Wing-in-ground effect oscillating foils for marine propulsion

Development of propulsion systems for both marine vehicles and aircrafts experienced a magnificent evolution in the past and will not slow the pace in the future. Remarkable performance with respect to thrust generation and maneuverability from the natural world using high-aspect-ratio appendages had inspired human beings to pursue more novel techniques for propulsion community. Owing to superior advantages in various aspects, the oscillating foil propulsor with optimized geometry and locomotion is considered as an alternative technology to conventional screw propellers. The configuration of interest in the current study is the auto-pitch wing-in-ground effect oscillating foil propulsors (APWIGs) which is characterized as a combination of biplane arrangement and flow-induced pitching motion. The biplane configuration with imposed counter-phase heaving motion can produce a beneficial dynamic wing-in-ground (WIG) effect. The lumped flexibility modelled by torsional spring attached to pitching axis is capable of improving the surrounding flow and simplifying the control mechanism. Based on the established and well-validated unsteady Reynolds-Averaged Navier-Stokes (URANS) model, a numerical study was undertaken to quantitatively evaluate and systematically analyze the propulsive characteristics of current concerned oscillating foil propulsor. The initial focus was the effect of individual parameters on the hydrodynamic behaviors of APWIGs. The torsional spring stiffness is a dominant parameter that determines both dynamic responses and propulsive performance. Hence, the hydro-elasticity characteristics of APWIGs with the variation of torsional flexibility was firstly investigated. It was found that there exists an optimum spring stiffness for APWIGs that produces the highest propulsive efficiency under certain conditions. As an indication of non-dimensional spring stiffness, the frequency ratio was employed to distinguish the locomotion states and depict the performance metrics. Simulations showed that the forward-motion regime and stable locomotion state with periodic dynamical behaviors can only be obtained by a specific range of frequency ratio. Especially, the optimal propulsive efficiency is achieved at a frequency ratio in the range of 1.1 to 1.4. In addition, a parametric study with the consideration of various kinematic and geometry parameters was conducted. It was noted that the oscillating frequency highly affects the vortex shedding along both leading and trailing edges of each foil. The parameters including heaving amplitude, equilibrium distance between two foils and position of pitching axis have a strong effect on the pitch-leading phase difference and maximum pitching angle. Based on an appropriate parameter combination, the APWIGs can produce a high efficiency of over 70% within the considered parametric space. A comparative study between the current configuration of interest and the widely examined oscillating foil systems was performed to demonstrate the advantages of APWIGs in both thrust production and efficiency enhancement. The contrasted propulsion systems consist of the single auto-pitch oscillating foil, biplane heave-only configuration and biplane fully-prescribed configuration. The maximum increase of propulsive efficiency of over 10% was obtained as a result of favorable WIG effect. It was revealed that the flow-adapted pitching motion can substantially reduce the flow separation, resulting in a significant improvement of propulsive performance. The efficiency of APWIGs was found to be independent of advance speed compared with a fully-prescribed configuration that is dramatically sensitive to the inflow velocity. To completely understand the physical mechanisms of oscillating foil propulsors, a series of three-dimensional simulations on the hydrodynamic characteristics of biplane configuration were carried out. The effect of aspect ratio (AR) on the propulsive performance and wake topology of WIG effect oscillating foil propulsors with fully-prescribed kinematics was firstly investigated by covering an adequately large range of AR from 1 to 10. Three-dimensional effect was found to be dominant for the flow behaviors of oscillating foils with an AR of below 2. Considering the compromise among multiple factors including efficiency deterioration, manufacturing cost, navigation and maneuvering of target vehicle, an AR range of 3-5 is recommended for the practical application of biplane oscillating foil propulsors. Furthermore, the preceding two-dimensional studies on the APWIGs were extended to the real three-dimensional flow. The hydro-elasticity characteristics of finite-span APWIGs was found to be analogical to two-dimensional predictions with an averaged efficiency loss of around 10% due to low-aspect-ratio effect. Numerical visualization of wake topologies showed that the flow pattern of finite-span oscillating foil is characterized as two sets of intertwined vortex rings, which largely differ from the wake structure of reverse K‚àö¬8rm‚àö¬8n vortex street in the two-dimensional flow. Based on the framework of comprehensive computations and analyses, a design procedure for APWIGs was proposed to guide parameter selection regarding both kinematic aspect and geometric consideration. In order to satisfy specific requirements of target vehicles, several approaches were developed to perform the parameter optimization that aims to maximize propulsive performance. A case study on the application of APWIGs to an autonomous underwater vehicle (AUV) was conducted to demonstrate detailed process of the design procedure. In general, the presented research in this thesis makes original contributions to the development of novel propulsors and the understanding of fundamental flow physics. The highlighted findings provide significant insights into hydrodynamics of oscillating foils with biplane arrangement and flow-adapted pitch. The outcomes of the current study are regarded as an essential reference for the design, optimization and prototyping of WIG effect propulsors and a considerable supplement to the engineering solutions of unconventional marine propulsion.