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  • Cell & Developmental Biology
  • Humans
  • Inorganic Chemistry
  • SPACE SCIENCES
  • Underwater acoustics
  • Massachusetts Institute of Technology and Woods Hole Oceanographic Institution  (2)
  • 2015-2019  (2)
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  • 1
    Publication Date: 2022-05-25
    Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2015
    Description: The spatial and temporal evolution of stratified shear instabilities is quantified in a highly stratified and energetic estuary. The measurements are made using high-resolution acoustic backscatter from an array composed of six calibrated broadband transducers connected to a six-channel high-frequency (120-600 kHz) broadband acoustic backscatter system. The array was mounted on the bottom of the estuary and looking upward. The spatial and temporal evolution of the waves is described in terms of their wavelength, amplitude and turbulent dissipation as a function of space and time. The observed waves reach an arrested growth stage nearly 10 times faster than laboratory and numerical experiments performed at much lower Reynolds number. High turbulent dissipation rates are observed within the braid regions of the waves, consistent with the rapid transition to arrested growth. Further, it appears that the waves do not undergo periodic doubling and do not collapse once their maximum amplitude is reached. Under some conditions long internal waves may provide the perturbation that decreases the gradient Richardson number so as to initiate shear instability. The initial Richardson number for the observed instabilities is likely between 0.1 and 0.2 based on the slope and growth rate of the shear instabilities.
    Keywords: Underwater acoustics ; Underwater acoustic telemetry
    Repository Name: Woods Hole Open Access Server
    Type: Thesis
    Format: application/pdf
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  • 2
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    Massachusetts Institute of Technology and Woods Hole Oceanographic Institution
    Publication Date: 2022-05-26
    Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2014
    Description: Motivated by inspection of complex underwater environments, we have developed a system for multi-sensor SLAM utilizing both structured and unstructured environmental features. We present a system for deriving planar constraints from sonar data, and jointly optimizing the vehicle and plane positions as nodes in a factor graph. We also present a system for outlier rejection and smoothing of 3D sonar data, and for generating loop closure constraints based on the alignment of smoothed submaps. Our factor graph SLAM backend combines loop closure constraints from sonar data with detections of visual fiducial markers from camera imagery, and produces an online estimate of the full vehicle trajectory and landmark positions. We evaluate our technique on an inspection of a decomissioned aircraft carrier, as well as synthetic data and controlled indoor experiments, demonstrating improved trajectory estimates and reduced reprojection error in the final 3D map.
    Keywords: Underwater acoustics ; Underwater acoustic telemetry
    Repository Name: Woods Hole Open Access Server
    Type: Thesis
    Format: application/pdf
    Location Call Number Expected Availability
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