Flapping Foil Propulsion for Conventional Ships
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- Institutt for marin teknikk 
Feasibility of flapping foil propulsion was discussed with respect to design in the project thesis. The study showed that flapping foil propulsion is feasible for conventional ships, but the performance predictions were insufficient to give any hydrodynamical recommendations.The study described in this Master s thesis takes the performance predictions to the nextlevel. In the master s thesis, a two-dimensional panel code has been implemented from scratch bythe author. The panel code includes both vortex shedding, nonlinear wake deformation andquasi-steady effects due to pitch motion and time-dependent geometry. Three-dimensionaleffects, viscous drag and added mass are added separately by use of expressions adapted fromthe literature. The code has been evaluated quantitatively by linear theory, convergence tests and momentum change, and qualitatively by use of boundary condition tests. Performance of flapping foils with different flapping characteristics are studied. A number of explicit parameter studies are discussed in order to extract helpful knowledge about flapping propulsion. There is a remarkable correlation between optimal Strouhal number found by the panel code and the Strouhal number executed by real fish. The optimal Strouhal numberis approximately St = 0.3 for all cases, but the existence of such an optimal value is not fully understood yet. An average wake profile study has been performed in this thesis in order to find eventual preferences on Strouhal number with respect to momentum. The study showed that an optimal Strouhal number might exist from a momentum perspective, but the optimal Strouhal number predicted here was St = 1. A reduced frequency approach would howeverrecommend St = 0. The author suggests that St = 0.3 is a reasonable compromise betweenheave- and pitch-motion effects, in addition to viscous drag and 3D effects. A built-in particle swarm optimization toolbox in MATLAB has been applied for multi-parametric optimization. All the optimization cases give optimal Strouhal numbers approximately equal to St = 0.3. Several studies during the last century have dealt with flapping foil propulsion. This thesisdiffers from the others because it relates propulsive performance to the propulsion requirements of a specific ship. A medium sized general cargo ship has been considered as acase study with respect to thrust requirements. The foil dimensions are considered as feasible with respect to design implementation, i.e. the foils are within the length, breadth and draught of the ship when they are in resting position. The present theoretical study gives efficiencies in the order of 0.85-0.9 with optimized flapping motion. These results are questionable due to the limitations of a panel code. There is however reason to believe that efficiencies above 0.8 are realistic with flapping foils based on this thesis. That is a significant increase relative to standard propellers. Acceleration tests from 2 to 19 knots also favored flapping foils over propellers. Acceleration from 0 to 2 knots is considered as a highly viscous problem, and must be analyzed by more sophisticated methods in future studies. The overall conclusion is that flapping foil propulsion is feasible and can provide a significant increase in efficiency compared to propellers for conventional ships. It must therefore be considered as attractive from a hydrodynamical point of view.