Edward R. Levine

On Measuring the Terms of the Turbulent Kinetic Energy Budget from an AUV

Abstract The terms of the steady-state, homogeneous turbulent kinetic energy budgets are obtained from measurements of turbulence and fine structure from the small autonomous underwater vehicle (AUV) Remote Environmental Measuring Units (REMUS). The transverse component of Reynolds stress and the vertical flux of heat are obtained from the correlation of vertical and transverse horizontal velocity, and the correlation of vertical velocity and temperature fluctuations, respectively. The data were obtained using a turbulence package, with two shear probes, a fast-response thermistor, and three accelerometers. To obtain the vector horizontal Reynolds stress, a generalized eddy viscosity formulation is invoked. This allows the downstream component of the Reynolds stress to be related to the transverse component by the direction of the finescale vector vertical shear. The Reynolds stress and the vector vertical shear then allow an estimate of the rate of production of turbulent kinetic energy (TKE). Heat flux ...

Turbulence Measurement from an Autonomous Underwater Vehicle

Abstract Horizontal profiles of the microstructure of velocity and temperature were obtained with a large autonomous underwater vehicle (AUV) using two piezoelectric shear probes, an FP07 thermistor, and three orthogonal accelerometers mounted on a sting at the forward end of the vehicle. A winter field trial in Narragansett Bay provided a run in the midwater pycnocline at 8-m depth that contained a thermal front, an ascending profile to 3-m depth, and a run in the weakly stratified surface layer at this depth. Although shear spectra were strongly contaminated by narrowband vibrations produced by the motor and actuators, this contamination was highly coherent with the measured acceleration and was removed with standard signal processing techniques. The corrected spectra agreed well with the Nasmyth universal spectrum for wavenumbers up to 40 cpm. The estimated rate of dissipation of kinetic energy varied from 0.8 to 250 × 10−8 W kg−1 and was consistent with the rate of production of turbulence by surface ...

Vertical Motion of Neutrally Buoyant Floats

Abstract The vertical motion of a neutrally buoyant float is determined from the solution to the nonlinear forced harmonic oscillator equation originally set forth by Voorhis. Float response to forced vertical oscillations is characterized by the response ratio, r = ξr/ξw, where ξr, is the vertical displacement of an isopycnal relative to the float, and ξw is the vertical displacement of an isopycnal relative to its initial equilibrium position. For isopycnal displacements with frequencies much less than the resonant frequency of the float, the goat can be considered to be in near dynamic equilibrium with the forcing, and r is a function of the relative compressibility between the float and seawater, s = γf/ γw, and the normalized buoyancy frequency N = N/Ω, where Ω is a characteristic float frequency defined by Ω2 = gξw[1 − (αfαw−1)]− 1, where αf, αw are the coefficients of thermal expansion of the float and water, respectively. For the new dynamic equilibrium case, data obtained from a float deployment ...

Autonomous Underwater Vehicle–Based Hydrographic Sampling

Abstract An autonomous underwater vehicle (AUV), the U.S. Navy’s Large Diameter Unmanned Underwater Vehicle (LDUUV), was used as a stable platform for rapid, repeated, near-synoptic CTD measurements of estuarine variability in Narragansett Bay. Surveys were made in lawnmower-like patterns at middepth to obtain horizontal profiles and maps, and in vertical yo-yo patterns to obtain vertical profiles. These observations were ground-truthed using standard CTDs on the fixed position of the launch cage and on ship-based surveys around the perimeter of the study area before and after the runs. For the horizontal surveys, a comparison of temperature and salinity time series from the LDUUV and the launch cage CTDs shows that differences are within the range of lateral variability around the study area observed at run depth from the ship. For the yo-yo surveys, a comparison of LDUUV CTD and standard CTD profiles shows indications of hysteresis in the vehicle-obtained data, which can be minimized with improved sampl...

Subsurface Observations of Surface Waves From an Autonomous Underwater Vehicle

In this study, we will show how ocean surface wave displacement can be estimated from sensors onboard a subsurface moving autonomous underwater vehicle (AUV). The approach is to use a high-resolution vertical accelerometer along with a highresolution pressure sensor mounted coincidently on an AUV. We apply this approach to data collected in summer 2005, in northeast Monterey Bay, CA, during an engineering experiment as a part of the Layered Organization in the Coastal Ocean (LOCO) program. The AUV used was the School for Marine Science and Technology, University of Massachusetts Dartmouth T-REMUS vehicle. Data were collected while the vehicle traveled at a constant speed 1.2 m/s and a constant depth of 10 m in water of depth 19.5 m. The local wind-generated surface wave field was relatively weak and dominated by surface wave swell of frequencies of 1/12 and 1/6 Hz, respectively. Surface waves of these frequencies in this depth of water have wavelengths of 150 and 55 m, respectively, both values being much larger than the length of the T-REMUS, which is 2 m. This condition, along with the fact that the AUV is moving over an order of magnitude slower than the phase speed associated with these surface waves, allows estimation of the frequency spectrum of surface waves by sensors onboard the AUV as well as interpretation of how the AUV responds to the surface wave field. From our estimation technique, we have confirmed that the two frequencies of 1/12 and 1/6 Hz were the dominant surface wave frequencies present and that the AUV-based estimated spectral values agreed very closely with in situ observations made by a fixed slow rise profiler located 200 m away. Pitch spectra indicated that the AUV responded to the higher frequency swell component of 1/6 Hz by adjusting its depth to try to follow the surface-wave-induced pressure. At the lower frequency, the AUV tended to follow the surface wave coherently.

Turbulence and optics sampling from an autonomous underwater vehicle

The adaptation of synergistic sensor technologies to AUV platforms enables a new class of sampling strategies not available with older technologies. The combined turbulence, optics, and hydrographic data acquisition capabilities can be utilized for near-synoptic horizontal mapping in a variety of estuarine and continental shelf process studies. Horizontal profiles of turbulence are obtained using two piezoceramic shear probes orthogonally to the direction of motion, a single ultrafast thermistor, and three orthogonal accelerometers, mounted on a sting. Horizontal profiles of optical absorption and attenuation are obtained with a WET Labs AC-9 instrument mounted in a wet well, with the proper flow rate provided by a pump. Field trials in the Narragansett Bay tidal estuary in January 1996 resulted in obtaining turbulence and optical data from the UUV during near-spring ebb tide. UUV-based observations were made in the deep pycnocline, as well as in the ocean surface layer during wind row occurrences. A portion of the data has been examined for evaluation of noise effects on the turbulence data. In general, this analysis that meaningful estuarine turbulence data can be obtained from an autonomous underwater vehicle, and its relation to the optics data can be studied.

AUV-based swell characterization

In this study we show that ocean surface wave swell can be measured from a subsurface transiting AUV. Data were obtained onboard an extended REMUS 100, 2m long, which carried a RGL turbulence package and a SeaBird CTD cantilevered off of the port and starboard sides of the bow, respectively. The data reported in this manuscript was obtained in the summer of 2005, in northeast Monterey Bay, as part of the Layered Organization of the Coastal Ocean (LOCO) experiment. The AUV traveled at 1.2 m/s, sampling at 10 m depth, in the region of the 20 m isobath. In the study region, surface wave swell have a wavelength and phase velocity an order of magnitude greater than the AUV length and transit speed, respectively, with swell wave height often no larger than a meter. A simple model for the T-REMUS response to swell displacement is presented. The model allows use of pressure and vertical acceleration measured on board T-REMUS as input parameters. Four legs, both crossing and following the bathymetry, were performed. We observed a well defined peak in the estimated spectra at .08-.09 Hz (T ~ 11.8 s), typical of swell. A secondary peak also occurs at ~.17 Hz (T~ 5.9 s), twice the swell frequency, and corresponds to the AUV pitch response. For ground truth, swell observations where made with a pressure sensor at a fixed station within 2 km of the AUV transit. Comparison between these two data sets showed excellent agreement at the swell peak frequency.