Hydrodynamics

Aquatic organisms are able to achieve swimming efficiencies that are much higher than any underwater vehicle that has been designed by humans. This is mainly due to the adaptive swimming patterns that they display in response to changes in their environment and their behaviors, i.e., hunting, fleeing, or foraging. In this work, we explore these adaptations from a hydrodynamics standpoint, using numerical simulations to emulate self-propelled artificial swimmers in various flow fields. Apart from still or uniform flow, the most likely flow field encountered by swimmers are those formed by the wakes of solid objects, such as roots of aquatic vegetation, or underwater structures. Therefore, a simplified bio-inspired design of porous structures consisting of nine cylinders was considered to identify arrangements that could produce wakes of varying velocities and enstrophy, which in turn might provide beneficial environments for underwater swimmers. These structures were analyzed using a combination of numerical simulations and experiments, and the underlying flow physics was examined using a variety of data-analysis techniques.
Subsequently, in order to recreate the adaptations of natural swimmers in different flow regimes, artificial swimmers were positioned in each of these different types of flow fields and then trained to optimize their movements to maximize swimming efficiency using deep reinforcement learning. These artificial swimmers utilize a sensory input system that allows them to detect the velocity field and pressure on the surface of their body, which is similar to the lateral line sensing system in biological fish. The results demonstrate that the information gleaned from the simplified lateral line system was sufficient for the swimmer to replicate naturally found behaviors such as K´arm´an gaiting. The phenomenon of schooling in underwater organisms is similarly thought to provide opportunities for swimmers to increase their energy efficiency, along with the other associated benefits. Thus, multiple swimmers were trained using multi-agent reinforcement learning to discover optimal swimming patterns at the group level as well as the individual level.

A computational investigation of the hydrodynamic and seakeeping performance of a catamaran in calm, and in the presence of transforming head and following seas in waters of constant and varying depths is described. Parametric studies were conducted for a selected WAM-V 16 catamaran geometry using OpenFOAM® to uncover the physical phenomena. In the process a methodology has been developed for simulating the interactions between the vehicle and the shallow water environment akin to that in the coastal environment. The multiphase flow around the catamaran, including the six degrees-of-freedom motion of the vehicle, was modeled using a Volume of Fluid (VoF) method and solved using a dynamic mesh. The numerical approach was validated through computing benchmark cases and comparing the results with previous work. It is found that in a calm shallow water environment the total resistance, dynamic trim and sinkage of a catamaran in motion can be significantly impacted by the local water depth. The variations of the impact with depth and length-based Froude numbers are characterized. The impact varies as the vehicle moves from shallow waters to deep water or vice versa. In the presence of head and following small-amplitude seas, interesting interactions between incident waves and those generated by the vehicle are observed and are characterized for their variation with Froude number and water depth.

Hydrodynamics interaction is a factor in the performance of fish schooling or underwater vessels in close formation. In this work, we visualized the wake structure of pitching hydrofoils using an inclined soap film. We considered one-, two-, three- and nine-foil configurations with different spacing and actuation parameters: amplitude (A), frequency (f), phase difference (), and flow speed (U). The wake structures were recorded with a high-speed camera and analyzed to measure the vortex angle created. The wake structure of two- and three-foil configurations were compared with the Strouhal number, St = fA/U, of a single foil. For the nine-foil configuration, the wake velocity and the standard deviation of the velocity were used to interpret the hydrodynamic interaction. It was found that both spacing and phase difference between foils are relevant in the hydrodynamic interaction. Qualitative observations are also made, and vortex street behavior characteristics are identified.

Noise prediction methods are necessary in aspects of aerodynamic and hydrodynamic engineering. Predictive models of noise from rotating machinery ingesting turbulence is of much interest and relatively recently studied. This thesis presents a numerical method processed in a series of three codes that was written and edited to receive input for geometrical features of rotating machinery, as well as, adjustments to turbulent operating conditions. One objective of this thesis was to create a platform of analysis for any rotor design to obtain five parameters necessary for noise prediction; 1) the hydrodynamic inflow angle to each blade section, 2) chord length as a function of radius, 3) the cylindrical radius of each blade section, 4) & 5) the leading edge as a function of span in both the rotor-plane and as a function of axial distance downstream. Another objective of this thesis was to use computational fluid dynamics (CFD), specifically by using a Reynold’s-Averaged Navier-Stokes (RANS) Shear Stress Transport (SST) 𝑘 − 𝜔 model simulation in ANSYS Fluent, to obtain the turbulent kinetic energy distribution, also necessary in the noise prediction method presented. The purpose of collecting the rotor geometry data and turbulent kinetic energy data was to input the values into the first of the series of codes and run the calculation so that the output spectra could be compared to experimental noise measurements conducted at the Stability Wind Tunnel at Virginia Tech. The comparison shows that the prediction method results in data that can be reliable if careful attention is payed to the input parameters and the length scale used for analysis. The significance of this research is the noise prediction method presented and used simplifies the model of turbulence by using a correlation function that can be determined by a one-dimensional function while also simplifying the iterations completed on rotor blade to calculate the unsteady forces.

In recent decades, ecohydrology has received renewed attention because of the impacts of groundwater withdrawal on ecosystems. Growing population and urban expansion in Palm Beach County, FL. place pressure to eradicate natural areas, such as Florida scrub habitats, and increase groundwater withdrawal. This study presents preliminary results of soil and hydrological characterization of an ecological preserve surrounded by changing land use. Soil moisture and water levels were monitored to assess the effects of precipitation as influenced by plants and soil analysis determined the suitability of current soil conditions for hosting native vegetation habitats. Hydrologic and soil conditions on the preserve fall within values expected for native Florida scrub habitats. Hydrologic response to precipitation varied due to factors including antecedent conditions and vegetation types. These results provide a better understanding of the interactions between soil proper ties, hydrologic cycle, and plants, and assist with establishing a baseline to monitor changes over time.

In this study, a laminar flow hull shape is implemented on an Autonomous Underwater Vehicle (AUV), with boundary layer suction at the aft end of the hull to prevent separation. The hull shape has the largest diameter of the vehicle near the aft end of the hull resulting in an accelerating flow over the majority of the hull's surface. The problem of axially symmetrical flow around the AUV is solved using a potential flow analysis. A finite difference algorithm evaluates the stream function, leading to the computation of fluid velocity and pressure fields. The boundary layer characteristics are analyzed to predict the risk of separation. The numerical results are compared with laboratory measurements of the flow using a Particle Image Velocimetry system. Fuzzy Logic Sliding Mode Controllers are implemented to control the vectored thruster vehicle, and are simulated using a six-degree of freedom dynamic model of the vehicle.

The drag reduction by vortex fusion was investigated. A comparison of flow over a bundle of cylinders in uniform and in disturbed currents was performed in a water channel. The model was subjected to cross flow. A thin cylindrical wire located nearby upstream and leveled at half the height of the test model was used as a source of disturbance. A hydrogen bubble technique was utilized to observe the flow pattern. The accumulation of vortices at stagnating regions in front of a bundle of cylinders transformed into a counter-rotated curl at leading edges of each leading cylinder in the bundle. Measurements were carried out by a computerized data acquisition system. Drag coefficient measurements, digital spectral and fourier analyses were also performed. Results have shown that a drag reduction can be obtained by introducing a thin cylindrical wire in front of the stagnation.

The sound field associated with the motion of 2-dimensional finite core vortex past a forward facing step is obtained. A numerical scheme using Contour Dynamics technique and incompressible, inviscid equations of motion is developed to determine the evolution of the structure of the vortex, its path over the step and the radiated sound. An appropriate low-frequency Green's function is derived and the expression for the far field acoustic pressure as formulated by Mohring is used. The vortex structure evolves in the non-uniform flow in the vicinity of the step and under certain conditions is found to undergo significant deformation of its core structure. The far field acoustic pressure is found to be a strong function of vortex motion in the vicinity of the step. Results for the vortex trajectory and the associated acoustic pressure are presented for a variety of flow parameters.

A Computer Automated Radioactive Particle Tracking (CARPT) facility was designed and implemented for the investigation of hydrodynamics in two phase flows. This facility was complemented by a versatile fluidized bed facility capable of handling high air flow rates. Solids mean dynamic behavior and heat transfer to internals in a 29.21 cm diameter fluidized bed were investigated for different operating conditions. Different flow parameters like the solids ensemble-averaged velocity, stagnancy and the phase density in the presence of horizontal tubes were determined using the CARPT facility. Local circumferential variations of heat transfer coefficients at the surface of horizontal tubes were measured at different locations in a large particle fluidized bed using a miniature heat transfer probe assembly. The influence of solids hydrodynamics on the heat transfer coefficient in gas-fluidized beds was investigated. The data obtained in the present study was compared to current heat transfer models for large particle gas-fluidized beds.

Edge waves are the longshore periodic wave motions that are trapped at the edge of water bodies and play an important role in coastal hydrodynamics. This study presents the experimental investigation of the excitation of synchronous edge waves by waves normally and obliquely incident on a uniformly sloping beach. The experimental results show that the edge wave amplitude is linearly proportional to that of the reflected waves. For a perfectly reflecting beach, the conclusion is consistent with the Rockliff model. The experimental results also indicate that the ratio of the edge wave amplitude to reflected amplitude is linearly proportional to the approach angle.