Speaker: Samriddhi Sankar Ray, International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore, India

Time : 15:00 - 16.00 CET (Rome/Paris)

Hosted at: SISSA, International School of Advanced Studies, Trieste, Italy

Zoom :  A zoom meeitng link will appear here, one hour before the talk

Organizers : Pavan Pranjivan Mehta* (pavan.mehta@sissa.it) and Arran Fernandez** (arran.fernandez@emu.edu.tr)

* SISSA, International School of Advanced Studies, Italy

** Eastern Mediterranean University, Northern Cyprus

Keywords: Turbulence, Multifractals, Intermittency

Abstract: Recent ideas [1] of many-body chaos in classical Hamiltonian systems — namely decorrelators or the classical analogues of OTOCs — have opened an interesting window to revisit problems in non linear, out of equilibrium flows.

In this talk we will introduce this idea through a classical spin chain model [1] and then motivate how the same can be applied to nonlinear, Hamiltonian, thermalised fluids [2] to show how the Lyapunov exponent scales with temperature in a classical many-body system. We then take two further examples from non-Hamiltonian, out-of-equilibrium systems — namely fully developed high Reynolds number classical turbulence [5] and low Reynolds active turbulence of bacterial suspensions [3] — to further exploit such decorrelators in order to see how the Lyaponov exponent λ scales with the Reynolds number Re in the first and activity in the second case. In particular, for the high Reynolds number fully developed turbulence problem, we show λ ∝ Reα and investigate the interplay of the competing effects of viscous dissipation and nonlinearity. We obtain a precise value of α = 0.59 ± 0.04 and show that the departure from the Kolmogorov mean field result λ ∝ √Re is a consequence of the intermittent fluctuations [4] in the velocity gradient tensor. The robustness of our results are further confirmed in a local, dynamical systems model for turbulence.

Biography: Samriddhi Sankar Ray is an Associate Professor at the International Centre for Theoretical Sciences of the Tata Institute of Fundamental Research (ICTS-TIFR) in Bangalore, India. Prior to this Samriddhi obtained his PhD in Physics from the Indian Institute of Science (IISc), Bangalore and then a short post-doctoral stint at Observatoire de la Cote d'Azur in Nice, France. His research interests, lying at the interface of statistical physics, applied mathematics and computational fluid dynamics, focusses on high Reynolds number fully-developed turbulence and turbulent transport. He has also worked extensively on problems of thermalisation of finite-dimensional ideal hydrodynamics as well as, most recently, low Reynolds number bacterial suspensions.

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