PhD Thesis by Fabio Jessen Werneck de Almeida Martins: “Characterization of Near-Wall Turbulent Flows by Tomographic PIV”

The fundamental study of near-wall turbulent structures is extremely important due to its dominant role in several different engineering applications. In turbulent flows, structures with different scales interact among themselves through a complex phenomenon, characterized by a continuous and self-sustainable process. Although the presence of coherent patterns seems to be responsible for the maintance of turbulence, the evolution and interaction of these structures are not yet completely understood. In order to investigate such phenomena, experimental tests of turbulent flow over a flat plate were conducted in a wind tunnel. Experiments were also conducted in a square channel flow, representing a couple of near-wall flow studies. The present work employed a tomographic particle image velocimetry technique, TomoPIV, with high temporal resolution for measuring 3D velocity fields, allowing the determination of the complete velocity gradient tensor, as well as the reconstruction of the temporal history of the turbulent structures. Processing procedures for the TomoPIV technique were optimized in order to improve the algorithm performance and precision of the measured vectorial fields. The conducted measurements allowed for the attainment of conditional statistics for the typical turbulent boundary layer coherent structures, such as (high and low) velocity streaks, vortices sweeps and ejections, thus providing quantitative information on such structures. The statistical analyses were accomplished through the use of a data post-processing code, especially developed for the current research. Results have shown vortices distributed all over the flow; those were concentrated, however, in the neighborhood of low velocities and ejections. These vortices have presented a variety of angles (inclinations) and a wide range of size scales. The results have indicated that the vortices were indirectly correlated with the turbulence production process, despite being the mainly accountable for dissipation of turbulent energy. Ejections and sweeps, associated to the regions of low and high velocities, respectively, were seen as the structures which contribute the most for turbulence production.