Splash and surface craters (cavities) are ubiquitous phenomena when a mass impacts an air-liquid interface, penetrating the liquid phase from the air side—a process known as water entry. Depending on the impact velocity, the formed splash and cavity might result in four types of water entry: quasi-static, shallow, deep, and surface seal. Although numerous studies have been conducted to investigate different types of water entry, surface seal water entry is not well understood yet due to the complex interaction of the splash curtain with the cavity. This research employs high-fidelity computational fluid dynamics simulations to study the characteristics of surface seal water entry and develop formulations of the time scaling and pressure scaling laws for low and high impact velocities. CFD studies were conducted to analyze surface seal dynamics across low and high-speed regimes (U = 6 to 50 m/s). Our findings suggest that the pressure inside the cavity can be scaled based on the impact velocity, and the dimensionless surface seal time can be scaled by the pressure within the cavity. We propose new scaling laws for pressure and time regarding surface seal cavities, and we also explore the pressure, velocity, and vorticity distributions inside and outside the air cavity, alongside the characteristics of splash dynamics.

This dissertation aims to contribute toward the understanding of water-entry and -exit behaviors in nature. Since water is nearly a thousand times denser than air, transitioning between the two mediums is often associated with significant changes in force. Three topics with implications in water-entry are discussed, along with a fourth topic on water-exit. For a plunge-diving seabird, the first two stages of water-entry (initial impact and air-cavity formation) create large stresses on the bird's neck. Linear stability analysis of a cone-beam system impacting water shows buckling and non-buckling behaviors on the beam, which is extended to the diving birds. The next topic is related to the third stage of water-entry (air-cavity pinch-off), in which the chest feathers come in contact with the water. Here, the elasticity of Northern Gannet contour feathers is calculated using the nonlinear bending equation. The third topic will explore the formation of ripples along air cavity walls and their resulting force after pinch-off. An acoustic model predicts the observed wavelengths of the ripples. The final topic will delve into the mechanics of how animals leap out of water. A scaling law that balances the power of thrust and drag will predict the height of the jump. Finally, a bio-inspired robot was built to help identify physical conditions required to jump out of water. / Ph. D. / In nature, animals use enter and exit water (water-entry and water-exit, respectively) as a strategy for hunting prey and/or escaping predators. In this dissertation, we focus on the fluid mechanics of water-entry and water-exit phenomena as it pertains to animals. First, we study how seabirds plunge-dive into water at high speeds without neck injuries. Second, we discuss calculating the elasticity of bird feathers. Next, the rippling behavior of air-cavities is studied in the context of force production. Finally, we study the water-exit phenomenon of animals leaping out of water. The topics of this dissertation have implications in the water-entry and -exit of vehicles and autonomous robotics.

In this thesis the impact of inverted cones on a liquid surface is studied. It is known that with the right combination of velocity, geometry, and surface treatment, a cavity of air can be formed behind an impacting body and extended for a considerable distance. Other investigators have shown that the time and depth of the cone when this cavity collapses and seals follows a different power law for flat objects such as disks, then it does for slender objects such as cylinders. Intuitively it can be expected that a more slender body will have less drag and that the streamlined shape will not push the fluid out of it's way at impact to the same extent as a more blunt body, therefore forming a smaller cavity behind it. With a smaller initial cavity, the time and depth of it's eventual collapse can be expected to be less than that of a much more blunt object, such as a flat disk. To study this, a numerical model has been developed to simulate cones with the same base radius but different angles impacting on a liquid surface over a range of velocities, showing how the seal depth, time at cavity seal, and drag forces change. In order to ensure the numerical model is accurate, it is compared with experimental data including high speed video and measurements made of the force with time. It is expected that the results will fall inside the power law exponents reported by other authors for very blunt objects such as disks on one end of the spectrum, and long slender cylinders on the other. Furthermore, we expect that the drag force exerted on the cones will become lower as the L/D of the cone is increased. / Master of Science / In this thesis the impact of inverted cones on a liquid surface is studied. It is known that with the right combination of velocity, geometry, and surface treatment, a cavity of air can be formed behind an impacting body and extended for a considerable distance. Other investigators have shown that the time and depth of the cone when this cavity collapses and seals follows a different power law for flat objects such as disks, then it does for slender objects such as cylinders. Intuitively it can be expected that a more slender body will have less drag and that the streamlined shape will not push the fluid out of it’s way at impact to the same extent as a more blunt body, therefore forming a smaller cavity behind it. With a smaller initial cavity, the time and depth of it’s eventual collapse can be expected to be less than that of a much more blunt object, such as a flat disk. To study this, a numerical model has been developed to simulate cones with the same base radius but different angles impacting on a liquid surface over a range of velocities, showing how the seal depth, time at cavity seal, and drag forces change. In order to ensure the numerical model is accurate, it is compared with experimental data including high speed video and measurements made of the force with time. It is expected that the results will fall inside the power law exponents reported by other authors for very blunt objects such as disks on one end of the spectrum, and long slender cylinders on the other. Furthermore, we expect that the drag force exerted on the cones will become lower as the <i>L/D</i> of the cone is increased.

<p>  </p> <p>Cavitation, liquid slosh, and splashes are ubiquitous in science and engineering. However, these phenomena are not fully understood. Yet to date, we do not understand when or why sometimes the splash seals, and other times does not. Regarding cavitation, a high temporal resolution method is needed to characterize this phenomenon. The low temporal resolution of experimental data suggests a model-based analysis of this problem. However, high-fidelity models are not always available, and even for these models, the sensitivity of the model outputs to the initial input parameters makes this method less reliable since some initial inputs are not experimentally measurable. As for sloshing, the air-liquid interface area and hydrodynamic stress for the liquid slosh inside a confined accelerating cylinder have not been experimentally measured due to the challenges for direct measurement.</p>