水溶性聚合物添加剂与超疏水表面协同减阻及其失效机理研究

A tiny amount of polymer additive can suppress turbulent flow and reduce friction drag. Polymer additive is an important way for drag reduction and flow control. However, the polymeric moleculars suffer from scission of the high molecular chain by the high shear rate of the intensive turbulent flow. Consequently, it leads to the failure or reduced performance of polymer, which restricts its application severely. An effective slip is expected on the flow over superhydrophobic surface. Therefore, superhydrophobic surface has extensive applications such as anti-fouling and drag reduction. However, the high pressure and strong diffusion ambient condition could reduce the longevity of air layer and result in the failure of the material. Based on the flow characteristics of each technology, we aim to study the influence of synergetic drag reduction of polymer additives and superhydrophobic surface on the failure of both technologies. On one hand, the polymer additive could suppress the turbulent diffusion of air layer of superhydrophobic material to the ambient liquid, and reduce the peak pressure so that the failure of superhydrophobic material could not happen. On the other hand, the effective slip of superhydrophobic surface could reduce the peak shear rate in turbulent flow. Then the degradation of polymer could be avoided. In this project, the competitive mechanism of the maximum drag reduction and the range of application of the both technology on drag reduction will be studied by numerical simulation. Furthermore, Level-Set and immersed boundary method as well as the influence of surface tension would be implemented in numerical software to simulate the real physical characteristics of superhydrophobic surface in order to study the stability condition and the evolution of gas-liquid interface. Experimental study will be carried out in order to evaluate the numerical results and validate the failure mechanism of both technologies.

流体中微量的水溶性聚合物添加剂可抑制湍流并大幅降低摩擦阻力，是流动控制与减阻的重要手段，然而高分子链易在高剪切率湍流环境中剪断而失效或降低性能，限制其应用范围；超疏水材料在水流过时可在其表面产生有效滑移，在流动减阻、表面防污等领域具有十分广阔的应用前景，而高压强、高扩散流动环境严重影响其表面气体的长存性而使其失效。因此，本项目拟结合二者的流动特性，添加聚合物抑制超疏水材料表面气体的湍流扩散，并降低其湍流压力的峰值；另一方面利用超疏水表面的有效滑移，降低最大剪切率；从而在协同工作中增加二者的失效阈值。项目拟通过数值模拟，研究二者对减阻的竞争性机理、最大减阻量和减阻适用性。随后将依据超疏水材料表面的真实物理特性，结合水平集与浸入边界法开发考虑表面张力和微结构变形下的数值计算程序，研究超疏水表面的气-液界面失稳条件与演化规律，并将通过实验研究对数值模拟结果以及二者的失效机理进行验证。

水下航行器的减阻具有十分重要的高速航行器开发的工程意义，同时可以降低温室气体排放应对全球变暖。采用高分子聚合物添加剂和采用超疏水表面材料是两种非常具有潜力的水下航行器减阻技术。但由于二者的减阻机理不同，且二者在减阻中缺陷也不相同，本项目结合数值模拟与试验探索了二者协同作用时的减阻互惠性。项目中首先通过数值模拟，研究了两种常见的减阻技术的减阻能力提升关键参数研究，其次开展了二者混合使用时的减阻量变化，数值研究表面二者的协同作用可以进一步提升航行器的减阻量，但总的减阻量没有达到两种技术单独使用所取得的减阻量的总和。考虑到这两种减阻技术在实际使用中的限制与环境因素，开展了基于二者的减阻试验。试验分别选取了聚合物添加剂PEO和PAM；以及可大面积扩展的随机纹理超疏水表面和规则化加工的凹槽表面进行组合探索。结果表明，PAM相较于PEO在超疏水表面上展现出更好的协同效应。PAM与可扩展随机纹理超疏水表面在实验中实现的协同减阻减小在所测试的雷诺数50000范围内令人满意结果，例如在该雷诺数下采用典型50PPM的二者协同方式下的总减阻量大于任意单一技术的减阻量，且超过了二者单独试验减阻量之和的75%以上。另一方面，PEO链的严重表面活性剂效应可能会损害空气-水界面，并降低随机纹理SHSs的滑移长度，这可能是之前研究中协同滑移减小受到PEO添加剂限制的原因。本项目的探索为进一步提升水下航行器减阻提出了有效发展路径。