An investigation of the hydrodynamic forces on a jointed oscillating eel-like structure

Abstract

This study is an investigation of the forces developed by a jointed oscillating structure resembling a mechanical eel. The structure consists of rigid segments that oscillate from side to side to facilitate self-propulsion; this study aims to develop a simple numerical method that can be used for the engineering design of such a structure. The project was undertaken as a natural first step towards the design and construction of autonomous underwater vehicles (AUV) based on this concept.Published literature on fishlike propulsion and its mechanical implementation is investigated before a brief examination of the flow field surrounding such a structure is presented. Simple numerical methods, which try to predict the forces from this type of structure are then evaluated before the numerical implementation of some of them are presented.To evaluate the various numeric methods a physical structure was built and tested in a towing tank. The design of this structure is presented along with the physical measurements from it. The structure had two moving segments and a head segment. The structure was tested in three different scenarios:o The two segments moving as one with no forward speedo The two segments moving independently without forward speedo The two segments moving independently with forward speedAs all the numerical methods depend on quasi-empirical force coefficients, theirsolutions cannot be directly compared to the measured values. The force measurements were therefore used to compute these force coefficients. The consistency of these force coefficients can then be seen as a validation of the numerical method. The method that was found to predict the forces best was the Morison Equation with Keulegan-Carpenter number dependent force coefficients (Graham 1980).With the best-fit force coefficients, the Morison Equation was found to be able to predict the thrust from the scenario when the two segments moved together to within 15 % of the measured value. However, in the second scenario with the two segments moving independently, the generated thrust was substantially over-predicted, in some cases by nearly 200 %. The self-propulsion speed was however only over-predicted by about 30 % in the worst cases and in most cases by significantly less.The Morison Equation with Keulegan-Carpenter number dependent force coefficients can therefore be seen to predict the average forces acting on a single segment oscillating structure well. It can also be seen to predict the self-propulsion speed of a two segment oscillating structure quite well and can thus be used for design purposes. This study however shows that the time history of the forces developed by such a structure is not well predicted by the Morison Equation and neither is the thrust developed in the bollard-pull condition for a two-segment structure