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

当管道或通道内的流动从层流状态过渡到湍流状态时，其摩擦阻力急剧增加，即湍流状态下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输油、区域供热/制冷系统以及船舶、飞机和车辆的推进。因此，紊流减阻（DR）技术可以带来巨大的经济效益（降低成本）和环境效益（减少自然资源消耗和温室气体排放）。通过简单地将聚合物添加剂溶解到流体中，可以实现高达80%的DR。尽管聚合物在实际应用中并不总是被允许的，但对其DR效应的理解将激发新的流量控制技术设计，使其在简单流体（无聚合物）中达到相当的节能水平。该领域最突出的问题是所谓的最大减阻（MDR）：这是一个普遍的渐近极限，限制了所有不同聚合物溶液达到的最大减阻水平。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未解决问题。揭示其机制不仅具有重大的基础意义，而且在接近MDR状态的聚合物溶液中清晰的动力学图像对于一般DR技术的未来发展也至关重要。提出的程序侧重于紊流中磁阻的数值计算，短期目标是研究磁阻的机理。我们将采用的方法受到我们最近在简化模型流研究中提出的一个新的概念框架的启发。除了为耐多药研究引入一个独特的视角外，该框架还为耐多药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动态的机械描述；另一个重点是在现实尺度上验证我们对湍流的物理理解。沿着这条道路的进展将为我们的长期目标做好准备：开发下一代技术，在简单的牛顿流体中进行高水平的DR。聚合物在许多情况下是不溶的、被禁止的或不受欢迎的；因此，简单流体中的DR技术可以达到更广泛的应用范围。然而，这种现有技术所达到的节能水平远低于聚合物。随着对MDR附近聚合物湍流的更好理解，可以设想新的方法，其中可以模仿聚合物的作用，以便在没有这些添加剂的情况下将湍流动力学束缚在MDR上。最后，该计划的设计也考虑到高素质人才（HQP）的培训。拟议的研究分为几个项目，适用于研究生和本科生的多个学生。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，他们还将接受系统的数值计算和算法开发方面的培训。除学术界外，这些技能在广泛的工业部门，特别是石油、化学和制造业中也受到高度重视。

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.