Browsing by Author "Gilbert, Christine Marie"
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- An Analysis on Hydrodynamic Loads for Surface-Piercing Propellers Using Computational Fluid DynamicsBrookshire, Kaleb (Virginia Tech, 2022-07-18)A surface piercing propeller (SPP) is a propeller that is partially submerged in water and is considered a possible solution to high-speed vessels (greater than 50 knots) where cavitation plays a vital role due to its ever-increasing detrimental effects. Computational Fluid Dynamics (CFD) has become a more prevalent solution in recent years due to lower costs and the ability to evaluate varying setups. However, Computational Fluid Dynamics has had problems accurately solving the hydrodynamic loads for an SPP as recently as a few years ago. Accurately predicting these loads is of great importance because it will allow future simulations to add more effects such as cavitation, shaft inclination effects, multiple propellers, and fluid-structure interaction. Using FINE/Marine, a CFD software specifically designed for marine applications simulations with the 841-B SPP model and changing the Froude number (Fn) and advance coefficient (J), an in-depth validation process and extending upon previous results found when combining CFD and surface-piercing propellers was performed. Several cases between J = 0.6 to J = 1 and Fn = 2 to Fn = 6 are first performed to validate the models against experiments, then more complex features such as multiple propellers and shaft inclination angles were included to extend upon previous work of CFD for surface-piercing propellers. This analysis of the results suggests that CFD models could genuinely be validated against current experimental setups, and therefore more complex additions could also be made and with stronger accuracy than in previous years.
- CFD-informed Lumped Parameter Models Result In High-Fidelity Maneuvering Predictions of AUVsMiller, Lakshmi Madhavan (Virginia Tech, 2023-07-11)Recent developments in autonomous underwater vehicles (AUV) have created the need for a low cost AUV that is comparable in class and payload capabilities to existing, commercially available, expensive and sub-optimal crafts. The Navy is active in research of autonomous, unmanned, highly efficient, high speed underwater craft. Small, low cost AUVs capable of swarm control are of special interest for military mine applications. No matter the nature of the application or class of craft, a common challenge is the accuracy of maneuvering predic- tions. Maneuvering predictions not only affect design, but also the real time understanding of mission capabilities and endurance. Thus the proliferation of AUVs in recent times for commercial and defense applications have led to the need of higher fidelity of physics based lumped parameter models. The sensor data, along with maneuvering model data can tie into a more accurate trajectory. Multiple such incremental advances in the literature for prediction of maneuvering shall lead to a more accuracy. This work hopes to bridge some important gaps that ensure the creation of such a non-linear LPM to predict the maneuver- ing characteristics of an AUV using non linear hydrodynamic derivatives obtained through static and dynamic CFD. This model shall be implemented for the craft designed for DIVE technologies, our industrial sponsor and an in-house craft, the 690. This model shall also be made generalized for most submerged craft with a torpedo or slender hull form, with cruciform or X configuration of fins. This dissertation looks to provide the framework to identify CFD informed high fidelity dynamic model for AUVs. The model thus created shall be spe- cialized to account for specific important effects such as flow interaction among appendages, effect of using steady and unsteady maneuvers as CFD information and kinematic charac- teristics of captive maneuvers. The specific, innovative contributions in this dissertation are listed below: 1. Definition of a new stability index to incorporate effects of gravity at low-moderate speeds 2. Novel method for identification of hydrodynamic derivatives 3. Systematic and comprehensive study on the parameters affecting VPMM
- Computationally-effective Modeling of Far-field Underwater Explosion for Early-stage Surface Ship DesignLu, Zhaokuan (Virginia Tech, 2020-03-23)The vulnerability of a ship to the impact of underwater explosions (UNDEX) and how to incorporate this factor into early-stage ship design is an important aspect in the ship survivability study. In this dissertation, attention is focused on the cost-efficient simulation of the ship response to a far-field UNDEX which involves fluid shock waves, cavitation, and fluid-structural interaction. Traditional fluid numerical simulation approaches using the Finite Element Method to track wave propagation and cavitation requires a high-level of mesh refinement to prevent numerical dispersion from discontinuities. Computation also becomes quite expensive for full ship-related problems due to the large fluid domain necessary to envelop the ship. The burden is aggravated by the need to generate a fluid mesh around the irregular ship hull geometry, which typically requires significant manual intervention. To accelerate the design process and enable the consideration of far-field UNDEX vulnerability, several contributions are made in this dissertation to make the simulation more efficient. First, a Cavitating Acoustic Spectral Element approach which has shown computational advantages in UNDEX problems, but not systematically assessed in total ship application, is used to model the fluid. The use of spectral elements shows greater structural response accuracy and lower computational cost than the traditional FEM. Second, a novel fully automatic all-hexahedral mesh generation scheme is applied to generate the fluid mesh. Along with the spectral element, the all-hex mesh shows greater accuracy than the all-tetrahedral finite element mesh which is typically used. This new meshing approach significantly saves time for mesh generation and allows the spectral element, which is confined to the hexahedral element, to be applied in practical ship problems. A further contribution of this dissertation is the development of a surrogate non-numerical approach to predict structural peak responses based on the shock factor concept. The regression analysis reveals a reasonably strong linear relationship between the structural peak response and the shock factor. The shock factor can be conveniently employed in the design aspects where the peak response is sufficient, using much less computational resources than numerical solvers.
- Development of Diagnostic Tools for Use in a Gas Turbine Engine Undergoing Solid Particulate IngestionOlshefski, Kristopher Thomas (Virginia Tech, 2023-05-30)Aircraft propulsion systems can be exposed to a variety of solid particulates while operating in either arid or other hazardous environments. For conventional takeoff and landing aircraft, debris can be ingested directly into the gas turbine powerplant which is exposed to the ambient environment. For helicopters and other vertical takeoff and landing (VTOL) aircraft, rotor down wash presents a particular threat during takeoff and landing operations as significant amounts of groundlevel particles can be entrained in the surrounding air and subsequently ingested into the engine. Prolonged exposure to particle ingestion events leads to premature engine wear and, in extreme cases, rapid engine failure. Expanding our current understanding of these events is the first step to enabling engine manufacturers to mitigate these damage mechanisms through novel engine designs. The work described in this dissertation is aimed at increasing the scientific understanding of these ingestion events through the development of two distinct diagnostic instruments. First, an anisokinetic particle sampling probe is designed to be used for in-situ particle sampling inside of a gas turbine engine compressor. Offtake of particles during engine operation in dusty conditions will provide researchers with an improved understanding of particle breakage tendency and component erosion susceptibility. Both experimental and numerical investigations of the probe present a comprehensive realization of probe performance characteristics. Secondly, a novel particle visualization technique is developed to provide users with particle distribution and particle mass flow estimates at the inlet of a gas turbine engine. This technique yields both time-resolved and time-averaged quantities, allowing users to have a comprehensive account of particles entering the engine.
- Experimental and Numerical Investigation of Forward and Aft Swept Stepped Planing Hulls in Calm Water and Regular WavesHusser, Nicholas Alexander (Virginia Tech, 2023-02-22)Stepped hull forms are hulls with a vertical step in the hull bottom to improve performance at top speed. Stepped hulls are well documented anecdotally and scientifically to improve calm water performance at high speeds, but commonly demonstrate dangerous and unexpected dynamic instabilities during initial trials. These hulls also operate practically in waves, but their performance characteristics in waves are not well understood and rarely evaluated prior to full scale trials. To expand the scientific understanding of stepped hull performance, a systematic set of experiments and Reynolds Averaged Navier Stokes (RANS) computational fluid dynamics (CFD) simulations are used to evaluate the calm water performance, dynamic stability, and regular head wave response of two stepped hull models. Calm water experiments on two stepped hull models at varying displacement, longitudinal center of gravity location and forward speed offer data which can be used in the design to interpolate hull performance throughout expected operating conditions. CFD simulations in calm water are validated using the experimental results and numerical modeling approaches for stepped hull simulations are recommended. The calm water dynamic stability of both stepped hulls is investigated experimentally and numerically and procedures to evaluate the dynamic stability using both approaches are recommended. The performance of both stepped hulls in regular head waves is studied through experiments, which are used to validate CFD simulations of the hull in regular waves. System identification is used on five calm water CFD simulations to identify a reduced order model for the prediction of stepped hull response in waves.
- An Experimental Investigation into the Passive Reconfiguration of Flexible Plates Near a Free SurfaceScianna, Nicholas Alexander (Virginia Tech, 2022-05-26)Reconfiguration refers to the ability of a flexible structure to change its shape, allowing it to reduce its area perpendicular to the flow, to reduce drag. Decreasing the flexural rigidity of human-made structures can lead to improved designs that operate at higher propulsive efficiencies. The work presented in this thesis examines the physics surrounding a flexible plate under prescribed oscillatory heaving motions. White light movies were recorded at constant frequency and varying proximity to the free surface to investigate the change in reconfiguration as the plate approaches the free surface. Results, analyzed in terms of deformed plate shape, deflection, and plate tip kinematics, found that free surface effects increase the deflection of the plate as the plate approaches the free surface. Expanding on the initial experiments, a variety of frequencies were tested. The results show that each heaving frequency has a different critical height to the free surface in which deep water behavior is distinguished from shallow water behavior. At the critical depth, the plate deflection becomes asymmetric due to free surface effects. The second stage of experiments focused on measuring the fluid loading and fluid flow surrounding the flexible plate. The fluid loading, or drag force, acting on the plate was estimated by using a strain gauge load cell. Results of these experiments found that the drag force is equivalent on plates with lower heaving frequencies when compared to the highest heaving frequency tested due to increased reconfiguration at the higher frequency. The fluid moved from the keel to the edge of the plate as seen in the particle image velocimetry experiments. Higher heaving frequencies created faster fluid flow off the plate and stronger tip vortices being shed from the plate. When the flexible plate operated at large distances from the free surface, the fluid dynamics showed the same behavior for the upstroke and downstroke of the plate. Whereas, when the plate operated close to the free surface, a vortex only forms on the upstroke, leading to asymmetric loading and deformations.
- An Experimental Investigation on the Performance of a Shape Changing, Bio-inspired F2MC PanelJohansson, Oscar (Virginia Tech, 2024-05-23)The purpose of this thesis is to explore the performance of a bio-inspired plate undergoing oscillatory heave motions and active shape change. The shape change will be achieved using a panel embedded with Fluidic Flexible Matrix Composite (F2MC) tubes for actuation. A beam, or plate strip, model is presented as a means of verifying that F2MC tubes can effectively serve as a means of actuation. This model was actuated in air and water at several internal tube pressures. The static experimental deflections were compared to two beam models relying on Euler-Bernoulli and Timoshenko beam theories with concentrated tip moments and a distributed moment. It was found that the Euler-Bernoulli model with a concentrated tip moment best approximated the static experimental deflections. Following the success of the plate strip, and panel with 10 embedded F2MC tubes was manufactured. The plate panel was constructed with Dragon Skin Silicone and embedded with two rows of five F2MC tubes which provide the means of shape actuation. Experimental results from actuating the panel in static conditions showed that F2MC tubes are an effective means of prescribing a repeatable shape change to a silicone panel. Then, Classical Plate Theory and First-Order Shear Deformation Plate Theory were used with a concentrated tip moment at the free edge to provide a means of modeling the full panel. When comparing the static experimental results to the numerical models, it was found that the deflected plate shape could be most accurately predicted at lower pressures for upward deflection and higher pressures for downward deflections. When tested in unsteady conditions in a heaving experiment (0.5 Hz to 2.3 Hz), the force measured at frequencies above 1.5 Hz were up to 3.6 times greater than those measured for frequencies below 1.5 Hz. Additionally, the phase difference between the tip deflection and force with respect to the keel position decreased for force as frequency increased, while the opposite was true for the tip deflection. At 1.5 Hz, the tip deflection and force were equally out of phase with the keel. When the panel was subjected to an oscillatory heaving motion while asymmetrically actuated, it was found that faster heaving frequencies resulted in higher maximum force values for all actuation pressures, actuation directions, and depths below the free surface. However, when subjected to dual actuation by pressurizing the top and bottom tubes at the same pressure, the tip amplitude was highly dependent on specific combinations of heaving frequency, actuation pressure, and depth below the free surface. This indicates that the actuation pressure must be tuned to the depth and frequency of operation to obtain the desired tip amplitude for a given application. These findings further the knowledge of shape-changing F2MC panels operating near a free surface and lay a groundwork for developing flapping propulsors that mimic marine animals.
- An Experimental Study of the Fluid- Structure Interactions of Water Entry of Compliant StructuresJavaherian Hamedani, Mohammad Javad (Virginia Tech, 2021-09-03)Water entry of compliant structures is a major area of interest within different fields of engineering. In the case of highly flexible panels, an application for this topic is on drag reduction due to shape reconfiguration of panels near the free surface to support further development of undulatory propulsors. Moreover, it has been an important concept in the study of the slamming of small high-speed craft with flexible bottom structures, such as those made of composites. In this work, this fluid-structure interaction problem is experimentally investigated in different stages. In Stage I, free-falling water entry experiments are conducted on wedges that have bottom panels with different flexural rigidities. Kinematics, hydrodynamics, spray root propagation, and structural response of the model are measured during the experiments. Results are interpreted to evaluate the effect of flexural rigidity on the slamming characteristics. The comparison between the rigid and flexible wedges shows that the evolution of the spray root on a flexible wedge is influenced due to fluid-structure interaction. In Stage II, a hybrid approach is proposed that incorporates spray root measurements with the existing analytical models in order to estimate the hydrodynamic loads in water entry of wedges with different boundary conditions. The validity of this approach is evaluated using a case study of a flexible wedge drop experiment. The results of this analysis show that the proposed approach can reasonably predict the wedge kinematics and hydrodynamic pressure due to impact. Future components of this study will further develop this tool to be used for highly-flexible structures, where it is not easy to install traditional pressure sensors. Stage III of this work is on analysis of a tow-tank test of a rigid composite planing-hull model performed at the U.S. Naval Academy. Experiments conducted in regular waves were examined in terms of their kinematics and pressure loads. The goal of this analysis is to begin planning of the towing-tank tests that will be conducted at the VT Advanced Towing Tank Facility. These future VT experiments will combine the flexible composite panel with the hull form and motions, which are analyzed in the tow-tank study to investigate the fluid-structure interaction in the slamming of a flexible-planing hull. In stage IV, The findings of experimental investigations on wedge water entry are utilized in a 2D+t method to predict the hydrodynamics and motions of a prismatic planing craft. In this approach, the hydrodynamic loading on each V-type section of the vessel is calculated employing wedge water entry experiments (Stage I) and existing theoretical models (Stage II). A modified strip theory, also known as 2D+t, is then implemented to use these data and solve for the hydrodynamics and motion of the high-speed craft in calm water. Results show a good agreement with that of Savitsky prediction method and existing towing tank measurements.
- A Hybrid Framework of CFD Numerical Methods and its Application to the Simulation of Underwater ExplosionsSi, Nan (Virginia Tech, 2022-02-08)Underwater explosions (UNDEX) and a ship's vulnerability to them are problems of interest in early-stage ship design. A series of events occur sequentially in an UNDEX scenario in both the fluid and structural domains and these events happen over a wide range of time and spatial scales. Because of the complexity of the physics involved, it is a common practice to separate the description of UNDEX into early-time and late-time, and far-field and near-field. The research described in this dissertation is focused on the simulation of near-field and early-time UNDEX. It assembles a hybrid framework of algorithms to provide results while maintaining computational efficiency. These algorithms include Runge-Kutta, Discontinuous Galerkin, Level Set, Direct Ghost Fluid and Embedded Boundary methods. Computational fluid dynamics (CFD) solvers are developed using this framework of algorithms to demonstrate the computational methods and their ability to effectively and efficiently solve UNDEX problems. Contributions, made in the process of satisfying the objective of this research include: the derivation of eigenvectors of flux Jacobians and their application to the implementation of the slope limiter in the fluid discretization; the three-dimensional extension of Direct Ghost Fluid Method and its application to the multi-fluid treatment in UNDEX flows; the enforcement of an improved non-reflecting boundary condition and its application to UNDEX simulations; and an improvement to the projection-based embedded boundary method and its application to fluid-structure interaction simulations of UNDEX problems.
- Hydroelasticity of High-Speed Planing Craft Subject to Slamming Events: An Experimental and Numerical Investigation of Wedge Water EntryRen, Zhongshu (Virginia Tech, 2020-08-27)High-speed planing craft operating in waves are subject to frequent water impact, or slamming, as a portion or whole of the craft exits the water and re-enters at high velocity. The global load induced by slamming can cause fatigue-related damages to structures. The local slamming can cause local damage to structures and its induced acceleration can cause damage to equipment and personnel aboard. Therefore the slamming loads in high-speed craft are critical design loads. Nowadays, due to the increasing use of composite materials in high-speed craft, the interaction between the hydrodynamic loading and structural response, or hydroelasticity, must be considered. In this work, a flexible V-shaped wedge, which vertically enters the calm water with an impact velocity, was examined experimentally and numerically to characterize the slamming of a representative cross-section of high-speed craft. Physical quantities of interest include rigid-body kinematic motions, spray root propagation, hydrodynamic loading, and structural response. In the experimental work, with varied impact velocity and flexural rigidity of the wedge bottom plate, a wide range of hydroelasticity factors were investigated. The intersection between the bottom plate and side plate is called chine. The phases before and after the spray root reached the chine are called chine-unwetted and chine-wetted phase, respectively. It was found that the maximum deflection and strain occur in the chine-unwetted phase while a structural vibration with rapidly decaying magnitude is observed in the chine-wetted phase. Furthermore, the kinematic effect of hydroelasticity changes the spray root propagation and hence the pressure, while the inertial effect elongates the natural period of the plate. Inspired by the experimental work, a computational framework was proposed to focus on the chine-unwetted phase. Several hydroelastic models can be obtained from this framework. The hydroelastic models were validated to show reasonable agreement with experiments. Various parameters were studied through the computational framework. The hydroelasticity factor was modified to account for the mass and boundary conditions. It was found that the nondimensional rigid-body kinematic motions and maximum deflection showed little dependence on the hydroelasticity factor. Hydroelastic effects increased the time it takes for the peak maximum deflection to be reached for small values of the hydroelasticity factor. Hydroelastic effects also have little influence on the magnitude of the maximum deflection. These discoveries further the understanding of hydroelastic slamming and show the potential to guide the structural optimization and design of high-speed craft.
- Induction Infrared Thermography for Non-Destructive Evaluation of Alloy SensitizationRoberts, Matthew Thomas (Virginia Tech, 2019-06-26)The sensitization of stainless steel describes the process by which a high-carbon steel alloy is heated above a certain threshold (either naturally or artificially) followed by a cooling period during which chromium (one of the elements most responsible for providing stainless steel with its corrosion-inhibiting properties) forms new compounds with the carbon present in the steel. With the chromium being taken from the parent material to form these compounds, the corrosion-resistant properties are compromised, which can lead to corrosion, cracking, and broader failure. Currently, the accepted techniques used to test for the presence of sensitization are qualitative and/or destructive in nature. Attempts have been made to non-destructively detect and characterize sensitization through various means, but all with mixed results. With the use of these high-carbon alloys in a range of industries, a comprehensive, in-place process is desirable. This thesis will focus specifically on non-destructive evaluation of sensitization seen as a result of welding steel plates using induction infrared thermography (IIRT). This process uses an induction coil to generate heat within a sample whose resulting heat signature can then be detected with an infrared (IR) camera and analyzed. Previous IIRT experimental results have shown higher levels of heating in the HAZ when sensitization is present as it modifies the original microstructure of the material. New IIRT experiments have been conducted on both welded and unwelded 440C alloy samples to establish quantitative data on the heating profiles. These results (in conjunction with the appropriate experimental parameters) were then used to create a numerical model to replicate them. Despite some limitations in populating the model with accurate parameters, the results obtained were in good agreement with the experiments and provide a foundation for future work. Future work will focus on establishing a predictive tool that can detect and quantify the level of sensitization in an arbitrary steel sample in the field.
- An Investigation of Phase Change Material (PCM)-Based Ocean Thermal Energy HarvestingWang, Guangyao (Virginia Tech, 2019-06-10)Phase change material (PCM)-based ocean thermal energy harvesting is a relatively new method, which extracts the thermal energy from the temperature gradient in the ocean thermocline. Its basic idea is to utilize the temperature variation along the ocean water depth to cyclically freeze and melt a specific kind of PCM. The volume expansion, which happens in the melting process, is used to do useful work (e.g., drive a turbine generator), thereby converting a fraction of the absorbed thermal energy into mechanical energy or electrical energy. Compared to other ocean energy technologies (e.g., wave energy converters, tidal current turbines, and ocean thermal energy conversion), the proposed PCM-based approach can be easily implemented at a small scale with a relatively simple structural system, which makes it a promising method to extend the range and service life of battery-powered devices, e.g, autonomous underwater vehicles (AUVs). This dissertation presents a combined theoretical and experimental study of the PCM-based ocean thermal energy harvesting approach, which aims at demonstrating the feasibility of the proposed approach and investigating possible methods to improve the overall performance of prototypical systems. First, a solid/liquid phase change thermodynamic model is developed, based on which a specific upperbound of the thermal efficiency is derived for the PCM-based approach. Next, a prototypical PCM-based ocean thermal energy harvesting system is designed, fabricated, and tested. To predict the performance of specific systems, a thermo-mechanical model, which couples the thermodynamic behaviors of the fluid materials and the elastic behavior of the structural system, is developed and validated based on the comparison with the experimental measurement. For the purpose of design optimization, the validated thermo-mechanical model is employed to conduct a parametric study. Based on the results of the parametric study, a new scalable and portable PCM-based ocean thermal energy harvesting system is developed and tested. In addition, the thermo-mechanical model is modified to account for the design changes. However, a combined analysis of the results from both the prototypical system and the model reveals that achieving a good performance requires maintaining a high internal pressure, which will complicate the structural design. To mitigate this issue, the idea of using a hydraulic accumulator to regulate the internal pressure is proposed, and experimentally and theoretically examined. Finally, a spatial-varying Robin transmission condition for fluid-structure coupled problems with strong added-mass effect is proposed and investigated using fluid structure interaction (FSI) model problems. This can be a potential method for the future research on the fluid-structure coupled numerical analysis of AUVs, which are integrated with and powered by the PCM-based thermal energy harvesting devices.
- Multiphase Fluid-Material Interaction: Efficient Solution Algorithms and Shock-Dominated ApplicationsMa, Wentao (Virginia Tech, 2023-09-05)This dissertation focuses on the development and application of numerical algorithms for solving compressible multiphase fluid-material interaction problems. The first part of this dissertation is motivated by the extraordinary shock-resisting ability of elastomer coating materials (e.g., polyurea) under explosive loading conditions. Their performance, however, highly depends on their dynamic interaction with the substrate (e.g., metal) and ambient fluid (e.g., air or liquid); and the detailed interaction process is still unclear. Therefore, to certify the application of these materials, a fluid-structure coupled computational framework is needed. The first part of this dissertation developes such a framework. In particualr, the hyper-viscoelastic constitutive relation of polyurea is incorporated into a high-fidelity computational framework which couples a finite volume compressible multiphase fluid dynamics solver and a nonlinear finite element structural dynamics solver. Within this framework, the fluid-structure and liquid-gas interfaces are tracked using embedded boundary and level set methods. Then, the developed computational framework is applied to study the behavior a bilayer coating–substrate (i.e., polyurea-aluminum) system under various loading conditions. The observed two-way coupling between the structure and the bubble generated in a near-field underwater explosion motivates the next part of this dissertation. The second part of this dissertation investigates the yielding and collapse of an underwater thin-walled aluminum cylinder in near-field explosions. As the explosion intensity varies by two orders of magnitude, three different modes of collapse are discovered, including one that appears counterintuitive (i.e., one lobe extending towards the explosive charge), yet has been observed in previous laboratory experiments. Because of the transition of modes, the time it takes for the structure to reach self-contact does not decrease monotonically as the explosion intensity increases. Detailed analysis of the bubble-structure interaction suggests that, in addition to the incident shock wave, the second pressure pulse resulting from the contraction of the explosion bubble also has a significant effect on the structure's collapse. The phase difference between the structural vibration and the bubble's expansion and contraction strongly influences the structure's mode of collapse. The third part focuses on the development of efficient solution algorithms for compressible multi-material flow simulations. In these simulations, an unresolved challenge is the computation of advective fluxes across material interfaces that separate drastically different thermodynamic states and relations. A popular class of methods in this regard is to locally construct bimaterial Riemann problems, and to apply their exact solutions in flux computation, such as the one used in the preceding parts of the dissertation. For general equations of state, however, finding the exact solution of a Riemann problem is expensive as it requires nested loops. Multiplied by the large number of Riemann problems constructed during a simulation, the computational cost often becomes prohibitive. This dissertation accelerates the solution of bimaterial Riemann problems without introducing approximations or offline precomputation tasks. The basic idea is to exploit some special properties of the Riemann problem equations, and to recycle previous solutions as much as possible. Following this idea, four acceleration methods are developed. The performance of these acceleration methods is assessed using four example problems that exhibit strong shock waves, large interface deformation, contact of multiple (>2) interfaces, and interaction between gases and condensed matters. For all the problems, the solution of bimaterial Riemann problems is accelerated by 37 to 87 times. As a result, the total cost of advective flux computation, which includes the exact Riemann problem solution at material interfaces and the numerical flux calculation over the entire computational domain, is accelerated by 18 to 81 times.
- Slamming of High Speed Craft: A Parametric Study of Severe CasesVan Erem, Robert John (Virginia Tech, 2024-05-29)High-speed planing craft slamming into waves can cause structural damage to the vessel as well as hinder or injure personnel onboard. As a result, it is one of the primary constraints that limit the operating envelope of high-speed surface vessels. The controlled motion experiments presented in this thesis were designed to be an intermediate step between vertical water entry tests of a wedge and a traditional tow tank experiment of a planning hullform in waves. This allowed a deeper study of the hydrodynamic loads that occur during slamming. A planing hull model was subjected to controlled motions in the vertical plane to replicate the types of slamming motions that a vessel may experience in the ocean. The slamming events investigated were chosen based on towing tank experiments previously conducted at the U.S. Naval Academy. Hydrodynamic forces were measured globally and also at particular locations near the bow. The vertical motions were programmed into a pair of linear actuators that were rigidly mounted to the towing carriage. The towing carriage prescribed the horizontal motion. Each actuator was independently controlled and capable of moving at 1.3 m/s and 15 m/s^2. Pressure sensors were used to measure the pressure time history at discrete points on the model. Force sensors mounted beneath the actuators were used to compute the overall slamming load and moments induced by the slam event. A combination of other sensors were used to verify the accuracy of the prescribed motion profile. The results suggested that total impact velocity is correlated with the load growth rate. In addition, the velocity normal to the keel was found to be most impactful on the magnitude of the peak force.
- Slamming of High-Speed Craft: A Machine Learning and Parametric Study of Slamming EventsShepheard, Mark William (Virginia Tech, 2022-05-27)Slamming loads are the critical structural design load for high-speed craft. In addition to damaging the hull structure, payload, and injuring personnel, slamming events can also significantly limit operating envelopes and decrease performance. To better characterize slamming events and the factors affecting their severity, a parametric study will be carried out in the Virginia Tech Hydroelasticity Lab. This thesis provides the groundwork for this longitudinal project through meticulous analysis of irregular wave tow tank experiments. Through the modification of machine learning techniques and taking inspiration from facial recognition algorithms, key parameters were identified to form an experimental matrix which captures intricacies of the complex interdependent relation of variables in the slamming problem. The independent effects of parameters to be evaluated include hull flexural rigidity, LCG location, heave and surge velocity, and impact trim, angular velocity and acceleration. In preparation for this parametric study, an innovative experimental setup was designed to simulate the impact of a deep-vee planing hull into waves, through a controlled motion slam into calm water. To provide a baseline to compare data from future controlled motion experiments to, a model drop experiment was completed to characterize the relationships of impact velocity and trim to slamming event severity. During this experiment, the position, acceleration, strain, and pressure were measured. These measurements illustrated a decrease in peak acceleration, pressure, and strain magnitude with an increase in impact trim. Additionally, as trim was increased a delay in the time of peak magnitude for all measurements was observed. These results are attributed to the change in buoyancy with the change in impact angle. At non-zero angles of trim, a pitching moment was generated by the misalignment of the longitudinal center of buoyancy and center of gravity. This moment caused racking in the setup which was observed in the acceleration time histories immediately after impact. This finding furthers the need to investigate the angular velocity and acceleration of the model at impact, through the proposed series of experiments, as they are crucial naturally occurring motions inherent to slamming events.
- Structure and Turbulence of the Three-Dimensional Boundary Layer Flow over a HillDuetsch-Patel, Julie Elizabeth (Virginia Tech, 2023-01-31)Three-dimensional (3D) turbulent boundary layers (TBLs) are ubiquitous in most engineering applications, but most turbulence models used to simulate these flows are built on two-dimensional turbulence theory, limiting the accuracy of simulation results. To improve the accuracy of turbulence modeling capabilities, a better understanding of 3DTBL physics is required. This dissertation outlines the experimental investigation of the attached 3D TBL flow over the Benchmark Validation Experiments for RANS/LES Investigations (BeVERLI) Hill using laser Doppler velocimetry in the Virginia Tech Stability Wind Tunnel. The mean flow and turbulence behavior of the boundary layer are studied and compared with turbulence theories to identify the validity of these assumptions in the BeVERLI Hill flow. It is shown that the pressure gradients and curvature of the hill have a significant effect on the turbulence behavior, including significant history effects at all stations due to the changing pressure gradient impact through the height of the boundary layer. Supplementing the experimental results with analysis from rapid distortion theory and simulations, it is shown that the stations lower on the hill are significantly affected by the non-linear history effects due to the varying upstream origins of the flow passing through those stations. Stations closer to the hill apex pass through a region of extremely strong favorable pressure gradient and hill constriction, resulting in behavior that matches qualitatively with the results from rapid distortion theory and provides insights into the physical mechanisms taking place in these regions of the flow. Despite the misalignment of the mean flow angle (γFGA) and turbulent shear stress angle (γSSA) throughout all of the profiles, the proposed 3D law of the wall of van den Berg (1975), which incorporates pressure gradient and inertial effects and relies on the assumption that γFGA=γSSA, is able to predict the flow behavior at more mildly non-equilibrium stations. This suggests that models that currently rely on assumptions founded on the two-dimensional law of the wall could be improved by incorporating van den Berg's model instead. The total shear stress distribution at selected stations on the BeVERLI Hill are all significantly reduced below equilibrium two-dimensional (2D) levels, indicating that turbulence models built on this assumptions will not be able to accurately simulate the 3D turbulence behavior.