In this work, original experimental data on the fluid flow instabilities associated with the oscillation of the liquid free surface in low liquid height-to-tank diameter ratio stirred tanks are presented. The local flow features of this type of tanks, which are commonly adopted as (bio)-reactors in important industrial productions such as drugs and bio-energy, are investigated by Particle Image Velocimetry. The local mean flow field and the turbulent characteristics are found to depend critically on the periodic variations of the liquid free surface, whose features vary as a function of the fill ratio. The frequency analysis of the dynamic pressure time series collected by a Pitot system shows that the frequency of the flow instabilities is mainly affected by the geometrical characteristics of the system. Therefore, the observed instabilities do not arise from the same origin of the so-called macro-instabilities previously identified in stirred vessels. A novel interpretation based on the liquid sloshing dynamics in cylindrical vessel is suggested. (C) 2016 Elsevier B.V. All rights reserved.