Experimental and numerical investigation of turbulent entrainment in dilute polymer solutions

Dilute polymer effects on the inter-scale energy transfer in turbulent flows is studied in this work with a major focus on the problem of turbulent entrainment across turbulent/non-turbulent interface. Polymers alter this region of flow significantly due to the large gradients at the interface and strong interaction of multiple scales - large scales that deflect the interface and the small scales that diffuse the vorticity and strain. An experimental (PIV) and numerical (DNS with FENE-P model) study has been performed to characterize the basic mechanisms of turbulent entrainment in Newtonian vs poly(ethylene oxide) solutions. We work on a localized patch of turbulent flow created numerically or by a small spherical oscillating grid, isolating the effects of boundary friction effects from the bulk effects. We analyze the patch initial growth, a steady state and the decay phase. The effects are quantified in terms of the reduced growth rates, turbulent kinetic energy and enstrophy balance, curvature of the interface and summarized by the reduced entrainment rates. Numerical model allows to reveal the underlying mechanism that controls the rates of turbulent energy transfer towards and across the interface and to further improve models of turbulent entrainment.