<p> Air-cushion vehicles (ACV) are defined as surface vehicles that utilize air pressure for partial-or total support over the operational surface. An outline of the history of the five widely known ACV concepts and an analysis of the mode of operation of each is given, with their advantages and disadvantages.</p> <p> A sixth type, called the thrust-cushion vehicle (TCV), is a promising but unknown concept which, as yet, has not received much study or recognition. A preliminary theoretical analysis for design purposes is made, and the test results of a static model and a model running on a radial tether are given to establish a research and design basis for future work.</p> / Thesis / Master of Engineering (MEngr)

A system of ordinary differential equations (ODE) is constructed for an air cushion vehicle (ACV). The system is simplified to an equation for the balance of the vertical forces and the equation for the adiabatic compression of the air in the cushion. Air pressure is constantly supplied into the system, but can leak out from underneath the edges of the cushion. A series of regular waves encounters the air cushion, causing a change in volume. Additionally, a computational analysis of the seakeeping of a Surface Effect Ship (SES) is performed using the commercial software WAMIT, which uses low-order, linear potential panel method. The model of the T-Craft consists of catamaran hulls, rigid end skirts, and the interface between the air cushion and the water surface. Beyond the six rigid body degrees of freedom of the T-Craft, additional modes are added for the motion of the interface panels. To verify the method used, the model is benchmarked using computational data for a small-scale barge model and experimental data for a T-Craft model. A comparison is performed for the T-Craft with and without its cushion. The solution for the nonlinear time-domain system is found numerically, and the stability of the system is studied by observing bifurcations with the incoming wave amplitude as the bifurcation parameter. The system experiences a period-doubling bifurcation, from a periodic orbit, to a subharmonic orbit, to a solution with multiple periods. Further increasing the wave amplitude increases the period doubling, eventually leading to chaotic behavior. As a result of the linear-potential simulations, significant differences are found in the seakeeping of the T-Craft when on and off the cushion. These differences are caused by the direct and indirect effects of the cushion (added aerodynamics and a decreased draft). The RAO's of the craft experience changes in amplitude and phase, which will affect the multi-body relative motions. The time-domain model shows very chaotic behaviour that is presented visually in a bifurcation diagram. These linear potential and time-domain methods illustrate the complexity and importance of modelling air-cushion effects.

Air cushion vehicle flexible seals operate in a complex and chaotic environment dominated by fluid-structure interaction. An efficient means to explore interdependencies between various governing parameters that affect performance is through high fidelity numerical simulation. As previous numerical efforts have employed separate iterative partitioned solvers, or have implemented simplified physics, the approaches have been complex, computationally expensive, or of limited utility. This research effort performs numerical simulations to verify and validate the commercial multi-physics tool STAR-CCM+ as a stand-alone partitioned approach for fluid-structure interaction problems with or without a free surface. A dimensional analysis is first conducted to identify potential non-dimensional forms of parameters related to seal resistance. Then, an implicit, Reynolds-averaged Navier-Stokes finite volume fluid solver is coupled to an implicit, nonlinear finite element structural solver to successfully replicate benchmark results for an elastic beam in unsteady laminar flow. To validate the implementation as a seal parameter exploratory tool, a planer bow seal model is developed and results are obtained for various cushion pressures and inflow speeds. Previous numerical and experimental results for deflection and resistance are compared, showing good agreement. An uncertainty analysis for inflow velocity reveals an inversely proportional resistance dependency. Using Abaqus/Explicit, methodologies are also developed for a two-way, loosely coupled explicit approach to large deformation fluid-structure interaction problems, with and without a free surface. Following numerous verification and validation problems, Abaqus is ultimately abandoned due to the inability to converge the fluid pressure field and achieve steady state. This work is a stepping stone for future researchers having interests in ACV seal design and other large deformation, fluid-structure interaction problems. By modeling all necessary physics within a verified and validated stand-alone approach, a designer's ability to comprehensively investigate seal geometries and interactions has never been more promising. / Ph. D. / Air cushion vehicles are specialized marine craft that utilize flexible seals to enable improved performance and fully amphibious operation. An efficient means to explore interdependencies between various seal design parameters that affect performance is through computer modeling of the fluid-structure interaction between the seal and the sea. This research effort performs numerical simulations to verify and validate the commercial multi-physics tool STAR-CCM+ as a single computer program for fluid-structure interaction problems occurring on the water surface. A dimensional analysis is first conducted to identify parameters related to seal resistance. Then, a fluid model is coupled to a structural model to successfully replicate benchmark results for a flexible beam in an oscillating fluid flow. To validate the implementation as a seal parameter exploratory tool, a model of an ACV bow seal is developed and results are obtained for various operational conditions and inflow speeds. Previous numerical and experimental results for seal deflection and seal resistance are compared, showing good agreement. This work is a stepping stone for future researchers having interests in ACV seal design and other large deformation, fluid-structure interaction problems. By modeling all necessary physics within a verified and validated stand-alone computer program, a designer’s ability to comprehensively investigate seal geometries and interactions has never been more promising.

Thesis (M.S.)--Air Force Institute of Technology, 2005. / "September 2005" Also available as a PDF file on the Air Force Institute of Technlogy website.