UNS: Collaborative research: the onset of turbulence in viscoelastic wall-bounded shear flows

这项研究的目标是利用理论和模拟相结合的方法来理解非牛顿流体（如聚合物和生物流体）从层流到湍流的转变。这项工作的动机是，非牛顿流体被用于许多工业环境中（例如，聚合物加工），其中不稳定性和过渡可能导致制造缺陷。此外，复杂流体流动中的过渡机制对于微/纳米流体器件的发展非常重要。理解、预测和控制向湍流过渡的能力对于许多技术应用和科学上重要的过程都是重要的。本文主要探讨粘弹性流体向湍流过渡的物理机制。虽然在牛顿流体中已经进行了多年的大量研究、实验、计算和理论分析，但非牛顿流体中的过渡是一个新的、充满活力的领域，具有巨大的技术影响，需要探索。这些pi组的初步结果表明，在粘性流动中，向湍流的过渡可以在比牛顿流体低得多的雷诺数下发生，并且另一个无量纲数，即Weissenberg数，是一个重要参数。这个数字表示流体弛豫时间尺度与流动特征时间尺度的关系。作者以前的工作主要集中在粘弹性流动中的线性机制不稳定性。顺流方向上的条纹已被确定为可以被放大并导致湍流的过渡结构。提出的工作重点是识别在过渡过程的后期阶段涉及的非线性机制。在提议的工作中，通道流动几何将被用来检验最放大的线性扰动的二次接受性和不稳定性。导致光谱展宽的最后过渡阶段将通过直接数值模拟进行检验。

The goal of the proposed study is to use a combination of theory and simulations to understand the transition from laminar to turbulent flows in the case of non-Newtonian fluids, like polymers and biological fluids. The work is motivated by the fact that non Newtonian fluids are used in many industrial settings (e.g., polymer processing) where instability and transition could lead to manufacturing defects. In addition, transition mechanisms in flows of complex fluids are very important for the development of micro/nano-fluidic devices. The ability to understand, predict, and control the transition to turbulence is important for a multitude of technological applications and scientifically important processes. This proposal is focused on exploring the physical mechanism of transition to turbulence in the flow of viscoelastic fluids. While in Newtonian fluids a lot of research, experiments, computations, and theoretical analyses have been done over several years, transition in non-Newtonian fluids is a new, vibrant area with tremendous technological impact that needs to be explored. Preliminary results from the group of these PIs have indicated that in viscous flows the transition to turbulence can occur at much lower Reynolds number than for Newtonian fluids, and that another dimensionless number, the Weissenberg number, is an important parameter. This number characterizes the fluid relaxation time scale in relation to the characteristic time scale of the flow. Previous work by the authors has focused on the linear mechanism instability in viscoelastic flow. Streaks in the streamwise flow direction have been identified as the transition structure that can be amplified and lead to turbulence. The proposed work is focused on discerning the nonlinear mechanisms involved in the later stages of the transition process. In the proposed work, channel flow geometry will be used to examine the secondary receptivity and instability of the most amplified linear disturbance. The final stages of transition that lead to spectral broadening will be examined via direct numerical simulation.