Experimental study of the boundary layer separation and transition processes under turbine-like conditions by means of advanced post-processing techniques

In this work the transition process of the boundary layer (BL) evolving under turbine-like conditions has been experimentally investigated in details. The effects of the Reynolds number (Re), the free-stream turbulence intensity (Tu) and the adverse pressure gradient (APG) imposed to the flow have been studied for a large variation of these parameters, since they are known to strongly influence the separation and transition processes of the boundary layer. Emphasis has been put on both the statistical and the dynamic behaviour of the flows at hand, that have been experimentally characterized by means of advanced and ad-hoc developed post processing techniques. The study of the effects of the Reynolds number and the Tu level on the development of laminar separation bubbles (LSB) under fixed APG is presented in the first part of this work. The mechanisms by which the variations of Re and Tu act on the bubble size were found to be substantially different and the coexistence of different amplification mechanisms has been observed in the LSBs for high Tu levels. In case of by-pass transition, the effects of the APG has been investigated with respect to the zero pressure gradient condition. The transition process has been found to be more rapid due to the APG imposed to the flow with respect to the zero pressure gradient case. The profiles of the mean streamwise velocity and velocity fluctuation rms obtained by means of Hot-Wire instrumentation showed a self-similar behavior in the laminar part of the boundary layer. For what concern the effects of the Tu level on the velocity and rms of velocity fluctuation profiles, the high free-stream turbulence has been found to reduce the effects of the pressure gradient on the curvature of the mean velocity profiles and shifting the maximum of the turbulence peak towards the wall. In order to shed light on the effects of the APG variation on the statistical and dynamic behaviour of LSB, as well as to provide a complete experimental database containing information about the effects of Re, Tu and APG in case of both attached and separated flows, a new test section has been designed in the second part of this work allowing the continuous variation of the pressure gradient imposed to the flow. In case of separated flows, the separation position was found to move downstream when the APG is reduced and the bubble becomes longer. However, the bubble thickness is reduced with respect to the higher APGs conditions. Proper Orthogonal Decomposition (POD) has been adopted to reduce the large amount of experimental data collected, obtaining a statistical treatment of the main dynamics at hand in terms of their energy content. Moreover, with the aim of characterizing the coexistence of structures with different energy within the flow (e.g., boundary layer streaks, Kelvin- Helmholtz and free-stream vortices) a variant of the classical POD procedure has been proposed. The application of this technique in case of both attached and separated flows highlighted the presence of free-stream structures near the edge of the boundary layer where the transition process has been found to occur, suggesting that free-stream structures can actually play a crucial role in the evolution and breakdown of structures growing into the boundary layer, thus leading transition. Finally, the analysis of the statistical quantities of the flows at hand (i.e. BL integral parameters) has been carried out for all the acquired conditions with the aim of developing new empirical correlations for the prediction of the transition onset and length in case of separated flows. Data collected during both the measuring campaigns allowed the tuning of the proper coefficients in order to take into account for the variation of all the parameters considered in this work. The proposed correlations have been found to fit both the collected data as well as other experimental data available in literature.