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

这项拟议的研究的目标是使用理论和模拟相结合的方法来了解非牛顿流体，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，在许多工业环境(如聚合物加工)中使用非牛顿流体，在这些环境中，不稳定和过渡可能导致制造缺陷。此外，复杂流体流动中的相变机理对于微纳流体器件的发展具有重要意义。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是重要的。这一建议的重点是探索粘弹性流体流动向湍流转变的物理机制。虽然在牛顿流体中已经进行了大量的研究、实验、计算和理论分析，但在非牛顿流体中的相变是一个新的、充满活力的领域，需要探索其巨大的技术影响。这组PI的初步结果表明，在粘性流动中，向湍流转变的雷诺数比牛顿流体的雷诺数低得多，而另一个无量纲数Weissenberg数是一个重要的参数。该数字描述了流体松弛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。沿流向流动方向的条纹被认为是可以放大并导致湍流的过渡结构。拟议的工作重点是识别过渡过程后期阶段所涉及的非线性机制。在拟议的工作中，沟道流几何将被用来研究放大最大的线性扰动的二次感受性和不稳定性。导致光谱展宽的过渡的最后阶段将通过直接的数值模拟来检验。

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.