The dynamics of fibers dispersed in viscoelastic turbulent flows

Abstract

This study explores the dynamics of finite-size fibers suspended freely in a viscoelastic turbulent flow. For a fiber suspended in Newtonian flows, two different flapping regimes were identified previously by Rosti et al (2018). Here we explore, how the fiber dynamics is modified by the elasticity of the carrier fluid by performing Direct Numerical Simulations of a two-way coupled fiber-fluid system in a parametric space spanning different Deborah numbers, fiber bending stiffness and the linear density difference between fiber and fluid. We examine how these parameters influence various fiber characteristics such as the frequency of flapping, curvature, and alignment with the fluid strain and polymer stretching directions. Results reveal that the neutrally-bouyant fibers, depending on their flexibility, oscillate with large and small time scales transpiring from the flow, but the smaller time-scales are suppressed as the polymer elasticity increases. Polymer stretching is uncommunicative to denser-than-fluid fibers, which flap with large time scales from the flow when flexible and with their natural frequency when rigid. Thus, the characteristic elastic time scale has a subdominant effect when the fibers are neutrally-bouyant, while its effect is absent when the fibers become more inertial. Additionally, we see that the inertial fibers have larger curvatures and are less responsive to the polymer presence, whereas the neutrally-bouyant fibers show quantitative changes. Also, the neutrally-bouyant fibers show a higher alignment with the polymer stretching directions compared to the denser ones. In a nutshell, the polymers exert a larger influence on neutrally-bouyant fibers compared to the denser ones. The study comprehensively addresses the interplay between polymer elasticity and the fiber structural properties in determining its response behaviour in an elasto-inertial turbulent flow.