Date of Award
Doctor of Philosophy in Ocean Engineering
Jason M. Dahl
This dissertation focuses on the experimental and numerical modelling of fluid structure interaction (FSI) problems. The main objective of this work was to develop state-of-art experimental and numerical tools to investigate a variety of Fluid-Structure Interaction problems. This topic is of critical importance to Naval Hydrodynamics and offshore applications, such as in ship design, floating offshore wind platforms and offshore ocean energy systems.
On the experimental modelling of FSI, this dissertation includes detailed of state-of-art hydrodynamic testing tank system for studying biomimetic fluid-structure interaction problems. This system was employed to study unsteady ground effect for pitching and heaving flapping foil propulsors in the near presence of a wall. More than 2000 experiment were conducted demonstrating the dynamic ground effect on lift and thrust with flapping foil propulsor as a function of Strouhal number, distance from the wall, and foil kinematics. It was demonstrated that 2D and 3D ground effect in a dynamically flapping system are fundamentally different.
The dissertation also focus on the development and improvement of a numerical wave tank based on fully nonlinear potential flow method accelerated with the Fast Multipole Method (FMM) for advanced FSI problems. In the past 30 years, increasingly accurate and efficient models have been developed to simulate nonlinear wave propagation and transformations over a varying nearshore bathymetry as well as their interactions with submerged and surface piercing fixed or floating structures. One successful approach has been based on models solving Fully Nonlinear Potential Flow (FNPF) theory, by a higher-order Boundary Element Method (BEM), in 2D and 3D. Such models can accurately simulate overturning waves and have been used to investigate their physical properties just before breaking. In this thesis, an improved Numerical Wave Tank (NWT) based on BEMFNPF is discussed with developed improvements for implementation with a fast hybrid BEM-LBM solver for ship seakeeping simulations. In particular, improvements are developed to incorporate compatibility condition in 3D corner intersections and implementation method to suppress wave breaking in 3D potential flow simulations.
In many naval hydrodynamics and ocean/coastal engineering applications, it is desirable to prevent steep waves from overturning as this eventually leads to instabilities and stops computations. A number of methods have been proposed to do so, some based on specifying an absorbing surface pressure", similar to the method used in absorbing beaches. A method is implemented in the NWT using Hilbert transform tracking of wave crest to implement slope and curvature based criteria to identify and suppress breaking waves. Impending breaking is detected based on local maximum free surface slope/steepness criterion, and wave energy absorbed using local "absorbing pressure" patch whose strength is calibrated with a physical criterion. The method is validated for a submerged hydrofoil generating waves at the free surface with experiments from Duncan (1981).
Mivehchi, Amin, "EXPERIMENTAL AND NUMERICAL SIMULATIONS FOR FLUID BODY INTERACTION PROBLEMS" (2018). Open Access Dissertations. Paper 819.