Webb Douglas C.

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Webb
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Douglas C.
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  • Article
    Underwater tunable organ-pipe sound source
    (Acoustical Society of America, 2007-08) Morozov, Andrey K. ; Webb, Douglas C.
    A highly efficient frequency-controlled sound source based on a tunable high-Q underwater acoustic resonator is described. The required spectrum width was achieved by transmitting a linear frequency-modulated signal and simultaneously tuning the resonance frequency, keeping the sound source in resonance at the instantaneous frequency of the signal transmitted. Such sound sources have applications in ocean-acoustic tomography and deep-penetration seismic tomography. Mathematical analysis and numerical simulation show the Helmholtz resonator's ability for instant resonant frequency switching and quick adjustment of its resonant frequency to the instantaneous frequency signal. The concept of a quick frequency adjustment filter is considered. The discussion includes the simplest lumped resonant source as well as the complicated distributed system of a tunable organ pipe. A numerical model of the tunable organ pipe is shown to have a form similar to a transmission line segment. This provides a general form for the principal results, which can be applied to tunable resonators of a different physical nature. The numerical simulation shows that the “state-switched” concept also works in the high-Q tunable organ pipe, and the speed of frequency sweeping in a high-Q tunable organ pipe is analyzed. The simulation results were applied to a projector design for ocean-acoustic tomography.
  • Article
    A sound projector for acoustic tomography and global ocean monitoring
    (IEEE, 2003-07-09) Morozov, Andrey K. ; Webb, Douglas C.
    Long-range underwater acoustic systems, such as those used in ocean acoustic tomography, require low-frequency signals covering a broad frequency band. To meet this requirement, a novel design based on of a tunable narrow-band high-efficiency sound projector has been exploited. The projector transmits a frequency sweep signal by mechanically tuning a resonator tube (or organ pipe) to match the frequency and phase of a reference signal. The resonator tube projector consists of a symmetrical pressure-balanced Tonpilz driver placed between two coaxially mounted tubes. The Tonpilz acoustical driver is composed of two pistons separated by preloaded ceramic stacks. The resonant tube is a simple. efficient, narrow-band, medium-output projector that operates at any ocean depth. Both projector tubes have slots (or vents), which are progressively covered or uncovered by sliding coaxial tubular sleeves. The frequency varies with the sleeves position. A computer-controlled electromechanical actuator moves the cylindrical sleeves along the tubes, keeping the projector in resonance at the instantaneous frequency of a swept frequency signal. The actuator smoothly tunes the frequency of the resonator tube in the bandwidth of 200 to 300 Hz during a 135-s transmission. A computer synthesizes the linear frequency-modulated signal; compares the phase between transmitted and reference signals; and, using a phase-lock loop (PLL) system, keeps the resonator tube frequency in resonance with the driver frequency. The estimated PLL precision is better than 3 phase error. The system was analyzed by means of finite element analysis and electrical equivalent circuit simulation. The projector prototype was first tested at theWoods Hole Oceanographic Institution (WHOI) dock inWoods Hole, MA and later in the Pacific Ocean during a voyage of the R/V “Point Sur,” November 2001.
  • Technical Report
    A quasi-Lagrangian study of mid-ocean variability using long range SOFAR floats
    (Woods Hole Oceanographic Institution, 1976-04) Rossby, H. Thomas ; Voorhis, Arthur D. ; Webb, Douglas C.
    Twenty neutrally buoyant SOFAR floats were used in the Mid-Ocean Dynamics Experiment (MODE) to study the structure and variability of the deep ocean currents. The floats were clustered so that the pattern of motions could be resolved (mapping and pattern recognition). A number of float trajectories are shown and the very individual character of their signature is emphasized. Some floats remain nearly stationary for a year whereas others will cover hundreds of kilometers to the south or west in just a few months. Superposition of all trajectories in the spaghetti diagram is shown to reveal considerable organization of the "eddy" field in the MODE area and is thought to be caused by the near presence of the Blake-Bahama Outer Ridge to the west. There is considerable asymmetry to the float dispersal with floats rapidly scattering to the south and west, but not to the north and east even though the r .m.s. velocities are a factor 3 to 6 times greater than the mean drift. The evolution and dispersal of the float cluster is illustrated in a set of figures in each of which a 12 day segment of all float trajectories is displayed. At times their mobility and relative motion is shown to be associated with onset of sudden swirls and regions of large horizontal shear, features that are not evident from the analysis of individual trajectories. Cluster averages of the float velocities and kinetic energy, computed weekly and plotted as a function of time, show substantial variability. Much better averages are obtained by limiting the cluster to floats within a geographical region. As the spaghetti diagram indicates and the following paper discusses in more detail there exist substantial geographical variations in the average kinetic energy levels. These may be in some way caused topographically by the close proximity to the continental margin. Whatever the reason they caution us to reexamine the notion that the scale of variation of the second order eddy statistics is large compared to the eddies themselves, at least in the MODE-I area. Ten floats also contained a system to record the local pressure, temperature and vertical currents. The pressure and temperature yield data concerning low frequency vertical displacements and the vertical current meters measure the internal wave sea state which is shown to be remarkably constant.
  • Article
    Highly resolved observations and simulations of the ocean response to a hurricane
    (American Geophysical Union, 2007-07-07) Sanford, Thomas B. ; Price, James F. ; Girton, James B. ; Webb, Douglas C.
    An autonomous, profiling float called EM-APEX was developed to provide a quantitative and comprehensive description of the ocean side of hurricane-ocean interaction. EM-APEX measures temperature, salinity and pressure to CTD quality and relative horizontal velocity with an electric field sensor. Three prototype floats were air-deployed into the upper ocean ahead of Hurricane Frances (2004). All worked properly and returned a highly resolved description of the upper ocean response to a category 4 hurricane. At a float launched 55 km to the right of the track, the hurricane generated large amplitude, inertially rotating velocity in the upper 120 m of the water column. Coincident with the hurricane passage there was intense vertical mixing that cooled the near surface layer by about 2.2°C. We find consistent model simulations of this event provided the wind stress is computed from the observed winds using a high wind-speed saturated drag coefficient.
  • Technical Report
    Shearmeter floats in the area of the WHOI Brazil Basin Tracer Release Experiment : technical and oceanographic data
    (Woods Hole Oceanographic Institution, 2002-01) Duda, Timothy F. ; Guest, Brian J. ; Wooding, Christine M. ; Jones, Clayton M. ; Lelievre, Scott ; Webb, Douglas C.
    Six drifting floats designed to measure shear were deployed in the vicinity of the Brazil Basin Tracer Release Experiment. The one-year long time series of oceanographic conditions obtained by the floats are for direct comparison with long-term tracer dispersion. The purpose of the tracer dispersion experiment was to study mixing of Antarctic Bottom Water at approximately 4000 m depth with less dense water above. Two of the floats returned shear records, one from about 1660 m depth and one from about 2800 m depth. Mean shear at 1660 m was 2.2 x 10 -3 s-1 with N = 1.1 cph, about 1.9 times the Garrett-Munk model amount. Mean shear at 2800 m was 1.1 x 10-3 with N = 0.5 cph, about 2.2 times Garrett-Munk. There was no apparent depth structure to the shear recorded by the near-bottom float moving over the mountainous seafloor. The two shear time series and the local tidal velocities were not strongly correlated, but the tide and shear series did have some similarities. Some variability in the 1660-m shear may be due to atmospheric forcing. Three floats deeper than 2800 m returned one-year long trajectories. Two trajectories were persistently eastward.