in response to the triangular wave pattern with gradual change characteristics. Increasing the period enlarged and then stabilized the peak DR, while diminishing and stabilizing the trough DR. Thus, the average DR effect remained unchanged, and always comparable to the continuously releasing method. Increasing the slow-release rate promoted and then stabilized both the peak and trough DRs. The increasing trend of the overall DR efficiency versus slow-release rate nearly overlapped with that achieved by the continuously releasing method. This indicated that releasing polymer by triangular waves could not only provide equivalent DR to the continuously slow-release method but also offer greater environmental robustness. It is conjectured that other gradient wave patterns for periodically releasing polymer solution may also result in improved DR performance, potentially surpassing the efficiency of the continuous releasing method. This study offers valuable insights into optimizing polymer release strategies for the purpose of enhancing DR in external flows.