UNS: Origins of maximum drag reduction in viscoelastic turbulence

拟议的研究集中在湍流中发生的减阻的调查及其发生的原因。虽然这是湍流中非常重要的基本问题，但它在长距离运输流体的能源成本方面也具有非常重要的应用领域。在以高速率输送流体或当固体穿过流体时（例如，空军或海军舰船）是为了减小流体阻力。在内部流的情况下（即，在管道内），湍流阻力可以通过低水平的聚合物或表面活性剂添加剂显著降低。虽然这种效应是众所周知的，但关于它是如何发生的仍然存在争议。湍流减阻的一个特征是存在所谓的“最大减阻”（MDR）渐近线。 对于给定压降下的给定流动几何形状，存在可通过添加聚合物实现的渐近流速。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--它是通用的。PI小组先前的结果表明，粘弹性有助于抑制活跃的湍流，并产生一种不太活跃的间歇性湍流状态，称为“冬眠”。该建议的主要假设是，这种冬眠的湍流状态是一个负责的MDR渐近线的发生。拟议的研究将探讨在最小的通道湍流在高Weissenberg数（无量纲Weissenberg数是流体松弛时间尺度的流动的特征时间尺度的比率）的粘弹性流体的湍流流动，并在这样做可能也获得洞察力关于近壁湍流生产牛顿流。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研究最小湍流单元对大区域的有效性。

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.