Graduate Thesis Or Dissertation
 

Numerical Studies on Fluid-Structure Interaction and Applications

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  • Fluid-structure interaction (FSI) has been of significant research interest worldwide over the past several decades because of its variety of applications in both offshore and coastal engineering. Researchers analyzing FSI systems rely heavily on experimental tests in model scale in laboratories or large-scale sea trials. However, these tests are often very costly and not well suited for parametric studies of design variables. Hence application of numerical modeling techniques to analyze real-world FSI problems has become more attractive and widespread. With major advancements in computer technology and processing speed, it has become easy and highly efficient, in both time and costs, to perform parametric studies. This dissertation studies selected practical FSI problems using different coupling schemes - one-way coupling and two-way coupling - with multiple scales (global-scale for field modeling and local-scale for laboratory test simulation). The problems modeled and analyzed in this dissertation focused on offshore renewable energy and breaking wave impact effect on an elevated coastal structure. In general, the scope of this work includes model development and validation, characterization and optimization of model parameters for different scales and complexity of problems, evaluation of predictive capabilities and computational performance of several models, and analysis of the selected problems based on numerical results. This dissertation is divided into three parts. In the first part, a combined nonlinear mooring-line and umbilical cable dynamics model (MDB) with cable bending stiffness, seabed contact, buoy or weight interconnections, and implicit time marching capabilities was developed and validated against experimental data and numerical predictions from the literature. The close matching of MDB predictions with select available results demonstrated outstanding performance in resolving the cable loads (with and without bending stiffness) in both static and dynamic analyses. The newly developed model was integrated into an open-source wave energy converter simulator, WEC-Sim, and used to simulate and analyze representative offshore WEC and WEC array systems. The dynamics of a representative two-body floating point absorber (FPA) WEC system was simulated and analyzed. Accurate predictions in both statics and dynamics demonstrated the outstanding capabilities of the model applied in analyzing the WEC dynamic response and WEC design. The integrated model was applied to simulate a section of advanced WEC array design of the FPA system by adding a power cord to the FPA spar connected to a floating static hub. The overall maximum and minimum bending moments distribution along the power cord is skewed to the hub. The added power cord is found to reduce the overall tensions of the mooring lines when the device motions do not significantly change, compared to the case without a power cord. In the second part, a novel offshore binary species free-floating longline macroalgae farming system (MFS) was designed and modeled numerically. The hydrodynamic load effect on the system was analyzed under extreme waves and current conditions in different growth periods and associated locations. The models show that the maximum tensions on the longline for all stages are far less than the longline breakage limit, and that all of the maximum tensions at the holdfasts of bull kelp plants and sugar kelp clusters are below the breaking limit obtained from laboratory tests. The drag force due to the waves and current is the dominant load applied on the sugar kelp clusters. On the other hand, the net buoyancy force effect is more significant on the bull kelp plants. Potential damage of the young kelp may occur if the longline stretches out of the water, exposing the kelp to the air (hence to gravity force) without buoyancy support. Buoy-longline contact interaction may damage the buoy, resulting in total loss of the system by sinking. Finally, the model results show that kelp-longline, kelp-supporting line, and kelp-kelp entanglements may potentially occur, causing kelp damage and biomass loss. In the third part, a combined two-dimensional (2D) and three-dimensional (3D) reduced-domain fluid-structure interaction (FSI) model is developed coupling CSD and CFD in a two-way FSI approach to simulate surge waves with different wave breaking conditions (non-breaking, impulsive breaking, and broken) and their impact on an elevated coastal structure. The resulting wave elevation, velocity, uplift pressure, and vertical force are compared with experimental data. For the 2D wave simulations, close agreements on wave elevation and velocity are achieved for non-breaking and breaking wave conditions. Fairly close agreements are obtained for broken wave simulations. The accurately predicted velocity field at the pre-selected cross-section obtained from the wave modeling can provide reliable velocity profile input for the reduced-domain 3D FSI model. The overall uplift pressure distribution pattern achieved from numerical simulation matches experimental measurements on the bottom of the elevated structure for multiple breaking conditions. However, matching measured peak pressure from breaking wave data is challenging for numerical computations as experimental results are not always repeatable. The vertical force is calculated by integrating the pressure along the middle of the specimen and, as expected, the distribution pattern computed from the numerical results matches the experimental data well. In general, the non-breaking wave and FSI simulation results show the closest agreement among all experimental measurements. On the other hand, the broken wave and FSI predictions show the largest deviations because the broken wave condition is significantly more sensitive to mesh size and imposed velocity input compared to the non-breaking and breaking cases.
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  • Financial support from the US Department of Energy, Advanced Research Projects Agency-Energy (ARPA-E) Macroalgae Research Inspiring Novel Energy Resources (MARINER) program, award number 17/CJ000/09/01 and 17/CJ000/09/02 and is gratefully acknowledged.
  • Financial support from the US Department of Energy Grant No. DE-EE0006816 is gratefully acknowledged.
  • Supported by the National Science Foundation under Grant Nos. 1519679 and 1661315, and is gratefully acknowledged.
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  • Pending Publication
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  • 2021-09-20 to 2024-01-11

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