Table-top rotating turbulence : an experimental insight through Particle Tracking

The influence of the Earth background rotation on oceanic and atmospheric currents, as well as the effects of a rapid rotation on the flow inside industrial machineries like mixers, turbines, and compressors, are only the most typical examples of fluid flows affected by rotation. Despite the Coriolis acceleration term appears in the Navier-Stokes equations with a straightforward transformation of coordinates from the inertial system to the rotating non-inertial one, the physical mechanisms of the Coriolis acceleration are subtle and not fully understood. Several fluid flows affected by rotation have been studied by means of numerical simulations and analytical models, but the experimental data available is scarce and purely of Eulerian nature. The present work addresses experimentally the topic, focusing on a class of fluid flows of utmost importance: confined and continuously forced rotating turbulence. Experiments of the same turbulent flow (maximum Re ¿¿ 110 for O = 0) subjected to different background rotation rates ( O ¿ {0; 0.2; 0.5; 1.0; 2.0; 5.0} rad/s) are performed, visualised by optical means, and measured quantitatively with Particle Tracking Velocimetry. The measurement system is designed and implemented around the experimental setup, using innovative solutions. The data collected is processed in the Lagrangian frame, where the trajectories are filtered and the 3D time-dependent signals of position, velocity, acceleration, temporal velocity derivatives, and full velocity gradient tensor are extracted. The data is further interpolated over a regular grid, in order to analyse it in the Eulerian frame. The background rotation is found to decrease the kinetic energy and the energy dissipation of the turbulent field, and to damp the coupling between large-scale flow and small-scale turbulence. Interesting large-scale features of the flow field are revealed: the increase of rotation rate induces vertical coherency of the fluid motion (in terms of velocity, velocity derivatives, Eulerian spatial and temporal auto-correlations of velocity), till at the maximum rotation rate of 5 rad/s a quasi-2D flow is measured, dominated by stable counter-rotating vertical tubes of vorticity. Exception is the 2 rad/s run, for which an anomalous behaviour of all the investigated flow features is observed: at this rotation rate, the vertical vortex tubes fluctuate in the measurement domain with much higher amplitude and on a longer time scale than for any other run. The estimated values for the critical Rossby number indicate that the stability of the large-scale anticyclonic vortices may be compromised for 1.0 <O <5.0 rad/s. No indications of resonant oscillations in the container, triggered by inertial waves, are instead recognised in the data. Further investigations are necessary to explain the anomaly measured for this run, but the present data suggest the possibility that anticyclone instabilities significantly alter the large-scale flow. The (non-)rotating turbulent flow is also investigated in terms of Eulerian spatial correlations of the velocity field, and – for the first time – of Lagrangian correlations of the velocity, acceleration, and vorticity vectors extracted along fluid particle trajectories. The increase of vertical (parallel to the rotation vector) and horizontal velocity correlations induced by rotation is measured in the Eulerian and the Lagrangian frames. Rotation is seen to strongly enhance the correlation of the vertical vorticity component, characteristic of a flow dominated by columnar vortex structures. It is also seen to enhance the longitudinal horizontal acceleration component, confirming the direct role played by the Coriolis acceleration in the amplification of the Lagrangian acceleration correlations in turbulence. In the same Lagrangian frame, the turbulent dispersion process at short times in the presence of rotation is investigated. The data permits to describe the initial ballistic dispersion regime, and the beginning of the inertial range regime. A more pronounced effect is observed on single-particle dispersion statistics, which are influenced by rotation in a non-monotonic way, strongly anisotropic only for the fastest rotating runs. Two-particle dispersion is monotonically reduced with increasing rotation rate, and the anisotropy is revealed only for maximum rotation rate. Some of the results presented in this thesis are completely new. Other results confirmed well-known features of rotating turbulent flows, further quantifying them on the basis of state-of-the-art Particle Tracking experimental data. Surely this work opened new questions. Concluding remarks give suggestions about possible future measurements in the same turbulence setup, as well as in view of the design of a new experimental setup specifically devoted to Lagrangian flow analysis and/or to the investigation of rotating steady turbulence. The results obtained in this study have been presented at international conferences and workshops, and will be submitted for publication to international journals