High Reynolds Number Experiments in the Long Pipe at CICLoPE

Mascotelli, Lucia (2020) High Reynolds Number Experiments in the Long Pipe at CICLoPE, [Dissertation thesis], Alma Mater Studiorum Università di Bologna. Dottorato di ricerca in Meccanica e scienze avanzate dell'ingegneria, 32 Ciclo. DOI 10.6092/unibo/amsdottorato/9455.
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In aerospace applications 50% of the drag is due to the friction experienced in the turbulent boundary layer. To reduce drag, it is important to rely on accurate numerical codes to correctly predict friction. The logarithmic-law, the wall function describing the flow in the near-wall region, and in particular its governing parameter k (Von Karman constant), are still object of debate. If there is now agreement on k=0.39 for pipes and other canonical flows, the matching of wake discussed by Coles has not received enough attention. It requires that in the centerline evolution of the logarithmic law, U+CL=1/kCL ln(Ret)+C, with kCL being its Von Karman constant, k=kCL. The requirement is violated for pipes where kCL is higher than 0.42, not accounted for in numerical models. This thesis is aimed at investigating the anomalies in the estimate of kCL, through centerline measurements in Long Pipe at CICLoPE, for Reynolds numbers ranging from 8000 to 40000. The Long Pipe is suited to assess if the differences between k and kCL are due to experimental uncertainties or rooted in physics, given the nontrivial nature of the boundary layer. Despite being chaotic, it hides a hierarchy of differently-sized eddies whose interactions are still unclear and challenge the classical view. Then, single-wire measurements were performed in the range of Re from 10000 to 40000, covering up to y/R=0.93 to quantify the contributions to the turbulence intensity for the different scales. Using a spectral cut-off filter, the large scales showed a growing streamwise variance with increasing Reynolds number, while the small scales exhibited the universality stated by the classical theory. Then, 2-point measurements allowed to investigate the nature of the turbulent eddies. The wall-attached and self-similar behaviour postulated by Townsend's attached eddy hypothesis is observed for the coherent structures with an aspect ratio of around 2.

Tipologia del documento
Tesi di dottorato
Mascotelli, Lucia
Dottorato di ricerca
Settore disciplinare
Settore concorsuale
Parole chiave
Wall turbulence, pipe flow, high Reynolds number, hot-wire, coherent structures, logarithmic law, von Karman constant, friction
Data di discussione
6 Aprile 2020

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