UNS: Origins of maximum drag reduction in viscoelastic turbulence

建议的研究集中在研究湍流中发生的减阻及其产生的原因。虽然这是湍流中一个非常重要的基本问题，但它在长距离输送流体的能源成本方面也有非常重要的应用领域。在以高速率输送流体或当固体通过流体(例如，空军或海军舰艇)时，一个重要的技术问题是减小流体阻力。在内部流动(即管道内部)的情况下，低水平的聚合物或表面活性剂添加剂可以显著降低湍流阻力。虽然这种影响是众所周知的，但关于它是如何发生的仍存在争议。湍流减阻的一个特征是存在所谓的“最大减阻”(MDR)渐近线。对于给定的流动几何形状，在给定的压降下，存在通过添加聚合物可以获得的渐近流速。改变添加剂的浓度、分子量甚至化学结构对这个渐近值没有影响--这是普遍存在的。PI的研究小组先前的研究结果表明，粘弹性有助于抑制活跃的湍流，并导致一种不太活跃的间歇性湍流状态，这被称为“冬眠”。该提议的主要假设是，这种冬眠湍流状态是导致MDR渐近线发生的原因之一。这项研究将探索粘弹性流体在高Weissenberg数(无量纲Weissenberg数是流体松弛时间尺度与流动的特征时间尺度之比)的最小通道湍流中的湍流流动，这样做还可能获得关于牛顿流动中近壁湍流产生的洞察力。PI将与利物浦大学的一个小组合作进行实验，他还将通过大规模直接数值模拟来研究最小湍流单元在大区域的有效性。

The proposed research is focused on the investigation of drag reduction that occurs in turbulent flows and on the reason for its occurrence. While this is a very important fundamental problem in turbulent flows, it also has very significant application areas in energy costs for the transportation of fluids across long distances. An important technical problem in transporting fluids at high rates or when a solid body travels through a fluid (e.g., an air or naval vessel) is to reduce the fluid drag. In the case of internal flows (i.e., inside conduits), turbulent drag can be dramatically reduced by low levels of polymer or surfactant additives. While this effect is well-known, there is still debate about how it happens. A feature of turbulent drag reduction is the existence of a so-called "maximum drag reduction" (MDR) asymptote. For a given flow geometry at a given pressure drop, there is an asymptotic flow rate that can be achieved through addition of polymers. Changing the concentration, molecular weight or even the chemical structure of the additives has no effect on this asymptotic value -- it is universal. Prior results from the PI's group have showed that viscoelasticity serves to suppress active turbulence and gives rise to a less active, intermittent turbulence state which is termed 'hibernating'. The main hypothesis of the proposal is that this hibernating turbulence state is the one that is responsible for the occurrence of the MDR asymptote. The proposed research will explore turbulent flow of viscoelastic fluids in minimal channel turbulence at high Weissenberg number (the dimensionless Weissenberg number is the ratio of the fluid relaxation time scale to the characteristic time scale of the flow), and in so doing possibly also gain insight regarding near-wall turbulence production in Newtonian flows. The PI would collaborate with a group at the University of Liverpool to perform experiments and he will also investigate the validity of the minimal turbulence cell to large domains via large scale direct numerical simulations.