Ananthakrishnan, Palaniswamy

Linear and non-linear hydrodynamic coefficients of single and multiple hulls are
obtained using the boundary-integral method. For linear frequency-domain analysis, the
boundary-integral method based on a simple source distribution (Yeung [50] was used.
The nonlinear time domain simulations were carried out using a boundary-integral
algorithm based on the mixed Eulerian-Lagrangian (MEL) formulation (Longuet-Higgins
and Cokelet (19] ). Also, linear time domain simulations were carried out by utilizing a
simplified mixed Eulerian-Lagrangian formulation and the steady-state results compared
with that obtained from linear-frequency domain analysis. Both 2D and 3D results were
obtained for a range of parameters such as beam/draft, hull-separation/beam ratios and
frequency and amplitude of hull motions. The results shed light on complex wave-body
interactions involved in multi-hull ships and identifY critical hydrodynamic and
geometric parameters affecting their sea keeping performance. The computational tools
developed and the findings thus contribute to design of multi-hull ships for improved atsea
performance.

This thesis presents two-dimensional hydrodynamic analysis of flapping foils for the propulsion of underwater vehicles using a source-vortex panel. Using a simulation program developed in MatLab, the hydrodynamic forces (such as the lift and the drag) as well as the propulsion thrust and efficiency are computed with this method. The assumptions made in the analysis are that the flow around a hydrofoil is two-dimensional, incompressible and inviscid. The analysis is first considered for the case of a deeply submerged hydrofoil followed by the case where it is located in shallow water depth or near the free surface. In the second case, the presence of the free surface and wave effects are taken into account, specifically at high and low frequencies and small and large amplitudes of flapping. The objective is to determine the thrust and efficiency of the flapping –foils under the influence of added effects of the free surface. Results show that the free-surface can significantly affect the foil performance by increasing the efficiency particularly at high Frequencies.

To determine the effect of body shape on the response of underwater vehicles to
surface waves in shallow water, the wave radiation hydrodynamic forces are evaluated
for a family of (i) prolate spheroidal hull forms and (ii) cylindrical bodies with
hemispherical nose and conical tail sections by systematically varying the geometric
parameters but keeping displacement constant. The added-mass and wave damping
coefficients are determined using a frequency-domain, simple-source based boundary
integral method. Results are obtained for a range of wave frequencies and depths of
vehicle submergence all for a fixed water depth of 10 m. With the wave exciting force
and moment determined using the Froude-Krylov theory, the response transfer functions
for heave and pitch are then determined. The heave and pitch response spectra in actual
littoral seas are then determined with the sea state modeled using TMA spectral relations.
Results show that vehicle slenderness is a key factor affecting the hydrodynamic coefficients and response. The results show two characteristics that increase the radiation
hydrodynamic forces corresponding to heave and pitch motions: namely, vehicle length
and further-away from mid-vehicle location of the body shoulder. The opposite is true for
the oscillatory surge motion. By utilizing these observed characteristics, one can design
the lines for maximum radiation forces and consequently minimum hull response for the
critical modes of rigid-body motion in given waters and vehicle missions. In the studies
carried out in the thesis, a hull with a long parallel middle body with hemispherical nose
and conical tail sections has better heave and pitch response characteristics compared
prolate spheroid geometry of same volume. The methodology developed herein, which
is computationally efficient, can be used to determine optimal hull geometry for minimal
passive vehicle response in a given sea.

The research presented in this thesis utilizes Blade Element Momentum (BEM) theory with a
dynamic wake model to customize the OrcaFlex numeric simulation platform in order to allow
modeling of moored Ocean Current Turbines (OCTs). This work merges the advanced cable modeling
tools available within OrcaFlex with well documented BEM rotor modeling approach creating a
combined tool that was not previously available for predicting the performance of moored ocean
current turbines. This tool allows ocean current turbine developers to predict and optimize the
performance of their devices and mooring systems before deploying these systems at sea. The BEM
rotor model was written in C++ to create a back-end tool that is fed continuously updated data on the
OCT’s orientation and velocities as the simulation is running. The custom designed code was written
specifically so that it could operate within the OrcaFlex environment. An approach for numerically
modeling the entire OCT system is presented, which accounts for the additional degree of freedom
(rotor rotational velocity) that is not accounted for in the OrcaFlex equations of motion. The properties
of the numerically modeled OCT were then set to match those of a previously numerically modeled
Southeast National Marine Renewable Energy Center (SNMREC) OCT system and comparisons were
made. Evaluated conditions include: uniform axial and off axis currents, as well as axial and off axis wave fields. For comparison purposes these conditions were applied to a geodetically fixed rotor, showing nearly identical results for the steady conditions but varied, in most cases still acceptable accuracy, for the wave environment. Finally, this entire moored OCT system was evaluated in a dynamic environment to help quantify the expected behavioral response of SNMREC’s turbine under uniform current.

The hydrodynamic interactions between ships in confined waters, restricted in width and depth, is analysed using a numerical surface singularity method called the Panel method. Based on potential flow and rigid free surface assumptions, a source distribution is utilized over the boundary surfaces to model the flow. The strength of the source distribution is obtained on satisfying the normal kinematic boundary conditions, i.e. as a solution of an integral equation. Discretization of the boundary surfaces yields a system of linear algebraic equations corresponding to the boundary integral equation, which then is solved numerically. From the singularity distribution, the velocity components, pressure coefficients and finally the interaction loads are calculated.

The response of single- and multi-module floating platforms to surface waves is investigated theoretically. Wave exciting forces are computed using methods based on the Morrison equation and Froude-Krylov hypothesis. The radiation forces are obtained from experimental results of Vugt and where possible diffraction forces using the Haskind reciprocity relation. Heave and pitch response of a one-module platform and hinge-connected two-module platform are determined by integrating the corresponding equations of rigid-body motion. A structural dynamic analysis is also carried out using the Green's function method to determine the elastic flexural response of the platform to waves. The results are compared with the experimental and numerical findings of others. The thesis contributes to a better understanding of rigid-body and elastic response of large ocean platforms subject to wave forces. The methodology is computationally less intensive and therefore can be effectively used for the design of platforms and the validation of numerical algorithms.

A three-dimensional nonlinear time-dependent boundary-integral algorithm is developed to compute wave forces on an underwater vehicle. The effect of viscosity is neglected and the cases for which the effects could be important are discussed. The present algorithm is however an efficient tool to determine wave forces on a submerged body and can also be integrated into a viscous flow algorithm. A numerical wave tank is constructed for the simulation. A damping layer is introduced to minimize spurious reflection of scattered waves at the open boundary. A sinusoidal progressive pressure patch is used to generate incident waves. Wave forces are determined using four different methods: viz., (1) Froude-Krylov volume integration method, (2) Froude-Krylov surface pressure integration method, (3) Linear diffraction analysis and (4) Nonlinear diffraction analysis for a range of parameters including incident wavelength and wave height. Results are compared to quantify effects of nonlinearity and diffraction effect of the body.

The inviscid hydrodynamic coefficients of an underwater vehicle (Ocean EXplorer), including the nonlinear effects of the wave surface, are computed using a boundary-integral method. A mixed Eulerian-Lagrangian formulation (Longuet-Higgins and Cokelet, 1976) is used for the treatment of nonlinear free-surface conditions. The algorithm is validated using the work-energy theorem (Yeung, 1982) and experimental data. Results, in the form of free-surface elevations and hydrodynamic coefficients, are obtained for a range of body geometries and maneuvers. The open-loop dynamics of underwater vehicles are then investigated by solving the 3DOF rigid-body equations of motion (OXY plane). The advantages and possible usage of the developed methodology for the design and control of underwater vehicles, as well as topics for further research, are addressed in the conclusion chapter of the thesis.

Boundary integral algorithms are developed to analyze three-dimensional inviscid fluid-body interactions, including the nonlinear free-surface effects. Hydrodynamic coefficients are computed for various body geometries, some corresponding to that of small underwater vehicles, in deep waters and near the free surface. The fully nonlinear unsteady wave-radiation problem corresponding to forced submerged-body oscillations and forward translation are solved using the mixed Eulerian-Lagrangian formulation (Longuet-Higgins and Cokelet, 1976). By implementing the leading-order free-surface conditions on the calm surface, linear time-domain solutions are also obtained. The nonlinear and linear results are compared to quantify the nonlinear free-surface effects. Linear frequency-domain analysis of the wave-body interactions is also carried out using a boundary-integral method based on the simple-source distribution (Yeung, 1974). The linear time-domain and the latter frequency-domain results are also compared for a validation of the algorithms.