Date of Award

Fall 2018

Project Type

Thesis

Program or Major

Mechanical Engineering

Degree Name

Master of Science

First Advisor

Diane Foster

Second Advisor

Barry Fussell

Third Advisor

Thomas Lippmann

Abstract

As a result of military testing and training around the United States, the property potentially containing military weapons in underwater environments exceeds 10 million acres. The weapons, called munitions, are difficult to locate, can move suddenly, and are a danger to marine life and the public. Before safe and cost-effective munition recovery efforts can be developed, a better understanding of their mobility in underwater environments is needed. Specifically, this research looks to resolve the role of dynamic pressure gradients surrounding the munition that can impact its position or orientation. To do so, a pressure-mapped model munition (PMM) has been designed, fabricated, and tested in laboratory and field settings. The PMM is an untethered instrument, containing all electronics necessary to retrieve, time, and store data. The PMM is capable of detecting and measuring surface pressure gradients and orientation and positional changes and uses an acoustic tracker for retrieval purposes after a deployment.

The surface pressure mapping was accomplished with an array of 16 small diaphragm pressure sensors. The instrument also contains an Inertial Measurement Unit (IMU) to record orientation changes. All data is stored to an on-board microSD card and recorded on the same time stamp. After wiring and constructing the PMM, the instrument was evaluated through several laboratory experiments to determine its accuracy in detecting hydrostatic pressure changes, orientation changes, passing waves, and environment changes, such as being submerged in a sand bed.

The diaphragm pressure sensors were sampled at 13 Hz with battery and disk storage of up to 3 days. Individual pressure sensors can experience drift up to 1 mbar/hour, which is the hydrostatic equivalent of 1 cm of water per hour, and showed offsets of up to 50 mbar, or 50 cm of hydrostatic pressure. The rolling experiment and the overnight drift experiment proved that a reference pressure sensor, in combination with the orientation data from the IMU on board the instrument, can be used to recreate the steady state pressures. While the pressure sensors have an internal temperature measurement, a reference temperature sensor installed on the surface would allow for better characterization of the instrument’s response to environment changes.

The pressure sensors accurately recorded changes in pressure due to hydrostatic changes and passing waves with a noise rms of only 0.25 mbar, or 2.5 mm of hydrostatic pressure. This result was promising for this research because the interest is in recording dynamic pressure gradients due to passing waves and local vortices. The instrument resolved wave motions as accurately as standard field instruments.

Horizontal pressure gradients across the PMM were in phase with the local acceleration as measured with an acoustic Doppler velocimeter as would be predicted by linear wave theory. However, the pressure gradients across the munition were larger than the local acceleration, providing evidence for the influence of vortex shedding. Additional evidence for vortex shedding was evident in the local deviations of pressure surrounding the munition during a passing wave. The observations show that during 100% exposure, the offshore face of the munition experiences a magnified signal as the crest passes, while the onshore side experiences a pressure deficit. Moreover, spectra of the PMM pressure observations showed significant energy at the first harmonic frequency that was not present in the nearby free stream pressure measured by the Vector. The largest KC value for the flow in the laboratory experiments was 3.1, meaning that, if the cylinder were in the freestream, there would be vortices shedding twice a wave period. However, comparisons with theory are challenging due to vortex growth limitations due to the percent exposure of the short cylinder.

The laboratory experiments have demonstrated the current PMM’s capabilities to capture passing waves or vortex shedding schemes causing dynamic pressure gradients on the surface of the cylinder. To date, full scale laboratory and field conditions have been limited to wave environments that do not exceed the threshold for momentary liquefaction. Planned field studies will provide for observations in the higher energy conditions that can lead to significant positional state change of the short cylinder. This instrument will allow for quantitative measurements of vortices in controlled, laboratory experiments and in nearshore environments, improving the understanding of fluid around a cylinder.

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