Date of Award

Fall 2023

Document Type

Open Access Thesis

Department

Marine Science

First Advisor

George Voulgaris

Abstract

The study of near surface processes in plumes and other stratified environments requires the collection of hydrodynamic and hydrographic data near the sea surface at high temporal and spatial (in the vertical) resolution. This need led to the design and development of a low-cost autonomous surface vehicle (ASV) equipped with a 5-beam acoustic Doppler current profiler (Nortek 1000 Signature) capable of measuring the vertical structure of both mean flow and turbulence dissipation rate (ε) as close as 30 cm below the sea surface. The ASV is a small catamaran-like platform propelled by four thrusters arranged in an Omni-X configuration, to minimize flow contamination while in operation. It is controlled by a CubeOrange Pilot autonomous vehicle control system that integrates an inertial measurement unit (IMU) and a GNSS GPS. The control unit uses the Ardupilot open-source software that enables operation in remote control, autonomous, and loitering mode. The latter allows the ASV to resemble a semi-stationary buoy for data collection within a predefined location radius. The evaluation of the system determined that it is suitable for studies of plume dynamics in the coastal environment over a range of wind and wave conditions. The developed ASV was used in a plume kinematics and mixing study, conducted at a site offshore of Winyah Bay, SC (USA). Designed to capture the propagation and evolution of a freshly discharged buoyant plume. Data collected, over a period of ~4 hrs, included vertical structure of the mean flow throughout the water column extending from 0.3 m below the sea surface extending to the seabed. In addition, high-frequency pulse coherent data from the 5th beam were used to estimate dissipation rate profiles using the structure function method. A secondary surface platform with a vertical thermistor array resolving temperature structure in the upper 3 m of the water column provided additional data. In addition, dissipation rate and T-S data obtained using a microCTD sensor, manually deployed in the vicinity of the platforms, were incorporated in the analysis. The analysis of the data revealed the propagation of a freshly discharged plume over a pre-existing 2-layer structure formed during the discharge of a plume at the previous tidal cycle. A comparison of theoretical Ekman current profiles to measured flows revealed that the pre-existing plume was driven by Ekman dynamics. Once the new plume arrived, a 3-layer structure was formed with the propagation speed of the new plume (top layer, 0.3 m/s) found to be in agreement with that of gravity current (0.27 m/s) defined as C_p=√(g'*D) , where g' is reduce gravity and D the depth of the plume (~2.2m). Finally, areas of active mixing were estimated by the gradient Richardson number indicating active mixing, while the front was passing, as well as on the base of the new plume, where dissipation rates were increased as well reaching values in the O(10^(-5) ) m^2/s^3. A conceptual model of the structure of the water column is presented as a new plume was propagating, with average layer values of hydrographic and turbulent parameters (dissipation rates, eddy viscosity and diffusivity).

Rights

© 2024, Christopher Tom Papageorgiou

Download

Share

COinS