NSF-BSF: Explaining the Mismatch of Experiments and Simulations for Viscoelastic Flows

复杂流体是具有悬浮颗粒的材料，这些悬浮颗粒构成的微观结构在流动过程中发生变化，并使材料具有粘弹性，或者更一般地说，是非牛顿响应，即材料既可以表现为粘性液体又可以表现为弹性固体。对于各种各样的复杂流体，如聚合物溶液、乳液等，本构模型可以将材料中的应力与材料变形和流动特征的应变和应变速率联系起来。然而，数值模拟的结果很少与实验细节进行定量比较。因此，尽管经过了几十年的研究，模拟工具箱已经得到了验证，也就是说，数值计算机代码解决了用来描述材料的方程，但方法还没有得到验证，也就是说，材料行为方程的解通常不能再现在通道流动中所做的实验观察，因此需要新的理解来确定适当的方程。提出的研究旨在提供一个验证管道，使用稀聚合物溶液的共同宏观描述，但现在纳入了聚合物的不同微观结构特征。特别地，将通过理论、模拟和实验相结合的方法研究非均匀通道形状下的压降与流速的关系。这些拓展工作是PI参与、教学和指导未来研究科学家的方法的特点，包括继续举办自2002年以来的年度“假日”科学讲座。复杂流体在自然环境中流动的例子并不缺乏，例如粘液、在含聚合物的水溶液中游动的细胞、用于家庭和个人护理的乳剂，以及工业应用，例如聚合物加工。粘弹性流动的数值模拟也不乏发展，使用已建立的本构方程，其中大多数具有代表各种物理效应的参数。然而，即使是具有相当好特征的流体，如聚合物溶液，也会产生结果，例如，非均匀通道中的压降与流速的关系，这是众所周知的连续级数值模拟无法很好地预测的。拟议的研究计划将利用扩展的Oldroyd-B描述稀聚合物溶液的框架来解决这一差异，包括受微观结构启发的有限聚合物可扩展性（FENE）术语，微观结构拉长时的构象相关阻力，以及微观结构相对于流体的相对应变/旋转。理论、模拟和实验将结合在一起，研究非均匀通道形状下的压降与流速的关系，同时也表征了微观结构的构象。PI在研究活动的广泛影响方面有着重要的记录，包括在工程和物理学的主要期刊上发表文章，将基础研究与应用联系起来，接待来自不同学科和教育机构的访客，为本科生、研究生和博士后同事领导专业发展活动，并鼓励来自代表性不足群体的年轻研究人员参与。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Complex fluids are materials with suspended particles, which constitute a microstructure that changes during flow and gives the material a viscoelastic, or more generally, a non-Newtonian response, i.e., the material can behave as both a viscous liquid and an elastic solid. For a wide variety of complex fluids, such as polymer solutions, emulsions, etc., constitutive models are available that relate the stress in the material to the strain and rate of strain characteristic of the materials deformation and flow. However, the results of numerical simulations are rarely compared quantitatively with details of experiments. As a consequence, in spite of many decades of research, the simulation toolbox has been verified, i.e., the numerical computer codes solve the equations assumed to describe the materials, but the methodology has not been validated, i.e., the solutions to the equations for the material behavior do not generally reproduce the experimental observations made in channel flows, which thus requires new understanding to identify appropriate equations. The proposed research aims to provide a validation pipeline using a common macroscopic description for dilute polymer solutions but now incorporating different microstructural features of polymers. In particular, the pressure drop versus flow rate relation in non-uniform channel shapes will be studied through a combination of theory, simulations, and experiments. The outreach efforts characterize the PI’s approaches to engaging with, teaching, and mentoring future research scientists, including continuing an annual “holiday” science lecture that has been delivered since 2002.There is no shortage of examples of the flow of complex fluids in natural environments, e.g., mucus, swimming cells in polymer-laden aqueous fluids, emulsions used for home and personal care, and industrial applications, e.g., polymer processing. There is also no shortage of developments in the numerical simulation of viscoelastic flows using well-established constitutive equations, most of which have parameters representing various physical effects. Nevertheless, even reasonably well-characterized fluids, such as polymer solutions, produce results, e.g., pressure drop versus flow rate in a non-uniform channel, that are not well-predicted by well-known continuum-level numerical simulations. The proposed research program will address this discrepancy by utilizing the framework of the extended Oldroyd-B description of dilute polymer solutions, including microstructurally inspired terms for finite polymer extensibility (FENE), conformation-dependent drag as the microstructure is elongated, and relative strain/rotation of the microstructure relative to the fluid. Theory, simulations, and experiments will be integrated to study the pressure drop versus flow rate relation in non-uniform channel shapes, while also characterizing the conformation of the microstructure. The PI has a significant track record in the broader impacts of research activities, including publishing in leading journals in engineering and physics, linking fundamental research to applications, hosting visitors from different disciplines and educational institutions, leading professional development activities for undergraduate, graduate, and postdoctoral colleagues, and encouraging participation of young researchers from under-represented groups.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.