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

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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.