Experimental study of Multi-Particle Lagrangian Dynamics in Intense Turbulent Flows

We report experimental measurements of Lagrangian dynamics following multiple particles in intense turbulent flows with Taylor microscale Reynolds number up to 800. We focus on the course grained-velocity gradient measured from 4 fluid particles -- the tetrads. We present data obtained from tetrads with sizes in a wide range of the inertial range. For any size of tetrads in the inertial range, the probability distribution of the coarse-grained velocity gradient is skewed towards the bi-axial extensional flows. We also measured the evolution of the coarse-grained velocity gradients following the Lagrangian tetrads, which reveals the importance of pressure in this problem. Finally, we report the effect of polymer additives on the coarse-grained velocity gradients and the comparison with theoretical and numerical work.