Ken Holappa

An AUV survey in the littoral zone: small-scale subsurface variability accompanying synoptic observations of surface currents

A survey of small-scale subsurface variability within the synoptic observational field of an ocean surface current radar (OSCR) using an autonomous underwater vehicle (AUV) is described. The survey involved observation of a developing upper mixed layer in a littoral zone off southeast Florida, on the edge of a strong Florida current during the summer of 1999. Complimentary in situ observations from a bottom-mounted acoustic Doppler current profiler (ADCP), conductivity-temperature (CT) chain arrays, atmospheric measurements from a surface buoy, and CTD and ADCP observations from a surface ship provided the background to the survey. The AUV, the Ocean Explorer, equipped with a CTD, downward and upward looking ADCPs, and a small-scale turbulence package, was used to conduct a continuous 12-h survey of small-to-fine-scale variability within a few grid cells of the surface current radar field. The vehicle repeatedly sampled the same grid in a set pattern at a fixed mid-water depth. Maps of developing spatial distribution of current, salinity, temperature, and rate of dissipation have been developed using the AUV-based observations. The observed features in the current field compare well with the OSCR and the bottom-mounted ADCP measurements.

An Autonomous Ocean Turbulence Measurement Platform

Abstract The use of a small autonomous underwater vehicle (AUV) as a platform for making in situ flow measurements in the ocean environment is described. Two high-wavenumber shear probes and a dynamic Pitot tube, housed in a pressure vessel mounted on the nose of the AUV, allow measurements, in the dissipation range, of all three components of velocity. Microstructure temperature and possible body vibrations are monitored using auxiliary local probes. A conductivity–temperature–depth package, an acoustic Doppler current profiler, and a Marsh–McBirney current meter on board allow measurement of mean background conditions. Gathered data are stored on an onboard computer. The AUV can survey 7–11-km regions at a speed of 1.5–2 m s−1, its motion being uncoupled from that of any surface mother ship. The vehicle has relatively low manufacture and operational costs and can potentially operate in stormy conditions. The small-scale measurements allow the determination of estimates of in situ energy dissipation rate...

Ocean flow measurement using an autonomous underwater vehicle

In this paper, we discuss making flow measurements in a shallow-water ocean environment using Florida Atlantic Universitys 2.2 m long autonomous underwater vehicle (AUV). An AUV has the advantage over tethered vehicles in that its self motion is not coupled to the motion of the sea surface and that of a mother ship, and it can potentially operate in stormy conditions. Two high-wavenumber shear probes and a Pitot tube, housed in a pressure-vessel mounted on the nose of the AUV, allow measurements, in the dissipation range, of all three components of velocity. Salinity, temperature, mean velocity gradients and possible body vibrations are monitored using auxiliary local probes. The AUV allows surveying 3-5 mile square regions at a speed of 3-4 knots. The influence of the self motion of the AUV on the flow measurements is discussed.

Failure analysis of an injection molded modular tail section on an autonomous underwater vehicle

Injection molded plastic (Ultem) pressure vessels are used as modular underwater vehicle body sections on the morpheus autonomous underwater vehicle at Florida Atlantic University. The bulk properties of the material were used in the design and analysis due to the lack of effective property data of the Ultem injection module which are actually dependent on the molding process. A series of test specimens were prepared and tested to determine the tensile strength, bending strength, shear strength and Youngs modulus achieved in the final modules. Implementation of the control surface actuators that penetrate the plastic pressure vessel requires an assessment of the desired failure mode of the system and an accurate structural model of the system components. A nonlinear finite element analysis of the actuator system including the plastic modules is developed as a quarter section in order to take advantage of the symmetry of the problem. Finally, failure modes are found using von Mises analysis.

A cross-platform microstructure turbulence measurement package

Ambient turbulence in the ocean is responsible for dissipating the kinetic energy in the ocean at very small scales. Small autonomous underwater vehicles (AUVs) provide versatile mobile platforms for surveying a shallow water coastal environment at low operational costs. The platforms are uncoupled from the low frequency vibrations associated with platforms towed from surface ships. Measurement of turbulent microstructure and the associated dissipation rate using an AUV requires accommodation of the inherent vibration characteristics of a self-propelled platform. Often such vibrations are in the range of flow measurements of interest and may corrupt the data. At Florida Atlantic University, a turbulence measurement package has been developed which implements vibration isolation from the AUV platform. The sensors in the package include two shear probes, a fast response temperature probe, a fast response conductivity probe and accelerometers for monitoring vibrations. In addition, the turbulence package developed is modular and interoperable in the sense that it may be mounted on a variety of platforms in a simple robust fashion without changes to the platform. The AUV platforms used thus far include the Ocean Explorer vehicle at Florida Atlantic University and the Autosub at the Southampton Oceanography Center. These vehicles vary in size from 2 meters long and under 200 kg for the OEX to over 7 meters long and over 1500 kg for the Autosub. The design of the turbulence package and results from implementation aboard these very different autonomous underwater vehicles are described.

An autonomous turbulence measurement platform

Turbulence measurements in a shallow-water ocean environment using Florida Atlantic Universitys 2.2m long autonomous underwater vehicle (AUV) are described. Two high-wavenumber shear probes and a pitot tube, housed in a pressure-vessel mounted on the nose of the AUV, allow measurements, in the dissipation range, of all three components of velocity. Salinity, temperature, mean velocity gradients and possible body vibrations are monitored using auxiliary local probes. The AUV allows surveying 3-5 mile square regions at a speed of 3-4 knots. Preliminary measurements in shallow water off the east coast of Florida are presented.