Effect of ship motions on propeller-hull interaction

Ali, Md Ashim (2020) Effect of ship motions on propeller-hull interaction. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

It is important to consider the detailed hydrodynamics of the propeller, the associated flow conditions and the hull interaction with effective wake distribution for developing the best ship propulsion system design. Model experiments are conventionally used to assess propeller flow and performance but are subject to scale effect and difficulties in flow measurement. Computational Fluid Dynamics (CFD), using a viscous flow solver, provides an alternate way to simulate propeller flow conditions. Although simplified propeller flow models are currently available, this thesis extends the CFD approach to develop a more comprehensive model that includes detailed propeller geometry, ship motions and wave action. Numerical self-propulsion simulations with propeller action and ship motions are performed using a newly developed dynamic motion class based on the sliding mesh method in OpenFOAM. Validation and verification are carried out in a step by step manner. As a first step, a model incorporating bare hull resistance, wave elevation on the hull surface, free surface wave contour, and axial velocity distribution at several transverse planes for a fixed ship condition and dynamic ship condition was developed and validated using available experimental data, and the comparison between the simulations and the experiments showed good agreement. Propeller open water simulations with detailed propeller geometry and using the body-force method were developed and validated with experimental data in the second step. Because of the highly non-linear and rotational nature of the flow near to the propeller region, curvature correction was implemented to the standard k − ω SST turbulence model to improve the numerical predictions. Prior to the third stage self-propulsion simulation, which combined the stage 1 and 2 simulations, a numerical wave tank was developed and validated. Numerical self-propulsion simulations were performed for a ship at an even keel condition with constant propeller rotational speed. The numerical calculation of the skin friction correction, thrust coefficient, torque coefficient, and axial velocity distribution downstream of the propeller, confirmed good accuracy against the available experimental data. Then the developed dynamic motion class was extended to models with ship sinkage and trim. Comparison with available data showed a excellent agreement for all self-propulsion parameters. An expected increase in total resistance and decrease in propeller thrust was observed due to ship motion. As an alternative of detailed propeller geometry modelling, two body-force models were developed for self-propulsion simulation when detailed flow features are not required. The effective local velocity body-force model showed better numerical predictions than nominal average velocity body-force method. The self-propulsion simulation was performed in waves with forward speed using the developed dynamic motion class and it was able to incorporate the rotating propeller with large ship motions. The thrust generated by a propeller varied with encounter wave period and was reduced compared to the even keel condition.

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