Browsing by Subject "Ocean waves"

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    Adjoint-based Data Assimilation for Ocean Waves
    (2022-05) Wu, Jie
    The exchange between the atmosphere and oceans, which regulates weather and climate processes and upper ocean dynamics, critically depends on the spatial and temporal evolution of ocean surface waves. In reality, directly measuring the entire wave state is an expensive and difficult task. Firstly, we propose a method for the reconstruction and prediction of nonlinear wave fields from coarse-resolution measurement data. We adopt the adjoint-based data assimilation framework to search for the optimal wave states to match the given measurement data and governing physics. The performance of our method is assessed by considering a variety of wave steepness and noise levels for the nonlinear irregular wave fields. It is found that our method shows significantly improved performance in the reconstruction and prediction of instantaneous surface elevation, surface velocity potential, and high-order wave statistics. Secondly, we propose a method to infer coastal bathymetry from the spatial variations of surface waves by combing a high-order spectral method for wave simulation and an adjoint-based variational data assimilation method. The proposed bottom detection method is applied to a realistic coastal environment involving complex two-dimensional bathymetry, non-periodic incident waves, and nonlinear broadband multi-directional waves. We also address issues related to surface wave data quality, including limited sampling frequency and noise. Both laboratory-scale and field-scale bathymetries with monochromatic and broadband irregular waves are tested, and satisfactory detection accuracy is obtained. Last, we investigate the impact of oceanic internal wave and surface wave parameters on the surface roughness signature based on the two-fluid solver. We use the ratio of the mean surface slope between the rough and smooth bands, which are identified in the simulated surface field, to systematically investigate their response to the internal wave forcing across all our simulation cases. We find that the strongest surface heterogeneity is caused by varying upper-lower layer density ratios. Our results also show that it is necessary to include the wave steepness statistics to account for the internal wave-induced surface wave modulation.
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    Fluid-Structure Interaction Simulation of Complex Floating Structures and Waves
    (2015-11) Calderer Elias, Antoni
    A novel computational framework for simulating the coupled interaction of complex floating structures with large-scale ocean waves and atmospheric turbulent winds has been developed. This framework is based on a domain decomposition approach coupling a large-scale far-field domain, where realistic wind and wave conditions representative from offshore environments are developed, with a near-field domain, where wind-wave-body interactions can be investigated. The method applied in the near-field domain is based on a partitioned fluid-structure interaction (FSI) approach combining a sharp interface curvilinear immersed boundary (CURVIB) method with a two-phase flow level set formulation and is capable of solving free surface flows interacting non-linearly with complex real life floating structures. An aspect that was found critical in FSI applications when coupling the structural domain with the two-fluid domain is the approach used to calculate the force that the fluid exerts to the body. A new force calculation approach, based on projecting the pressure on the surface of the body using the momentum equation along the local normal to the body direction, was proposed. The new approach was shown, through extensive numerical tests, to greatly improve the ability of the method to correctly predict the dynamics of the floating structure motion. For the far-field domain, a large-scale wave and wind model based on the two-fluid approach of Yang and Shen (JCP 2011), which integrates a viscous Navier-Stokes solver with undulatory boundaries for the motion of the air and an efficient potential-flow based wave solver, was employed. For coupling the far-field and near-field domains, a wave generation method for incorporating complex wave fields into Navier-Stokes solvers has been proposed. The wave generation method was validated for a variety of wave cases including a broadband spectrum. The computational framework has been further validated for wave-body interactions by replicating an experiment of floating wind turbine model subject to different sinusoidal wave forces. The simulation results, which agree well with the experimental data, have been compared with other numerical results computed with available numerical codes based on lower order assumptions. Despite the higher computational cost of our method, it yields to results that are in overall better accuracy and it can capture many additional flow features neglected by lower order models. Finally, the full capabilities of the framework have been demonstrated by carrying out large eddy simulation (LES) of a floating wind turbine interacting with realistic ocean wind and wave conditions.