Experimental analysis of turbulent flows at high Reynolds numbers in the CICLoPE "Long Pipe"

This thesis aims to investigate wall shear stress uncertainties and asymptotic scaling laws in turbulent pipe flows, with a focus on their implications for active flow control strategies. The experiments were conducted mainly at the Long Pipe facility of the CICLoPE laboratory at the University of Bologna, a state-of-the-art facility designed to reproduce canonical turbulent pipe flow across a wide range of Reynolds numbers while minimizing measurement uncertainties related to spatial resolution. This research include the development and refinement of the Oil Film Interferometry (OFI) as a direct local measurement technique for wall shear stress in the Long Pipe and a comprehensive uncertainty analysis comparing OFI with static pressure drop methods, classical methodology to obtain global wall shear stress in pipe flows. These findings enabled the reduction of overall uncertainties in determining asymptotic scaling laws. Additionally, OFI was applied to active flow control scenarios, demonstrating its versatility. Further investigations into wall-pressure fluctuations revealed their scaling behavior and coherence with near-wall velocity fluctuations both streamwise and wall-normal. A consistent linear correlation in the logarithmic region of the flow highlights the potential for scalable, real-time flow control applications. These results affirm the CICLoPE facility’s significant role in advancing the understanding of wall-bounded turbulence and its application to active drag reduction strategies.