Date of Award

January 2021

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Forrest Ames

Abstract

The air-sea UAV is designed to fly, transition into water, and move through water submerged. It is however subjected to a huge impact force during its transition from air to sea. This high impact force poses the risk of causing damage to the UAV structure and components. Therefore, it is very important to carry out quantitative research on understanding the forces and accelerations involved in the transition process. Computational fluid dynamics (CFD) was used to develop a method capable of handling the water entry process. The Fluent software package under ANSYS Inc. was used for the simulation and computations.The study considered the center body and the wing body of the UAV separately and independently. The center body analyses were performed using three dimensional (3-D) models while the wing body analyses were done with two dimensional (2-D) models. Two approaches were considered in determining the impact load a body experience during transition into water: the transition method and the submerged flow method. The transition method which used a transient-time analysis proved to be reliable, as this method was validated using experimental data from Nisewanger [27] and Baldwin and Steves [28]. The steady state analysis of the submerged flow method can quickly provide a useful understanding of the velocity and pressure distribution over a body submerged in or transitioning into water. However, the steady state simulation underestimates the drag force as it does not capture the initial acceleration of the water upon entry. Results from the submerged flow method, show that a sharp nose center body is more effective in reducing drag. Computations for the slender body using transition method indicate manageable impact and drag forces. The study also reveals that wedge shaped leading edges for the wing body provide reduced impact however they may not be the best when considering lift in air. This study therefore provides useful reference information for air-sea UAV structural design and movement conditions.

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