Archie T. Morrison

Determination of the hydrodynamic parameters of an underwater vehicle during small scale, nonuniform, 1-dimensional translation

The hover and tight maneuvering regime of an underwater vehicle is difficult to characterize hydrodynamically. Characterization in a reliable model is necessary for successful control of the vehicle during task performance. A 1-dimensional system identification procedure for this regime is described. The experimental set up is simple and noncritical. The analysis is performed by minimizing the error between the trajectories of the vehicle and several numerical models during a free decay. Minimization is controlled by the Nelder-Meade simplex algorithm. Agreement with the work of other researchers is strong. The procedure is flexible and may be extended to coupled models with 4 degrees of freedom.<<ETX>>

Motion tracking in an acoustic point-measurement current meter

Measurements of velocity structure in the water column under Arctic ice from an Ice-Tethered Profiler (ITP) employed an acoustic point-measurement current meter, MAVS (Modular Acoustic Velocity Sensor) [1]. With the velocity sensor it becomes the Ice-Tethered Profiler with Velocity (ITPV). The profiler, containing a Seabird CTD, MAVS, batteries, an inductive modem, and a wire crawling engine, integrated by McLane Labs, was constrained to be deployed through a 24″ diameter hole drilled in the ice. The anchor to the ice via a buoy with a satellite transmitter fixed the top of the mooring to a drifting but GPS tracked location while the profiler descending to a depth of 800m measured velocity relative to the moving mooring and the climbing profiler. A large current-orienting alignment fin was not possible on the ITP due to the limit of the ice hole diameter yet it was known that vortex shedding by the profiler body in the current would cause the instrument to swing and the current sensor to measure horizontal velocities due to the rotation of the profiler around the center of gyration of the package. To remove this platform motion from the current measurement, an inertial sensor, Analog Devices ADIS 16355 [2], was added to the MAVS current meter and three axes of angular velocity and three axes of linear acceleration were added to each data record of time, velocity, temperature, three-axis magnetic vector components and two axes of tilt. From the rate gyro value of angular velocity around the vertical axis, the platform rotation is determined and using the distance that the velocity sensor is displaced from the axis of rotation the horizontal current sensor velocity can be subtracted from the horizontal velocity measured by the sensor. This ITP was deployed in October, 2009 and data from the first profile indicates expected and unexpected performance.

Calibration of the BASS acoustic current meter with carrageenan agar

The BASS current meter can measure currents down to the millimeter per second range. Due to the dependence of zero offset on pressure, determining a sensor referenced velocity requires accurate in situ zeroing of the meter. Previously, flow was restricted during calibration by placing plastic bags around the acoustic volume. In this paper, bacterial grade and carrageenan agars are used in the laboratory to create a zero flow condition during calibration and are shown to be acoustically transparent. Additionally, the results of open ocean and dockside carrageenan and plastic bag comparisons are presented. Carrageenan is shown to reliably provide a low noise, zero mean flow environment that is largely independent of ambient conditions. The improved zeros make millimeter per second accuracy possible under field conditions.<<ETX>>

Multiplexer design for the BASS Rake acoustic transducer array

The BASS Rake is an acoustic current meter designed to measure velocity profiles through the continental shelf wave bottom boundary layer. The instrument uses BASS electronics with a new transducer geometry to resolve the primary spatial and temporal flow scales of the wave boundary layer. The necessary characteristics of an interface satisfying the requirements of both the BASS transmit/receive circuit and the BASS Rake sensor design are extremely low through resistance, O(1 /spl Omega/), rapid switching, O(10 /spl mu/s), flexible cross connection of the transducers, and strong isolation between channels, >40 dB. The layered multiplexer described is designed to meet these requirements.

The McLane WTS-LV: a large volume, high accuracy, oceanographic sampling pump

McLane Large Volume Water Transfer System (WTS-LV) oceanographic pumps are single-sample instruments normally deployed from the hydrowire of a research vessel. In the standard configuration, samples are acquired on 142 mm filters. 293 mm filters, adsorption cartridge filters, and extraction columns are available options. The disk filter holders were designed at McLane to ensure even distribution of collected material over the entire face of the filter. Flow rates from 1 lpm (liters/minute) to 50 lpm are programmed by the operator to match both the type of filter and the capacity of the modular pump head installed on the system. Sample volumes as large as 45000 liters can be filtered in a single cast with a volumetric error less than 5%. When equipped with appropriate filters, the WTS-LV can be used to collect suspended particles, particulate trace metals, phytoplankton, and zooplankton. Equipped with extraction columns, the system can be used to collect dissolved compounds, dissolved radio-nucleides, and dissolved trace metals.

Shallow-water messenger-line recovery system

Shelf and estuarine deployments of bottom mounted instruments generally require complete recovery of the instrument, including anchors. Subsurface instruments may have lift lines for recovery, often on acoustically commanded release of a float. The lift line and float are large for heavy instruments and this creates a flow disturbance that distorts the environment being measured. When redundancy in recovery lines is added, the volume of lines and floats may become unacceptable. Light weight messenger lines with small messenger floats are less flow disturbing and can be added to provide redundancy with less compromise to the measurement. A set of four messenger lines with floats was used in the Hudson River in 1995 to recover a massive quadrapod deployed on the bottom for several weeks. The messenger lines, with ample scope, were used to pull, by hand, one end of a short, strong lift line to the surface for recovery of the quadrapod. In this deployment, each messenger line went to an independent lift line, but several messenger lines could be joined to a single lift line. Redundancy is needed for the most vulnerable elements of a system and in shallow water this is the lift line itself. In deeper water, the extra complexity of connecting several messenger lines to a single lift line is offset by the substantial savings in volume by eliminating a redundant lift line. Experience in two recoveries with this recovery system shows that the burnwire used to release the messenger line float works very well but can become fatigued in shipping, line fouling can trap the float in the launch silo, and floats can rise but fail to surface in strong current because Froude drag increases near the surface. The authors have yet to learn if biofouling and heavy sediment deposition are a problem The benefit of redundancy has been noted in numerous other experiments where loss occurred when lift lines were cut by propellers, bilge keels, and guard buoys, and when tangles prevented a float from coming all the way to the surface. The benefit of small, low drag messenger lines would have been substantial in deployments in deep, high current regimes where a scope of two to one made the lift line package quite large. Four independent messenger line recovery packages took less space than the single primary lift line system that it replaced.

Measurements of Waves and Current in Support of Coastal Projects on Nantucket and Martha's Vineyard

MAVS acoustic velocity sensors have been deployed in the surf zone at Madaket beach, Nantucket and on a dock piling at Oak Bluffs, Marthas vineyard since mid-winter of 2006. Power is supplied to each instrument through a dedicated cable connection and data are returned through this tether to PCs for logging. The current meters report velocity, pressure, temperature, and turbidity in real-time to support continuous monitoring of wave and sediment conditions as they impact beach processes or velocity and pressure for wave conditions in support of marine construction.

Preliminary tow tank and flume tests of a prototype BASS Rake wave bottom boundary layer sensor

The BASS Rake is an acoustic travel time current meter designed to make spatially and temporally dense velocity profile measurements in the continental shelf wave bottom boundary layer. The vertical extent of the WBBL is typically one to several centimeters, varying with water depth and wave conditions. The thinness of the layer is responsible for high levels of bottom shear stress which are important contributors to the sediment entrainment process and which enhance turbulent dissipation of flow energy. The BASS Rake is a modification of BASS, the Benthic Acoustic Stress Sensor, using a new geometry to image flow in the WBBL. An analysis of the flow distortion due to the sensor is presented suggesting some dependence of the gain on flow speed. Tow tank tests demonstrate the suggested dependence and are used to calibrate the sensor. In flume tests, the horizontal velocity vector is measured at 0.5 cm, 2.4 cm, and 5.0 cm above a sand bottom at nominal flow speeds of 10 cm/s, 20 cm/s, and 34 cm/s and a depth of 8.6 cm. The 34 cm/s test included a significant bedload with no observed degradation of sensor performance. Velocity profiles are acquired within a 4 ms window at a 1 Hz rate. The flume measurements are compared to concurrent measurements made with an LDV.

Measurements of Surf Zone Currents and Waves In Support of Madaket and Sankaty Head, Nantucket, Beach Nourishment

Coastal sand bluffs and cliffs on the Atlantic seaboard of North America frequently regress, forcing home drawbacks and lighthouse relocation such as that facing homeowners at Madaket and the Sankaty Light on Nantucket. Whether the dominant cause of bluff and cliff calving is erosion of beach at the base from high tide waves or exceptional high water currents or if it is rainfall induced, increased sand on the beach is unlikely to be harmful and may reduce the rate of regression. Evidence of sand spit building from a wrecked clam boat at Sankaty Head beneath the lighthouse suggests that the regression rate might be reduced to the point where relocation of the light is less urgent if barges could be sunk to cause sand to accrete and build the beach at this point. Measurement of suspended sand turbidity, along shore current, and waves at 3 m and 4 m depth is being undertaken with two monopod emplacements of MAVS current and wave sensors. Presently these are installed off Madaket at the western end of the island where beach regression is severe. These instruments are powered from shore and return data by cable at 4 Hz for access by Internet. A webcam at the Madaket site supplements the data record on the Internet so that conditions of tidal state, sand bar building and exposure, breaker line, and sand nourishment can be monitored remotely. Surf zone wave and current measurements are generally high maintenance installations with wave forces tending to uproot and overturn support structures. Cable connections can be dragged alongshore and broken. In this installation, a monopod was employed to reduce the horizontal area of the support structure subject to upward force by waves and chain-weighting buried the cable to reduce lateral drag. Two locations offshore from the region of concern permit shear in the alongshore current to be detected and difference in wave states captured. Winter storms are important causes of beach sand loss or possibly deposition and this set of observations is designed to correlate current and wave state with such events. Northeast storms are assumed to be the most serious drivers of beach processes here on the coast of Massachusetts but these are typical of sand beaches from Maine to Florida. Observation of the Valentines Day northeast storm of 2007 represents these processes at Madaket and the extension of these measurements to Sankaty Head may be useful for other locations where cliff regression has endangered lighthouses. Remediation by barge sinking for beach nourishment is planned to follow with similar monitoring of current, waves, and beach condition.

Waves and seiches in Flathead Lake, Montana: measurements of a quiet lake by differential travel-time current measurements

Measurements of velocity and pressure at the south end of Flathead Lake, Montana, reveal lake level fluctuations due to wind set up and seiches. Short period wind wave events, typically of single day duration, were recorded several times during the study period in the fall of 2004. The wind waves are fetch limited, in most cases with periods below 3 second but with significant wave heights that can exceed 50 cm during episodic storm events. Pronounced diurnal and longer period changes due to river influx and controlled drawdown through a hydroelectric dam across the exit river are clearly evident in the record. Broad scale upward fluctuations in lake level due to lake level regulation and seiches have exposed fragile portions of the shore to wind waves that have caused extensive lake wide erosion. The background motion of the lake is typically less than 2 cm/s, the waves are often relatively small with short periods, and the water is clear, making this a difficult environment for current and wave measurement. MAVS (modular acoustic velocity sensor) acoustic differential travel-time measurements detected and recorded these small signals cleanly and well above the instrument noise level