Amador Ramírez, André

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    Probing the hydrodynamics of plunging gravity waves through Lagrangian observations of inertial particle dynamics
    (2013-07) Amador Ramírez, André; Canals Silander, Miguel F.; College of Engineering; Leonardi, Stefano; Cancelos, Silvina; Department of Mechanical Engineering; Mercado, Aurelio
    Understanding wave breaking phenomena is a challenging problem that still lacks satisfactory understanding. The present study is concerned with the development of novel instrumentation adapted to obtain Lagrangian field measurements of essential variables that are intimately related to the physics of wave breaking. In this study we describe the design, development and laboratory testing of miniature Lagrangian drifters equipped with inertial measurement units. We show that the developed instrumentation is able to collect useful data of inertial particle dynamics in a controlled flow generated in the laboratory and designed to mimic the wave breaking process. We then apply filtering, calibration and error reduction methods to obtain the best possible estimates of particle orientation, acceleration and rotation. These post-processed variables are then projected onto a reference frame aligned with the mean flow direction using quaternion-based methods, and the resulting accelerations are then integrated to estimate particle trajectories via dead reckoning. The trajectories obtained from the dead reckoning qualitatively coincide with the expect trajectories given the geometry of the laboratory generated flow. Encouraged by the promising laboratory results, we then carry out field observations in actual breaking waves with heights on the order of two meters, and obtain the first ever Lagrangian field observations of inertial particle dynamics in breaking waves. After application of the error reduction and reference frame rotation techniques, we analyze the resulting inertial particle accelerations in a reference frame aligned with the wave crest. A transition from a highly anisotropic acceleration signature during the initial entrainment to a more isotropic field in the post-breaking region is observed, suggesting that the large vortex generated by the wave overturning event degenerates into smaller three-dimensional vortices. We then take a step further and actually estimate, using dead reckoning of the post-processed accelerations, the inertial particle trajectories in the initial entrainment event. The computed trajectories and the spatial structure of the acceleration fields agree qualitatively with published numerical simulations. We expect that further development of the technology and analysis tools presented in this thesis could revolutionize our understanding of the hydrodynamics of breaking waves. Besides the obvious implications regarding the fundamental hydrodynamics of such a complicated flow, further work on this subject could lead to better numerical models of wave energy dissipation. This could have very significant implications for coastal and ocean science and engineering, since coastal sediment transport models currently use very simple parameterizations of wave energy dissipation, and it is well known that wave-induced sediment suspension is the leading term in nearshore sediment transport.