Date of Award

Fall 2022

Project Type

Thesis

Program or Major

Ocean Engineering

Degree Name

Master of Science

First Advisor

Diane L Foster

Second Advisor

Carl Friedrichs

Third Advisor

Peter Howd

Abstract

\noindent The causes of munition mobility in nearshore regions are not well understood phenomena; and in situ measurements of positional state, orientation, and the local flow field are complicated by the high energy nature of the surf and swash zones. A pressure mapped munition (PMM) has been developed to autonomously measure the surface pressure and attitude to overcome some of these challenges. The PMM is a self-contained, Lagrangian instrument representative of 105-155~mm shells, that is capable of autonomously recording surface pressure and attitude for up to 100~h. The PMM utilizes 16 high resolution pressure sensors distributed around its surface to measure deviations in the local pressure field caused, in part, by the formation of and eventual shedding of vorticies. An internal inertial measurement unit (IMU) is used to make synchronous measurements of the PMM's attitude. Nearby vertical pressure variability is measured using a second instrument, the pressure stick (PS), to characterize the presence of momentary liquefaction events that can also factor into positional state changes of unexploded ordinance (UXO).

This thesis summarizes three deployments of the PMM at Wallis Sands Beach, a medium sloping beach in New Hampshire, with a relatively uniform alongshore profile, then data collected in October 26-27, 2021 during a large nor'easter, is examined in detail. Pressure time series collected with the PMM and PS are used to characterize the nearshore wave climate, and offshore conditions are measured at the Jeffreys Ledge waverider buoy. The pressure and attitude records from the PMM are used along with shallow water approximations for linear wave theory to calculate the average depth of the PMM, peak wave period, and significant wave height to broadly determine under which conditions the PMM is mobile. Time series of the surface pressure gradients are approximated by taking the difference between sensor values on opposing sides of the PMM, while the location of sensors is determined by the attitude time series. Non-dimensional characterizations of the horizontal mobilizing forces acting on the PMM are given in a form similar to the Sleath parameter with a wave component, $S_{\text{wave}}$, and residual component, $S_{\text{residual}}$. The resolution of the pressure sensors is 0.06~cm, and the resulting surface pressure gradient estimates are measured with a total error of $\pm{}\epsilon{}=1.4$~cm.

Positional state of the PMM is inferred based on changes in attitude and on characterizations of the surface pressure variability, where transport occurs when the PMM is rolling multiple times about its long axis at more than $360^\circ$/s.Smaller wobbling motions are defined between 0.5-360$^\circ{}$/s. The PMM is assumed proud when the standard deviation of the vertical and/or horizontal pressure gradients exceed a critical threshold of 0.15. Similarly, if the standard deviation of the pressure gradients are below 0.07, the PMM is assumed to be buried.

Offshore motion of the PMM is highly correlated to the presence of vertical and horizontal surface pressure gradients indicative of vortex development around the PMM, corresponding with upward and offshore forcing.The effects of vortex shedding are seen in both the vertical and horizontal pressure gradients, evidenced by deviations from the expected hydrostatic pressure field around the surface of the PMM that reach 2-3 times. These occurrences are commensurate with long period (15-40~s) wave draw-down and swash oscillations, attributable to infragravity motions while momentary bed liquefaction is indicated. Onshore motion is seen during times when the PMM is partially or fully exposed to free stream flows and $S_{\text{wave}}$ exceeds a critical threshold as oncoming waves and wave bores pass over the PMM. At these times the vertical pressure gradients are elevated and there is a horizontal pressure gradient acting in the onshore direction.

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