Tochko John Steven

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Tochko
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John Steven
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  • Technical Report
    An acoustic sensor of velocity for benthic boundary layer studies
    (Woods Hole Oceanographic Institution, 1978-04) Williams, Albert J. ; Tochko, John Steven
    The techniques of flow measurement which have been successful in laboratory studies of boundary layer turbulence are difficult to use in the ocean; and the current meters penerally used in the ocean are not suited to measuring bottom boundary layer flow . A suitable sensor for bottom turbulence measurements should measure vector components, respond linearly to these components, maintain an accurate zero point, disturb the flow negligibly or in a well predicted way, and sense a small enough volume to represent the important scales of the flow. We have constructed an acoustic travel time sensor in a configuration that will allow vector components of the flow to be measured with sufficient accuracy to compute Reynolds stress at a point 50 cm above the bottom. This sensor responds linearly to horizontal and vertical flows in flume tests. When the flow is neither horizontal nor vertical, the wake from one acoustic transducer may interfere with the measurement along one sensing path but there is sufficient redundancy in the determination to reject this path and still resolve the vector velocity. An instrument· using four of these sensors is being designed to measure Reynolds stress in the lower six meters of the ocean.
  • Thesis
    A study of the velocity structure in a marine boundary layer : instrumentation and observations
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1978-02) Tochko, John Steven
    The design and operation of a unique flow measuring instrument for bottom boundary layer studies in the marine environment is documented. The effectiveness of the instrument in acquiring data with which models of near bottom flows in the ocean can be tested is demonstrated by the results of a field experiment in Vineyard Sound. The instrument uses four sensors which measure the mean and fluctuating parts of the three components of the velocity vector at four heights above the sea bed. The sensors employ the acoustic travel time difference technique, and are designed to minimize sensor-induced flow disturbances. BASS, an acronym for Benthic Acoustic Stress Sensor, has a resolution of .033 cm/sec per least bit, a range of ±62 cm/sec, noise of .07 cm/sec in 10 sec, and an estimated accuracy of ±.5 cm/sec, referred to an in situ zero point. A complete set of velocity measurements is made every .750 seconds, each measurement being the vector component averaged over 15 cm. The data is internally recorded on digital cassette tape. Eight hours of continuous data can be recorded. BASS was deployed in a tidal flow in Vineyard Sound at a depth of 10 m where a time series of u, v, and w velocities at 26 cm, 46 cm, 96 cm, and 210 cm above the bottom was recorded. The mean velocity was determined by fitting each 6 hour series with a sixth order polynomial and the deviations from the polynomial, the fluctuating velocity components, were correlated to produce Reynolds stress profiles. The stress series shows very few negative stress events while the dominant positive events have an average duration of 5 seconds and exceed 30 dynes/cm2. Zero offset was removed from the mean by assuming a log profile at maximum ebb. Deviations from a log profile developed when the current dropped below 40% of maximum, i.e., when the flow could no longer be considered steady. A break in the Reynolds stress profile at 1 m suggested a larger length scale than the 1 cm bottom roughness was present in the flow. A value of u* was determined by using the quadratic drag law (u* = 1.56 cm/sec), the log profile method (u* = 1.60 cm/sec), and the eddy correlation method (u* = 1.91 cm/sec). Integral length scales of 5 m cross-stream, and 2.5 m vertically were identified by correlation calculations. Two length scales were present in the downstream direction, 5 m within 1 meter of the wall and 8 m further from the wall.