Small scale dynamics of a shearless turbulent/non-turbulent interface in dilute polymer solutions

We study the physics of turbulent/non-turbulent interface of an isolated turbulent region in dilute polymer solutions and Newtonian fluid. The performance of a FENE-P model with a localized homogeneous forcing is verified using the specially designed experimental setup of a turbulent patch growing in water/dilute polymer solution, without mean shear and far from the walls. The results of the small scale dynamics of vorticity and strain help to reveal the key mechanism of polymer action in turbulent flows without mean shear. Modified degrees of alignment between vorticity, the polymer conformation and the rate-of-strain tensors found especially near the interface explain the reduced vorticity stretching and increased vorticity compression terms. These small scale alignments in the non-Newtonian turbulent flow thus lead to a reduced production of enstrophy and consequently to a reduced entrainment (seen as propagation or dispersion).