Mechanical Engineering - Theses

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Author: Romero, Sylvia × End date: 2023 × Start date: 2020 × Subject: turbulence ×

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    Measurements and analysis of turbulent boundary layers subjected to streamwise pressure gradients
    Romero, Sylvia ( 2021)
    This thesis details the changes in structure between an adverse pressure gradient turbulent boundary layer (APG TBL), zero pressure gradient (ZPG) TBL, and channel flow by means of the mean momentum balance (MMB). In order to understand the effects of a pressure gradient on a turbulent boundary layer, aspects of the physical flow are studied via mean statistics, turbulence measurements, and spectra analysis. This study uses new experimental measurements that are conducted along an APG ramp as well as measurements downstream of the ramp insert to study the flow as it relaxes towards equilibrium. In the present experimental set-up the boundary layer is under modest APG conditions, where the Clauser pressure-gradient parameter $\beta$ is $\leq 1.8$. Well-resolved hot-wire measurements are obtained at the Flow Physics Facility (FPF) at the University of New Hampshire. Comparisons are made with ZPG TBL experimental data at similar Reynolds number and computational data at lower Reynolds number. Present measurements are also compared to existing APG TBL lower Reynolds number experimental and computational data sets. Finally, it is shown how these findings relate to an analytical transformation. The primary takeaways from the MMB analysis presented herein are $(i)$ distance-from-the-wall scaling can result from an assumption of self-similar mean dynamics, and does not require primacy of a single velocity scale, and $(ii)$ distance-from-the-wall scaling does not necessarily imply a logarithmic mean velocity profile; a power-law velocity scale hierarchy along with self-similar mean dynamics simultaneously produces distance-from-the-wall scaling \textit{and} a power law mean velocity profile. The choice to refer to the (potentially) self-similar subdomain as the `inertial sublayer' in the present study (rather than the `log' layer) is therefore deliberate.