Date of Award

Fall 2020

Project Type

Thesis

Program or Major

Ocean Engineering

Degree Name

Master of Science

First Advisor

Diane Foster

Second Advisor

Thomas Lippmann

Third Advisor

Donya Frank-Gilchrist

Abstract

Military testing and training around the United States has resulted in over 10 million acres of property in underwater environments, potentially containing military weapons. The weapons, hereafter referred to as ``munitions," are difficult to locate, capable of sudden movement, and a danger to marine life and the public. Improved understanding of their mobility in underwater environments is vital for safe and cost-effective munition recovery. A pressure-mapped model munition (PMM) was designed and fabricated to resolve the role of dynamic pressure gradients on munition mobility.

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, orientation and positional changes, and uses an acoustic tracker for retrieval after deployments. The surface pressure mapping was accomplished using an array of sixteen 3mm-diameter diaphragm pressure sensors. Orientation changes were recorded using an Inertial Measurement Unit (IMU).

Performing field deployments at multiple locations provides a range of conditions for munition behavior observations. Considering the data acquired and the environmental conditions experienced during each deployment, the Wallis Sands Beach deployment with the newest design iteration of the PMM is the focus of the data analysis and findings. During low tide with 0% burial observations, PMM mobility is commensurate with PMM-Sleath parameter exceeding a threshold of +/-0.1. Additionally, sediment-Shields parameter values exceed a threshold of 0.05 indicating live bed scour during these observations.

Estimations of the horizontal momentum show discrepancies between the inertia and the pressure terms, especially during large accelerations in the flow which could be due to the simplifications made to the momentum equation, and/or pressure gradients due to vortex shedding off the PMM. Cross-spectral analysis reveals a phase relationship between the average pressure and the pressure gradient recorded by the PMM, where the phase is zero at low frequencies and approaches ~90 degrees at higher frequencies. The phase shift could be resolving a physical process including, boundary layer dynamics, and/or nonlinearities in the flow caused by wave shape or vortex shedding.

Estimations of the horizontal and vertical wave-induced pressure forces acting on the PMM-2.0 show forcing in the upwards and onshore directions commensurate with onshore PMM motion. Phase-space-average plots reveal that the onshore forcing occurs during the passing of the wave crest, and the PMM-2.0 onshore directed mobility occurs just after the crest. Additionally, phase-space-average plots indicate that PMM-2.0 Shields and Sleath parameters are highest during the wave crests.

Analyzing the data recorded by the PMM provides insight into the mechanisms responsible for munition mobility in the coastal environment. The results of this research suggest that vortex shedding may significantly contribute to the burial of free-standing objects. The deployment datasets provide valuable insight for furthering munition mobility investigations, such as developing predictive models. Additionally, these results can be more broadly applied to relationships between fluid, sediment and other types of cylindrical structures on the seafloor.

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