Small scale statistics of turbulent fluctuations close to a stagnation point

Abstract

Experimental data measured with a 3d Shadow-Particle Tracking Velocimetry (S-PTV) setup in fully developed turbulence (Reλ=[175-225]> ) is presented. The underlying flow is of the von Kármán type and as other similar flows, its mean flow is bistable, the two states having the topology of a stagnation point with one contracting and two dilating directions. Tracer particle trajectories permit the investigation of the inhomogeneity and anisotropy of the smallest scales, namely acceleration statistics. The local variance and time-scale of acceleration components are shown to mimic the large scale properties of the flow, the time-scales being more anisotropic than the variances. We explain the hierarchy of time-scales by investigating the Lagrangian Taylor micro-scale which is related to acceleration and velocity variances, and discuss the very high Reynolds number regime.