EXPERIMENTAL ANALYSIS OF TRANSITIONAL FLOWS UNDER TURBINE-LIKE CONDITIONS VIA APPLICATION AND DEVELOPMENT OF ADVANCED POST-PROCESSING TECHNIQUES

The present thesis is primarily devoted to developing and applying advanced post-processing techniques to inspect complex transitional boundary layer (BL) flows evolving under variable inflow conditions. A large amount of data has been experimentally acquired utilizing particle image velocimetry (PIV) and laser Doppler velocimetry (LDV) in a test section consisting of a flat plate installed between two adjustable endwalls. Depending on the Reynolds number (Re), the free-stream (FS) turbulence intensity (Tu) and the adverse pressure gradient (APG) imposed to the flow, attached or separated boundary layer transition was obtained. The effects of the inflow parameters variation have been studied in detail, focusing on the flow statistical and dynamical behavior. Due to the complexity and variety of the transitional phenomena, data-driven modal decomposition techniques have been employed to reduce the large amount of experimental data collected here. Moreover, new variants of well-established post-processing techniques have been developed to identify the main features embedded in the extensive databases. In the case of separated flow transition, the modal decomposition procedures allowed a deep insight into the instability mechanism developing in the shear layer. Dynamic Mode Decomposition (DMD) was used to analyze the most unstable wavelengths related to the Kelvin-Helmholtz (K-H) vortices driving transition. Proper Orthogonal Decomposition (POD) was applied to PIV data, inspecting the main flow structures developing in the different regions of the LSB. Subsequently, an Extended Proper Orthogonal Decomposition (E-POD) procedure was applied, highlighting the correlation between the main dynamics observed in the forward part of the bubble and the breakup events occurring in the reattachment region. Regarding the data reduction, the extensive database was used to develop new empirical correlations predicting the transition process regarding the geometry of a LSB and the related shedding process. The transition process was systematically analyzed using decomposition techniques in the context of the free-stream turbulence induced transition. In order to inspect BL receptivity to free-stream disturbances, a variant of the E-POD was proposed, based on the correlating events between the FS and the BL. Low-order reconstructions of the original data were used to highlight the most correlating events directly linked to the formation and the breakup of streaky structures. Moreover, a turbulent spot recognition algorithm was implemented to identify the BL statistical response to the inflow parameters through the probability density function (PDF) of spot nucleation. Thus, a model for the PDF of spot nucleation is proposed as a function of the main flow parameters involved in the transition process. Based on the results of the previous analyses, engineering correlations for predicting the free-stream turbulence induced transition are also introduced. Independently on the transition type, results obtained employing the aforementioned procedures allowed a fruitful characterization of the different instability mechanisms developing in the first stage of transition, the description and evolution of coherent structures, and the correlation between their dynamics.