Soft Seminars

Novel transition to elastic turbulence in a straight channel flow

by Dr Victor Steinberg (Weizmann Institute )

Europe/Stockholm
Description

zoom link : 

https://stockholmuniversity.zoom.us/j/622224375

Meeting ID: 622 224 375

Abstract:

By now, it is well understood that wall-bounded shear flows of Newtonian fluid are not described by a linear stability analysis involving normal modes. Despite the linear stability of a laminar flow, turbulence starts at finite Re and, in sustained turbulent state, coherent structures (CSs) arise, which are attributed to the non-normal nature of the transition to turbulence due to finite-size perturbations. Nonlinear interactions of sufficiently amplified non-normal CSs are organized into cycling self-sustained process (SSP) of emerging and decomposing CSs during the cycling period. This by-pass route to turbulence via the weakly unstable CSs as the ingredients of cycling SSP are discovered and identified numerically and confirmed experimentally in turbulent shear flows. Here I will present a discovery of cycling SSP of weakly unstable CSs of co-existing streaks and stream-wise vortices in elastic turbulence (ET) in a straight channel shear flow of viscoelastic fluid that remarkably resembles one observed in Newtonian turbulence in a
channel shear flow. The sequence of CSs periodically repeats itself every cycle by their regeneration and breakdown via a secondary instability, which interface dynamics remarkably recalls a Newtonian Kelvin-Helmholtz instability, resulting in chaotic steady state. The surprising novel ingredient in ET is the observation of elastic waves that are critical for pumping energy into CSs and synchronizing SSP cycle, which frequency is completely defined by elastic wave frequency and equal it, the key and only distinction with Newtonian turbulent shear flow. Otherwise, the observed similarity suggests the universality in stochastically steady state of CSs cycling in SSP in Newtonian as well as elastic turbulence.