Long-range ordering of turbulent stresses in 2D turbulence

etc15 Tracking Number 211

We use filter-space techniques to study the geometric alignment of turbulent stresses and strain rates in an experimental quasi-two-dimensional weakly turbulent flow. When these stresses and strains are misaligned, the usual turbulent energy cascade can be suppressed; and more generally, the relative alignment of these two tensors determines the direction of the cascade. We show that as a function of length scale, the turbulent stress undergoes a transition to system-spanning order. However, by exploring analogously defined quantities in a field built from random Fourier modes, we see qualitatively similar behavior, suggesting that at least some of this ordering is purely kinematic. By comparing our results from the experiment and the random field, we highlight the role played by the orientation of the rate of strain tensor in the energy transfer process; additionally, our results allow us to pose several intriguing conjectures.