Energy conservation in turbulent flows of complex and simple fluids: mechanisms and new approaches

当管道或通道中的流动从层流状态转变为湍流状态时，其摩阻突然增加，即后者的能量耗散要高得多。与此同时，湍流在工程应用中无处不在；常见的例子包括管道石油输送、区域供暖/冷却系统，以及船舶、飞机和车辆的推进。因此，湍流减阻技术可以带来巨大的经济效益(较低的成本)和环境效益(较少的自然资源消耗和温室气体排放)。只需将聚合物添加剂溶解到流体中，即可实现高达80%的高DR。虽然聚合物在实际应用中并不总是被允许的，但对其DR效应的了解将启发新的流动控制技术的设计，以便在简单流体(不含聚合物)中达到类似的节能水平。**这一领域最突出的问题是所谓的最大减阻(MDR)：即限制所有不同聚合物溶液达到的DR水平的通用渐近极限。尽管经过了几十年的研究，MDR仍然是粘弹性湍流领域中最重要的悬而未决的问题。揭示其机理不仅具有重大的基础意义，而且清楚地了解MDR区域附近聚合物溶液中的动力学也是未来DR技术发展的关键。**拟议的程序侧重于湍流中DR的数值计算，短期目标设定在MDR机制上。我们将采取的方法是受到我们最近对简化模型流的研究中提出的一个新的概念框架的启发。除了为MDR研究引入一个独特的视角外，该框架还对关于MDR的所有定性观察提供了一致的解释，显示出指导未来研究的巨大希望。在这一进展的基础上，我们的计划分为两个方向：一个旨在构建MDR动力学的机械描述；另一个专注于在现实尺度上验证我们对湍流的物理理解。**在这条道路上的进展将为我们的长期目标做好准备：开发用于简单牛顿流体中高水平DR的下一代技术。聚合物在许多情况下是不溶的、禁用的或不受欢迎的；因此，在简单流体中进行DR的技术可以达到更广泛的应用范围。然而，现有的这类技术实现的节能水平远远低于聚合物。随着对MDR附近聚合物湍流的更好了解，可以设想新的方法，模拟聚合物的作用，以便在没有这些添加剂的情况下将湍流动力学与MDR联系起来。**最后，该计划的设计也考虑到了高素质人员(HQP)的培训。这项拟议的研究分为几个项目，面向研究生和本科水平的多名学生。从事这些项目的学生将在流体动力学、聚合物物理、流变学和应用数学方面建立坚实的背景。同时，他们还将接受数值计算和算法开发方面的系统培训。除学术界外，这些技能在广泛的工业部门也受到高度重视，特别是石油、化工和制造业。

When flow in a pipe or channel transitions from the laminar state to turbulence, its friction drag increases abruptly: i.e., energy dissipation is much higher in the latter. Turbulence, meanwhile, is ubiquitous in engineering applications; common examples include pipeline oil transferring, district heating/cooling systems, and the propulsion of ships, airplanes and vehicles. Techniques for turbulent drag reduction (DR) can therefore bring about massive benefit both economically (lower cost) and environmentally (less consumption of natural resources and less greenhouse gas emissions). Substantial DR - up to 80% - can be achieved by simply dissolving polymer additives into the fluid. Although polymers are not always permissible in practical applications, understanding of their DR effect will inspire new designs of flow-control techniques for reaching comparable levels of energy saving in simple fluids (without polymers).**The most prominent problem in this area is the so-called maximum drag reduction (MDR): a universal asymptotic limit that bounds the level of DR reached by all different polymer solutions. Despite decades of research, MDR has remained the most important unsolved problem in the area of viscoelastic turbulence. Revealing its mechanism is not only of great fundamental interest, a clear picture of the dynamics in polymer solutions near the MDR regime is also pivotal for future developments in DR techniques in general.**The proposed program focuses on the numerical computation of DR in turbulence, with a short-term goal set at the mechanism of MDR. The approaches we will take are inspired by a new conceptual framework proposed in our recent studies of simplified model flows. In addition to introducing a distinct perspective to MDR research, the framework also offers consistent explanations for all qualitative observations about MDR, showing great promise of guiding future research. Building on this progress, our plan divides into two directions: one aims at constructing a mechanistic depiction of MDR dynamics; the other focuses on validating our physical understanding in turbulence at realistic scales.**Progress along this path will prepare us for our long-term goal: developing next-generation techniques for high levels of DR in simple Newtonian fluids. Polymers are insoluble, prohibited or undesirable in many situations; techniques for DR in simple fluids can thus reach a much broader range of applications. The level of energy saving achieved by existing techniques of such kind is however much lower than that of polymers. With better understanding of polymeric turbulence near MDR, new approaches can be envisioned in which the role of polymers is imitated so that turbulent dynamics can be tethered to MDR in the absence of these additives.**Finally, the program is also designed with the training of highly qualified personnel (HQP) in mind. The proposed research breaks down into several projects for multiple students at both the graduate and undergraduate levels. Students working on these projects will build a solid background in fluid dynamics, polymer physics, rheology and applied mathematics. Meanwhile they will also receive systematic training in numerical computation and algorithm development. In addition to the academia, these skills are highly valued in a broad range of industrial sectors, especially the petroleum, chemical and manufacturing industries.