Geotropic tracers in turbulent flows: a proxy for fluid acceleration

We investigate the statistics of orientation of small, neutrally buoyant, spherical tracers whose centre of mass is displaced from the geometrical centre. If appropriate-sized particles are considered, a linear relation can be derived between the horizontal components of the orientation vector and the same components of acceleration. Direct numerical simulations are carried out, showing that such relation can be used to reconstruct the statistics of acceleration fluctuations up to the order of the gravitational acceleration. Based on such results, we suggest a novel method for the local experimental measurement of accelerations in turbulent flows.