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

Princeton University, Princeton NJ

Investigators

Abstract

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.