Turbulent boundary layers over biofouling-type topographies
Open Access DepositedA common and complex roughness type of high importance to the naval engineering and fluid dynamics community is biofouling. Biofouling is the undesirable accumulation of ocean and marine organisms on the hull of naval vessels that eventually leads to an excess of fuel consumption and greenhouse gas emissions, as a result of the increased hydrodynamic resistance. It is estimated that for a specific destroyer class of the US Navy alone the economic cost of biofouling can rise up to $1B over a 15 year period. Despite that, very few studies have focused on biofouling-type surfaces, which are characterized by multi-scale terrains and specific topographical properties. Thus, the application of current drag-topography correlations based on canonical roughness can lead to erroneous predictions. To address that, in the present work synthesized biofouling topographies are generated via our in-house surface generation/analysis package, while the resulting fluid-flow field is studied by means of direct numerical simulation (DNS) over spatially developing turbulent boundary layers. The dissertation focuses on two main objectives: i) the identification of the main topographical parameters that result in the drag-producing physics and ii) the anatomy of the rough-wall fluid-flow in both its statistically-stationary, as well as dynamical state. Emphasis is given on the evolution of turbulent boundary layers over various biofouling arrangements and coverages with respect to both the first and higher order statistics. The DNS results are validated against peer experimental measurements, while a direct comparison is made with respect to the mean flow structure between a boundary layer and a dynamically and geometrically similar channel configuration under the same rough-wall conditions. The three-dimensional structure of the boundary layer flow is investigated in both its mean and instantaneous component by performing conditional averages, two-point correlations and modal analysis in order to study important mean flow properties, such as the outer-layer similarity, as well as to elucidate the dynamical structure of the near-wall flow. Furthermore, a discussion is held regarding the effects of the streamwise evolving nature of boundary layers on the equivalent-sandgrain roughness height. Finally, an attempt towards the generalization of Schlichting's equations in rough-wall flows is proposed.
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Kaminaris_gwu_0075A_17256.pdf | 2025-04-09 | Open Access |
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