Evaluation of motion characteristics of ships and offshore structures at the early stage of design as well as during operation at the site is very important. Strip theory based programs and 3D panel method based programs are the most popular tools used in industry for vessel motion analysis. These programs use different variations of the Green’s function or Rankine sources to formulate the boundary element problem which solves the water wave radiation and diffraction problem in the frequency domain or the time domain. This study presents the development of a 3D frequency domain Green’s function method in infinite water depth for predicting hydrodynamic coefficients, wave induced forces and motions. The complete theory and its numerical implementation are discussed in detail. An in house application has been developed to verify the numerical implementation and facilitate further development of the program towards higher order methods, inclusion of forward speed effects, finite depth Green function, hydro elasticity, etc. The results were successfully compared and validated with analytical results where available and the industry standard computer program WAMIT v7.04 for simple structures such as floating hemisphere, cylinder and box barge as well as complex structures such as ship, spar and a tension leg platform.

University of Macau / Faculty of Science and Technology / Department of Electromechanical Engineering

Plumes are formed when a continuous buoyant forcing is supplied at a localized source. Buoyancy can be created by either a heat flux, a compositional difference between the fluid coming from the source and its surroundings, or a combination of both. In this thesis, two types of laminar plumes with different forcing mechanisms were investigated: forced plumes and autocatalytic plumes. The forced plumes were compositionally buoyant and were injected with inertial forcing into a fluid filled tank. The autocatalytic plumes were produced without mechanical forcing by buoyancy that was entirely the consequence of a nonlinear chemical reaction -- the iodate-arsenous acid (IAA) reaction. This reaction propagates as a reacting front and produces buoyancy by its exothermicity, and by the compositional difference between the reactant and product. Both the forced and autocatalytic plumes were examined in starting and steady states. The starting, or transient, state of the plume occurs when it initially rises through a fluid and develops a plume head on top of a trailing conduit. The steady state emerges after the plume head has risen to the top of a fluid filled tank leaving only a persistent conduit. Plume behaviour was studied through experimentation, simulation, and by using simple theoretical analysis. We performed the first ever study of plumes as they crossed over the transition between buoyancy-driven to momentum-driven flow. Regardless of the driving mechanism, forced plumes were found to exhibit a single power law relationship that explains their ascent velocity. However, the morphology of the plume heads was found to depend on the dominating driving mechanism. Confined heads were produced by buoyancy-driven plumes, and dispersed heads by momentum-driven plumes. Autocatalytic plumes were found to have rich dynamics that are a consequence of the interplay between fluid flow and chemical reaction. These plumes produced accelerating heads that detached from the conduit, forming free vortex rings. A second-generation head would then develop at the point of detachment. The detachment process for plumes was sensitively dependent on small fluctuations in their initial formation. In some cases, head detachment could occur multiple times for a single experimental run, thereby producing several generations of autocatalytic vortex rings. Head detachment was reproduced and studied using autocatalytic plume simulations. Autocatalytic flame balls, a phenomenon closely related to autocatalytic plumes, were also simulated. Flame balls were found to have three dynamical regimes. Below a critical radius, the smallest flame balls experienced front death. Above this radius, they formed elongating, reacting tails. The largest flame balls formed filamentary tails unable to sustain a reaction.

Hurricanes are one of the primary threats to the Texas coastal environment and economy. They generate large wave and storm surges that have caused much damage on the Texas coast in the past. Understanding both the hydrodynamic processes that damage coastal habitats and hurricane hazard and risk are critical to preserve coastal vegetation and quantify its benefits to coastal storm protection. The goal of this project is to quantify the impact of wave attenuation and wave refraction as well as the development of coherent structures in marsh fringes and the formation of a rip current system over wetlands on storm protection. The 3D Shallow Water Wave Basin at Texas A&M University hosted a series of large-scale experiments considering an idealized wetland model to pursue this goal. Study of the marsh geometry of the Texas coast was done in order to scale the experiments to the size of the Haynes Laboratory 3D-Water Wave Basin using a Froude and a Reynolds scalings. Particularly, averaged size and idealized shape of marsh segments in the area of Dalehite Cove in Galveston Bay were considered. Three sets of different wave conditions and water levels were tested to approximate different intensities of storm surge. Identical tests with both vegetated and non-vegetated marshes were run to compare the influence of the vegetation in storm conditions, and three different spacings between marsh segments were tested. In the basin, normally incident regular waves were generated at three water circulation structures. Data analysis allows the determination of the impact of discontinuous marsh segments on wave attenuation and wave refraction. Coherent structures such as rip current and the circulation pattern were analyzed to study the change in the flow field during passage of the waves. The experimental measurements were able to describe the wave transformations over the marsh segments. The influence of coastal wetlands was identified to affect the hydrodynamic process and reduce the total wave energy which is dissipated and redistributed by vegetation. The presence of the mounds induced an important decrease in the wave height, in addition to the damping of the waves by the vegetation stems. The variation in spatial coverage of the wetland model has been shown to highly affect the flow dynamics by generating offshore directed flow in the channel and onshore directed flow on the marsh mounds. This experimental approach provides a further understanding of flow dynamics by waves and surge in wetlands, at a large scale.

The effect of vegetation on the dissipation of waves is important in understanding the vegetation's role in protecting coastal communities during extreme events such as hurricanes and tsunamis. Numerical modeling makes it possible to study the flow through vegetation fields, but it is important to understand the flow dynamics around one piece of vegetation and validate the numerical model used, before the dynamics of an entire vegetated patch can be modeled and understood. This project validated GPUSPH, a Lagrangian mesh-free numerical model, by determining the optimal characteristics to obtain accurate simulations for flow through a flume with and without an obstruction. The validation of GPUSPH and determination of optimal characteristics was accomplished by varying model particle spacing, sub-particle scale (SPS) turbulence inclusion in the conservation of momentum equation, and kernel weighting function for two test cases. The model particle spacing sets the initial distance between the moving grid points, known as particles, in the system. The SPS turbulence term is intended to account for turbulence generated at the sub-particle scale between the particles. The kernel weighting functions used are the quadratic kernel and the cubic spline kernel. These kernels determine how much influence surrounding particles have on the flow characteristics of an individual particle. The numerical results of these tests were compared with experimental results to obtain conclusions about the accuracy of these simulations. Based on comparisons with experimental velocities and forces, the optimal particle spacing was found to occur when the number of particles was in the high 100,000s for single precision calculations, or mid-range capabilities, for the hardware used in this project. The sub-particle scale turbulence term was only necessary when there was large-scale turbulence in the system and created less accurate results when there was no large-scale turbulence present. There was no definitive conclusion regarding the best kernel weighting function because neither kernel had overall more accurate results than the other. Based on these conclusions, GPUSPH was shown to be a viable option for modeling free-surface flows for certain conditions concerning the particle spacing and the inclusion of the subparticle scale turbulence term.

A time-domain model was developed to predict the fluid/structure interaction of a three-dimensional deformable body in a fluid domain subject to long-crested finite amplitude waves. These nonlinear waves induce transient motion in the body. In turn, the interaction of the body with the waves modifies the wave field, causing additional motion in the body. A time-domain simulation was required to describe these nonlinear motions of the body and the wave field. An implicit three-dimensional time-domain boundary element model of the fluid domain was developed and then coupled iteratively with a finite element model of the deformable body. Large body hydrodynamics and ideal fluid flow are assumed and the diffraction/radiation problem solved. Either linear waves or finite amplitude waves can be treated in the model. Thus the full nonlinear kinematic and dynamic free surface boundary conditions are solved in an iterative fashion. To implicitly include time in the governing field equations, Volterra's method was used. The approach is similar to that of the typical boundary element method for a fluid domain where the boundary element integral is derived from the governing field equation. The difference is that in Volterra's method the boundary element integral is derived from the time derivative of the governing field equation. The transient membrane motions are treated by discretizing the spatial domain with curved isoparametric elements. Newton-Raphson iterations are used to account for the geometric nonlinearities and the equations of motion are solved using an implicit numerical method. Examples are included to demonstrate the validity of the boundary element model of the fluid domain. The conditions in a wave channel were numerically modeled and compared to sinusoidal waves. The interaction of a submerged rigid horizontal cylinder with water waves was modeled and results compared to experimental and numerical results. The capability of the model to predict the interaction of highly deformable bodies and water waves was tested by comparing the numerical model to large-scale physical model experiment of a membrane cylinder placed horizontally in a wave channel. / Graduation date: 1992

Brownian dynamics (BD) simulation method has been widely used for studying problems in dispersed systems, such as polymer solutions, colloidal suspensions and more recently, complex (dusty) plasmas. The main problem addressed with this simulation technique is that of time scale separation, which occurs when one form of motion in the system is much faster than the other. This can be a serious problem in Molecular dynamics (MD) simulation where very short time steps are needed to handle the fast motions and thus, requiring very long time runs for the proper evolution of slower modes making the simulation very expensive. More importantly, the fast motions may not be of much interest within themselves, as will be the case in a dusty plasma. The motion of neutral atoms or molecules comprising the plasma occurs at a very fast time scale with respect to the motion of dust particles, and is usually of very little interest, though a large number of such neutrals are present. In such cases, an approximate method is usually adopted, whereby the neutral particles are omitted from the simulation and their effect upon the dynamics of dust particles modeled by a combination of random forces and frictional terms. This leads to a recasting of the Newton's Equation of motion solved in MD, to a Langevin equation, solved in BD. Adopting this approach, we simulate a system of charged dust particles interacting via Yukawa potential in a 2-Dimensional layer, and extract relevant equilibrium statistical features such as the radial distribution function, static structure factor and the low frequency dust wave modes. We then propose the use of a Generalized Hydrodynamical (GH) approach to provide a semi-analytical model for the dust collective modes, which not only provides us with good predictions of the wave dispersion but also provides reasonable estimates for wave-number dependent wave damping, both of which will be compared against the results obtained from BD simulation. Finally, through our simulations, we also observe the equilibrium configuration of dust particles in the presence of cold ions streaming perpendicularly into the 2-Dimensional layer of dust particles. This provides us with novel results in the regime of sub-sonic ion flow speeds.

This dissertation describes a multidimensional relativistic hydrodynamic code which solves the special relativistic hydrodynamic equations as a hyperbolic system of conservation laws based on the total variation diminishing (TVD) scheme. Several standard tests and test simulations are presented to demonstrate the accuracy, robustness and flexibility of the code. Using this code we have studied three-dimensional hydrodynamic interactions of relativistic extragalactic jets with two-phase ambient media. The deflection angle of the jet is influenced more by the density contrast of the cloud than by the beam Mach number of the jet, and a relativistic jet with low relativistic beam Mach number can eventually be slightly bent after it crosses the dense cloud. Relativistic jet impacts on dense clouds do not necessarily destroy the clouds completely, and much of the cloud body can survive as a coherent blob due to the combination of the geometric influence of off-axis collisions and the lower rate of cloud fragmentation through the Kelvin-Helmholtz instability for relativistic flows. We find that relativistic jets interacting with clouds can produce synchrotron emission knots similar to structures observed in many VLBI-scale radio sources and the synchrotron emission peaks right before the jet passes through the cloud.

Brownian dynamics (BD) simulation method has been widely used for studying problems in dispersed systems, such as polymer solutions, colloidal suspensions and more recently, complex (dusty) plasmas. The main problem addressed with this simulation technique is that of time scale separation, which occurs when one form of motion in the system is much faster than the other. This can be a serious problem in Molecular dynamics (MD) simulation where very short time steps are needed to handle the fast motions and thus, requiring very long time runs for the proper evolution of slower modes making the simulation very expensive. More importantly, the fast motions may not be of much interest within themselves, as will be the case in a dusty plasma. The motion of neutral atoms or molecules comprising the plasma occurs at a very fast time scale with respect to the motion of dust particles, and is usually of very little interest, though a large number of such neutrals are present. In such cases, an approximate method is usually adopted, whereby the neutral particles are omitted from the simulation and their effect upon the dynamics of dust particles modeled by a combination of random forces and frictional terms. This leads to a recasting of the Newton's Equation of motion solved in MD, to a Langevin equation, solved in BD. Adopting this approach, we simulate a system of charged dust particles interacting via Yukawa potential in a 2-Dimensional layer, and extract relevant equilibrium statistical features such as the radial distribution function, static structure factor and the low frequency dust wave modes. We then propose the use of a Generalized Hydrodynamical (GH) approach to provide a semi-analytical model for the dust collective modes, which not only provides us with good predictions of the wave dispersion but also provides reasonable estimates for wave-number dependent wave damping, both of which will be compared against the results obtained from BD simulation. Finally, through our simulations, we also observe the equilibrium configuration of dust particles in the presence of cold ions streaming perpendicularly into the 2-Dimensional layer of dust particles. This provides us with novel results in the regime of sub-sonic ion flow speeds.

Fluidized bed dryers (FBDs) are used in the pharmaceutical industry to remove excess moisture from granule prior to tablet formation. As granule moisture content is reduced from its initial to final state, the velocity required to fully fluidize the granule decreases and the bed voidage decreases. The change in these fluidization properties are attributed to the decrease in the interparticle force load created by a reduction in liquid bridging as moisture is removed. During constant velocity drying, these fluidization properties result in a bubbling fluidization state, which evolves into a bubble coalescing regime as drying proceeds. This behaviour was identifiable using pressure fluctuation time-series analysis techniques.<p> Distributor design studies using dry and wet granule in a conical fluidized bed suggest that the punched plate design limits bubble coalescence when compared to the perforated plate and Dutch weave mesh designs. Furthermore, the Dutch weave results in extensive segregation, which is undesirable from a fluidization perspective. Local drying hydrodynamic measurements using x-ray densitometry found that the punched and perforated plates generate a centralized bubbling core region during drying with a defluidized bed periphery. This fluidized core region grows as drying proceeds until the defluidized region disappears. Under the same operating conditions, a porous plate distributor creates extensive channelling and defluidization across the entire bed cross-section during the constant rate period of drying. These poor fluidization characteristics are a result of the porous plate introducing the gas into the bed as a fine dispersion.<p> Lastly, the hydrodynamics associated with the conical vessel geometry improves the circulation and mixing patterns in fluidized bed dryers. This is especially the case in the entry region of the conical bed where the high inlet gas velocity prevents defluidization around the periphery of the bed. The straight walled geometry of the cylindrical bed resulted in defluidization in this area. As a result, the hydrodynamics associated with bubbling differ significantly between the geometries over the course of drying.